(729b) Tuning the Rate of Aromatic Hydrogenation By Systematic Variation of the Solvent and Surface Polarity in Mesoporous Organosilicas

In liquid phase reactions, catalytic activity and selectivity can be influenced by solvent selection as well as by modification of the solid catalyst surface. In this presentation, I will describe how catalyst surface polarity in combination with solvent choice affects the activity for hydrogenation of aromatics by modulating the extent of reactant/product adsorption on the catalyst surface. A series of ordered mesoporous organosilicas with similar surface textural properties but a wide range of surface polarities was prepared via the incorporation of oxo, phenylene, and biphenylene linkers in various ratios. The surface polarity was probed by measuring the fluorescence of a solvatochromic dye adsorbed onto each organosilica. The surface polarities range from a value similar to methanol for the most polar pure silica material, to a value similar to DMSO for the least polar biphenylene-bridged organosilica. Pd nanoparticles with similar dispersions were created on each support materials through appropriate choice of Pd precursor. Phenol hydrogenation was conducted in various solvents, including water, methanol, acetonitrile, and n-decane. For the Pd catalyst supported on a surface with intermediate polarity, the catalytic activities measured in either water or n-decane are an order of magnitude higher compared to those in methanol and acetonitrile. This surprising trend may be because phenol has limited solubility in both water and n-decane, leading to higher adsorption of phenol onto the catalyst surface. In addition, the activity of the least polar catalyst is 2-4 times higher than the most polar catalyst in both water and n-decane. Quantification of adsorbed phenol in these solvent shows that the rate enhancement is correlated with stronger interactions of phenol with less polar surfaces. These findings show that the rate of aromatic hydrogenation for a given type of active site can be optimized by tuning the surface polarity in combination with choice of solvent.