Preparation of carboxylic acid anhydrides

A carboxylic acid anhydride, such as acetic anhydride, is prepared from a carboxylate ester or a hydrocarbyl ether in carbonylation processes comprising the use of an iodide, carbon monoxide and a molybdenum-nickel or tungsten-nickel component in the presence of a promoter comprising an organo-phosphorus compound or an organo-nitrogen compound wherein the phosphorus and nitrogen are trivalent.

This invention relates to the preparation of anhydrides of carboxylic 
acids, more particularly mono-carboxylic acids, and especially the 
anhydrides of lower alkanolic acids, such as acetic anhydride, by 
carbonylation. 
Acetic anhydride has been known as an industrial chemical for many years 
and large amounts are used in the manufacture of cellulose acetate. It has 
commonly been produced on an industrial scale by the reaction of ketene 
and acetic acid. It is also known that acetic anhydride can be produced by 
the decomposition of ethylidene diacetate, as well as by the oxidation of 
acetaldehyde, for example. Each of these "classic" processes has 
well-known drawbacks and disadvantages and the search for an improved 
process for the production of acetic anhydride has been a continuing one. 
Proposals for producing anhydrides by the action of carbon monoxide upon 
various reactants (carbonylation) have been described, for example, in 
Reppe et al U.S. Pat. Nos. 2,729,561, 2,730,546, and 2,789,137. However, 
such prior proposals involving cobalt or nickel catalysts have required 
the use of very high pressures. In later patents, carbonylation at lower 
pressures has been proposed but only as a route to the preparation of 
acetic acid. French Pat. No. 1,573,130, for example, describes the 
carbonylation of methanol and mixtures of methanol with methyl acetate in 
the presence of compounds of Group VIII noble metals such as iridium, 
platinum, palladium, osmium and ruthenium and in the presence of bromine 
or iodine under more moderate pressures than those contemplated by Reppe 
et al. Similarly, South African Pat. No. 68/2174 produces acetic acid from 
the same reactants using a rhodium component with bromine or iodine. 
Schultz (U.S. Pat. Nos. 3,689,533 and 3,717,670) has disclosed a 
vapor-phase process for acetic acid production employing various catalysts 
comprising a rhodium component dispersed on a carrier. None of these later 
carbonylation disclosures, however, refers to or contemplates the 
preparation of acetic anhydride or other carboxylic acid anhydrides. 
Most recently, improved processes for preparing carboxylic acid anhydrides, 
including acetic anhydride, have been disclosed in British Pat. No. 
1,468,940 and in U.S. Pat. No. 4,115,444. In all of these recent 
processes, however, a Group VIII noble metal is an essential catalyst 
component. Consequently, while entirely effective, these processes suffer 
from the need to employ expensive, relatively rare metals. 
U.S. Pat. No. 4,002,678 discloses the use of a non-noble metal system 
involving a promoted nickel-chromium catalyst. While effective, this 
catalyst system is not completely satisfactory from the standpoint of 
reaction rate. 
It is an object of the present invention to provide an improved process for 
the manufacture of carboxylic acid anhydrides, especially lower alkanoic 
anhydrides, such as acetic anhydride, which requires neither high 
pressures nor Group VIII noble metals. 
In accordance with the invention, carbonylation of a carboxylic ester 
and/or a hydrocarbyl ether is carried out by using a molybdenum-nickel or 
a tungsten-nickel co-catalyst in the presence of a promoter comprising an 
organ-phosphorus compound or an organo-nitrogen compound wherein the 
phosphorus and nitrogen are trivalent, and in the presence of an iodide. 
The surprising discovery has been made that this co-catalyst in 
combination with the promoter-iodide system of the character indicated 
makes possible carbonylation of esters and ethers not only at relatively 
low pressures but with rapid, high yield production of carboxylic acid 
anhydrides. 
