Hierarchical low-silica SAPO-34 with enhanced MTO performance: Mesopore template- and fluoride-free synthesis

Abstract

The inherent mass transport limitations and undesired acidity of silicoaluminophosphate SAPO-34 catalysts are two key factors that result in a fast deactivation rate in the methanol-to-olefin (MTO) reaction. Herein, we demonstrate that SAPO-34 with hierarchical pore architecture and different silicon contents can be rationally synthesized by hydrothermal treatment of homogeneous amorphous silicoaluminophosphate precursors in the absence of mesopore templates and fluoride. It reveals that the hierarchical architecture is formed by in situ alkaline TEAOH etching of framework atoms. The silicon content (acidity) of the products can be facilely tuned in wide range. 29Si MAS NMR spectra reveal silicon mainly exists in the form of Si(4Al) in low-silica SAPO-34 catalysts free from silica islands. The hierarchical SAPO-34 catalyst with a moderate silicon content (nSi/n(Si + Al + P) = 0.048) and small particle size (265 ± 94 nm) exhibits a prolonged catalytic lifetime as well as a higher selectivity in MTO reaction, attributing to the synergetic effect of a fast diffusion speed and an appropriate acidic density.