Copper oxides supported sulfur-doped porous carbon material as a remarkable catalyst for reduction of aromatic nitro compounds

Synthesis and manufacturing of metal–organic framework derived carbon/metal oxide nanomaterials with an advisable porous structure and composition are essential as catalysts in various organic transformation processes for the preparation of environmentally friendly catalysts. In this work, we report a scalable synthesis of sulfur-doped porous carbon-containing copper oxide nanoparticles (marked CuxO@CS-400) via direct pyrolysis of a mixture of metal–organic framework precursor called HKUST-1 and diphenyl disulfide for aromatic nitro compounds reduction. X-ray diffraction, surface area analysis (BET), X-ray energy diffraction (EDX) spectroscopy, thermal gravimetric analysis, elemental mapping, infrared spectroscopy (FT-IR), transmission electron microscope, and scanning electron microscope (FE-SEM) analysis were accomplished to acknowledge and investigate the effect of S and CuxO as active sites in heterogeneous catalyst to perform the reduction-nitro aromatic compounds reaction in the presence of CuxO@CS-400 as an effective heterogeneous catalyst. The studies showed that doping sulfur in the resulting carbon/metal oxide substrate increased the catalytic activity compared to the material without sulfur doping.


Devices
Identifying organic and inorganic materials was accomplished by the Furrier Transform Infrared Spectroscopy (FT-IR, 8400S).The X-ray diffraction (XRD, Bruker D8-ADVANCE diffractometer: Cu/Kα (λ = 0.15406 nm)) was utilized to analyze the crystal structure of solid specimens.The elemental combination and homogeneous dispensation of instances were distinguished via energy-dispersive X-ray (EDX) and elemental mapping procedures.The morphology and structural attributes of the specimens were appraised per the passing electron microscope or magnifier (TEM, Philips EM208S 100 kV) and scanning electron microscope (FE-SEM, TESCAN MIRA3).The Braeuer-Emmett-Teller (BET) analysis for the measurement of the specific area and porosity distribution of the solid sample was provided at a low temperature (150 °C) via N 2 adsorption-desorption isotherms (Belsorp Mini II).

The procedure for the synthesis of, porous carbon/copper oxides catalyst derived from MOFs
Synthesis of HKUST-1 HKUST-1 was synthesized from copper (II) nitrate trihydrate (Cu(NO 3 ) 2 3H 2 O) and 1,3,5-benzenetricarboxylic acid (C 9 H 6 O 6 ) based upon the reported methodology 47 .Specifically, Cu (NO 3 ) 2 3H 2 O (2.174 g, 8.998 mmol) was dissolved in 30 mL distilled water; in the meantime, benzene-1,3,5-tricarboxylic acid (1.05 g, 4996 mmol) as an organic ligand was dissolved in 30 mL ethanol.Subsequently, the solutions were blended together and transferred to a 100 mL round bottom flask, and the mixture was continuously agitated for 30 min under an intense stirring situation at ambient temperature.After stirring for 30 min, the mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and kept at 120 °C for 12 h.Eventually, after cooling to room temperature, the blue powder was collected by centrifugation, and washed several times with deionized water and ethanol alternately, and dried at 60 °C in a vacuum for 12 h, to modify the synthesized HKUST-1, the obtained blue powder was chemically purified with ethanol and dichloromethane, and finally, in order to remove solvent molecules and activate the synthesized HKUST-1, the solid powder was heated in an oven at 150 °C for 12 h.

Synthesis of Cu x O@CS-400
The activated MOF powder (HKUST-1) and Diphenyl disulfide at different ratios (H:D 1:2, 1:3, and 1:5) were dissolved in 15 mL of N,N-dimethylformamide.The mixture was stirred for 24 h and dried in an oven at 60 °C.The precursor prepared was carbonized straight under the nitrogen atmosphere at 400 °C for 2 h with a thermal rapidity of 10 °C min −1 under the N 2 atmosphere.Finally, the above-carbonized sample was labeled Cu x O@ CS-400 and was used as a catalyst with excellent efficiency in organic reactions, and as expected, the catalyst has significant recyclability and can run eight times without any obvious decline in catalytic activity.Figure 1 epitomizes the schematic representation of the synthesis methodology of the carbon-based copper oxide catalyst.The Cu x O@C-400 was synthesized accordingly, without diphenyl disulfide.
Catalytic test 1mmol of substrate, 3 mL of deionized water and 7 mg of Cu x O@CS-400 were stirred for a few minutes at ambient temperature.Following, NaBH 4 (2 mmol) as the reductant was added to the above mixture, and the reaction mixture was stirred at 55 °C.The progression of the hydrogenation reaction was followed by TLC.At the end of the reaction time, the Cu x O@CS-400 was separated, rinsed, and reused.The catalyst was isolated, the resultant catalyst-free mixture was extracted by EtOAc as an organic phase (3 × 5 mL).The organic (EtOAc) layer was separated and dried by sodium sulphate.Eventually, by vaporizing the solvent, the pure product was gained (Table 1).

