A study of commercial transition aluminas and of their catalytic activity in the dehydration of ethanol

Conversion of ethanol was investigated over four commercial aluminas prepared by different industrial procedures and one commercial silica-alumina. Characterization was performed by TEM, XRD, SBET and porosity measurements, and IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. Different features are attributed to different phases (γ-, δ-, θ-Al2O3) and different impurities (Na+, Cl−). Total conversion of ethanol with >99% selectivity to ethylene is achieved at 623 K over the purer Al2O3 catalyst (Na < 0.002 wt%). The most active sites are believed to be Lewis acidic Al3+ sites in a tetrahedral environment located on edges and corners of the nanocrystals. Ethanol adsorbs dissociatively on Lewis acid-base pair sites but may also displace water and/or hydroxyl groups from Lewis acidic Al3+ sites forming the active intermediate ethoxy species. Surface ethoxy groups are supposed to be intermediate species for both diethyl ether and ethylene production. Silica-alumina also works as a Lewis acid catalyst. The slightly lower activity on surface area basis of silica-alumina than aluminas attributed to the lower density of Lewis acid sites and the absence of significant basicity.