Carbonylation of methanol to produce acetic acid: A versatile reaction to study

Carbonylation reaction has aroused much interest in industry for the production of acetic acid. The first methanol to acetic acid carbonylation process was commercialized in 1960 by BASF, which involved iodide-promoted active cobalt catalyst [HCo(CO)(4)] and required very high pressures (600 atm) as well as high temperatures (230 degrees C), and the selectivity was 90% only. In order to reduce the severe process parameters, Monsanto commercialized in 1970, a rhodium based homogeneous liquid-phase catalyst [Rh12(C0)2]- which operated at a pressure of 30-60 bar and at temperatures of 150-200 degrees C with the high selectivity over 99% based on methanol. Subsequently, in 1996, BP Chemicals developed and commercialized the most efficient Cativa process for production of acetic acid, which was based on active iridium catalyst [IrI2(CO)(2)](-) (with Ru-complex activator) and was able to operate at reduced water content (less than 8 wt.%), compared to that about 14-15 wt.% required by Monsanto process. In order to develop more advantageous catalysts for the carbonylation processes, several efforts are already put forward in the laboratory scale process developments based on metal-complex catalysts of functionalized ligands, such as complexes of the types [Rh(CO)(2)CI(P boolean AND O)] and [Rh(CO)CI(P-O)(2)] (P-O = 2-Ph2PC6H4COOCH3) which showed efficient catalytic carbonylation reactions with high Turn Over Number (TON). Iridium based catalytic carbonylation of methanol, on the other hand, is gaining much industrial importance compared to rhodium based catalyst because of several advantages. Interesting heterogeneous catalytic process for carbonylation of methanol are also in progress. This brief review focuses on the importance and progress of carbonylation reaction of methanol to produce acetic acid starting from the initial processes to the state-of-the-art technology and also laboratory scale processes based on metal-complex or heterogeneous catalysts including our recent research results and implications.