TEAOH-Templated SAPO-34 Zeolite with Different Crystallization Processes and Silicon Sources: Crystallization Mechanism and MTO Performance
Graphical Three-stage SAPO-34s have higher purity, smaller crystal size, more acidic sites, longer catalytic lifetime and better olefin selectivity. The crystallization mechanism and the relationship between properties and performance are discussed. Acidity and 2–4 nm mesopores have little effect on MTO performance, which mainly depends on the size and dispersity of the crystallites and the purity of SAPO-34. SAPO-34 zeolites were synthesized by one-, two-, and three-stage crystallization processes with fumed silica (FS), acidic silica sol (AS), basic silica sol (BS) and tetraethoxysilane (TEOS) as silicon sources. Properties of samples were characterized by XRD, SEM, EDX, N 2 -adsorption/desorption, NH 3 -TPD and 29 Si NMR. In contrast with one- and two-stage processes, the three-stage one favors the formation of SAPO-34s with higher purity, higher Si/Al ratio, smaller crystallite/crystal size, better dispersion and more acidic sites. Crystallization mechanism was proposed. It is supposed that three-stage process favors the formation of high supersaturated SBUs (secondary building units) and CBUs (composite building units) before nucleation. Thus massive nuclei are formed during nucleation and well-dispersed small size SAPO-34 crystals are formed in three-stage process. Three-stage SAPO-34s are longer catalytic lifetime and better olefin selectivity in MTO process. TEOS-3 has the best MTO performance. It was found that acidity of sample and mesopores in 2–4 nm range have little effect on the MTO performance of SAPO-34. Catalytic lifetime and olefin selectivity are mainly depending on the size of crystallites, the dispersity of crystallites, and the purity of SAPO-34.
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