WO2008086897A1

WO2008086897A1 - Anti-oxidant fuel additives

Info

Publication number: WO2008086897A1
Application number: PCT/EP2007/050550
Authority: WO
Inventors: Willy Van Brussel; Geert Schelkens; Patrick Ricquier
Original assignee: Ajinomoto Omnichem S.A.
Priority date: 2007-01-19
Filing date: 2007-01-19
Publication date: 2008-07-24

Abstract

The present invention is related to the use of natural polyphenols and fatty acid esters thereof as additives in liquid fuels i.e. petroleum-based fuels and bio-fuels so as to thermally stabilize said fuels, and in particular so as to reduce their oxidation. The polyphenols which are hydrolysable tannins and/or condensed tannins, and their fatty acid esters, can be used as such (solid form) or under a formulated form. Possible applications namely include engine industry as a whole, and more particularly car and aircraft industry.

Description

ANTI-OXIDANT FUEL ADDITIVES

Field of the invention

[0001] The present invention concerns the field of fuel industry.

[0002] More precisely, the present invention is related to additives which can advantageously be used in fuel for their anti-oxidant (anti-oxidative) and thermal stability properties .

Background of the invention

[0003] Liquid fuels, such as for example diesel or jet fuel, are known to undergo degradation reactions in the presence of oxygen, consisting essentially in oxidation and polymerization reactions. These degradation reactions become more prominent with increasing temperatures and extended storage times. This results in the formation of undesired oxidized resinous reaction by-products or precipitates that can damage fuel injection systems, and namely clog fuel lines as well as filters or other parts of engines.

[0004] This problem has become increasingly pressing due to increasingly strict environmental requirements regarding maximum allowed levels of sulphur in liquid fuels. Indeed, the presence of sulphur derivatives can have a marked positive effect on stability. Because sulphur levels are continuously being reduced in fuels due to these environmental requirements, the reduced native presence of sulphur compounds has reduced the inherent stability of hydrocarbon fuels. [0005] However to compensate this increased instability, no sulphur-containing stabilizers can be used and other alternative stabilizers are very much needed.

[0006] Furthermore, the increased use of hydrotreated fractions and the use of Fisher-Tropsch feedstocks in the formulation of diesel fuels has also resulted in an increased occurrence of stability problems in such fuels.

[0007] Similarly, it is known that so-called bio-diesel fuels such as methyl or ethyl esters of vegetable oils such as rapeseed, soy or palm oil, are also very susceptible to oxidative degradation. Due to such oxidative degradation the storage stability of non-stabilized bio-diesel is severely limited.

[0008] A further problem arises from the fact that such stability problems are in a number of cases, i.e. bio-diesel or jet fuel, caused by or related to the presence of dissolved metal salts, and more specifically copper salts.

[0009] Anti-oxidants currently in use provide only partial or no protection against the detrimental effects of metal contaminants in fuel.

[0010] In other words, there is an obvious need for additives that can neutralize or limit these negative effects in liquid fuels.

[0011] A number of additives have been developed over the years or are currently in use. Examples of such additives are hindered phenols (for diesel fuels) . Condensates of phenols, aldehydes, a polyamine and a succinic acid polyolefine derivative have also been described as additives to improve thermal stability of aircraft fuels.

[0012] However, these additives present different drawbacks. For example, some of these additives present solubility problems in the fuel. Furthermore, additives comprising nitrogen become less desirable due to increasing strict regulation regarding environmental emissions of NO_x by combustion engines.

[0013] There is thus a high need for other alternative additives in order to improve thermal stability of liquid fuels .

Aims of the invention

[0014] The present invention aims to provide alternative additives to liquid fuels which do not present drawbacks of the additives of the prior art.

[0015] More precisely, the present invention aims to provide an additive able to enhance thermal stability of a liquid fuel to which it is added, while satisfying the regulations regarding environmental emissions.

[0016] In particular, the present invention aims to provide an additive able to reduce the phenomenon of oxidation to which liquid fuel are naturally susceptible.

[0017] Another aim of the present invention is to provide an additive which is adapted to any kind of liquid fuels, that is petroleum-based fuels as well as bio-fuels. [0018] The present invention also aims to provide an additive which can easily be produced or obtained and at low costs .

[0019] Another aim of the present invention is to provide an additive which can easily be added to liquid fuels, with minimal requirements in terms of proceedings.

Summary of the invention:

[0020] Although anti-oxidant (anti-oxidative) properties of natural polyphenols are well established in a number of applications such as foods, beverages or boiler water treatment, the Applicant now provides for the first time evidence supporting the use of natural polyphenols and of fatty acid esters thereof as anti-oxidants in apolar media such as liquid fuels (including hydrocarbon fuels and bio- fuels) .

[0021] This specific use of natural polyphenols and of fatty acid esters thereof, was not obvious for the man skilled in the art. Indeed most polyphenols, and namely natural polyphenols, are easily dissolved in aqueous media but are little or not soluble in apolar solvents.

[0022] The Applicant has surprisingly found that such polyphenols can also be used as anti-oxidant in non-aqueous media as such. The Applicant has also found that natural polyphenols can also be used as anti-oxidant in non-aqueous media, such as in liquid fuels, possibly using appropriate means for dispersing, dissolving or emulsifying. In particular, the Applicant has found that fatty acid esters of polyphenols can be used as anti-oxidant in such non-aqueous media .

