CA1146971A - Method of preparing aliphatic carboxylic acids - Google Patents

Abstract

D. 75,758-1-FB

METHOD OF PREPARING ALIPHATIC
CARBOXYLIC ACIDS

ABSTRACT

Carboxylic acids having 2 to 6 carbon atoms can be converted into their higher homologues by heating the acids with synthesis gas (carbon monoxide and hydrogen) at a pressure of at least 500 psi (35.5 bars) in the presence of a catalyst comprising a palladium or nickel compound (e.g. an oxide or salt of palladium or nickel, or a nickel carbonyl or hydrocarbonyl ) and a group VB ligand (e.g. triphenylphosphine), and an iodide or bromide promoter (e.g. a palladium or nickel halide, or a separately-included alkyl halide).
Homologous acids having one more carbon atoms than the starting acid are formed with good selectivity.

Description

D.75,758-1-FB

METHOD-OF PREPARING_ALIPHATIC
CARBOXYLIC ACIDS

This invention concerns a process for the preparation of carboxylic acids by homologation of aliphatic carboxylic acids with synthesis gas using a specific catalyst system.
There i5 an ever increasing ne~d for a wide variety of aliph~tic carboxylic acids of dif~ering carbon numbers and structures which have become important articles of commerce. The many processes leading to the preparation of these acids include oxidation of saturated ~nd unsaturated hydrocarbons, the carboxy-lation of monole~ins, particularIy ~-olefins, and dienes such a~ con~ugated dienes like 1,3-butadiene, and the carbonylation of lower aliphatic ~lcohols.
We now disclose a new preparative rcute to short-claim aliphatic acids involving the homologation of luwer molecular weight aliphatic carboxylic acids.
Homologation is effected by treatment of said carboxylic acids with synthesis gas (also known as syngas, a mixture of carbon monoxide and hydrogen).
-The homologation of carboxylic acids by means of synthesis gas in the presence of the speai~ic catalyst system of this invention has not, to our knowledge, been disclosed previously, but in co-pending application No. 356,050 th~ homologation of these same acids by means o~ synthesis gas and in the presence of a ruthenium-containing catalyst and an iodide. or bromide promoter has been set out. The homologation of saturated alkyl benzyl alcohols, and substituted benzyl alcohols, by synthesis gas to yield the corresponding higher molecular weight alcohols has been extensively studied.
Pertinent examples include the homologation of methanol to ethanol, and the conversion of ethanol to propanol, butanol and pentanol isomers (see: "Carbon Monoxide ~k .

in Organic Synthesis" by J. Falbe, pages 59-62 and 1. Wender, Catal. Rev. Sci~ Eng., 14, 97-129 (1976))~
Cobalt carbonyls, with or without phosphine or metal modifiers, are commonly used as catalysts in such alcohol homologation reactions (see: &erman Offenlegung-sschri~t No. 2,625,627 and U.S. Patent No. 4,111,837).
Related homogeneous cobalt carbonyl catalysts are ~lso effective for the synthesis of aliphatic carboxylic acid by carbonylation of lower aliphatic alcohols. More re~ently, soluble rhodium catalysts have become the c~talysts of choice in, for example, the synthesis of acetic acid by methanol carbonylation (Chem. Tech., p. 605, October 1971).
Other relevant homologation technology includes the recently reported homologation of dimethyl ether and methyl acetate to ethyl acetate (see: G. Braca et. al. 9, Amer. Chem. Soc., 100, 6238 (1978)).
One of the objects of this invention is to pro~ide . a no~el process of homologation of short-chain aliphatlc carboxylic acids to the hlgher homologues thereof by means of a unique catalyst system. The feedstock utili~ed in this process comprises synthesis gas along with the acid which is homologized~
This invention provides a process of preparing higher homologues of aliphatic carboxylic acids having

2-6 carbon ato~s which comprises heating the aliphatic carboxylic acid starting material with carbon monoxide and hydrogen in the presence of a catalytic amount of a palladium-containing or nickel-containing compound in combination with a Group VB tertiary donor ligand and in the presence of an iodide or bromide promoter at a superatmospheric pressure of at least 500 psi (35.5 bars).
The process of this invention can be illustrated by the homologation of acetic acid to higher acids according to equation 1:

CH COOH + CO/H - -C H 1 COOH (1) . . ~

-.

