Unveiling the Role of Copper Valence States in Enhancing the Catalytic Performance of Copper-Modified ZSM-5 for Direct Methane Conversion

Abstract

The conversion of methane (CH₄) to methanol (CH₃OH) under mild conditions remains a significant challenge in catalysis. In this study, we introduce a method to adjust the surface valence states of copper species in Cu-ZSM-5 catalysts by annealing under different atmospheres (N₂, air, and H₂). Among these, the 10% Cu-ZSM-5 catalyst calcined in H₂ showed outstanding performance, achieving a methanol productivity of 8.08 mmol/(g_cat·h) and 91% selectivity at 70 °C and 3 MPa using H₂O₂ as the oxidant. Comprehensive characterization revealed that H₂ annealing optimized the Cu surface to a lower valence state (predominantly Cu⁺), enhancing CH₄ adsorption and promoting H₂O₂ activation to generate ·OH and ·CH₃ radicals, which drive selective CH₃OH formation. In situ DRIFTS and radical trapping experiments further confirmed the critical role of Cu⁺ in facilitating C-H bond cleavage and suppressing overoxidation.

1. Introduction

Methane (CH₄), a principal constituent of natural gas, emerges as a viable alternative energy source for chemical production. The recent uncovering of substantial shale gas, methane hydrate, and natural gas reserves presents an opportunity to diminish reliance on petroleum, underscoring the strategic importance of CH₄ in the energy landscape [1,2,3]. Concurrently, CH₄ poses a more potent greenhouse effect than carbon dioxide (CO₂), with approximately 30 times the global warming potential in the atmosphere, despite its significantly shorter atmospheric lifetime compared to CO₂. Consequently, it is of paramount importance to achieve a 75% reduction in CH₄ emissions from fossil fuel sources by 2030 in order to meet the ambitious goal of limiting global warming to 1.5 °C [4,5].

However, CH₄ is a highly symmetrical hydrocarbon characterized by low polarity and a significant dissociation energy of 439.3 kJ/mol [6]. Consequently, its conversion under mild conditions is often referred to as the “Holy Grail” within the realm of chemistry [7]. At present, the transformation of CH₄ into other products predominantly occurs under harsh conditions, where it is initially converted into syngas and subsequently into methanol (CH₃OH) or other oxygenated compounds. For instance, dry reforming processes typically operate at temperatures above 750 °C, which can then be followed by Fischer–Tropsch synthesis and methanol production pathways [8,9,10]. Thus, achieving the high-value conversion of CH₄ to CH₃OH and other C1 oxygenates under mild conditions remains a significant challenge that needs to be addressed.

In the natural world, CH₄ monooxygenase proteins (MMOs) are capable of activating the highly stable CH₄ and converting it into CH₃OH, with remarkable 100% selectivity [11,12]. Drawing inspiration from the enzyme’s structure, researchers have opted for zeolite as a substrate to engineer catalysts that mimic the enzyme’s functionality [13,14,15,16]. Upon calcination, ZSM-5 and MOR zeolites exhibit the formation of more robust reaction cores compared to USY, FER, BEA, and analogous zeolites, with ZSM-5 demonstrating superior performance as a substrate over MOR [17]. Noble metals possess exceptional chemical properties that enable them to exhibit remarkable productivity in the synthesis of CH₃OH. Cao et al. [18] found that Au/ZSM-5 + XC72R showed excellent activity, generating CH₃OH, while Qi et al. [19] obtained Au-ZSM-5 that can achieve high selectivity of oxygenates. While noble metals exhibit exceptional catalytic activity, their high cost renders them economically unfeasible. In recent years, transition metal-based catalysts, particularly those containing copper (Cu), have garnered significant research interest due to their commendable resistance to overoxidation and affordability. However, the activity and selectivity of Cu catalysts have posed challenges to their advancement. Consequently, we propose that enhancing the catalytic performance could be achieved by altering the surface properties of Cu atoms and increasing the utilization rate of Cu. Studies have indicated that catalyst surface modification can redirect the conversion pathway of CH₄ and enhance the product selectivity [20]. Pappas et al. [21] presented the Cu-CHA treated in high-temperature oxygen, resulting in forming more CuII active sites to reach a higher yield of product. Pokhrel et al. [22] found that Cu-SSZ-39 exhibited better activity than Cu-SSZ-13 due to the pore structure displaying better steric effects to generate more CH₃OH. Burnett et al. [23] found that the copper nanoparticles were deposited on the support structure, leading to the declining of the efficiency of the materials with low Si: Al, while Tang et al. [24] applied modified adsorption methods synthesizing catalysts with atomically dispersion, displaying higher activity characteristics than others [21,25,26,27]. While prior studies emphasized structural confinement and nuclearity effects, the interplay between Cu oxidation states and radical-mediated mechanisms in H₂O₂-driven systems remains underexplored—a gap that limits rational catalyst design. Notably, conventional synthetic approaches often yield mixed Cu⁺/Cu²⁺ populations, obscuring the precise role of low-valent Cu species in stabilizing reactive intermediates or suppressing parasitic overoxidation. Therefore, it is necessary to further study the effect of valence state on the production of free radicals and the effect on the performance of CH₄ conversion.

