(342d) Ordered Mesoporous Organosilica Materials with Systematically Controlled Surface Polarity

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) [1-3], and may affect the stability of the catalyst under operating conditions [4]. Surface polarity must be varied systematically in order to construct structure-activity correlations. We prepared a series of ordered mesoporous organosilica materials with similar surface textural properties but a wide range of surface polarities 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 IR, 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 maximum 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 water for the pure silica material, to acetonitrile for biphenylene-bridged organosilicas. Surface functionalization with TEMPO allowed us to probe surface polarity using electron paramagnetic resonance (EPR) spectroscopy, since broad signal became stronger as hydrophobicity increased. Surface water diffusivity was explored using Overhauser Dynamic Nuclear Polarization (ODNP) and correlated with the degree of surface hydrophobicity. At room temperature, the adsorption capacity for phenol from water changes by a factor of 1.5 with surface hydrophobicity. The ability of these materials to catalyze reactions of organic compounds will be explored next, with a view to systematically correlating their activity with their hydrophobicity.

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