WO2013093100A1 - Organic nitrates as ignition enhancers - Google Patents

Organic nitrates as ignition enhancers Download PDF

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Publication number
WO2013093100A1
WO2013093100A1 PCT/EP2012/076841 EP2012076841W WO2013093100A1 WO 2013093100 A1 WO2013093100 A1 WO 2013093100A1 EP 2012076841 W EP2012076841 W EP 2012076841W WO 2013093100 A1 WO2013093100 A1 WO 2013093100A1
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WO
WIPO (PCT)
Prior art keywords
nitrate
fuel composition
methyl
cetane number
diesel fuel
Prior art date
Application number
PCT/EP2012/076841
Other languages
French (fr)
Inventor
Matthias Eggenstein
Andrew David Horton
Renate UITZ
Widyai Prenish BANSIE
Original Assignee
Shell Internationale Research Maatschappij B.V.
Shell Oil Company
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Publication of WO2013093100A1 publication Critical patent/WO2013093100A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L10/00—Use of additives to fuels or fires for particular purposes
    • C10L10/12—Use of additives to fuels or fires for particular purposes for improving the cetane number
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L1/00—Liquid carbonaceous fuels
    • C10L1/10—Liquid carbonaceous fuels containing additives
    • C10L1/14—Organic compounds
    • C10L1/22—Organic compounds containing nitrogen
    • C10L1/23—Organic compounds containing nitrogen containing at least one nitrogen-to-oxygen bond, e.g. nitro-compounds, nitrates, nitrites
    • C10L1/231—Organic compounds containing nitrogen containing at least one nitrogen-to-oxygen bond, e.g. nitro-compounds, nitrates, nitrites nitro compounds; nitrates; nitrites
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L2270/00—Specifically adapted fuels
    • C10L2270/02—Specifically adapted fuels for internal combustion engines
    • C10L2270/026—Specifically adapted fuels for internal combustion engines for diesel engines, e.g. automobiles, stationary, marine
    • C—CHEMISTRY; METALLURGY
    • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
    • C10L—FUELS NOT OTHERWISE PROVIDED FOR; NATURAL GAS; SYNTHETIC NATURAL GAS OBTAINED BY PROCESSES NOT COVERED BY SUBCLASSES C10G, C10K; LIQUEFIED PETROLEUM GAS; ADDING MATERIALS TO FUELS OR FIRES TO REDUCE SMOKE OR UNDESIRABLE DEPOSITS OR TO FACILITATE SOOT REMOVAL; FIRELIGHTERS
    • C10L2300/00—Mixture of two or more additives covered by the same group of C10L1/00 - C10L1/308
    • C10L2300/20—Mixture of two components

Definitions

  • the present invention relates to a method of
  • composition to give improvements in fuel combustion and cetane number.
  • the cetane number of a fuel composition is a measure of its ease of ignition and combustion. With a lower cetane number fuel a compression ignition (diesel) engine tends to be more difficult to start and may run more noisily when cold; conversely a fuel of higher cetane number tends to impart easier cold starting, to lower engine noise, to alleviate white smoke ("cold smoke”) caused by incomplete combustion after.
  • cetane boost additives also known as cetane boost additives or cetane (number) improvers / enhancers, to ensure compliance with such specifications and generally to improve the combustion characteristics of the fuel.
  • Organic nitrates have been known for some time as ignition accelerants in fuels, and some are also known to increase the cetane number of diesel fuels.
  • Such organic nitrates generally include short- and medium-chain linear and branched alkanols and nitrates of cycloalkanols, such as those described in US4479905.
  • 2-ethylhexyl nitrate (2-EHN)
  • 2-EHN is also a radical initiator, and can potentially have an adverse effect on the thermal stability of a fuel. Poor thermal stability in turn results in an increase in the products of instability reactions, such as gums, lacquers and other insoluble species. These products can block engine filters and foul fuel injectors and valves, and consequently can result in loss of engine efficiency or emissions control.
  • WO2006/067234 relates to the use of fatty acid alkyl esters (FAAEs) in diesel fuels to increase the cetane number .
  • FAEs fatty acid alkyl esters
  • this invention aims to overcome or alleviate at least one of the problems associated with the prior art .
  • an organic nitrate in an diesel fuel composition for the purpose of reducing the ignition delay (ID) of the diesel fuel composition, wherein the organic nitrate is selected from the group consisting of:
  • each of R% to R 9 is independently selected from H or Ci-C 6 alkyl, or nitrate ( ⁇ ON0 2 ) , wherein optionally one of R4 and R5 forms an optionally substituted alkylene bridge with one of Rs and R 9 , which may be substituted by one or more Ci ⁇ C 6 alkyl, and/or nitrate ⁇ -ONO2) ; wherein at least one of Ri to R 9 is not H, and provided that no more than one R 2 to R9 comprises a nitrate group.
  • the organic nitrate has the effect of increasing the cetane number of fuel, such as to a desired or target cetane number.
  • One or more additional organic nitrates may be used.
  • the addition organic nitrates are also defined in accordance with the invention.
  • composition which method comprises adding to the
  • composition an amount of an organic nitrate according to the invention.
  • the method may involve increasing the cetane number of the diesel fuel composition to achieve a target cetane number.
  • the method may involve adding one or more additional organic nitrate to the fuel composition.
  • the method may further be for reducing the amount of 2 ⁇ ethylhexyl nitrate (2-EHN) or any other known cetane enhancer in the diesel fuel composition to achieve the target cetane number.
  • a further aspect of the invention is directed to a diesel fuel composition for use in a compression ignition engine, which comprises an organic nitrate in accordance with the invention.
  • the diesel fuel composition has a cetane number of 40 or more, 50 or more, 60 or more, or 70 or more.
  • the diesel fuel composition of this or any other aspect may comprise a biofuel, and optionally may
  • FAAEs such as FAMEs.
  • the organic nitrate may be present in the diesel fuel composition at a concentration of: (a) between
  • the organic nitrate is selected from: bornyl nitrate, fenchyl nitrate and menthly nitrate.
  • the uses and methods of the present invention may additionally or alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with cetane number, for example, to improve the combustion performance of the fuel composition, e.g. to shorten ignition delays (i.e. the time being fuel injection and ignition in a combustion chamber during use of the fuel) , to facilitate cold starting or to reduce incomplete combustion and/or associated emissions in a fuel-consuming system running on the fuel composition) and/or to improve fuel economy or exhaust emissions generally .
  • an organic nitrate in a diesel fuel composition for the purpose of improving the fuel economy of an engine into which the fuel composition is or is intended to be introduced, or of a vehicle powered by such an engine, wherein the organic nitrate is defined herein.
  • a method for the preparation of a diesel fuel composition having a target cetane number for use in a compression ignition engine comprising adding an organic nitrate, as defined elsewhere herein, to the diesel fuel composition; and blending the organic nitrate with the diesel fuel composition to provide a diesel fuel composition having the target cetane number.
  • Still yet another aspect of the invention relates to a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition as defined elsewhere herein, or as obtained by the uses and methods of the invention.
  • Figure 1 depicts the structures of representative organic nitrates of the invention
  • Figure 2 illustrates the reduction in ignition delay (ID) as a percentage for a diesel base fuel comprising organic nitrates of the invention.
  • the graphs show the data obtained at organic nitrate concentrations of 1.0% w/w (left, graphs A, C, E and G) and 0.1% w/w (right, graphs B, D, F and H) at representative combustion conditions (first row, condition a03, graphs A and B;
  • nitro-substituted methyl oleate (column 6) ; cholesterol nitrate (column 7); 1-octyl nitrate (column 8); 1- tetradecyl nitrate (column 9) ; 2-ethylhexyl nitrate
  • FIG. 3 illustrates the derived ignition quality (DIQ) for a diesel base fuel comprising exo-fenchol nitrate at concentrations of 0.1% w/w (left graph) and 1.0% w/w (right graph), measured under all test
  • DIQ derived ignition quality
  • Figure 4 is a graph illustrating correlations between the measured ignition delay at the various combustion conditions (aOl to all - see Key) against the organic nitrate used as a cetane enhancer in a diesel fuel composition according to the invention. The results are shown against molecular weight of the cetane
  • cetane (number) improver and “cetane (number) enhancer” are used interchangeably to encompass any component that, when added to a fuel composition at a suitable concentration, has the effect of increasing the cetane number of the fuel composition relative to its previous cetane number under one or more engine
  • cetane number ⁇ cetane number
  • a cetane number improver or enhancer may also be referred to as a cetane number increasing additive / agent or the like.
  • the cetane number of a fuel composition may be determined in any known manner, for instance using the standard test procedure ASTM D613 (ISO 5165, IP 41) which provides a so-called "measured" cetane number obtained under engine running conditions. More preferably the cetane number may be determined using the more recent and accurate
  • IQT ignition quality test
  • the methods / uses encompass adding a cetane enhancer of. the invention to a fuel composition so as to adjust the cetane number or to achieve or reach a desired target cetane number.
  • a target cetane number can also embrace exceeding that number.
  • the target cetane number may be a target minimum cetane numbe .
  • the present invention suitably results in a fuel composition which has a derived cetane number (IP 498) of 50 or greater, more preferably of 51, 52, 53, 54 or 55 or greater.
  • IP 498 derived cetane number
  • the resultant fuel composition may have a cetane number of 60 or greater, 65 or greater or even 70 or greater.
  • the present invention may additionally or
  • any property of the fuel composition which is equivalent to or associated with cetane number for example, to improve the combustion performance of the fuel composition, e.g. to shorten ignition delays ⁇ i.e. the time between fuel injection and ignition in a combustion chamber during use of the fuel), to facilitate cold starting or to reduce incomplete combustion and/or associated emissions in a fuel- consuming system running on the fuel composition ⁇ and/or to improve fuel economy or exhaust emissions generally.
  • cetane number improvers also encompass additives that increase the combustability of the fuel to which it is added and, as such, decrease the ignition delay. Therefore, as used herein, an organic nitrate that increases the
  • combustability i.e. a "combustion enhancer / improver”
  • ignition delay i.e. an "ignition enhancer / improver”
  • Cetane number improvers of the invention may be used to increase the cetane number of a fuel composition.
  • an "increase" in the context of cetane number embraces any degree of increase compared to a previously measured cetane number under the same or equivalent conditions. Thus, the increase is suitably compared to the cetane number of the same fuel
  • composition prior to incorporation of the cetane number increasing (or improving) component or additive prior to incorporation of the cetane number increasing (or improving) component or additive.
  • the cetane number increase may be measured in comparison to an otherwise analogous fuel composition (or batch or the same fuel composition) that does not include the cetane number enhancer of the invention.
  • an increase in cetane number of a fuel relative to a comparative fuel may be inferred by a measured increase in combustability or a measured decrease in ignition delay for the comparative fuels.
  • the increase in cetane number (or the decrease in ignition delay, for example) may be measured and/or reported in any suitable manner, such as in terms of a percentage increase or decrease.
  • the percentage increase or decrease may be at least 1%, such as at least 2%.
  • the percentage increase in cetane number or decrease in ignition delay is at least 5%, at least 10%, at least 15% or at least 20%.
  • the increase in cetane number or decrease in ignition delay may be at least 25%, at least 30%.
  • the engine in which the fuel composition of the invention is used may be any appropriate engine.
  • the fuel is a diesel or biodiesel fuel composition
  • the engine is a diesel or compression ignition engine.
  • any type of diesel engine may be used, such as a turbo charged diesel engine, provided the same or equivalent engine is used to measure fuel economy with and without the cetane number increasing component.
  • the invention is applicable to an engine in any vehicle.
  • the cetane number improvers of the invention are suitable for use over a wide range of engine working conditions.
  • some organic nitrates of the invention may provide optimal effects under a particular narrow range of engine working conditions, such as under mild conditions and more suitably under harsh conditions.
  • Cetane number enhancers are known and commercially available, and may also be known (in the context of diesel fuels) as “cetane (number) improvers", “combustion improvers” and “ignition improvers” etc. as previously decribed . Cetane enhancers are often added to diesel fuels, at additive levels (typically 0.1 to 2.0% w/w) , to improve the combustion properties of the fuel. They function to reduce the ignition delay, i.e. the period between the time of injection of the fuel and the start of combustion (ignition) . This, in turn, leads to better engine
  • cetane enhancers such as their effect in increasing the heating rate of the fuel, it is generally accepted that they act as sources of chain-initiating radicals .
  • a fuel with a high CN has a short ignition delay.
  • molecules with high octane numbers, which confer a resistance to spontaneous ignition in gasoline spark ignition engines have low cetane numbers.
  • the addition of small amounts of cetane enhancers to a diesel fuel may, therefore, result in improved fuel properties based on the shorter ignition delay.
  • cetane number enhancers include: a) certain organic nitrates (e.g. isopropyl nitrate, 2-ethylhexyl nitrate (2-EHN) , cyclohexyl nitrate, and methoxyethyl nitrate); b) organic peroxides and hydroperoxides (e.g. di-tert-butyl peroxide ⁇ ; and c) organic peracids and peresters.
  • organic nitrates e.g. isopropyl nitrate, 2-ethylhexyl nitrate (2-EHN) , cyclohexyl nitrate, and methoxyethyl nitrate
  • organic peroxides and hydroperoxides e.g. di-tert-butyl peroxide ⁇
  • organic peracids and peresters e.g. di-tert-butyl peroxide ⁇
  • cetane enhances are dialkylperoxides (ROOR, di-t-butylperoxide) and organic nitrates (R-ON0 2 ) , of which the most important is 2- ethylhexylnitrate (2-EHN) .
  • ROOR dialkylperoxides
  • R-ON0 2 organic nitrates
  • 2-EHN 2- ethylhexylnitrate
  • 2-EHN is produced industrially by the nitration of 2-ethylhexanol, and in Europe this consumes almost a quarter of the production of this alcohol.
  • the nitration of the alcohol involves reaction with a 1/1 mixture of undiluted nitric and sulfuric acids (using stoichiometric amounts of alcohol and nitric acid) .
  • cetane number improvers which may provide benefits over the known cetane enhancer. Any useful benefit may be achieved, such as in relation to their synthesis, storage, transportation; or in use, e.g. under certain operating conditions or in certain diesel fuels. Particular benefits of the invention are directed to one or more of the following: increased stability under storage and transport conditions; at least equal and more suitably greater effectiveness as a cetane enhancer;
  • the present invention provides alternative organic nitrates for use as cetane number improvers in diesel fuels and optionally for achieving one or more associated benefit.
  • the cetane number improver of the invention may be selected from nitrated:
  • terpene alcohols particularly monoterpene alcohols and most suitably monocyclic and bicyclic molecules including pinenes, such as borneol, fenchol and menthol;
  • fatty acid esters e.g. the synthetic alcohol mixture, Neodol 23, derived from the SHF process
  • linear Cio-Cie alkanols and diols such as 1-octanol, 1,10,- decanediol, 1-dodecenol, 1-tridecanol, 1-tetradecanol, 1- hexadecanol, 1-octadecanol and mixtures thereof.
  • the cetane number improver has one or two nitrate (N0 3 ) groups in place of the hydroxyl groups in the above-mentioned compounds.
  • N0 3 nitrate
  • the alcohol feedstocks are commercially available at scales of at least 10 kt/a, which would potentially be compatible with production of new useful cetane enhancers for worldwide consumption.
  • 10-decanediol may be
  • the cetane number improver of the invention has the Formula (1): R-ONO 2 , wherein R is a terpene or an oxygenated (saturated) terpene.
  • R is a terpene or an oxygenated (saturated) terpene.
  • the terpene may be natural or substituted by up to three (e.g. 1, 2 or 3) Ci-Ce alkyl groups or a further nitrate (-ON0 2 ) group .
  • Terpenes are classified according to the number of units of the basic structure methylbuta ⁇ -l , 3-diene or isoprene, which make up the terpene.
  • Monoterpenes contain two isoprene units and are generally considered to have the chemical formula CioH 16 .
  • monoterpenes are particularly sensitive to oxygenation at the carbon- carbon double bond and so monoterpenes are typically saturated hydrocarbons lacking the carbon-carbon double bond. These oxygenated, saturated molecules are sometimes also referred to as monoterpenes, and are encompassed by the term "monoterpene" as used in the context of this invention.
  • Monoterpenoid is another term that is
  • Monoterpenes may be acyclic such as myrcene and ocimene or cyclic such as limonene and pinene.
  • the terpene is a monocyclic or a bridged-monocyclic (i.e. bicyclic) alkyl.
  • the cetane enhancer of the invention has the Formula (2): CioHi 5 -ON02 , wherein X is selected from H, C x -C6 alkyl and O O2. More suitably, X is selected from H, methyl, ethyl and ON02; and still more suitably X is H. Most suitably, R of Formula (1) is a monoterpene selected from menthyl, fenchyl and bornyl, which may optionally be substituted by X as defined above .
  • cetane number improver ma be a compound of Formula (3) :
  • each of R a to R 9 is independently selected from H or Ci-C 6 alkyl, or nitrate ( -ONO2 ) , wherein optionally two of Ri to Rg may be connected together to form a bridge, which may be substituted by one or more Ci-C 6 alkyl, and/or nitrate (-ON0 2 ⁇ ; provided that no more than 1 R x to R 9 comprises a nitrate group.
  • Ri to Rg are independently selected from H or Ci ⁇ Cg alkyl, wherein optionally one of R4 and R 5 forms an optionally
  • At least one of Ri to R 9 is not H; more preferably 1, 2, 3, 4 or 5 of R x to Rg is not H.
  • the C1-C6 alkyl may be straight chain (i.e. linear) or branched chain, wherein the number 1 to 6 refers to the total number of carbon atoms in the group. Suitable
  • Ci-Ce alkyl radicals include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, sec-butyl, n- pentyl, isopentyl, tert-pentyl, sec-pentyl, n-hexyl, 2- ethylbutyl, and 2 , 3-dimethylbutyl .
  • i to Rg are independently selected from H, methyl, ethyl, n ⁇ propyl, isopropyl, n-butyl, isobutyl and tert-butyl, wherein optionally one of R4 and R 5 forms an optionally substituted alkylene bridge with one of Rg and Rg.
  • the alkylene bridge has the formula ⁇ ⁇ CR a Rb) n - / wherein R a and R b are independently selected from. H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2.
  • R a and R b are independently selected from H, methyl and ethyl, and n is 1; and yet more suitably selected from H and methyl, and n is 1.
  • Ri, Rg and R 7 are H.
  • Ri, R 6 and R 7 are H
  • one of R4 and R5 is H
  • one of R 8 and Rg is H
  • the other of R 4 and R 5 , and R 8 and R 9 is selected from methyl, ethyl, n- propyl, isopropyl, n-butyl, isobutyl and tert-butyl or they are connected together to form an alkylene bridge of the formula -CR a R b -, wherein R a and R b are defined above.
  • one of R4 and R 5 is H, and the other is methyl; and one of Rs and Rg is H, and the other is isopropyl; wherein Ri, R 2 , R 3f e and R 7 are as defined in any of the above embodiments
  • Ri, R 6 and R 7 are B, and R 2 and R3 are as defined in any of the above embodiments; and most preferably wherein i, R 2 , R3, e and R 7 are H.
  • one of R3 ⁇ 4 and R5 is H
  • one of Re and R 9 is H or methyl
  • the other of R ⁇ j and R 5 and of R 8 and Rg are connected together to form an alkylene bridge of the formula -CR a R b -, wherein R a and R b are H or methyl.
  • one of Rs and R 9 is methyl
  • Ri, Re and R 7 are H
  • R a and R b are H or methyl
  • R 2 and R 3 are as defined in any of the above embodiments.
  • at least two of R a , Rbr R2 and R 3 are methyl.
