Nitrogen-Doped Carbon Modified Cobalt Nanoparticles Catalyzed

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Nitrogen-Doped Carbon Modified Cobalt Nanoparticles Catalyzed Oxidative Cleavage of Lignin #-O-4 Model Compounds under Mild Conditions Huihui Luo, Lianyue Wang, Guosong Li, Sensen Shang, Ying Lv, Jingyang Niu, and Shuang Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02802 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Sustainable Chemistry & Engineering

Nitrogen-Doped Carbon Modified Cobalt Nanoparticles Catalyzed Oxidative Cleavage of Lignin β-O-4 Model Compounds under Mild Conditions Huihui Luo,†,‡ Lianyue Wang,*, ‡ Guosong Li,‡ Sensen Shang,‡ Ying Lv,‡ Jingyang Niu,*, † Shuang Gao*, ‡ †

Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal

Engineering, College of Chemistry and Chemical Engineering Henan University Kaifeng, 475004, China ‡

Dalian National Laboratory for Clean Energy Dalian Institute of Chemical Physics, the

Chinese Academy of Sciences Dalian, 116023, China Corresponding Author * E-mail: [email protected]. Fax: 0086+371-23886876; Tel: 0086+371-23886876 *E-mail: [email protected]. [email protected]. Fax: 0086+411-84379248; Tel: 0086+41184379248

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Abstract: A noble-metal-free Co-based catalyst, derived from pyrolysis of natural vitamin B12 on activated carbon, is developed for the first time for one-pot oxidative cleavage of lignin linkages to phenols and aromatic esters with molecular oxygen as the oxidant under mild reaction conditions. High yields of phenol were obtained, and no oxidative coupling of phenol was produced based on the present cobalt catalyst. Compared to the previous report, the present catalyst can achieve the oxidative cleavage of β-O-4 ketones even at room temperature using dioxygen balloon. The heterogeneous catalyst shows robust recyclability and can be conveniently recovered and reused up to eight times without an appreciable loss of catalytic activity. Moreover, this catalyst system can realize the bond cleavage of organosolv lignin. The evidence of depolymerization was given by 2D HSQC NMR and gel permeation chromatography (GPC). Characterization of catalyst by ICP, N2 adsorption-desorption, Raman spectroscopy, SEM, TEM, HRTEM, HAADF-STEM, EDX, XPS and control experiments provide fundamental understanding of the catalytic materials and reaction pathway. Co3O4 in situ supported on N-doped carbon matrix by the way of high-temperature pyrolysis might be catalytic active specie. Two reaction intermediates are detected and confirmed by GC-MS.

KEYWORDS: lignin, oxidative cleavage, heterogeneous catalyst, model compounds, cobalt oxides INTRODUCTION Cleavage of lignin linkages for obtaining high value-added aromatic compounds represents a real challenge in terms of utilization of renewable energy nowadays.1-4 Various C-C (e.g., β-5 and 5-5) and C-O (e.g., β-O-4, α-O-4 and 4-O-5) bonds are the main connections in lignin. Among those linkages, the β-O-4 linkage is a significant component and its abundance is up to 50% in the


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hardwood and softwood.4 Therefore, the cleavage of β-O-4 linkage is the key points and breakthrough points to depolymerize the lignin. Many methods such as solvolysis,5,6 pyrolysis,7 reduction,8,9 oxidation10-12 and other efficient alternative strategies13-17 have been developed for its cleavage. Among these, we are particular interested in using the oxidation strategy to break down the linkages. There exists a secondary benzylic alcohol at the Cα position in β-O-4 linkage. Beckham and co-workers have discovered that the dissociation energy of the Cβ-O could be reduced by the oxidation of the Cα-hydroxyl group of the β-O-4 model compound into a ketone, thereby facilitating subsequent bond cleavage reactions.18 Based on the DFT calculations, several groups reported on the two-step depolymerisation pathways. The first step is the oxidation of Cα alcohol in native lignin to ketones. The second step is cleavage of the oxidized lignin. For example, Stahl and co-workers reported a metal-free catalytic system, consisting of 4-acetamido-TEMPO, HNO3 and HCl for the aerobic oxidation of Cα alcohol, providing Cα ketone, and a subsequent oxidation step to achieve Cα-Cβ bond cleavage.19 Westwood and co-workers performed the oxidation of Cα-hydroxyl group in lignin using DDQ/tBuONO/O2 catalytic system, and Zn/NH4Cl was used for the reductive cleavage of β-O-4 linkages.20 Wang and co-workers reported a VOSO4 /TEMPO catalyst for the β-O-4 alcohol oxidation to ketone, and then Cu(OAc)2/1,10-phenanthroline catalyst was used for the oxidative cleavage of β-O-4

