Nitrogen-Doped Biomass Carbons Meet with Polyoxometalates

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Nitrogen-doped biomass carbons meet with polyoxometalates: synergistic catalytic reductant-free aerobic hydroxylation of benzene to phenol Zhouyang Long, Yadong Zhang, Guojian Chen, Jiang Shang, Yu Zhou, Jun Wang, and Liming Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05920 • Publication Date (Web): 19 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Sustainable Chemistry & Engineering

Nitrogen-doped biomass carbons meet with polyoxometalates: synergistic catalytic reductant-free aerobic hydroxylation of benzene to phenol Zhouyang Long,† Yadong Zhang,† Guojian Chen*,†, Jiang Shang,† Yu Zhou,‡ Jun Wang‡ and Liming Sun*,† † School

of Chemistry and Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional

Materials, Jiangsu Normal University, No.101, Shanghai Road, Tongshan District, Xuzhou 221116, Jiangsu, China ‡State

Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing

Tech University, No. 5 Xinmofan Road, Nanjing 210009, Jiangsu, China * Corresponding authors, E-mail: [email protected], [email protected]

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Abstract A nitrogen-doped biomass carbon catalyst SFNC(800) was prepared by carbonizing the cheap and readily available soybean flour as the starting material at 800 oC. By combing the SFNC(800) with the Keggin-type V-containing polyoxometalate Ch5PMoV2, a facilely recyclable catalytic system towards liquid-phase reductant-free aerobic oxidation of benzene to phenol was built. The combined catalyst SFNC(800)-Ch5PMoV2 exhibited a high activity with 11.2% phenol yield, 14.04 h−1 turnover frequency (TOF) and high potential for reusability under the optimized reaction conditions. The present TOF value surpassed almost all the ones of other similar previously reported reductant-free LBTP-O2 systems and was even higher than the previous noble metal involved system. Based on the characterization results and density functional theory calculations, the structure-activity analysis revealed that the graphitic N-containing moiety in the SFNC(800) is critical to absorb and activate the substrate benzene. KEYWORDS: Nitrogen-doped, Biomass carbon, Polyoxometalate, Reductant-free, Hydroxylation of benzene

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 INTRODUCTION The existing industrial process for producing phenol, an important intermediate in the chemical industry, suffers from high energy-consumption, low phenol yield (~5%) and lack of environmental favorability.1 The direct hydroxylation of benzene to phenol is a recognized promising alternative to replace the current three-step cumene process for phenol production.2-5 Liquid-phase aerobic oxidation of benzene to phenol with the cheap and clean molecular oxygen (O2) as oxidant (denoted as LBTP-O2) receives much attention because of its atom economy and economic superiority.2,6-16 Unfortunately, due to the considerable stability of benzene and O2, the good performance over most catalysts for LBTP-O2 strongly relies on the presence of sacrificial reductants (H2, CO or ascorbic acid);6,7 otherwise, noble metal catalysts are required.8-10 From green chemistry and sustainable developing perspectives, reductant-free non-noble metal catalytic system for LBTP-O2 is much more attractive. However, the previously reported ones are confronted with the drawbacks of inferior catalytic activities and/or complex recycling procedures.11-16 Besides, those reported catalysts generally involve complicated preparation processes by using expensive and/or harmful precursors.14,16 Therefore, it is urgently demanded to build a highly efficient reductant-free catalytic system for LBTP-O2 using facilely prepared low-cost and easily recyclable catalysts. In recent years, carbon materials have received ever-increasing interest in several fields, such as nanoelectronics, sensors, nanocomposites, and catalysis.17 Compared with the undecorated carbons, nitrogen-doping carbons (NCs) have exhibited significantly enhanced catalytic performance in the focused application in catalysis.18 For instance, NCs have been used as fascinating heterogeneous catalysts for selective oxidation of organic substrates including propane, ethylbenzene, styrene, cyclohexane, cyclohexene, benzyl alcohol and etc.18,19 It has been proposed that the variety and amount of the doped-N species are crucial to enhance the catalytic performance.18 As for the aerobic oxidation of benzene to phenol, NC-supported polyoxometalates (POMs) and NC-supported Fe2O3 nanoparticles have been reported as the dual-site catalysts, where the NCs activate the 3



