Boosting photocatalytic oxidative coupling of amines by a Ru-complex-sensitized metal-organic framework

Journal of Catalysis 378 (2019) 248–255

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Boosting photocatalytic oxidative coupling of amines by a Ru-complexsensitized metal-organic framework Xue Yang a,b, Tao Huang a, Shuiying Gao a,b,⇑, Rong Cao a,b,⇑ a b

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China University of Chinese Academy of Science, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 7 July 2019 Revised 22 August 2019 Accepted 26 August 2019

Keywords: Metal-organic frameworks Photocatalysis Active oxygen species Amines Heterogeneous photocatalysis

a b s t r a c t Visible-light-driven selective photocatalytic organic synthesis has recently become a topic of great interest due to its environmental friendliness and sustainability. It is demanding for photocatalysis to utilize the wider range of light, such as visible light, and its performance is often plagued by the sluggish separation of photogenerated charge carriers. An approach is now reported to address these issues by incorporating light harvesting RuII-polypyridyl complexes into a semiconductor-type metal-organic framework (MIL-125). Delightedly, the obtained Ru(bpy)3@MIL-125 photocatalyst presents a remarkably stable and high photoactivity toward the selective oxidative coupling of amines under ambient air with visible light irradiation (k > 440 nm). The mechanistic investigation unveiled that both effectively photoexcited electrons transfer from [Ru(bpy)3]Cl2 to MIL-125 and the interaction of CAH bonds with superoxide radical (O
2 ) play a critical role in photo-catalyzing selective aerobic oxidative coupling of amines. This work highlights a significant role of MOFs as heterogeneous photocatalysts in photocatalytic organic transformations. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Aerobic couplings of primary amines to corresponding imines is important in organic synthesis [1-3]. However, it still remains a great challenge [4-6]. Traditional oxidants, such as N-tertbutylphenylsulfinimidoyl chloride or 2-iodoxybenzoicacid, are neither environmental-friendly nor economical, because they often demand relatively harsh reaction conditions and lead to undesired side products [7]. Alternatively, the visible light-driven organic transformation has especially attracted growing interest due to its promising potential for solar energy utilization [8-12]. Moreover, photocatalytic organic transformations also facilitate higher selectivity. It is desirable to facilitate the charge separation in photocatalysis and inhibit the strong recombination of photogenerated e -h+ pair [13-16]. Metal-organic frameworks (MOFs) are emerging functional inorganic-organic hybrid materials that feature as well-ordered pore structures, large surface areas, and structural flexibility [13,17-20]. Recently, increasing work focused on the applications of MOFs to propel the sunlight-driven organic transformations ⇑ Corresponding authors at: State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China. E-mail addresses: [email protected] (S. Gao), [email protected] (R. Cao). https://doi.org/10.1016/j.jcat.2019.08.038 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

[21–26]. However, its practical application was inevitably restricted, because most MOFs can merely harvest high energy UV light [27,28], which constitutes only 4% of the solar light, thus not being a safe process [29,30]. Several strategies have been proposed to improve the sunlight utilization of MOFs [31,32], including decoration of organic linker or metal center [33], combining MOFs with other narrow band gap semiconductors [34], and incorporating dyes or other additional catalytic species into MOFs [35-38]. Li and co-workers reported the first case of using NH2-MIL-125 to catalyze the aerobic selective oxidation of amines to imines under visible light [39]. Unfortunately, their low photocatalytic efficiency is not as good as that of traditional semiconductor materials. The reports related to the oxidative coupling of amines by MOFs are still rare. [RuII(bpy)3]Cl2 is a well-known catalyst and demonstrates extensively applications in photocatalytic organic transformations [40,41]. However, [RuII(bpy)3]Cl2 is a homogeneous catalysts that not only exists the common issues of difficulties of separation and recyclability but also often causes aggregation and suppresses the process of reaction [42-44]. It is still a challenge to realize the easily recovery and recycle of RuII-polypyridyl complexes in photocatalysis. Considering the well-defined cavities of metal-organic frameworks (MOFs), it is a rational design to incorporate the RuII-polypyridyl complexes into MOFs to develop heterogeneous photocatalysts. Yuan et al. [45] reported the photocatalysis of aerobic CDC reactions of

