Carbonylation of Methanol and Methyl Acetate

CH3OH + CO CH3COOH G = -75 KJ/mol

Acetic acid

Monsanto developed the rhodium-catalyzed process for the carbonylationof methanol to produce acetic acid in the late sixties.

At standard conditions the reaction is thermodynamically allowed, butwithout a catalyst, so as many carbonylation reactions, it would takeplace at all.

• The rhodium and iridium catalyst have several distinct advantages over the cobalt catalyst; they are much faster and far more selective.

• For years now the Monsanto process (now owned by BP) has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATIVA process, developed by BP, has come on the stream.

The percentage of the selectivity in methanol is in the highninties. But the selectivity in carbon monoxide may be as low as 90%. This is due to the shift reaction.

COCH3OH +

RhX3

HI

CH3CO2HRhI2(CO)2

-or

The mechanism of the shift reaction in this catalyst system involves the attack of hydroxide anion at coordinated carbon monoxide, forminga metallacarboxylic acid. Elimination of CO2 gives a rhodium hydridespecies that can react with the proton stemming from water to givedihydogen.

While water is an indispensable ingredient for the organic cycle (1 and 5),a high concentration of water causes the major loss of one of thefeedstocks. Water is also made in situ together with methyl acetate frommethanol and acetic acid. Not only water, but also HI is the cause of by-product formation:

• Other companies (e. g. Hoechst,, now Celanese) have developed a slightly different process in which the water content is low in order to save CO feed stock. In the absence of water it turned out that the catalyst precipitate. Also, the regeneration of rhodium(III) is much slower.

Rate limiting step

The rate-determining step is the oxidative addition of methyl iodideto 1. As for other nucleophiles, the reaction is much slower with methylbromide or methyl chloride,

The CATIVA Process

• BP has announced that an improved process became operative in 1996 using iridium (or a combination of iridium and another metal, usually ruthenium). The new system shows high rates at low water concentrations. The catalyst system exhibits high stability allowing a wide range of process conditions and compositions to be accessed without catalyst precipitation. In 2003 four plants are in operation using this new catalyst.

• In general the oxidative addition to iridium is much faster than that to the corresponding rhodium complexes.

• The reductive elimination may be slower for iridium. Migration is now the slowest step. In third low metals, the metal to carbon -bonds are stronger, more localized and more covalent than those in second-row metal complexes.

• Two distinct classes of promoters have been identified for the reaction: simple iodide complexes of zinc, cadmium, mercury, and carbonyl complexes of tungsten, rhenium, ruthenium, and osmium. The promoters exhibit a unique synergy with iodide salts, such as lithium iodide, under low water concentrations. Both main group and transition metal salts can influence the equilibria of iodide species involved.

• The salts abstract iodide from the ionic methyl-iridium species and that in the resulting neutral species the migration is 800 times faster.

• about 25% faster than the Monsanto rhodium catalyst.

In the 1970’s Halcon (now Eastman) and Hoest (now Celanese) developeda process for the conversion of methyl acetate and carbon monoxide to acetic anhydride.The reaction scheme follows that of the Monsanto process except forthe “organic” cycle, in which acetic acid replaces water, and methylacetate replaces methanol.

H3C I

O+

H3C O-Li

O

H3C O

O

CH3

O+ LiI

H3C O-CH3

O+ LiI CH3-I +

H3C O-Li

O