CA1212688A - Process for preparing alkyl esters of carboxylic acids from an acid and syngas using a novel catalyst composition - Google Patents

Abstract

PROCESS FOR PREPARING ALKYL ESTERS OF
CARBOXYLIC ACIDS FROM AN ACID AND
SYNGAS USING A NOVEL CATALYST COMPOSITION
ABSTRACT
Lower alkyl esters of carboxylic acids are prepared in good yield from a carboxylic acid and syngas by con-tacting a mixture of the carboxvlic 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 carboxy-lic acid ester, and then recovering the same from the reaction mixture. Methanol is an additional and preferred reactant.

Description

~231 Z6138 PROCESS FOR PREPARING ALKYL ESTERS OF
CARBOXYLIC i!5CIDS FP~OM AN ACID hND
S~NGAS USING A NOVEL CP.TALYST COr~lPO~ITION
This invention relakes to a ne~r process ~o~ prepa~
ing lower alkyl esters of carboxylic acids. r,~ore particularly, the invention relates to a new process for preparing lower alkyl esters of carboxylic acids ~rom zn acid and svngas using a novel catalyst composition.
Specifically, the invention provides a new and i~proved process for preparing lower alkvl esters of car-boxvlic acids, such as ethyl and propyl propionate, in good yield from the acid, such as propionic acid, carbon monoxide and hydrogen which comprises contactins a mixture of the carboxylic acid, carbon monoxide and hydrogen with a catalyst composition com,prising a ruthenium-containing compound, a cobalt-containing compound and a ~uaternary onium sa~t or base, and heating the resulting mixture at an elevated temperature and pressure for sufficient time to produce the desired alkyl carboxylic aci~ 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 eY.ample, in the production of anhydrides and in the production of ethylene and propylene. They may also be used as solvents and dil-uents 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 com-monly obtained from petroleum or agrichemical feedstocks.
A direct synthesis of the esters from syngas would be potentiallv more economical and highly desirable~
It has been proposed to prepare the lower alkyl .~

