In situ-DRIFTS study: influence of surface acidity of rhenium-based catalysts in the metathesis of various olefins for propylene production

Abstract

Various olefins including 1- and 2-butene, 2-pentene, and ethylene were used as the reactants for producing propylene by self- and cross-metathesis reactions at 60 °C on supported Re-based catalysts (4 wt% Re). A similar surface structure of rhenium oxides in the form of isolated ReO₄ species was observed on Al₂O₃ and SiO₂–Al₂O₃ supported rhenium oxide catalysts. The catalysts, however, differed in terms of the types (Lewis/Bronsted) and strengths of the acidic sites, as revealed by ammonia temperature programmed desorption (NH₃-TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of NH₃ adsorption and thermodesorption results. Weak Lewis acidity, especially high ratios of Lewis acid bands at 1280 cm⁻¹/1622 cm⁻¹, led to high metathesis activity of the Re-based catalysts. The additional isomerization activity, however, required strong Bronsted acid sites. The product distribution from the various feeds depended largely on the size of the olefins, and not only on the acid properties of the catalysts. Re₂O₇/SiO₂–Al₂O₃ appeared to be a versatile catalyst for obtaining a high yield of propylene using either a single reactant (1- or 2-butene) or an ethylene mixed feed.