Hierarchical P-ZSM-5 zeolites in situ synthesized using home-made asymmetric quaternary phosphonium for the methanol-to-propylene reaction

Abstract

Phosphorization and hierarchization of industrially important ZSM-5 zeolites as one of the primary catalysts for the methanol-to-propylene (MTP) reaction have increased their catalytic stability and C₃H₆ selectivity, but they still suffer from issues of poor phosphorus stabilization efficiency, uneven dispersion, and cumbersome synthesis and modification. In this work, we prepared a series of hierarchical P-ZSM-5 zeolites in situ in a one-step process by utilizing home-made asymmetric tributyl-(R)-phosphonium (R = propyl-, octyl-, or hexadecyl-) hydroxides with adjustable basicity and spatial configurations. Results revealed that the higher intercrystalline mesopore volume in the hierarchical P-ZSM-5 zeolites was generated with an increase in the R-alkyl chain length and the basicity of asymmetric tributyl-(R)-phosphonium. Moreover, fabricating tetracoordinated-phosphorus species could not only decrease the density of Brønsted acid sites and reinforce its acidity but could also stabilize them. Notably, the strong Brønsted acidity decreased with an increase in the number of mesopores. MTP tests showed that the hierarchical PZ-P catalyst synthesized using tributylpropylphosphonium exhibited an appreciable catalytic lifetime (23 h) and higher C₃H₆ selectivity (51.2%) in comparison to the conventional ZSM-5 zeolite. Given this, a positive relationship among the C₃H₆ selectivity, hierarchy factor and strong Brønsted acidity was further revealed. This work gives valuable information for developing an efficient MTP catalyst and on phosphorus–zeolite chemistry.