Z6~3~

alkanoates by carbonylation techniques, but these ~ethods up to the present have not been entirelv satisactory as they give low yields o the desired esters or us~ ~xpen-sive catalysts or catalysts that are difficlllt to utilize on a large scale. For example, U.S. 4,270,015 and re~er-ences cited therein disclose various catalyst s~stem~ for use in produciny esters by carbonylation. U.S. 4,270,015 discloses the ~reparation of ethyl esters from syngas using a ruthenium-Group VA ligand catalyst com~lex as catalyst. While this process produces the ethyl esters, there is a great deal to be clesired 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 anc 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 hydro-gen and possibly methanol with a catalyst co~position comprising a ruthenium-containing compoun~, a cobalt-containing compound and a quate~nary onium salt or base, and heating the resultin~ mixture at an elevated tempera-ture and pressure for sufficient ti~e to produce thedesired 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-containinq compound as cocatalyst gives improve~ selec-tivity in the ormation of the desired ethyl and propylcarboxylic 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 35 the equations:
(1) 2 CO + 4 H + RCOOH - ~ C H OOCR + 2 H20 ~2 ) CO + 2 H2 + RCOOH + CH30H ~ C2H500CR ~ 2 H20 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% ~o 53~ in the absence o methanol. In the presence of methanol~ typical converslon of the carboxylic acid ranges from 65~ to about 84%, with the total yield of the ethyl and n-pro.~yl 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 by-products such as the methyl and butyl esters, are produced concur-rently from ~he carboxylic acid and svngas 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 rutheniu~-containing compoun~, a cobalt-containing com-pound and a quaternary onium salt or base, (b) heating the saicl mixture to an elevated tern-perature, above 150C, 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 substan-tial formation of the desired ester has been achieved~ and ~5 (c) preferably isolating the said ester an~ minor by-products from the reaction mixture, as by distillation.
In order to present the inventi~e concept of the present invention in the greatest possible detail, the following supplementary disclosure is submitted. The process of the invention i5 practiced as follows:
As noted, the new catalyst system used in the pro-cess of the invention contains a ruthenium-containing compound, a cobalt-containing compound and a auaternary 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 mix-ture in an oxide forrn, as in the case of J for ex~nple, Z6~8 ru-thenium(IV) oxide hydrate, anhydrous ruthenium(IV) dioxide and ruthenium(VIII) tetraoxide. Alternatively, it may be added as the salt of a mineral aaid, a.s ~n the case of ruthenium(III) ehloride hydrate, ruthenl~(III) bromide, rutheniuM-(III) iodide, tricarbonylruthenium ni-trate, or a~ ~he sal-t of a s~litable oryanic carboxylic acid, f.or example, ruthenium(III) acetate, ruthenium naphthenate, ruthenium valerate and ruthenium complexes with carbonyl~containing liyands such as ruthenium(III) acetylacetonate. The ruthenium may also be added to the reae-tion zone as a carbonyl or hydrocarbonyl derivative. Here, suitable examples include, among others, triruthenium dodecacarbonyl and other hydrocarbonyls such as H2Ru4(CO~13 and H4Ru4(C0)12, and substituted carbonyl species such as the tri-carbonylruthenium(II) chloride dimer, (Ru(CO)3C12)2.
Preferred ruthenium~containing compounds include oxides of ruthenium, ruthenium salts of an organic carboxylic aeid and ruthenium carbonyl or hydrocar-bonyl derivatives. Among these, partieularly preferred are ruthenium(IV~ dioxide hydrate, ruthenium(VIII) tetraoxide, anhydrous ruthenium(IV) oxide, ruthenium acetate, ruthenium propionate, ruthenium(III) 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, Co3O4, CoO, cobalt-(II) bromide, cobalt(II) iodide, cobalt(II) thiocyanate, cobalt(II) hydroxide, cobalt(II) carbonate, cobalt(II) nitrate, cobalt(II) phosphate, cobalt acetate, cobalt(III) acetoacetonate, cobalt naphthenate, cobalt benzoate, cobalt valerate, cobalt cyclohexanoate, cobalt carbonyls, such as dicobalt octacarbonyl Co2(CO)8, tetracobalt dodecacarbonyl Co4(CO)12 and hexacobalt hexadocacarbonyl Co6(CO)16 and derivatives thereof by reaction with ligands, and preferably group V donors, ~'Z~Z6f~8 such as the phosphines, arsines and stibi.ne derivatives suah as (Co~CO)3L)2 wherein L is PR3, ASR3 and SbR3 wherein R is a hydrocarbon radical, aobal-t car-bonyl hydrides, cobal-t carbonyl halides, cobalt nitrosyl carbonyls as Co~O~C0)3, Co(NO)(CO)2PPh3, cobalt nitrosyl halides, a cycloalkad:ienyl cobalt carbon~1, a cobalt salt of an organic carboxylic acid, organometallic cornpounds obtained by reacting cobalt carbonyls ~ith olefins, allyl and acetylene compounds, such as bis(lr-cyclopentandienyl) cobalt (lrC5H5)2Co, 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, tetracobalt 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, perchlor-ates, 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 composi-tion may be any onium salt or base, but are preferably those containing phosphor-ous or nitrogen, such as those of the formula 26~3~

R2 Y R~J X-~.~
wherein Y is phosphorous or nitro~en, ~ R2~ R3 and R4 are oryanic radicals preferably alkyl, aryl or alkaryl radicals, and X is an anionic 5pecies. The organic rac1i-cals use~ul in this instance include those raaicals havingfrom 1 to 20 carbon atoms in a branched or linear alkvl chain, such as methyl, ethyl, n-butyl, isobutvl, octyl,

