Unsymmetrical dialkyl carbonate fuel additives

Abstract

 Unsymmetrical dialkyl carbonates are blended with hydrocarbon liquid fuels such as gasoline to provide fuel compositions having improved octane number and anti-knock characteristics. The unsymmetrical dialkyl carbonate may be methyl t-butyl carbonate, ethyl t-butyl carbonate, methyl t-amyl carbonate, or ethyl t-amyl carbonate.

Description

[0001] This invention pertains to improved hydrocarbon fuel compositions containing minor amounts of at least one unsymmetrical dialkyl carbonate. In particular, the invention relates to blends of gasoline and tertiary alkyl substituted carbonates such as methyl t-butyl carbonate, ethyl t-butyl carbonate, methyl t-amyl carbonate, and ethyl t-amyl carbonate.

BACKGROUND OF THE INVENTION

[0002] To improve efficiency, modern spark ignition internal combustion engines such as those used in automobiles typically are designed to operate at a relatively high compression ratio. The gasoline used in such engines must have a high octane number throughout its entire distillation range in order to eliminate fuel-wasting and potentially damaging engine knock at all engine speeds and loads. Because of this requirement, gasoline compositions are normally formulated to have a Research Octane Number (RON) greater than about 80 and more preferably greater than about 90. Premium grade fuels may have octane ratings that are even higher. Because of the difficulties and expense associated with refining crude oil to directly yield a high octane fuel, it has been common practice to incorporate an additive into the fuel to increase the octane rating. At one time, lead compounds such as tetraethyl lead were widely used as octane rating improvers. However, the environmental and health problems associated with the discharge of lead into the atmosphere from internal combustion engines have led to a gradual removal of such additives from gasoline.

[0003] Alternative octane rating improvers that have been proposed for use include ethers such as methyl t-butyl ether and methyl t-amyl ether and alcohols such as methanol, ethanol, and t-butyl alcohol. Although blending components of these types are currently being used in commercial practice, certain disadvantages are associated with their use.

[0004] For example, alcohols such as methanol and ethanol have limited compatibility with gasoline. In addition, they tend to absorb water as a result of their polar hydrophilic character. Phase separation is commonly observed in a gasoline-alcohol blend once even a small amount of water is absorbed. This can result in the accelerated corrosion of a fuel system as well as plugging of fuel filters by the corrosion products. Extraordinary precautions must therefore be taken to avoid water contamination of alcohol-containing fuels.

[0005] In addition, adding methanol or ethanol to gasoline increases its vapor pressure and distorts or flattens the front half of the gasoline's distillation curve. Hard starting and vapor lock in hot weather may result. The distorted distillation curve and higher heat of vaporization can also have a combined deleterious effect on cold weather warm-up.

[0006] While methyl t-butyl ether has a high blending RON, it has a relatively low boiling point (55°C) and so its use as an octane improver has the disadvantage that the amount of butane that can be included in the fuel is reduced. This may tend to offset the octane enhancement effect of the ether since butane is a high-octane blending component in its own right. The high volatility of methyl t-butyl ether also limits the maximum amount that can be blended into gasoline due to the driveability problems such as vapor lock which can result at high concentrations.

[0007] Another class of oxygenated organic compounds proposed for use as blending agents to increase the octane rating of gasoline are carbonates (i.e., diesters of carbonic acid). Fuel compositions of this type are described, for example, in Jpn. Pat. No. 60-46473, European Pat. Appl. Nos. 98,691, 112,172, and 82,688, and U.S. Pat. Nos. 2,331,386, 3,001,941, 3,382,181, 4,302,215, 4,380,455, 4,600,408, 4,891,049 and 4,904,279. To date, attention has been focussed on symmetrical dialkyl carbonates in which the alkyl groups are derived from lower unbranched primary alcohols such as methanol and ethanol. Dimethyl carbonate and diethyl carbonate, while having suitably high Blending Octane Values (BOV), do suffer from certain disadvantages which would tend to discourage their use as octane improvers in gasoline compositions. Most noticeably, these compounds are very susceptible to hydrolysis, particularly in the presence of acidic substances which can serve as catalysts. Thus, although dimethyl carbonate and diethyl carbonate tend to be somewhat more compatible with gasoline than alcohols in the presence of water, gasoline compositions containing such compounds may degrade to an unacceptable degree upon prolonged storage or exposure to an acidic environment. Hydrolysis of the carbonates would generate methanol or ethanol; as described previously, the presence of these materials in gasoline is known to result in phase separation, corrosion, and driveability problems.