Other lower aliphatic acids, such as propionic acid ~d others having 2-6 inclusiYe carbon atoms may also be homologized by a similar procedure.
The process of this invention which involves preparing higher homologues of aliphatic carboxylic acids having ~-6 carbon atoms comprises the steps of contacting the aliphatic acid starting materials with at least a catalytic amount of a palladium-containing or nickel-containing compound in combination with a Group YB tertiary donor ligand c~d in the presence of an iodide or bromide promoter, and heating the resultant reaction mixture at a pressure of at least 500 psi ~35.5 bars) with carbon monoxide and hydrogen until substantial formation of the desired acids containing at least 3 carbon atoms has been achieved.
In carrying out the homologation reaction of this invention selectively~ to produce the higher homologues of the charged aliphatic carboxylic acids, it is de3irable to supply at least sufficient carbon monoxide and hydrogen to satis~y the stoichiometry of the desired higher carbo~ylic acid homologues, although excess carbon monoxide or hydrogen over the stoichiometric amounts may be present.
It has been found that the homologation reaction ~5 is effected only with a synthesis gas mixture, and carbon mono~ide alone is not sufficient ~contrary to prior art processes involving carbonylation of lower aliphatic alcohols to carboxylic acids).
In addition it has been found here that a iodide or bromide promoter ls necessary for acid homologation to take place according to the general scheme outlined ab~ve. Lastly, and surprisingly, it has been found that lower alkyl organic iodide or bromide promoters are much more effective than alkali metal iodides or bromides, such as cesium iodide.
The following discloses in greater detail the process of the present invention.

.
- .

Catalysts that are suitable for use in the practice of this invention contain palladium or nickel. They may be chosen from a wide variety of organic or inorganic compounds9 complexes, etc., as will be shown and illustrated below. It is only necessary that the catalyst precursor actually employed should contain palladium or nickel in any of its ionic states. The actual catalytically active species i5 then believed to comprise palladiwm or nickel in complex combination with carbon monoxide and hydrogen. The most effective catalyst is achieved where the palladium or nickel-hydrocarbonyl species is solubilized in the carboxylic acid co-reactant --employed to satisfy the stoichiometry of eq~ l.
The palladium catalyst precursors may take many different forms. For instance, the palladium may be added to the reaction mixture in an oxide form, e.~. palladium~II) oxide (PdO). Alternatively, it - may be added as the salt of a mineral acid, e.g. as palladium(TI) chloride (PdCl 2) palladium(II) bromlde (Pd~r2)~ palladium~ iodide (Pdl2), anhydrous palladium (11) ohloride (PdCl 2 ) or palladium nitrate (Pd (NO 2)2 XH 2)' or as the salt of a suitable organic carboxylic acid, for example, palladium(II) acetate or palladium(II) acetylacetonate.
Preferred palladium-containing compounds include oxides of palladium, palladium salts of a mineral acid and palladium ~alts or organic carboxylic: acids.
Among these, particularly preferred are palladium(II) acetate, palladium acetylacetonate and palladium oxide.
The nickel catalyst precursors may also take many dif~erent forms. For in~tance, the nickel may be added to the reaction mixture in an oxide form, e.g. nickel(II) oxide (NiO), nickel(III) oxide (Ni 23 6HA O) and nickel(II,III) oxide (Nio, Ni203).
Alternatively, it may be added as the salt of a mineral acid, e.g. nickel(II) chloride (NiCl 2 ~ nickel(II) , ,:
-~ ' ' ' .