2-ethylhexyl and ~o~ecyl radicals. etraethvlphosphonium bromide and tetrabutylphosphonium bromide are typical exarnples presently in cormercial production. The corres-ponding quaternary phosphonium or a~nonium acetates, hydroxides, nitrates, chromates, tetrafluoroborates an~
other halides, such as the corresponding chlorides, and iodides, are also satisfactory.
Equally useful are the phsophoni~n and ammonium salts containing phosphorous or nitrogen bonde~. to a mix-ture of alkyl, aryl and alkaryl radicals, which radicals preferably contain from 6 to 20 carbon atoms~ The aryl ra~i~al is most commonly phenyl. The alkaryl group may comprise phenyl substituted with one or more Cl to C10 alkyl substituents, bonded to phosphorous or nitrogen tihrough the aryl function.
Illustrative examnles of suitable quaternarv oni~n salts or bases include tetrabutylphosphonium bromide, hep-tyltri~henylphosphonium bromide, tetrabutylphosphoni~niodide, tetrabutylammonium chloride, tetrabutylphosphonium nitrate, tetrabutyl~hosphonium hydroxi~e, tetrabutylphos-phonium chromate, tetraoctylphosphoni~m tetrafluoroborate, ~etrahexylphosphonium acetate and tetraoctyl~nonium bromide.
The preferred quaternary oniurn salts and bases to - be used in the process comprise the tetralkylphosphonium ~Z~;~6~38 salts containing alkyl groups having 1 to 6 carbon atom~, such as methyl, ethyl, butyl, hexyl, hepkyl and isobutyl.
Tetralkylphosphonium salts, such as ~he halides, bromides, chlorides and iodides, and the acetate and chromate salts and hydroxide base, are the most preferred.
The quanti.ty of the ruthenium-contalning compouncl and the cobalt-containiny compound to be used in the pro-cess 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 com-pound and the active cobalt-containing compound which gives the desired product in a reasonable yield. The reac-tion proceeds when employing as little as about 1 x 10 6 weight percent, and even lesser amounts of the ruthenium-containin~ oompound, together with as little as about1 x 10 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 pres-sures of carbon monoxide, operating temperature, etc. Aruthenium-containing compound concentration of from about 1 x 10 5 to about 10 weight percent in conjunction with a cobalt-containing compolmd concentration of from about 1 x 10 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 pro-cess of the invention, the molar ratio of the ruthenium-containing compound to the quaternary onium salt or basewill 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 guarte~nary onium salt or base 0.4 to .2~

60 moles, and still more preerably when the component~
are combined in the following molar ratios: ruthenium-containing compound 1 to 4 moles, cobal~ containlng com-pound Q.25 to 1.0 moles a~d the quaternary onium b~se or salt 10 to 50 moles.
The carboxylic acid used in the process of the i~ven-tion forms the acid moiety of the desirec1 alkyl ester.
Suitable carboxylic acids include the ali.phatic aci~s, alicyclic monocarboxylic acids, heterocyclic acids and aromatic acids, both substituted ~nd unsubstituted.
Examples of such aci~s include, among others, the lower monoaliphatic carboxylic acids, such as formic acid, acetic, propionic, butyric, isobutyxic, valeric, caprioic, capric, perlargonic and lauric aci~s, tog0ther with ~icarboxylic acids, such as oxalic, malonic, 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 incluae acetoacetic acid, dichloroacetic acid and trifluoroacetic acid, chloropropionic acid, trichloro-acetic aci~, mono~luoroacetic acid and the like. A~.ong ~he suitable aromatic acids contemplated are benzoic acid, naphthoic aci2s, toluic acids, chlorobenæoic acids, amino-benzoic acids and phenylacetic acid. The acyclic mono-carboxylic acids may contain from 3 to 6 carbon atoms in the ring, both substitute~ or unsubstituted, and may con-tain one or more carboxyl groups, such as cyclopentane-carboxylic acid and hexahydrobenzoic acids. The hetero-cyclic acids may contain 1 to 3 fused rings both substi-tuted and unsubstituted together with one or more carboxy-lic 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. Anhydriaes of the acids can also be used.
Preferred carboxylic acids include the lower mono-carboxylic acids containing from 1 to 12 carbon atoms, and ~26~3~