[0008] Higher dialkyl carbonates, i.e., those containing alkyl groups of more than two carbon atoms, may be more resistant to hydrolysis than diethyl or dimethyl carbonate. However, it is recognized in the art that the octane enhancement value of dialkyl carbonates tends to decrease as the total number of carbon atoms in the molecule is increased. EP 98,691, for example, teaches that the average blending octane number of a fuel is lowered as the normal alkyl group of a symmetrical carbonate is varied from methyl to ethyl to n-propyl to n-butyl. Di-n-butyl carbonate, in fact, provides little or no enhancement of the octane value of a typical gasoline fuel composition.

SUMMARY OF THE INVENTION

[0009] We have now found that the octane number of a liquid hydrocarbon fuel composition may be significantly increased by the addition of minor amounts (i.e., less than 50 weight percent) of an unsymmetrical dialkyl carbonate selected from the group consisting of methyl t-butyl carbonate, ethyl t-butyl carbonate, methyl t-amyl carbonate, and ethyl t-amyl carbonate. Surprisingly, the octane ratings of the fuel compositions of this invention meet or exceed the octane ratings of the prior art fuel compositions containing unbranched symmetrical dialkyl carbonates. This result was highly unexpected in view of the recognized trend towards lower octane enhancement as the number of carbon atoms in a dialkyl carbonate fuel additive is increased. We have found, for example, that a gasoline composition of this invention containing methyl t-butyl carbonate has a blending octane value which is not only much higher than that of a composition containing an equivalent amount of the analogous straight chain carbonate (methyl n-butyl carbonate) but which is even somewhat higher than a fuel containing either dimethyl carbonate or diethyl carbonate.

[0010] These unsymmetrical carbonate additives are highly compatible with the hydrocarbon fuel and are surprisingly resistant to hydrolysis. Exposure of the modified fuel composition to water thus does not result in an undesirable extraction of the carbonate octane enhancer into the water phase or gradual decomposition of the carbonate. The carbonates of this invention, unlike dimethyl carbonate and diethyl carbonate, are nearly completely insoluble in water (<0.5% at 70°C); fuel blends containing the unsymmetrical carbonates exhibit a reduced tendency to absorb water. The unsymmetrical carbonates have a mild pleasant odor that would not be objectionable upon blending into gasoline.

[0011] A further advantage of this invention is that the addition of the unsymmetrical dialkyl carbonate to a gasoline fuel does not adversely affect the Reid vapor pressure of the fuel. That is, the Reid vapor pressure is desirably lowered when the unsymmetrical dialkyl carbonates of this invention are added to a gasoline fuel. This is a distinct advantage since low cost, highly volatile fuel components such as butanes can then be blended into the fuel without exceeding the desired vapor pressure limit. In contrast, the use of certain conventional octane enhancers having a high volatility such as methyl t-butyl ether may preclude the incorporation of large amounts of butanes.

[0012] The use of the unsymmetrical dialkyl carbonates of this invention in a fuel is additionally expected to provide a cleaner burning fuel composition as compared to a fuel that does not contain any oxygen-containing additives. That is, by analogy to known oxygenated fuel additives such as ethers, alcohols, and diethyl carbonate, the production of undesirable and harmful engine emissions such as carbon monoxide is expected to be significantly suppressed by the incorporation of an unsymmetrical dialkyl carbonate into a fuel.

[0013] This invention provides an improved liquid fuel composition comprising a major proportion of a hydrocarbon liquid fuel base and a minor proportion of a carbonate having the general structure


wherein R is methyl or ethyl and R′ is tertiary butyl or terti amyl.

