Synergistic catalysis of Fe2O3 nanoparticles on ... - ACS Publicationshttps://www.researchgate.net/.../Synergistic-Catalysis-of-Fe2O3-Nanoparticles-on-Mesop...

Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 12289-12296

pubs.acs.org/IECR

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* 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, PR China S Supporting Information *

ABSTRACT: Carbon-supported ferric oxide 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-3vinylimidazolium 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 good reusability in the reductant-free aerobic oxidation of benzene to phenol with O2. The synergistic effect of Fe2O3 NPs and carbon accounted for the high performance. This work delivered the first efficient and environmentally friendly heterogeneous catalyst for reductant-free benzene hydroxylation with O2. (NPs).24−28 Compared to PILs, MPILs are able to deliver internal porosity of metal−PIL precursors26,27,29 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 Fe2O3 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 nontoxic active site to catalyze many reactions such as benzene hydroxylation with H2O2/N2O and oxidation of phenol.1,30,31 Nevertheless, none of the Fe-based catalysts are involved in the aerobic oxidation of benzene to phenol with O2, and the Fe-containing carbon of this work served as the first environmentally benign robust heterogeneous catalyst in this reaction. The 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. Carbonsupported 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 good 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.

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 threestep cumene process suffered from high energy consumption, formation of byproduct 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 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 nontoxic heterogeneous catalysts is still a challenge for the benzene hydroxylation with O2, particularly under reductant-free conditions. Carbons are widely used catalyst supports with many advantages and can be produced from numerous precursors such as small molecules, polymers, fossil fuels, and biomassbased materials.9,15−20 In this context, ionic liquids (ILs) and poly(ionic liquid)s (PILs) are attractive carbon precursors that have drawn growing attention.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 lacking for nonionic polymers and has been applied in the preparation of supported noble metal nanoparticles © 2017 American Chemical Society

Received: Revised: Accepted: Published: 12289

June 22, 2017 September 30, 2017 October 3, 2017 October 3, 2017 DOI: 10.1021/acs.iecr.7b02566 Ind. Eng. Chem. Res. 2017, 56, 12289−12296

Article

Industrial & Engineering Chemistry Research

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 was analyzed by an OPTMA 20000 V 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 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 microscopy (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 spectroscopy (XPS) was conducted on a PHI 5000 Versa Probe X-ray photoelectron spectrometer equipped with Al Kα radiation (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 a 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 of P123, 1 g of PEG 20000, and 1.7 g of [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 assynthesized material in ethanol. The obtained sample was named 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 led to carbon material MC-500. 2.2.4. Preparation of Ferric Functional Carbons. Ferric functional carbons FeC(n)s, where n denotes 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 the mixture was 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 K3[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 of 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 that of FeC(5) except for the use of PIL PAV-N. 2.3. Catalysis Assessment. 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 of catalyst, 0.6 g of LiOAc, 4 mL of benzene, and 25 mL of solvent (an aqueous solution of acetic acid, 60 vol %) were mixed in the reactor, followed with charging 2.2 MPa O2 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 a gas chromatography (GC) instrument (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. 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. 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.

3. RESULTS AND DISCUSSION 3.1. Catalyst Preparation 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 cotemplate to improve the porosity. The target MPIL PAV exhibited a C/N molar ratio similar 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 (Figures S2 and S3 and Table S2). SEM and elemental mapping images revealed a high dispersion of ferric precursors on [PAV][Fe(CN)6] (Figures S4 and S5). Carbon12290

DOI: 10.1021/acs.iecr.7b02566 Ind. Eng. Chem. Res. 2017, 56, 12289−12296

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthetic Route and Proposed Pathway for Benzene Hydroxylation with Dioxygen

