Hierarchical SAPO-34/18 zeolite with low acid site density for converting methanol to olefins
•A hierarchical SAPO-34/18 zeolite with well-ordered macropores and low Si/Al ratio was synthesized.•The hierarchical structure effectively reduced the diffusion distance.•Simultaneous measurement of the coking rate and the gaseous product production rate was made by TG–GC.•The improved catalytic performance is due to the combination of a low acid site density and a hierarchical morphology.
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.
Graphical abstractA hierarchical CHA/AEI intergrowth zeolite with well-ordered macropores and improved MTO catalytic performance was synthesized by simply using a decreased Si content in the gel mixture.Figure optionsDownload full-size imageDownload high-quality image (133 K)Download as PowerPoint slide
Journal: Catalysis Today - Volume 233, 15 September 2014, Pages 2–7