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

摘要

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 beta-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 (similar to 10(3)-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 (E-vdw) 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 beta-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 egr