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Nitrogen-enriched Carbon Nanotube Supported Palladium as a Catalyst for Desulfurization of Dibenzothiophene and Reduction of Nitroarenes

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10 May 2024

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10 May 2024

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Abstract
In this study, a new nitrogen-enriched multiwall carbon nanotube is prepared and utilized to stabilize palladium species. After the characterization of this new composite, it was applied as a catalyst in the reduction of nitroarenes and hydrodesulfurization of dibenzothiophene. Using this newly established catalyst, a wide range of aromatic compounds were reduced to the corresponding amines under a very low amount of Pd (0.005 mol%) in short reaction times. Also, dibenzothiophene (a major pollutant in fuel oil) was very efficiently transformed using this catalyst. Its robustness was studied in the recycling process where, after 20 runs in the reduction of 4-nitrotoluene and 5 times in the desulfurization of dibenzothiophene, the activity of the catalyst remained intact. According to the E-factor calculations, the present Pd catalyst meets the standards of green chemistry.
Keywords: 
Subject: Chemistry and Materials Science  -   Materials Science and Technology

1. Introduction

Carbon nanotubes with outstanding physical properties have different promising applications in drug delivery, biosensors, renewable energy storage, solar cells, electronic materials, etc. [1,2]. Due to the high chemical and physical stability of CNTs, recently, they can extensively be used in the design of heterogeneous catalysts [3,4,5]. Despite their unique properties, CNTs are chemically ineffective, hydrophobic, and have little activity on their own. However, after functionalization or modification of CNT with different ligands and functional groups, they become suitable substrates for the stabilization of active metal species and the preparation of heterogeneous catalysts. Various functional groups and ligands have been employed for the modification of the surface of carbon nanotubes, being ionic liquids of great importance due to their special properties such as high stability, assisting to dissolve reactants, increasing selectivity, and stabilizing metal nanoparticles. Furthermore, with the functionalization of CNT incorporating different ionic liquid moieties, the desired hydrophilicity and hydrophobicity can be modulated simply by exchanging the corresponding anion of IL [6,7]. However, ionic liquids are expensive and their use in large amounts is not economically worthwhile. One way to solve the problem is to support them onto solid surfaces allowing their easy recycling, and multiple uses [8,9].
Nitroaromatic compounds have extensive applications in different industries, explosives, pharmaceuticals, and in the synthesis of complex organic molecules. Nitroarenes such as nitrophenols are considered potent toxic, carcinogenic, and not biodegradable compounds, and their release into the environment and water is a hazard to living organisms and human being health [10,11]. On the other hand, reduction of nitro compounds to less toxic and useful amines is an efficient and promising method for transforming them. Compared to traditional methods, which are run in harsh acidic conditions, transition metal catalyzes the reduction of nitroarenes to corresponding amines in are very efficient, fast, and selective manner [12,13].
In addition, sulfur-containing organic compounds such as dibenzothiophene, present in fuel oils, are source of air pollution (acid rain generation) and engine corrosion [14,15]. Very recently, the governments around the world have planned to reduce the sulfur content of fuel to a very low level. Different strategies such as oxidative desulfurization [16], biodesulfurization [17], extractive desulfurization [18] and catalytic hydrodesulfurization (HDS) [19,20] have been performed for lowering sulfur concentration in fuel oils. Among them, catalytic HDS processes in the presence of heterogeneous catalyst and hydrogen sources is the most commonly method for producing cleaner fuels. However, this method operates using harsh reaction condition such as high temperature and high pressure with parallel deactivation of the catalyst which has encouraged the researchers to design of new catalysts for this purpose.
In this work, we are preparing a new nitrogen-enriched carbon nanotube for the immobilization of Pd species for the application in two very important reactions namely reduction nitroarenes and desulfurization of dibenzothiophene.

