English

• The selective conversion of CH₄ to complex carbon-based compounds under mild conditions is one of the most important transformations both in nature and for the chemical industry [1]. However, the inherent inertness of C-H bonds in methane poses great challenges for catalytic systems, not only in terms of activation, but also in controlling chemical selectivity and over functionalization under harsh necessary conditions (high temperatures, ultra-acidic media, strong oxidants). In addition, the low solubility of gaseous substrates in most solvents has presented substantial practical difficulties [2]. Elegant catalytic systems using noble metals such as Pd, Au, and Rh have recently been reported [3-5]. However, the challenge remains to develop efficient and non-noble catalytic systems under gentle conditions. Methane catalysis using non-precious metals, such as Mo [6], Fe [7], Cu [8], and W [9], usually requires high temperatures and pressures, which leads to high energy consumption and complex process [10]. Therefore, it is an urgent task to realize methane conversion under mild conditions.

  Photoredox catalysis with non-noble metal has recently emerged as a powerful platform for the direct activation and functionalization of organic molecules. Cerium(Ⅲ) salts has been used as pre-catalysts for the challenging oxidation of a variety of alcohols [2]. The cerium(Ⅲ) salts are ideal catalyst for sustainable, large-scale photocatalytic systems to achieve C-H bond activation due to the readily available abundance and high activity. Recently, metal-organic frameworks (MOF) have received considerable attention due to their high porosity, large surface area and functionalization [11]. MOF has a low catalytic activity as a catalyst, but can provide a coordination environment for active site binding [12-17]. POMs are typically used as co-catalysts [18] to enhance the catalytic activity of MOFs due to their well-defined structure and rich redox properties [19-22]. Therefore, it is one of the current research topics to synergistically couple Ce-MOF as a carrier for POMs in catalyzing methane activation.

  Among them, decatungstate anion (W₁₀O₃₂^4-, noted as W₁₀) is one of the most promising POMs, due to its high photocatalytic activity in the selective oxidation with molecular oxygen [23,24]. In 2018, Davide's group studied the synergistic control of the hydrogen-absorbing transition state to achieve selective functionalization of C(sp³)-H through polarity and spatial effects in W₁₀O₃₂^4- photocatalysis, which can selectively functionalize the C-H bonds of alkanes, alcohols, ethers, ketones, amides, esters, nitriles and pyridinolanes [25]. Timothy reported a general and mild strategy to activate C(sp³)-H bonds in methane, ethane, propane, and isobutane through hydrogen atom transfer using W₁₀ as photocatalyst at room temperature [26]. W₁₀ can also be used to generate a chlorine radical from chloride ion for the photochemical partial oxidation of methane and achieves methane to methyl trifluoroacetate conversion through the W₁₀-chloride-iodine ensemble [27]. These studies demonstrated the feasibility of W₁₀ in catalytic C-H bond breaking.

  Here, the W₁₀@Ce-bpdc were fabricated by combining Ce-MOF with W₁₀, for the conversion of methane to HCOOH under mild conditions. The yield of HCOOH under light conditions with oxygen as the oxidant can reach 155 µmol/g_cat. The catalytic performance was improved by 12.3 and 3.69 times compared to that of W₁₀ and Ce-bpdc, respectively. The electron paramagnetic resonance (EPR) results indicated that the combination of the catalyst W₁₀@Ce-bpdc could promote the generation of ·OH with O₂ as the reactant, which significantly improved the methane conversion.

  W₁₀ was fabricated with reference to the previous literature with improvements [28]. Ce-bpdc was synthesized by [Ce₆(OH)₄(NH₃CH₂COO)₈(NO₃)₄(H₂O)₆]Cl₈·8H₂O (hereinafter referred to as Ce₆) [29] and 4,4′-biphenyldicarboxylic acid (H₂bpdc). The Ce₆ and H₂bpdc were reacted in a mixture of water and DMF (v/v, 1:3) at 100 ℃ for 15 min, and naturally cooled to room temperature with a yellowish solid product. The synthesis of W₁₀@Ce-bpdc was similar to Ce-bpdc, but with different amounts of W₁₀ (10 mg, 20 mg, 30 mg, 40 mg, 50 mg, designated as W₁₀@Ce-bpdc-1, 2, 3, 4, 5, respectively) during the synthesis process (Scheme 1) [30].

