SAPO-34 and ZSM-5 nanocrystals’ size effects on their catalysis of methanol-to-olefin reactions

The catalysis of methanol-to-olefin (MTO) reactions over SAPO-34 and ZSM-5 was strongly shown to depend on the crystallites sizes. Four samples (average size of 50–5000 nm) of each catalyst were characterized for shape, size, acidity and diffusion properties. Materials adsorbed and occluded on them during the reaction were directly monitored in situ by IR spectroscopy and analyzed ex situ by GC–MS after extraction. Crystallites of 200–500 nm showed the best conversions and lifetimes because of their large external surfaces and short diffusion lengths. SAPO-34 with the smallest crystallites (<150 nm) deactivated rapidly due to pore blocking by heavy coke deposits; similarly sized ZSM-5 exhibited low conversion due to the low number of strong Brönsted acid sites in its micropores. The adverse effects of the nanocrystallity of zeolite catalysts are discussed in terms of their framework topology, acidity, and diffusivity.

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► Four catalysts each of 20–5000 nm SAPO-34 and ZSM-5 were prepared.
► The smallest (<150 nm) catalysts exhibited poor activity in the MTO reaction.
► The largest (>5000 nm) catalysts also exhibited poor catalytic activity.
► The catalyst with moderate crystallites (200–500 nm) show high catalytic activity.
► Extremely small nanocrystallites (<150 nm) significantly hindered catalysis.