GB2123411A - Preparation of a mixture of ethers useful as a gasoline additive - Google Patents

Abstract

A mixture of ethers, which is useful as a gasoline additive, is prepared by synthesising an alcohol mixture containing lower alcohols, i.e. methanol and ethanol, and higher alcohols, dehydrating the higher alcohols to olefins and etherifying the olefins with the lower alcohols or with dialkyl ethers obtained by dehydration of the lower alcohols.

Description

SPECIFICATION Gasoline additives This invention relates to gasoline additives, and in particular to additives which improve the antiknock rating of a hydrocarbon based gasoline composition.

Whereas it has been common for many years to improve the anti-knock rating of gasoline by adding lead alkyls and organic halogen compounds, this is now recognised to result in exhaust gases that endanger health. In the absence of such additives it is possible to improve the antiknock rating by increasing, relative to straight chain hydrocarbons, the proportion of branched, cyclic, and aromatic hydrocarbons, but this results in a decrease in the rate of extraction of useful fuel from petroleum. More recently it has been found that methyl t-butyl ether (MTBE) is a very effective anti-knock additive. Following the development of highly efficient methanol production processes it has become economic to manufacture MTBE on a large scale from methanol and isobutene, which until recently was a by-product from petroleum refining with few uses.There is now, however, a prospect that further manufacture of MTBE may be limited by a shortage of isobutene which is normally obtained from a C4 hydrocarbon refinery stream.

It has been proposed in the Preprints of the Division of Petroleum Chemistry Inc. of the American Chemical Society Vol. 20 No. 1 February 1975 at pages 261 and 265 to make the isobutene, for MTBE manufacture, by synthesising a mixture of methanol and higher alcohols from a synthesis gas, separating isobutanol from the other higher alcohols, and dehydrating the isobutanol.

We have found that the isobutanol is advantageously not separated from the other higher alcohols but rather the whole higher alcohol mixture is subjected to dehydration and the resulting mixture of olefins is used to form ethers. This gives rise to a mixture of MTBE and other ethers. These mixtures are particularly useful gasoline additives: in some cases the Research Octane Number (RON) of the gasoline containing the additive ether mixture is greater than that which would be expected from the use of the individual ethers. Also, by using a mixture of ethers, the boiling range of the octane enhancing components of the gasoline is advantageously broadened.

It has been proposed in published British Patent Application 2031886 to employ mixtures of MTBE and t-amyl methyl ether as gasoline additives. However such mixtures were obtained from a C4-C5 hydrocarbon refinery fraction and were essentially only binary mixtures.

Accordingly we provide a process for the manufacture of a mixture of ethers useful as gasoline additives comprising (i) synthesising, from synthesis gas comprising hydrogen, carbon monoxide, and, optionally, carbon dioxide, a mixture of alcohols consisting of methanol, ethanol, and higher alcohols, (ii) dehydrating said higher alcohols to give a mixture of olefins, and (iii) etherifying said olefin mixture by reaction with (a) methanol, and optionally ethanol, including at least part of said methanol, and optionally said ethanol, in said alcohol mixture and/or with (b) di-alkyl ethers resulting from dehydration of methanol, and optionally ethanol, including at least part of said methanol, and optionally said ethanol, in said alcohol mixture.

Thus ethyl, as well as methyl, ethers can be produced by using some or all of the ethanol or the di-alkyl ether dehydration product thereof, together with methanol or the di-alkyl ether dehydration product thereof, for the reaction with the olefin mixture.

Various processes have been described for the synthesis of a mixture of alcohol mixtures from a H2/CO synthesis gas, using a variety of catalysts, for example in GB-A-1547687, GB-A2037179, ZA-A-81/0981 and DE-A-3113838.

The composition of the alcohol mixture (which will mainly comprise alcohols containing up to 5 carbon atoms) and the structure, i.e. branched or straight chain, of the alcohols higher than ethanol produced by these processes will depend on the precise catalyst and synthesis conditions, including the H2/CO ratio, employed.

Thus a typical alcohol mixture produced by the procedures of GB-A-2037179 has the composition % by weight methanol 20-25 ethanol 30-45 n-propanol 10-25 n-butanol 9-1 7 branched alcohols < 1 0 whereas the alcohol synthesis step described as an intermediate step in DE-A-3113838 typically gives a larger proportion of methanol and little ethanol, but the proportion of methanol may be reduced by recycling some of the methanol to the synthesis step with the synthesis gas. The higher alcohols produced by the alcohol synthesis step of DE-A-3113838 are mainly branched chain.

