GB2113206A - Process for preparing alkyl esters of carboxylic acids from an acid and syngas using a novel catalyst composition - Google Patents

Abstract

Lower alkyl esters of carboxylic acids are prepared in good yield from a carboxylic acid and syngas by contacting a mixture of the carboxylic acid, carbon monoxide and hydrogen with a catalyst composition comprising a ruthenium-containing compound, a cobalt-containing compound and a quaternary onium salt or base, and heating the resulting mixture at an elevated temperature and pressure for sufficient time to produce the desired alkyl carboxylic acid ester, and then recovering the same from the reaction mixture. Methanol is an additional and preferred reactant.

Description

SPECIFICATION Process for preparing alkyl esters of carboxylic acids from an acid and syngas using a novel catalyst composition This invention relates to a new process for preparing lower alkyl esters of carboxylic acids, More particularly, the invention relates to a new process for preparing lower alkyl esters of carboxylic acids from an acid and syngas using a novel catalyst composition.

Specifically, the invention provides a new and improved process for preparing lower alkyl esters of carboxylic acids, such as ethyl and propyl propionate, in good yield from the acid, such as propionic acid, carbon monoxide and hydrogen which comprises contacting a mixture of the carboxylic acid, carbon monoxide and hydrogen with a catalyst composition comprising a ruthenium-containing compound, a cobaltcontaining compound and a quaternary onium salt or base, and heating the resulting mixture at an elevated temperature and pressure for sufficient time to produce the desired alkyl carboxylic acid esters, and then recovering the same from the reaction mixture. In a preferred form of the invention, methanol is present as an additional reactant.

Lower alkyl alkanoates, such as ethyl propionate and propyl propionate, are chemicals which have found wide use in industry. They may be used, for example, in the production of anhydrides and in the production of ethylene and propylene, They may also be used as solvents and diluents and as plasticizers and softeners for resins.

Various methods have'been used in the past for the production of these esters. The esters can be produced, for example, by the reaction of an alkanol, such as ethanol, with an alkanoic acid.

Both components are commonly obtained from petroleum or agrichemical feedstocks. A direct synthesis of the esters from syngas would be potentially more economical and highly desirable.

It has been proposed to prepare the lower alkyl alkanoates by carbonylation techniques, but these methods up to the present have not been entirely satisfactory as they give low yields of the desired esters or use expensive catalysts or catalysts that are difficult to utilize on a large scale. For example, U.S. 4,270,015 and references cited therein disclose various catalyst systems for use in producing esters by carbonylation. U.S.

4,270,015 discloses the preparation of ethyl esters from syngas using a ruthenium-Group VA ligand catalyst complex as catalyst. While this process produces the ethyl esters, there is a great deal to be desired as to the selectivity and yield of the desired products.

It is an object of the invention, therefore, to provide a new and improved process for preparing the lower alkyl esters of carboxylic acids. We use a new and improved catalyst system and obtain improved selectivity and yield. The catalyst system is suitable for use on large scale operations.

The process of the invention comprises contacting a mixture of a carboxylic acid, carbon monoxide and hydrogen and possibly methanol with a catalyst composition comprising a ruthenium-containing compound, a cobaltcontaining compound and a quaternary onium salt or base, and heating the resulting mixture at an elevated temperature and pressure for sufficient time to produce the desired alkyl carboxylic acid ester, and then recovering the same from the reaction mixture. It was surprising to find that this new catalyst system using the cobalt-containing compound as cocatalyst gives improved selectivity in the formation of the desired ethyl and propyl carboxylic acid esters and improved conversion rates.

The process of the invention is particularly characterized by the high selectivity in the conversion of the alkanoic acids to the desired esters as according to the equations:

Typical conversion of the alkanoic acid ranges from 27% to about 78%, with the total yield of the ethyl and propyl esters generally ranging from 22% to 53% in the absence of methanol. In the presence of methanol, typical conversion of the carboxylic acid ranges from 65% to about 84%, with the total yield of the ethyl and n-propyl esters ranging from 49% to 63%. With the formation of the desired ethyl and propyl esters, other esters, such as the methyl and butyl esters are formed as minor by-products.

In the operation of the process of the invention, the ethyl and propyl esters, along with minor byproducts such as the methyl and butyl esters, are produced concurrently from the carboxylic acid and syngas by a process which may comprise the following steps: (a) contacting a mixture of the carboxylic acid, carbon monoxide and hydrogen with a catalyst comprising a ruthenium-containing compound, a cobalt-containing compound and a quaternary onium salt or base, (b) heating the said mixture to an elevated temperature, above 1 500C, and an elevated pressure, e.g. above 500 psi (34.5 bars) with sufficient carbon monoxide and hydrogen to satisfy the stoichiometry of the formation of the esters as noted in equation 1 above, until substantial formation of the desired ester has been achieved, and (c) preferably isolating the said ester and minor by-products from the reaction mixture, as by distillation.

In order to present the inventive concept of the present invention in the greatest possible detail, the following supplementary disclosure is submitted. The process of the invention is practiced as follows: As noted, the new catalyst system used in the process of the invention contains a rutheniumcontaining compound, a cobalt-containing compound and a quaternary onium salt or base.

