Highly Selective Visible-Light Photocatalytic Benzene Hydroxylation to Phenol Using a New Heterogeneous Photocatalyst UiO-66-NH2-SA-V

  • Yunxia Fang
  • Liuxue ZhangEmail author
  • Qianqian Zhao
  • Xiulian Wang
  • Xu Jia


A new heterogeneous catalyst UiO-66-NH2-SA-V was prepared by the vanadium oxyacetylacetonate anchored on the Schiff base UiO-66-NH2-SA with chemical bonds. The liquid-phase oxidation of benzene to phenol over UiO-66-NH2-SA-V as a heterogeneous photocatalyst was studied under visible light. A mixed solvent of acetonitrile and acetic acid was chosen as the preferred solvent and hydrogen peroxide was chosen as an ecofriendly oxidant. Under optimized conditions, the photocatalyst showed benzene conversion of 15.3% at a phenol selectivity of 100%, giving no dihydroxylated byproducts. The synthesized material was characterized by scanning electron microscopy, transmission electron microscopy, FT-IR, X-ray powder diffraction and nitrogen adsorption–desorption. The effects of different parameters on the catalytic performance of UiO-66-NH2-SA-V were investigated. The anchor effect of vanadium oxyacetylacetonate stabilized and inhibited the leak of vanadium, leading to good catalytic recyclability with almost unchanged catalytic efficiency after five recycling tests in the acid reaction condition. The excellent catalytic performance of UiO-66-NH2-SA-V was due to the integration of vanadium species with high catalytic activity and the UiO-66-NH2 support in their interaction with the benzene substrate.

Graphic Abstract


Phenols Heterogeneous catalysis Photocatalysis Benzene hydroxylation Porous materials 

1 Introduction

Phenol is an important organic intermediate for the synthesis of phenol resins, dyes, antioxidation agents and pharmaceuticals [1]. The well-known Hock process [2] via an explosive cumene hydroperoxide intermediate in liquid phase currently employed for phenol production is energy-intensive, low phenol yield and not environmentally friendly. From the viewpoints of economy and the environment, the one-step hydroxylation of benzene to phenol is highly attractive compared to the three-step cumene process. Nowadays, remarkable progress in the direct hydroxylation of benzene to phenol has been witnessed involving different oxidant such as nitrous oxide, hydrogen peroxide, molecular oxygen, and a mixture of oxygen and hydrogen [3, 4, 5, 6, 7]. Among all of the oxidants, hydrogen peroxide is increasingly used in the liquid-phase reactions because it not only has environmentally friendly feature but also has water as its only byproduct.

A class of transition metal/oxides and V-containing composites heterogeneous photocatalysts has presented a promising performance on phenol production [8, 9]. A more efficient strategy is to immobilize catalytic metal/oxides species into mesoporous materials, such as carbon, metal oxides, molecular sieves [10, 11, 12, 13, 14], and an enhancement of both product selectivity and catalytic ability could be realized due to the stabilization of the active centers and prevention of phenol from over-oxidation [15]. However, the durability of composite catalysts is insufficient and the catalytic efficiency decreased due to the loss of loaded particles in the recycling process. Therefore, the design and preparation of highly efficient catalysts for direct hydroxylation of benzene with high stability and recycling performance is still a key issue.

Metal–organic framework (MOF) materials with high surface areas and large pore volumes in uniform pores, and organic functionalization is attainable via diverse post-synthetic chemical treatments. Zr-based MOF (such as UiO-66-NH2) possesses high porosity combined with high thermal, chemical, and mechanical stability [16], which contributed that the Zr–MOF is the good supports for catalyst. Herein, we chose UiO-66-NH2 as the support and reacted with salicylaldehyde to form the Schiff base, which could be employed to anchor VO(acac)2 with strong chemical bond. The prepared UiO-66-NH2-SA-V was applied as a heterogeneous catalyst in benzene hydroxylation, and the reaction conditions were optimized. Upon an optimization of different factors including the reaction time, temperature and the ratio between H2O2 and benzene, a good benzene conversion as well as excellent phenol selectivity had been obtained. These results implied that the UiO-66-NH2-SA-V has a great potential on the hydroxylation of benzene to phenol.

