Zinc enhances catalytic performance of pyridine-adsorbed HMOR: Dimethyl ether carbonylation

Abstract

Dimethyl ether (DME) carbonylation to methyl acetate (MA) on H-mordenite (HMOR) zeolite is of critical importance for ethanol synthesis from syngas in industry, but it is generally accepted that carbonaceous deposits at Brønsted acid sites (BASs) in the twelve-membered ring (12-MR) lead to rapid deactivation of HMOR. Pyridine (Py) significantly prolong the catalytic lifetime of HMOR by selectively enter into the 12-MR to occupy the BASs. Moreover, loading Zn species into HMOR dramatically improve the MA formation. However, the role, active type and location of Zn species in HMOR are unclear. Herein, we comparatively dissected the catalytic mechanisms of DME carbonlyation in HMOR, pyridine-adsorbed HMOR (Py-HMOR), and three types of Zn-modified Py-HMOR systems by density functional theory (DFT) calculations, and confirmed Zn2+ ion doped Py-HMOR (Zn2+-Py-HMOR) system with the Zn2+ located between T3O31/T3O31′ and T3O33 sites in the 8-MR shows the optimal catalytic activity for DME carbonylation to MA. During the whole cycle of DME carbonylation, only the reaction step of acetyl intermediate with CH3OH is able to restore the active T3O31/T3O31′. Our Bader charge and charge density differences analyses revealed that the catalytic activity improvement of Zn2+-Py-HMOR system for DME decomposition and MA formation is stem from the enhancement of electrostatic interaction, more framework O sites in 8-MR involving in electron transfer, and matched spatial confinement. This work will provide indispensable inspiration for further modification of zeolite catalysts with 8-MR for DME carbonylation reaction.