Browsing by Subject "Metal Oxides"

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    Kinetics, mechanisms, and site requirements
    (2016-08) DeWilde, Joseph
    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.
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    Theoretical Insights into Metal Promoted Oxides for Alkane Activation Chemistries: Tuning Catalyst Reactivity via Metal Doping
    (2020-02) Chen, Thomas
    Metal-doped refractory oxides show promise for the upgradation of light alkanes to higher-value chemicals. With the advent of shale gas, direct dehydrogenation routes catalyzed by metal-doped oxides provides an alternative and selective route to propene from propane that is unattainable by thermal routes. Further, alkanes transformation to aromatics with its myriad of reactions may be aided by the bifunctional sites of these catalysts. However, the lack of understanding of the mechanisms, active sites, and structure-reactivity relationships of metal-doped oxides presents outstanding challenges towards their development. Density functional theory (DFT) calculations can assist in gathering fundamental insight and practical understanding of these catalysts through atomistic simulations and microkinetic modeling. Comparing pure and Y-doped ZrO2 has allowed elucidation of the metal dopant’s role in enabling formation of catalytically active oxygen vacancies. Vacancy-O sites effectively catalyze dehydrogenation where the stronger Lewis acidity of oxygen vacancies compared to terrace (Zr-O) sites aids in C-H activation without overly binding H intermediates that can poison the catalyst. Through the catalyst formulation, an “ideal” catalyst may be developed that balances the opposed reactions (C-H activation and H2 recombination) of dehydrogenation. Different oxides (TiO2, ZrO2, and CeO2) and dopants (Sc, Y, La, Ti, Ce, V, Cr) were examined to study their effect on the structure of the catalyst, i.e. oxygen vacancy formation; on the electronic structure, i.e. vacancy’s electron density distribution; and on dehydrogenation reactivity, i.e. Lewis acidity and apparent barriers. Changing the oxide affects the catalyst’s inherent Lewis acid-base strength, and changing the dopant alters the oxide’s propensity to form vacancies and the electronic structure at the vacancy site, which allows for the “tuning” of the catalyst’s relative C-H activation and H removal ability. DFT calculations show the maximal concomitant rate of reactions (C-H activation and H2 recombination) as determinant of the catalyst dehydrogenation activity. Lastly, these catalysts were extended to C6-C8 alkane dehydrocyclization where 1,6-ring closure was found to be dominantly rate limiting in the conversion of alkanes to aromatics. Thus dehydrocyclization, unlike dehydrogenation, is controlled by a single step which is catalyzed by strong Lewis acids, tunable by the dopant and host oxide. This work shows the ability of metal-doped oxides to catalyze various reactions and provides insights to aid in the catalyst development for targeted pathways.