J. Cent. South Univ. (2021) 28: 1279-1290

DOI: https://doi.org/10.1007/s11771-021-4696-8

ZIF-67 derived CoS_x/NC catalysts for selective reduction of nitro compounds

ZHANG Guang-ji(张广吉)¹, TANG Fei-ying(唐飞鹰)¹, WANG Li-qiang(王立强)²,YANG Wen-jie(杨文杰)², LIU You-nian(刘又年)^{1, 3}

1. Hunan Provincial Key Laboratory of Micro & Nano Materials Interface Science, College of

Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Henan Province Industrial Technology Research Institute of Resources and Materials, School of

Material Science and Engineering, Zhengzhou University, Zhengzhou 450001, China;

3. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: Transition metal sulfides (TMSs)-based materials have been extensively investigated as effective non-noble catalysts for various applications. However, the exploration of TMSs-based catalysts for hydrogenation of nitro compounds is limited. Herein, CoS_x/NC catalysts were prepared by solvothermal sulfurization of ZIF-67, followed by high-temperature annealing (300–600 °C) under NH₃ atmosphere. It was found that the structures and compositions of the as-prepared CoS_x/NC can be readily tuned by varying the annealing temperature. Particularly, CoS_x/NC-500, which possesses higher degree of S defects and larger specific surface areas, can achieve high conversion, selectivity and stability for catalytic reduction of nitro compounds into amines under mild reaction conditions.

Key words: transition metal sulfides; catalytic hydrogenation; crystalline phase; selective reduction

Cite this article as: ZHANG Guang-ji, TANG Fei-ying, WANG Li-qiang, YANG Wen-jie, LIU You-nian. ZIF-67 derived CoS_x/NC catalysts for selective reduction of nitro compounds [J]. Journal of Central South University, 2021, 28(5): 1279-1290. DOI: https://doi.org/10.1007/s11771-021-4696-8.

1 Introduction

Catalytic hydrogenation is an essential reaction in the organic industry [1, 2]. For example, hydrogenation offers an important approach towards for reduction of nitro compounds into the corresponding amines which have been extensively used for synthesis of dye, medicine, pigment, etc [3-5]. Nowadays, noble metal-based catalysts play a dominating role in hydrogenation of nitro compounds. Nevertheless, the high price, low selectivity and easy to be poisoned features seriously limit their industrial-scale applications [6]. As an alternative, carbon-supported non-noble catalysts have received tremendous attention recently [7], since Beller found that carbon- supported FeO_x or CoO_x exhibited decent activity and high selectivity towards hydrogenation of nitro compounds [8-10]. Especially, transition metal sulfides (TMSs) based materials have been investigated as low cost and highly active non-noble catalysts for various applications [11]. DUAN et al [12] prepared a hybrid non-noble FeS₂/NSC catalyst, and the catalyst shows excellent activity, selectivity and stability in the hydrogenation of nitro compounds. SORRIBES et al [13] prepared a nanolayered molybdenum disulfide cobalt-promoted materials (Co-Mo-S), which possess high selectivity for catalytic hydrogenation of nitro compounds into the corresponding amines under mild conditions. Our group also developed an N-doped porous carbon-supported TMSs-based catalyst that can afford high activity and selectivity for hydrogenation of nitroarenes in water/methanol mixture [14]. Though several TMSs-based catalysts have been reported in hydrogenation of nitro compounds into the corresponding amines, the catalysts with improved performance are still high required.

Recently, MOFs (metal-organic frameworks) have been proved excellent precursors towards preparing carbon-supported catalysts [15-23]. The MOFs-based catalysts often have a uniform porous structure and uniformly dispersed metal active sites, which offer the catalysts excellent catalytic activity [24-26]. For instance, MOF-derived transition metal sulfides (TMSs) have been proved excellent candidates for electrocatalysts [27-31]. To the best of our knowledge, there are only a few reports on the exploration of MOF-derived TMSs-based materials for catalytic hydrogenation of nitro compounds. Especially, there are few reports on the influence of the crystalline of TMSs on the catalytic performance of hydrogenation [32, 33].

Herein, hollow porous cobalt sulfides supported on N-doped carbon are developed as hydrogenation catalysts for the reduction of nitro compounds. Firstly, Co₃S₄/NC was obtained by solvothermal sulfuration of ZIF-67 (as a precursor). Co₃S₄/NC was annealed under NH₃ atmosphere at various temperatures, affording CoS_x/NC-X (X represents annealing temperature) with tunable structures and compositions (see Scheme 1). Then,the effects of annealing temperature on the compositions and structures of the catalysts were investigated. Finally, the catalytic hydrogenation performance of CoS_x/NC-X was evaluated.

