N–C Cataylsts: Efficient

Oct 9, 2017 - As shown in Figure 1, all of them were characteristic of a Type-IV isotherm according to the International Union of Pure and Applied Che...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9894-9902

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In Situ Synthesis of Chitin-Derived Rh/N−C Cataylsts: Efficient Hydrogenation of Benzoic Acid and Derivatives Yueling Cao, Minghui Tang, Mingming Li, Jiang Deng, Fan Xu, Lei Xie, and Yong Wang* Advanced Materials and Catalysis Group, Department of Chemistry, Zhejiang University, Hangzhou 310028, P. R. China S Supporting Information *

ABSTRACT: A novel N-doped carbon supported Rh catalyst was developed via one-pot pyrolysis of chitin and (NH4)3RhCl6. The catalyst exhibited excellent catalytic activity and recyclability for the hydrogenation of benzoic acid to cyclohexane carboxylic acid. Characterization indicated that the high catalytic performance of Rh/N−C-700 is mainly attributed to the proportion of Rh0 to Rh3+ and Rh particle size. More importantly, this novel synthesis strategy significantly increased the interaction between Rh nanoparticles and N-doped carbon in contrast with the conventional impregnation and NaBH4 reduction methods, thus preventing the Rh nanoparticles from migration, aggregation, and leaching from the support surface and therefore improving the reusability of the catalyst. This synthetic method may pave a new way for producing N-doped carbon supported metal catalysts from chitin on a large scale, which is attractive for industrial applications. KEYWORDS: One-pot pyrolysis, N-Doped carbon, Benzoic acid, Chemoselective hydrogenation, Heterogeneous catalyst

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INTRODUCTION Noble metal nanoparticle (NP) catalysts, which normally exhibits high dispersion and excellent catalytic activity, have been widely utilized in catalysis, such as hydrogenation,1,2 oxidation,3,4 coupling,5 electrocatalysis,6 fuel cells,7,8 and photocatalysis.9,10 However, the stability of noble metal NPs severely limits their wide application in industrial processes in view of the following fact. It is well-known that decreasing NPs down to nano- or subnanoscale makes the surface atoms chemically active; however, this makes metal NPs unstable. Thermodynamically, these nano or subnanoscale NPs easily tend to aggregate into larger particles, which could lead to a remarkable decrease in catalytic activity. Therefore, the development of novel catalyst synthetic techniques of fabricating stable noble metal catalysts is of extreme importance for their applications in catalysis. Recently, many strategies have been developed to enhance the stabilities of NPs, such as the premodification of supports before metal NPs loading,11,12 the nanoconfinement of metal NPs within porous channels,13,14 and the creation of a strong metal−support interaction between metal NPs and supports by substrate coatings.15−19 Even though significant progress has been achieved through these approaches, some issues remain to be addressed. For example, the Ostwald ripening process of metal NPs cannot be well suppressed by premodification and nanoconfinement, and they still suffer from the growth and aggregation of metal NPs.11 As for the substrate coatings approach, although it has shown to be particularly effective in improving thermal stability of metal NPs, the coatings such as © 2017 American Chemical Society

oxide or carbon shells also reduce the activity of the catalyst by blocking active sites and/or causing diffusion limitations.20 Even worse, the catalyst would be deactivated if the process for the oxide or carbon shell growth is not well controlled.17,18 On the other hand, it should be noted that among various supports supported catalyst, the deactivation of carbon supported metal NPs is most likely to happen because of the high chemical inertness of carbon materials, which leads to the low interaction between metal and support.21,22 Therefore, the development of a new synthetic strategy capable of stabilizing metal NPs on carbon materials without sacrificing their activity is highly desirable but more challenging. Chitin, the second most abundant biopolymer on earth after cellulose, is an N-containing biomass material widely existing in the exoskeletons of insects and crustaceans.23 The abundance, low price, and environmental sustainability of natural chitin make it highly practical for various applications.24−26 More importantly, there are a wide variety of oxygen- and nitrogencontaining groups on the surface of chitin, which exhibit high binding affinity for metal ions.27,28 Therefore, chitin displays vast potential as both carbon and nitrogen precursors for the preparing metal/N−C catalysts by taking advantage of its strong coordination ability with metal ions, where both high activity and stability may be expected. Received: June 9, 2017 Revised: August 21, 2017 Published: October 9, 2017 9894