  • R a and R b are methyl
  • R 2 and R 3 are methyl.
  • the cetane number improver is defined by Formula (4) :
  • each of Ri to R 9 is as defined in connection with Formula (3).
  • at least one of Ri to Rg is not H and sill more suitably, 1, 2, 3, 4 or 5 of Ri to R9 is not H.
  • R 8 and R4 are connected together to form an alkylene bridge of the formula - ⁇ iCR a R b ) r wherein R a and R b are independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2.
  • the alkylene bridge has the formula -CR a R b - / wherein R a and R b are H or methyl.
  • R a and R b are methyl
  • R2 and R 3 are methyl.
  • cetane number improver is defined by Formula (5) :
  • Ri to R 3f R 5 to R 7 , R 9 , R a and R b are as defined in connection with any embodiment of Formulas (3) or ⁇ 3A) , or as defined in connection with any embodiment of Formulas (4) or (4A) .
  • Ri, R6 and R 7 are H; and R2, R3, R5, Rg, R a and are independently selected from H, methyl and ethyl.
  • R 2 , R3, R5 , Rg, R a and R b is not H, and preferably 1, 2 or 3 of R2, R3, sr Rg, R a and Rb is not H.
  • Most preferred are compounds wherein three of R 2/ R3, R5, R9, R a and are methyl.
  • the nitrate may be arranged in an axial or equatorial position relative to the alkyl ring. In a preferred embodiment, however, the nitrate is attached in an equatorial position on the ring. Most preferred cetane improvers of this embodiment of the invention are bornyl nitrate, fenchyl nitrate and menthly nitrate.
  • the cetane number improver is a compound of the Formula (6) : R x -ON0 2 , wherein R x is a linear
  • (straight-chain) aliphatic group having 8 to 24 carbon atoms.
  • the aliphatic group has between 10 and 20 carbon atoms, and more suitably between 10 and 18 carbon atoms.
  • particularly suitable R x groups have 8, 10, 12, 13, 14, 16 or 18 carbon atoms; and a most suitable R x group has 18 carbon atoms.
  • aliphatic group may be saturated or unsaturated. When unsaturated it is preferably mono-unsaturated.
  • the R x group may optionally be substituted with a nitrate (- ON0 2 ) group to form a dinitrate compound.
  • a nitrate (- ON0 2 ) group Preferably the one to two nitrate groups are attached to the terminal carbon atoms of the aliphatic group.
  • Formula (6) include 1-octyl nitrate, 1,10,-decyl
  • the R x group a fatty acid derivative, particularly a fatty alcohol derivative.
  • a preferred fatty alcohol derivative is monounsaturated, and is more preferably a cis- monounsaturated fatty alcohol derivative (i.e. wherein adjacent hydrogen atoms are on the same side of the double bond) .
  • the cetane number improver of the present invention has the Formula (7) :
  • the carbon-carbon double bond is in the cis formation.
  • a preferred monounsaturated aliphatic nitrate of the invention is oleyl nitrate.
  • the cetane number improver of the invention is a nitro-substituted fatty acid ester of the Formula (7) : R y -[0 s N0 t ] u , wherein R y is a alkyl ester of a fatty acid, i.e. a fatty acid alkyl ester (FAAE) moiety, S is 0 or 1, t is 1 or 2, and u is 1 to 4 or 1 to 3.
  • R y is a alkyl ester of a fatty acid, i.e. a fatty acid alkyl ester (FAAE) moiety
  • S is 0 or 1
  • t is 1 or 2
  • u is 1 to 4 or 1 to 3.
  • the FAAE group is a linear (straight- chain) aliphatic group having 8 to 24 carbon atoms, more suitably between 10 and 22 carbon atoms, still more suitably between 12 and 20 carbon atoms, and most
  • R y groups suitably between 14 and 18 carbon atoms.
  • particularly suitable R y groups have 14, 16 or 18 carbon atoms; and advantageously the R y group has 18 carbon atoms.
  • the aliphatic group may be saturated or
  • R y is mono-unsaturated, and more preferably the R y group has a cis double-bond.
  • the R y group may optionally be substituted with one or two additional nitro-substituted groups of formula O s NO t .
  • a preferred nitro-substituted fatty acid ester of the invention is a methyl oleate substituted with 1, 2 or 3 O s NOt groups.
  • the cetane number improver of the invention is a nitro-substituted diterpene of the Formula (8): R z - [O s NO t ] u , wherein R z is a diterpene or an alkyl ester of a diterpene, and wherein [O s NO t ] u is as defined above.
  • the compound of Formula (8) is an alkyl ester of abietic acid. More suitably the ester is a methyl or ethyl ester.
  • the R E group may optionally be substituted with one or two additional nitro-substituted groups of formula O s NO t .
  • the nitro-substituted diterpene is nitro-substituted ethyl abietate substituted with 1, 2 or 3 O s NO t groups.
  • the cetane number improver of the invention is a nitro-substituted steroid containing from 1 to 3 nitrate groups, such as cholesterol nitrate.
  • the cetane number improving additive of the invention may be pre ⁇ dissolved in a suitable solvent, for example an oil such as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture; a fuel component (which again may be either mineral or Fischer-Tropsch derived ⁇ compatible with the diesel fuel composition in which the additive is to be used ⁇ for example a middle distillate fuel component such as a gas oil or kerosene) ; a poly alpha olefin; a so-called biofuel such as a fatty acid alkyl ester (FAAB), a Fischer-Tropsch derived biomass-to- liquid synthesis product, a hydrogenated vegetable oil, a waste or algae oil or an alcohol such as etnanol ; an aromatic solvent; any other hydrocarbon or organic solvent; or a mixture thereof.
  • a suitable solvent for example an oil such as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture
  • a fuel component which again may be either mineral or Fischer-Tropsch derived ⁇ compatible with the diesel fuel composition
  • the cetane enhancer will be part of an additive (performance) package additionally containing other additives such as detergents, anti-foaming agents, corrosion inhibitors, dehazers etc.
  • the cetane enhancing agent of the invention may be blended directly with the base fuel.
  • the concentration of the cetane number enhancing additive used may depend on desirable fuel
  • the concentration of the cetane number improving additive in the fuel composition may be up to 2% w/w and suitably up to 1.0% w/w.
  • the concentration of the cetane number improver may be from 0.025% w/w to 2% w/w, or from 0.05% w/w to 1% w/w.
  • the concentration of the cetane number improver is from 0.05% w/w to 1.0 w/w, such as 0.05% w/w, 0.1% w/w, 0.25% w/w, 0.5% w/w, 0.75% w/w or 1.0% w/w based on the total weight of the fuel
  • cetane number improving additives it will be appreciated that amounts / concentrations may also be expressed as ppm, in which case 1% w/w corresponds to 10, 000 ppm w/w.
  • composition will typically consist of one or more automotive base fuels optionally together with one or more fuel additives, for instance as described in more detail below.
  • the relative proportions of the cetane number enhancer, fuel components and any other components or additives present in a diesel fuel composition prepared according to the invention may also depend on other desired properties such as density, emissions performance and viscosity.
  • olefinic substrates may afford nitrates
  • C-N0 2 bond homolysis like nitrates, they are potential sources of radicals and may, therefore, also be useful as cetane enhancers.
  • Alcohol nitration involves attack of the -OH functionality on N0 2 + , and may be generated using the following reagents: (i) nitric acid for secondary alcohols; (ii) a mixture of nitric acid, sulfuric acid and urea for primary or secondary alcohols; and (iii) a mixture of nitric acid and acetic anhydride (precursor of reactive acetyl nitrate) in acetic acid for unsaturated alcohols.
  • reagent used to synthesise the nitrate or nitro-compounds of the invention may depend on the reactivity of the alcohol starting material and the desirability to avoid side reactions, such as oxidation of the alcohol to a ketone, or sulfation of the olefin.
  • Reactive olefins such as pinenes
  • nitric acid e.g. Canoira et al. ⁇ 2007 ⁇ Fuel, 86, 965; Bakhvalov et al. (2000) J. Organic Chem. , 36, 1601; Bakhvalov et al. (2002) J. Organic Chem., 38,
  • olefins containing ester functionalities may undergo partial hydrolysis of the ester under these conditions, affording free acid, which is not desirable in a diesel fuel component.
  • N 2 0 4 may suitably be used instead of nitric acid. Reaction with olefins normally, therefore, results in a mixture of products, containing nitro (R-N0 2 ) , as well as nitrate (R-ONO 2 ) and hydroxyl functionalities .
  • the most common method of converting primary / secondary alcohols to nitrates involves the use of mixed nitric and sulfuric acids (molar ratio substrate: nitric acid: sulfuric acid generally 1:3:8), together with urea (0.25 equivalent based on substrate).
  • the urea is used to remove any HN0 2 (nitrous acid) formed.
  • the mixed acid is a more powerful nitrating agent than nitric acid alone, due to the higher concentration of N0 2 + , which is the reactive species.
  • a general method is shown in Scheme 1 below .
  • the primary alcohols were nitrated at 0°C for 2 hours. Following neutralisation, extraction with
  • secondary alcohols may be nitrated using the same reagent mixture and conditions as used for primary alcohols.
  • nitric acid may be used, which is a less powerful oxidising agent than the mixed acid.
  • One exemplary secondary alcohol is endo-borneol, which may be converted into a bornyl nitrate.
  • the secondary alcohols may be nitrated by reacting with nitric acid at room temperature (RT) for approximately 24 hours.
  • RT room temperature
  • exo ⁇ borneol was reacted with nitric acid at RT to obtain the exo-nitrate of borneol (compound 7, yield 87%; see reaction Scheme 2 below and Figure 1 ⁇ .
  • the MPV reaction is an aluminium-catalyzed hydride shift (of RCHOH) , from the alcohol (isopropanol) to the carbonyl carbon (L ⁇ fenchone) .
  • Isopropanol may be used as a hydride donor, because the acetone formed can be easily removed by distillation.
  • the reaction was refluxed at 95 °C for 7 days followed by an extraction.
  • ⁇ -pinene may be reacted with nitric acid, e.g. at -15 °C in dichloromethane, according to methods known to the person of skill in the art (see reaction Scheme 5 below) .
  • nitric acid e.g. at -15 °C in dichloromethane
  • the product may be isolated by extraction with dichloromethane. NMR may be used to confirm the identit of the product.
  • Tertiary nitrates are generally known to be less thermally stable than primary and secondary nitrates.
  • tertiary nitrates such as a-terpineol nitrate
  • unsaturated alcohols can be nitrated selectively at the alcohol position to leave the double bond intact, provided that the mode of introducing the reactants and the amount of nitrating agent is suitably controlled, and the use of sulfuric acid is avoided (as this may react with the double bond to give a sulfate) .
  • any suitable method can be used.
  • oleyl alcohol e.g. cis-9- octadecen-l-ol
  • acetic acid solvent at 15°C
  • this method avoids build-up of significant concentrations of acetyl nitrate (which may undergo runaway decomposition above 60 °C) .
  • Acetyl nitrate is a powerful nitrating agent, being a good source of N0 2 + , which reacts with the alcohol.
  • the product (compound 10; Figure 1) can then be extracted .with a suitable solvent, such as diethyl ether. 1H-NMR confirmed the clean formation of the desired single product without significant by-products.
  • Nitration of unsaturated secondary alcohols, such as cholesterol may be performed similarly so as to form cholesterol nitrate (compound 11) shown in Figure 1.
  • nitrate or nitro derivatives of olefins may be carried out using any appropriate reaction scheme.
  • conditions used will be different to those described above for non-olefinic esters. For example, it is desirable to avoid sulfuric acid (to reduce risk of olefin sulfation) .
  • Exemplary conditions include: (i) HN0 3 (70%) and acetic anhydride (precursor of acetyl nitrate); (ii) fuming HN0 3 , acetic acid and
  • reaction scheme (ii) has been found to result in low conversion rates of unsaturated FAMEs, with nitro-olefins formed in low yields. Therefore, reaction scheme (iii) may be preferred, since it is expected to avoid ester
  • N2O may conveniently be used as a dilute solution (boiling point 30-100°C) , and excess reagent removed under low pressure at the end of the procedure. Chloroform and hexane were both used as solvents.
  • N2O4 was removed in vacuo and the product quenched in an ice bath.
  • the products can be extracted in a suitable solvent, such as ether, and dried.
  • a mixture of products was identified by X H and 13 C NMR analysis.
  • Nitration products of abietic acid - the most prevalent of the organic acids that form the largest constituent of rosin - such as ethyl abietate were also synthesised in similar fashion. Reactions were carried out using N 2 0 4 in either hexane or chloroform and a similar mixture of nitrogen-containing products was formed. Hexane is a preferred solvent for this reaction. The NMR data was consistent with the formation of dinitro and nitro-alcohol products containing a single double bond (e.g. 1, 4 ⁇ dinitro-2-alkene) .
  • thermal stability assays e.g. differential scanning calorimetry /
  • thermogravimetric analysis DSC/TGA
  • MS mass spectrometry
  • DSC measures heat flow resulting from evaporation (endotherm) or thermal decomposition of the compound (exotherm) .
  • TGA measures the weight loss on evaporation or decomposition. MS of the gas space allows the identity of the volatile decomposition products to be determined.
  • the associated exotherm may, in principle, be determined.
  • a diesel fuel composition which comprises a cetane number improver of the invention.
  • the cetane number improver is present at a concentration sufficient and appropriate for achieving a desired cetane number in the resultant fuel composition.
  • a diesel fuel composition prepared in accordance with the present invention may in general be any type of diesel fuel composition suitable for use in a compression ignition (diesel) engine; and it may itself comprise a mixture of diesel fuel components.
  • a diesel fuel composition prepared according to the present invention may comprise one or more diesel fuel components of conventional type. It may, for example, include a major proportion of a diesel base fuel, for instance of the type described below.
  • a "major proportion" means at least 50% w/w, and typically at least 85% w/w based on the overall composition. More suitably, at least 90% w/w or at least 95% w/w. In some cases at least 98% w/w or at least 99% w/w of the fuel composition consists of the diesel base fuel.
  • the base fuel may itself comprise a mixture of two or more diesel fuel components of the types described below.
  • Typical diesel fuel components comprise liquid hydrocarbon middle distillate fuel oils, for instance petroleum derived gas oils.
  • Such base fuel components may be organically or synthetically derived, and are suitably obtained by distillation of a desired range of fractions from a crude oil. They will typically have boiling points within the usual diesel range of 150 to 410 °C or 170 to 370 °C, depending on grade and use. They will typically have densities from 0.75 to 0.9 g/cm 3 , such as from 0.8 to 0.86 g/cm 3 , at 15°C (IP 365) and measured cetane numbers (ASTM D613) of from 35 to 80, more preferably from 40 to 75. Their initial boiling points will suitably be in the range 150 to 230 * C and their final boiling points in the range 290 to 400 'C. Their kinematic
  • viscosity at 40 °C ⁇ ASTM D445) might suitably be from 1.5 to 4.5 centistokes.
  • Such fuels are generally suitable for use in compression ignition (diesel) internal combustion engines, of either the indirect or direct injection type.
  • the fuel composition may have a density from 0.82 to 0.845 g/cm 3 at 15°C; a T 95 boiling point (ASTM D86) of 360°C or less; a cetane number (ASTM D613) of 45 or greater; a kinematic viscosity (ASTM D445 ⁇ from 2 to 4.5 mm 2 /s at 40 °C; a sulphur content (ASTM D2622) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons ( PAH) content (IP391 (mod)) of less than 11% w/w.
  • PAH polycyclic aromatic hydrocarbons
  • the measured cetane number will preferably be from 45 to 70, to 75 or to 80, more preferably from 50 to 65, or at least greater than 50, greater than 55, greater than 60, or greater than 65.
  • a petroleum derived gas oil e.g. obtained from refining and optionally (hydro) rocessing a crude
  • diesel fuel composition may be incorporated into a diesel fuel composition. It may be a single gas oil stream obtained from such a refinery process or a blend of several gas oil fractions obtained in the refinery process via different processing routes. Examples of such gas oil fractions are straight run gas oil, vacuum gas oil, gas oil as obtained in a thermal cracking process, light and heavy cycle oils as obtained in a fluid catalytic
  • a petroleum derived gas oil may comprise some petroleum derived kerosene fraction.
  • gas oils may be processed in a hydro- desulphurisation (HDS) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition. This also tends to reduce the content of other polar species such as oxygen- or nitrogen- containing species.
  • the fuel composition will include one or more cracked products obtained by splitting heavy hydrocarbons.
  • the base fuel may be or contain another so-called “biodiesel” fuel component, such as a vegetable oil, hydrogenated vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester, FAME) , or another oxygenate such as an acid, ketone or ester.
  • biodiesel fuel component
  • a vegetable oil hydrogenated vegetable oil or vegetable oil derivative
  • e.g. a fatty acid ester, in particular a fatty acid methyl ester, FAME e.g. a fatty acid methyl ester, FAME
  • oxygenate such as an acid, ketone or ester.
  • Such components need not necessarily be bio- derived.
  • the biodiesel component may be present in quantities up to 100%, such as between 1% and 99% w/w, between 2% and 80% w/w, between 2% and 50% w/w, between 3% and 40% w/w, between 4% and 30% w/w, or between 5% and 20% w/w.
  • the biodiesel component may be FAME.
  • a diesel base fuel may consist of or comprise a Fischer-Tropsch derived diesel fuel component, typically a Fischer-Tropsch derived gas oil.
  • a Fischer-Tropsch derived diesel fuel component typically a Fischer-Tropsch derived gas oil.
  • the terra "Fischer-Tropsch derived" means that a material is, or is obtained from, a synthesis product of a Fischer- Tropsch condensation process-
  • a Fischer-Tropsch derived fuel or fuel component will therefore be a hydrocarbon stream in which a substantial portion, except for added hydrogen, is derived directly or indirectly from a
  • Fischer-Tropsch fuels may be derived by converting gas, biomass or coal to liquid (XtL) , specifically by gas to liquid conversion (GtL) , or from biomass to liquid conversion (BtL) . Any form of Fischer-Tropsch derived fuel component may be used as a base fuel in accordance with the invention.
  • the base fuel suitably has a low sulphur content, for example at most 1000 mg/kg (1000 parts per million by weight / ppmw) . More suitably it will have a low or ultra low sulphur content, for instance at most 500 mg/kg (500 ppmw) , such as no more than 350 mg/kg (350 ppmw) , and still more suitably no more than 100 or 50 or 10 or even 5 mg/kg (5 ppmw) of sulphur. It may be a so-called "zero- sulphur" fuel; although in some cases it may be desired that the base fuel is not a sulphur free (“zero sulphur”) fuel. Ideally a fuel composition which results from carrying out the present invention will also have a sulphur content falling within these limits.
  • the diesel fuel composition which results from the method of the present invention may, if desired, contain no, or only low levels of additional cetane improving (ignition improving) additives such as 2-ethylhexyl nitrate (2-EHN) .
  • additional cetane improving (ignition improving) additives such as 2-ethylhexyl nitrate (2-EHN) .
  • the present invention embraces the use of certain organic nitrates in a diesel fuel composition for the purpose of reducing the level of a second (or further) cetane improving additive in the composition.
  • a fuel composition prepared according to the present invention, or a base fuel used in such a composition may contain one or more fuel additives, or may be additive-free. If additives are included (e.g.
