English

• 0.   Introduction

  Accelerating catalysts with enhanced activity and selectivity can minimize or eliminate the formation of by‑products, thereby aligning with the principles of energy conservation, emission reduction, and sustainable development^[1]. However, when reactions involve competitive pathways that result in the generation of multiple intermediates or products, achieving high selectivity for the target product becomes crucial and difficult^[2-4]. One typical example is the selective hydrogenation of nitroaromatics to functional amines^[5], such as the selective hydrogenation of halogenated nitroaromatics to halogenated aromatic amines which are important chemical raw materials and intermediates for the synthesis of pharmaceuticals, dyes, and pesticides^[6]. However, the complex hydrogenation process occurs through both direct and condensation routes, with the formation of up to five intermediates (nitroso, hydroxylamine, azoxy, azo, and hydrazo) together with the desired amine product^[7]. Furthermore, it entails a competitive reaction of dehalogenation in the hydrogenation, which results in the formation of dehalogenated by-products (Fig. 1). This not only reduces the purity of the product but also generates strong acids (HX, where X=F, Cl, Br, I). These acids can cause severe corrosion of the reactor, posing a significant safety hazard. The highly selective hydrogenation of halogenated nitroaromatics is thus both essential and challenging for obtaining the desired amine products.

  Figure 1

  

  Figure 1.  Reaction network of the hydrogenation of halogenated nitroaromatics, exemplified by 4-chloronitrobenzene (4-Cl-NB)

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  In general, the selective catalytic hydrogenation of halogenated nitroaromatics can be enhanced by modulating the structure of the catalyst active site, as this allows to regulation of the adsorption energy and conformation of the reactants/intermediates^[8-10]. One of the most prevalent strategies is the introduction of organic dehalogenation inhibitors by modifying the metal surface with organic ligands. The ligands typically comprise coordination atoms, such as P, S, N, and so forth^[11-15]. The steric or electronic effects generated by the organic ligands can effectively manipulate the structure and strength of the interaction between the substrate and the active site, thereby achieving high selectivity^[16-19]. However, organic ligands on metal surfaces have been observed to significantly impair or even deactivate the catalyst in numerous instances^[20-21].

  Furthermore, halogens act as electron-withdrawing groups, which frequently result in a notable accumulation of hydroxylamines. The hydrogenation of hydroxylamine to aniline occurs at a relatively slow rate, which further leads to a reduction in the overall reaction rate^[22]. The hydroxylamine accumulation also serves to enhance the extent of the condensation. The formation of azo compounds not only slows the reaction rate but also negatively affects the product quality, as azo compounds possess a particularly dark color^[2]. The introduction of vanadium species has been documented as an effective strategy to resolve the hydroxylamine accumulation issue, as a catalytic amount of vanadium enables the rapid conversion of hydroxylamine to amine via disproportionation^{[2, 22-23]}. Therefore, it is expected that the selective catalytic hydrogenation of halogenated nitroaromatics to their corresponding halogenated aromatic amines can be achieved by effectively combining the benefits of both organic and inorganic modifications while minimizing their respective drawbacks.

  Herein, we achieved highly selective catalytic hydrogenation of halogenated nitroaromatics by employing a strategy of co-modification of the Pd/C catalyst with both organic ligand (PPh₃) and inorganic species (NaVO₃). The catalyst applied was a heterogeneous Pd/C catalyst with Pd nanoparticles (NPs) supported on activated carbon. The Pd NPs contain high-valence Pd species at the metal-support interface and zero-valence Pd species on the metal surface. A systematic investigation demonstrated that the coordination of PPh₃ on the zero-valence Pd sites inhibited the coplanar adsorption of halogenated nitroaromatics on Pd, thereby inhibiting dehalogenation. The preferential coordination of metavanadate species to high-valence Pd at the carbon-metal interface helps to activate H₂ heterolytically for the selective hydrogenation of nitro-groups and inhibits the accumulation of hydroxylamine intermediates. With the mechanism insights, the selective hydrogenation of a range of halogenated nitroaromatics has been successfully achieved by the co-modified Pd catalyst developed in this work.

  1.   Experimental

  1.1   Reagents

  4-Cl-NB (AR), 2-chloronitrobenzene (AR), 3‑chloronitrobenzene (AR), 3, 4‑dichloronitrobenzene (AR), 1, 2, 3-trichloro-5-nitrobenzene (AR), 4-nitro-1, 2, 3-trichloro-benzene (AR), 2-chloro-5-nitrobenzotrifluoride (AR), 3-chloro-4-fluoro-1-nitrobenzene (AR), 1-fluoro-4-nitrobenzene (AR), 1-fluoro-3-nitrobenzene (AR), 1‑bromo‑4‑nitrobenzene (AR), 1‑bromo‑3‑nitrobenzene (AR), 4-iodonitrobenzene (AR), and benzaldehyde (AR) were purchased from Alfa Aesar Chemical Reagent Co., Ltd. (Tianjin, China). Pd/C catalyst, PPh₃ (AR), NaVO₃ (AR), and ethanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Carbon black (Cabot vulcan XC-72) was purchased from Acmec Biochemical Co., Ltd. (Shanghai, China). H₂ (99.999%) and D₂ (99.999%) were purchased from Linde Gas. The water employed in all experiments was of an ultrapure quality (18.25 MΩ·cm). All reagents were used without any further purification process.

  1.2   Measurements

  The transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energy dispersive spectroscopy (EDS) elemental mapping measurements were performed on a JEOL JEM-F200 transmission electron microscope. The resolution of TEM and STEM was 0.10 and 0.14 nm at an operating voltage of 200 kV, respectively. The energy resolution of EDS was no greater than 133 eV (Mn Kα). The samples were prepared by dropping ethanol dispersion of samples onto 300-mesh carbon-coated copper grids and immediately evaporating the solvent. The oxy