English

• Nowadays, the efficient conversion of hydrocarbons derived from fossil fuel resources into high additional-value compounds by selectivity oxidation has been become one of the most important reactions of modern petrochemical industry [1-5]. Benzaldehyde and styrene oxide are important intermediates of pharmaceutical, pesticide and functional materials, which can be produced by selective oxidation of styrene [6-8]. Photocatalytic oxidation technology is receiving considerable attention as one of the most promising strategies to obtain compounds for having an inherent advantage in environmental protection, energy conservation and high efficiency by utilizing environmentally singlet oxygen (¹O₂) and solar energy source to drive the redox reaction [9-12]. However, as a sole oxidant ¹O₂, its reaction efficiency still needs to be improved. Superoxide radicals (O₂^·‒) has displayed excellent catalytic performance in many reactions, which can be produced by heterogeneous photocatalysts through photogenerated electrons transferring to O₂ [13-15]. Therefore, in order to achieve the synergy of ¹O₂ and O₂^·‒, it is imperative to develop charge transfer units and to promote multi-electron transfer reactions.

  The combination of photosensitizer and oxidation catalyst within one framework has attracted much attention as a powerful generation photocatalysts for prospective practical applications [16-18]. Beyond that, the major challenge to achieve high photocatalytic efficiency and selectivity lies in the synergy of the multiple catalytic sites and the efficient electron transfer. Inspired by the high efficient, specificity and stereoselective of natural enzymes, chemists have attempted to construct chemical photocatalysis system capable of forming long-lived charge-separated states for advances in solar energy conversion [19-21]. By assembling the well-chosen electron donor and acceptor in an analogous coordination mode to metalloenzymes make them possess highly intrinsic peroxidase-like activity, good stability and long excited state lifetimes [22, 23].

  Functional naphthalenediimide (NDI) derivatives as n-type semiconductors meet the evaluation of design principles because of their efficient electron-transfer properties, high redox activity and strong π-acidity [24, 25]. Additionally, it can be easily introduced into framework by using non-covalent or covalent strategies to enhance the light absorption capacity of catalysts act as a photosensitizer. Owing to the excellent reversible photo- and electro-chromism, thermal stability and redox-active properties, polyoxometalates (POMs) are well-known catalysts studied in the olefin epoxidation [26, 27]. Especially for multi-electron transfer processes, POMs are promising candidates because they are stable against oxidative degradation and can receive a lot of electrons and protons without deforming their structures [28-30].

  In this context, a new polyoxometalate-based metal-organic framework (POMOF), Ni₂(H₂O)₇(DPNDI)_2.5 [H₂ZnW₁₂O₄₀]·2H₂O (NiW-DPNDI) was synthesized by solvothermal method in presence of nickel(II) ion, [WZn₃(H₂O)₂(ZnW₉O₃₄)₂]^12‒ and DPNDI ligand. In this structrue, the strong anion⋯π interactions between the trapped [ZnW₁₂O₄₀]^6‒ anions and the electron-deficient naphthalenic ring centroids and the π⋯π interactions between the DPNDI moieties in the solid-state superstructure can promote electron–hole separation and create a short charge transport pathway. NiW-DPNDI was explored in the reaction of oxidation of styrene under thermocatalysis and photocatalysis systems, respectively, which displayed high efficiency and diverse chemselectivity. Under irradiation, benzaldehyde is the main product, while styrene oxide is by-product in the reaction of oxidation of styrene. However, the main product in thermocatalysis is opposite to that in photocatalysis. In addition, because the carbonic anhydrase (CA)-mimicking Ni sites and the negative electron-enriched POM are well aligned in the pores, which can activate epoxide and promote the capture and activation of CO₂. Thus, NiW-DPNDI shows good effect for CO₂ cycloaddition reactions at a mild temperature. Moreover, it gave a yield of 78% in one-pot tandem reaction (Scheme 1).

  Scheme 1

  

  Scheme 1.  The design concept of NiW-DPNDI for photocatalysis styrene oxidation.

