Chemoselective Hydrogenation of Halonitroaromatics over Platinum on Carbon as Catalyst in Supercritical Carbon Dioxide

The chemoselective hydrogenation of 2‐chloronitrobenzene to 2‐chloroaniline over platinum on carbon (Pt/C) as catalyst in supercritical carbon dioxide (scCO2) was investigated. In the scCO2 phase, undesired dehalogenation was significantly suppressed to yield 2‐chloroaniline in 99.7% selectively. The utilization of scCO2 resulted in an about two‐fold increase in the rate of the reaction compared to the solvent‐free conditions. Visual inspection of phase behaviour of the reaction mixture revealed that the formation of a single supercritical phase including substrate, hydrogen, and products, is the crucial factor for the facile hydrogenation of nitro groups. In situ diffuse reflectance FT‐IR spectroscopy revealed that carbon monoxide generated in situ by the reverse water‐gas shift reaction (CO2+H2→CO+H2O) poisoned selectively platinum active sites that promoted undesired side reactions including dehalogenation and hydrogenation of aromatic rings, resulting in the excellent product selectivity in scCO2. The untouched active sites efficiently catalyzed the nitro group hydrogenation in scCO2, thereby realizing the highly chemoselective hydrogenation to haloanilines. The present chemoselective hydrogenation is applicable to other chlorinated nitrobenzenes to give the desired products in 100% conversion and with over 99% selectivity.