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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 Suboch, Olga Yu. Podyacheva, Olga A. Stonkus, Vladimir I. Zaikovskii, Yurii A. Chesalov, Lidiya S. Kibis, and Oxana A. Kholdeeva ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03933 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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ACS Catalysis

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)

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 divanadiumsubstituted 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 at% 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 2,3,6-trimethylphenol (TMP) with H2O2 under mild reaction conditions (60 ºC, 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 applying the optimal catalyst V2-POM/N-CNTs enclosing 15 wt% of V2-POM and 1.8 at% of N, TMBQ could be obtained with 99% yield and 80% oxidant utilization efficiency. The catalyst demonstrated 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 1

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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 economic and ecological route to the production of vital BQs, in particular trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate).4-7 Nowadays, TMBQ is manufactured through oxidation of 2,3,6trimethylphenol (TMP) with molecular oxygen in the presence of quasi-stoichiometric amounts of copper chloride.4 The inherent drawbacks of this process are the formation of Cl-containing by-products and the need for corrosion-resistant equipment. A recent attempt of reducing the amount of CuCl2 involved the use of an ionic liquid, 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), coupled with n-butanol as co-solvent.8 Co(II)-Schiff base complexes were suggested as catalysts for oxidation of substituted phenols to BQs with molecular oxygen,9,10 but salen ligands suffer oxidative destruction under turnover conditions. Recently, gold nanoparticles 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 1,2-quinones using atmospheric air as the sole oxidant, but the reaction with 2,6-di-t-butylphenol led to the formation of C–C coupling product, 3,3,5,5tetra-t-butyl-4,4-diphenoquinone, rather than corresponding BQ.11 Although molecular oxygen remains the oxidant of choice from the economic viewpoint, catalysis based on the activation of the alternative green oxidant – aqueous H2O2 – is a rapidly 2


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developing field.12-16 Several homogeneous catalyst systems, including methyltrioxorhenium MeReO3,17,18 ruthenium and iron compounds/complexes,19-22 have been developed for the selective oxidation of functionalized phenols with H2O2. A 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, as well as fairly good hydrolytic and solvolytic stability in typical conditions of liquid-phase oxidation.24-27 All these provide advantages for POMs over conventional catalysts containing organic and organometalic ligands that are prone to oxidative degradation and/or hydrolysis. Commercially available heteropoly acids 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 corresponding BQs in moderate yields.28 Alkylphenols could be also converted to BQs with reasonably good yields through oxidation with molybdovanadophosphoric heteropoly acids H3+nPMo12−nVnO40 (HPA-n, n = 2–6) and their re-oxidation with molecular oxygen.29-31 Recently,

Mizuno

and

coworkers

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 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 be in principle recycled, but the presence of traces of metals in the quinone product cannot be excluded. 3


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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 oxidation of alkyl-substituted phenols,43-49 but catalyst stability and oxidant utilization efficiency still leave a room for improvements.6,23,50,51 Therefore, the development of a truly heterogeneous and recyclable catalyst for the selective oxidation of alkylphenols to corresponding BQs would have a major impact in industrial applications as well as in 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 hydroxides69,70 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, non-specific interactions with the support.52,8284

The application of POM-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 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 polar reaction media typically used for phenols oxidation.54,85 In this work, we explored different approaches to immobilization 4


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of V2-POM and assessed the elaborated solid catalysts in the practically important H2O2-based oxidation of TMP to TMBQ (Scheme 1).

O

OH V 2-POM/Support H2O 2 TMP

O TMBQ

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 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 singlewall carbon nanotubes (SWCNTs).93 Few reports were devoted to the application of POM/CNTs composites as catalysts for oxidations with H2O2.92,94 Unfortunately, hot filtration test95 was not reported to prove unambiguously 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), that provide a variety of adsorption sites.96,97 Arrigo et al. successfully employed N-doped 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 aerobic 5


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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 immobilization of a catalytically active POM, namely V2POM, on N-CNTs, provide characterization of the new materials by physicochemical techniques (N2 adsorption, SEM, TEM, XPS, and FTIR spectroscopy) and evaluate 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 catalysis was addressed and advantages of V2-POM/N-CNTs, in terms of activity, selectivity and recyclability, over solid V2-POM catalysts prepared by other immobilization techniques have been demonstrated.