  Scheme 1

  

  Scheme 1.  Schematic for the fabrication of W₁₀@Ce-bpdc. W₁₀: bule polyhedron; Ce node: yellow polyhedron; bpdc: silver stick.

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  The PXRD pattern of Ce-bpdc MOF was similar to that of simulated Ce-UiO-67 MOF, confirming its UiO-topology constructed from Ce₆ nodes and H₂bpdc bridges, which is composed of octahedral and tetrahedral cages with internal pores [31]. Moreover, the structure of the framework was not changed after the introduction of W₁₀. In Fig. 1a, it is shown that there is no signal peak of W₁₀ (7.77°) after the addition of W₁₀, indicating that W₁₀ is not on the surface of the MOF and no agglomeration is formed. The characteristic peaks of MOF were not shifted after the addition of W₁₀, indicating that the structure of MOF remained intact.

  Figure 1

  

  Figure 1.  (a) PXRD, and (b) FTIR spectra of W₁₀, Ce-bpdc, W₁₀@Ce-bpdc-2. (c) UV-vis DRS of different samples. (d) TG analysis of W₁₀@Ce-bpdc-2.

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  In order to prove the integrity of W₁₀ after reconstitution, the material was characterized by Fourier transform infrared spectroscopy (FTIR). As shown in Fig. 1b, the peaks near 960, 890 and 804 cm^-1 in W₁₀@Ce-bpdc-2 correspond to the stretching vibration of W=O_t, W=O_b=W (O_b: Corner shared bridging oxygen) and W=O_c=W (O_c: Side shared bridging oxygen) respectively, indicating the presence of W₁₀ in the composite material [28]. Furthermore, the peaks at 1579 cm^-1 and 1400 cm^-1 are attributed to the asymmetric and symmetrical tensile vibration of H₂bpdc connector O-C-O. A weak band around 500-700 cm^-1 is also observed due to Ce-O tensile vibration [32], providing further evidence for the existence of Ce-bpdc in W₁₀@Ce-bpdc-2.

  The optical properties of the W₁₀, Ce-bpdc, and W₁₀@Ce-bpdc-2 composites in the range of 300-700 nm were characterized by the UV-vis absorption spectrum. As shown in Fig. 1c, the optical absorption range of the sample is mainly in the ultraviolet region. The absorption belt edge of W₁₀, Ce-bpdc, and W₁₀@Ce-bpdc-2 composite was at 516, 440 and 520 nm, respectively. Ce-bpdc possessed an intense absorption in the UV region at wavelengths of < 380 nm due to the p-p* and n-p* transition of the bpdc linker [33]. And the formed W₁₀@Ce-bpdc-2 has a good light absorption ability, which is favorable for photocatalysis.

  In the thermogravimetry (TG) pattern (Fig. 1d), the first step of degradation of W₁₀@Ce-bpdc-2 corresponds to the loss of solvent about 5.43%, which usually occurs in the range of 40-180 ℃. The second stage occurs at 200-300 ℃ with a weight loss of about 5.36%, which can be attributed to the unreacted H₂bpdc present in the framework. The MOF frame began to collapse after 320 ℃, indicating that the synthesized W₁₀@Ce-bpdc-2 was stable below 320 ℃.

  The morphology and structure of Ce-bpdc and W₁₀@Ce-bpdc-2 were examined by high magnification scanning electron microscopy (SEM) images. Ce-bpdc and W₁₀@Ce-bpdc-2 have a polyhedron structure and the microparticles have unequal sizes of 50-100 nm. Besides, the particles of the MOF showed obvious uneven phases on their edges and rough surfaces. The particle size of W₁₀@Ce-bpdc-2 was about 50 nm, as shown in high magnification SEM (Fig. 2a) and transmission electron microscopy (TEM) (Fig. 2b). While the morphology for Ce-bpdc without W₁₀ was observed as irregular particles of about 100 nm in SEM (Fig. S1 in Supporting information). Energy dispersive X-ray spectroscopy (EDS) Mapping (Figs. 2c-h) revealed that W₁₀@Ce-bpdc-2 is composed of the elements W, Ce, C, N, and O. The uniform distribution of W indicates that W₁₀ does not agglomerate in W₁₀@Ce-bpdc-2.