Thus, in accordance with the invention, carbon monoxide is reacted with a 
carboxylate ester, especially a lower alkyl alkanoate, or a hydrocarbyl 
ether such as a lower alkyl ether, to produce a carboxylic anhydride, such 
as a lower alkanoic anhydride, the carbonylation taking place in the 
presence of an iodide e.g., a hyrdocarbyl iodide, especially a lower alkyl 
iodide, such as methyl iodide. Thus, acetic anhydride, for example, can be 
effectively prepared in a representative case by subjecting methyl acetate 
or dimethyl ether to carbonylation in the presence of methyl iodide. In 
all cases, the carbonylation is carried out under anhydrous conditions in 
the presence of the co-catalyst promoter-system described above. Moreover, 
an ester-ether mixture can be carbonylated if desired. 
It will be understood that the iodine moiety does not have to be added to 
the system as a hydrocarbyl iodide but may be supplied as another organic 
iodide or as the hydroiodide or other inorganic iodide, e.g., a salt, such 
as the alkali metal or other metal salt, or even as elemental iodine. 
Following the reaction the organic components of the reaction mixture are 
readily separated from one another, as by fractional distillation. 
In like manner, other lower alkanoic anhydrides, i.e., anhydrides of lower 
alkanoic acids, such as propionic anhydride, butyric anhydrides, and 
valeric anhydrides, can be produced by carbonylating the corresponding 
lower alkyl alkanoate or a lower alkyl ether. Similarly, other carboxylic 
acid anhydrides, e.g., the anhydrides of other alkanoic acids, such as 
those containing up to 12 carbon atoms, for example capric anhydrides, 
caprylic anhydrides and lauric anhydrides, and like higher anhydrides are 
produced by carbonylating the corresponding ester, e.g., alkyl akanoates 
containing up to 11 carbon atoms in the alkyl group and up to 12 carbon 
atoms in the carboxylate group, or aryl esters, or the corresponding 
ether, such as heptyl caprylate, nonyl decanoate, undecyl laurate, phenyl 
benzoate, heptyl ether, nonyl ether, phenyl ether, and the like. 
It is preferred that the reactants be selected so that the resulting 
anhydride will be a symmetrical anhydride, i.e., having two identical acyl 
groups, viz., wherein R in equations (1) and (2) is the same in each 
instance, but it is within the scope of the invention to produce 
non-symmetrical or mixed anhydrides and this can be readily effected by 
using different combinations of reactants, e.g., by using compounds having 
different R groups in the foregoing reactions, as will be obvious to 
persons skilled in the art. 
The above-described reactions can be expressed as follows: 
EQU CO+RCOOR.fwdarw.(RCO).sub.2 O (1) 
EQU 2CO+ROR.fwdarw.(RCO).sub.2 O (2) 
wherein R is a hydrocarbyl radical which may be saturated, e.g., alkyl of 1 
to 11 carbon atoms, or monocyclic aryl, e.g., phenyl or aralkyl, e.g., 
benzyl. Preferably, R is lower alkyl i.e., an alkyl group of 1 to 4 carbon 
atoms, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl and 
t-butyl. 
The hydrocarbyl radical may be substituted with substituents which are 
inert in the reactions of the invention. 
The more volatile components such as alkyl iodide and unreacted ether or 
ester in the final product mixture can be readily removed, as by 
distillation, for recycling, and the net yield of product is substantially 
exclusively the desired carboxylic anhydride. In the case of liquid-phase 
reaction which is preferred, the organic compounds are easily separated 
from the metal-containing components, as by distillation. The reaction is 
suitably carried out in a reaction zone to which the carbon monoxide, the 
ester or ether, the iodide, the specified co-catalyst and the promoter are 
fed. No water is produced in the abovedescribed reactions and anhydrous or 
substantially anhydrous conditions are employed. 
