Visible-Light-Promoted Selective Oxidation of Alcohols Using a

Letter pubs.acs.org/acscatalysis

Visible-Light-Promoted Selective Oxidation of Alcohols Using a Covalent Triazine Framework Wei Huang, Beatriz Chiyin Ma, Hao Lu, Run Li, Lei Wang, Katharina Landfester, and Kai A. I. Zhang* Max Planck Institute for Polymer Research, D-55128 Mainz, Germany

ACS Catal. 2017.7:5438-5442. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/27/19. For personal use only.

S Supporting Information *

ABSTRACT: The formation of aldehydes and ketones via selective oxidation of alcohols is an essential transformation in organic synthesis. However, the usually harsh reaction conditions using toxic metal catalysts or corrosive reagents lead to undesired side products and wastes. Environmentally friendly and mild reaction conditions using metal-free catalysts remain a huge challenge. Herein, we report the use of a thiophene-based covalent triazine framework (CTF) as pure organic and visible-light-active photocatalyst for the selective oxidation of alcohols at room temperature. Molecular oxygen was activated as a clean and selective oxidant. The high selectivity and efficiency of the pure organic photocatalyst could be demonstrated and were comparable to those of the state-of-art metal or nonmetal catalytic systems reported. KEYWORDS: covalent triazine framework, photocatalysis, selective oxidation, alcohol, visible light

T

ic semiconductors for visible light-promoted photocatalytic reactions. Among the developed macromolecular systems, porous carbon nitrides,26−33 which represent a nitrogen-rich state-of-art example, were used for the selective oxidation of alcohols into aldehyde at elevated temperatures.34 A recent report showed the use of a graphene/carbon nitride composite for selective oxidation of saturated hydrocarbons at high temperature (>130 °C) under high oxygen pressure (10 bar).35 Further examples using metal-free photocatalysts have barely been reported for this type of reaction so far. The poor performance in the selective oxidation reaction of the organic photocatalysts could likely be caused by their insufficient photogenerated oxidation potential, as a result of the usually high highest occupied molecular orbital (HOMO) levels. To overcome this limitation of pure organic photocatalysts, a rational design on the electron donor−acceptor arrangement in the backbone structure of the organic semiconductor could lower the energetic band level, improve the electron delocalization, and thus enhance the photocatalytic efficiency.36 Covalent triazine-based frameworks (CTFs)37−40 could be a promising candidate for this purpose, because of their highly robust nature and large structure design variety. By carefully varying the building blocks connected to the electronwithdrawing triazine unit, the energy band structure of the CTFs could be modified. However, the traditional ionothermal preparation conditions lead to the inevitable carbonization of the skeleton, making the energetic bands of CTFs unfavorable for photocatalytic processes.37 Optimized ionothermal conditions at decreased temperature have been reported.41 A

he selective oxidation of organic compounds is a primary reaction in organic synthesis and industrial chemistry.1 Especially, the formation of aldehydes and ketones via direct and selective oxidation of alcohols is an essential and important reaction. However, to overcome the usually high oxidation potential of the substrates, expensive and toxic noble-metal catalysts or highly corrosive reagents under thermal conditions have been used, which leads to undesired side products and is troublesome for product purification.2,3 Extensive efforts have been devoted to the search for a greener reaction pathway. Photochemistry offers a promising alternative. In particular, active oxygen species can be produced as a clean and terminal oxidant under light irradiation and, thus, provide environmentally benign reaction conditions.4,5 So far, mainly metalbased photocatalytic systems have been used for the oxidation of the organic compounds. For example, Zhao et al. reported on the use of TiO2 as an effective photocatalyst for selective aerobic oxidation of alcohols, amines, and alkanes.6 Shiraiishi et al. reported on Pt nanoparticle-functionalized metal oxide semiconductors for selective oxidation of alcohols to the corresponding aldehydes or ketones.7 Jiang et al. reported a Ptfunctionalized porphyrinic MOF for selective oxidation of alcohols.8 Nevertheless, the low quantum yield and inefficient photocatalytic activity of TiO2 in the visible-light regime, as well as the employment of precious metals, significantly limit their practical application. To overcome these limitations, an alternative photocatalytic system is strongly needed. Organic semiconductor-based photocatalysts offer a more sustainable and environmentally friendly alternative to the traditional metal photocatalysts, because of their absorption in the visible range, nonmetal nature, and highly tunable optic and electronical properties.9,10 Recent research activities have employed small molecular11−15 or macromolecular16−25 organ© 2017 American Chemical Society