chloride hydrate (NiGl 26H 2 )~ nickel(ll) bromide, (Ni~r2), nickel(ll) bromide hydrate (NiBr2X~120), nickel iodide (Nil 2 )~ or nickel(ll) nitrate hydrate (Ni(N0 3)2 6H 20); or as the salt of a suitable organlc carboxylic acld, ~or example, nickel(ll) formate, nickel~ll) acetate, nickel(ll) propionate, nickel(ll) naphthenate, nickel(lll) acetylacetonate, etc. The nickel may also be added to the reaction zone as a carbonyl or hydrocarbonyl derivative~ Here 7 suitable examples include n~cke~l carbonyl (Ni(C0)4), hydrocarbonyls and substituted carbonyl species such as bis-(triphenyl-phosphine) nickel dicarbonyl, and bis-~triphenylphosphite) nickel dicarbonyl.
Pre*erred nickel-containing compounds include oxides of nickel, nickel salts of a mineral acid, nickel salts of organic carboxyllo acids and nickel carbonyl or hydrocarbonyl deri~atives. Among these, particularly preferred are nickel~ll) acetylaceto~ate, nickel(ll~ acetate, nickel~ll) propionate, and nlckel carbonyl.
In this in~ention palladium or nickel can, if desired, be added to the reaction zone as one or more oxide, salt or carbonyl derivative specles as a complex with one or more Group VB tertiary donor llgands.
If de~ired, however, the ligand may be added separately to the reaction mixture. The key elements of the group VB ligands inolude nitrogen, phosphorus9 arsenic and antimony. These elements 7 in their trivalent oxidation states, particularly tertiary phosphorus and nitrogen, may be bonded to one or more aliphatic, cycloaliphatic, aromatic, substituted aromatic, aryloxy, alkoxy or mixed aliphatic-aromatic radicals, each containlng up to 12 carbon atoms, ~r they may be part of a heterocyclic ring system~ or he mixtures thereof.
Illustrative examples of suitable ligands that may be used in this invention include: triphenylphosphine, tri-n-butylphosphine, triphenylphosphite, triethyl-.. ' ' - , - .

; ~
' phosphite, trimethylphosphite, trimethylphosphine 9 tri-p-methoxyphenylphosphine, triethylphosphine, tri-methylarsine, triphenyLarsine~ tri-~-tolylphosphine, tricyclohexylphosphine, dimethylphenylphosphine, trioctyl-phosphine, tri-o-tolylphosphine, 19 2-bis(diphenyl-phosphino)ethane, triphenylstibine, trimethylamine) triethylamine, tripropylamine, tri-n-octyl~mine~ pyridine, 2,2'-dipyridyl, 1 t 10-phenanthrollne, quinoline, N,NI-dimethylpipera~ine, 1,8-bis(dimethylamino)naphthalene and N,N-dimethylaniline.
One or more of these palladium- or nickel-tertiary group VB donor ligand combinations may be preformed~
before addition to the reaction zone~ as in thle case, for example, of bis(triphenylphosphine)palladium chloride, bis(triphenylphosphine~palladium(ll) acetate and tetrakis (triphenylphosphine)palladium(O), bis(l~2-diphenyl-phosphino)ethane nickel(ll) chloride, dicarbonyl bis(triphenylphosphine)nickel and tetrakis (triphenyl-phosphite) nickel(O). AlternatiYely, the complexes may be ~ormed in situ, with the palladium- or nickel-containing co~pound and the ligand being added separately.
The preparation of such palladium- or nickel-tertiary group VB donor ligand combinations in complex form is more completely described in U.S. Patents 3,102,899 and 3.560,539.
The amounts of the group VB tertiary donor ligand employed with the palladium or nickel compound can be varied widely, e~g. from the stoichiometric amount required to form a complex wi-th the palladium or nickel compound, up to 5 or more ~imes the molar amount needed to form the complex.
The iodide or bromide promoter found necessary to effect khe desired acid homologation reaction may be in combined form with the palladium or nickel, as ~or instance in palladium(l~) chloride or nickel iodide but it is generally preferred to have an excess of halogen present in the catalyst system as a promoting - .