the halo, alkoxy, ¢yano, alkylthio and amino-substituted monocarboxyli~ acids containing up to 12 carbon atoms, and the dicarboxylia acids con-tainirly up to 12 carbon atoms.
rrhe amoun-t o~ the carboxylic acid to be used in the process oE ~he ~nv~
ention may vary over a wide rancJe. In general, the amoun-t o~ ~aid -to be used should be sufficient -ko satisfy ~he stoichiometry of the formation of -the esters as shown in e~uation 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 pre-paration of the desired alkyl esters of carboxylic acid. Suitable solvents mayinclude 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 reac-tion conditions, should be relatively non-polar. Preferably, the solvent will have a boiling point greater than that of the ester and other products of the reaction 50 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 ben-zoate, 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 cyc-lohexanone, 2-methylcyclohexanone, as well as acyclic ketones, such as 2-penta-none, butanone, acetophenone, etc. Ethers which may be utilized as solvents inc-lude cyclic, acyclic, and heterocyclic materials. Preferred ethers are the hetero-cyclic ethers as illustrated by l,4-dioxane and 1,3-dioxane. Other suitable e-th-_9_.

. ;

6~38 ers include isopropyl propyl ether, diethylene glycol, dimethyl e-ther, di~utyl ether, diphenyl -9a-~2~Z6~

ether, ~ibutyl ether, heptyl phenyl ether, anisole, te~ra-hydrofuran, etc. The most useful solvents o all o khe above ~roups include the ethers, as diphenyl ether and 1,4-dioxane, etc.
The amount of the solvent emplovecl may varv as desired. In general, it is desirable to use suf~icient solvent to 1ui~ize the catalyst system.
The relative amounts of carbon monoxide and hydro-gen which can be initially present in the svngas 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 monoxide-hydrogen gaseous mixtures may also be used in con-junction 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, hyarocarbons, such as methane, ethane, ~ropane and the ~ike, ethers such as dimethyl ether, methylethyl ether and dimethyl ether, and higher alcohols.
The temperature ran~e which can usefully be employed in the process of the in~ention may vary over a consider-able range de~ending upon experimental facts, including the choice of catalyst, pressure and other variables. The preferred temperatures are above 150C and more preferably between 150C and 350C when superatmospheric pressures of syngas are employed. Coming under special consideration are the temperatures ranging from about 180C to about 250C.
Superatmospheric pressures of about 500 ~si (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 ~Z~%688 7500 (517.5 bars) also provlde useful yields of the desir-ed products. The pressures reerred to herein represent the total pressure generated bv all the reactants, although they are substan~ially due to the carbon monoxid~
and hydro~en reactants.
The desired products o ~he reaction, the ethyl and prop~l esters of the desired alkanoic acids, will be for~ed in significant auantities varying generally from about 22~ to about 63% in yield. Also formed ~ill be minor by-products, such as the ~.ethyl and butyl esters of those alkanoic acids as well as other oxvgenated products. The desired products can be recovered from the reaction mixture by conventional means, such as fractional aistillation in vacuo, etcO
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 batch-wise~ or it may be continuously or intermittently intro-duced into such a zone during the course of the synthesis reaction. Operating conditions can be adjusted to optimize the formation of the desired estersl and said material can be recovered by methods known to the art, such as distil-lation, fractionation, extraction and the like. A frac-tion rich in the catalyst Gomponents maV then be recycled to the reaction zone, if desired, and additional product generated.
The products ha~e 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 po~nds per square inch (psi) and bars.