Preparation of Cu x O@CS-400
Figure 1 illustrates the process to prepare Cu x O@CS-400 catalyst, which was synthesized by thermal pyrolysis of a mixture of HKUST-1 and diphenyl disulfide at 400 °C for 2 h.As shown in Figure 1, firstly, the HKUST-1 was constructed according to the literature 47 .In the second step, the sulfur heteroatom was infiltrated into porous HKUST-1 by thermally annealing the precursor, which had been prepared by the wet-impregnation technique of a mixture of Ph 2 S 2 and HKUST-1in three ratios.After carbonization of the pre-materials in a stationary atmosphere, further organic connections were thermally transmuted into a porous carbon matrix, and Cu x O@ CS-400 was constructed as a heterogeneous catalyst.

FT-IR analysis
In order to explain the architecture of the Cu x O@CS-400-2 catalyst, the FT-IR spectrometric study was carried out.In Fig. 2, the FT-IR spectrum of HKUST-1 is presented.Peaks related to C=O asymmetric and symmetric stretching vibrations of carboxylic ligands appeared in 1646 cm −1 and 1375 cm −1 .The C=C groups of the aromatic rings were observed at 1431 cm −1 , which demonstrated that HKUST-1 has been successfully synthesized 47,48 .
On the other hand, obviously, the XRD analysis confirmed the prosperous preparation of HKUST-1.After the pyrolysis of the precursor, due to the production of carbonaceous material, the intensity of C=O was decreased by a considerable amount, and some peaks appeared in the distinctness of 1100-1200 cm −1 , which was associated with the C-O and C-S bonds in the as-synthesized Cu x O@CS-400 catalyst.

XRD measurement
The crystallinity, phase behavior, and diffraction peaks of HKUST-1 and Cu x O@CS-400 materials at different ratios (H: D 1:2, 1:3, and 1:5) were recognized by XRD analysis, as depicted in Fig. 3 and 4. The XRD diffraction peaks of HKUST completely matched with literature reports 47 .After pyrolysis the crystalline nature of the HKUST-1 has changed and new peaks have been created as follow: the as-prepared catalysts exhibited a broad peak at 2θ = 20-30˚, which is attributed to the carbon amorphous part of the as-synthesized samples (Fig. 4A-C is the XRD image of Cu x O@CS-400 material).Matching to the JCPDS standard card, the peaks at )002) (200) (220), and (113) crystal planes are the diffraction peaks of CuO (JCPDS: 48-1548), and the peaks appearing at (111) and (113) crystal planes in Fig. 4 belong to the diffraction peaks of Cu 2 O (JCPDS: 05-0667), which were clearly and precisely indicated in the XRD schemes of the samples 49 .It is noteworthy that the Bragg diffraction peaks of all oxides have a low vehemence, informing that Cu x O indicates negligible crystallinity with a small granule scale of 34 nm.For samples in Fig. 4A-C, the characterization results show a transient shift in the refraction summit circumstances and relative intensities.These are caused by varying copper oxide loading ratios in the final materials as a result of varying HKUST-1 and diphenyl disulfide ratios used as precursor materials for sulfur doping in catalyst structures.
The sample with a ratio of 1:2 (H:D) was chosen as the required catalyst, and the subsequent analyses were carried out on it since the catalytic activity of the three manufactured samples (with varying ratios of diphenyl disulfide) had the same catalytic activity.

EDX analysis
The elemental composition of Cu x O@CS-400 was studied using EDX analysis, and the results manifested that copper, carbon, oxygen, and sulfur are presented in the particular selected area of the catalyst.This is a verification of the prosperous synthesis of the desired catalyst with high purity (Fig. 5).Moreover, the exact loading of copper in Cu x O@CS-400 using the AAS (atomic absorption spectroscopy) technique was found to be 65.5%,   www.nature.com/scientificreports/which increased upon pyrolysis of pre-material in the N 2 atmosphere in comparison with HKUST-1.In addition, the amount of copper is in agreement with the EDX analysis.Elemental mapping images of Cu x O@CS-400 are shown in Fig. 6, which obviously shows a homogeneous distribution of the desired elements Cu, C, O, especially S, in the structure of the catalyst.