  • additives include (but are not limited to) :
  • enhancing additives e.g. ester- and acid-based
  • viscosity improving additives or viscosity modifiers e.g. styrene-based copolymers, zeolites, and high viscosity fuel or oil derivatives
  • dehazers e.g. alkoxylated phenol formaldehyde polymers ⁇
  • anti-foaming agents e.g. polyetherTMmodified polysiloxanes
  • anti-rust agents e.g. a propane-1, 2 ⁇ diol semi-ester of
  • phenolics such as 2 , 6-di-tert-butylphenol) ; metal
  • composition may for example contain a
  • detergent containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove or slow the build up of engine deposits. In some embodiments, it may be advantageous for the fuel
  • composition to contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity enhancing additive.
  • composition contains such additives (other than the cetane number increasing components of the invention) , it suitably contains a minor proportion (such as 1% w/w or less, 0.5% w/w or less, 0.2% w/w or less), of the one or more fuel additives, in addition to the cetane number increasing component (s) .
  • the (active matter) concentration of each such additive component in the fuel composition may be up to 10000 ppmw, such as in the range of 0.1 to 1000 ppmw; and advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw .
  • one or more additive components may be co-mixed ⁇ e.g. together with suitable diluent) in an additive concentrate, and the additive concentrate may then be dispersed into a base fuel or fuel composition.
  • suitable diluent suitable diluent
  • the cetane number improving additive may be pre-diluted in one or more such fuel components, prior to its incorporation into the final automotive fuel composition.
  • Such a fuel additive mixture may typically contains a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a mineral oil, a solvent such as those sold by Shell companies under the trade mark "SHELLSOL", a polar solvent such as an ester and, in particular, an alcohol ⁇ e.g. hexanol, 2TM
  • ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark "LINEVOL", especially LINEVOL 79 alcohol which is a mixture of C 7 _ 9 primary alcohols, or a C12-1 alcohol mixture which is commercially available) .
  • the total content of the additives in the fuel composition may be suitably between 0 and 10000 ppmw and more suitably below 5000 ppmw.
  • amounts e.g. concentrations, ppmw and %w/w
  • active matter i.e.
  • the present invention involves adjusting the cetane number of the fuel composition, using the cetane number enhancing component, in order to achieve a desired target cetane number.
  • the maximum cetane number of an automotive fuel composition may often be limited by relevant legal and/or commercial specifications, such as the European diesel fuel specification EN 590 that stipulates a cetane number of 51.
  • relevant legal and/or commercial specifications such as the European diesel fuel specification EN 590 that stipulates a cetane number of 51.
  • typical commercial automotive diesel fuels for use in Europe are currently manufactured to have cetane numbers of around 51.
  • the present invention may involve manipulation of an otherwise standard
  • diesel fuel composition using a cetane number enhancing additive, to increase its cetane number so as to improve the combustability of the fuel, and hence reduce engine emissions and even fuel economy of an engine into which it is, or is intended to be,
  • the cetane number improver increases the cetane number of the fuel composition by at least 3 cetane numbers.
  • the cetane number increase may be up to approximately 9, or any value in between these ranges. Accordingly, in other embodiments , the cetane number of the resultant fuel is between 51 and 60.
  • an automotive diesel fuel composition prepared according to the present invention will suitably comply with applicable current standard specification (s) such as, for example, EN 590 (for Europe) or ASTM D-975 (for the USA) .
  • the overall fuel composition may have a density from 820 to 845 kg/m 3 at 15°C (ASTM D- 4052 or EN ISO 3675) ; a T95 boiling point (ASTM D-86 or EN ISO 3405) of 360°C or less; a measured cetane number
  • diesel fuel composition prepared according to the present invention may contain fuel components with properties outside of these ranges, since the properties of an overall blend may differ, often significantly, from those of its individual constituents.
  • cetane number improver of the invention to achieve a desired cetane number of the resultant fuel composition.
  • desired cetane number is achieved or intended to be achieved under a specified set or range of engine working conditions, as described elsewhere herein.
  • cetane number enhancers of the invention may be suitable for reducing the combustion delay of a fuel composition under all engine running conditions, or under mild, or under harsh engine conditions. Cetane number enhancers of the invention may serve to improve combustion and, hence, improve associated engine factors, such as exhaust emissions and/or engine deposits under a range of engine operating conditions - particularly under harsh engine conditions when fuel emissions might otherwise be
  • a cetane number improver in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more fuel components (typically diesel base fuels) and optionally with one or more fuel additives.
  • the cetane number improver is preferably
  • the viscosity increasing component may be dosed directly into (e.g. blended with) one or more components of the fuel
  • composition or the base fuel at the refinery may be pre-diluted in a suitable fuel component, which subsequently forms part of the overall automotive fuel composition.
  • a suitable fuel component which subsequently forms part of the overall automotive fuel composition.
  • it may be added to a diesel fuel composition downstream of the refinery.
  • it may be added as part of an additive package containing one or more other fuel additives. This can be particularly advantageous because in some circumstances it can be inconvenient or
  • the blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles.
  • the "use" of the invention may also encompass the supply of a cetane number improver together with instructions for its use in a diesel fuel
  • the cetane number increasing component may therefore be supplied as a component of a formulation which is suitable for and/or intended for use as a fuel additive, in particular a diesel fuel additive.
  • the cetane number improver may be incorporated into an additive formulation or package along with one or more other fuel additives.
  • the one or more fuel additives may be selected from any useful additive, such as detergents, anti-corrosion additives, esters, poly-alpha olefins, long chain organic acids, components containing amine or amide active centres, and mixtures thereof, as is known to the person of skill in the art.
  • automotive fuel composition which process involves blending a diesel base fuel (or base fuel mixture) with a cetane number improver of the invention.
  • the blending may be carried out for one or more of the purposes described herein.
  • cetane number improver of the invention may not be suitable for pre-mixing with other fuel additives and may, therefore, be dosed directly into the fuel composition from a concentrated (100%) or pre- diluted stock.
  • two or more cetane number increasing additives may be used in a diesel fuel composition to provide one or more of the effects of the invention described herein.
  • the present invention can provide an effective way of improving fuel combustion /
  • certain organic nitrate molecules of the invention can, at relatively low concentrations, increase the cetane number of a diesel fuel composition by an amount greater than known organic nitrate cetane enhancers under some engine operating conditions.
  • cetance number enhancing agents of the invention may be capable of providing greater benefits than some prior art cetane number improvers, particularly under harsh engine working conditions (e.g. high engine speeds and powers).
  • cetane number increasing component for use in accordance with the invention may vary depending of fuel type and/or engine working
  • a further benefit of the invention is that under some engine conditions the amount of cetane number improver needed to observe the benefit of the invention may be surprisingly low, such as at the level of typical fuel additives.
  • the blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles.
  • Another aspect of the invention provides a method of operating an internal combustion engine and/or a vehicle powered by such an engine, which comprises introducing into a combustion chamber of the engine a fuel
  • the fuel composition prepared in accordance with the invention.
  • the fuel composition is advantageously introduced for one or more of the purposes described in connection with this invention.
  • the engine is preferably operated with the fuel composition for the purpose of improving ease of fuel ignition during use of the engine (by increasing fuel combustability) and, for example, associated
  • the engine is in particular a diesel engine, and may be a turbo charged diesel engine.
  • the diesel engine may be of the direct injection type, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or of the indirect
  • injection type It may be a heavy or a light duty diesel engine.
  • EUDI electronic unit direct injection
  • emission levels may be measured using standard testing procedures such as the European R49, ESC, OICA or ETC (for heavy- duty engines) or ECE+EUDC or MVEG ⁇ for light-duty
  • Cetane enhancers of the invention may provide various benefits, in use, that are associated with increased combustability and a reduction in ignition delay, such as reducing engine noise, reducing build-up of engine deposits, reducing engine emissions, and may even ' improve fuel economy in some cases.
  • Organic nitrates of the invention were also tested for thermal stability to obtain useful information on necessary storage and transportation conditions.
  • Organic nitrate synthesis processes may also help in determining suitability of the organic nitrates for use as cetane enhancers in diesel fuel at a commercial level (e.g. with respect to ease of synthesis and cost of production) .
  • N2O4 gaseous yellow/brown NO 2 N2O (Sigma-Aldrich) was slowly bubbled into the solvent (chloroform or hexane) in a round-bottomed-flask in an ice bath.
  • the dew point of the N 2 0 4 is around 20 °C, so the heavy gas condenses into the cold solvent (at ca . 4°C) .
  • the amount of N 2 0 4 added was determined by weighing the flask before and after addition of N 2 0 4 .
  • Nitration Product of 1-tetradecanol A mixture of concentrated nitric acid ⁇ 27 ml, 0.58 mol) , concentrated sulfuric acid (81 ml, 1.42 mol) and urea (3 g, 0.05 mol) were placed in a 500 ml three-necked flask and cooled to 0°C. 1-Tetradecanol (48 ml, 0.19 mol) in dichloromethane (30 ml) was added from a dropping funnel for 45 min. The temperature was maintained at 0 to 5°C with an ice-salt bath After 2 hours the reaction mixture was poured in an ice bath (quenched) and
  • Infrared spectra were measured on a Nicolet 6700 FTTM IR Spectrometer (Fourier transform infrared spectroscopy) from Thermo Scientific.
  • the oven started at 35 'C for the first 5 min, then increased with 10°C/min to 300 °C followed by a hold time of 10 min at 300 °C.
  • organic nitrate (or nitro) cetane number The organic nitrate (or nitro) cetane number
  • enhancing agents were performance tested in diesel fuel blends.
  • the assessment of their ability to increase the cetane number of the fuel was carried out indirectly by measuring changes in ignition delay (ID) of the blended fuels using a combustion research unit (CRU) .
  • the CRU is operated to determine ignition quality in a similar fashion to the ignition quality tester (IQT;
  • the cetane number is given as a dimensionless number, which describes the ignition behaviour of a fuel in comparison to primary reference fuels (PRFs) with a defined cetane number.
  • test compounds were blended into an EN590 compliant diesel ⁇ European specification) , zero-sulfur fuel (i.e. ZSD base-fuel from Stanlow refinery; density 837.4 kg/m 3 at 15°C; viscosity 2.89 mm 2 /s at 40°C) at concentrations of 0.1% w/w and 1.0% w/w. Some test compounds were also blended at concentrations of 0.05% w/w and 0.5% w/w. The mixtures were stirred at RT for 1 hour, after which all solid compounds were dissolved in the base fuel.
  • diesel-like fuel is injected into a high temperature, high pressure chamber where it mixes with the hot air and ignites, thus mimicking combustion in a compression-ignition engine.
  • the combustion process is monitored via a pressure sensor inside the chamber.
  • the CRU delivers p-t-charts of the ignition process from which the ignition delay, the burn rate and the maximum pressure increase (MPI) can be determined.
  • MPI maximum pressure increase
  • the engine conditions selected for CRU tests are designed to simulate a wide range of engine operating conditions so as to assess the cetane enhancers under mild, intermediate and harsh conditions; e.g. temperature and pressure are both varied from low to high. This allows the temperature and pressure dependence of ignition delay to be demonstrated.
  • the slope of the isothermal or isobaric charts plotted from the data obtained from these tests provide direct information about how the response of the fuel blend changes from mild to harsh conditions.
  • the primary data obtained from. CRU measurements are pressure-time traces, from which the ignition delay (ID), burn period (BP) and maximum pressure increase (MPI) can be determined.
  • ID ignition delay
  • BP burn period
  • MPI maximum pressure increase
  • ignition delay There are different definitions for ignition delay. Typically, ID 5% or ID 0 " 2 measurements are used, which are defined as the time taken for the pressure in the
  • combustion chamber to rise to its initial value plus 5% of the MPI, or to 0.2 bar above its initial value, respectively.
  • both values are used, although ID 0,2 is preferred due to the lower standard deviations observed.
  • a derived ignition quality is obtained from the ignition delay via comparison with primary reference fuels ⁇ PRFs) , which have a known cetane number. Since ignition delay correlates with the cetane number of the fuel, a calibration model can be obtained for all
  • DIQ can be determined for any unknown fuel and derived as a pseudo-cetane number .
  • exo-fenchyl nitrate was generally the most effective cetane enhancer at low concentrations (e.g. 0.1% w/ ⁇ , while it was generally slightly less effective than menthyl nitrate at higher concentrations (e.g. 1.0% w/w) .
  • the effect was most pronounced under the milder test conditions.
  • the fatty acid derived nitrates displayed slight reductions in
  • the effective additives can be used to good effect at low concentrations, which is useful in many respects, such as production level, storage, cost, handling and safety.
  • the derived ignition quality (DIQ) was determined for each of the organic nitrates at concentrations of 0.1% w/w and 1.0% w/w.
  • the DIQ for the diesel base fuel comprising exo-fenchyl nitrate at different
  • the isothermal and isobaric responses of the fuel blends were measured and compared to see the effect of temperature and pressure on ignition delay for each organic nitrate.
  • the experimental design that was chosen for CRU measurements allowed for isobaric and isothermal comparison of the additives, which
  • the ignition delay was the shortest. This may be due to faster injection rate and mixing.
  • thermogravimetric analysis was used to evaluate the thermal stabilities of the nitrates and other compounds prepared in the study; and also to provide information on the decomposition mechanism from mass spectrometry (MS) of the volatile products.
  • the line beginning in the top left of the graph illustrates the weight loss of the product on decomposition.
  • the two traces showing sharp peaks towards the right hand side of the graph represent the exotherm (right-hand peak: maximum at about 200°C) and the concentration of the decomposition product ⁇ left-hand peak) .
  • the rate- determining step in the decomposition of primary and secondary nitrates is thought to be the homolysis of the R0-N0 2 bond to give the alkoxyl radical and N0 2 . This may be followed by one or more competing reactions of the alkoxyl radical, e.g.: ⁇ -cleavage of the alkoxyl radical to liberate formaldehyde (CH 2 O) and form an alkyl
  • the DSC/TGA method was initially used to confirm that product samples were sufficiently stable to allow safe transportation and storage. However, the methods also provide detailed information on the thermal
  • the DSC method provides some qualitative information on the relative stability of different cetane enhancers, but, as it involves a temperature scan, it does not directly reflect the relative thermal stability at any particular temperature. Hence, the stability order derived from the temperature of exotherm maximum may differ from that determined at a particular temperature in kinetic experiments if the activation energy of the compounds differ. Components exhibiting a broad exotherm would be expected to have a lower activation energy than those with a narrow exotherm.
  • organic nitrates of the invention in general are more stable than some known cetane enhancers (e.g. Di-tert-butylperoxide, DTBP) , tetradecyl nitrate is a particularly attractive cetane enhancer of the invention that appears to be more stable than 2-EHN.
  • cetane enhancers e.g. Di-tert-butylperoxide, DTBP
  • tetradecyl nitrate is a particularly attractive cetane enhancer of the invention that appears to be more stable than 2-EHN.
  • the Phi-Tec equipment is usually operated in a heat- wait-search mode, which means that the temperature of the reactor is increased stepwise until the decomposition onset temperature (DOT) of the reaction is detected.
  • Onset temperatures are graphically derived by searching for the temperature at which deviation from linear temperature rise takes place due to self-heating of the sample.
  • the apparatus than automatically tracks the runaway exotherm to 490°C maximum (at short exposure time; otherwise 400°C) .
  • the decomposition onset is detected when the self heating rate exceeds 0.02°C/min.
  • Intake sample ca . 66 g
  • Test cell material SS-316 (STRCA type, 1*
  • Void cell volume ca. 110 ml
  • TDN tetradecyl nitrate
  • MN menthyl nitrate
  • DTBP di-tert- butylperoxide
  • Cetane enhancers were diluted to approx. 15% w/w in toluene for thermal measurements.
  • Tetradecyl nitrate was also blended at approx. 27% w/w for PhiTec measurements due to its high molecular weight.
  • variable heat capacity (Cp) for the novel nitrates is assumed to be 2.54 J/gK.
  • the ranking described for thermal stability of the compounds is also reflected in the activation energies (Ea) , which indicates that compounds with lower reaction onset temperature also exhibit lower activation energy.
  • the reaction enthalpies (not shown) for the respective compounds correlate with the adiabatic temperature rise.
  • DTBP demonstrated the lowest reaction enthalpy and the lowest temperature rise, whereas the organic nitrates compared well, with both the reaction enthalpy and the adiabatic temperature rise are in the same order of magnitude.

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Abstract

The use of an organic nitrate in a diesel fuel composition is described for the purpose of reducing the ignition delay of the fuel and/or for increasing its cetane number to a defined level. The organic nitrate may be a terpene nitrate. Methods of using a organic nitrate for achieving a desired cetane number, diesel fuel compositions comprising the organic nitrate, and methods of operating a compression ignition engine are also described.

Description

ORGANIC NITRATES AS IGNITION ENHANCERS
Field of the Invention
The present invention relates to a method of
improving diesel fuels; and in particular to the use of organic nitrates as additives in a diesel fuel
composition to give improvements in fuel combustion and cetane number.
Background of the Invention
The cetane number of a fuel composition is a measure of its ease of ignition and combustion. With a lower cetane number fuel a compression ignition (diesel) engine tends to be more difficult to start and may run more noisily when cold; conversely a fuel of higher cetane number tends to impart easier cold starting, to lower engine noise, to alleviate white smoke ("cold smoke") caused by incomplete combustion after.
There is a general preference, therefore, for a diesel fuel composition to have a high cetane number, a preference which has become stronger as emissions
legislation grows increasingly stringent, and as such automotive diesel specifications generally stipulate a minimum cetane number. To this end, many diesel fuel compositions contain ignition improvers, also known as cetane boost additives or cetane (number) improvers / enhancers, to ensure compliance with such specifications and generally to improve the combustion characteristics of the fuel.
Organic nitrates have been known for some time as ignition accelerants in fuels, and some are also known to increase the cetane number of diesel fuels. Such organic nitrates generally include short- and medium-chain linear and branched alkanols and nitrates of cycloalkanols, such as those described in US4479905.
Perhaps the most commonly used diesel fuel ignition improver is 2-ethylhexyl nitrate (2-EHN) , which operates by shortening the ignition delay of a fuel to which it is added. However, 2-EHN is also a radical initiator, and can potentially have an adverse effect on the thermal stability of a fuel. Poor thermal stability in turn results in an increase in the products of instability reactions, such as gums, lacquers and other insoluble species. These products can block engine filters and foul fuel injectors and valves, and consequently can result in loss of engine efficiency or emissions control.
The organic nitrates described in the prior art as combustion improvers and/or cetane number improvers have a series of disadvantages, especially lack of thermal stability, excessively high volatility and insufficient efficacy. However, it may be expected that by decreasing the volatility of a cetane enhancer, e.g. by using a molecule of higher molecular weight, its efficacy as a combustion improver and/or cetane number improver may then decline.
There are also health and safety concerns regarding the use of 2-EHN, which is a strong oxidising agent and is also readily combustible in its pure form. It can also be difficult to store in concentrated form as it tends to decompose, and so is prone to forming potentially explosive mixtures. Furthermore, it has been noted that 2-EHN functions most effectively under mild engine conditions .
These disadvantages, taken together with the often significant cost of incorporating 2-EHN as an additive into a fuel composition, mean that it would be generally desirable to reduce or eliminate the need for 2-EHN and other known cetane number improvers in diesel fuel compositions, whilst at the same time maintaining
acceptable combustion properties.
WO2008/000778 describes one such approach to
reducing the amount of an ignition enhance required in a diesel fuel by using a Fischer-Tropsch derived fuel component, in a fuel composition containing an ignition improver, which acts to enhance the effect of the
ignition improver and thus reduce the amount required to achieve the same desired cetane number.
WO2006/067234 relates to the use of fatty acid alkyl esters (FAAEs) in diesel fuels to increase the cetane number .
Thus, this invention aims to overcome or alleviate at least one of the problems associated with the prior art .