The ruthenium-containing compounds employed as a catalyst may take many different forms. For instance, the ruthenium may be added to the reaction mixture in an oxide form, as in the case of, for example, ruthenium(lV) oxide hydrate, anhydrous ruthenium(lV) dioxide and ruthenium(VI II) tetraoxide. Alternatively, it may be added as the salt of a mineral acid, as in the case of ruthenium(lil) chloride hydrate, ruthenium(lll) bromide, ruthenium(lil) iodide, tricarbonylruthenium nitrate, or as the salt of a suitable organic carboxylic acid, for example, ruthenium(lil) acetate, ruthenium naphthenate, ruthenium valerate and ruthenium complexes with carbonyl-containing ligands such as ruthenium(lil) acetylacetonate.The ruthenium may also be added to the reaction zone as a carbonyl or hydrocarbonyl derivative. Here, suitable examples include, among others, triruthenium dodecacarbonyl and other hydrocarbonyls such as H2Ru4(CO),3 and H4Ru4(CO)12, and substituted carbonyl species such as the tricarbonylruthenium(ll) chloride dimer, (Ru(CO)3C12)2.

Preferred ruthenium-containing compounds include oxides of ruthenium, ruthenium salts of an organic carboxylic acid and ruthenium carbonyl or hydrocarbonyl derivatives. Among these, particularly preferred are ruthenium(lV) dioxide hydrate, ruthenium(VII I) tetraoxide, anhydrous ruthenium(lV) oxide, ruthenium acetate, ruthenium(lil) acetylacetonate, and triruthenium dodecacarbonyl.

The cobalt-containing compound to be used in the catalyst composition may take many different forms. For instance, the cobalt may be added to the reaction mixture in the form of an oxide, salt, carbonyl derivative and the like. Examples of these include, among others, cobalt oxides Co203, Co304, CoO, cobalt(ll) bromide, cobalt(ll) iodide, cobalt(ll) thiocyanate, cobalt(ll) hydroxide, cobalt(ll) carbonate, cobalt(ll) nitrate, cobalt(ll) phosphate, cobalt acetate, cobalt naphthenate, cobalt benzoate, cobalt valerate, cobalt cyclohexanoate, cobalt carbonyls, such as dicobalt octacarbonyl Co2(CO)8, tetracobalt dodecacarbonyi Co4(CO),2 and hexacobalt hexadocacarbonyl Co6(CO),6 and derivatives thereof by reaction with ligands, and preferably group V donors, such as the phosphines, arsines and stibine derivatives such as (Co(CO)3L)2 wherein L is PR3, AsR3 and SbR3 wherein R is a hydrocarbon radical, cobalt carbonyl hydrides, cobalt carbonyl halides, cobalt nitrosyl carbonyls as CoNO(CO)3, Co(NO)(CO)2PPh3, cobalt nitrosyl halides, organometallic compounds obtained by reacting cobalt carbonyls with olefins, allyi and acetylene compounds, such as bis(Tcyclopentandienyl) cobalt (TCsHs)2Cot cyclopentadienyl cobalt dicarbonyl, bis(hexamethylenebenzene)cobalt.

Preferred cobalt-containing compounds to be used in the catalyst system comprise those having at least one cobalt atom attached to carbon, such as the cobalt carbonyls and their derivatives as, for example, dicobalt octacarbonyl, tetracoba It dodecacarbonyl, (Co(CO)3P(CH3)3)2, organometallic compounds obtained by reacting the cobalt carbonyls with olefins, cycloolefins, allyl and acetylene compounds such as cyclopentadienyl cobalt dicarbonyl, cobalt carbonyl halides, cobalt carbonyl hydrides, cobalt nitrosyl carbonyls, and the like, and mixtures thereof. Additionally the cobalt salts, such as the halides, nitrates, perchlorates, acetates, valerates, and the like, may be used.

Particularly preferred cobalt-containing compounds to be used in the catalyst comprise those having at least one cobalt atom attached to at least three separate carbon atoms, such as for example, the dicobalt octacarbonyls and their derivatives and the cobalt halides, such as cobalt iodide, cobalt bromide, cobalt chloride, cobalt salts of nitric and perchloric acid and cobalt salts of monocarboxylic acids containing 1 to 10 carbon atoms.

The quaternary onium salt or base to be used in the catalyst composition may be any onium salt or base, but are preferably those containing phosphorous or nitrogen, such as those of the formula

wherein Y is phosphorous or nitrogen, R1, R2, R3 and R4 are organic radicals preferably alkyl, aryl or alkaryl radicals, and X is ananionic species. The organic radicals useful in this instance include those radicals having from 1 to 20 carbon atoms in a branched or linear alkyl chain, such as methyl, ethyl, n-butyl, isobutyl, octyl, 2-ethylhexyl and dodecyl radicals. Tetraethylphosphonium bromide and tetrabutylphosphonium bromide are typical examples presently in commercial production.