~ZlZG1~8 To îllustrate ~he process o~ the inven~ion, the following exa~ples are given. It is to be understood, however, that the ex~mples are given in the way o illus-tration and are not to be regarded as limiting the ln~en-tion in any way.
E X ~ M P L E
This example illustrates the proved selestivity to ethyl and propyl esters from synthesis gas plus the appropxiate carboxylic acid that may be achieved using the new class of catalyst compositions comprising a ruthenium-containing compound, a cobalt-containing compound an~. a quaternary onium salt.
A glass liner was charged with hydrated ruthenium oxide hydrate (0.19 grams, 1.0 mmole), n-heptyltriphenyl-15 phosphonium bromide (4.25 grams, 10 mmoles), dicobaltoctacarbonyl (0.085 grams, 0.25 mmole) and propionic acid (10.0 grams, 135 mmoles). The glass liner was placed in a stainless steel reactor an~ purged of air with hydrogen and carbon monoxide (1:1 molar ratio), then pressured to 2000 psi (138 bars) and heated to 220C. The pressure was brought up to 6280 psi ~433.3 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 (3950 psi - 272.6 bars) noted, the excess gas sampled and vented and 16.9 g of the liquid products recovered.
Analysis of ~he pro~uct liquid fraction by gas-liqui~ chromatography (g/c~ showed the presence of:
30.3% ethyl propionate 3015.6~ n-propyl propionatP
2.4% methyl propionate 1.9% n-butyl propionate 41.4% unreacted propionic acid Ethyl and propyl propionate selectivities were cal-culated to be:
ethyl propionate 56 mole % selectivity n-propyl propionate 25 mole % selectivity ~Z~6~8 , Total ethyl ~ n-propyl propionate selectiv-ity - 81 mole %.
Ethyl and n-propyl propionate yields, ~a~ic pro-pionate acid charged, were calculate~. to be:
ethyl propionate 27 mole n-propyl propionate 12 mole %
The total ethyl and n-propyl nropionate yield was 33 mole %. Conversion of propionic acid is estimated to be ~9 mole %.
C O M P P R A T I V E E X A M P I. E A
In this comparative example the synthesi~ of eth~l and n-propyl propionate esters from synthesis g~s plus propionic acid is illustrated using a two component cat-alyst system comprising a rutheniu~-containing compound and a quaternary onium salt. There is no cobalt-containing compound present in this comparative example.
The results are substantially the same as those disclosed in U.S. Patent 4,270,015, Example lo To an 850 ml glass-lined autoclave reactor equip-ped for pressuring, heating, cooling and means of agita-tion is charged 0.764 gm of ruthenium(IV) oxide, hy~rate (4.0 mmole), 17.64 gm of heptyl(trihenyl)~hosphoni~m bromide (40 mmole) and propionic acid (50 gm). Upon stirring under a nitrogen atmosphere mo~t of the solids dissolve to give a deep-red solution. The reactor is then sealed, flushed with C0/H2, pressured to ~00~ psi (138 bars~ with synthesis gas ta 1:1 mixture of hydrogen and car~on monoxide1 and heated to 220C with agitation. At temperature~ the pressure within the reactor is raised to 6300 psi (435 bars) with C0/H2 mix, and the pressure held constant throughout the 18 hour run by automatic a~dition o~ more synthesis gas from a large surge tank. Upon cool-ing, the excess gases are sampled ~nd vented, and the deep-yellow liquid product t73.8 ~m) removed for analysis.
35 There is no solid product fraction.
Analysis of the liquid fraction by gas-liquid chromatography (glc) shows the presence of:

~2~Z~8 38.2 wt ~ ethyl propionate 16.5 wt ~ methyl propionate 8.4 wt ~ n-propyl propionate 0.8 wt % n butyl p~opionate 0.9 wt % glycol dipropionate 2.7 wt % water 27.8 wt % unreacte~. propionic acia.
H~re the calculated ethyl and propyl propionates selectivitles were estimated to be for this example:
ethyl propionate 47 mole % sPlectivity n-propyl propionate 9 mole % selectiYity Total ethyl plus n-propyl propionate selec tivity - 56 mole %.
It may be noted that:
The total selectivity to ethyl and propyl propion-ate (56%~ in this comparativ2 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 ~icobalt octacar-bonyl.

