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Catalytic Reaction Mechanism for Hydroxylation of Benzene to Phenol with H2O2/O2 as Oxidants

Mingjue Zhang , Changpo Fan , Long Wang , Xuejing Wu , Yu Zhou , Jun Wang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1026 -1041.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1026-1041 DOI: 10.7536/PC210501
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Catalytic Reaction Mechanism for Hydroxylation of Benzene to Phenol with H2O2/O2 as Oxidants
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Abstract

Activation of C—H bond is one of the most important scientific topics in organic synthesis area. Hydroxylation of benzene to phenol with environmentally friendly oxidants like H2O2 and O2 is a challenge in this filed for tens of years, since it involves not only the fundamental issue of the $\text C_{\text s \text p^{2}}— \text H $ bond activation for benzene, but also other issues such as the activation of H2O2/O2, decomposition of H2O2, and deep oxidation of phenol product. More importantly, in the context of green chemical industry, this green process becomes more attractive due to the promising alternative to replace the current cumene route. In this review, recent researches on metal-based catalysts and newly emerging non-metal catalysts are summarized, clued by the catalytic reaction mechanism of hydroxylation of benzene to phenol. The structure-activity relationship between catalysts’ composition and structure with their reactive activity and selectivity is analyzed in detail based on radical and non-radical mechanisms. Outlook and suggestions on the further development of rational design of catalysts and deeper insights into reaction mechanism in this field are proposed. This review is hopefully to benefit the exploring of novel catalysts with higher activity and better stability for hydroxylation of benzene to phenol.

Contents

1 Introduction

2 Metal-based catalysts for hydroxylation of benzene

2.1 H2O2-mediated radical mechanism

2.2 H2O2-mediated non-radical mechanism

2.3 O2-mediated radical mechanism

2.4 O2-mediated non-radical mechanism

2.5 O2-mediated synergistic catalytic mechanism

3 Non-metal materials for catalyzing hydroxylation of benzene

3.1 Radical mechanism over carbon materials catalysts

3.2 Radical mechanism over polymers catalysts

4 Conclusion and outlook

Graphical abstract

Keywords

benzene / phenol / hydroxylation reaction / metal-based catalysts / non-metal catalysts / reaction mechanism

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Mingjue Zhang, Changpo Fan, Long Wang, Xuejing Wu, Yu Zhou, Jun Wang. Catalytic Reaction Mechanism for Hydroxylation of Benzene to Phenol with H2O2/O2 as Oxidants. Progress in Chemistry, 2022, 34(5): 1026-1041 DOI:10.7536/PC210501

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1 Introduction

The activation and functionalization of aromatic C — H bonds in aromatic compounds is a well-known problem in academia for a long time. Through the activation of C — H bonds, relatively cheap hydrocarbons can be converted into oxygenates with high added value. Among them, the selective oxidation of benzene to phenol is one of the most attractive reactions in the C — H bond activation of aromatic rings. Phenol is an important and widely used bulk chemical raw material, which is mainly used in the production of phenolic resin, bisphenol A, caprolactam, aniline and other chemicals[1,2]. In recent years, the demand for phenol has continued to grow at a rate of about 3% per year, and the total global production of phenol is expected to reach 13.6 million tons by 2021[3]. Industrially, 90% of phenol is produced by the cumene process through the following three steps: (1) benzene alkylation to obtain cumene; (2) oxidizing cumene to obtain cumene peroxide; (3) Cumene peroxide redecomposes to produce phenol and acetone. However, there are many problems in the production process, such as high energy consumption, low phenol yield (about 5%), co-production of a large amount of acetone, potential safety hazards due to the production of explosive intermediates, equipment corrosion and environmental pollution caused by sulfuric acid used[4⇓~6]. In contrast, one-step hydroxylation of benzene to phenol with environmentally friendly oxidants such as H2O2 and O2 is an expected green process, which has been one of the research hotspots in the field of chemical industry for a long time[1,7]. However, the conjugated π bond of the benzene ring has high chemical stability, so it is difficult to activate the C — H bond on the benzene ring, resulting in the low reactivity of the catalyst. In addition, the kinetic study showed that the oxidation of benzene was a continuous series reaction process[8,9]. In the initial stage of the reaction, the conversion of benzene and the yield of phenol will increase with the extension of the reaction time, but the phenol produced is easier to be oxidized than benzene, and the phenol will continue to be converted into by-products such as diphenol, benzoquinone, and even carbon black and tar, which is the main reason for the low phenol selectivity of the benzene hydroxylation reaction and the difficulty to improve. Therefore, the efficient directional conversion of benzene to phenol remains one of the most challenging problems in organic synthesis.
In the study of hydroxylation of benzene to phenol, it has been the focus to explore catalysts with high activity and selectivity to improve the reaction activity and product selectivity. At the same time, the understanding of the reaction mechanism is also deepening. Many studies have shown that in the absence of external catalyst, no active oxygen species are formed in the reaction system under any conditions, and the yield of phenol is basically zero[4,9⇓⇓~12]. When a suitable catalyst is added, the catalyst first reacts with the reactant to form an intermediate species, thus reducing the activation energy of the target reaction, which is the most critical factor affecting the reaction results[13⇓~15]. Therefore, the rational design of catalyst based on the understanding of catalytic mechanism becomes the key in benzene hydroxylation. Moreover, the requirements of energy saving and green catalysis are also the key factors to be considered in catalyst design.
There have been some review articles summarizing the research progress of catalysts for benzene hydroxylation to phenol in different periods. For example, in 2013, Jiang et al. Reviewed the research progress of benzene hydroxylation catalyzed by metal-doped molecular sieve catalysts with H2O2 as oxidant[2]; In 2014, Wang Weitao et al. Summarized the research progress of benzene hydroxylation catalyzed by various metal-based catalysts using O2 as oxidant[1]; In 2020, Ottenbacher et al. Summarized the progress of homogeneous and heterogeneous catalysts for aromatic C — H bond oxidation in the past decade, including some benzene hydroxylation catalysts[16]. In recent years, the study of heterogeneous catalysts for benzene hydroxylation has made a lot of important progress. The abundance of characterization methods and the rapid development of computational chemistry have also enabled researchers to conduct a more in-depth analysis of the catalytic mechanism and reaction process of benzene hydroxylation. Moreover, the published review articles seldom take the catalytic mechanism of benzene hydroxylation as the main clue for induction and analysis. In this review, the reaction mechanism and structure-activity relationship of benzene hydroxylation catalyzed by various metal-based catalysts using environmentally friendly H2O2/O2 as oxidants are summarized, and the literatures involved are mainly published in the past decade. Especially, the reaction and mechanism of benzene hydroxylation catalyzed by non-metallic catalysts, which have emerged in recent years, are also summarized and expounded. In this paper, the catalytic mechanism of benzene hydroxylation and the latest progress in the design of corresponding heterogeneous catalysts are emphatically summarized and analyzed, and on this basis, the understanding of the catalytic reaction system is deepened.It provides a useful reference for further research and development of heterogeneous catalysts to improve the activity and selectivity of benzene hydroxylation, and inspires the practical application process of one-step benzene hydroxylation to phenol.

