Abstract
Melamine based polymer (MT) was prepared and then reacted with a mixture of glucose (Glu) and β-cyclodextrin (CD) under hydrothermal conditions to afford, MT/Glu-CD. Then, the adsorption of Pd salt was realized on MT/Glu-CD. The resulting compound was subsequently carbonized to furnish Pd/MT/C that exhibited high catalytic activity for the hydrogenation of nitroarenes in aqueous media. To elucidate the roles of CD, Glu, the molar ratio of Glu:CD and the carbonization in the catalytic activity, several control catalysts have been prepared and their performances for a model hydrogenation reaction were compared with that of Pd/MT/C. The results confirmed the importance of the carbonization as well as the presence of CD for achieving high catalytic activity. Moreover, it was found that the molar ratio of Glu:CD could affect the catalytic activity of the final catalyst and the optimum molar ratio of Glu:CD was 30:70. The recycling test as well as measurement of Pd leaching demonstrated high recyclability and low Pd leaching of Pd/MT/C.
Introduction
In recent years, a new class of carbon materials, porous carbon materials, has attracted considerable research interests in numerous fields [1], [2], [3], [4]. Among these carbons, N-doped porous carbon materials that benefit from the advantages of high surface area, low cost, rigid molecular backbone, high nitrogen content and good mechanical properties are attractive candidates for numerous applications such as catalysis [5], [6]. These materials can be fabricated by using N-containing precursors such as aniline [7], pyrrole [8], acrylonitrile [9], [10], [11] and especially melamine [12], [13].
The outstanding features of β-cyclodextrin, β-CD, including, low price, availability, biocompatibility, tunability of the surface chemistry and the size of its cavity that can encapsulate various guest molecules render CD a good candidate not only for the catalytic purposes, but also for designing smart delivery systems [14], [15], [16]. Moreover, CD is a desired carbon precursor, which can be dehydrated and decomposed into a satisfactory graphitic sp2 network upon carbonization [17]. Notably, the cavity of CD may contribute to the porosity of the subsequent carbon materials and affect the textural properties of the obtained carbon.
The utility of anilines for the organic synthesis and pharmaceutical applications triggered many attempts to develop efficient and economic protocols for their large scale synthesis. Among various reported methodologies, hydrogenation of nitroarenes is of great importance. This catalytic reaction proceeds in the presence of precious metals such as Pd and a reducing agent such as hydrogen gas or sodium borohydride [18], [19], [20], [21]. Considering the environmental concerns, the use of heterogeneous and recyclable catalysts as eco-friendly and cost-effective agents is very demanding [22], [23], [24], [25], [26], [27], [28]. In this regard, various heterogeneous catalysts have been developed for the hydrogenation reaction [27], [28].
In the continuation of our research on the utility of carbon materials as well as CD for the catalytic purposes [27], [28], [29], [30], herein we wish to disclose a novel palladated nitrogen-doped porous carbon material as an efficient and heterogeneous catalyst for the hydrogenation of nitroarenes in the aqueous media. The carbon material was synthesized through multi-step procedure. More precisely, melamine-based mesoporous polymer was prepared and then reacted with glucose (Glu) and CD under hydrothermal conditions. In the next step, Pd nanoparticles were immobilized and the resulting compound was carbonized. The roles of carbonization, introduction of Glu, CD and the molar ratio of Glu:CD in the catalytic activity of the final catalyst were also investigated. Moreover, the generality of the developed protocol, recyclability and Pd leaching were studied.
Experimental
Materials
All the chemicals and solvents including, melamine, terephthalaldehyde, D-(+)-glucose, β-CD, NaBH4, Pd(OAc)2, DMSO, THF, dichloromethane, MeOH, EtOH, deionized water and toluene were received from Sigma-Aldrich. The hydrogenation reaction was performed by using nitroarenes. All nitroarenes were of analytical reagent grade and used without further purification.
Instruments
The prepared catalyst was analyzed using BET, TGA, TEM, FTIR, Raman, ICP techniques and ζ potential analyses. The details of the used apparatus are provided in SI.
