Bio-inspired noble metal-free reduction of nitroarenes using NiS_2+x/g-C₃N₄

Abstract

We introduce the concept of bio-inspired catalytic hydrogenation of nitroarenes by mimicking the catalytic behavior of enzymes with NiS_2+x nanoparticles and polymeric melon (g-C₃N₄). The g-C₃N₄-supported NiS_2+x nanoparticles functioned as ligand-free and noble metal-free catalysts and offered high efficiency, comparable to noble metal-based catalysts, but at a much better selectivity.

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Reduction/hydrogenation of nitroarenes to corresponding substituted anilines is of great importance, due to the wide applications of aniline as one of the most important intermediates in industry for fabricating polymers and fine chemicals. Typically, the hydrogenation of nitroarenes is catalyzed by either supported noble metal nanoparticles or metal complexes with expensive ligands.^1–5 The majority of those catalysts investigated in the literature were involved with expensive noble metal components or specific ligands, limiting their applications in large scale. Moreover, only limited samples of Au or Pd nanocatalysts and Fe complexes can access the hydrogenation of nitroarenes under ambient conditions.^6–8 Recently, the M. Beller group reported that a series of iron complexes exhibit remarkable activities for catalytic or photocatalytic dehydrogenation of FA under mild conditions with very high hydrogen evolution rates.^9,10 However, practical utilization of those homogeneous catalysts for releasing H₂ from FA was obviously prevented by the cost of expensive ligands and the difficulty in reuse.

Heterogeneous catalysts are usually preferred for practical applications because of their better stability, ease of separation and recyclability as compared to homogeneous counterparts. Despite numerous developments of heterogeneous catalysts for the reduction of nitroarenes, there still exists a great interest in novel, inexpensive, active and selective catalysts. Cobalt oxide catalysts were recently introduced as an excellent heterogeneous catalyst for the hydrogenation of nitroarenes into the corresponding anilines, whilst specific ligands were still involved in the fabrication of the catalysts.^11,12 For practical applications, abundant, cheap and reusable catalysts are appreciated without the involvement of noble metal or expensive organic ligands.

In nature, enzymes composed of amino acids and [Ni–Fe–S] clusters (Scheme 1) can use binding and proximity effects to achieve an astounding rate for specific reactions (e.g. hydrogenation of CO) with high selectivity under mild conditions. Enzyme mimics have long been an important branch of bioinorganic chemistry, which is inspired by nature and aims to follow the general principles of natural enzymes using alternative materials. Simply mimicking the binding and proximity effects of enzymes, by using some inorganic clusters or nanoparticles as model materials, can make artificial enzyme progress for catalysis or photo-catalysis.^13,14

Scheme 1 Chemical structures of typical enzymes with [Ni–S] or [Ni–N] centers and the proposed structure of NiS_2+x/g-C₃N₄ nanohybrids (only one unit cell of NiS_2+x was depicted for simplification).

Although important reports have been made recently regarding the use of metal oxide and manganese sulfide materials as robust catalysts for photo-splitting and electro-splitting of water,^15–17 few such materials exist for hydrogenation of organic compounds. Thus, we turned towards a noble metal-free heterogeneous catalyst based on transition metal sulfide nanoclusters and amine-group-rich polymeric carbon nitride (g-C₃N₄). The g-C₃N₄ materials have recently been described as an excellent support to embed and activate metallic nanoparticles for both photocatalysis and organic synthesis.^18–25 More importantly, the unique heterocyclic macrocycle structure of g-C₃N₄ with an N–C–N-bonding pattern resembling porphyry in Scheme 1 can principally act as in situ “solid ligands” to provide strong coordination to metal ions and thus can tune the electronic environment of the supported nanoclusters.^26–29 We here establish that highly selective reduction of nitroarenes to the corresponding anilines is efficiently catalysed by g-C₃N₄ supported NiS_2+x nanoclusters (NiS_2+x/CN) under mild conditions. A commercially available reductant, NaBH₄, was selected as a model hydrogen source, because NaBH₄ has been widely used with metallic nanoparticle-based catalysts for the reduction of nitroarenes at an ambient temperature.^8,10,30,31 The selectivity of the reduction of –NO₂ over NiS_2+x/g-C₃N₄ is very high, even in the presence of other functional groups.

