MOLYBDENUM CATALYZED CARBONYLATION OF ETHYLENE TO PROPIONIC ACID AND ANHYDRIDE
Joseph R. Zoeller
Eastman Chemical Company Research Labs
Tuesday, December 2, 1997, 3:30 pm
Ben Bandy Conference Center
Center for Applied Energy Research
We have discovered a low pressure and low temperature process for the single step conversion of ethylene and carbon monoxide to propionic acid, methyl propionate, or propionic anhydride utilizing surprisingly active, inexpensive Cr group based catalysts. The optimal catalyst is generated from a mixture of Mo(CO)6, an iodide salt, and ethyl iodide with the addition of small amounts of hydrogen to maintain activity. This combination allows the process to operate at very high rates (exceeding turnover frequencies of 240 moles of propionic acid equivalents/mole of catalyst/hour) under low to moderate pressures (30-70 atm.) and temperatures (150-200 degrees C).
The reaction kinetics for the process are very peculiar in that the reaction is independent of ethylene, demonstrates greater than first order inverse dependence in carbon monoxide, and is fractional order in several of the catalyst components. Detailed mechanistic investigations of the Mo based process imply that catalysis is initiated by a sequence involving an initial rate limiting dissociation of CO from Mo(CO)6 to form Mo(CO)5, which is followed by halide abstraction from ethyl iodide using the resultant coordinatively unsaturated Mo(CO)5 species.
The ethyl radicals generated by this process are believed to be rapidly scavenged by Mo(CO)6 to form very reactive odd electron Mo species. These odd electron Mo species would be expected to undergo rapid catalysis via classical olefin carbonylation mechanisms. The involvement of radical species in this process, which is well supported by the kinetics, by-product formation, EPR, and inhibition by radical scavengers, provides a plausible rationale for the complex behavior of the system.
This discovery not only represents the first case of an efficient carbonylation process based on the Cr group metals, but also represents a unique method for initiating a catalytic carbonylation. A general description of this process and the mechanistic proposal, which is based upon detailed kinetics, spectroscopy, and model reactions, will be presented.