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Synergistic catalysis of Fe2O3 nanoparticles on mesoporous poly(ionic liquid) derived carbon for benzene hydroxylation with dioxygen Qin Qin, Yangqing Liu, Wanjian Shan, Wei Hou, kai wang, Xingchen Ling, Yu Zhou, and Jun Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02566 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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Industrial & Engineering Chemistry Research

Synergistic catalysis of Fe2O3 nanoparticles on mesoporous poly(ionic liquid) derived carbon for benzene hydroxylation with dioxygen Qin Qin, Yangqing Liu, Wanjian Shan, Wei Hou, Kai Wang, Xingchen Ling, Yu Zhou*, Jun Wang* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University (former Nanjing University of Technology), No. 5, Xinmofan Road, Nanjing, 210009, P. R. China E-mail: [email protected] (Y. Zhou); [email protected] (J. Wang) Tel: +86-25-83172264, Fax: +86-25-83172261

ABSTRACT: Carbon supported ferric oxides nanoparticles (Fe2O3 NPs) were constructed via directly carbonizing mesoporous poly(ionic liquid) with [Fe(CN)6]3- anions, which was prepared through the free radical self-polymerization of ionic liquid monomer 1-allyl-3-vinylimidazolium chloride in a soft-template route and successive ion-exchange with potassium ferricyanide. The unique mesoporous ionic networks enabled the molecular dispersion of ferric precursors and therefore resulted in highly dispersed Fe2O3 NPs on carbon. The catalyst exhibited high yield, remarkable turnover number and well reusability in the reductant-free aerobic oxidation of benzene to phenol with O2. The synergistic effect of Fe2O3 NPs and carbon accounted for above high performance. This work delivered the first efficient and environmentally-friendly heterogeneous catalyst for the reductant-free benzene hydroxylation with O2.

KEYWORDS:

poly(ionic

liquid),

iron-doped

carbon,

heterogeneous

hydroxylation

1

ACS Paragon Plus Environment

catalysis,

benzene


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1. INTRODUCTION Phenol is one of the most important chemicals for the production of resins, fungicides, pharmaceuticals and so on.1-6 The industrial production of phenol through traditional three-step cumene process suffered from high energy consumption, formation of by-product acetone and low phenol yield (5%).2,7 Benzene hydroxylation with O2 is one of the most attractive alternatives because of the high atom-efficiency and the easily available, cheap oxidant.2,8-14 Because both benzene and O2 are inert reactants, the present catalytic systems usually involved the uneconomic sacrificial reducing agents.9,12,13 More importantly, these catalysts normally relied on either expensive noble metals or poisonous transition metal of vanadium (V).9-14 The significant leaching caused rapid deactivation and serious potential pollution of the environment.9,11 Therefore, the fabrication of efficient, stable and non-toxic heterogeneous catalyst is still one challenge for the benzene hydroxylation with O2, particularly under reductant-free condition. Carbons are widely used catalyst supports with multi-advantages and can be produced from numerous precursors such as small molecules, polymers, fossil fuels and biomass based materials.9,15-20 In this context, ionic liquids (ILs) and poly(ionic liquid)s (PILs) are attractive carbon precursors that have drawn growing attentions. 18,19,21,22 Mesoporous PILs (MPILs) with IL moieties in the network combine the features of ILs, polymers and mesoporous materials.23,24 MPILs have high mechanical durability and thermal stability, and thus minimize the mass loss of precursors and exclude the potential complete decomposition of ILs under high temperature.21,22,24 Noticeably, molecularly dispersed metal precursors can be incorporated into MPILs as anions. This unique feature is lack for nonionic polymers and has been applied in the preparation of supported noble metal nanoparticles (NPs).24-28 Compared to PILs, MPILs are able to deliver internal porosity of metal-PILs precursors.26,27,29 and thus benefit the pore formation of subsequent carbons, which is important for the heterogeneous catalysis. Nonetheless, rare MPILs are used as the precursor of metal containing carbons. Herein, we report the synthesis of carbon supported Fe 2O3 NPs via carbonizing MPILs with ferricyanate [Fe(CN)6]3- anions. Among transition metals, Fe is generally regarded as one low-cost, abundant and relatively non-toxic active site to catalyze many reactions such as 2


