Kinetics and mechanism of alcohol dehydration on γ-Al₂O₃: Effects of carbon chain length and substitution

Steady-state rates of ether formation from alcohols (1-propanol, 2-propanol, and isobutanol) on γ-Al₂O₃ at 488 K increase at low alcohol pressure (0.1-4.2 kPa) but asymptotically converge to different values, inversely proportional to water pressure, at high alcohol pressure (4.2-7.2 kPa). This observed inhibition of etherification rates for C₂-C₄ alcohols on γ-Al₂O₃ by water is mechanistically explained by the inhibiting effect of surface trimers composed of two alcohol molecules and one water molecule. Unimolecular dehydration of C₃-C₄ alcohols follows the same mechanism as that for ethanol and involves inhibition by dimers. Deuterated alcohols show a primary kinetic isotope effect for unimolecular dehydration, implicating cleavage of a C-H bond (such as the C_β-H bond) in the rate-determining step for olefin formation on γ-Al₂O₃. Bimolecular dehydration does not show a kinetic isotope effect with deuterated alcohols, implying that C-O or Al-O bond cleavage is the rate-determining step for ether formation. The amount of adsorbed pyridine estimated by in situ titration to completely inhibit ether formation on γ-Al₂O₃ shows that the number of sites available for bimolecular dehydration reactions is the same for different alcohols, irrespective of the carbon chain length and substitution. 2-Propanol has the highest rate constant for unimolecular dehydration among studied alcohols, demonstrating that stability of the carbocation-like transition state is the primary factor in determining rates of unimolecular dehydration which concomitantly results in high selectivity to the olefin. 1-Propanol and isobutanol have olefin formation rate constants higher than that of ethanol, indicating that olefin formation is also affected by carbon chain length. Isobutanol has the lowest rate constant for bimolecular dehydration because of steric factors. These results implicate the formation and importance of di- and trimeric species in low-temperature dehydration reactions of alcohols and demonstrate the critical role of substitution and carbon chain length in determining selectivity in parallel unimolecular and bimolecular dehydration reactions.