2. Results and Discussions

For the preparation of the catalyst, initially, imidazole was allowed to react with propargyl bromide, and resulting 1-(prop-2-yn-1-yl)-1H-imidazole (A) was reacted with 1,2-dibromopropane to produce IL(B). The multiwall carbon nanotube was treated with m-CPBA to produce epoxy-functionalized CNT (1). Next, CNT-epoxy was reacted with sodium azide followed by a reaction azido-CNT (2) with IL(B) under CuAAC to produce (3). Furthermore, IL-modified CNT was reacted with sodium azide and resulting compound (4) was treated with 1-(prop-2-yn-1-yl)-1H-imidazole under CuAAC (click reaction) to produce the nitrogen-enriched CNT (5). Finally, modified CNT was mixed with Na2PdCl4 to afford a CNT-supported palladium catalyst (6) which was referred as CNT-TZ-IL@Pd throughout the text (Scheme 1).
The physical morphology and structure of the prepared materials were characterized by different techniques. Transmission electron microscopy (TEM) images of CNT-TZ-IL@Pd showed the presence of uniform carbon nanotubes and also the presence of some Pd NPs which could be generated in the presence of nitrogenous CNT (Figure 1). Furthermore, the typical entangled morphology was clearly observed in the scanning electron microscopy (SEM) images of CNT@IL-Pd (Figure 2).
An energy-dispersive X-ray analysis (EDX) also confirmed the presence of elements such as C, N, and Pd in the structure (Figure 3).
To ensure the placement of organic compounds on the substrate, the Fourier Transform Infrared Spectroscopy (FTIR) was taken from all steps of the catalyst synthesis (Figure 4). The results showed that in the spectrum of CNT-epoxy composition, the peak observed in the region of 1128-1139 cm−1 is related to C-O-C stretching vibration [21]. Also, the appearance of a sharp peak at 2120 cm−1 in the spectra of CNT-N3 (2) and CNT-IL-N3 (4) compounds corresponded to the stretching vibration of the azide functional group, which indicates the successful azidation of the mentioned materials [22]. Also, the peak located at 2120 cm−1 in the spectrum of CNT-IL (3) and CNT-TZ-IL (5) compounds disappeared confirming the successful CuAAC-mediated click reaction between azide and alkyne. The related peaks to C=N appeared in the region of 1690-1640 cm−1 and peaks due to the C=C of imidazole ring appeared around 1650 cm−1, which overlaps with the surface –OH bending vibration. The observed peak at 1120 cm−1 in both CNT-IL (3) and CNT-TZ-IL (5) compounds is related to the N-N stretching vibration in the triazole ring. Also, the peaks around 12940 cm−1 were related to the symmetric and asymmetric stretching vibration of CH2, and peaks located around 3600-3300 corresponded to the possible surface O-H stretching vibration [22,23,24,25,26].
X-ray photoelectron spectroscopy (XPS) of CNT@Pd in C1s, N 1s and Pd 3d regions were studied (Figure 5). XPS in C1s showed peaks located at 284.6, 285.6, 286.4, and 288.1 eV related to C-H/C=C, C-C, C-N, C-O-C, C=O/-C=N+ forms of carbon [27]. Peaks centered at 399.2, 400.6, and 401.8 eV confirm N=N, C-N, and +NR3 forms of nitrogen [28]. Also, XPS of the Pd 3d region showed a doublet at 339.2 (Pd 3d3/2) and 344.5 eV (Pd 3d5/2) due to the presence of Pd(II) species [29].
Thermal gravimetric analysis (TGA) of CNT, modified CNT (3), and (5) in temperatures between 25-700 °C were carefully surveyed (Figure 6). TGA obtained from CNT-IL (3) showed two major weight losses in the temperature range of 150-350 and 350-420 °C. The first weight reduction (21%) was attributed to the destruction of organic groups placed on the carbon nanotube surface. The second weight loss of 57% was attributed to the destruction of the CNT structure. Analogously for sample (5), a 16% and a 57% were detected for the same ranges of the temperature, respectively. According to this data, and considering quantitative yield in all the steps required for the preparation of carbon materials (3) and (5), the functionalization was estimated in 0.94 and 0.70%, respectively.
X-ray diffraction analysis (XRD) of CNT-TZ-IL@Pd showed Bragg reflection related to carbon nanotubes in the region in 2θ = 25, 43, and 53 [30]. Considering that Pd is present in the ionic form, the corresponding peaks related to Pd did not appear (Figure 7).
Very uniform dispersion of Pd on CNT-IL was confirmed by EDX and elemental mapping images (Figure 8).
Next, the application of the prepared catalyst was assessed in the reduction of nitroarenes. For this purpose, the reduction of 4-nitrotoluene was selected as a model reaction, and the effect of different factors on the reaction was studied. Results showed that using 0.01 mol% catalyst, NaBH4 as the reducing agent, in solvents such as EtOH, H2O, and H2O:EtOH mixture, quantitative yields were achieved (Table 1, entries 1-3). However, other solvents as ethyl acetate, acetonitrile, DMSO, DMF or THF, gave poor or low yields (Table 1, entries 4-8), therefore, H2O was selected as the most appropriate solvent. Other reducing agents such as formic acid, ammonium formate, glycerol, isopropyl alcohol, and hydrazine in H2O gave poor yields (Table 1, entries 9-13). Using a 0.005 mol% of the catalyst, the chemical yield was not altered. However, an important lowering of the yield (61%) was obtained employing a 0.003 mol% of catalyst loading (Table 1, entries 14 and 15). Consequently, the optimum reaction conditions were H2O as a solvent, NaBH4 as a reducing agent, and 0.005 mol% catalyst as the suitable catalyst amount.