In some cases, for efficient operation it may be desirable to recycle a large proportion of the methanol, and optionally ethanol, in the alcohol mixture to the alcohol synthesis step so that the amount of unrecycled methanol, and optionally ethanol, is insufficient for the etherification of the olefin mixture. In this case the methanol, and optionally ethanol, required for etherification, may be supplemented from a separate source, e.g. an alcohol synthesis step that produces methanol, and optionally ethanol, more selectively.

However, it is preferred that the alcohol synthesis conditions, including the amount of recycle, are adjusted such that no supplementary supply of methanol or ethanol is required for the etherification step.

Following synthesis of the alcohol mixture at least the higher alcohols i.e. those containing 3 or more carbon atoms are subjected to a dehydration step, by passing over an alchol dehydration catalyst. This is very suitably alumina, preferably in a form having a surface area when anhydrous of at least 50 m2/g. Those gamma alumina and activated aluminas having surface areas in the range 100-500 m2/g can be used. Other dehydrating agents include amorphous aluminosilicates, crystalline aluminosilicates such as zeolites of large (over 7 Angstrom units), medium (5-7) or small (under 5) pore diameter and solid or solid-supported acids such as isopolyacid, heteropolyacids and phosphoric acids on silica.If a crystalline zeolite of medium or large pore diameter is used it should preferably not be in an acidic form such as the hydrogen or rare earth form, since this would result in deposition of higher hydrocarbon derivatives including possibly solid polymers and carbon at the surface of the dehydration catalyst and synthesis catalyst, which would not withstand the treatments necessary to remove such deposits.

Depending on the nature of the process and conditions employed for the dehydration and subsequent etherification process, the methanol (and, in some cases, the ethanol) may be separated from the higher alcohols prior to dehydration. Alternatively, where the dehydration is selective, e.g. by the use of selective dehydration catalysts, and/or the etherification is conducted using dialkyl ethers produced by dehydration of methanol, or methanol and ethanol, the methanol, or methanol and ethanol, as the case may be, need not be separated prior to dehydration.

The etherification is preferably effected by reacting the mixture of olefins produced by dehydration of the mixture of higher alcohols with methanol, or methanol and ethanol, in the presence of a suitable catalyst. Examples of such catalysts include mineral acids e.g. sulphuric acid; heteropolytungstic or molybdic acids doped with phosphorus or boron; acidified alumina; acidified ion-exchange resins; and certain zeolites such as high silica zeolites having an XO2/Y203 ratio equal to or greater than 10, wherein X is silicon and/or germanium and Y is one or more of aluminium, iron, chromium, vanadium, molybdenum, arsenic, manganese, gallium or boron.

Preferred zeolites have acid sites within zeolite pore systems active in the catalysis of tertiary alkyl ether formation. The entry port size in these preferred zeolites is such that reactants and products could move into and out of the pore systems i.e. with a Lennard Jones diameter a (A) of from about 5.0 to 10.0 (the Lennard Jones diameter a (A) is defined by D W Breck in "Zeolite Molecular Sieves", Wiley Interscience, 1947, page 636).

The preferred zeolites for use as catalyst are based on XO2 as silica (six2) and Y203 as alumina (Al2O3).

By the present invention the ether mixture is readily produced without the need for isolating the individual olefins.

In the accompanying drawings, various alternative flowsheets are shown for the production of the ether mixtures.

In Fig. 1 the reaction products from the alcohol synthesis are passed to a separator where the unreacted synthesis gas is separated from the alcohol mixture and is recycled. The alcohol mixture is distilled to produce a lower alcohols stream (methanol, or methanol and ethanol) and a higher alcohols stream. Part of the lower alcohols stream is recycled to the alcohol synthesis reactor while the higher alcohols stream is dehydrated, subjected to a water separation step and then the resultant olefin mixture, together with the remainder of the lower alcohols stream is fed to the etherification stage.

In the scheme shown in Fig. 2, after separation of the unreacted synthesis gas, the alcohol mixture is selectively dehydrated to give a mixture of methanol and ethanol, water and olefins derived from higher alcohols. The olefins are then separated and fed to the etherification stage.

Water is distilled from the methanol/ethanol stream. Part of the latter may be recycled to the alcohol synthesis while the remainder is fed to the etherification stage.

The scheme of Fig. 3 is similar to that of Fig. 2 except that the products from the alcohol synthesis stage are fed directly to the selective dehydration stage and the unreacted synthesis gas is separated, and recycled to the alcohols synthesis, after the etherification stage.