It is therefore an object of the invention to provide alternative organic nitrates which are effective as combustion improvers or cetane number improvers. It is another object of the invention to provide alternative organic nitrates which have similar or lower volatility than known cetane number improvers, or which meet acceptable safety levels for use in commercial diesel fuels. It is yet another object of the invention to provide alternative organic nitrates for use as ignition / combustion improvers and that are most cost-effective and/or more convenient to manufacture than known organic nitrate cetane number improvers. It is still another object of the invention to provide alternative organic nitrates for use as cetane number enhancers that work well under harsh engine conditions (for example, some known cetane enhancers may undesirably over-advance combustion. Yet another object of the invention is to provide cetane enhancers derived from renewable (or waste) feedstocks or by-products. It is another object of the invention to provide methods for producing organic nitrates useful as cetane number improvers by nitration of corresponding organic alcohols.
Summary of the Invention
It has now surprisingly been found that certain long chain linear organic nitrates, certain cyclic terpene organic nitrates, and certain nitrates derived from fatty alcohols and fatty acid alkyl esters can serve to reduce the ignition delay and/or as effective cetane number improvers in diesel fuels.
Accordingly, in a first aspect of the invention, there is provided the use of an organic nitrate in an diesel fuel composition for the purpose of reducing the ignition delay (ID) of the diesel fuel composition, wherein the organic nitrate is selected from the group consisting of:
a cyclic nitrate of Formula (4):
Figure imgf000006_0001
or Formula (4A)
Figure imgf000007_0001
e wherein each of R% to R9 is independently selected from H or Ci-C6 alkyl, or nitrate (~ON02) , wherein optionally one of R4 and R5 forms an optionally substituted alkylene bridge with one of Rs and R9, which may be substituted by one or more Ci~C6 alkyl, and/or nitrate {-ONO2) ; wherein at least one of Ri to R9 is not H, and provided that no more than one R2 to R9 comprises a nitrate group.
Suitably, the organic nitrate has the effect of increasing the cetane number of fuel, such as to a desired or target cetane number.
One or more additional organic nitrates may be used. Beneficially, the addition organic nitrates are also defined in accordance with the invention.
In another aspect of the invention, there is
provided a method for reducing the ignition delay and/or increasing the cetane number of a diesel fuel
composition, which method comprises adding to the
composition an amount of an organic nitrate according to the invention.
The method may involve increasing the cetane number of the diesel fuel composition to achieve a target cetane number. In some embodiments, the method may involve adding one or more additional organic nitrate to the fuel composition. Suitably, the method may further be for reducing the amount of 2~ethylhexyl nitrate (2-EHN) or any other known cetane enhancer in the diesel fuel composition to achieve the target cetane number.
A further aspect of the invention is directed to a diesel fuel composition for use in a compression ignition engine, which comprises an organic nitrate in accordance with the invention. Suitably, the diesel fuel composition has a cetane number of 40 or more, 50 or more, 60 or more, or 70 or more.
The diesel fuel composition of this or any other aspect may comprise a biofuel, and optionally may
comprise FAAEs, such as FAMEs.
The organic nitrate may be present in the diesel fuel composition at a concentration of: (a) between
0.025% and 2.0% w/w; (b) between 0.05% and 1.0% w/w; or (c) one of 0.05% w/w, 0.1% w/w, 0.5% w/w or 1.0% w/w; based on the total weight of the fuel composition.
Most suitably, the organic nitrate is selected from: bornyl nitrate, fenchyl nitrate and menthly nitrate.
The uses and methods of the present invention may additionally or alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with cetane number, for example, to improve the combustion performance of the fuel composition, e.g. to shorten ignition delays (i.e. the time being fuel injection and ignition in a combustion chamber during use of the fuel) , to facilitate cold starting or to reduce incomplete combustion and/or associated emissions in a fuel-consuming system running on the fuel composition) and/or to improve fuel economy or exhaust emissions generally . Accordingly, in one further aspect of the invention there is provided the use of an organic nitrate in a diesel fuel composition for the purpose of improving the fuel economy of an engine into which the fuel composition is or is intended to be introduced, or of a vehicle powered by such an engine, wherein the organic nitrate is defined herein.
In yet another aspect of the invention there is provided a method for the preparation of a diesel fuel composition having a target cetane number for use in a compression ignition engine. The method comprising adding an organic nitrate, as defined elsewhere herein, to the diesel fuel composition; and blending the organic nitrate with the diesel fuel composition to provide a diesel fuel composition having the target cetane number.
Still yet another aspect of the invention relates to a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition as defined elsewhere herein, or as obtained by the uses and methods of the invention.
Brief Description of the Drawings
The invention is further illustrated by the
accompanying drawings in which:
Figure 1 depicts the structures of representative organic nitrates of the invention;
Figure 2 illustrates the reduction in ignition delay (ID) as a percentage for a diesel base fuel comprising organic nitrates of the invention. The graphs show the data obtained at organic nitrate concentrations of 1.0% w/w (left, graphs A, C, E and G) and 0.1% w/w (right, graphs B, D, F and H) at representative combustion conditions (first row, condition a03, graphs A and B;
second row, condition a05, graphs C and D; third row, condition a07, graphs E and F; and fourth row, condition a08, graphs G and H) . Data for organic nitrates is as follows: exo bornyl nitrate (column 1); menthly nitrate (column 2); oleyl nitrate {column 3); 1,10-decyl
dinitrate (column 4}; 1-octadecyl nitrate (column 5) ;
nitro-substituted methyl oleate (column 6) ; cholesterol nitrate (column 7); 1-octyl nitrate (column 8); 1- tetradecyl nitrate (column 9) ; 2-ethylhexyl nitrate
(positive control, column 10} ; 1-hexadecyl nitrate
(column 11} ; dodecyl / tridecyl nitrate mixture (column 12) ; nitro-substituted ethyl abietate (column 13) ; and exo fenchyl nitrate (column 14};
Figure 3 illustrates the derived ignition quality (DIQ) for a diesel base fuel comprising exo-fenchol nitrate at concentrations of 0.1% w/w (left graph) and 1.0% w/w (right graph), measured under all test
combustion conditions, aOl to a08 (columns 1 to 8, respectively) and a09 to all (columns 10 to 12,
respectively) . In each graph, for comparative purposes the DIQ of the diesel base fuel under reaction condition a08 is illustrated (column 9) ;
Figure 4 is a graph illustrating correlations between the measured ignition delay at the various combustion conditions (aOl to all - see Key) against the organic nitrate used as a cetane enhancer in a diesel fuel composition according to the invention. The results are shown against molecular weight of the cetane
enhancer: bornyl nitrate (1), menthyl nitrate (2), 1,10- decyl dinitrate (3), oleyl nitrate (4), hexadecyl nitrate (5), nitro-substituted methyl oleate (6), and nitro- substituted ethyl abietate (7}; and Figure 5 is a differential scanning calorimetry (DSC) / thermogravimetric analysis (TGA) plot
illustrating the thermal decomposition of 1,10-decyl dinitrate .
Detailed Description of the Invention
In order to assist with the understanding of the invention several terms are defined herein.
The terms "cetane (number) improver" and "cetane (number) enhancer" are used interchangeably to encompass any component that, when added to a fuel composition at a suitable concentration, has the effect of increasing the cetane number of the fuel composition relative to its previous cetane number under one or more engine
conditions within the operating conditions of the
respective fuel or engine. The term cetane number
improvers / enhancers of the invention are organic nitrates as described herein. As used herein, a cetane number improver or enhancer may also be referred to as a cetane number increasing additive / agent or the like.
In accordance with the present invention, the cetane number of a fuel composition may be determined in any known manner, for instance using the standard test procedure ASTM D613 (ISO 5165, IP 41) which provides a so-called "measured" cetane number obtained under engine running conditions. More preferably the cetane number may be determined using the more recent and accurate
"ignition quality test" (IQT; ASTM D6890, IP 498), which provides a "derived" cetane number based on the time delay between injection and combustion of a fuel sample introduced into a constant volume combustion chamber. This relatively rapid technique can be used on laboratory scale (ca 100 ml) samples of a range of different fuels. Alternatively, cetane number may be measured by near infrared spectroscopy (NIR) , as for example described in USS349188. This method may be preferred in a refinery environment as it can be less cumbersome than for
instance ASTM D613. NIR measurements make use of a correlation between the measured spectrum and the actual cetane number of a sample. An underlying model is
prepared by correlating the known cetane numbers of a variety of fuel samples with their near infrared spectral data .
In some embodiments, the methods / uses encompass adding a cetane enhancer of. the invention to a fuel composition so as to adjust the cetane number or to achieve or reach a desired target cetane number. In the context of the invention, to "reach" a target cetane number can also embrace exceeding that number. Thus, the target cetane number may be a target minimum cetane numbe .
The present invention suitably results in a fuel composition which has a derived cetane number (IP 498) of 50 or greater, more preferably of 51, 52, 53, 54 or 55 or greater. For example, in some embodiments the resultant fuel composition may have a cetane number of 60 or greater, 65 or greater or even 70 or greater.
The present invention may additionally or
alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with cetane number, for example, to improve the combustion performance of the fuel composition, e.g. to shorten ignition delays {i.e. the time between fuel injection and ignition in a combustion chamber during use of the fuel), to facilitate cold starting or to reduce incomplete combustion and/or associated emissions in a fuel- consuming system running on the fuel composition} and/or to improve fuel economy or exhaust emissions generally.
In accordance with the invention, therefore, cetane number improvers also encompass additives that increase the combustability of the fuel to which it is added and, as such, decrease the ignition delay. Therefore, as used herein, an organic nitrate that increases the
combustability (i.e. a "combustion enhancer / improver") and/or decreases the ignition delay (i.e. an "ignition enhancer / improver") is also considered to be a cetane number improver or enhancer.
Cetane number improvers of the invention may be used to increase the cetane number of a fuel composition. As used herein, an "increase" in the context of cetane number embraces any degree of increase compared to a previously measured cetane number under the same or equivalent conditions. Thus, the increase is suitably compared to the cetane number of the same fuel
composition prior to incorporation of the cetane number increasing (or improving) component or additive.
Alternatively, the cetane number increase may be measured in comparison to an otherwise analogous fuel composition (or batch or the same fuel composition) that does not include the cetane number enhancer of the invention.
Alternatively, an increase in cetane number of a fuel relative to a comparative fuel may be inferred by a measured increase in combustability or a measured decrease in ignition delay for the comparative fuels.
The increase in cetane number (or the decrease in ignition delay, for example) may be measured and/or reported in any suitable manner, such as in terms of a percentage increase or decrease. By way of example, the percentage increase or decrease may be at least 1%, such as at least 2%. Suitably, the percentage increase in cetane number or decrease in ignition delay is at least 5%, at least 10%, at least 15% or at least 20%. In some embodiments the increase in cetane number or decrease in ignition delay may be at least 25%, at least 30%.
However, it should be appreciated that any measurable improvement in cetane number or ignition delay may provide a worthwhile advantage, depending on what other factors are considered important, e.g. availability, cost, safety and so on.
The engine in which the fuel composition of the invention is used may be any appropriate engine. Thus, where the fuel is a diesel or biodiesel fuel composition, the engine is a diesel or compression ignition engine. Likewise, any type of diesel engine may be used, such as a turbo charged diesel engine, provided the same or equivalent engine is used to measure fuel economy with and without the cetane number increasing component.
Similarly, the invention is applicable to an engine in any vehicle. Generally, the cetane number improvers of the invention are suitable for use over a wide range of engine working conditions. However, some organic nitrates of the invention may provide optimal effects under a particular narrow range of engine working conditions, such as under mild conditions and more suitably under harsh conditions.
Cetane dumber Enhancers / Ignition Improvers
Cetane number enhancers are known and commercially available, and may also be known (in the context of diesel fuels) as "cetane (number) improvers", "combustion improvers" and "ignition improvers" etc. as previously decribed . Cetane enhancers are often added to diesel fuels, at additive levels (typically 0.1 to 2.0% w/w) , to improve the combustion properties of the fuel. They function to reduce the ignition delay, i.e. the period between the time of injection of the fuel and the start of combustion (ignition) . This, in turn, leads to better engine
performance, for example, in terms of higher fuel
efficiency, lower emissions, reduced combustion noise and improved cold starting. Addition of a cetane enhancer to a diesel fuel allows the point in the diesel cycle at which heat is released to be advanced, which results in improved thermodynamic efficiency (maximum efficiency at about 10° after top dead centre.
Although there are various explanations of the working of cetane enhancers, such as their effect in increasing the heating rate of the fuel, it is generally accepted that they act as sources of chain-initiating radicals .
The cetane number (CN) of a fuel is defined by reference to the ignition properties of standard mixtures of n-hexadecane (cetane, CN = 100) and 2,2,4,4,6,8,8- hepta-methylnonane (CN = 15} . A fuel with a high CN has a short ignition delay. Typically, molecules with high octane numbers, which confer a resistance to spontaneous ignition in gasoline spark ignition engines, have low cetane numbers. The addition of small amounts of cetane enhancers to a diesel fuel may, therefore, result in improved fuel properties based on the shorter ignition delay.
Known cetane number enhancers include: a) certain organic nitrates (e.g. isopropyl nitrate, 2-ethylhexyl nitrate (2-EHN) , cyclohexyl nitrate, and methoxyethyl nitrate); b) organic peroxides and hydroperoxides (e.g. di-tert-butyl peroxide} ; and c) organic peracids and peresters. The most commonly used cetane enhances are dialkylperoxides (ROOR, di-t-butylperoxide) and organic nitrates (R-ON02) , of which the most important is 2- ethylhexylnitrate (2-EHN) . European consumption of 2-EHN grew from 75 kt/a to 101 kt/a from 2000 to 2008, and an average annual growth of approximately 3.5% has been predicted from 2008 to 2013.
The consumption of 2-EHN in North America {USA: 7.2 kt/a in 2008) is much lower than in Europe.
2-EHN is produced industrially by the nitration of 2-ethylhexanol, and in Europe this consumes almost a quarter of the production of this alcohol. The nitration of the alcohol involves reaction with a 1/1 mixture of undiluted nitric and sulfuric acids (using stoichiometric amounts of alcohol and nitric acid) .
There are some safety concerns surrounding the production, transport and use of 2-EHN. For example, it is conceivable that 2-EHN drums exposed to high
temperatures during transport could be subject to runaway decomposition reaction. The low flash point of 2-EHN (76°C) is also a concern. Furthermore, the auto-ignition temperature of 2-EHN of 130 "C is lower than normal hydrocarbons. There have been very many studies of the thermal stability of 2-EHN (e.g. Pritchard (1989),
Combustion and Flame, 75, 415; Bornemann & Scheldt
(2000), F. Int. J. Chem. Kinetics, 34, 34; and Zeng et al. (2008), J, Thermal Analysis & Calorimetry, 91, 359).
Accordingly, it was desired to identify further alternative cetane number improvers, which may provide benefits over the known cetane enhancer. Any useful benefit may be achieved, such as in relation to their synthesis, storage, transportation; or in use, e.g. under certain operating conditions or in certain diesel fuels. Particular benefits of the invention are directed to one or more of the following: increased stability under storage and transport conditions; at least equal and more suitably greater effectiveness as a cetane enhancer;
organic nitrates derivable from renewable feedstocks or waste streams; effectiveness under harsh engine
conditions; and effectiveness at component or more suitably at additive concentration levels.
Thus, the present invention provides alternative organic nitrates for use as cetane number improvers in diesel fuels and optionally for achieving one or more associated benefit.
The cetane number improver of the invention may be selected from nitrated:
(a) terpene alcohols, particularly monoterpene alcohols and most suitably monocyclic and bicyclic molecules including pinenes, such as borneol, fenchol and menthol;
(b) fatty alcohols, particularly obtained by
hydrogenation of fatty acid esters (e.g. the synthetic alcohol mixture, Neodol 23, derived from the SHF process)
(c) unsaturated fatty esters, particularly FAMEs, such as methyl oleate;
(d) tall oil derived resin esters, particularly abietate esters such as ethyl abietate; and
(e) long-chain linear alkanols, particularly linear Cio-Cie alkanols and diols, such as 1-octanol, 1,10,- decanediol, 1-dodecenol, 1-tridecanol, 1-tetradecanol, 1- hexadecanol, 1-octadecanol and mixtures thereof.
Accordingly, the cetane number improver has one or two nitrate (N03) groups in place of the hydroxyl groups in the above-mentioned compounds. With the exception of 1, 10, -decanediol, generally the alcohol feedstocks are commercially available at scales of at least 10 kt/a, which would potentially be compatible with production of new useful cetane enhancers for worldwide consumption. 1 , 10-decanediol may be
obtained by the hydrogenation of sebacic acid {1,8- octanedicarboxylic acid) , which itself is produced via the alkali fusion of ricinoleic acid, the major
constituent of castor oil.
Thus, in accordance with a first embodiment of the invention, the cetane number improver of the invention has the Formula (1): R-ONO2, wherein R is a terpene or an oxygenated (saturated) terpene. Optionally, the terpene may be natural or substituted by up to three (e.g. 1, 2 or 3) Ci-Ce alkyl groups or a further nitrate (-ON02) group .
Terpenes are classified according to the number of units of the basic structure methylbuta~-l , 3-diene or isoprene, which make up the terpene. Monoterpenes contain two isoprene units and are generally considered to have the chemical formula CioH16. However, monoterpenes are particularly sensitive to oxygenation at the carbon- carbon double bond and so monoterpenes are typically saturated hydrocarbons lacking the carbon-carbon double bond. These oxygenated, saturated molecules are sometimes also referred to as monoterpenes, and are encompassed by the term "monoterpene" as used in the context of this invention. Monoterpenoid is another term that is
understood to include the monoterpenes and other related compounds having the monoterpene skeleton, and such monoterpenoid structures are also encompassed within the definition of monoterpene used herein. Monoterpenes may be acyclic such as myrcene and ocimene or cyclic such as limonene and pinene. Suitably the terpene is a monocyclic or a bridged-monocyclic (i.e. bicyclic) alkyl.
In one suitable embodiment the cetane enhancer of the invention has the Formula (2): CioHi5 -ON02 , wherein X is selected from H, Cx-C6 alkyl and O O2. More suitably, X is selected from H, methyl, ethyl and ON02; and still more suitably X is H. Most suitably, R of Formula (1) is a monoterpene selected from menthyl, fenchyl and bornyl, which may optionally be substituted by X as defined above .
According to the invention, therefore, the cetane number improver ma be a compound of Formula (3) :
or Formula (
Figure imgf000019_0001
wherein each of Ra to R9 is independently selected from H or Ci-C6 alkyl, or nitrate ( -ONO2 ) , wherein optionally two of Ri to Rg may be connected together to form a bridge, which may be substituted by one or more Ci-C6 alkyl, and/or nitrate (-ON02} ; provided that no more than 1 Rx to R9 comprises a nitrate group. Suitably, Ri to Rg are independently selected from H or Ci~Cg alkyl, wherein optionally one of R4 and R5 forms an optionally
substituted alkylene bridge with one of Rg and Rg.
Preferably at least one of Ri to R9 is not H; more preferably 1, 2, 3, 4 or 5 of Rx to Rg is not H.
The C1-C6 alkyl may be straight chain (i.e. linear) or branched chain, wherein the number 1 to 6 refers to the total number of carbon atoms in the group. Suitable
Ci-Ce alkyl radicals include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, sec-butyl, n- pentyl, isopentyl, tert-pentyl, sec-pentyl, n-hexyl, 2- ethylbutyl, and 2 , 3-dimethylbutyl .
