EP4034614A1

EP4034614A1 - Tracers and method of marking liquids

Info

Publication number: EP4034614A1
Application number: EP20764711.6A
Authority: EP
Inventors: Duncan MCCALLIEN
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2019-09-23
Filing date: 2020-08-18
Publication date: 2022-08-03
Also published as: GB202012865D0; GB2589681B; US20220298439A1; BR112022002497A2; GB201913663D0; GB2589681A; WO2021058935A1

Abstract

A method of marking a hydrocarbon liquid comprising adding to said hydrocarbon liquid a tracer compound, the tracer compound being a substituted biphenol ether having a core structure of Formula I, wherein the two R groups are the same or different and selected from straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or the two R groups form a single substituent linked intramolecularly to both oxygen atoms, and wherein one or both of the aromatic rings of the core structure is further substituted with at least one non-planar group.

Description

Tracers and Method of Marking Liquids

Field

The present specification concerns marking liquids, especially hydrocarbon liquids, with tracer materials. The present specification in particular concerns marking hydrocarbons which are taxable or liable to be subject to tampering or substitution, such as gasoline and diesel fuels for example.

Background

It is well-known to add tracers to hydrocarbon liquids. A typical application is the tagging of hydrocarbon fuels in order to identify the fuel at a subsequent point in the supply chain. This may be done for operational reasons, e.g. to assist in distinguishing one grade of fuel from another, or for other reasons, in particular to ensure fuel quality, deter and detect adulteration and to provide a means to check that the correct tax has been paid. Apart from fuels, other products, such as vegetable oils may be marked to identify the product produced at a particular source, or certified to a particular standard.

One problem which is known to exist with the marking of fuel liquids in particular is the potential for the tracer to be removed, by evaporation from the fuel, by degradation of the tracer through ageing or exposure to environmental conditions such as heat, sunlight or air or alternatively by deliberate removal of the tracer for unlawful purposes such as for avoidance of tax. Methods for deliberate removal of tracers include adsorption of the tracer onto common adsorbent materials such as charcoal or clays, exposure to radiation, such as ultraviolet light, oxidation etc. A useful fuel tracer therefore needs to be resistant to removal by these common methods and also to more sophisticated treatments such as treatment with acids and/or bases. It is an aim of the invention to provide tracer compounds and methods of marking hydrocarbon liquids which are more resistant to removal of the tracer than other known tracers.

It is known from WO2013/003573 to use biphenol ether compounds as markers for liquid hydrocarbons and other fuels and oils. W02013/003573 discloses a method of marking a petroleum hydrocarbon or a liquid biologically derived fuel, the method comprising adding to said petroleum hydrocarbon or liquid biologically derived fuel at least one compound having the formula illustrated below:

wherein R represents C₁ - C₁₈ alkyl, C₃ - C₁₈ alkenyl or C₃ - C₁₈ alkynyl.

Summary of invention

A method of marking a hydrocarbon liquid is provided comprising adding to said hydrocarbon liquid a tracer compound, the tracer compound being a substituted biphenol ether having a core structure of Formula I: wherein the two R groups are the same or different and selected from straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or the two R groups form a single substituent linked intramolecularly to both oxygen atoms, and wherein one or both of the aromatic rings of the core structure is further substituted with at least one non-planar group.

The substituted biphenol ether tracer compounds as defined above have several advantages over prior art tracers as discussed below.

It has been noted that the two aromatic rings in the biphenol ether of WO2013/003573 can rotate around the central bond to be co-planar. The planar structure is susceptible to adsorption by active charcoal, which is a common laundering agent as mentioned in the background section. As such, to solve this problem and increase the resistance to laundering of such biphenol ether tracer compounds, the biphenol ether compounds have been modified such that one or both of the aromatic rings of the biphenol ether is additionally functionalized with a non-planar group (in addition to its ether functionalization). Such a non-planar functionalization restricts rotation about the carbon-carbon bond connecting the two aromatic rings, particularly if it is ortho to the carbon-carbon bond between the rings. This means that the two aromatic rings cannot be co- planar, which reduces the compounds susceptibility to adsorption by activated charcoal, which is a common laundering reagent. The presence of bulky non-planar groups on the aromatic rings further reduces the planarity of the molecule improving its resistance to adsorption. As such, substituted biphenol ethers as described herein have improved resistance to laundering.

In addition to reducing the planarity of the biphenol ether, non-planar substituents on the aromatic rings can also serve to improve the non-polar nature of the tracer molecule and protect the ether linkage from potential reaction.

