GB2150560A - Production of alcohols by hydrogenolysis of esters - Google Patents

Abstract

A process for effecting hydrogenolysis of a carboxylic acid ester comprises contacting a vaporous mixture containing the ester, hydrogen and a minor amount (e.g. about 0.2 moles up to about 1 mole per 100 moles of hydrogen) of carbon dioxide, with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 150 DEG C up to about 240 DEG C and at a pressure in the range of from about 5 bar up to about 50 bar, preferably about 15 bar to about 40 bar. The ester is preferably an alkyl acetate, or a dialkyl oxalate or maleate.

Description

SPECIFICATION Process This invention relates to the hydrogenolysis of carboxylic acid esters, more particularly to the hydrogenolysis of esters of mono- and dicarboxylic acids.

Hydrogenolysis of carboxylic acid esters has been described on numerous occasions in the literature.

Typically in such a reaction the -CO-O - linkage of the ester group is cleaved so that the acid moiety of the ester group is reduced to an alcohol whilst the alcohol moiety is released as free alcohol according to the following equation: R1COOR2 + 2H2 = R,CH20H + HOR2 (I) where R1 and R2 are each alkyl radicals, for example. According to page 129 etseq of the book "Catalytic Hydrogenation in Organic Synthesis" by M. Freifelden, published by John Wiley and Sons Inc (1978), the catalyst of choice for this reaction is said to be barium promoted copper chromite. Typical reaction conditions include use of temperatures in the region of 250"C and pressures in the range of 225-250 atmospheres.Although a good yield of alcohol is often obtained using this technique for hydrogenolysis of an ester, the temperature necessary for conversion of the ester to alcohol is also conducive to side reactions.

For example, the resulting alcohol may undergo further hydrogenolysis to hydrocarbon or may react with starting material to produce a higher molecular weight ester that is more difficult to hydrogenolyse.

Besides these side reactions copper chromite catalysts are reported to have other disadvantages for commercial scale operation. In particular, the use of copper chromite catalysts has been said to be environmentally hazardous and to necessitate the adoption of special and costly handling techniques on account of the toxicity of chromium. Moreover it is apparently difficult to produce successive batches of copper chromite with reproducible catalyst activity.

United States Patent Specification No. 2079414 describes a process for catalytic hydrogenation of esters using catalysts such as fused copper oxide, either wholly or partially reduced, which may be promoted with oxide promoters such as manganese oxide. Particularly recommended catalysts are those comprising copper oxide promoted by chromium oxide, e.g. copper chromite. According to page 3, right-hand column line 57 etseq.: "In operating in the vapour phase it is preferred to use temperatures within the range of 300"C to 400"C". It is also stated that: "The best conversions to alcohols are obtained at the highest pressure obtainable in the available equipment and at the lowest temperatures consistent with obtaining a practical rate of reaction" (page 4, right-hand column, line 2 etseq.).The Examples describe batch reactions and in all cases the pressure is 2500 psia or higher (172.5 bar or higher), whilst in all cases the temperature is 250"C or higher; in most cases it exceeds 300"C. A limitation to the process is that methyl esters cannot be used because methanol, which would be a hydrogenation product from a methyl ester, is subject to gaseous decomposition (see page 5, right-hand column, line 58 etseq). Similar considerations would appear to prevent the application of the process to esters of formic acid since the formic acid moiety would also be likely to yield methanol.

Further teaching of the use of chromites as catalysts for hydrogenation of esters will be found in United States Patent Specification No.2109844.

Example 4 of United States Patent Specification No.3197418 discloses the preparation of a copper-zinc catalyst which can be used in the liquid phase hydrogenation of oils and fats at pressures in excess of about 120 bar and at a temperature of 320"C.

United States Patent Specification No. 2241417 teaches the production of higher aliphatic alcohols by liquid phase hydrogenation of glycerides in the presence of copper-containing catalysts at temperatures of 200"C to 400"C and at pressures of about 60 to about 500 bar.

Hydrogenolysis of esters to saturated hydrocarbons using catalysts having as essential ingredients as indium or rhodium component and a halogen component is described in United States Patent Specification No.4067900.

Catalytic hydrogenolysis of formate esters present in oxo reaction products using Ni catalysts is described in East German Patent Specification No.92440 (see Chem. Abs., 124069j, Vol 78 (1973), page 439). Other references to hydrogenation offormatesincludea paper by E. Lederle, Anales Real Soc. Espan. Fis y Quim.

(Madrid) 57B, pages 473-5 (1961). Also West German Patent Specification No. 902375 describes the production of methanol by hydrogenation of alkyl formates at pressures of about 20 to 50 bar using copper chromite catalysts; there is a passing suggestion to incorporate zinc oxide in the catalyst.

Catalytic cleavage of formic acid esters is described in British Patent Specification No. 1277077. According to this proposal a hydrogenation catalyst containing copper and nickel is used but the formyl radical is reported to be dehydrogenated in the course of the reaction and appears as carbon monoxide.

Production of ethylene glycol by hydrogenolysis is suggested by some references including United States Patent Specification No. 4113662 which teaches hydrogenation of esters to alcohols at temperatures of 1 50"C to 4500C and pressures of 500-10,000 psig (34.5-690 bar) using catalysts comprising cobalt, zinc and copper.

Examples IV, V and VIII describe comparative experiments using polyglycolide and methyl glycolate with Cu-Zn oxides as catalyst at 250"C and at pressures of at least 2800 psig (194 bar), i.e. conditions under which the ester (polyglycolide or methyl glycolate) is in the liquid phase. United States Patent Specification No.