BRIEF DESCRIPTION OF THE DRAWING

[0014] Figure 1 graphically illustrates the results obtained in Examples 4-8 wherein the octane blending values of several different symmetrical and unsymmetrical carbonates were measur using an unleaded gasoline base fuel. The horizontal axis of figure represents the molecular weight of the carbonate, while the vertical axis represents the blending octane value.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Base fuels to which the carbonate compounds of this invention may be added to improve the anti-knock properties include all of the volatile liquid fuels known to be suitable for spark ignition internal combustion engines. Preferably, the hydrocarbon liquid fuel base comprises gasoline, e.g., a hydrocarbon liquid having a boiling range from about 90°F to about 430°F. The liquid fuel base may comprise straight chain or branched chain paraffins, cyclic paraffins, olefins, and substituted or unsubstituted aromatic hydrocarbons or mixtures thereof. This fuel may be produced by any known method, including, for example, distillation or fractionation yielding straight-run product, thermal and catalytic cracking, alkylation, reforming, polymerization, isomerization, and dehydrocyclodimerization. Straight-run naphtha, alkylate gasoline, polymer gasoline, natural gasoline, isomerized or hydrotreated stocks, catalytically cracked or thermally cracked hydrocarbons, catalytically reformed stocks and synthetic liquid hydrocarbon fuels derived from carbonaceous materials such as coal or oil shale are suitable for use in this invention.

[0016] Also suitable for use are liquid hydrocarbon fuels heavier than gasoline such as residual fuels, kerosene, jet fuels, heating oils, diesel fuels, light gas oils, heavy gas oils, light cycle gas oils, heavy cycle gas oils, vacuum gas oils, petroleum middle distillate fuels, and diesel fuels.

[0017] The carbonates useful in the compositions of this invention have the general structure


wherein R is methyl or ethyl and R′ is tertiary butyl or tertiary amyl. Specific examples of suitable carbonates include methyl t-butyl carbonate, ethyl t-butyl carbonate, methyl t-amyl carbonate, and ethyl t-amyl carbonate. Mixtures of these carbonates may be used if desired. All of these compounds are characterized in having one C₁-C₂ primary alkyl group and one C₄-C₅ tertiary alkyl group attached to a carbonate

moiety and have boiling points at atmospheric pressure of from about 140°C to 190°C (284°F to 374°F). The carbonates may be prepared by any of the methods known in the art. For example, a tertiary alkoxide prepared by reacting an alkali metal such as potassium with a tertiary alcohol such as t-butyl alcohol or t-amyl alcohol may be reacted with carbon dioxide and subsequently an alkyl halide such as methyl bromide or ethyl bromide to yield the unsymmetrical carbonate. Alternatively, the carbonates may be produced by reacting a tertiary alkoxide with a haloester such as ethyl chloroformate or methyl chloroformate. Such methods are described in Carpino, J. Am. Chem. Soc. 82, 2725(1960).

[0018] Although any proportion of the carbonate less than 50 percent may be present in the liquid fuel compositions of this invention, the amount of carbonate is preferably from about 1 to 15 weight percent. Lower amounts will not have a significant effect on the octane number of the fuel composition. Higher amounts will likely be uneconomical. Owing to the high solubility of the carbonates in hydrocarbon fuels, the fuel compositions may be readily prepared by simply blending or mixing the carbonate into the hydrocarbon liquid fuel base.

[0019] The liquid fuel compositions of this invention may contain, in addition to the unsymmetrical dialkyl carbonate, any of the additives normally employed in fuels such as anti-icing agents, detergents, demulsifiers, corrosion inhibitors, dyes, deposit modifiers, anti-oxidants, metal deactivators, and upper cylinder lubricants, as well as other anti-knock additives such as organometallic compounds (e.g., tetraethyl lead, tetramethyl lead, cyclopentadienyl tricarbonyl manganese), alcohols (e.g., methanol, ethanol, t-butyl alcohol, isopropyl alcohol), ethers (e.g., methyl t-butyl ether, methyl t-amyl ether), and other types of carbonates (e.g., symmetrical dialkyl carbonates such as dimethyl carbonate, diethyl carbonate, di-n-butyl carbonate, di-t-butyl carbonate, and dipropyl carbonate, dicarbonates such as dimethyl dicarbonate and diethyl carbonate, alkyl phenyl carbonates such as t-butyl phenyl carbonate, and cyclic alkylene carbonates such as propylene carbonate, ethylene carbonate, and butylene carbonate, as well as other organic carbonates such as isopropyl cyclohexyl carbonate and diisopropyl carbonate). In one preferred embodiment, however, the liquid fuel composition contains no significant amount of any added octane-enhancer other than the unsymmetrical dialkyl carbonate. Unleaded gasoline (i.e., gasoline that is essentially free of organo-lead additives) is particularly preferred for use as the hydrocarbon liquid fuel base, although leaded gasoline may also be employed if desired.