2. Experimental 2.1. Materials. 2,3,6-Trimethylphenol (TMP, 97+%) was obtained from Fluka. 2,3,5trimethylphenol (2,3,5-TMP, 99%), 2,6-di-tert-butylphenol (DTBP, 99%), 2,6-dimethylphenol (2,6-DMP, 99%), 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ, 98%), 3,5-dimethylphenol (3,5DMP, 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. Catalysts and supports preparation. V2-POM in the form of the acid TBA salt TBA3.5H1.5[γ-PW10O38V2(µ-O)(µ-OH)] (TBA stands for n-Bu4N) was prepared following the procedure reported previously.40 The compound purity was confirmed by FTIR (Figure S1 in Supporting Information (SI)), 51V and 31P NMR spectroscopy (Figures S2 and S3, respectively). NH2-modified silica was obtained by post-synthetic modification of SiO2.53 MIL-101 was synthesized according to the method described by Férey et al.101 with some modifications.102 6


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ACS Catalysis

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 a cationic form NH3+-SiO2. A 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 the reasons of the 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 pH reached 2. The resulting mixture was allowed to stay for one hour at 25 oC and then the temperature was raised to 60 oC. After 16 h, a hydrogel was formed. The hydrogel was dried in vacuum at 80 oC for 2 h. Then the solid was washed with hot water and again dried in vacuum at 80 oC. The resulting material was thoroughly washed with MeCN until the liquor became colorless. All 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 a Fe-Ni-Al2O3 catalyst by decomposition of a pure C2H4 or C2H4/NH3 mixtures at 700 oC, 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. Afterwards 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 Ar flow at 170 oC. The water capacity of N-CNTs was measured by a standard incipient wetness impregnation method. Prior to use, CNTs and N-CNTs were extra dried in vacuum at 100 oC. Adsorption of V2POM was carried out at room temperature from MeCN solution. To enhance the adsorption capacity, HClO4 was added (1 equiv to POM). The completion of the adsorption process was

7


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controlled by UV–vis. The resulting solid material was separated by filtration, washed with MeCN three times and dried in vacuum at 60 oC. 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) preliminary dried in vacuum at 100 oC was placed into the vessel, and a solution of V2POM in acetonitrile (2 mL, 0.1–2 mmol/L) was added. The mixture was stirred at 500 rpm and samples of the solution were taken by a syringe (100 µL) after 1 h (this time is enough to reach the adsorption/desorption equilibrium), diluted with MeCN (900 µL), and the V2-POM concentration in the solution was determined by UV–vis (λ =34 2 n m, l = 0 . 1 c m ) 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 and a sample of the solution was taken by a syringe (100 µL) after 1 h, diluted with MeCN (900 µL), and the concentration of V2POM 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 V2POM and preliminary dried in vacuum at 60 oC, 1 mL MeCN, 60 oC, 10–20 min. The reactions were started with the addition of H2O2. The reaction products were identified by GC/MS and 1H NMR (see SI) and quantified by GC using biphenyl as internal standard. Before reuse, the catalyst was separated by filtration, washed with acetonitrile and evacuated at 60 oC. 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 oC, and samples of the solution (300 µL) were obtained 8


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ACS Catalysis

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 samples characterization. GC analyses were performed using a gas chromatograph Chromos GC-1000 equipped with a flame ionization detector and a quartz capillary column BPX5 (30 m × 0.25 mm). 1H,

31

P 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 (JЕОL 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 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 JSM-6460 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 a non-monochromatic AlKα radiation (hv 1486.6 eV). The Au4f7/2 and Cu2p3/2 core-level lines with binding energies 84.0 eV 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 corresponded 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: C1s, N1s, O1s, P2p, V2p, W4f were collected. The atomic sensitivity factors for each element105 were taking into account for the quantitative estimations. The analysis of the spectra was performed using the XPS-Calc

9


ACS Catalysis

program106,107 after Shirley background subtraction. The nitrogen content in N-CNTs was defined as N/C ratio (at%) from XPS data.

3. Results and discussion 3.1. Adsorption studies and immobilization of V2-POM 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 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.