[0023] More precisely, the present invention is related to the use of at least one natural polyphenol and/or at least one fatty acid ester thereof as additive in a liquid fuel. [0024] The present invention also concerns a method for enhancing the anti-oxidative properties of a liquid fuel, comprising the step of adding to said liquid fuel at least one natural polyphenol and/or at least one fatty acid ester thereof .

[0025] In other words, the present invention also concerns a method for reducing the susceptibility to oxidation of a liquid fuel, comprising the step of adding to said liquid fuel at least one natural polyphenol and/or at least one fatty acid ester thereof.

[0026] The present invention also concerns a method for enhancing the thermal stability properties of a liquid fuel, comprising the step of adding to said liquid fuel at least one natural polyphenol and/or at least one fatty acid ester thereof . [0027] Furthermore, the present invention also covers a composition comprising at least one liquid fuel and at least one natural polyphenol and/or at least one fatty acid ester thereof .

[0028] According to the invention, the liquid fuel can be a petroleum-based fuel, and especially a diesel fuel. [0029] The liquid fuel can also be a bio-fuel, and especially bio-diesel.

[0030] The liquid fuel can also be a mixture of at least one petroleum-based fuel and at least one bio-fuel. [0031] Preferably, the natural polyphenol is selected from the group consisting of hydrolysable tannins, condensed tannins, and mixtures thereof.

[0032] According to one preferred embodiment, the natural polyphenol is a hydrolysable tannin comprising at least one hydroxybenzoic acid or at least one hydroxycinnamic acid, said hydroxybenzoic acid and said hydroxycinnamic acid being esterified on a central nucleus, wherein said hydroxybenzoic acid is preferably selected from the group consisting of gallic acid and oligomers thereof, and elagic acid and oligomers thereof, wherein said hydroxycinnamic acid is preferably selected from the group consisting of caffeic acid, ferulic acid and synaptic acid, and wherein said central nucleus comprising one or more units of a monomer is preferably selected from the group consisting of carbohydrates, and is more preferably selected from the group consisting of sugars and acids, and is more more preferably selected from the group consisting of glucose, glycerol and quinic acid.

[0033] According to another preferred embodiment, the natural polyphenol is a condensed tannin selected from the group consisting of flavonoids, stilbenes and phloroglucinols . [0034] Preferably, the condensed tannin corresponds to the general formula (I) or (II) :

wherein :

- A and B are carbon atoms connected by a single or by a double bond;

- D is hydrogen, hydroxide, or a hydroxide esterified with gallic acid or ellagic acid;

- E is hydrogen, hydroxide, 0-glucose or another condensed tannin corresponding to Formula (I) or (II) to produce dimeric, trimeric, tetrameric or higher condensation products;

- R is hydrogen, hydroxide, 0-glucose, O-alkyl group comprising 1-3 carbon atoms, or O-gallic acid group.

[0035] Preferably, in the present invention, the natural polyphenol is provided for addition into the liquid fuel as such or under a formulated form, said formulated form being selected from the group consisting of solutions, dispersions, emulsions, and mixtures thereof.

[0036] Preferably, the concentration at which the polyphenol is provided for addition into the liquid fuel is such that the final concentration of said polyphenol after addition to the liquid fuel is comprised between about 0% and about 5% mass/volume.

[0037] Preferably, the concentration at which the polyphenol is provided for addition into the liquid fuel is such that the final concentration of said polyphenol after addition to the liquid fuel is comprised between about 1 ppm and 2500 ppm, preferably, between about 5 ppm and about 1000 ppm. [0038] Preferably, the polyphenol presents a purity greater than 80% or is part of a plant extract.

[0039] Preferably, the polyphenol is provided for addition into the liquid fuel under the form of a solution in an adequate solvent, said adequate solvent being selected from the group consisting of esters, ethers, polyethers, alcohols, ketones and aliphatic hydrocarbon mixtures, aromatic hydrocarbon mixtures, and mixtures thereof.

[0040] Preferably, the adequate solvent is selected from the group consisting of high boiling esters, esters of glycerol, and esters of glycol and mixtures thereof, and preferably in the subgroup consisting of fatty acid methyl esters, fatty acid ethyl esters and fatty acid propyl esters.

[0041] In one embodiment of the invention, the polyphenol is provided for addition into the liquid fuel under the form of a solution in an adequate solvent, said adequate solvent being the liquid fuel itself or analogues thereof, or a solvent comprising said fuel.

[0042] According to another embodiment, the polyphenol is provided for addition into the liquid fuel under the form of a concentrated liquid solution having a polyphenol concentration ranging from about 5 and 60% (m/V) , and preferably ranging from about 15 and 50% (m/V) , and more preferably ranging from about 20 and 40% (m/V) .

[0043] The present invention thus also concerns compositions comprising the natural polyphenol and/or the fatty acid ester and the adequate solvent as described hereabove .

[0044] In the present invention, the polyphenol can be used in combination with another additive, preferably selected from the group consisting of emulsifiers and miscibility enhancers .

[0045] The present invention thus also concerns compositions comprising the natural polyphenol and/or the fatty acid ester and at least one other additive as described hereabove .