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benzene hydroxylation with H2O2/N2O and oxidation of phenol.1,30,31 Nevertheless, none of Fe-based catalyst is involved in the aerobic oxidation of benzene to phenol with O 2, and Fe-containing carbon of this work served as the first environmentally benign robust heterogeneous catalyst in this reaction. MPIL precursor was prepared through the free radical self-polymerization of IL monomer by using a soft-template method, followed with the ion-exchange with potassium ferricyanide (K3[Fe(CN)6]) to incorporate molecularly dispersed Fe precursors. Carbon supported Fe2O3 NPs were achieved in a direct carbonization process. The catalyst was active in the benzene hydroxylation to phenol with O2. No reductant was involved in the reaction. Moreover, the catalyst can be facilely separated and reused with well reusability. A potential catalytic route was proposed to understand this catalytic process, indicating that the high performance was attributable to the synergistic catalysis of Fe2O3 NPs and carbon.

2. EXPERIMENTAL SECTION 2.1 Materials and methods All of the reagents were commercially available and used as received without further purification. Ionic liquid (IL) monomer 1-allyl-3-vinylimidazolium chloride ([AVIm]Cl, ≥99%) was purchased from Lanzhou Greenchem ILS, LICP, Chinese Academy of Sciences. Triblock copolymer poly(ethylene oxide)-block-poly(propylene oxide)-block-poly-(ethylene oxide) (EO20PO70EO20, P123; Mav=5800) was purchased from Sigma-Aldrich. Ammonium persulfate (APS, >98%) and potassium ferricyanide (K3[Fe(CN)6], >98%) was purchased from Shanghai Lingfeng Chemical Reagent CO., LTD. Elemental analyses were performed on a CHNS elemental analyzer Vario EL cube. Metal content were analyzed by an OPTMA 20000V inductively coupled plasma (ICP) spectrometer. Thermogravimetric (TG) analysis was performed with an STA409 instrument in oxygen atmosphere at a heating rate of 10°C min-1. The nitrogen sorption isotherms and pore size distribution curves were tested at 77 K on a BELSORP-MINI analyzer, the samples were degassed at 473 K to a vacuum of 10-3 Torr before analysis. The pore size distribution (PSD) curves were calculated from the adsorption branch of the isotherm using the 3



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Barrett-Joyner-Halenda (BJH) algorithm. X-ray diffraction (XRD) patterns from 5 to 50°(0.2° s-1) were collected on a Smart Lab diffractmeter from Rigaku equipped with a 9 kW rotating anode Cu source at 45 kV and 200 mA. Scanning electron microscopic (SEM) images were recorded on a Hitachi S-4800 field-emission scanning electron microscope. Energy Dispersive X-ray spectrometry (EDS) was obtained on this instrument with an acceleration voltage of 20 kV. Transmission electron microscopy (TEM) analysis was performed on a JEM-2100 (JEOL) electron microscope operating at 200 kV. The X-ray photoelectron spectra (XPS) was conducted on a PHI 5000 Versa Probe X-ray photoelectron spectrometer equipped with Al Karadiation (1486.6 eV). Electron spin-resonance (ESR) spectra were recorded on a Bruker EMX-10/12 spectrometer at X-band. Raman Spectra were obtained using a horiba HR 800 spectrometer. Temperature-programmed reduction (TPR) was carried out using JAPAN BELCAT-Analyzer.

2.2 Materials synthesis 2.2.1 Preparation of mesoporous poly(ionic liquid). Mesoporous poly(ionic liquid) was prepared by free radical self-polymerization of IL monomer [AVIm]Cl. Typically, 2 g P123, 1 g PEG 20000 and 1.7 g [AVIm]Cl were dissolved in water. The polymerization occurred at 40 °C for 10 h and then 50 °C for 14 h after the addition of APS as initiator. The solid was obtained by filtration. Template was removed by extracting the as-synthesized material in ethanol. The obtained sample was named as PAV.

