Highly Efficient Catalysts Based on Divanadium ... - ACS Publications

Research Article Cite This: ACS Catal. 2018, 8, 1297−1307

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Highly Efficient Catalysts Based on Divanadium-Substituted Polyoxometalate and N‑Doped Carbon Nanotubes for Selective Oxidation of Alkylphenols Vasiliy Yu. Evtushok,†,‡ Arina N. Suboch,†,‡ Olga Yu. Podyacheva,†,‡ Olga A. Stonkus,†,‡ Vladimir I. Zaikovskii,†,‡ Yurii A. Chesalov,†,‡ Lidiya S. Kibis,†,‡ and Oxana A. Kholdeeva*,†,‡ †

Boreskov Institute of Catalysis, Lavrentieva ave. 5, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia

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‡

S Supporting Information *

ABSTRACT: Alkyl-substituted benzoquinones serve as versatile building blocks for a variety of biologically active compounds. In this work, we present an approach for the environmentally benign synthesis of alkyl-p-benzoquinones, in particular trimethyl-p-benzoquinone (TMBQ, vitamin E precursor), which employs aqueous hydrogen peroxide as oxidant and a divanadium-substituted γ-Keggin polyoxotungstate, [γ-PW10O38V2(μ-O)(μ−OH)]4− (V2-POM), immobilized on nitrogen-doped carbon nanotubes (N-CNTs) as heterogeneous catalyst. A series of supported catalysts V2-POM/N-CNTs containing 5−25 wt % of V2-POM and 0−4.8 atom % of N has been prepared and characterized by elemental analysis, N2 adsorption, SEM, TEM, XPS, and FTIR techniques. The catalytic performance of V2-POM/N-CNTs was assessed in the selective oxidation of 2,3,6trimethylphenol (TMP) with H2O2 under mild reaction conditions (60 °C, MeCN). The presence of nitrogen in the support ensures strong adsorption and molecular dispersion of V2-POM on the carbon surface, leading to highly active and selective heterogeneous catalysts, which do not suffer from metal leaching and can be used repeatedly without loss of the catalytic performance. By application of the optimal catalyst V2-POM/N-CNTs enclosing 15 wt % of V2-POM and 1.8 atom % of N, TMBQ could be obtained with 99% yield and 80% oxidant utilization efficiency. The catalyst demonstrated the truly heterogeneous nature of the catalysis and high turnover frequencies (500 h−1) and space−time yield (450 g L−1 h−1). FTIR and XPS techniques confirmed the stability of V2-POM and N-CNT support under the turnover conditions. KEYWORDS: alkylphenols, benzoquinones, selective oxidation, heterogeneous catalysis, hydrogen peroxide, polyoxometalate, carbon nanotubes, nitrogen-doped

1. INTRODUCTION Functionalized benzoquinones (BQ) are frequently occurring structural moieties in a wide range of biologically active compounds.1−3 The selective catalytic oxidation of alkylphenols with atomically efficient, cheap, and readily available oxidants is the most economical and ecological route to the production of vital BQs, in particular trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate). 4−7 Currently, TMBQ is manufactured through the oxidation of 2,3,6-trimethylphenol (TMP) with molecular oxygen in the presence of quasistoichiometric amounts of copper chloride.4 The inherent drawbacks of this process are the formation of Cl-containing byproducts and the need for corrosion-resistant equipment. A recent attempt to reduce the amount of CuCl2 involved the use of an ionic liquid, 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), coupled with n-butanol as cosolvent.8 Co(II)Schiff base complexes were suggested as catalysts for the oxidation of substituted phenols to BQs with molecular oxygen,9,10 but salen ligands suffer oxidative destruction under turnover conditions. Recently, gold nanoparticles © 2018 American Chemical Society

supported on carbon nanotubes (CNTs) were employed as heterogeneous catalysts for aerobic oxidation of phenolic substrates.11 Dihydroxybenzenes with electron-donating substituents were converted to corresponding 1,4- and 1,2quinones using atmospheric air as the sole oxidant, but the reaction with 2,6-di-tert-butylphenol led to the formation of a C−C coupling product, 3,3,5,5-tetra-tert-butyl-4,4-diphenoquinone, rather than the corresponding BQ.11 Although molecular oxygen remains the oxidant of choice from an economic viewpoint, catalysis based on the activation of an alternative green oxidantaqueous H2O2is a rapidly developing field.12−16 Several homogeneous catalyst systems, including methyltrioxorhenium (MeReO3)17,18 and ruthenium and iron compounds/complexes,19−22 have been developed for the selective oxidation of functionalized phenols with H2O2. A Received: November 17, 2017 Revised: December 28, 2017 Published: January 2, 2018 1297

DOI: 10.1021/acscatal.7b03933 ACS Catal. 2018, 8, 1297−1307

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ACS Catalysis

based materials in liquid-phase selective oxidations has been recently reviewed.54 The development of an efficient POM-based heterogeneous catalyst for the selective oxidation of alkylphenols to the corresponding BQs remains a challenging goal. Attempts to immobilize HPA-n and perform heterogeneous oxidation catalysis using these materials were unsuccessful because the solids suffered from POM leaching in the polar reaction media typically used for phenol oxidation.54,85 In this work, we explored different approaches to the immobilization of V2POM and assessed the elaborated solid catalysts in the practically important H2O2-based oxidation of TMP to TMBQ (Scheme 1).