It has also been found that while the additional functionalization of biphenol ether molecules with non-planar groups increases their mass, the molecules are surprisingly quick-eluting by gas chromatography for their mass. The combination of higher mass while remaining relatively quick-eluting is a very useful combination of properties as it means the tracer molecules elute at least with some of the components of the hydrocarbon liquid in which they are disposed but can still be resolved from those components by virtue of their mass. For example, the tracer molecules as described herein are heavier than most of the components of a typical fuel (gasoline or diesel fuel) but are still readily distinguishable from the fuel components which elute at a similar rate as the tracer molecules.

Furthermore, it is often the case that prior art tracer molecules operate best on one or other of gasoline and diesel but not both. Due to the combination of properties as outlined above, the substituted biphenol ether tracer molecules as described herein can operate well in both fuels while satisfying the other critical requirement of non-launderability.

Further still, the tracer molecules of the present invention can consist of atoms selected only from the group carbon, hydrogen, and oxygen which is a specified requirement for certain fuel marking applications. Additionally, the tracer molecules do not contain reactive functional groups or fused-ring structures which would otherwise decrease their resistance to laundering.

Finally, the basic substituted biphenol ether structure enables a family of related tracer molecules to be derived. That is, forming a substituted biphenol ether confers the advantage that a suite of molecular tracers can be produced simply by varying the species that is reacted with the biphenol core. The R groups of the biphenol ether, while typically being C₁ to C₂₀ groups, can be intentionally varied to provide a suite of tracer compounds. As each biphenol ether will possess a different mass or affinity to the separation column, they can all be distinguishable from each other by gas chromatography mass spectrometry (GC-MS). Such a suite of tracer compounds is very useful for marking hydrocarbon liquids (e.g. fuels) from different sources and/or for marking a hydrocarbon liquid with a combination of different tracer molecules.

A method of marking a hydrocarbon liquid, such as a gasoline or diesel fuel, a liquified petroleum gas fuel, or a biofuel, is provided comprising adding a tracer compound as defined above to the hydrocarbon liquid.

Further still, there is also provided a hydrocarbon liquid, such as a gasoline or diesel fuel, a liquified petroleum gas fuel, or a biofuel, comprising a tracer compound as defined above.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 shows the structure of a substituted ortho-biphenol ether;

Figure 2 shows a reaction scheme for the synthesis of a substituted ortho-biphenol from a substituted phenol;

Figure 3 shows two possible reaction schemes for the synthesis of a substituted ortho-biphenol ether from a substituted ortho-biphenol;

Figures 4(a) and 4(b) show examples of substituted ortho-biphenol ethers;

Figure 5 shows a reaction scheme for the synthesis of 2,2'-dipropyloxy-3,3',5,5'-tetra-(tert)-butyl- biphenyl from 3,3',5,5'-tetra-(tert) butyl 2,2'-dihydroxy-biphenyl;

Figure 6 shows a section from the GC-MS results for the 2,2'-dipropyloxy-3,3',5,5'-tetra-(tert) butyl- biphenyl product;

Figure 7 shows a reaction scheme for the synthesis of substituted para-biphenols;

Figure 8 shows alkylation of a substituted phenol;

Figure 9 shows the structure of substituted para-biphenol ethers;

Figure 10 shows a reaction scheme for the synthesis of 3,5,3',5'-tetra-(tert)-butyl-4,4'- diphenoquinone; Figure 11 shows a reaction scheme for the synthesis of 3,5,3',5'-tetra-(tert)-butyl-4,4'- dihyhydroxybiphenyl;

Figure 12 shows a section from the GC-MS results for the 3,5,3',5'-tetra-(tert)-butyl-4,4'- dihydroxy-biphenyl;

Figure 13 shows a reaction scheme for the synthesis of 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert) butyl 4,4'-biphenyl; and

Figure 14 shows a section of the GC-MS results for the 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert) butyl 4,4'-biphenyl.

Detailed Description

As described in the summary section, the present specification provides a tracer compound for marking a hydrocarbon liquid, the tracer compound being a substituted biphenol ether having a core structure of Formula I:

Formula I wherein the two R groups are the same or different and selected from straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or the two R groups form a single substituent linked intramolecularly to both oxygen atoms, and wherein one or both of the aromatic rings of the core structure is further substituted with at least one non-planar group.

