An in situ study of dimethyl ether conversion over HZSM-5/Al2O3 zeolite catalysts by high-temperature diffuse reflectance infrared fourier transform spectroscopy

Abstract

Isolated and associated OH groups have been in situ identified by high-temperature diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of the surface of zeolites calcined until complete removal of water. It has been shown that the Bronsted acid sites (BASs) involved in dimethyl ether (DME) conversion on the zeolite surface exhibit two characteristic bands in the DRIFT spectra. It has been found that the composition of DME conversion products on HZSM-5 zeolite catalysts modified with Mg or Rh is temperature-dependent. Ethanol and a small amount of methanol were detected on an Rh-HZSM-5/Al2O3 zeolite catalyst at T = 270°C and were not observed on Mg-HZSM-5/Al2O3 under the same conditions. Spectral features of ketene were detected in the DRIFT spectra of the surface of Mg-HZSM-5/Al2O3 in a stream of DME. The spectrum of Rh-HZSM-5/Al2O3 calcined at 200°C exhibited spectral features of ethyl alcohol and bands characterizing a strong Lewis acid site (LAS) capable of hydrogenating ketene to ethyl alcohol. The spectra of the surface of all the zeolite catalysts tested in a DME stream at 320°C contain bands characterizing olefins and alkanes and do not exhibit the spectral features of alcohols and ketene.