2 Hydroxylation of Benzene Catalyzed by Metal-Based Catalyst

Metal-based catalysts for phenol hydroxylation are mainly transition metal compounds, including metal-doped molecular sieves and titanium silicalite molecular sieves, vanadium catalysts, copper catalysts, iron catalysts, heteropolyacid catalysts, precious metal catalysts and other catalysts[2,4,13][17][6,18⇓~20][12,21,22][23][24⇓⇓⇓~28][29⇓~31][32,33]. The d orbital of the transition metal valence shell has an unpaired single electron that is easy to gain or lose, and has a variable valence state of oxidation or reduction, so it can catalyze redox reactions.
There are two ways for the selective oxidation of the C — H bond of the aromatic ring: (1) the oxidation is induced by the activation of the reaction substrate; (2) Reactive oxygen species produced by activated oxidants attack the reaction substrate. Because the C — H bond energy of the benzene ring is as high as 473.1 kJ/mol, it is difficult for the benzene hydroxylation reaction to produce phenol by activating the benzene ring first, so pathway (2) is usually dominant[34]. Benzene hydroxylation reactions are mostly carried out under mild liquid phase conditions, and the reaction system consists of benzene (reaction substrate), oxidant (H2O2/O2+ reductant), catalyst and solvent. In this section, the mechanism of benzene hydroxylation catalyzed by metal-based catalysts is introduced, and the relationship between the composition and structure of the catalysts and their catalytic properties is discussed. Considering that the difficulty and mechanism of catalyst activation are different when H2O2 and O2 are used as oxidants, they are discussed separately. According to the types of reactive intermediates produced in the reaction, free radical mechanism and non-free radical mechanism are involved when H2O2 is used as oxidant. When O2 is used as oxidant, free radical mechanism, non-free radical mechanism and two-active center coordinated catalytic mechanism are involved. Based on the above different reaction mechanisms, Table 1 summarizes the activities of typical metal-based catalysts for benzene hydroxylation in recent years, including phenol yield (or benzene conversion) and selectivity, as well as the corresponding reaction conditions.