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N-phenyltetrahydroisoquinoline derivatives with phosphite esters catalyzed by FJI-Y2 ([Mn6(BINDI)4(H2O)2Cl2]
[Ru(bpy)3]
4Me2NH2). Although some systems based on Ru/MOFs composites were previously reported, the applications of RuII-polypyridyl-MOF composites in photocatalytic organic reactions are very limited. Moreover, those systems did not show photoredox of [RuII(bpy)3] Cl2, and the related charge-separation dynamics still remains elusive. Herein, RuII-polypyridyl complex was successfully incorporated into a representative semiconductor-like metal–organic framework (MIL-125) (hereafter named as Ru(bpy)3@MIL-125) [46-48]. The resulting Ru(bpy)3@MIL-125 catalyst exhibits satisfactory activity and selectivity toward photooxidative coupling of amines using open-air as the terminal oxidant. This work combines the merits of its homogeneous counterpart [Ru(bpy)3]Cl2 and MIL125 and avoids the disadvantages of each component. Mechanistic investigation reveals that O2
and e were the main reactive species during photocatalytic process. Additionally, the deeper understanding of the reaction mechanisms is also discussed in detail, which for the first time unveils that effective electrons transfer from excited [Ru(bpy)3]Cl2 to semiconductor-type MIL-125 plays a critical role in efficient and selective catalysis in organic chemistry. 2. Experimental section 2.1. Materials and instrumentation All chemicals were purchased from commercial sources and used as received without further treatment. De-ionized water with the specific resistance of 18.25 MX cm was obtained by reversed osmosis followed by ion-exchange and filtration. Powder X-ray diffraction (PXRD) data were collected on a Rigaku MiniFlex 600 diffractometer working with Cu Ka radiation. The N2 sorption isotherms were operated on an automatic volumetric adsorption equipment (Micromeritics ASAP 2460). 2.2. Catalyst preparation 2.2.1. Synthesis of the MIL-125 (Ti) MIL-125 was synthesized according to the previous report with minor modifications [46]. Typically, terephthalic acid (0.6 g), anhydrous methanol (1.2 mL), and anhydrous DMF (9 mL) were mixed well in glovebox. The mixture was then transferred into a 25 mL autoclave containing Ti(OiPr)4 (0.312 mL), and heated at 130 °C for 15 h. Finally, white MIL-125 were harvested by filtration and washed with ethyl alcohol and acetone for several times. 2.2.2. Synthesis of the Ru(bpy)3@MIL-125 The encapsulation of [Ru(bpy)3]Cl2 in MIL-125 was prepared by the same procedure as that for pure MIL-125, except for that 0.312 mmol [Ru(bpy)3]Cl2 was added into the mixture during hydrothermal process. 2.3. Characterization methods 2.3.1. ESR detection of O2 over Ru(bpy)3@MIL-125 40 mL of 5-Tert-Butoxycarbonyl-5-Methyl-1-Pyrroline N-oxide (DMPO)/methanol (20 mL/mL) was added into 0.4 mL acetonitrile solution of Ru(bpy)3@MIL-125 (5 mg). The capillary was enclosed in an EPR tube for test. The electron paramagnetic resonance (EPR) signal of the O
2 was measured by a Bruker-BioSpin E500 spectrometer at room temperature under open air conditions. The light source was the same as that for photocatalytic experiments described.

