(40a) “Finding the Right Fit”: Catalysis and Confinement At the Nanoscale

Gounder, R., University of California at Berkeley
Iglesia, E., University of California at Berkeley

Dispersion forces within voids of molecular dimensions solvate transition states that mediate chemical reactions. These interactions give rise to ubiquitous enthalpy-entropy compromises for confined species, which influence, in turn, the Gibbs free energy differences between transition states and reactants. Kinetically-relevant transition states for dimethyl ether carbonylation (438 K) are more effectively stabilized by van der Waals interactions within eight-membered ring (8-MR) pockets than in larger 12-MR channels of MOR zeolites. At low temperatures, enthalpic contributions dominate free energies and the tighter fit in 8-MR pockets decreases activation barriers and leads to higher turnover rates. Turnover rates for monomolecular alkane (C3H8. n-C4H10, i-C4H10) activation (748 K) are also higher on 8-MR protons in spite of higher barriers, which reflect less effective van der Waals contacts that result from partial confinement of larger transition states within shallow 8-MR pockets. Partially-confined transition states have higher entropies than their fully confined analogs, leading to higher rates because entropy effects contribute to free energies more strongly at higher temperatures. The preeminent effects of entropy and partial confinement at high temperatures are selective for reactions involving looser transition states; they are also consequential for C3H6 hydrogenation with H2 and C2H4 alkylation with CH4, which involve the same ion-pair transition states as their C3H8 monomolecular dehydrogenation and cracking counterparts. These routes occur at rates constrained by gas-phase thermodynamics and with the same site requirements as alkane activation, because catalytic surfaces remain vacant with varying reactant pressures and distances from chemical equilibrium. These findings highlight the role of temperature in mediating contributions of transition state enthalpy and entropy, which depend strongly on confinement, to free energies and turnover rates in catalysis.