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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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01719 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017
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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 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 with the state-of-art metal or non-metal catalytic systems reported. KEYWORDS: covalent triazine framework, photocatalysis, selective oxidation, alcohol, visible light
The 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 an environmentally benign reaction condition.4,5 So far, mainly metal-based 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, amine 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 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 due to their ab-
sorption in the visible range, non-metal nature and highly tunable optic and electronical properties.9,10 Recent research activities have employed small molecular11-15 or macromolecular16-25 organic semiconductors for visible light-promoted photocatalytic reactions. Among the developed macromolecular systems, porous carbon nitrides,26-33 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 kind of reaction so far. The poor performance in the selective oxidation reaction of the organic photocatalysts could likely be caused by their insufficient photo-generated 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-accepter 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 offer a promising candidate for this purpose due to their highly robust nature and large structure design variety. Via 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 inevitably carbonization of the skeleton, making the energetic bands of CTFs unfavorable for photocatalytic processes.37 Opti-
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mized ionothermal conditions at decreased temperature have been reported.41 A recent report on metal-free synthetic routes using trifluoromethanesulfonic acid (TfOH) as catalyst in solution at elevated temperatures provided a huge advantage towards metal-free preparation of the CTFs.42,43 Nevertheless, the hydrolysis of the terminal groups of the precursors often caused the poor polymerization degree and low surface area.44 Lately, our group has developed a solid state synthesis of highly porous CTFs under TfOH vapor.45 Herein, we report the use of a thiophene-containing CTF (CTF-Th) as photocatalyst for metal-free, visible light-promoted 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 support. The catalytic efficiency of CTF-Th was comparable with most of the state-of-art metal or non-metal catalytic systems reported.
Figure 1. Illustrated design and formation pathway of the thiophene-based covalent triazine framework CTF-Th on SBA-15 as mesoporous nanoreactor.
As illustrated in Figure 1, via cyclization polymerization of 2,5-dicyanothiophene (DCT) under TfOH vapor, CTFTh was directly synthesized onto mesoporous silica SBA15, obtaining the mesoporous nanoreactor CTF-Th@SBA15 as insoluble yellow powder (Figure S1). The scanning electron microscopy (SEM) revealed the hexagonal cylinder morphology of the CTF-Th@SBA-15 with a dimeter of ca. 500 nm and a length of 800 nm (Figure 2a). The high resolution transmission electron microscopy (HR-TEM) confirmed the presence of 2D hexagonal channels (Figure 2b). The elemental mapping indicated that all of the elements (C, N and S) are evenly localized in the material (Figure 2c-2f), which suggested the uniformly 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 relative pressure (0.499% selectivity (entry 2). Additionally, 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 % (entry 3 and Figure S12). The decreased activity was probably caused by the structural collapse and aggregation when removing the silica support, resulting in the low surface area (Figure S2/S3).
Table 1. Photocatalytic selective oxidation of benzyl alcohol with oxygen under visible light.a
Entry
Catalyst
Atm.
Conv. (%)
Sel. (%)
1
CTF-Th@SBA-15
O2
>99
>99
2
CTF-Th@SBA-15
Air
92
>99
CTF-Th
O2
17
>99
CTF-Th@SBA-15
O2
0
n.d.
No Catalyst
O2
0
n.d.
3 4
b
5 6
CTF-Th@SBA-15
N2
8
>99
c
CTF-Th@SBA-15
O2
38
95
d
CTF-Th@SBA-15
O2
15
97
e
CTF-Th@SBA-15
O2
>99
>99
10
f
CTF-Th@SBA-15
O2
42
>99
g
CTF-Th@SBA-15
O2
39
>99
CTF-Th@SBA-15
O2
>99
>99
7
8
9
11
h
12
a
Reaction condition: benzyl alcohol (0.1 mmol), photocatalyst (10 mg), benzotrifluoride (1.5 ml), O2, blue LED lamp (λ=460 nm, 0.16 W/cm²), R.T, 4 h; conversion and selectivity b c were determined by GC-MS. In dark; benzoquinone as d e superoxide scavenger; CuCl2 as electron scavenger; tertf butyl alcohol as hydroxyl radical scavenger; ammonium g oxalate as hole scavenger; NaN3 as singlet oxygen scavenger; h n.d.: not detected; Catalase as H2O2 scavenger.