[0046] In addition, the present invention also concerns compositions comprising at least one liquid fuel as disclosed hereabove, the natural polyphenol and/or the fatty acid ester as described hereabove and at least one other additive as described hereabove.

[0047] In addition, the present invention also concerns compositions comprising at least one liquid fuel as disclosed hereabove, the natural polyphenol and/or the fatty acid ester as described hereabove, the adequate solvent as disclosed hereabove and at least one other additive as described hereabove .

[0048] The present invention preferably finds applications in engine industry, including car and aircraft industry. The present invention can also be used in heating applications .

Definitions

[0001] The present invention concerns liquid fuels i.e. petroleum-based fuels, including petroleum-based diesels, as well as bio-fuels, including bio-diesels.

[0002] The term "bio-fuel" refers to any fuel that is derived from biomass including living organisms and their metabolic byproducts.

[0003] More precisely, the term "bio-diesel" refers to a diesel-equivalent, processed fuel derived from biological sources, such as vegetable oils or animal fats. A "bio- diesel" can be defined as a fuel comprising mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats. It can be used per se as fuel or as an additive, generally in a blend with petroleum-based diesel fuel. [0049] It is meant by "natural polyphenol" or

"polyphenol" in the sense of the present invention any hydrolysable tannin or condensed tannin or mixtures thereof. [0050] A "tannin" is defined as a naturally occurring large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) so that they may form strong complexes with proteins and other macromolecules . Tannins have molecular weights ranging from 500 to over 20,000. They are naturally present in different parts of plants i.e. bark, wood, fruit, fruitpods, leaves, roots, and plant galls. Tannins are usually divided into hydrolysable tannins and condensed tannins (also known as proanthocyanidins) .

[0051] The terms "hydrolysable tannins" refer to compounds as disclosed in the standard work " Chemistry of vegetable tannins" by E. Haslam, Ed. Academic Press, London, 1966.

[0052] More precisely, hydrolysable tannins are complex molecules occurring in natural products, comprising a central nucleus with hydroxybenzoic acids or hydroxycinnamic acids esterified onto the central nucleus or esterified onto an aromatic hydroxyl function of an hydroxybenzoic acid or hydroxycinnamic acid moiety already esterified onto the central nucleus of the molecule. This last type of bond is known as a "depsidic bond". Examples of products comprising one or two depsidic bonds are digallic acid and trigallic acid. Examples of these hydroxybenzoic acids are gallic acid and oligomers thereof and ellagic acid and oligomers thereof. Examples of hydroxycinnamic acids are caffeic acid, ferulic acid or synaptic acid, and their respective oligomers. [0053] Hydrolysable, naturally occurring tannins all comprise such acids (hydroxybenzoic acids or hydroxycinnamic acids) or a mixture of these acids esterified on a central nucleus comprising a carbohydrate, preferably selected from the group consisting of sugars and acids. In particular, the hydrolysable, naturally occurring tannins all comprise such acids or a mixture of these acids esterified on a central nucleus comprising glucose, glycerol, or quinic acid. [0054] The most abundant central nuclei are glucose such as in tannin from Chinese gallnuts and Aleppo nuts, and quinic acid such as in Tara tannin. The most abundant organic acids esterified onto these central nuclei are gallic acid and oligomers thereof and/or ellagic acid and oligomers thereof. Hydrolysable tannins comprising ellagic acid are known as ellagitannins .

[0055] Hydrolysable tannins based on glucose as central nucleus and gallic acid as hydroxybenzoic acid typically comprise, in different proportions depending on the plant origin, gallic acid and/or oligomers thereof, monogalloylglucose, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, undeca-, and/or dodecagalloylglucose, and ellagitannins in relatively smaller quantities. Higher substituted polygalloylglucose derivatives may also occur, but are rare .

[0056] A hydrolysable tannin according to the invention may thus, for example, comprise a central nucleus such as glucose or quinic acid esterified with a number of molecules such as gallic, digallic, trigallic and ellagic acid or combinations thereof.

[0057] The terms "condensed tannins" refer to compounds as disclosed in the standard work "Chemistry of vegetable tannins" by E. Haslam, Ed. Academic Press, London, 1966. [0058] More precisely, condensed tannins are polyphenols comprising at least two aromatic rings, each substituted with at least one hydroxyl function, and with at least two of these aromatic rings attached to each other by one or more single or double carbon-carbon bonds and/or an ether linkage of the form (CH₂) aO (CH₂) b with a and b independently from each other between

0 and 3.

[0059] Examples of condensed tannins are flavonoids, stilbenes and phloroglucinols .

[0060] A condensed tannin according to the invention can be represented or corresponds to the general formula:

wherein :

- A and B are carbon atoms connected by a single or by a double bond;

- E is hydrogen, hydroxide, O-glucose or another condensed tannin corresponding to Formula (I) or (II) to produce dimeric, trimeric, tetrameric or higher condensation products;

- R is hydrogen, hydroxide, O-glucose, O-alkyl group comprising 1-3 carbon atoms, or O-gallic acid group.

[0061] The polyphenols according to this invention are extracted from natural sources, such as vegetable sources, by means of extraction processes known to those skilled in the arts .