2305104 describes hydrogenation of alkyl glycolates using catalysts containing Zn, Cr, and Cu to produce ethylene glycol. Vapour phase hydrogenation of oxalate esters at temperatures of 150 C to 3005C for the production of ethylene glycol has been described in United States Patent Specification No. 4112245; this process uses a copper chromite or copper zinc chromite catalyst and requires that the oxalate ester has a sulphur content of less than 0.4 ppm.

International Patent Publication No. WO 82!03854 describes a process for effecting hydrogenolysis of a carboxylic acid ester which comprises contacting a vaporous mixture containing the ester and hydrogen with a catalyst comprising a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 75"C up to about 300"C and at a pressure in the range of from about 0.1 kg/cm2 absolute (about 9.8 kPa) up to about 100 kgicm2 absolute (about 9813 kPa).

In International Patent Publication No. WO 83/03409 there is described a process for the production of ethanol which includes a hydrogenolysis step in which an acetate ester, such as methyl acetate or ethyl acetate, is subjected to hydrogenolysis using the same catalyst as described in International Patent Publication No. WO 82/03854 and using the same general temperature and pressure conditions. In this hydrogenolysis step there may be used hydrogen containing significant amounts of carbon monoxide, for example hydrogen containing about 1% by volume of carbon monoxide up to a 1:2 molar ratio CO:H2 mixture. As described therein, however, the gas used as make up gas for the hydrogenolysis step (see page 16, line 15) is subjected to CO2 removal.Hence the teaching is that the gas used in the hydrogenolysis step of International Patent Publication No. WO 83103409 has had CO2 removed therefrom.

It is an object of the present invention to provide an improved process for effecting hydrogenolysis of esters, more particularly esters of aliphatic mono- and di-carboxylic acids.

It is a further object of the invention to provide an improved process for the production of ethanol which involves hydrogenolysis of an acetate ester.

Yet again the invention seeks to provide an improved method of hydrogenolysing oxalate esters to yield ethylene glycol.

It also seeks to provide an improved method of hydrogenolysing maleate esters.

According to the present invention there is provided a process for effecting hydrogenolysis of an ester of an aliphatic carboxylic acid which comprises contacting a vaporous mixture comprising the ester, a minor amount of carbon dioxide and hydrogen, with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 1500C up to about 240"C and at a pressure in the range of from about 5 bar up to about 50 bar.

We have found surprisingly that the addition of even a small quantity of carbon dioxide to the vaporous mixture has a profound effect upon the activity of the Cu/Zn hydrogenolysis catalyst. Thus, for example, addition of an amount as low as about 0.2 parts by volume of carbon dioxide per 100 parts by volume of hydrogen to the vaporous mixture has been shown to increase significantly the catalyst activity. For example, we have found that under otherwise identical hydrogenolysis conditions the conversion of methyl acetate to a mixture of methanol and ethanol can be increased from about 60% to about 80%, an increase of approximately one third, by addition of about 0.2 moles of CO2 per 100 moles of H2 used for the hydrogenolysis.This means that, either a lower catalyst volume can be employed for a given throughput of ester, or that the pla nt operator can operate at a lower temperature (perhaps about 30"C lower) to ach ieve the same throughput that he would obtain from a given catalyst volume when using pure hydrogen. In the latter case this means cheaper production costs.

This activating effect of carbon dioxide is surprising as carbon dioxide is reported to be "inert" with respect to reduced copper oxide'zinc oxide catalysts in European Patent Publication No. 0008767A (see page 9, lines 4 and 5).

The ester may be essentially any vaporisable ester of a mono- or di-carboxylic acid, especially esters of aliphatic mono- and di-carboxylic acids. Preferably the ester is an alkyl ester.

Such esters may be derived, for example, from the following acids: formic acid; acetic acid; propionic acid; n- and iso-butyric acids; n- and iso-valeric acids; caproic acid; caprylic acid; capric acid; 2-ethylhexanoic acid; lauric acid; myristic acid; palmitic acid; oleic acid; linoleic acid; stearic acid; glycolic acid; pyrutic acid; lactic acid; citric acid; oxalic acid; malonic acid; succinic acid; glutaric acid; adipic acid; maleic acid; fumaric acid; acrylic acid; methacrylic acid; alpha- or beta-crotonic acid; acetylene-dicarboxylic acid; methoxyacetic acid; and the like. Preferably the acid contains from 1 to about 12 carbon atoms.

As mentioned above, the ester is preferably an alkyl ester or diester. The alkyl residue or residues may thus be, for example, methyl, ethyl, nor iso-propyl, n-butyl, n-hexyl, 2-ethylhexyl, cetyl, lauryl, or the like.

Preferably the or each alkyl residue contains not more than about 12 carbon atoms.