common drawback of most of these catalysts is insufficiently high BQ selectivity and oxidant utilization efficiency.6,23 Early-transition-metal oxygen anion nanosize clusters or polyoxometalates (POMs) have received growing attention as oxidation catalysts due to a unique combination of properties, such as totally inorganic metal oxide like structure, thermodynamic stability to oxidation, tunable solubility, acid−base and redox properties, and fairly good hydrolytic and solvolytic stability under typical conditions of liquid-phase oxidation.24−27 All of these provide advantages for POMs over conventional catalysts containing organic and organometallic ligands that are prone to oxidative degradation and/or hydrolysis. Commercially available heteropolyacids HnXM12O40 (X = P, Si; M = W, Mo; n = 3, 4) of the Keggin structure have been employed as catalysts for H2O2-based oxidation of alkylated phenols in acetic acid to produce the corresponding BQs in moderate yields.28 Alkylphenols could be also converted to BQs with reasonably good yields through oxidation with molybdovanadophosphoric heteropolyacids H3+nPMo12−nVnO40 (HPA-n, n = 2−6) and their reoxidation with molecular oxygen.29−31 Recently, Mizuno and co-workers revealed that a divanadium-substituted polyoxotungstate of the γ-Keggin structure, [γ-PW10O38V2(μ-O)(μ-OH)]4− (V2-POM), is a highly efficient homogeneous catalyst for selective oxidation of a large scope of organic substrates using aqueous H2O2 as oxidant.32−36 This POM is able to perform aromatic hydroxylation of alkylbenzenes37,38 and transformation of alkylphenols/naphtholes39 and methoxytoluenes40 to the corresponding BQs. Specifically, TMP oxidation in the presence of V2-POM affords TMBQ with a nearly quantitative yield, high oxidant utilization efficiency (80−90%), and unprecedentedly high turnover frequency (TOF 500−1000 h−1).39 The homogeneous catalyst can in principle be recycled, but the presence of traces of metals in the quinone product cannot be excluded. The use of heterogeneous catalysts offers clear benefits of catalyst separation and recycling, purity of products, and amenability to continuous processing and thus better meets the requirements of sustainable chemistry.15,16 The widely known microporous titanium silicalite TS-1 is employed in the commercial process of phenol oxidation to hydroquinone and pyrocatechol with aqueous H2O2;41,42 however, the small pore size of TS-1 (0.53 × 0.56 nm) imposes limitations on the substrate scope. Mesoporous titanium silicates prepared by various approaches are effective catalysts in the oxidation of alkyl-substituted phenols,43−49 but catalyst stability and oxidant utilization efficiency still leave room for improvement.6,23,50,51 Therefore, the development of a truly heterogeneous and recyclable catalyst for the selective oxidation of alkylphenols to the corresponding BQs would have a major effect on industrial applications as well as on scientific research. Several approaches to the preparation of POM-based heterogeneous catalysts have been developed.26,52−57 These methodologies involve elaboration of insoluble POM salts using Cs+, Ag+, K+, NH4+, and some organic polycations,58−60 entrapment within carrier materials during their synthesis,61−63 electrostatic attachment through cationic functional groups using amine or ionic liquid modified silica64−67 or anion exchange with a solid matrix (e.g., cationic silica nanoparticles,66−68 layered double hydroxides,69,70 or metal−organic frameworks (MOF)71−73), covalent or dative anchoring,74−78 inclusion of POM into the structure of MOF through covalent bonding,79−81 and finally, immobilization by other, nonspecific interactions with the support.52,82−84 The application of POM-

Scheme 1. TMP Oxidation with Hydrogen Peroxide over Supported V2-POM

In addition to the conventional immobilization techniques, such as embedding into silica, electrostatic attachment to amine-modified SiO2 and metal−organic framework MIL-101, and adsorption on active carbon,53 our attention was directed to promising solid supports, carbon nanotubes (CNTs), which have received much recent attention for the preparation of hybrid inorganic materials, including heterogeneous catalysts and electrocatalysts.86 Composites based on POMs and carbon nanomaterials have been widely used in electrocatalysis.87−91 Immobilization of POMs was accomplished by impregnation92 or adsorption on the carbon surface modified with cationic groups.87,89,91 Giusti et al. have demonstrated adsorption of a sandwich POM on single-walled carbon nanotubes (SWCNTs).93 Few reports have been devoted to the application of POM/CNT composites as catalysts for oxidations with H2O2.92,94 Unfortunately, a hot filtration test95 was not reported to prove unambiguously the heterogeneous nature of the catalysis. On the other hand, nitrogen-doped carbon nanomaterials offer a range of different surface N species, including pyridine-like (NPy), pyrrole-like (NPyr), and quaternary (NQ), which provide a variety of adsorption sites.96,97 Arrigo et al. successfully employed Ndoped CNTs (N-CNTs) as supports for Pd nanoparticles in the liquid-phase oxidation of benzyl alcohol to benzaldehyde.98 It was demonstrated also that N-doped graphene nanosheets can act as metal-free catalysts for the aerobic selective oxidation of benzylic alcohols.99 Recently, wet air phenol oxidation was realized using Ru nanoparticles supported on N-doped carbon nanofibers (N-CNFs).100 In this work, we first report the immobilization of a catalytically active POM, namely V2-POM, on N-CNTs, provide characterization of the new materials by physicochemical techniques (N2 adsorption, SEM, TEM, XPS, and FTIR spectroscopy), and evaluate the catalytic performance of the supported catalysts in the industrially relevant oxidation of TMP and other alkylphenols with aqueous H2O2. The role of nitrogen in the CNT supports for V2-POM immobilization and 1298


