c-Axis-oriented sheet-like Cu/AEI zeolite contributes to continuous direct oxidation of methane to methanol

Abstract

In the continuous direct oxidation of methane to methanol (cDMTM), suppression of tandem reactions of the produced methanol to olefins is one of the key problems that must be solved to improve the methanol formation rate, selectivity, and catalytic stability. Herein, a strategy to reduce the thickness along the c-axis of AEI-type crystals was adopted to fulfill a short diffusion path along the straight channel. A c-axis-oriented sheet-like Cu-exchanged AEI zeolite (Cu/AEI) was prepared by adding C16-cationic surfactant into the synthesis gel. The effect of the thickness of the straight channels in the c-axis on the catalytic properties was investigated. Comparative results demonstrated that the sheet-like Cu/AEI zeolite exhibited a higher methanol formation rate and reaction performance with greater stability compared to the typical cubic-shaped zeolite. This advantage was ascribed to the decreased straight channel length that significantly facilitates the diffusion of reactants and products, decreases the accessibility of acid sites, and thus suppresses the secondary reaction of methanol to olefins on the Brønsted acid sites. Furthermore, the influence of the acid content in the framework of the cubic crystal AEI zeolite was investigated by means of post-calcination at high temperature. Thus, lower methanol formation rates and decreased stability were obtained with post-calcined Cu/AEI zeolites as compared to sheet-like Cu/AEI zeolite. The acidic property of the sheet-like Cu/AEI zeolite was optimized by post-treatments involving dealumination followed by desilication. Thus, treated sheet-like Cu/AEI zeolite achieved a methanol formation rate of 34 μmol g⁻¹ min⁻¹ (2022 μmol g⁻¹ h⁻¹) (i.e., space time yield (STY)) with 77% selectivity, which was higher than that for the cubic zeolite (28 μmol g⁻¹ min⁻¹, 49%) and the original sheet-like Cu/AEI zeolite (33 μmol g⁻¹ min⁻¹, 45%). This improvement was caused by the modified acidic properties and the introduction of secondary pores.