Mechanistic Study of Alcohol Dehydration on γ‑Al₂O₃

The acid sites on γ-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 × 10¹⁸ molecules/m². 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 γ-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 Brønsted sites on the γ-Al₂O₃ based on FTIR of pyridine or TPD of 2-propanamine.