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

摘要

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 (<150nm) deactivated rapidly due to pore blocking by heavy coke deposits; similarly sized ZSM-5 exhibited low conversion due to the low number of strong Bronsted 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.