The procedure of Example 1 was repeated with the exception that the catalyst ~omponents were utilized as follows: ruthenium oxide hydrate (1 mmole, 0.19 g), n-heptyltriphenylphosphonium bromide (10 mmoles, 4.25 g),cobalt(II) 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, 220C and 1~ hours.
The recovered liquid product (19.1 g) was analysed 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 pxoduct selec-~ivities to ethyl and. n-propyl propionate were calculated to be:

.. ; , .~ -L2~8~

53 mole % ethyl propionate 28 mole ~ n~propvl propionate.
The combined se'Lectivlty o~ ethyl- and n-propyl propionate was 81% at 27% pro~ionic acid conversion The estimated total yie.ld of ethyl and propyl p~opionates ~base propionic acid charged) is 22 mole %.

_ A glass liner was charged with hydrated ruthenium oxide hyflrate (1 mmole, 0.19 g), n-heptyltriphenylphos-phonium bromide (10 mmoles, 4.25 g), dicobalt octacarbonyl (o.25 mmole, 0.085 g), p~opionic acid (12.0 g) and 1,4-dioxane 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 220C. The pressure was brought up to 6500 psi (44B.5 bars) and maintained by using a surge bnk. 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 analysed 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-aioxane The results were calculated in terms of pro~uct selectivities and absolute yields.
ethyl propionate 4~ mole % selectivity n-propyl propionate 24 mole ~ selectivity.
The total molar sel'ectivi~y to ethyl ana n-propyl propionate was 66 mole ~ and the conversion of propionic acid was 35 mo'le %.
E X A M P ~ E 4 A glass liner was charged with hydrated ruthenium oxide (2 m~oles, 0.38 g), tetra-n-butylphosphonium bromide (20 mmoles, 6.8 g), cobalt(III) acetylacetonate (1.0 mmole, 0.36 g) and 25 g of propionic acid. The glass liner was placed in a stainless steel reackor and purged o~ air ~7i~h hydrogen and carbon monoxide (1:l ratlo~, then pre~5ured to 2000 psi ~138 bars) and heated to 220C. The pre~sure was brought up to 6600 psi (455.4 bar~) and maintained by using a surge tank. A~ter 18 hours, the reactor was allowed to cool, the gas pressure (3420 psi - 236 bars) noted, the excess gas was ven~ed and the liquid product recovered (44.5 g).
The liquid products were analysed by glc as follows:
6.8 wt ~ methyl propionate 32.8 wt ~ ethyl propionate 15.8 wt % n-propyl propionate 1.5 wt ~ n-butyl propionate 15.2 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 selecti~ity of ethyl and n~propyl propionate was 63 mole % and the conversion of propionic acid was 78 mole %.

.
The procedure of Example 4 was repeated with the exception that the cataly~t components were utilized as follows: ru~henium oxide hydrate ~1 mmole, 0.19 g), tetra-n-butylphosphonium bromide (10 mmoles, 3.4 gl, cobalt~II) perchlorate (0.125 mmole, 0.046 g), cobalt(III) acetyl-acetonate (0.125 mmole, 0.046 g) and 12 ml of propionic acid. The reaction conditions were at 220C, 6400 - 5800 psi ~441.6 - 400 bars) o~ C0/H2 = 1:1 syngas and 18 hour reaction period. The liquid product ~19.7 g) was recov-ered and gc analysis showed:
37.2 wt % ethyl pro~ionate 10.4 wt % n-propyl propionate 15.6 wt % methyl propionate 26.3 wt % unreacted propionic aci~

~%6~8 Ethyl and n-propyl propionate selectivi~y ~lere calculated to be:
51 mole % ethyl propionate 12 mole % n-propyl propionate.
5The co~ined selectivity was ~2~ and the conversivn of propionic acid was calculated to he 67 mole %.

The procedure of Example ~ was re eated with the exceptlon that the catalyst components were utilized as follows: ruthenium oxide hydrate (2 mm, 0.38 g), tetra-n-bu~ylphosphonium bromide ~20 mmole, 6.8 g), cobalt(III) 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, 220C and 18 hours. The liquid product (32.4 g) were recovered and gc analysis showedO
29.8 wt % ethyl propionate 12.3 wt % n-propyl propionate 2.0 wt % methyl propionate.
20 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 2515 mole % n-propyl propionate The conversion of propionic acid was calculated to be 66 mole %.

