(545e) Inhibiting Role of Dimer Surface Species in the Dehydration of Alcohols Over ?-Alumina

Steady state kinetic measurements verify that surface dimer species composed of alcohol, water, and alcohol-water co-adsorbed surface species inhibit the synthesis of olefins and ethers from the unimolecular and bimolecular dehydration of ethanol and n-propanol on gamma-alumina at 488K.  Exposure of the catalyst surface to water prior to reaction lowered the synthesis rates for olefins and ethers, demonstrating that water irreversibly poisons a fraction of the active sites for alcohol dehydration over gamma-alumina. Kinetic isotope studies demonstrated that the rate-limiting step for the formation involves the either cleavage of the C_β-H bond or desorption of water after formation of the olefin molecule (KIE = 2.4), while the synthesis of ethers are limited by cleavage of either the C-O bond of the alcohol molecule or an O-Al bond of a surface-bound reaction intermediate. Different degrees of inhibition of the rates of olefin and ether synthesis by pyridine indicate that the active sites for olefin and ether formation are both acidic and non-equivalent. In-situ titrations of the active sites for ether synthesis identify an active site density of 0.1 sites nm^-2 at 488K. The selectivity for olefin formation is a factor of ten larger for 1-propanol dehydration than for ethanol dehydration, indicating a carbocation-like transition state for the unimolecular dehydration of alcohols on gamma-alumina. Dimer-inhibited mechanisms for the unimolecular and bimolecular dehydration pathways of ethanol and 1-propanol on gamma-alumina consistent with measured steady state and isotopic kinetic measurements and titrations are presented and evaluated.