2.2.2 Preparation of nonporous PIL PAV-N. Nonporous PIL PAV-N was synthesized by free radical self-polymerization of IL monomer [AVIm]Cl in the absence of P123 and PEG with the same synthetic procedure as PAV.

2.2.3 Preparation of MC-500. Directly carbonizing PAV at 500 °C for 2 h (2 °C/min) under nitrogen atmosphere leaded to a carbon material MC-500.

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2.2.4 Preparation of ferric functional carbons. Ferric functional carbons FeC(n)s, n denoted the Fe content, were prepared through the carbonization of [PAV][Fe(CN) 6], which was prepared through the anion-exchange of PAV with Fe precursor (K3[Fe(CN)6]). Typically, PAV (2 g) was mixed with an aqueous K3[Fe(CN)6] solution (50 mL, 0.36 g), and then stirred at room temperature for 48 h. After that, the solid [PAV][Fe(CN) 6] was separated by filtration and washed with water. Carbonizing [PAV][Fe(CN) 6] at 500 °C for 2 h (2 °C/min) under nitrogen atmosphere gave the carbon sample FeC(5). Other FeC(n) samples were prepared by varying the concentration of the aqueous K 3[Fe(CN)6] solution.

2.2.5 Preparation of Fe2O3@MC-500. Fe2O3@MC-500 was prepared through a conventional impregnation method by using MC-500 as the support. MC-500 and 30 mL aqueous K3[Fe(CN)6] solution were stirred at room temperature for 8 h, and then evaporated at 70°C. The obtained solid was calcined at 300 °C for 2 h (2 °C /min) under nitrogen atmosphere, giving the sample Fe2O3@MC-500 with the Fe content of 4.7 wt%.

2.2.6 Preparation of FeC-N. The preparation of nonporous carbon FeC-N was same as FeC(5) except by using PIL PAV-N.

2.3 Catalysis assess Hydroxylation of benzene with dioxygen (O2) was carried out in a temperature controllable pressured titanium reactor (100 mL) equipped with a mechanical stirrer. Typically, 0.2 g catalyst, 0.6 g LiOAc, 4 mL benzene and 25 mL solvent (an aqueous solution of acetic acid, 60 vol%) were mixed in the reactor, followed with charging 2.2 MPa O 2 at room temperature. The reaction was conducted at 150 °C for 30 h with vigorous stirring. After reaction, 1,4-dioxane was added as an internal standard. The product was analyzed by gas chromatograph (GC Agilent 7890B) equipped with a FID detector and a capillary column (HP-5, 30 m×0.25 mm×0.25 μm). Under the employed conditions, the phenol was the sole product detected by GC.

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Hot filtration was carried out by stopping the reaction at 25 h. After the filtration of the solid catalyst, the filtrate was further stirred at the reaction temperature for another 5 h. The liquid phase was monitored by GC at the reaction time of 25, 26, 28, and 30 h, respectively. The reusability was assessed in a five-run recycling test. After each run, the catalyst was recovered by centrifugation, washed with aqueous acetic acid to remove the potential adsorbed organic compounds, dried in vacuum and then charged into the next run.

Scheme 1. Synthetic route and proposed pathway for benzene hydroxylation with dioxygen.