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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purged with H2 to remove air. Subsequently, the reactor was filled with 1.0 MPa hydrogenation pressure at room temperature and finally heated to 55 °C with a stirring speed of 1000 rpm. After the reaction, the contents of products and substrate were determined by GC-FID and the products were identified by GC-MS. In order to examine stability of the catalysts, they were recycled in repeated runs. First, benzoic acid (60 mg), deionized water (5 mL) and an excess of Rh/N−C-700 (200 mg) were added into the batch reaction and the catalytic hydrogenation was carried out at 55 °C and 1 MPa H2 for 1 h. After the first run, the recovered catalyst was washed with ethanol and dried overnight at 40 °C under vacuum, and then 10 mg of catalyst was taken from them and was used for the next run (this is the second cycle). Meanwhile, the remanent Rh/N−C-700 catalyst was tested under same reaction conditions: Rh/N−C-700, benzoic acid (60 mg), deionized water (5 mL) at 55 °C and 1 MPa H2 for 1 h. Afterward, the recovered catalyst was washed with ethanol and dried overnight at 40 °C under vacuum, and then 10 mg of catalyst was taken from them and was used for the next run (this is the third cycle). Subsequent reusability tests were carried out on the same material by following the same procedure.

Herein, we report a novel and straightforward method to prepare a N-doped carbon supported Rh catalyst (Rh/N−C) through in situ pyrolysis of chitin and (NH4)3RhCl6. The catalytic performance of Rh/N−C catalysts was tested in the hydrogenation of benzoic acid (BA) to cyclohexane carboxylic acid (CAA), which is an important industrial transformation29−33 and an effective model hydrogenation reaction for the evaluation of the stability of supported noble metal catalysts due to the acidic reaction condition.21,22 For comparison, Rh/ N−C catalysts prepared from traditional postloading methods such as wet impregnation and NaBH4 reduction methods were also tested.

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EXPERIMENTAL SECTION

Materials. (NH4)3RhCl6 (Rh = 27.5%), chitin, 4-methylbenzoic acid (AR, 98%), 4-methxoybenzoic acid (AR, 98%), 4-chlorobenzoic acid (AR), 4-bromobenzoic acid (AR, 98%), 4-hydroxybenzoic acid (AR, 99%), and 3-phenylpropionic acid (AR, 99%) were used as received from Aladdin Chemistry Co., Ltd. Benzoic acid (AR, > 99.5%) and NaBH4 (AR, > 96%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Phenyl acetic acid (AR) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Catalyst Synthesis. The synthesis of Rh/N−C composites involved one-pot pyrolysis of chitin and (NH4)3RhCl6. In a typical experiment, 2 g of chitin and 2 mL of aqueous solution of (NH4)3RhCl6 (10 mg/mL) were added into ethanol solution, and then, the as-made solution was heated by oil bath at 60 °C until completely drying to form a powder. Then the dried powder was transformed into a furnace and heated at suitable temperature (550− 750 °C) for 2.0 h (with a heating rate of 5 °C/min) under nitrogen flow of 400 mL/min. The obtained catalysts were denoted as Rh/N− C-X, where X stands for the pyrolysis temperature. For comparison, chitin was directly pyrolyed at 700 °C for 2.0 h by the standard calcination procedure, hereafter referred to as N−C-700. Then, two kinds of catalysts were prepared using N−C-700 as the support. Rh/ N−C−IM catalysts were prepared via conventional impregnation method. Rh/N−C−NaBH4 catalysts were prepared via a simple ultrasound assistant method according to the previous work.34 Characterizations. The structure of the as-obtained catalysts was characterized by X-ray diffraction (XRD, Ultima TV, Cu Kα irradiation, operated at 40 kV and 30 mA). The primary crystallite sizes of the catalysts were determined using Scherrer’s equation L=

0.89λKα B(2θ) cos θmax

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RESULTS AND DISCUSSION The porosity of the as-prepared N−C and Rh/N−C samples pyrolyzed at different temperatures was measured by N2 sorption. As shown in Figure 1, all of them were characteristic

Figure 1. N2 adsorption−desorption isotherms of Rh/N−C composites prepared at various pyrolysis temperatures.