Next, the catalytic activity of CNT-TZ-IL@Pd (6) was studied in the reduction of structurally different nitroarenes to corresponding amines. Results showed that reductions of aromatic nitro compounds having both electron-donating and electron-withdrawing groups proceeded very well in short reaction times. In the case of nitroarenes having a formyl group both carbonyl and nitro functional groups were reduced to alcohol and amine, respectively, while carbonyl groups of ketones, esters, and amide were intact (Table 2, entries 25-30). The result of the high chemoselectivity leaving the ketone unaltered is a very important feature of this transformation. Reductions of heterocyclic nitroarenes were performed very well and the corresponding amines were obtained in excellent yields (Table 2, entries 30 and 31). Pharmaceutically active nitroarenes having higher molecular weights were reacted under longer reaction times, in the presence of TPGS-750-M, as a surfactant and THF as a co-solvent (Table 2, entries 32-35).
Finally, the environmental factor (E) for the reduction of the 4-nitrobromobenzene compound was obtained according to Shelden’s method [31]. The value of the E factor for this reaction was determined about 4.1, which according to Sheldon’s classification range of E factors, this catalytic system is suitable for industrial use.
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Next catalytic activity of CNT-TZ-IL was studied in the catalytic hydro-desulfurization reaction of dibenzothiophene (DBT). For this purpose, the effect of different hydrogen sources, reaction time, and temperature were studied. Results showed desulfurization of DBT (0.2 mmol) in the presence of 0.01 mol% catalyst and DBT 10 mmol gave poor yields and a 17% conversion was achieved. Increasing the amount of NaBH4 to 20 mmol, the yield was improved to 65% (Figure 9a). Also, results showed that increasing reaction temperature to 100 and 130 °C did not improve the reaction yield, while decreasing the temperature reaction to room temperature the yield decreased to 5% (Figure 9b).
Next, we studied the effect of other reducing agents such as formic acid, isopropanol, and tetralin in the hydrodesulfurization of DBT. Results indicated that formic acid and isopropanol afforded very poor conversion while 80% conversion was obtained using tetralin as a hydrogen source (Figure 10a). Finally, yields were improved to 93% conversion at 130 °C.
One of the most important aspects of heterogeneous catalysts is the possibility of recovering the catalyst from the reaction media and reusing it in successive cycles. In this regard, the recyclability of the CNT-TZ-IL@Pd catalyst for the reduction reaction of 4-nitrotoluene under optimal reaction conditions was investigated. After the completion of the reaction, the catalyst was separated from the reaction mixture using a centrifuge, washed with water and ethyl acetate, and used again in the same reaction with the addition of the raw materials. The results showed that this catalyst was recycled and reused up to 20 steps without a significant decrease in catalytic activity (Figure 11). The recyclability of the catalyst was also studied in the hydro-desulfurization of DBT using tetralin as the hydrogen source and results indicated that the catalyst can be recyclable for 4 times without decreasing of efficiency.
The structure of the reused catalyst after 20 runs was studied with different analyses such as TEM, XPS, EDX, and XRD.
It was interesting that the TEM images of the recycled catalyst after 20th run showed the presence of palladium nanoparticles with an average size of 1.6 nm, which were uniformly dispersed on the surface of the modified CNT (Figure 12). The presence of palladium nanoparticles was generated, possibly, by the use of sodium borohydride as reductant in the reaction process of reduction of nitro compounds. In addition, the presence of abundant nitrogen atoms in the substrate stabilized produced palladium nanoparticles.
The presence of Pd nanoparticles was further confirmed by XRD by appearing peaks at 2Ɵ = 40.1, 46.8, 68.2, and 82.2 [30] (Figure 13).
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Figure 11. XRD of reused catalyst in the reduction of 4-nitrotoluene.
Figure 11. XRD of reused catalyst in the reduction of 4-nitrotoluene.
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EDX analysis was studied on the recycled CNT-TZ-IL@Pd catalyst, the results showed that the structure of the catalyst was preserved and the patterns were very similar to the original catalyst (Figure 14).
XPS spectra of the reused catalyst in C1s and N1s regions showed very similar patterns to the fresh catalyst, however, the XPS spectrum of Pd confirms the formation of Pd(0) appearing a new doublet peak at 335.2 and 340.6 eV (Figure 15).