  Figure 2

  

  Figure 2.  (a) Ultrahigh-resolution SEM, and (b) TEM of W₁₀@Ce-bpdc-2. (c-h) EDS mapping of Ce, N, C, O and W in W₁₀@Ce-bpdc-2.

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  The oxidation of methane was catalyzed using W₁₀@Ce-bpdc containing 10, 20, 30, 40, and 50 mg for 12 h with O₂ as the oxidant at room temperature (25 ℃) and pressure (1 atm), with the Xe lamp light intensity 200 mW/cm² (Fig. 3a). The highest yield of HCOOH was 155.0 µmol/g_cat for W₁₀@Ce-bpdc-2 (20 mg of W₁₀). And the HCOOH yield decreased with the increase amount of W₁₀ during the fabrication. As shown in PXRD images (Fig. 1a), the MOF structure collapse after the addition of excessive W₁₀, which leads to the gradual reduction of HCOOH yield.

  Figure 3

  

  Figure 3.  HCOOH yield of (a) W₁₀@Ce-bpdc loaded with different contents of W_10. (b) The control experiments. (c) Time-dependent HCOOH yield over W₁₀@Ce-bpdc-2 and Ce-bpdc. (d) Recycle experiments for CH₄ photooxidation.

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  In order to prove the required conditions in the catalytic process, a series of controlled tests were conducted as shown in Fig. 3b. When no catalyst or methane was added, the output of formic acid was almost 0 µmol/g_cat, indicating that C in HCOOH comes from methane. At the same time, in the absence of light or oxygen, the yield of formic acid was 1.19 µmol/g_cat and 21.29 µmol/g_cat, respectively, indicating that both light and O₂ were necessary conditions for catalytic oxidation of methane to formic acid.

  In order to determine the optimal reaction time, the HCOOH yield was measured with the extension of the reaction time for 16 h. the HCOOH produced by CH₄ photooxidation gradually increased with the increase of irradiation amount, and its yield reached 155.0 µmol/g_cat within 12 h, which was 3.7 times than that of the original Ce-bpdc (42.0 µmol/g_cat) and 12.3 than times that of the original W₁₀ (Fig. 3c and Fig. S2 in Supporting information). To confirm the stability of the catalyst, the cycle experiments were performed on the catalyst, and the yield of HCOOH was basically unchanged after cycling W₁₀@Ce-bpdc-2 for four times (Fig. 3d). The 4^th recycled catalyst were characterized by PXRD (Fig. S3 in Supporting information), and there is no sign collapsing sign for the basic framework of the MOF, indicating that the catalyst can remain stable during the photocatalytic process.

  In order to further determine the chemical state of the elements in the catalyst W₁₀@Ce-bpdc-2, the elemental compositions of the material were analyzed to gain insights into the nature of the Ce sites in the catalyst. XPS full spectrum testing determined the presence of the elements Ce, C, N, O and W (Fig. 4a). The presence of a mixed valence state of Ce^Ⅲ/Ⅳ was revealed in the XPS analysis of pristine Ce-bpdc (Fig. 4b). The peaks observed at 917.4, 908.5, 902.0, 888.1 and 885.9 eV were attributed to Ce^Ⅳ, while those at 905.2, 898.8, 887.1 and 882.8 eV were attributed to Ce^Ⅲ [34-36]. After illumination, the peaks of 908.5, 905.2, 889.6, 887.1 and 882.8 eV on the Ce 3d orbit shift to the higher binding energy by 1.5, 0.8, 1.5, 1.0 and 0.5 eV, respectively [34,37]. At the same time, the ratio of Ce^Ⅲ/Ce^Ⅳ content in W₁₀@Ce-bpdc-2 after illumination decreases from 1.22 before illumination to 1.12, the content of Ce³⁺ increases from 53% to 55%, and the content of Ce⁴⁺ also decreases. This is due to the low altitude property of empty 4f orbit of Ce [38]. Under highly favorable illumination conditions, the content of Ce⁴⁺ also decreases. The loss of an electron causes part of Ce⁴⁺ to be reduced to form Ce³⁺, and the accompanying oxygen vacancy of Ce³⁺ formed also plays a key role in the oxidation mechanism of CH₄. Additionally, the W 4f_5/2 and W 4f_7/2 peaks, initially located at 38 eV and 35.9 eV, were observed to shift to lower energies by 0.1 eV after illumination (Fig. 4c) [39,40]. This phenomenon indicates that under the condition of illumination, electron transfer occurs in the Ce element processing itself under the condition of illumination.

