Kinetics and mechanism of ethanol dehydration on γ-Al ₂O₃: The critical role of dimer inhibition

Steady state, isotopic, and chemical transient studies of ethanol dehydration on γ-alumina show unimolecular and bimolecular dehydration reactions of ethanol are reversibly inhibited by the formation of ethanol-water dimers at 488 K. Measured rates of ethylene synthesis are independent of ethanol pressure (1.9-7.0 kPa) but decrease with increasing water pressure (0.4-2.2 kPa), reflecting the competitive adsorption of ethanol-water dimers with ethanol monomers; while diethyl ether formation rates have a positive, less than first order dependence on ethanol pressure (0.9-4.7 kPa) and also decrease with water pressure (0.6-2.2 kPa), signifying a competition for active sites between ethanol-water dimers and ethanol dimers. Pyridine inhibits the rate of ethylene and diethyl ether formation to different extents verifying the existence of acidic and nonequivalent active sites for the dehydration reactions. A primary kinetic isotope effect does not occur for diethyl ether synthesis from deuterated ethanol and only occurs for ethylene synthesis when the β-proton is deuterated; demonstrating olefin synthesis is kinetically limited by either the cleavage of a C_β-H bond or the desorption of water on the γ-alumina surface and ether synthesis is limited by the cleavage of either the C-O bond of the alcohol molecule or the Al-O bond of a surface bound ethoxide species. These observations are consistent with a mechanism inhibited by the formation of dimer species. The proposed model rigorously describes the observed kinetics at this temperature and highlights the fundamental differences between the Lewis acidic γ-alumina and Brønsted acidic zeolite catalysts.