Chemoselective hydrogenation of 3-nitrostyrene over a Pt/FeO_x pseudo-single-atom-catalyst in CO₂-expanded liquids

Chemoselective hydrogenation of substituted nitroarenes containing two reducible groups in one molecule is a highly desired approach to the synthesis of functionalized anilines. To make this process environmentally benign, we used a pseudo-single-atom-catalyst Pt/FeO_x and investigated the reaction in supercritical CO₂ and CO₂-expanded toluene. The results showed that supercritical CO₂ afforded excellent selectivity but low reactivity due to the limited substrate solubility in the reaction medium. By contrast, when the reaction proceeded in CO₂ expanded toluene, both the conversion of 3-nitrostyrene and the selectivity of 3-vinylaniline reached above 95% under optimum conditions while the organic toluene amount could be reduced by 90% compared to that without CO₂. The thermodynamic calculations revealed that the solubility of H₂ increased while the viscosity of the reaction system decreased with the CO₂ pressure, which facilitated the mass transfer and therefore increased the reaction rate meanwhile keeping the selectivity at a high level.