In accordance with a preferred embodiment, i to Rg are independently selected from H, methyl, ethyl, n~ propyl, isopropyl, n-butyl, isobutyl and tert-butyl, wherein optionally one of R4 and R5 forms an optionally substituted alkylene bridge with one of Rg and Rg. More suitably, the alkylene bridge has the formula ~{CRaRb)n-/ wherein Ra and Rb are independently selected from. H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2. Still more suitably, Ra and Rb are independently selected from H, methyl and ethyl, and n is 1; and yet more suitably selected from H and methyl, and n is 1.
In a preferred embodiment, Ri, Rg and R7 are H. In another preferred embodiment Ri, R6 and R7 are H, one of R4 and R5 is H, one of R8 and Rg is H, and the other of R4 and R5, and R8 and R9 is selected from methyl, ethyl, n- propyl, isopropyl, n-butyl, isobutyl and tert-butyl or they are connected together to form an alkylene bridge of the formula -CRaRb-, wherein Ra and Rb are defined above. In one particularly preferred embodiment, one of R4 and R5 is H, and the other is methyl; and one of Rs and Rg is H, and the other is isopropyl; wherein Ri, R2, R3f e and R7 are as defined in any of the above embodiments
preferably wherein Ri, R6 and R7 are B, and R2 and R3 are as defined in any of the above embodiments; and most preferably wherein i, R2, R3, e and R7 are H.
In yet another particularly preferred embodiment of Formula (3) or (3A) , one of R¾ and R5 is H, one of Re and R9 is H or methyl, and the other of R<j and R5 and of R8 and Rg are connected together to form an alkylene bridge of the formula -CRaRb-, wherein Ra and Rb are H or methyl. Beneficially in this preferred embodiment, one of Rs and R9 is methyl, Ri, Re and R7 are H, Ra and Rb are H or methyl, and R2 and R3 are as defined in any of the above embodiments. In one preferred group of compounds of this embodiment at least two of Ra, Rbr R2 and R3 are methyl. For example, in one embodiment Ra and Rb are methyl, and in another embodiment R2 and R3 are methyl.
Accordingly, in another preferred embodiment of the invention, the cetane number improver is defined by Formula (4) :
Figure imgf000021_0001
Re Formula (4A)
Figure imgf000022_0001
wherein each of Ri to R9 is as defined in connection with Formula (3). Suitably, at least one of Ri to Rg is not H and sill more suitably, 1, 2, 3, 4 or 5 of Ri to R9 is not H. In a preferred structure of Formula {4) or Formula (4A) R8 and R4 are connected together to form an alkylene bridge of the formula -~ iCRaRb) r wherein Ra and Rb are independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2. More preferably the alkylene bridge has the formula -CRaRb-/ wherein Ra and Rb are H or methyl. Also preferred are compounds of Formula (4) and Formula (4A) in which Ri, R6 and R7 are H. Still more preferred are compounds of Formula {4) and Formula (4A) wherein Ri, R6 and R7 are H and at least two of Ra, Rb, R2 and R3 are methyl. In one particular embodiment Ra and Rb are methyl, and in another particular embodiment R2 and R3 are methyl.
In yet another preferred embodiment, the cetane number improver is defined by Formula (5) :
Figure imgf000023_0001
Figure imgf000023_0002
wherein each of Ri to R3f R5 to R7, R9, Ra and Rb are as defined in connection with any embodiment of Formulas (3) or {3A) , or as defined in connection with any embodiment of Formulas (4) or (4A) . Preferably, in the compound of Formulas (5) and (5A) , Ri, R6 and R7 are H; and R2, R3, R5, Rg, Ra and are independently selected from H, methyl and ethyl. Beneficially at least one of R2, R3, R5 , Rg, Ra and Rb is not H, and preferably 1, 2 or 3 of R2, R3, sr Rg, Ra and Rb is not H. Most preferred are compounds wherein three of R2/ R3, R5, R9, Ra and are methyl.
In the cetane number improver compounds of the invention, the nitrate may be arranged in an axial or equatorial position relative to the alkyl ring. In a preferred embodiment, however, the nitrate is attached in an equatorial position on the ring. Most preferred cetane improvers of this embodiment of the invention are bornyl nitrate, fenchyl nitrate and menthly nitrate.
In accordance with a second embodiment of the invention, the cetane number improver is a compound of the Formula (6) : Rx-ON02, wherein Rx is a linear
(straight-chain) aliphatic group having 8 to 24 carbon atoms. Suitably, the aliphatic group has between 10 and 20 carbon atoms, and more suitably between 10 and 18 carbon atoms. For example, particularly suitable Rx groups have 8, 10, 12, 13, 14, 16 or 18 carbon atoms; and a most suitable Rx group has 18 carbon atoms. The
aliphatic group may be saturated or unsaturated. When unsaturated it is preferably mono-unsaturated. The Rx group may optionally be substituted with a nitrate (- ON02) group to form a dinitrate compound. Preferably the one to two nitrate groups are attached to the terminal carbon atoms of the aliphatic group.
Suitable cetane enhancers of the invention of
Formula (6) include 1-octyl nitrate, 1,10,-decyl
dinitrate, 1-dodecyl nitrate, l~tridecyl nitrate, 1- tetradecyl nitrate, 1-hexadecyl nitrate, and 1-octadecyl nitrate .
In accordance with one embodiment, the Rx group a fatty acid derivative, particularly a fatty alcohol derivative. A preferred fatty alcohol derivative is monounsaturated, and is more preferably a cis- monounsaturated fatty alcohol derivative (i.e. wherein adjacent hydrogen atoms are on the same side of the double bond) . Thus, in some embodiments, the cetane number improver of the present invention has the Formula (7) :
CH3 (CH2 ) XCH=CH {CH2) yON02 wherein X is between 3 and 8 and Y is between 4 and 9. Suitably, X is between 5 and 7 and Y is between 6 and 8. More suitably, X is 5 or 7 and Ϊ is 6 or 8. In a
preferred embodiment, the carbon-carbon double bond is in the cis formation.
A preferred monounsaturated aliphatic nitrate of the invention is oleyl nitrate.
In accordance with a third embodiment of the
invention, the cetane number improver of the invention is a nitro-substituted fatty acid ester of the Formula (7) : Ry-[0sN0t]u, wherein Ry is a alkyl ester of a fatty acid, i.e. a fatty acid alkyl ester (FAAE) moiety, S is 0 or 1, t is 1 or 2, and u is 1 to 4 or 1 to 3.
Suitably, the FAAE group is a linear (straight- chain) aliphatic group having 8 to 24 carbon atoms, more suitably between 10 and 22 carbon atoms, still more suitably between 12 and 20 carbon atoms, and most
suitably between 14 and 18 carbon atoms. For example, particularly suitable Ry groups have 14, 16 or 18 carbon atoms; and advantageously the Ry group has 18 carbon atoms. The aliphatic group may be saturated or
unsaturated. Preferably Ry is mono-unsaturated, and more preferably the Ry group has a cis double-bond. The Ry group may optionally be substituted with one or two additional nitro-substituted groups of formula OsNOt.
A preferred nitro-substituted fatty acid ester of the invention is a methyl oleate substituted with 1, 2 or 3 OsNOt groups.
In accordance with a fourth embodiment of the invention the cetane number improver of the invention is a nitro-substituted diterpene of the Formula (8): Rz- [OsNOt]u, wherein Rz is a diterpene or an alkyl ester of a diterpene, and wherein [OsNOt]u is as defined above. Suitably the compound of Formula (8) is an alkyl ester of abietic acid. More suitably the ester is a methyl or ethyl ester. The RE group may optionally be substituted with one or two additional nitro-substituted groups of formula OsNOt. Most suitably the nitro-substituted diterpene is nitro-substituted ethyl abietate substituted with 1, 2 or 3 OsNOt groups.
In accordance with a fifth embodiment of the
invention the cetane number improver of the invention is a nitro-substituted steroid containing from 1 to 3 nitrate groups, such as cholesterol nitrate.
In use, the cetane number improving additive of the invention may be pre~dissolved in a suitable solvent, for example an oil such as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture; a fuel component (which again may be either mineral or Fischer-Tropsch derived} compatible with the diesel fuel composition in which the additive is to be used {for example a middle distillate fuel component such as a gas oil or kerosene) ; a poly alpha olefin; a so-called biofuel such as a fatty acid alkyl ester (FAAB), a Fischer-Tropsch derived biomass-to- liquid synthesis product, a hydrogenated vegetable oil, a waste or algae oil or an alcohol such as etnanol ; an aromatic solvent; any other hydrocarbon or organic solvent; or a mixture thereof. Preferred solvents for use in this context are mineral oil based diesel fuel
components and solvents, and Fischer-Tropsch derived components such as the "XtL" components referred to below. Biofuel solvents may also be preferred in certain cases. Typically, the cetane enhancer will be part of an additive (performance) package additionally containing other additives such as detergents, anti-foaming agents, corrosion inhibitors, dehazers etc. Alternatively, the cetane enhancing agent of the invention may be blended directly with the base fuel.
The concentration of the cetane number enhancing additive used may depend on desirable fuel
characteristics / properties, such as: the desired combustability of the overall fuel composition; the combustability of the composition prior to incorporation of the additive; the combustability and/or stability of the additive itself; and/or the properties of any solvent in which the additive is used. By way of example, the concentration of the cetane number improving additive in the fuel composition may be up to 2% w/w and suitably up to 1.0% w/w. Thus, the concentration of the cetane number improver may be from 0.025% w/w to 2% w/w, or from 0.05% w/w to 1% w/w. In some cases, the concentration of the cetane number improver is from 0.05% w/w to 1.0 w/w, such as 0.05% w/w, 0.1% w/w, 0.25% w/w, 0.5% w/w, 0.75% w/w or 1.0% w/w based on the total weight of the fuel
composition.
Where a combination of two or more cetane number improving additives is used in the fuel composition, the same concentration ranges may apply to the total
combination of cetane number improving additives. It will be appreciated that amounts / concentrations may also be expressed as ppm, in which case 1% w/w corresponds to 10, 000 ppm w/w.
The remainder of the composition will typically consist of one or more automotive base fuels optionally together with one or more fuel additives, for instance as described in more detail below.
The relative proportions of the cetane number enhancer, fuel components and any other components or additives present in a diesel fuel composition prepared according to the invention may also depend on other desired properties such as density, emissions performance and viscosity.
The synthesis of the cetane enhancers of the
invention is described further below.
Synthesis of Cetane Number Enhancers
A range of alcohols and olefins were evaluated as precursors of cetane enhancers. The conversion of
alcohols to nitrates is generally well-known in the art (Olah et al. Nitration: Methods and mechanisms, Chapter
4, Aliphatic nitration in Organic nitro chemistry series, VCH, New York, 1989, p. 219; Boschan et al. (1955) Chem. Rev. r 55, 485), and any appropriate procedure can be used to produce the organic nitrates of the invention.
Although olefinic substrates may afford nitrates
(i.e. R-0N02) on reaction with typical nitrating agents (HN03f N2O4, N205 etc.), more typically they react to give products with nitro (R-N02) and other substituents . Since nitroalkanes are inherently unstable (decomposing via both non-radical, HONO elimination; and radical
mechanisms, C-N02 bond homolysis) ; like nitrates, they are potential sources of radicals and may, therefore, also be useful as cetane enhancers.
It was anticipated that, in most cases, the alcohols could be easily converted to the corresponding nitrates by reaction with nitric acid. Alcohol nitration involves attack of the -OH functionality on N02 +, and may be generated using the following reagents: (i) nitric acid for secondary alcohols; (ii) a mixture of nitric acid, sulfuric acid and urea for primary or secondary alcohols; and (iii) a mixture of nitric acid and acetic anhydride (precursor of reactive acetyl nitrate) in acetic acid for unsaturated alcohols. The choice of reagent used to synthesise the nitrate or nitro-compounds of the invention may depend on the reactivity of the alcohol starting material and the desirability to avoid side reactions, such as oxidation of the alcohol to a ketone, or sulfation of the olefin.
Reactive olefins, such as pinenes, can be cleanly
converted to nitrates, for example, by (ring-opening) reaction with nitric acid (e.g. Canoira et al. {2007} Fuel, 86, 965; Bakhvalov et al. (2000) J. Organic Chem. , 36, 1601; Bakhvalov et al. (2002) J. Organic Chem., 38,
507). However, olefins containing ester functionalities (such as unsaturated constituents of FAME) may undergo partial hydrolysis of the ester under these conditions, affording free acid, which is not desirable in a diesel fuel component. In these cases N204 may suitably be used instead of nitric acid. Reaction with olefins normally, therefore, results in a mixture of products, containing nitro (R-N02) , as well as nitrate (R-ONO2) and hydroxyl functionalities .
Synthesis of the cetane enhancing agents of the invention were initially performed at small scale (ca. 2 g) to investigate optimum conditions (e.g. yield,
purity), and for safety reasons. The reactions were then scaled up to afford 20 to 40 g of the cetane enhancers, which (following thermal stability tests) were sent for assays in a combustion research unit (CRU) .
Following the synthesis of the organic nitrates, a differential scanning calorimetry / thermal gravimetric analysis (DSC/TGA measurement) was performed to determine the thermal decomposition behaviour of the molecules, before shipping for further analysis. Conversion of Primary Alcohols
The most common method of converting primary / secondary alcohols to nitrates involves the use of mixed nitric and sulfuric acids (molar ratio substrate: nitric acid: sulfuric acid generally 1:3:8), together with urea (0.25 equivalent based on substrate). The urea is used to remove any HN02 (nitrous acid) formed. The mixed acid is a more powerful nitrating agent than nitric acid alone, due to the higher concentration of N02 +, which is the reactive species. A general method is shown in Scheme 1 below .
Figure imgf000030_0001
urea
0 °C
Scheme 1: Preparation of Nitrate Esters from
Primary Alcohols
The primary alcohols were nitrated at 0°C for 2 hours. Following neutralisation, extraction with
dichloromethane and drying, the primary nitrates were obtained in a yield of 80-98%. Where a di-nitrate is to be synthesised from a di-hydroxy starting material the relative proportions of reaction components are selected to achieve two equivalents of HNO3 to ensure complete conversion of hydroxyl groups. In some cases lower than expected yields of nitrated alkyls were obtained, which may be due to dissolution of part of the nitrated product in the water layer.
Exemplary nitrated alkyls (compounds 1 to 6) of the invention are illustrated in Figure 1.
Conversion of Secondary Alcohols
In some cases secondary alcohols may be nitrated using the same reagent mixture and conditions as used for primary alcohols. Alternatively, nitric acid may be used, which is a less powerful oxidising agent than the mixed acid.
One exemplary secondary alcohol is endo-borneol, which may be converted into a bornyl nitrate.
By way of example, the secondary alcohols may be nitrated by reacting with nitric acid at room temperature (RT) for approximately 24 hours. In another example, exo~ borneol was reacted with nitric acid at RT to obtain the exo-nitrate of borneol (compound 7, yield 87%; see reaction Scheme 2 below and Figure 1} .
Figure imgf000031_0001
e o-borneoi
Scheme 2: Preparation of Nitrate Ester of Exo-borneol Nitrated products from exo-fenchol and endo-fenchol may be obtained in a similar manner to the borneol compounds (see reaction Scheme 3} . Unlike endo-fenchol, exo-fenchol is not commercially available, but may be prepared by the reduction of L-fenchone via a Meerwein- Ponndorf-Verley reduction (MPV; see e.g. Huckel & Rohrer (I960) Che . Ber. , 93, 1053; Mojtahedi et al. (2007) Org. Lett., 9, 2791) . This was found to give a mixture of the endo™ and exo-fenchol in the ratio 1:3. The MPV reaction is an aluminium-catalyzed hydride shift (of RCHOH) , from the alcohol (isopropanol) to the carbonyl carbon (L~ fenchone) . Isopropanol may be used as a hydride donor, because the acetone formed can be easily removed by distillation. The reaction was refluxed at 95 °C for 7 days followed by an extraction. This mixture of secondary alcohols was allowed to react with nitric acid for 24 hours at RT, to give a mixture of two nitrated compound (compounds 8a, 8b; see Figure 1) as the main products, together with unreacted endo~fenchol . The sterically crowded endo-isomer appears not to react with the milde nitrating reagent, whereas the exo-fenchol affords the two isomeric nitrates, as confirmed by 1R and 13C NMR spectroscopy.
Figure imgf000032_0001
-isofenchylnitrate Menchy Initiate endo-fencho!
Scheme 3: Preparation of Fenchol Isomers from Fenchone and Conversion of Exo-fenchol to Isomeric Nitrate Esters The secondary alcohol, menthol (e.g. L-isomer) , may be nitrated using the same conditions as for primary alcohols in general (i.e. mixed acid; see Scheme 4).
After extraction with diethyl ether, the nitrated product (compound 9; see Figure 1) was obtained in a yield of
Figure imgf000033_0001
i-Menthol
Scheme 4: Preparation of Nitrate Ester of L-menthol
Conversion of -Terpineol
β-pinene may be reacted with nitric acid, e.g. at -15 °C in dichloromethane, according to methods known to the person of skill in the art (see reaction Scheme 5 below) . After neutralisation, the product may be isolated by extraction with dichloromethane. NMR may be used to confirm the identit of the product.
Figure imgf000033_0002
Scheme 5: Preparation of Nitrate Ester of Alpha-terpineol from β-pinene
As illustrated, this reaction proceeds via
protonation of the double bond in β-pinene, followed by rearrangement ( β-fragmentation) to relieve ring strain and attack of NO3 "" on the tertiary carbonium ion. Tertiary nitrates are generally known to be less thermally stable than primary and secondary nitrates.
Therefore, it may be necessary to store and ship tertiary nitrates, such as a-terpineol nitrate, with particular
G3.J €¾
Conversion of Unsaturated Alcohols
In general, unsaturated alcohols can be nitrated selectively at the alcohol position to leave the double bond intact, provided that the mode of introducing the reactants and the amount of nitrating agent is suitably controlled, and the use of sulfuric acid is avoided (as this may react with the double bond to give a sulfate) . Thus, any suitable method can be used.
By way of example, oleyl alcohol (e.g. cis-9- octadecen-l-ol) was nitrated slowly adding nitric acid to a mixture of the alcohol and acetic anhydride in acetic acid solvent at 15°C (see Scheme 6) . Advantageously, this method avoids build-up of significant concentrations of acetyl nitrate (which may undergo runaway decomposition above 60 °C) . Acetyl nitrate is a powerful nitrating agent, being a good source of N02 +, which reacts with the alcohol. The product (compound 10; Figure 1) can then be extracted .with a suitable solvent, such as diethyl ether. 1H-NMR confirmed the clean formation of the desired single product without significant by-products.
Nitration of unsaturated secondary alcohols, such as cholesterol may be performed similarly so as to form cholesterol nitrate (compound 11) shown in Figure 1.
Figure imgf000034_0001
Oleyi alcohoi
Scheme 6: Preparation of Nitrate Ester of Oleyl Alcohol Conversion of Olefinic Esters
The formation of nitrate or nitro derivatives of olefins may be carried out using any appropriate reaction scheme. Typically, conditions used will be different to those described above for non-olefinic esters. For example, it is desirable to avoid sulfuric acid (to reduce risk of olefin sulfation) . Exemplary conditions include: (i) HN03 (70%) and acetic anhydride (precursor of acetyl nitrate); (ii) fuming HN03, acetic acid and
NaN02" (iii) N2O4 in chloroform or hexane .
When used in the nitration of FAMEs, processes (i) and (ii) have been reported to result in the partial hydrolysis of the ester. For example, scheme (i) has been found to form nitro/nitrate and nitro/acetate products as well as nitro-substitution of the allylic position.
