The direct introduction of hydroxy groups into arenes is an important process in both fundamental and applied chemistry across a wide variety of fields ranging from resins and plastics to agrochemicals and pharmaceuticals ^[1]. Traditionally, phenols with one or more functional groups have been produced industrially in multistep processes. The direct hydroxylation of arenes to phenols would therefore be a great improvement and attracts much attention despite being a challenging subject ^{[2, 3]}. To meet the needs of a green catalytic process, environmentally-friendly oxidants (such as H₂O₂ ^{[4, 5, 6, 7, 8, 9, 10, 11, 12]}, N₂O ^{[13, 14]}, and O₂ ^{[15, 16]}) have been investigated for the hydroxylation of arenes to phenols. Vanadium complexes are active towards the oxidation of a hydrocarbon with O₂, H₂O₂, and other oxygen donors ^{[17, 18, 19, 20]}. As O₂ is abundant and cheap, many investigations have targeted efficient catalysts for the hydroxylation of benzene to phenol that can use molecular oxygen as the oxidant ^{[21, 22, 23, 24, 25, 26]}. However, the hydroxylation reaction has been limited to the hydroxylation of benzene to phenol, and few reports have been made on the hydroxylation of arenes containing functional groups. Using H₂O₂ as oxidant, the hydroxylation of arenes with one or more substitutes has been achieved with a divanadium-substituted phosphotungstate catalyst under extreme reaction conditions, using a molar ratio of arene to H₂O₂ of 50:1 ^[27]. However, the reaction system was limited to the arenes with electron-donating groups. Some substituted 2-phenylpyridines have been regioselectively oxidized to the substituted 2-(pyridin-2-yl)phenols when catalyzed by PdCl₂ and using N-hydroxyphthalimide (NHPI) and O₂ ^[28]. However, both the noble metal Pd catalyst and NHPI are expensive and are required for the hydroxylation. Pd-catalyzed ortho-& lt; span lang="EN-US" style='font-size:2.0pt; color:black'> hydroxylation of benzoic acids has also been developed ^[29]. Oxygenase has been used in the oxidation of arenes, with the hydroxylation of electron-deficient arenes being carried out at room temperature under air ^[30], but an excess strong base was used. H₅PMo₁₀V₂O₄₀ has been found to be active for the ortho-hydroxylation of nitrobenzene with molecular oxygen, although the activity was low ^[31]. The hydroxylation of arenes remains limited when functional groups are present. Recently, we reported on immobilized polyoxometalate catalysts containing vanadium, which were reusable and highly active for the selective hydroxylation of benzene ^{[32, 33]}.

More recently, there has been growing interest in the use of carbon nitrides. Mesoporous carbon nitride (mpg-C₃N₄) is predicted to be the most stable allotrope in ambient conditions, and numerous articles have been published on the synthesis of mpg-C₃N₄ and its derivatives ^{[34, 35, 36]}. mpg-C₃N₄ can be directly used for oxidation ^{[37, 38, 39]} and Friedel-Crafts reactions ^[40]. Moreover, metal doped mpg-C₃N₄ has been explored for use as catalysts ^{[41, 42]}. V@mpg-C₃N₄ was found to be highly active in the hydroxylation of benzene to phenol ^{[43, 44]}. Based on these results, a vanadium-doped graphitic carbon nitride (V@CN) catalyst was prepared using dicyandiamide as the carbon nitride source and vanadiumoxy acetylacetonate as the vanadium precursor. V@CN was investigated for the direct hydroxylation of functional arenes using O₂. Benzenes with electron- withdrawing groups such as CN, NO₂, COOH, CF₃, and COCH₃ were oxygenated to the corresponding phenols. The catalyst was also tested for applicability for the hydroxylation of aromatic halides (F, Cl, and Br).

All reagents were purchased from the Aladdin Reagent Company, Sigma-Aldrich Company, and Alfa-Aesar Company and used without further purification. X-ray diffraction (XRD) patterns were collected on the Bruker D8 Advance powder diffractometer using Ni-filtered Cu K_α radiation source at 40 kV and 20 mA, from 5° to 80° at a scan rate of 0.5°/min. The BET surface areas were measured at −196 °C with liquid nitrogen using a Micromeritics ASAP 2010 analyzer. The samples were degassed at 150 °C under a vacuum of 10⁻³ Torr before analysis. The CHN elemental analysis (EA) was performed on an elemental analyzer Vario EL cube. Thermogravimetric (TG) analysis was carried out with a STA409 instrument under dry air at a heating rate of 10°C/min. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al K_α radiation. The base pressure was about 3×10⁻⁹ mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. ESR spectra were recorded on a Bruker EMX-10/12 spectrometer at the X-band. The amount of vanadium species was measured using a Jarrell-Ash 1100 ICP-AES spectrometer (inductively coupled plasma-atomic emission spectrometry).