The procedure of Example 4 W25 repeated with the exception that the catalyst componen-ts were utilized as follows: ruthenium oxide hydrate (1 mm, 0.19 g), tetra-n-butylphosphonium bromide (10 mm, 3.4 g), cobalt(II) iodide (o.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, 220C and 18 hours. The liquid product recovered (25.7 g) was analysed by glc.

~Z12688 .

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-pro~yl 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 Ibasis prop-ionic acid charged) is 53 mole %.

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 oni~m salt or base, under con-ditions almost identical to tho e used in Example 1.
A glass liner was charged with ruthenium oxide hydxate tl mmole, 0.19 g), n-heptyltriphenylphosphonium bromide (10 mmole, 4.25 g), dlcobalt 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 alr with hydrogen and carbon monoxide (1:1 ratio), then pres-sured up to 2000 psi (138 bars) and heated to 220C. Thepressure was brought up to 6000 psi (4~4 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.
~ he liquid products ~21.8 g) were analysed by glc 6~8 as follows:
43 weight percent ethyl propionate 7.9 weight percent n-propyl propionate 4.1 weight percent methyl propionate

3.9 we.ight percent ethanol 0.4 weight percent unreac~ed 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 ~ropionate selectiv-ity = 80 mole ~.
Ethyl and n~propyl propionate yields, basis on 5 propionic acid charged were calculated to be:
ethyl propionate 45 mole %
n-propyl propionate 7 mole %
~otal ethyl and n-propyl propionate yield = 52 mole ~. 0 The conversion of propionic acid was 65 mole %.
It may be noted that:
1. T~e total yield of ethyl and n-propyl pro~ion-ate (39 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.

A glass liner was charged with ruthenium oxide hydrate (1 mmole, 0.19 g), n-heptyltriphenylphosvhonium 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 (lO.o 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 220C. The pressure was brought up to 6300 psi (435 6~8 baxs) and during the reactive period, the constant pres-sure was maintained by using a surge kank. After 18 hours, the reactor was allowed to cool, khe yas pressure (3$00 psi - 242 bars) noted, the excess gas vented and the llquid products recovered ~30.3 g).
rrhe li~lid producks were analysed by glc a~ follows:
30.4 weight percent ethyl proplonate 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 selec~ivities were calculated to be:
ethyl propionate 57 mole %
n-propyl propionate lO mole %
Ethyl and n~propyl propionate yields, based on propionic acid charged, were calculated to be:
ethyl propionate 4S mole %
n-propyl propionate 8 mole %
The conversion of propionic acid was 78%.

Following the procedure of Example 8, the synthesis of ethyl and propyl propionate was repeatea with the exception that 10 grams of diphenyl ether was included in the reaction mixture as inert solvent. ~he pressure in the reactor during the desired synthesis was maintained at 6100 psi (421 bars) and the temperature was maintained at 220C. The liquid product (31.7 g) was recovered at the conclusion of the reaction, and analysis by glc showed the following resultso ethyl propionate selectivity 67 mol ~
n-propyl propionate selectivitv 21 mol %
methyl propionate selecti~ity 7 mol %
Total ethyl and n-propyl propionate selectivity is therefore 89 mol %. Ethyl and n-propyl propionate yields ~Z~88 based on propionlc acid charged) were calculated to be:
ethyl propionate 48 mol ~
propyl propionate 15 mvl %
Propionic acid conversion was 72%.
~ LI
Example 8 was repeated with the exception that the catalyst sy~t~m contalned 1 mmole of ruthenlum oxide hydrate (0.19 g), 10 mmole of n-tetrabutylphosPhonium bromide (304 g) and 1 mmole of cobalt~III) acetylaceton-ate tO.36 g) and the reaction mixture containecl 7~8 g ofmethanol and 10 g of propionic acid. Pressure was main-tained at 6575 psi (454 bars) and the temperature at 221C
for 18 hours. The liquid product (23.8 g) obtained at the conclusion of the reaction was analysed and results were 15 as follows:
ethyl propionate selectivity 52 mole %
n-propyl propionate selectivity 6 mole %
ethyl propionate yield 44 mole %
n-propyl propionate yield 5 mole %
Total ethyl plus propyl propionate yield = 49 mole %
Propionic acid conversion was estimated to be 84~.