The or each non-planar group can consist of atoms selected from the group carbon, hydrogen, and oxygen. Furthermore, the R groups can also consist of atoms selected from the group carbon, hydrogen, and oxygen. As such, for applications which specify that the tracer must only contain carbon, hydrogen, and/or oxygen atoms, embodiments of the tracer compound as described herein can fulfil this requirement. The or each non-planar group can be a C₄ to C₂₀ non-planar group and can be a non-planar alkyl group such as a branched alkyl group, e.g. a tert-butyl group. Furthermore, one or both of the aromatic rings of the core structure can be substituted with at least two of the non-planar groups. For example, in certain embodiments the tracer compound has two non-planar substituents provided on each aromatic ring of the biphenol ether to inhibit rotation of the rings to a planar configuration, reduce planarity, and increase mass of the tracer.

The substituted biphenol ether can be an ortho-biphenol ether, a meta-biphenol ether, or a para- biphenol ether. Ortho-biphenol ethers and para-biphenol ethers are preferred with ortho- biphenol ethers being particularly preferable as they can be manufactured at lower cost.

A method of marking a hydrocarbon liquid is also provided comprising adding a tracer compound as described herein to the hydrocarbon liquid. The resultant product is a hydrocarbon liquid, such as a gasoline or diesel fuel, comprising the tracer compound. The hydrocarbon liquid may be a pure compound such as hexane or octane or it may comprise a mixture of compounds such as a distillation fraction having a particular range of boiling points. The hydrocarbon liquid may be intended for use as a chemical, a solvent or a fuel. The tracer compounds as described herein are of particular use for marking liquid hydrocarbon fuels such as gasoline and diesel fuels or liquified petroleum gas. In one particular application a low-tax fuel such as an agricultural diesel may be marked in order to detect any subsequent sale and use for purposes such as road-vehicle fuel which would normally be taxed more highly. In such cases unlawful dilution or substitution of a more highly taxed fuel with the low-taxed fuel may be detected by analysis of the highly taxed fuel to determine whether the tracer is present. Therefore, in these cases, it is highly beneficial to use a tracer compound in the low-taxed fuel which is not easily removed, or laundered, from the fuel to a level at which it can no longer be detected. We have found that compounds as described herein are resistant to removal from hydrocarbon fuels by multiple known methods of fuel laundering.

The tracer compound is added to the hydrocarbon liquid in such an amount as to provide a concentration of the tracer compound which is detectable by readily available laboratory methods capable of identifying the tracer compound in the liquid at the concentrations used. Suitable methods include but are not limited to gas chromatography coupled with a suitable detector such as a mass spectrometer. Typical concentrations are within the range 1 mg/l to 10000 mg/l with the specific amount dependent on the detection method and limit of detection of the particular tracer compound used. The tracer compound may be present at a higher concentration than 1000 μg/l although when the product to be marked is a high-volume commodity such as a motor-fuel, economic considerations usually favour lower levels of tracer compound. The tracer compound may be supplied in the form of a concentrated dosing solution (or master-batch) of the tracer compound in a solvent. In this case the preferred solvent is a liquid which is similar to the liquid to be marked, although a different solvent, e.g. a single or mixed component aliphatic or aromatic solvent, may be used provided the presence of such a solvent can be tolerated in the hydrocarbon liquid to be marked. A preferred solvent is naphtha. The concentrated dosing solution can be added to the hydrocarbon liquid to be marked so as to produce the required final concentration of the tracer compound by dilution. More than one tracer compound may be added to the liquid.

Examples

1. Substituted ortho-biphenol ethers

The general structure of a substituted ortho-biphenol ether is shown in Figure 1. R1 and R2 can be the same or different, straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups. R1 and R2 can also be the same substituent linked intramolecularly to both oxygen atoms.

Ortho-biphenols, for example 3, 5, 3', 5'-tetra-(tert) -butyl2,2'-dihydroxy-biphenyl, can be generated from the reaction scheme in Figure 2. A method as described in WO2017/175582 can be utilized. An alternative method of coupling 2,4-di-tert-butylphenol has been reported using manganese dioxide in boiling heptane (paragraph 42 of US2012/0259126). Further methods have also been reported: using basic hydrogen peroxide (JP 2001/097908); or using potassium hydroxide, potassium hexacyanoferrate III and methanol (Adv. Synth and Catalysis 2004, 346(8), 993). Alternatively, ortho-biphenols, more specifically 5, 5' -alkyl-2, 2' -dihydroxy-biphenyls, can also be synthesized in good yield using copper II chloride, amine and air (see JP 2007/326798 and JP 2007/230975).

