Abstract: The reaction mechanism of methanol carbonylation catalyzed by pyridine carbonylic acid rhodium cation ([MRh(CO)₂]⁺, where M = pyridine carbonylic acid ligand) catalyst was studied by the ab initio method with the effective core potential approximation in the HF/LANL2DZ level. The results indicated that the whole reaction process was composed of four steps: (1) CH₃I oxidative addition; (2) carbonyl rearrangement reaction; (3) carbonyl coordination; (4) reductive elimination of CH₃COI. With the consideration of zero-point energy correction, the activation barriers of the four steps were 167.78, 110.67, 0, and 62.94 kJ/mol, respectively. The CH₃I oxidative addition was a rate-controlling step, which corresponded to the highest activation barrier. The reaction mechanism of methanol carbonylation catalyzed by [MRh(CO)₂]⁺ was similar to that by [Rh(CO)₂I₂]^-. The reaction activation barriers over [MRh(CO)₂]⁺ were lower than those over [Rh(CO)₂I₂]^- in all four steps.

Key words: reaction mechanism, methanol, carbonylation, acetic acid, rhodium, theoretical calculation