2 Experimental

2.1 Synthesis of UiO-66NH2-SA-V

0.3 g of UiO-66-NH2, which synthesized according to the reported method [17] with a slight modification, was dispersed into 30 mL methanol solution containing 0.2 mL salicylaldehyde and refluxed under stirring for 24 h. The obtained sharp yellow crystallites (named as UiO-66-NH2-SA) were recovered by filtration and washed with abundant ethanol for several times and then dried under vacuum overnight at 60 °C. To a solution of VO(acac)2 (0.21 g) in methanol (30 mL), UiO-66-NH2-SA(0.3 g) was added and heated at 60 °C for 24 h. The product was separated by filtration, washed with ethanol and dried under vacuum overnight at 60 °C. The obtained pale green product was named as UiO-66-NH2-SA-V (Scheme 1).
Scheme 1

Synthesis of UiO-66-NH2-SA-V

2.2 Photocatalytic Hydroxylation Experiments

Hydroxylation experiments were carried out in a quartz tube connected with a condenser under visible light. Typically, 10 mg catalyst and 1 mL benzene were added into a mixture solution (5 mL of acetonitrile and 1 mL of acetic acid). The mixture was heated to 60 °C, followed by the dropwise addition of hydrogen peroxide (30 wt%) within 30 min and stirred magnetically for 4 h. To carry out the photochemical reactions, a 300 W Xe lamp with a 420 nm UV-cut filter was used as a visible light source. Control experiments in the dark were carried out with the reaction vessel wrapped in tinfoil. The reusability of the UiO-66-NH2-SA-V catalyst for the benzene hydroxylation reaction was tested under the optimized reaction condition. At the end of the hydroxylation reaction, the catalyst was separated by centrifugation, washed with methanol, and dried under vacuum overnight at 60 °C. Then, the catalyst sample was used in the next run under the same reaction conditions. The oxidized products were identified and quantified with a GC–MS and a gas chromatograph (FID) with toluene as the internal standard.

3 Results and Discussion

3.1 Characterization of Catalysts

The morphology of the samples was investigated by scanning electron microscopy (SEM) and the images were shown in Fig. 1. The results showed that UiO-66-NH2 was containing with spherical and octahedral crystals (Fig. 1a). The morphology of the crystals obtained after reaction with salicylaldehyde (Fig. 1b) and VO(acac)2 (Fig. 1c) had a slight change, but it still maintained polyhedral or spherical structure. The transmission electron microscopy (TEM) images of UiO-66-NH2 (Fig. 2a, b) revealed a sponge-like structure with uniformly distributed pores. The materials retained the textural properties even after reacted with salicylaldehyde (Fig. 2c, d) and VO(acac)2 (Fig. 2e, f), which indicated the chemical stability of the material.
Fig. 1

SEM images of a UiO-66-NH2 and b UiO-66-NH2-SA and c UiO-66-NH2-SA-V

Fig. 2

TEM images of a, b UiO-66-NH2 and c, d UiO-66-NH2-SA and e, f UiO-66-NH2-SA-V

The phase purity and crystallinity of the prepared powder were confirmed by Powder X-ray diffraction. As shown in Fig. 3, the characteristic peaks positions of UiO-66-NH2 matched well with the as-prepared UiO-66-NH2 in peak positions, all the diffraction peaks of the samples (2θ = 7.42°, 8.62° and 25.82°, labeled with ‘*’) were well correspondent to those reported in previous literature [18]. Compared with UiO-66-NH2, the pattern of UiO-66-NH2-SA-V had no apparent difference after imine formation and coordination with V, which indicated the basic lattice structure of UiO-66-NH2 was well maintained after post-synthesis, and this was consistent with the results of SEM.
Fig. 3

PXRD patterns of UiO-66-NH2, UiO-66-NH2-SA and UiO-66-NH2-SA-V

The FT-IR spectra of UiO-66-NH2, UiO-66-NH2-SA and UiO-66-NH2-SA-V were shown in Fig. 4. The uncoordinated free –NH2 was appeared in the range of 3300–3500 cm−1 and 1600–1500 cm−1. And the sharp peaks of 1338 cm−1 and 1261 cm−1 were attributed to the C–N stretching vibrations of NH2–BDC [19]. The appeared band at 1653 cm−1 in UiO-66-NH2-SA and UiO-66-NH2-SA-V was ascribed to the azomethine group (C=N) stretching vibration. Moreover, the appeared peak at 1096 cm−1 can be assigned to the C–O of VO(acac)2 in UiO-66-NH2-SA-V [20]. These results indicated that the VO(acac)2 was successfully anchored into the Schiff base UiO-66-NH2-SA.
Fig. 4