As examples of specific esters there may be mentioned: alkyl formates (e.g. methyl, ethyl, n- and iso-propyl, n-, iso-, sec- and t-butyl formates); alkyl acetates (e.g. methyl, ethyl, n- and iso-propyl, and n-, iso-, sec- and t-butyl acetates); alkyl propionates (e.g. n-propyl propionate); alkyl n-butyrates (e.g. n-butyl n-butyrate); alkyl iso-butyrates (e.g. iso-butyl iso-butyrate); alkyl n-valerates (e.g. n-amyl valerate); alkyl iso-valerates (e.g. methyl iso-valerate); alkyl caproates (e.g. ethyl caproate); alkyl caprylates (e.g. methyl caprylate); alkyl caprates (e.g. ethyl caprate); alkyl 2-ethylhexanoates (e.g. 2-ethyl hexyl 2-ethylhexanoate); alkyl alkoxyacetates (e.g. methyl methoxyacetate); alkyl glycolates (e.g. methyl and ethyl glycolates);; alkyl lactates (e.g. ethyl lactate); alkyl pyruvates (e.g ethyl pyruvate); alkyl laurates (e.g. methyl laurate, ethyl laurate); alkyl myristates (e.g. methyl myristate, ethyl myristate); alkyl palmitates (e.g. methyl palmitate, ethyl palmitate); alkyl oleates (e.g. methyl oleate, ethyl oleate); alkyl lineoleates (e.g. methyl linoleate, ethyl linoleate); alkyl stearates (e.g. methyl stearate, ethyl stearate); dialkyl oxalates (e.g. dimethyl, diethyl and di-n-butyl oxalates); dialkyl succinates (e.g. dimethyl and diethyl succinates); dialkyl adipates (e.g. dimethyl and diethyl adipates); cyclic aliphatic esters (e.g. glycolide and ethylene oxalate); dialkyl maleates (e.g. dimethyl maleate, diethyl maleate); dialkyl fumarates (e.g. dimethyl fumarate, diethyl fumarate); and the like.

In the process of the invention the vaporous mixture to be contacted with the catalyst contains, in addition to the ester, hydrogen and a minor amount of carbon dioxide, possibly in admixture with one or more other gases (desirably gases inert to the ester and the catalyst). Such other gases may include carbon monoxide and/or one or more inert gases such as nitrogen, methane, argon, and/or helium. Although the gas present in the vaporous mixture preferably comprises, in addition to a minor amount of carbon dioxide, a major amount of hydrogen and not more than a minor amount of carbon monoxide, it is also within the scope of the invention to carry out the process using a gas mixture that contains a major amount of inert gas and a minor amount only of hydrogen in addition to the minor amount of carbon dioxide.Preferably the hydrogen-containing gas contains at least about 50% by volume of hydrogen and not more than about 25% by volume each of carbon monoxide and carbon dioxide.

The vaporous mixture may contain, for example, from about 0.02 moles up to about 5 moles or more of carbon dioxide per 100 moles of hydrogen. Preferably, however, the vaporous mixture contains from about 0.2 moles up to about 1 mole of carbon dioxide per 100 moles of hydrogen.

If the vaporous mixture contains carbon monoxide then there should be present at least about 1 mole of carbon dioxide for every 100 moles of carbon monoxide. Hence the CO2:CO molar ratio is desirably at least about 1:100. For example, if a hydrogen-containing gas contains about 20% by volume of CO, then it should also contain at least about 0.2% by volume of CO2. If steps are taken deliberately to recover CO from the H2-stream, then the CO2:CO molar ratio becomes, theoretically at least, infinite. Normally when CO is present the CO2:CO molar ratio will be in the range of from about 1:100 to about 10000:1 or more, e.g. about 1:50 to about 1000:1 Vaporization of the ester can be effected in any convenient manner.For example, liquid ester can be sprayed into an ascending stream of hot hydrogen or hydrogen-containing gas to form a vapour stream that is partially saturated with ester vapour which can then be contacted with the hydrogenolysis catalyst, possibly after dilution with further hydrogen or hydrogen-containing gas to produce the desired H2:ester molar ratio. In the case of an ester that is solid at normal temperatures, such as methyl stearate, it may be convenient to dissolve it in a suitable inert solvent prior to vaporization in order to prevent danger of blockage of supply lines. Suitable solvents include alcohols such as methanol or ethanol and hydrocarbons such astoluene.

The hydrogenolysis process of the present invention is conducted at a temperature of between about 150"C and about 240"C. The preferred temperature range is from about 160"C to about 220 C. The total pressure is between about 5 bar and about 50 bar, and preferably between about 15 bar and about 40 bar.

The catalyst is a mixed metal oxide catalyst that consists essentially of a reduced mixture of copper oxide and zinc oxide. By the term "consists essentially of" we mean that the catalyst includes as essential ingredients in the mixture before reduction copper oxide and zinc oxide, but may also include amounts of other metal oxides that do not materially alter the basic characteristics of the catalyst as well as inert fillers or supports, such as carbon.Such characteristics include the ability to catalyse hydrogenolysis of alkyl esters of aliphatic monocarboxylic acids, in the presence of an at least stoichiometric amount of hydrogen at moderate temperatures, for example in the range of from about 1 600C to about 220 C, and low pressures, for example in the range of from about 5 kg/cm2 absolute to about 50 kg/cm2 absolute, with high selectivity to the desired alcohol or alcohols, typically in excess of 90%, and with a marked absence of significant amounts of by-products, typically less than 1% in total of by-products, such as alkanes, alkenes, ethers, acids, aldehydes, and "heavies", and traces amounts, if any, of CO. Such "heavies" may include, for example, higher alcohols having twice or three times as many carbon atoms as the desired alcohol.

The catalyst may be derived from a mixture which contains only copper oxide and zinc oxide. Alternatively it may include one or more other materials, such as an inert support or other material that is effectively catalytically inactive in the ester hydrogenolysis reaction.

