Facile in situ hydrothermal crystallization synthesis of SAPO-34/ZSM-5 composite catalyst for methanol to olefin reaction

Abstract

The SAPO-34/ZSM-5 composite catalyst (SZ-TS) was successfully fabricated using a two-step in situ crystallization method. X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, N2 adsorption–desorption, and ammonia temperature-programmed desorption measurements were used to characterize the synthesized catalysts and evaluate their catalytic performances on methanol-to-olefin (MTO) reaction. The synthesis mechanism of two-step in situ crystallization was further investigated. Results showed that the SZ-TS synthesized by two-step in situ crystallization featured an intimately bonded composite structure owing to the synergistic effects of the interface. However, the physical mixture of SAPO-34 and ZSM-5 (SZ-PM) presented two discrete phase structures. By using the composite catalyst in MTO reaction, the SZ-TS exhibited the longest life of 1400 min, which was much higher than that of SZ-PM (720 min). Methanol conversion reached 97.3%, and the selectivity for light olefins totalled 93.0%. The excellent catalytic performance of SZ-TS was attributed to the hierarchical pore structure and suitably weak acid site distribution. Via pre-crystallization, alkalinity can be adjusted in the adjacent ranges of SAPO-34 and ZSM-5. By adding the pre-crystallized alkaline SAPO-34 slurry into the strong alkaline system of the parent ZSM-5 slurry, phase dissolution of SAPO-34 in the secondary crystallization solution was avoided. The external surface of SAPO-34 was etched in the alkaline environment of the secondary crystallization solution, resulting in hierarchical pore distribution, which benefited the MTO reaction. This in situ synthesis method features the advantages of simplicity, low time consumption, and low cost. Therefore, this approach possesses the potential to satisfy the demands of the MTO market.