Effect of the Acidity of HZSM-5 Zeolite and the Binder in the DME Transformation to Olefins

Abstract

The effect of the catalyst properties, particularly matrix acidity and its properties, has been studied in the transformation of dimethyl ether (DME) to light olefins. The catalysts used were prepared by agglomerating HZSM-5 zeolite with a different SiO2/Al2O3 ratio (30, 80, and 280) with bentonite or boehmite as a binder and α-Al2O3 as inert filler. The experiments have been carried out in a fixed bed reactor, under the following reaction conditions: 350 and 400 °C; space-time, 0.4 gzeolite h molC–1; time on stream, 4 h. The lower content of H2O in the reaction medium compared to that of the methanol transformation explains the advancement on the reaction scheme (with the transformation of olefins in other hydrocarbons) and more rapid deactivation by coke, which advises the use of catalysts with lower acidity and shaping the catalyst particles with a porous structure that attenuates the blocking of acid sites. The acidic properties of the zeolite have a significant impact on the DME conversion, yield and selectivity of light olefins, and catalyst stability. A good compromise of these rates is obtained with a catalyst prepared with HZSM-5 zeolite of moderate acidity (SiO2/Al2O3 = 280) and using boehmite as binder. In the calcination of catalyst the boehmite is converted into γ-Al2O3, whose weak acidity and mesoporous structure contribute to increase activity and stability of the catalyst.