Hence the catalyst may be derived from a mixture of CuO and ZnO, which before reduction contains from about 5 to about 95% by weight preferably from about 20 to 85% by weight, and typically from about 25 to about 70% by weight, of CuO and from about 95 to about 5% by weight, preferably from about 80 to about 15% by weight and typically from about 75 to about 30% by weight, of ZnO. Hence the mixture may contain, for example, from about 20 to about 40% by weight of CuO and from about 60 to about 80% by weight of ZnO. A preferred mixture, for example contains from about 30to about 36% by weight of CuO and from about 62 to about 68% by weight of ZnO.Other particularly preferred mixtures contain from about 65 to about 85% by weight of CuO and from about 35 to about 15% by weight of ZnO, for example mixtures containing from about 60 to about 75% (e.g. about 68 to about 75%) by weight of CuO and from about 40 to about 25% (e.g. about 32 to about 25%) by weight of ZnO.

As already mentioned the catalyst may contain minor amounts of at least one carrier material such as carbon, titanium oxide, zirconium oxide, manganese dioxide, silica, diatomaceous earth, kieselguhr, or aluminium oxide. Such other materials do not usually comprise in total more than about 20% by weight of the catalyst, calculated (except in the case of carbon) as oxide. The presence of minor amounts of other materials can be tolerated in the mixture from which the catalyst is derived, provided that these do not significantly affect the catalyst performance. For example, it is known that basic oxides such as magnesium oxide or barium oxide tend to promote alcohol condensation and dehydration reactions.Hence incorporation of excessive quantities of alkaline oxides in the mixture to be reduced will tend to promote "heavies" formation and to produce one or more alkenes as by-products. For example, in the hydrogenolysis of ethyl acetate, inclusion of increasing amounts of basic oxides in the mixture from which the catalyst is derived will tend to produce increasing amounts of n-butanol and ethylene, particularly at the higher end of the temperature range. For this reason, in the case of sodium it is best not to exceed about 0.5% by weight, calculated as oxide. For other less basic oxides, such as magnesium oxide or barium oxide, higher amounts may be tolerated in the oxide mixture, for example up to about 5% by weight or more.

Preferably, however, the total content of basic oxides is no more than about 1% by weight of the catalyst.

Other preferred catalysts include mixtures containing from about 40 to about 50 weight percent each of CuO and ZnO and from 0 to about 20 weight percent, typically from about 5 to about 20 weight percent, of alumina. Hence preferred catalysts of this type comprise from about 40 to about 47.5 weight percent each of ZnO and CuO and from about 5 to about 20 weight percent of alumina.

The catalyst is, however, preferably essentially free from other metals, particularly from metals of Group VIII of the Periodic Table, such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt, as well as from Group VIB metals, such as Cr, Mo, and W, from the metals Tc, Ag, Re, Au and Cd, and also from elements of atomic number 80 and above, e.g. Hg and Pb. By the term "essentially free' we mean that the catalyst contains not more than about 0.1 wt% (i.e. not more than about 1000 ppm), and preferably not more than about 250 ppm, of the element in question. Such metals include metals which, under the reaction conditions used in the process of the invention can cause side reactions, such as decomposition of product alcohols, andorformation of methane, or which lack catalytic activity or are catalyst poisons.

The catalyst may be prepared by any of the methods known in the art of forming a composite of copper oxide and zinc oxide. The catalyst may be prepared by fixing the separate oxides, by coprecipitation of the oxalates, nitrates, carbonates, or acetates, followed by calcination. The coprecipitation method is preferred.

Generally, the mixture of CuO and ZnO is reduced by hydrogen or carbon monoxide at a temperature in the range of between about 160"C and about 250do for several hours, peferably for 8 to 24 hours, prior to contact with the vaporous mixture containing by-product ester and hydrogen. If the catalyst is charged in a pre-reduced form the period required for reduction can be reduced accordingly.

The mixture of CuO and ZnO is reduced prior to its use as catalyst in the hydrogenolysis step. Hydrogen or CO, or mixtures thereof, are generally mixed with a diluent gas such as steam, nitrogen, or combustion gas, to maintain the catalyst bed temperature and to carry away the heat of reduction.

Reduction of the mixture of CuO and ZnO is complete when no more hydrogen or carbon monoxide is being reacted as shown by analysis of the inlet and outlet gas. When using hydrogen complete reduction of the mixture occurs when the total amount of water produced in the reduction is equal to the stoichiometric value of water which should be produced when a given amount of copper oxide is reduced to copper. This value is about 0.079 kg of water per kg of catalyst for a mixture containing 35 weight percent of CuO.

An inert carrier material such as alumina may be included in the hydrogenolysis catalyst composition. The catalyst is generally formed into pellets, tablets, or any other suitable shape prior to use, by conventional techniques.

It is advantageous that the mixture of CuO and ZnO have an internal surface area of from about 25 to about 50 sq.m. per gram. The internal surface area may be determined by the well-known BET method.

The process of the present invention is most conveniently carried out in a continuous manner, although semi-continuous or batch operations may also be employed. In the preferred method of continuous operation, an ester, or a mixture of esters, a gas mixture containing hydrogen and possibly also a minor amount of carbon monoxide, a minor amount of carbon dioxide, and optionally, a carrier gas such as nitrogen or methane, may be brought together and, under the desired pressure contacted in the vaporous state with the catalyst. The reaction zone can be operated under adiabatic reaction conditions or isothermal or substantially isothermal conditions, e.g. in a cooled tubular reactor.

In the hydrogenolysis process of the invention a monocarboxylic ester yields an alcohol product which usually comprises a mixture of alcohols, one derived from the carboxylic acid moiety and one derived from the alcohol moiety. However, in certain cases a single alcohol is the primary product; for example, ethanol is the primary product for hydrogenolysis of ethyl acetate, the same alcohol being derived both from the carboxylic acid moiety and from the alkyl moiety. Esters of dicarboxylic acids give a mixture of an alcohol and a corresponding diol. Thus, for example, dialkyl oxalates yield ethylene glycol and the corresponding alkyl alcohol.