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

supported Fe2O3 NPs were obtained by directly carbonizing [PAV][Fe(CN)6]. 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 Fecontaining carbons with varied Fe content. SEM images indicated that FeC(n)s were irregular aggregations of micrometer-leveled primary particles (Figures 1A and S6−S8). Elemental mapping analyses (Figure 1B) showed 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 oxide particles, indicating the high dispersion of iron oxide nanoparticles. FeC(n)s showed type I and II isotherms (Figure 2A), revealing the existence of micromeso-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 large surface area and pore volume, and the surface area and pore volume decreased with increasing Fe content (Table 1). By contrast, carbon (FeC-N) obtained from the nonporous PIL (PAV-N) presented much lower surface area (5 m2 g−1; Figure S9 and Table S2), indicating that the porosity of MPIL is important for the pore formation of the carbons. XRD patterns of each FeC(n) exhibited a interplane (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 antiferromagnetically 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

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

cm−1 (G-band) corresponds to splitting of the E2g stretching mode of graphite, which reflects the structural intensity of the sp2-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 the FeC(n) series first 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 in the low-temperature 12291


Article

Industrial & Engineering Chemistry Research Table 1. Aerobic Oxidation of Benzene to Phenola entry 1 2g 3 4 5 6 7

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)

Vpd (cm3 g−1)

− −

− −

426 425 308 274 152

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

a

Reaction conditions: 45 mmol (4 mL) benzene, 25 mL of aqueous solution of acetic acid (60 vol %), 0.2 g of catalyst, 0.6 g of LiOAc, 2.2 MPa O2, 150 °C, 30 h. bFe content. cBET surface area. dTotal pore volume. eThe yield of phenol. fTurnover number (TON): [mmol (phenol)]/[mmol (Fe2O3)]. gReaction conditions: 13.4 mg of Fe2O3 NPs with the average size of 30 nm.

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

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

region is related to the reduction of Fe2O3 to Fe3O4, while the high-temperature region is assigned to the reduction of Fe3O4 to FeO. FeC(5) showed the lowest reduction temperature, indicating Fe3+ species in this sample are most easily reduced. The surface chemical composition and the nature of chemical bonding of constituent elements of FeC(n)s were investigated by XPS analysis (Figures 4 and 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) were fitted with three peaks at around 284.8, 286.2, and 288.9 eV, 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 peaks corresponding to Fe 2p3/2 and Fe 2p1/2 were observed at relatively high binding energies in the Fe 2p 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) were 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 Fe2O3 NPs plus the defect density of carbons. FeC(n) series were used as heterogeneous catalysts in the aerobic hydroxylation of benzene to phenol under reductantfree conditions (Tables 1 and S3). (It should be pointed out that herein the reductant-free condition specifies that no external sacrificial reducing agent such as ascorbic acid is added). The spontaneous reaction without a catalyst gave no 12292


Article


demonstrates 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 efficient additive. LiOAc acted as the buffer agent, reducing the overpowering oxygen radicals and therefore enhancing the selective hydroxylation process.47,48 Figure 5F displays the effect of the solvent composition and indicates that a suitable aqueous solution of acetic acid is important for high activity. Furthermore, in a separate test, the reaction was stopped at 25 h by an immediate hot filtration to remove the catalyst. The yield remained unchanged with further reaction of the filtrate (Figure 5D). No detectable Fe by ICP was found in the filtrate. All of these results 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

phenol yield (Table 1, entry 1). Fe2O3 NPs or Fe-free porous carbon MC-500 obtained from the carbonization of PAV (the structure information is provided in Figures 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 Fe2O3 NPs and carbon. FeC(5) afforded a delightful phenol yield of 14.2% [turnover number (TON): 76], about 35% higher than that of the previous best catalyst [DiBimCN]2HPMoV2@NC-580 (yield: 10.5%). Highest TON of 148 was achieved by using FeC(1), more than ten times that of [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 those of all previous catalysts involving reductants or noble metal catalysts under similar reaction conditions.9−13 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

Figure 5. Influence of (A) temperature, (B) catalyst dosage, (C) oxygen pressure, (D) reaction time (red 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 (4 mL) benzene, 25 mL of aqueous solution of acetic acid, 0.2 g of catalyst, 0.6 g of LiOAc, 2.2 MPa O2, 150 °C, 30 h.