Dicyandiamide (3.0 g, 35.7 mmol) and vanadiumoxy acetylacetonate (0.3 g, 1.1 mmol) were dissolved in 200 mL of absolute ethanol with stirring at 50 °C for 12 h. The solvent was removed by rotary evaporation and a white solid was obtained. The solid was transferred into a tube furnace and heated to 150 °C under a flow of N₂, and then heated to 550 °C at a rate of 4 °C/min and kept at 550 °C for 4 h. The resulting black powder (V@CN) was cooled to room temperature under N₂. Other metal doped CN catalysts, including Cu@CN, Fe^Ⅲ@CN, and Fe^Ⅱ@CN were synthesized in a similar process. The metal precursors used for preparing these catalysts were cupric(II) acetylacetonate, iron(III) acetylacetonate, and iron(II) acetylacetonate, respectively.

Hydroxylation reactions of aromatics to the corresponding phenols were carried out in a 50-mL stainless steel autoclave with a Teflon liner and magnetic stirring. Typically, 10 mmol of the arene, 100 mg of V@CN (0.11 mmol V), 1.5 mmol of ascorbic acid, and 10 mL of aqueous acetic acid (50 vol%) were sealed in the autoclave. The autoclave was then flushed three times with O₂, pressurized with O₂ to 1.5 MPa, and transferred into 105 °C oil bath with stirring. The reactor was kept at this temperature for 11 h. After the reaction mixture was cooled to room temperature, dioxane (100 μL) was added to the mixture as an internal standard for product analysis. The mixture was analyzed by gas chromatography (GC). The yields of phenols were calculated as mmol phenol/mmol initial benzene.

After the reaction was finished, the reaction mixture was cooled to room temperature. The catalyst was separated by centrifugation and washed with diethyl ether three times. The obtained catalyst was then used directly for the next run.

V@CN and the catalyst precursor (the mixture of dicyandiamide and vanadiumoxy acetylacetonate) were analyzed by XRD (Fig. 1). A number of sharp Bragg peaks appeared for the catalyst precursor, which meant that it contained at least one crystalline phase. These peaks disappeared in V@CN, and a single broad peak at 2θ = 27.5° was observed. This was assigned to the interlayer diffraction from a graphitic structure ^{[45, 46]}. There was no diffraction peak found for a vanadium species in V@CN, suggesting a lack of crystalline vanadium species in the carbon nitride material. These observations show that the vanadium was uniformly doped into the composite during the formation of a carbon nitride material. The N₂ adsorption-desorption isomer is shown in Fig. 2. V@CN had a BET surface area of about 18 m²/g. The EA results were consistent with graphitic carbon nitride (found: C, 26.86 wt%; H, 2.32 wt%; N, 32.33 wt%). Results from ICP-AES revealed that the amount of vanadium in the catalyst was 1.1 mmol/g (10 wt%), which was consistent with the TG result (Fig. 3).

Fig. 1. XRD patterns of the mixture of dicyandiamide and vanadiumoxy acetylacetonate (1), recycled V@CN after four runs (2), and V@CN (3).

Fig. 2. N2 adsorption-desorption isotherm of V@CN.

Fig. 3. TG curve of the catalyst V@CN.

To understand the surface composition of the catalyst, in particular the valence state of the vanadium, V@CN was characterized by XPS and ESR. Figure 4(a) shows the high resolution V 2p XPS spectrum. The two peaks centered at 524.5 and 517.6 eV corresponded to V 2p_1/2 and V 2p_3/2, suggesting the presence of a V⁵⁺ species. The other two peaks located at 523.4 and 514.9 eV corresponded to V 2p_1/2 and V 2p_3/2, indicating the presence of a V⁴⁺ species ^[47]. The C 1s spectrum (Fig. 4(b)) has two peaks at 284 and 287 eV. The peak at 284 eV is assigned to carbon bonded to other carbon atoms. The peak at 287 eV is attributed to carbon atoms bonded to three nitrogen atoms in the g-C₃N₄ lattice ^[47]. The XPS spectra of N 1s (Fig. 4(c)) showed two nitrogen species. The peak at 398.7 eV is attributed to nitrogen bonded to two carbon atoms (C-N-C). The peak at 399.8 eV is assigned to an amino functional groups ^[43]. To obtain more information on the valence state of the vanadium, the catalyst V@CN was also characterized by ESR. As shown in Fig. 5, the sharp and strong ESR signals for V@CN confirm again the coexistence of V⁵⁺ and V⁴⁺ species. The coexistence of V⁵⁺ and V⁴⁺ species may be highly useful for the hydroxylation of benzene. Generally, V⁴⁺ species can react with O₂ to generate active peroxide intermediate species V⁵⁺-O₂ for the hydroxylation of benzene. After the hydroxylation, V⁵⁺ species are left, which need to be reduced back to V⁴⁺ species by the ascorbic acid.

Fig. 4. High resolution V 2p (a), C 1s (b), and N 1s (c) XPS spectra and proposed deconvolution.

Fig. 5. ESR pattern of the catalyst V@CN.

The optimization of the reaction conditions (including the amount of catalyst and ascorbic acid, acetic acid concentration, and reaction time) was performed and the results are shown in Fig. 6. The phenol yields (at 105 °C with 1.5 MPa O₂) were depended on the amount of catalyst, acetic acid concentration, amount of ascorbic acid, and reaction time. The optimized conditions are as follows: 100 mg of catalyst (0.11 mmol V), 1:1 of H₂O/acetic acid volumetric ratio (5 mL), 1.5 mmol of ascorbic acid, and an 11-h reaction.