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room temperature from MeCN solution. To enhance the adsorption capacity, HClO4 was added (1 equiv to POM). The completion of the adsorption process was controlled by UV− vis. The resulting solid material was separated by filtration, washed with MeCN three times, and dried under vacuum at 60 °C. 2.3. Adsorption/Desorption Studies. Adsorption measurements were carried out in a glass vessel at 25 °C following the methodology described earlier.71 The adsorbent (CNTs or N-CNTs, 20 mg) preliminarily dried under vacuum at 100 °C was placed in the vessel, and a solution of V2-POM in acetonitrile (2 mL, 0.1−2 mmol/L) was added. The mixture was stirred at 500 rpm, samples of the solution were taken by a syringe (100 μL) after 1 h (this time is enough to reach the adsorption/desorption equilibrium) and diluted with MeCN (900 μL), and the V2-POM concentration in the solution was determined by UV−vis (λ 342 nm, l = 0.1 cm) using a calibration curve. Desorption measurements were carried out at 25 °C using mixtures of MeCN solutions of V2-POM containing 20 mg of N-CNTs that remained after the adsorption measurements. An aliquot of the solution (1 mL) was taken, and the volume was restored by addition of fresh MeCN. The mixture was stirred at 500 rpm, a sample of the solution was taken by a syringe (100 μL) after 1 h and diluted with MeCN (900 μL), and the concentration of V2-POM was determined by UV−vis. The amount of irreversibly adsorbed V2-POM was determined from the point at which the desorption curve cut the axis (Y). 2.4. Catalytic Oxidation and Product Analysis. Catalytic experiments were carried out in thermostated glass vessels at 60 °C under vigorous stirring (500 rpm). Typical reaction conditions were as follows: 0.1 M TMP, 0.35−0.45 M H2O2, catalyst containing 1.5 mM of V2-POM and preliminarily dried under vacuum at 60 °C, 1 mL of MeCN, 60 °C, 10−20 min. The reactions were started with the addition of H2O2. The reaction products were identified by GC/MS and 1H NMR (see the Supporting Information) and quantified by GC using biphenyl as internal standard. Before reuse, the catalyst was separated by filtration, washed with acetonitrile, and evacuated at 60 °C. 2.5. H2O2 Decomposition. A support (CNTs or N-CNTs, 10 mg) was placed into a glass reactor containing 5 mL of MeCN, and then 50 μL of 11 M H2O2 was added. The reaction mixture was vigorously stirred at 50 °C, and samples of the solution (300 μL) were obtained periodically by a syringe. The concentration of H2O2 was determined by titration with 0.002 M KMnO4. Initial rates of H2O2 decomposition, W0, were determined from initial portions of kinetic curves. 2.6. Instrumentation and Sample Characterization. GC analyses were performed using a Chromos GC-1000 gas chromatograph equipped with a flame ionization detector and a BPX5 quartz capillary column (30 m × 0.25 mm). 1H, 31P, and 51 V NMR spectra were recorded on a Brüker AVANCE-400 spectrometer at 400.130, 161.67, and 105.24 MHz, respectively. Chemical shifts for H, P and V (δ) were determined relative to tetramethylsilane, 85% H3PO4, and VOCl3, respectively. Infrared spectra were recorded in KBr pellets on an Agilent 660 FTIR spectrometer. UV−vis spectra were recorded on a Varian Cary 60 UV−vis spectrophotometer. High-resolution transmission electron microscopy (HRTEM) was performed using a JEM-2200FS (JEOL Ltd., Japan) electron microscope operated at 200 kV for obtaining TEM images. Images with a high atomic number contrast were acquired using a high angle

catalysis is addressed, and the advantages of V2-POM/N-CNTs, in terms of activity, selectivity and recyclability, over solid V2POM catalysts prepared by other immobilization techniques have been demonstrated.

