Fig. 1. (a) Bimolecular ethanol dehydration turnover rate (per H+, 373 K) on H-Al-Beta-F (synthesized in HF medium) as a function of the C2H5OH/H2O pressure ratio, in different ranges of H2O pressures. (b) Apparent first-order bimolecular ethanol dehydration rate constant (per H+, 373 K) as a function of H2O pressure on H-Al-Beta-F (■), H-Al-TON (▲), H-Al-FAU (◆), H-Al-MFI (●), H-Al-AEI (□), H-Al-CHA (○), and HPW/Si-MCM-41 (). (c) Free energy diagram illustrating the effects of solvation by extended hydrogen-bonded H2O networks. Adapted with permission from Ref. [31]. Copyright 2020, Royal Society of Chemistry.

Fig. 2. (a) Effects of calcination temperature of MoO3-ZrO2 catalysts (Mo/Zr = 0.1) on the conversion of 2-butanol dehydration in the absence of water (○) and in the presence of water at 44 kPa (●) at 423 K. (b) Reversible influence of 44 kPa water on the MoO3-ZrO2 catalyst calcined at 1073 K. (c) Effects of partial pressures of water on the 2-butanol dehydration and esterification over the MoO3-ZrO2 catalyst calcined at 1073 K (○) and SiO2-Al2O3 (●). All reactions were performed in the gas phase with He as balance. Adapted with permission from Ref. [28]. Copyright 2002, Springer Nature.

Fig. 3. Increasing hydration of the BAS leads to a shift in the identity of the rate-determining step (RDS) in E1-type elimination of an alkanol (ROH) to an alkene ([R-H]=). The left figure illustrates a plausible E1-type elimination path for alcohol dehydration on a dry BAS (H+Z-), for which the C-O scission step is the RDS, while the right figure shows a plausible E1-type path for alcohol dehydration over hydrated hydronium ions (H3O+hydr.), for which the C-H scission step becomes the RDS. Note the energy levels of TS2, TS3 and that of the intermediate prior to C-H cleavage (hydrated carbenium ion or surface alkoxide), relative to the respective initial state in either case, are most dramatically impacted by the solvation of water, causing such a shift in the identity of the rate-determining step and the kinetically relevant transition state.

Fig. 4. Effects of intrapore ionic strength on aqueous-phase dehydration of cyclohexanol over zeolites. (a) Comparison of TOFs of aqueous-phase cyclohexanol dehydration catalyzed by hydrated hydronium ions (H3O+hydr.) in H-MFI and H-BEA pores at 423 K. (b) Reaction free-energy barriers and excess chemical potential of the ground state (GS) and transition state (TS) under the ideal condition and under an ionic strength. (c) Schematic illustration of H3O+hydr. and cyclohexanol in H-MFI micropore channels and the mean distance dh-h between two neighboring H3O+hydr. and the mean distance db-b and volume Vb-b between the boundaries of neighboring H3O+hydr.. (d) Enthalpy of the ground and transition states as a function of db-b and Vb-b. (e) Gibbs free energy landscape of aqueous-phase cyclohexanol dehydration catalyzed by intrapore H3O+hydr. under ideal and nonideal (with non-negligible ionic strength) conditions. Adapted with permission from Ref. [65]. Copyright 2021, American Association for the Advancement of Science.

Fig. 5. Solvation effects on acid-catalyzed reactions in monophasic mixtures of water and polar aprotic solvent. (a) Gibbs free energy surface in H2O and polar aprotic organic solvents of the conversion of reactant R into product P catalyzed by a Brønsted acid. (b) Ratio of the turnover frequencies (TOF) for xylose conversion in gamma-valerolactone (GVL) and H2O versus the pKa value for homogeneous Brønsted acid catalysts. Experimental (?) and theoretical (—) TOF ratios are given. (c) TOFs for the dehydration of xylose to furfural in purely aqueous phase and in the GVL(90 wt%)-H2O (10 wt%) solvent mixture for heterogeneous acid catalysts (silicotungstate dissolves in both liquids). Note the TOFs of aqueous-phase dehydration of xylose has been multiplied by a factor of 5. Adapted wither permission from Ref. [71]. Copyright 2014, Wiley-VCH.

Fig. 6. (a) DFT calculations of H2O molecule adsorption on an amorphous SiO2 surface functionalized with SO3H groups and (b) an illustration of the concept of water-extended remote bond polarization, where the color codes indicate: Si (blue); O (red); C (silver); H (white); S (cream); H (green) of SO3H group; transferred H (cerulean). Adapted with permission from Ref. [87]. Copyright 2021, Elsevier.

Fig. 7. Adsorbed water fragments (OH groups) affect the adsorption strength and deformation severity of DAA on the Zn1Z11O12 surface: (upper panel) side views of diacetone alcohol adsorption at four sites in the vicinity of an adsorbed OH group on a hydrated surface type; (lower panel) side views of diacetone alcohol adsorption at four sites on the dehydrated surface. Zr, Zn, C, O, and H atoms or ions are represented by green, gray, black, red, and white spheres, respectively. Partial charge densities are presented by yellow isosurfaces for electron gains and by blue isosurfaces for electron losses. Adapted with permission from Ref. [90]. Copyright 2021, American Chemical Society.

