Elsevier

Molecular Catalysis

Volume 452, June 2018, Pages 175-183
Molecular Catalysis

NH2-MIL-125(Ti)/TiO2 composites as superior visible-light photocatalysts for selective oxidation of cyclohexane

https://doi.org/10.1016/j.mcat.2018.04.004Get rights and content

Highlights

  • The NH2-MIL-125(Ti)/TiO2 composites are used as visible-light photocatalysts with good stability for selective oxidation of cyclohexane.

  • The composites have good catalytic performance due to energy bands coupling and intimate interfacial contact between NH2-MIL-125(Ti) and TiO2.

  • The possible reaction mechanism has been proposed based on controlling experiments using different radical scavenger techniques.

Abstract

Semiconductor-metal-organic framework (MOF) hybrid photocatalysts have aroused increasing interest because of their enhanced photocatalytic activity. In this work, NH2-MIL-125(Ti)/TiO2 composites with different molar ratios were successfully prepared by a facile one-pot solvothermal method and used for visible-light driven photocatalytic selective oxidation of cyclohexane. The as-prepared NH2-MIL-125(Ti)/TiO2 composites presented better catalytic performances compared with NH2-MIL-125(Ti). This is due to the energy bands coupling and intimate interfacial contact between NH2-MIL-125(Ti) and TiO2, which can improve the photon-generated carrier transfer and minimize the recombination of electron-hole pairs. For the optimum composite photocatalyst, the activity was three times higher than that of NH2-MIL-125(Ti) in visible-light driven photocatalytic cyclohexane oxidation using molecular oxygen as oxidant at room temperature and under ambient pressure. Additionally, the effects of illumination time on the photocatalytic performance and reusability of the optimum composite photocatalyst were investigated in detail. Finally, a possible reaction mechanism has been proposed based on controlling experiments using different radical scavenger techniques, which prove that the photogenerated holes are significantly involved in the oxidation process.

Introduction

Liquid phase selective oxidation of cyclohexane is an important reaction for the production of cyclohexanol and cyclohexanone (known as KA oil), which are the intermediates to fabricate nylon-6 and nylon-66 polymers [1,2]. In recent years, more and more attention has been devoted to photocatalytic selective oxidation of cyclohexane because photocatalysis technique is green, mild and has shorter reaction sequences which can reduce the possibility of side reactions [3]. Titanium dioxide is considered to be the most promising photocatalyst because of its superior photocatalytic activity, chemical stability, low cost, and nontoxicity [4,5]. In 1989, Mu with co-workers used Degussa P25 TiO2 (consisting of approximately 70% anatase and 30% rutile) as photocatalyst into the selective oxidation of cyclohexane, and high selectivity to cyclohexanone was observed [6]. Subsequently, Almquist and Biswas [7] investigated the effects of different solvents on the catalytic activity and selectivity in cyclohexane photooxidation over TiO2. Mul and co-workers [8] studied neat cyclohexane photooxidation applying different wavelengths using different reactor setups with varying TiO2 dosage and constitution. However, some shortages of TiO2 limit its practical application as photocatalyst for cyclohexane oxidation. One main drawback is its wide band gap (∼ 3.2 eV) which causes it to have no photocatalytic activity in visible light, and another main drawback is poor photocatalytic performance because of the high recombination of photogenerated electron-hole pairs and poor tunability characteristics.

To enable purely UV-active TiO2 have visible light activity, various modification methods such as impurity doping [[9], [10], [11], [12]], semiconductor coupling [13,14], dye sensitization [15,16], etc., have been employed in recent years. However, the dye-sensitized photocatalysts are usually found to be unstable under irradiation and the metal ion doping of TiO2 also shows several drawbacks, such as thermal instability and increased electron/hole recombination [17]. By comparison, the coupling with semiconductors with matched band gaps is an efficient method to achieve photocatalysis in visible light. Moreover, this approach is convenient and the matched semiconductors are extensive.

