Theoretical calculation study on the reaction mechanism of methanol/dimethyl ether carbonylation catalyzed by B/Al/Ga-MOR Zeolites

The DFT theoretical calculations were used to comprehensively compare the commonalities and differences in the mechanism of methanol and dimethyl ether carbonylation reactions when B, Al and Ga respectively substitute for the eight-membered ring side pocket T3 and the twelve-membered ring channel T4 sites of the MOR zeolite. The results indicate that CO inserting into methoxy to form acetyl groups follows the SN2 mechanism and is a rate-determining step in the carbonylation reaction. Under the condition of 473K, when methanol or dimethyl ether is used as feedstock, the formed acetyl group prefers to interact with CH3O in methanol to form methyl acetate. For the carbonylation reaction and the side reaction of catalyst deactivation by generation of aromatics from trimethoxonium ions, the T3 sites show better carbonylation selectivity whereas T4 sites show better trimethoxonium ions selectivity. Comparing with Al-MOR, the introduction of B or Ga at the T3 site increases the energy barrier of carbonylation reaction and weakens catalytic activity, while the introduction of B or Ga especially B at the T4 site can substantially increase the energy barrier of generating trimethyloxyonium ions, which effectively suppresses the side reaction and improves the stability of the catalyst. This work contributes to understanding the differences in the carbonylation reaction mechanism when isomorphous replacement occurs at acidic sites in the different channels of the MOR zeolite and provides certain theoretical support for tailoring and designing efficient MOR zeolite catalysts.