We reported that phosphorus-doped carbon nanotubes (P-CNTs), showing metal-like properties, can efficiently promote metal-free hydrogenation of nitrobenzene (1a) to aniline (2a) using molecular hydrogen (H2) as a reducing reagent under very mild conditions with a reaction temperature of only 50 °C. The kinetics of 1a hydrogenation over P-CNT reveals that the hydrogenation rate of 1a is a first-order dependence on the H2 pressure and the P-CNT loading level, and a zero-order dependence on 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 the P-CNT-promoted metal-free hydrogenation process is further demonstrated by applying various functionalized nitroaromatics with wide industrial interest. The P-CNT shows both excellent yields and selectivities to hydrogenation with respect to reducible, labile, and strong leaving groups on the nitroaromatics molecules. The stability and reusability of the P-CNT demonstrate up to 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 using diverse hydrogen sources, including hydrazine hydrate (N2H4·H2O), carbon monoxide/water (CO/H2O), and formic acid/triethylamine (HCOOH/Et3N).

Co nanoparticles (NPs) encapsulated in N‐doped carbon nanotubes (Co@NC900) are systematically investigated as a potential alternative to precious Pt‐group catalysts for hydrogenative heterocyclization reactions. Co@NC900 can efficiently catalyze hydrogenative coupling of 2‐nitroaniline to benzaldehyde for synthesis of 2‐phenyl‐1H‐benzo[d]imidazole with >99 % yield at ambient temperature in one step. The robust Co@NC900 catalyst can be easily recovered by an external magnetic field after the reaction and readily recycled for at least six times without any evident decrease in activity. Kinetic experiments indicate that Co@NC900‐promoted hydrogenation is the rate‐determining step with a total apparent activation energy of 41±1 kJ mol−1. Theoretical investigations further reveal that Co@NC900 can activate both H2 and the nitro group of 2‐nitroaniline. The observed energy barrier for H2 dissociation is only 2.70 eV in the rate‐determining step, owing to the presence of confined Co NPs in Co@NC900. Potential industrial application of the earth‐abundant and non‐noble transition metal catalysts is also explored for green and efficient synthesis of heterocyclic compounds.

A bifunctional role of carbon dioxide (CO 2 ) was developed in this research as a medium for catalyst preparation and as a soft oxidant for the subsequent coupling reaction. A highly dispersed and low-content Cu catalyst (Cu/NPC) was obtained by homogeneously loading copper oxide (CuO) on porous nitrogen−phosphorus co-doped carbon (NPC) under supercritical-CO 2 (sc-CO 2 ) conditions. Cu/NPC shows an irregular and highly crosslinked three-dimensional (3D) morphology which is assembled by ultrathin and lamellar-like nitrogen−phosphorus co-doped carbon nanosheets. The resulting Cu/NPC catalyst can efficiently promote dehydrogenative homocoupling of terminal alkynes by using CO 2 as a soft oxidant for 1,3-diyne syntheses. Quantitative 1,3-diyne yields up to 99% were achieved with a broad terminal alkyne scope by using Cu/NPC. Additionally, Cu/NPC can be readily reused in the coupling reaction. This research thus demonstrates dehydrogenative homocoupling of terminal alkynes with CO 2 as a renewable, green, readily available, and soft oxidant by using a Cu-based catalyst.

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