2.1 Free radical reaction mechanism with H2O2 as oxidant

H2O2 itself has high activity and is easy to generate hydroxyl radical · OH or peroxy radical · OOH by homolysis under the action of catalyst, or is bonded with metal to form metal peroxy radical M-O-O ·[27,35,36][23,37][38]. Free radical is the active species of the reaction, and the formation of free radical is generally the rate-determining step in benzene hydroxylation[22,39]. The type, composition and structure of the transition metal in the reaction system, and the reaction conditions (such as the type of solvent, the acidity and basicity of the solution, etc.) Will affect the type of free radicals produced. Fig. 1 shows a typical hydroxyl radical reaction mechanism. Firstly, H2O2 is homolyzed into active radical · OH in the presence of catalyst, and then attacks the benzene ring to form hydroxycyclohexadiene radical, which is finally converted into product phenol by hydrogen transfer. There is an interconversion cycle between the oxidized state and the reduced state in the reaction process of the metal-based catalyst. Early benzene hydroxylation reactions catalyzed by Fenton reagents or Fenton-like reagents with Fe2+ and H2O2 as the main components followed the hydroxyl radical mechanism[35,36].
Jian et al. Used sodium metavanadate as a catalyst and proposed a mechanism for the production of metal peroxyl radicals M-O-O · during the reaction[38]. The reaction was carried out in acidic medium, and the vanadium species existed in the form of VO2+. With the addition of hydrogen peroxide, the VO2+ formed monoperoxovanadium species and diperoxovanadium species in turn, and then the vanadium-oxygen bond was homolyzed to form V(Ⅳ)-O-O· radicals, which attacked the benzene ring to form phenol, thus completing the catalytic cycle. Tsuji et al. Synthesized a homogeneous dinuclear copper molecular catalyst, and proposed a radical reaction mechanism by calculating the kinetic isotope effect (KIE) value (Fig. 2)[12]. In the catalytic process, under the action of hydrogen peroxide, the dinuclear copper first generates an active species precursor (CuO2H)2 with electrophilic properties, and further through the hydrogen bonding of water, ((CuO2H)2 is reversibly dissociated into Cu(Ⅱ)-O· and Cu(Ⅱ)-O-O· radicals, and the former attacks the benzene ring to generate phenol. The catalyst has high efficiency, and the phenol turnover number (TON) is more than 12 000 after 40 H of reaction.
Although the homogeneous catalyst has a high efficiency in catalyzing benzene to phenol, it is difficult to recycle the catalyst. Over the years, researchers have continued to explore ways to improve the recycling of catalysts, and developed a series of heterogeneous catalysts and heterogeneous catalytic systems for benzene hydroxylation. For example, Peng et al. Used iron dodecyl sulfonate as a catalyst and H2O2 as an oxidant to introduce ionic liquid into the reaction system to form a heterogeneous catalytic system[40]. The reaction process follows the Fenton hydroxyl radical mechanism, and the ionic liquid mediates the phase transfer process: the catalyst and benzene are dissolved in the ionic liquid, the H2O2 is mainly dissolved in the aqueous phase, and the phenol produced by the reaction quickly enters the aqueous phase, which avoids the deep oxidation of the product and improves the selectivity and conversion of the reaction. It is worth pointing out that when the molar ratios of benzene /H2O2 and catalyst/benzene in the reaction system are as low as 1 ∶ 1 and 0.002 ∶ 1, respectively, the selectivity of H2O2 to benzene conversion is as high as 90%. This liquid-liquid two-phase heterogeneous catalytic system shows the characteristics of high efficiency and environmental protection, but the recovery of catalyst is still a difficult problem.
Wang Jun et al. Designed a class of heteropolyacid salt heterogeneous catalysts modified by organic ions, such as binuclear ionic solid catalyst [Dmin]2.5PMoV2, phase transfer catalyst [C3CNpy]4HPMoV2, polyionic liquid supported catalyst P-[DVB-VBIM]5PMoV2, etc., by using the strategy of organic group or functionalized ionic liquid to solidify phosphorus-molybdenum-vanadium H3+xPMo12-xVxO40, and made systematic research in benzene hydroxylation reaction[25,27,41,55⇓⇓~58][25][27][41]. Through FT-IR, UV-vis and other characterization analysis, they believe that this kind of organic ionic heteropolyacid salt heterogeneous catalyst follows the classical hydroxyl radical reaction mechanism in the reaction process, and the reduced V4+ in the H3+xPMo12-xVxO40 is the active site of the reaction. The ionic liquid cation in the catalyst interacts with the heteropolyanion, and the modification of the organic component not only makes the catalyst present a heterogeneous phase or phase transfer phenomenon in the reaction system, which is convenient for catalyst recovery, but also accelerates the reversible redox cycle of the V4+/V5+ and improves the redox catalytic performance of the phosphomolybdovanadate anion.
Loading catalytic active species on porous supports is the most common strategy for the preparation of heterogeneous catalysts, which is also widely used in benzene hydroxylation. Porous support plays two important roles in the reaction: on the one hand, it can support or disperse metal active sites, reduce their loss rate, improve mass transfer and enhance catalyst reusability[59]; On the other hand, the physical properties of the support itself (such as hydrophobicity, acidity, etc.) are used to cooperate with the metal active sites to improve the reactivity and selectivity[8,9,18][57].
Borah et al. Synthesized the supported catalyst PMO-1 by copolycondensation of the active precursor vanadyl acetylacetonate with tetraethyl orthosilicate (TEOS) using CTAB as a template, and found that PMO-1 formed V(Ⅴ)-O-O· peroxovanadium radicals with H2O2, and then attacked the benzene ring to produce phenol, with a phenol selectivity of up to 100%[19]. This is because the silanol group on the PMO carrier is weakly acidic, and the weakly basic benzene is easily adsorbed on the acidic skeleton through electrostatic interaction, and once benzene is converted into phenol, the acidic phenol is easily desorbed from the acidic skeleton to prevent the formation of over-oxidation products. Leng et al. Used F127 as a template and 3-aminophenol and tannin as precursors to encapsulate the H5PMo10V2O40 in an ordered mesoporous polymer (OMP) by calcination. The material has a narrow distribution of mesopores (5 – 10 nm) and a high surface area (150~250 m2/g), and shows high catalytic performance and stable catalyst reuse performance in the catalytic benzene hydroxylation reaction[42]. This is due to the unique confinement effect of the carrier and the strong interaction between organic components and PMoV.
Sun et al. Synthesized a heterogeneous composite catalyst Cu2O-rGO by pyrolyzing metal organic framework (Cu-MOF) and graphene, and proposed a radical trapping mechanism (Fig. 3)[22]. Firstly, the Cu(I) is oxidized by H2O2 to generate Cu(Ⅱ)-OH and OH, and the OH reacts with a benzene ring,Phenyl radical generation via the hydrogen atom transfer mechanism (HAT) ·C,Cu(Ⅱ)-OH can rapidly capture the produced · C to generate a Ar-Cu(Ⅲ)-OH intermediate, which is then reductively eliminated from this high valence Cu(Ⅲ) intermediate to give phenol. They pointed out that the radical trapping mechanism overcomes the difficulty of activating the inert C — H bond and makes the reaction proceed faster. In addition, the carrier also plays an extremely important role in controlling the formation rate of hydroxyl radicals and improving the selectivity of the reaction. Through comparative experiments, they found that the decomposition rate of hydrogen peroxide by pure Cu2O NPs was too fast, and only 6.3% of phenol yield was obtained, while the decomposition rate of H2O2 on the Cu2O-rGO was moderate after the carrier graphene rGO was compounded with Cu2O NPs, and at the same time, graphene increased the adsorption of benzene. The two synergetic effects resulted in a higher phenol yield of 21.2%.
ElMetwally et al. Prepared a layered compound FeOCl by chemical vapor phase conversion, and used this commonly used cathode material for lithium batteries as a heterogeneous catalyst to catalyze benzene hydroxylation (Fig. 4)[23]. In the layered structure of FeOCl, iron exists in the form of R. D. S, and the rate-determining step (r.d.s) is the reduction of R. D. S to Fe2+. The reduced Fe2+ can adsorb H2O2 and rapidly produce hydroxyl radical · OH and peroxy radical · OOH, which attack the benzene ring to complete the hydroxylation reaction, respectively. A large number of unsaturated Fe atoms are distributed on the (020) crystal face of FeOCl, which are uniformly arranged in the form of O-Fe-Cl. The coordination of chlorine and oxygen atoms can improve the self-redox ability of iron, resulting in higher conversion and selectivity.
In the free radical reaction mechanism discussed above, researchers generally believe that the initial and most critical step is the reduction of high-valence metal active sites to low-valence States by H2O2, followed by the oxidation of low-valence metal active sites back to high-valence States and the production of hydroxyl radicals · OH in the second step (Fig. 5). Because the H2O2 is an oxidant, the reducibility is very weak, so the first step of the reduction process is usually slow, which is the rate-determining step of the reaction[20,23,25,60]. In addition, the generated hydroxyl radicals have strong oxidizability and poor selectivity, and are easy to further oxidize phenol to generate deep oxidation by-products, thereby reducing the effective utilization rate of the H2O2[18,27]. Therefore, this kind of radical reaction mechanism is not conducive to the improvement of the activity and selectivity of benzene hydroxylation to some extent.