Two methods of alkylating the hydroxyl groups in biphenols have been reported: reaction with sodium hydride and a bromoalkene (see US 5,498,797); or reaction with potassium carbonate, sodium iodide and chloroacetonitrile (see S Higuchi, K Ito, Lett in Org. Chem. 2007, 4(6), 404- 408 and S Higuchi et al. J. Incl. Phen. And Mac. Chem. 2008, 62, (3-4), 215-222). Figure 3 shows example reaction schemes. By extending the alkylation procedure shown in Figure 3 to a variety of substituents it is possible to make molecules such as 3,5,3',5'-tetra-(tert) -butyl-2,2'-dialkoxy-biphenyl as shown in Figure 4(a). R could be a straight or branched alkyl group, a cyclic aliphatic group, a benzyl group, or a substituted benzyl group. Figure 4(b) shows an alternative molecule in which a common substituent is provided to link the either groups. The common linker can be a straight or branched alkyl, a cyclic aliphatic group, or a substituted or unsubstituted aromatic group.

Figure 5 shows a reaction scheme for the synthesis of 2,2'-dipropyloxy-3,3',5,5'-tetra-(tert) butyl- biphenyl from 3,3',5,5'-tetra-(tert)-butyl-2,2'-dihydroxy-biphenyl. The synthesis procedure is summarized in the experimental section below.

Experimental

A three necked 50ml round bottom flask was charged with 3,3',5,5'-tetra-(tert)-butyl-2,2'- dihydroxybiphenyl (0.750 g, 1.84 mmol) and pulverised potassium hydroxide pellets (0.409g, 7.3 mmol, 4.0 eq.). Dimethylsulfoxide (10 ml) was then added. The flask was warmed to approx. 100°C for about 60 min. The biphenol dissolved rapidly but the potassium hydroxide remained undissolved and the solution was colourless. After about 20 minute the solution had become a pale-yellow colour.

Bromopropane (1.669 ml, p= 1.35 g/cc, 18.3 mmol, 10 eq.) was added by syringe, keeping the contents of the reaction vessel under nitrogen. The temperature was reduced to about 80°C to prevent the bromopropane from being lost too rapidly by evaporation as it boils at 71°C. On addition of the bromopropane the reaction became turbid and white in colour. The reaction was then left heating under nitrogen over night at about 90°C. The reaction was sampled and analysed by GC-MS. The reaction was now a pale turbid yellow with white precipitate.

Analysis revealed the reaction was incomplete, so the reaction mix was dissolved in a small quantity of toluene, washed with water, dried over magnesium sulfate and then evaporated down. The resultant yellow oil was dissolved in dimethyl sulfoxide (10 ml) and tetrahydrofuran (4 ml) giving a deep yellow solution. Potassium hydroxide (0.352 g, 6.29 mmol) was added and the reaction was heated under nitrogen for about 30 minutes. It turned dark brown. Bromopropane (2 ml, 21.9 mmol, 11.9 eq.) was added. The reaction was stirred at 80°C under nitrogen over two days and then re-analysed by GC-MS.

The reaction was diluted in toluene, washed with water (2x), washed with brine (1x), dried over magnesium sulfate and evaporated down to give a yellow solid. Traces of dimethyl sulfoxide were removed from the mixture by adding methanol to the flask during evaporation. The product was analysed by GC-MS with a section from the results indicated in Figure 6. The di-alkylated product has mass 494.80. The solid can be recrystallized from acetone. Yield of the solid product was 0.663 g (73%). As such, this method can be successfully used to synthesise 2,2'-dipropyloxy- 3,3',5,5'-tetra-(tert)-butyl-biphenyl from 3,3',5,5'-tetra-(tert)-butyl-2,2'-dihydroxy-biphenyl. The product was then subjected to a range of launder test as summarized the following section.