Received: May 25, 2017 Revised: June 29, 2017 Published: July 17, 2017 5438

DOI: 10.1021/acscatal.7b01719 ACS Catal. 2017, 7, 5438−5442

Letter

ACS Catalysis recent report on metal-free synthetic routes using trifluoromethanesulfonic acid (TfOH) as a catalyst in solution at elevated temperatures provided a huge advantage toward metalfree preparation of the CTFs.42,43 Nevertheless, the hydrolysis of the terminal groups of the precursors often caused a poor degree of polymerization and low surface area.44 Lately, our group has developed a process involving the solid-state synthesis of highly porous CTFs under TfOH vapor.45 Herein, we report the use of a thiophene-containing CTF (CTF-Th) as a photocatalyst for metal-free, visible-lightpromoted selective oxidation of alcohols into the corresponding aldehydes and ketones, using molecular oxygen as a clean oxidant at room temperature. By incorporating thiophene units into the CTF backbone, a significantly low HOMO level at +1.75 V vs SCE could be achieved, resulting into a high photooxidation potential. Furthermore, to elucidate the morphology effect and enlarge the active area of CTF-Th during the photocatalytic process, mesoporous silica was employed as a support. The catalytic efficiency of CTF-Th was comparable with most of the state-of-art metal or nonmetal catalytic systems reported. As illustrated in Figure 1, via cyclization polymerization of 2,5-dicyanothiophene (DCT) under TfOH vapor, CTF-Th was

Figure 2. (a) Typical SEM and (b, c) HR-TEM images of CTF-Th@ SAB-15; corresponding element mapping images in (d) carbon, (e) sulfur, and (f) nitrogen; (g) gas sorption isotherms and (h) pore size distribution of pure SBA-15 and CTF-Th@SBA-15; (i) Fourier transform infrared (FT-IR) spectra of the monomer DCT and the polymer CTF-Th; (j) N 1s and (k) C 1s XPS spectra; (l) UV-vis diffuse reflectance (DR) spectrum; and (m) HOMO/LUMO band positions of CTF-Th.

15 could be calculated (Figure 2h). In contrast, after the removal of SBA-15, the obtained pure CTF-Th exhibited a significantly decreased BET surface area of 57 m2/g (see Figure S2 and Table S1 in the Supporting Information), because of the collapse of the mesoporous channels (Figure S3 in the Supporting Information). The weight content of pure CTFTh in CTF-Th@SBA-15 was estimated to be 32.6% via thermogravimetric analysis (TGA) under an oxygen atmosphere (see Figure S4 in the Supporting Information). Note that the CTF-Th shows high thermal resistance up to ca. 268 °C under an oxygen atmosphere, demonstrating its excellent stability. Solid-state 13C cross-polarization magic-angle-spinning (CPMAS) NMR verified the presence of sp2 carbons at 167 ppm in the triazine and thiophene ring at 147, 132 ppm (see Figure S5 in the Supporting Information). A small amount of sp carbons from terminal cyano groups at 115 ppm could be determined. Fourier transform infrared (FT-IR) spectroscopy (Figure 2i) showed two intensive peaks at 1483 and 1341 cm−1, which can be ascribed to the aromatic C−N stretching and breathing modes in the triazine unit. The minimal peak at 2230 cm−1, typically for terminal cyano groups, confirmed a high degree of cyclization of DCT during the CTF formation. Notably, CTFTh showed high chemical stability and tolerance to NH4HF4 while etching the silica support. This was supported by the elemental analysis, solid-state 13C NMR, and the FT-IR spectra of both materials (see Table S2 and Figures S5 and S6 in the Supporting Information). The X-ray photoelectron spectroscopy (XPS) revealed the structural insight of CTF-Th (see Figures 2j and 2k). The high-resolution N 1s spectrum can be deconvoluted into three peaks. The dominant peak centered at