Scheme (ii) has been found to result in low conversion rates of unsaturated FAMEs, with nitro-olefins formed in low yields. Therefore, reaction scheme (iii) may be preferred, since it is expected to avoid ester
hydrolysis. N2O may conveniently be used as a dilute solution (boiling point 30-100°C) , and excess reagent removed under low pressure at the end of the procedure. Chloroform and hexane were both used as solvents.
Three unsaturated FAMEs and a resin ester were nitrated using this procedure. In a typical reaction, methyl oleate was added to a stock solution of N2O4 in chloroform at 0C and stirred for 48 hours. After,
N2O4 was removed in vacuo and the product quenched in an ice bath. The products can be extracted in a suitable solvent, such as ether, and dried. A mixture of products was identified by XH and 13C NMR analysis.
Analysis indicated that the main products of the nitration reaction were the 1,2-dinitro compound, (02N) C (R) H-C {R' )H(N02) and nitro alcohols (e.g. N02...OH) . It is likely that addition of a nitro radical to the double bond is followed by reaction of the resulting radical with a second nitro radical to give the 1,2- dinitroalkane and l~nitro-2-nitritoalkane . The nitrite group (R-ONO) in the l-nitro-2-nitrito product tends to undergo hydrolysis during work-up to give the nitro- alcohol. Only small amounts of nitroalkene (RCH-C (R' ) N02] and the allylic nitro products were observed. The reaction and the resultant mixture of products is illustrated in Scheme 7 below.
Figure imgf000036_0001
instead of double bond
Figure imgf000036_0002
Scheme 7: Product Mixture Obtained from the Reaction of Methyl Oleate with NgOg (groups shown in lower half of scheme replace carbon-carbon double bond) Similar reactions were carried out to convert other FAMEs, e.g. methyl linoleate (methyl cis,cis~9, 12- octadecadienoate ) and methyl linolenate {methyl
cis, cis,cis-9f 12, 15-octadecatrienoate ) in chloroform and hexane as solvents.
Nitration products of abietic acid - the most prevalent of the organic acids that form the largest constituent of rosin - such as ethyl abietate were also synthesised in similar fashion. Reactions were carried out using N204 in either hexane or chloroform and a similar mixture of nitrogen-containing products was formed. Hexane is a preferred solvent for this reaction. The NMR data was consistent with the formation of dinitro and nitro-alcohol products containing a single double bond (e.g. 1, 4~dinitro-2-alkene) .
DSC/TGA analysis showed that the product mixture underwent an exothermic decomposition reaction in the temperature range 150-250°C, making a full analysis of the products difficult.
Since the relative thermal stability of the organic nitrates of the invention is one of the key
characteristics for determine (and explain) how rapidly they combust and, hence, how effective they may be as combustion improvers (cetane number enhancers) when used in a fuel within a diesel engine, thermal stability assays (e.g. differential scanning calorimetry /
thermogravimetric analysis (DSC/TGA) ) of the products were also conducted, as described in the Examples. Mass spectrometry (MS) may also be used to provide information on the decomposition mechanism.
DSC measures heat flow resulting from evaporation (endotherm) or thermal decomposition of the compound (exotherm) . TGA measures the weight loss on evaporation or decomposition. MS of the gas space allows the identity of the volatile decomposition products to be determined.
As well as the decomposition temperature, the associated exotherm may, in principle, be determined. These
measurements on organic nitrates according to the invention were intended to provide an initial assessment of their relative thermal decomposition behaviour.
Diesel Fuel Compositions
In one aspect of the invention, there is provided a diesel fuel composition, which comprises a cetane number improver of the invention. In particular, the cetane number improver is present at a concentration sufficient and appropriate for achieving a desired cetane number in the resultant fuel composition.
A diesel fuel composition prepared in accordance with the present invention may in general be any type of diesel fuel composition suitable for use in a compression ignition (diesel) engine; and it may itself comprise a mixture of diesel fuel components.
Thus, in addition to the cetane enhancer, a diesel fuel composition prepared according to the present invention may comprise one or more diesel fuel components of conventional type. It may, for example, include a major proportion of a diesel base fuel, for instance of the type described below. In this context, a "major proportion" means at least 50% w/w, and typically at least 85% w/w based on the overall composition. More suitably, at least 90% w/w or at least 95% w/w. In some cases at least 98% w/w or at least 99% w/w of the fuel composition consists of the diesel base fuel.
Accordingly, in some embodiments, the base fuel may itself comprise a mixture of two or more diesel fuel components of the types described below.
Typical diesel fuel components comprise liquid hydrocarbon middle distillate fuel oils, for instance petroleum derived gas oils. Such base fuel components may be organically or synthetically derived, and are suitably obtained by distillation of a desired range of fractions from a crude oil. They will typically have boiling points within the usual diesel range of 150 to 410 °C or 170 to 370 °C, depending on grade and use. They will typically have densities from 0.75 to 0.9 g/cm3, such as from 0.8 to 0.86 g/cm3, at 15°C (IP 365) and measured cetane numbers (ASTM D613) of from 35 to 80, more preferably from 40 to 75. Their initial boiling points will suitably be in the range 150 to 230 *C and their final boiling points in the range 290 to 400 'C. Their kinematic
viscosity at 40 °C {ASTM D445) might suitably be from 1.5 to 4.5 centistokes. Such fuels are generally suitable for use in compression ignition (diesel) internal combustion engines, of either the indirect or direct injection type.
An automotive diesel fuel composition which results from carrying out the present invention will also
suitable fall within these general specifications.
Accordingly, it will generally comply with applicable current standard specification {s) such as for example EN 590 (for Europe) or ASTM D975 (for the USA). By way of example, the fuel composition may have a density from 0.82 to 0.845 g/cm3 at 15°C; a T95 boiling point (ASTM D86) of 360°C or less; a cetane number (ASTM D613) of 45 or greater; a kinematic viscosity (ASTM D445} from 2 to 4.5 mm2/s at 40 °C; a sulphur content (ASTM D2622) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons ( PAH) content (IP391 (mod)) of less than 11% w/w.
Relevant specifications may, however, differ from country to country and from year to year and may depend on the intended use of the fuel composition. In particular, its measured cetane number will preferably be from 45 to 70, to 75 or to 80, more preferably from 50 to 65, or at least greater than 50, greater than 55, greater than 60, or greater than 65.
A petroleum derived gas oil, e.g. obtained from refining and optionally (hydro) rocessing a crude
petroleum source, may be incorporated into a diesel fuel composition. It may be a single gas oil stream obtained from such a refinery process or a blend of several gas oil fractions obtained in the refinery process via different processing routes. Examples of such gas oil fractions are straight run gas oil, vacuum gas oil, gas oil as obtained in a thermal cracking process, light and heavy cycle oils as obtained in a fluid catalytic
cracking unit, and gas oil as obtained from a
hydrocracker unit. Optionally a petroleum derived gas oil may comprise some petroleum derived kerosene fraction. Such gas oils may be processed in a hydro- desulphurisation (HDS) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition. This also tends to reduce the content of other polar species such as oxygen- or nitrogen- containing species. In some cases, the fuel composition will include one or more cracked products obtained by splitting heavy hydrocarbons.
In some embodiments of the present invention, the base fuel may be or contain another so-called "biodiesel" fuel component, such as a vegetable oil, hydrogenated vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester, FAME) , or another oxygenate such as an acid, ketone or ester. Such components need not necessarily be bio- derived. Where the fuel composition contains a biodiesel component, the biodiesel component may be present in quantities up to 100%, such as between 1% and 99% w/w, between 2% and 80% w/w, between 2% and 50% w/w, between 3% and 40% w/w, between 4% and 30% w/w, or between 5% and 20% w/w. In one embodiment the biodiesel component may be FAME.
A diesel base fuel may consist of or comprise a Fischer-Tropsch derived diesel fuel component, typically a Fischer-Tropsch derived gas oil. As used herein, the terra "Fischer-Tropsch derived" means that a material is, or is obtained from, a synthesis product of a Fischer- Tropsch condensation process- A Fischer-Tropsch derived fuel or fuel component will therefore be a hydrocarbon stream in which a substantial portion, except for added hydrogen, is derived directly or indirectly from a
Fischer-Tropsch condensation process.
Fischer-Tropsch fuels may be derived by converting gas, biomass or coal to liquid (XtL) , specifically by gas to liquid conversion (GtL) , or from biomass to liquid conversion (BtL) . Any form of Fischer-Tropsch derived fuel component may be used as a base fuel in accordance with the invention.
The base fuel suitably has a low sulphur content, for example at most 1000 mg/kg (1000 parts per million by weight / ppmw) . More suitably it will have a low or ultra low sulphur content, for instance at most 500 mg/kg (500 ppmw) , such as no more than 350 mg/kg (350 ppmw) , and still more suitably no more than 100 or 50 or 10 or even 5 mg/kg (5 ppmw) of sulphur. It may be a so-called "zero- sulphur" fuel; although in some cases it may be desired that the base fuel is not a sulphur free ("zero sulphur") fuel. Ideally a fuel composition which results from carrying out the present invention will also have a sulphur content falling within these limits.
The diesel fuel composition which results from the method of the present invention may, if desired, contain no, or only low levels of additional cetane improving (ignition improving) additives such as 2-ethylhexyl nitrate (2-EHN) . In other words, the present invention embraces the use of certain organic nitrates in a diesel fuel composition for the purpose of reducing the level of a second (or further) cetane improving additive in the composition.
Furthermore, a fuel composition prepared according to the present invention, or a base fuel used in such a composition may contain one or more fuel additives, or may be additive-free. If additives are included (e.g.
added to the fuel at the refinery) , it may contain minor amounts of one or more additives. Selected examples or suitable additives include (but are not limited to) :
anti-static agents; pipeline drag reducers; flow
improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers) ; lubricity
enhancing additives (e.g. ester- and acid-based
additives) ; viscosity improving additives or viscosity modifiers (e.g. styrene-based copolymers, zeolites, and high viscosity fuel or oil derivatives); dehazers (e.g. alkoxylated phenol formaldehyde polymers}; anti-foaming agents (e.g. polyether™modified polysiloxanes) ; anti-rust agents (e.g. a propane-1, 2~diol semi-ester of
tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative) ; corrosion inhibitors; reodorants; anti-wear additives; antioxidants (e.g.
phenolics such as 2 , 6-di-tert-butylphenol) ; metal
deactivators; combustion improvers; static dissipator additives; cold flow improvers (e.g. glycerol monooleate, di-isodecyl adipate) ; antioxidants; and wax anti-settling agents. The composition may for example contain a
detergent. Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove or slow the build up of engine deposits. In some embodiments, it may be advantageous for the fuel
composition to contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity enhancing additive.
Where the composition contains such additives (other than the cetane number increasing components of the invention) , it suitably contains a minor proportion (such as 1% w/w or less, 0.5% w/w or less, 0.2% w/w or less), of the one or more fuel additives, in addition to the cetane number increasing component (s) . Unless otherwise stated, the (active matter) concentration of each such additive component in the fuel composition may be up to 10000 ppmw, such as in the range of 0.1 to 1000 ppmw; and advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw .
If desired, one or more additive components, such as those listed above, may be co-mixed {e.g. together with suitable diluent) in an additive concentrate, and the additive concentrate may then be dispersed into a base fuel or fuel composition. In some cases, it may be possible and convenient to incorporate the cetane number increasing component of the invention into such an additive formulation. Thus, the cetane number improving additive may be pre-diluted in one or more such fuel components, prior to its incorporation into the final automotive fuel composition. Such a fuel additive mixture may typically contains a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a mineral oil, a solvent such as those sold by Shell companies under the trade mark "SHELLSOL", a polar solvent such as an ester and, in particular, an alcohol {e.g. hexanol, 2™
ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark "LINEVOL", especially LINEVOL 79 alcohol which is a mixture of C7_9 primary alcohols, or a C12-1 alcohol mixture which is commercially available) .
The total content of the additives in the fuel composition may be suitably between 0 and 10000 ppmw and more suitably below 5000 ppmw.
As used herein, amounts (e.g. concentrations, ppmw and %w/w) of components are of active matter, i.e.
exclusive of volatile solvents/diluent materials.
In one embodiment, the present invention involves adjusting the cetane number of the fuel composition, using the cetane number enhancing component, in order to achieve a desired target cetane number.
The maximum cetane number of an automotive fuel composition may often be limited by relevant legal and/or commercial specifications, such as the European diesel fuel specification EN 590 that stipulates a cetane number of 51. Thus, typical commercial automotive diesel fuels for use in Europe are currently manufactured to have cetane numbers of around 51. Thus, the present invention may involve manipulation of an otherwise standard
specification diesel fuel composition, using a cetane number enhancing additive, to increase its cetane number so as to improve the combustability of the fuel, and hence reduce engine emissions and even fuel economy of an engine into which it is, or is intended to be,
introduced.
Suitably, the cetane number improver increases the cetane number of the fuel composition by at least 3 cetane numbers. In some particular embodiments, the cetane number increase may be up to approximately 9, or any value in between these ranges. Accordingly, in other embodiments , the cetane number of the resultant fuel is between 51 and 60.
An automotive diesel fuel composition prepared according to the present invention will suitably comply with applicable current standard specification (s) such as, for example, EN 590 (for Europe) or ASTM D-975 (for the USA) . By way of example, the overall fuel composition may have a density from 820 to 845 kg/m3 at 15°C (ASTM D- 4052 or EN ISO 3675) ; a T95 boiling point (ASTM D-86 or EN ISO 3405) of 360°C or less; a measured cetane number
(ASTM D-613) of 51 or greater; a VK 40 (ASTM D-445 or EN ISO 3104) from 2 to 4.5 mm2/s; a sulphur content (ASTM D- 2622 or EN ISO 20846) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons (PAH) content (IP 391 (mod) ) of less than 11% w/w. Relevant specifications may, however, differ from country to country and from year to year, and may depend on the intended use of the fuel composition .
It will be appreciated, however, that diesel fuel composition prepared according to the present invention may contain fuel components with properties outside of these ranges, since the properties of an overall blend may differ, often significantly, from those of its individual constituents.
Uses and Methods
In accordance with one aspect of the invention, there is provided the use of a cetane number improver of the invention to achieve a desired cetane number of the resultant fuel composition. In some embodiments the desired cetane number is achieved or intended to be achieved under a specified set or range of engine working conditions, as described elsewhere herein. Accordingly, an advantage of the present invention is that cetane number enhancers of the invention may be suitable for reducing the combustion delay of a fuel composition under all engine running conditions, or under mild, or under harsh engine conditions. Cetane number enhancers of the invention may serve to improve combustion and, hence, improve associated engine factors, such as exhaust emissions and/or engine deposits under a range of engine operating conditions - particularly under harsh engine conditions when fuel emissions might otherwise be
expected to increase.
In the context of the present invention, "use" of a cetane number improver in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more fuel components (typically diesel base fuels) and optionally with one or more fuel additives.
The cetane number improver is preferably
incorporated into the fuel composition before the composition is introduced into an engine which is to be run on the composition. Accordingly, the viscosity increasing component may be dosed directly into (e.g. blended with) one or more components of the fuel
composition or the base fuel at the refinery. For instance, it may be pre-diluted in a suitable fuel component, which subsequently forms part of the overall automotive fuel composition. Alternatively, it may be added to a diesel fuel composition downstream of the refinery. For example, it may be added as part of an additive package containing one or more other fuel additives. This can be particularly advantageous because in some circumstances it can be inconvenient or
undesirable to modify the fuel composition at the refinery. For example, the blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles.
Accordingly, the "use" of the invention may also encompass the supply of a cetane number improver together with instructions for its use in a diesel fuel
composition to achieve one of the benefits of the present invention. The cetane number increasing component may therefore be supplied as a component of a formulation which is suitable for and/or intended for use as a fuel additive, in particular a diesel fuel additive. By way of example, the cetane number improver may be incorporated into an additive formulation or package along with one or more other fuel additives. As described above, the one or more fuel additives may be selected from any useful additive, such as detergents, anti-corrosion additives, esters, poly-alpha olefins, long chain organic acids, components containing amine or amide active centres, and mixtures thereof, as is known to the person of skill in the art.
According to another aspect of the invention, there is provided a process for the preparation of an
automotive fuel composition, which process involves blending a diesel base fuel (or base fuel mixture) with a cetane number improver of the invention. The blending may be carried out for one or more of the purposes described herein.
In some cases the cetane number improver of the invention may not be suitable for pre-mixing with other fuel additives and may, therefore, be dosed directly into the fuel composition from a concentrated (100%) or pre- diluted stock.
In accordance with one embodiment of the present invention, two or more cetane number increasing additives may be used in a diesel fuel composition to provide one or more of the effects of the invention described herein.
For example, the present invention can provide an effective way of improving fuel combustion /
combustability in an internal combustion engine.
It has surprisingly been found that certain organic nitrate molecules of the invention can, at relatively low concentrations, increase the cetane number of a diesel fuel composition by an amount greater than known organic nitrate cetane enhancers under some engine operating conditions. In particular, cetance number enhancing agents of the invention may be capable of providing greater benefits than some prior art cetane number improvers, particularly under harsh engine working conditions (e.g. high engine speeds and powers).
While the amount of the cetane number increasing component for use in accordance with the invention may vary depending of fuel type and/or engine working
conditions to be used; a further benefit of the invention is that under some engine conditions the amount of cetane number improver needed to observe the benefit of the invention may be surprisingly low, such as at the level of typical fuel additives.
This in turn can reduce the cost and complexity of the fuel preparation process. For example, it can allow a fuel composition to be altered in order to improve fuel combustability, by the incorporation of additives downstream of the refinery, rather than by altering the content of the base fuel at its point of initial preparation. The blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles. This in particular may be achievable where the cetane number improver is sufficiently stable to allow it to be transported under suitable conditions without taking unnecessary safety risks. Of course, in some case it may not be appropriate due to safety factors to transport the cetane number improver .
Moreover, an additive which is to be used at a relatively low concentration can naturally be
transported, stored and introduced into a fuel
composition more cost effectively than can a fuel
component which needs to be used at concentrations of the order of tens of percent by weight.
Another aspect of the invention provides a method of operating an internal combustion engine and/or a vehicle powered by such an engine, which comprises introducing into a combustion chamber of the engine a fuel
composition prepared in accordance with the invention. The fuel composition is advantageously introduced for one or more of the purposes described in connection with this invention. Thus, the engine is preferably operated with the fuel composition for the purpose of improving ease of fuel ignition during use of the engine (by increasing fuel combustability) and, for example, associated
benefits such as reduced engine emissions, engine noise, etc. The engine is in particular a diesel engine, and may be a turbo charged diesel engine. The diesel engine may be of the direct injection type, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or of the indirect
injection type. It may be a heavy or a light duty diesel engine. For example, it may be an electronic unit direct injection (EUDI) engine.
Where relevant to a particular assessment, emission levels may be measured using standard testing procedures such as the European R49, ESC, OICA or ETC (for heavy- duty engines) or ECE+EUDC or MVEG {for light-duty
engines} test cycles. Ideally emissions performance is measured on a diesel engine built to comply with the Euro II standard emissions limits (1996) or with the Euro III (2000), IV (2005) or even V (2008) standard limits.
Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as
singularity, unless the context requires otherwise.
Thus features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the present invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. Thus, features of the "uses" of the invention are directly applicable to the "methods" of the invention. Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose .
The invention will now be further illustrated by way of the following non-limiting examples. Examples
Introduction
Various organic nitrates were synthesised and assayed for their potential to act as cetane enhancers in diesel fuels. Cetane enhancers of the invention may provide various benefits, in use, that are associated with increased combustability and a reduction in ignition delay, such as reducing engine noise, reducing build-up of engine deposits, reducing engine emissions, and may even' improve fuel economy in some cases.
