Reaction kinetics and phase behavior in the chemoselective hydrogenation of 3-nitrostyrene over Co-N-C single-atom catalyst in compressed CO2

Single-atom catalysts (SACs) have demonstrated excellent performances in chemoselective hydrogenation reactions. However, the employment of precious metals and/or organic solvents compromises their sustainability. Herein, we for the first time report the chemoselective hydrogenation of 3-nitrostyrene over noble-metal-free Co-N-C SAC in green solvent — compressed CO2. An interesting inverted V-curve relation is observed between the catalytic activity and CO2 pressure, where the conversion of 3-nitrostyrene reaches the maximum of 100% at 5.0 MPa CO2 (total pressure of 8.1 MPa). Meanwhile, the selectivities to 3-vinylaniline at all pressures remain high (> 99%). Phase behavior studies reveal that, in sharp contrast with the single phase which is formed at total pressure above 10.8 MPa, bi-phase composed of CO2/H2 gas-rich phase and CO2-expanded substrate liquid phase forms at total pressure of 8.1 MPa, which dramatically changes the reaction kinetics of the catalytic system. The reaction order with respect to H2 pressure decreases from ~0.5 to zero at total pressure of 8.1 MPa, suggesting the dissolved CO2 in 3-nitrostyrene greatly promotes the dissolution of H2 in the substrate, which is responsible for the high catalytic activity at the peak of the inverted V-curve.