(1)

of a Type-IV isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification, which were reasonably associated with the presence of mesoporous structure, possibly due to the chitin with a designed nanorod structure.35 Moreover, the proportion of mesopores in all samples is higher than 60% (Table S1), indicating that chitin is a good precursor to prepare mesoporous N-doped carbon. The BET surface area of Rh/N−C catalysts, as shown in Table S1, slightly increased from 377 to 449 m2 g−1 when the pyrolysis temperature increased from 550 to 700 °C, then significantly increased to 617 m2 g−1 when the pyrolysis temperature was up to 750 °C. The pore volume of Rh/N−C pyrolyzed at various temperatures showed the same trend. However, it should be noted that both the BET surface area and pore volume of Rh/ N−C-700 sample were almost double that of N−C-700, indicating that the presence of metal salt can function as chemical activator during pyrolysis process, which is beneficial to increase the surface area and pore volume.36,37 More structural information was further revealed by highresolution TEM (HR-TEM) characterization, as shown in Figure 2. The well-dispersed Rh nanoparticles can be clearly seen (Figure 2A and B), and the average size of the Rh

where L denotes the average particle size, 0.89 is the value when B(2θ) is the full width at half-maximum (fwhm) of the peak broadening in radians, λKα is the wavelength of the X-ray radiation (0.15418 nm), and θmax is the angular position at the (111) peak maximum of Rh. The actual loading of Rh in various catalysts was measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Plasma-Spec-II spectrometers). Then the BET surface area was tested on an ASAP 2020 HD88 instrument, BET equation was used to calculate the surface area and pore volume. Raman spectra were also collected on a Raman spectrometer (JY, HR 800) using 514 nm laser. CO chemisorption was carried out at 40 °C on a Quantachrome Autosorb-IQ Chemisorb apparatus. Prior to measurements, the catalysts were reduced in situ for 1 h at 400 °C in H2. The size and dispersity of Rh particles in the samples were measured using transmission electron microscopy (TEM) with a JEM-2000 and averaged over 150 monodispersed Rh particles. The X-ray photoelectron spectra (XPS) were obtained by an ESCALAB MARK II spherical analyzer using an aluminum−magnesium binode (Al. 1486.6 eV; Mg, 1253.6 eV) X-ray source. Catalytic Tests. The hydrogenation reaction was carried out in a 50 mL stainless-steel autoclave. In a typical procedure, benzoic acid (60 mg), catalyst (10 mg), and deionized water (5 mL) were introduced into the stainless-steel autoclave, and then the reactor was 9895

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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ACS Sustainable Chemistry & Engineering

Figure 2. HRTEM images (A, B, and C) and particle size distribution (D) of Rh/N−C700. Particle size distributions were calculated based on about 150 particles.

Figure 3. HAADF-STEM image and EDS mapping of Rh/N−C700.

crystallite is measured to be 3.3 nm (Figure 2D). Figure 2C presents the crystal planes of Rh nanoparticles, and the crystal plane spacing is measured as 0.220 nm, corresponding to the Rh (111) planes.38 Additionally, scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping was employed to obtain elemental distribution of Rh, C,

and N in the Rh/N−C-700 sample. As shown in Figure 3, the Rh, C, and N atoms were homogeneously distributed in the whole sample, verifying the uniform dispersion of three elements. The TEM images of Rh/N−C prepared at different pyrolysis temperatures are presented in Figure S1. It can be seen that the 9896

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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Figure 4. XRD patterns (A) and XPS spectra (B and C) of Rh/N−C composites prepared at various pyrolysis temperatures.

increased continuously with increasing pyrolysis temperature from 550 to 750 °C. According to Scherrer’s formula and the half-width of (111) peak, the calculated particle sizes of Rh crystallites in Rh/N−C samples increased from 4.2 to 8.4 nm with pyrolysis temperature (Table 1), which exhibits the same