  DownLoad: Full-Size Img PowerPoint

  NiW-DPNDI was synthesized by NiCl₂·6H₂O, DPNDI and Na₁₂[WZn₃(H₂O)₂(ZnW₉O₃₄)₂]·46H₂O precursor under hydrothermal conditions. Single-crystal structural analysis revealed that NiW-DPNDI crystallizes in a space group P-1. The asymmetric unit of NiW-DPNDI consists of a [H₂ZnW₁₂O₄₀]^4‒ unit, a [Ni₂(DPNDI)_2.5(H₂O)₇]⁴⁺ cation, and two free H₂O molecules (Fig. 1a). [ZnW₁₂O₄₀]^6‒ shows the Keggin type structure, which connected with two nickel ions (Ni1 and Ni2) by two terminal oxygen atoms, respectively. The Ni1 ion adopts a distorted octahedral geometry and coordinates with one terminal oxygen atoms, one N atom of DPNDI and four H₂O molecules. And Ni2 ion coordinated with two N atoms of two DPNDI molecules, one terminal oxygen atoms and three H₂O. The lability of the coordinated water molecules and free protons allows NiW-DPNDI to act as both Lewis acid catalyst and Brønsted acid catalyst. Ni1 and Ni2 are linked by DPNDI to form a fragment. The fragments are then stacked in a paralleled pattern by π⋯π interactions (about 3.3~3.4 Å) to form a 3D porous structure (Fig. 1b). The H-type aggregate between the DPNDI moieties in the solid-state superstructure is beneficial for enhanced electron transport. The [ZnW₁₂O₄₀]^6‒ anions are localized on the electron-deficient naphthalenic ring centroid with centroid distance 3.0 Å. Hydrogen bonds were found between the C atoms and the imbedded [ZnW₁₂O₄₀]^6‒ anion with separations between C⋯O about 3.07~3.30 Å. With these hydrogen bonds interacted [ZnW₁₂O₄₀]^6‒ anion, the sheets were further stacked in a parallel fashion with both the active sites of the Lewis acid catalysts and the oxidation catalysts aligned in the channels. The accessible empty space of the framework is approximately 232.9 ų (5.0%), as calculated from PLATON analysis, suggesting the possibility that the MOF-based material NiW-DPNDI can absorb substrates within its pores [31].

  Figure 1

  

  Figure 1.  (a) Ball-and-stick representation of NiW-DPNDI. (b) 3D network of NiW-DPNDI viewed down the a axis.

  DownLoad: Full-Size Img PowerPoint

  The morphology of NiW-DPNDI was characterized by scanning electron microscopy (SEM), which showed quadrilateral shaped blocks with coarse surfaces. The mapping images of NiW-DPNDI reflected the element distribution (Fig. S2 in Supporting information). In the FTIR spectrum, the peaks at 938−768 cm^‒1 are attributed to v(W=O_d) and v(W–O_b/O_c) vibrations of [ZnW₁₂O₄₀]^6‒ (Fig. S3a in Supporting information) [32]. The peaks of 1710−1597 cm⁻¹ corresponded to C−H, C=O, N-H stretching vibrations of the DPNDI. Elemental analyses and powder X-ray analysis (PXRD) indicated the pure phase of its bulk sample (Fig. S3b in Supporting information). Thermo-gravimetric analysis (TGA) indicated that there is a weight loss of 3.43% (calcd. 3.77%) from 30 ℃ to 128 ℃, which are corresponding to the loss of two free H₂O molecules and seven coordinated water molecules. From 128 ℃ to 490 ℃, there was a weight loss of 5.10% corresponding to half a DPNDI in the framework (calcd. 5.21%). From 490 ℃, the skeleton starts to collapse with loss of two DPNDI. The results proved NiW-DPNDI has good thermal stability that meets most of the prerequisites as an ideal platform for heterogeneous catalysts (Fig. 2a).

  Figure 2

  

  Figure 2.  Characterizations of NiW-DPNDI, (a) TG. (b) TPD−CO₂ curve. (c) Solid UV–vis absorption spectra (inset: Tauc plots). (d) Mott−Schottky plots. (e) Electrochemical impedance spectroscopy (EIS) Nyquist plots in 0.1 mol/L Na₂SO₄ aqueous solution without bias versus Ag/AgCl electrode irradiated with white light. (f) Transient photocurrent response in 0.1 mol/L Na₂SO₄ aqueous solution without bias versus Ag/AgCl electrode irradiated with white light.

  DownLoad: Full-Size Img PowerPoint

  TPD−CO₂ (temperature-programmed desorption of CO₂) study on CO₂ sorption was carried out [33]. Generally, the different desorption peaks stand for different bonding strength between the CO₂ and the sorbent. In Fig. 2b, two desorption peaks indicated there are two sorption sites at 194 ℃ and 392 ℃, respectively. As seen, there was a desorption peak at 194 ℃, meanwhile, there also exited a weight loss in TGA at the similar temperature. This phenomenon verifed that abundant water molecules in the structure can effectively enhance the adsorption of CO₂. The other desorption peak at 392 ℃ is corresponding to the skeleton collapses. In order to explore the universality of NiW-DPNDI, it was soaked in a series of solution with pH value of 2, 6 and 10 for 24 h, respectively. The IR spectra of soaked crystals are consistent with that of fresh samples, which proved NiW-DPNDI has the good acid-base stability (Fig. S4 in Supporting information).

