Isomerization and β-scission reactions of alkanes on bifunctional metal-acid catalysts: Consequences of confinement and diffusional constraints on reactivity and selectivity

Abstract

Small voids stabilize transition states through van der Waals contacts in solid acid catalysts and lead to higher reactivity as inorganic hosts and organic carbocations at transition states become similar in size and shape. Such voids also impose diffusional hurdles that can strongly influence selectivities. Bifunctional mixtures of Pt/SiO2 with mesoporous or microporous aluminosilicates (Al-MCM-41, FAU, SFH, BEA, MFI) with acid sites of similar strength are used in this study to describe the consequences of confinement and diffusional constraints on isomerization and β-scission turnover rates and selectivities for n-heptane reactants. First-order rate constants (per H+) for n-heptane isomerization (to primary methylhexane products) reflect free energy differences between confined carbenium ions at transition states and their gaseous alkenes precursors; they increase (∼103-fold) as van der Waals contacts between carbocations and voids become more effective. Maximum turnover rates are achieved when their diameters, based on spherical constructs, become similar. Such diameters represent, however, incomplete assessments of fit, because they neglect matters of shape essential for effective van der Waals contacts. These geometric descriptors are replaced here by van der Waals interaction energies (Evdw) determined by statistical sampling of the void space in each framework using representative DFT-derived carbocation structures. These Evdw values are similar for the transition states that mediate primary and secondary isomerization and secondary β-scission reactions within each given void environment, leading to intrinsic selectivities that cannot depend on confinement effects. The marked differences in selectivity among aluminosilicates frameworks reflect instead diffusional enhancements of secondary transformations. Primary methylhexene isomers, which are more reactive and diffuse more slowly than n-heptenes, isomerize to dimethylpentenes as they egress from zeolite crystals before hydrogenating at the extracrystalline Pt function, and dimethylpentenes represent the predominant precursors to β-scission products. These diffusional effects are strongest for small voids, because they impose the most effective confinement and the most severe diffusional hurdles, and among these, on larger acid domains at higher proton densities. Reaction-transport formalisms show that Thiele moduli for these systems accurately describe selectivities with rate constants for β-scission and primary and secondary isomerization events affected similarly by a given framework, consistent with the similar Evdw values for their respective transition states. The effects of intracrystalline density of accessible protons within acid domains, varied systematically as pre-adsorbed NH3 titrants desorbed, on selectivities confirm these mechanistic interpretations. These data show that void structures affect selectivity, but not because of preferential stabilization of some transition states or of acid sites that vary in strength among aluminosilicate framework, as proposed previously to account for such effects. The conceptual and mathematical framework developed and used here for n-heptane isomerization on bifunctional metal-acid catalysts is general; it is also essential for rigorous mechanistic and practical assessments of catalysis on microporous catalysts, for which diffusive and confinement properties are in fact inseparable.