Synthesis of Hierarchical Chabazite Zeolite via Interzeolite Transformation of Coke-containing Spent MFI

Abstract

Nowadays, it’s highly desirable to develop a sustainable strategy for utilizing the wasted zeolite catalysts to selectively produce value-added and useful zeolites in industry. Herein, using the coke-containing spent MFI zeolite as the raw materials, a robust interzeolite transformation by a steam-assisted dry gel conversion method was reported to synthesize the hierarchical SSZ-13 zeolites (CHA type) with high crystallinity and well-developed mesoporosity (mesopore volume = 0.38 m3/g). The obtained hierarchical SSZ-13 exhibited a marked improvement of catalytic lifetime in methanol to olefins (MTO) reaction, showing about 2.5 times longer than that of the conventional SSZ-13 in bulk single-crystal structure. The catalyst also presented long-term stability after 3 consecutive cycles of reaction and regeneration. It was found that the coke in the spent MFI could function not only as a qualified hard template for mesoporosity formation but also as a restrictor to hinder the zeolite grain growth. The ordered aluminosilicate fragments decomposed from MFI zeolites at the initial period may act as nutrients, which were then rearranged to form CHA zeolites at a high crystallization rate. Besides, the enhanced diffusion rate and adsorption capacity arising from the hierarchical pores in SSZ-13 greatly decreased the coking rate and improved its coke tolerance by lowering the probability of secondary reaction inside zeolite channels. This work will provide a general and efficient way to transform waste zeolite catalysts into more valuable hierarchical zeolites.