  The optical properties of NiW-DPNDI were researched with UV–vis diffuse reflectance spectroscopy (DRS). As shown in Fig. 2c, it exists optical absorption showed in UV−vis band from 250 nm to 450 nm, which proved it has good light absorption ability. The band gap (E_g) was calculated to be 2.66 eV by using Kubelka−Munk (KM) method based on Tauc plot [34]. As shown in Mott−Schottky plots, NiW-DPNDI is a typical n-type semiconductor [35]. As can be seen from Fig. 2d, the flat band potential of NiW-DPNDI was estimated to be ‒1.32 V (vs. Ag/AgCl). From the equation E_CB = E(vs. Ag/AgCl) + 0.2 V, the negative position of the lowest unoccupied molecular orbital (LUMO) of NiW-DPNDI is estimated to be –1.12 V (vs. NHE). The highest occupied molecular orbital (HOMO) potential value is 1.54 V calculate by the equation of E_VB = E_CB + E_g. According to Nyquist plots, NiW-DPNDI has a minimal charge transfer resistance, which indicated that NiW-DPNDI has higher photochemical activity for electron and hole separation (Fig. 2e). It is obvious that NiW-DPNDI has steady and strong photocurrent response than DPNDI, which in harmony with the result of EIS (Fig. 2f). The result further verified the incorporation of the redox-active DPNDI into POM-based MOF can promote the production of abundant of photogenic electrons.

  Selective oxidation of styrene was a subject of experimental significance. The photocatalytic reactions were performed on WATTCAS Parallel Light Reactor (WP-TEC-1020HSL) with 10 W 6000‒6500 K white LED. In this work, NiW-DPNDI as catalyst were explored in the styrene oxidation under the thermocatalytic and photocatalytic conditions, respectively. We first investigated NiW-DPNDI with t-butylhydroperoxide (TBHP, 70% in decane) as the oxidant at 80 ℃ for 72 h, it gave a 74.9% conversion and 95.7% selectivity for styrene epoxide (Table 1, entry 1). At the same conditions, it gave a 55.75% of conversion and 59.53% selectivity for epoxide without NiW-DPNDI. Combined with the above photoelectrochemical properties, it gave a 66.63% yield of benzaldehyde using O₂ as oxidant under irridiation for 72 h at room temperature. At the same time, there is a 13.02% yield of styrene oxide in photocatalytic reactions (Table 1, entry 2). The blank control experiment gave a 24.20% of conversion and 72.73% selectivity for benzaldehyde. The use of such a catalyst can be extended to the chlorostyrene with comparable activity and selectivity (Table 1, entry 3). However, the substrates with electron-donating groups (as -OCH₃ and -C₄H₉) gave an inferior efficency under the same reaction conditions, which revealed that the substrates are too large to be adsorbed in the channels. It is suggested that the reaction indeed occur in the channels of the POMOF, not on the external surface (Table 1, entries 4 and 5).

  Table 1

  

  Table 1.  Epoxidation of styrene by NiW-DPNDI under thermocatalysis and photocatalysis^b.

  DownLoad: CSV

  A possible reaction mechanism for NiW-DPNDI catalyzed epoxidation of styrene was proposed in Scheme 2. In order to confirm the reactive oxygen species (ROS) contributing to the catalysis, trapping experiments were performed (Table 2, entries 6–8). Isopropanol (IPA) as the ·OH scavenger introduced decreased the conversion from the 79.65% to 46.62%. However, when 1, 4-diazabicyclo[2.2.2]octane (DABCO) and benzoquinone were added as ¹O₂ and O₂^·‒ radical traps, the reactions were almost inert. So we surmised that ¹O₂ and O₂^·‒ play major roles in oxidation of styrene under the light. Furthermore, electron paramagnetic resonance (EPR) employing TEMPO and 5, 5-dimethylpyrroline-N-oxide (DMPO) as the classical ¹O₂ and O₂^·‒ probes further revealed that both active oxygen species (¹O₂ and O₂^·‒) were involved in the photocatalytic reaction (Figs. 3a and b) [36, 37]. The formed ROS O₂^·− further combined protons that generated from the oxidation of methanol by photogenerated-hole to form H₂O₂ (Fig. 3c), and H₂O₂ further converted into ¹O₂, which play a supportive role in the oxidation of styrene.