ketone to acids and phenols. Besides, some groups reported

efficient catalytic systems for the oxidative cleavage of β-O-4 linkage in one step. For example, Toste’s group and Hanson’s group utilizing the vanadium-based catalysts exhibited high performances for the oxidative cleavage of β-O-4 linkage models in one step.21-23 Bozell and oworkers reported Co-Schiff base catalysts for the oxidative cleavage of monomeric and dimeric lignin models to the to the corresponding benzoquinones.24 Xu and co-workers reported


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vanadium-catalyzed oxidative C-C bond cleavage of 2-phenoxy-1-phenylethanol in the presence of acetic acid.25 Corma and co-workers reported copper-and vanadium-catalyzed oxidative cleavage of lignin using dioxygen as the terminal oxidant.26 Although these homogeneous catalytic systems have achieved the cleavage of β-O-4 linkage very well, these systems suffer from the pervasive difficulty in recovery and reuse of the catalyst. Moreover, addition of ligands, organic bases, and other promoters often bring separation problems. Heterogeneous catalysis can address these limitations, but most of the current processes suffer several drawbacks in terms of experimental point such as the required harsh conditions (high temperatures and pressures), the use of expensive catalysts, as well as sometimes long reaction time.27 For example, Pd/CeO2 catalyst was reported for the one-pot oxidative conversion of 2-phenoxy-1-phenylethanol in methanol under O2 at 458 K for 24 h. Despite the harsh reaction conditions, five products were obtained such as 2-phenoxy-1-phenylethanone, phenol, acetophenone, methyl benzoate and benzoic acid.28Some other catalysts such as Co3O4,29,30 Mn–Co(mixed oxide),31 and Co-ZIF-932 use simple model compounds (e.g., veratryl alcohol) that lack key lignin structural features, in addition to the harsh reaction conditions and low the product yield. Moreover, the stability and recyclability of heterogeneous catalysts are scarcely investigated as concerns the oxidation of lignin and derived representative model compounds. Therefore, the development of sustainable and efficient heterogeneous catalysts with mild reaction conditions is highly desirable. During the last decade, carbon materials or carbon-supported metal catalysts have exhibited great promising alternative as sustainable catalysts for organic synthesis. 33-35 The prominent catalytic performance is related to special electronic property of these materials. It is found that the doping of heteroatoms such as nitrogen, boron, phosphorus, and sulphur or the introduction of non-noble transition metals (Fe, Co, etc.) into the carbon skeleton can provide an efficient


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method to improve catalytic activity of the materials with the tunable electron feature and generating more active sites.36-,39Among these catalysts, it has been demonstrated that N-doped carbon materials produce defects in graphene stacking, which lead to the formation of the O 2 absorption active sites. On the basic of the properties of the N-doped carbon materials to activate O2 and our recent investigations on the use of cobalt-based catalysts and N-doped mesoporous carbon heterogeneous materials in the oxidative self-coupling of amines to imines,40 the crosscoupling of alcohols with amines to imines,39 and the selective oxidation of alcohols to nitriles,42,43 we started to explore their application in the more challenging catalytic oxidation cleavage of the β-O-4 linkages. To the best of our knowledge, N-doped carbon materials have not been applied in the oxidative cleavage of the β-O-4 linkages. Herein, we report for the first time that Co-based nanoparticles supported on nitrogen-doped carbon materials as sustainable heterogeneous catalysts for the onepot oxidative cleavage of the β-O-4 linkages under mild conditions with molecular oxygen as the environmentally benign terminal oxidant. It is particularly notable that high yields of the phenol were obtained. No oxidative coupling of phenols occurred with the present Co catalyst. Notably, the oxidative cleavage of β-O-4 ketone model compounds can be achieved with the present catalyst at room temperature under dioxygen balloon. Moreover, it has a significant advantage that the inexpensive catalyst is easily recovered and can be conveniently reused. Characterization of the catalyst and control experiments provide insights into the catalytic materials and fundamental understanding of the reaction pathway. EXPERIMENTAL SECTION Chemicals. All commercially available reagents and solvents were used without any further purification.


