Temperature programmed desorption-FTIR investigation of C1-C5 primary alcohols adsorbed on γ-alumina

Temperature programmed desorption (TPD)-FTIR was used to investigate the products chemical reactions produced when methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol desorp from gamma-alumina into vacuum at elevated temperatures. Methanol and dimethyl ether desorbed for methanol and the alcohol, the ether, and the 1-alkene desorbed from the C-2-C-5 alcohols clearly indicating that some chemical reactions had occurred before the compounds desorbed. The mass of ether produced per gram of alumina decreased and the mass of alkene produced per gram of alumina increased as the number of carbons in the alcohol increased. The activation energies for the production of the ether and the alkene were identical within experimental error for the C2-C5 alcohols. With the exception of the formation of dipropyl ether, the activation energies found for the desorption of the reaction products agreed with the activation energies reported from kinetic measurements for the C-1-C-3 alcohols. PM3 calculations indicated that the preferred mode of bonding for undissociated methanol was the Lewis acid interaction between the oxygen in the methanol and the aluminum on the model surface. Bonding to neighboring Lewis acid-base sites had the lowest reaction enthalpy when the methanol dissociated into methoxy radicals and hydrogen atoms while adsorbed on the surface. (C) 2003 Elsevier B.V. All rights reserved.