Octane Enhancers for Fuel Compositions

The present invention relates to an anti-block additive for fuel compositions, primarily gasoline compositions.

The petroleum industry has long recognized a need for greater fuel economy and efficiency in the operation of gasoline powered spark ignition engines. In many instances, high compression ratios are desired in order to provide for superior engine performance under various driving conditions. In order to provide high performance in high compression engines without the risk of knock damage, fuels which will be used in such engines require a high octane number and good anti-knock characteristics.

While octane ratings of fuels can be improved by blending appropriate refining streams, the necessary additional refining and blending operations needed to obtain a fuel having the desired high octane rating are costly. In lieu of these various defining and blending processes the petroleum industry sometimes blends anti-knock additives into fuels to increase the octane number of the fuel. For many refineries the use of anti-knock compounds is essential due to the lack of the refining and blending facilities to produce the high octane fuels.

Numerous compounds have been suggested as anti-knock additives for fuel compositions. The most successful of these anti-knock compounds additives are organo-lead compounds. However, the future use of organo-lead compounds as anti-knock additives is severely limited by recent legislation and is completely prohibited in the future.

As a replacement for lead-containing additives, numerous non-lead, anti-knock compounds have been suggested as octane improvers. Among these are rare earth beta-keto-enolate compounds, the lithium and sodium salts of organo-amino-cresols, various other organo metallic compounds, in particular organo-iron and organo-manganese compounds, such as iron pentacarbonyl and methylcyclopentadienyl manganese tri-carbonyl. In addition, it is known to improve the anti-knock and octane properties of gasoline by blending alcohol therewith.

These anti-knock additives have their own associated problems when blended into fuels for use in internal combustion engines. The numerous organo-iron compounds increase the potential of wear in internal combustion engines and the organo-manganese compounds, in addition to causing wear problems, may affect the catalytic converters used on most cars today to reduce air pollution for exhaust emissions. Fuel compositions of gasoline and alcohol have many problems, including separation if water is admixed with the composition. As a result, there is a need for additives for increasing the octane value of gasoline without causing any detrimental effects, particularly the detrimental effects of lead, iron and other metal octane improvers or the miscibility problems of alcohol.

The present invention is founded on the unexpected discovery that organic compounds containing a tert-butyl, a trimethyl silyl group, or an organic radical containing a trimethyl silyl group bonded to a carbon or nitrogen atom which in turn is bonded by unsaturated double or triple bonds to another atom, provide octane enhancement to gasoline. Alternatively expressed, it has been found that compounds containing a carbon or nitrogen atom bonded to another atom by at least one pi bond are useful for improving the octane value of gasoline, provided that there is also bonded to said carbon or nitrogen atom at least one of the following: (1) a substituted or unsubstituted tertiary butyl group; (2) a substituted or unsubstituted trimethyl silyl group; and (3) an organic radical containing a substituted or unsubstituted silyl trimethyl group.

Accordingly, the invention provides a fuel composition comprising a base fuel, such as gasoline, and an octane improver comprising one or more of the above described compounds, except where the unsaturated bonding is (1) between two carbon atoms in an unsubstituted phenyl ring of formula --C.sub.6 H.sub.5 or (2) between two carbon atoms where one of the carbon atoms is bonded only to species selected from the group consisting of hydrogen and methyl.

In addition, the invention provides a method for operating an internal combustion engine, such as an automotive engine, wherein the fuel employed to power the engine is a fuel composition of the invention.

The invention is directed to enhancing the octane of gasoline by the addition of certain additives. As stated earlier, there are many known compounds which enhance the octane value of gasoline, the most well known undoubtedly being tetraethyllead. However, it is also known that t-butyl benzene and 3,3-dimethyl-1-butene can provide octane enhancement to gasoline, but heretofore it was unknown that compounds having either a tert-butyl group or an organic radical containing a trimethyl silyl group bonded to an unsaturated carbon or nitrogen atom would, as a class, have the common property of enhancing the octane value of gasoline. In the present invention, however, it has been found that such compounds do in fact share this common property.

The class of most useful compounds discovered in the present invention to provide octane enhancement to gasoline are represented by the following generic formulae: ##STR1## wherein X.dbd.Y is C.dbd.O, C.dbd.N--R3, C.dbd.C.dbd.W, ##STR2## or C.dbd.S; X.tbd.Y is C.tbd.N or C.tbd.C--R6; N.dbd.Z is N.dbd.O, N.dbd.C.dbd.W, or ##STR3## W is oxygen or sulfur; A is an aryl-containing group with R.sup.1 bonded to the carbon atom of an aromatic ring; R.sup.1 is either a substituted or unsubstituted tert-butyl group, a substituted or unsubstituted trimethyl silyl group, or an organic radical containing a substituted or unsubstituted trimethyl silyl group; and R2, R3, R4, R5, R6, R7, and R8 are the same or different organic or inorganic species. (As used herein, the term "organic" refers to any chemical species containing one or more carbon atoms. As a corollary, the term "inorganic" refers to any chemical species not containing at least one carbon atom, e.g., --H, --Cl, --Br, etc.)