Fig. 6. The influences of the amounts of catalyst (a) and ascorbic acid (b), acetic acid concentration (c), and reaction time (d) on phenol yields in the hydroxylation of benzene. Reaction conditions: benzene (5 mmol), catalyst (0.11 mmol metal), solvent (5 mL), O2 (1.5 MPa), 105 °C.

Cu, Fe, and V based catalysts have been widely investigated for the hydroxylation of benzene to phenol with H₂O₂. Thereafter, we synthesized the Cu, Fe, and V doped carbon nitrides catalysts. These catalysts were tested for the hydroxylation of benzene to phenol with O₂. No phenol was obtained in the absence of catalyst and ascorbic acid (Table 1, entry 1). When 1.5 mmol ascorbic acid was added into the reaction, phenol was detected in 4.5% yield (Table 1, entry 2). The hydroxylation of benzene with metal-free CN also gave phenol in 4.3% yield in the absence of ascorbic acid (Table 1, entry 3). Cu^Ⅱ, Fe^Ⅱ, and Fe^Ⅲ doped CN catalysts were found to be active for the direct hydroxylation of benzene to phenol (Table 1, entries 4-6) with the highest phenol yield of 16.9% using the Fe^Ⅱ@CN catalyst (Table 1, entry 6). Compared with the other catalysts, under the same conditions, V@CN was much more active, and phenol was obtained in 23.0% yield (Table 1, entry 7). The results showed that V@CN was suitable for the direct hydroxylation of benzene to phenol with O₂. Additionally, homogeneous VO(acac)₂ was also active, and yielding phenol at 18.1%.

Table 1 The hydroxylation of benzene to phenol over various catalysts. Entry Catalyst Conversion (%) Phenol yield (%) 1 None 0 0 2 Ascorbic acid 4.5 4.5 3 CN * 4.3 4.3 4 Cu@CN 10.9 10.9 5 FeⅢ@CN 10.6 10.6 6 FeⅡ@CN 16.9 16.9 7 V@CN 23.0 23.0 8 VO(acac)2 18.1 18.1 Reaction conditions: benzene (5 mmol), catalyst (0.11 mmol metal), ascorbic acid (1.5 mmol), H2O (5 mL), acetic acid (5 mL), O2 (1.5 MPa), 105 °C, 11 h. * 100 mg CN, without ascorbic acid.

Table 1 The hydroxylation of benzene to phenol over various catalysts.

Because the hydroxylation of functional arenes has been only rarely reported, we tried to extend the hydroxylation reaction to functionalized arenes, especially those with electron- withdrawing groups. With the optimal reaction conditions, we examined the hydroxylation of a series of arenes using V@CN with O₂, and the results are listed in Table 2. Aryl halides such as the fluorobenzene, chlorobenzene, and bromobenzene were transformed into the corresponding phenols in moderate yields in a distribution of ortho- (o) and para- (p) isomers, and no detectable by-products (Table 2, entries 1-3). Moreover, no meta-substituted phenols were detected (Table 2, entries 1-3). Functional arenes with electron-withdrawing groups such as NO₂, CN, CH₃CO, CF₃, and COOH were also oxygenated to the corresponding phenols in moderate yields (Table 2, entries 4-7). But the isomeric distributions in these reactions (o-, m-, and p-) were irregular. Interestingly, no reaction was observed for the hydroxylation of anisole under the same reaction conditions (Table 2, entry 9).

Table 2 Hydroxylation of arenes to phenols with O2 catalyzed by V@CN.

Table 2 Hydroxylation of arenes to phenols with O2 catalyzed by V@CN.

3.4. Catalyst reusability

During the reaction, the catalyst V@CN was practically insoluble in the reaction system, and the heterogeneous catalysts can be recovered conveniently by centrifugation after each reaction run. The reusability of V@CN was tested and the results are shown in Fig. 7. No significant loss of conversion and selectivity was observed after four runs, which shows that V@CN can be reused at least four times. To test if the vanadium species was leaching into solution, we measured the vanadium content of the catalyst after the 4th reaction by ICP-AES, and about 15% of the vanadium was lost after four reaction cycles.

Fig. 7. Catalytic reusability of V@CN in the hydroxylation of benzene to phenol. Reaction conditions: benzene (5 mmol), catalyst (100 mg, 0.11 mmol V), ascorbic acid (1.5 mmol), H2O (5 mL), acetic acid (5 mL), 105 °C, 11 h.

Scheme 1. The proposed reaction mechanism.

The vanadium doped carbon nitride catalyst (V@CN) was designed and prepared and shown to be active for the direct hydroxylation of arenes to phenols with O₂. Functional arenes with electron-withdrawing groups such as CN, NO₂, COOH, CF₃, and COCH₃ were oxygenated into the corresponding phenols in moderate yields. The catalyst was also applicable for the hydroxylation of aromatic halides (F, Cl, and Br). This catalyst showed the potential for vanadium in the activation of C-H bonds in functionalized arenes with electron-withdrawing groups.