Rh particle sizes on the catalysts were slightly increased and the size distributions were widened with the increase of pyrolysis temperature from 550 to 700 °C. However, the Rh NPs became significantly aggregated with the average size of 6.0 nm when the pyrolysis temperature was up to 750 °C. These results suggested that the thermolysis temperature played an important role in controlling the particle size of Rh particles. Based on the above discussion, it can be concluded that the one-pot pyrolysis is a promising method to in situ form N-doped carbon supported Rh catalysts. Raman spectra of various Rh/N−C catalysts are shown in Figure S2. The D-band (∼1350 cm−1, the vibrations of sp3hybrized carbon atoms in defects or disordered carbon), Gband (∼1580 cm−1, the vibrations of sp2-hybrized carbon atoms in graphitic hexagonal lattices) were detected in all samples.39 The relative intensities of the D-band (ID) and Gband (IG) provide the evidence for studying the characteristics of the carbon materials. Carbon materials with high quality have a low ID/IG ratio, which means low defects and highly dispersed carbon layers.40 It was found that the ratio of ID/IG increased with the increase of pyrolysis temperature (as shown in Table S1). However, it should be noted that the ID/IG value is still very low (1.7) even for the sample pyrolyzed at the highest temperature (750 °C), indicating that the N-doped carbon obtained from pyrolysis of chitin possesses many structural defects in the carbon skeleton. X-ray diffraction (XRD) patterns of Rh/N−C composites that were prepared at various pyrolysis temperatures are shown in Figure 4A. The XRD patterns of all samples exhibited three weak peaks at 41.1, 47.8, and 69.9°, which corresponded to the characteristic diffractions of (111), (200), and (220) crystalline planes of metallic Rh (JCPDS 05-0685). This indicated that the Rh3+ adsorbed on chitin had been thermally reduced to metal Rh and anchors itself on the carbon matrix via pyrolysis. More importantly, it was found that the peak intensity of Rh

Table 1. Structural Parameters of Various Rh Catalysts particle size (nm)

relative percentage (%)

catalyst

XRD

TEM

Rh0/Rh3+

N/(C + N)

Rh/N−C-550 Rh/N−C-600 Rh/N−C-650 Rh/N−C-700 Rh/N−C-750

4.2 4.8 5.2 6.1 8.4

2.1 2.4 2.9 3.3 6.0

46/54 64/36 70/30 76/24 63/37

7.2 7.3 11.8 5.6 6.5

trend with TEM results. However, it should be noted that there are many other peaks appeared in all XRD patterns, which resulted from the impurity of chitin such as CaCO3. It is wellknown that chitin extracted from crustacean shells normally contain some mineral, mainly calcium carbonate.41 Based on above discussion, it could be concluded that the pyrolysis temperature played a crucial role in the dispersion of Rh NPs. To gain further insight into the catalyst structure, X-ray photoelectron spectroscopy (XPS) analysis was performed on the samples, as shown in Figure 4B. In the Rh 3d region, peaks characteristic of both Rh0 and Rh3+ were observed with typical binding energies of 307.0 and 309.4 eV, assigning to Rh0 3d5/2 and Rh3+ 3d5/2, respectively.42,43 Moreover, according to the semiquantitative analysis of XPS (Table 1), it is was found that the ratio of Rh0 to Rh3+ in Rh/N−C samples increased with the rise of pyrolysis temperature, and reached the highest (76%) at 700 °C. However, further increasing in pyrolysis temperature led to a slight decline in the portion of metallic Rh. According to the literature, the reduction of metal Rh is mainly attributed 9897

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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In order to highlight the advantages of the one-pot pyrolysis method, Rh/N−C catalysts prepared by conventional impregnation method (Rh/N−C-IM) and ultrasound-assisted NaBH4 reduction method (Rh/N−C-NaBH4) have been tested under identical conditions. The textural properties of the different catalysts are given in Table 3. Rh/N−C-700 showed the highest

to the emission of reducing gas such as H2 and CO during the pyrolysis process of carbon precursors.44−46 On the other hand, it should be noted that the electronic interaction between Rh and N may be also responsible for the difference in Rh0 proportion of various Rh/N−C samples. It can be seen from Figure 4C that the binding energy of graphitic N gradually shifted to the high-energy side with the increase of pyrolysis temperature, while the binding energy of pyridinic N showed the opposite trend, which is consistent with previous report.47,48 The graphitic nitrogen atom in carbon material uses four valence electrons to form σ and π bonds, and the fifth electron was in the higher-energy π* state, which leads to electron donor behavior of N-doped carbon.47 In contrast with graphitic nitrogen, the pyridinic nitrogen uses two electrons to fill σ-bonds with carbon neighbors, and two electrons to form a lone pair in the graphene plane, and the remaining electron occupies the N π-state, which results in a π-electron missing of pyridinic nitrogen, functioning as an electron acceptor.48 Based on above discussion, it can therefore be concluded that the pyrolysis temperature played a crucial role in the valence state of Rh NPs. The catalytic activities of the prepared materials were tested in the hydrogenation of BA to CCA at 55 °C under 1 MPa H2. The results are summarized in Table 2. The blank run (without