The corresponding quaternary phosphonium or ammonium acetates, hydroxides, nitrates, chromates, tetrafluoroborates and other halides, such as the corresponding chlorides, and iodides, are also satisfactory.

Equally useful ure the phosphonium and ammonium salts containing phosphorous or nitrogen bonded to a mixture of alkyl, aryl and alkaryl radicals, which radicals preferably contain from 6 to 20 carbon atoms. The aryl radical is most commonly phenyl. The alkaryl group may comprise phenyl substituted with one or more C, to C,O alkyl substituents, bonded to phosphorous or nitrogen through the aryl function.

Illustrative examples of suitable quaternary onium salts or bases include tetrabutylphosphonium bromide, heptyltriphenylphosphonium bromide, tetrabutylphosphonium iodide, tetrabutylammonium chloride, tetrabutylphosphonium nitrate, tetrabutylphosphonium hydroxide, tetrabutylphosphonium chromate, tetraoctylphosphoniu m tetrafluoroborate, tetrahexylphosphonium acetate and tetraoctylammonium bromide.

The preferred quaternary onium salts and bases to be used in the process comprise the tetralkylphosphonium salts containing alkyl groups having 1 to 6 carbon atoms, such as methyl, ethyl, butyl, hexyl, heptyl and isobutyl.

Tetralkylphosphonium salts, such as the halides, bromides, chlorides and iodides, and the acetate and chromate salts and hydroxide base, are the most preferred.

The quantity of the ruthenium-containing compound and the cobalt-containing compound to be used in the process of the invention may vary over a wide range. The process is conducted in the presence of a catalytically effective quantity of the active ruthenium-containing compound and the active cobalt-containing compound which gives the desired product in a reasonable yield. The reaction proceeds when employing as little as about 1 xl 0-6 weight percent, and even lesser amounts of the ruthenium-containing compound, together with as little as about 1 x 1 0-6 weight percent of the cobalt-containing compound, or even lesser amounts, based on the total weight of the reaction mixture. The upper concentration is dictated by a variety of factors including catalyst cost, partial pressures of carbon monoxide, operating temperature, etc.A rutheniumcontaining compound concentration of from about 1 x 10-5 to about 10 weight percent in conjunction with a cobalt-containing compound concentration of from about 1 x10-5 to about 5 percent, based on the total weight of the reaction mixture is generally desirable in the practice of this invention. The preferred ruthenium to cobalt atomic ratios are from about 10:1 to 1:10.

Generally, in the catalyst system used in the process of the invention, the molar ratio of the ruthenium-containing compound to the quaternary onium salt or base will range from about 1:0.01 to about 1:100 or more, and preferably will be from about 1:1 to about 1:20.

Particularly superior results are obtained when the above-noted three components of the catalyst system are combined in a molar basis as follows: ruthenium-containing compound 0.1 to 4 moles, cobalt-containing compound 0.025 to 1.0 mole and the quaternary onium salt or base 0.4 to 60 moles, and still more preferably when the components are combined in the following molar ratios: ruthenium-containing compound 1 to 4 moles, cobalt-containing compound 0.25 to 1.0 moles and the quaternary onium base or salt 10 to 50 moles The carboxylic acid used in the process of the invention forms the acid moiety of the desired alkyl ester. Suitable carboxylic acids include the aliphatic acids, alicyclic monocarboxylic acids, heterocyclic acids and aromatic acids, both substituted and unsubstituted.Examples of such acids include, among others, the lower monoaliphatic carboxylic acids, such as formic acid, acetic, propionic, butyric, isobutyric, valeric, caprioic, capric, perlargonic and lauric acids, together with dicarboxylic acids, such as oxalic, masonic, succinic and adipic acids. The invention further contemplates the use of substituted monoaliphatic acids containing one or more functional substituents, such as the lower alkoxy, chloro, fluoro, cyano, alkylthio, and amino functional groups, examples of which include acetoacetic acid, dichloroacetic acid and trifluoroacetic acid, chloropropionic acid, trichloroacetic acid, monofluoroacetic acid and the like. Among the suitable aromatic acids contemplated are benzoic acid, naphthoic acids, toluic acids, chlorobenzoic acids, aminobenzoic acids and phenylacetic acid.The acyclic monocarboxylic acids may contain from 3 to 6 carbon atoms in the ring, both substituted or unsubstituted, and may contain one or more carboxyl groups, such as cyclopentane-carboxylic acid and hexahydrobenzoic acids. The heterocyclic acids may contain 1 to 3 fused rings both substituted and unsubstituted together with one or more carboxylic groups, examples include quinolinic, furoic and picolinic acids. Mixtures of said classes of carboxylic acids, in any ratio, may also be used in the process of the invention.

Anhydrides of the acids can also be used.

Preferred carboxylic acids include the lower monocarboxylic acids containing from 1 to 12 carbon atoms, and the halo, alkoxy, cyano, alkylthio and amino-substituted monocarboxylic acids containing up to 12 carbon atoms, and the dicarboxylic acids containing up to 12 carbon atoms.