Figure 6. (A) Reusability of FeC(5) in the aerobic oxidation of benzene to phenol. Reaction conditions: 4 mL of benzene, 25 mL of aqueous solution of acetic acid (60 vol %), 0.2 g of catalyst, 0.6 g of 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 the fifth run.

yield as a function of reaction temperature displayed a first increasing and then decreasing trend, reaching the highest phenol yield of 14.2% at 150 °C (Figure 5A). Figure 5B shows the influence of catalyst dosage, revealing that the moderate amount of catalyst gave the highest phenol yield. Figure 5C indicates the activity was sensitive to the oxygen pressure, and the optimal value was 2.2 MPa. The yield of phenol first increased and then decreased along with reaction time (Figure 5D). The decline of the phenol yield at long reaction time is assigned to the overoxidation of formed phenol. Figure 5E

obtained in the fifth run. Such reusability is greatly superior to that of previous catalytic systems.10,11 The recovered catalyst had porosity (Figure 6B) similar to that of the fresh catalyst. TEM and elemental mapping images (Figure 6C,D) of recovered FeC(5) after the fifth run indicate that Fe2O3 NPs were still highly dispersed on carbon. The Fe content of the recovered FeC(5) after the fifth run was almost the same as the that of the fresh catalyst. All of these account for the good reusability. The slight deactivation comes from the weak 12293


Article


acts as the final oxygen source, which is reflected by the low yield of FeC(5) in the absence of O2 and XPS analysis (Fe2+−□−Fe2+ species were observed in the recovered catalyst from N2-mediated reaction, but they were absent in the recovered catalyst from O2-mediated reaction). Both benzene and O2 are inert reactants; therefore, the benzene hydroxylation with O2 requires the simultaneously activation of them. According to the results of the 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 were almost inactive in the reaction. The yield was only 1.3% (Table 1, entry 2), even inferior to the that (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 tens 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 were 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 sufficient activation of benzene that can be subsequently oxidized by Fe3+−O−Fe3+. As aforementioned, without activation of O2 by the Fe3+/Fe2+ ion pair, carbon itself was also inactive in the reaction. Moreover, the phenol yield was 1.3% over the neat carbon, while it was 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 reports,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.

aggregation of Fe2O3 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 enable 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, Fe 2O 3@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 a large surface area and electronic state and content of Fe2O3 NPs that are similar to those of FeC(5) (Figures S19−S29 and Tables 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 Fe2O3 NPs with weak affinity toward the support. Furthermore, FeC-N prepared from the nonporous MPIL showed a much lower yield (8.1%, Table S3, entry 2). These results strongly reflect the advantage of MPIL toward highperformance carbon catalyst. The activity evaluation of FeC(n)s by using the same molar ratio of benzene to Fe indicated that FeC(5) still had the best performance (Table S3, entries 3−5), suggesting that the Fe state significantly affects the activity. This phenomenon suggests that the electronic state of Fe2O3 can be facilely adjusted by varying the concentration of Fe anions in the MPIL precursor to reach a high performance. To gain deeper insight, the reaction was carried out under a 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 in the reaction. This is further reflected by comparing the XPS spectra of the recovered FeC(5) isolated from the O2- and N2-mediated reactions (Figures S31−S40). Fe2+ species and decreased lattice O species (530.6 eV) were observed (Figures 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+). In addition, 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 a 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+ (where □ 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 the Mars−van Krevelen mechanism is proposed for FeC(5) catalyzed aerobic oxidation of benzene to phenol (Scheme 1). First, benzene is adsorbed on the surface of carbon with sufficient activation. Immediately, the lattice O in Fe3+−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

4. CONCLUSION Carbon-supported Fe2O3 NPs were constructed via carbonizing MPIL 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 good reusability in the oxidation of benzene to phenol with O2 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 attractive carbon precursors.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02566. 12294