Figure 1. Illustrated design and formation pathway of the thiophenebased covalent triazine framework CTF-Th on SBA-15 as a mesoporous nanoreactor.

directly synthesized onto mesoporous silica SBA-15, obtaining the mesoporous nanoreactor CTF-Th@SBA-15 as an insoluble yellow powder (see Figure S1 in the Supporting Information). Scanning electron microscopy (SEM) revealed the hexagonal cylinder morphology of the CTF-Th@SBA-15 with a diameter of ca. 500 nm and a length of 800 nm (see Figure 2a). Highresolution transmission electron microscopy (HR-TEM) confirmed the presence of two-dimensional (2D) hexagonal channels (Figure 2b). The elemental mapping indicated that all of the elements (C, N and S) are evenly localized in the material (Figures 2c−f), which suggested the uniform formation of CTF-Th inside the mesopores of SBA-15. Similar to the initial silica support SBA-15, the nitrogen gas sorption isotherms of CTF-Th@SBA-15 showed a typical hysteresis at a relative pressure of 0.4 < P/P0 < 0.8 for mesopores (Figure 2g). The Brunauer−Emmett−Teller (BET) surface area of CTF-Th@SBA-15 was determined to be 548 m2/g, with a pore volume of 0.7 cm3/g, which is lower than the BET surface area of pristine SAB-15, which is 863 m2/g with a pore volume of 1.2 cm3/g. CTF-Th@SBA-15 exhibited a narrow pore size distribution at ca. 3.8 nm. From the main pore size of pristine SBA-15 of ca. 5.7 nm, an average thickness of ca. 1.9 nm for the formed CTF-Th layer in the mesopores of SBA5439

DOI: 10.1021/acscatal.7b01719 ACS Catal. 2017, 7, 5438−5442

Letter

ACS Catalysis 398.8 eV clearly indicates the successful formation of pyridinic nitrogen in triazine units (C−NC). The weak emissions at 400.2 and 402.1 eV can be attributed to the residual nitrile groups (CN) and amnion (N−H) formed by slightly hydrolysis of nitrile in the presence of TfOH. Correspondingly, the high-resolution C 1s spectrum exhibited three peaks at 284.9 for carbon atoms (C−C) in thiophene, 286.9 eV for C− N in triazine ring, and a weak one at 288.5 for CO in imide groups. Moreover, sulfur was also detected in the S 2p XPS spectrum, with an energy value of 164.5 eV (C−S−C) in thiophene units (Figure S7 in the Supporting Information). The powder X-ray diffraction (PXRD) spectrum of the CTFTh revealed its amorphous character with unordered molecular skeletons (see Figure S8 in the Supporting Information). The UV-vis diffuse reflectance (DR) spectrum of the CTFTh@SBA-15 showed a broad absorption up to ca. 520 nm and an intense green fluorescence with the maximum at ca. 530 nm. (see Figure 2l, as well as Figure S1). From the Kubelka−Munktransformed reflectance spectrum, an optical band gap of 2.47 eV could be calculated (Figure S9 in the Supporting Information). Cyclic voltammetry revealed the lowest unoccupied molecular orbital (LUMO) of CFT-Th at −0.72 V vs SCE (Figure S10 in the Supporting Information). The corresponding HOMO level at +1.75 V vs SCE could be estimated by subtracting the LUMO level from the optical band gap (Figure 2m), indicating a possible high oxidizing nature of CTF-Th. The electron paramagnetic resonance (EPR) spectra showed an enhanced signal upon light irradiation, relative to that taken in darkness, indicating photogenerated radicals, i.e., electron−hole pairs in CTF-Th (see Figure S11 in the Supporting Information). We first chose the selective oxidation of benzyl alcohol to evaluate the photocatalytic activity of CTF-Th@SBA-15. The results are listed in Table 1. The photocatalytic oxidation of