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Synthesis of mesoporous catalysts. The sustainable heterogeneous cobalt catalyst was prepared by impregnating VB12 on commercially available activated carbon powder and then pyrolysis of the resulting catalyst precursors at different temperatures (600-1000 oC) under N2 atmosphere for 2 h. The as-synthesized materials are labelled as VB12@C-X, where X represents the pyrolysis temperature. Other materials obtained from the supports such as SiO2, α-Al2O3, and ZrO2 are denoted as VB12@SiO2, VB12@α-Al2O3 and VB12@ZrO2. Typical procedure for the cleavage of lignin. 84 mg VB12@C-900 catalyst (15 mol% Co) and 6.8 mg K2CO3 (0.05 mmol) were added to 25 mL flask. Then, 53.5 mg 2-phenoxy-1phenylethanol (0.25 mmol) and 4mL methanol were added sequentially. The resulting mixture was transferred to an autoclave. After autoclave was closed, dioxygen was charged to 0.1 MPa. After reaction, the reactor was cooled to room temperature, biphenyl as internal standard was added and the mixture was diluted with methanol. Then, the catalyst was filtered off, and a sample of the mixture was directly subjected to GC analysis. Quantitative and qualitative analysis of all products were made by GC, GC–MS and identified by comparison with authentic samples. Catalytic oxidative cleavage of lignin. Lignin 40 mg, VB12@C-900 catalyst (15 mol% Co), 6.8 mg K2CO3 (0.05 mmol) and 4mL methanol were added to 25 mL flask. Then the reactor was charged with 0.1 MPa of O2 and heated to 80 oC kept for 24 h under magnetic stirring. After the reaction was complete, the reaction mixture was centrifuged, solid catalyst was washed several times with methanol and the liquid was all collected and evaporated. DMSO-d6 was added to determine the 2D HSQC NMR spectrum. Catalyst Characterization. Nitrogen sorption isotherms were determined at 77K using a QuadraSorb SI4 Station. Pirorto the measurement, the samples were degassed in vacuum at


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300 °C for 3 h, and the BrunauerEmmett-Teller (BET) surface areas of the samples were calculated using adsorption data. Raman spectroscopy was performed at ambient conditions using a 532 nm laser on Nano Wizard Ultra Speed & in Via Raman Scanning

electron

microscope (SEM) images were conducted on a JSM-7800F microscope operated at 20 kV landing energy. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were acquired with JEM-2100 microscope. High-angle annular dark-ﬁeld scanning transmission electron microscopy (HADDF-STEM) images were obtained from JEM-2100F microscope. X-ray photoelectron spectroscopy (XPS) analysis of the samples were performed on Thermo Scientific ESCALAB 250Xi instrument with Al Ka radiation anode (hυ = 1486.6 eV). The C 1s line (284.6 eV) was used as the reference to correct the binding energies (BE). 2D HSQC spectra were recorded on a Bruker AVAVCE III HD 700 MHz spectrometer at 25 oC. GPC analysis was recorded on a Viscotek TDAmax equipped with three detectors (i.e., refractive index, light scattering, and viscometry detectors), a guard column (40 mm length 7.8 mm inside diameter) from Polymer Laboratories and a Viscotek column (300 mm length 7.8 mm inside diameter, modified porous styrene-divinylbenzene copolymer). Quanturn chemical calculations are performed by using Gaussian 09 program suite. Complexes are optimized at the M06-2X level of density functional theory using the def2-TZVP basis set on all atoms (C, H, O). The reported structures are true minima without imaginary vibrational frequencies. Bond energies are calculated as single point energy differences between optimized complexes and their corresponding dissociative fragments without optimization after interested bonds are cleaved. RESULTS AND DISCUSSION The catalytic activities of as-obtained materials were evaluated in the oxidative cleavage of the lignin β-O-4 model compounds. To optimize the reaction conditions, 2-phenoxy-1-phenylethanol