Launder Tests

A range of fuel laundering tests have been performed on 2,2'-dipropoxy-3,3',5,5'-tetra-tert- butyl-biphenyl. 2,2'dipropoxy-3,3',5,5'-tetra-tert-butyl-biphenyl (32.3 mg, referred to as 'the taggant') was dissolved in toluene (25 ml) giving a solution containing 1292 mg/L of the taggant. Taggant solution (154 microliters) was added to gasoline (200 ml) giving gasoline tagged at 1 mg/L. Taggant solution (386 microliters) was added to diesel (500 ml) giving diesel tagged at 1 mg/L. Samples of the tagged fuel were subjected to a series of launder tests where the fuel was subjected to commonly used laundering reagents. The concentration of the taggant in laundered fuel was compared after a particular launder test with the concentration of the taggant in a sample of the same fuel which had not been subjected to a launder test. A sample of tagged fuel that had not been subjected to laundering is referred to as tagged reference. A typical GC sequence included tagged reference, untagged fuel, samples of laundered fuel, tagged reference. Reference samples were run at the beginning and end of any GC sequence to help eliminate instrument drift over the course of the sequence. All those launder tests involving a washing procedure were carried out in sealed brown glass bottles to minimise evaporation over a four-hour stirring period. All launder tests involving stirring were allowed to separate before sampling. The fuel layer from any launder test containing an aqueous reagent was separated into a scintillation vial where it was dried over anhydrous magnesium sulfate or potassium carbonate before being filtered through a cotton wool plug and finally transferred to a GC vial. All tests involving the passage of fuel through a column of solid absorbent were carried out by applying reduced pressure to the outlet of the column rather than a positive pressure to the mouth of the column. This was achieved by fitting the column outlet via a close fitting seal to a receptacle, such as a Buchner flask, collecting the liquid that elutes from the column into the flask whilst attaching a pump to reduce the pressure in the Buchner flask via the side arm. Fuels containing obvious particulate matter were filtered before sampling into a GC vial. The fuel from all other launder tests was sampled into GC vials without further clean-up.

Launder tests included the following:

Hydrochloric acid wash - Concentrated hydrochloric acid (36% cone., aq., 30 ml) was diluted with de-ionised water (70 ml). The diluted acid (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Sulfuric acid wash - Concentrated sulfuric acid (95% cone., aq., 10 ml) was diluted with de-ionised water (90 ml). The diluted acid (25ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Nitric acid wash - Concentrated nitric acid (70% cone., aq., 15 ml) was diluted with de- ionised water (85 ml). The diluted acid (25ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Sodium hydroxide wash - Sodium hydroxide pellets (40 g, 1 mole) were added to a beaker which was placed in a water bath. Deionised water (100 ml) was added to the beaker and the pellets were dissolved giving a 10M sodium hydroxide solution. Sodium hydroxide solution (25 ml, 10M) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Potassium hydroxide wash - Potassium hydroxide pellets (56 g, 1 mole) were added to a beaker which was placed in a water bath. Deionised water (100 ml) was added to the beaker and the pellets were dissolved giving a 10M potassium hydroxide solution. Potassium hydroxide solution (25 ml, 10M) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Water wash - Water (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature.

Methanolic potassium hydroxide wash - Potassium hydroxide pellets (11.2 g, 0.2 moles) were added to a beaker. Deionised water (10 ml) was added to the beaker giving a 20 M solution of potassium hydroxide. Methanol (190 ml) was added to the potassium hydroxide solution giving a 1 M methanolic potassium hydroxide solution.

Hydrogen peroxide wash - Hydrogen peroxide solution (35% cone., aq., 25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature. Methanol wash - Methanol (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature. When separating the phases the fuel was found to be the lower layer.

Acetonitrile wash - Acetonitrile (25 ml) was mixed with tagged fuel (25 ml) and stirred for four hours at room temperature. When separating the phases the fuel was found to be the lower layer.

Fuller's earth column - A 10 cm glass column with 1 cm internal diameter was packed with Fuller's earth. Tagged fuel (50 ml) was passed through the column. Two repeat passes of the fuel through the column were undertaken using fresh adsorbant.

Alumina column - A 10 cm glass column with 1 cm internal diameter was packed with basic activated alumina (0.063 - 0.2 mm particle size). Tagged fuel (50 ml) was passed through the column. Two repeat passes of the fuel through the column were undertaken using fresh adsorbant.

Activated charcoal column - A 10 cm glass column with 1 cm internal diameter was packed with activated charcoal (Norit RBAA-3 rod). Tagged fuel (50 ml) was passed through the column. Two repeat passes of the fuel through the column were undertaken using fresh adsorbant. Crushed charcoal was not used as minimal fuel will pass through even under reduced pressure over an hour.

Crushed sepiolite column - A 10 cm column with 1 cm internal diameter was packed with sepiolite. Tagged fuel (50 ml) was passed through the column. Two repeat passes of the fuel through the column were undertaken using fresh adsorbant.