2.2 Non-radical reaction mechanism with H2O2 as oxidant

In recent years, the benzene hydroxylation of H2O2 based on non-radical mechanism has developed rapidly. In the non-radical reaction mechanism, the intermediate produced is no longer the hydroxyl radical · OH, but the metal-oxygen active intermediate, such as M = O, M-O-M, M-O-O, etc[45][43][61]. KIE value is an important means to judge the mechanism of the reaction[13,43]. It has been reported that the KIE value of Fenton type hydroxyl radical is generally in the range of 1.7 ~ 1.8, while the KIE value of metal-oxygen active intermediate is often in the range of 0.9 ~ 1.1[43,44,62][13,63]. Fig. 6 shows a typical non-radical reaction mechanism of benzene hydroxylation, in which the H2O2 first reacts with the metal catalyst HO-M-L to form a metal-peroxide active species to attack the benzene ring, followed by oxygen transfer from the intermediate to form a C — O bond with the benzene ring, and finally phenol is obtained by hydrogen transfer. In general, the oxygen transfer step (i.e., the formation of the C — O bond) is the rate-determining step of the non-radical mechanism[45,61]. Non-radical mechanism is an effective method to improve the activity and selectivity of benzene hydroxylation, which can promote the rapid activation of — H bond in aromatic C s p 2 and inhibit the over-oxidation of phenolic products.
A series of transition metal homogeneous complexes are capable of leading a non-radical mechanism in the hydroxylation of benzene. [43]. The KIE value measured in the benzene hydroxylation reaction was 1.0, and they speculated that the dinickel dioxygen complex was the reaction intermediate, which underwent hydroxylation by electrophilic attack on the aromatic ring to produce phenol (Fig. 7). When H 2 18 O2 was used as the oxidant, 18O labeled phenol was mainly obtained, indicating that hydrogen peroxide was the main oxygen source. Recently, Anandababu et al. Synthesized a tetradentate N-liganded cobalt complex (L)CoⅢ(OOH)]2+. The active intermediate [(L)CoⅢ(OOH)]2+ formed in the reaction process also hydroxylated to phenol by electrophilic attack on the aromatic ring, and hydrogen peroxide was also the main oxygen source[44]. This kind of metal complex can catalyze the reaction to obtain phenol with a high yield of 21% ~ 29%, but the catalyst is not easy to recover and reuse.
Single-atom Catalysts (SACs) have emerged rapidly in the field of catalytic oxidation reactions due to their high activity and selectivity. Dispersing SACs on the defect sites of carbon materials is one of the effective methods to prevent the agglomeration of single atoms. Recently, some researchers have tried to use these supported SACs as efficient catalysts for benzene hydroxylation[64⇓~66]. Pan et al. Synthesized single-atom iron-based catalysts Fe-NxCy w ith different coordination numbers of nitrogen atoms by manipulating the polymer pyrolysis strategy[45]. Among them, the four-nitrogen coordinated catalyst Fe-N4 gave benzene hydroxylation up to 78.4% benzene conversion and 100% phenol selectivity, and the catalytic activity of benzene hydroxylation gradually decreased when the N atom of Fe-N4 was replaced by one or two C atoms to become Fe-N3C and Fe-N2C2, respectively. In order to explore the relationship between structure and reactivity at the atomic level, they performed density functional theory (DFT) calculations on Fe-NxCySAs/N-C catalytic system.The results further confirm that the atomically dispersed "metal-N-C" structure is the effective active site for the catalytic hydroxylation of benzene, and the difference of space structure and electron density caused by the Fe-NxCy of different nitrogen atom coordination environment is the main reason for the change of catalytic activity. The energy barrier for the activation path of the H2O2 by the tetra-coordinated nitrogen atom anchored Fe-based monoatomic site Fe-N4 is moderate, and the generated reaction intermediate O = Fe = O has the highest activity, which is beneficial to attacking the C — H bond on the benzene ring and rapidly generating an oxygen transfer reaction (rate-determining step) to form a stable C — O bond, and then generating phenol through hydrogen transfer to finally complete the catalytic cycle. The SACs are relatively well defined, which provides a new opportunity to use computational chemistry to further understand the mechanism of heterogeneous catalysis. The theoretical calculation results of this work confirm that the coordination mode not only determines the structural and electronic properties of Fe-NxCy catalysts,But also affects the reaction pathway and the formation of key oxidation species. Adjusting the coordination environment of SACs is an effective means to improve the catalytic performance of the catalyst, but the reuse performance of the single-atom catalyst was not investigated.
Wang Jun et al. Synthesized V-Si-ZSM-22 heterogeneous molecular sieve catalyst by ionic liquid-assisted dry gel method, which effectively confined V clusters in the zeolite cavity and realized efficient catalysis of benzene hydroxylation[13]. The formation of monoperoxovanadium species (C) and diperoxovanadium species (D) during the reaction was confirmed by radical trapping experiments, KIE, reaction kinetics analysis, and activation energy calculation (Fig. 8A). The reaction follows a non-radical mechanism. Unlike the previously reported electrophilic oxidation path of benzene in the non-radical mechanism, this reaction mechanism leads to a nucleophilic oxidation path (Fig. 8B), and the reaction is completed at a very high speed within 30 s, which is the shortest reaction time of benzene hydroxylation process so far[43,44]. The catalytic path is beneficial to obtaining phenol with high selectivity and effectively avoids the over-oxidation of the product phenol.
To sum up, the non-free radical mechanism can promote the rapid oxidation of C — H on the benzene ring, and can avoid the deep oxidation of phenol compared with the free radical process. The construction of heterogeneous catalysts with high phenol yield and high hydrogen peroxide utilization is still a challenging research difficulty, and it is also the bottleneck to realize the industrialization of the reaction process. Based on the reaction mechanism of benzene hydroxylation with hydrogen peroxide catalyzed by transition metals, the modification methods and effects of carriers, organic groups and ligands were summarized. It is worth pointing out that in order to improve the conversion rate of the substrate, most studies often need to add excessive H2O2 in the reaction, which is not conducive to improving the effective utilization of H2O2, and the transition metal will still be more or less lost in the reaction system, thus affecting the stability of the catalyst.