In some cases an alcohol produced as primary product may undergo further reaction. For example, hydrogenolysis of diethyl maleate or of diethyl succinate may yield not the 1 ,4-butanediol perhaps per ps to be expected as primary product but tetrahydrofuran.

The alcohol product or products from the hydrogenolysis reaction may be separated from the hydrogen and other gas or gases by condensation and the excess gas can be compressed and recycled to the reaction zone. The crude alcohol product may be used in this form or it can be further purified in a conventional manner such as by fractional distillation. In one or more stages under normal, reduced or elevated pressure.

In some instances two or more of the products produced may form an azeotrope; appropriate care must be taken in designing a suitable product separation section of the plant. If desired, any unconverted portion of the ester or ester mixture may be separated from the reaction product and recycled to the reaction zone and, preferably, admixed with frexh feed gases prior to entering the reaction zone.

In operating the process of the invention the partial pressure of the ester may vary within wide limits, e.g.

from about 0.05 bar or less up to about 10 bar or more. Care must however be taken to ensure that at all times the temperature of the vaporous mixture in contact with the catalyst is above the dew point of the ester or of any other condensible component present under the prevailing pressure conditions.

The vaporous mixture preferably contains at least an amount of hydrogen corresponding to the stoichiometric quantity of hydrogen required for hydrogenolysis. Usually an excess of hydrogen over the stoichiometric quantity will be present. In this case the excess hydrogen remaining after product recovery can be recycled to the catalytic reaction zone. As will be apparent from equation (I) above, 2 moles of hydrogen are required for hydrogenolysis of each carboxylic acid ester group present in the ester molecule.

If the ester contains non-aromatic unsaturation (i.e. carbon-carbon double or triple bonds) such unsaturated linkages may also undergo hydrogenation under the hydrogenolysis conditions employed. Hence the stoichiometric quantity of hydrogen required for reduction of 1 mole of an unsaturated mono-ester may correspond to 3,4 or more moles of hydrogen.

Diesters require 4 moles of hydrogen per mole of diester, if saturated, for hydrogenolysis; nonaromatically unsaturated diesters may require 5 or more moles of hydrogen for hydrogenolysis of 1 mole of diester.

The hydrogen :ester molar ratio within the vaporous mixture may vary within wide limits, e.g. from about 2:1 to about 100:1 or more, for example up to about 200:1, for a monoester or from about 4:1 to about 200:1 or more, for example up to about 400:1 or even 800:1, for a diester. This ratio will depend, at least to some extent, on the volatility of the ester used as well as on the number of ester groups in the ester substrate to be reduced. Typically the hydrogen-ester molar ratio is at least about 10:1.

Although the process of the invention is generally applicable to carboxylic acid esters best results will usually be obtained with esters boiling at temperatures of not more than about 3000C at atmospheric pressure. Whilst it is possible to utilise esters having still higher boiling points, the use of higher boiling point materials limits the partial pressure of the ester that can be used in the vaporous mixture and hence limits the rate of hydrogenolysis. If extremely high boiling esters are used then rates of reaction will be correspondingly reduced.

Certain esters may undergo thermal decomposition at temperatures approaching 300"C and possibly at temperatures below their boiling point at atmospheric pressure. When using such esters the temperature during hydrogenolysis should not be so high that significant thermal decomposition of the ester occurs.

In general it is preferred to use monocarboxylic acid esters, preferably aliphatic monocarboxylic acid esters of aliphatic alcohols, containing from 2 to about 20 carbon atoms, or dicarboxylic acid diesters, preferably aliphatic dicarboxylic acid diesters, containing from 4to about 16 carbon atoms.

According to a particularly preferred aspect of the invention there is provided a process for the production of ethylene glycol which comprises effecting hydrogenolysis of a dialkyl oxalate by contacting a vaporous mixture comprising the dialkyl oxalate, a minor amount of carbon dioxide and hydrogen, with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 150"C up to about 240"C and at a pressure in the range of from about 5 bar up to about 50 bar, and recovering resulting ethylene glycol.

According to a further aspect of the present invention there is provided a process for the production ethanol in which a vaporous mixture containing an alkyl acetate, a minor amount of carbon dioxide and hydrogen is contacted with a catalyst comprising a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 1 500C up to about 240"C and at a pressure in the range of from about 5 bar up to about 50 bar and recovering resulting ethanol.

In each case recovery of the hydrogenolysis products can be effected in conventional manner, e.g. by condensation followed, if desired, by fractional distillation in one or more stages under normal, reduced or elevated pressure. If the products form an azeotrope, then particular care must be taken in designing the product recovery section.

The invention is further illustrated in the following Examples.

Example 1 Di-ethyl oxalate was pumped at a rate of 10.0 mllhr to an electrically heated gas/liquid mixing device to which a gas mixture (0.27% COp, 10.41% CH4 and 89.32% H2) was also supplied at a controlled rate and pressure via an electrically heated line. The resulting vaporous mixture was passed through a lagged, electrically heated line to a pre-heating coil prior to passage through a tubular stainless steel reactor packed with 20 ml of a crushed catalyst. Both the tubular reactor and the pre-heating coil were immersed in a fluidised sand bath. The temperature of the sand bath was adjusted until the temperature of the vaporous mixture, as detected by a thermocouple positioned immediately upstream from the catalyst bed, was 219 C.

