Ethanol dehydration on alumina catalysts: I. The thermal desorption of surface compounds

Abstract

The surface ethoxide on alumina made by ethanol adsorption was desorbed by heat treatment, with the desorbed species analyzed by mass spectrometry. Below 135 °C the main components of the desorbed species by the thermal treatment were ethanol, diethyl ether, and ethylene, whereas only ethylene was obtained above 135 °C. There was no difference in the infrared spectra which correlates with the change in product distribution. As the concentration of the surface ethoxide was increased, the formation of diethyl ether was favored at the low-temperature region. On the contrary, the formation of ethylene was suppressed with the increased concentration of both the surface ethoxide and the surface hydroxyl group. At the high-temperature region, however, ethylene was the only product without regard to the surface concentration. A diethyl ether molecule is formed from two surface ethoxide groups. The ethylene formation may take place by hydrogen abstraction from the methyl group of the surface ethoxide by the exposed oxide ion adjacent to the surface ethoxide. The structure of the ethanol adsorption on silica gel was investigated for comparison. The dehydration of ethanol over silica-alumina is also discussed.