3. RESULTS AND DISCUSSION 3.1 Catalysts preparations and characterization Scheme 1 illustrates the synthetic procedure involving the polymerization of IL, anion-exchange and carbonization. MPIL precursor PAV was prepared through a free radical polymerization of IL monomer [AVIm]Cl by using P123 as a soft template. PEG-20000 was used as a co-template to improve the porosity. The target MPIL PAV exhibited the similar 6


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C/N molar ratio to that of IL precursor, suggesting complete removal of the template (Table S1). The fact is also reflected by the clear difference in TG curves of PAV and its template-bearing precursor (Figure S1). Fe-containing MPIL [PAV][Fe(CN)6] was prepared through the ion-exchange of PAV with potassium ferricyanide (K3[Fe(CN)6]). PAV and [PAV][Fe(CN)6] exhibited type IV nitrogen sorption isotherm, indicating that they have abundant meso-porosity with the surface area of 100-200 m2 g-1 (Figure S2-S3 and Table S2). SEM and elemental mapping images revealed a high dispersion of ferric precursors on [PAV][Fe(CN)6] (Figure S4 and S5). Carbon supported Fe2O3 NPs were obtained by directly carbonizing [PAV][Fe(CN)6].

Figure 1. (A) SEM, (B) EDS elemental (Fe, O and N) mapping images of FeC(5), TEM images of (C) FeC(1), (D) FeC(3), (E) FeC(5), (F) FeC(7).

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Varying Fe content in [PAV][Fe(CN)6] precursors afforded a series of Fe-containing carbons, which were named as FeC(n)s (n denotes the target Fe content, wt%). The chemical compositions of FeC(n)s were measured by ICP and CHN elemental analysis (Table S1), validating the formation of Fe-containing carbons with varied Fe content. SEM images indicated that FeC(n)s were irregular aggregations of micrometer-leveled primary particles (Figure 1A and Figure S6-8). Elemental mapping analyses (Figure 1B) shown the homogeneously distribution of Fe, O and N for the typical sample FeC(5). TEM image of FeC(n)s (Figure 1C-F) showed no obvious metal oxides particles, indicating the highly dispersion of iron oxides nanoparticles. FeC(n)s shown the type I+II isotherms (Figure 2A), revealing the existence of micro-meso-macroporosity. The pore size distribution curves (Figure 2B) further revealed the existence of mesopores with a pore size distribution centered at 2-4 nm. FeC(n)s have the large surface area and pore volume and the surface area and pore volume are decreased with the increasing of Fe content (Table 1). By contrast, carbon (FeC-N) obtained from the nonporous PIL (PAV-N) presented much low surface area (5 m2 g-1, Figure S9, Table S2),

Figure 2. (A) N2 sorption isotherms and (B) pore size distribution curves. 8


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indicating that the porosity of MPIL is important for the pore formation of the carbons. XRD pattern of each FeC(n) exhibited a inter-plane (002) diffraction peak at around 25°,19,32 attributable to the graphitic carbon (Figure 3A). No observable signal for Fe2O3 crystal and other impurities reflected the high dispersion of Fe2O3 NPs, which is consistent with TEM images. ESR spectra of FeC(n)s (Figure 3B) showed a narrow signal at g≈2 attributable to the antiferromag-netically coupled Fe(III) oxides.33,34 The defect densities were detected by Raman spectra (Figure 3C). The peak near 1352 cm-1 is attributable to the disordered graphite structure (D-band). The peak at high wavenumber near 1565 cm-1 (G-band) corresponds to splitting of the E2g stretching mode of graphite, which reflects the structural intensity of the SP 2-hybridized carbon atoms.35,36 The relative intensity of D and G modes (ID/IG) were calculated to demonstrate the variety of defect density. The ID/IG values of FeC(n) series firstly increased and then decreased along with the increase of Fe content, reaching the maximum (1.32) for FeC(5). This phenomenon indicates that FeC(5) possesses the highest defect density. Figure 3D shows the H2-TPR curves of FeC(n)s, revealing their redox properties. Each sample presented two peaks, in which the one at low temperature region is related to the reduction of Fe2O3 to Fe3O4, while high temperature

Figure 3. (A) XRD patterns, (B) ESR spectra, (C) Raman spectra and (D) TPR curves. 9



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Figure 4. (A) C 1s, (B) Fe 2p, (C) N 1s and (D) O1s XPS spectra.