The vaporous mixture exiting the reactor was passed through a water cooled condenser and then through a second refrigerated condenser through which coolant at -5 C was passed. The resulting condensate was collected in a refrigerated knock out pot also kept at -50C. The exit gas pressure was controlled to 41.31 bar.

The non-condensed gases were then passed through a let-down valve, the gas flow rate being monitored downstream from this valve in a wet gas meter. A gas flow rate of 330 litres/hr (measured at atmospheric pressure) was maintained throughout the experiment. The liquid space velocity of the di-ethyl oxalate was 0.5 hr-1.

The liquid condensate was analysed by gas chromatography using a 2 metre stainless steel column (6mm outside diameter) packed with polyethylene glycol (nominal molecular weight 20,000) on Chromosorb PAW, a helium gas flow rate of 30 ml/minute and a thermal conductivity detector. The instrument was fitted with a chart recorder having a peak integrator and was calibrated using a mixture of ethanol, ethyl glycolate, ethylene glycol and di-ethyl oxalate of known composition. The condensate was shown to contain a mixture of 42.80 wt% ethanol, 3.23 wt% ethyl glycolate, 14.67 wt% ethylene glycol, and 33.23 wt% di-ethyl oxalate.

The catalyst used in this Example was charged to the reactor as a co-precipitated mixture of CuO and ZnO contaiing 67+3% CuO and 33+3% ZnO having particle size in the range of 1.2 mm to 2.4 mm and an internal surface area of about 45 sq.m. per gram. This was pre-reduced in the reactor using a 5 vol % H2 in N2 gas mixture at 200C for 16 hours followed by pure hydrogen at 200DC for 16 hours, the gas flow rate in each case being about 20 litres/hr (measured at atmospheric pressure) and the gas pressure being 15.18 bar. After this pre-reduction stage the catalyst was at all times maintained in a hydrogen-containing atmosphere.

In a comparative run using pure hydrogen and an inlet temperature of 220"C but otherwise identical conditions the liquid condensate contained 25.15 wt% ethanol, 2.23 wt% ethyl glycolate, 7.79 wt% ethylene glycol, and 62.61 wt% di-ethyl oxalate.

The above results indicate that, despite the presence of 10.41% by volume methane, the addition of CO2 in an amount corresponding to 0.3 moles per 100 moles of hydrogen substantially doubled the rate of conversion of di-ethyl oxalate to ethylene glycol under otherwise identical conditions.

Example 2 The general procedure of Example 1 was repeated using, in place of di-ethyl oxalate, an ester feed containing 93.35 wt% ethyl acetate, 6.60 wt% ethanol and 0.05 wt% water, at a feed rate of 41.0 mllhr. The reactor pressure was 41.39 bar and the inlet temperature was 213"C. The gas mixture used contained 92.2% hydrogen, 7.59% methane and 0.2% carbon dioxide and the gas flow rate was 210 litres/hr (measured at atmospheric pressure). The liquid hourly space velocity was 2.05 hr-'.

Gas chromatographic analysis showed that the conversion of ethyl acetate to ethanol was 86.9%.

When the experiment was repeated under identical conditions except that the gas mixture contained 93.08% hydrogen, 6.00% methane and 0.92% carbon dioxide, the observed conversion of ethyl acetate to ethanol was 85.3%.

Upon replacing the gas by pure hydrogen in a comparative run the conversion of ethyl acetate to ethanol was only 75.6%.

These experiments demonstrate clearly that the incorporation of a minor amount of carbon dioxide in the vaporous ester-containing mixture has an extremely beneficial effect on catalyst activity.

Example 3 The procedure of Example 2 was repeated using a gas mixture containing 0.91% carbon dioxide, 2.04% carbon monoxide, 7.28% methane, and 89.77% hydrogen. Otherwise the same reaction conditions were used as in Example 2. The observed conversion of ethyl acetate to ethanol was 83.7%, showing that carbon monoxide had no significant effect on the catalyst activity under these conditions.

Example 4 The procedure of Example 1 was repeated, using in place of di-ethyl oxalate, a feed ester mixture containing 93.62 wt% methyl acetate, 6.33 wt% methanol and 0.05 wt% water. This was supplied at 41.8 ml/hr. As reducing gas there was used a mixture of 0.21% carbon dioxide, 0.59% carbon monoxide, 8.93% methane and 90.27% hydrogen. The gas flow rate was 236 litres/hr (measured at atmospheric pressure).At an operating pressure of 41.39 bar the following conversions of methyl acetate to ethanol were observed: Temperature PC) Conversion sol 193 50.9 207 73.4 220 82.7 In a comparison run using pure hydrogen at 35.88 bar and 220"C, but otherwise identical conditions, the observed conversion of methyl acetate to ethanol was 60.3%.

It will thus be seen that the presence of a minor amount of carbon dioxide increases the catalyst activity by approximately one third at 2200C.

Example5 Diethyl maleate was used in place of diethyl oxalate following the general procedure of Example 1, but using a 50 ml charge of a catalyst containing, before reduction, 33t3% CuO and 66+3% ZnO. The conditions were as follows: Ester flow rate 18.5 ml/hr Inlet temperature 201"C Gas pressure 41.39 bar Gas composition 0.22% CO2, 1.46% CH4,98.32% H2 Gas flow rate : 360 litres/hr (at atmospheric pressure) Gas chromatographic analysis showed that the liquid condensate contained 3.74 wt% tetrahydrofuran, 38.17 wt% ethanol, 0.10 wt% n-butanol, 33.32 wt% diethyl succinate, 11.10 wt% butyrolactone and 13.06wt% butane-1,4-diol.No diethyl maleate was detected.