The amount of the carboxylic acid to be used in the process of the invention may vary over a wide range. In general, the amount of acid to be used should be sufficient to satisfy the stoichiometry of the formation of the esters as shown in equation 1 above, although larger or small amounts may be used as desired or necessary.

Inert solvents may also be added to the reaction media during the preparation of the desired alkyl esters of carboxylic acid. Suitable solvents may include the oxygenated hydrocarbons, e.g. compounds possessing only carbon, hydrogen and oxygen and one in which the oxygen atom present is in an ether, ester, ketone carbonyl or hydroxyl group or groups.

Generally, the oxygenated hydrocarbon will contain from about 3 to 12 carbon atoms and preferably a maximum of three oxygen atoms. The solvent must be substantially inert under the reaction conditions, should be relatively nonpolar. Preferably, the solvent will have a boiling point greater than that of the ester and other products of the reaction so that recovery of the solvent by distillation is facilitated.

Preferred ester type solvents are the aliphatic, cycloaliphatic and aromatic carboxylic acid esters as exemplified by methyl benzoate, isopropyl benzoate, butyl cyclohexanoate, as well as dimethyl adipate. Useful alcohol-type solvents include the monohydric alcohols as cyclohexanol and 2-octanol, etc. Suitable ketone-type solvents include, for example, cyclic ketones, such as cyclohexanone, 2-methylcyclohexanone, as well as acyclic ketones, such as 2-pentanone, butanone, acetophenone, etc. Ethers which may be utilized as solvents include cyclic, acyclic, and heterocyclic materials. Preferred ethers are the hetetocyclic ethers as illustrated by 1,4-dioxane and 1,3-dioxane. Other suitable ethers include isopropyl propyl ether, diethylene glycol, dibutyl ether, diphenyl ether, dibutyl ether, heptyl phenyl ether, anisole, tetrahydrofuran, etc.The most useful solvents of all of the above groups include the ethers, as diphenyl ether and 1 ,4-dioxane, etc.

The amount of the solvent employed may vary as desired. In general, it is desirable to use sufficient solvent to fluidize the catalyst system.

The relative amounts of carbon monoxide and hydrogen which can be initially present in the syngas mixture are variable, and these amounts may be varied over a wide range. In general, the mole ratio of CO:H2 is in the range from about 20:1 to about 1 :20; and preferably from about 5:1 to 1:5, although ratios outside these ranges may also be employed with good results.

Particularly in continuous operations, but also in batch experiments, the carbon monoxidehydrogen gaseous mixtures may also be used in conjunction with up to 50% by volume of one or more other gases. These other gases may include one or more inert gases such as nitrogen, argon, neon, and the like, or they may include gases that may, or may not undergo reaction under carbon monoxide hydrogenation conditions, such as carbon dioxide, hydrocarbons, such as methane, ethane, propane and the like, ethers such as dimethyl ether, methylethyl ether and dimethyl ether, and higher alcohols.

The temperature range which can usefully be employed in the process of the invention may vary over a considerable range depending upon experimental facts, including the choice of catalyst, pressure and other variables. The preferred temperatures are above 1 500C and more preferably between 1 500C and 3500C when superatmospheric pressures of syngas are employed. Coming under special consideration are the temperatures ranging from about 1 800C to about 2500C.

Superatmospheric pressures of about 500 psi (34.5 bars) or greater lead to substantial yield of the desired esters. A preferred range is from about 1000 psi (69 bars) to about 7500 psi (517.5 bars) although pressures above 7500 (517.5 bars) also provide useful yields of the desired products. The pressures referred to herein represent the total pressure generated by all the reactants, although they are substantially due to the carbon monoxide and hydrogen reactants.

The desired products of the reaction, the ethyl and propyl esters of the desired alkanoic acids, will be formed in significant quantities varying generally from about 22% to about 63% in yield.

Also formed will be minor by-products, such as the methyl and butyl esters of those alkanoic acids as well as other oxygenated products. The desired products can be recovered from the reaction mixture by conventional means, such as fractional distillation in vacuo, etc.

The process of the invention can be conducted in a batch, semicontinuous or continuous manner.

The catalyst can be initially introduced into the reaction zone batchwise, or it may be continuously or intermittently introduced into such a zone during the course of the synthesis reaction. Operating conditions can be adjusted to optimize the formation of the desired esters, and said material can be recovered by methods known to the art, such as distillation, fractionation, extraction and the like. A fraction rich in the catalyst components may then be recycled to the reaction zone, if desired, and additional product generated.

The products have been identified in this work by one or more of the following analytical procedures: viz, gas-liquid phase chromatography (glc), infrared (ir) mass spectometry, nuclearmagnetic resonance (nmr) and elemental analyses, or combination of these techniques.

Analyses have, for the most part, been by parts by weight; all temperatures are in degree centigrade and all pressures in pounds per square inch (psi) and bars.

To illustrate the process of the invention, the following examples are given. It is to be understood, however, that the examples are given in the way of illustration and are not to be regarded as limiting the invention in any way.

