Effect of Hydrothermal Treatment on the Acidity and Catalytic Performance of Nanosized HZSM-5 Zeolites for the Conversion of Methanol to Propene

Abstract

A series of modified nanosized HZSM-5 zeolites were prepared by hydrothermal treatment at different temperatures and characterized by the adsorption and desorption of N2,X-ray diffraction (XRD),Al solid state magic angle spinning nuclear magnetic resonance (27Al MAS NMR),temperature-programmed desorption of ammonia (NH3-TPD),and Fourier transform infrared (FT-IR) spectroscopy of adsorbed pyridine techniques. The conversion of methanol to propene was tested using a continuous flow fixed-bed microreactor at atmospheric pressure,500 ℃,and a methanol weight hourly space velocity (WHSV) of 1.0 h-1. Results showed that after the appropriate hydrothermal treatment,some of Al was removed from the framework of zeolite and extracted out of the zeolite channels after treatment with an aqueous solution of citric acid. This led to a decrease in the strength and amount of acidic sites and an increase in the volume and diameter of the pores. As a result,propene selectivity and the reaction time continued to maintain the total conversion of methanol (i.e.,life time of catalyst) increased significantly to 38.9% and 160 h from 30.1% and 75 h over the parent nanosized HZSM-5 zeolite. However,severe hydrothermal treatment decreased the acidity remarkably and the strongly acidic sites were almost totally destroyed,resulting in a remarkable decrease both in propene selectivityand catalyst stability.