[0062] It should be understood that the polyphenols can be used in substantially pure form, i.e. at a purity of about 80% or greater, i.e. preferably of more than 85%, more preferably of more than 90%, and more more preferably of more than 95%, or they can also be used as a part of a plant extract .

[0063] Different vegetable raw material sources for preparing the polyphenols (and their fatty acid esters) according to the invention are suitable. Virtually every plant comprises some form of hydrolysable and condensed polyphenol, but there are certain plants or plant extracts that are recognized as being particularly rich sources of polyphenols .

[0064] Examples of plants which may produce extracts useful in the compositions include plants of the genera: Gingko, Lespedeza, Pass!flora, Silybum, Citrus, Hamamelis, Thymus, Chamaemelum, Achillea , Equisetum, Sophora , Fagopyrum, Eucalyptus, Sambucus, Betula, Vitis, Pinus, Crataegus, Quercus, Ratanhia , Lythrum, Acacia, Cupressus, Vaccinium, Ribes, Centaurea, Rosa, Hibiscus, Camellia, Malva, Podophyllum, Schizandra , Gaiacum, Theobroma , Arctostaphylos, Glycine, Cynara, Rosmarinus, Orthosiphon, Solidago, Lithospermum, Curcuma , Aesculus, Melilotus, Ammi , Hieracium, Angelica , and Asperula . These raw material sources are provided only as examples and are not intended to be limiting. [0065] Particularly rich sources of polyphenols include red wine, grape juice, grape skins, grape seeds, blueberries, persimmon, eucalyptus, cocoa, green tea, black tea, white tea, pomegranate, and Chinese gallnut.

[0066] Concerning more specifically condensed tannins, examples of suitable natural extracts include tannin extracts from trees (wood and/or brak) such as Eucalyptus, Acacia, Schinopsis (Quebracho tannin) , Castanea, Quebracho, Quercus, Rhizophora , Picea, Pinus or Larix, or from other plant sources, such as grape seed extracts, green tea extracts, black tea extracts, cocoa extracts, cocoa nips, wine polyphenols, tannins from fruit or vegetables such as Persimmon or Kaki tannin, grapes, berries, citrus fruits or soy beans, or tannins from herbs and spices such as rosemary. These raw material sources are provided only as examples and are not intended to be limiting.

[0067] Concerning more specifically hydrolysable tannins, examples of suitable natural extracts include tannin extracts from Acacia, Wattle and Mimosa wood; Tara or Teri pods, Divi-Divi, Myrobalans, Aleppo or Chinese gallnuts. These raw material sources are provided only as examples and are not intended to be limiting.

[0068] Furthermore, as used herein the terms "fatty acid ester of hydrolysable tannin" (or "fatty acid ester derivative of hydrolysable tannin") refer to a compound obtained from hydrolysable tannins as described above with at least one covalently bound fatty acid residue, preferably selected from the group consisting of lauric, myristic, palmitic, stearic, arachidic, linolenic, linoleic, arachidonic and oleic acid, although other natural occurring or synthetic fatty acid are also suitable. These fatty acids are provided as examples and are not intended to be limiting.

[0069] These fatty acid residues are bound to the hydrolysable tannin core by means of ester bounds. [0070] Reference is made to the patent application WO 09/11268A1 in the name of J. Brepoels et al . , wherein examples of such fatty acid ester derivatives have previously been described.

[0071] In addition, as used herein the terms "fatty acid ester of condensed tannin" (or "fatty acid ester derivative of condensed tannin") refer to a compound obtained from condensed tannins as described above with at least one covalently bound fatty acid residue preferably selected from the group consisting of lauric, myristic, palmitic, stearic, arachidic, linolenic, linoleic, arachidonic and oleic acid, although other natural occurring or synthetic fatty acids are also suitable. These fatty acids are provided as examples and are not intended to be limiting.

[0072] Reference is made to patent application EP

0698595A1 in the name of J. Nkiliza, wherein examples of such fatty acids are disclosed.

[0073] An example of a fatty acid ester of condensed tannin according to the invention is represented by or corresponds to the general formula:

wherein :

- A and B are carbon atoms connected by a single or by a double bond;

- D is hydrogen, hydroxide, or a hydroxide esterified with a long chain fatty acid comprising 6-30 carbon atoms, or a hydroxide esterified with gallic acid or ellagic acid;

- E is hydrogen, hydroxide, 0-glucose or another O-alkyl group (with 6-30 carbon atoms) comprising condensed tannin corresponding to Formula (I) or (II);

- R is hydrogen, hydroxide, O-glucose, or a hydroxide esterified with a long chain fatty acid comprising 6-30 carbon atoms .

[0074] Oxidation stability of a polyphenol is evaluated in the present invention using "Rancimat test" as defined by EN14112 European standard. This method is known as a reliable and sensitive method by the man skilled in the art. [0075] The general principle of the Rancimat test is to induce a premature aging of a test sample by thermal heating (between 50⁰C and 220⁰C), which results in the progressive oxidation of the test sample. The resulting degradation products are transferred by a stream of air into a measuring vessel containing deionised water, whose conductivity is continually being measured. Plotting conductivity against time produces oxidation curves, whose point of inflection is known as the "induction time". The Rancimat test apparatus as used in the present application was a Rancimat 743 test apparatus purchased from Metrohm AG (Switzerland) .