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Table 1. Optimization conditionsa OH

O

OH O

catalyst, MeOH

OMe

+

O2, 80 oC, 24 h 1

3

2

b

Entry

Catalysts

Conversion (%)

1

VB12

2

Yieldb (%) 2

3

-

-

-

C

-

-

-

3

VB12+C

-

-

-

4c

-

-

-

-

5

VB12@C-600

59

46

28

6

VB12@C-700

63

63

44

7

VB12@C-800

80

83

57

8d

VB12@C-900

96

96

73

9

VB12@C-1000

82

82

58

10e

VB12@C-900

77

77

51

11f

VB12@C-900

90

85

63

12g

VB12@C-900

-

-

-

13h

VB12@C-900

19

19

19

14

VB12@C-900-H+

-

-

-

15

VB12 @SiO2-900

51

48

23

16

VB12@α-Al2O3-900

47

47

29

17

VB12@ZrO2-900

15

15

7

a

Reaction conditions: 0.25 mmol 2-phenoxy-1-phenylethanol, 15 mol% catalyst, 20 mol% K2CO3, 80 oC, 0.1 MP O2, 24 h. bconversion and yield were determined by GC using biphenyl as the internal standard. c Only 20 mol% K2CO3. d 26% yield of benzoic acid was obtained. e 10 mol% catalyst, f 0.1 MP air. g 0.1 MP N2. h Without K2CO3.


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was tested as a model substrate. Typically, this model reaction was performed using MeOH as the solvent under simply 0.1 MPa O2 at 80 oC with catalytic amounts of K2CO3 as base. The results are summarized in Table 1. Firstly, a series of control experiments were carried out with only commercially available vitamin B12 and activated carbon or a physical mixture of both. The results indicated that the catalyst precursors showed no reactivity without the high temperature pyrolysis process (Table 1, entries 1-3). Only in the presence of K2CO3 the reaction did not proceed (Table 1, entry 4). Much to our delight, catalyst materials from the pyrolysis of the vitamin B12 supported on carbon gave a high activity for the oxidative cleavage of 2-phenoxy-1phenylethanol and VB12@C-900 exhibited the best catalytic performance to produce phenol and methyl benzoate in 96% and 73% yields, respectively, with 26% yield of benzoic acid as the major by-product (Table 1, entries 5-9). It is particularly notable here, because Co-based catalyst tends to carry out one-electron oxidations which produce oxidative coupling of phenols or overoxidation to quinone. When the amount of catalyst was reduced, the yields of the target products were also decreased (Table 1, entry 10). The high activity of the present catalytic material has also been demonstrated by using air as the oxidant in place of pure dioxygen (Table 1, entry11). No reactivity was observed under N2 atmosphere (Table 1, entry12). Only 19% of conversion was obtained in the absence of K2CO3 (Table 1, entry13). These results suggested that the presence of base and O2 was necessary for the transformation. When the VB12@C-900 was immersed in aqua regia and the resulting material is denoted as VB12@C-900-H+. VB12@C-900H+ showed no activity under the same reaction conditions (Table 1, entry14). When the vitamin B12 was supported on SiO2, α-Al2O3 and ZrO2, the conversions were obtained in 51%, 47% and 15%, respectively (Table 1, entries 15-17). These results show that the special structure of the carbon materials might provide an available reaction environment with high activity.


