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

Abstract

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).

H2 Activation with Co Nanoparticles Encapsulated in N‐Doped Carbon Nanotubes for Green Synthesis of Benzimidazoles

“…However, a zero‐order kinetic dependence of the initial 1 a hydrogenation rate on 1 a concentration is observed (Figure 5c), suggesting a quick adsorption of 1 a on the Co@NC 900 surface. Generally, the kinetic behaviors of Co@NC 900 ‐promoted hydrogenation of 1 a are very close to our recently reported nitrobenzene hydrogenation with phosphorus‐doped carbon nanotubes as a metal‐free catalyst [68] . Therefore, a pseudo‐first‐order kinetics is applied to hydrogenation of 1 a with respect to H 2 pressure if considering Co@NC 900 loading level as a constant in the reaction.…”

P‐doped Carbon as the Efficient Support of Nickel Catalysts for Hydrodechlorination of Chlorodifluoromethane

“…As displays in Figure 3(a), the high-resolution spectra of P 2p could be deconvoluted into two peaks located at 135.1 and 133.7 eV, which are corresponded to PÀ O and PÀ C binds, respectively. [17,18,24] Through integrating the specific area, the percentage of PÀ C accounts for 33.6 %, which means that P species is successfully doped into carbon materials. Moreover, the existence of PÀ C bond is also evident from the C 1s XPS (Figure 3(b)) accompanied with a clearly CÀ P signal at 285.7 eV.…”

“…), [7][8][9][10] supported transition metal (Ni, Mo, Fe, etc.) [11][12][13][14][15][16] and heteroatom doped carbon-based materials (eg., N, S, P or B), [17,18] which exhibited metal-like catalytic properties. Noble metal catalysts present good catalytic performance, but they experience poor stability due to severely deposition of carbonaceous species and high cost, which both limit the large-scale application of these catalysts.…”

“…[20] As for heteroatom-doped carbonbased materials, due to the incorporation of heteroatoms into porous carbon could modify the electronic structure of the carbon matrix, these catalysts showed excellent activity in HDC or hydrogenation. [17,18] For supported transition metal catalysts, carbonaceous materials are widely used as the supports due to their high specific surface areas, tunable surface chemistry and high stability towards corrosive gas HCl byproduct from HDC reaction. [15,21] However, the carbon materials supported transition metal catalyst usually undergo aggregation during preparation or high temperature reaction, leading to the low utilization efficiency.…”

“…The activity of supported transition metal‐based catalysts is inferior to the noble metal catalysts, but they hold good product selectivity and stability [20] . As for heteroatom‐doped carbon‐based materials, due to the incorporation of heteroatoms into porous carbon could modify the electronic structure of the carbon matrix, these catalysts showed excellent activity in HDC or hydrogenation [17,18] …”