Title: "Tuning the surface hydrophobicity of SBA-15-type materials for controlled molecular adsorption, hydration dynamics, and heterogeneous catalysis"
Abstract: Alternative routes to produce chemicals from renewable raw materials are needed in order to establish a sustainable supply of chemical feedstocks. Suitable catalysts to effectively convert renewable materials such as lignocellulosic biomass to platform chemicals are being investigated. One approach to achieve high catalytic activity and selectivity in these low temperature, liquid phase processes is to vary the surface polarity of catalyst and thereby tune the relative affinity of the reactive surface for reactants, products, and spectator molecules (e.g., solvent). Surface polarity can also affect catalyst stability under hydrothermal operating conditions. Systematic variation of surface polarity aids in constructing structure-activity correlations, although creating such materials presents a synthetic challenge and is rarely undertaken. In addition, there are few methods to assess surface polarity directly or to describe molecular behavior at solid-liquid interfaces in heterogeneous catalysis.
This dissertation describes the preparation of a series of ordered mesoporous organosilica materials with a wide range of surface polarities. Surface properties such as polarity, hydration dynamics, and adsorption affinity were assessed at the molecular level. Surface polarity was assessed by measuring the fluorescence of a solvatochromic dye adsorbed onto the organosilicas from water. Polarities were observed to range from values similar to methanol for the pure silica material, to DMSO for a biphenylene-bridged organosilica. Pure silica becomes less polar with thermal treatment to convert silanol groups to siloxanes, and the diffusivity of near-surface water increases accordingly. In contrast, organosilica surfaces show gradually decreasing surface water diffusivity as their polarity decreases. This surprising finding indicates that non-polar silica surfaces present low entropy spots that promote molecular adsorption. The materials were modified with Pd nanoparticles in order to explore the effects of tuning surface polarity on molecular adsorption and activity/selectivity in phenol hydrogenation. The reaction was investigated using operando NMR spectroscopy coupled with adsorption measurements. The rate and selectivity to cyclohexanone increase with decreasing surface polarity, due to changes in adsorption that precedes hydrogen addition.