2. EXPERIMENTAL SECTION 2.1. Materials. 2,3,6-Trimethylphenol (TMP, 97+%) was obtained from Fluka. 2,3,5-Trimethylphenol (2,3,5-TMP, 99%), 2,6-di-tert-butylphenol (DTBP, 99%), 2,6-dimethylphenol (2,6DMP, 99%), 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ, 98%), 3,5-dimethylphenol (3,5-DMP, 98%), and 2,3-dimethylphenol (2,3-DMP, 99%) were purchased from Aldrich. Acetonitrile (Panreac, HPLC grade) was dried and stored over activated 4 Å molecular sieves. All the other compounds were the best available reagent grade and were used without further purification. The concentration of H2O2 (35 wt % in water) was determined iodometrically prior to use. 2.2. Catalyst and Support Preparation. V2-POM in the form of the acid TBA salt TBA3.5H1.5[γ-PW10O38V2(μ-O)(μOH)] (TBA stands for n-Bu4N) was prepared by following the procedure reported previously.40 The compound purity was confirmed by FTIR (Figure S1 in the Supporting Information) and 51V and 31P NMR spectroscopy (Figures S2 and S3, respectively). NH2-modified silica was obtained by postsynthetic modification of SiO2.53 MIL-101 was synthesized according to the method described by Férey et al.101 with some modifications.102 Immobilization of V2-POM on NH2-SiO2, MIL-101, and active carbon Sibunit was carried out following protocols described in ref 53. NH2-SiO2 was treated with HClO4 to obtain the cationic form NH3+-SiO2. The composite V2-POM/ SiO2 was prepared by the sol−gel method using tetraethoxysilane (TEOS) as the silica precursor. A modified literature protocol was used for reasons of POM solubility.103 A solution of 4 wt % V2-POM in MeCN (13 mL) was mixed with 5 mL of n-BuOH and 5 mL of TEOS. Then 5 mL of H2O was added and the acidity was adjusted by aqueous HCl until the pH reached 2. The resulting mixture was held for 1 h at 25 °C, and then the temperature was raised to 60 °C. After 16 h, a hydrogel was formed. The hydrogel was dried under vacuum at 80 °C for 2 h. Then the solid was washed with hot water and again dried under vacuum at 80 °C. The resulting material was thoroughly washed with MeCN until the liquor became colorless. All of the liquors were combined and evaporated, and the total amount of leached V2-POM was determined. As a result, 1.7 g of V2-POM/SiO2 composite containing 10 wt % of V2-POM was obtained. The synthesis of CNTs and N-CNTs was carried out in a flow-type quartz reactor on an Fe-Ni-Al2O3 catalyst by decomposition of pure C2H4 or a C2H4/NH3 mixture at 700 °C, respectively.104 To remove the catalyst from N-CNTs, the materials were treated with concentrated HCl several times for several weeks at room temperature and then additionally boiled in 2 M HCl for 12 h. Afterward the materials were washed with distilled water until no chloride ions were detected in the rinsing liquid. This severe treatment reduced the content of the initial catalyst to 1−2 wt %, while the remaining catalyst particles were encapsulated within the carbon framework. The washed samples were dried in an Ar flow at 170 °C. The water capacity of N-CNTs was measured by a standard incipient wetness impregnation method. Prior to use, CNTs and N-CNTs were dried again under vacuum at 100 °C. Adsorption of V2-POM was carried out at 1299


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ACS Catalysis annular dark field (HAADF) detector in scanning-TEM (STEM) mode. The samples for the TEM study were prepared on perforated carbon film mounted on a copper grid. Scanning electron microscopy images were obtained using a JEOL JSM6460 LV microscope. Nitrogen adsorption measurements were carried out at 77 K using an ASAP-2400 instrument. X-ray photoelectron spectra were collected on a KRATOS ES300 photoelectron spectrometer with nonmonochromatic Al Kα radiation (hν 1486.6 eV). The Au 4f7/2 and Cu 2p3/2 core-level lines with binding energies 84.0 and 932.7 eV, respectively, were used for the spectrometer calibration. The survey spectra in the range of 0−1100 eV were recorded with an energy resolution corresponding to the maximum sensitivity for qualitative analysis of the surface composition of the samples. For quantitative analysis and analysis of the charging states of the elements on the surface the core-level spectra C 1s, N 1s, O 1s, P 2p, V 2p, and W 4f were collected. The atomic sensitivity factors for each element105 took into account quantitative estimations. The analysis of the spectra was performed using the XPS-Calc program106,107 after Shirley background subtraction. The nitrogen content in N-CNTs was defined as N/C ratio (atom %) from XPS data.

No irreversible adsorption occurred in the absence of acid: i.e., all POM could be washed out using MeCN. With the addition of HClO4, the amount of irreversibly adsorbed V2POM markedly increased and attained 22 and 17% for NCNTs and CNTs, respectively (Figure 2). Therefore, the

Figure 2. Adsorption (■ and ●) and desorption (□ and ○) isotherms for V2-POM on N-CNTs (4.8 atom % N) and CNTs. Conditions: MeCN, 25 °C, 1 equiv of HClO4 added.

3. RESULTS AND DISCUSSION 3.1. Adsorption Studies and Immobilization of V2POM on CNTs. To evaluate the potential of undoped and nitrogen-doped carbon nanotubes as supports for immobilization of the catalytically active POM, we first studied the adsorption of V2-POM on CNTs and N-CNTs from MeCN solutions where the TBA salt of V2-POM is completely soluble. The adsorption process was monitored by UV−vis. Adsorption curves are shown in Figure 1.

presence of nitrogen in CNTs is not an indispensable factor required for the adsorption of POM but it allows one to attain a higher amount of irreversibly immobilized POM. On the other hand, additives of acid are crucial for POM immobilization on the carbon surface. Previously, acid additives were used to accomplish adsorption of a POM on SWCNTs from water,93 but the amount of irreversibly adsorbed POM and the role of acid were not addressed. The promoting affect of acid in the case of N-CNTs might indicate that electrostatic interactions between the POM anion and protonated pyridinic nitrogen of N-CNTs contribute to the POM immobilization. However, in contrast to other supported catalysts where POM immobilization was realized through Coulombic interactions with the surface,53 V2-POM could not be re-extracted from the N-CNT support by anion exchange using a 1 M solution of TBAClO4 in acetonitrile. This implies that electrostatic interactions are not solely responsible for the strong binding of POM to N-CNTs. Moreover, the ability of undoped CNTs to bind POM irreversibly allows one to suggest that other noncovalent interactions, such as hydrogen bonding and van der Waals contacts, may take place. FT-IR spectroscopy was used to verify the retention of the V2-POM structure after immobilization. FT-IR spectra of NCNTs (spectrum 1) and 8 wt % V2-POM/N-CNTs (spectra 2 and 3) are shown in Figure 3a. The spectrum of N-CNTs is similar to that reported previously.108 The band at 3420 cm−1 is due to ν(H2O) mode. The bands at 1590 and 1260 cm−1 are associated with the stretching vibrations of CC and CN bonds. The band at 1590 cm−1 is assigned to the E1u vibrational mode of graphite, whereas the appearance of the band at 1260 cm−1 in the spectra can be related to the translational symmetry disturbance caused by the defectiveness of the graphite-like structure.108 Very weak bands in the range 1000−750 cm−1 are also observed in the spectra of 8 wt % V2-POM/N-CNTs. The structure of the spectra in this region is clearer in the difference spectra (Figure 3b, spectra B and C). The spectrum of V2-POM is also shown in Figure 3b for comparison (spectrum A). The main vibration features in the characteristic POM region (955, 870, and 800 cm−1) can be clearly distinguished in the spectra