Upon repeating this experiment using pure hydrogen as the reducing gas, the condensate still contained no diethyl maleate and 3.37 wt% tetrahydrofuran, 30.42 wt% ethanol, 0.07 wt% n-butanol, 46.36 wt% diethyl succinate, 9.36 wt% butyrolactone, and 10.14 wt% butane-1,4-diol.

It is readily apparent from these results that the presence of a small proportion of CO2 in the reducing gas has a beneficial effect on catalyst activity.

Example 6 Using the same catalyst charge as used in Example 5, diethyl oxalate gave the following results: Ester flow rate 20.4 ml/hr Inlet temperature : 224"C Gas pressure : 41.39 bar Gas composition 0.23% CO2, 1.30% CH4, 98.47% H2 Gas flow rate : 566 litres/hr (at atmospheric pressure) The condensate gave the following analysis: 45.85 wt% ethanol, 5.43 wt% ethyl glycolate, 18.04 wt% ethylene glycol, and 26.49 wt% diethyl oxalate.

When pure H2 was used in a comparative run at an inlet temperature of 222"C but otherwise using the same conditions, the analysis of the condensate was: 28.62 wt% ethanol, 5.35 wt% ethyl glycolate, 11.42 wt% ethylene glycol and 50.58 wt% diethyl oxalate.

Again the beneficial effect on catalyst activity of a minor amount of CO2 is most marked, the increased in catalyst activity being almost 100%.

Example 7 Using the same general procedure as described in Example 1, the hydrogenolysis of methyl acetate was investigated under the following conditions: Liquid hourly space velocity 0.55 hr-' Feed gas 80%H2/20%N2 Temperature 185 C H2:ester molar ratio 20:1 Pressure 29.0 bar.

After a "bedding in" period, the observed conversion of methyl acetate to ethanol was steady at approximately 60%. After about 24 hours the feed gas was changed to an'80%H2/20%CO mixture, whereupon the observed conversion of methyl acetate fell off to about 25% as shown in Figure 1 of the drawings, which plots % conversion of methyl acetate against time. After about 60 hours the gas supply was changed back to 80%H2/20%N2, and the rate of conversion increased again until it reached approximately its original level. This demonstrates that the deactivating effect on catalyst activity of CO is reversible.

Example 8 The procedure of Example 1 was repeated, using methyl acetate as the feed ester, under the following conditions: Liquid hourly spaced velocity of ester 1.0 hF1 Gas feed 100%H2 H2:ester molar ratio 20:1 Temperature 1900C Pressure 29.0 bar.

After an initial "bedding in" period the observed conversion of methyl acetate to ethanol was steady at approximately 70%. After approximately 30 hours of operation the gas feed supply was changed to a hydrogen stream to which had been added 2000 ppm CO2. The rate of conversion of methyl acetate to ethanol gradually rose until it reached a peak of about 85%. After 90 hours into the test the gas supply was changed back to a pure H2 supply and the conversion declined again to approximately the initial 70% level.

These results are illustrated in Figure 2 of the drawings.

Example 9 The procedure of Example 8 was repeated. After operating initially at a conversion of about 70% under the initial conditions of Example 8, a feed gas consisting of H2 and 2000 ppm CO2 was used. Upon lowering the temperature from 190 C to 165 C the rate of conversion of methyl acetate was maintained at about 70%.

Example 10 Methyl acetate was subjected to hydrogenolysis under the same initial conditions as were used in Example 7. After steady state operation had been achieved, with a conversion of methyl acetate at a steady level of approximately 70%, the feed gas was changed from an 80%H2/20%N2 mixture to an 80%H2/20%CO mixture. As in Example 7 the conversion of methyl acetate declined over a period of about 36 hours to about 25%. Upon adding 2000 ppm of CO2 to the gas feed the conversion rate gradually increased again until it restabilised at about the 60% level again. These results are shown in Figure 3 of the drawings.

Example ii Methyl acetate was subjected to hydrogenolysis under the same initial conditions as were used in Example 8. Upon changing the pure H2 supply to a mixture containing, in addition to H2, also 5000 ppm of CO and 2000 ppm of CO2 the observed conversion rate rose to about 85%, as shown in Figure 4 of the drawings.

When a gas feed consisting of an 80%H2/20%CO mixture to which had been added 2000 ppm CO2 was used the conversion rate stabilised at about 70% as is also indicated in Figure 4 of the drawings.

Claims (36)

1. A process for effecting hydrogenolysis of an ester of an aliphatic carboxylic acid which comprises contacting a vaporous mixture comprising the ester, a minor amount of carbon dioxide and hydrogen, with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 1 50"C up to about 240"C and at a pressure in the range of from about 5 bar up to about 50 bar.

2. A process according to claim 1, in which the vaporous mixture contains from about 0.02 moles up to about 5 moles of carbon dioxide per 100 moles of hydrogen.

3. A process according to claim 1 or claim 2, in which the vaporous mixture contains from about 0.2 moles up to about 1 mole of carbon dioxide per 100 moles of hydrogen.

4. A process according to any one of claims 1 to 3, in which the vaporous mixture contains also a minor amount of carbon monoxide, and in which the molar ratio of carbon dioxide to carbon monoxide is at least about 1:100.

5. A process according to any one of claims 1 to 4, in which the pressure is in the range of from about 15 barto about40 bar.