Scheme 1 Schematic illustration of preparation of CoS_x/NC-X for reducing nitro compounds (RT: Room temperature)

2 Results and discussion

CoS_x/NC was prepared by solvothermal sulfuration of ZIF-67, followed by high- temperature anneal (see Supporting information). ZIF-67 prepared according to reported method exhibits a high degree of crystallinity (Figure S1(a)) [34, 35]. After sulfuration by thioacetamide, ZIF-67 was converted into Co₃S₄. It was found that some residual organic materials were left and coated on the surface of Co₃S₄. Notably, at low sulfuration temperature, Co₃S₄ shows a low crystallinity. The XRD patterns of NH₃-treated samples are shown in Figure 1. After treatment at 300 °C, the main crystal phase of Co₃S₄/NC-300 is still Co₃S₄, but possesses a higher degree of crystallinity. This is because high temperatures can remove the residual organic molecules and promote the crystallization (see Figure 1(a)). Raising the processing temperature to 400 °C can further improve crystallinity (see Figure 1(b)). However, when the temperature was raised to 500 °C, CoS_x/NC-500 displays XRD peaks of a mixed crystal phase of Co₉S₈ and Co_1-xS (see Figure 1(c)). As illustrated in Figure 1(d), when the pyrolysis temperature was increased from 500 °C to 600 °C, Co₃S₄ was completely converted into Co₉S₈ with a high degree of crystallinity, indicating that CoS_x/NC can gradually change from Co₃S₄ to Co₉S₈ with the increase of the temperature. Such a phase transition is mainly due to the reducibility of NH₃. NH₃ can interact with Co within CoS_x, and drive the crystal to form thermodynamically stable Co₉S₈ under high temperature. Of note, such a process leads to S defect, which can be explored to tune the catalytic activity of the gained CoS_x.

SEM and TEM measurements were performed to provide a direct view of the morphology of CoS_x/NC-X. As shown in Figure 2(a), ZIF-67 exhibits a solid regular dodecahedron structure. After sulfuration, Co₃S₄/NC possesses a polyhedron-like frame but with more wrinkles on its surface (see Figure 2(b)). After annealing under the NH₃ atmosphere, the morphology displays negligible changes.

Specifically, Co₃S₄/NC-300, Co₃S₄/NC-400 and CoS_x/NC-500 maintain the polyhedron morphology, but partial collapses occurred, and more collapses produced when the pyrolysis temperature was increased to 600 °C (see Figures 2(c)-(f)). The structure collapse is mainly caused by chemical etching effect of ammonia, which is aggravated by raising pyrolysis temperature. Such collapse is likely to bury the active sites, and consequently sacrifice the activity. Energy dispersive results of X-ray analysis (EDS) reveal that Co, S, N and C are homogeneously distributed through the whole CoS_x/NC-X catalysts (see Figures 2(g), (h) and (i)).

Figure 1 XRD patterns of Co₃S₄/NC-300 (a), Co₃S₄/NC-400 (b), CoS_x/NC-500 (c), and Co₉S₈/NC-600 (d)

TEM images of the samples are shown in Figure 3. All CoS_x/NC-X samples possess hollow structures, consistent with the results reflected by SEM. The hollow structure can work as a reactor, which can improve the reaction kinetics by accelerating mass transfer and increasing the local concentration of the reactants. Similarly, NH₃- caused deformation was observed in the TEM images. Meanwhile, the high-resolution transmission electron microscopy (HR-TEM) image reveals that the lattice spacing of 0.30 nm indexing to the (311) crystal plane of Co₉S₈ appears, meaning that there exists Co₉S₈ within CoS_x-500.

X-ray photoelectron spectroscopy (XPS) was used to probe the chemical composition and state of CoS_x/NC catalysts. As shown in Figure 4(a), the peaks of C 1s, N 1s, O 1s, S 2p and Co 2p can be observed in the XPS profiles of CoS_x/NC. It shows that C 1s is divided into three peaks at 284.8, 286.4, and 288.8 eV (see Figure 4(b)), corresponding to sp² hybridized C=C, C—N/C—O and C=O, respectively [22, 33, 34]. It can also be observed that N 1s is divided into three peaks, i.e., pyridine N (398.4 eV), Co-N_x (399.8 eV) and graphite N (401.6 eV) [35-38]. Usually, N atoms coordinate with metal ions, thus can adjust the electronic structure of the catalyst, and stabilize the metal-based nanoparticles (see Figure S4). XPS of S 2p of CoS_x/NC is shown in Figure 4(c). Peaks at 161.8 and 163.2 eV can be observed, which are attributed to S 2p_3/2 and S 2p_1/2 of S^2–, respectively [14, 32]. The peak at 168.8 eV is attributed to an oxidized sulfur species, indicating the presence of Co sulfide [27]. In the Co 2p spectra of CoS_x/NC,the peak at 778.8 eV belongs to Co 2p_3/2 of Co₉S₈. The corresponding peak intensity follows the order of Co₃S₄/NC-400 < CoS_x/NC-500 < Co₉S₈/NC-600, indicating that the Co₃S₄ phase in the catalyst gradually changes to Co₉S₈ with the increase of annealing temperature. Peaks at 781.0, 793.8 and 797.2 eV are attributed to Co 2p_3/2 of Co²⁺, Co 2p_1/2 of Co³⁺ and Co²⁺, respectively (see Figure 4(d)) [20, 39-43].