Metal-organic frameworks (MOFs) consisting of metal clusters interconnected with organic linkers have received considerable attention because of their large surface area, pore volume and tunable pore structure, which have shown potential applications for gas storage, separation, catalysis, electrical conductivity, sensor devices and drug delivery [[18], [19], [20]] etc. In addition, semiconductor MOFs also draw great attention on their utilization as photocatalysts. To date, several representative semiconductor MOFs (such as MOF-5, UiO-66(Zr), ZIF-8, and MIL-125(Ti) et al.) have been widely served as photocatalysts, but these semiconductor MOFs showed limited efficiency under solar illumination because of the lack of visible light response. Fortunately, for the semiconductor MOFs, the effective use of solar light can be facilely achieved by the modification of their organic linkers or metal centers [21]. For example, NH2-MIL-125(Ti) (denoted as NH2-M125), as the isostructural MOF of MIL-125(Ti), constitutes of cyclic Ti8O8(OH)4 oxoclusters and 2-aminoterephthalate ligands, and it shows extended absorption spectra into the visible-light region (around 550 nm) because of its amino functionality moieties [22]. In 2012, Li et al. [22] first reported that NH2-M125 reduced CO2 to HCOO under visible light irradiation but unfortunately with low activity. In 2015, Yuan’s group [23] employed two Ti-based MOFs (MIL-125(Ti) and its derivative NH2-M125) as multifunctional photocatalysts to reduce Cr(VI). Compared with MIL-125(Ti), the NH2-M125 exhibited higher photocatalytic activity for Cr(VI) reduction in aqueous solution under visible-light irradiation.

Recently, MOFs as matched semiconductors coupling with traditional inorganic semiconductors such as TiO2 and CdS, have been proven to be a feasible and effective approach to reduce the recombination probability of photogenerated electrons and holes thus improving the photocatalytic activity [24,25]. In 2014, Lin and co-workers [25] discovered UiO-66/CdS with a photocatalytic hydrogen evolution rate 11.2 times as high as that of pure commercial CdS. The high photocatalytic activity of UiO-66/CdS could be attributed to the increased catalytic sites and reaction centers and the minimized recombination of charge carriers. In 2016, Luo and co-workers [26] prepared TiO2/ZIF-8 nanofibers which displayed much better performance for the photocatalytic degradation of RhB than either TiO2 or ZIF-8. The excellent photocatalytic performance was attributed to the formed N-Ti-O chemical bond under sonochemical treatment between TiO2 and ZIF-8, which may result in reducing recombination of the electron-hole pairs.

In this paper, we have realized the semiconductor coupling of NH2-M125 and TiO2. The main purposes are listed below: (i) Narrow the band gap of TiO2 and extend its absorption spectra toward visible light region; (ii) Reduce recombination of the electron-hole pairs through the transportation of photo-generated electrons of NH2-M125 to TiO2 [27]; and (iii) Promote the dispersion of TiO2 nanoparticles to produce more active centers by taking advantage of the porous structure and large surface area of NH2-M125. Previously, Jin and co-workers [28] used NH2-M125/TiO2 modified glassy carbon electrode (GCE) for the photoelectrochemical detection of pesticide clethodim, and the results implied that the fast photoelectronic communication among clethodim, TiO2, NH2-M125 and GCE led to an effective method for the photoelectrochemical detection of clethodim with good analytic performance. In this study, a series of NH2-M125/TiO2 composites were prepared via a modified solvothermal method according to the Jin’s work [28]. The photocatalytic performance of as-synthesized photocatalysts in the photocatalytic oxidation of cyclohexane were studied in detail, including the effects of illumination time and reusability. The enhanced photocatalytic performance was observed compared with pure NH2-M125 or TiO2. The possible reaction mechanism for the selective visible-light photocatalytic oxidation of cyclohexane over NH2-M125/TiO2 composites were also proposed and discussed.