FT-IR spectra of UiO-66-NH2, UiO-66-NH2-SA and UiO-66-NH2-SA-V

UV–Vis diffuse reflectance spectra (DRS) of UiO-66-NH2, UiO-66-NH2-SA and UiO-66-NH2-SA-V were characterized by a UV-2600 UV–Vis spectrometer with an integrating sphere attachment, using BaSO4 as reference (Fig. 5). The UV–Vis spectrum of UiO-66-NH2 exhibited an intense peak located at 200–440 nm. Compared with UiO-66-NH2, the optical adsorption of both UiO-66-NH2-SA and UiO-66-NH2-SA-V moved towards longer wavelength, suggesting the decrease in band gap and the increase of reaction activity in visible light field.
Fig. 5

a UV–Vis DRS spectra and b the band gaps of UiO-66-NH2, UiO-66-NH2-SA and UiO-66-NH2-SA-V

The nitrogen adsorption–desorption isotherms of UiO-66-NH2 and UiO-66-NH2-SA-V were shown in Fig. 6. The Langmuir surface area of UiO-66-NH2 was 721 m2/g with pore volume of 0.32 cm2/g. After a series of reaction, the Langmuir surface area of UiO-66-NH2-SA-V decreased to 603 m2/g with pore volume of 0.30 cm2/g, which indicated the occupation of internal channels in Zr-based MOF structure.
Fig. 6

N2 adsorption and desorption isotherm of UiO-66-NH2 and UiO-66-NH2-SA-V

3.2 Photocatalytic Ability of the Catalyst

The catalytic efficiency of the heterogeneous UiO-66-NH2-SA-V was tested in the photooxidation of benzene to phenol in liquid phase by using hydrogen peroxide as the oxidant under visible-light irradiation. Initially, we studied the effect of various parameters to optimize the reaction conditions. The reaction temperatures were found to be sensitive to the catalytic activity. At room temperature the reaction was found to be slow, upon increasing the temperature, the conversions of benzene were enhanced progressively, 60 °C was found to be optimum (Fig. S1). However, much higher temperatures above 60 °C led to a negative impact on the catalytic activity, mainly due to the rapid decomposition of H2O2. The catalytic activity was also dependant on reaction time. Under the conditions adopted in the present study, the highest activity was received at 4 h, and further prolonging the hydroxylation reaction showed no apparent improvement for its productivity (Fig. S2). The decomposition of hydrogen peroxide was inevitable during the hydroxylation reaction, in order to improve the utilization of hydrogen peroxide, the ratio of H2O2 to benzene was investigated (Table 1). The conversion of benzene and yield of phenol increased rapidly from 2.4 to 15.3% as the molar ratio of H2O2 to benzene was increased from 1:2 to 3:2 (Table 1 entry 1 to entry 4). However, when extra hydrogen peroxide was added and the the molar ratioes of H2O2 to benzene were 2:1 and 3:1, the reaction mixture changed into two layers, and resulting in a decrease in reaction efficiency and a decrease in phenol yield (entry 5, 6). Surprisingly, no other by-products were detected in the reaction, and the results showed that the synthesized catalyst had a very high selectivity for benzene hydroxylation to phenol. As shown in Table 1 entry 7, the yield of phenol catalyzed by UiO-66-NH2 was 1.86%, which was much lower than that catalyzed by UiO-66-NH2-SA-V. After the adsorption of benzene, the pores of Zr–MOF were occupied by benzene, which greatly reduced the adsorption of phenol on the catalyst and prevented the phenol from being further oxidized, thereby realizing the highly selective benzene hydroxylation to phenol. Moreover, for UiO-66-NH2-SA-V catalyst, the VO(acac)2 anchored on the MOFs promoted H2O2 to generate ·OH radicals under illumination, thereby promoting hydroxylation of benzene adsorbed on the catalyst to form phenol, resulting in relatively high yield of phenol [21]. Therefore, the synergetic effect between the Zr–MOFs and the vanadium complex resulted the excellent catalytic performance of UiO-66-NH2-SA-V. Catalytic reactions without irradiation were also performed, and under the dark conditions, slight amount of phenol (5.8%) was observed (entry 8). No reaction took place in a blank experiment carried out by simply mixing H2O2 with benzene (3:2) and stirring the resulting mixture at 60 °C in the absence of catalyst (entry 9). This suggested that a photoactivation of catalytic species was involved in the catalytic cycle of hydroxylation mechanism.
Table 1

Screening of catalyst for hydroxylation of benzene to phenol with H2O2



Conversion (%)

Selectivity (%)



