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region is assigned to the reduction of Fe3O4 to FeO. FeC(5) shown the lowest reduction temperature, indicating Fe3+ species in this sample are most easily to be reduced. The surface chemical composition and the nature of chemical bonding of constituent elements of FeC(n)s were investigated by XPS analysis (Figure 4 and Figure S10). Full scan XPS spectra indicated the existence of core levels of C, Fe, N and O (Figure S10). The high resolution C 1s XPS spectra (Figure 4A) was fitted with three peaks at around 284.8 eV, 286.2 eV and 288.9eV, corresponding to C-C, C-O and C=O/C=N respectively, which are in agreement with previous reports.37,38 Two obvious sets of doublet peak corresponding to Fe2p3/2 and Fe2p1/2 were observed at relatively high binding energies in the Fe2p XPS spectrum of each FeC(n) sample (Figure 4B), revealing the presence of Fe3+ originating from Fe2O3. This is further confirmed by the satellite peak at about 719 eV. 39,40 FeC(n) series displayed varied binding energy of Fe3+ species, with the highest value for FeC(5). The high resolution N 1s XPS spectra (Figure 4C) revealed the presence of pyridinic-N, pyrrolic-N and graphitic-N at around 398, 400 and 401 eV, respectively. 41,42 Because of the very small difference between the binding energies of pyridinic-N and Fe–N, the signal around 398 eV may include a contribution from Fe–N.42,43 The high resolution O 1s XPS spectra (Figure 4D) was fitted with three peaks at around 530, 532 and 533.5 eV corresponding to Fe-O, C-O/C=O and C-O-C, respectively.40 FeC(5) shows the lowest binding energy of Fe-O, in line with the highest binding energy in the Fe 2p XPS spectra. This phenomenon suggests the highest mobility of oxygen species for FeC(5), which is further reflected by the lowest reduction temperature of FeC(5) in the H2-TPR curves (Figure 3D). This can be explained by the variation of defect density in these FeC(n) samples. Defects corresponding to N-doping have a profound impact on the electronic transport properties and produce acceptor-like states within graphitic materials.10 The high defect density can promote the electron transfer from Fe to the defect sites, which enables more positive Fe and the easier reduction of O species in Fe2O3 NPs. The above results indicate that varying Fe content in [PAV][Fe(CN)6] can facilely adjust both the content and electronic state of Fe 2O3 NPs plus the defect density of carbons. FeC(n) series were used as heterogeneous catalysts in the aerobic hydroxylation of benzene to phenol under reductant-free condition (Table 1 and S3). (It should be pointed out that herein the reductant-free condition is specified that no external sacrificial reducing agent 11



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such as ascorbic acid is added). The spontaneous reaction without a catalyst gave no phenol yield (Table 1, entry 1). Fe2O3 NPs or Fe-free porous carbon MC-500 obtained from the carbonization of PAV (the structure information was seen in Figure S11-S18) alone gave the low phenol yield of 1.3% and 2.1% respectively. FeC(n) effectively catalyzed the oxidation of benzene (entries 4-7), reflecting a synergistic catalysis of Fe 2O3 NPs and carbon. FeC(5) afforded a delightful phenol yield of 14.2% (TON: 76), about 35% higher than previous best catalyst [DiBimCN]2HPMoV2@NC-580 (yield: 10.5%). Highest TON of 148 was achieved by using FeC(1), more than ten times of the one over [DiBimCN] 2HPMoV2@NC-580 (14.1).10 Though various catalytic systems have been fabricated,9,14,44-46 the yield and TON over FeC(5) were even higher than all the previous catalysts involving reductants or noble metal catalysts under similar reaction conditions.9-13 Table 1. Aerobic oxidation of benzene to phenola Entry 1 2g 3 4 5 6 7 a

Catalyst None Fe2O3 NPs MC-500 FeC(1) FeC(3) FeC(5) FeC(7)