Figure 1. Adsorption of V2-POM on CNTs and N-CNTs (4.8 atom % N) from MeCN at 25 °C (additives, 1 equiv with respect to POM).

Without any additives, the maximum amount of V2-POM adsorbed on the preliminarily dried CNTs and N-CNTs was quite similar and did not exceed 6−8 wt %. On the other hand, the addition of HClO4 greatly increased the maximum adsorption, which reached 35 and 31 wt % for N-CNTs and CNTs, respectively. The use of TBAClO4 instead of HClO4 led to reduction of the maximum POM sorption (see Figure 1). The wetness of N-CNTs turned out to be a critical parameter that greatly affected the POM adsorption process and catalysis (vide infra). Without predrying of CNTs under vacuum, the value of V2-POM adsorption reached only 22 wt %, even in the presence of HClO4. 1300


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Figure 3. (a) FT-IR spectra of (1) N-CNTs (4.8 atom % N), (2) 8 wt % V2-POM/N-CNTs, and (3) 8 wt % V2-POM/N-CNTs after TMP oxidation. (b) FT-IR spectra of (A) V2-POM, (B) 8 wt % V2-POM/N-CNTs, and (C) 8 wt % V2-POM/N-CNTs after TMP oxidation. The spectrum of N-CNTs (4.8 atom % N) was used for subtraction in (B) and (C).

of the immobilized V2-POM, indicating preservation of its structure. One can notice some red-shifting of the peak maxima relative to the initial POM, which could be due to a strong interaction between POM and the solid support. The values of POM loading in the catalysts evaluated by elemental analysis were in good agreement with UV−vis measurements on the MeCN solution remaining after completion of the POM adsorption process. In agreement with the literature,97,109−111 XPS identified five types of nitrogen species present in N-CNTs (Figure S4 in the Supporting Information): N Py (398.3−398.5 eV), N Pyr (399.6−400.2 eV), NQ (400.8−401.3 eV), NOx (oxidized nitrogen, 402.5−402.8 eV), and NN2 (molecular nitrogen, 404.6−404.9 eV). The content of NPy increased with an increase in the total N content and reached 26% in N-CNTs containing 4.8 atom % of N. The molar ratio of N to POM varied from 45 to 120 in the catalyst samples. In particular, the ratio of NPy to POM was in the range of 6.3−31. Textural characteristics of representative N-CNT supports and supported V2-POM catalysts acquired by low-temperature N2 adsorption are presented in Table 1 along with elemental analysis data.

Figure 4. HRTEM (a) and HAADF-STEM (b and c) images of 8 wt % V2-POM/N-CNTs (4.8 atom % N).

Since the POM contains 10 W atoms, the Z-contrast method can be used reliably to detect POM particles on the surface of the carbon material. Figure 4b,c shows images obtained in the HAADF-STEM mode, where POM particles give a bright light contrast. It is possible to distinguish individual particles of about 1 nm in size. Since the diameter of a Keggin POM is 1.2 nm, we can conclude that no POM agglomeration occurs during the immobilization process and single polyanions are attached to the surface of the N-CNTs. It is interesting that, in the case of the thinnest nanotubes with a diameter of about 10 nm, we can observe regions with a size of up to 5 nm, where POM particles are virtually absent. Such local inhomogeneity in the POM distribution could be related both to the heterogeneity of the nitrogen distribution in the N-CNT structure and to the presence of different types of nitrogen on the surface of the nanotubes. Importantly, HAADF-STEM images of V2-POM supported on N-free CNTs (Figure 5) show lengthy sections without POM particles as well as some aggregation of V2-POM species on the surface. This observation points out the crucial role of nitrogen doping for molecular distribution of POM on NCNTs. 3.2. TMP Oxidation over V2-POM/N-CNTs. The catalytic performance of V2-POM supported on N-CNTs was first evaluated in the practically important TMP oxidation with aqueous H2O2 under standard reaction conditions previously employed for homogeneous V2-POM-catalyzed TMP oxidation.39 Since wetness of the CNT supports turned out to be a critical factor in the catalyst preparation, the catalytic

Table 1. Physicochemical Properties of N-CNTs before and after V2-POM Immobilization N,a atom % 0 0 1.8 1.8 4.8 4.8

POM,a wt % 15 15 8

SBET,b m2/g

Vpore,c cm3/g

Dpore,d nm

167 132 170 145 157 147

0.86 0.64 0.59 0.64 0.53 0.53

20 19 14 15 14 14

a

Determined by elemental analysis. bSpecific surface area. cMesopore volume. dMean pore diameter.