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2.3.2. Electrochemical characterization All the electrochemical measurements were performed on a Zahner Instruments electrochemical station in a traditional three electrode quartz cell with a 0.5 M aqueous Na2SO4 solution as electrolyte. The catalyst-coated FTO (exposed area 0.25 cm2) acted as the working electrode, a Pt plate as the counter electrode, and Ag/AgCl as the reference electrode. The photocurrent signals were measured using the same light source as that for photocatalytic experiments described above. Mott-Schottky plots of photocatalyst were measured on an electrochemical workstation at frequencies of 500, 1000, and 1500 Hz, respectively. Cyclic voltammetry measurements were performed in a traditional three electrode with glassy carbon as the working electrode. A 0.1 M tetra-nbutylammonium hexafluorophosphate (TBAPF6) in CH3CN solution was used as the electrolyte, which was purged with N2 for a while before the measurement. Ferrocene was employed as an internal standard. 2.4. Catalytic reaction In a typical procedure, 5 mg prepared photocatalyst was added into 3 mL CH3CN solution containing 0.1 mmol of amine in a 20 mL quartz vial. The sample was stirred vigorously and irradiated with a 300 W xenon lamp equipped with an ultraviolet cut-off filter (>440 nm). After reacted for a certain period of time, 100 mL of the reaction was diluted by ethyl alcohol to remove photocatalyst for gas chromatograph analysis (Agilent 7890A equipped with an FID detector). The products quantification was confirmed by comparing the rendition time with that commercial samples and further confirmed by gas chromatography mass spectrometry (GC-MS) (QP2020). When photocatalysis was finished, the reaction solutions were filtered by filter membrane and washed with acetonitrile. The photocatalyst was dried at 70 °C under vacuum conditions. Then the collected photocatalyst was reused for another catalytic cycle under the optimized reaction conditions. 3. Results and discussion Photoactive Ru(bpy)3@MIL-125 was prepared from the similar method of MIL-125 in the presence of [Ru(bpy)3]Cl2. The Ru content in Ru(bpy)3@MIL-125 was measured to be 0.58% by inductively coupled plasma (ICP) measurement. Fig. 1a. showed that the XRD spectra of Ru(bpy)3@MIL-125 agrees well with its host frameworks (i.e., MIL-125) upon incorporation of RuII-polypyridyl complex [49]. In consistent with previous reports, the BET specific surface area of the pristine MIL-125 was determined to be 1102.84 m2/g, while the Ru(bpy)3@MIL-125 showed a significantly decreased BET surface area of 486.42 m2/g (Fig. 1b). In addition, the pore size distribution analysis also showed a decreased pore volume of the Ru(bpy)3@MIL-125 (Fig. 1c). These results indicated the successful incorporation of RuII-polypyridyl complexes into MIL-125. Interestingly, the incorporation of light-harvesting [Ru (bpy)3]Cl2 greatly broadened the light absorption of MIL-125 into visible light region (Fig. 1d), which was accompanied by a distinct samples colour change from white to orange, indicating its potential application for the target reactions in visible light range. The significant visible light absorbing ability of Ru(bpy)3@MIL125 encourages us to explore its activity toward oxidative coupling of amines under visible light (k > 440 nm). Here, benzylamine was used as a probe substrate to optimize the reaction conditions (Table 1). To our delight, the conversion of benzylamine could reach 75% under standard conditions (entry1, Table 1), which was superior to the reported system [39,50-52]. When no catalysts or light was present (entries 2 and 5), 0% of conversion of benzylamine was observed, indicating that the photocatalytic processes

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Fig. 1. (a) The XRD patterns of Ru(bpy)3@MIL-125 and MIL-125. (b) Nitrogen adsorption–desorption isotherm, and (c) pore size distribution analysis of MIL-125 and Ru (bpy)3@MIL-125, (d) UV–vis spectra of the fresh MIL-125, [Ru(bpy)3]Cl2 and Ru(bpy)3@MIL-125.

Table 1 Oxidative coupling of benzylamine under various conditions a.

a b c d e f g

Entry

Catalyst

Time/h

Conversion/%b

1 2c 3d 4e 5 6f 7 8 9g

Ru(bpy)3@MIL-125 Ru(bpy)3@MIL-125 Ru(bpy)3@MIL-125 Ru(bpy)3@MIL-125 No catalyst Ru(bpy)3@MIL-125 P25 MIL-125 Ru(bpy)3@MIL-125

3 3 3 3 3 12 3 3 12

75 0 100 15 0 trace 0 0 43

Reaction conditions: 0.1 mmol benzylamine, 5 mg Ru(bpy)3@MIL-125 in 3 mL CH3CN, in the open air with visible light irradiation (>440 nm) at room temperature. Determined by GC analysis. In darkness. Under O2 atmosphere. Under a 1 atm N2 atmosphere. The reaction in the presence of Ru(bpy)3@MIL-125 was stirred under the dark at 70 °C. 5 mmol benzylamines, without solvent.