Feb (wt%) 0 70 0 0.8 2.6 4.7 6.6

SBETc (m2 g-1) 426 425 308 274 152

Vpd (cm3 g-1) 0.26 0.29 0.28 0.23 0.07

Yielde (%) 0 1.3 2.1 4.7 9.1 14.2 11.9

TONf 7 148 88 76 44

Reaction conditions: 45 mmol (4 mL) benzene, 25 mL aqueous solution of acetic acid (60

vol%), 0.2 g catalyst, 0.6 g LiOAc, 2.2 MPa O2, 150 oC, 30 h. bFe content. cBET surface area. d

Total pore volume. eThe yield of phenol. fTurnover number (TON): [mmol (phenol)]/[mmol

(Fe2O3)]. gReaction condition: 13.4 mg Fe2O3 NPs with the average size of 30 nm.

Activities of FeC(5) under different reaction conditions were investigated by varying the reaction temperature, catalyst dosage, oxygen pressure, reaction time, lithium acetate (LiOAc) dosage and solvent composition (Figure 5). The yield as a function of reaction temperature displayed a first increase and then decrease trend, reaching the highest phenol yield of 14.2% at 150 °C (Figure 5A). Figure 5B showed the influence of catalyst dosage, revealing that the moderate amount catalyst gave the highest phenol yield. Figure 5C indicated the activity was 12


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sensitive to the oxygen pressure and the optimal one was 2.2 MPa. The yield of phenol firstly increased and then decreased along with reaction time (Figure 5D). The decline of the phenol yield at long reaction time is assigned to the over-oxidation of formed phenol. Figure 5E demonstrated that the concentration of LiOAc affected the activity. However, the yield of phenol was 8.9% in the absence of LiOAc, suggesting that LiOAc in this reaction acts as an

Figure 5. Influence of (A) temperature, (B) catalyst dosage, (C) oxygen pressure, (D) reaction time (dotted line: the hot filtration result), (E) LiOAc dosage and (F) HAc concentration on aerobic oxidation of benzene to phenol over FeC(5). Reaction conditions: 45 mmol (4mL) benzene, 25 mL aqueous solution of acetic acid, 0.2 g catalyst, 0.6 g LiOAc, 2.2 Mpa O2, 150 °C, 30 h. 13



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Figure 6. (A) Reusability of FeC(5) in the aerobic oxidation of benzene to phenol. Reaction conditions: 4 mL benzene, 25 mL aqueous solution of acetic acid (60 vol%), 0.2 g catalyst, 0.6 g LiOAc, 2.2 MPa O2, 150 °C, 30 h. (B) N2 sorption isotherm (the inset figure is the corresponding pore size distribution curve), (C) TEM image, (D) EDS elemental mapping images, (E) SEM image, (F) XRD pattern of recovered FeC(5) after 5th run.