The NiS_2+x/CN catalysts were prepared by a modified self-catalytic method previously described in our group (see the ESI† for details).^32,33 A typical TEM image revealed the stability of mesoporous g-C₃N₄ after the deposition of the NiS_2+x components on the surface (Fig. 1A). After the deposition of NiS_2+x, both the surface area and the pore size distribution of mesoporous g-C₃N₄ remained unchanged (Fig. S1†). Due to the similar contrasts of both g-C₃N₄ and NiS_2+x in the view range of TEM, it is difficult to distinguish NiS_2+x from g-C₃N₄ at a relatively low resolution. A high resolution TEM (HRTEM) image (Fig. 1B), however, ambiguously displays a fringe with lattice spacing of approximately 2.52 Å, corresponding to the (210) plate of NiS₂. The successful deposition of NiS₂ can also be confirmed by XRD (See Fig. S2†). The main peaks in the X-ray diffraction pattern can be indexed to NiS₂ (JCPDS no. 11-0099).

Fig. 1 TEM (A) and HRTEM (B) images of NiS_2+x/CN. XPS spectra of Ni 2p (C) and S 2p (D). Two shake-up satellites of Ni²⁺ (denoted as “Sat.”)³⁴ were also observed in the Ni 2p_3/2 XPS spectrum.

The X-ray photoelectron spectroscopy (XPS) spectra of the samples further demonstrated the contents and chemical status of the NiS_2+x nanoclusters. As revealed by the XPS analysis result (Fig. S3†), the atomic number ratio of nickel and sulphur is 1 : 2.2, which is slightly lower than the theoretical value (1 : 2) of NiS₂. Consequently, the typical sample was named as NiS_2+x/CN, due to the non-stoichiometric composition of our sample. The high resolution Ni 2p XPS spectrum of the typical sample (Fig. 1C) revealed that small parts of Ni²⁺ were oxidized into Ni³⁺ during the process of depositing NiS_2+x nanoclusters on the surface of the g-C₃N₄ support. The fact that the self-catalytic method for depositing metal sulphides was conducted at 80 °C rather suggests a photocatalytic oxidation of Ni²⁺ using g-C₃N₄ in air. Such a photocatalytic oxidation reaction has been well defined for g-C₃N₄ with a specific nanostructure for organic synthesis, where the formation of O²⁻ ions was detected.³⁵ The possibility of a photocatalytic oxidation reaction during the formation process of NiS₂ nanoclusters was further confirmed by the detailed S 2p XPS spectrum (Fig. 1D), with two strong peaks at 168.56 eV and 169.6 eV attributed to sulfur oxoanions (denoted as S–O_x in Fig. 1D).³⁶ The typical peaks at 162.55 eV and 163.68 eV suggested the presence of S–S bonds of NiS₂ in the samples, again revealing the formation of NiS₂ on the surface of the g-C₃N₄ support.^37,38 All these observations revealed a functional group-rich surface of the NiS₂ nanoparticles, well explaining the slightly lower Ni/S ratio. Obviously, these groups will benefit from the formation of a coupled interface between g-C₃N₄ and the NiS_2+x nanoclusters, and thus shows a potential application of NiS_2+x/CN as a catalyst for organic synthesis.

Aromatic amines are key intermediates in the synthesis of fine chemicals, biologically active compounds and more. As a typical aromatic amine, aniline, with a worldwide production exceeding four million tons per year, was obtained mainly (∼97%) from hydrogenation of nitrobenzene. The excess amount of H₂ gas (>100 times in moles) was usually used for the catalytic transformation of nitrobenzene to aniline at high pressure (greater than 5 atm) and high temperature (>200 °C), and the process is thus of “high-energy consumption” and unsafe in terms of H₂ handling. Thus, chemoselective hydrogenation of nitro compounds to aniline and the corresponding substituted derivates in a sustainable manner is of great importance for both industrial applications and fundamental researches. We tested the possibility of hydrogenation of nitrobenzene to aniline for the NiS_2+x/CN samples at room temperature by using water as the green solvent and NaBH₄ as the only hydrogen source.