[0020] In one embodiment of this invention, the liquid fuel composition comprises from about 80 to 98 weight percent gasoline and from about 2 to 20 weight percent of an additive mixture. The additive mixture is comprised of from about 1 to 19 weight percent (based on the total weight of the liquid fuel composition) of at least one unsymmetrical dialkyl carbonate of the type described hereinabove and from about 1 to 19 weight percent (based on the total weight of the composition) of at least one additional oxygenated compound. The unsymmetrical dialkyl carbonate is most preferably methyl t-butyl carbonate, although ethyl t-butyl carbonate, methyl t-amyl carbonate, ethyl t-amyl carbonate, or mixtures thereof can also be used. The oxygenated compound is selected from the group consisting of alcohols, ethers, symmetrical dialkyl carbonates, and cyclic alkylene carbonates, but most preferably is methyl t-butyl ether.

[0021] In another embodiment of this invention, the fuel composition contains from about 1 to 15 weight percent of a mixture of symmetrical and unsymmetrical carbonates corresponding to the following structural formulae:


wherein R is the same in carbonates A and B and is methyl or ethyl and R′ is the same in A and C and is tertiary butyl or tertiary amyl, For example, the carbonate mixture may comprise methyl t-butyl carbonate, dimethyl carbonate, and di-t-butyl carbonate. Preferably, the unsymmetrical carbonate A is the predominate component of the carbonate mixture (i,e., at least about 34 mole percent).

[0022] The incorporation of alkyl carbonate into a fuel composition may reduce the effectiveness of some conventional corrosion inhibitors that are used to control the tendency of fuel system components to rust or otherwise corrode when placed in contact with the fuel. However, the addition of one or more corrosion inhibitors of the type typically used in conventional oxygenate-containing fuels will provide adequate protection against rusting. An example of a suitable corrosion inhibitor is "DCI 11", available from E. I. du Pont de Nemours. Addition of this inhibitor at a 20 ppm level into a gasoline-based fuel containing indolene and methyl t-butyl carbonate yielded a composition having an NACE rust rating (ASTM D665) of A.

[0023] The following examples serve to further illustrate the particular advantages of this invention and instruct one skilled in the art in the best mode of practicing the invention and are not intended to be construed as limiting the invention thereto.

EXAMPLE 1

[0024] This example demonstrates the greater hydrolytic stability of the liquid fuel compositions of this invention as compared to the stabilities of compositions containing dimethyl carbonate.

[0025] Toluene (100 parts by volume) was employed as the hydrocarbon liquid fuel base in a composition containing 10 parts by volume methyl t-butyl carbonate. Two phases formed when 5 parts by volume water was added, indicating that the methyl t-butyl carbonate does not significantly increase the hydrophilicity of the fuel composition. The two phase mixture was allowed to stand in a stoppered glass volumetric flask for three weeks at room temperature with occasional agitation. No hydrolysis of the methyl t-butyl carbonate was detectable by gas chromatographic analysis of the organic and aqueous phases. The aqueous phase did not extract any of the methyl t-butyl carbonate from the organic layer.

COMPARATIVE EXAMPLE 2

[0026] A fuel composition prepared as described in Example 1 but using 10 parts by volume dimethyl carbonate instead of methyl t-butyl carbonate exhibited gradual hydrolysis of the dimethyl carbonate to methanol when exposed to 5 parts by volume water for three weeks at room temperature.

EXAMPLES 3-8

[0027] Fuel compositions were prepared by blending 10 volume percent of various organic carbonates into a sample of regular unleaded gasoline obtained from a commercial source. The research and motor octane values of each composition were determined using modifications of ASTM methods D2699 and D2700 developed by the Pittsburgh Applied Research Corporation and its predecessor Gulf Oil. These modified test methods have been in use for over 40 years in the industry, have been tested and verified using National Exchange Group samples, and have been proven to be as reliable as the standard ASTM procedures. A constant pressure micro method carburetor is installed on the fourth bowl of a four bowl carburetor CFR-octane test unit using a special holder attached to the sight glass. A glass micro-bowl holding the fuel sample to be tested is inserted into the holder. The standard CFR horizontal metering jet is replaced with a modified micro-bowl jet (different jets are required for the RON and MON tests).