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Phenol

100

Methyl benzoate

80

Yield/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 0

1

2

3

4

5

Recycle number

6

7

8

Figure 1. Recycling of VB12@C-900 catalyst in the oxidative cleavage of the 2-phenoxy-1phenylethanol. The stability and recyclability are scarcely investigated in the heterogeneous catalytic oxidation of lignin and its model compounds. On the basic of the promising results above, we cannot wait to examine the stability and recyclability of the VB12@C-900, which is significant issue of our concern. After reaction with VB12@C-900, the catalyst was filtrated off, washed with methanol and dried under vacuum, and then reused for the next run under the identical reaction conditions. As shown in Figure 1, the catalyst can be reused for eight times without significant loss of activity. Furthermore, the reused VB12@C-900 catalyst was characterized to examine the structure and morphology. The N2 isotherm and TEM images of VB12@C-900 catalyst after nine runs indicated that the mesoporous structure and particle size of Co 3O4 had no obvious change (Figure S1). The specific surface was reduced to 434 m2 g-1, which might be due to partial blocking the pores. The ICP-AES results indicated that the content of Co decreased from 2.8 wt% to 2.4 wt%, but there was only slight change in activity of the reused catalyst. A leaching experiment result showed that no active species leaching occurred to the VB12@C-900 and the heterogeneous nature of the reaction was verified. These results demonstrated that the


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Figure 2. (a) N2 adsorption-desorption isotherm of VB12@C-900; (b) Raman

spectra of

VB12@C-X (X = 600 oC, 700 oC, 800 oC, 900 oC, 1000 oC) highly active catalyst was stable and reusable under mild reaction conditions and the catalytic process was truly heterogeneous. BET surface area and pore structure of the VB12@C-900 materials were determined by N2 adsorption-desorption at 77 K. As shown in Figure 2a, the adsorption isotherm curve shows a typical type- Ⅳ isotherm pattern with H4 hysteresis loop, which indicates the presence of mesoporous structure in VB12@C-900 material with BET surface area 458 m2g-1. Total pore volume of 4.9 cm3g-1 was obtained from volume of N2 adsorbed at P/Po=0.98. Raman spectroscopy was used to study the graphitization process (Figure 2b). All resulting materials have an obvious D band at 1328 cm-1, which is associated with defects, and a G band at 1593 cm1

, which is due to graphitic carbon. The intensity ratio of D and G bands, ID/IG, is used to

evaluate the defect density in the materials. The intensity ratio of ID/IG decreased with the temperature (700-1000 oC) of pyrolysis increased. The lower value ID/IG at 600 oC indicates that the defect begins to form. The structure and morphology of the VB12@C-900 composites were investigated by SEM and TEM observation (Figure 3 and more images see Figure S2 and S3). SEM observation showed


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Figure 3. SEM (a, b), Elemental Mapping (c), TEM (d), HRTEM (e), and HADDF-STEM (f) images of VB12@C-900. the surface morphology of the VB12@C-900. The mesoporous structure could be observed on the surface in VB12@C-900, which was in good accordance with the N2 adsorption-desorption (Figure 3a). EDX spectrum exhibited the existence of C, O, N and Co (Figure S4). The elemental mapping analysis for VB12@C-900 confirmed that the elements were uniform distribution throughout the N-doped carbon materials (Figure 3c). The TEM observations indicate that the size of the Co-based nanoparticles was 10-30 nm in the VB12@C-900 sample (Figure 3d). Furthermore, the high-resolution TEM (HRTEM) image of VB12@C-900 revealed that Co-based nanoparticles were embedded in N-doped graphitized carbon. It can be clearly observed that a lattice spacing was 2.82 Å, which is attributed to the (220) plane of Co3O4 (Figure 3e).44 HAADF-STEM image further evidence that the cobalt oxide nanoparticles are dispersed uniformly on the support and surrounded by N-doped graphitized carbon (Figure 3f and More images see Figure S5). In the subsequent heat treatment, cobalt accumulates into larger particles could be greatly hindered; because of the cobalt ions were well coordinated at the centre of the VB12 precursor. In contrast, when commercial Co3O4 was used as catalyst under the same