6. A process according to any one of claims 1 to 5, in which the temperature is in the range of from about 160"C to about 220"C.

7. A process according to any one of claims 1 to 6, in which the carboxylic acid ester is an ester of an aliphatic monocarboxylic or dicarboxylic acid.

8. A process according to any one of claims 1 to 7, in which the carboxylic acid ester is an alkyl acetate and ethanol is a product of hydrogenolysis.

9. A process according to claim 8, in which the carboxylic acid ester is methyl acetate or ethyl acetate.

10. A process according to any one of claims 1 to 7, in which the carboxylic acid ester is dialkyl oxalate and ethylene glycol is a product of hydrogenolysis.

11. A process according to claim 10, in which the carboxylic acid ester is di-ethyl oxalate.

12. A process according to any one of claims 1 to 7, in which the carboxylic acid ester is a dialkyl maleate orfumerate and hydrogenolysisylelds 1,4-butanediol and/ortetrahydrofuran.

13. A process according to claim 12, in which the carboxylic acid ester is diethyl maleate.

14. A process according to any one of claims 1 to 13, in which the catalyst consists essentially of a reduced mixture of copper oxide and zinc oxide derived from a mixture comprising, before reduction, from about 10 to about 70 percent by weight CuO and about 90 to about 30 percent by weight ZnO.

15. A process according to claim 14, in which the mixture comprises from about 20 to about 40 percent by weight CuO and from about 60 to 80 percent by weight ZnO.

16. A process according to any one of claims 1 to 13, in which the catalyst consists essentially of a reduced mixture of copper oxide and zinc oxide derived from a mixture comprising, before reduction, from about 65 to about 85 percent by weight CuO and about 15 to about 35 percent by weight ZnO.

17. A process for the production of ethylene glycol which comprises effecting hydrogenolysis of a dialkyl oxalate by contacting a vaporous mixture comprising the dialkyl oxalate, a minor amount of carbon dioxide and hydrogen, with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 1 50"C up to about 240"C and at a pressure in the range of from about 5 bar up to about 50 bar and resulting ethylene glycol is recovered.

18. A process according to claim 17, in which the vaporous mixture contains from about 0.02 moles up to about 5 moles of carbon dioxide per 100 moles of hydrogen.

19. A process according to claim 17 or claim 18, in which the vaporous mixture contains from about 0.2 moles up to about 1 mole of carbon dioxide per 100 moles of hydrogen.

20. A process according to any one of claims 17 to 19, in which the vaporous mixture contains also a minor amount of carbon monoxide and in which the molar ratio of carbon dioxide to carbon monoxide is at least about 1:100.

21. A process according to any one of claims 17 to 20, in which the pressure is in the range of from about 15 bar to about 40 bar.

22. A process according to any one of claims 17 to 21, in which the temperature is in the range of from about 160"C to about 220 C.

23. A process according to any one of claims 17 to 22, in which the dialkyl oxalate is di-ethyl oxalate.

24. A process according to any one of claims 17 to 23, in which the catalyst consists essentially of a reduced mixture of copper oxide and zinc oxide derived from a mixture comprising, before reduction, from about 10 to about 70 percent by weight CuO and about 90 to about 30 percent by weight ZnO.

25. A process according to claim 24, in which the mixture comprises from about 20 to about 40 percent by weight CuO and from about 60 to 80 percent by weight ZnO.

26. A process according to any one of claims 17 to 23, in which the catalyst consists essentially of a reduced mixture of copper oxide and zinc oxide derived from a mixture comprising, before reduction, from about 65 to about 85 percent by weight of CuO and about 15 to about 35 percent weight ZnO.

27. A process for the production of ethanol in which a vaporous mixture containing an alkyl acetate, a minor amount of carbon dioxide and hydrogen is contacted with a catalyst consisting essentially of a reduced mixture of copper oxide and zinc oxide at a temperature in the range of from about 1500C up to about 240"C and at a pressure in the range of from about 5 bar up to about 50 bar, and resulting ethanol is recovered.

28. A process according to claim 27, in which the vaporous mixture contains from about 0.02 moles up to about 5 moles of carbon dioxide per 100 moles of hydrogen.

29. A process according to claim 27 or claim 28, in which the vaporous mixture contains from about 0.2 moles up to about 1 mole of carbon dioxide per 100 moles of hydrogen.

30. A process according to any one of claims 27 to 29, in which the vaporous mixture includes also a minor amount of carbon monoxide and in which the molar ratio of carbon dioxide to carbon monoxide is at least about 100:1.

31. A process according to any one of claims 27 to 30, in which the pressure is in the range of from about 5 barto about 40 bar.

32. A process according to any one of claims 27 to 31, in which the temperature is in the range of from about 160 C to about 220"C.

33. A process according to any one of claims 27 to 32, in which the acetic acid ester is methyl acetate or ethyl acetate.

34. A process according to any one of claims 27 to 33, in which the catalyst consists essentially of a reduced mixture of copper oxide and zinc oxide derived from a mixture comprising, before reduction, from about 10 to about 70 percent by weight CuO and about 90 to about 30 percent by weight ZnO.

35. A process according to claim 34, in which the mixture comprises from about 20 to about 40 percent by weight CuO and from about 60 to 80 percent by weight ZnO.

36. A process according to one of claims 27 to 33, in which the catalyst consists essentially of a reduced mixture of copper oxide and zinc oxide derived from a mixture comprising, before reduction, from about 65 to about 85 percent by weight CuO and about 15 to about 35 percent by weight ZnO.