Constructing single-crystalline hierarchical plate-like ZSM-5 zeolites with short b-axis length for catalyzing MTO reactions

Abstract

ZSM-5 zeolites with hierarchical and lamellar structure are highly desirable in industrial applications. In this paper, we report an efficient additive, tetramethylguanidine (TMG), for the modification of crystal growth of zeolites to obtain this morphology. The mechanism underlying the modification of the crystal growth by TMG involves the conversion of the gel into a colloid and subsequent attachment of the colloidal particles to little crystals produced nearby mainly in its a- and c-axis directions. Due to the oriented attachment, the little crystals could orientally grow in a solid phase, endowing the produced crystals with a plate-like morphology, hierarchical structure but single-crystalline nature. The ZSM-5 crystals with a thickness of 85 nm in the b-axis direction and an L_c/L_b aspect ratio of 10 displayed higher activity, higher C⁼₂ + C⁼₃ selectivity, longer catalytic lifetime, and higher regeneration stability in the MTO reaction in comparison with the conventional ones with a spherical morphology because the hierarchical structure is more favorable for reaction products escaping from the zeolite channels and the single-crystalline nature is thermally more stable.