Parallel Alkane Dehydrogenation Routes on Brønsted Acid and Reaction-Derived Carbonaceous Active Sites in Zeolites

Abstract

Alkane dehydrogenation rates on acidic zeolites measured in the presence of co-fed H2 during initial contact with reactants solely reflect protolytic reactions at Brønsted acid sites, while rates measured without co-fed H2 and at later reaction times reflect additional contributions from an extrinsic dehydrogenation function derived from reactants and products. This extrinsic function consists of unsaturated organic residues that catalyze dehydrogenation turnovers by accepting H-atoms from alkanes and recombining them as H2. Such hydrogen transfer routes are inhibited by alkenes and H2 products and proceed with activation barriers much lower than for protolytic dehydrogenation at H+ sites, causing them to become more prevalent at lower temperatures and for zeolites with lower H+ densities. The number, composition, and reactivity of these extrinsic carbonaceous active sites depend on the local concentrations of reactants and products, which vary with alkane and H2 pressure, bed residence time, and axial mixing. These extrinsic catalytic moieties form within H2-deficient regions of catalyst beds but can be removed by thermal treatments in H2, which fully restore zeolite catalysts to their initial state. Carbonaceous deposits do not catalyze alkane cracking reactions; thus, cracking rate constants serve as a reporter of the state of proton sites, and their invariance with product pressure, residence time, and axial mixing confirms that protons remain unoccupied and undisturbed as extrinsic organic residues change in number, composition, and reactivity. The rates of the reverse reaction (alkene hydrogenation) under H2-rich conditions inhibit the formation and the reactivity of these organic residues, and taken together with formalisms based on nonequilibrium thermodynamics, they confirm that alkane dehydrogenation occurs solely via protolytic routes only at the earliest stages of reaction in the presence of added H2. These findings provide a coherent retrospective view of the root causes of the literature discord about alkane dehydrogenation turnover rates and activation barriers on acidic zeolites, variously attributed to extraframework Al or radical active sites and to turnovers limited by alkene desorption instead of protolytic steps. Importantly, these findings also prescribe experimental protocols that isolate the kinetic contributions of protolytic dehydrogenation routes, thus ensuring their replication, while suggesting strategies to deposit or remove extrinsic organocatalytic functions that mediate hydrogen transfer reactions.

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