Selective hydrogenation of nitrostyrene to aminostyrene over Pt/TiO 2 catalysts: Effects of pressurized carbon dioxide and catalyst preparation conditions

Abstract

The present work has studied the influence of CO 2 pressurization on the hydrogenation of nitrostyrene (NS) using Pt/TiO 2. With CO 2 pressurization up to 12 MPa (CO 2-dissolved expanded liquid (CXL) phase), the over-all reaction rate and the selectivity to aminostyrene (AS) increased. At higher CO 2 pressures where all NS dissolved in scCO 2 (scCO 2-homogeneous phase), both of them decreased with increasing the CO 2 pressure. The phase behavior was an important factor for the present reaction system. It was also found that the presence of pressurized CO 2 gave higher selectivity to AS than in its absence at any conversion level. This was ascribed to retardation effects of dense phase CO 2 on the hydrogenation of AS to ethylaniline. Competitive adsorption of nitro and vinyl groups was suggested to determine the product selectivity. FTIR measurements showed that the pressurized CO 2 lowers the reactivity of the nitro group, and this effect is stronger in scCO 2-homogeneous phase than in CXL phase, resulting in the lower AS selectivity in the former phase. Lower Pt loadings and higher catalyst reduction temperatures gave Pt/TiO 2 catalysts more selective for the AS formation. FTIR measurements of adsorbed CO over the catalysts suggested that the presence of less-coordinated Pt atoms on edge, corner and kink sites was significant for the selective hydrogenation of NS to AS.