agent. By excess is meant an amount of halogen greater than three atoms of halogen per atom of palladium or nickel in the catalyst systemO This promoting component of the catalyst system may consist o~ a halogen9 and/or a halogen compound, that may be introduced into the reaction zone in a gaseous or liquid form or saturated in a suitable solvent or react,ant.
Satisfactory halogen promoters include hydrogen halides such as hydrogen iodide and gaseous hydriodic acid9 alkyl, aryl and ara~kyl halides containing up to 1?
carbon atoms such as methyl iodide, ethyl iodide, l-iodopropane, 2-iodobutane, l-iodobutane, ethyl bromide, iodob~nzene and b~nzyl iodide as well as acyl iodides such as acetyl iodide. Also suitable as halogen co-reactants are the quaternary armmonium and phosphoniumhalides; examples include tetramethylammonium iodide and tetrabutylphosphonium iodide. Alkali metal and alkaline ear~h metal halides, such as ceslum iodide, ma~ also be used but are generally not as effective as other listed promoters ~or this homologation.
Alkyl iodide or bromide promoters haYing 1 to 6 carbon atoms are the prcferred promoters for the pallad~um- or nickel-catalyzed acid h~mologation reaction of this in~ention. Most preferred are methyl iodide and ethyl iodide. .
Starting carboxylic acids useful in the process of this invention are aliphatic acids conta:ining 2 to 6 carbon atoms. Preferably, said acids are also useful as solvents for the palladi~m or nickel catalysts.
Suitable carboxylic acids include acetic, propionic, butyri~ iso~utyric~ valeric, trimethylacetic and caproic, together with aliphatic dicarboxylic acids having 2 to 6 carbon atoms, such as oxalic, malonic, succinic and adipic acids. The invention further includes the use of substituted aliphatic acids containing one or more functional substituents, such as the lower alkoxy, chloro, fluoro, phenyl, substituted phenyl, cyano, alkylthio, and amino functional groups, examples . ~ .

of whieh include acetoacetic acid, dichloroacetic and trifluoroacetie acid, chloropropionic acidl trichloro-acatic acid, monofluoroacetic acid and the like.
Mixtures of said carboxylic acids, in ~ny ratio, may also be used in the inventive process. The preferred carboxylic acids homologized in accordance with this invention are acetic aeid and propionic acid, with acetic acid being most preferred.
The quantity of palladium or nickel compound -- 10 employed in the in~tant invention is not critical and may vary over a wide range. In general, the no~el process is desirably conducted in the presence of a eatalytically effective quantity of one or more of the active palladium or nickel species whieh glves the desired products in reasonable yields. The reaetion proeeeds when employing as little as 1 x 10 6 weight percent and even lesser amounts of palladium or nickel, basis the total weight of the reaction mixture~ The upper concentrakion is dictated by a variety of factors including catalyst cost, partial pressures of carbon mo~oxide and hydrogen, operatin~ temperature and choice o* carboxylic acid diluent/reactant. A catalyst concentration of from 1 x 10 5 to 10 wei8ht percent of palladium or nickel, based on the total weight o~ reaction mixture, i5 generally desirable in the practice of this invention.
The temperature range which can usefully be employed in these syntheses is a variable dependent upon other experlmental factors, includlng the choice of carboxylic acid co-reactant, the pressure, and the concentration and choice of particular sp~cies of catalyst, among other things~ The range of operability is from 100 to about 350C, when superatmospheric pressures of syngas are employed. A narrower range of about 180 to about 250C represents the preferred temperature range~

.
- , , , . . -- ~
.