[0028] The modified jet is equipped with a "T" to drain fuel and is connected to the micro-bowl with tubing for fuel flow to the CFR cylinder. Constant pressure is maintained by the use of a rubber stopper, drilled to hold a piece of copper tubing, and an air vent valve inserted into the micro-bowl. The procedure used to measure octane values is otherwise the same as that of the standard ASTM method

[0029] The results obtained using five different alkyl carbonates as fuel additives are shown in Table I and in Figure 1. As expected from the teachings of the prior art, the blending octane values for fuel compositions containing unbranched dialkyl carbonates were found to decrease as the molecular weight and the number of carbon atoms in the carbonate additive increased. That is, in the series dimethyl carbonate:diethyl carbonate:methyl n-butyl carbonate, the blending octane value of the fuel decreased from 101.7 to 96.2. Isobutylene carbonate, a cyclic alkylene carbonate, yielded a much lower octane number improvement than its acyclic unbranched symmetrical analogue, diethyl carbonate.

[0030] In contrast, methyl t-butyl carbonate was found to have an octane blending value considerably higher than methyl n-butyl carbonate (105.5 vs. 96.2). This result was unexpected since methyl t-butyl carbonate and methyl n-butyl carbonate are isomers and differ only with respect to the structure of the butyl group. Additionally, the blending octane value of methyl t-butyl carbonate was significantly greater than that of dimethyl carbonate. This finding was particularly surprising in view of the general expectation from the prior art that octane value decreases as the molecular weight and number of carbons per molecule increase.

[0031] Reid Vapor Pressure values for the fuel compositions were also determined using ASTM method D4953; the experimental values are given in Table 1. The results demonstrate that the incorporation of 10 volume percent of methyl t-butyl carbonate into a standard gasoline fuel has the desirable effect of decreasing the vapor pressure of the fuel composition.

Claims

1. A liquid fuel composition comprising a major proportion of a hydrocarbon liquid fuel base and a minor proportion of at least one carbonate having the general structure

wherein R is methyl or ethyl and R′ is tertiary butyl or tertiary amyl.

2. A composition as claimed in Claim 1, wherein the amount of the carbonate is from about 1 to 15 weight percent of the composition.

3. A liquid fuel composition comprising from about 85 to 99 weight percent of a hydrocarbon liquid fuel base and about 1 to 15 weight percent of a carbonate mixture comprising carbonates of structural formula

wherein R is the same in carbonate A and carbonate B and is methyl or ethyl and R′ is the same in carbonate A and carbonate C and is tertiary butyl or tertiary amyl.

4. A composition as claimed in Claim 3, wherein carbonate A is at least about 34 mole percent of the carbonate mixture.

5. A composition as claimed in any one of Claims 1 to 4, wherein the hydrocarbon liquid fuel base is leaded gasoline or unleaded gasoline.

6. An improved liquid fuel composition comprising from about 80 to 98 weight percent gasoline and from about 2 to 20 weight percent of an additive mixture comprising

a) from about 1 to 19 percent based on the total weight of the liquid fuel composition of at least one unsymmetrical dialkyl carbonate having the general structure

wherein R is methyl or ethyl and R′ is tertiary butyl or tertiary amyl; and

b) from about 1 to 19 percent based on the total weight of the liquid fuel composition of at least one oxygenated compound selected from the group consisting of alcohols, ethers, and carbonates other than those having the general structure described in (a).

7. A composition as claimed in Claim 6, wherein the oxygenated compound is an ether.

8. A composition as claimed in Claim 7, wherein the ether is methyl t-butyl ether.

9. A composition as claimed in any one of Claims 1 to 8, wherein R is methyl and R′ is tertiary butyl.

10. A liquid fuel composition as claimed in Claim 1 comprising from about 85 to 99 weight percent unleaded gasoline and from about 1 to 15 weight percent methyl t-butyl carbonate.

11. A method for improving the antiknock properties of a leaded or unleaded gasoline composition, said method comprising incorporating therein a carbonate having the general structure

herein R is methyl or ethyl and R′ is tertiary butyl or tertiary amyl.

12. A method as claimed in Claim 11, wherein R is methyl and R′ is tertiary butyl.

13. The method as claimed in Claim 11 or Claim 12, wherein the amount of the carbonate is from 1 to 15 parts by weight per 100 parts by weight of the total gasoline composition.

14. The use as an antiknock agent in a hydrocarbon liquid-based liquid fuel composition of a carbonate having the general structure

wherein R is methyl or ethyl and R′ is t-butyl or t-amyl.

Drawing