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

Kinetics and Mechanism of Ethanol Dehydration on γ‑Al₂O₃: 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

Kinetics of Direct Olefin Synthesis from Syngas over Mixed Beds of Zn–Zr Oxides and SAPO-34

A packed bed containing a physical mixture of both Zn–Zr mixed oxide catalyst and SAPO-34 converts syngas directly into a mixture of C2–C5 olefins and paraffins. Specifically, the mixed oxide catalyst is responsible for intermediate oxygenate synthesis from syngas while the molecular sieve catalyzes olefin synthesis from the oxygenate intermediates. Kinetic measurements with cofed propylene over each catalyst independently confirm olefin hydrogenation activity over both components of the composite bed. The addition of either water or CO to the feed drops the activity of propylene hydrogenation over the Zn–Zr oxide. In sum, under reaction conditions of syngas feed and produced water, olefin hydrogenation predominantly occurs over the SAPO-34 catalyst, rather than over the catalyst responsible for hydrogenating CO into oxygenate intermediates. A developed kinetic model consistent with this conclusion describes measurements at differing feed compositions, temperatures, space velocities, and bed catalyst mixing ratios. Technoeconomic analysis of the process indicates that the olefin-to-paraffin ratio is a key performance metric for commercial scale syngas conversion and highlights the importance of considering olefin hydrogenation rates over the molecular sieve component