[0076] In the present invention, it was decided, for an easy comparison of different series of test results, to use a relative measure rather than an absolute value in order to evaluate the performance of the different test additives. For that matter, a "relative induction time" has been defined as follows :

relative induction time = induction time sample x 100 induction time reference

wherein the induction time reference corresponds to the induction time as measured for a non-stabilised fuel or substrate used as reference.

[0077] It is meant by "good oxidation stability" that the risks of generating hydroperoxide is diminished, preventing the peroxidation chain mechanism which damages the fuel system components. It is also an indication that the fuel darkening and the formation of insoluble resins and gums will be substantially attenuated.

Brief description of the drawings

[0078] Figure 1 represents a titration curve of the formation of a coloured Fe (III) -Tapa-7-complex as obtained by measuring the absorbance at 710 nm for a 1 % (m/V) solution of Tapa-7 in MeOH to different Fe (III) -concentrations of an Fe(III) solution has been added (909.1 ppm Tapa-7). Detailed description of the invention

[0079] The additives according to the invention act as thermal stabilizers of liquid fuels, and more precisely as anti-oxidants . Theoretically, anti-oxidants may work through different pathways. They can prevent free radical formation by chelating redox-active metals, or they can neutralise radicals by reacting with these radicals to form stable components. The Applicant has determined that the additives according to the present invention offer the advantage that they exhibit both mechanisms. They are active as both radical scavengers and as metal chelating agents.

[0080] Different pathways are available for explaining anti-oxidant (anti-oxidative) properties: hydroquinone-quinone redox reactions, phenolic oxidative coupling reactions or true radical scavenging reactions as well as quenching of metal catalysts such as Fe or Cu. Unlike synthetic anti-oxidants acting through only one of these possible pathways, natural polyphenols or fatty acid ester derivatives thereof according to the invention have the ability to act according to all of these mechanisms depending upon reaction conditions such as pH, concentration, temperature, etc . It is clear that such a "multiple pathway" additive can act in a much broader set of application circumstances than an additive relying on one particular pathway with its own (narrow) set of application parameters (e.g. pH, concentration,...).

[0081] As already mentioned hereabove, one way of administering the additives according to the invention into the liquid fuels is by means of solutions.

[0082] To this purpose, solvents can be used miscible with the fuel needing stabilisation. Suitable solvents are - but are not limited to the following list - esters, ethers or polyethers, alcohols, ketones and aliphatic or aromatic hydrocarbon mixtures. More particularly, high boiling esters or esters of glycerol or glycol are particularly suitable. Among these solvents, fatty acid methyl, ethyl or propyl esters are preferably used.

[0083] Nevertheless, fuels themselves or analogues of such fuels are the most suitable solvents.

[0084] As also already mentioned here above, if needed, other additives can be added in combination with the polyphenols according to the invention, in order to confer a complementary function to the liquid fuel. For example, in order to improve miscibility of the anti-oxidant solution with the fuel to be stabilised, emulsifiers or similar compounds (miscibility enhancers) can be added in combination with the polyphenols according to the invention. Such additives are well known to those skilled in the arts.

[0085] As also already mentioned hereabove, although the proposed polyphenols or their fatty acid ester derivatives can directly be added to the fuel to be stabilised, a more preferred embodiment of the current invention is to add them as concentrated liquid solution or emulsions. Such solutions contain between about 5 and about 60 % m/V, more particularly between about 15 and about 50 % m/V, and even more preferably between about 20 and about 40 % active agent (polyphenol or ester) .

[0086] Such solutions can be prepared by direct dissolution of the active in the carrier solvent under agitation if necessary. It can also be necessary to heat the carrier solution to accelerate the dissolution of the polyphenol or fatty acid ester derivative thereof. Heating should not exceed temperatures above 200⁰C.

[0087] Another method for preparing solutions of additives according to the invention involves the use of a solvent switch. The active agent (polyphenol or ester) is dissolved in a volatile solvent that consequently is stripped under vacuum while being replaced by the final higher boiling solvent. For instance, Tapa-7 can be dissolved in EtOAC and is consequently transferred into bio-diesel.

[0088] When the additives according to the invention - in solution or as solid products - are added to the fuel, sufficient precautions should be taken to ensure adequate and thorough mixing of the additive with the fuel. To this purpose stirring or in-line dosing are prime examples of suitable techniques, but other methods known to those skilled in the arts can also be used.

[0089] As already mentioned hereabove, concentrations of the additives (dosage levels) needed to sufficiently stabilize a liquid fuel will depend on the desired degree of stabilisation, the type of fuel and the exact nature of the natural polyphenol or fatty acid ester derivative thereof used. Typically, said concentrations vary preferably from about 0 to about 5 % percent mass/volume, and more preferably from about 1 ppm to about 2500 ppm, and more more preferably from about 5 ppm to about 1000 ppm.

Examples of embodiments:

Example 1: Comparison of some traditional anti-oxidants in bio-diesel .

[0090] Different anti-oxidants were dissolved in bio- diesel or PEG 300 to produce a 1 % stock solution. From this stock solution aliquots were added to bio-diesel to obtain an anti-oxidant concentration of 150 ppm.