Example 1 This example illustrates the improved selectivity to ethyl and propyl esters from synthesis gas plus the appropriate carboxylic acid that may be achieved using the new class of catalyst compositions comprising a rutheniumcontaining compound, a cobalt-containing compound and a quaternary onium salt.

A glass liner was charged with hydrated ruthenium oxide hydrate (0.19 grams, 1.0 mmole), n-heptyltriphenyl-phosphonium bromide (4.25 grams, 10 mmoles), dicobalt octacarbonyl (0.085 grams, 0.25 mmole) and propionic acid (10.0 grams, 135 mmoles). The glass liner was placed in a stainless steel reactor and purged of air with hydrogen and carbon monoxide (1 :1 molar ratio), then pressured to 200 psi (138 bars) and heated to 2200C. The pressure was brought up to 6280 pis (433.3 bars) and during the reaction period, the constant pressure was maintained by using surge tank. After 18 hours, the reactor was allowed to cool, the gas pressure (3950 psi-272.6 bars) noted, the excess gas sampied and vented and 16.9 g of the liquid products recovered.

Analysis of the product liquid fraction by gasliquid chromatography (g/c) showed the presence of: 30.3% ethyl propionate 15.6% n-propyl propionate 2.4% methyl propionate 1.9% n-butyl propionate 41.4% unreacted propionic acid Ethyl and propyl propionate selectivities were calculated to be: ethyl propionate: 56 mole % selectivity n-propyl propionate: 25 mole % selectivity Therefore: Total ethyl +n-propyl propionate selectivity-8 1 mole %.

Ethyl and n-propyl propionate yields, basic propionate acid charged, were calculated to be: ethyl propionate: 27 mole % n-propyl propionate: 12 mole % The total ethyl and n-propyl propionate yield was 30 mole %. Conversion of propionic acid is estimated to be 49 mole %.

Comparative Example A In this comparative example the synthesis of ethyl and n-propyl propionate esters from synthesis gas plus propionic acid is illustrated using a two component catalyst system comprising a ruthenium-containing compound and a quaternary onium salt. There is no cobaltcontaining compound present in this comparative example. The results are substantially the same as those disclosed in U.S. Patent 4,270,01 5, Example 1.

To an 850 ml glass-lined autoclave reactor equipped for pressuring, heating, cooling and means of agitation is charged 0.764 gm of ruthenium(lV) oxide, hydrate (4.0 mmole), 17.64 gm of heptyl(triphenyl)phosphonium bromide (40 mmole) and propionic acid (50 gm). Upon stirring under a nitrogen atmosphere most of the solids dissolve to give a deep-red solution. The reaction is then sealed, flushed with CO/H2, pressured to 2000 psi (138 bars) with synthesis gas (a 1:1 mixture of hydrogen and carbon monoxide) and heated to 2200C with agitation. At temperature, the pressure within the reactor is raised to 6300 psi (435 bars) with CO/H2 mix, and the pressure held constant throughout the 1 8 hour run by automatic addition of more synthesis gas from a large surge tank.Upon cooling, the excess gases are sampled and vented, and the deep-yellow liquid product (73.8 gm) removed for analysis.

There is no solid product fraction.

Analysis of the liquid fraction by gas-liquid chromatography (glc) shows the presence of: 38.2 wt % ethyl propionate 1 6.5 wt % methyl propionate 8.4 wt % n-propyl propionate 0.8 wt % n-butyl propionate 0.9 wt % glycol dipropionate 2.7 wt % water 27.8 wt % unreacted propionic acid.

Here the calculated ethyl and propyl propionates selectivities were estimated to be for this example: ethyl propionate; 47 mole % selectivity n-propyl propionate: 9 mole % selectivity Total ethyl plus n-propyl propionate selectivity-56 mole %.

It may be noted that: The total selectivity to ethyl and propyl propionate (56%) in this comparative Example A is lower than the 81 mole % achieved in Example 1 using the three-component catalyst system comprising ruthenium oxide, hydrate, n-heptyltriphenylphosphonium bromide and dicobalt octacarbonyl.

Example 2 The procedure of Example 1 was repeated with the exception that the catalyst components were utilized as follows: ruthenium oxide hydrate (1 mmole, 0.19 g), n-heptyltriphenylphosphonium bromide (10 mmoles, 4.25 g), cobalt(ll) iodide (0.125 mmole, 0.040 g) and propionic acid (162 mmoles, 12.0 g). The reaction conditions were 6950 psi (480 bars) of CO:H2=1:1, 2200C and 18 hours. The recovered liquid product (19.1 g) was analyzed by gc as follows: 16.0 wt % ethyl propionate 9.6 wt % n-propyl propionate 2.0 wt % methyl propionate 59.8 wt % unreacted propionic acid The product selectivities to ethyl and n-propyl propionate were calculated to be: 53 mole % ethyl propionate 28 mole % n-propyl propionate.

The combined selectivity of ethyl- and n-propyl propionate was 81% at 27% propionic acid conversion. The estimated total yield of ethyl and propyl propionates (base propionic acid charged) is 22 mole %.