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substrate benzene while the POMs or Fe2O3 nanoparticles activate the oxidant O2.14,16 Nonetheless, the above reported two catalysts only offered low turnover frequency (TOF) values under harsh reaction conditions, and the microcosmic mechanism for the enhanced effect of the doped-N species is still unclear. Herein, the soybean flour, a cheap natural biomass, is used as the precursor to prepare the nitrogen-doped biomass carbon catalyst SFNC(800) via a direct carbonization process at 800 oC. The SFNC(800) behaves a remarkable catalytic performance toward LBTP-O2 reaction when combined with another catalyst Ch5PMoV2 prepared by modifying Keggin-type V-containing POM anions (PMoV2) with choline (Ch) cations. The hybrid catalyst Ch5PMoV2 exhibits an interesting temperature-controlled phase-transfer feature that leads to a facile process for recycling the catalysts.15 Under reductant-free reaction conditions, a very impressive TOF value and high potential for reusability are obtained in LBTP-O2. In particular, we focus on exploring the structure-activity relationship of the N-doped carbon catalyst and the microcosmic mechanism for the enhanced catalytic activities by the control experiments, comprehensive characterizations and density functional theory (DFT) calculations.

 EXPERIMENTAL SECTION Materials and Methods. All solvents and reagents were purchased from commercial source and used without further purification. XRD patterns were collected on the Bruker D8 Advance powder diffractometer using Ni-filtered Cu Ka radiation source at 40 kV and 20 mA, from 5 to 90o with a scan rate of 0.2o s-1. The Raman spectra were recorded using a Jobin Yvon Labram HR800 spectrometer (Horiba/Jobin. Yvon, Longjumeau, France), with an excitation laser at 532 nm. Signals were recorded in the range from 200-2500 cm-1. Field emission scanning electron microscope (FESEM; Hitachi SU8010, accelerated voltage: 15 kV) was used to study the morphology of the catalysts and the element distribution. The Brunauer-Emmett-Teller (BET) surface area and pore structure of the samples were obtained by nitrogen adsorption–desorption isotherm measurements (ASAP2020, USA). Fourier 4


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transform infrared (FT-IR) spectra were recorded on a Bruker Vertex 80V instrument (KBr disks) in the 4000-500cm-1 region. The surface states were analyzed by X-ray photoelectron spectroscopy (XPS, AXIS-NOVA, Kratos Inc.) using an X-ray source of mono-chromatic Al Ka (1486.6 eV) 150 W. Catalyst Preparation. Preparation of Carbon Materials. Soybean flour (SF) was bought from the local market and used without pre-treatment. 2.0 g SF was carbonized at the required temperature for 2 h in a nitrogen flow of 100 mL/min with a heating rate of 10 oC/min. After cooling, the black power was ground, washed by water with ultrasonic treatment for 30 min and dried at 80 oC for 24 h. The SFNC series is named as SFNC(T)s, where T denotes the carbonization temperature (T=700, 800, 900). N-free biomass carbon cellulose-C(800) was prepared with microcrystalline cellulose as raw material through the same process as SFNC(800). Preparation of H5PV2Mo10O40 (H5PMoV2) and Ch5PMoV2. H5PMoV2 and Ch5PMoV2 were synthesized prepared according to previous literatures.15,20 Catalyst Performance. Aerobic of benzene to phenol was carried out in a customer-designed temperature controllable pressured titanium reactor (10 mL). In a typical run, 0.39 g (5 mmol) benzene, 0.01 g SFNC(800), 0.03 g Ch5PMoV2, 0.06 g LiOAc, and 2 ml aqueous acetic acid solution (50%) were added into the reactor successively. After the reactor was charged with 2.0 MPa O2 at room temperature, the reaction was conducted at 120 oC for 3.0 h with vigorous stirring. After reaction, the reaction mixture was cooled to room temperature, and then the solid was collected by centrifugation. The solid was washed with acetone for three times and dried in vacuum oven for the next use. 1,4-dioxane was as an internal standard and added into the isolated liquid phase of the reaction mixture. The mixture was analyzed by gas chromatograph (GC) with a FID and a capillary column (SE-54; 30 m × 0.32 mm × 0.25 μm). Because only the phenol GC peak was detected as the product when the reaction was performed around the above typical conditions, the GC-measured selectivity for phenol is reasonably estimated to be above 99%. The 5



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average values of three parallel reaction tests were given for phenol yields. Theoretical calculations. In order to explore the catalytically activated process of benzene on SFNC, periodic density functional theory (DFT) computations were performed by using a plane-wave method implemented in the Cambridge Sequential Total Energy Package (CASTEP) code.21 The exchange-correlation potential was described by generalized gradient approximation with the Perdew-Burke-Ernzerhof (GGA-PBE) scheme.22 Interaction between the valence electrons and the ion core was substituted by an ultrasoft pseudopotential.23 A 240eV cutoff for the plane-wave basis set was adopted in all computations. The self-consistent convergence accuracy was set at 2× 10−5 eV/atom, the convergence criterion for the force between atoms was 0.05eV/Å, and the maximum displacement was 0.002Å.