Section snippets

Materials

All reagents were analytical grade. 2-Aminoterephthalic acid (H2ATA) was obtained from Beijing J&K Scientific LTD. Tetrabutyl titanate (TBT) and benzyl chloride were supplied from Tianjin Guangfu Fine Chemical. N,N-dimetyl formamide (DMF) and methanol (MeOH) were purchased from Beijing Chemical Works. P25 (TiO2) was purchased from Degussa. Ammonium oxalate (AO), potassium persulphate (K2S2O8) and tert-butanol (TBA) were obtained from Tianjin Fuchen Chemical Reagents Factory. Benzoquinone (BQ)

Characterization

The NH2-MIL-125(Ti) MOF, an extension of Ti-based MOF of MIL-125 (Ti), has been first reported by Walsh et al. [5] in 2013 with similar crystal lattice structure to MIL-125(Ti). The XRD patterns of the as-prepared NH2-M125, P25 and NH2-M125/P25 composites are shown in Fig. 1A. The well-defined diffraction peaks of NH2-M125 reveal the high crystallinity of the NH2-MIL-125(Ti). As can be seen by viewing these curves, all of the NH2-M125/TiO2 composites samples exhibit all characteristic peak of

Conclusions

In conclusion, we have successfully prepared a series of NH2-M125/P25 composites with different P25 contents via a one-pot hydrothermal method. The as-prepared NH2-M125 and different NH2-M125/P25 composites can perform as visible-light driven photocatalysts for photocatalytic cyclohexane selective oxidation at room temperature and under ambient pressure. Moreover, all the obtained NH2-M125/P25 composites have higher photocatalytic activity than bare NH2-M125, which can be attributed to the

Acknowledgements

We acknowledge the support of this work by the National Natural Science Foundation of China (Grant No. 21676296) and National Key Research and Development Plan (Grant No. 2016YFC0303700).

References (51)

  • P. Du et al.

    J. Catal.

    (2006)
  • A. Islam et al.

    J. Photochem. Photobiol. A

    (2001)
  • Z.W. Yang et al.

    Appl. Catal. B

    (2016)
  • X. Zeng et al.

    ACS Appl. Mater. Interfaces

    (2016)
  • J.C.S. Wu et al.

    J. Catal.

    (2006)
  • L. Zeng et al.

    Appl. Catal. B

    (2016)
  • X. Jian et al.

    Appl. Surf. Sci.

    (2016)
  • A.V. Vinogradov et al.

    Chem. Commun.

    (2014)
  • X. Liu et al.

    Appl. Catal. B

    (2017)
  • Y. Shiraishi et al.

    ACS Catal.

    (2017)
  • J. Li et al.

    Ind. Eng. Chem. Res.

    (2010)
  • J. Li et al.

    Catal. Lett.

    (2010)
  • H. Li et al.

    Front. Chem. Sci. Eng.

    (2012)
  • X.B. Chen et al.

    Chem. Rev.

    (2007)
  • C.H. Hendon et al.

    J. Am. Chem. Soc.

    (2013)
  • W. Mu et al.

    Catal. Lett.

    (1989)
  • C.B. Almquist et al.

    Appl. Catal. A

    (2001)
  • F. Zuo et al.

    J. Am. Chem. Soc.

    (2010)
  • A.M. Czoska et al.

    Phys. Chem. Chem. Phys.

    (2011)
  • M. Janusab et al.

    Appl. Surf. Sci.

    (2015)
  • S. Hoang et al.

    J. Am. Chem. Soc.

    (2012)
  • H. Tada et al.

    Nature Mater.

    (2006)
  • M. Miyauchi et al.

    Adv. Mater.

    (2000)
  • Y. Tachibana et al.

    J. Phys. Chem.

    (1996)
  • J. Schneider et al.

    Chem. Rev.

    (2014)
  • Cited by (112)

    View all citing articles on Scopus
    View full text