Reaction conditions: catalyst (10 mg),benzene (1 mL, 11.2 mmol), acetonitrile (5 mL), acetic acid (1 mL), H2O2 (30 wt%), T = 333K, and t = 4 h

aThe catalyst is UiO-66-NH2

bWithout visible light

cWithout catalyst

As a heterogeneous catalyst, UiO-66-NH2-SA-V was insoluble in acetonitrile and its recycle abilities were evaluated. After completion of one cycle reaction, the catalyst was recovered by centrifugation, washed with methanol and dried in vacuum. A new reaction was then performed with fresh benzene, under similar conditions. The UiO-66-NH2-SA-V could be recycled at least five times without losing its activity (Fig. S3). The results were attributed to vanadium oxyacetylacetonate was anchored on the Schiff base UiO-66-NH2-SA with chemical bonds, and the anchor effect stabilized and prevented the leak of vanadium oxyacetylacetonate, leading to the high catalytic recyclability. The metal leaching of UiO-66-NH2-SA-V was studied by ICP–AES analysis after completion of the reaction. The ICP–AES of the mother liquor did not show the presence of vanadium metals (Table S1). The FT-IR spectra (Fig. S4) and XRD pattern (Fig. S5) of the recovered UiO-66-NH2-SA-V showed no significant differences compared with the fresh catalyst, indicating the good reusability of the catalyst in the hydroxylation of benzene. This economically and environmentally friendly approach with UiO-66-NH2-SA-V is expected to find applications in the hydroxylation of aromatics.

4 Conclusion

A new heterogeneous catalyst UiO-66-NH2-SA-V was prepared by the vanadium oxyacetylacetonate anchored on the Schiff base UiO-66-NH2-SA with chemical bonds. This catalyst could give 15.3% yield of phenol with high selectivity. The high selectivity and activity was the synergetic effect between the Zr–MOF and the vanadium complex. The V complex provided the active site while the MOF guaranteed the selectivity of phenol. This catalyst also had good stability and high recoverability in the hydroxylation of benzene to phenol.



This project was granted financial support from the Henan Province program for science and technology development (Grant No. 16210221247) and the Program of Henan Province Department of Education (Grant No. 15A430053).

Supplementary material

10562_2019_2842_MOESM1_ESM.docx (1 mb)
Supplementary material 1 (DOCX 1049 kb)