 RESULTS AND DISCUSSION

Scheme 1. Preparation of the catalysts of Ch5PMoV2 and the SFNC(T)s. Characterization of Catalysts. Scheme 1 describes the preparation procedure of Ch5PMoV2 and the N-doped biomass carbon SFNC(T)s, in which T =700, 800 and 900. Ch5PMoV2 is synthesized via a simple precipitation reaction between ChCl and H5PMoV2 in aqueous solution. SFNC(T)s are prepared by carbonization of soybean flour (SF) under a N2 atmosphere. The renewable soybean flour is selected as the raw material due to the existence of enriched nitrogen elements in proteins of the soybean flour.24,25 During the carbonization procedure, a high 6


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temperature leads to the decomposition of carbonaceous species, and meanwhile, the N element can be in-situ doped into the resultant carbon skeleton24-26. Consequently, the N-doped carbon materials SFNCs are obtained without using additional nitrogen sources.

Figure 1. (A) XRD patterns and (B) Raman spectra of SFNC(T)s.

Figure 2. (A), (B) and (C) SEM images of SFNC(800); (D) N2 adsorption-desorption isotherms and (E) corresponding BJH pore size distributions of SFNC(T)s. 7



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Figure 1A shows the XRD patterns of the SFNC(T)s prepared at 700, 800 and 900 oC. All SFNC(T)s samples exhibit two broad peaks centered at 24.3o and 43.8o, attributable to the (002) and (100) lattice planes of typical graphitic carbon, suggesting the formation of graphitic carbon structure.27-29 Two prominent peaks centered around 1338 and 1595 cm−1 are observed in the Raman spectra of the samples shown in Figure 1B, which represent for the characteristic D and G bands of carbon materials, respectively.24,26,27,30 The D band indicates the presence of disordered graphite carbons, while the G band demonstrates the existence of graphitic carbons in the SFNC(T)s,30 corresponding to the XRD results. Besides, the intensity ratio of the D and G bands (ID/IG) of SFNC(T)s is estimated to be 2.83, 2.65 and 2.47 for SFNC(700), SFNC(800) and SFNC(900), indicating the improved graphitization and less disordered carbon atoms caused by increasing the carbonization temperature.24,26,29-32 From the SEM images of SFNC(T)s shown in Figure 2A-C and Figure S1, it can be seen that all SFNC(T)s exhibit the bulk morphology with the sizes about tens of microns (Figure 2A and S1). The typical sample SFNC(800) has a rough surface with many observable holes (Figure 2B and Figure 2C), which may be resulted from the decomposition of carbonaceous species of SF at a high carbonization temperature.24 Elemental mapping analysis (Figure S2) shows a homogeneously distribution of N for the typical sample SFNC(800). The pore structures of the as-prepared products SFNC(T)s are characterized by N2 adsorption-desorption experiment. As shown in Figure 2D, the samples SFNC(800) and SFNC(900) exhibit type IV adsorption isotherms with H1 type hysteresis loops, indicative of their mesoporous structures.33 The BET surface area and pore volume of SFNC(700) attained at 700 oC are only 3.8 m2 g-1 and 0.004 cm3 g-1 (Table 1, entry 1), suggesting that few pores exist in the SFNC(700). When the carbonization temperature increases to 800 oC and 900 oC, the obtained SFNC(800) and SFNC(900) exhibit much larger surface areas of 71.6 m2 g-1 and 55.3 m2 g-1, and pore volumes of 0.044 cm3 g-1 and 0.043 cm3 g-1 (Table 1, entries 2 and 3), respectively, indicating the existence of inside mesopores in the SFNC(800) and SFNC(900). The smaller surface area of SFNC(900) than SFNC(800) may be due to the partial collapse of the 8


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pores in the carbon material at a higher carbonization temperature. Figure 2E gives the pore size distribution of SFNC(T)s calculated by the Barrett-Joyner-Halenda method. Relatively narrow pore size distributions are observed with the most probable pore sizes centered at 3.1 nm, 3.7 nm and 3.7 nm for SFNC(700), SFNC(800) and SFNC(900), respectively. These results show that SFNC(800) has the largest BET surface area with an abundant mesoporosity. Table 1. Textural properties and nitrogen contents for the SFNC(T)s, where T=700, 800, and 900 (the carbonization temperature, oC).