~ . . ... ~

Superatmospheric pressures of at least 500 psi (35.5 bars) lead to substantial ~ields of desirable alipha-tic carboxylic acid higher homolo~ues by the process of thls invention. A preferred operating 5 range i5 from 1000 to 7500 psi ~69.9 to 518.2 bars), althnugh pressures above 7500 psi (518.2 bars) also provide useful yields of the desired acid. The pressures referred to here represent the total pressure generaked - by all the reactants, although they are substantially due to the carbon monoxide ancl hydrogen fractions in these examples.
The relative amounts of carbon monoxide and hydrogen which may be initially present in the syngas mixture are varlable~ and these amounts may be varied over a wide range. In general 9 the mole ratio of C0 H2 is in the range from 20:1 up to 1:20, preferably from 5:1 to 1:5, although ratios outside these ranges may also be employed. Particularly in continuous operations, but also in batch experiments 9 the carbon monoxide-hydro~en gaseous mixtures may also be used ~n con~unction wlth up to 50% by volume o~ one or more other gases.
These other gases may include one or more inert gases such as nitrogen 9 argon, neon and the like, or they may include gases that may, or may not, ~ndergo reaction under C0 hydrogenation conditions, such as carbon dioxide, hydrocarbons such as methane, ethane, propane and the like, ethers such as dimethyl ether, methylethyl ether and diethyl ether, alkanols such as methanol and esters such as methyl acetate.
In all these syntheses in order to achie~e a high degree of selectivity the amount of carbon monoxide and hydrogen present in the reaction mixture should be su*ficient to at least satisfy the stoichiometry of eq (1). Excess carbon monoxide and/or hydrogen over the stoichiometric amounts may be present, if desired.
As far a~ can be determined, without limiting the invention thereby, the one-step acid homologation process disclosed herein leads to the formation of .

.

' L~7~

acid products primarily containing one carbon atom more than the starting materi~l. Minor amounts of higher acid homologues containing two or three additional carbon atoms are also usually present. When acetic acid is the co-reacta~t, the principal products are propionic acid, butyric acid- and valeric acid. By-products such as water and ethyl acetate are also detected in the liquid product fraction. When propionic acid is the reactant acid, the principal products are _-butyric acid and iso-butyric acid. The ratio of isomeric n to iso acids is commonly about 3:1.
._ The novel process of this invention can be conducted in a batch, semi-continuous or continuous fashion.
The catalyst may 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 synth~sis reaction. Operating conditions can be adjusted to optimize the formation of the desired ester product, and said material ma~ be recovered by methods well known in the art, such as distillation, fractionation, extraction and the like. A f'raction rich in palladium or nickel catalyst components may then be r~cycled to She reaction zone, if desired, and additional products generated.
The products ha~e been identified in this work by one or more of the following analytical procedures, viz, gas-liquid phase chromato~raphy (glc), infrared (ir), mass spectrometry9 nuclear magnetic resonance (nmr) and elemental analyses, or a combination of these techniques. Analyses h~ve, for the most part, been by parts in weight; all temperatures are in degrees centigrade and all pressures in pounds per square inch gauge (psi) and bars.
The following example which illustrates one embodiment of the invention is to be considered not limitative.

EXAMPLE I

To a N2-flushed liquid mix of acetic acid ~25g) and methyl iodide (8.0g, 56 mmole) set in a glass-liner there was added 0.4 g of palladium acetate ( 1. 8 mmole Pd) and 4.0g. of triphenylphosphine ~15 mmole).
The mixture was stirred so as partially to dissolve the palladium acetate, and the glass liner plus contents were charged to a 450 ml rocking autoclave. The reactor was flushed with a gaseous mixture containing equi molar amounts of carbon monoxide and hydrogen, pressurized 10to 2000 psi (138.9 bars) with the same gaseous mixture and heated with rocking to 220C. At temperature, the pressure was further raised to 6300 psi (435.5 bars ) using the gaseous carbon monoxide/hydrogen mixture and held constant throughout the remainder o~ the run by incremental addition o~ the gaseous mixture from a large surge tank.
Upon eooling; depressurl~ing the reactor and sampling o~ the off-gas, 32.6g o~ clear, deep-red liquid product was recovered from the glass liner.
A small quantity- (<1 ml) o~ lighter liquid phase was also detected. Analysis of the bulk phase by glc showed the presenc~ Or:
13.2% propionic acid 1.1% butyric acid 0.5~ valeric acid l&.0X water 47 . 8$ unreacted acetic acid Typical off-gas samples showed the presence of:
45% carbon monoxide 3022X hydrogen 13% carbon dioxlde 15h methane EXAMPLES 2_to 4 Following the general procedure of Example 1, additional catalyst combinations were employed. Speci-fically:

.

.
:
.