The surface area of N-CNTs just slightly decreased after deposition of V2-POM, while the mesopore volume and average pore diameter remained nearly intact. Studies by SEM (Figures S5 and S6 in the Supporting Information) demonstrated no evident changes in the N-CNTs particle size and morphology after POM immobilization. Figure 4a shows an HRTEM image of a section of N-CNTs with a typical bamboo-like structure. The presence of POM on the surface leads to a local change in the contrast of the image. 1301


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N-CNTs was negligible and the main oxidation product in this case was a C−C coupling product, 2,2′,3,3′,5,5′-hexamethyl4,4′-bisphenol. The catalytic performance changed drastically when V2-POM was supported on the N-CNTs. With only 5 wt % of V2-POM, TMBQ selectivity attained 83% at 98% phenol conversion after 15 min. The quinone selectivity could be further improved by increasing POM loading. With 15 wt % of V2-POM, it reached >99% and then remained constant (Figure 6). The effect of N content in CNTs on TMP oxidation with H2O2 is shown in Figure 7. The absence of nitrogen in the support leads to deterioration of TMBQ selectivity. However, only 1.8 atom % of N is enough to accomplish TMP oxidation with excellent selectivity. Therefore, the presence of both V2POM and nitrogen are crucial for superior catalytic performance. Taking into account that the yield of N-doped CNTs normally decreases with increasing N content,104 the optimal composition of the supported V2-POM/N-CNTs catalyst, from the point of view of both catalytic performance and catalyst cost, can be inferred as 1.8 atom % N and 15 wt % V2-POM. In order to verify how catalyst wetness affects catalytic performance, several catalytic tests were also carried out without predrying of the catalysts using the same series of 15 wt % V2-POM/N-CNTs differing in N content. Interestingly, TMBQ selectivity decreased for the wet catalysts relative to the dried catalysts, except for the catalyst with 1.8 atom % N (compare parts a and b of Figure 7). To understand this phenomenon, we studied the dependence of water capacity of N-CNTs on nitrogen content. Figure S7 in the Supporting Information shows that the increase in nitrogen content in NCNTs and, in particular, NPy/NQ ratio, leads to an increase in the water capacity of the carbon nanotubes. Therefore, we may assume that the surface hydrophilicity of N-CNTs increasing with N content disfavors adsorption of the phenol substrate and, in contrast, favors adsorption of H 2 O 2 and its unproductive decomposition. To verify this hypothesis, hydrogen peroxide decomposition over N-CNTs was studied. 3.3. Hydrogen Peroxide Decomposition over N-CNTs and Oxidant Efficiency. Figure 8 shows that the initial rate of hydrogen peroxide decomposition linearly increases with the growth of N content in N-CNTs. No activity was observed for N-free CNTs, which confirms that the residual catalyst used for their synthesis is indeed encapsulated within CNT particles and

Figure 5. HAADF-STEM images of 8 wt % V2-POM/CNTs.

experiments were carried out after catalyst drying under vacuum at 60 °C. Figure 6 shows the effect of V2-POM content on the catalytic performance of V2-POM/N-CNTs materials along with a blank

Figure 6. Effect of V2-POM content on TMP oxidation with H2O2 over V2-POM/N-CNTs (4.8 atom % N). Reaction conditions: 0.1 M TMP, 0.45 M H2O2, catalyst 1.5 mM V2-POM, 1 mL of MeCN, 15 min, 60 °C.

experiment. The latter shows that the support itself is able to catalyze TMP oxidation with H2O2 (80% TMP conversion was attained under standard reaction conditions). Indeed, the catalytic activity of N-CNTs in various reactions owing to the presence of different nitrogen species on the surface has been documented.96 However, selectivity to TMBQ over POM-free

Figure 7. Effect of N content on TMP oxidation with H2O2 over 15 wt % V2-POM/N-CNTs: (a) catalysts preliminarily dried under vacuum at 60 °C and (b) catalysts without predrying. Reaction conditions: 0.1 M TMP, 0.45 M H2O2, catalyst 40 mg (1.5 mM V2-POM), 1 mL of MeCN, 15 min, 60 °C. 1302


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Table 2. Oxidation of Various Alkylphenols with H2O2 Catalyzed by V2-POM/N-CNTsa

Figure 8. Effect of nitrogen content in N-CNTs on H 2 O 2 decomposition rate. Reaction conditions: 10 mg of N-CNT, 0.11 M H2O2, 5 mL of MeCN, 50 °C.

is not available for reactants. Previous studies on N-CNTs by Raman spectroscopy revealed that the concentration of structural defects in the nanotubes is proportional to the N content.112 Hence, we may tentatively suppose that decomposition on the structural defects may contribute to the process of H2O2 decomposition. Another reason could be the increase in wetting ability of N-CNTs caused by the growing amount of N, in particular NPy (Figure S7), which facilitates H2O2 adsorption and decomposition. Indeed, while a 2.25-fold excess of H2O2 was required to achieve 100% TMP conversion over V2-POM immobilized on N-CNTs with 4.8 atom % N, only 1.75-fold excess of the oxidant (relative to the stoichiometry for transformation of phenol to quinone, which is 1:2) was enough in the case of NCNTs with 1.8 atom % N. Therefore, oxidant utilization efficiency can be significantly improved without reduction of the quinone product yield if we reduce the amount of N in the CNT support (Figure 9). With the optimal catalyst (15 wt % V2-POM and 1.8 atom % N), the oxidant efficiency reaches 80%, the value previously attained with homogeneous V2POM.39

a

Reaction conditions: 0.1 M TMP, 0.45 M H2O2, catalyst 40 mg (15 wt % V2-POM; N 1.8 atom %), 1 mL of MeCN, 15 min, 60 °C.