Pore volume (cm3 g-1)

Dpb (nm)

Total N content (at%)c

Entry

Catalyst

SBETa (m2 g-1)

1

SFNC(700)

3.8

0.004

3.1

2

SFNC(800)

71.6

0.044

3

SFNC(900)

55.3

0.043

The N species categories and corresponding contentsc graphitic N (401.3 ± 0.1 eV)

pyrrolic N (400.0 ± 0.1 eV)

pyridinic N (398.4 ± 0.3 eV)

7.29

1.01%

2.01%

4.27%

3.7

9.43

2.51%

4.36%

2.56%

3.7

6.13

2.23%

1.85%

2.05%

a

Specific surface area determined according to BET (Brunauer-Emmett-Teller) method.

b

The most probable pore diameter.

c

The data collecting from the results of XPS.

X-ray photoelectron spectroscopy (XPS) is used as a powerful technique for the characterization of elemental composition and bonding configuration on the materials surface with a depth of several nanometers.34 The contents and types of nitrogen species in these three SFNC(T)s samples are investigated by the XPS. As shown in Table 1, the total N contents (atomic percentages, at%) detected by the XPS are 7.29%, 9.43% and 6.13% for SFNC(700), SFNC(800) and SFNC(900), respectively, demonstrating that the SFNC(800) has the highest N content (Figure 3A). The high-resolution N1s spectra of these three samples are shown in Figure 3B, C and D, and a summary of the results is also listed in Table 1. For each sample, there are three sub-peaks centered at 401.3 ± 0.1 eV, 400.0 ± 0.1 eV and 398.4 ± 0.3 eV, corresponding to the graphitic N (denoted as N(g)), pyrrolic N (denoted as N(pl)) and 9



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pyridinic N(denoted as N(pd)) (Figure 3 BCD).14,34-37 The atomic percentages (at%) of each type of these N species are 1.01, 2.01 and 4.27% for SFNC(700), 2.51, 4.36 and 2.56% for SFNC(800) and 2.23, 1.85 and 2.05% for SFNC(900). It can be seen that the N(g) contents of the three samples follow the order (Figure 3A): SFNC(800)> SFNC(900)> SFNC(700); the N(pl) contents follow the order: SFNC(800)> SFNC(700)> SFNC(900); and the N(pd) contents follow the order: SFNC(700)> SFNC(800)> SFNC(900).

Figure 3. (A) the relationships between the carbonization temperature and the N species contents; High-resolution N 1s spectra of the SFNC(700) (B), SFNC(800) (C) and SFNC(900) (D) catalysts. (a) N(g), (b) N(pl) and (c) N(pd). Catalytic Assessments. Catalytic performances of SFNC(T)s, when combined with the V-containing POM catalyst Ch5PMoV2, toward reductant-free aerobic oxidation of benzene to phenol are investigated, as listed in Table 2. It can be seen that no phenol is observed by using the individual homogeneous catalyst H5PMoV2 or the phase-transfer catalyst Ch5PMoV2 (Table 2, entry 1). The same result is also observed by using SFNC(T)s as the 10


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single catalyst (Table 2, entry 2). When SFNC(T)s meet with Ch5PMoV2, the combined catalysts SFNC(700)-Ch5PMoV2, SFNC(800)-Ch5PMoV2 and SFNC(900)-Ch5PMoV2 dramatically afford the desired phenol yields of 5.5%, 11.2% and 9.4% (Table 2, entries 3-5). These results imply that the synergistic catalytic effect between SFNC(T)s and Ch5PMoV2 is very critical for achieving the reaction. It is noted that the phenol yield obtained over the SFNC(T)s-Ch5PMoV2 increases with the carbonization temperature raising from 700 oC to 800 oC,

then decreases at 900 oC. Therefore, SFNC(800) is regarded to be more active than SFNC(700) and SFNC(900).