No conversion was observed without a catalyst (entry 1, Table 1) or NaBH₄ (entry 2, Table 1). Bare carbon nitride offered no conversion at all (entry3, Table 1). Unsupported NiS₂ nanoparticles and carbon black (C, surface area: 62.5 m² g⁻¹) or nitrogen doped layered carbon (N-LC, surface area: 190 m² g⁻¹)³⁹ supported NiS_2+x nanoparticles could only offer moderate conversions and selectivity (entry 4–6, Table 1). Under fixed conditions, NiS_2+x/CN (9 wt%) offered a conversion of nearly 100% with a selectivity to aniline of up to 99% (entry 7, Table 1). Note that the content of NiS_2+x was calculated from the amounts of precursors used in the synthetic process for a better clarification, whilst the real content was further confirmed by XPS (Fig. S3†) analysis. Obviously, carbon nitride has played an important role in increasing both the activity and selectivity of the NiS₂ nanoparticles (Scheme S1†).

Table 1 Hydrogenation of nitrobenzene^a

Entry	Catalyst	Conv. [%]	Sel. [%]
a Reaction conditions: 0.5 mmol nitrobenzene, 50 mL of water, 10 mg of catalyst, 0.5 mmol NaBH4, room temperature, 1 h.b Without catalyst.c Without NaBH4.d The byproduct is nitrosobenzene.
1^b	—	—	—
2^c	NiS2+x/CN	—	—
3	CN	—	—
4	NiS2/N-LC	48	79^d
5	NiS2/C	99	77^d
6	NiS2	42	53^d
7	NiS2+x/CN	>99	>99
8	Pd/CN	>99	>99
9	Au/CN	>99	56^d
10	Pt/CN	>99	54^d

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To determine the effect of the content of the NiS_2+x nanoparticles in the catalyst on the catalytic performance, the NiS_2+x/CN catalyst with weight percentages of the NiS_2+x species from 1 to 15% were prepared under the same conditions and used for the hydrogenation of nitrobenzene. The corresponding conversions are summarized in Table S1.† When the weight percentage of the NiS₂ nanoparticles was only 1% (1 wt%-NiS_2+x/CN), the catalyst gave a minor conversion (8%) and less selectivity to aniline. The conversion and selectivity have been both gradually improved to >99% as the weight percent of NiS₂ was increased to 9%. When more NiS_2+x species (>12 wt%) were introduced, the selectivity decreased via 95% to 86%. This is to say that too much NiS₂ nanoparticles deposited on the surface of the g-C₃N₄ support disturbed the synergetic effect between the two components. The synthetic temperature was optimized to be 80 °C (Table S2†). As the best catalyst here, NiS_2+x/CN, obtained at 80 °C with 9 wt% of the NiS_2+x nanoparticles, was selected for all the following hydrogenation reactions of nitrobenzene.

Some noble metal-based control catalysts (with a weight percentage of 9 wt%) were tested under standard conditions. Although full conversions (entry 8–10, Table 1) were also achieved using all these noble metal catalysts, only Pd/CN gave a selectivity of nearly 100% to aniline. Due to the same efficiency of NiS_2+x/CN and Pd/CN in the reduction of nitrobenzene for aniline, we further studied the reduction of halogenated nitrobenzene using the two catalysts. As shown in Table 2, the reduction of 4-chloronitrobenzene was found to have a yield of 96% for 4-chloroaniline and 4% for aniline using Pd/CN. Significant dehalogenation occurred in the reduction of 4-bromine nitrobenzene using Pd/CN with only a selectivity of 5% to the target product. Moreover, 4-iodonitrobenzene was completely reduced to aniline using Pd/CN. Such a dehalogenation reaction was normal for noble metal-based nanocatalysts, suggesting their limitation in the hydrogenation of halogenated compounds. Surprisingly, NiS_2+x/CN could completely transfer halogenated nitrobenzene into substituted halogenated aniline without obvious dehalogenation under standard conditions. The cheap and mild NiS_2+x/CN catalyst has obvious advantages for practical applications in industry.