were involved and entirely responsible for benzylamine conversion. This could be further proved by entry 6 where no conversion of benzylamine was observed even the system was stirred under the dark at 70 °C for 12 h. Obvious conversion of benzylamine was only observed in the presence of air (entry 3), while a much lower conversion was obtained in N2 atmosphere (entry 4). This result indicated that oxygen was required to be involved in the photocatalytic oxidative coupling of benzylamine. For comparison, as a widely studied commercial photocatalyst, P25 showed no conversion under standard conditions due to its wide band gap (entry

7). More interestingly, even without using any solvent, Ru(bpy)3@ MIL-125 still showed remarkable catalytic activity and excellent selectivity (entry 9). To demonstrate that the reaction was truly processed in a heterogeneous manner [53], Ru(bpy)3@MIL-125 was filtered out during the reaction (of the third cycle), and the results showed that keeping the reaction under the identical conditions for another 2 h did not lead to any conversion of benzylamine. It is necessary to explore the activity of Ru(bpy)3@MIL-125 photocatalyst on oxidative coupling reactions of various amines under

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the optimal reaction conditions. As shown in Table 2, using openair as the terminal oxidant, various amines derivatives bearing different steric and electronic moieties were converted to corresponding imines with almost absolute reaction selectivity. In addition, the benzylamines para-substituted with electron-donating groups proceeded slightly faster than those with electronwithdrawing groups (entries 1–6). More strikingly, amines containing nitrogen or sulfur atoms, which are usually known to poison metal catalysts, could also be well tolerated and displayed excellent yields (entries 7, 8). Among all the selected amines, 3-(Aminomethyl) pyridine showed the best conversion to its product. It is very meaningful to enhance the life time of photocatalyst for its practical applications. In the present study, Ru(bpy)3@ MIL-125 can be recycled by simple centrifuge and deal with acetonitrile. The result showed that the photocatalytic activity and selectivity of Ru(bpy)3@MIL-125 are almost maintained after 5 cycles (Fig. 2). The conversion decreased after first cycle, which probably due to the dissociated of the [Ru(bpy)3]Cl2 from the very surface of the MIL-125. In this work, MIL-125 promotes the photostability and recyclability of the [Ru(bpy)3]Cl2 of the homogeneous [Ru(bpy)3]Cl2 and also makes it more convenient to be easily recycled. Furthermore, the XRD patterns (Fig. 2b) of Ru(bpy)3MIL-125 remained well before and after the reaction, indicating its excellent recyclability.

To better understand the photocatalytic mechanism of the reaction, the spin trap DMPO (5, 5-dimethyl-pyrroline-N-oxide) was used to confirm the presence of reactive oxygen species. As Fig. 3a showed, there was no EPR signal for the Ru(bpy)3@MIL125 in the dark. In contrast, distinct EPR signals owing to the O
2 were detected under visible light irradiation. We also further explore the origin of the enhanced photocatalytic behavior. As Fig. 3b showed, photocurrent transient response over several on/ off cycles of intermittent irradiation was observed to have good reproducibility. The much higher photocurrent response of Ru (bpy)3@MIL-125 demonstrated its superiority for promoting more efficient electrons transfer, which will contribute to the photocatalytic activity towards aerobic coupling oxidation of benzylamine under visible light irradiation. Under visible light irradiation (>440 nm), the RuII-polypyridyl complexes can serve as antennas to absorbed energy and is then excited to form electronically excited states. Based on the MottSchottky plots at different frequencies (Fig. 4a), the lowest unoccupied molecular orbital (LUMO) of [Ru(bpy)3]Cl2 is located at 0.88 V vs normal hydrogen electrode (NHE). Furthermore, the bandgap of [Ru(bpy)3]Cl2 was measured to be 1.72 eV resorting to the Tauc plot (Fig. 4b). It thus can be concluded that the highest that the occupied molecular orbital (HOMO) of [Ru(bpy)3]Cl2 was 0.84 V vs NHE. Mott-Schottky measurements at different frequencies showed a positive slope of the MIL-125, indicating its typical

Table 2 Scope of the Photocatalytic Oxidative Coupling Reactions of Amines Using Ru(bpy)3@MIL-125 as a Photocatalyst a.