Moreover, the Ch5PMoV2 dissolves into the reaction medium when the temperature reaches 120 oC; while after the reaction system cools down to room-temperature, the temperature-controlled Ch5PMoV2 precipitates as a solid (Figure S3),15 leading to a facile recycling process of the combined catalyst SFNC(800)-Ch5PMoV2. When using the homogeneous H5PMoV2 instead of Ch5PMoV2, the combined catalyst SFNC(800)-H5PMoV2 offers a lower phenol of 8.1% (Table 2, entry 6). Meanwhile, H5PMoV2 dissolves in the solvent and results in a complicated recycling procedure due to its homogeneous character. The N-free catalyst cellulose-C(800) is tested for comparison, and only 0.9% phenol yield is given over the combined catalyst cellulose-C(800)-Ch5PMoV2 (Table 2, entry 7), which is much lower than the ones offered by the N-containing SFNC(T)s-Ch5PMoV2. These results powerfully demonstrate that the N species in the SFNC(T)s play very key roles in gaining the desired activity. The temperature-controlled Ch5PMoV2 performs better than the homogeneous H5PMoV2, which is consistent with the previous report.15 Particularly, it is observed that the more graphitic-type N species in the SFNC(T)s catalysts afford the higher phenol yields, which would be discussed later. The reaction conditions are systematically optimized by adjusting the concentration of acetic acid aqueous solution, amount of the catalysts, amount of the additive LiOAc, reaction temperature, reaction time and initial oxygen pressure (Figure S4-S10). The effect of the concentration of acetic acid aqueous solution on the phenol yield is investigated as displayed in Figure S4. When pure water is used as the solvent, no phenol forms; then the 11



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yield of phenol increases from 0.5% to 11.2% as the acetic acid concentration increases from 20% to 50%. Further increase of the acetic acid concentration results in a decline in the phenol yield. The nonpolar benzene molecule hardly dissolves in water, causing the contact problem between the substrate benzene and the catalysts dispersed in water. It is known that acetic acid can dissolve benzene easily and the concentration of acetic acid aqueous solution affects the solubility of the substrate and the dispersion of the catalysts. As a result, the contact between the substrate and the catalytic species leads to the different catalytic performances. However, high concentration acetic acid aqueous solution usually aggravates excessive oxidation of phenol, leading to the decrease in the phenol yields at a high acetic acid concentration, which may be caused by radical processes.38 It is very interesting to see that no phenol is observed when glacial acetic acid is employed as the solvent even with the simultaneous addition of SFNC(800) and Ch5PMoV2.

Table 2. Reductant-free oxidation of benzene to phenol with O2 as oxidant over various catalysts. Entry

Catalysts

Phenol yield (%)

1

--

H5PMoV2 or Ch5PMoV2

0

2

SFNC (T)s (T=700, 800, 900)

--

0

3

SFNC (700)

Ch5PMoV2

5.5%

4

SFNC (800)

Ch5PMoV2

11.2%

5

SFNC (900)

Ch5PMoV2

9.4%

6

SFNC (800)

H5PMoV2

8.1%

7

cellulose-C(800)

Ch5PMoV2

0.9%

Reaction conditions: carbonaceous catalyst 0.01 g; Ch5PMoV2 (or H5PMoV2) 0.03 g; benzene 5 mmol; LiOAc 0.06 g; aqueous acetic acid solution (50%) 2 mL; O2 2.0 MPa; 120 oC; 3.0 h.

The influences of the catalysts amount on the phenol are shown in Figure S5 and S6. As seen, the highest phenol yield of 11.2% is obtained when using 0.01 g SFNC(800) and 0.03 g Ch5PMoV2 together. The phenol yield 12


