Catalytic Consequences of Spatial Constraints and Acid Site Location for Monomolecular Alkane Activation on Zeolites

Abstract

The location of Brønsted acid sites within zeolite channels strongly influences reactivity because of the extent to which spatial constraints determine the stability of reactants and of cationic transition states relevant to alkane activation catalysis. Turnover rates for monomolecular cracking and dehydrogenation of propane and n-butane differed among zeolites with varying channel structure (H-MFI, H-FER, H-MOR) and between OH groups within eight-membered ring (8-MR) side pockets and 12-MR main channels in H-MOR. Measured monomolecular alkane activation barriers depended on catalyst and reactant properties, such as deprotonation enthalpies and proton affinities, respectively, consistent with Born-Haber thermochemical cycles that define energy relations in acid catalysis. Monomolecular alkane cracking and dehydrogenation turnovers occurred with strong preference on acid sites contained within smaller 8-MR pockets in H-MOR, while rates on sites located within 12-MR channels were much lower and often undetectable. This strong specificity reflects transition states that are confined only partially within 8-MR pockets; as a result, entropic gains compensate for enthalpic penalties caused by their incomplete containment to give a lower free energy for transition states within small 8-MR side pockets. These effects of entropy are stronger for dehydrogenation, with a later and looser transition state, than for cracking in the case of both propane and n-butane; therefore, selectivity can be tuned by the selective positioning or titration of OH groups within specific environments, the number of which was assessed in H-MOR by rigorous deconvolution of their infrared spectra. Specifically, cracking-to-dehydrogenation ratios for propane and n-butane were much smaller and terminal-to-central C-C bond cleavage ratios for n-butane were much larger on 8-MR than on 12-MR acid sites as a result of partial confinement, a concept previously considered phenomenologically as pore mouth catalysis. These marked effects of spatial constraints and of entropic factors on acid site reactivity and selectivity, also inferred for MFI from titration of OH groups by Na(+), have not been previously proposed or recognized and appear to be unprecedented in hydrocarbon catalysis. These findings and their conceptual interpretations open opportunities for the design of microporous solids by the rational positioning of acid sites within specific channel locations and with predictable consequences for catalytic rates and selectivities.