IDENTIFICATION OF ALCOHOL ADSORPTION SITES ON GAMMA-ALUMINA

A novel temperature-programmed desorption technique in which the probe of a mass spectrometer is used as the reactor has been developed. FT-IR and this new technique have been used to study the adsorption and dehydration reactions of a series of aliphatic alcohols on γ-alumina. It was found that the facility with which the various alkoxides undergo elimination processes is dependent on the size of the alkyl groups and the degree of branching. Reactions involving [18O[ethanol and [18O]methanol produce two isotopomeric ether products (R18OR and R16OR), which shows that bound alkoxides are formed by two routes; (a) dissociative adsorption on Lewis acid sites and (b) nucleophilic attack by a surface oxide on an alcohol that is probably activated toward CO cleavage. Poisoning experiments using 2,6-dimethylpyridine confirm that the ether isotopomers are formed from different alkoxide species. Time studies indicate that there is no scrambling of the 16O and 18O alkoxides between the different types of surface sites. © 1992.