Design and in-situ synthesis of hierarchical SAPO-34@kaolin composites as catalysts for methanol to olefins
This article reports a novel strategy to design and synthesize hierarchical SAPO-34@kaolin composites as catalysts for methanol to olefins. This strategy includes two key steps: the first is the top-down depolymerization of kaolin microspheres into active silicon and aluminium species via thermal treatment; and the second is the bottom-up rearrangement and reorganization of the resulting active silicon and aluminium species and the external phosphorous source into hierarchical SAPO-34@kaolin composites both outside and inside the calcined kaolin microspheres which is co-templated by hexadecyltrimethyl-ammonium bromide (CTAB) and 3-(trimethoxysilyl)propyl dimethyl octadecyl ammonium chloride (TPOAC). It was found that: CTAB functions as an in-situ growth promoter to increase the SAPO-34 content in the resulting composites; while TPOAC serves as a mesoporogen to generate mesopores in the SAPO-34 phase of the SAPO-34@kaolin composites; and both CTAB and TPOAC play a role of crystal growth inhibitor leading to the formation of SAPO-34 with small crystal size. Distinctly different from the semi-synthesis products that were obtained by physically mixing and binding SAPO-34 with kaolin in appropriate proportions, the SAPO-34@kaolin composite synthesized involving both CTAB and TPOAC exhibits outstanding performance in methanol to olefins, with the selectivity to ethylene and propylene being increased by 20% and the catalyst lifetime being remarkably prolonged from 50 to 200 min. This is attributed to the high content, small crystal size, and hierarchical micro-mesoporous structure of the SAPO-34 crystallites in the composites.