Mechanistic insights into the catalytic role of various acid sites on ZSM-5 zeolite in the carbonylation of methanol and dimethyl ether†
The catalytic role of various acid sites located at the straight channel, sinusoidal channel and intersection cavity on ZSM-5 zeolite in the carbonylation of methanol and dimethyl ether (DME) was investigated through combined DFT-D calculation and micro-kinetic analysis. The results indicate that the formation of acetate takes priority in the straight channel, whereas the acid sites in the intersection cavity may induce the conversion of methanol and DME into unexpected by-products that lead to the rapid deactivation of the ZSM-5 catalyst. The catalytic behavior of various acid sites in carbonylation is strongly influenced by their local environment and the surrounding framework structure which determine the space confinement and electrostatic stabilization effects. Meanwhile, DME as feedstock shows a higher carbonylation activity than methanol in the straight channel, producing preferably methyl acetate. As a result, the catalytic activity and product selectivity of ZSM-5 in carbonylation can be finely tuned through purposely regulating the location of acid sites. Moreover, the carbonylation reaction rate can be further elevated through incorporating Cu into the ZSM-5 zeolites, especially mononuclear copper (Cu) species, due to the strong electrostatic interaction between the nucleophilic CH3+–CO intermediate and electrophilic Cu sites. This work helps to clarify the catalytic role of various acid sites on ZSM-5, as well as the nature of active Cu species in the carbonylation reaction, which is of great benefit to the design of efficient zeolite catalysts for the carbonylation process.