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reaction conditions, no desired products were obtained. Nitrogen-free catalyst obtained from the pyrolysis of carbon-supported Co(OAc)2 showed little activity and produced methyl benzoate in 10% yield, and no phenol was detected. These results suggested that the N-doped carbon structure improved the catalytic activity of Co3O4. To gain more information of elemental composition and surface chemical states of VB12@C900 XPS analysis was performed (Figure 4). As shown in survey that C, N, Co and O are present on the surface of VB12@C-900 (Figure 4a), which was consistent with EDX. The atomic percentages of C, N. Co and O were quantified as 89.14%, 1.70%, 0.29% and 8.87%, respectively. N 1s spectrum can be deconvoluted to four peaks, namely, pyridinic (389.2 eV), pyrrolic (399.9 eV), graphitic (401.0 eV), oxidized nitrogen (403.4 eV) (Figure 4b).45 The C1s spectrum shows the existence of four types of carbon species: C=C at 284.6 eV, C=N at 285.7 eV, C-N 286.6 eV, and O-C=O at 289.2 eV, indicating that N atoms were successfully doped in the VB12 derived carbon materials (Figure 4c).46 In the cobalt region, low-energy band at 780.8 eV (Co 2p3/2) and high-energy band at 795.8 eV (Co 2p1/2) are detected, which is attributed to oxidic Co (Figure 4d). The satellite peaks at 786.8eV and 802.9 eV further suggested that the oxidic Co was mainly composed of Co3O4,47 which is consistent with the results of TEM. From the atomic percentages of Co and O for all samples (Table S1), the higher O/Co ratio was obtained for VB12@C-900,

suggesting more active species existed, which might give an

explanation for the high catalytic activity. Use the optimized conditions that have been obtained, we turned our attention to estimate this catalytic oxidation system for other β-O-4 lignin model compounds with different substituents. The results are summarized in scheme 1. The substrates with different methoxyl groups proceeded smoothly to produce the corresponding valuable phenol in good yields and aromatic


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Figure 4. XPS spectra of VB12@C-900 composites. (a) Survey; (b) N1s; (c) C1s; (d) Co 2p esters in moderate yields. Lignin models with Cγ-OH were also investigated. The moderate yields were obtained under the present mild conditions. Some reports directly employed β-O-4 ketone model compounds to investigate the cleavage of Cα-Cβ bond and Cβ-O bond with harsh oxidation conditions.48 The control experiments have demonstrated that ketone is an intermediate in the present catalytic process. A kinetic study of the oxidative cleavage of 2-phenoxy-1-phenylethanol and its corresponding ketone indicated that the intermediate ketone was fast converted to the target products (Figure 5). This result prompted us to study whether this transformation can be achieved under more mild conditions. To our delight, even at room temperature, the target bonds can be cleaved with decreasing the amount of catalyst (Scheme 2). Without VB12@C-900, the target products were not detected. For all ketone lignin model compounds, phenols were obtained in good to excellent yields, even the methoxy substituted


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Scheme 1. Catalytic oxidative cleavage of lignin β-O-4 model compounds. Reaction conditions: 0.25 mmol model compound, 15 mol% VB12@C-900, 20 mol% K2CO3, 4 mL MeOH. Yields were determined by GC analysis with biphenyl as an internal standard.

Figure 5. (a) Kinetic study of the oxidative cleavage of 2-phenoxy-1-phenylethanol. (b) Kinetic comparison of the oxidative cleavage of 2-phenoxy-1-phenylethanol and its corresponding ketone under the standard reaction conditions. phenol could also get a good yield. The esters got a lower yield with Methyl benzoylformate as corresponding major by-product. These results indicated that Cβ-O bond is very easy to break in the case of ketone as substrate with the present catalytic system.


