Fast synthesis of hierarchical CHA/AEI intergrowth zeolite with ammonium salts as mineralizing agent and its application for MTO process

Abstract

Fast solvent-free synthesis of hierarchical SAPO-34/18 zeolite was realized by combining oil-bath heating and the addition of catalytic amount of ammonium salt. The obtained zeolite samples were systematically characterized by XRD, SEM, XRF, BET, NH3-TPD, and NMR. XRD and SEM results showed that high-quality SAPO-34/18 nano-crystals with plate morphology can be obtained in only 60 min. The addition of ammonium salt served as mineralizing agent which speeded up the crystallization process in solvent-free precursor. The fast heat transfer of oil-bath heating and seeding also contributed to the ultrafast synthesis. Nitrogen adsorption–desorption analysis indicated the presence of high BET surface area and micropore volume, as well as abundant mesopores and macropores. NH3-TPD and 29Si NMR results showed that the samples exhibited appropriate acid strength and concentration. The obtained SAPO catalysts showed excellent performance in MTO reaction with significantly longer catalytic lifetime and higher propylene and butylene selectivities, compared with micron-sized SAPO-34 catalyst made with the conventional hydrothermal method. The extended lifetime could be attributed to the thin-plate crystal morphology and hierarchical pore structure, which greatly alleviated the diffusion constrain and coke deposition in MTO reaction. Higher propylene and butylene selectivities could be ascribed to the larger size of the AEI cage, which allows the easy diffusion of longer alkenes. The realization of ultrafast synthesis with solvent-free precursor not only reduces the waste disposal and energy consumption, but also increases the autoclave efficiency significantly.