Example 3 A glass liner was charged with hydrated ruthenium oxide hydrate (1 mmole, 0.19 g), n heptyltriphenylphosphonium bromide (10 mmoles, 4.25 g), dicobalt octacarbonyl (0.25 mmole, 0.085 g), propionic acid (12.0 g) and 1,4dioxane solvent (12.0 g). The glass liner was placed in a stainless steel reactor and purged of air with hydrogen and carbon monoxide (1:1 ratio), then pressured to 2000 psi (138 bars) and heated to 2200 C). The pressure was brought up to 6500 psi (448.5 bars) and maintained by using a surge tank. After 18 hours, the reactor was allowed to cool, the gas pressure (3735 psi 257.7 bars) noted, the excess gas was vented and the liquid products recovered (29.5 g).

The liquid products were analyzed by glc as follows: 1.3 wt % methyl propionate 14.7 wt % ethyl propionate 8.6 wt % n-propyl propionate 1.2 wt % n-butyl propionate 46.8 wt % unreacted propionic acid 22.2 wt % p-dioxane The results were calculated in terms of product selectivities and absolute yields.

ethyl propionate: 42 mole % selectivity n-propyl propionate: 24 mole % selectivity.

The total molar selectivity to ethyl and n-propyl propionate was 66 mole % and the conversion of propionic acid was 35 mole %.

Example 4 A glass liner was charged with hydrated ruthenium oxide (2 mmoles), 0.38 g), tetra-n-butyl phosphonium bromide (20 mmoles, 6.8 g), cobalt(lil) acetylacetonate (1.0 mmole, 0.36 g) and 25 g of propionic acid. The glass liner was placed in a stainless steel reactor and purged of air with hydrogen and carbon monoxide (1:1 ratio), then pressure to 2000 psi (138 bars) and heated to 2200C. The pressure was brought up to 6600 psi (455.4 bars) and maintained by using a surge tank. After 1 8 hours, the reactor was allowed to cool, the gas prssure (3420 psi236 bars) noted, the excess gas was vented and the liquid product recovered (44.5 g).

The liquid products were analyzed by glc as follows: 6.8 wt % methyl propionate 32.8 wt % ethyl propionate 1 5.8 wt % n-propyl propionate 1.5 wt % n-butyl propionate 1 5.2 wt % unreacted propionic acid 8.8 wt % ethanol The results were calculated as follows: ethyl propionate: 44 mole % selectivity n-propyl propionate: 19 mole % selectivity.

The combined selectivity of ethyl and n-propyl propionate was 63 mole % and the conversion of propionic acid was 78 mole %.

Example 5 The procedure of Example 4 was repeated with the exception that the catalyst components were utilized as follows: ruthenium oxide hydrate (1 mmole, 0.19 g), tetra-n-butylphosphonium bromide (10 mmoles, 3.4 g), cobalt(ll) perchlorate (0.125 mmole, 0.046 g), cobalt(lil) acetylacetonate (0.125 mmole, 0.046 g) and 12 ml of propionic acid. The reaction conditions were at 2200 C, 6400-5800 psi (441.6-400 bars) of CO/H2=1 :1 syngas and 18 hour reaction period.

The liquid product (19.7 g) was recovered and gc analysis showed: 37.2 wt % ethyl propionate 10.4 wt % n-propyl propionate 15.6 wt % methyl propionate 26.3 wt % unreacted propionic acid Ethyl and n-propyl propionate selectivity were calculated to be: 51 mole % ethyl propionate 12 mole % n-propyl propionate.

The combined selectivity was 62% and the conversion of propionic acid was calculated to be 67 mole %.

Example 6 The procedure of Example 4 was repeated with the exception that the catalysts components were utilized as follows: ruthenium oxide hydrate (2 mm, 0.38 g), tetra-n-butylphosphonium bromide (20 mmole, 6.8 g), cobalt(lil) acetylacetonate (0.25 mm, 0.090 g) and propionic acid (270 mm, 20 g). The reaction conditions were 6300 psi syngas pressure of CO:H2=1 :1, 2200Cand 18 hours.The liquid product (32.4 g) were recovered and gc analysis showed: 29.8 wt % ethyl propionate 12.3 wt % n-propyl propionate 2.0 wt % methyl propionate 8.9 wt % ethanol 26.8 wt % unreacted propionic acid The selectivities to ethyl and n-propyl propionate were calculated to be: 42 mole % ethyl propionate 15 mole % n-propyl propionate The conversion of propionic acid was calculated to be 66 mole %.

Example 7 The procedure of Example 4 was repeated with the exception that the catalyst components were utilized as follows: ruthenium oxide hydrate (1 mm, 0.19 g), tetra-n-butylphosphonium bromide (10 mm, 3.4 g), cobalt(ll) iodide (0.25 mm, 0.080 g) and propionic acid (162 mmole, 12.0 g). The reaction conditions were 6350 psi (438.2 bars) pressure of CO:H2=1 :1, 2200C and 18 hours. The liquid product recovered (25.7 g) was analyzed by glc.

