Novel group 9 catalysts for the carbonylation of methanol

Abstract

The carbonylation of methanol catalysed by Group 9 metals is the principle industrial route to acetic acid. It has been one of the most important applications of homogeneous catalysts for thirty years. In that time the preferred catalytic species has descended the group from cobalt, through rhodium, and recently to iridium with the introduction of B.P. Chemical's Cativa process. Rhodium and iridium are precious metals, it would be advantageous to develop a catalytic system which does not depend on a rare metal. One way this could be achieved is by improving cobalt catalysed carbonylation. Work has concentrated on the ability of the cyclopentadienyl, pentamethylcyclopentadienyl and triethyl phosphine ligands to promote cobalt catalysts. Several novel cobalt catalysts and one novel rhodium catalyst have been discovered for the carbonylation of methanol to methyl acetate. CH₃OH + ROH Co/Catalyst-→ CH₃COOR + H₂O

At 120°C using [Cp*Co(CO)₂] and Pet₃ as catalyst precursors rates of methanol carbonylation have been achieved which are, to our knowledge, far greater than any previously reported for cobalt catalysts. The initial rate of carbonylation compares favourably with that of rhodium based systems. High Pressure Infrared Spectroscopy has been utilised extensively as a tool for investigating the solution behaviour of the novel catalyst precursors [CpCo(CO)PMe₂ph], [CpCo(CO)₂], [Cp*Co(CO)₂] and [Cp*Rh(C0)₂].