[0091] These stabilized samples were evaluated in a Rancimat 743 test apparatus operated at 110⁰C using an air flow of 20 1/h. The corresponding results are presented in Table 1.

Anti-oxidant Relative induction time (% - vs. non-stabilized bio-diesel)

Table 1: Relative induction time of bio-diesel stabilized with 150 ppm anti-oxidant

[0092] It should be noted that a number of these antioxidants experienced solubility problems (e.g. propyl gallate, ascorbyl palmitate) making them of no practical value notwithstanding the fact that they do exhibit anti-oxidant properties .

[0093] From the tabulated values it can be concluded that induction time and therefore storage stability is increased if products such as BHT or TBHQ are used as stabilizer in bio-diesel. Tocopherol however is hardly active.

Example 2: Comparison of some traditional anti-oxidants in bio-diesel contaminated with copper.

[0094] Different anti-oxidants were dissolved in bio- diesel or PEG 300 to produce a 1 % stock solution. From this stock solution aliquots were added to bio-diesel that was previously doped with 0.5 ppm Cu to obtain an anti-oxidant concentration of 150 ppm. Copper concentrations in this and following examples are expressed as amounts elemental copper. Copper palmitate was used as a source of copper to dope the bio-diesel .

[0095] These stabilized samples were evaluated in a Rancimat 743 test apparatus operated at 110⁰C using an air flow of 20 1/h. The corresponding results are presented in Table 2.

Table 2 : Relative induction time of bio-diesel contaminated with 0.5 ppm Cu and stabilized with 150 ppm anti-oxidant

[0096] Comparison of the results from table 2 with results established in the previous example (see table 1) clearly show that even 0.5 ppm Cu has a significant effect on storage stability of non-stabilized bio-diesel as is reflected in a 96 % decrease of the relative induction time. Such a decrease in Rancimat induction time basically makes this bio- diesel unsuitable for further use in diesel engines. [0097] It can also be observed that most of the synthetic anti-oxidants evaluated in this experiment do not exhibit any protective action in the presence of even low amounts of Cu. Especially the total failure of the currently used additives BHT and TBHQ in bio-diesel are noteworthy. [0098] It should also be noted that background levels of Cu determined in commercial bio-diesel samples can be significantly higher than the test amount of 0.5 ppm used in these examples. Example 3: Use of condensed tannins in bio-diesel

[0099] A number of condensed tannins were dissolved in bio-diesel or PEG 300 depending on their solubility and if necessary heated up to 120⁰C to obtain 1 % (m/v) stock solutions .

[0100] Aliquots of these solutions were added to both fresh bio-diesel and bio-diesel contaminated with 0.5 ppm Cu

(added as copper palmitate) in order to obtain dosage level of

150 ppm anti-oxidant in the fuel.

[0101] These stabilized samples were evaluated in a

Rancimat 743 test apparatus operated at 110⁰C using an air flow of 20 1/h. The corresponding results are presented in

Table 3.

Table 3: Relative induction time of bio-diesel stabilized with 150 ppm condensed tannin of different origins

[0102] All evaluated condensed tannins exhibited antioxidant properties in fresh bio-diesel similar to BHT. However, unlike BHT, they are also able to (partially) protect bio-diesel contaminated by Cu against oxidation , while under these conditions BHT no longer exhibited any protective effect

(see example 1) . Example 4: Use of hydrolysable tannins in bio-diesel

[0103] A number of hydrolysable tannins were dissolved in bio-diesel or PEG 300 depending on their solubility and if necessary heated up to 120⁰C to obtain 1 % (m/v) stock solutions .

[0104] Aliquots of these solutions were added to both fresh bio-diesel and bio-diesel contaminated with 0.5 ppm Cu

(added as copper palmitate) in order to obtain dosage level of

150 ppm anti-oxidant in the fuel.

[0105] These stabilized samples were evaluated in a

Table 4.

Table 4: Relative induction time of bio-diesel stabilized with 150 ppm hydrolysable tannin of different origins

[0106] ALSOK, Brewtan, Tanal 02C, Tanal SC and Tanex A FCC are commercially available hydrolysable tannins produced and marketed by Ajinomoto OmniChem NV/SA. Brewtan, Tanal 02C and Tanal SC are high molecular weight polygalloyl glucose tannins. ALSOK is a medium molecular weight hydrolysable polygalloyl glucose tannin and Tanex A FCC is a low molecular weight polygalloyl quinic acid tannin. [0107] The data clearly show that these compounds all exhibit very strong anti-oxidant properties in fresh bio- diesel, of the same order of magnitude as obtained with TBHQ. However unlike TBHQ this strong effect is largely retained in Cu-contaminated bio-diesel fuel while under these circumstances TBHQ no longer exhibits any protective effect

(see example 1) . This clearly demonstrates the dual action - anti-oxidant and metal sequestering - of these polyphenolic compounds .

Example 5: Use of palmitate esters of condensed tannins in bio-diesel

[0108] A number of palmitate esters of Quebracho tannin were dissolved in bio-diesel or PEG 300 depending on their solubility and if necessary heated up to 120⁰C to obtain 1 % (m/v) stock solutions. Aliquots of these solutions were added to both fresh bio-diesel and bio-diesel contaminated with 0.5 ppm Cu (added as copper palmitate) in order to obtain dosage level of 150 ppm anti-oxidant in the fuel.

