Technology
Catalyst behind new route to acetic acid New carbonylation catalyst leads to Monsanto's process with carbon monoxide and methanol feeds Probably the best-known carbonylation reaction is the oxo reaction, producing aldehydes and alcohols from carbon monoxide and olefins. Lesser known but commercially important is the reaction between carbon monoxide and methanol to produce acetic acid, a reaction translated by Monsanto into a commercial plant at Texas City a year ago. Behind the successful commercialization lies a homogeneous carbonylation catalyst that easily activates carbon monoxide at very low pressures. Commercialization of the reaction to make acetic acid had been hindered by the tough operating conditions required: pressures up to 10,000 p.s.i. and temperatures up to 300° C. Acid compounds such as boron trifluoride and phosphoric acid in combination with other metallic salts had been tried as catalysts. But the severe operating conditions presented corrosion problems and unwanted by-products from side reactions. At Monsanto's central research department, the problem was solved with a highly specific catalyst system that functions at very low pressures— about 1 atm. The system was developed by a research team headed by Dr. James F. Roth and including Dr. J. H. Craddock, Dr. A. Hershman, and Dr. F. E. Paulik. The company's hydrocarbon and polymers division carried out development of the technology for the commercial process. Promoted. The catalyst system employs iodide-promoted rhodium compounds that convert methanol and carbon monoxide to acetic acid with molar selectivities as high as 99%. There are no significant by-products formed from side reactions, even in the presence of hydrogen. This is an important consideration, since byproducts from hydrogenation reac-
tions have been major contaminants in other acetic acid processes. The carbonylation catalyst is a twocomponent system containing a rhodium moiety as the active catalyst along with a halogen promoter. An iodide promoter is preferable. The actual catalytic species is believed to be a coordination complex of rhodium with carbon monoxide and halogen ligands. The Monsanto scientists tried a variety of rhodium and promoter compounds but all produced about the same reaction rates. This suggests to them that all the catalyst combinations tried formed the same active catalytic species. The solvent medium chosen depends on solubility and compatibility of the species in the reacting system. Results indicate that faster reaction rates are achieved with more polar solvents, suggesting that the active catalytic species may be ionic. According to Dr. Roth, who will present further details next month at the national meeting of the American Chemical Society in Washington, D.C., the reaction is believed to proceed by way of a five-step mechanism. The mechanism is consistent with the results if it is assumed that the ratedetermining step is the oxidative ad-
dition of CH 3 I and that all other steps in the mechanism are very rapid. Severe. Previous studies of the catalytic carbonylation of methanol to acetic acid with Group VIII metal catalysts have required severe conditions in the reactor. In Europe a process employed by Badische Anilin-& Soda-Fabrik utilizes cobalt catalysts with iodine promoters originally developed by Reppe in the early 1940's. The BASF process operates at carbon monoxide pressures of about 7500 p.s.i. and a temperature of 210° C. The molar selectivities of methanol and carbon monoxide to acetic acid are about 90%. The major by-product is carbon dioxide produced by the water-gas shift reaction. Other byproducts are ethanol, propionic acid, propionaldehyde, butyraldehyde, and butanol. Unlike the cobalt-catalyzed systems, the Monsanto process is not strongly dependent on either carbon monoxide or methanol concentration. This extreme dissimilarity suggests to Dr. Roth that cobalt and rhodium catalysts either operate by different mechanisms or that different rate-determining steps are involved. The rhodiumcatalyzed reactions are zero order with respect to the reactants.
Acetic acid reaction may have five-step mechanism CH3OH
+
HI
— CH3I +
H20 ChL3
CH3I
+
[Rh complex]-
1
Rh complex I
CH, 3 I Rh complex | + CO
CH, 3 1 Rh complex • CO
j CH, 3 I Rh complex
•
CO
CH, 3 I C= 0 Rh complex
I
I CH3 C =0 Rh complex
H
2°
CH3COH + [Rh complex] +
HI
O
AUG.
30, 1971 C&EN 19
