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

摘要

Alkane dehydrogenation rates on acidic zeolites measured in the presence of co-fed H-2 during initial contact with reactants solely reflect protolytic reactions at Bronsted acid sites, reflect additional contributions from an extrinsic dehydrogenation while rates measured without co-fed H-2 and at later reaction times 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 H-2. Such hydrogen transfer routes are inhibited by alkenes and H-2 products and proceed with activation barriers much lower than for protolytic dehydrogenation at H+ sites, causing them to become more prevalent at lower temper- attires 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 H-2 pressure, bed residence time, and axial mixing. These extrinsic catalytic moieties form within H-2-deficient regions of catalyst beds but can be removed by thermal treatments in H-2, 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 H-2-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 H-2. 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.