Origin of High Selectivity of Dimethyl Ether Carbonylation in the 8-Membered Ring Channel of Mordenite Zeolite

The mechanisms of dimethyl ether (DME) carbonylation in the 8-membered ring (MR) and the 12-MR channels of H-mordenite (H-MOR) zeolite were separately investigated by quantum-chemical methods to unravel the origin of high selectivity of the methyl acetate product in the 8-MR channel. It is shown that not only the formation of surface methoxy species (SMS) but also its following carbonylation reaction are more energetically feasible in the 8-MR channel. Based on the kinetic analysis of the carbonylation reaction in the two channels, the effective rate constants were determined to reveal the reactivity difference. It was found that at 423 K, the effective rate constant in the 8-MR channel was ca. 13 magnitudes higher than that in the 12-MR channel, revealing the unique selectivity for acetyl intermediate formation in the 8-MR channel. The calculated host-guest interaction energies (E-int) provided evidence that the DME carbonylation reaction exclusively occurring in the 8-MR channel can be ascribed to the strong pore confinement. Among the different components of the E-int, the Pauli repulsive interaction (E-Pauli) is responsible for the SMS formation route, while the electrostatic (E-es) and orbital terms (E-oi) dictate the high selectivity of the carbonylation step in the 8-MR channel. These results suggested that the experimental observation of acetyl intermediate species solely in the 8-MR channel could be mainly ascribed to the difference of the Pauli repulsion, electrostatic interaction, and orbital interaction in the two channels. The theoretical results provide new insights into the DME carbonylation reaction mechanism over H-MOR zeolite.