Letter pubs.acs.org/journal/ascecg
Biomass-Derived Carbon Nanospheres with Turbostratic Structure as Metal-Free Catalysts for Selective Hydrogenation of o‑Chloronitrobenzene Peng Zhang,†,‡ Xuedan Song,† Chang Yu,† Jianzhou Gui,‡ and Jieshan Qiu*,† †
Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ State Key Laboratory of Separation Membranes and Membrane Processes, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China S Supporting Information *
ABSTRACT: Nowadays, metal-free catalysts are sought after for synthesis of fine chemicals. Herein, we utilized the natural biomass xylose as the precursor to fabricate carbon nanospheres with turbostratic carbon structure via hydrothermal treating and hightemperature calcinating. These green and sustainable carbon materials were used as metal-free catalysts in selective hydrogenation of o-chloronitrobenzene (o-CNB), exhibiting a significantly enhanced catalytic activity with the increasing calcination temperature, as well as 100% selectivity to o-chloroanilines (o-CAN). In terms of the systematic characterization and simulated calculation, it is found turbostratic carbon formed in the calcination process might act as the catalytic active center in this reaction, and its selective reaction with −NO2 in o-CNB results in the enhanced catalytic activity and fully selectivity to o-CAN. KEYWORDS: Metal-free catalyst, Hydrothermal carbon, Turbostratic carbon, Selective hydrogenation
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INTRODUCTION Selective hydrogenation of chloronitrobenzene (CNB) to corresponding chloroanilines (CAN) is an important industrial reaction for producing many chemicals such as dyes, drugs, herbicides, and pesticides. However, besides the target product, many toxic byproducts are also concomitantly generated during this process, owning to the intrinsic complicated pathways and mechanisms (Scheme S1),1−3 which not only wastes a lot of industrial cost to purify products but also pollutes the environment. To effectively convert CNB to CAN, many metal catalysts with both high catalytic activity and CAN selectivity are studied as promising catalysts by now.4−8 In spite of great catalytic property, the high price, rareness and unsustainability of those metal catalysts limit their practical application to a great degree. As a result, many inexpensive and eco-friendly metal-free catalysts with good catalytic performance are eagerly needed to be developed to replace the existing metal catalysts. Benefiting from the good environmental compatibility, versatile structure and stable property, carbon nanomaterials (such as nanodiamond,9 C60,10,11 CNF,12 and CNT13−15) have been widely sought after as metal-free catalysts, and showcase the great catalytic performance in various chemical reactions.16 As a novel isolated signal-layer material, graphene recently has attracted tremendous interests due to its remarkable electronic, mechanical, and chemical properties, thus many works have been © 2017 American Chemical Society
carried out to investigate the catalytic performance of graphene or surface-modified graphene.17−25 However, the synthetic procedure of graphene is usually time-consuming, complex and harshness; more importantly, it is hard to produce high-quality graphene to a large scale until now.26 As such, it is highly necessary to develop sustainable and low-cost carbon materials via a simple and controllable synthetic method to replace graphene as possible green catalysts. Among proposed carbon materials alternatives, hydrothermal carbon (HTC) derived from glucose, xylose, and other monosaccharide or polysaccharide has recently attracted significant attention. This features not only a facile and scalable synthetic method27,28 but also a highly controllable pore size, structure, and surface chemistry.29−31 Besides, Baccile and his coworkers32 found the local graphene structure (inset of Figure 1) would be formed inside HTC, when it was thermally treated at a high temperature. This suggests HTC-calcined products might show the similar catalytic property with graphene to some degree. Herein, we adopt xylose, a natural biomass, as the feedstock to prepare well-dispersed carbon nanospheres (denoted as CNS) via a facile low-temperature hydrothermal Received: April 24, 2017 Revised: August 9, 2017 Published: August 18, 2017 7481
DOI: 10.1021/acssuschemeng.7b01280 ACS Sustainable Chem. Eng. 2017, 5, 7481−7485