In carrying out the process of the invention, a wide range of temperatures, 
e.g., 25.degree. to 350.degree. C. are suitable but temperatures of 
100.degree. to 250.degree. C. are preferably employed and the more 
preferred temperatures generally lie in the range of 125.degree. to 
225.degree. C. Temperatures lower than those mentioned can be used but 
they tend to lead to reduced reaction rates, and higher temperatures may 
also be employed but there is no particular advantage in their use. The 
time of reaction is also not a parameter of the process and depends 
largely upon the temperature employed, but typical residence times, by way 
of example, will generally fall in the range of 0.1 to 20 hours. The 
reaction is carried out under super-atmospheric pressure but, as 
previously mentioned, it is a feature of the invention that excessively 
high pressures, which require special high-pressure equipment, are not 
necessary. In general, the reaction is effectively carried out by 
employing a carbon monoxide partial pressure which is preferably 15 to 
2000 psi and most preferably 30 to 1200 psi, although carbon monoxide 
partial pressures of 1 to 10,000 psi can also be employed. By establishing 
the partial pressure of carbon monoxide at the values specified, adequate 
amounts of this reactant are always present. The total pressure is, of 
course, that which will provide the desired carbon monoxide partial 
pressure and preferably it is that required to maintain the liquid phase 
and in this case the reaction can be advantageously carried out in an 
autoclave or similar apparatus. The final reaction mixture will normally 
contain volatile components such as a hydrocarbyl iodide, unreacted ester 
or ether along with the product anhydride and these volatile components, 
after separation from the anhydride, can be recycled to the reaction. At 
the end of the desired residence time the reaction mixture is separated 
into its several constituents, as by distillation. Preferably, the 
reaction product is introduced into a distillation zone which may be a 
fractional distillation column, or a series of columns, effective to 
separate the volatile components from the product anhydride and to 
separate the product anhydride from the less volatile catalyst and 
promoter components of the reaction mixture. The boiling points of the 
volatile components are sufficiently far apart that their separation by 
conventional distillation presents no particular problem. Likewise, the 
higher boiling organic components can be readily distilled away from the 
metal co-catalyst components and any organic promoter which may be in the 
form of a relatively non-volatile complex. The co-catalyst components, and 
promoter can then be combined with fresh amounts of ester or ether and 
carbon monoxide and reacted to produce additional quantities of anhydride. 
The process is advantageously carried out in the presence of a solvent or 
diluent, particularly when the reactant has a relatively low boiling 
point, as in the case of dimethyl ether. The presence of a higher boiling 
solvent or diluent, which may be the product anhydride itself, e.g., 
acetic anhydride in the case of dimethyl ether, or which may be the 
corresponding ester, e.g., methyl acetate, again in the case of methyl 
ether, will make it possible to employ more moderate total pressure. 
Alternatively, the solvent or diluent may be any organic solvent which is 
inert in the environment of the process such as hydrocarbons, e.g., 
octane, benzene, toluene, or carboxylic acids, e.g., acetic acid, and the 
like. The carboxylic acid, when used, should preferably correspond to the 
anhydride being produced. A solvent or diluent is suitably selected which 
has a boiling point sufficiently different from the desired product in the 
reaction mixture so that it can be readily separated, as will be apparent 
to persons skilled in the art. 
The carbon monoxide is preferably employed in substantially pure form, as 
available commercially, but inert diluents such as carbon dioxide, 
nitrogen methane, and noble gases can be present if desired. The presence 
of inert diluents does not effect the carbonylation reaction but their 
presence makes it necessary to increase the total pressure in order to 
maintain the desired CO partial pressure. The carbon monoxide, like other 
reactants should, however, be essentially dry, i.e., the CO and the other 
reactants should be reasonably free from water. The presence of minor 
amounts of water such as may be found in the commercial forms of the 
reactants is, however, entirely acceptable. It is preferable that the 
amount of moisture be kept to a minimum since the presence of water has 
been found to have an adverse effect upon the activity of the co-catalyst 
promoter system. Hydrogen which may be present as an impurity is not 
objectionable and even may tend to stabilize the catalyst. Indeed, in 
order to obtain low CO partial pressures the CO fed may be diluted with 
hydrogen or any inert gas such as those above mentioned. 
The co-catalyst components can be employed in any convenient form, viz., in 
the zero valent state or in any higher valent form. For example, the 
nickel and the molybdenum or tungsten can be the metals themselves in 
finely divided form, or a compound, both organic or inorganic, which is 
effective to introduce the co-catalyst components into the reaction 
system. Thus, typical compounds include the carbonate, oxide, hydroxide, 
bromide, iodide, chloride, oxyhalide, hydride, lower alkoxide (methoxide), 
phenoxide, or molybdenum, tungsten or nickel carboxylates wherein the 
carboxylate ion is derived from an alkanoic acid of 1 to 20 carbon atoms 
such as acetates, butyrates, decanoates, laurates, benzoates, and the 
like. Similarly, complexes of any of the co-catalyst components can be 
employed, e.g., carbonyls and metal alkyls as well as chelates, 
association compounds and enol salts. Examples of other complexes include 
bis-(triphenylphosphine) nickel dicarbonyl, tricyclopentadienyl trinickel 
dicarbonyl, tetrakis (triphenylphosphite) nickel, and corresponding 
complexes of the other components, such as molybdenum hexacarbonyl and 
tungsten hexacarbonyl. 