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Scheme 2. Catalytic oxidative cleavage of β-O-4 ketone model compounds. Reaction conditions: 0.25 mmol model compound, 5 mol% VB12@C-900, 20 mol% K2CO3, 4 mL MeOH. Yields were determined by GC analysis with biphenyl as an internal standard. The promising results from the Co-based catalyst in the oxidative cleavage of the β-O-4 model compounds motivated us to learn more about the mechanistic details (Scheme 3, using 2phenoxy-1-phenylethanol as a typical substrate). Based on the previous reports, ketone might be the first intermediate formed by the oxidation of a secondary benzylic alcohol. Under the same conditions, ketone gave 82% phenol and 38% methyl benzoate (eqn, (1)). The methyl ether was used as substrate to block the oxidation of secondary benzylic alcohol to ketone, no Cβ-O and Cα-Cβ bond-cleavage products were observed (eqn, (2)). When the methylene position of 2phenoxy-1-phenylethanol was blocked, only the corresponding ketone was obtained (eqn, (3)). The results prove that the methylene group is essential to initiate the cleavage of Cβ-O and CαCβ bond. These results from control experiments gave the following conclusions that 1) ketone is one of the intermediates, 2) the secondary hydroxyl group is necessary for the cleavage of Cβ-O and Cα-Cβ bond to occur, and 3) the methylene group in substrate is indispensable for access to Cβ-O and Cα-Cβ bond cleavage pathway, which might exist methylene activation by the Cobased catalyst.


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Scheme 3. Control experiments. It has been reported that the oxidation of β-O-4 secondary alcohol to β-O-4 ketone lowers the Cβ-O bond energy and increases the Cα-Cβ bond energy, making the Cα-Cβ bond harder to be converted. In the present catalytic system, a minor amount of Methyl benzoylformate was detected from GC-MS, which might be the second reaction intermediate. When the Methyl benzoylformate was used as reactant, the quantitative methyl benzoate was obtained, suggesting that the Cβ-O in 1 was first cleaved, followed by Cα-Cβ oxidative cleavage to methyl benzoate (eqn, (4)). In contrast, the oxidative cleavage of Methyl benzoylformate did not occur in the absence of catalyst (eqn, (5)). Although benzoic acid as by-product was detected from GC-MS, benzoic acid as substrate did not give methyl benzoate. The result indicated that benzoic acid was not reaction intermediate (eqn, (6)). When the radical scavenger TEMPO or BHT was used,


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Scheme 4. Proposed reaction pathway for the VB12@C-900-catalyzed conversion of 2-phenoxy1-phenylethanol in MeOH. the yields of target products were decreased, indicating that this transformation may involve a radical process (Scheme S1). Based on the pertinent literatures and control experiments, a possible reaction process was inferred (Scheme 4). First, dioxygen is activated by the Co-based catalyst to superoxide species,48-51 which, with the assistance of K2CO3, oxidized Cα-OH into ketone. One possible reason could be given for the lower product yield of the left benzene ring. The C-H activation of methylene via radical process may form a low polymer, which does not affect the Cβ-O bond cleavage. The process of methylene activation is similar to that of ethylbenzene oxidation, which may produce three compounds.51 DFT calculations pointed out that methylene was oxidized to hydroxyl, which showed lower Cβ-O bond dissociation energy (Table S2), resulting in the Cβ-O bond to be more easily broken than Cα-Cβ bond. Then phenol and Methyl benzoylformate were formed. Finally, oxidative cleavage of C-C bond of Methyl benzoylformate generates methyl benzoate, along with the release of CO2, which was detected by the lime-water-test (Figure S6).


