Catalytic Alkylation Routes via Carbonium-Ion-Like Transition States on Acidic Zeolites

Brønsted acid sites in zeolites catalyze alkene hydrogenation with H2 via the same kinetically-relevant (C-H-H) + carboniumion-like transition states as those involved in monomolecular alkane dehydrogenation. Reactions between C3H6 and H2 selectively form C3H8 (>80% carbon basis) at high H2/C3H6 ratios (>2500) and temperatures (>700 K). Ratios of C3H8 dehydrogenation to C3H6 hydrogenation rate constants (718–778 K) were identical on H-FER, H-MFI, and H-MOR zeolites and equal to the equilibrium constant for the stoichiometric gas-phase reaction, consistent with De Donder non-equilibrium thermodynamic treatments of chemical reaction rates. 3] The seemingly fortuitous extensions of the principle of microscopic reversibility and the De Donder relations beyond their rigorous descriptions of chemical reaction dynamics at equilibrium and far from equilibrium but at identical (T, Pj), respectively, reflect the persistence of the same single kinetically-relevant step and the prevalence of unoccupied H sites at the different conditions used to measure forward and reverse rates. By inference, larger alkanes should also form in direct alkene-alkane addition steps via the same (C-C-H) carboniumion-like transition states involved in monomolecular alkane cracking. These chemical processes differ from alkylation mechanisms prevalent on liquid and solid acids (e.g. , HF, H2SO4, Hzeolites) and superacids (e.g. , HF-SbF5, HF-TaF5), which are mediated by carbenium-ion chain carriers that terminate as alkanes via hydride transfer. Carbonium-ions contain threeatom/two-electron centers and have been posited to mediate the formation of C3H8 in reactions of CH4-C2H4 mixtures on superacids at the low temperatures (<573 K) required for favorable alkylation thermodynamics. Here, we provide definitive kinetic and isotopic evidence that catalytic CH4-C2H4 alkylation reactions are mediated by the same transition states involved in monomolecular alkane cracking, even on zeolitic Brønsted acid sites at high temperatures (>700 K). Monomolecular alkane cracking routes prevail at high temperatures and low concentrations of alkene products ; they involve late (C-C-H) carbonium-ion-like transition states in kinetically-relevant C C bond cleavage steps and unoccupied H sites as most abundant surface intermediates (MASI). 7, 12–16] Minority species adsorbed on H sites are in quasi-equilibrium with gas phase reactants and products, leading to monomolecular C3H8 cracking rates given by Equation (1):