Synthesis of SAPO-34 with small and tunable crystallite size by two-step hydrothermal crystallization and its catalytic performance for MTO reaction

SAPO-34 crystallites with 160–550 nm were synthesized by a two-step hydrothermal crystallization and characterized by XRD, SEM, XRF, low-temperature N2 adsorption and NH3-TPD. The H2O2 aqueous solution was used to remove template. The size effects of SAPO-34 crystallites on its catalytic performance (the catalytic activity, lifetime and coking formation) were investigated for the conversion of methanol to olefins (MTO). Comparing with SAPO-34 sample (I) prepared by traditional one-step hydrothermal crystallization, the sample (II) prepared by a two-step hydrothermal crystallization possesses the similar CHA structure, cubic-like rhombohedra morphology, chemical composition and Si environment as sample (I); however, the sample (II) has smaller crystallite (160 nm), larger surface area (648 m2/g) and more acid sites than the sample (I) (1100 nm, 498 m2/g). For the MTO reaction, the olefins selectivity and lifetime strongly depend on the crystallite sizes of SAPO-34 catalyst, and the smaller the crystallite sizes of SAPO-34 and the better its catalytic performance is. Using the catalyst (II), the olefins selectivity and lifetime in single run can reach more than 94.9% and 10 h, respectively. Therefore, this two-step hydrothermal crystallization developed here offers a promising strategy to control the crystallite size of SAPO-34, and SAPO-34 with small crystallite size is an excellent candidate of catalyst for MTO process.