Kinetics, mechanisms, and site requirements

Abstract

We report the kinetics, mechanisms, and site densities of parallel ethanol dehydration and dehydrogenation over gamma-alumina (γ-Al2O3), a high surface area and thermally-stable metal oxide used both as a catalyst support and as a Lewis acid catalyst in industrial practice. We further extend our investigations to diethyl ether conversion over γ-Al2O3 to describe the reaction network for ethanol dehydration and dehydrogenation at conversions exceeding 10%. Steady state measurements demonstrate that unimolecular and bimolecular ethanol dehydration rates are inhibited by water-ethanol co-adsorbed complexes at 488 K. Reactive surface intermediates, rather than co-adsorbed complexes, inhibit the rates of ethanol dehydration and dehydrogenation at industrially-relevant temperatures (>623 K). Co-processing pyridine with ethanol/water feed mixtures results in a reversible inhibition of both unimolecular and bimolecular ethanol conversion pathways; the synthesis rates of ethylene and acetaldehyde are inhibited to a greater extent than diethyl ether synthesis rates, establishing that unimolecular reactions occur on a pool of catalytic sites separate from the pool for bimolecular dehydration reactions. An observed 1:1 ratio of acetaldehyde and ethane in the eluent verifies that ethanol dehydrogenation proceeds via a hydrogen transfer mechanism. We employ asymmetric ethers as probes to establish ether conversion on γ-Al2O3 occurs through a disproportionation pathway to form an olefin and an alcohol, rather than through a hydration pathway. Diethyl ether disproportionation rates were verified to (i) possess an intrinsic rate constant that is within a factor of two of that of unimolecular ethanol dehydration and (ii) be inhibited by pyridine to the same extent as ethylene synthesis rates from ethanol dehydration. These observations are consistent with a proposed mechanism in which ether disproportionation and unimolecular alcohol dehydration occur through a common alkoxide reaction intermediate and on a common pool of catalytic sites. Our combined investigations of alcohol and ether conversion establish the existence of two distinct pools of catalytic centers, verify all unimolecular pathways of alcohol dehydration, dehydrogenation, and ether disproportionation occur on a common set of active sites, and provide a rigorous kinetic description of these pathways.