~6~7~

1) Examples 2 and 3 demonstrate the use of different initial palladium(ll) acetate-to-triphenyl-phosphine molar ratios~ initially pressuring the reactor to 4000 psi (27~.8 bars) with a mixture containing equimolar amounts of carbon monoxide and hydrogen and effecting the homologation of acetic acid under variable pressure conditions.
b) Example 4 illustrates t]he use o~ bis(tri-phenylphosphine)-palladium(ll~ chloride as a catalyst precursor.

EXAMPLES A-H

Following the general procedure of Example a number of runs were carried out using a variety of additional catalysts with methyl iodide as the promoter and with acetic acid as the starting carboxylic acid. The results which are summarized in Table below show that platinum, cobalt, iron, manganese, rhenium, molybdenum and chromium &atalysts are ineffe&tive in the homologation of acetic acid to the higher carboxylic acids.

' 7~

~o o o o ~ o o o~
o .~1 o o o o o "1 51 . - .

CO ,",~1 U~ ~ 11 .
. ~10 0 .- - O o O ~ . . . S ~
O O . . . ~ r _ olr~ n ~ ~
~ ~ o . o ~ ~D~
~ . .o o ~ C~E~ ' ' ~ ' ~ ~ 0 3~ ~
8 ~ ~ ~ ~
o ' O ~ o ~ ' ' ~ ~ oq v ~ ~ ~
.. ; . ~ ~ S
J 1~ ~ 0 ~ ~ V '7 ~ N
=I~ u~ ~
-I ~ l ~
--~1 -- ¦~D o O o o ~ f~ `i O O` _ 11~ ~ ~ _I N ffl 8 ., h ~ ~ ~ ~ ~ C
u . . ii E E E ~`Z ~ c~ ~
. f,~
G -: ~ u u u ~ tJ u u U u U U ~ ~1 ~ ri O ~ O U~
~:1 'O ~ O t~
O O 0 00 '^~.
H ~ . .~ & ~ ~ ~ *
1.~ ~ ~ ~ S = S 2 :~ S N l.r) Ir~ S

o -- -- -- ~ ~ o ~n o ~ u~ o _ o~ ~ I I I I ~ I I I I I ~ '~ S
~ O O t~ ` ~ ^ ^
~ ¢ ¢ ~ ~d ~

1 . . O C3 O O ~ C ~
O. a~ ~ ~4 . .C,C;C~ O~a.. D.. O

o o rl ~ ¢ ¢ ¢ ¢ ~
;~U <~ O .t: O O .1 ~ ~ 'C ~
4 1~- X y o 4 ~ C ~
C ~ C C ~ g ~ 0 _ ~ ul u~ 0,l S l ~1 , ~ u~ ." ~ ; a :.

. ' ~ ' :
, :' .

- 13a-.

~ xample 1 was repeated 9 except that there were employed 0.50g o-f nickel(ll) acetate (2.0 mmole Ni) S and 1.43g of triphenylpho~phine, (5.4 mmole).
Upon cooling, depressuring the reactor and sampling o~ the off-gas9 33.9g of dark-b:rown liqu~d product was recovered from the glass liner. Analysis by glc showed the presence o~:
7.6% propionic acid 0.~% butyric acid 0.4~ valeric acid ~.2% water 75.1% ~nreacted acetic acid Typical off-gas samples showed the presence of;
57% carbon monoxide 34% hydrogen 2% carbon dioxide 2X methane EX~MPLE 6 Following the general procedure of Examples and S, a second example was carried out ~sing a complex of triphenylphosphine and . nickel carbonyl, viz.
(Ni~PPh 3) 2~C0)~). The results are summarized in Table 2 which follows.

:

,~ ~.
cl o .
~o :~ c~ :;~
.~ O C~
.
~e~ ' ~1 ' .

3 . . :~
o Cl ~ .
o . . .... 0 o ' :~ . . oo '~ "I c~ ~ . E ~ o ~. ~ o ~ ,. 00 ~ .
E~ . .
o ol , ~ ~1 . ' .
. C~ t~ O
. ~ .`
l ~ ~ o o o . .
E~ O ~ ~ , ~0 ~ p _ I ~ U'. ; t,, ~ U~

.
's 't: ~: ~ oo - ~ o- o o .. ~ h CO ~0 .~10 . ....
e ~1 C ~rl ~rl O ~ ' . d; ~ X ~ ' a, . ~ ~ E C
C ~ . t~ t~ O O
a. ' ~. . C C

~ ~ ~,' . .... '.,I ' ' ' ,.. ~ .
O a.
, _ _, h ~ ~cl ~ ." ,- . 1~
t~ 2 Z SS O
t.) C.) C.) E I E I C~
x ~ X l~l l E l _ ~
. . .

