In Situ Aluminum Migration into Zeolite Framework during Methanol-To-Propylene Reaction: An Innovation to Design Superior Catalysts

This work involves methanol-to-propylene (MTP) conversion over aluminosilicate MFI (ZSM-5), borosilicate MFI (B-ZSM-5), and all-silica MFI (Silicate-1). Both B-ZSM-5 and Silicate-1 were inactive in MTP reaction. However, extruded Silicate-1 sample prepared by extrusion with Al₂O₃ binder, followed by acid washing treatment, displayed a rapid increase in methanol conversion in the initial 30 h time on stream (TOS), and then retained 99% methanol conversion, higher propylene selectivity (52.2%), and higher propylene/ethylene ratio (11.3) for 400 h TOS. Silicate-1 modified with AlCl₃ or Al(NO₃)₃ showed similar catalytic performance as the extruded samples. Several AlCl₃-modified Silicate-1 samples after various TOS values were regenerated and characterized by NH₃-termperature-programmed desorption (TPD), Fourier transform infrared (FT-IR) spectroscopy, and ²⁷Al magic angle spinning nuclear magnetic responance (MAS NMR). The results point to continuous aluminum insertion into Silicate-1 framework during reaction in situ, which rationalizes the superior catalytic performance. By analyzing catalytic performances of AlCl₃-modified samples with different amounts of defect sites, we concluded that Al migration is related to the defect sites. Finally, a catalyst with a much longer lifetime of 960 h and 53.2% propylene selectivity was developed by incorporating Al migration into hierarchical samples.