Silica column - A 10 cm column with 1 cm internal diameter was packed with silica (Davisil grade 710, pore size 50-76Å). Tagged fuel (50 ml) was passed through the column. Two repeat passes of the fuel through the column were undertaken using fresh adsorbant.

Activated charcoal stir- Activated charcoal powder (2.5 g, powder not pellets) was mixed with tagged fuel (50 ml) and stirred forfour hours at room temperature. The charcoal was filtered off using filter paper.

Fuller's earth stir - Fuller's earth (2.5 g) was mixed with tagged fuel (50 ml) and stirred for four hours at room temperature. The Fuller's earth was filtered off using filter paper.

Stirring - Tagged fuel (50 ml) was placed in a beaker and stirred for four hours at room temperature.

Heat - Fuel (50 ml) was placed in a beaker and heated at 60°C for four hours.

Air sparging - Tagged fuel (50 ml) was placed in a brown glass bottle and air was passed through at about 200 ml/min. The fuel was tested after 24 hours and 48 hours. UV treatment - Three scintillation vials were each filled with tagged fuel (25 ml). Two of the vials were placed under a bench top UV lamp (15W, 365nm). One vial was carefully laid on its side under the lamp whilst another was placed upright, without any lid, so that the lamp was directly over it. The distance of these vials from the light bulb was about 12 cm. The third vial was placed on a window sill. The fuel samples were tested after varying degrees of exposure.

Results

Results of the launder tests in diesel fuel are summarized in the table below indicating fuel type, launder test, and amount of tracer remaining after the test in terms of a percentage of the initial concentration of tracer in the fuel.

As can be seen, even when reference samples were run at the beginning and end of the GC sequence to help eliminate instrument drift over the course of the sequence, many of the results indicate tracer concentrations of slightly over 100% after the launder test. As such, it is evident that there is some error in the measurement method as is always the case for analytical measurements. That said, it should be noted that no problem or interference was experienced in analysing by GC-MS for the taggant molecule, 2,2'-dipropoxy-tetra-(tert)-butyl-biphenyl. In some GC analyses, small peaks were observed of much lower intensity than that of the taggant, and at different retention times. However, when the molecular ion of the taggant (m/e = 494) was extracted from the chromatogram, no peak other than that of the taggant was observed. It is concluded that the 2,2'-propoxy-tetra-t-butyl-biphenyl seems resistant to all common laundering methods in diesel fuel.

The properties of 2,2'-propoxy-tetra-t-butyl-biphenyl in gasoline would appear to be similar to when it is in diesel. Complete test results have not yet been obtained but the results of preliminary testing are show in the table below.

The errors in these preliminary results would appear to be larger for the gasoline fuel when compared to the diesel fuel. Despite the errors in the measurement method, clearly the majority if not all of the tracer/taggant is being retained the fuel after the launder tests.

2. Substituted para-biphenol Ethers Para-biphenols can be generated from the reaction scheme in Figure 7. The reaction scheme in Figure 7 uses manganese III acetylacetonate as the oxidant. The synthesis of tetra-methyl biphenyl-diol and tetra-(tert)-butyl-biphenyl-diol using this method is described in MJS Dewar, T Nakaya, J. Am. Chem. Soc. 1968,90, 7134. Examples of other commercially available starting materials include 2,6-di-iso-propyl phenol and 2,6-diphenyl phenol.

Figure 8 shows alkylation of 2,6-tert-butyl phenol as described in N. Kornblum, R. Selzer J. Am. Chem. Soc. 1961, 83, 3668. By applying the procedure in figure 8 to the reaction products from the procedure in figure 7, it is possible to create molecules such as that shown in figure 9. An alternative route to the type of molecule in Figure 9 is described in D. Mirk, B. Wibbeling, R. Froehlich, S. Waldvogel, Synlett 2004,11, 1970. It involves the oxidative coupling of ortho-alkyl substituted methoxy benzenes to form a bis-alkyl biphenol ether.

From the prior art, biphenol, phenylphenol and phenol-based marker chemicals are all detectable by GC-MS or GC-FID at concentrations from 1-10 ppm. By analogy, the molecules shown in Figure 9 are also detectable by GC-MS or GC-FID in fuels at similar concentrations.