Included among the catalyst component listed above are complexes of the 
metal co-catalyst component with organic promoter ligands derived from the 
organic promoters hereinafter described. Particularly preferred are the 
elemental forms, compounds which are iodides, and organic salts, e.g., 
salts of the monocarboxylic acid corresponding to the anhydride being 
produced. It will be understood that the foregoing compounds and complexes 
are merely illustrative of suitable forms of the several co-catalyst 
components and are not intended to be limiting. 
The specified co-catalyst components employed may contain impurities 
normally associated with the commercially available metal or metal 
compounds and need not be purified any further. 
The organo-phosphorus promoter is preferably a phosphine, e.g., a phosphine 
of the formula 
##STR1## 
wherein R.sup.1, R.sup.2 and R.sup.3 may be same or different, and are 
alkyl, cycloalkyl, aryl groups, amide groups or halogen atoms, preferably 
containing up to 1 to 20 carbon atoms in the case of alkyl and cycloalkyl 
groups and 6 to 18 carbon atoms in the case of aryl groups. Typical 
phosphines include trimethylphosphine, tripropylphosphine, 
tricyclohexylphosphine and triphenylphosphine. 
Preferably the organo-nitrogen promoter is a tertiary amine or a 
polyfunctional nitrogen-containing compound, such as an amide, a hydroxy 
amine, a keto amine, a di-, tri and other polyamine or a 
nitrogen-containing compound which comprises two or more other functional 
groups. Typical organo-nitrogen promoters include 2-hydroxypyridine, 
8-quinolinol, 1-methylpyrrolidinone, 2-imidazolidone, 
N,N-dimethylacetamide, dicyclohexylacetamide, dicyclohexyl-methylamine, 
2,6-diaminopyridine, 2-quinolinol, N,N-diethyltoluamide, and imidazole. 
Although generally the organic promoters are added separately to the 
catalyst system, it is also possible to add them as complexes with the 
co-catalyst metals such as bis(triphenylphosphine) nickel dicarbonyl, and 
tetrakis (triphenyl phosphite) nickel. Both free organic promoters and 
complexed promoters can also be used. Indeed, when a complex of the 
organic promoter and the nickel and/or co-catalyst components are used, 
free organic promoter can also be added as well. 
The amount of each co-catalyst component is in no way critical and is not a 
parameter of the process of the invention and can vary over a wide range. 
As is well known to persons skilled in the art, the amount of catalyst 
used is that which will provide the desired suitable and reasonable 
reaction rate since reaction rate is influenced by the amount of catalyst. 
However, essentially any amount of catalyst will facilitate the basic 
reaction and can be considered a catalytically-effective quantity. 
Typically, however, each component of the co-catalyst is employed in the 
amount of 1 mol per 10 to 10,000 mols of ester or ether, preferably 1 mol 
per 100 to 5,000 mols of ester or ether, and most preferably 1 mol per 500 
to 1,000 mols of ester or ether. 
The ratio of nickel to its co-catalyst component can vary. Typically, it is 
one mol of nickel per 0.01 to 100 mols of the other co-catalyst component, 
preferably the nickel component is used in the amount of 1 mol per 0.1 to 
20 mols, most preferably 1 mol per 1 to 10 mols of the other co-catalyst 
component. 
The quantity of organic promoter can also vary widely but typically it is 
used in the amount of 1 mol per 0.1 to 10 mols of co-catalyst components, 
preferably 1 mol per 0.5 to 5 mol, most preferably 1 mol per 1 to 5 mols 
of co-catalyst components. 
As previously mentioned, in the working up of the reaction mixtures, e.g., 
by distillation, the promoter components can be readily recovered and 
recycled to the reaction. The nickel and co-catalyst metal generally 
remain as the least volatile components, and are recycled or otherwise 
handled together. They may, however, distill with the volatile components, 
e.g., in the case of nickel carbonyl. The same is true of the promoter 
components. 
