Metal-Free H2 Activation for Highly Selective Hydrogenation of Nitroaromatics Using Phosphorus-Doped Carbon Nanotubes.

We reported that phosphorus-doped carbon nanotubes (P-CNT), showing metal-like property, can efficiently promote metal-free hydrogenation of nitrobenzene (1a) to aniline (2a) using molecular hydrogen (H2) as reducing reagent under very mild conditions with reaction temperature only 50C. The kinetics of 1a hydrogenation over P-CNT reveals that the hydrogenation rate of 1a is a first-order dependence to H2 pressure and P-CNT loading level, and zero-order dependence to 1a concentration, demonstrating the rate-determining step of H2 adsorption and activation over P-CNT. The activation energy of P-CNT catalyzed 1a hydrogenation is 433 kJ mol1 with the turnover frequency around 3.600.12 h1 at 50C. In addition to 1a, the general applicability of P-CNT promoted metal-free hydrogenation process is further demonstrated by applying to various functionalized nitroaromatics with widely industrial interests. The P-CNT shows both excellent yields and selectivities to the hydrogenation with respect to reducible, labile, and strong leaving groups on the nitroaromatics molecules. The stability and reusability of the P-CNT demonstrates an up to an eight-time recycling without evident loss of activity and selectivity. In addition to hydrogenation, metal-free catalytic transfer hydrogenation of 1a is achieved with P-CNT by using diverse hydrogen sources including hydrazine hydrate (N2H4H2O), carbon monoxide/water (CO/H2O) and formic acid/triethyl amine (HCOOH/Et3N).