Thermogravimetric analysis
Thermal gravimetric analysis data at HKUST-1 and Cu x O@CS-400 were measured under the air with the heating speed of 10 °C min −1 , and outcomes are shown in Fig. 7.The HKUST-1 exhibited three weight loss stages.Two primary degradations, nearly 22 and 19%, befell at 30-400˚C and were ascribed to the evaporation of organic solvents inside the pores, either adsorbed or coordinated water.The second degradation was observed from 400 to 550 °C, nearly 60%, owing to the destruction of the HKUST-1 framework 47 .The TGA and DGA analysis of prepared Cu x O@CS-400 concluded that the catalyst exhibited three weight loss stages.Two primary degradations within 50-450 °C with weight-loss about 4% and 6.75% was due to the loss of adsorbed water as well as the coordinated water molecules in the carbon substrate.The third destruction was coupled with a weight-loss about 10.23% between 450 and 750 °C suggesting that residue organic substances has started to decompose and produce some volatiles materials at 450 °C.
The TGA and DTG analyses of prepared Cu x O@CS-400 concluded that the catalyst is stable up to 750 °C without any significant weight loss (Fig. 7), which lays a solid foundation for convenient catalytic high temperature synthesis reactions.

FE-SEM and TEM analysis
Figure 8 exhibits the scanning electron microscopy (FE-SEM) images of HKUST-1 and Cu x O@CS-400.The FESEM image of HKUST-1 shows a octahedral shaped morphology, as previously reported in the literature 47 .Upon incorporation of diphenyl disulfide and annealing of the pre-material at 400 °C, the structure of HKUST-1 is partially maintained and presents a rough surface due to the decoration of a number of Cu x O nanoparticles and holes that are produced during the carbonization process.
The structural examination was then assessed in depth using TEM measurements.The scattered Cu x O nanoparticles enclosed in the carbonaceous matrix are plainly visible in the concentrated black region.The porous carbon matrix that encircles the framework of dense metal oxides is indicated by the lighter portion of the catalyst (Fig. 9).

N 2 adsorption-desorption isotherm
According to IUPAC, the N 2 adsorption-desorption isotherms of HKUST-1 and catalyst belonged to type IV isotherms, with the H 1 hysteresis loop within the relative pressure of 0.1-0.95 as presented in Fig. 10.The specific surface areas of HKUST-1 and as-synthesized catalyst were 1021.2 and 2.4221 m 2 /g, respectively.It may be due to the agglomeration of Cu x O nanoparticles during the pyrolysis.The pore size was mainly distributed at 0.4173 and 0.01067 m 3 /g indicating a mesoporous structure.Finally, the average pore size of the catalysts (with different ratios of diphenyl disulfide) was calculated as 1.6345 and 26.826 nm, respectively.

Catalytic performance of Cu x O@CS-400 in the nitroarenes hydrogenation
To better understand the catalytic efficacy of Cu x O@CS-400 nano-catalyst, the 4-nitro aniline has been selected as a catalytic model and subjected to reduction of the nitro aromatic compound in the presence of NaBH 4 .In order to check the progress of the reaction after the supplementation of the reaction, soft stratum chromatography (TLC) was performed.Different reaction parameters, including the solvent, the reaction temperature, dosage of the catalyst, and the reductant, were optimized.The reaction consequences are exhibited in Table 1.
Among the examined solvents (entries 8, 9, 10, and 11), the best conversion is obtained in the presence of H 2 O (entry 6, 100% conversion).According to a literature review, the presence of water molecules has a significant impact on the conversion of 4-nitro aniline to 1,4-diaminobenzene 50 .The reducing agent screening and its amount show that NaBH 4 is present for the optimal conversion (entry 6).Higher or lower values of the reducing agent result in a reduction in the reaction yield (entries 12-14).Additionally, the impact of reaction temperature was examined (entries 12).The ideal temperature was determined to be 55 °C for the process.The ideal amount of catalyst for the reduction of 4-nitro aniline in the studied process was found to be 7 mg, or 0.072 mol% Cu, based on catalyst screening (entry 6).It should be noted that the reaction stops and no yield is produced in the absence of a catalyst (entry 1).Additionally, the decreased catalytic activity of CuxO@C-400 and HKUST-1 in the absence of the heteroatom demonstrated the critical role that sulphur and copper oxides played in the reaction's progression and final product yield (entries 3 and 4).Furthermore, while employing the sulfur-doped carbon substrate as the catalyst, no product was seen (entry 2).
The catalytic activity of Cu x O@CS-400 is examined in relation to other reactants.Different types of substituents on nitro aromatic compounds were transferred to aromatic amines under optimal reaction conditions.
In order to investigate the generality of the transformation, different types of aromatic nitro compounds, derivatives containing electron-donating (OMe, OH, NH 2 ) and electron-withdrawing (Br, CF 3 , Cl, CN) were reduced under the optimized reaction terms.The supreme returns of the obtained aromatic amines in a very short reaction time indicate the good efficiency of the catalyst in both electron-poor and electron rich substituted nitro aromatic compounds (Table 2).Unfortunately, the yields of the corresponding reduced products were low for aliphatic nitro compounds.For a more detailed investigation of the catalytic activity of Cu x O@CS-400 nano-catalyst, the investigated reaction was performed in the presence of 10 mmol of 4-nitro aniline under optimal reaction conditions, and the yield of the product was reported to be 95%.