POM.39 In contrast, 3,5-DMP and 2,3,5-TMP showed only 10−15% conversion. Actually, 2,3,5-TMP was less reactive than 2,3,6-TMP under the same conditions when homogeneous V2POM was employed.39 This finding agrees with the previous results acquired for hydroxylation of alkylbenzenes catalyzed by V2-POM37,38,113 and supports a mechanism that involves electrophilic oxygen transfer from a sterically hindered peroxo vanadium species to phenol substrate. However, the difference in reactivity of 2,3,5- and 2,3,6-TMP was enhanced for the supported V2-POM/N-CNTs catalyst. We may tentatively suppose that this is due to an additional steric hindrance that arises upon immobilization of V2-POM. On the other hand, the poorer reactivity of 2,3-DMP relative to 2,3,6-TMP may be caused by the smaller amount of electron-donating CH3 groups in the former with a comparable steric factor. 3.5. Catalyst Stability and Recyclability. The recyclability and stability of the optimal catalyst with 15 wt % V2-POM and 1.8 atom % N in N-CNTs was assessed under standard reaction conditions. The catalyst could be reused, at least six times, without appreciable loss of activity and selectivity (Figure 10). Retention of the V2-POM structure under the turnover conditions of TMP oxidation was confirmed by FT-IR spectroscopy (see Figure 3, curve C). The hot filtration test demonstrated that the reaction stopped completely after removal of the catalyst and proved the truly heterogeneous nature of the catalysis (Figure 11). In addition, elemental analysis showed that that amount of V and W in the filtrate did not exceed 1 ppm. Importantly, the surface of N-CNTs appeared to be stable toward oxidation under the conditions employed for TMP oxidation, as demonstrated by XPS. The O/C and N/C ratios remained nearly the same before and after the catalysis (Table 3). Earlier, XPS revealed that, during wet phenol oxidation with O2 in the presence of Ru/N-CNF catalyst,100 the O/C ratio

Figure 9. Effect of N content in N-CNTs on hydrogen peroxide efficiency and TMBQ yield.

3.4. Substrate Scope. To evaluate the scope of the V2POM/N-CNTs/H2O2 catalyst system, oxidation of some other representative alkylphenols has been studied. The results are summarized in Table 2. The oxidation of 2,6-DMP and 2,6DTBP readily proceeded to give the corresponding p-BQs with high yields close to those reported for homogeneous V21303


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Table 4. TMP Oxidation with H2O2 over Various Supported V2-POM Catalystsa

Table 3. N/C and O/C Ratios in the Initial Support N-CNTs (4.8 atom % N), after Immobilization of V2-POM (8 wt %) and after TMP Oxidation Obtained by XPS O/C, atom %

N-CNTs V2-POM/N-CNTs V2-POM/N-CNTs after catalysis

4.8 4.7 4.8

4.4 5.8 5.5

TMBQ selectivity, %

100 100 22 68 86 (50)b 98c

99 99 68 68 90 (23)b 98

deprotonation of V2-POM, resulting in the catalytically inactive form [γ-PW10O38V2(μ-O)2]5−. The solid catalysts prepared by POM encapsulation within silica or adsorption on the active carbon Sibunit exhibited higher conversion and selectivity. However, the composite V2-POM/SiO2 synthesized by the sol−gel methodology suffered from substantial V2-POM leaching during the catalytic reaction, and the catalysis was in effect homogeneous rather than heterogeneous, as verified by the hot filtration test. Previously, some of us showed that the sol−gel approach may lead to the preparation of highly active and leaching-tolerant POM-containing microporous catalysts.53,62,67 However, since V2-POM is not soluble in the methanol/ethanol solvents conventionally used in the sol−gel technique, we had to modify the preparation procedure and employed MeCN/n-BuOH to introduce the POM into the sol−gel synthesis. The average pore size of this material was about 1.5 nm, which is comparable to the size of the POM (1.2 nm), and as a result, the silica matrix could not prevent POM leaching into the reaction mixture. In agreement with our previous findings,53 the Sibunit-supported V2-POM showed poor recyclability because of strong adsorption of the oxidation products on the carbon surface. Already in the first reuse, TMBQ selectivity and TMP conversion decreased dramatically from 90 to 50% and from 86 to 23%, respectively. Treatments with boiling MeCN and MeOH did not allow the catalyst to be regenerated. Hence, the data acquired in this work clearly demonstrate the advantages of N-doped CNTs, in terms of achievable conversion and selectivity as well as stability and reusability, as supports for immobilization of V2-POM. 3.7. Comparison with Other Heterogeneous Catalysts. Few attempts at the use of heterogeneous catalysts for the selective oxidation of alkylphenols have been reported.6 Mesoporous vanadium silicates (V-MMM, V-HMS and others) prepared by direct synthesis or grafting of vanadium species onto the silica surface could accomplish the oxidation of alkylphenols to BQs with moderate selectivity, but substantial vanadium leaching was observed.114,115 MCM-41 substituted with Cu(II) and Al(III) catalyzed the oxidation of TMP to TMBQ with 72% selectivity,116 but the nature of the catalysis remained unclear. Supported P-Mo-V Keggin heteropolyacids also suffered from leaching and/or revealed relatively low selectivity or phenol conversion.117,118 Methyl rhenium trioxide supported on poly(4-vinylpyridine) (PVP) demonstrated high selectivity and conversion toward p-BQs;119 however, the catalyst stability was rather low and the overall turnover number (TON) did not exceed 30. Note that, at least, 360 TON could be achieved after 6 recycles with V2-POM supported on N-CNTs.