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 4-5). And the conversion rate of benzyl alcohol toward benzaldehyde dramatically decreased to 8 % under N2 atmosphere, indicating the mandatory role of oxygen as oxidant (entry 6). By adding CuCl2 and benzoquinone as 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 (entry 7-8). The LUMO level of CTF-Th lay at 0.72 V vs SCE, which is sufficient enough to transform molecular oxygen into its reduced activated superoxide radical form, where the reduction potential of O2/·O2¯ lies at -0.56 V vs SCE.46 In comparison, for the other activated oxygen species, hydroxyl radical, 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 enough. This could be confirmed by another control experiment under addition of tert-butyl alcohol as hydroxyl radical scavenger, no effect on the conversion was observed (entry 9). This could explain the extremely high selectivity of the oxidation reaction. Under addition of ammonium oxalate as a hole scavenger, a reduced conversion of 42 % was observed, indicating an active part
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of the photo-generated hole (entry 10). Moreover, the spin trap EPR experiment confirmed the formation of ·O2¯ using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as trapping agent (Figure S13). Singlet oxygen (1O2), another selective oxidant, has also been detected when 2,2,6,6,tetramethylpiperidine (TEMP) was used as the trapping agent (Figure S14). 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). Thus, the alcohols oxidation 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 photo-generated 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).47 Given that fact that H2O2 could be also a potential oxidant for the oxidation reaction, an additional control experiment with Catalase as H2O2 scavenger was conducted under the standard conditions. An almost quantitative conversion was obtained with a selectivity of >99% toward benzaldehyde (entry 12). This result clearly indicated the formed H2O2 did not play a crucial role during 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 the rate-determining step should be the hydrogen elimination in the reaction process (Figure S16). The turnover frequency (TOF) of the photocatalyst was 7.6 × 10-3 mol/g/h. It is worth to point out that the catalytic efficiency of CTF-Th@SBA-15 was highly comparable with that of most of state-of-art metal or non-metal catalysts reported (Table S3). Furthermore, the repeating experiments demonstrated that CTF-Th@SBA-15 could remain its 73% catalytic efficiency after 5 extra reaction cycles without loss of the selectivity (Figure S17). No apparent change of the UV-Vis DR and FT-IR spectra of the photocatalyst after 5 reaction cycles were observed, indicating its high stability and recyclability as heterogeneous photocatalyst (Figure S18, S19).
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Figure 3. Proposed reaction mechanism of the selective oxidation of alcohols using CTF-Th@SBA-15 as photocatalyst. Next, the scope of the alcohols was tested. As shown in the Table 2, the photocatalytic oxidation of primary alcohols was obtained in almost quantitative conversion with >99% selectivity after 4 h, with excellent group tolerance (entry 15). The oxidation of secondary alcohols was achieved with conversions higher than 90 % and high or moderate selectivity (entry 6-7). Additionally, the allylic alcohol could be oxidized to the corresponding α,β-unsaturated aldehyde with a conversion of 78% and selectivity of 72% (entry 8).
Table 2. Scope of the photocatalytic selective oxidation of alcohols using CTF-Th@SBA-15 as photocatalyst.a Entry
Substrate
Product
Conv. (%)
Sel. (%)
1
>99
>99
2
>99
>99
3
>99
>99
4
>99
>99
5
>99
>99
6
90
>99
7
93
87
8
78
72
b
c
a
Reaction condition: benzyl alcohol (0.1 mmol), CTFTh@SBA-15 (10 mg), benzotrifluoride (1.5 ml), O2, blue light (λ=460 nm, 0.16 W/cm²), RT, 4 h; conversion and selectivity b were determined by GC-MS. 1,1,2,2-tetraphenylethan-1-ol as c side product (13%). Benzaldehyde as side product (27%).
In conclusion, we reported the employment of a thiophen-containing covalent traizine framework (CTF) as 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 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 non-metal catalysts reported. We believe that this study could boost the feasibility of CTFs as highly efficient metal-free photocatalyst for a broader application field in organic synthesis under mild and environmentally benign conditions.
ASSOCIATED CONTENT Supporting Information. Experiment details and characterization of the monomer and CTFs, comparison with the
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state-of-art catalysts reported etc. This material is available free of charge via the Internet at http://pubs.acs.org.
Corresponding Author *
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT The authors acknowledge the Max Planck Society for financial support. W.H. thanks the Chinese 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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