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ACS Sustainable Chemistry & Engineering
in TG curve of the CNS in N2, a continuous weight-loss process could be observed in a wide range of 200−900 °C, as shown in Figure S2. Catalytic performance of five carbon nanospheres for the selective hydrogenation of o-CNB is tested under various reaction conditions, of which the detailed results have been summarized in Table 1. Compared with the blank experiment, it is inferred the CNS has little catalytic activity, which only enhances about 1.4% conversion of o-CNB under the same reaction condition. However, the elevating calcination temperature will gradually improve the catalytic activity of carbon nanospheres (Entry 3−6, Table 1), indicating a temperaturedependent catalytic property. Hence, among five catalysts synthesized in this work, the CNS-900 with the highest calcination temperature is found to have the fastest o-CNB conversion, which can reduce 45% o-CNB in 3 MPa H2 and 12 h (Entry 6, Table 1). Meanwhile, the similar trend could also be proved by recording reaction processes of various catalysts at 3 MPa H2. As shown in Figure S3, all the carbon spheres exhibit the pseudo-first-order relationship between reaction time and oCNB conversion, and slopes refer to their conversion rates (k in Figure S3). On the basis of the o-CNB conversion rates, reaction rates of the CNS, CNS-300, CNS-500, CNS-700, and CNS-900 can be estimated to be 0.55, 1.09, 1.89, 2.11, and 2.24 mmol/g·h (Table 2), respectively. Therefore, the catalytic activity of CNS-T samples can be continuously increased with the calcination temperature. In addition, H2 pressure generally are well-known as an important parameter affecting the hydrogenation process. At the initial H2 pressure of 5 MPa, the CNS-900 can finish over half conversion of o-CNB in 12 h (Entry 9, Table 1), further fully hydrogenating o-CNB to o-CAN in 24 h (Entry 10, Table 1), whereas the CNS-900 only reach 14.8% conversion of substrate when H2 pressure drops to 2 MPa (Entry 8, Table 1). This result demonstrates high H2 pressure will accelerate o-CNB hydrogenation.
Figure 1. Synthetic illustration of carbon nanospheres, their structure and application on the o-CNB hydrogenation as metal-free catalysts.
method, as shown in Figure 1. Followed by the calcination at different temperatures, a series of carbon nanospheres (denoted as CNS-T) are obtained and used as metal-free catalysts for the selective hydrogenation of o-CNB, showing discrepant catalytic activities and 100% selectivity to the target product o-CAN. In terms of systematic characterizations and simulated calculations, the structure of as-obtained catalysts is analyzed in detail and a possible catalytic mechanism is also proposed in the present study.
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RESULTS AND DISCUSSION From representative SEM images of the CNS and CNS-T samples (Figure 2a−e), it can be clearly seen CNS exhibits an
Table 1. Catalytic Performance of Carbon Nanospheres for Selective Hydrogenation of o-CNBa
Figure 2. Typical SEM images of (a) CNS, (b) CNS-300, (c) CNS-500, (d) CNS-700, and (e) CNS-900. (f) Average diameters of various carbon nanospheres.
uniform nanospheric morphology with high dispersity, and its diameter varies in a range of 150−450 nm (Figure S1a). After high-temperature treating, all of CNS-T samples maintain the sphere-shaped structure with high dispersity, although their sizes get smaller with the gradually elevating calcination temperature due to the pyrolysis and removal of functional groups. Statistical average diameters of the CNS, CNS-300, CNS-500, CNS-700, and CNS-900 are 295, 245, 209, 208, and 191 nm, respectively, as shown in Figure 2f and Figure S1. Obviously, the average size of CNS-T samples dramatically shrinks below 500 °C, whereas slightly decreases from 500 to 900 °C. It is indicated a considerable structural variation of the CNS would occur when the thermal-treating temperature is lower than 500 °C. Besides,
Entry
Catalyst
H2 (MPa)
Time (h)
Con (%)
Sec (%)
1 2 3 4 5 6 7 8 9 10
No catalyst CNS CNS-300 CNS-500 CNS-700 CNS-900 CNS-500 CNS-900 CNS-900 CNS-900
3 3 3 3 3 3 3 2 5 5
12 12 12 12 12 12 24 12 12 24
10.1 11.5 19.9 36.1 41.1 45 73.2 14.8 52.1 100
100 100 100 100 100 100 100 100 100 100
a
General reaction conditions: 0.50 g o-CNB, 0.05 g catalyst, 50 mL ethanol, 140 °C.
