Theoretical Insights into Metal Promoted Oxides for Alkane Activation Chemistries: Tuning Catalyst Reactivity via Metal Doping

Abstract

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.