(535c) Computational Studies of Supported Single-Atom Catalysts for Alkane Dehydrogenation

Authors: 
Das, U. - Presenter, Argonne National Laboratory
Getsoian, A. - Presenter, Argonne National Laboratory
Stair, P. C. - Presenter, Northwestern University
Miller, J. T. - Presenter, Argonne National Laboratory
Curtiss, L. A. - Presenter, Argonne National Laboratory

Supported single-atom catalysts are ideal for highly selective chemical transformation while maintaining very high atom-efficiency. In this presentation, we will talk about supported single-atom catalysts for dehydrogenation of light alkanes to olefins, an important chemical reaction used to produce feedstocks for polymer industries. While thermal cracking of alkanes also produces olefins, catalytic conversion is desired to achieve higher selectivity. Acidic zeolites modified by gallium ions convert light alkanes to aromatics, and are used as catalysts in the commercial Cyclar process. The catalyst is bifunctional wherein the metal cations are catalytic for dehydrogenation of alkanes to olefins, which then oligomerize and subsequently form aromatics over Brønsted acid sites of the zeolite. The reaction product distribution depends on the Ga to acid site ratio, and for catalysts containing few acid sites, olefin is the main reaction product. Despite the progress, the true nature of the Ga active species in zeolite catalyst is not well understood. While Ga is present in a +3 oxidation state in the as prepared catalyst, under reaction conditions, the metal is believed to be reduced into Ga+1 and exists either as charge compensated isolated Ga+1 ion or GaH species. Interestingly, such reduction of Ga+3 is not observed in similar conditions when amorphous silica instead of zeolite is used as support. In this presentation, we will talk about our theoretical studies investigating the origin of such an apparent discrepancy of Ga+3 ion reducibility on two different support materials, i.e., zeolite versus silica. Furthermore, our recent theoretical, XAS, and Raman spectroscopic results indicating that Ga+3 perhaps does not reduce to Ga+1 in zeolite catalysts will be discussed.