Mechanistic sight for the hydrogenation of the nitroarenes by CuxO@CS-400
According to the previous papers 43 , the plausible mechanistic pathway of the hydrogenation process of nitrobenzene was presented (Fig. 11).Firstly, the reaction between the reducing agent (NaBH 4 ) and the solvent (H 2 O) can generate hydrogen (H 2 ), which moved to the surface of the catalyst and the catalyst surface activation was done while NaBO 2 was formed as the by-product at room temperature.In the next step, the adsorption of the substrate onto the activated CuxO@CS-400 surface was done to transfer the hydride from the catalyst to the nitrobenzene.Then the nitro reduction was happened and the nitroso group was produced.Subsequently, the hydroxylamine was generated according to the addition reductive process on nitroso group.In final step, the primary amine was released from the surface of the catalyst by hydrogenation of the hydroxylamine and the also the CuxO@CS-400 surface was set-free and for the next hydrogenation catalytic runs.

Performance comparison of Cu x O@CS-400 in hydrogenation of aromatic nitro compounds with other reported MOF-derived Cu-based catalysts
In this part of the research work, the catalytic efficiency and performance of Cu x O@CS-400 in the hydrogenation reaction of nitroarenes were reconciled with the previously published MOF-derived Cu-based catalysts, and the outcomes are succinct in Table 3.However, every one of these catalysts shown in Table 3 has its own advantages.It is clear that the above catalyst has shown significant activity and efficiency due to reaction conditions such as short reaction time, green solvent (H 2 O), low temperature, appropriate amount of reductant, recovery and reusability, as well as time and product yield.

Recyclability and stability of Cu x O@CS-400
Considering the environmental and economic advantages and the reusable characteristics of a catalyst, it is very important for economic and industrial applications.Accordingly, the reusability of Cu x O@CS-400, under optimal reaction conditions was examined for the transformation of 4-nitroaniline to 1,4-phenylenediamine.The reaction vessel was supplemented with a fresh sample of 4-nitroaniline and the repurposed catalyst.When the reaction mix was stirred under ideal circumstances, 90% yield was produced after eight cycles, suggesting that Cu x O@CS-400 could be recycled for the reduction of 4-nitro aniline (Fig. 12).After the eight cycles, the product formation efficiency decreased to 86%, which could be due to copper oxide leaching.So, atomic absorption www.nature.com/scientificreports/spectroscopy was performed on recycled catalyst and the percentage of copper was determined to be 64.1%, which indicates 1.4% decrease compared to the fresh catalyst.
To prove the stability and high activity of the Cu x O@CS-400 catalyst, several stages of recovery were done, and finally, the structural characteristics and morphology of the recovered catalyst were compared with those of the fresh catalyst by accomplishing XRD and TEM analysis following the reusability examination (Fig. 13).This analysis indicated that the morphology of the Cu x O@CS-400 catalyst used was relatively similar to the catalyst without recovery, which proves and confirms that it is a powerful catalyst (Fig. 13).

Conclusion
In conclusion, a sulfur doped HKUST-1 material was subjected to pyrolysis at 400 °C in a nitrogen atmosphere, resulting in the creation of an exquisite heterogeneous nano-catalyst.This novel heterogeneous catalyst, named Cu x O@CS-400, underwent comprehensive characterization and exhibited outstanding performance in the reduction reaction of nitro aromatic compounds.It also demonstrated excellent recovery capability, with a yield of 90% over eight consecutive cycles.Furthermore, even after the eighth cycle, Cu x O@CS-400 keeps its original morphology, indicating its long-lasting nature.The catalyst offers several advantages, including its simple synthesis, short reaction time, eco-fraternal disposition, thermal resistance, and remarkable catalytic activity and reusability.These attributes make Cu x O@CS-400 a highly desirable catalyst for various catalytic applications.

Figure 6 .
Figure 6.Elemental mapping images of Cu x O@CS-400 for Cu, C, N, O, and S elements.

Figure 8 .
Figure 8.The FE-SEM images of HKUST-1 and Cu x O@CS-400 catalyst in different magnifications.

Figure 13 .
Figure 13.TEM image and XRD patterns of fresh and recovered Cu x O@CS-400 after the 8th cycle.