Kinetics and mechanisms of alcohol dehydration on metal oxide catalysts

Abstract

Alcohol dehydration on metal oxide catalysts produces valuable chemicals, olefins and ethers. The surface characteristics of the metal oxide formulation under reaction environments are important to understand the versatile deoxygenation, dehydrogenation, and carbon-chain growth reactions that occur in parallel. In this dissertation, I studied the kinetics, mechanisms, and site requirements of alcohol dehydration on metal oxides including alumina polymorphs (alpha, gamma, and eta), zirconia, and chlorinated alumina, to probe the evolution and function of these metal oxides under reaction environments. 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 presence of surface mutlimers composed of two alcohol molecules and one water molecule under reaction conditions. Mono-alcohol dehydration of C3-C4 alcohols follows the same mechanism as that for ethanol and involves inhibition by dimers. 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 di-alcohol dehydration reactions is the same for different alcohols, irrespective of the carbon chain length and substitution. 2-Propanol has the highest rate constant for mono-alcohol dehydration among studied alcohols, demonstrating that stability of the carbocation-like transition state is the primary factor in determining rates of mono-alcohol dehydration which concomitantly results in high selectivity to the olefin. 1-Propanol and isobutanol have higher olefin formation rate constants than ethanol indicating that olefin formation is also affected by carbon chain length. Isobutanol has the lowest rate constant for di-alcohol dehydration because of steric factors. The steady state rates of ethene and diethyl ether formation in parallel ethanol dehydration reactions at 573 and 623 K are mechanistically and kinetically described by the same rate expression on different alumina materials (α-, γ-, and η-Al2O3), implying that alumina materials have similar surface sites under reaction environments. In-situ chemical titration using pyridine as a titrant elucidates similar site densities (~0.12 sites nm-2 and ~0.07 sites nm-2 for ethene formation and ~0.14 sites nm-2 and ~0.09 sites nm-2 for diethyl ether formation on γ- and η-Al2O3, respectively) on γ- and η-Al2O3 indicating that similar surface features exist on both γ- and η-Al2O3. Pyridine-ethanol co-feed experiments show that pyridine inhibited the formation of ethene to a greater extent than diethyl ether suggesting that the two parallel dehydration reactions are not catalyzed by a common active site. The mechanisms and rate expressions that describe the kinetics of mono- and di-alcohol dehydration by alumina polymorphs are also valid for other solid acid catalysts such as ZrO2 and Cl-Al2O3. A comparison of the rates (per g) for alumina polymorphs, ZrO2, and Cl-Al2O3 reveals that rates are lower for ZrO2 and higher for Cl-Al2O3. ZrO2 formulations show ~50% carbon selectivity to ethene at 573 K with 2.2 kPa of ethanol and 1.1 kPa of water partial pressure while the selectivity to ethene at these process conditions on γ-Al2O3 is ~30%. Chlorinated alumina and γ-Al2O3 show same selectivity to ethene at 488 K when water pressure is negligible.