The compounds most suitable for use in the invention are gasoline-soluble and conform to the above formulae (I) through (IV) except that X.dbd.Y is other than C.dbd.CR.sup.4 R.sup.5 where R.sub.4 is hydrogen or a methyl group and A is other than a phenyl ring, i.e., --C.sub.6 H.sub.5. The preferred compounds are those of formulae (I), (II), and (III), with preferred compounds of formula (III) having N.dbd.Z equal to N.dbd.C.dbd.O. The most preferred compounds, however, conform to formula (I) or (II) wherein X.dbd.Y is C.dbd.O and X.tbd.Y is C.tbd.N. Preferably, R1 is either an unsubstituted tert-butyl group or trimethylsilylacetate group, with the unsubstituted tert-butyl group presently being most preferred. In the preferred embodiment, R2 is an organic radical of about 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, with the following groups presently being preferred: substituted or unsubstituted alkyl, carbyloxy, alkoxy, hydroxy, amino, acetyl, and acetyl-containing species of formula: ##STR4## where R9 is an organic radical, usually an alkyl group of 1 to 5 carbon atoms, and most preferably a methylene group. Also, while R3 and R5 to R8 may be any inorganic or organic species, it is preferred that R3, R5, R6, R7, and R8 either be alkyl groups of 1 to 10 carbon atoms or hydrogen, with hydrogen being more preferred, R4, as stated above, may be any inorganic or organic constituent with the exception of hydrogen or methyl, with alkyl groups of 2 to 10 carbon atoms being preferred, and ethyl being most preferred.

A trimethyl silyl group is of chemical formula: ##STR5## and an organic radical containing a trimethyl silyl group is of formula: ##STR6## wherein R10 is an organic radical, preferably a straight or branched chain alkyl group of 1 to 15 carbon atoms, more preferably an alkyl group of between 1 and 3 carbon atoms, and most preferably a methylene or dimethylene group, and most highly preferred of all, a methylene group. It will be understood, of course, that the trimethyl silyl group may be substituted, any of the hydrogens in the --CH.sub.3 groups being readily available for substitution by organic or inorganic species. However, in the preferred embodiment, the trimethyl silyl group is unsubstituted. Moreover, when a trimethyl silyl group or an organic radical containing a trimethyl silyl group is selected from R1, the preferred compounds conform to formula (I), with X.dbd.Y being a C.dbd.O group and R2 being an alkyl group or an alkoxy group of 1 to 10 carbon atoms. At present, the most preferred compound for use when R1 is or contains a trimethyl silyl group is t-butyl trimethyl silyl acetate.

Among the specific compounds which prove useful in the invention as octane improvers are pivalonitrile, methyl trimethyl acetate, pinacolone, 2,2,6,6-tetra-methyl hexa-3,5-dione, pivalic anhydride, pivalic acid, t-butyl isocyanate, and t-butyl trimethyl silyl acetate. The foregoing compounds have all been found to enhance the octane value of gasoline, some to an extent greater than that presently provided by commercial additives, such as tert-butyl methyl ether. In addition, these compounds are fully soluble in gasoline at a concentration of 5 volume percent, and it is noted that preferred compounds for use in the invention are those which are soluble in gasoline at this level. Further still, no detrimental effects have been found to result from the use of these compounds as octane improvers in unleaded gasoline. (However, silyl-containing compounds may leave an inorganic residue, and if this is not acceptable in certain applications, then a compound containing no ash-forming inorganic constituents is recommended.)

Anti-knock characteristics of an additive are typically evidenced by an increase in the motor and research octane numbers of the base fuel when the additive is admixed therewith. The motor (MON) and research (RON) octane numbers of fuel compositions are typically measured by the method described in ASTM D 2700 and ASTM D 2699, respectively.

The fuel composition may be comprised of any amount of the additive compound of this invention which enhances the anti-knock characteristics of the fuel. In the usual instance, the compositions of the invention are prepared simply by dissolving the desired additive in the fuel in a concentration sufficient to increase the octane value of the fuel. Normally the anti-knock additive comprises a minor amount (i.e., less than 50 percent by volume) of the fuel composition. Usually, the additive is gasoline-soluble (herein defined as soluble at 25.degree. C. to the extent of at least 0.5 grams per 100 ml of gasoline), and, as stated before, is preferably soluble to the extent of at least 5 volume percent. Preferably the fuel composition comprises from about 1 volume percent to about 15 volume percent of the additive compound of this invention, more preferably from about 3 to about 10 volume percent, and most preferably from about 5 to about 10 volume percent of the additive compound.