'' ' ' ' :

Claims (16)

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

1. A process of preparing higher homologues of aliphatic carboxylic acids having 2-6 carbon atoms which comprises heating the aliphatic carboxylic acid starting material with carbon monoxide and hydrogen in the presence of a catalytic amount of a palladium-containing or nickel-containing compound in combination with a group VB tertiary donor ligand and in the presence of an iodide or bromide promoter at a superatomospheric pressure of at least 500 psi (35.5 bars).

2. A process as claimed in Claim 1 wherein heating is carried out at a temperature of from 100 to 350°C.

3. A process as claimed in Claim 1 wherein the temperature is from 180 to 250°C.

4. A process as claimed in Claim 1, 2 or 3 wherein the pressure is from 1000 to 7500 psi (69.9 to 518.2 bars).

5. A process as claimed in Claim 1, 2 or 3 wherein the palladium-containing compound is an oxide of palladium, a palladium salt of a mineral acid or a palladium salt of an organic carboxylic acid.

6. A process as claimed in Claim 1, 2 or 3 wherein the pressure is from 1000 to 7500 psi (69.9 to 518.2 bars) and the palladium-containing compound is an oxide of palladium, a palladium salt of a mineral acid or a palladium salt of an organic carboxylic acid.

7. A process as claimed in Claim 1, 2 or 3 wherein the palladium-containing compound is palladium (II) acetate, palladium (II) acetylacetonate, palladium (II) chloride or palladium oxide.

8. A process as claimed in Claim 1, 2 or 3 wherein the pressure is from 1000 to 7500 psi (69.9 to 518.2 bars) and the palladium-containing compound is palladium (II) acetate, palladium (II) acetylacetonate, palladium (II) chloride or palladium oxide.

9. A process as claimed in Claim 1, 2 or 3 wherein the nickel-contain-ing compound is an oxide of nickel, a nickel salt of a mineral acid, a nickel salt of an organic carboxylic acid or a nickel carbonyl or hydrocarbonyl derivative.

10. A process as claimed in Claim 1, 2 or 3 wherein the pressure is from 1000 to 7500 psi (69.9 to 518.2 bars) and the nickel-containing compound is an oxide of nickel, a nickel salt of a mineral acid, a nickel salt of an organic carboxylic acid or a nickel carbonyl or hydrocarbonyl derivative.

11. A process as claimed in Claim 1, 2 or 3 wherein the nickel-containing compound is nickel (II) chloride, nickel (II) oxide, nickel (II) acetylacetonate, nickel (II) acetate, nickel (II) propionate or nickel carbonyl.

12. A process as claimed in Claim 1, 2 or 3 wherein the pressure is from 1000 to 7500 psi (69.9 to 518.2 bars) and the nickel-containing compound is nickel (II) chloride, nickel (II) oxide, nickel (II) acetylacetonate, nickel (II) acetate, nickel (II) propionate or nickel carbonyl.

13. A process as claimed in Claim 1, 2 or 3 wherein the group VB
tertiary donor ligand is triphenylphosphine, trimethylphosphine, tri-n-butyl-phosphine, triphenylphosphite, triethylphosphite, triphenylarsine, trimethyl-amine, triethylamine, tripropylamine or tri-n-octylamine.

14. A process as claimed in Claim 1, 2 or 3 wherein the iodide or bromide promoter is an alkyl iodide or bromide having 1-6 carbon atoms.

15. A process as claimed in Claim 1, 2 or 3 wherein the iodide or bromide promoter is methyl iodide, methyl bromide, ethyl iodide or ethyl bromide.

16. A method as claimed in Claim 1, 2 or 3 wherein the aliphatic carboxylic acid starting material is acetic acid.