Cracking and isomerization reactions shorten chains and vary branching of acyclic-alkane-rich streams. These reactions involve alkene intermediates and steps mediated by solid Brønsted acids, requiring protons within molecular diffusion distances of hydrogenation-dehydrogenation sites. Here, we probe isomerization and β-scission reactions of n-heptane and 2,4-dimethylpentane on zeolitic acids of different acid strength, framework structure, and metal-acid site proximity (crystal size and proton density), mixed with Pt/SiO<sub>2</sub> aggregates. Isomerization and β-scission turnover rates depend uniquely on alkane/H<sub>2</sub> ratio and are invariant with Pt loading, indicating alkenes are in equilibrium with respective alkanes for MFI (10-membered ring (MR)), BEA (12-MR), FAU (12-MR), SFH (14-MR), and Al-MCM-41 (mesoporous). n-Heptane isomerization turnover rates are unaffected by crystal size or proton density on all large-pore materials and on small MFI crystals (&lt;50 nm), consistent with rigorous reaction-transport formalisms indicating absence of intrapellet gradients and thus the true chemical origins of measured turnover rates, where first-order n-heptane isomerization rate constants reflect free energy differences between ion-pair transition states, stabilized both by their conjugate anions and van der Waals interactions with the confining framework, and gas-phase reactants. Thus, rate constants, for a given framework (e.g., MFI), decrease with decreasing acid strength as rigorously described by deprotonation energy, and for a given acid strength, increase as voids become smaller, until voids become too small to stabilize the transition state, consistent with sizes and energies derived from density functional theory. Measured product selectivities reflect a single surface sojourn rather than intrinsic selectivities because product isomers have higher reactivities for isomerization and β-scission reactions and lower diffusivities than reactant n-heptane, consistent with observed 2,4-dimethylpentane concentration gradients in the same materials. Thus, these results suggest that while intrinsic reactivity depends only on zeolite framework and heteroatom identity, metal-acid site proximity can be exploited to direct selective product formation through diffusion-enhanced secondary reactions.

Implications of acid strength, confinement, and site proximity for reactivity and selectivity in bifunctional metal-acid catalysis

251st National Meeting (2016)

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