2.3 Free radical reaction mechanism with O2 as oxidant

The H2O2 single bond length is 1.4848 Å, and the bond dissociation energy is 213 kJ·mol-1. In contrast, the double bond length of O2 is only 1.208 208 Å, and the double bond energy is as high as 497 kJ · mol, which is difficult to be activated during the reaction-1-[67]. Therefore, the hydroxylation of benzene with O2 as the oxidant is more difficult, and it is often necessary to increase the reaction temperature, pressure or time, and add reducing agents (ascorbic acid, CO, H2, zinc powder, NH3, etc.) to assist the catalyst to activate O2[33,68][28][37,69][70][14].
In the reaction process, the reducing agent first reduces the catalyst, and then the reduced catalyst reduces the O2 to produce H2O2, and then catalyzes the benzene hydroxylation reaction following the hydroxyl radical mechanism (Fig. 9a)[70,71]; In another case, the O2 is activated under acidic conditions to directly produce · OH, which then attacks the benzene ring to produce phenol (Fig. 9b)[37,72].
Heterogeneous vanadium-based catalysts have been widely studied in the hydroxylation of benzene due to the high catalytic activity of vanadium-containing compounds for the oxidation of hydrocarbons[13]. The structure and performance of vanadium-based catalysts can be modulated by supports, such as supported vanadium-based catalysts with VOx immobilized on different supports, vanadium-substituted polyoxometalate-based catalysts, vanadium-doped carbon nitride polymers, etc[68,71][48][73].
Ma Yangmin et al. Synthesized the supported catalyst VxOy@C by hydrothermal method using biomass (sucrose, fructose, glucose, etc.) as carbon source and NH4VO3 as vanadium source, and the phenol yield reached 9. 3% after 10 H of reaction at 80 ℃ in acetonitrile solvent[9]. BHT (2,6-di-tert-butyl-4-methylphenol) was added to the reaction system to confirm that the reaction followed the hydroxyl radical mechanism. The ascorbic acid reducing agent in the system assists the VxOy@C to activate the O2 to generate H2O2, and then generates hydroxyl radicals to attack the benzene ring to generate phenol, but the active sites of the supported catalyst are seriously lost in the reaction system. In order to improve the reusability of the catalyst, they grafted VO(acac)2 onto the carrier UiO-66-NH2(Zr-MOF) to prepare the catalyst V/UiO-66-NH2, which catalyzed the hydroxylation of benzene to obtain 22% yield of phenol[46]. During the reaction, the active site V4+ activates the O2 to produce hydroxyl radicals, and the positively charged carrier UiO-66-NH2 with L acidity is beneficial to the rapid adsorption of the reactants benzene and hydroxyl radicals, and is also beneficial to the timely desorption of the product phenol. In addition, the V/ZrO2 obtained by further calcining the V/UiO-66-NH2 has better reusability, and the yield of phenol can also reach 16. 2%.
Gao Xiaohan et al. Synthesized nitrogen-doped vanadyl oxalate catalyst VOC2O4-N-5 by hydrothermal method, and detected the generation of superoxide radical [ O 2 - ·] during the reaction by real-time in situ FT-IR detection (Fig. 10)[47]. The low-valence vanadium V4+ first activates oxygen to generate high-valence vanadium V5+ and superoxide radical [ O 2 - ·], and the superoxide radical [ O 2 - ·] further combines with acetic acid to generate peroxyacetic acid intermediate,At the same time, V5+ can be reversibly converted into V4+, and finally peracetic acid inserts active oxygen into the C — H bond of the benzene ring to produce phenol.

2.4 Non-radical reaction mechanism with O2 as oxidant

The catalytic oxidation mechanism involving lattice oxygen is the most common mechanism in the hydroxylation of benzene with O2 as the oxidant[26,55]. The general process is as follows: (1) The reactant is oxidized to a product by combining with the lattice oxygen in the catalyst, and at the same time, the catalyst produces oxygen vacancies and becomes a reduced catalyst; (2) The reduced catalyst activates oxygen and fills oxygen atoms into oxygen vacancies, and the catalyst is oxidized again to regenerate. This process is also known as the classical Mars-van Krevelen mechanism[74,75]. Since both the benzene ring and oxygen are inert reactants, it is usually necessary to add a sacrificial reductant in this reaction system (Fig. 11). The metal oxide is reduced under the assistance of a reducing agent, and the oxygen vacancy M-;O2 formed on the surface is activated and filled into the oxygen vacancy on the surface to generate a metal-oxygen active intermediate M-O 2 *; The intermediate attacks the benzene ring to produce phenol, while the catalyst returns to the initial state to complete the catalytic cycle.
It has been reported that the adjacent vanadium atoms (V-O-V structure) in vanadium-doped polyoxometalate H3+xPMo12-xVxO40 are easy to form bridged oxygen defect sites, which are highly active in oxygen transfer reactions[74]. Ishii et al. Studied the hydroxylation of benzene using H7PMo8V4O40 as a homogeneous catalyst and CO as a reducing agent[28,76]. In the reaction process, CO abstracts oxygen atoms in the H7PMo8V4O40structure to generate oxygen vacancies, and meanwhile, V5+ in the Keggin framework structure is reduced to V4+,Subsequent activation of O2 by V4+ generates a reactive oxygen intermediate, V5+- O 2 *, which in turn attacks the benzene ring to yield phenol. Isotopic tracking by 16O2 and H 2 18 O confirmed that hydrogen peroxide (H16O18OH) or radicals were not generated in situ during the reaction. A small amount of PhOH in the product comes from the oxygen in the framework of H7PMo8V4O40, and the reaction process follows the classical Mars-van Krevelen mechanism.
Gao et al. Designed a metal-organic framework MOF-199 coated H6[PMo9V3O40] composite structure, and supported it on mesoporous material SBA-15 to obtain heterogeneous catalyst POM @ MOF @ SBA-15 (PMS)[48]. The results showed that the composition and structure of the catalyst were stable in the process of benzene hydroxylation, and the activity of PMS decreased slightly after four times of reuse due to the decrease of crystallinity, and no loss of active component was detected in the reaction system after the reaction.
Chen et al. Prepared the supported catalyst PMoV @ PCIF-1 by loading PMo10V2 O 40 5 - anions in the polyhedral oligomeric silsesquioxane (POSS) -based porous ionic framework PCIF-n with rich ionic structure[49]. The catalytic material has an ultra-high specific surface area (1025 cm2/g), an ultra-large pore volume (0.9 cm3/g), a high catalytic activity for catalyzing the benzene hydroxylation reaction, and a stable catalyst reuse performance. The hydrophobic property of the POSS group in the carrier is beneficial to the timely release of the hydrophilic product phenol and the improvement of the selectivity of phenol.
It is worth noting that in the above free radical or non-free radical reaction mechanism in which the O2 is the oxidant, the sacrificial reductant is often indispensable in the system, and the catalyst can activate oxygen to produce intermediates to promote the reaction only after it is reduced to low valence active sites by the reductant. Sacrificial reductants are irreversibly transformed in this process, which reduces the atom economy of the reaction and greatly increases the cost, which is not conducive to the practical application of the reaction. Therefore, researchers are committed to developing a benzene hydroxylation reaction system without sacrificial reductants[77,78]. For example, Guo et al. Used the high-cost noble metal complex PdⅡ(bpym) to catalyze the oxidation of benzene ring, and introduced AlⅢ into the system to effectively avoid the coupling side reaction of benzene ring through the coordination of alkaline metal, so as to obtain higher phenol selectivity[31]. Neumann et al. Found that in aqueous solution with sulfuric acid concentration > 50%, the electron of benzene ring was transferred to the H5PV2Mo10O40structure, which was activated to produce benzene ring cation radical, and then reacted with O2 to produce phenol[50]. The theoretical calculation confirmed that the reaction process followed the electron transfer (ET) mechanism (Fig. 12), and the reaction should be carried out in a sufficient O2 environment. Under anaerobic conditions, the thermodynamic calculation results show that the low free energy intermediate formed by the benzene ring and POM can not continue to react to form phenol. The benzene conversion rate can be as high as 65% without adding a reducing agent, but the catalytic system also has obvious disadvantages, for example, the catalyst is dissolved in the reaction medium, which is not conducive to recovery and reuse, the reaction conditions are harsh, and the strong acidic conditions corrode the equipment and have a serious impact on the environment.

