Dry-gel conversion synthesis of SAPO-14 zeolites for the selective conversion of methanol to propylene

SAPO-14 (AFN topology) zeolite has been considered as an ideal catalyst for the methanol-to-propylene (MTP) reaction with a very high one-pass propylene selectivity, due to its unique framework structure containing narrow 8-MR pore openings and ultra-small AFN cages. However, the conventional synthesis of SAPO-14 by hydrothermal treatments generally suffers from issues relating to low-efficiency, difficulty in product collection, and being unfriendly to the environment. Herein, we showcase a facile strategy for the fast and efficient synthesis of highly crystalline SAPO-14 via dry-gel conversion (DGC) for the first time. The crystallization time can be shortened from 2 days to 8 hours at a lower temperature, and the product yield after 8-hour synthesis can be increased from 41.8 to 71.6% compared with the conventional hydrothermal synthesis. Influencing factors in the synthesis, including the molar ratio of dry gel, the time of crystallization, and the ratio of water-to-dry gel (WTDG), on the crystallinity, morphology, and textural properties of the product zeolites are systematically discussed, and the possible crystallization mechanism is also proposed. Successful attempts using four types of silica sources, including silicic acid, fumed silica, Ludox HS-40, and TEOS, confirm the flexibility and adaptability of the proposed synthesis strategy. The superior catalytic performance with not only a high propylene selectivity (76.2%) but also a large P/E ratio (7.19) demonstrates that the SAPO-14 produced by the DGC method is a high-performance catalyst candidate for the selective conversion of methanol to propylene.