Synthesis of the catalyst
Synthesis of MT (1)
The preparation of melamine-based mesoporous polymer network was achieved using the previous method with slight modification [31], [32]. Typically, melamine (3 g) was dissolved in 150 mL DMSO and then terephthalaldehyde (9.44 g) was added. The solution was heated to 180°C under vigorous stirring under argon atmosphere for 96 h. Upon completion of the polymerization reaction, the mixture was allowed to cool down to room temperature, and then filtered over a Buchner funnel. The precipitate was successively washed with excess amount of DMSO, THF and CH2Cl2 and dried under vacuum at 100°C for 10 h to furnish the pure product in the yield of 83 %.
Preparation of MT/Glu-CD (30:70)
A mixture of glucose (0.5404 g) and CD (7.9450 g) with molar ratio of 30:70 was added to an aqueous suspension of MT (0.1 g) in water (10 mL). The resulting mixture was thoroughly dispersed by sonication with power of 200 W for 0.5 h. In the following, the mixture was transferred into a Teflon-lined stainless steel autoclave (150 mL). The container was then sealed and maintained under hydrothermal conditions at 200°C for 24 h. Subsequently, the Teflon reactor was cooled down to room temperature and the as-prepared black-brown precipitate was filtered, rinsed with ethanol, centrifuged for five times and dried in an oven at 80°C. Finally, the product was filtrated, washed with distilled water/EtOH and dried under vacuum at 60°C.
Immobilization of Pd nanoparticles on MT/Glu-CD (30:70)
Pd/MT/Glu-CD (30:70) (4) was synthesized by conventional impregnation-reduction method. Typically, MT/Glu-CD (3 g) was dispersed in 75 mL of toluene in a flask, and then 30 mL solution of Pd(OAc)2 in toluene (0.06 g) was added slowly to the suspension. After stirring overnight, 15 mL of an aqueous solution of NaBH4 (0.2 M) as chemical reducing agent was added slowly into the above-mentioned suspension under inert atmosphere. Then the mixture was stirred vigorously for 6 h. After filtration, the solid product 4 was washed with deionized water and EtOH for several times and was dried at 60°C in an oven for 12 h.
Synthesis of Pd/MT/C (30:70)
To prepare palladated nitrogen-enriched porous carbon Pd/MT/C (30:70) (5), Pd/MT/Glu-CD (30:70) (4) was carbonized at 450°C with a heating rate of 3°C·min−1 for 7 h in a tubular furnace under argon flow. Carbonization was completed by further thermal treatment at 750°C for 1 h.
The processes for the synthesis of control catalysts, compounds 2, 3, 7, 9 and 11 are depicted in Fig. 1. The details of the synthesis of the control catalysts are presented in SI.
The synthetic procedure for the preparation of the Pd/MT/C (30:70) and the control samples.
Hydrogenation of nitrobenzene
The mixture of nitroarene (1 mmol) and Pd/MT/C catalyst (1 mol %) in 5 mL of water was heated up to 60°C, subjected to H2 (1 atm) and stirred vigorously (350 rpm) for 1.5 h. The progress of the hydrogenation reaction was traced by thin layered chromatography. After completion of the reaction, Pd/MT/C was filtrated and the resulting filtrate was collected, diluted with ethanol. All the obtained anilines were known products and their formation was confirmed by FTIR spectroscopy and the comparison of their melting/boiling points with that of the authentic compounds. The recovered Pd/MT/C was washed several times with distilled water and absolute ethanol, dried in oven and reused for the next run of the hydrogenation reaction.
Results and discussion
Catalyst characterization
To study the thermal stability of Pd/MT/C (30:70), the thermogram of Pd/MT/C (30:70) was recorded (Fig. 2c). Moreover, to investigate the thermal stability of MT and confirming the conjugation of Glu-CD, the TGA thermograms of MT polymer, Pd/MT/Glu-CD (30:70) were also recorded (Fig. 2b). As shown, MT polymer exhibited the lowest thermal stability among the aforementioned samples. The TGA thermogram of MT showed two weight losses. The one below 100°C is related to the loss of water. Another one at 330°C is due to the degradation of MT polymer. In the TGA thermogram of Pd/MT/Glu-CD (30:70), two weight losses can be observed. Similar to MT polymer, the first weight loss can be assigned to the loss of the structural water. The second weight loss (at 390°C) can be attributed to the degradation of MT/Glu-CD. Notably, the conjugation of Glu-CD improved the thermal stability of MT. The thermogram of the catalyst, Pd/MT/Glu-CD (30:70) indicated that carbonization can significantly increase the thermal stability of the catalyst.