Entry

a b

Amine

Product

Conv. (%)

b

Sel. (%)

1

75

>99

2

61

>99

3

65

>99

4

54

>99

5

57

>99

6

70

>99

7

100

>99

8

65

>99

9

19

>99

10

60

>99

Reaction conditions: 0.1 mmol of amine, 5 mg of Ru(bpy)3@MIL-125, 3 mL of CH3CN, in the open air, visible light (k > 440 nm), 3 h. Determined by GC using dodecane as an internal standard.

b

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Fig. 2. (a) Recycling test on Ru(bpy)3@MIL-125 for aerobic coupling oxidation of benzylamine under visible light irradiations for 3 h, (b) The XRD patterns of the recycled photocatalyst.

Fig. 3. (a) The electron paramagnetic resonance (EPR) spectrum of O
2 captured by DMPO. (b) Photocurrent transient response of Ru(bpy)3@MIL-125 and MIL-125 under visible light irradiation.

Fig. 4. Mott-Schottky plots for [Ru(bpy)3]Cl2 (a) and MIL-125 (c) at frequencies of 500, 1000 and 1500 Hz. Tauc plot and its bandgap for [Ru(bpy)3]Cl2 (b) and MIL-125 (d).

X. Yang et al. / Journal of Catalysis 378 (2019) 248–255

n-type semiconductor like character. The energy diagrams of MIL125 was concluded with assisted of UV–Vis spectra (Fig. 4c, d). Given the more negative potential of the LUMO (-0.63 V vs. NHE) in MIL-125, it is theoretically feasible for electrons transfered to O2 to produce related reactive oxygen species (ROS, E(O2/O2 ) = 0.33 V vs. NHE) [54]. It is reasonable to conclude that MIL-125 exhibits semiconducting properties and the appropriate band positions between [Ru(bpy)3]Cl2 and MIL-125 enable the thermodynamically possible for the photoinduced electrons transfer. Eventually, the amines effectively convert to its corresponding imines. It is worth mentioning that the well-defined crystalline structure of the MIL-125 creates short migration paths for charge carriers transfer to the surface for reacting with substrates, thus improving the transfer of photogenerated charge carriers. The molecular oxygen would act as a container for trapping and transferring of the photoinduced electrons to produce reactive oxygen species (as indicated above). Furthermore, the oxidation potentials of benzylamine was measured by cyclic voltammetry. The voltammogram of benzylamine showed the first oxidative potential at ca. + 0.46 V (vs Ag/AgCl, similarly hereinafter), which was assigned to the oxidation of benzylamine to benzalaniline (Fig.S1). It suggests that, the HOMO value of candidate photocatalyst for oxidative cou-

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pling of benzylamine should be more positive than 0.46 V to trigger the reaction. Obviously, the oxidation of benzylamine can be synergistically and theoretically catalyzed by Ru(bpy)3@MIL-125. The electrons transfer process for the photocatalytic oxidative coupling of benzylamine was further proposed (Scheme 1). On the basis of above analysis and the previously reported mechanisms [6,55] the transfer of photoexcited electrons to reaction species was further proposed as the following pathway (Scheme 2): firstly, and [Ru(bpy)3]Cl2 was employed as an visible-light-sensitizer and was excited to produce excited Ru2+*. With open air as an oxidant, Ru2+ was regenerate from Ru+ through a novel single electron transfer (SET) process. The photoinduced electrons transfer to the molecular oxygen to produce superoxide radical species, which was also confirmed by EPR measurement (Fig. 3a). And accordingly, the as-formed Ti3+ was oxidized back to Ti4+ in the reductive quenching cycle. Amines can donate an electron, and be oxidized into PhCH2NH+2. The superoxide radical can abstract a hydrogen atom at the benzylic position of a benzylamine radical cation to produce H2O2, which is a superoxide radical participating catalytic reaction. The efficient exciton-O2 energy transfer contribute to the oxidative coupling of benzylamine to produce the desired product after addition of another amine and

Scheme 1. The proposed mechanism for the charge transfer process for the photocatalytic oxidative coupling of benzylamine under ambient conditions. (LUMO = lowest unoccupied orbital, HOMO = highest occupied orbital).