Figure 2 SEM images of ZIF-67 (a), Co₃S₄/NC (b), Co₃S₄/NC-300 (c), Co₃S₄/NC-400 (d), CoS_x/NC-500 (e, g),Co₉S₈/NC-600 (f), elemental mapping of Co, S, N and C in CoS_x/NC-500 (h, i)

Figure 3 TEM images of Co₃S₄/NC-300 (a), Co₃S₄/NC-400 (b), CoS_x/NC-500 (c), Co₉S₈/NC-600 (d), HR-TEM images of CoS_x/NC-500 (e, f)

N₂ adsorption/desorption measurements were carried out to investigate the porous structure of CoS_x/NC-X. As shown in Figure 5(a) and S5, the adsorbed volume of ZIF-67 is much larger than that of CoS_x/NC-X, indicating that both the sulfuration and pyrolysis can decrease the porosity of CoS_x/NC-X by breaking the micropore structure. There exists a hysteresis loop between 0.6 and 0.9 (P/P₀), suggesting the existence of the mesoporous structure in the sample. The pore size distribution is shown in Figure 5(b). All CoS_x/NC-X samples possess mesopore structures, consistent with the results of nitrogen adsorption/desorption isotherms. Besides, there are also some micropores with sizes of 0.5 to 2 nm in the samples. The details of the pore volume and specific surface area of CoS_x/NC-X are listed in Table S1. The specific pore volumes of Co₃S₄/NC-300, Co₃S₄/NC-400, CoS_x/NC-500 and Co₉S₈/NC-600 are 0.03, 0.06,0.03 and 0.03 cm³/g, respectively. The specific surface area of CoS_x/NC decreases along with raising the pyrolysis temperature, i.e., Co₃S₄/NC- 300 (44.5 m²/g)>Co₃S₄/NC-400 (38.7 m²/g)>CoS_x/ NC-500 (24.6 m²/g)>Co₉S₈/NC-600 (9.6 m²/g), which can be contributed to the collapse of the structure of the catalysts. The micropore surface areas of Co₃S₄/NC-300, Co₃S₄/NC-400, CoS_x/NC- 500 and Co₉S₈/NC-600 are 6.4, 12.9, 2.3 and 4.1 m²/g, respectively, demonstrating that mesopore is the main pore structure of the catalyst. Mesopore structure is conducive to the mass transfer, hence promoting the reaction.

Figure 4 XPS spectra of CoS_x/NC-X:

Figure 5 CoS_x/NC-X of N₂ adsorption/desorption curves (a) and pore size distribution (b)

The catalytic performance of the catalysts for the reduction of nitro compounds was then evaluated. First, CoS_x/NC-X catalysts were used for 4-nitrophenol hydrogenation. As listed in Table 1, The precursor ZIF-67 has none catalytic activity, and Co₃S₄/NC exhibits poor catalytic activity (Conversion: 10%; Selectivity: 83%). The poor catalytic performance is possible because there are organic residues on the surface of Co₃S₄, which can poison the active sites. After annealing under the NH₃ atmosphere, CoS_x/NC-X samples exhibit significantly improved catalytic performance. Specifically, Co₃S₄/NC-400 performs better than Co₃S₄/NC-300 (Conversion: 93% vs. 60%; Selectivity: 99% vs. 98%). This is because high temperature is beneficial for removing the remaining organic molecules, hence improving the crystallinity of Co₃S₄. When the annealing temperature was raised to 500 °C, the as-prepared CoS_x/NC-500 reveals reduced specific surface area but exhibits higher catalytic activity (Conversion: 99%; Selectivity: 99%), probably because there are more S defects and high crystallinity in CoS_x, which could afford more active sites for adsorption and activation of the nitro compounds. The specific surface area of Co₉S₈/NC-600 is much lower than that of CoS_x/NC-500, so is the porosity. Consequently, the catalytic activity of Co₉S₈/NC-600 is lower than that of CoS_x/NC-500. However, the isolated Co₉S₈ particles only exhibit common catalytic performance. This phenomenon suggests that nitrogen-doped carbon substrates are vital to the excellent catalytic performance of CoS_x/NC-500, which can modulate the electron structure of the active site for improved catalytic activity. Overall, CoS_x/NC-500 performs the best among the as-prepared catalysts due to the combined effect of rich S defects and relatively large specific surface areas.