In spite of different catalytic activities, all of catalysts involved in this work have 100% selectivity to o-CAN under versatile reaction conditions, probably due to the prior reaction of carbon nanospheres with −NO2 in o-CNB. The relationship between the catalytic performance and structure of carbon nanospheres will be further investigated in this work behind, and a possible reaction mechanism is shown in Figure 5. As summarized in Table 2, the as-obtained CNS is very different from traditional carbon materials, exhibiting a large 7482
DOI: 10.1021/acssuschemeng.7b01280 ACS Sustainable Chem. Eng. 2017, 5, 7481−7485
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ACS Sustainable Chemistry & Engineering
the CNS-900 (Figure 3d), it can be observed turbostratic-type carbon mainly exists on inner edge of the sample, while little could be found deep inside. XPS is utilized to further explore the detailed structure of CNS-T samples, and the XPS spectra of the CNS-900 are displayed in Figure 4. From the XPS survey spectrum (Figure 4a), it is found only C and O are present in the CNS-900, and their content percentages keep consistent with the element analysis results above. The C 1s XPS spectrum (Figure 4b) demonstrates various carbon structures correspond to CC (284.5 eV), CC (284.9 eV), COH and COC (285.3 eV), CO (286.2 eV), and OCO (288.5 eV).35 Three O peaks could be found in the O 1s XPS spectrum (Figure 4c), which represent CO (532.0 eV), CO (532.8 eV), and O H (533.9 eV), respectively.36 In addition, 81% of C atoms in the CNS-900 can be assigned to CC, CC, COH, and C OC, whereas only 19% of C atoms come from CO and O CO. In consideration of H2O and CO2 absorbed on the surface, we can estimate only hydroxy and epoxy groups still remain in the carbonized product at a high temperature of 900 °C, which is consistent with the FTIR result. Consequently, it is demonstrated the resultant turbostratic carbon in CNS-T samples should be very close to the fragmentized graphene modified by hydroxy and epoxy groups, as shown in Figure 5a. It is noted the high temperature calcination also leads to form the porous structure in CNS-T samples, of which their corresponding N2 adsorption−desorption isotherms are shown in Figure 3c. From the depressed isotherms, we can confirm there is almost no pore in the CNS and CNS-300. In contrast, for the CNS-500, CNS-700, and CNS-900, the ultrahigh N2 adsorption quantity at relative pressure of 0−0.1 suggests a large amount of micropores would be generated at 500 °C or above. Besides micropores, from the pore size distributions (Figure S5), abundant macropores have also been clearly observed in the three CNS-T samples. Consistent with the N2 adsorption analysis, no pore in a wide range of 0.3−100 nm could be found in the CNS and CNS-300 (Figure S5). As summarized in Table 2, the average pore diameters of CNS-T samples with the calcination temperature over 500 °C are dramatically smaller than that of the two samples below 500 °C, indicating the thermal-generated pores mainly are micropores. During the reaction process, these micropores will provide free channels for the diffusion of reactants and products, thus enhancing the catalytic performance of CNS-T catalysts. On the other hand, the elevating calcination temperature can also gradually enlarge the specific surface areas of samples (Table 2), whereas the CNS-900 has inversely a decreasing specific surface areas (365.9 m2/g), resulting from the destruction of unstable pores by the overhigh temperature.37 In terms of all information available, it is concluded CNS-900 has the high turbostratic carbon content, large microporous volume, as well as best catalytic performance for the selective hydrogenation of o-CNB. Consequently, we can conjecture turbostratic carbon distributed in micropores of
amount of O content (about 28.7%). It is postulated those O are attributed to the abundant oxygen-containing groups in the CNS, further proved by its FTIR spectrum (Figure 3a), in which
Figure 3. (a) FTIR spectra, (b) XRD patterns, and (c) N2 adsorption− desorption isotherms of carbon nanospheres. (d) High resolution TEM image of CNS-900.