N-CNTs with HClO 4

0.40

CNTs with HClO 4

0.35

V2-POM content, g/g

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

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0.30 wet CNTs with HClO 4

0.25 0.20

CNTs with TBAClO 4

0.15 N-CNTs

0.10

CNTs

0.05 0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

[V 2-POM], mM

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

Without any additives, the maximal amount of V2-POM adsorbed on the preliminary 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 maximal adsorption that reached 35 and 31 wt% for NCNTs and CNTs, respectively. The use of TBAClO4 instead of HClO4 led to reduction of the maximal POM sorption (see Figure 1). Wetness of N-CNTs turned out a critical parameter that greatly affected the POM adsorption process and catalysis (vide infra). Without pre-drying of 10


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ACS Catalysis

CNTs in vacuum, the value of V2-POM adsorption could reach only 22 wt%, even in the presence of HClO4. 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 V2-POM markedly increased and attained 22 and 17%, for N-CNTs and CNTs respectively (Figure 2). Therefore, the presence of nitrogen in CNTs is not an indispensable factor required for 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 interaction between POM anion and protonated pyridinic nitrogen of N-CNTs contribute into the POM immobilization. However, in contrast to other supported catalysts where POM immobilization was realized through columbic 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 not only electrostatic interactions are 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 non-covalent interactions, such as hydrogen bonding and van der Waals contacts, may take place.

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0.35

N-C NT C NTs

0.30

V2-POM content, g/g

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0.25 0.20 0.15 0.10 0.05 0.00 0.0

0.2

0.4

0.6

0.8

1.0

1.2

[V 2 -POM], m M

Figure 2. Adsorption (■ and ●) and desorption (□ and ○) isotherms for V2-POM on N-CNTs (4.8 at% N) and CNTs. MeCN, 25 °C, 1 equiv. of HClO4 was added. FT-IR spectroscopy was used to verify the retention of the V2-POM structure after immobilization. FT-IR spectra of N-CNTs (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 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 of the immobilized V2-POM, indicating preservation of its structure. One can notice some redshifting of the peak maxima relative to the initial POM, which could be due to a strong interaction between POM and the solid support. 12


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a)

b) A

B

1260

1590

3420

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ACS Catalysis

C

3 2 1 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber, cm

1000

950

900

850

Wavenumber, cm

800

750

-1

Figure 3. (a) FT-IR spectra of (1) N-CNTs (4.8 at% 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% V2POM/N-CNTs, and (C) 8 wt% V2-POM/N-CNTs after TMP oxidation. The spectrum of N-CNTs (4.8 at% N) was used for subtraction in B and C.

The values of POM loading in the catalysts evaluated by elemental analysis were in good agreement with UV-vis measurements on the MeCN solution remained 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 SI): NPy (398.3–398.5 eV), NPyr (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 increasing the total N content and reached 26% in N-CNTs containing 4.8 at% 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.

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Table 1. Physicochemical properties of N-CNTs before and after V2-POM immobilization

a

Na

POMa

SBETb

Vporec

Dpored

at%

wt%

m2/g

cm3/g

nm

0

-

167

0.86

20

0

15

132

0.64

19

1.8

-

170

0.59

14

1.8

15

145

0.64

15

4.8

-

157

0.53

14

4.8

8

147

0.53

14

Determined by elemental analysis; b specific surface area; c mesopore volume; d mean pore

diameter.

The surface area of N-CNTs just slightly decreased after deposition of V2-POM, while mesopore volume and average pore diameter remained nearly intact. Studies by SEM (Figure S5 and S6 in SI) demonstrated no evident changes in the N-CNTs particles 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. 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. Figures 4b and 4c show images obtained in the HAADF-STEM mode, where POM particles give a bright light contrast. It is possible to distinguish individual particles 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 14


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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.

a)

b)

c)

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

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 N-CNTs.

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

3.2. TMP oxidation over V2-POM/N-CNTs 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 the critical factor in the catalyst preparation, the catalytic experiments were carried out after catalyst drying in vacuum at 60 oC. Figure 6 shows the effect of V2-POM content on the catalytic performance of V2-POM/ N-CNTs materials along with a blank 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, 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 N-CNTs was negligible and the main oxidation product in this case was a C–C coupling product, 2,2’,3,3’,5,5’-hexamethyl-4,4’-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 16


Page 16 of 37

Page 17 of 37

quinone selectivity could be further improved by increasing POM loading. With 15 wt% of V2POM, it reached >99% and then remained constant (Figure 6).