TGA thermograms of (a) MT polymer, (b) Pd/MT/Glu-CD (30:70) and (c) Pd/MT/C (30:70).
The formation of Pd/MT/C (30:70) was also confirmed by XRD analysis (Fig. 3). In the XRD pattern of Pd/MT/C (30:70), two types of characteristic bands can be observed. Pd characteristic bands (denoted as #) can be detected at 2θ=41.1°, 45.5°, 68.7°, 79.6° and 87° [33]. The bands labeled as “o” (2θ=25.2°, 42.5°, 43.4°, 73.8°) can be assigned to the nitrogen doped porous carbon. According to the literature, these bands are representative of diamond–lonsdaleite system [34].
XRD pattern of Pd/MT/C (30:70).
The FTIR spectrum of MT sheet is presented in Fig. 4. The characteristic bands of MT can be listed as follow: 3411 cm−1 that can be assigned to the –OH functionality, 2927 cm−1 that is due to the aliphatic C–H stretching [35], [36], 1651 and 1627 cm−1 that are representative of –C=N and –C=C functionalities respectively, 1461 cm−1 that can be attributed to the –C–N stretching [35], [36], [37]. The FTIR spectra of Pd/MT/Glu-CD (30:70) and Pd/MT/C (30:70) also showed the same characteristic bands, indicating that the formed carbon possesses various functionalities. According to the literature, the absence of the characteristic bands of the carbonyl functionalities confirms complete carbonization [38].
FTIR spectra of (a) MT sheet, (b) Pd/MT/Glu-CD (30:70) and (c) Pd/MT/C (30:70).
To characterize the nature of the formed carbon material, Pd/MT/C (30:70) was subjected to Raman spectroscopy. The recorded Raman spectrum of Pd/MT/C (30:70) (Fig. 5) indicated that the prepared carbon material has a graphitic nature. More precisely, the band at 1378 cm−1 is a D-band and representative of sp3 configuration, while the G-band observed at 1594 cm−1 can be assigned to the graphitic carbon [39], [40], [41]. The ID/IG value calculated from Raman spectrum was 0.86, confirming high amounts of defective or disordered graphitic structures [42].
Raman spectrum of the Pd/MT/C (30:70).
To measure the specific surface area of Pd/MT/C (30:70) and shed light to the textural properties of the catalyst, the N2 adsorption-desorption of Pd/MT/C (30:70) was recorded (Fig. S1). As illustrated, the catalyst exhibited type IV isotherm, indicating the porous nature of the formed carbon material. Using BET result, the specific surface area of the catalyst was measured to be 2578 m2·g−1.
In Fig. 6a–c, the TEM images of Pd/MT/C (30:70) are illustrated. As shown, the fine graphitic like sheet of MT/C can be observed in the TEM images. The dark spots are indicative of Pd nanoparticles. The mean diameter of Pd nanoparticles was calculated to be 46.0±25.8 nm (Fig. 6d) with a standard deviation about 25 nm. This observation clearly established that carbonization effectively induce aggregation of Pd nanoparticles.
TEM images of Pd/MT/C (30:70) at magnification of (a) ×19 000; (b) ×29 000; (c) ×50 000 and (d) the corresponding size distribution.
Catalytic activity
In the next part of this study, the catalytic activity of Pd/MT/C has been investigated. In this line, hydrogenation of nitrobenzene under H2 pressure and in water was selected as a standard chemical transformation. First, this reaction was performed in the presence of very low amount of the catalyst (0.5 mol %) at 60°C. Gratifyingly, the results confirmed high catalytic activity of Pd/MT/C (Table S1). Motivated by this result, the effects of the reaction variables, i.e. the amount of used catalyst and solvent, were evaluated. The optimization experiments (Table S1) showed that the reaction in water at 60°C with 1 mol % of the catalyst led to the desired product with yield and conversion higher than 99 %. In the following, to elucidate the generality of this methodology, the hydrogenation of various nitroarenes was carried out under the same reaction conditions. The results (Table 1) confirmed that nitroarenes with electron withdrawing and electron donating groups could undergo hydrogenation to afford the corresponding product in high to excellent yields. Moreover, hydrogenation of steric substrate, 1-nitronaphtalene, was also successful. Notably, hydrogenation of 4-nitroacetophenone that contained both nitro and ketone functionalities in its structure led to the formation of 4-aminoacetophenone. This result indicated high selectivity of the catalyst towards nitro group.