2.5 Two-site Synergistic Catalytic Mechanism with O2 as Oxidant

In the field of synergistic catalysis, the synergistic catalysis of two reaction substrates by two active sites can effectively reduce the difficulty of reactant activation, improve the reaction rate, and produce the effect of "1 + 1 > 2"[53,79⇓~81]. In the benzene hydroxylation reaction, the dual-site catalyst with activated benzene ring and O2 was designed and constructed from the electron transfer mechanism (ET, activated benzene ring) and the lattice oxygen mechanism (M-v K, activated oxygen), which was helpful to start the reaction quickly and complete the catalytic cycle without reducing agent. For example, Liu et al. Formed a synergistic catalytic system with palladium acetate and vanadium-substituted heteropolyacid, in which the noble metal palladium activated the benzene ring into Pd2+, and the vanadium-containing heteropolyacid re-oxidized the Pd2+ into Pd4+, and the reduced heteropolyacids further activated oxygen to generate a metal-oxygen active intermediate, which further attacked the activated benzene ring to generate phenol[29]. In the absence of reducing agent, the system gave a phenol yield of 9.4%. The main disadvantage of this reaction is the use of expensive and unrecyclable noble metal catalysts.
Wang Jun et al. Have made continuous exploration in the construction of dual-active center synergistic catalytic benzene hydroxylation reaction[26,51⇓⇓~54]. Long et al. Constructed a non-noble metal catalytic system of "mixed type" of g-C3N4 with melamine structure and vanadium heteropolyacid PMoV2“, and proposed a two-center synergistic catalytic reaction mechanism (Fig. 13): the benzene ring is adsorbed on the g-C3N4, and the g-C3N4 transfers "electron density" to the benzene ring through π electron orbital overlap.An electron-rich benzene re transition state intermediate is formed, meanwhile, that PMoV 2 [ o x ] attack an electron-rich benzene active species, bridge oxygen on V-O-V is transferred to the activated benzene re to form phenol, and then an oxygen vacancy-containing transition state PMoV 2 [ r e d ] is generated on the catalyst[26]; Subsequently, molecular oxygen oxidizes PMoV 2 [ r e d ], and the insertion of the oxygen vacancy of PMoV 2 [ r e d ] restores PMoV2 to the initial state PMoV 2 [ o x ]. The catalytic system gave a phenol yield of 13.6% without the addition of sacrificial reductant. However, the catalytic system also has limitations. In order to make the benzene ring active intermediate and the PMoV2 containing active oxygen contact and react quickly and reduce the mass transfer difficulty between the two reaction intermediates, the PMoV2 in the "mixed" catalyst is dissolved in the reaction system during the reaction, which is conducive to close contact with the solid g-C3N4, but it is not convenient for recycling.
Long et al. Further designed two types of dual-site catalysts, namely SFNC(800)+Ch5PMoV2 and g-C3N4+Ch5PMoV2, in order to improve the recycling effect of dual-site catalysts in benzene hydroxylation reaction[51,52]. The modification of PMoV2 by choline chloride (Ch) not only enhanced the ability to activate oxygen to produce active oxygen, but also showed a temperature-controlled phase transfer phenomenon in the reaction process, which was conducive to the recovery and reuse of the catalyst. After density functional calculations, they pointed out that nitrogen-doped carbon SFNC (800) can transfer more "electron density" to the benzene ring than g-C3N4, further reducing the difficulty of active oxygen attacking the benzene ring. In addition, by comparing the adsorption energy of benzene ring adsorbed on different sites of SFNC (800), it is found that the more the content of graphitic nitrogen, the more favorable the adsorption activation of benzene ring.
Cai et al. Compounded N-doped ordered mesoporous carbon (NC-580) and heteropolyacid-based organic ion salt ([DiBimCN]2HPMoV2) to prepare a "one-piece" dual-active center heterogeneous catalyst, and under the optimal reaction conditions, the phenol yield could reach 10.5%, and the catalyst could be recovered and reused by simple filtration, but it was difficult to completely avoid the loss of heteropolyacid active components from the surface of the carrier to the reaction system[53].
Qin et al. Used potassium ferricyanide as iron source and polyionic liquid as carbon source to obtain carbon material FeC with highly dispersed iron atoms by high temperature calcination[54]. The graphite-like carbon in FeC (5) has a continuous π-electron conjugated structure, and the benzene ring is adsorbed to form a benzene active intermediate through π-π interaction, and then the lattice oxygen atom of the Fe3+-O-Fe3+ in FeC (5) directly oxidizes the benzene active intermediate to form phenol, and the catalyst is reduced to a reduced state Fe2+--Fe2+ containing an oxygen vacancy, and finally the oxygen is filled into the Fe2+--Fe2+ to regenerate the oxygen vacancy. In the absence of reducing agent, FeC (5) -catalyzed hydroxylation of benzene ring gave phenol in 14.2% yield. The catalyst could be recovered by filtration after the reaction, and had good reusability.
The hydroxylation of benzene to phenol using O2 as oxidant is a green process with the highest atom economy, energy saving and environmental protection. From the above review of the research progress of transition metal-based catalysts and reaction mechanism, it can be seen that it is very difficult to complete the catalytic cycle of the reaction, especially without the participation of sacrificial reductants. On the one hand, the design of transition metal active sites and supports, the use of organic functional groups and other modification methods, and the improvement of catalyst pore structure and hydrophilicity and hydrophobicity will be the main research contents that researchers will pay attention to for a long time. On the other hand, from the perspective of synergistic catalytic mechanism, it is still a goal to explore heterogeneous catalysts with dual active sites, avoid the addition of sacrificial reductants in the system, and improve the convenience of catalyst recovery and reuse stability.