When an ether is used as the reactant, the corresponding ester is formed as 
an intermediate, e.g., methyl acetate is formed when dimethyl ether is 
carbonylated in accordance with the invention. The intermediate ester may 
be recovered from the reaction mixture, if desired, e.g., by fractional 
distillation, for example during the separation of the volatile components 
of the reaction mixture as described above. 
The amount of iodide component may also vary widely but, in general, it 
should be present in an amount of at least 10 mols (expressed as I) per 
hundred mols of ester or ether. Typically, there are used 10 to 50 mols of 
the iodide per 100 mols of ester or ether, preferably 17 to 35 mols per 
100 mols. Ordinarily, more than 200 mols of iodide per 100 mols of ester 
or ether are not used. 
It will be apparent that the above-described reactions lend themselves 
readily to continuous operation in which the reactants and catalyst, 
preferably in combination with the promoter, are continuously supplied to 
the appropriate reaction zone and the reaction mixture continuously 
distilled to separate the volatile organic constituents and to provide the 
desired product or products, e.g., carboxylic acid anhydride, with the 
other organic components being recycled and, in the case of liquid-phase 
reaction, a residual nickel co-catalyst-containing (and 
promoter-containing) fraction also being recycled. 
A particular embodiment of the catalyst comprising the molybdenum-nickel or 
tungsten-nickel co-catalyst component, the organic promoter component and 
the iodide component can be represented by the following formula X:T:Z:Q, 
wherein X is molybdenum or tungsten, T is nickel, X and T being in zero 
valent form or in the form of a halide, an oxide, a carboxylate of 1 to 20 
carbon atoms, a carbonyl or an hydride; Z is an iodide source which is 
hydrogen iodide, iodine, an alkyl iodide wherein the alkyl group contains 
1 to 20 carbon atoms or an alkali metal iodide, and Q is an 
organo-phosphorus compound or an organo-nitrogen compound wherein the 
phosphorus and the nitrogen are trivalent. Preferred are the nitrogen and 
phosphorus compounds previously indicated as being preferably used and in 
the most preferred form Q is a phosphine of the formula 
##STR2## 
as hereinbefore defined, especially hydrocarbyl phosphines, the molar 
ratio of X to T being 0.1-10:1, the molar ratio of X+T to Q being 
0.05-20:1 and the molar ratio of Z to X+T being 1-1,000:1. 
It will also be apparent that the catalytic reaction involved in the 
process of the invention can be carried out in the vapor phase, if 
desired, by appropriate control of the total pressure in relation to the 
temperature so that the reactants are in vapor form when in contact with 
the catalyst. In the case of vapor-phase operation, and in the case of 
liquid-phase operation, if desired, catalyst components may be supported 
i.e., they may be dispersed on a carrier of conventional type such as 
alumina, silica, silicon carbide, zirconia, carbon, bauxite, attapulgus 
clay, and the like. The catalyst components can be applied to the carriers 
in conventional manner, e.g., by impregnation of the carrier with a 
solution of the catalyst component. Concentrations upon the carrier may 
vary widely, e.g., 0.01 weight percent to 10 weight percent, or higher. 
Typical operating conditions for vapor-phase operation are a temperature 
of 100.degree. to 350.degree. C., preferably 150.degree. to 275.degree. C. 
and most perferably 175.degree. to 255.degree. C., a pressure of 1 to 
5,000 p.s.i.a., preferably 59 to 1,500 p.s.i.a. and most preferably 150 
to 500 p.s.i.a., with space velocities of 50 to 10,000 hr..sup.-1, 
preferably 200 to 6,000 hr..sup.-1 and most preferably 500 to 4,000 
hr..sup.-1 (STP). In the case of a supported catalyst, the iodide 
component is included with the reactants and not on the support. 
The following examples will serve to provide a fuller understanding of the 
invention, but is it to be understood that they are given for illustrative 
purposes only, and are not to be construed as limitative of the invention. 
In the examples, all parts are on a molar basis and all percentages are by 
weight, unless otherwise indicated. The various reactants and catalyst 
components are charged to the reaction vessel which is then closed and 
brought to the reaction temperature indicated.