Conversion Selectivity

100 80 60 %

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

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40 20 0 0

5

10

15

25

V 2 -POM content, wt% Figure 6. Effect of V2-POM content on TMP oxidation with H2O2 over of V2-POM/N-CNTs (4.8 at% N). Reaction conditions: 0.1 M TMP, 0.45 M H2O2, catalyst 1.5 mM V2-POM, 1 mL MeCN, 15 min, 60 oC.

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 at% of N is enough to accomplish TMP oxidation with excellent selectivity. Therefore, the presence of both V2-POM and nitrogen are crucial for the 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 both catalytic performance and catalyst cost, can be inferred as 1.8 at% N and 15 wt% V2-POM. In order to verify how catalyst wetness affects catalytic performance, several catalytic tests were also carried out without pre-drying 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 ones, except for the catalyst with 1.8 at% N (compare Figure 7a and 17


ACS Catalysis

b). To understand this phenomenon, we studied the dependence of water capacity of N-CNTs on nitrogen content. Figure S7 in the SI shows that the increase in nitrogen content in N-CNTs and, in particular, NPy/NQ ratio, leads to the increase in water capacity of the carbon nanotubes. Therefore, we may assume that surface hydrophilicity of N-CNTs increasing with N content disfavors adsorption of phenol substrate and, on the contrary, favors adsorption of H2O2 and its unproductive decomposition. To verify this hypothesis hydrogen peroxide decomposition over N-CNTs was studied.


(a)


(b)

100

80

80

60

60 %

100

%

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

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40

40

20

20

0

0

0

1.8

4.8

Nitrogen content, at%

0

1.8

3.1

4.8


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

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 growing 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 CNTs particles and is not available for reactants. Previous studies on N-CNTs by Raman spectroscopy revealed that 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 18


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ability of N-CNTs caused by the growing amount of N and, in particular NPy (Figure S7), which facilitates H2O2 adsorption and decomposition.

6

W0, mM/min

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

ACS Catalysis

4

2

0 0

2

4

6

8

10


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

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 at% 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 N-CNTs with 1.8 at% 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 CNTs support (Figure 9). With the optimal catalyst (15 wt% V2-POM and 1.8 at% N), the oxidant efficiency reaches 80%, the value previously attained with homogeneous V2POM.39

19


ACS Catalysis

H2O2 efficiency TMBQ Yield 100

H2O2 efficiency/TMBQ Yield, %

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

Page 20 of 37

75

50

25

0 V2-POM/N-CNT V2-POM/N-CNT (4.8 at% N)

V2-POM

(1.8 at% N)

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 V2-POM/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 2,6-DTBP readily proceeded to give the corresponding p-BQs with high yields close to those reported for homogeneous V2-POM.39 On the contrary, 3,5-DMP and 2,3,5-TMP showed only 10–15% conversion. Actually, 2,3,5-TMP was less reactive than 2,3,6TMP under the same conditions when homogeneous V2-POM was employed [39]. This finding agrees with the previous results acquired for hydroxylation of alkylbenzenes catalyzed by V2POM37,38,113 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 has 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 at a comparable steric factor .

20


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Table 2. Oxidation of various alkylphenols with H2O2 catalyzed by V2-POM/N-CNTs

Substrate

Conversion, %

Selectivity, %

100

99

15

10

10

0

90

90

30

88

94

90

OH

OH

OH

OH

OH

OH t-Bu

t-Bu

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

3.5. Catalyst stability and recyclability Recyclability and stability of the optimal catalyst with 15 wt% V2-POM and 1.8 at% N in N-CNTs was assessed under standard reaction conditions. The catalyst could be reused, at least 6 times, without appreciable losses of activity and selectivity (Figure 10).

21


ACS Catalysis

Conversion Selectivity 100 80

%

60 40 20 0 1

2

3

4

5

6

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

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 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.