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decreases with increasing the catalysts amount. In the oxidative environment, phenol is more reactive than the substrate benzene, thus the desired product phenol may be oxidized excessively during the reaction when the catalysts is overcommitted.6,33,39 The over-oxidation product may be tar which is undetectable by GC. Therefore, 0.01 g SFNC(800) and 0.03 g Ch5PMoV2 are used in the following tests. LiOAc is usually used as an additive agent for this reaction, 8,10,13,14,16 and its amount is optimized as shown in Figure S7. It can be seen that 2.9% phenol is still obtained in the absence of LiOAc. However, the resulted reaction mixture is dark, implying the formation of by-product tar resulted from the deep-oxidation. With the LiOAc amount increasing, the phenol yield increases continuously and reaches the maximum of 11.2% at 0.06 g of LiOAc; and meanwhile, the dark tar is not visualized any more. It is known that LiOAc can buffer the aqueous acetic acid and depresses the formation of the overpowering oxygen radicals in the reaction system, thus leads to the enhanced selectivity of the hydroxylation process.8,10,13,14,16 Nevertheless, using excess LiOAc (0.08 g) causes a slight decrease of phenol, which may be due to the depressed generation of reactive oxygen species. Similar trends are observed by varying the reaction temperature, reaction time and initial oxygen pressure (Figure S8-S10). Accordingly, the optimized conditions are revealed to be SFNC(800) 0.01 g, Ch5PMoV2 0.03 g, benzene 5 mmol, LiOAc 0.06 g, aqueous acetic acid solution (50%) 2 mL, O2 2.0 MPa, 120 oC, 3.0 h, under which SFNC(800)-Ch5PMoV2 offers the highest phenol yield of 11.2% in this work. The present SFNC(800)-Ch5PMoV2 gives a high turnover frequency (TOF) of 14.04 h-1, where TOF is expressed as mmol phenol/(mmol POM × h reaction time). This TOF value surpasses almost all the ones of other similar previously reported reductant-free LBTP-O2 systems, such as C3N4-Ch5PMoV2 (8.94 h-1, 120 oC, Table S1, entry 1),15 C3N4-H5PMoV2 (5.91 h-1, 130 oC, Table S1, entry 2)13 and [DiBimCN]2HPMoV2@NC (1.66 h-1, 140 oC, Table S1, entry 3)14. The TOF value of SFNC(800)-Ch5PMoV2 is even higher than the one of the noble metal-POM catalysis system Pd(OAc)2-PMoVx (4.09 h-1, Table S1, entry 4),8 and the ones of the POM-catalyzed systems with 13



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CO or ascorbic acid as the sacrificial reducing agents (1.50 h-1, 6.65 h-1 and 0.86 h-1, Table S1, entries 5-7).6,7,39 From a practical synthetic viewpoint, it is vital to evaluate the reusability of the catalyst. Thus, the reusability tests for the combined catalyst SFNC(800)-Ch5PMoV2 are conducted under the optimized reaction conditions. As seen in Figure 4, the phenol yields are 9.6%, 8.4% for the second and third reaction runs, and 7.2% phenol yield is still observed in the fourth reaction run. Compared with the previously reported similar catalytic systems for reductant-free LBTP-O2,10,11,13-16 SFNC(800)-Ch5PMoV2 offers a better or comparable reusability (Table S2). As shown in Figure S12, the FT-IR spectrum of the recovered Ch5PMoV2 shows nearly the same characteristic bands as the fresh Ch5PMoV2,15 indicating that the structure of the recovered Ch5PMoV2 retains well after the reactions. The XRD pattern and SEM images of the recycled SFNC(800) shown in Figure S13 are very similar to those for the fresh one, indicating that the structure of SFNC(800) changes little after being recycled. During the recover process of the catalysts by centrifugation for each run, inevitable loss of the catalysts was observed, which may be an important cause for the decrease in phenol yields.7 Moreover, the coverage of the active sites on the solid catalyst SFNC(800) by the formed by-product tar may be another cause for the decrease in phenol yields.13,15

Figure 4. Catalytic reusability of SFNC(800)-Ch5PMoV2 for aerobic oxidation of benzene to phenol; reaction conditions: benzene (5 mmol), aqueous acetic acid solution (2 mL, 50%), SFNC(800) (0.01 g), Ch5PMoV2 (0.03 g), LiOAc (0.06 g), O2 (2.0 MPa) 120 oC, 3 h. 14


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Insights into the catalytic behavior of SFNC(T)s. As evaluated above, SFNC(800)-Ch5PMoV2 effectively catalyzes the reductant-free LBTP-O2 reaction and the control experiments indicate that the N species contained in the SFNC(T)s are crucial for their activity. In order to study the microcosmic mechanism for the effect of the doped-N species in the present catalysts, DFT calculations were performed. Three models of the N-doped carbon moieties which respectively contain the N(g), N(pd) and N(pl) species are built and the optimized structures of adsorption benzene on the three moieties were shown in Figure 5. The calculated adsorption energies of these three structures were -5.73 eV, -3.81 eV, and -2.19 eV, respectively, indicating that benzene prefer to adsorbs on the N(g)-containing moiety than the other two. It is considered that this adsorption initiates the benzene activation via electron density transfer.28,40-42 Initially, the total Mulliken charge population of the six carbon atoms of benzene is calculated to be -1.74e;15 and after benzene adsorbs on the N(g), N(pd) and N(pl)-containing moieties, -1.83e, -1.80e, and -1.73e are respectively observed for the adsorbed benzene. That is, the electron density of the benzene ring increases when it adsorbs on the N(g) and N(pd)-containing moieties, while rarely changes on the N(pl)-containing moiety. The above results given by DFT calculations show that N(g)-containing moiety is the best to absorb and activate the substrate benzene in the present work. This well explains that the phenol yields obtained over the three combined catalysts of SFNC(700)-Ch5PMoV2, SFNC(800)-Ch5PMoV2 and SFNC(900)-Ch5PMoV2 follow the order of the N(g) contents in the three SFNCs catalysts, SFNC(800)> SFNC(900)> SFNC(700).