Synthesis of 3,3',5,5'-tetra-(tert)-butyl-4,4'-diphenoquinone

Synthesis of 3,3',5,5'-tetra-(tert)-butyl-4,4'-diphenoquinone was via the reaction scheme shown in Figure 10. 2,6-di-tert-butyl phenol (2.005 g, Mw=206.3, 9.7mmol) was added to a 50 ml round bottom flask. Ethanol (25 ml) was added to dissolve the substituted phenol giving a 0.39M solution. A solution of potassium ferricyanide (8.72g, Mw=329.2, 26.4mmol) and potassium hydroxide (5.01g, Mw= 56, 0.090mmol) was dissolved in water (50 ml). The concentration of the ferricyanide was 0.53M and of the hydroxide 1.8M. The alkaline ferricyanide solution (45 ml) was added quickly to a stirred solution of 2,6-di-tert-butylphenol. The reaction mix immediately turned into a yellow solid, so xylene (25 ml) was added. The reaction mix consisted of a yellow slurry suspended in a dark brown liquid. It was stirred for approximately three hours. The reaction mix was then extracted with xylene and washed with water. The organic phases were combined and then dried over anhydrous magnesium sulfate.

The unpurified reaction mix was evaporated to dryness giving a red/purple solid. This was analysed by GC-MS. Two major peaks were observed at 9.61 min (no clear mass, area = 20k) and at 10.61 min (Mw = 408/ 410, area = 8845k). The peak at 10.61 min corresponds to the product (yield = 1.886g; 95%; purity approx. 99.8%). The solid was dissolved in boiling cyclohexane (8g or 10ml) giving a dark orange/ brown solution. This solution crystallised at room temperature and was then cooled further to -18°C in the freezer. It was filtered through a sintered glass funnel and washed with chilled methanol. On drying, a red coloured solid results (yield = 1.6157 g; 81.6%).

Synthesis of 3,3',5,5'-tetra-(tert)-butyl-4,4'-dihydroxy-biphenyl

Synthesis of 3,3',5,5'-tetra-(tert)-butyl-4,4'-dihydroxybiphenyl was via the reaction scheme shown in Figure 11. Tetra-(tert)-butyl-4,4'-diphenoquinone (1.0 g, Mw = 408.6, 2.45 mmol) and ethanol (50 ml) were added to a 250 ml round bottom flask. The quinone dissolved giving a rust coloured solution. The flask was heated in a crystallizing dish containing silicone oil. The contents of the flask were warmed to 57°C (hotplate dial read 90°C). A solution of sodium dithionite (10 g, Mw = 174, 57 mmol) was prepared in water (50 ml) and then added to the reaction mix. The reactants were then stirred for 2.5 hours at 60°C during which time the colour changed to a dirty brown. On allowing the contents of the flask to settle there was a layer of white solid at the bottom, a fine orange suspension and a clean yellow solution. The temperature in the oil bath was increased to 80°C and additional sodium dithionite (2 x 5g, 57 mmol) was added. Once reduction of the quinone had occurred the colour of the reaction flask changed from deep orange brown to dirty yellow. On allowing the contents of the flask to settle there was a layer of white solid and a yellow supernatant.

The supernatant crystallised as yellow/ orange needle shaped crystals. These were filtered and analysed by GC-MS as shown in Figure 12. The crystals contain 98.9% tetra-(tert)-butyl-4,4'- dihydroxy-biphenyl (yield = 0.939 grams; 93.4%). As such, is was concluded that 2,6-di-tert-butyl- phenol can be coupled to itself giving 3,3',5,5'-tetra-tert-butyl biphenol.

Synthesis of 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert)-butyl-biphenyl

Synthesis of 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert)-butyl-biphenyl was via the reaction scheme shown in Figure 13. A two necked 100 round bottom flask was charged with bis-tetra -(tert)- butyl-4,4'-dihydroxy-biphenyl (0.754 g, 1.8 mmol) and pulverised potassium hydroxide pellets (0.328g, 5.9 mmol, 3.4 eq.). Dimethylsulfoxide (20 ml) was then added. On addition of DMSO the reagents instantly turned a dirty yellow/ green colour. The reagents were heated to 70°C under nitrogen for 3.5 hours. The colour was unchanged. It was then cooled to 40°C and bromopropane (4.52 g, 36.7 mmol) was added. The reaction flask was then stirred overnight becoming yellow/ orange in colour. The reaction was quenched with water (100 ml) and extracted with toluene (3x50 ml). The organic phases were combined and dried over anhydrous sodium sulfate.