100 80

TMBQ yield, %

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

Page 22 of 37

Filtration

60 40 20 0 0

5

10

15

20

25

Time, min

Figure 11. Hot filtration test for TMP oxidation over 15 wt% V2-POM/N-CNTs (1.8 at% N). Reaction conditions: 0.1 M TMP, 0.35 M H2O2, 1.0 mM V2-POM, 1 mL MeCN, 50 oC. 22


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Importantly, the surface of N-CNTs appeared stable toward oxidation under the conditions employed for TMP oxidation, as demonstrated by XPS. The ratios of О/C and N/C 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 ratio of О/C greatly increased and new functional O-containing groups appeared on the N-CNF surface. The observed discrepancy can be explained by much harsher conditions employed for the wet phenol oxidation (160 oC, 10 bar O2) and different structure of N-CNFs (herringbone) and N-CNTs (bamboo-like).

Table 3. Ratios of N/C and O/C in the initial support N-CNTs (4.8 at% N), after immobilization of V2-POM (8 wt%), and after TMP oxidation obtained by XPS. Sample

N/С, at%

О/C, at%

N-CNTs

4.8

4.4

V2-POM/N-CNTs

4.7

5.8

V2-POM/N-CNTs after catalysis

4.8

5.5

3.6. Comparison of various immobilization techniques The catalytic performance of V2-POM supported on N-CNTs 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.

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Table 4. TMP oxidation with H2O2 over various supported V2-POM catalysts

Catalyst

TMP conversion, % TMBQ selectivity, %

V2-POM (homogeneous)

100

99

V2-POM/N-CNTs

100

99

V2-POM/MIL-101

22

68

V2-POM/NH2-SiO2

68

68

V2-POM/Sibunit

86 (50)a

90 (23)a

V2-POM@SiO2

98b

98

Reaction conditions: 0.1 M TMP, 0.35 M H2O2, catalyst 40–60 mg (1.5 mM V2-POM), 1 mL MeCN, 15 min, 60 oC. a

After reuse; b substantial POM leaching (ca. 15%) and homogeneous catalysis were observed.

One can see that V2-POM electrostatically attached to the surface of MIL-101 and NH2SiO2 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 [γPW10O38V2(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 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 V2POM/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 sol24


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gel approach may lead to the preparation of highly active and leaching-tolerate POM-comprising microporous catalysts.53,62,67 However, since V2-POM is not soluble in 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 with the size of 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 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 of using 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 silica surface could accomplish 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 oxidation of TMP to TMBQ with 72% selectivity,116 but the nature of the catalysis remained unclear. Supported PMo-V Keggin heteropolyacids also suffered from leaching and/or revealed relatively low selectivity or phenol conversion.117,118 Methyl rhenium trioxide supported on poly(4vinylpyridine) (PVP) demonstrated high selectivity and conversion toward p-BQs;119 however, the catalyst stability was rather low and 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. 25


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Page 26 of 37

Finally, we should compare V2-POM/N-CNTs with a mesoporous titanium-silicate catalyst, which was considered so far as the most effective heterogeneous catalyst for TMP oxidation with hydrogen peroxide.6,49 Figure 12 shows a comparison of the most important characteristics of the two catalyst systems. One can see that conversion and selectivity are quite similar (both >99%), but V2-POM/N-CNTs demonstrates clear benefits in H2O2 utilization efficiency, activity (TOF), and space-time yield (STY).

TMP conversion, % TMBQ selectivity, % H2O2 efficiency, %

500

TOF, h

400

-1

STY, g L

-1 -1 h

Volume yield, g L

300

-1

200 100 0 V2-POM/N-CNT

Ti-MMM-E

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

4. Conclusion The divanadium-substituted γ-Keggin phosphotungstate V2-POM was for the first time immobilized on undoped and N-doped CNTs by adsorption from MeCN solution. The key factors that ensure successful immobilization of V2-POM are pre-drying of the supports and 26


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ACS Catalysis

addition of mineral acid. Both factors strongly affect the maximal 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 selective oxidation of alkylphenols to corresponding p-benzoquinones using aqueous H2O2 as green oxidant. Using the optimal catalyst (15 wt% V2-POM and 1.8 at% N), the vitamin E precursor, TMBQ, can be produced with the yield as high as 99% and 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.

AUTHOR INFORMATION Corresponding Author *Oxana A. Kholdeeva. Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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ASSOCIATED CONTENT Supporting Information. 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. This material is available free of charge via the Internet at http://pubs.acs.org.

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.

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TOC graphic OH

Oxidant Efficiency 80%

O

H2O2 O

Selectivity 99%

Conversion 100%

N

N


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

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