Mechanistic Study of Alcohol Dehydration on γ-Al2O3

The acid sites on gamma-Al2O3 were characterized using FTIR spectroscopy of adsorbed pyridine and temperature programmed desorption (TPD) of 2-propanamine, ethanol, 1-propanol, 2-propanol, and 2-methyl-2-propanol, together with density functional theory (DFT) calculations. Following room-temperature adsorption and evacuation, the surface coverages of the adsorbed alcohols were between 2 and 3.2 x 10(18) molecules/m(2). For each of the adsorbed alcohols, reaction to olefin and water products occurred in a narrow peak that indicated reaction is a first-order process with a well-defined activation energy, which in turn depended strongly on the particular alcohol. DFT calculations on an Al8O12 cluster are in excellent agreement with the experimental observations and show that the transition states for dehydration had carbenium-ion character. The carbenium ion stability in terms of proton affinity (of alkenes) matches well with the activation energy of the dehydration reaction. Adsorption of water on the gamma-Al2O3, followed by evacuation at 373 K, demonstrated that water simply blocks sites for the alcohols without affecting the reaction activation energy. There was no evidence for Bronsted sites on the gamma-Al2O3 based on FTIR of pyridine or TPD of 2-propanamine.