Organic nitrates of the invention were also tested for thermal stability to obtain useful information on necessary storage and transportation conditions.
Organic nitrate synthesis processes may also help in determining suitability of the organic nitrates for use as cetane enhancers in diesel fuel at a commercial level (e.g. with respect to ease of synthesis and cost of production) .
Reagents and Chemicals
In general, standard reagents, chemicals and
solvents were purchased from Sigma-Aldrich. Ethyl abietate (70%) was obtained from TCI and ABCR. Methyl linoleate was purchased from TCI. Neodol 23 (C12-C13 alcohol mixture) was obtained from Shell Chemicals.
To form the solution of N2O4, gaseous yellow/brown NO2 N2O (Sigma-Aldrich) was slowly bubbled into the solvent (chloroform or hexane) in a round-bottomed-flask in an ice bath. The dew point of the N204 is around 20 °C, so the heavy gas condenses into the cold solvent (at ca . 4°C) . The amount of N204 added was determined by weighing the flask before and after addition of N204. General
The following general synthesis methods were used for the preparation of nitrate and nitro compounds of the invention :
9 Nitric acid, for secondary alcohols
• Mixture of nitric acid, sulfuric acid and urea, for primary or secondary alcohols
• Mixture of nitric acid and acetic anhydride
(precursor of reactive acetyl nitrate) in acetic acid, for unsaturated alcohols
• N204, for olefins
A 500 ml three-necked round-bottomed flask, equipped with a "bubbler' , thermometer, a dropping funnel and a magnetic stirring bar was used in all the experiments. Reactions were performed under nitrogen.
Synthesis of Organic Nitrates
Nitration Product of 1-octanol
Figure imgf000052_0001
1
A mixture of concentrated nitric acid (45 ml, 0.96 mol) , concentrated sulfuric acid (134 ml, 2.35 mol) and urea (5 g, 0.08 mol) were placed in a 500 m three-necked flask and cooled to 0°C. 1-Octanol (48 ml, 0.30 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5°C with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and 1
- 51 - the solvent was evaporated by a rotorvap (40 °C bath temperature, 5 mbar vacuum) . The yield was 43 g (81%)
1H NMR (CDC13, 300 MHz): 4.4 (t, 2H) , 1.8-1.6 (m, 2H) , 1.4-1.1 (m, 10 H) , 0.9-0.8 (m, 3H)
13C NMR (CDCI3) : 73.7, 32.0, 29.5, 29.4, 27.1, 26 23.0, 14.4
Nitration Product of 1 , 10-decanediol
Figure imgf000053_0001
A mixture of concentrated nitric acid (67 ml, 1.44 mol) , concentrated sulfuric acid (200 ml, 3.50 mol) and urea (7 g, 0.12 mol) were placed in a 500 ml three-necked flask and cooled to 0°C. 1, 10-octanediol (48 ml, 0.30 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5°C with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and
dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40°C bath temperature, 5 mbar vacuum). The yield was 49 g (98%) .
XH NMR (CDCI3, 300 MHz): 4.4 (t, 4H) , 1.8-1.6 (m, 4H) , 1.4-1.2 (m, 12H)
13C NMR (CDCI3) : 73.7, 29.5, 29.3, 27.0, 25.9
Nitration Product of 1-tetradecanol
Figure imgf000053_0002
A mixture of concentrated nitric acid {27 ml, 0.58 mol) , concentrated sulfuric acid (81 ml, 1.42 mol) and urea (3 g, 0.05 mol) were placed in a 500 ml three-necked flask and cooled to 0°C. 1-Tetradecanol (48 ml, 0.19 mol) in dichloromethane (30 ml) was added from a dropping funnel for 45 min. The temperature was maintained at 0 to 5°C with an ice-salt bath After 2 hours the reaction mixture was poured in an ice bath (quenched) and
dichloromethane {100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40°C bath temperature, 5 mbar vacuum) . The yield was 38 g (79%) .
lli NMR (CDC13, 300 MHz): 4.4 (t, 2H) , 1.8-1.6 (m, 2H) , 1.5-1.2 (m, 22H) , 0.9-0.8 (m, 3H)
13C NMR (CDCI3) : 73.7, 32.3, 30.0r 29.9, 29.8, 29.7,
2!9 * 5 ^ δ · 0 ^ 23*l/ JL4**3
Nitration Product of Hexadec l Alcohol
Figure imgf000054_0001
4
A mixture of concentrated nitric acid (24 ml, 0.52 mol), concentrated sulfuric acid (72 ml, 1.26 mol) and urea (2 g, 0.04 mol) were placed in a 500 ml three-necked flask and cooled to 0°C. Hexadecyl alcohol {40 g, 0.16 mol) in dichloromethane {30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5°C with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and
dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap {40°C bath temperature, 5 mbar vacuum). The yield was 39 g (80%).
H NMR (CDC13, 300 MHz): 4.5 (t, 2H) , 1.8-1.6 (m, 2H) , 1.4-1.2 (m, 26H) , 0.9-0.8 (m, 3H)
1C NMR (CDC13) : 73.6, 32.3, 30.1, 29,8, 27.0, 26.0, 23.1, 14.5
Nitration Product of 1-octadecanol
Figure imgf000055_0001
A mixture of concentrated nitric acid (16 ml, 0.35 mol) , concentrated sulfuric acid (48 ml, 0.85 mol) and urea (2 g, 0.03 mol) were placed in a 500 ml three-necked flask and cooled to 0°C. 1-Octadecanol (36 g, 0.11 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5°C with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and
dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40°C bath temperature, 5 mbar vacuum). The yield was 28 g (80%).
aH NMR (CDCI3, 300 MHz): 4.5 (t, 2H) , 1.8-1.6 (m, 2H) , 1.4-1.2 (m, 30H) , 0.9-0.8 (m, 3H)
13C NMR {CDCI3) : 73.7, 32.3, 30.1, 30.0, 29.9, 29.7, 29.5, 27.1, 26.0, 23.1, 14.5
Nitration Product of Neodol 23
A mixture of concentrated nitric acid (31 ml, 0.66 mol), concentrated sulfuric acid (92 ml, 1.61 mol) and urea (3 g, 0.05 mol) were placed in a 500 ml three-necked flask and cooled to 0°C. Neodol 23 (48 ml, 0.21 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5°C with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40 ° C bath temperature, 5 mbar vacuum) . A mixture of dodecyl and tridecyl nitrates was obtained with a yield of 45 g
(93%) .
XH NMR (CDC13, 300 MHz): 4.5 (t, 2H) , 1.8-1.6 (m, 2H) , 1.4-1.2 (m, 16H) , 0.9-0.8 (m, 3H)
13C NMR (CDCI3) : 73.7, 32.3, 30.0, 29.9, 29.7, 29.5, 27.1, 26.0, 23.1, 14.5
Nitration Product of Exo-borneol
Figure imgf000056_0001
In a 500 ml three neck-flask filled with nitric acid (150.21 ml, 3.24 mol) was added exo-borneol (50 g, 0.32 mol) slowly (3.5 hours) at room temperature. After 2 hours the reaction mixture was poured in an ice bath (quenched) and diethyl ether (200 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40 'C bath temperature and +5 mbar vacuum. The yield was 57 g (87%) . XH NMR {CDCI3, 300 MHz): 4.8 (t, 1H) , 2.0-1.9 (m, 2B) , 1.8-1.6 (m, 3H) , 1.3-1.1 (m, 2H) , 1.0-0.9 (m, 6H) , 0.8 (s, 3H)
13C NMR (CDCI3) : 90.5, 49.9, 47.4, 45.1 38.3, 34.5, 27.2, 20.2, 11.5
Nitration Product of Exo-fenchol
Figure imgf000057_0001
8a Sb
In a 500 ml three neck-flask was filled L-Fenchone (40 g, 0.26 mol) , aluminium isopropoxide (2 g) and isopropyl alcohol (300 ml) . This mixture was refluxed (during reflux the acetone was evaporated and filled with isopropyl alcohol) for 168 hours, extracted with diethyl ether, dried and evaporated. 25 g exo/endo-fenchol in ratio 3:1 was isolated.
In a 500 ml three neck-flask filled with nitric acid (75.10 ml, 1.62 mol) was added the exo/endofenchol mixture (25 g, 0.16 mol) slowly (2 hours) at room
temperature. After 2 hours the reaction mixture was poured in an ice bath (quenched) and diethyl ether {100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated using a rotorvap, 40°C bath temperature and ± 5 mbar vacuum. The yield was 26 g (82%) .
JH NMR (CDCI3, 300 MHz): 4.8-4.6 (m, 1H) , 4.2-4.2 (m, 1H) , ratio 1:0.47
13C NMR (CDCI3) : 97.2, 88.2 Nitration Product of L-menthol
Figure imgf000058_0001
9
A mixture of concentrated nitric acid (15.79 ml, 0.34 mol) , concentrated sulphuric acid (47.37 ml, 0.83 mol) and urea (1.62 g, 0.03 mol) were placed in a 500 ml three necked flask and cooled to 0°C. L-menthol (17 g, 0.11 mol) in diethyl ether (20 ml) was added from a dropping funnel for 60 min. The temperature was
maintained at 0 to 5"C with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40 °C bath temperature and + 5 mbar vacuum. The yield was 41 g (95%).
1H NMR (CDCI3, 300 MHz): 4.9-4.8 (m, 1H) , 2.2-1.9 {m, 2H) , 1.8-1.7 (m, 2H) , 1.6-1.4 (m, 2H) , 1.2-1.0
(m, 2H), 1.0-0.9 (m, 6H) , 0.9-0.8 (m, 3H)
13C RMR {CDCI3) : 84.4, 45.8, 39.7, 34.2, 31.8, 26.5, 24.2, 22.2, 20.8, 16.8
Nitration Product of Oleyl Alcohol
Figure imgf000058_0002
10 To a mixture of acetic anhydride (80 mL) , acetic acid {80 ml) and oleyl alcohol (40 g) at 15°C, was added nitric acid (10 ml) drop wise (60 min) . After 30 min the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40 'C bath temperature and ± 5 mbar vacuum. The yield was 34 g (74%) .
ΧΗ NMR (CDC13, 300 MHz): 5.4-5.2 (m, 2H) , 4.4 (t, 2H) , 2.1-1.9 (m, 4H) , 1.4-1.2 (24H) , 1.0-0.8 (m, 3H)
13C NMR (CDCI3) : 130.2, 129.8, 73.7, 32.3, 30.1, 30.0, 29.9, 29.7, 29.6, 29.5, 29.4, 27.6, 27.5, 27.1,
26.0, 23.1, 14.5
Nitration Product of Cholesterol
Figure imgf000059_0001
i1
To a mixture of cholesterol (35 g) in chloroform (10 ml) and acetic anhydride (90 ml) in chloroform (10 ml) at 15 °C, was added drop wise (60 min) a mixture of nitric acid (12.6 ml) in acetic acid (45 ml). After 60 min the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40 °C bath temperature and ± 5 mbar vacuum. The yield was 24 g (63%) . Cholesterol nitrate is a solid at RT.
XH NMR (CDCI3, 300 MHz}: 5.5-5.4 (m, 1H) , 4.9-4.7 {m, 1H) , 2.6-2.3 (m, 2H) , 2.1-1.1 (m, 25H) , 1.1-0.9
(m, 15H) , 0.8-0.6 (m, 3H)
13C NMR (CDCI3) : 138.3, 122.3, 83.5, 57.6, 56.5, 50.2, 42.5, 40.2, 40.1, 37.8, 37.5, 31.8, 29.5, 26.8,
23.4, 18.5, 17.5, 11.2
Nitration Product of Methyl Oleate
To a solution of N204 (13.96 g, 0.15 mol) in
chloroform (20 ml) at 0°C was added drop wise (3 hours) methyl oleate (30 g, 34,48 ml). The reaction mixture was stirred for 48 hours with a low nitrogen flow. The reaction mixture was poured in an ice bath (quenched) and diethyl ether (50 ml) was added. The organic phase was washed for three times with water. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40 °C bath temperature and ± 5 mbar vacuum. The yield was '31.2 g.
Nitration Product of Ethyl Abxetate
To a solution of 2O4 (12.75 g, 0.14 mol) in hexane (20 ml) at 0°C was added drop wise (2 hours) ethyl abxetate (30 g, 34.48 ml). The reaction mixture was stirred for 48 hours with a low nitrogen flow. The reaction mixture was poured in an ice bath (quenched) and diethyl ether (50 ml) was added. The organic phase was washed for three times with water. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40 'C bath temperature and ± 5 mbar vacuum. The yield was 33.2 g. The products of the reaction are solid at RT . Analytical Methods
1H and 13C NMR spectra were recorded on a Varian Mercury 300 MHz or a Varian Inova 400 MHz system. All NMR samples were measured in CDCI3.
Infrared spectra were measured on a Nicolet 6700 FT™ IR Spectrometer (Fourier transform infrared spectroscopy) from Thermo Scientific.
Gas chromatography-mass spectrometry (GC-MS)
analyses were performed on a Trace GC Ultra chromatograph from Interscience equipped with a 50 m x 0.2 mm x 0.5 pm RTX-1 PONA column and an DSQII mass-selective EI
detector. The following temperature profile was used in the GC-MS for measuring the components. The oven started at 35 'C for the first 5 min, then increased with 10°C/min to 300 °C followed by a hold time of 10 min at 300 °C.
DSC/TGA was measured on a STA 409 PC (Simultaneous
Thermal Analysis) with QMS 403 C (quadrupole mass spectrometers) from NETZSCH.
Testing of Blended Fuels
The organic nitrate (or nitro) cetane number
enhancing agents were performance tested in diesel fuel blends. The assessment of their ability to increase the cetane number of the fuel was carried out indirectly by measuring changes in ignition delay (ID) of the blended fuels using a combustion research unit (CRU) . The CRU is operated to determine ignition quality in a similar fashion to the ignition quality tester (IQT;
International Standard EN 15195 : 2007 : E) known to the person of skill in the art, i.e. using a heated,
pressurised constant volume chamber. Typically, the cetane number is given as a dimensionless number, which describes the ignition behaviour of a fuel in comparison to primary reference fuels (PRFs) with a defined cetane number. PRFs are bimodal mixtures of n-hexadecane {or "'cetane"; CN = 100) and heptamethylnonane {CN = 15} .
Fuel Blending
All test compounds were blended into an EN590 compliant diesel {European specification) , zero-sulfur fuel (i.e. ZSD base-fuel from Stanlow refinery; density 837.4 kg/m3 at 15°C; viscosity 2.89 mm2/s at 40°C) at concentrations of 0.1% w/w and 1.0% w/w. Some test compounds were also blended at concentrations of 0.05% w/w and 0.5% w/w. The mixtures were stirred at RT for 1 hour, after which all solid compounds were dissolved in the base fuel.
For purposes of comparison, similar fuel blends containing 2-EHN were prepared.
Combustion Research Unit (CRU) Testing
In CRU tests, diesel-like fuel is injected into a high temperature, high pressure chamber where it mixes with the hot air and ignites, thus mimicking combustion in a compression-ignition engine. The combustion process is monitored via a pressure sensor inside the chamber.
The CRU delivers p-t-charts of the ignition process from which the ignition delay, the burn rate and the maximum pressure increase (MPI) can be determined. A comparison of blended fuels with standard fuels
demonstrates changes in ignition delay / combustability of the fuels, and may also allow determination of cetane numbers under different operating conditions.
The engine conditions selected for CRU tests are designed to simulate a wide range of engine operating conditions so as to assess the cetane enhancers under mild, intermediate and harsh conditions; e.g. temperature and pressure are both varied from low to high. This allows the temperature and pressure dependence of ignition delay to be demonstrated. Thus, the slope of the isothermal or isobaric charts plotted from the data obtained from these tests provide direct information about how the response of the fuel blend changes from mild to harsh conditions.
Each of the fuel blends (at each concentration of cetane enhancer used) and the base fuels were tested for ignition quality on the CRU under the 11 different sets of parameters (engine operating conditions) illustrated in Table 1,
Working Twall / Pchamb / PFuel / Pre-Inj . Main Inj . Delay EGR / % No.
point °C range bar bar Period / Period / Pre- range 0- Injections
350-590 range range usee 0- usee Main 100
10-75 200|l600 1400 range 100 (usee)
- 1500 range
100-
3000
01 590 30 900 0 900 100 0 10
02 590 50 900 0 900 100 0 10
03 590 75 900 0 900 100 0 10
04 560 30 900 0 900 100 0 10
05 560 50 900 0 900 100 0 10
06 560 75 900 0 900 100 0 10
07 530 30 900 0 900 100 0 10
08 570 21.4 200 0 1500 100 0 10
09 530 75 900 0 900 100 0 10
10 530 50 900 0 900 100 0 10
11 590 65 1600 0 1500 100 0 10
Table 1: Conditions 01 to 11 Used for CRU Measurements of Fuel Blends
Ignition Delay
The primary data obtained from. CRU measurements are pressure-time traces, from which the ignition delay (ID), burn period (BP) and maximum pressure increase (MPI) can be determined.
There are different definitions for ignition delay. Typically, ID5% or ID0"2 measurements are used, which are defined as the time taken for the pressure in the
combustion chamber to rise to its initial value plus 5% of the MPI, or to 0.2 bar above its initial value, respectively. In these studies both values are used, although ID0,2 is preferred due to the lower standard deviations observed.
A derived ignition quality (DIQ) is obtained from the ignition delay via comparison with primary reference fuels {PRFs) , which have a known cetane number. Since ignition delay correlates with the cetane number of the fuel, a calibration model can be obtained for all
conditions. With such calibrations, DIQ can be determined for any unknown fuel and derived as a pseudo-cetane number .
The PRFs used in this study are shown in Table 2.
Heptamethylnonan CW
PRF Cetane
e (by
number <Vol%)
{Vol%) definition)
1 40.0 60.0 49
2 100.0 0.0 100
3 64.7 35.3 70
4 88.2 11.8 90
5 52.9 47.1 60
6 76.5 23.5 80
7 35.3 64.7 45
Table 2: PRFs Used to Determine the ID- -DIQ Calibration
Models
The effect of the different organic nitrates in fuel blends was compared by measuring ignition delay and calculating the percentage ID-reduction relative to the respective base fuel. The results for exemplary
combustion conditions 03, 05, 07 and 08 are illustrated in Figure 2.
These data show that all of the organic nitrates provided a benefit compared to the base fuel by achieving a shorter ignition delay. In general, menthyl nitrate achieved a greater reduction in ignition delay than the known cetane enhancer, 2-EHN, under all test conditions, and was the most effective combustion enhancer tested. Under some test conditions exo-fenchyl nitrate or 1,10- decyl dinitrate gave the greatest reductions in ignition delay. In general, 1,10-decyl dinitrate, neodol23 nitrate (mixture of dodecyl and tridecyl nitrates) , and 1-octyl nitrate achieved similar reductions in ignition delay to 2-EHN under all test conditions. Some organic nitrates displayed a concentration-dependent effect. For example, exo-fenchyl nitrate was generally the most effective cetane enhancer at low concentrations (e.g. 0.1% w/}, while it was generally slightly less effective than menthyl nitrate at higher concentrations (e.g. 1.0% w/w) . Furthermore, the effect was most pronounced under the milder test conditions. Interestingly, the fatty acid derived nitrates displayed slight reductions in
effectiveness as chain length increased. Of the saturated and unsaturated nitrates of the same carbon-chain length, it appears that the saturated nitrates provide a stronger cetane boost than unsaturated molecules. The results also suggest that the nitrated molecules are more effective cetane enhancers than the nitro molecules.
