Consequences of Acid Strength and Diffusional Constraints for Alkane Isomerization and β-Scission Turnover Rates and Selectivities on Bifunctional Metal-Acid Catalysts

Crystalline silicates with a given structure but different framework heteroatoms (e.g., Al, Ga, Fe, B in MFI) provide similar confining voids but sites with different acid strength. Their known structure allows systematic mechanistic inquiries into the role of acid strength on reactivity through theory (DFT) and experiments, as illustrated here for isomerization and β-scission of linear and branched alkanes. Stronger acids lead to higher turnover rates for all reactants because of their more stable conjugate anions at ion-pair transition states. As acid strength decreases, β-scission transition states become preferentially stabilized over those for isomerization because of the differences in charge distributions at their carbocations, leading to higher scission selectivities on weaker acids; these findings contradict prevailing paradigms based on observations that reflect the higher proton reactivity in stronger acids, which, in turn, leads to the diffusion-enhanced secondary β-scission of primary isomer products. The small voids in zeotypes lead to high reactivity through transition state stabilization by confinement but also hinder diffusion, leading primary isomers to undergo secondary reactions before egressing into the extracrystalline fluid phase. These diffusional effects lead to the observed high selectivities for β-scission on stronger acids, as shown by reaction-transport formalisms underpinned by experiments that systematically vary intracrystalline proton densities through the gradual desorption of preadsorbed NH₃ titrants during catalysis. These strategies allow intrinsic selectivities (single sojourn at an acid site) to be assessed separately from ubiquitous effects of diffusion-enhanced interconversions. Single-sojourn selectivities are similar on mesoporous and large-pore aluminosilicates (Al-MCM-41, FAU, BEA), reflecting confinement effects that influence isomerization and β-scission transition state carbocations to the same extent. In contrast, single-sojourn selectivities on medium-pore three-dimensional aluminosilicates (SVR, MFI, MEL) are influenced by secondary reactions even as intracrystalline proton densities decrease to very low values, because such reactions are enhanced by diffusional constraints even within a single cage, as a result of the undulating motifs prevalent in these frameworks.