  Scheme 2

  

  Scheme 2.  Proposed catalytic mechanism of photocatalysis of oxidation of styrene.

  DownLoad: Full-Size Img PowerPoint

  Table 2

  

  Table 2.  Cycloaddition of CO₂ with epoxides by NiW-DPNDI^a.

  DownLoad: CSV

  Figure 3

  

  Figure 3.  (a) EPR spectra of ¹O₂ trapped by TEMPO with O₂. (b) EPR spectra of O₂^·− trapped by DMPO. (c) UV–vis absorption spectra of the NiW-DPNDI system after light irradiation for 4 h.

  DownLoad: Full-Size Img PowerPoint

  Owing to the excellent CO₂ adsorption and active ability, NiW-DPNDI was also explored to promote the reaction of styrene oxide and CO₂. As a model reaction, the reaction was conducted in CH₃CN under CO₂ atmosphere at 80 ℃ with an excellent yield of > 99% (Table 2, entry 1). The N atoms of DPNDI as basic sites can activate CO₂, and TBAB as nucleophile attacks epoxides to accelerate ring-opening. When the reaction in the absence of TBAB, only a few yield of desired carbonate produced (Table 2, entry 2). When in the absence of NiW-DPNDI, it gave a 65.24% conversion. To demonstrate the catalytic universality of NiW-DPNDI for the reaction with other epoxide substrates under the standard conditions. Compare with the styrene oxide, 2-(4-bromophenyl)oxirane with –Br as electron withdrawing group also showed high conversion (Table 2, entry 3). The lower conversion of trans-stilbene oxide is due to its large steric hindrance impeded the reaction with CO₂ existed in micropores (Table 2, entry 5). Cyclohexene oxide as substrates only obtain 33% yield bacause of high steric hindrance blocked the opening of epoxides (Table 2, entry 6). The stability of NiW-DPNDI plays a critical role in practical use. As shown in Fig. S7a (Supporting information), the XRD peaks intensity of NiW-DPNDI with a slight decrease after 3^rd run. The reusability of NiW-DPNDI was investigated in the progress of cycloadditions of CO₂ from styrene oxide, the yield has a little change after 3^rd run (Fig. S7b in Supporting information).

  We also explored the one-pot tandem catalysis to realize the conversion from styrene to styrene carbonate with NiW-DPNDI as catalyst, TBAB as cocatalyst and TBHP as oxidant under 0.1 MPa CO₂ atmosphere for 5 days at 80 ℃, it gave a 78% yield. The experimental result demonstrates catalytic ability of NiW-DPNDI for the one-pot synthesis from styrene to styrene carbonate. In the blank control experiment for cycloaddition of CO₂ from styrene, it gave a 18.6% conversion. Ground on the previous reported mechanism of CO₂ and epoxides activation, a possible reaction mechanism was proposed (Scheme S1 in Supporting information). First, styrene was converted into styrene epoxide with TBHP as oxidant. Next, Ni(II) centers as Lewis acid sites activated ring opening of epoxide to promote interact with the oxygen atom of epoxide. Whereafter, the Br^‒ of TBAB hit the less blocking of C atoms in the epoxide by nucleophilic attack. Then, combine CO₂ with the active oxygen anions to form alkylcarbonate salts. The final step, styrene carbonate was formed by closing-ring, and NiW-DPNDI was regenerated.

  In summary, a new photoactive POMOF NiW-DPNDI was obtained by solvothermal method. The strong anion⋯π interactions between [ZnW₁₂O₄₀]^6‒ anions and the electron-deficient naphthalenic ring centroid and the π⋯π interactions between DPNDI moieties promote electron–hole separation and create a short charge transport pathway. NiW-DPNDI gave high efficiency and diverse chemselectivity in the reaction of oxidation of styrene under thermocatalysis and photocatalysis systems. In addition, the carbonic anhydrase-mimicking Ni sites and the negative electron-enriched POM aligned in the pores can activate epoxide and CO₂, thus displaying a good efficiency for cycloadditions of CO₂ with styrene oxide under mild conditions. This work provide a new route for design and construction of excellent solid catalysts and expands the scope of MOFs as heterogeneous photocatalysts.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the Natural Science Foundation of Henan (Nos. 202300410043 and 222102230091), the Key Scientific Research Project of Henan Higher Education Institutions (No. 20ZX006), the National Natural Science Foundation of China (No. 21601048).