In the scheme of Fig. 4 the products of the alcohols synthesis stage are fed to a non-selective dehydration stage, so that in addition to the higher alcohols being dehydrated to olefins, the methanol and ethanol are dehydrated to the corresponding di(lower alkyl) ethers. Water is then separated and the etherification reaction using the di(lower alkyl) ethers is then performed. The unreacted synthesis gas, together with any excess of di(lower alkyl) ether, is then separated from the etherification reaction products and recycled. In this etherification reaction the amount of water present has to be carefully controlled: too much water produces alcohols and little ether while too little water gives rise to high proportions of di(lower alkyl) ethers and olefins in the recycled gas.

The ether mixture produced by the aforementioned processes predominantly comprises a mixture of ethers of the formula R1-O-R2 where R1 is methyl or ethyl, at least some of the groups R1 being methyl, and R2 corresponds to the alkyl groups derived from the olefins. R2 will generally comprise t-butyl in admixture with at least one of isopropyl, 2-methylbut-2-yl, and 3methylpent-3-yl. MTBE will generally comprise between 40 and 80% by weight of the ether mixture.

Gasoline compositions may be made by adding the ether mixture to a suitable hydrocarbon or hydrocarbon/alcohol, e.g. methanol and/or ethanol, mixture. The amount of ether mixture is preferably 1-1 5% by volume based on the total volume of the gasoline composition.

To illustrate the synergistic effect on the RON given by the use of mixtures of ethers of the type present in the mixtures in accordance with the invention, the RON was measured for a number of mixtures of 10 parts by volume of a specified ether and 90 parts by volume of a hydrocarbon base fuel of RON 80. From the experimental RON data, the blending RON for each ether was calculated. The results are shown in Table 1.

Table 1 RON found by Ether experiment Blending RON MTBE 85.4 134 diisopropyl 83.1 111 t-butyl ethyl 85.7 137 iso-amyl ether (3-methyl-butyl ethyl) 71.3 - 7 sec-amyl ethyl (pent-2-yl ethyl) 74.1 21 sec-butyl ethyl (1-methyl-propyl ethyl) 72.1 1 Blends were then made from 5 parts by volume of the specified ether, 5 parts by volume of MTBE, and 90 parts by volume of the RON 80 base fuel and the RON determined for each blend. The RON for these blends was calculated from the blending RON data of Table 1 and compared with the RON determined experimentally. The results are shown in Table 2.

Table 2 calculated RON Ether RON found by experiment from Table 1 data diisopropyl 84.4 84.25 t-butyl ethyl 86.1 85.55 3-methyl-butyl ethyl 80 78.35 pent-2-yl ethyl 80.7 79.75 1-methyl-propyl ethyl 80.6 79.75 In all these cases it is seen that the experimental RON is greater than that expected by calculation from the blending RON.

Claims (8)

1. A process for the manufacture of a mixture of ethers useful as gasoline additives comprising (i) synthesising, from synthesis gas comprising hydrogen, carbon monoxide and, optionally, carbon dioxide, a mixture of alcohols consisting of methanol, ethanol, and higher alcohols.

(ii) dehydrating said higher alcohols to give a mixture of olefins, and (iii) etherifying said olefin mixture by reaction with (a) methanol, and optionally ethanol, including at least part of said methanol, and optionally said ethanol, in said alcohol mixture, and/or with (b) di-alkyl ethers resulting from dehydration of methanol, and optionally ethanol, including at least part of said methanol, and optionally said ethanol, in said alcohol mixture.

2. A process according to claim 1 wherein part of the methanol and the ethanol in the alcohol mixture is recycled to the alcohol synthesis step.

3. A process according to claim 1 or claim 2 wherein the methanol, and optionally the ethanol, is separated from the alcohol mixture prior to dehydration of the higher alcohols.

4. A process according to claim 1 or claim 2 wherein the methanol and ethanol are dehydrated to the corresponding di-alkyl ethers at the same time as the higher alcohols are dehydrated to the olefins, and said olefins are etherified by reaction with said di-alkyl ethers.

5. A mixture of ethers produced by the process of any one of claims 1 to 4.

6. A mixture of ethers according to claim 5 comprising ethers having the formula R,-O-R2 wherein R1 is methyl or ethyl with at least some of the groups R1 being methyl and R2 is t-butyl in admixture with at least one of isopropyl, 2-methylbut-2-yl, and 3-methylpent-3-yl.

7. A mixture of ethers according to claim 5 or claim 6 containing 40 to 80% by weight of methyl t-butyl ether.

8. A gasoline composition comprising a hydrocarbon mixture containing 1 to 15% by volume, based on the total volume of the composition, of a mixture of ethers according to any one of claim 5 to 7.