Tuning the surface hydrophobicity of SBA-15 type materials for controlled molecular adsorption, hydration dynamics, and heterogeneous catalysis

Surface polarity may impact the rate and selectivity of heterogeneous catalytic reactions because of the different affinities of the surface for reactants, products, and spectator molecules (e.g., solvent), and may affect the stability of the catalyst under operating conditions. Surface polarity must be varied systematically in order to construct structure-activity correlations. In addition, the lack of methods to assess surface polarity and solid-liquid interfaces makes it difficult to construct such a correlation in heterogeneous catalysis. To explore the effect of surface polarity, a series of ordered mesoporous organosilica materials with similar surface textural properties but a wide range of surface polarities was prepared via the incorporation of oxo, phenylene, and biphenylene bridging groups in various ratios. The SBA-15-type materials were synthesized through co-condensation and were characterized using TGA, powder XRD, and 13C CP/MAS NMR. The surface polarity was probed by measuring the fluorescence of a solvatochromic dye, Prodan, adsorbed onto the organosilica surfaces from water. By comparing the emission maxima of the fluorescence from the dry materials to that of the dye dissolved in various solvents, the surface polarities were observed to range from values similar to methanol for the pure silica material, to DMSO for biphenylene-bridged organosilicas. Surface functionalization with TEMPO allowed us to probe surface polarity using electron paramagnetic resonance (EPR) spectroscopy, since the line broadening of the EPR signal increases with decreasing surface polarity. In order to study the role of silica surface chemistry and polarity on the properties of near-surface water, the translational dynamics of surface water were investigated using Overhauser dynamic nuclear polarization. In pure silicas, condensation of polar silanols to give moderately nonpolar siloxane groups leads to an increase in translational water diffusivity due to the weak interactions between the surface and water. In contrast, increasingly nonpolar surfaces achieved by incorporating organic groups, such as phenylene, biphenylene and ethylene bridges, lead to a gradual decrease in surface water diffusivity. The opposite trend for water diffusivity observed for the nonpolar, organic surfaces is likely due to the formation of a strong hydrogen-bonding network at the organic-water interface. Hydrophilic catalysts with surface silanol groups and hydrophobic catalysts with biphenylene linkers were synthesized by incorporating Pd into the ordered mesoporous silicas. Phenol hydrogenation was studied using operando NMR spectroscopy. In addition to the signals for solution-phase molecules, additional peaks representing each of the key molecules (phenol, cyclohexanone, and cyclohexanol) interacting with the catalyst surface were observed. The latter peaks shifted during the reaction due to the changing composition of the adsorbed layer the surface. Quantitative analysis of the NMR arrays and measurements of adsorption suggest that an increased local phenol concentration on the surface leads to an increase in Pd active site-phenol interactions, accelerating the rate of phenol hydrogenation while suppressing the rate of cyclohexanone hydrogenation despite increased cyclohexanone adsorption onto the support. The effect of solvent polarity on phenol adsorption and hydrogenation was also investigated as a function of surface type. Solvents that do not solvate phenol well and therefore allow appreciable phenol adsorption, such as cyclohexane and water, lead to significantly faster phenol conversion compared to organic solvents that solvate phenol well, such as acetonitrile, THF, and p-dioxane. The use of hydrophobic catalysts in cyclohexane and water further improved the rate of phenol conversion (by a factor of ca. 2) and selectivity to cyclohexanone (by ca. 20 %), due to the increased interaction between phenol and the surface compared with the hydrophilic catalyst. Overall, this research demonstrates how different methods can be used to assess surface properties such as polarity and hydration dynamics at the molecular level, and shows that tuning surface polarity is an effective strategy to modulate molecular adsorption and activity/selectivity in heterogeneous catalysis