Dominant Role of Entropy in Stabilizing Sugar Isomerization Transition States within Hydrophobic Zeolite Pores

Lewis acid sites in zeolites catalyze
aqueous-phase sugar isomerization
at higher turnover rates when confined within hydrophobic rather than
within hydrophilic micropores; however, relative contributions of
competitive water adsorption at active sites and preferential stabilization
of isomerization transition states have remained unclear. Here, we
employ a suite of experimental and theoretical techniques to elucidate
the effects of coadsorbed water on glucose isomerization reaction
coordinate free energy landscapes. Transmission IR spectra provide
evidence that water forms extended hydrogen-bonding networks within
hydrophilic but not hydrophobic micropores of Beta zeolites. Aqueous-phase
glucose isomerization turnover rates measured on Ti-Beta zeolites
transition from first-order to zero-order dependence on glucose thermodynamic
activity, as Lewis acidic Ti sites transition from water-covered to
glucose-covered, consistent with intermediates identified from modulation
excitation spectroscopy during in situ attenuated total reflectance
IR experiments. First-order and zero-order isomerization rate constants
are systematically higher (by 3–12×, 368–383 K)
when Ti sites are confined within hydrophobic micropores. Apparent
activation enthalpies and entropies reveal that glucose and water
competitive adsorption at Ti sites depend weakly on confining environment
polarity, while Gibbs free energies of hydride-shift isomerization
transition states are lower when confined within hydrophobic micropores.
DFT calculations suggest that interactions between intraporous water
and isomerization transition states increase effective transition
state sizes through second-shell solvation spheres, reducing primary
solvation sphere flexibility. These findings clarify the effects of
hydrophobic pockets on the stability of coadsorbed water and isomerization
transition states and suggest design strategies that modify micropore
polarity to influence turnover rates in liquid water.