29.2 wt % ethyl propionate 12.8 wt % n-propyl propionate 1.8 wt % n-butyl propionate 4.8 wt % methyl propionate 14.2 wt % ethanol 14.0 wt % unreacted propionic acid The product selectivities to ethyl and n-propyl propionate were calculated to be: 52 mole % ethyl propionate 20 mole % n-propyl propionate The conversion of propionic acid was 74 mole %.

The combined selectivity to ethyl and n-propyl propionate was estimated to be 72 mole %. The calculated total yield of ethyl and propyl propionate (basis propionic acid charged) is 53 mole %.

Example 8 This example illustrates an improved synthesis of ethyl and propyl propionate from synthesis gas, propionic acid and methanol using the catalyst system comprising the ruthenium-containing compound, a cobalt-containing compound and a quaternary onium salt or base, under conditions almost identical to those used in Example 1.

A glass liner was charged with ruthenium oxide hydrate (1 mmole, 0.19 g), n-heptyltriphenylphosphonium bromide (10 mmole, 4.25 g), dicobalt octacarbonyl (0.25 mmole, 0.085 g) and 5.2 grams of methanol (0.16 mole) and 12 grams of propionic acid (.16 mole). The glass liner was placed in a stainless steel reactor and purged of air with hydrogen and carbon monoxide (1 :1 ratio), then pressured up to 2000 psi (138 bars) and heated to 2200 C. The pressure was brought up to 6000 psi (414 bars) and during the reaction period, the constant pressure was maintained by using a surge tank. After 18 hours, the reactor was allowed to cool, the gas pressure (3300 psi-228 bars) noted, the excess gas vented and the liquid products recovered.

The liquid products (21.8 g) were analyzed by glc as follows: 43 weight percent ethyl propionate 7.9 weight percent n-propyl propionate 4.1 weight percent methyl propionate 3.9 weight percent ethanol 0.4 weight percent unreacted methanol 24.6 weight percent unreacted propionic acid.

Ethyl and n-propyl propionate selectivities were calculated to be: ethyl propionate 69 mole % n-propyl propionate 11 mole % Total ethyl and n-propyl propionate selectivity=80 mole %.

Ethyl and n-propyl propionate yields, basis on propionic acid charged were calculated to be: ethyl propionate 45 mole % n-propyl propionate 7 mole % Total ethyl and n-propyl propionate yieid=52 mole %.

The conversion of propionic acid was 65 mole %.

It may be noted that: 1. The total yield of ethyl and n-propyl propionate (30 mol %) in Example 1 is lower than the 52 mol % achieved in Example 8 using methanol as the coreactant.

2. Selectivity to ethyl and n-propyl propionate (81 mol % total) in Example 1 is similar to the figure (80 mol %) in Example 8.

Example 9 A glass liner was charged with ruthenium oxide hydrate (1 mmole, 0.19 g), n-heptyltriphenylphosphonium bromide (10 mmole, 4.25 g), dicobalt octacarbonyl (0.25 mmole, 0.085 g), methanol (162 mmole, 5.2 g), propionic acid (162 mmole, 12.0 g) and p-dioxane (10.0 g). The glass liner was placed in a stainless steel reactor and purged of air with hydrogen and carbon monoxide (1:1 ratio), then pressured to 2000 psi (138 bars) and heated to 2200C. The pressure was brought up to 6300 psi (435 bars) and during the reactive period, the constant pressure was maintained by using a surge tank. After 18 hours, the reactor was allowed to cool, the gas pressure (3500 psi-242 bars) noted, the excess gas vented and the liquid products recovered (30.3 g).

The liquid products were analyzed by glc as follows: 30.4 weight percent ethyl propionate 6.2 weight percent n-propyl propionate 3.9 weight percent methyl propionate 2.3 weight percent ethanol 11.4 weight percent unreacted propionic acid 0 weight percent unreacted methanol 34.5 weight percent p-dioxane Ethyl and n-propyl propionate selectivities were calculated to be: ethyl propionate 57 mole % n-propyl propionate 10 mole % Ethyl and n-propyl propionate yields, based on propionic acid charged, were calculated to be: ethyl propionate 45 mole % n-propyl propionate 8 mole % The conversion of propionic acid was 78%.

Example 10 Following the procedure of Example 8, the synthesis of ethyl and propyl propionate was repeated with the exception that 10 grams of diphenyl ether was included in the reaction mixture as inert solvent. The pressure in the reactor during the desired synthesis was maintained at 6100 psi (421 bars) and the temperature was maintained at 2200C. The liquid product (31.7 g) was recovered at the conclusion of the reaction, and analysis by glc showed the following results: ethyl propionate selectively 67 mol % n-propyl propionate selectivity 21 mol % methyl propionate selectivity 7 mol % Total ethyl and n-propyl propionate selectivity is therefore 89 mol %. Ethyl and n-propyl propionate yields (based on propionic acid charged) were calculated to be: ethyl propionate 48 mol % Propyl propionate 15 mol % Propionic acid conversion was 72%.