3 Benzene Hydroxylation Catalyzed by Nonmetallic Materials

In the hydroxylation of benzene catalyzed by metal-based catalysts, the metal active components in the catalyst will be more or less dissolved in the liquid phase reaction system, resulting in unsatisfactory recovery and reuse performance and causing environmental pollution[9,49,55,77]. In contrast, non-metallic catalysts do not contain metal atoms or ions, which can avoid the loss of metal active components, and are a promising class of environmentally friendly heterogeneous catalysts. In recent years, non-metallic catalysts, such as carbon catalysts, have become one of the most active research objects in benzene hydroxylation reaction, and it is of great research value to explore green, efficient, recyclable and stable non-metallic catalysts. It has been reported that the oxygen-containing functional groups and defect structures on the surface of non-metallic materials, such as carbon materials, can act as catalytic active sites to activate oxidants and produce free radical active intermediates, thus driving various oxidation reactions such as benzene hydroxylation[82]. Table 2 summarizes the reactivity of typical non-metallic materials for the catalytic hydroxylation of benzene. From the results reported so far, whether H2O2 or O2 is used as the oxidant, these catalytic processes follow the free radical mechanism.

3.1 Free radical mechanism of benzene hydroxylation catalyzed by carbon materials

Carbon materials are widely distributed in nature, which have the advantages of low cost, non-toxicity, light weight, easy functionalization and adjustable porosity, and are the most widely studied and applied inorganic non-metallic materials, mainly including graphene, carbon nanotubes, mesoporous ordered carbon materials, heteroatom doped carbon materials and so on[89][90][72][91]. Defect sites and oxygen-containing groups on the surface of carbon materials can act as electron donors and acceptors, showing electron gain and loss behavior similar to metals in the oxidation process, and realizing the interconversion cycle between low valence States and high valence States[87,92,93]. Therefore, more and more researchers have used carbon materials as non-metallic catalysts to catalyze redox reactions[89,94,95]. The surface area and pore volume of carbon materials, the doped heteroatoms (N, P, B, etc.), the type of defect sites (such as zigzag and armchair), the oxygen-containing groups on the surface, and the residual trace metals may affect their catalytic oxidation properties[85][91,96][84][72]. Benzene hydroxylation catalyzed by carbon materials generally follows a free radical mechanism. For example, the sp2 hybrid surface of graphitic carbon has a large π conjugated electron system, which can activate oxidants to produce peroxo or superoxide intermediates[97,98]. The edges or defects of nanocarbon materials such as graphene have high chemical reaction activity, and the unpaired electrons on the edges can activate oxidants to produce superoxide intermediates[15,85,89]. Oxygen-containing groups on the surface of carbon materials, such as quinone carbonyl and phenolic hydroxyl groups, can also act as active site activating oxidants to produce hydroxyl radical active intermediates[86,87,93]. Carbon materials can not only activate oxidants (O2 or H2O2); Moreover, the large π electron conjugated system can adsorb the benzene ring and increase the electron density on the surface of the benzene ring to activate the benzene ring[51,52]. It can be seen that carbon materials also belong to heterogeneous catalysts with two active centers in a sense.
The edge localization of graphene nanocarbon materials adds additional electronic States, which are superimposed between the bonding π and antibonding π* orbitals, and increases the Fermi level electron density, so the graphene edge sites have chemical reaction activity to activate the oxidant[89]. At the same time, the large π-electron system of graphene has the ability to adsorb the reactant benzene ring. Martin et al. Reported a chemically treated graphene catalyst CCG, which catalyzed the hydroxylation of benzene to obtain 18% yield of phenol[11]. During the reaction, the reactants benzene and H2O2 are simultaneously adsorbed on the surface of CCG, following the Langmuir-Hinshelwood mechanism. The H2O2 adsorbed on the surface of CCG first dissociates into reactive intermediates, which in turn react with the benzene ring adsorbed in the vicinity to produce the product phenol at a rate of k1 (Fig. 14). At the same time, reactive oxygen intermediates will also decompose to water and oxygen at a rate of k2. CCG has the ability of kinetic control to balance the k1 and k2 rate, which effectively avoids the deep oxidation of phenol and promotes the high selectivity of benzene to phenol.
The defect sites of carbon materials have an important influence on the catalytic activity. Song et al. Obtained MWCNTs with different defect densities by concentrated nitric acid solution and ultrasonic treatment, which were directly used to catalyze the hydroxylation of benzene with H2O2 as the oxidant[83]. The defect sites on MWCNTs adsorb hydrogen peroxide to produce active oxygen intermediates, which attack the benzene ring to produce phenol. They found that the higher the defect site density, the faster the MWCNTs decomposed H2O2 to generate active oxygen. Wen et al. Used carbon nanotubes CNT7000 treated with hydrochloric acid to remove metal residues to catalyze the benzene hydroxylation reaction in a H2O2 system, and obtained a phenol yield of 13.6%[84]. Through the characterization of small molecule model catalyst, XPS, Raman spectroscopy, SIMS and in situ infrared spectroscopy, it is proved that the defect sites on the surface of carbon materials are the active centers of the reaction, especially the armchair defect sites are conducive to the adsorption of hydrogen peroxide and the production of active oxygen intermediates, which attack the benzene ring molecules adsorbed on the adjacent carbon surface to produce phenol. Wei et al. Could also increase the defect sites on porous carbon HPC by H2 reduction and other means, and they also found that the higher the number of defect sites, the higher the catalytic activity of HPC[85]. In order to deeply understand the catalytic nature of the surface defect sites of carbon materials for benzene hydroxylation in acetonitrile medium, Lyu et al. Carried out molecular simulation calculations. The results showed that both the zigzag defect sites and the armchair defect sites were effective active sites for the production of the reaction intermediate · OH, and the formation of hydroxyl radicals was the rate-determining step of the reaction[15]. Through further activation strain model (ASM) analysis, they believed that the zigzag defect site had a stronger stabilizing effect on the reaction transition state (TS), and the HOMO-LUMO energy gap of the triplet activation strain model was narrower, so it showed higher catalytic performance.
A large number of oxygen-containing groups on the surface of carbon materials treated by concentrated HNO3 and hydrogen peroxide have also been proved to be the catalytic active sites in the oxidation reaction. It is reported that the carbon-oxygen double bond is similar to the metal-oxygen double bond, and there is a phenomenon of electron gain and loss in the reversible transformation of quinone carbonyl and phenolic hydroxyl on the surface of carbon materials, which is similar to the metal-oxygen intermediate with redox properties and can catalyze redox reactions[99,100][92,101]. Xu et al. Used nitric acid and hydrogen peroxide oxidation, high temperature heat treatment and other methods to modify the commercial wooden activated carbon. The modified activated carbon was used to catalyze the hydroxylation of benzene with H2O2 as the oxidant, and the yield of phenol was 13.5%. The catalyst had stable reusability[86]. Characterization and experiments confirmed that the reversible cycle between phenolic hydroxyl groups and quinone carbonyl groups on the surface of activated carbon promoted the activation of H2O2 to generate hydroxyl radicals, and then attacked the benzene ring to generate phenol (Fig. 15). They observed that an appropriate concentration of H+ in solution favors the establishment of a tautomeric equilibrium of quinone/phenol.
Wang Jun et al. Studied the surface functional group modification of carbon materials and its catalytic mechanism in benzene hydroxylation reaction with O2 as oxidant[72,87]. Shan et al. Used melamine-modified phenolic resin as raw material to synthesize N-doped mesoporous carbon material NOCs with oxygen-rich surface by high temperature carbonization method, and the surface area reached 620 m2/g[72]. In the presence of O2 as an oxidant and lithium acetate promoter (LiOAc), NOCs can efficiently catalyze the hydroxylation of benzene to phenol with a phenol yield of up to 12.5%, which is comparable to the results of partial transition metal-based catalysts. The performance of the catalyst was stable after 6 times of reuse, and the phenol yield was basically unchanged. The characterization, comparison test and DFT calculation show that the carbonyl group (cyclic ketone/quinone carbonyl C = O) on the surface of NOCs first extends the O — O bond by chemisorption of O2, and then the O2 is activated to form · OH with the assistance of the system H+, and then attacks the benzene ring to form phenol (Fig. 16). They pointed out that the tautomerism of quinone/phenol groups provided a prerequisite for electron transfer, and nitrogen doping not only improved the stability of NOCs, but also increased the number of quinone/phenol groups on the surface of NOCs.