[0109] These stabilized samples were evaluated in a Rancimat 743 test apparatus operated at 110⁰C using an air flow of 20 1/h. The corresponding results are presented in Table 5.

Identification fatty acid Relative induction time (% - ester vs. non-stabilized bio-diesel, without Cu contamination)

Degree of Bio-diesel Bio-diesel palmitate + 0.5 ppm Cu substitution

Quebracho palmitate

329-006-DS Low 111 31

329-002-DS Medium 112 25

301-198-DS high 109 22

Quebracho None 113 4

Bio-diesel 100 4 no additive

Table 5: Relative induction time of bio-diesel stabilized with 150 ppm of different fatty acid ester derivatives of condensed tannins

[0110] Remarkably palmitate esters of Quebracho tannin exhibit similar anti-oxidant properties as the parent compound in fresh bio-diesel. However, if the bio-diesel is contaminated with Cu, it can be seen that the fatty acid ester derivative still retains some anti-oxidant properties while Quebracho tannin is not active.

Example 6: Use of palmitate esters of hydrolysable tannins in bio-diesel

[0111] A number of fatty acid ester derivatives of hydrolysable tannins were dissolved in bio-diesel or PEG 300 depending on their solubility and if necessary heated up to

120⁰C to obtain 1 % (m/v) stock solutions.

[0112] Aliquots of these solutions were added to both fresh bio-diesel and bio-diesel contaminated with 0.5 ppm Cu

(added as copper palmitate) in order to obtain dosage level of

150 ppm anti-oxidant in the fuel.

[0113] These stabilized samples were evaluated in a

Rancimat 743 test apparatus operated at 110⁰C using an air flow of 20 1/h. The corresponding results are presented in Table 6.

Table 6: Relative induction time of bio-diesel stabilized with 150 ppm of different fatty acid ester derivatives of hydrolysable tannins

[0114] Tapa-4 and Tapa-7 are palmitate derivatives of hydrolysable tannin extracted from Chinese gallnuts. Tapa-4 has a degree of substitution of 4 eq. palmitate / mole tannin; Tapa-7 of 7 eq. palmitate / mole tannin.

[0115] TTP-5-301-140 and TTP-8-301-144 are palmitate derivatives of a Tara derived hydrolysable tannic acid. TTP-5- 301-140 has an approximate degree of substitution of 5 eq. palmitate / mole tannin and TTP-8-301-144 has a degree of substitution of approximately 8 eq. palmitate / mole tannin. [0116] Again substantial anti-oxidant properties are observed both when fatty acid ester derivatives of hydrolysable tannins are used in fresh bio-diesel and in bio- diesel contaminated with trace amounts of copper. [0117] Depending on the degree of substitution, it can be seen that anti-oxidant properties are also modified: higher degrees of substitution result in decreased anti-oxidant activity. This could be explained by the fact that active aromatic OH-groups are sacrificed to accommodate additional fatty acid residues. On the other hand, the introduction of apolar fatty acid residues also greatly improves solubility in apolar media such as bio-diesel.

Example 7: Use of palmitate esters of hydrolysable tannins in bio-diesel exposed to metallic copper.

[0118] Samples of bio-diesel with and without polyphenolic stabilizer were stored at room temperature for 1 week open to the air in contact with metallic copper. After this period of time samples were evaluated using a Rancimat 743 test apparatus operated at 110⁰C and an air flow of 20 1/h. The corresponding results are presented in Table 7.

Table 7: Relative induction time of bio-diesel stabilized with 1000 ppm of different palmitate esters of hydrolysable tannins exposed to copper

[0119] The above tabulated results clearly show that storage of non-stabilized bio-diesel in contact with metallic copper even for the very short period of 1 week at ambient conditions results in unacceptable degradation of the fuel rendering it basically unsuitable for combustion in diesel engines .

[0120] However the addition of tannin and especially the addition of tannin palmitate ester provides a sufficient protection against the negative effects of storage of bio- diesel in contact with copper open to the air. Example 8: use of mixed fatty acid esters of Tara hydrolysable tannin in bio-diesel .

[0121] Starting from Tara tannin and Rape Seed oil based bio-diesel a trans-esterification was carried out resulting in a mixed fatty acid ester of Tara tannin.

[0122] Depending upon the variety of rape seed, rape seed oil primarily consists of triglycerides of erucic, oleic, linoleic and Ct-linoleneic acid. Other fatty acids such as palmitic and stearic acid can also be present in minor amount. A bio-diesel produced from such a feed thus contains methyl- esters of the above mentioned fatty acids and consequently the trans-esterification product of such bio-diesel and tannic acid also is a mixture of said fatty acids attached to the tannin core by means of ester-bonds.

[0123] This mixed fatty acid ester derivative was presented in bio-diesel. An aliquot of this solution was added to both fresh bio-diesel and bio-diesel contaminated with 0.5 ppm Cu (added as copper palmitate) in order to obtain a dosage level of 150 ppm anti-oxidant in the fuel.

[0124] These stabilized samples were evaluated in a Rancimat 743 test apparatus operated at 110⁰C using an air flow of 20 1/h.