Figure 5. The optimized structures of adsorption benzene on the three N-doped carbon moieties with the N(g) (a), 15



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N(pd) (b) and N(pl) (c) species, respectively.

Figure 6. Proposed pathway for SFNC(800)-Ch5PMoV2-catalyzed benzene hydroxylation with O2. Based on the above experimental results, DFT calculations and previously reported studies,13-16 the present catalytic process is described in Figure 6. SFNC(800) and Ch5PMoV2 are solid before the reaction starts. At the initial stage of the reaction when the temperature reaches 120 oC, the phase-transfer catalyst Ch5PMoV2 dissolves in the reaction mixture. The substrate benzene is absorbed and catalytically activated by the N(g)-containing moiety in the solid SFNC(800); then the dissolved Ch5PMoV2 selectively oxidizes the activated benzene to phenol via transfer the lattice oxygen in its V-O-V structure, which is accompanied by the reduction of V5+ to V4+; finally, O2 oxidizes the V4+ back to V5+ and the V-O-V structure is regenerated to complete the catalytic cycle.15,43 Once the reaction mixture is cooled to room temperature, Ch5PMoV2 precipitates as a solid, which facilitates its recovery with the solid catalyst SFNC(800). It is clear that the synergistic catalytic effect between SFNC(800) and Ch5PMoV2 is critical for the present reaction going smoothly. A direct evidence for the proposed synergistic effect is that, when the glacial acetic acid is employed as the solvent, no phenol is detected even with the presence of SFNC(800)-Ch5PMoV2 as described before. This is because the catalyst Ch5PMoV2 hardly dissolves in the glacial acetic acid and behaves as a solid catalyst, which sequentially results in bad contact with the SFNC(800). The 16


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synergy between Ch5PMoV2 and SFNC(800) is thus disabled, and as a result, the catalytic cycle is breaks down. As mentioned above, it is interesting to observe that the present SFNC(800)-Ch5PMoV2 exhibits a higher catalytic activity than the recently reported C3N4-Ch5PMoV2 under the same reaction conditions.15 It is clear that the difference between the above two dual-catalysis systems is reasonably attributed to the difference between SFNC(800) and C3N4. By the DFT calculation, when benzene absorbs on the melem unites of C3N4, the adsorption energy is -0.91 eV, much lower than the one of -5.73 eV when benzene absorbs on the N(g)-containing moiety of SFNC(800). This result implies an easier adsorption of benzene on the latter one. For further comparison, the TOF values of the two combined catalysts of SFNC(800)-Ch5PMoV2 and C3N4-Ch5PMoV2 is calculated by another method : mmol of phenol/(g carbonaceous catalyst × h reaction time), which is denoted as TOF(b). As shown in Table S3, the TOF(b) value for SFNC(800)-Ch5PMoV2 is 18.67, which is about as two times as the one of 9.91 for C3N4-Ch5PMoV2. That is, the SFNC(800) performs better than C3N4 in activating the substrate benzene and consequently leads to a higher activity of the combined catalyst SFNC(800)-Ch5PMoV2. Besides, the BET surface area of SFNC(800) (71.6 m2 g-1) is larger than the one of C3N4 (8.4 m2 g-1),13,15 which promotes the mass transfer during the reaction and further improves the activity of SFNC(800)-Ch5PMoV2. To get more insights into the feature of the present SFNC(800)-Ch5PMoV2, it is further compared with other similar dual-catalysis systems for this reaction involving the previously reported combined catalyst C3N4-H5PMoV2, the heterogeneous composite catalysts [DiBimCN]2HPMoV2@NC and Fe2O3/NC. In the combined catalyst of C3N4-H5PMoV2, C3N4 is heterogeneous while H5PMoV2 dissolves in the reaction mixture and acts homogeneously.13 The homogeneous feature of H5PMoV2 enables the efficient contact between the two catalytic active sites of C3N4 and H5PMoV2, and consequently facilitates the synergistic effect between them.13 However, the recovery of the homogeneous H5PMoV2 for reuse involves a complicated process.13 By contrast, in the present combined catalyst of SFNC(800)-Ch5PMoV2, Ch5PMoV2 acts as a thermoregulated phase-transfer catalyst that is 17