Article


■

(12) Long, Z. Y.; Zhou, Y.; Chen, G. J.; Zhao, P. P.; Wang, J. 4,4′Bipyridine-modified molybdovanadophosphoric acid: A reusable heterogeneous catalyst for direct hydroxylation of benzene with O2. Chem. Eng. J. 2014, 239, 19−25. (13) Yang, H.; Li, J.; Wang, L.; Dai, W.; Lv, Y.; Gao, S. Exceptional activity for direct synthesis of phenol from benzene over PMoV@ MOF with O2. Catal. Commun. 2013, 35, 101−104. (14) Long, Z. Y.; Zhou, Y.; Chen, G. J.; Ge, W. L.; Wang, J. C3N4H5PMo10V2O40: a dual-catalysis system for reductant-free aerobic oxidation of benzene to phenol. Sci. Rep. 2014, 4, 3651. (15) Gong, J.; Michalkiewicz, B.; Chen, X.; Mijowska, E.; Liu, J.; Jiang, Z.; Wen, X.; Tang, T. Sustainable conversion of mixed plastics into porous carbon nanosheets with high performances in uptake of carbon dioxide and storage of hydrogen. ACS Sustainable Chem. Eng. 2014, 2, 2837−2844. (16) Nandi, M.; Okada, K.; Dutta, A.; Bhaumik, A.; Maruyama, J.; Derks, D.; Uyama, H. Unprecedented CO2 uptake over highly porous N-doped activated carbon monoliths prepared by physical activation. Chem. Commun. 2012, 48, 10283−10285. (17) Gong, J.; Yao, K.; Liu, J.; Jiang, Z.; Chen, X.; Wen, X.; Mijowska, E.; Tian, N.; Tang, T. Striking influence of Fe2O3 on the “catalytic carbonization” of chlorinated poly(vinyl chloride) into carbon microspheres with high performance in the photo-degradation of Congo red. J. Mater. Chem. A 2013, 1, 5247−5255. (18) He, F.; Chen, X. H.; Shen, Y. F.; Li, Y.; Liu, A.; Liu, S. Q.; Mori, T.; Zhang, Y. J. Ionic liquid-derived Fe−N/C catalysts for highly efficient oxygen reduction reaction without any supports, templates, or multi-step pyrolysis. J. Mater. Chem. A 2016, 4, 6630−6638. (19) Li, Z. L.; Li, G. L.; Jiang, L. H.; Li, J. L.; Sun, G. Q.; Xia, C. G.; Li, F. W. Ionic Liquids as Precursors for Efficient Mesoporous IronNitrogen-Doped Oxygen Reduction Electrocatalysts. Angew. Chem., Int. Ed. 2015, 54, 1494−1498. (20) Sun, F.; Gao, J.; Yang, Y.; Zhu, Y.; Wang, L.; Pi, X.; Liu, X.; Qu, Z; Wu, S.; Qin, Y. One-step ammonia activation of Zhundong coal generating nitrogen-doped microporous carbon for gas adsorption and energy storage. Carbon 2016, 109, 747−754. (21) Gong, J.; Lin, H. J.; Antonietti, M.; Yuan, J. Y. Nitrogen-doped porous carbon nanosheets derived from poly(ionic liquid)s: hierarchical pore structures for efficient CO2 capture and dye removal. J. Mater. Chem. A 2016, 4, 7313−7321. (22) Gong, J.; Antonietti, M.; Yuan, J. Y. Poly(Ionic Liquid)-Derived Carbon with Site-Specific N-Doping and Biphasic Heterojunction for Enhanced CO2 Capture and Sensing. Angew. Chem. 2017, 129, 7665. (23) Gao, C. J.; Chen, G. J.; Wang, X. C.; Li, J.; Zhou, Y.; Wang, J. A hierarchical meso-macroporous poly(ionic liquid) monolith derived from a single soft template. Chem. Commun. 2015, 51, 4969−4972. (24) Qian, W. J.; Texter, J.; Yan, F. Frontiers in poly(ionic liquid)s: syntheses and applications. Chem. Soc. Rev. 2017, 46, 1124−1159. (25) Zhang, P. F.; Qiao, Z. A.; Jiang, X.; Veith, G. M.; Dai, S. Nanoporous Ionic Organic Networks: Stabilizing and Supporting Gold Nanoparticles for Catalysis. Nano Lett. 2015, 15, 823−828. (26) Sun, J. K.; Kochovski, Z.; Zhang, W. Y.; Kirmse, H.; Lu, Y.; Antonietti, M.; Yuan, J. Y. A General Synthetic Route Towards Highly Dispersed Metal Clusters Enabled by Poly(ionic liquid)s. J. Am. Chem. Soc. 2017, 139, 8971. (27) Wang, Q.; Cai, X. C.; Liu, Y. Q.; Xie, J. Y.; Zhou, Y.; Wang, J. Pd nanoparticles encapsulated into mesoporous ionic copolymer: Efficient and recyclable catalyst for the oxidation of benzyl alcohol with O2 balloon in water. Appl. Catal., B 2016, 189, 242−251. (28) Dani, A.; Crocella, V.; Maddalena, L.; Barolo, C.; Bordiga, S.; Groppo, E. Spectroscopic Study on the Surface Properties and Catalytic Performances of Palladium Nanoparticles in Poly(ionic liquid)s. J. Phys. Chem. C 2016, 120, 1683−1692. (29) Han, H. L.; Jiang, T.; Wu, T. B.; Yang, D. X.; Han, B. X. VxOy Supported on Hydrophobic Poly(Ionic Liquid)s as an Efficient Catalyst for Direct Hydroxylation of Benzene to Phenol. ChemCatChem 2015, 7, 3526−3532.