Table 3. Physicochemical Properties of Catalysts Prepared by Different Methods catalyst Rh/N−C-700 Rh/N−C-IM Rh/N−CNaBH4

catalyst

conversion (%)

TOFb (h−1)

1 2 3 4 5 6 7 8 9

none N−C-700 Rh/N−C-550 Rh/N−C-600 Rh/N−C-650 Rh/N−C-700 Rh/N−C-750 Rh/N−C-IM Rh/N−C-NaBH4

0 0 26 24 66 76 58 52 47

160b 166b 312b 320b (3039)c 287b b 371 (1139)c 309b (1685)c

Vt a (cm3/g)

V mb (cm3/g)

Dc (nm)

dispersiond (%)

449 255 276

0.34 0.19 0.21

0.12 0.07 0.07

3.0 3.0 3.0

18 35 20

a Total pore volume. bMicropore volume. cPore size. dMeasured by CO chemisorption.

BET surface area (449 m2/g) and pore volume (0.34 cm3/g) but the lowest Rh dispersion (18%). However, among the three catalysts, Rh/N−C-IM was found to give the most active system, with the highest TOF of 371 h−1. For the Rh/N−C700 catalyst, although it afforded a high conversion, the TOF of 320 h−1 was a little lower. This might be due to partial Rh NPs completely embedded in carbon matrix, thus resulting in them failing to work. The Rh/N−C-NaBH4 catalyst ranked third in activity, with a TOF of 309 h−1 and a low conversion of BA (47%). However, Rh/N−C-700 became the best catalytic system with a TOF of 3039 h−1 when the TOF calculated based on the surface Rh atoms measured via CO chemisorption were used to compare the difference of catalytic activity. Similar phenomenon was also observed for Ru/C catalysts by the Li group.50 They found that the Ru-MC-g with a semiembedded structure gave the highest TOF (calculated based on the surface Ru atoms), which is far higher than that of Ru-MC-i prepared by a conventional impregnation method. To illustrate the general applicability of Rh/N−C-700, the method was extended to ring hydrogenation of as many as 7 benzoic acid derivatives, and the results are presented in Table S5. First, the hydrogenations of para-substituted benzoic acids (−OH, −CH3, −Cl, and −Br) were investigated. The hydrogenations of para-chlorobenzoic acid and para-bromobenzoic acid exhibited conversion of 81% and 55% within 1 and 1.5 h, respectively; however, we noticed that the hydrogenations of para-halo benzoic acids did not give the corresponding para-halo cyclohexane carboxylic acids as the final products, but rather, nonsubstituted cyclohexanoneboxylic acid was the product. It is known that the halo moiety of the aromatics is usually easy to be reduced under hydrogen pressure, and this is applicable to the above-mentioned case.51 For the hydrogenations of para-methoxybenzoic acid and paramethylbenzoic acid, the conversions were 55% and 23%, which were much lower than that of nonsubstituted benzoic acid, suggesting that the electron-withdrawing group may restrain the hydrogenation. Hydrogenation of para-hydroxybenzoic acid gave a conversion of 92% within 1.5 h, possibly owing to the Hbond between −OH and solvent water, which is believed to be positive for the hydrogenation reaction. Additionally, the Rh/ N−C-700 catalyst also showed high activity for the hydrogenation of phenylacetic acid and 3-phenylpropionic acid, given the conversion of 96% and 87% within 3 h, respectively.