  Figure 4

  

  Figure 4.  (a) XPS full spectra of W₁₀@Ce-bpdc-2. (b) Ce 3d, (c) W 4f, and (d) O 1s of W₁₀@Ce-bpdc-2.

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  Furthermore, the O 1s spectra were deconvoluted into three peaks at 530.5, 531.7 and 533.3 eV, corresponding to Ce-O-Ce, Ce-O-C, and COOH bonds, respectively (Fig. 4d). The peaks of C 1s spectra overlapped and were fitted into six Gaussian lines at approximately 283.8, 284.3, 284.7, 285.3, 286.2 and 288.6 eV, which is assigned to the C 1s signal of C-C, C-H, C=O, O-C=O, C-N and C=O, respectively (Fig. S3 in Supporting information) [32,36]. The presence of Ce^Ⅲ/Ⅳ were shown in the XPS of Ce-bpdc (Fig. S4 in Supporting information) and the W 4f and O 1s XPS spectra of W₁₀ are shown in Fig. S5 (Supporting information) [37,41].

  In order to explore the charge-separation efficiency of the catalyst, photocurrent measurements (I-t) and electrochemical impedance spectroscopy (EIS) were performed. As shown in Fig. 5a, the photocurrent for W₁₀@Ce-bpdc-2 enhanced in comparison with that for pristine Ce-bpdc. In Fig. 5b, W₁₀@Ce-bpdc-2 showed a smaller diameter of circle than Ce-bpdc, indicating smaller electron-transfer resistance, which is consistent with the results in the I-t mapping. The results demonstrate that the incorporation of W₁₀ into the W₁₀@Ce-bpdc-2 facilitates the separation of photogenerated electron-hole pairs, thereby enhancing the light-harvesting capacity and charge transfer-separation rate of W₁₀@Ce-bpdc-2.

  Figure 5

  

  Figure 5.  (a) Photocurrent responses for W₁₀@Ce-bpdc-2 and Ce-bpdc. (b) EIS Nyquist plots for W₁₀@Ce-bpdc-2 and Ce-bpdc. (c) Mott-Schottky plots of W₁₀@Ce-bpdc-2. (d) Tauc plots of W₁₀@Ce-bpdc-2.

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  The Mott-Schottky plots and Tauc plots were performed to study the flat-band potential and energy band structure of W₁₀@Ce-bpdc-2 for studying the mechanism for oxidation of W₁₀@Ce-bpdc-2. The optical band gap energy (E_g) of the samples can be calculated by the formula: (αhυ)_1/2 = A(hυ - E_g), hυ = 1240/λ (where α, h, λ and A are absorption coefficient, Planck constant, absorption edges, and a constant, respectively). As shown in Figs. 5c and d, the potentials of the conduction and valence bands of W₁₀@Ce-bpdc-2 are -0.146 V and 2.90 V vs. normal hydrogen electrode (NHE), respectively. And W₁₀@Ce-bpdc-2 has a corresponding highest occupied molecular orbital (HOMO) of 2.754 V vs. NHE, which allows the reduction of O₂ (O₂ + 2H⁺ + 2e^- → H₂O₂, O₂/H₂O₂: 0.695 vs. NHE, H₂O₂ + H⁺ + e^- → ·OH, H₂O₂/·OH: 1.14 V vs. NHE) and the oxidization of H₂O (H₂O + h⁺ → ·OH + H⁺, ·OH/H₂O: 2.38 V vs. NHE) respectively. Since the reduction potential of H₂O₂/·OH (1.14 V vs. NHE) is lower than that of O₂/H₂O₂ (0.695 V vs. NHE), the generated H₂O₂ can be rapidly reduced to ·OH in situ (Fig. S6 in Supporting information) [42-44].