Base fuels to which the additive of this invention may be included to improve the anti-knock properties include all of the volatile liquid fuels suitable for spark-ignition, internal combustion engines, particularly automotive engines. Suitable liquid hydrocarbon fuels of the gasoline boiling range as described in ASTM D-439 are mixtures of hydrocarbons boiling in the range from about 25.degree. C. (77.degree. F.) to about 225.degree. C. (437.degree. F.), and often comprise mixtures of saturated hydrocarbons, olefinic hydrocarbons and aromatic hydrocarbons. Preferred are gasoline blends consisting of or consisting essentially of a saturated hydrocarbon content ranging from about 40 to about 80 percent by volume, an olefinic hydrocarbon content from about 0 to about 30 percent by volume and an aromatic hydrocarbon content rnaging from about 10 to about 60 percent by volume. The base fuel can be derived from straight run gasoline, alkylate gasoline, polymer gasoline, natural gasoline, dimer and trimerized olefins, synthetically-produced hydrocarbon mixtures, thermally or catalytically reformed hydrocarbons, isomerized and/or hydrotreated stocks, or catalytically cracked or thermally cracked stocks, and mixtures of these. The ultimate source of the base fuel is not critical, i.e., the fuel may be derived from petroleum or hydrocarbons derived from coal, oil shale, natural gas, etc. The hydrocarbon composition and octane level of the base fuel are not critical. In general, any conventional motor fuel base may be employed in the practice of this invention.

The base fuel may contain other additives normally employed in fuels, e.g., anti-icing agents, detergents, demulsifiers, corrosion inhibitors, dyes, deposit modifiers, anti-knock improvers, multi-purpose additives and the like. However, since this invention relates to anti-knock compounds useful for admixture into base fuels, the base fuel used will preferably be essentially free of other anti-knock compounds, particularly the organo-metallic compounds, e.g., organo-lead and organo-manganese compounds, and other anti-knock compounds used in base fuels, specifically, alcohols such as methanol. Thus, the preferred composition of this invention comprises a major portion of a base fuel and an anti-knock enhancing amount of the compound of this invention, with the composition being essentially free of compounds such as organo-lead and organo-manganese compounds and completely free of alcohol. By "essentially free of" it is meant that the composition will comprise less than 0.5 grams each of organo-lead and organo-manganese compounds, per gallon of fuel.

The following examples serve to further illustrate the invention and are not intended to be construed as limiting thereof.

The following Examples 1-7 illustrate the superior performance of pivalic anhydride (containing a tert-butyl group) over six other anhydrides. Various additives were blended into a base fuel at the levels indicated in Table 1. The base fuel was a gasoline containing 33.5 volume percent aromatics, 7.5 volume percent olefins and 59 volume percent saturates having an A.P.I. gravity of 58.4, vapor pressure of 8.6, a sulfur content of 296 ppm, and less than about 0.05 grams of lead/gallon of fuel. The gasoline base fuel had a research octane number of 94.4 and a motor octane number of 84.1. Also indicated in Table 1 are the organic radicals of each anti-knock anhydride tested, said anhydrides being of formula R--CO--O--CO--R', where R and R' in each anhydride are the same.

As shown by the data in Table 1, pivalic anhydride outperformed all other anhydrides for increasing octane. The closest competitor was acetic anhydride, which, at the 5% level, showed a comparable MON increase but a match smaller RON increase than pivalic anhydride. In addition, when the average increase is evaluated, that is (.DELTA.RON+.DELTA.MON)/2, as is usually considered important in fuel performance, it will be seen that pivalic anhydride is clearly superior to acetic anhydride at the 5 vol. % level. The average increase was 1.0 for pivalic anhydride and only 0.8 for acetic anhydride.

Because the data in Examples 1 to 7 showed that the tert-butyl group adjacent the C.dbd.O group of anhydrides proved the best, with a methyl group being the closest competitor, a series of experiments was performed comparing several components containing a tert-butyl group adjacent a pi system against a methyl group adjacent a pi system for octane improvement. The test method was the same as described for Examples 1 to 7, and the data obtained are summarized in the following Table 2:

In these examples, pivalic acid of formula (CH.sub.3).sub.3 C--COOH was tested for octane performance by the same method as in the previous examples. Also tested was tert-butyl methyl ether, a commercial octane enhancer of formula (CH.sub.3).sub.3 C--O--CH.sub.3 known as MTBE. The data are summarized in Table 3:

As shown in Table 3, a typical additive of the invention is superior to present, commercial oxygenated compounds for octane improvement. Also, if the data in Example 17 are compared against those of Examples 1, 9, 11, 13, and 15, it will be seen that pivalic anhydride, pivalonitrile, and pinacolone also prove superior to the commercial additive MTBE.

In these examples, t-butyl trimethyl silyl acetate is compared against t-butyl acetate for octane enhancement by the method of the previous examples. (t-Butyl acetate differs from t-butyl trimethyl silyl acetate by the replacement of a hydrogen atom in the methyl group of the acetate for a trimethyl silyl group.) A summary of the data obtained are shown in Table 4:

The data in Table 4 clearly reveal that t-butyl trimethyl silyl acetate substantially increases the octane value of gasoline. The data for this compound also prove superior to MTBE, the commercial additive tested in Example 17.

While the preferred embodiments have been described and illustrated, various modifications and substitutions may be made thereto without departing from the spirit and the scope of the present invention. The invention has been described by way of illustration and not limitation, and thus no limitation should be imposed other than those as indicated in the following claims.