benzyl alcohol toward benzaldehyde was obtained in almost quantitative conversion, with >99% selectivity after 4 h of blue light irradiation (entry 1 in Table 1). Remarkably, even when the reaction was performed under air, the excellent catalytic activity still remained well, with 92% conversion and >99% selectivity (entry 2 in Table 1). In addition, the morphology effect of the mesoporous CTF-Th@SBA-15 as a nanoreactor could be demonstrated by removing the SBA-15 support, leading to reduced conversion of 17% (see entry 3 in Table 1 and Figure S12 in the Supporting Information). The decreased activity was probably caused by the structural collapse and aggregation when removing the silica support, resulting in the low surface area (see Figures S2 and S3). To unveil the mechanistic insight of the photocatalytic selective oxidation reaction, we then conducted a series of control experiments. It could be clearly shown that no product was determined in the absence of photocatalyst or light (entries 4 and 5 in Table 1). Moreover, the conversion rate of benzyl alcohol toward benzaldehyde dramatically decreased to 8% under N2 atmosphere, indicating the mandatory role of oxygen as an oxidant (entry 6 in Table 1). By adding CuCl2 and benzoquinone as an electron scavenger and superoxide (•O2−) scavenger, the conversion was significantly inhibited to 38% and 15%, respectively, indicating the role of photogenerated electron to active the molecular oxygen to generate the oxidant species •O2− during the catalytic process (entries 7 and 8 in Table 1). The LUMO level of CTF-Th resides at −0.72 V vs SCE, which is sufficient to transform molecular oxygen into its reduced activated superoxide radical form, where the reduction potential of O2/•O2− resides at −0.56 V vs SCE.46 In comparison, for the other activated oxygen species, hydroxyl radical, which is known as a strong nonselective oxidant with the oxidation potential E(−OH/•OH) = +2.56 V vs SCE,46 the HOMO of CTF-Th is not sufficient. This could be confirmed by another control experiment under the addition of tert-butyl alcohol as a hydroxyl radical scavenger; no effect on the conversion was observed (entry 9 in Table 1). This could explain the extremely high selectivity of the oxidation reaction. Under the addition of ammonium oxalate as a hole scavenger, a reduced conversion of 42% was observed, indicating an active part of the photogenerated hole (entry 10 in Table 1). Moreover, the spin trap EPR experiment confirmed the formation of •O2− using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent (see Figure S13 in the Supporting Information). Singlet oxygen (1O2), which is another selective oxidant, has also been detected when 2,2,6,6,-tetramethylpiperidine (TEMP) was used as the trapping agent (see Figure S14 in the Supporting Information). The addition of NaN3 as a 1O2 scavenger resulted in a decreased conversion of 39%, demonstrating its active role for the oxidation process (entry 11 in Table 1). Thus, the oxidation of alcohols in the present system is associated with combined action of •O2− and 1O2. Based on the observations from the control experiments, we propose a reaction mechanism, as illustrated in Figure 3. Under light irradiation, the photogenerated electron activates molecular oxygen into its activated forms (•O2− and 1O2), which extracts one proton of benzyl alcohol into its anionic form by obtaining •OOH species. The benzyl anion is initially oxidized by the photogenerated hole into an anionic radical, which is subsequently oxidized by •OOH into benzaldehyde as the final product. As side product, H2O2 could be determined by UV-vis absorption monitoring of triiodide in aqueous solution (Figure S15 in the Supporting Information).47 Given the fact that H2O2

Table 1. Photocatalytic Selective Oxidation of Benzyl Alcohol with Oxygen under Visible Lighta

entry

catalyst

atmosphere

conversion (%)

selectivity (%)

1 2 3 4b 5 6 7d 8e 9f 10g 11h 12i

CTF-Th@SBA-15 CTF-Th@SBA-15 CTF-Th CTF-Th@SBA-15 no catalyst CTF-Th@SBA-15 CTF-Th@SBA-15 CTF-Th@SBA-15 CTF-Th@SBA-15 CTF-Th@SBA-15 CTF-Th@SBA-15 CTF-Th@SBA-15