Dose Rate Response
The effectiveness of the terpene nitrates, exo- bornyl nitrate and L-menthyl nitrate, were tested at different dose levels (i.e. 0.05% w/w, 0.1% w/w, 0.5% w/w and 1.0% w/w). The results (not shown) illustrate that there is a saturation effect for cetane enhancement.
Thus, at the low concentrations tested (0.05% w/w and 0.1% w/w) there was a clear concentration-dependent reduction in ignition delay. However, a relatively small additional reduction in ignition delay was observed in the concentration range of 0.1% w/w to 0.5% w/w and 1.0% w/w. This saturation effect was found under all
combustion conditions tested.
Importantly, therefore, the effective additives can be used to good effect at low concentrations, which is useful in many respects, such as production level, storage, cost, handling and safety.
Derived Ignition Quality ¾DIQ)
The derived ignition quality (DIQ) was determined for each of the organic nitrates at concentrations of 0.1% w/w and 1.0% w/w.
By way of example, the DIQ for the diesel base fuel comprising exo-fenchyl nitrate at different
concentrations (i.e. 0.1% w/w and 1.0% w/w) was
determined relative to the non-modified base fuel composition and relative to blends with the known cetane enhancer, 2-EHN. Studies were carried out under all 11 sets of combustion conditions and the results are shown in Figure 3.
By way of example, these data show that at IQT- conditions 08, as specified in cetane number measurement according to EN15195 (Oxley et al. (2000) Energy Fuels, 14, 1252-1264), the 0.1% w/w exo-fenchyl nitrate fuel blend achieved a DIQ of 52.2, while the 1.0% w/w exo- fenchyl nitrate fuel blend had a DIQ of 63.7. This represents a significant increase on the DIQ of 44.1 for the base fuel composition alone. These results thus show a cetane number enhancement of 7.1 at 1000 ppm, which is similar to that expected for 2-EHN.
Isothermal and Isobaric Response
The isothermal and isobaric responses of the fuel blends were measured and compared to see the effect of temperature and pressure on ignition delay for each organic nitrate. In particular, the experimental design that was chosen for CRU measurements allowed for isobaric and isothermal comparison of the additives, which
provided an opportunity to follow the response of
additives as combustion conditions changed from mild to harsh.
The data, not illustrated, demonstrated that in general all additives showed the same response pattern irrespective of their specific performance as ignition enhancers. Higher temperature and higher pressure (harsh conditions) resulted in reduced ignition delays (i.e. each additive demonstrated a parallel down-shift compared to the base fuel as pressure and/or temperature
increased. Thus, under combustion condition 11 (maximum power), typically, the ignition delay was the shortest. This may be due to faster injection rate and mixing.
However, it was found that the relative difference in ignition delay in comparison to the base fuel increased with reducing temperature. In other words, relative to base fuel the reduction in ignition delay is greater at mild conditions.
Thermal Stability
Differential scanning calorimetry /
thermogravimetric analysis (DSC/TGA) was used to evaluate the thermal stabilities of the nitrates and other compounds prepared in the study; and also to provide information on the decomposition mechanism from mass spectrometry (MS) of the volatile products.
The experiments were carried out under argon flow, with a temperature profile of 5°C/min up to 200~350°C. Assays were carried out under atmospheric pressure and, hence, the thermal degradation of products that evaporate prior to thermal decomposition could not be studied (e.g. 1-octyl nitrate) .
An exemplary DSC/TGA assay trace for 1,10-decyl dinitrate is illustrated in Figure 5. As illustrated, the product begins to decompose at a temperature of
approximately 170°C. The line beginning in the top left of the graph illustrates the weight loss of the product on decomposition. The two traces showing sharp peaks towards the right hand side of the graph represent the exotherm (right-hand peak: maximum at about 200°C) and the concentration of the decomposition product {left-hand peak) .
Typically several ions are observed in MS studies of the organic nitrates under thermal decomposition (e.g. M
(or M/2) = 12, 14, 18, 28, 30, 44, 46 and 56).
Without wishing to be bound by theory, the rate- determining step in the decomposition of primary and secondary nitrates is thought to be the homolysis of the R0-N02 bond to give the alkoxyl radical and N02. This may be followed by one or more competing reactions of the alkoxyl radical, e.g.: β-cleavage of the alkoxyl radical to liberate formaldehyde (CH2O) and form an alkyl
radical, which may undergo β-cleavage or other reaction; loss of a hydrogen radical (via the alternative - cleavage) to give an anhydride; and hydrogen abstraction from a suitable hydrogen donor (e.g. PhC¾R) to give the alcohol . The degradation mechanism and product distribution for a particular nitrate depends on whether it occurs in the liquid or gas phase and the presence of other
components. Without being bound by theory, it is expected that the compound 1,10-decyl dinitrate would decompose in a similar fashion to the mononitrates, except perhaps, more quickly as the two nitrate functionalities are effectively isolated.
The DSC/TGA method was initially used to confirm that product samples were sufficiently stable to allow safe transportation and storage. However, the methods also provide detailed information on the thermal
degradation of potential cetane enhancers, including on the mechanism of decomposition, at least for those products that do not (partially) evaporate below the degradation temperature (linear organic nitrates having less than 14 carbon atoms may require a modified method) .
The DSC method provides some qualitative information on the relative stability of different cetane enhancers, but, as it involves a temperature scan, it does not directly reflect the relative thermal stability at any particular temperature. Hence, the stability order derived from the temperature of exotherm maximum may differ from that determined at a particular temperature in kinetic experiments if the activation energy of the compounds differ. Components exhibiting a broad exotherm would be expected to have a lower activation energy than those with a narrow exotherm.
Methods are known for determining kinetic parameters from single scan DSC heat flow data, from the dependence of the temperature of maximum exotherm on the heating rate, and using isothermal DSC methods. For example, the activation energy and pre-exponential factor for the decomposition of di-t-butylperoxide have been determined from DSC. It should, in principle, be possible (e.g.
using adiabatic measurements) to extend these methods for the determination of Arrhenius kinetic constants to cetane enhancers based on organic nitrates and related compounds .
The identification of more stable cetane enhancers is an attractive prospect as it may then be possible to transport and supply such cetane enhancers in more concentrated stocks than is possible for the current commonly used cetane enhancers, such as 2-EHN. This may then provide a number of advantages over existing
formulations, such as by reducing the volume of chemicals that need to be added to fuels and, thus, transported and stored. It may also reduce or eliminate the need for some specialist storage, transportation and/or handling equipment to avoid combustion hazards. While the organic nitrates of the invention in general are more stable than some known cetane enhancers (e.g. Di-tert-butylperoxide, DTBP) , tetradecyl nitrate is a particularly attractive cetane enhancer of the invention that appears to be more stable than 2-EHN.
Adiabatic Thermal Stability Measurements
To obtain a quantitative measurement of the thermal stability of some cetane enhancers adiabatic heat-wait search (HWS) experiments were carried out using a Phi- Tech calorimeter, which gives an accurate determination of kinetic and thermodynamic parameters of the molecules decomposition reaction.
The Phi-Tec equipment is usually operated in a heat- wait-search mode, which means that the temperature of the reactor is increased stepwise until the decomposition onset temperature (DOT) of the reaction is detected. Onset temperatures are graphically derived by searching for the temperature at which deviation from linear temperature rise takes place due to self-heating of the sample. The apparatus than automatically tracks the runaway exotherm to 490°C maximum (at short exposure time; otherwise 400°C) . The decomposition onset is detected when the self heating rate exceeds 0.02°C/min.
Typical Phi-Tec reaction conditions :
Intake sample: ca . 66 g;
Test cell material: SS-316 (STRCA type, 1*
1/8" tube connection) ; Void cell volume: ca. 110 ml;
Start temperature: ca 20 °C below Tonset (ex thermal screening / TSu) ; Detection limit: 0.02°C/min;
Step: 5 to 20°C;
Maximum search temperature: 270°C;
Maximum track temperature : 300 °C;
Maximum pressure: 80 bara.
The organic nitrates tetradecyl nitrate (TDN) , menthyl nitrate (MN) - both of the invention, 2- Ethylhexylnitrate (2-EHN; Aldrich) , and di-tert- butylperoxide (DTBP; Merck) - known in the prior art were assessed in consecutive experiments. Cetane enhancers were diluted to approx. 15% w/w in toluene for thermal measurements. Tetradecyl nitrate was also blended at approx. 27% w/w for PhiTec measurements due to its high molecular weight. Experiments with DTBP were conducted in an inert atmosphere (under N2) , which is expected to increase the onset temperature compared to an oxygen- containing atmosphere. Other experiments were carried out under air to mimic typical conditions in use. The data obtained from Phi-Tec measurements are summarised in Table 3. The actual start temperature at which the Phi-Tec switched to adiabatic tracking mode (i.e. first detection of decomposition onset temperature, DOT) , is the temperature of the sample at which the self- heating rate exceeds 0.02°C/m.in. From the final tracking temperature, the adiabatic temperature rise can be calculated by multiplication with Phi, the heat
distribution factor for the reaction (as is known by the skilled person in the art) . The variable heat capacity (Cp) for the novel nitrates is assumed to be 2.54 J/gK.
As indicated in Table 3, in these experiments MN was found to be slightly less stable than 2-EHN, which correlates with its effectiveness as a cetane enhancer described above. Overall, TDN was found to be the most stable molecule, such that the stability of the compounds was in the order DTBP « MN < EHN « TDN.
Since the maximum temperature rise is within the Phi-Tec operating window (i.e. below 400°C), 1st order kinetic data can be obtained from the exothermic
decomposition reaction by plotting in an Arrhenius plot In ( (dT/dt) /60) / (T-Tmax) ) against 1/T. The frequency factor k (intercept) and activation energy Ea (slope)' can thus be derived. It is reasonable to assume the
decomposition reaction as 1st order, as it is solely dependent on the concentration of the nitrate or peroxide respectively, which approach has been documented by other investigators (Oxley et al. (2000) Energy & Fuels, 14, 1252) . Compound Cone. tmos . Phi Ea Phi-tec (w%) (kJ/mol) Onset
Temp.
(°c>
DTBP 15.0 N2 1.10 160.2 111
DTBP 14.6 N2 1.10 162.0 110
DTBP 14.9 N2 1.10 157.6 110
_JJ, EtJET 14.9 N2 1.10 158.4 112
2-EHN 15.2 air 1.10 178.2 132
2-EHN 14.9 ί XT 1.10 177.5 135
TDN 15.1 air 1.10 190.3 142
TDN 27.2 air 1.10 179.1 136
TDK 27.3 ¾X X? 1.11 179.5 137
MN 14.9 air 1.10 171.1 123
MN 14.9 air 1.10 171.1 127
Table 3: Kinetic and Thermodynamic Data Derived from Phi-
Tec HWS Experiments
As illustrated in the data, the ranking described for thermal stability of the compounds is also reflected in the activation energies (Ea) , which indicates that compounds with lower reaction onset temperature also exhibit lower activation energy. The reaction enthalpies (not shown) for the respective compounds correlate with the adiabatic temperature rise. DTBP demonstrated the lowest reaction enthalpy and the lowest temperature rise, whereas the organic nitrates compared well, with both the reaction enthalpy and the adiabatic temperature rise are in the same order of magnitude.
Conclusions
The synthesis of possible alternative cetane
enhancers to the commonly used 2-ethylhexylnitrate (2- EHN) was investigated, with the focus on the use of renewable feedstocks. There are some safety concerns surrounding the production, transport and use of this current compound. Furthermore, 2-EHN functions most effectively under mild engine conditions and a cetane enhancer that also works well under harsher engine conditions is desirable. Potential cetane enhancers were prepared via the nitration of various bio-feedstocks: terpene alcohols - borneol, fenchol and menthol (and pinene) ; fatty
alcohols; unsaturated FAME' s ; 1, 10-Decandiol, ultimately derived from ricinoleic acid (castor oil) ; and ethyl abietate, a resin ester derived from tall oil.
Following determination of the optimum experimental conditions for nitration in small-scale experiments, different nitrated products were prepared in 25-50 g scale. The majority of the samples were well-defined nitrates (R-ON02) , prepared from alcohol precursor by reaction with nitric acid, optionally in combination with other reagents. Treatment of the two olefin feedstocks with dinitrogen tetroxide (N204) led to mixtures of compounds. All samples were characterised by NMR and IR spectroscopy. DSC/TGA analyses was used to provide information on the exothermic decomposition of the products. As expected, most nitrates underwent exothermic decomposition in a narrow temperature range, around
210 °C, whereas products derived from olefins decomposed over a much broader temperature range.
Following confirmation (from DSC/TGA and NMR
monitoring of storage stability) that the samples were sufficiently stable to transport, products were shipped for evaluation of their effectiveness as cetane enhancers in fuel compositions at a Combustion Research Unit.
A thermal screening and calorimetric assessment of the organic nitrates for use as cetane enhancers has been conducted for comparison with cetane enhancers known in the art (e.g. 2-EHN and DTBP) .

Claims

C L A I M S
1. A diesel fuel composition for use in a compression ignition engine, which comprises an organic nitrate selected from the group consisting of:
a cyclic nitrate of Formula (4) :
Figure imgf000076_0001
Formula {4A)
Figure imgf000076_0002
Re wherein each of Ri to Rg is independently selected from H or Ci-Cg alkyl, or nitrate ( -ON02) , wherein optionally one of R4 and R¾ forms an optionally substituted alkylene bridge with one of R8 and R9r which may be substituted by one or more Ci-C6 alkyl, and/or nitrate (-ON02) ; wherein at least one of Ri to Rg is not H, and provided that no more than one R2 to R9 comprises a nitrate group.
2. The diesel fuel composition of Claim 1, wherein at least one of Rx to Rg is selected from methyl, ethyl, n- propyl, isopropyl, n-butyl, isobutyl and tert-butyl.
3. The diesel fuel composition of Claim 1 or Claim 2, wherein one of R4 and R5 is methyl and one of R8 and Rg is isopropyl .
4. The diesel fuel composition of Claim 1 or Claim 2, wherein one of R4 and R5 forms an optionally substituted alkylene bridge with one of Rs and R9; and wherein the alkylene bridge has the formula ~ ( RaRb} n-, wherein Ra and Rfa are independently selected from H, methyl, ethyl, n- propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2.
5. The diesel fuel composition of Claim 4, wherein the cetane number improver is defined by Formula (5) :
Figure imgf000077_0001
Figure imgf000077_0002
wherein (i) at least two of Ra/ Rhr ¾ i ¾ and R9 are methyl; (ii) Ra and Rb are methyl; (iii) R2 and R3 are methyl; (iv) Ra, Rb and R9 are methyl; or (iv) R2, 3 and R9 are methyl.
6. The diesel fuel composition of any preceding claim, wherein the cetane number of the diesel fuel composition containing the organic nitrate is higher than the cetane number of the diesel fuel composition lacking the organic nitrate.
7. The diesel fuel composition of any preceding claim, which has a cetane number of between 52 and 58.
8. The diesel fuel composition of any preceding claim, wherein the diesel fuel composition comprises one or more additional organic nitrate.
9. A method for reducing the ignition delay and/or increasing the cetane number of a diesel fuel
composition, which method comprises adding to the
composition an amount of an organic nitrate, wherein the organic nitrate is selected from the group consisting of: a cyclic nitrate of Formula (4) :
Figure imgf000078_0001
Re or Formula (4A) :
Figure imgf000079_0001
R6 wherein each of Ri to Rg is independently selected from H or Ci-Ce alkyl, or nitrate (-ON02) , wherein optionally one of R,s and R5 forms an optionally substituted alkylene bridge with one of R8 and Rg, which may be substituted by one or more Ci-C6 alkyl, and/or nitrate (-0N02) ; wherein at least one of Ri to R9 is not H, and provided that no more than one R2 to Rg comprises a nitrate group.
10. The method of Claim 9, wherein at least one of Ri to R9 is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl.
11. The method of Claim 9 or Claim 10, wherein one of 4 and R5 is methyl and one of Rg and R9 is isopropyl.
12. The method of Claim 9 or Claim 10, wherein the cetane number improver is defined by Formula (5) :
(5>
Figure imgf000079_0002
Formula (5A)
Figure imgf000080_0001
wherein (i) at least two of Ra, ¾, R2, ¾ and R9 are methyl; (ii) Ra and Rb are methyl; (iii) R2 and R3 are methyl; (iv) Ra, Rb and R9 are methyl; (iii) R2, R3 and R9 are methyl.
13. The method of any of Claims 9 to 12, which is for increasing the cetane number of the diesel fuel
composition to achieve a target cetane number.
14. The method of any of Claims 9 to 13, which comprises adding one or more additional organic nitrate to the fuel composition.
15. The method of any of Claims 9 to 14, which is for reducing the amount of 2-ethylhexyl nitrate (2-EHN) in the diesel fuel composition to achieve the target cetane number.
16. Use of an organic nitrate in a diesel fuel
composition for the purpose of reducing the ignition delay (ID) of the diesel fuel composition, wherein the organic nitrate is selected from the group consisting of: a cyclic nitrate of Formula (4}
Figure imgf000081_0001
or Formula
Figure imgf000081_0002
RP wherein each of R to Rg is independently selected from H or Ci-Ce alkyl, or nitrate { -ON02 ) , wherein optionally one of R4 and R5 forms an optionally substituted alkylene bridge with one of Rg and R9, which may be substituted by one or more Ci-Cg alkyl, and/or nitrate (-ΟΝ<¾) ; wherein at least one of Ri to R9 is not H, and provided that no more than one R2 to R9 comprises a nitrate group.
17. The use of Claim 16, wherein at least one of Ri to Rg is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl.
18. The use of Claim 16 or Claim 17, wherein one of 4 and R5 is methyl and one of Rs and R9 is isopropyl.
19. The use of Claim 16 or Claim 17, wherein one of R4 and R5 forms an optionally substituted alkylene bridge with one of Rs and R9; and wherein the alkylene bridge has the formula - (CRaRb} n~, wherein Ra and Rb are
independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2.
20. The use of Claim. 19, wherein the cetane number improver is defined by Formula (5) :
Figure imgf000082_0001
wherein (i) at least two of Ra, Rb, R r R3 and R9 are methyl; (ii) Ra and b are methyl; (iii) R2 and R3 are methyl; (iv) Ra, Rb and R9 are methyl; (iii) R2, R3 and R9 are methyl.
21. The use of any of Claims 16 to 20, wherein the cetane number of the diesel fuel composition is increased by addition of the organic nitrate.
22. The use of any of Claims 16 to 21, which is for increasing the cetane number of the diesel fuel
composition to achieve a target cetane number.
23. The use of any of Claims 16 to 22, wherein the diesel fuel composition comprises one or more additional organic nitrate.
24. The diesel fuel composition of any of Claims 1 to 8, the method of any of Claims 9 to 15, or the use of any of Claims 16 to 23, wherein the organic nitrate is present in the diesel fuel composition at a concentration of: (a) between 0.025% and 2.0% w/w;
(b) between 0.05% and 1.0% w/w; or
(c) 0.05% w/w, 0.1% w/w, 0.5% w/w or 1.0% w/w;
based on the total weight of the fuel composition.
25. The diesel fuel composition, method, or use of any of Claims 1 to 24, wherein the organic nitrate is further defined as follows:
(i) Ri, Re and R7 are H;
(ii) Ri, R6 and R7 are H; R2 and R3 are methyl;
(iii) Ri, R6 and R7 are H; R5 is H and R9 is methyl; or
(iv) Rx, R6 and R7 are H; R2 and R3 are methyl; R5 is H and R9 is methyl.
26. The diesel fuel composition, method, or use of any of Claims 1 to 25, wherein the organic nitrate is selected from: bornyl nitrate, fenchyl nitrate and menthly nitrate.
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