Example 11 Example 8 was repeated with the exception that the catalyst system contained 1 mmole of ruthenium oxide hydrate (0.19 g), 10 mmole of ntetrabutylphosphonium bromide (3.4 g) and 1 mmole of cobalt(lil) acetylacetonate (0.36 g) and the reaction mixture contained 7.8 g of methanol and 10 g of propionic acid. Pressure was maintained at 6575 psi (454 bars) and the temperature at 221 0C for 18 hours. The liquid product (23.8 g) obtained at the conclusion of the reaction was analyzed and results were as follows: ethyl propionate selectivity 52 mole % n-propyl propionate selectivity 6 mole % ethyl propionate yield 44 mole % n-propyl propionate yield 5 nrole % Total ethyl plus propyl propionate yield=49 mole % Propionic acid conversion was estimated to be 84%.

Example 12 Example 8 is repeated with the exception that the ruthenium dioxide hydrate is replaced with equivalent amounts of triruthenium dodecacarbonyl, ruthenium acetate and ruthenium(lll) acetylacetonate. Related results are obtained.

Example 13 Example 8 is repeated with the exception that the propionic acid is replaced with equivalent amounts of acetic acid. Related results are obtained.

Example 14 Example 8 is repeated with the exception that the cobalt carbonyl is replaced with equivalent amounts of cobalt(ll) acetate and cobalt(lil) acetylacetonate. Related results are obtained.

Claims (21)

Claims

1. A process for preparing lower alkyl esters of carboxylic acids which comprises contacting a reaction mixture of the desired carboxylic acid, carbon monoxide and hydrogen with a catalyst composition comprising a ruthenium-containing compound, a cobalt-containing compound and a quaternary onium salt or base, and heating the resulting mixture at an elevated temperature and pressure for sufficient time to produce the desired alkyl ester of the carboxylic acid.

2. A process as claimed in claim 1, wherein the reaction mixture also comprises methanol.

3. A process as claimed in claim 1 or 2, wherein the carboxylic acid is an aliphatic monocarboxylic acid containing from 1 to 12 carbon atoms.

4. A process as claimed in any preceding claim, wherein the carboxylic acid is an aliphatic dicarboxylic acid containing up to 12 carbon atoms.

5. A process as claimed in any preceding claim, wherein the ester to be formed is an ethyl or propyl ester.

6. A process as claimed in any preceding claim, wherein the ruthenium-containing compound is one or more oxides of ruthenium, a ruthenium complex of carbonyl-containing ligands, a ruthenium salt of an organic acid or a rutheniumcarbonyl or hydrocarbonyl compound.

7. A process as claimed in claim 6, wherein the ruthenium-containing compound is anhydrous ruthenium(lV) dioxide, ruthenium(lV) dioxide hydrate, ruthenium(VI II) tetraoxide, ruthenium acetate, ruthenium propionate, ruthenium(lil) acetylacetonate, or triruthenium dodecacarbonyl.

8. A process as claimed in any preceding claim, wherein the cobalt-containing compound is a cobalt halide, cobalt nitrate, cobalt perchlorate, a cobalt salt of a monocarboxylic acid containing up to 10 carbon atoms, or a cobalt oxide.

9. A process as claimed in any of claims 1 to 7, wherein the cobalt-containing compound is a cobalt carbonyl or a derivative thereof obtained by reacting the carbonyl with a group V donor ligand, a cobalt carbonyl hydride, a cobalt carbonyl halide, a cobalt nitrosyl carbonyl, a cycloalkadienyl cobalt carbonyl, or a cobalt salt of an organic carboxylic acid.

1 0. A process as claimed in any preceding claim, wherein the cobalt-containing compound is a cobalt compound having at least one cobalt atom linked to at least three separate carbon atoms.

11. A process as claimed in any preceding claim, wherein the cobalt-containing compound is a cobalt carbonyl.

12. A process as claimed in any preceding claim, wherein the cobalt-containing compound is a cobalt halide.

13. A process as claimed in any preceding claim, wherein the cobalt-containing compound is cobalt perchlorate.

14. A process as claimed in any preceding claim, wherein the cobalt-containing compound is cobalt(lll) acetylacetonate.

1 5. A process as claimed in any preceding claim, wherein the quaternary onium salt or base is a quaternary phosphonium salt.

1 6. A process as claimed in any preceding claim, wherein the quaternary onium salt or base is a quaternary ammonium salt.

1 7. A process as claimed in any preceding claim, wherein the reaction is carried out in the presence of an inert solvent.

18. A process as claimed in claim 17, wherein the inert solvent is selected from 1 ,3-dioxane, 1,4-dioxane, dipropyl ether, diethylene glycol dimethyl ether and dibutyl ether.

1 9. A process as claimed in any preceding claim, wherein the catalyst components are utilized in the following molar ratios: rutheniumcontaining compound 0.1 to 4 moles; cobaltcontaining compound 0.025 to 1.0 moles; quaternary onium salt or base 0.4 to 60 moles.

20. A process as claimed in any preceding claim, wherein the reaction is conducted at a temperature between 1 500C and 3500C.

21. A process as claimed in any preceding claim, wherein the process is conducted at a superatmospheric pressure between 69 and 51.8 bars.