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homogeneous during the reaction at a high temperature and becomes heterogeneous after the reaction at room-temperature. This merit of Ch5PMoV2 not only brings the efficient contact between Ch5PMoV2 and SFNC(800), but also largely facilitates the catalysts recovery. Furthermore, the modification effect of Ch+ cation on the PMoV2 anion enhances the redox ability of PMoV2 anion, which accounts for the higher catalytic activity of Ch5PMoV2 than H5PMoV2.15 As for the other two heterogeneous catalysts, [DiBimCN]2HPMoV2@NC and Fe2O3/NC, they are insoluble in the reaction mixture and could be recovered by simple centrifugation. However, higher reaction temperature (140 oC or 150 oC) and much longer reaction time (17 h or 30 h) are demanded.14,16 Even so, the TOFs of the two heterogeneous catalysts are only 1.66 h-1 for [DiBimCN]2HPMoV2@NC (mmol phenol / (mmol POM catalyst × h reaction time)) and 2.53 h-1 for Fe2O3/NC (mmol phenol / (mmol Fe2O3 × h reaction time)), implying their lower catalytic efficiencies than SFNC(800)-Ch5PMoV2. The two heterogeneous catalysts [DiBimCN]2HPMoV2@NC and Fe2O3/NC are fabricated with two solid components, i.e. the N-doped carbon and the [DiBimCN]2HPMoV2 or Fe2O3.14,16 The contact interface between the two solid components is considered to be the active site where the aforementioned synergistic effect takes places. Although the BET surface areas of the two composite catalysts of [DiBimCN]2HPMoV2@NC and Fe2O3/NC are relatively large (121 m2 g-1 and 274 m2 g-1),14,16 the desired contact interface may be not large enough to endow sufficient site for the synergistic effect to take place. By comparison, the phase-transfer catalyst Ch5PMoV2 is homogeneous during the reaction and contacts the solid catalyst SFNC(800) molecularly, which maximizes the contact interface and accounts for the high activity of SFNC(800)-Ch5PMoV2.

 CONCLUSIONS N-doped biomass carbon catalyst SFNC(800) is prepared by carbonizing the raw material of commercial soybean flour in N2 at 800 oC. The combined catalyst SFNC(800)-Ch5PMoV2 offers a high phenol yield of 11.2% with a high TOF value, and exhibits high potential for reusability with a facile recovery process. Based on the experiments 18


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and DFT calculations, the graphitic N species is regarded as the essential catalytic active site where benzene can be adsorbed and activated, and more graphitic N species afford the higher phenol yield in this work. The efficient contact between SFNC(800) and Ch5PMoV2 leads to the improved synergistic effect between the two catalysts, accounting for the high catalytic activity SFNC(800)-Ch5PMoV2. This work suggests that the plenty graphitic N species-containing N-doped carbons and the lattice oxygen-containing catalysts can work together for the reductant-free aerobic oxidation of benzene to phenol.

 ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website.

 AUTHOR INFORMATION Corresponding Authors *G. J. Chen, E-mail: [email protected] *L. M. Sun, Email: [email protected] ORCID Zhouyang Long: 0000-0002-5520-6263 Guojian Chen: 0000-0003-1773-8741 Liming Sun: 0000-0002-3819-8417 Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENTS We are grateful for financial support from the National Nature Science Foundation of China (Nos. 21503098, 21603089, 21476109, 21136005, and U1662107), TAPP, Jiangsu Province Science Foundation for Youths (BK20160209 and BK20150237), the Natural Science Foundation of the Jiangsu Higher Education Institutions of 19



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China (16KJB150014) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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TOC

A nitrogen-doped biomass carbon derived from soybean flour was used to activate benzene for its subsequent reductant-free hydroxylation to phenol.

25


Nitrogen-Doped Biomass Carbons Meet with Polyoxometalates

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