Table 1 Catalytic performance of various catalysts for hydrogenation of 4-nitrophenol

Generally, the solvent has a great impact on the catalytic performance. Ethanol (EtOH), methanol (MeOH), tetrahydrofuran (THF), acetonitrile, dioxane and water (H₂O) were used as solvents for the hydrogenation. As listed in Table S2, CoS_x/NC-500 performs the best in both EtOH and H₂O, and the conversions of 4-aminophenol in EtOH and H₂O are 99% and 99.9%, respectively. Meanwhile, the selectivity in both solutions exceeds 99%. The kinetics of catalytic reduction was further investigated. As shown in Figure S3, the conversion increases upon prolonging the reaction time (16 h, conversion 100%).

To verify the generality of CoS_x/NC-500 in hydrogenation reduction of nitro compounds, different types of nitro compounds were used as reaction substrates. As listed in Table 2, catalyst CoS_x/NC-500 delivers high conversion and selectivity for different nitro compounds. For example, for halogenated nitro compounds, the conversion and the selectivity are 98.9% and 99%, respectively (substrates 4 and 13), and no dehalogenation was observed. Unsaturated groups such as aldehyde groups are often sensitive to the hydrogenation and can be easily reduced on Pt/C catalysts. Comparatively, CoS_x/NC-500 exhibits high selectivity and conversion,4-nitrobenzaldehyde can be efficiently converted into 4-amino benzaldehyde (Conversion: 97%; Selectivity: 99%, Entry 11) without aldehyde group being reduced. For nitro compounds containing other functional groups such as ether, ester, alkyl, carboxyl, and amino, CoS_x/NC-500 can catalyze the reduction of nitro group, selectively. Similarly, nitrogen-containing heterocyclic nitro compounds can also be selectively reduced to the corresponding amines.

Table 2 Catalytic performance of CoS_x/NC-500 for various substrates

The stability of the catalyst was also evaluated. As shown in Figure 6, after eight catalytic cycles, the conversion of 4-aminophenol is above 95%, and the selectivity is around 99%, meaning that CoS_x/NC-500 is of high stability. The crystal structure of the recycled catalysts was monitored by XRD, where negligible changes of XRD profiles were observed.

Figure 6 Catalytic recyclability of CoS_x/NC-500

3 Conclusions

In summary, diverse CoS_x/NC-X catalysts were prepared by sulfidation of ZIF-67, followed by annealing at different temperatures (300-600 °C) under NH₃ atmosphere. During the annealing process, Co₃S₄ was gradually converted into CoS_x phase with a higher degree of crystallinity. The catalytic activity of CoS_x/NC-X is highly related to its crystal structure and specific area. Particularly, CoS_x/NC-500 with more S defects and relatively larger specific surface areas, displays excellent performance in catalytic hydrogenation of nitro compounds. CoS_x/NC-500 catalyst shows high conversion and selectivity for production of amines under mild reaction conditions (100 °C, 1.6 MPa H₂). Our findings provide a facile strategy for the fabrication of N-doped porous carbon-supported TMSs-based catalysts with a tunable crystalline phase, which has a great potential for industrial applications.

Supporting information

1 General information

All nitroarenes were purchased from Aladdin Chemicals (Shanghai, China). Cobalt chloride hexahydrate (CoCl₂·6H₂O), ethanol absolute was obtained from Adamas-beta. Chromatographic methanol and 2-methylimidazole were purchased from Tansoole under Shanghai Titan Technology. All chemicals were of analytical grade and used directly without further purification.

2 Catalyst preparation

ZIF-67 was prepared as follows: 2-methylimidazole (96 mmol) was dissolved in 200 mL methanol, then 12 mmol cobalt chloride (CoCl₂·6H₂O) was dispersed into 200 mL methanol. The above two solutions were mixed and stirred at room temperature for 6 h. The resulting solid was collected by centrifugation. Then it was washed 5 times with methanol and dried in an oven at 60 °C for 12 h to afford ZIF-67.