Additional characterizations for catalytic materials and various controls (Figures S1−S40); textural properties, element analysis, and additional catalysis results of aerobic oxidation of benzene to phenol with O2 (Tables S1−S3) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-25-83172264. Fax: +86-25-83172261. *E-mail: [email protected]. Tel: +86-25-83172264. Fax: +86-25-83172261. ORCID

Jun Wang: 0000-0002-7669-9992 Notes

The authors declare no competing financial interest.

■

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. (2) Hirose, K.; Ohkubo, K.; Fukuzumi, S. Catalytic Hydroxylation of BenzenetoPhenol by Dioxygen with an NADH Analogue. Chem. - Eur. J. 2016, 22, 12904−12909. (3) Niwa, S. I.; Eswaramoorthy, M.; Nair, J.; Raj, A.; Itoh, N.; Shoji, H.; Namba, T.; Mizukami, F. A One-Step Conversion of Benzene to Phenol with a Palladium Membrane. Science 2002, 295, 105−107. (4) Morimoto, Y.; Bunno, S.; Fujieda, N.; Sugimoto, H.; Itoh, S. Direct Hydroxylation of Benzene to Phenol Using Hydrogen Peroxide Catalyzed by Nickel Complexes Supported by Pyridylalkylamine Ligands. J. Am. Chem. Soc. 2015, 137, 5867−5870. (5) Makgwane, P. R.; Ray, S. S. Hydroxylation of benzene to phenol over magnetic recyclable nanostructured CuFe mixed-oxide catalyst. J. Mol. Catal. A: Chem. 2015, 398, 149−157. (6) Verma, S.; Baig, R. B. N.; Nadagouda, M. N.; Varma, R. S. Hydroxylation of Benzene via C-H Activation Using Bimetallic CuAg@g-C3N4. ACS Sustainable Chem. Eng. 2017, 5, 3637−3640. (7) Yamada, M.; Karlin, K. D.; Fukuzumi, S. One-step selective hydroxylation of benzene to phenol with hydrogen peroxide catalysed by copper complexes incorporated into mesoporous silica−alumina. Chem. Sci. 2016, 7, 2856−2863. (8) Sarma, B. B.; Carmieli, R.; Collauto, A.; Efremenko, I.; Martin, J. M. L.; Neumann, R. Electron Transfer Oxidation of Benzene and Aerobic Oxidation to Phenol. ACS Catal. 2016, 6, 6403−6407. (9) Wang, W. T.; Ding, G. D.; Jiang, T.; Zhang, P.; Wu, T. B.; Han, B. X. Facile one-pot synthesis of VxOy@C catalysts using sucrose for the direct hydroxylation of benzene to phenol. Green Chem. 2013, 15, 1150−1154. (10) Cai, X. C.; Wang, Q.; Liu, Y. Q.; Xie, J. Y.; Long, Z. Y.; Zhou, Y.; Wang, J. Hybrid of Polyoxometalate-Based Ionic Salt and N-Doped Carbon toward Reductant-Free Aerobic Hydroxylation of Benzene to Phenol. ACS Sustainable Chem. Eng. 2016, 4, 4986−4996. (11) Chen, G. J.; Zhou, Y.; Wang, X. C.; Li, J.; Xue, S.; Liu, Y. Q.; Wang, Q.; Wang, J. Construction of porous cationic frameworks by crosslinking polyhedral oligomeric silsesquioxane units with Nheterocyclic linkers. Sci. Rep. 2015, 5, 11236. 12295