EXAMPLE 1 
A magnetically-stirred Hastelloy Parr bomb is employed as the reaction 
vessel. The bomb is charged with methyl acetate (250 parts), methyl iodide 
(250 parts), bis-triphenylphosphine nickel dicarbonyl (8 parts) plus 
molybdenum carbonyl (15 parts) as co-catalyst, and triphenyl phosphine (25 
parts), is swept out with argon and is pressured to 600 psig with carbon 
monoxide. The vessel is heated to 200.degree. C. with stirring during 
which time the pressure rises to about 1,200 psig. The temperature is 
maintained at 180.degree. C. and the pressure is maintained at 1,200 psig 
by recharging with carbon monoxide when needed. After 4 hours reaction 
time, G. C. analysis of the reaction product shows it to be composed of 30 
mol % methyl acetate and 70 mol % acetic anhydride. 
EXAMPLE 2 
Example 1 is repeated except that the original gas charge contains 5% 
hydrogen and 95% CO. Subsequent recharges of gas are made with pure carbon 
monoxide. After 1.75 hours reaction time, G. C. analysis of the reaction 
product shows it to be composed of 18 mol % methyl acetate and 82 mol % 
acetic anhydride. 
EXAMPLE 3 
Example 1 is again repeated with the exception that the temperature is 
160.degree. C. After 3 hours of reaction time, G. C. analysis of the 
reaction product shows it to be composed of 45 mol % methyl acetate and 55 
mol % acetic anhydride. 
EXAMPLE 4 
Example 1 is repeated once again except that the bistriphenylphosphine 
nickel dicarbonyl is replaced with nickel acetate. After 6 hours of 
reaction time, G. C. analysis of the reaction product shows it to contain 
22 mol % methyl acetate and 75 mol % acetic anhydride. 
EXAMPLE 5 
When Example 1 is repeated again, except that the bistriphenylphosphine 
nickel dicarbonyl is replaced with nickel iodide, after 5 hours of 
reaction time, G. C. analysis of the reaction product shows it to contain 
18 mol % methyl acetate and 81 mol % acetic anhydride. 
EXAMPLE 6 
Again repeating Example 1 but with the molybdenum carbonyl being replaced 
with molybdenum acetate, after 5 hours of reaction time, G. C. analysis of 
the reaction product shows it to contain 20 mol % methyl acetate and 78 
mol % acetic anhydride. 
EXAMPLE 7 
Example 1 is repeated with the exception that molybdenum hexacarbonyl is 
replaced with an equivalent quantity of tungsten hexacarbonyl. The 
reaction product, after a reaction time of 8 hours, corresponds to that 
described in Example 1. 
EXAMPLE 8 
A reactor as described in Example 1 is charged with 250 parts methyl 
acetate, 250 parts methyl iodide, 15 parts triphenylphosphine, 7 parts 
bis-triphenylphosphine nickel dicarbonyl and 5 parts molybdenum 
hexacarbonyl. The reactor is pressured with 100 psig of hydrogen and up to 
600 psig with carbon monoxide and is then heated to 185.degree. C. with 
stirring. The pressure is adjusted to 1,200 psig with carbon monoxide and 
is kept at that level by recharging carbon monoxide when needed. After 4 
hours of reaction time, G. C. analysis shows that the reaction product to 
conain 70 mol % acetic anhydride. 
EXAMPLE 9 
Example 8 is repeated with the exception that molybdenum carbonyl is 
replaced with an equal weight of chromium carbonyl. After 4 hours of 
reaction time, G. C. analysis shows that the reaction product contains 
only 39.6 mol % acetic anhydride. 
EXAMPLE 10 
A pressure reactor as described in Example 1 is charged with 250 g. methyl 
iodide, 250 g. methyl acetate, 25 g. imidazole, 10 g. nickel iodide, 10 g. 
molybdenum hexacarbonyl. The reactor is pressured with 200 psig hydrogen 
and then to 600 psig carbon monoxide. The reactor is heated up to 
180.degree. C. with stirring. The pressure increases to 950 psig and is 
maintained at that pressure by recharging carbon monoxide when needed. 
After 4 hours reaction time, G. C. analysis of the reaction mixture shows 
that it contains 42 mol % acetic anhydride.