Table 2. Hydrogenation of Benzoic Acid over Various Catalystsa entry

SBET (m2/g)

a

Reaction conditions: BA 60 mg, catalyst 10 mg, H2O 5 mL, H2 1.0 MPa, 55 °C, 1 h. bTOF calculated based on the total Rh loadings. c TOF (in parentheses) calculated based on the surface Rh atoms measured via CO chemisorption under low BA conversion (Table S3).

any catalysts) gave essentially no activity in this system (even using N−C as catalyst) after 1 h of reaction. These results indicated that the Rh NPs in Rh/N−C were the active sites for this reaction. Compared with other samples, the Rh/N−C-700 pyrolyzed at 700 °C exhibited the highest catalytic activity under the same reaction conditions, showing 76% conversion of BA in 1 h. To the best of our knowledge, such a high reactivity and selectivity in the hydrogenation of BA under mild conditions has not been reported to date (as shown in Table S6). As mentioned before, the poor catalytic performances of Rh/N−C pyrolyzed at low temperatures may be resulted from their low proportion of Rh0 to Rh3+. Based on previous work, the metal in catalyst with high percentage of metallic state is favor to promoting the hydrogenation of aromatic rings.49 Additionally, there may be more part of Rh NPs completely embedded into carbon matrix, which results in the loss of part catalytic activity. In contrast, the decreased BA conversion over Rh/N−C-750 is mainly attributed to large diameters and poor dispersion of Rh particles as shown in TEM results. 9898

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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make the Rh/N−C-700 catalyst attractive for both fundamental research and practical applications. To further understand the deactivation mechanism of the carbon-supported Ru catalysts, the inductively coupled plasma optical emission spectrometry (ICP-AES) was performed to estimate the Rh contents in the fresh and the spent catalysts. As shown in Table 4, the Rh content in Rh/N−C-700 decreased

Considering the fact that the stability of the catalyst is an important issue from a practical viewpoint, the recyclability of the three N-doped carbon supported Rh catalysts was investigated under the same reaction conditions (Figure 5).

Table 4. Rh Content in Different Catalysts Rh contenta

a

catalyst

fresh

used

Rh loss (%)

Rh/N−C-700 Rh/N−C-IM Rh/N−C-NaBH4

1.20 0.71 0.78

1.05 0.36 0.45

12.5 49.3 42.3

Calculated by ICP-AES.

from an initial 1.20 to 1.05 wt % after 14 cycles, with 12.5% leaching of Rh particles. This may be resulted from that part of unstable Rh particles suffered from loss during reaction. In contrast, Rh leaching was considerably higher for the other two Rh-based catalysts during the recycling, with a leaching of 49.3% for Rh/N−C-IM (after seven cycles) and 42.3% for Rh/ N−C-NaBH4 (after six cycles). The results clearly suggest that the one-pot pyrolysis method can greatly inhibit the leaching of Rh particles because those particles receive the largest acting force originating from the interaction between Rh particles and carbon support. Among the tested catalysts, both Rh/N−C-IM and Rh/N−C-NaBH4 prepared by conventional postloading methods exhibited serious leaching of Rh. This may be mainly because of the weak interaction between metal particles and carbon support. It is well-known that decreasing the metal particle size down to several nanometers not only makes the surface atoms chemically active, but also makes the metal NPs unstable.52 For the carbon supported metal catalysts synthesized by traditional postloading method, the metal particles are well dispersed on the surface of carbon frames, but they normally show low interaction with the support and thus are easy to leach during the reaction. The morphology and corresponding particles size distributions of the fresh and spent catalysts were also investigated by TEM (Figure 7). The morphology of the Rh/N−C-700 catalyst did not change after 14 cycles, retaining a similar particle distribution as before of 3.2 nm, and no clear particle aggregation was observed, indicating again the advantage of the one-pot pyrolysis method. In the spent Rh/N−C-IM and

Figure 5. Recyclability results of Rh/N−C catalyst prepared by various methods. Reaction conditions: BA (60 mg), solvent (5 mL), catalyst 10 mg, 55 °C, 1.0 MPa H2, and 1 h.