Scheme 2. Schematic illustration of electron transfers for the oxidative coupling of benzylamine photocatalyzed by Ru(bpy)3@MIL-125.

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removal of ammonia. As an additional proof to confirm the proposed mechanism, the generation of benzaldehyde during photocatalysis has been directly evidenced by GC-MS. More interestingly, the pores of MIL-125 are beneficial for its interactions with substrate during the reaction. 4. Conclusions In summary, photocatalytic oxidation of amines to imines under visible light irradiations (k > 440 nm) using open-air as oxidant were achieved on the Ru(bpy)3@MIL-125 photocatalyst. The photoinduced electrons transfer occurs from excited [RuII(bpy)3] Cl2 to MIL-125, which significantly facilitates the migration of the photogenerated carriers and ultimately improves the photocatalytic activity for oxidative coupling of amines. The structure and photocatalytic reactivity of Ru(bpy)3@MIL-125 also remained negligible changes after several cycles. This work combines the merits of both [RuII(bpy)3]Cl2 and MIL-125 and overcomes the disadvantages of each component. The specific reaction mechanism proposed in this work opens a new avenue for applying MOFs based photocatalysts toward high selective organic synthesis under visible light. Acknowledgement The authors acknowledge the financial support of National Key Research and Development Program of China (2017YFA0700100), financial support of the NSFC (21520102001, 51572260 and 21571177), Key Research Program of Frontier Sciences, CAS (Grant No. QYZDJ-SSW-SLH045), and Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.08.038. References [1] M.T. Schümperli, C. Hammond, I. Hermans, Developments in the aerobic oxidation of amines, ACS Catal. 2 (2012) 1108–1117. [2] X. Zhang, K.P. Rakesh, L. Ravindar, H.-L. Qin, Visible-light initiated aerobic oxidations: a critical review, Green Chem. 20 (2018) 4790–4833. [3] N. Zhang, X. Li, H. Ye, S. Chen, H. Ju, D. Liu, Y. Lin, W. Ye, C. Wang, Q. Xu, J. Zhu, L. Song, J. Jiang, Y. Xiong, Oxide defect engineering enables to couple solar energy into oxygen activation, J. Am. Chem. Soc. 138 (2016) 8928–8935. [4] H. Liu, C. Xu, D. Li, H.L. Jiang, Photocatalytic hydrogen production coupled with selective benzylamine oxidation over MOF composites, Angew. Chem. Int. Ed. 57 (2018) 5379–5383. [5] C. Xu, H. Liu, D. Li, J.H. Su, H.L. Jiang, Direct evidence of charge separation in a metal-organic framework: efficient and selective photocatalytic oxidative coupling of amines via charge and energy transfer, Chem. Sci. 9 (2018) 3152– 3158. [6] J.D. Xiao, H.L. Jiang, Metal-organic frameworks for photocatalysis and photothermal csatalysis, Accounts Chem. Res. 52 (2019) 356–366. [7] K.C. Nicolaou, C.J. Mathison, T. Montagnon, o-Iodoxybenzoic acid (IBX) as a viable reagent in the manipulation of nitrogen-and sulfur-containing substrates: scope, generality, and mechanism of IBX-mediated amine oxidations and dithiane deprotections, J. Am. Chem. Soc. 126 (2004) 5192– 5201. [8] X. Lang, X. Chen, J. Zhao, Heterogeneous visible light photocatalysis for selective organic transformations, Chem. Soc. Rev. 43 (2014) 473–486. [9] C. Parmeggiani, F. Cardona, Transition metal based catalysts in the aerobic oxidation of alcohols, Green Chem. 14 (2012) 547–564. [10] J. Chen, J. Cen, X. Xu, X. Li, The application of heterogeneous visible light photocatalysts in organic synthesis, Catal. Sci. Technol. 6 (2016) 349–362. [11] X. Lang, W.R. Leow, J. Zhao, X. Chen, Synergistic photocatalytic aerobic oxidation of sulfides and amines on TiO2 under visible-light irradiation, Chem. Sci. 6 (2015) 1075–1082. [12] D. Ravelli, M. Fagnoni, A. Albini, Photoorganocatalysis. What for?, Chem Soc. Rev. 42 (2013) 97–113. [13] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535.

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