Table 8: Relative induction time of bio-diesel stabilized with 150 ppm of a mixed fatty acid ester derivative of hydrolysable Tara tannin

[0125] These data show that this mixed fatty acid ester of hydrolysable Tara tannin presents interesting anti-oxidant properties not only in fresh bio-diesel, but also in bio- diesel contaminated with trace amounts of copper.

Example 9: metal complexlng properties of palmltate esters of hydrolysable tannins In non-aqueous solvent

[0126] Metal chelating properties of fatty acid ester derivatives of hydrolysable tannin can be exemplified with the following experiment.

[0127] To a 1 % (m/V) solution of Tapa-7 in MeOH, 200 μl of an Fe(III) solution was added. This resulted in the formation of a coloured Fe (III) -Tapa-7-complex . By measuring the absorbance at 710 nm and using different Fe(III)- concentrations a titration curve has been established (see figure 1) .

Claims

1. Use of at least one natural polyphenol and/or at least one fatty acid ester thereof as additive in a liquid fuel.

2. The use according to claim 1, characterized in that the liquid fuel is a petroleum-based fuel.

3. The use according to claim 2, characterized in that the petroleum-based fuel is diesel fuel.

4. The use according to claim 1, characterized in that the liquid fuel is a bio-fuel.

5. The use according to claim 4, characterized in that the bio-fuel is bio-diesel.

6. The use according to any one of the preceding claims, characterized in that the natural polyphenol is selected from the group consisting of hydrolysable tannins, condensed tannins, and mixtures thereof.

7. The use according to any one of the preceding claims, characterized in that the natural polyphenol is a hydrolysable tannin comprising at least one hydroxybenzoic acid or at least one hydroxycinnamic acid, said hydroxybenzoic acid and said hydroxycinnamic acid being esterified on a central nucleus, wherein said hydroxybenzoic acid is preferably selected from the group consisting of gallic acid and oligomers thereof, and elagic acid and oligomers thereof, wherein said hydroxycinnamic acid is preferably selected from the group consisting of caffeic acid, ferulic acid and synaptic acid, and wherein said central nucleus comprising one or more units of a monomer is preferably selected from the group consisting of carbohydrates, and is more preferably selected from the group consisting of sugars and acids, and is more more preferably selected from the group consisting of glucose, glycerol and quinic acid.

8. The use according to any one of the preceding claims, characterized in that the natural polyphenol is a condensed tannin selected from the group consisting of flavonoids, stilbenes and phloroglucinols .

9. The use according to claim 8, characterized in that the condensed tannin corresponds to the general formula (I) or (II) :

wherein :

- A and B are carbon atoms connected by a single or by a double bond;

10. The use according to any one of the preceding claims, characterised in that the natural polyphenol is provided for addition into the liquid fuel as such or under a formulated form, said formulated form being selected from the group consisting of solutions, dispersions, emulsions, and mixtures thereof.

11. The use according to claim 10, characterised in that the concentration of the polyphenol is provided for addition into the liquid fuel is such that the final concentration of said polyphenol after addition to the liquid fuel is comprised between about 0% and about 5% mass/volume.

12. The use according to claim 11, characterised in that the concentration of the polyphenol is provided for addition into the liquid fuel is such that the final concentration of said polyphenol after addition to the liquid fuel is comprised between about 1 ppm and 2500 ppm, preferably, between about 5 ppm and about 1000 ppm.

13. The use according to claim 12, characterized in that the polyphenol presents a purity greater than 80% or is part of a plant extract.

14. The use according to any one of the preceding claims, characterized in that the polyphenol is provided for addition into the liquid fuel under the form of a solution in an adequate solvent, said adequate solvent being selected from the group consisting of esters, ethers, polyethers, alcohols, ketones and aliphatic hydrocarbon mixtures, aromatic hydrocarbon mixtures, and mixtures thereof.

15. The use according to claim 14, characterized in that the adequate solvent is selected from the group consisting of high boiling esters, esters of glycerol, and esters of glycol and mixtures thereof, and preferably in the subgroup consisting of fatty acid methyl esters, fatty acid ethyl esters and fatty acid propyl esters.

16. The use according to any one of claims 1 to 14, characterized in that the polyphenol is provided for addition into the liquid fuel under the form of a solution in an adequate solvent, said adequate solvent being the liquid fuel itself or analogues thereof, or a solvent comprising said fuel.

17. The use according to any one of the preceding claims, characterized in that the polyphenol is provided for addition into the liquid fuel under the form of a concentrated liquid solution having a polyphenol concentration ranging from about 5 and 60% (m/V) , and preferably ranging from about 15 and 50% (m/V) , and more preferably ranging from about 20 and 40% (m/V) .

18. The use according to any one of the preceding claims, characterized in that the polyphenol is used in combination with another additive, preferably selected from the group consisting of emulsifiers and miscibility enhancers.

19. The use according to any one of the preceding claims in engine industry, including car and aircraft industry.

20. The use according to any one of the preceding claims in heating application.

21. A composition comprising at least one liquid fuel and at least one natural polyphenol and/or at least one fatty acid ester thereof, said natural polyphenol being preferably selected from the group consisting of hydrolysable tannins, condensed tannins, and mixtures thereof.

22. The composition according to claim 21, characterised in that the liquid fuel is selected from the group consisting of a petroleum-based fuels, bio-fuels and mixtures thereof.