As expected, the Rh/N−C-700 catalyst maintained high activity after 14 cycles, with BA conversion larger than 50% for each cycle. The slight activity decrease after nine cycles may be ascribed to the small amount of Rh loss during the testing as mentioned below. For the Rh/N−C-IM catalyst, the activity sharply decreased to approximately 3% after seven cycles. Furthermore, the BA conversion decreased to 27% even after six cycles for the Rh/N−C-NaBH4 catalyst. Based on the XPS results, the highest stability of Rh/N−C-700 catalyst may mainly result from the strongest interaction between Rh and carbon supports. As shown in Figure 6, compared with other two catalysts, the binding energy of Rh 3d5/2 of Rh/N−C-700 catalyst showed a negative shift (about 0.1−0.3 eV), while that of N 1s showed the opposite trend (The detailed information can be seen in Table S4). To be exact, among the three catalysts, for the Rh/N−C prepared from one-pot pyrolysis method the electronic interaction between Rh and N is the highest, which lead to N atoms donate partial electron to Rh atoms and thus reconstruct the electron density of Rh NPs. These results demonstrated that the Rh/N−C-700 prepared via one-pot pyrolysis method was more stable than those prepared by conventional methods under the investigated conditions. Its excellent catalytic performance and good recyclability should

Figure 6. Rh 3d and N 1s spectra of Rh catalysts prepared by different methods. 9899

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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Figure 7. HRTEM images and particle size distribution of fresh (A) Rh/N−C-700, (C) Rh/N−C-IM, and (E) Rh/N−C-NaBH4 and spent (B) Rh/ N−C-700, (D) Rh/N−C-IM, and (F) Rh/N−C−NaBH4 catalysts. Particle size distributions were calculated based on about 150 particles.

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CONCLUSION A simple, green, and low-cost synthetic strategy involving the one-pot pyrolysis has been developed to in situ fabricate a Rh/ N−C catalyst with well-dispersed Rh particles using renewable chitin as both the carbon and nitrogen precursors. Both the valence state and particle size of Rh on the N-doped carbon matrix can be tuned by controlling the pyrolysis temperature, which has great influence on the catalytic performance of Rh/ N−C catalysts for the selective hydrogenation of BA to CCA. It has been found that the Rh/N−C-700 catalyst with high portion of Rh0 and well-dispersed Rh particles of 3.3 nm in diameter has high catalytic performance for almost completely converting BA in 1.5 h even at low 55 °C. Additionally, the one-pot pyrolysis method significantly prevented the Rh nanoparticles from migrating on the carbon surface or leaching,

Rh/N−C-NaBH4 catalysts, their particle sizes of 2.0 and 3.3 nm are slightly larger than that of the corresponding fresh catalysts (1.8 and 3.0 nm, respectively). This might be resulted from the interaction between Rh particles and carbon supports are not strong enough to inhibit their migration and leaching. Abdelrahman et al. found that the turnover frequency (TOF) of levulinic acid hydrogenation over Ru/AC decreased rapidly even at a mild temperatures (50 °C).53 After detailed analysis, they concluded that aggregation of Ru nanoparticles from roughly 3.6 nm in the fresh sample to 6.8 nm in the spent sample was the main reason for the irreversible deactivation of Ru/AC. Together with the ICP-AES results, it might be concluded that for these Rh catalysts the serious leaching of deposited Rh was the key factor for the decreased hydrogenation activity in the recycling experiment. 9900

DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902

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which contributes to the excellent stability of the catalyst. This synthetic method is easy to scale up to produce the eco-friendly Rh/N−C catalysts that may hold significant promise in large scale applications.

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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.7b01853. Representative TEM images and corresponding particle size distributions (Figure S1) for Rh/N−C pyrolized at various temperatures; Raman spectra (Figure S2) of Rh/ N−C pyrolized at various temperatures; (Table S1) Physicochemical properties of Rh/N−C pyrolyzed at various temperatures; (Table S2) Rh content of Rh/N− C pyrolyzed at various temperatures; (Table S3) Hydrogenation of benzoic acid over various Rh-based catalysts prepared by different methods; (Table S4) Binding energies and curve-fitting results of Rh 3d and N 1s XPS spectra; (Table S5) Hydrogenation of benzoic acid derivatives; (Table S6) Productivity of different catalysts in the hydrogenation of benzoic acid to cyclohexane carboxylic acid (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. ORCID

Minghui Tang: 0000-0002-3384-8628 Yong Wang: 0000-0001-8043-5757 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Key R&D Program of China (2016YFA0202900), the National Natural Science Foundation of China (21622308, 91534114, 21376208), Key Program Supported by the Natural Science Foundation of Zhejiang Province, China (LZ18B060002), and the Fundamental Research Funds for the Central Universities are greatly appreciated.

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.7b01853 ACS Sustainable Chem. Eng. 2017, 5, 9894−9902