Modification of MWW Layer Structure to investigate the Effect of Acidity and Zn-type sites on Ethane Dehydroaromatization

Abstract

MWW derivatives zeolite of MCM-22 with 3D layer structure, MCM-56 with partial disordered layer, MCM-36 with pillared layer and ITQ-2 with delaminated layer were prepared by hydrothermal synthesis and further modification to investigate the effect of acidic and interlayer structure on the catalytic performance of ethane dehydroaromatization and ethylene aromatization. With the retention of the MWW structure and confirmation of the intercalation or delamination efficiency, it was found that the increased exposed half-cup pocket and aluminum site on the external MWW layer structure can improve the xylene selectivity and catalytic stability. Although the amounts of total and strong acid site of H-form ITQ-2, MCM-36, MCM-56 are lower than those of MCM-22, the ratio of Lewis(L)/Brönsted(B) acid sites increases, which indicates that the exposed tri-coordinated framework aluminum in the half-cup pocket of external surface generated by exfoliation and pillaring can acted as the L acid sites. After loading Zn species, the strong acid amount and L acid sites amount are improved by the interaction between the loaded Zn species and MWW framework aluminum. The impregnated Zn species exist on the MWW zeolite with the state of ZnO and ZnAl2O4. The Zn species play roles of both ethane dehydrogenation and ethylene aromatization. The strong B acid sites play an important role for the oligomerization and aromatization of light olefins, while the excess amount of strong acid sites causes the increasing of cracking step which favors for light olefins production.