O2 air O2 O2 O2 N2 O2 O2 O2 O2 O2 O2

>99 92 17 0 0 8 38 15 >99 42 39 >99

>99 >99 >99 n.d.c n.d.c >99 95 97 >99 >99 >99 >99

a

Reaction conditions: benzyl alcohol (0.1 mmol), photocatalyst (10 mg), benzotrifluoride (1.5 mL), O2, blue LED lamp (λ = 460 nm, 0.16 W/cm2), room temperature (RT), 4 h; conversion and selectivity were determined by GC-MS. bIn darkness. cn.d. = not detected. dWith benzoquinone as a superoxide scavenger. eWith CuCl2 as an electron scavenger. fWith tert-butyl alcohol as a hydroxyl radical scavenger. g With ammonium oxalate as a hole scavenger. hWith NaN3 as a singlet oxygen scavenger. iWith catalase as a H2O2 scavenger. 5440

DOI: 10.1021/acscatal.7b01719 ACS Catal. 2017, 7, 5438−5442

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ACS Catalysis

Table 2. Scope of the Photocatalytic Selective Oxidation of Alcohols Using CTF-Th@SBA-15 as a Photocatalysta

Figure 3. Proposed reaction mechanism of the selective oxidation of alcohols using CTF-Th@SBA-15 as a photocatalyst.

could be also a potential oxidant for the oxidation reaction, an additional control experiment with catalase as a H2O2 scavenger was conducted under the standard conditions. An almostquantitative conversion was obtained with a selectivity of >99% toward benzaldehyde (entry 12 in Table 1). This result clearly indicated that the formed H2O2 did not play a crucial role in the photocatalytic oxidation of benzyl alcohol. Furthermore, the intermolecular competing kinetic isotope effect (KIE) of this reaction was also investigated. A mild KH/KD value of 2.2 could be revealed, indicating that the rate-determining step should be the hydrogen elimination in the reaction process (Figure S16 in the Supporting Information). The turnover frequency (TOF) of the photocatalyst was 7.6 × 10−3 mol g−1 h−1. Note that the catalytic efficiency of CTF-Th@SBA-15 was highly comparable to that of most of the state-of-art metal or nonmetal catalysts reported (Table S3 in the Supporting Information). Furthermore, the repeating experiments demonstrated that CTFTh@SBA-15 could retain its 73% catalytic efficiency after five extra reaction cycles without a loss of the selectivity (Figure S17 in the Supporting Information). No apparent change of the UV-vis DR and FT-IR spectra of the photocatalyst was observed after five reaction cycles, indicating its high stability and recyclability as a heterogeneous photocatalyst (see Figures S18 and S19 in the Supporting Information). Next, the scope of the alcohols was tested. As shown in Table 2, the photocatalytic oxidation of primary alcohols was obtained in almost-quantitative conversion with >99% selectivity after 4 h, with excellent group tolerance (see entries 1−5 in Table 2). The oxidation of secondary alcohols was achieved with conversions of >90% and high or moderate selectivity (entries 6 and 7 in Table 2). In addition, the allylic alcohol could be oxidized to the corresponding α,β-unsaturated aldehyde with a conversion of 78% and selectivity of 72% (entry 8 in Table 2). In conclusion, we have reported the employment of a thiophen-containing covalent traizine framework (CTF) as an efficient, visible-light-active photocatalyst for direct and selective oxidation of alcohols into the corresponding aldehydes and ketones, with molecular oxygen as a clean oxidant. The CTF was formed on to a mesoporous silica (SBA-15) support, acting as a highly ordered and accessible nanoreactor with a pore size of ca. 4 nm. The catalytic efficiency of the CTF-Th@ SBA-15 was comparable with the state-of-art metal or nonmetal catalysts reported. We believe that this study could boost the feasibility of CTFs as a highly efficient metal-free photocatalyst for a broader application field in organic synthesis under mild and environmentally benign conditions.