Hydrogenation of nitroarene derivatives using Pd/MT/C (30:70) catalyst.a
| Entry | Reagent | Product | Time (h:min) | Yieldb (%) |
|---|---|---|---|---|
| 1 |
 |
 |
1:30 | >99 |
| 2 |
 |
 |
3:00 | 83 |
| 3 |
 |
 |
2:10 | 85 |
| 4 |
 |
 |
2:15 | 90 |
| 5 |
 |
 |
3:30 | 75 |
-
aReaction conditions: nitroarene (1 mmol), catalyst (1 mol %),water (5 mL), temperature, 60°C, 1 atm of H2, stirring=350 rpm. bIsolated yield.
In the next step, the effect of the structure of the hybrid material on the catalytic activity was investigated. In more detail, the roles of carbonization of MT, introduction of Glu, CD and the molar ratio of Glu:CD were evaluated. To this purpose, several control catalysts, including Pd/MT, Pd/C, Pd/MT/C(CD), Pd/MT/C(Glu) and Pd/MT/C (50:50) were prepared (the synthetic procedures for the synthesis of control catalysts are presented in the Experimental section). The ζ potential measurements related to the analysis of the dispersion of all of these materials has been done and gave ZP values ranging from −33.70 to −53.30 mV (Fig. S2). These ZP values are in correlation with a good dispersion of these materials into the aqueous phase, which is an important parameter in order to realize heterogeneous catalysis in water. The catalytic activities of these control samples were compared with that of Pd/MT/C (Table 2). As shown in Table 2, the yield obtained for Pd/MT (Table 2, entry 6) was only 65 %, while carbonization of Pd/MT significantly increased the catalytic activity (the yield obtained for Pd/C was 87 %, Table 2, entry 5). This observation confirms that carbonization of MT can improve the catalytic activity of the final catalyst. To shed light to the origin of this observation, the Pd loading and specific surface area (obtained from BET) of these two samples were compared (Table 2). As tabulated, carbonization significantly enhanced the specific surface area of the catalyst. Moreover, the loading of Pd nanoparticles in Pd/C was almost two times higher than that of Pd/MT. These different structural features can justify the superior catalytic activity of Pd/C compared to Pd/MT. Notably, apart from these parameters, other factors such as Pd dispersion and Pd average particle size could affect the catalytic activity. Next, it was studied whether the incorporation of Glu could affect the catalytic activity. The comparison of the catalytic activity of Pd/MT/C(Glu) (Table 2, entry 4) with that of Pd/C confirmed that incorporation of Glu had a detrimental effect on the catalytic activity of the final catalyst. Noteworthy, the specific surface area and Pd loading of Pd/MT/C(Glu) were lower than those of Pd/C. Next, the catalytic activity of Pd/MT/C(CD) was examined. It was revealed that this sample exhibited superior catalytic activity compared to Pd/MT/C(Glu). The higher catalytic activity of Pd/MT/C(CD) can be attributed to its higher specific surface area as well as higher Pd loading due to the capability of CD for capping Pd nanoparticles. Finally, the comparison of the catalytic activity of Pd/MT/C (50:50) (Table 2, entry 2) with that of Pd/MT/C (30:70) indicated that the catalyst with higher amount of CD showed the superior catalytic activity. Noteworthy, the specific surface area as well as Pd loading of Pd/MT/C (30:70) are higher than that of Pd/MT/C (50:50). This result further confirms the contribution of CD for anchoring Pd nanoparticles.
