Effect of chromium and boron incorporation methods on structural and catalytic properties of hierarchical ZSM-5 in the methanol-to-propylene process

Abstract

We introduced chromium and boron into the hierarchical structure of ZSM-5 by following an incipient wet impregnation and hydrothermal synthesis approach to enhance the performance of the catalysts in the methanol-to-propylene process. Crystal structure, preferred orientation, and crystal symmetry of synthesized zeolites were discussed using Rietveld refinement and revealed that all synthesized zeolites crystallized in a monoclinic structure, whereas the boron-incorporated sample exhibited an orthorhombic symmetry. Large-scale defect-free single-crystalline structures of hierarchical zeolite created by CTAB and F127 mesoporogens are confirmed by electron microscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The formation of surface metal oxides and extra-framework metal oxides caused changes in the electronic structure of the components as visible in Si 2p and O 1s spectra. The predominant presence of OH groups and the higher Cr (VI) /Cr (III) ratio account for the better performance of impregnated chromium-ZSM-5 especially in the production of light olefins. The introduction of boron by impregnation further preserved the preferred growth orientation, hierarchical structure with high crystallinity and caused surface acidity changes in favor of increasing propylene selectivity to 67% with a propylene/ethylene ratio of close to 8.