(79h) Acid Strength, Solvation, and Site Proximity Effects in Alkane Isomerization On Brønsted Solid Acids

Tungsten-based Keggin polyoxometalates (POM; H_8-nXⁿ⁺W₁₂O₄₀) and zeolites in their proton form are Brønsted solid acids with known structure and well-defined acid sites, making their acid strength accessible to theoretical estimates as deprotonation energies (DPE) [1]. DPE values are defined as those required to separate protons from conjugate bases to negligible interaction distances. Zeolites also exhibit diverse void structures, which can confer enhanced reactivity through van der Waals stabilization of confined reactants and transition states (TS). Here, we examine how electrostatic and dispersive forces act in concert to stabilize intermediates and TS structures in 2-methylpentane (2MP), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB), and n-hexane (nH) isomerization routes and, in doing so, give rise to observed acid strength and solvation effects on reactivity and selectivity. These reactions occur on bifunctional physical mixtures of POM clusters with different central atoms (X: P, Si, Al, and Co) or Pt impregnated zeolite H-Beta (BEA) with Pt/Al₂O₃, via alkene formation on metal (Pt) sites and subsequent isomerization of these alkenes mediated by ion-pairs at transition states on acid sites [2]. Slow diffusion of product alkenes out of acid domains allows them to undergo secondary isomerization reactions prior to their hydrogenation at Pt sites. The extent of these secondary reactions, which determines measured product selectivities, depends on the density and strength of sites in acid domains and the proclivity of product alkenes to undergo hydrogen transfer events with reactant alkanes.

Each reactant alkene forms multiple isomer products at acid sites via distinct ion-pair transition states. Measured first-order isomerization rate constants (including the formation of all isomers) (k_isom), normalized by the number of reactive protons,  decrease exponentially with increasing DPE values for all reactants on POM clusters, because stronger acids lead to more stable conjugate anions at ion-pair transition states. Thermochemical cycles indicate that electrostatic interactions between the anion and the cationic moieties at TS attenuate DPE effects on activation barriers because the geometry and charge distribution of the cationic fragments are similar among these catalysts. Isomerization rate constants for all reactants sense DPE changes similarly, in spite of their very different values, because cationic moieties at their respective TS exhibit similar charge distributions and thus benefit from electrostatic interactions with the conjugate anion to the same extent. The consequent absence of DPE effects on the ratio of these rate constants shows that acid strength cannot influence intrinsic isomerization selectivities, because all TS in this isomerization network contribute significantly to one or more of the measured overall rate constants. Acid sites within BEA zeolites give larger rate constants than predicted for POM acids for the calculated DPE values on zeolites because van der Waals forces, in addition to electrostatic interactions, stabilize kinetically-relevant transition states. These confinement effects are greater for reactions involving methyl shifts along the backbone than for those that vary the backbone length, because the transition states for methyl shifts are able to contact the zeolite framework more effectively and thus benefit from stronger van der Waals stabilization.

The size and diffusion properties of the acid domains in bifunctional catalysts and the concomitant metal-acid site proximity determine the concentration gradients of reactant and product alkenes inside and outside of acid catalyst pellets and influence rates and selectivities by changing the Thiele moduli for primary and secondary isomerization reactions. Measured first-order isomerization rate constants do not depend on acid site density or metal/acid site (Pt/H⁺) ratio for the physical mixtures used here, indicating that reactant alkenes and alkanes equilibrate at Pt sites and that reactant alkene concentration gradients are negligible within acid domains. Measured product selectivities, however, do not rigorously reflect the intrinsic formation rates of each isomer, because they can undergo subsequent isomerization events before forming less reactive alkanes, either via hydrogen transfer (from alkane reactants) at acid sites or reactions with H₂ at metal sites. Measured product selectivities depended on reactant alkane pressure, acid site density, and (Pt/H⁺) ratios, suggesting secondary isomerization events occur at rates comparable to those of hydrogen transfer and of alkene diffusion within acid domains. Secondary reactions are more consequential for measured selectivities on stronger acids because larger rate constants amplify the effects of acid site density on Thiele moduli for these reactions.

The results of this study suggest changes in acid strength alone cannot change intrinsic isomerization selectivities, because all isomerization transition states contain cations with similar charge distributions. However, selective isomerization conversion may be obtained by tailoring microporous environments to solvate specific backbone rearrangements, or by exploiting the effects of diffusion-enhanced secondary reactions.

References

1.     Macht, J., Janik, M. J., Neurock, M., and Iglesia, E. J. Am. Chem. Soc.130, 10369 (2008).

2.     Macht, J., Carr, R. T., and Iglesia, E. J. Am. Chem. Soc. 131, 6554 (2009).