In situ-formed cobalt embedded into N-doped carbon as highly efficient and selective catalysts for the hydrogenation of halogenated nitrobenzenes under mild conditions

Highlights

• Cobalt embedded into N-doped carbon was fabricated through one-pot pyrolysis of a mixture of Co (Ⅱ)-tannic acid coordination polymers and melamine.

• The dosage of tannin acid in catalyst preparation process was a crucial factor in affecting the dispersion and exposure degree of Co nanoparticles.

• The Co@CN catalyst exhibited an excellent catalytic performance for the liquid-phase hydrogenation of halogenated nitrobenzenes under 1 MPa H₂ at 60 °C.

Abstract

Inhibiting the dehalogenation is the main challenge when halogenated nitrobenzenes are hydrogenated using H₂ as hydrogen source by heterogeneous catalysis. Herein, the earth-abundant cobalt embedded into N-doped carbon (Co@CN) catalysts were fabricated via one-pot pyrolysis of tannic acid, Co(NO₃)₂·6H₂O and melamine, which can function as a highly efficient non-noble-metal-based heterogeneous catalyst for selective hydrogenation of halogenated nitrobenzenes. Chloroanilines, bromoanilines, and iodoanilines, including all regioisomers, could be obtained with excellent selectivity (typically >99 %) at 60 °C under 1 MPa H₂, at almost complete conversion of the substrates. Additionally, Co@CN demonstrated excellent catalytic stability and could be reused at least five times without obvious loss of catalytic activity and selectivity. Therefore, the Co@CN catalyst exhibits vast potential for future industrial application in the selective hydrogenation of halogenated nitrobenzenes.

Introduction

Halogenated anilines are series of crucial intermediates for the production of various high valued chemicals, such as organic dyes, pharmaceuticals, pesticides and photosensitive materials [[1], [2], [3]]. These widely applied halogenated anilines have been successfully synthesized from corresponding halogenated nitrobenzenes with stoichiometric reducing agents such as Fe, Zn, Sn, or hydride reagents [[4], [5], [6]]. Unfortunately, these processes are not environmentally sustainable because of the generation of stoichiometric amounts of harmful chemical wastes. Obviously, catalytic hydrogenation is the most environmentally friendly and cost-effective method for the preparation of halogenated anilines, using supported transition metals and gaseous hydrogen (H₂) as the catalyst and reducing agent, respectively. However, the most difficult challenge in this catalytic hydrogenation process is to inhibit the cleavage of the weak carbon-halogen (C–X) bond, forming aniline (Scheme 1) [3,7]. In fact, the undesired dehalogenation reaction may become more prominent from C-Cl to C–I bond because of the following reasons. Firstly, the amino group in the halogenated anilines can function as an electron-donating group, which could promote the hydrogenolysis of the C-Cl bond in chloroanilines, making the undesired dehalogenation reaction occur [[8], [9], [10]]. Additionally, the much weaker C–I bond relative to the C-Cl bond results in the much easier hydrogenolysis because of the large atomic diameter as well as low electronegativity of iodine [[10], [11], [12]]. However, compared with chloroanilines, the iodoanilines and bromoanilines with weaker C–X bonds are more valuable intermediates in various organic synthesis reactions [13,14]. So far, however, the hydrogenation of p-halogenated nitrobenzene is mainly focused on the chloro substituents [3,15]. Based on the above discussion, developing an efficient heterogeneous catalyst for halogenated nitrobenzenes hydrogenation with a wide substrate scope is not only more challenging but also highly desirable.

Recently, the most reported heterogeneous catalysts are mainly based on noble metal, such as Pt/TiO₂, Pt/FeO_x, Ir/TiO₂-FeO_x, and Ru/NPC [[16], [17], [18], [19], [20], [21], [22]]. However, there are several disadvantages impeding the large-scale application of this kind of catalysts. For example, the dehalogenation phenomenon cannot be completely avoided in the reaction. To improve the selectivity of these catalysts, modification by alloying or poisoning with other metal oxides or molecules are commonly used, but at the expense of catalytic activity [[23], [24], [25], [26]]. Furthermore, the high cost and scarcity of noble metal resources make them uneconomic for practical application [27]. In view of the drawbacks of noble metal catalysts, series of non-noble metal catalysts were developed successively [9,[28], [29], [30], [31], [32], [33], [34]]. For example, Raney nickel catalyst has been reported that it can be successfully applied in the selective hydrogenation of halogenated nitrobenzenes, but inhibitors are needed to inhibit the dehalogenation phenomenon [9]. Fe-based catalysts have also been found to be selective for this reaction. But only 97 % selectivity for the 5-chloro-2-methoxyaniline was obtained, and relatively harsh reaction conditions (120 °C, 5 MPa H₂) were needed [28]. Additionally, Beller et al. reported that the halogenated nitrobenzenes could be transformed into the corresponding anilines over cobalt-phenanthroline complexes derived Co oxide-N/C catalyst under 5 MPa H₂ at 110 °C [29]. However, for the 3-chloroaniline and 4-chloroaniline, only 95 % selectivity was achieved. Wang et al. also demonstrated that the Co°/Co₃O₄@NCNTs catalyst exhibited excellent catalytic performance for the hydrogenation of substituted nitroarenes with a wide scope under 3 MPa H₂ at 110 °C [30]. Similar problem, slight dehalogenation happened and only 96 % selectivity was obtained for the 3-iodoaniline. It can be seen that these non-noble metal catalysts also suffer from some limitations such as the occurrence of slight dehalogenation, relatively costly nitrogen-containing precursors, high reaction temperature (> 100 °C) and/or H₂ pressure (>2 MPa). Consequently, the development of a cost-effective non-noble metal catalyst system with both high selectivity and activity for halogenated nitrobenzenes hydrogenation is highly desirable.

Tannic acid (TA), extracted from plant tissue, contains abundant galloyl or catechol groups, making it possess a strong binding affinity for various metal ions. More importantly, low price, the abundance, and environmental sustainability of TA grant it highly practical for many applications [[35], [36], [37]]. For example, the metal-TA coordination polymers have been widely used as a versatile platform for the functional surface engineering [[38], [39], [40]]. Moreover, TA also exhibits great potential as carbon precursors for the fabricating metal/carbon composites using its strong chelating ability to metal ions.

Herein, we demonstrate the successful synthesis of N-doped carbon supported Co (Co@CN) catalysts through one-pot pyrolysis of a Co (Ⅱ)-TA coordination polymers with melamine. Co (Ⅱ)-TA coordination polymers play an important role in preventing the Co species from aggregation during pyrolysis process. Melamine, a common industrial chemical, can function as a soft template for the formation of sheet-like N-doped carbon and the dispersion of Co NPs (nanoparticles). The Co@CN catalyst displayed excellent catalytic performance for the liquid-phase hydrogenation of halogenated nitrobenzenes under 1 MPa H₂ at 60 °C, which is better than that of the most previous reported Co-based catalysts (Table S4). For the chloroanilines, bromoanilines, and iodoanilines, including all regioisomers, 99 % selectivity could be achieved at almost complete conversion of the substrates.