a

Reaction conditions: benzyl alcohol (0.1 mmol), CTF-Th@SBA-15 (10 mg), benzotrifluoride (1.5 mL), O2, blue light (λ = 460 nm, 0.16 W/cm2), RT, 4 h; conversion and selectivity were determined by GCMS. b1,1,2,2-tetraphenylethan-1-ol as side product (13%). cBenzaldehyde as side product (27%).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01719. Experimental details and characterization of the monomer and CTFs, comparison with the state-of-art catalysts reported, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Katharina Landfester: 0000-0001-9591-4638 Kai A. I. Zhang: 0000-0003-0816-5718 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Max Planck Society for financial support. W.H. thanks the China Scholarship Council (CSC) for the scholarship. Gunnar Glaßer and Kathrin Kirchhoff are acknowledged for HR-SEM, HR-TEM, and elemental mapping.

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REFERENCES

(1) Kochi, J.; Sheldon, R. Metal Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (2) Zhang, P.; Gong, Y.; Li, H.; Chen, Z.; Wang, Y. Nat. Commun. 2013, 4, 1593. (3) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. G. Nature 1999, 398, 227−230. (4) Lang, X.; Chen, X.; Zhao, J. Chem. Soc. Rev. 2014, 43, 473−486. (5) Colmenares, J. C.; Luque, R. Chem. Soc. Rev. 2014, 43, 765−778. (6) Lang, X.; Ma, W.; Chen, C.; Ji, H.; Zhao, J. Acc. Chem. Res. 2014, 47, 355−363. (7) Tsukamoto, D.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. J. Am. Chem. Soc. 2012, 134, 6309−6315. 5441

DOI: 10.1021/acscatal.7b01719 ACS Catal. 2017, 7, 5438−5442

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ACS Catalysis (8) Chen, Y.-Z.; Wang, Z. U.; Wang, H.; Lu, J.; Yu, S.-H.; Jiang, H.-L. J. Am. Chem. Soc. 2017, 139, 2035−2044. (9) Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355−360. (10) Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075− 10166. (11) Neumann, M.; Fuldner, S.; Konig, B.; Zeitler, K. Angew. Chem., Int. Ed. 2011, 50, 951−954. (12) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; König, B. Science 2014, 346, 725−728. (13) McTiernan, C. D.; Pitre, S. P.; Scaiano, J. C. ACS Catal. 2014, 4, 4034−4039. (14) Wang, L.; Huang, W.; Li, R.; Gehrig, D.; Blom, P. W.; Landfester, K.; Zhang, K. A. Angew. Chem., Int. Ed. 2016, 55, 9783− 9787. (15) McManus, J. B.; Nicewicz, D. A. J. Am. Chem. Soc. 2017, 139, 2880−2883. (16) Zhang, K.; Kopetzki, D.; Seeberger, P. H.; Antonietti, M.; Vilela, F. Angew. Chem., Int. Ed. 2013, 52, 1432−1436. (17) Kang, N.; Park, J. H.; Ko, K. C.; Chun, J.; Kim, E.; Shin, H. W.; Lee, S. M.; Kim, H. J.; Ahn, T. K.; Lee, J. Y.; Son, S. U. Angew. Chem., Int. Ed. 2013, 52, 6228−6232. (18) Luo, J.; Zhang, X.; Zhang, J. ACS Catal. 2015, 5, 2250−2254. (19) Wang, Z. J.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I. Adv. Mater. 2015, 27, 6265−6270. (20) Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2015, 137, 3265−3270. (21) Ghasimi, S.; Prescher, S.; Wang, Z. J.; Landfester, K.; Yuan, J.; Zhang, K. A. I. Angew. Chem., Int. Ed. 2015, 54, 14549−14553. (22) Yang, C.; Ma, B. C.; Zhang, L.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. Angew. Chem., Int. Ed. 2016, 55, 9202− 9206. (23) Sprick, R. S.; Bonillo, B.; Clowes, R.; Guiglion, P.; Brownbill, N. J.; Slater, B. J.; Blanc, F.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Angew. Chem., Int. Ed. 2016, 55, 1792−1796. (24) Zhang, G.; Lan, Z.-A.; Wang, X. Angew. Chem., Int. Ed. 2016, 55, 15712−15727. (25) Ko, J. H.; Kang, N.; Park, N.; Shin, H.-W.; Kang, S.; Lee, S. M.; Kim, H. J.; Ahn, T. K.; Son, S. U. ACS Macro Lett. 2015, 4, 669−672. (26) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76−80. (27) Wang, X.; Blechert, S.; Antonietti, M. ACS Catal. 2012, 2, 1596−1606. (28) Wang, Y.; Wang, X.; Antonietti, M. Angew. Chem., Int. Ed. 2012, 51, 68−89. (29) Zhang, J. S.; Chen, Y.; Wang, X. C. Energy Environ. Sci. 2015, 8, 3092−3108. (30) Lin, Z.; Wang, X. Angew. Chem., Int. Ed. 2013, 52, 1735−1738. (31) Martin, D. J.; Qiu, K.; Shevlin, S. A.; Handoko, A. D.; Chen, X.; Guo, Z.; Tang, J. Angew. Chem., Int. Ed. 2014, 53, 9240−9245. (32) Niu, P.; Zhang, L.; Liu, G.; Cheng, H.-M. Adv. Funct. Mater. 2012, 22, 4763−4770. (33) Shalom, M.; Inal, S.; Fettkenhauer, C.; Neher, D.; Antonietti, M. J. Am. Chem. Soc. 2013, 135, 7118−7121. (34) Chen, Y.; Zhang, J.; Zhang, M.; Wang, X. Chem. Sci. 2013, 4, 3244−3248. (35) Li, X.-H.; Chen, J.-S.; Wang, X.; Sun, J.; Antonietti, M. J. Am. Chem. Soc. 2011, 133, 8074−8077. (36) Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181−191. (37) Kuhn, P.; Antonietti, M.; Thomas, A. Angew. Chem., Int. Ed. 2008, 47, 3450−3453. (38) Katekomol, P.; Roeser, J. r. m.; Bojdys, M.; Weber, J.; Thomas, A. Chem. Mater. 2013, 25, 1542−1548. (39) Hao, L.; Ning, J.; Luo, B.; Wang, B.; Zhang, Y.; Tang, Z.; Yang, J.; Thomas, A.; Zhi, L. J. Am. Chem. Soc. 2015, 137, 219−225. (40) Li, L.; Fang, W.; Zhang, P.; Bi, J.; He, Y.; Wang, J.; Su, W. J. Mater. Chem. A 2016, 4, 12402−12406. (41) Schwinghammer, K.; Hug, S.; Mesch, M.; Senker, J.; Lotsch, B. Energy Environ. Sci. 2015, 8, 3345−3353.