Fig. 8. Baseline-corrected difference IR spectra of adsorbed water on a representative hydrophobic sample Ti-Beta-F-155 with 1.93 × 10-4 molSiOH g-1 (top, magnified for clarity) and a representative hydrophilic sample Ti-Beta-OH-46 with 7.02 × 10-4 molSiOH g-1 (bottom) at 298 K for (a) the δ(HOH) scissoring modes in the water bending region and (b) the ν(O-H) water-stretching region. Difference spectra reflect the subtraction of the spectrum measured on the sample under vacuum prior to water flow and corrected for background water adsorption onto the IR cell. Spectra for each sample displayed from bottom to top correspond to P/P0 = 0.1, 0.2, 0.5, and 0.75. The insets display the change in (a) the water bending peak area and (b) the water-stretching peak maximum with increasing water concentration for Ti-Beta-F-155 (●) and Ti-Beta-OH-46 (▲). Reproduced with permission from Ref. [102]. Copyright 2018, American Chemical Society.

Fig. 9. Stability of pristine and silylated H-Beta zeolites during cyclohexanol dehydration: (a) Correlation between the lifetime of a BEA catalyst and the concentrations of Brønsted acid sites during catalysis, measured at 443 K; (b) Estimated water concentration in the zeolite micropores as determined from measuring the uptake of cyclohexanol in an aqueous solution (0.33 mol/L) at room temperature. Note the linear correlation between Brønsted acid concentration and water uptake; any upward deviation is linked to adsorption on defect sites, shown by green arrows. Adapted with permission from Ref. [106]. Copyright 2017, American Chemical Society.

Fig. 10. Effects of dynamic reorganization of water clusters stabilized at hydrophilic silanol defects in Ti-zeolites, shown for olefin epoxidation in acetonitrile (in the presence of H2O2 and some water) as an example: (a) Free energy landscape with a set of elementary steps to form epoxidation transition states over (left) hydrophobic Ti zeolites that contain few (SiOH)x defects and H2O molecules near the Ti-active sites and (right) hydrophilic materials with many (SiOH)x defects near the active sites that entrain more H2O molecules proximate to the reaction centers. (b) Enthalpy (ΔHexcess) and entropy (ΔSexcess) compensation relationships for disruption of confined H2O structures within Ti-FAU (orange), Ti-BEA (blue), and Ti-MFI (purple) zeolites during 1-hexene (■), 1-octene (●) and 1-decene (▲) epoxidation reactions. The shaded region is intended to represent the span of enthalpy-entropy compensation expected within these different pore environments due to the differences in (SiOH)x for a given zeolite topology. Adapted with permission from Ref. [110]. Copyright 2021, Springer Nature.

Fig. 11. Impact of zeolite Si/Al and hydronium ion concentrations on the adsorption of cyclohexanol in water. (a) Schematic illustration of positively charged hydronium ions and negatively charged framework Al sites in H-MFI zeolite crystal in water. (b) Adsorption isotherm of cyclohexanol on H-MFI zeolite in aqueous phase at 298 K. (c) experimentally determined standard adsorption constants (Ka,exp○) and the associated heat of adsorption (Qa,exp) of cyclohexanol on H-MFI as a function of BAS concentration in H-MFI at 298 K. (d) Ka,exp○ of cyclohexanol on H-MFI as a function of the activity coefficient (or its inverse) of the intrapore phase of cyclohexanol. Adapted with permission from Ref. [48]. Copyright 2019, Wiley-VCH.

Fig. 12. Effects of pore condensation on catalysis at the interface between a porous catalyst and a vapor phase. (a) An illustration of a porous catalyst which is in contact with gas-phase water but forms inside its pore systems varying degrees of liquid-like water (the extent of pore filling depends on the pore size at a given pressure). (b) Pore volume fraction for small-pore Ni-Al-MCM-41 (pore diameter 1.7 ± 0.5 nm) filled with N2 at 77 K (continuous line) or alkenes at 248 K (dashed lines), and for large-pore Ni-Al-MCM-41 (pore diameter 3.5 ± 2.5 nm) filled with N2 at 77 K (dotted line) as a function of the corresponding relative saturation pressure. (c) First-order deactivation constants during olefin dimerization on small-pore Ni-Al-MCM-41 catalyst as a function of relative saturation pressures for olefin reactants. The (b) and (c) are reproduced with permission from Ref. [114]. Copyright 2017, Elsevier.

Fig. 13. In situ titration during liquid-phase reactions allow more rigorous comparisons of site-specific activities. (a) In situ titration of SO3H-functionalized MCM-41 catalysts with pyridine during the cyclopentanone aldol condensation. (b) TOF (turnover frequency) of MCM-41-SO3H catalysts with varying acid density [87]. (c) In situ titration of active sites using pyridine (which potentially binds to both BAS and LAS; blue filled diamonds) and 2,6-dimethylpyridine (which binds more selectively to BAS; red filled squares) in H-MFI and H-Beta zeolites during cyclohexanol dehydration in water [24]. In (a) and (b), the TOFpair and TOFsingle correspond to the site-specific activities of spatially close acid sites and isolated acid sites, respectively; in (c), the identical trends of rate decrease as a function of the cumulative titrant uptake for both base titrants and on both fresh and reused catalysts (green filled circles indicate a fresh catalyst prior to titration; open triangles indicate experimental data collected on catalysts recycled after its first service in this reaction) demonstrate that BAS (hydronium ions) are exclusive active sites for the alcohol dehydration reaction and that these zeolite catalysts did not deactivate after the aqueous-phase reaction. Adapted with permission from Refs. [24,87]. Copyright 2017, Springer Nature; Copyright 2021, Elsevier.