strong adsorption peaks at 3400, 2900, 1700, and 1600 cm−1 suggest the presence of hydroxy, carboxyl, epoxy, ester, or other groups.7 The oxygen-containing groups are generally formed via series of chemical reactions of xylose molecules under the hydrothermal conditions, including dehydration, condensation, polymerization and aromatization reactions.33 As for CNS-T samples, the O content is continuously decreasing accompanied by the elevating thermal-treated temperature, whereas the C content increases from 67% to 97% (Table 2). The removal of O content makes that little oxygen-containing groups are residual in the CNS-700 and CNS-900, thus there is nearly no any peak in a range of 4000−1500 cm−1 in their FTIR spectra (Figure 3a). It is considered pyrolysis and removal of functional groups make the size shrinkage of CNS-T samples during the calcination process. Besides the increase of C content, the C structure in carbon nanospheres has also been transformed during the hightemperature process. As XRD shown in Figure 3b, not only a broad peak (15°−30°) attributed to the amorphous carbon, but also a peak at about 44° could be found in the CNS-500, CNS700, and CNS-900, which belongs to the (100) in-plane diffraction of turbostratic-type carbon,34 and its intensity is increasing from the CNS-500 to the CNS-900. It is indicated turbostratic carbon could be formed at 500 °C or above, and its weight ratio gradually increases with the increasing calcination temperature. This trend is also confirmed by the high-resolution TEM images of various carbon nanospheres (Figure 3d and Figure S4). Moreover, from the high-resolution TEM image of
Table 2. Elemental Compositions, Specific Surface Areas, Average Pore Diameters, and Reaction Rates of Carbon Nanospheres Sample
C (%)
H (%)
O (%)
Specific surface area (m2/g)
Average pore diameter (nm)
Average reaction rate (mmol/g·h)
CNS CNS-300 CNS-500 CNS-700 CNS-900
67.0 67.4 84.2 86.7 97.0
4.4 4.1 3.3 1.9 0.3
28.7 28.5 12.5 11.5 2.8
18.8 17.2 443.4 520.4 365.9
15.2 17.0 3.15 2.18 2.75
0.55 1.09 1.89 2.11 2.24
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Figure 4. (a) XPS survey spectrum of CNS-900 and (b,c) high-resolution C 1s and O 1s XPS spectra of CNS-900.
during reaction process. As shown in Figure S6a, reacting with the unsaturated C, H2 dissociation energy (Ed) dramatically decreases to −16.9 kJ/mol from 467.3 kJ/mol (Ed of pure H2). It is indicated that, as reaction centers, the turbostratic carbon in carbon spheres not only active o-CNB molecules but also facilitates the H2 dissociation, both of which greatly accelerate the hydrogenation of o-CNB. Moreover, it is found epoxy groups located at the turbostratic carbon would also be possible active center for dissociating H2, owning to the calculated Ed of H2 (75.9 kJ/mol) reacted with epoxy group (Figure S6b).