Comparison of the catalytic activity of Pd/MT/C (30:70) catalyst with the other prepared catalysts in the hydrogenation of nitrobenzene.a
| Entry | Catalyst | Yieldb (%) | Time (min) | Loading of Pd NPs (mmol·g−1) | S BET (m2·g−1) | Leaching of Pd NPs (mmol·g−1) |
|---|---|---|---|---|---|---|
| 1 | Pd/MT/C (30:70) | >99 | 90 | 0.047 | 2578 | 0.0011 |
| 2 | Pd/MT/C (50:50) | 90 | 90 | 0.039 | 1899 | 0.0021 |
| 3 | Pd/MT/C(CD) | 92 | 90 | 0.040 | 1816 | 0.0053 |
| 4 | Pd/MT/C(Glu) | 64 | 90 | 0.011 | 302 | 0.0050 |
| 5 | Pd/C | 87 | 90 | 0.040 | 1801 | 0.0055 |
| 6 | Pd/MT | 65 | 90 | 0.022 | 49 | 0.0061 |
-
aReaction conditions: nitrobenzene (1 mmol), catalyst (1 mol %), water (5 mL), Temperature=60°C, 1 atm of H2, stirring=350 rpm. bIsolated yield.
Catalyst recyclability
To elucidate whether Pd/MT/C is a recyclable catalyst, the conventional recycling experiment was carried out. Typically, upon completion of the hydrogenation of nitrobenzene, Pd/MT/C was filtered off, rinsed, dried (as described in the Experimental section) and used for the next run of the same hydrogenation reaction under the similar reaction conditions. The recovered Pd/MT/C was consecutively recovered and reused. The yield of the reaction was determined for the seven reaction runs (Fig. 7). The results indicated that Pd/MT/C could be successfully recycled for four reaction runs with a slight decrease in the catalytic activity. Upon further recycling, the decrease in the catalytic activity increased and upon seven recycling, the yield of the desired product decreased from>99 % to 70 %.
Recyclability of Pd/MT/C catalyst for the nitrobenzene hydrogenation.
The study of the recycled Pd/MT/C was performed by comparing the FTIR spectra of fresh and recycled Pd/MT/C after seven reaction runs. The two FTIR spectra (Fig. S3) were identical and both exhibited the same characteristic bands. This result confirmed that recycling did not degrade the structure of the catalyst.
To elucidate whether the nature of the catalyst is truly heterogeneous, the reaction was halted and the catalyst was separated from the reaction mixture. Then, the progress of the reaction in the filtrate was monitored. The result confirmed that the reaction did not proceed in the filtrate, indicating the fact that the catalyst was truly heterogeneous and that Pd nanoparticles were not leached and re-deposited on the support in the course of the reaction.
Conclusion
A novel nitrogen-doped mesoporous carbon catalyst was designed and prepared through formation of MT followed by hydrothermal treatment with Glu and CD with molar Glu:CD ratio of 30:70 and subsequent Pd immobilization and carbonization. The catalyst could effectively promote hydrogenation of nitroarenes in aqueous media and under mild reaction conditions. Moreover, it was highly recyclable and could be recovered and reused with a slight loss of the catalytic activity. The comparison of the catalytic activity of the catalyst with that of several control catalysts proved that the use of CD as carbon precursor improved the efficiency of the final catalyst. Furthermore, the molar ratio of Glu:CD is an effective factor on the catalytic activity.
Article note
A collection of papers from the 18th IUPAC International Symposium Macromolecular-Metal Complexes (MMC-18), held at the Lomonosov Moscow State University, 10–13 June 2019.
Acknowledgments
The TEM in Lille (France) are supported by the Conseil Régional du Nord-Pas de Calais and the European Regional Development Fund (ERDF). This work has been supported by the Center for International Scientific Studies & Collaborations (CISSC) and French Embassy in Iran and Hubert Curien French-Iranian partnership “PHC GUNDISHAPUR 2018” n° 40870ZG. M.M. Heravi is also thankful to Iran National Science Foundation for the Individual given grant, No. 96010807. S. Sadjadi and M.M. Heravi appreciate Iran National Science Foundation for the Individual given grant (INSF), No. 97009384. The authors are grateful to Dr. Sébastien Noël for the zeta potential measurements.
References
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