3.2 Free radical mechanism of polymer-catalyzed benzene hydroxylation

In 2019, Wang et al. Synthesized a non-metallic quinone-amine polymer QAP, which also has a quinone/phenol interconversion cycle in the catalytic hydroxylation of benzene[88]. QAP catalyzed the reaction of benzene with O2 in the absence of reductant with the addition of LiOAc as a promoter gave phenol in high yield of 15.9%. In the catalytic process, the O2 is first activated by the phenolic hydroxyl group to produce a H2O2, which is homolytically cleaved into a · OH radical to attack the benzene ring to form a hydroxycyclohexadiene radical, and the phenolic hydroxyl group is reversibly oxidized to form a quinone carbonyl group, which promotes the hydrogen transfer of the hydroxycyclohexadiene radical to form phenol. The π-π conjugated system in QAP can improve the activity of benzene hydroxylation reaction by synergistic adsorption of benzene ring. However, the yield of phenol was greatly reduced to 6.6% after the QAP catalyst was reused twice.
In a word, non-metallic materials represented by carbon materials are a kind of environmentally friendly heterogeneous green catalysts, which can synergistically activate benzene rings and oxidants, and their large π-electron system and hydrophobicity promote the rapid adsorption of benzene rings and the timely desorption of phenol.And can provide appropriate activation and decomposition rates of H2O2, show good catalytic activity and selectivity in benzene hydroxylation, and even effectively catalyze the benzene hydroxylation reaction using O2 as an oxidant. Through the efforts of researchers, some carbon catalytic systems have shown the same or even more efficient catalytic effect as some metal-based catalysts. However, the surface structure of carbon materials is complex, and the active sites and specific properties at the molecular level are still controversial. It is still a long way to go to understand the reaction mechanism of benzene hydroxylation catalyzed by non-metallic catalysts at the molecular level[9,85,86].

4 Conclusion and prospect

The direct hydroxylation reaction of phenol by using H2O2/O2 as a green oxidant to oxidize the $\text C_{\text s\text p^{2}}—\text H$ bond of the benzene ring is very promising, but it is still very challenging. In this reaction, the $\text C_{\text s\text p^{2}}—\text H$ bond on the benzene ring is difficult to be activated, the oxidant, especially molecular oxygen, is difficult to be activated, the product phenol is easy to be over-oxidized, and the stability of catalyst reuse is difficult to improve.The construction of heterogeneous catalysts with high activity, selectivity and stability is the main research content, and it is also the key to promote the industrial application of benzene hydroxylation reaction. An in-depth understanding of the catalytic reaction mechanism and the structure-activity relationship of catalysts is the premise of rational design and construction of efficient catalysts.
For both metal-based and non-metallic catalysts, the classical free radical mechanism induced by activated oxidants (H2O2/O2+ reductants) has been the focus of research, and it is also the main basis for people to explore new catalysts. For example, for non-metallic catalysts, the existence of free radicals in the reaction process has also been observed recently, and it has been preliminarily proved that surface defect sites or oxygen-containing functional groups are the active sites leading to the production of free radicals. It is difficult for the radical intermediate to avoid the further oxidation of the target product phenol, so it is not conducive to improving the yield and selectivity of phenol. By contrast, the non-radical mechanism has the potential to improve phenol selectivity. The metal-oxygen intermediate produced by the interaction between the metal-based catalyst and the H2O2 can complete the reaction through an electrophilic or even nucleophilic oxidation path; For the reaction mediated by molecular oxygen, the lattice oxygen of the metal-based catalyst follows the Mars-van Krevelen mechanism to complete the catalytic cycle through the formation of oxygen defect sites. In order to avoid the addition of sacrificial reductants, dual-site catalysts that can activate both O2 and benzene rings and the corresponding synergistic catalytic mechanism have been paid attention to, and gratifying progress has been made. In the study of catalytic mechanism, the capture and analysis of reaction intermediates combined with in situ characterization techniques and the analysis of reaction transition States with the help of computational chemistry will greatly deepen people's understanding of the mechanism of benzene hydroxylation. At the same time, the rational design and precise preparation of catalyst active sites, the selection of supports and their synergistic effect with active sites, the modification of cocatalysts such as organic functional groups, the construction of catalyst pore structure and the improvement of hydrophilicity and hydrophobicity will still be the focus of long-term research. In the pursuit of efficient catalytic performance, special attention should also be paid to the problem of low effective utilization of hydrogen peroxide and how to improve the reaction effect when O2 is used as oxidant under mild conditions.

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Funding

National Natural Science Foundation of China(22072065)

National Natural Science Foundation of China(U1662107)

National Natural Science Foundation of China(21476109)

Youth Foundation of Southeast University ChengXian College(z0003)

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