(42) Ren, S.; Bojdys, M. J.; Dawson, R.; Laybourn, A.; Khimyak, Y. Z.; Adams, D. J.; Cooper, A. I. Adv. Mater. 2012, 24, 2357−2361. (43) Zhu, X.; Tian, C.; Mahurin, S. M.; Chai, S.-H.; Wang, C.; Brown, S.; Veith, G. M.; Luo, H.; Liu, H.; Dai, S. J. Am. Chem. Soc. 2012, 134, 10478−10484. (44) Bi, J.; Fang, W.; Li, L.; Wang, J.; Liang, S.; He, Y.; Liu, M.; Wu, L. Macromol. Rapid Commun. 2015, 36, 1799−1805. (45) Huang, W.; Wang, Z. J.; Ma, B. C.; Ghasimi, S.; Gehrig, D.; Laquai, F.; Landfester, K.; Zhang, K. A. I. J. Mater. Chem. A 2016, 4, 7555−7559. (46) Ghosh, S.; Kouamé, N. A.; Ramos, L.; Remita, S.; Dazzi, A.; Deniset-Besseau, A.; Beaunier, P.; Goubard, F.; Aubert, P.-H.; Remita, H. Nat. Mater. 2015, 14, 505−511. (47) Zou, Q.; Zhang, L.; Yan, X.; Wang, A.; Ma, G.; Li, J.; Möhwald, H.; Mann, S. Angew. Chem. 2014, 126, 2398−2402.

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DOI: 10.1021/acscatal.7b01719 ACS Catal. 2017, 7, 5438−5442