  1. 1.
    Carneiro L, Silva AR (2016) Selective direct hydroxylation of benzene to phenol with hydrogen peroxide by iron and vanadyl based homogeneous and heterogeneous catalysts. Catal Sci Technol 6:8166–8176. CrossRefGoogle Scholar
  2. 2.
    Hock H, Lang S (1944) Autoxydation von kohlenwasserstoffen, IX. mitteil.: über peroxyde von benzol-derivaten. Ber Dtsch Chem Ges 77:257–264. CrossRefGoogle Scholar
  3. 3.
    Wang Weitao, Li Na, Tang Hao, Ma Yangmin, Yang Xiufang (2018) Vanadium oxyacetylacetonate grafted on UiO-66-NH2 for hydroxylation of benzene to phenol with molecular oxygen. Mol Catal 453:113–120. CrossRefGoogle Scholar
  4. 4.
    Zhu C, Yun J, Wang Q, Hu Q, Yang G (2018) Facile and direct hydroxylation of benzene to phenol over graphene-based catalysts: integrated utilization of greenhouse nitrous oxide. Adv Theory Simul 1:1800005. CrossRefGoogle Scholar
  5. 5.
    Zhang Li, Qiu Shuhai, Jiang Guoqing, Jiang Guomin, Tang Ruiren (2018) A CuII-based metal-organic framework as an efficient photocatalyst for direct hydroxylation of benzene to phenol in aqueous solution. Asian J Org Chem 7:165–170. CrossRefGoogle Scholar
  6. 6.
    Sarma BB, Carmieli R, Collauto A, Efremenko I, Martin JML, Neumann R (2016) Electron transfer oxidation of benzene and aerobic oxidation to phenol. ACS Catal 6:6403–6407. CrossRefGoogle Scholar
  7. 7.
    Li Yan, Wang Zhi, Chen Rizhi, Wang Yong, Xing Weihong, Wang Jun, Huang Jun (2014) The hydroxylation of benzene to phenol over heteropolyacid encapsulated in silica. Catal Commun 55:34–37. CrossRefGoogle Scholar
  8. 8.
    Wang Hefang, Zhao Meng, Zhao Qian, Yang Yongfang, Wang Cunyue, Wang Yanji (2017) In-situ immobilization of H5PMo10V2O40 on protonated graphitic carbon nitride under hydrothermal condition: a highly efficient and reusable catalyst for hydroxylation of benzene. Ind Eng Chem Res 56:2711–2721. CrossRefGoogle Scholar
  9. 9.
    Murata Kazuhisa, Yanyong Ryu, Inaba Megumu (2005) Effects of vanadium supported on ZrO2 and sulfolane on the synthesis of phenol by hydroxylation of benzene with oxygen and acetic acid on palladium catalyst. Catal Lett 102:143–148. CrossRefGoogle Scholar
  10. 10.
    Wang C, Hu L, Wang M, Ren Y, Yue B, He H (2016) Vanadium supported on graphitic carbon nitride as a heterogeneous catalyst for the direct oxidation of benzene to phenol. Chin J Catal 37:2003–2008. CrossRefGoogle Scholar
  11. 11.
    Hu L, Wang C, Yue B, Chen X, He H (2017) Vanadium-containing mesoporous carbon and mesoporous carbon nanoparticles as catalysts for benzene hydroxylation reaction. Mater Today Commun 11:61–67. CrossRefGoogle Scholar
  12. 12.
    Shafigulin RV, Filippova EO, Shmelev AA et al (2019) Mesoporous silica doped with dysprosium and modified with nickel: a highly efficient and heterogeneous catalyst for the hydrogenation of benzene, ethylbenzene and xylenes[J]. Catal Lett 149(4):916–928. CrossRefGoogle Scholar
  13. 13.
    Jie Xu, Chen Ye, Hong Ying, Zheng Huan, Ma Dan, Xue Bing, Li Yong-Xin (2018) Direct catalytic hydroxylation of benzene to phenol catalyzed by vanadia supported on exfoliated graphitic carbon nitride. Appl Catal A 549:31–39. CrossRefGoogle Scholar
  14. 14.
    Verma S, NasirBaig RB, Nadagouda MN, Varma RS (2016) Photocatalytic C-H activation of hydrocarbons over VO@g-C3N4. ACS Sustain Chem Eng 4:2333–2336. CrossRefGoogle Scholar
  15. 15.
    Xu J, Chen Y, Hong Y et al (2018) Direct catalytic hydroxylation of benzene to phenol catalyzed by vanadia supported on exfoliated graphitic carbon nitride[J]. Appl Catal A 549:31–39. CrossRefGoogle Scholar
  16. 16.
    Vakili Reza, Shaojun Xu, Al-Janabi Nadeen, Gorgojo Patricia, Holmes Stuart M, Fan Xiaolei (2018) Microwave-assisted synthesis of zirconium-based metal organic frameworks(MOFs): optimization and gas adsorption. Microporous Mesoporous Mater 260:45–53. CrossRefGoogle Scholar
  17. 17.
    Han Yitong, Liu Min, Li Keyan, Zuo Yi, Wei Yingxu, Shutao Xu, Zhang Guoliang, Song Chunshan, Zhang Zongchao, Guo Xinwen (2015) Facile synthesis of morphology and size controlled zirconium metal–organic framework UiO-66: the role of hydrofluoric acid in crystallization. CrystEngComm 17:6434–6440. CrossRefGoogle Scholar
  18. 18.
    Shan Pu, Lei Xu, Sun Lin, Hongbin Du (2015) Tuning the optical properties of the zirconium-UiO-66 metal–organic framework for photocatalytic degradation of methyl orange. Inorg Chem Commun 52:50–52. CrossRefGoogle Scholar
  19. 19.
    Guan Q, Wang B, Chai X, Liu J, Gu J, Ning P (2017) Comparison of Pd-UiO-66 and Pd-UiO-66-NH2 catalysts performance for phenol hydrogenation in aqueous medium. Fuel 205:130–141. CrossRefGoogle Scholar
  20. 20.
    Shang S, Chen B, Wang L, Dai W, Zhang Y, Gao S (2015) High-performance recyclable V–N–C catalysts for the direct hydroxylation of benzene to phenol using molecular oxygen. RSC Adv 5:31965–31971. CrossRefGoogle Scholar
  21. 21.
    Hu L, Wang C, Yue B et al (2017) Vanadium-containing mesoporous carbon and mesoporous carbon nanoparticles as catalysts for benzene hydroxylation reaction[J]. Mater Today Commun 11:61–67. CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  • Yunxia Fang
    • 1
  • Liuxue Zhang
    • 1
    Email author
  • Qianqian Zhao
    • 1
  • Xiulian Wang
    • 2
  • Xu Jia
    • 1
  1. 1.School of Materials and Chemical EngineeringZhongyuan University of TechnologyZhengzhouPeople’s Republic of China
  2. 2.School of Energy and EnviromentZhongyuan University of TechnologyZhengzhouPeople’s Republic of China

Personalised recommendations