  Supplementary materials

  CCDC 2128021 contains the supplementary crystallographic data for this paper. Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107851.

  
  
• 1. [1]

     C.W.M. Murphy, G.B. Davis, J.L. Rayner, et al., J. Hazard. Mater. 430 (2022) 128482. doi: 10.1016/j.jhazmat.2022.128482

  2. [2]

     Y. Sun, M.Y. Gao, Y. Sun, et al., Inorg. Chem. 60 (2021) 13955–13959. doi: 10.1021/acs.inorgchem.1c02179

  3. [3]

     Q. Lan, S. Jin, B. Yang, et al., Trans. Tianjin Univ. 28 (2022) 214–225. doi: 10.1007/s12209-022-00324-z

  4. [4]

     Z. Wu, S. Guo, L.H. Kong, et al., Chin. J. Catal. 42 (2021) 1790–1797. doi: 10.1016/S1872-2067(21)63820-2

  5. [5]

     Y. Song, Y. Peng, S. Yao, et al., Chin. Chem. Lett. 33 (2022) 1047–1050. doi: 10.1016/j.cclet.2021.08.045

  6. [6]

     J. Liu, H. Wang, R. Ye, P. Jian, L. Wang, J. Colloid Interface Sci. 585 (2021) 61–71. doi: 10.5121/csit.2021.111406

  7. [7]

     Y. Zhang, H. Wang, S. Li, et al., Mol. Catal. 515 (2021) 111940. doi: 10.1016/j.mcat.2021.111940

  8. [8]

     K.O. Sulaiman, V. Sudheeshkumar, R.W.J. Scott, RSC Adv. 9 (2019) 28019–28027. doi: 10.1039/c9ra05566e

  9. [9]

     Y. Jiang, Z. Xiong, J. Huang, et al., Chin. Chem. Lett. 33 (2022) 415–423. doi: 10.1016/j.cclet.2021.06.058

  10. [10]

      Y. Nosaka, A.Y. Nosaka, Chem. Rev. 117 (2017) 11302–11336. doi: 10.1021/acs.chemrev.7b00161

  11. [11]

      X. Kan, J.C. Wang, Z. Chen, et al., J. Am. Chem. Soc. 144 (2022) 6681–6686. doi: 10.1021/jacs.2c01186

  12. [12]

      H. Wang, S. Jiang, W. Shao, et al., J. Am. Chem. Soc. 140 (2018) 3474–3480. doi: 10.1021/jacs.8b00719

  13. [13]

      X. Ma, H. Hao, W. Sheng, F. Huang, X. Lang, J. Mater. Chem. A 9 (2021) 2214–2222. doi: 10.1039/d0ta10757c

  14. [14]

      M. Huang, W. Xiang, C. Wang, et al., Chin. Chem. Lett. 31 (2020) 2769–2773. doi: 10.1016/j.cclet.2020.06.040

  15. [15]

      X. Xiao, M. Lu, J. Nan, et al., Appl. Catal. B 218 (2017) 398–408. doi: 10.1016/j.apcatb.2017.06.074

  16. [16]

      H. Li, S. Yao, H.L. Wu, et al., Appl. Catal. B 224 (2018) 46–52. doi: 10.1117/12.2296446

  17. [17]

      M.T.C. Martins-Costa, J.M. Anglada, J.S. Francisco, M.F. Ruiz-López, Chem. Sci. 13 (2022) 2624–2631. doi: 10.1039/d1sc06866k

  18. [18]

      Q. Wang, Q. Chen, G. Jiang, et al., Chin. Chem. Lett. 30 (2019) 1965–1968. doi: 10.1016/j.cclet.2019.08.037

  19. [19]

      Q. Chen, G.S. Luo, Y.J. Wang, Green Chem. 23 (2021) 7074–7083. doi: 10.1039/d1gc01887f

  20. [20]

      J. Amaro-Gahete, M.V. Pavliuk, H. Tian, et al., Coord. Chem. Rev. 448 (2021) 214172. doi: 10.1016/j.ccr.2021.214172

  21. [21]

      Y. Liu, R. Lv, S. Sun, et al., Chin. Chem. Lett. 33 (2022) 807–811. doi: 10.1016/j.cclet.2021.06.087

  22. [22]