Table 2 Hydrogenation of halogenated nitrobenzene using Pd/CN and NiS_2+x/CN^a

Substrate	Pd/CN		NiS2+x/CN
	Conv. [%]	Sel. [%]	Conv. [%]	Sel. [%]
a Standard conditions described in Table 1.b The byproduct is aniline.
	>99	96^b	>99	>99
	>99	5^b	>99	>99
	>99	0^b	>99	>99

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After knowing the superior catalytic activity of NiS_2+x/CN for the hydrogenation of nitrobenzene and halogenated nitrobenzene, we further tested its compatibility with various substituted nitroarenes. The methyl substituted substrates are completely reduced to the corresponding anilines in an excellent yield of 99% (entries 1–3, Table 3). The reaction of 2,6-dimethlnitrobenzene could also reach an excellent yield of the target product of up to 99% (entry 7, Table 3). Typical halogenated nitrobenzenes were efficiently reduced with both good conversion and selectivity (entries 4–6, Table 3). Our protocol also worked well for the reduction of other typical complex nitro compounds, such as 1-nitro-naphthalene and 6-nitroquinoline (entry 8, 9, Table 3). The activity of the NiS_2+x/CN catalyst will not be affected by steric hindrance and conjugate phenomenon. All these results suggest that the noble metal-free NiS_2+x/CN catalyst can be used for the reduction of nitroarenes with an excellent conversion and selectivity in a mild process.

Table 3 Hydrogenation of nitro compounds using NiS_2+x/CN^a

Entry	Substrate	Conv. [%]	Sel. [%]
a Standard conditions described in Table 1.
1		99	>99
2		99	>99
3		>99	>99
4		>99	99
5		>99	>99
6		>99	98
7		>99	99
8		>99	94
9		>99	>99

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The catalytic reduction of 4-nitrophenol was studied as a model reaction to investigate the stability and reusability of the catalyst. NiS_2+x/CN is successfully recycled and reused 2 times, almost without any significant loss of catalytic activity (Fig. S7†). The conversion of 4-nitrophenol decreased by 20% until the third round of reaction. This deactivation might be mainly attributed to the decomposition of the NiS_2+x nanoparticles, which was confirmed by the XRD analysis (Fig. S5†). This phenomenon is normal for metal sulfide based catalysts.^15,17 As the g-C₃N₄ support remained stable after multiple cycles of catalytic reactions (Fig. S6†), the regeneration of the used catalyst was facilely conducted via depositing additional NiS_2+x species. The regenerated catalyst offered a full conversion of 4-nitrophenol again (Fig. S7†). Further efforts will be focused on developing more stable noble metal-free heterogeneous catalysts by improving the stability of the metal sulfide nanoclusters or using other stable transition metal-based nanoparticles as active centers.

Conclusions

In summary, we developed an efficient g-C₃N₄ supported NiS_2+x nanoparticle (NiS_2+x/CN) catalyst system for the hydrogenation of nitroarenes under mild conditions. Our noble metal-free NiS_2+x/CN catalyst offered excellent catalytic activity, which was comparable to that of Pd nanocatalysts and much better than those of Au and Pt nanocatalysts. More importantly, the chemoselective hydrogenation of halogenated nitrobenzene to corresponding anilines using the NiS_2+x/CN catalyst is rather speaking for the advantages of the NiS_2+x/CN catalyst as a mild catalyst against noble metal counterparts. The strategy reported here may supply new possibilities for developing sustainable nanocatalysts for practical applications.

Acknowledgements

This work was supported by the National Basic Research Program of China (2013CB934102 and 2011CB808703) and the National Natural Science Foundation of China (21331004 and 21301116).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterizations, XPS, XRD, FTIR and PL spectra, and measurements. See DOI: 10.1039/c4ra10127h

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