Mechanism of alcohol decomposition over alumina. A dynamic treatment of chemisorbed species during the course of the reaction

Abstract

The mechanism of alcohol decomposition over alumina has been studied by means of infra-red techniques in the working condition. The decomposition of methanol (or ethanol) to ether, or that of isopropanol (or ethanol) to olefin, over alumina is zero order as to alcohol, the rate being independent of its pressure. When all the alcohol vapour was trapped out of the system, the evolution of ether or olefin first continued at the same rate through the decomposition of the adsorbed alkoxides. Consequently, the ether or olefin formation is the recombination reaction or the decomposition reaction of the alkoxide group on the surface: MeOH → MeO(ad.)+ H(ad.)→ MeOMe + H₂O [graphic omitted] i-PrOH → i-PrO(ad.)+ H(ad.)→ C₃H₆+ H₂O.

The evolution of ethylene from diethyl ether was also independent of ether pressure, and diethyl ether was adsorbed on alumina with dissociation to ethoxide groups. Consequently, diethyl ether and ethylene were formed from ethanol through the common intermediate, the ethoxide group on the surface.

Ether or olefin formation was retarded by the water produced. The decomposition rate decreased with the increasing amount of water adsorbed. The number of active sites for ether formation was estimated for the methanol decomposition. It was concluded that the adsorbates on the active surface behave uniformly in their working state.