3. Conclusion

In conclusion, a new carbon nanotube-supported palladium catalyst was prepared and characterized. Using this catalyst a wide range of nitroarenes were reduced to the corresponding amines using low amounts of catalyst and short reaction times. Also, dibenzothiophene as one of the major pollutants of fuel oil was removed under a catalytic hydro-desulfurization reaction. This catalyst showed high stability, recyclability, and turnover number in mentioned reactions.

Acknowledgments

The authors are grateful to IASBS Research Council and Alicante University for support of this work. This work is based upon research funded by Iran National Science Foundation (INSF) under the project no. 4022486. The authors would like to thank the (INSF-Grant number of 4022486), Ministerio de Ciencia, Innovación y Universidades (RED2018-102387-T, PID2019-107268GB-100), and Generalitat Valenciana (IDIFEDER/2021/013) for support of this work.

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Scheme 1. Sequence for the preparation of CNT-TZ-IL@Pd (6).
Scheme 1. Sequence for the preparation of CNT-TZ-IL@Pd (6).
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Figure 1. TEM images of CNT-TZ-IL@Pd in different magnifications.
Figure 1. TEM images of CNT-TZ-IL@Pd in different magnifications.
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Figure 2. SEM images of CNT-TZ-IL@Pd.
Figure 2. SEM images of CNT-TZ-IL@Pd.
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Figure 3. EDX analysis of CNT-TZ-IL@Pd.
Figure 3. EDX analysis of CNT-TZ-IL@Pd.
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Figure 4. FTIR spectra of the different synthesized materials.
Figure 4. FTIR spectra of the different synthesized materials.
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Figure 5. XPS spectra of CNT-TZ-IL@Pd in a) C1s, b) N1s, and c) Pd 3d regions.
Figure 5. XPS spectra of CNT-TZ-IL@Pd in a) C1s, b) N1s, and c) Pd 3d regions.
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Figure 6. TGA diagrams of a) CNT, b) modified MCNT (3), and c) modified CNT (5).
Figure 6. TGA diagrams of a) CNT, b) modified MCNT (3), and c) modified CNT (5).
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Figure 7. X-ray diffraction (XRD) pattern of CNT-TZ-IL@Pd (6).
Figure 7. X-ray diffraction (XRD) pattern of CNT-TZ-IL@Pd (6).
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Figure 8. Elemental mapping of CNT-TZ-IL@Pd (6).
Figure 8. Elemental mapping of CNT-TZ-IL@Pd (6).
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Figure 9. a) Conversion of DBT in the presence of different amounts of NaBH4. b) Effect of temperature versus conversion of DBT using NaBH4 and CNT-TZ-IL@Pd (6) catalyst.
Figure 9. a) Conversion of DBT in the presence of different amounts of NaBH4. b) Effect of temperature versus conversion of DBT using NaBH4 and CNT-TZ-IL@Pd (6) catalyst.
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Figure 10. a) effect of different hydrogen sources on hydro-desulfurization of DBT, b)effect of temperature hydro-desulfurization of DBT using tetralin as a hydrogen source in the presence of CNT-TZ-IL@Pd (6).
Figure 10. a) effect of different hydrogen sources on hydro-desulfurization of DBT, b)effect of temperature hydro-desulfurization of DBT using tetralin as a hydrogen source in the presence of CNT-TZ-IL@Pd (6).
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Figure 11. recycling of the catalyst for the reduction reaction of 4-nitrotoluene.
Figure 11. recycling of the catalyst for the reduction reaction of 4-nitrotoluene.
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Figure 12. TEM images of reused catalyst in the reduction of 4-nitrotoluene.
Figure 12. TEM images of reused catalyst in the reduction of 4-nitrotoluene.
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Figure 14. EDX analysis of reused catalyst in the reduction of 4-nitrotoluene.