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Encouraged by the promising results in the cleavage of the model compounds, the ability of such catalysts in real lignin fragmentation was studied. We decided to use two organosolv lignin sources to evaluate the ability of the present catalytic system. 2D-NMR (HSQC) experiments were conducted to monitor the cleavage of the characteristic interconnecting bonds within lignin. The attribution of the spectral data refers to the reported work by Sun and co-workers.52 The organosolv lignin from Birch was extracted in an ethanol-based organosolv process.53 Figure 6 showed the 2D-NMR spectra of organosolv lignin before (a) and after the reaction (b) under the standard reaction conditions. The β-O-4 linkages A-, and A`-signals were still present but with a

Figure 6. Partial 2D HSQC NMR spectra ((700 MHz, in DMSO-d6) of (a) ethanosolv brich lignin, (b) oxidized ethanosolv birch lignin with our catalytic reaction system, (c) dioxasolv brich lignin, (d) oxidized dioxasolv birch lignin with our catalytic reaction system, (e) assignment structures for the β-O-4` aryl ether linkages (A, A`), phenylcoumaran linkages (B) and resinol linkages (C).


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significantly reduced signal intensity. For example, based on the methoxy group integrals of the 2D HSQC NMR spectra, approximately 83% of β-O-4 linkages Aα, and Aα`were converted. No corresponding alpha ketone β-O-4 structure of A appeared. For the phenylcoumaran structures B the α-signal disappeared entirely and the β-and γ-were also significantly reduced. In contrast, the present catalytic system was able to completely degrade the resinol structures C. In order to further prove the general applicability of the present catalyst system, a dioxane-based organosolv lignin was subjected to the reaction conditions.54 To our delight, a complete degradation of the βO-4 linkages A, the phenylcoumaran structures B and the resinol structures C was observed in the HSQC spectra (Figure 6 c, d). These results proved strongly that VB12@C-900 catalyst exhibited excellent catalytic activity toward the oxidative conversion of lignin model compounds and organosolv lignin using molecular oxygen as the oxidant under mild conditions. Finally, the degree of depolymerisation for the organosolv lignin was determined by GPC analysis. The results showed that the molecular weight of the residual lignins decreased for both lignin sources relative to that of the corresponding fresh lignins, suggesting that depolymerization has occurred (Figure S7). CONCLUSION In summary, we have introduced the application of cobalt-based nanoparticles supported on nitrogen-doped carbon as sustainable heterogeneous catalysts for one-pot oxidative cleavage of lignin model compounds to phenols and aromatic esters with molecular oxygen as the oxidant under mild reaction conditions for the first time. The robust catalyst that is easily prepared by pyrolysis of vitamin B12 on activated carbon can be retained for at least seven cycles. A series of characterization shows that cobalt oxide surrounded by N-doped graphitized carbon is an active component. It is worth emphasizing that in the case of β-O-4 ketone model compounds as


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substrates, Optimized catalyst can achieve the target reaction at room temperature. Importantly, the catalyst was also effective in depolymerizing organosolv lignin. This work would provide an alternative to catalyst for the conversion of lignin to high-value-added chemicals. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1. TEM images and N2 adsorption-desorption isotherm of the reused VB12@C-900 after nine times Figure S2-5. SEM, TEM, EDX and HADDF-STEM images of VB12@C-900 Table S1. Distribution of element species obtained from XPS Scheme S1. Control experiments Figure S6. The lime-water-test Table S2. The bond energy of C-C and C-O bonds for the possible intermediates from density Functional theory (DFT) calculation Figure S7. GPC measurements for organosolv lignin samples

Substrate preparation Copies of the 1H-NMR and 13C-NMR spectra for all substrates


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: 0086+371-23886876; Tel: 0086+371-23886876 *E-mail: [email protected]. [email protected]. Fax: 0086+411-84379248; Tel: 0086+41184379248 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21773227, 21403219, and 21773232). REFERENCES (1) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B.M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110 (6), 3552-3599. (2) Xu, C.; Arancon, R. A. D.; Labidi, J.; Luque, R. Lignin depolymerisation strategies: towards valuable chemicals and fuels. Chem. Soc. Rev. 2014, 43 (22), 7485-7500.


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Table of Contents

SYNOPSIS Nitrogen-doped carbon modified cobalt nanoparticles as sustainable heterogeneous catalysts cleave C-C and C-O bonds in the lignin β-O-4 model compounds.


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Nitrogen-Doped Carbon Modified Cobalt Nanoparticles Catalyzed

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