      N. Zhang, L. Hong, A. Geng, et al., Chin. Chem. Lett. 29 (2018) 1409–1412. doi: 10.1016/j.cclet.2017.12.003

  23. [23]

      Z. Xi, K. Wei, Q. Wang, et al., J. Am. Chem. Soc. 143 (2021) 2660–2664. doi: 10.1021/jacs.0c12605

  24. [24]

      M. Tan, M. Takeuchi, A. Takai, Chem. Sci. 13 (2022) 4413–4423. doi: 10.1039/d2sc00035k

  25. [25]

      S.K. Keshri, T. Ishizuka, T. Kojima, Y. Matsushita, M. Takeuchi, J. Am. Chem. Soc. 143 (2021) 3238–3244. doi: 10.1021/jacs.0c13389

  26. [26]

      Q. Liu, Q. Zhang, W. Shi, et al., Nat. Chem. 14 (2022) 433–440. doi: 10.1038/s41557-022-00889-1

  27. [27]

      D. Hu, X. Song, S. Wu, et al., Chin. J. Catal. 42 (2021) 356–366. doi: 10.1016/S1872-2067(20)63665-8

  28. [28]

      S. Xing, J. Li, G. Niu, et al., Mol. Catal. 458 (2018) 83–88. doi: 10.1016/j.mcat.2018.08.011

  29. [29]

      Y. Dong, Q. Han, Q. Hu, et al., Appl. Catal. B 293 (2021) 120214. doi: 10.1016/j.apcatb.2021.120214

  30. [30]

      G. Hu, W. Chang, S. An, B. Qi, Y.F. Song, Chin. Chem. Lett. 33 (2022) 3968–3972. doi: 10.1016/j.cclet.2021.11.006

  31. [31]

      L.Q. Wei, B.H. Ye, Inorg. Chem. 58 (2019) 4385–4393. doi: 10.1021/acs.inorgchem.8b03525

  32. [32]

      J. Li, J. jiao, J. Chang, M. Li, Q. Han, Inorg. Chem. 60 (2021) 10022–10029. doi: 10.1021/acs.inorgchem.1c01311

  33. [33]

      J. Liang, R.P. Chen, X.Y. Wang, et al., Chem. Sci. 8 (2017) 1570–1575. doi: 10.1039/C6SC04357G

  34. [34]

      R. López, R. Gómez, J. Sol-Gel Sci. Technol. 61 (2012) 1–7. doi: 10.1007/s10971-011-2582-9

  35. [35]

      X.H. Li, M. Antonietti, Chem. Soc. Rev. 42 (2013) 6593. doi: 10.1039/c3cs60067j

  36. [36]

      J. Duan, L. Chen, H. Ji, et al., Chin. Chem. Lett. 33 (2022) 3172–3176. doi: 10.1016/j.cclet.2021.11.072

  37. [37]

      J. Wang, Z. Wu, H. Shen, G. Wang, Polym. Chem. 8 (2017) 7044–7053. doi: 10.1039/C7PY01683B

• 

  Scheme 1  The design concept of NiW-DPNDI for photocatalysis styrene oxidation.

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) Ball-and-stick representation of NiW-DPNDI. (b) 3D network of NiW-DPNDI viewed down the a axis.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Characterizations of NiW-DPNDI, (a) TG. (b) TPD−CO₂ curve. (c) Solid UV–vis absorption spectra (inset: Tauc plots). (d) Mott−Schottky plots. (e) Electrochemical impedance spectroscopy (EIS) Nyquist plots in 0.1 mol/L Na₂SO₄ aqueous solution without bias versus Ag/AgCl electrode irradiated with white light. (f) Transient photocurrent response in 0.1 mol/L Na₂SO₄ aqueous solution without bias versus Ag/AgCl electrode irradiated with white light.

  下载: 全尺寸图片 幻灯片
  

  Scheme 2  Proposed catalytic mechanism of photocatalysis of oxidation of styrene.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) EPR spectra of ¹O₂ trapped by TEMPO with O₂. (b) EPR spectra of O₂^·− trapped by DMPO. (c) UV–vis absorption spectra of the NiW-DPNDI system after light irradiation for 4 h.

  下载: 全尺寸图片 幻灯片

  Table 1.  Epoxidation of styrene by NiW-DPNDI under thermocatalysis and photocatalysis^b.

  下载: 导出CSV

  Table 2.  Cycloaddition of CO₂ with epoxides by NiW-DPNDI^a.

  下载: 导出CSV
•