Features of the Mechanism of the Dimethyl Ether to Light Olefins Conversion over MgZSM-5/Al2O3: Study by Vibrational Spectroscopy Experimental and Theoretical Methods

Features of the mechanism of dimethyl ether (DME) conversion to olefins over ZSM-5 zeolite catalysts were studied using experimental and theoretical methods of vibrational spectroscopy. A catalytic activity comparison of the catalysts Mg-HZSM-5 without a binder and Mg-HZSM-5/Al2O3 containing 1% Mg (by weight) and 33% Al2O3 (by weight) as a binder in the DME conversion was carried out. Using high-temperature diffuse reflectance IR (DRIR) spectroscopy in situ combined with quantum-chemical simulations in the temperature range of 25-450 degrees C in a stream of dry Ar and DME, the bands corresponding to OH bonds, including BAS and H3O+, were interpreted. Depending on the temperature and the presence of magnesium and a binder in the catalyst composition, the intensity and position of these bands maxima vary greatly. The intermediates of the catalytic DME conversion were discovered and identified. At low temperatures (below 200 degrees C), in a DME stream, methoxy groups (CH3O-Al-), ketene (CH2=C=O), and a carbocation (CH3+) were formed on the surface of the catalysts. As the temperature rises (above 300 degrees C), the bands from ketene completely disappear in the spectra of catalysts, and bands from oxonium cations or ylide particles appear, leading the process of DME conversion by the oxonium-ylide mechanism. In the presence of H3O+, the conversion of DME on the zeolite catalyst surface was more effective; however, selectivity for olefins was lower.