carbon nanospheres probably works as the catalytic active center in the hydrogenation of o-CNB. Integrating the structural feature of CNS-T samples with the according catalytic performance, in the present work, we suggest the unsaturated carbon atoms (C) on zigzag edges of the turbostratic carbon (Figure 5a) probably work as the active center and react with the heteroatomic groups in o-CNB, such as −NO2 and −Cl. As shown in Figure 5c,e, the binding energies (E) of CNO2 and CCl are −576.8 and 10.8 kJ/mol, indicating the unsaturated carbon atoms in carbon nanospheres would prior react with −NO2 in the selective hydrogenation of oCNB. As a result, the carbon nanospheres have exhibited a 100% selectivity to o-CAN, and no dechlorination byproduct (AN) has been detected. When −NO2 in o-CNB reacts with the unsaturated carbon atoms (Figure 5c), the NO band near −Cl is elongated from 1.23 to 2.70 Å, whereas the one far from −Cl is 1.45 Å. Thus, the NO band (especially the one near −Cl) is more easily attacked and fracture by H2, and the conversion process of o-CNB to o-CAN is successfully accelerated. Moreover, in DFT calculation of the intermediate structure of o-CNB reacting with graphene (Figure 5d), the
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CONCLUSIONS In summary, a series of sustainable xylose-derived carbon nanospheres yielded at different calcination temperatures are used as metal-free catalysts for selective hydrogenation of o-CNB to o-CAN. These samples have the temperature-dependent catalytic performance: the product obtained at a higher temperature will exhibit the superior catalytic activity, although all of catalysts have a 100% selectivity to o-CAN. Meanwhile, it is found the turbostratic structure and abundant micropores would be formed inside CNS-T catalysts during the calcination process, and the weight ratio of turbostratic carbon gradually increases with the calcination temperature. Therefore, the turbostratic carbon distributed in micropores has been inferred to work as catalytic active centers of the reaction. In terms of the DFT calculation, it is illuminated the unsaturated carbon atoms on zigzag edges of the turbostratic carbon could prior react with −NO2 in o-CNB, instead of -Cl, and elongate the NO band from 1.23 to 2.70 Å. Therefore, the carbon nanospheres exhibit the enhanced catalytic activity, as well as a 100% selectivity to oCAN.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01280. Possible reaction pathways of hydrogenation of CNB; diameter distribution of CNS, CNS-300, CNS-500, CNS700, and CNS-900; TG analysis of CNS in N2; timedependent o-CNB conversion over carbon nanospheres in 3 MPa H2; high-resolution TEM images of CNS, CNS300, CNS-500, and CNS-700; pore size distribution of carbon nanospheres; optimized reaction intermediates of unsaturated carbon atoms and epoxy groups in turbostratic carbon reacting with H2 (PDF)
Figure 5. Optimized structure of (a) turbostratic carbon in carbon nanospheres, (b) o-CNB molecule, (c) reaction intermediate of turbostratic carbon reacting with NO2 in o-CNB, (d) reaction intermediate of graphene reacting with NO2 in o-CNB, and (e) reaction intermediate of turbostratic carbon reacting with Cl in o-CNB.
binding energy of CNO2 (−494.1 kJ/mol) is higher than that of the turbostratic carbon (−576.8 kJ/mol). It is suggested the hydroxyl distributed on the turbostratic carbon can enhance the catalytic hydrogenation activity, which agrees with the conclusion drawn by Chen and his co-workers.21 Besides o-CNB activation, unsaturated C on zigzag edges of the turbostratic carbon could also catalyze the H2 dissociation
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AUTHOR INFORMATION
Corresponding Author
*Prof. Jieshan Qiu:
[email protected]. 7484
DOI: 10.1021/acssuschemeng.7b01280 ACS Sustainable Chem. Eng. 2017, 5, 7481−7485
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Peng Zhang: 0000-0001-5218-1673 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by the National Natural Science Foundation of China (No. 21336001) and the China Postdoctoral Science Foundation (No. 2016M590204).
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DOI: 10.1021/acssuschemeng.7b01280 ACS Sustainable Chem. Eng. 2017, 5, 7481−7485