Figure 14. EDX analysis of reused catalyst in the reduction of 4-nitrotoluene.
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Figure 15. XPS spectra of the recycled catalyst after 20 runs in a) C1s, b) N1s, and c) Pd 3d regions in the reduction of 4-nitrotoluene.
Figure 15. XPS spectra of the recycled catalyst after 20 runs in a) C1s, b) N1s, and c) Pd 3d regions in the reduction of 4-nitrotoluene.
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Table 1. Optimization of the reaction conditions to perform the reduction of 4-nitrotoluene to 4-aminotoluene.
Table 1. Optimization of the reaction conditions to perform the reduction of 4-nitrotoluene to 4-aminotoluene.
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Entry Solvent Reducing agent Yield
1 EtOH NaBH4 100
2 H2O NaBH4 99
3 H2O: EtOH NaBH4 100
4 Ethyl acetate NaBH4 10
5 CH3CN NaBH4 4
6 DMSO NaBH4 6
7 DMF NaBH4 63
8 THF NaBH4 5
9 H2O Formic acid 3
10 H2O Ammonium format 4
11 H2O Glycerol 2
12 H2O Isopropyl alcohol 7
13 H2O Hydrazine 2
14 H2O NaBH4 100
15 H2O NaBH4 61
Table 2. Reductions of structurally different nitroarenes using CNT-TZ-IL@Pd (6) catalysta.
Table 2. Reductions of structurally different nitroarenes using CNT-TZ-IL@Pd (6) catalysta.
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Entry ArNO2 Time (min) ArNH2 Yield(%)
1 Preprints 106161 i003 4 Preprints 106161 i004 99
2 Preprints 106161 i005 10 Preprints 106161 i006 96
3 Preprints 106161 i007 4 Preprints 106161 i008 99
4 Preprints 106161 i009 2 Preprints 106161 i010 98
5 Preprints 106161 i011 3 Preprints 106161 i012 95
6 Preprints 106161 i013 4 Preprints 106161 i014 93
7 Preprints 106161 i015 3 Preprints 106161 i016 96
8 Preprints 106161 i017 4 Preprints 106161 i018 93
9 Preprints 106161 i019 5 Preprints 106161 i020 97
10 Preprints 106161 i021 7 Preprints 106161 i022 95
11 Preprints 106161 i023 7 Preprints 106161 i024 98
12 Preprints 106161 i025 5 Preprints 106161 i026 94
13 Preprints 106161 i027 6 Preprints 106161 i028 90
14 Preprints 106161 i029 7 Preprints 106161 i030 90
15 Preprints 106161 i031 5 Preprints 106161 i032 99b
16 Preprints 106161 i033 6 Preprints 106161 i034 96b
17 Preprints 106161 i035 4 Preprints 106161 i036 90c
18 Preprints 106161 i037 10 Preprints 106161 i038 88
19 Preprints 106161 i039 5 Preprints 106161 i040 98
20 Preprints 106161 i041 7 Preprints 106161 i042 97
21 Preprints 106161 i043 8 Preprints 106161 i044 98
22 Preprints 106161 i045 5 Preprints 106161 i046 97
23 Preprints 106161 i047 6 Preprints 106161 i048 96b
24 Preprints 106161 i049 5 Preprints 106161 i050 98
25 Preprints 106161 i051 8 Preprints 106161 i052 97b
26 Preprints 106161 i053 8 Preprints 106161 i054 85
27 Preprints 106161 i055 8 Preprints 106161 i056 96
28 Preprints 106161 i057 21 Preprints 106161 i058 92
29 Preprints 106161 i059 60 Preprints 106161 i060 87f
30 Preprints 106161 i061 120 Preprints 106161 i062 85c,f
31 Preprints 106161 i063 8 Preprints 106161 i064 94
32 Preprints 106161 i065 540 Preprints 106161 i066 89d,f
33 Preprints 106161 i067 420 Preprints 106161 i068 83d,f
34 Preprints 106161 i069 180 Preprints 106161 i070 94d,f
35 Preprints 106161 i071 360 Preprints 106161 i072 91d,f
[a] Reaction conditions: ArNO2 (0.4 mmol), NaBH4 (0.4 mmol), CNT-TZ-IL@Pd (6) (7 mg, 0.005 mol%), H2O (1.5 mL). Yields determined by GC. [b] Reaction performed using 0.8 mmol NaBH4. [c] Reaction conditions: ArNO2 (0.4 mmol), NaBH4 (0.4 mmol), CNT-TZ-IL@Pd (6) (7 mg, 0..5 mol%), H2O (1.3 mL), and THF (0.2 mL). [d] Reaction performed using 1.6 mmol NaBH4. [e] Reaction conditions: ArNO2 (0.4 mmol), NaBH4 (1.6 mmol), CNT-TZ-IL@Pd (6) (9 mg, 0.006 mol%), 2 wt % TPGS-750-M, H2O (1.2 mL), EtOH (0.3 mL) and THF (0.15 mL). [f] Isolated yields.
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Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
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