  Electron paramagnetic resonance (EPR) tests using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a radical trapping agent also demonstrated the production of ·OH under light conditions (Fig. S7 in Supporting information) [45]. Notably, the test results showed that the presence of ·OH could not be detected in the absence of light conditions. In the presence of O₂ and light, distinct ·OH and ·CH₃ signal peaks were detected. Combined with the potentials data, the catalyst produces ·OH after reducing O₂ to H₂O₂ under light conditions, which in turn oxidizes CH₄ to produce products.

  In situ IR measurement was performed to explore the adsorption of radicals on W₁₀@Ce-bpdc-2 for the study of reaction mechanism. During the CH₄ photooxidation process, the two signals associated with CH₄ adsorption at 1301 and 3010 cm^-1 were gradually enhanced with the increase of irradiation time, indicating the benefit of W₁₀@Ce-bpdc-2 for CH₄ adsorption. In the amplified IR spectra, the peaks of *CH₃ (1355 cm^-1), *CH₂ (1539 cm^-1), *OH₂ (1643 cm^-1), *CH₃O (2823 cm^-1 and 1220 cm^-1), *HCOO (1344 cm^-1), *OOH (3178 cm^-1) and *OOH (3175 cm^-1), *OH (3192 cm^-1) were detected in the magnified IR spectrum (Figs. 6a and b) [46-48]. These observations are consistent with the experimental products of HCOOH products in pure water, and suggest the rationality of the catalytic mechanism.

  Figure 6

  

  Figure 6.  (a, b) In situ FTIR spectra of CH₄ photo-oxidation with W₁₀@Ce-bpdc-2 as the photo-catalyst.

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  In summary, the W₁₀@Ce-bpdc-2 was utilized to synthesized by the combination of Ce-bpdc and W₁₀, which served as the catalyst for converting methane into HCOOH under light and O₂ conditions. After four cycles, the catalyst exhibited a yield of 155 µmol/g_cat for 12 h. EIS and Mott-Schottky analysis confirmed that the charge separation efficiency of W₁₀@Ce-bpdc-2 improved after composite formation, thereby enhancing methane activation efficiency of W₁₀@Ce-bpdc-2. This straightforward approach represents an effective strategy for developing novel rare earth metal catalysts aimed at significantly enhancing the photoactivity of Ce-MOF by doping POMs.

  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.

  CRediT authorship contribution statement

  Yao Cheng: Writing – original draft, Data curation. Wen-Xiong Shi: Writing – review & editing, Supervision, Funding acquisition. Zhi-Ming Zhang: Writing – review & editing, Supervision, Funding acquisition.

  Acknowledgment

  This work was supported by the National Natural Science Foundation of China (Nos. 92261118, 92161103, 22071180).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110387.

  
  
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• 

  Scheme 1  Schematic for the fabrication of W₁₀@Ce-bpdc. W₁₀: bule polyhedron; Ce node: yellow polyhedron; bpdc: silver stick.

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  Figure 1  (a) PXRD, and (b) FTIR spectra of W₁₀, Ce-bpdc, W₁₀@Ce-bpdc-2. (c) UV-vis DRS of different samples. (d) TG analysis of W₁₀@Ce-bpdc-2.

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  Figure 2  (a) Ultrahigh-resolution SEM, and (b) TEM of W₁₀@Ce-bpdc-2. (c-h) EDS mapping of Ce, N, C, O and W in W₁₀@Ce-bpdc-2.

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  Figure 3  HCOOH yield of (a) W₁₀@Ce-bpdc loaded with different contents of W_10. (b) The control experiments. (c) Time-dependent HCOOH yield over W₁₀@Ce-bpdc-2 and Ce-bpdc. (d) Recycle experiments for CH₄ photooxidation.

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  Figure 4  (a) XPS full spectra of W₁₀@Ce-bpdc-2. (b) Ce 3d, (c) W 4f, and (d) O 1s of W₁₀@Ce-bpdc-2.

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  Figure 5  (a) Photocurrent responses for W₁₀@Ce-bpdc-2 and Ce-bpdc. (b) EIS Nyquist plots for W₁₀@Ce-bpdc-2 and Ce-bpdc. (c) Mott-Schottky plots of W₁₀@Ce-bpdc-2. (d) Tauc plots of W₁₀@Ce-bpdc-2.

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  Figure 6  (a, b) In situ FTIR spectra of CH₄ photo-oxidation with W₁₀@Ce-bpdc-2 as the photo-catalyst.

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