Osdas-Free Synthesis of High-Silica Mfi Zeolite by the Conventional Hydrothermal Route

Abstract

High-silica MFI zeolite can hardly be obtained by the conventional hydrothermal synthesis route in the absence of organic structure directing agents (OSDAs), which is ascribed to the competitive growth of quartz, magadiite, or kenyaite phase. In this work, these impurity phases were excluded from the synthetic gel through a low-temperature crystallization method, meanwhile the growth of zeolite phase was promoted by seed-directing and C2H5OH filling effects. Consequently, well-crystallized MFI samples with controllable SiO2/Al2O3 ratios from 92 to 410 and uniform particle sizes were successfully prepared, and the crystallization process was tracked by multiple characterization techniques, pointing toward a nonclassical particle attachment crystallization process. Interestingly, we found that the optimal C2H5OH/SiO2 ratio can be proportionally decreased with the decrease of the H2O/SiO2 ratio in the synthetic gel. More importantly, the obtained high-silica MFI zeolite exhibited much higher catalytic stability than the conventional sample synthesized with OSDAs during the methanol-to-propylene and n-butene cracking reactions, which may be attributed to its fewer defect sites determined by 1H magic angle spinning nuclear magnetic resonance. Overall, this work provides an efficient route for the large-scale production of high-silica MFI samples.

Keywords: High-silica ZSM-5, Green-synthesis, Low-temperature crystallization, defect sites