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 γalumina 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.

Steady state kinetics and measured pyridine inhibition of ethanol dehydration and dehydrogenation rates on γ-alumina above 623 K show that ethanol dehydrogenation can be described with an indirect hydrogen transfer mechanism to form acetaldehyde and ethane and that this mechanism proceeds through a shared surface intermediate with ethylene synthesis from ethanol dehydration. Ethane is produced at a rate within experimental error of acetaldehyde production, demonstrating that ethane is a coproduct of acetaldehyde synthesis from ethanol dehydrogenation. Steady state kinetic measurements indicate that acetaldehyde synthesis rates above 623 K are independent of co-fed water partial pressure up to 1.7 kPa and possess an ethanol partial pressure dependence between 0 and 1 (P ethanol = 1.0− 16.2 kPa), consistent with ethanol dehydrogenation rates being inhibited only by ethanol monomer surface species. The surface density of catalytically active sites for ethylene and diethyl ether production were estimated from in situ pyridine titration experiments to be ∼0.2 and ∼1.8 sites nm −2 , respectively, at 623 K. Primary kinetic isotope effects for ethylene and acetaldehyde are measured only when the C−H bonds of ethanol are deuterated, verifying that C−H bond cleavage is kinetically limiting for both products. The proposed indirect hydrogen transfer model for acetaldehyde synthesis is consistent with experimentally observed reaction rate dependences and kinetic isotope effects and highlights the complementary role of hydrogen adatom removal pathways in the formation of aldehydes on Lewis acidic systems.

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 Cβ–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.

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Direct conversion of syngas to hydrocarbons occurs over hybrid catalyst mixtures containing methanol synthesis and microporous acid components. In particular, both copper and zinc oxide-based as well as chromium-and zinc-based catalysts are active for methanol synthesis and can be used in the hybrid catalyst process. The choice of methanol synthesis catalyst alters product selectivity and distribution. In particular, reaction products of the Cu−Zn/SAPO-34 system include only saturated hydrocarbons, while the Cr−Zn/SAPO-34 catalyst enables light olefin production directly from syngas. Hydrogenation properties of the methanol synthesis catalyst influence the C 3 /C 2 yield ratios in the hydrocarbon products. We analyze the observed differences of selectivity with respect to olefin hydrogenation activities of the methanol synthesis components and their interaction with SAPO-34 for methanol-to-olefins conversion. A simplified kinetic model for the hybrid system is proposed to describe the observed selectivity patterns.