Hydrogenation of Nitrostyrene with a Pt/TiO2 Catalyst in CO2-Dissolved Expanded Polar and Nonpolar Organic Liquids: Their Macroscopic and Microscopic Features

Hydrogenation of nitrostyrene (NS) was studied with a Pt/TiO2 catalyst in CO2-dissolved ethanol and toluene phases at 323 K. The effects of CO2 pressure and organic solvents on the total conversion and the product distribution were examined. The conversion simply decreases with CO2 pressure in the both expanded solvents. The pressurization with CO2 varies the product selectivity in the nonpolar solvent but not in the polar solvent. The interactions among the substrate, solvent, and CO2 molecules were measured by in situ high-pressure FTIR methods in transmittance and attenuated total reflection modes. The reactivity of the nitro group of NS is lowered by interactions with CO2, which is responsible for the change in the product distribution. The local structures of an NS molecule in the expanded solvents are discussed from the results of FTIR and molecular dynamics simulations. The change in the product distribution is explained by the change of the local composition around the substrate molecule depending on CO2 pressure. The local composition is likely to change in the toluene but not in the ethanol, in accordance with the changes in the product selectivity in these two solvents. The substrate-CO2 interactions are important in toluene while those between the substrate and the solvent are predominant in ethanol even at high CO2 pressures.