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

Abstract

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.

Zeolite (In)Stability under Aqueous or Steaming Conditions

“…The importance of the synthesis procedure for zeolite hydrophilicity was highlighted already in the late 1990s by Blasco et al who showed that, for samples with similar Ti content, between 7 and 30 times more water is adsorbed in highly defective Ti-BEA samples prepared hydrothermally in an OH − medium (Ti-BEA(OH)) than in less defective samples synthetized in an F − medium (Ti-BEA(F)). [32,40] Similar ratios (10-40 times) in water uptake between Ti-BEA(OH) (postsynthetically exchanged in a dealuminated BEA zeolite) and Ti-BEA(F) samples were reported recently by Cordon et al [41] Such a drastic change in hydrophilicity due to synthetic procedure has been also confirmed for Ti-BEA samples. [39,42] Large hydrophilicity changes were convincingly assigned to differences in silanol densities, [41] showing that a larger silanol density results in larger water uptakes at low water vapor pressures (p/p 0 = 0-0.3).…”

“…[32,40] Similar ratios (10-40 times) in water uptake between Ti-BEA(OH) (postsynthetically exchanged in a dealuminated BEA zeolite) and Ti-BEA(F) samples were reported recently by Cordon et al [41] Such a drastic change in hydrophilicity due to synthetic procedure has been also confirmed for Ti-BEA samples. [39,42] Large hydrophilicity changes were convincingly assigned to differences in silanol densities, [41] showing that a larger silanol density results in larger water uptakes at low water vapor pressures (p/p 0 = 0-0.3). It has been demonstrated for Ti-BEA by Hoeven et al that even water sorption capacities (p/p 0 > 0.8) are proportional to silanol densities and that titanium content plays a much weaker role in water adsorption than the silanol defects.…”

Advances in porous and nanoscale catalysts for viable biomass conversion

“…490 Zeolite frameworks are typically composed of siloxane bonds, but they can also contain hydrophilic silanol (SiOH) groups, which originate from framework siloxy (i.e., SiO À ) defects formed during hydrothermal synthesis of zeolites. [513][514][515] These SiOH species are known to be the main active sites for the hydrolysis of zeolite frameworks in hot liquid water conditions (4150 1C and autogenous pressure). Hence, the formation of SiOH defects should be prevented to improve hydrothermal stability of the zeolites, This journal is © The Royal Society of Chemistry 2019 which can be done by using alternative charge-balancing anions (i.e., F À ).…”

Tighter Confinement Increases Selectivity of d‐Glucose Isomerization Toward l‐Sorbose in Titanium Zeolites

“…Lewis‐acid catalyzed glucose isomerization rates (373 K) were measured under acidic conditions (pH 3) to suppress background reactions from base‐mediated pathways that might occur in solution or at any minority titanium oxide domains that may be present on the samples . Initial glucose isomerization rates to fructose and sorbose (per Ti, 373 K) were measured at dilute glucose thermodynamic activities (<565 mol m −3 ; <10 % (w/w)), which correspond to kinetic regimes that are first‐order in initial glucose thermodynamic activity . A detailed derivation of glucose‐fructose and glucose‐sorbose isomerization rate expressions from accepted mechanisms can be found in our prior publication; we concisely summarize this here to allow for mechanistic interpretation of sorbose‐to‐fructose selectivities.…”

“…Catalytic routes to produce renewable chemicals from lignocellulosic biomass sources involve strategies that convert sugar derivatives (e.g., glucose) via isomerization pathways using heterogeneous Lewis acid catalysts . Silica‐based zeolite frameworks substituted with tetravalent metal heteroatoms (M 4+ =Sn, Ti, Zr, Hf) comprise a suite of catalysts that can isolate Lewis acid sites of different identity and coordination within secondary confining environments, providing two orthogonal design criteria to influence rates and selectivities. Confinement of active sites within microporous environments can prevent intrapore diffusion of large reactants (reactant shape selectivity), and influence product selectivity through relative diffusion rates of products of varying sizes (product shape selectivity) or through selective formation of specific transition states based on size restrictions (transition state selectivity) .…”