Ex~mple 8 is repeated with the exception that the ruthenium dioxide hydrate is replaced with equivalent amounts o~ triruthenium dodecacarbonyl, ruthenium acetate and ruthenium(III) acetylacetonate. Related results æ e obtained.

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

Example 8 is repeated with the exception that the cobalt carbonyl is repla~ed w~th-equivalent amount~ of cobalt(II) acetate and cobalt(III) acetylacetonate.
Related results are obtained.

Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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-contain-ing 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 com-prises methanol.

3. A process as claimed in claim 1, wherein the carboxylic acid is an ali-phatic monocarboxylic acid containing from 1 to 12 carbon atoms.

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

5. A process as claimed in claim 1, 2 or 3, wherein the ester to be formed is an ethyl or propyl ester.

6. A process as claimed in claim 1, 2 or 3, wherein the ruthenium-contain-ing compound is one or more oxides of ruthenium, a ruthenium complex of carbonyl-containing ligands, a ruthenium salt of an organic acid or a ruthenium-carbonyl or hydrocarbonyl compound.

7. A process as claimed in claim 1, 2 or 3, wherein the ruthenium-contain-ing compound is anhydrous ruthenium(IV) dioxide, ruthenium(IV) dioxide hydrate, ruthenium(VIII) tetraoxide, ruthenium acetate, ruthenium propionate, ruthenium-(III) acetylacetonate, or triruthenium dodecacarbonyl.

8. A process as claimed in claim 1, 2 or 3, 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 claim 1, 2 or 3, 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 car-bonyl halide, a cobalt nitrosyl carbonyl, a cycloalkadienyl cobalt carbonyl, or a cobalt salt of an organic carboxylic acid.

10. A process as claimed in claim 1, 2 or 3, wherein the cobalt-containing compound is a cobalt compound having at least one cobalt atom linkaged to at least three separate carbon atoms.

11. A process as claimed in claim 1, 2 or 3, wherein the cobalt-containing compound is a cobalt carbonyl.

12. A process as claimed in claim 1, 2 or 3, wherein the cobalt-containing compound is a cobalt halide.

13. A process as claimed in claim 1, 2 or 3, wherein the cobalt-containing compound is cobalt perchlorate.

14. A process as claimed in claim 1, 2 or 3, wherein the cobalt-containing compound is cobalt(III) acetoacetonate.

15. A process as claimed in claim 1, 2 or 3, wherein the quaternary onium salt or base is a quaternary phosphonium salt.

16. A process as claimed in claim 1, 2 or 3, wherein the quaternary onium salt or base is a quaternary ammonium salt.

17. A process as claimed in claim 1, 2 or 3, wherein the reaction is carr-ied out in the presence of an inert solvent.

18. A process as claimed in claim 1, 2 or 3, wherein the reaction is carr-ied out in the presence of an inert solvent and the inert solvent is selected from 1,3-dioxane, 1,4-dioxane, dipropyl ether, diethylene glycol, dimethyl ether and dibutyl ether.

19. A process as claimed in claim 1, 2 or 3, wherein the catalyst compon-ents are utilized in the following molar ratios: ruthenium-containing compound 0.1 to 4 moles; cobalt-containing compound 0.025 to 1.0 moles; quaternary onium salt or base 0.4 to 60 moles.

20. A process as claimed in claim 1, 2 or 3, wherein the reaction is con-ducted at a temperature between 150°C and 350°C.

21. A process as claimed in claim 1, 2 or 3, wherein the process is conduc-ted at a superatmospheric pressure between 69 and 517.5 bars.