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efficient additive. LiOAc acted as the buffer agent, reducing the overpowering oxygen radicals, and therefore enhances the selective hydroxylation process. 47,48 Figure 5F displayed the affection of the solvent composition and indicated that a suitable aqueous solution of acetic acid is important for high activity. Further, in a separate test, the reaction was stopped at 25 h by an immediate hot filtration to remove the catalyst. The yield kept unchanged with further reaction of the filtrate (Figure 5D). No detectable Fe by ICP was found in the ﬁltrate. All of these confirm the heterogeneous nature and satisfactory stability of FeC(5). Reusability of FeC(5) was investigated in a five-run recycling test. The catalyst was recovered conveniently by centrifugation after each run. The result indicated that FeC(5) was reused with slight deactivation (Figure 6A). A high yield of 10.1% was obtained in the 5 th run. Such reusability is greatly superior to previous catalytic systems. 10,11 The recovered catalyst had the similar porosity (Figure 6B) as the fresh one. TEM and elemental mapping images (Figure 6C, D) of recovered FeC(5) after 5th run indicates that Fe2O3 NPs were still highly dispersed on carbon. The Fe content of the recovered FeC(5) after 5th run was almost same as the one of the fresh catalyst. All of these account for the well reusability. The slight deactivation comes from the weak aggregation of Fe 2O3 NPs, as demonstrated by the XRD pattern (Figure 6F). The remarkable performance of FeC(5) is closely related to the employment of MPIL as the carbon precursor. The [Fe(CN)6]3- anions incorporated into a porous ionic network through ion-exchange enables the molecularly dispersed Fe precursors. The strong interaction between the [Fe(CN)6]3- anions with the imidazolium cations improves the high dispersion and stabilization of Fe2O3 NPs. For comparison, Fe2O3@MC-500 was prepared through a conventional impregnation method by using MC-500 as the support and K3[Fe(CN)6] as the Fe source. Fe2O3@MC-500 has the large surface area plus similar electronic state and content of Fe2O3 NPs as the one of FeC(5) (Figure S19-S29 and Table S1-S2). Nonetheless, inferior yield (11.0%, Table S3, entry 1) and significant deactivation during the recycling test (Figure S30) were observed, due to the aggregation and irregular dispersion of Fe 2O3 NPs with weak affinity towards the support. Further, FeC-N prepared from the nonporous MPIL shown a much low yield (8.1%, Table S3, entry 2). These results strongly reflect the advantage of MPIL towards high performance carbon catalyst. The activity evaluation of FeC(n)s by using 15



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the same molar ratio of benzene to Fe indicated that FeC(5) still had the best performance (Table S3, entry 3-5), suggesting that the Fe state significantly affects the activity. This phenomenon suggests that the electronic state of Fe 2O3 can be facilely adjusted by varying the concentration of Fe anions in the MPIL precursor to reach a high performance. To gain a deep insight, the reaction was carried out under nitrogen atmosphere. No phenol was formed by using carbon (Table S3, entry 6), while FeC(5) afforded a low phenol yield of 0.6% (Table S3, entry 7), suggesting that the lattice O species take part into the reaction. This is further reflected by comparing the XPS spectra of the recovered FeC(5) isolated from the O2 and N2-mediated reaction (Figure S31-S40). Fe2+ species and decreased lattice O species (530.6 eV) were observable (Figure S32 and S34), in the XPS spectra of the recovered FeC(5) after reaction under N2 atmosphere. By contrast, XPS analyses (Figure S37) indicated that the recovered sample after the reaction under O2 atmosphere returned back to its initial highest valence state (Fe3+). Besides, no deactivation was observed in the presence of either superoxide radical scavenger of butylated hydroxytoluene (Table S3, entry 8) or hydroxyl radical quencher of isobutanol (Table S3, entry 9). By contrast, phenol was produced with a low yield of 4.3% (Table S3, entry 10) in the presence of hole trap of hydroquinone. These phenomena indicate that the lattice O in the Fe–O–Fe units is involved in the catalysis process, causing the reduction of Fe3+–O–Fe3+ to Fe2+–□–Fe2+ (□ represents an oxygen vacancy).50 According to previous studies,10,51 carbon with a huge open π-electron system can activate benzene through chemical adsorption, which can be strengthened by N-doping. Owing to such sufficient activation, MC-500 alone afforded the phenol yield of 2.1% under O2 atmosphere. Based on these phenomena and previous corresponding studies, 10,52 a potential reaction pathway following Mars-van Krevelen mechanism is proposed for FeC(5) catalyzed aerobic oxidation of benzene to phenol (Scheme 1). Firstly, benzene is adsorbed on the surface of carbon with sufficient activation. Immediately, the lattice O in Fe 3+–O–Fe3+ attacks the activated benzene to produce phenol. The formed Fe2+–□–Fe2+ is further oxygenated to Fe3+–O–Fe3+ by O2 to complete the catalytic cycle. The O2 acts as the final oxygen source, which is reflected by the low yield of FeC(5) in the absence of O 2 and XPS analysis (Fe2+–□–Fe2+ species was observed in the recovered catalyst from N2 mediated reaction while absent in the recovered catalyst from O2 mediated reaction). 16