CoS_x/NC-X was prepared as follows: To 500 mL flask, 0.84 g ZIF-67, 350 mL ethanol, 1.57 g thioacetamide were added to afford a blue solution. After ultrasonication for 30 min, the obtained solution was placed into a high-pressure reactor, and the solvothermal reaction was carried out in an oven at a temperature of 120 °C for 4 h. After the reaction solution was cooled to room temperature, the solid was collected by centrifugation, followed by washing with ethanol several times, and drying in an oven at 75 °C overnight, to obtain black, solid Co₃S₄/NC. The black and solid Co₃S₄/NC was placed into a tube furnace and calcined at 300, 400, 500 and 600 °C for 2 h in an NH₃ atmosphere to give Co₃S₄/C-300, Co₃S₄/C-400, CoS_x/C-500 and Co₉S₈/C-600, respectively.

3 Catalyst characterization

X-ray diffraction (XRD) was carried out on Bruker D8 (diffractometer with Cu K_α (k=1.5418 ) radiation (50 kV, 30 mA)). Scanning electron microscopy (SEM, S-4800, Hitachi), and transmission electron microscopy (TEM, Tecnai G2-20, FEI) were explored to investigate the morphologies of the catalysts. The N₂ adsorption-desorption isotherms were measured to investigate the specific surface area and pore size of the catalysts (Aut absorb-IQ gas adsorption analyzer, Quantachrome). X-ray photoelectron spectra were explored to determine the compositions of materials (XPS, Thermo Scientific K-Alpha⁺ system).

4 Catalytic performance

The catalytic experiments were carried out in a 25 mL Teflon-lined high-pressure reactor. 0.3 mmol reaction substrate, 4 mL solvent and 7 mg catalyst were placed in the reaction kettle and then filled hydrogen of 1.6 MPa pressure. The reaction was kept at 100 °C for 16 h. After the reaction was completed, it was cooled to room temperature. GC-MS was used to detect the conversion and selectivity of the product of the reaction.

Figure S1 XRD patterns of ZIF-67 (a) and Co₃S₄/NC (b)

Figure S2 XRD pattern of reused CoS_x/NC-500

The catalyst was recovered by centrifugation and then washed several times with methanol. After drying, it was re-used to test the catalytic recyclability.

Figure S3 Kinetic curves of Co₃S₄/NC-400 and CoS_x/NC-500

Figure S4 XPS high-resolution N 1s spectra of Co₃S₄/NC-300 and CoS_x/NC-500

Figure S5 N₂ adsorption/desorption curves of ZIF-67

Table S1 Surface area and pore volume of CoS_x/NC

Table S2 Catalytic performance of CoS_x/NC-500 under various solvent

Contributors

ZHANG Guang-ji performed the experiments and wrote the first draft of the manuscript. ZHANG Guang-ji, TANG Fei-ying and WANG Li-qiang analyzed the measured data. YANG Wen-jie edited the draft of manuscript. LIU You-nian conceptualized and designed the study, coordinated and supervised data collection and analysis. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

There are no conflicts of interest to declare.

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(Edited by YANG Hua)

中文导读

ZIF-67衍生的CoS_x/NC催化剂的制备及其对硝基化合物的选择性还原

摘要：过渡金属硫化物作为高效的非贵金属催化剂，已在化工领域得到了广泛的应用。但是，很少将其应用于硝基化合物的催化加氢反应中。在本文中，通过硫化金属有机框架材料ZIF-67，并在氨气气氛下进行退火，制备得到了CoS_x/NC催化剂。研究发现，该类催化剂的结构和组成可通过改变退火温度进行调控。特别是，当退火温度为500 °C时，得到的催化剂CoS_x/NC-500由于具有高的硫缺陷和比表面积，可在温和反应条件下高选择性地将硝基化合物还原成相应的氨基化合物。

关键词：过渡金属硫化物；催化加氢；晶相；选择性还原

Foundation item: Projects(21636010, 21878342) supported by the National Natural Science Foundation of China; Project(2019JJ50758) supported by the Hunan Provincial Natural Science Foundation of China; Project(2019TP1001) supported by the Hunan Provincial Science and Technology Plan Project of China; Project(CX20190097) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-09-25; Accepted date: 2021-01-07

Corresponding author: YANG Wen-jie, PhD, Senior Engineer; Tel: +86-15378750416; E-mail: ywj2012@zzu.edu.cn; ORCID: https://orcid.org/0000-0002-5795-2278; LIU You-nian, PhD, Professor; Tel: +86-731-88879616; E-mail: liuyounian@csu.edu.cn；ORCID: https://orcid.org/0000-0002-7078-5937