Hierarchical SAPO-34/18 zeolite with low acid site density for converting methanol to olefins

The catalytic performance of a zeolite is largely determined by its framework topology, particle morphology and acidity. A hierarchical SAPO-34/18 zeolite with a low Si/Al ratio of 0.11-0.12 and well-ordered macropores was synthesized with the conventional hydrothermal method by just decreasing the Si content in the gel mixture, that is, the development of its hierarchical structure did not need the use of an extra template or post-processing demetallation. The hierarchical structure reduced the diffusion distance inside the zeolitic phase because the void space made it equivalent to being comprised of small zeolitic crystallites, which gave a catalytic performance akin to that of nanosized zeolite particles. Reactivity studies that used a thermogravimetry-GC combination that simultaneously measured coke in the zeolite in addition to the gaseous products from the methanol-to-olefins (MTO) process showed that the hierarchical structure and low acid site density resulted in a longer lifetime, lower selectivities to coke and propane and higher selectivities to propene and butene, that is, a better catalyst for the MTO process. (C) 2014 Elsevier B.V. All rights reserved.