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Both benzene and O2 are inert reactant, therefore the benzene hydroxylation with O2 requires the simultaneously activation of them. According to the results of N2 mediated reaction and XPS analysis of the recovered catalysts, O2 is activated by Fe2O3 NPs through the active sites of Fe3+/Fe2+ ion pair in this reaction. However, merely Fe2O3 NPs cannot effectively catalyze the benzene hydroxylation. For example, Fe2O3 NPs with the relatively uniform average size of 30 nm was almost inactive in the reaction. The yield was only 1.3% (Table 1, entry 2), even inferior to the one (11.0%, Table S3, entry 1) by using Fe2O3@MC-500 (supported Fe2O3 NPs on carbon), which contains irregular dispersion of Fe2O3 NPs with the size from several to ten of nanometers (Figure S24). This phenomenon suggests that carbon support was crucial for the activation of benzene. Owing to the lack of such assistance, Fe2O3 NPs alone was inert in the reaction. According to previous related studies,10,51 the graphite-like structure in carbons has a huge open π-electron system, providing the driving force for chemical adsorption of benzene. The π-π interaction between the carbon and benzene causes the sufficient activation of benzene that can be subsequently oxidized by Fe3+–O–Fe3+. As aforementioned, without activation of O2 by Fe3+/Fe2+ ion pair, carbon itself was also inactive in the reaction. Moreover, the phenol yield was 1.3% over the neat carbon, while 2.1% over Fe2O3 NPs. By contrast, FeC(5) showed the phenol yield of 14.2%. Such catalysis observation that the phenol yield of 14.2% over FeC(5) was dramatically much higher than the simple summation (3.3%) of those over carbon and Fe2O3 NPs, strongly suggests the existence of synergistic effect between Fe and carbon sites in the present composite catalyst of FeC(5). In previous works,10,49,53,54 defects in carbons from the electron-rich N species in the graphite domains can accelerate the reaction by offering enhanced electron transfer interaction between the graphene sheets of carbons and the benzene ring. As a result, the catalyst FeC(5) with high defect density (as demonstrated by the Raman spectra) favored high activity. Besides, FeC(5) has the highest mobility of O species (as reflected by H2-TPR and XPS analysis), which also accounts for its high activity according to the proposed mechanism.

4. CONCLUSION In conclusion, carbon supported Fe2O3 NPs were constructed via carbonizing the MPIL 17



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with [Fe(CN)6]3- anions. MPIL enables the high dispersion of molecularly dispersed Fe precursor with strong affinity to the support, delivering stable and highly dispersed Fe2O3 NPs. The synergistic catalysis of Fe2O3 NPs and carbon endowed the high yield/TON and well reusability in the oxidation of benzene to phenol with O 2 by using no reductant. This work provides the first efficient and environmentally benign heterogeneous catalyst for the reductant-free benzene hydroxylation with O2, and highlights the potential of MPILs as charming carbon precursors.

SUPPORTING INFORMATION The supporting Information is available free of charge on the ACS Publications website at DOI: Additional characterizations for catalytic materials and various controls (Figure S1-S40), and textural properties, element analysis and additional catalysis results of aerobic oxidation of benzene to phenol with O2 (Table S1-S3).

AUTHOR INFORMATION Corresponding Authors J. Wang. E-mail: [email protected]. Y. Zhou. E-mail: [email protected].

ACKNOWLEDGMENTS

The authors thank greatly the National Natural Science Foundation of China (Nos. 21476109, U1662107, 21136005 and 21303084), Jiangsu Provincial Science Foundation for Youths (No. BK20130921), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20133221120002) and PAPD. REFERENCES (1) Meng, L. Q.; Zhu, X. C.; Hensen, E. J. M. Stable Fe/ZSM‑5 Nanosheet Zeolite Catalysts for the Oxidation of Benzene to Phenol. ACS Catal. 2017, 7, 2709-2719. 18


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