Figure 11. Hot filtration test for TMP oxidation over 15 wt % V2POM/N-CNTs (1.8 atom % N). Reaction conditions: 0.1 M TMP, 0.35 M H2O2, 1.0 mM V2-POM, 1 mL of MeCN, 50 °C.

N/C, atom %

TMP conversion, %

a Reaction conditions: 0.1 M TMP, 0.35 M H2O2, catalyst 40−60 mg (1.5 mM V2-POM), 1 mL of MeCN, 15 min, 60 °C. bAfter reuse. c Substantial POM leaching (ca. 15%) and homogeneous catalysis were observed.

Figure 10. Recycling of 15 wt % V2-POM/N-CNTs (1.8 atom % N) in TMP oxidation. Reaction conditions: 0.1 M TMP, 0.45 M H2O2, catalyst 40 mg, 1 mL of MeCN, 15 min, 60 °C.

sample

Catalyst V2-POM (homogeneous) V2-POM/N-CNTs V2-POM/MIL-101 V2-POM/NH2-SiO2 V2-POM/Sibunit V2-POM@SiO2

greatly increased and new functional O-containing groups appeared on the N-CNF surface. The observed discrepancy can be explained by the much harsher conditions employed for the wet phenol oxidation (160 °C, 10 bar O2) and the different structures of N-CNFs (herringbone) and N-CNTs (bamboolike). 3.6. Comparison of Various Immobilization Techniques. The catalytic performance of V2-POM supported on NCNTs in TMP oxidation with H2O2 was compared with that of catalysts prepared by immobilization of this POM using other techniques. The results are provided in Table 4. One can see that V2-POM electrostatically attached to the surface of MIL-101 and NH2-SiO2 demonstrated low TMBQ selectivity (68%) and TMP conversion (22 and 68%, respectively). This can be rationalized if we remember that generation of active peroxo species from V2-POM and H2O2 strongly depends on the protonation state of V2-POM.37,40,113 For successful oxidation of TMP, it has to be present, at least, in the monoprotonated state [γ-PW 10 O 38 V 2 (μ-O)(μ− OH)]4−.39 Strong electrostatic binding of V2-POM with NH3+-SiO2 (the latter is generated upon acid treatment of NH2-SiO2) or anion exchange with MIL-101 may lead to 1304


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Finally, we should compare V2-POM/N-CNTs with a mesoporous titanium silicate catalyst, which has been considered so far as being the most effective heterogeneous catalyst for TMP oxidation with hydrogen peroxide.6,49 Figure 12 shows a comparison of the most important characteristics of

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03933. Characterization details of V2-POM (FTIR, 31P and 51V NMR spectra) and quinone products (GC/MS and 1H NMR spectra), XPS, SEM, and water capacity data for NCNTs (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for O.A.K.: [email protected]. ORCID

Olga Yu. Podyacheva: 0000-0001-7313-8682 Oxana A. Kholdeeva: 0000-0002-5315-5124 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The help of A. N. Serkova and T. Ya. Efimenko in SEM and N2 adsorption measurements, respectively, is greatly appreciated. This work was partially carried out in the framework of the budget project No. 0303-2016-0005 for the Boreskov Institute of Catalysis.

Figure 12. Comparison of catalytic performances of V2-POM/NCNTs and mesoporous titanium silicate Ti-MMM-E49 in TMP oxidation with H2O2 (TOF, turnover frequency; STY, space−time yield).

■

the two catalyst systems. One can see that conversion and selectivity are quite similar (both >99%), but V2-POM/NCNTs demonstrates clear benefits in H2O2 utilization efficiency, activity (TOF), and space−time yield (STY).

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4. CONCLUSION The divanadium-substituted γ-Keggin phosphotungstate V2POM was immobilized for the first time on undoped and Ndoped CNTs by adsorption from MeCN solution. The key factors that ensure successful immobilization of V2-POM are predrying of the supports and addition of mineral acid. Both factors strongly affect the maximum POM adsorption, which attains 31 and 35% for CNTs and N-CNTs, respectively. The addition of acid is also required to accomplish irreversible binding of V2-POM to the surface with retention of the POM structure and catalytic properties. Doping of CNTs by nitrogen enables molecular dispersion of V2-POM on the carbon surface, which in turn ensures excellent catalytic performance. The V2-POM/N-CNTs materials proved to be highly efficient catalysts for the selective oxidation of alkylphenols to the corresponding p-benzoquinones using aqueous H2O2 as a green oxidant. Using the optimal catalyst (15 wt % V2-POM and 1.8 atom % N), the vitamin E precursor TMBQ can be produced in the yield as high as 99% with 80% oxidant efficiency. The catalyst reveals unprecedentedly high TOF (500 h−1) and STY (450 g L−1 h−1). Moreover, the catalyst does not suffer POM leaching under the turnover conditions, demonstrates truly heterogeneous nature of the catalysis, and shows excellent recycling performance. The results acquired in this work clearly demonstrate advantages of N-doped CNTs, in terms of achievable conversion and selectivity as well as reusability, as supports for immobilization of V2-POM. We may suppose that the approach developed in this work might be useful for immobilization of other POMs that require preservation of the protonation state after immobilization in order to maintain their unique catalytic performance. Further studies are in progress in our group to verify this idea. 1305


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