The crude 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert)-butyl-biphenyl was added to acetone (10 ml). Full dissolution in the acetone was achieved by heating with a heat gun. The solution was cooled at -18°C for 110 min giving white crystals. The crystals were removed by filtration and dried in an oven (yield 0.271 g). These were analysed by GC-MS as shown in Figure 14. Mass of the product is 494 a.m.u (purity 98.9%). It was concluded that 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert) butyl 4,4'biphenyl can be synthesised from 3,3',5,5'-tetra-(tert) -butyl-4,4'-dihydroxy-biphenyl.

A similar procedure was also performed to synthesize 4,4'-dioctyloxy-3,3',5,5'-tetra-(tert)-butyl biphenyl.

Launder Tests

Both 4,4'-dipropyloxy-3,3',5,5'-tetra-(tert)-butylbiphenyl and 4,4'-dioctyloxy-3,3',,5'-tetra-(tert)-butyl-biphenyl were subjected to a range of launder test along the same lines as previously described for 2,2'-dipropoxy-3,3',5,5'-tetra-(tert)-butyl-biphenyl. Results are summarized in the tables below indicating percentage of tracer remaining in the fuel after various laundering treatments.

4,4'-dipropoxy-3,3',5,5'-tetra-(tert)-butyl-biphenyl 4,4'- ioctyloxy-3,3',5,5'-tetra-(tert) butyl 4,4'-biphenol

The properties of 4,4'-octoxy-tetra-(tert)-butyl-biphenyl in gasoline would appear to be similar to when it is in diesel. The results of preliminary testing are shown in the table below.

Gasoline evaporates very readily at ambient temperatures so tests involving the passage of the gasoline through an absorbent were not undertaken as a substantial proportion of the gasoline would be removed during the test procedure, which would render the apparent tracer concentration far greater than 100%. The errors in these preliminary results would appear to be larger for the gasoline fuel when compared to the diesel fuel. Despite the errors in the measurement method, clearly the majority if not all of the tracer/taggant is being retained the fuel after the launder tests.

As can be seen from the test results, the tracer compounds based on orf/io-biphenols and also those based on parab-iphenols performed well in all launder tests.

While this invention has been particularly shown and described with reference to certain embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of marking a hydrocarbon liquid comprising adding to said hydrocarbon liquid a tracer compound, the tracer compound being a substituted biphenol ether having a core structure of Formula I: wherein the two R groups are the same or different and selected from straight chain, branched or cyclic alkyl groups, phenyl or substituted phenyl groups, benzyl or substituted benzyl groups, or the two R groups form a single substituent linked intramolecularly to both oxygen atoms, and wherein one or both of the aromatic rings of the core structure is further substituted with at least one non-planar group.

2. A method according to claim 1, wherein the or each non-planar group consists of atoms selected from the group carbon, hydrogen, and oxygen.

3. A method according to claim 1 or 2, wherein the or each non-planar group is a C₄ to C₂₀ non-planar group.

4. A method according to any preceding claim, wherein the or each non-planar group is a non-planar alkyl group.

5. A method according to claim 4, wherein the non-planar alkyl group is a branched alkyl group.

6. A method according to claim 5, wherein the branched alkyl group is a tert-butyl group.

7. A method according to any preceding claim, wherein one or both of the aromatic rings of the core structure is substituted with at least two of the non-planar groups.

8. A method according to any preceding claim, wherein the substituted biphenol ether is an ortho-biphenol ether or a para-biphenol ether.

9. A method according to claim 8, wherein the substituted biphenol ether is an ortho-biphenol ether.

10. A method according to any preceding claim, wherein the R groups consist of atoms selected from the group carbon, hydrogen, and oxygen.

11. A method according to any preceding claim, wherein the R groups are C₁ to C₂₀ groups.

12. A method according to any preceding claim, wherein the tracer compound is added to the hydrocarbon liquid to yield a concentration of tracer compound in a range 1 mg/l to 10,000

13. A method of identifying a source of a hydrocarbon liquid comprising: analysing said hydrocarbon liquid to detect a tracer compound as defined in any preceding claim; and determining the source of the hydrocarbon liquid by the presence of the tracer compound.

14. A hydrocarbon liquid comprising a tracer compound as defined in any preceding claim.

15. A hydrocarbon liquid according to claim 14, wherein the hydrocarbon liquid is a diesel fuel, a gasoline fuel, a liquified petroleum gas fuel, or a biofuel in which the tracer compound is disposed.

16. A hydrocarbon liquid according to claim 14 or 15, wherein the tracer compound is present at a concentration in a range 1 mg/l to 10,000 mg/l.