Mechanistic insights into the effects of alkali metal ion on ZSM-11 zeolite synthesis and its catalytic alkylation performance

Abstract

Alkali metal cations showed significant effects on the zeolite synthesis process as well as its structure while the mechanism is still under debate. In this work, ZSM-11 zeolites were synthesized with the addition of four alkali metal cations (Li+, Na+, K+, Cs+). Except for different morphology and pore structure observed in the four samples, ZSM-11 synthesized with the presence of Li+ demonstrated low synthesis efficiency and weak acidic properties compared to those with the addition of Na+, K+, and Cs+ ions. Both DFT calculations and MD simulations were further performed to investigate the precursor reactions between silicates and aluminates as well as the polycondensation of two silicon-aluminum tetrahedral structural units. Except for the widely used factors including polarizable ability parameter (gi) and structural parameter (0W), two new parameters, i.e., polarizability (α0) and first hyperpolarizability (β0), were first employed as the descriptor to unravel the effects of alkali metal ion on ZSM-11 zeolite synthesis and properties. Obtained ZSM-11 samples exhibited the distinct catalytic performance for benzene alkylation with ethanol, which was attributed to the different structure and acidity properties. This work provides new insights into the deep understanding of the effects of alkali metal cations on zeolite synthesis in a closely mechanistic view.