Article


coupling versus hydroxylation. J. Mol. Catal. A: Chem. 2002, 185, 285− 290. (49) Luo, J.; Peng, F.; Wang, H.; Yu, H. Enhancing the catalytic activity of carbon nanotubes by nitrogen doping in the selective liquid phase oxidation of benzyl alcohol. Catal. Commun. 2013, 39, 44−49. (50) Das, T.; Nicholas, J. D.; Qi, Y. Long-range charge transfer and oxygen vacancy interactions in strontium ferrite. J. Mater. Chem. A 2017, 5, 4493−4506. (51) Yang, J. H.; Sun, G.; Gao, Y.; Zhao, H.; Tang, P.; Tan, J.; Lu, A. H.; Ma, D. Direct catalytic oxidation of benzene to phenol over metalfree graphene-based catalyst. Energy Environ. Sci. 2013, 6, 793−798. (52) Khenkin, A. M.; Weiner, L.; Wang, Y.; Neumann, R. Electron and Oxygen Transfer in Polyoxometalate, H5PV2Mo10O40, Catalyzed Oxidation of Aromatic and Alkyl Aromatic Compounds: Evidence for Aerobic Mars-van Krevelen-Type Reactions in the Liquid Homogeneous Phase. J. Am. Chem. Soc. 2001, 123, 8531−8542. (53) Yu, H.; Peng, F.; Tan, J.; Hu, X.; Wang, H.; Yang, J.; Zheng, W. Selective catalysis of the aerobic oxidation of cyclohexane in the liquid phase by carbon nanotubes. Angew. Chem., Int. Ed. 2011, 50, 3978− 3982. (54) Gao, Y.; Hu, G.; Zhong, J.; Shi, Z.; Zhu, Y.; Su, D. S.; Wang, J.; Bao, X.; Ma, D. Nitrogen-doped sp2-hybridized carbon as a superior catalyst for selective oxidation. Angew. Chem., Int. Ed. 2013, 52, 2109− 2113.

(30) Wang, D. K.; Wang, M. T.; Li, Z. H. Fe-Based Metal−Organic Frameworks for Highly Selective Photocatalytic Benzene Hydroxylation to Phenol. ACS Catal. 2015, 5, 6852−6857. (31) Wang, J.; Park, J. N.; Wei, X. Y.; Lee, C. W. Room-temperature heterogeneous hydroxylation of phenol with hydrogen peroxide over Fe2+, Co2+ ion-exchanged Naβ zeolite. Chem. Commun. 2003, 5, 628− 629. (32) Lin, L.; Zhu, Q.; Xu, A. W. Noble-Metal-Free Fe−N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136, 11027−11033. (33) Gomez, S.; Lerici, L.; Saux, C.; Perez, A. L.; Brondino, C. D.; Pierella, L.; Pizzio, L. Fe/ZSM-11 as a novel and efficient photocatalyst to degrade Dichlorvos on water solutions. Appl. Catal., B 2017, 202, 580−586. (34) Gao, F.; Zheng, Y.; Kukkadapu, R. K.; Wang, Y.; Walter, E. D.; Schwenzer, B.; Szanyi, J.; Peden, C. H. F. Iron Loading Effects in Fe/ SSZ-13 NH3-SCR Catalysts: Nature of the Fe Ions and Structure− Function Relationships. ACS Catal. 2016, 6, 2939−2954. (35) Cheng, Z. L.; Li, W.; Wu, P. R.; Liu, Z. A Strategy for Preparing Modified Graphene Oxide with Good Dispersibility and Transparency in Oil. Ind. Eng. Chem. Res. 2017, 56, 5527−5534. (36) Ferrero, G. A.; Preuss, K.; Marinovic, A.; Jorge, A. B.; Mansor, N.; Brett, D. J. L.; Fuertes, A. B.; Sevilla, M.; Titirici, M. M. Fe-NDoped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions. ACS Nano 2016, 10, 5922−5932. (37) Wu, Z. Y.; Xu, X. X.; Hu, B. C.; Liang, H. W.; Lin, Y.; Chen, L. F.; Yu, S. H. Iron Carbide Nanoparticles Encapsulated in Mesoporous Fe-N-Doped Carbon Nanofibers for Efficient Electrocatalysis. Angew. Chem., Int. Ed. 2015, 54, 8179−8183. (38) Zang, Y. P.; Zhang, H. M.; Zhang, X.; Liu, R. R.; Liu, S. W.; Wang, G. Z.; Zhang, Y. X.; Zhao, H. J. Fe/Fe2O3 nanoparticles anchored on Fe-N-doped carbon nanosheets as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries. Nano Res. 2016, 9, 2123−2137. (39) Fei, H. L.; Peng, Z. W.; Li, L.; Yang, Y.; Lu, W.; Samuel, E. L. G.; Fan, X. J.; Tour, J. M. Preparation of carbon-coated iron oxide nanoparticles dispersed on graphene sheets and applications as advanced anode materials for lithium-ion batteries. Nano Res. 2014, 7, 502−510. (40) Quan, H. Y.; Cheng, B. C.; Xiao, Y. H.; Lei, S. J. One-pot synthesis of a-Fe2O3 nanoplates-reduced graphene oxide composites for supercapacitor application. Chem. Eng. J. 2016, 286, 165−173. (41) Lin, Y.; Pan, X.; Qi, W.; Zhang, B.; Su, D. S. Nitrogen-doped onion-like carbon: a novel and efficient metal-free catalyst for epoxidation reaction. J. Mater. Chem. A 2014, 2, 12475−12483. (42) Chen, C.; Xu, J.; Jin, M.; Li, G.; Hu, C. Direct synthesis of phenol from benzene on an activated carbon catalyst treated with nitric acid. Chin. Chin. J. Chem. Phys. 2011, 24, 358−364. (43) Ma, J. Q.; Yang, Q. F.; Wen, Y. Z.; Liu, W. P. Fe-g-C3N4/ graphitized mesoporous carbon composite as an effective Fenton-like catalyst in a wide pH range. Appl. Catal., B 2017, 201, 232−240. (44) Bal, R.; Tada, M.; Sasaki, T.; Iwasawa, Y. Direct Phenol Synthesis by Selective Oxidation of Benzene with Molecular Oxygen on an Interstitial-N/Re Cluster/Zeolite Catalyst. Angew. Chem., Int. Ed. 2006, 45, 448−452. (45) Liu, Y.; Murata, K.; Inaba, M. Direct oxidation of benzene to phenol by molecular oxygen over catalytic systems containing Pd(OAc)2 and heteropolyacid immobilized on HMS or PIM. J. Mol. Catal. A: Chem. 2006, 256, 247−255. (46) Gao, X.; Lv, X.; Xu, J. Oxidation of benzene to phenol by dioxygen over vanadium oxide nano plate. Kinet. Catal. 2010, 51, 394− 397. (47) Li, X. H.; Wang, X.; Antonietti, M. Solvent-free and metal-free oxidation of toluene using O2 and g-C3N4 with nanopores: nanostructure boosts the catalytic selectivity. ACS Catal. 2012, 2, 2082−2086. (48) Burton, H. A.; Kozhevnikov, I. V. Biphasic oxidation of arenes with oxygen catalyzed by Pd (II)-heteropolyacidsystem: oxidative 12296