Based on the above, Cu-ZSM-5 catalysts are widely applied to CH₄ conversion and exhibit great activity, while the effect of the surface valence of Cu is seldom studied. Herein, to address this knowledge gap, the Cu valence states in ZSM-5 are systematically tailored through controlled annealing under N₂, air, and H₂ atmospheres. Combining advanced characterization with in situ DRIFTS and radical trapping experiments, we elucidate how Cu⁺-enriched surfaces enhance CH₄ adsorption, promote H₂O₂ activation to generate ·OH/·CH₃ radicals, and steer selectivity toward CH₃OH.

2. Results and Discussion

2.1. Structure and Chemical Properties of Catalysts

As shown in Figure 1, the X-ray diffraction (XRD) patterns of catalysts showed that the inherent structure of H-ZSM-5 remained unchanged after the introduction of Cu, and no diffraction peaks of Cu species were identified, indicating that the Cu had good dispersion and did not change the lattice structure of H-ZSM-5. Though the catalyst was still a well-ordered microstructure, the incorporation of copper reduced the intensity of the main diffraction peaks of the zeolite, which might ascribe to the high absorption coefficient of metallic compounds for X-ray radiation [28,29]. Furthermore, regardless of the calcination atmosphere, the peak intensities generally decreased with increasing copper content, as shown in Figure 1b,c. Notably, even the catalyst with the highest copper loading did not exhibit significant diffraction peaks for Cu, CuO, or Cu₂O, indicating that a small amount of copper was uniformly dispersed across the zeolite framework. This uniform dispersion suggests that the copper species were well integrated into the zeolite structure without forming distinct crystalline phases.

The scanning electron microscopy (SEM) images were shown in Figure 2a–c. The 10% Cu-ZSM-5 comprised all crystals with the size of about 100 nm. A very minute quantity of Cu species loading was observed in the catalysts that were calcinated in air, N₂, or H₂. There was no significant difference in the morphology of the catalysts [30]. With the purpose of gaining further insight into the distribution of copper crystallites, the catalysts were characterized by the transmission electron microscopy (TEM). It could be seen that, compared with 10% Cu-ZSM-5–air (Figure 2e) and 10% Cu-ZSM-5-N₂ (Figure 2f), the copper species on the 10% Cu-ZSM-5-H₂ (Figure 2d) was less agglomerated and more evenly dispersed. Regardless of the calcination atmosphere, the Cu particles remained below 4 nm in size. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Figure 2g) and energy-dispersive X-ray (EDX) mapping (Figure 2h) confirmed that Cu species were uniformly dispersed on the zeolite surface [31,32,33]. Thus, as shown in Supplementary Table S1, the increases in the Cu contents led to the decreases in the microporosity for the catalysts treated in H₂, indicating that the introduced Cu was pinned in the micropores of H-ZSM-5. The shapes of adsorption–desorption isotherm (Supplementary Figure S1) did not change after the copper doping, elucidating that the pore shapes remained the same. This was in accordance with the diffraction peaks and morphology observation of Cu species observed in XRD and SEM, being consistent with other research findings. The initial weight loss in thermo-gravimetric analysis (TGA) was attributed to the loss of adsorbed water; and even when heated up to 800 °C in an air or nitrogen atmosphere, the catalyst was basically stable and was not subjected to weight loss, as shown in Supplementary Figure S2.

The chemical composition and the various oxidation states of elements on the catalyst surface were analyzed using X-ray photoelectron spectroscopy (XPS). In the O 1s spectra (Supplementary Figure S3a), two distinct peaks at 532.3 eV and 536.9 eV were identified, suggesting the presence of lattice oxygen and adsorbed water molecules [34]. Figure 3a ascertained the presence of Cu⁺ and Cu²⁺, while the quantification was performed by analyzing the peak areas of the respective elements in the XPS spectra and converting these into atomic percentages, using the appropriate sensitivity factors provided by the instrument manufacturer. The Cu⁺ species could be observed at 932.9 eV and 952.6 eV, and the Cu²⁺ species could be identified by the peaks at 933.8 eV and 953.5 eV, which verified the existence of reduced copper species [35]. The XPS results showed that the capacity of Cu²⁺ of 10% Cu-ZSM-5-H₂ was lower than that of 10% Cu-ZSM-5-N₂ and 10% Cu-ZSM-5–air, while the Cu⁺ was higher, symbolizing the roasting in the H₂ atmosphere giving rise to the contents of Cu⁺ species [32]. The proportion of low chemical valence of 10% Cu-ZSM-5 under different atmospheres with the order of 10% Cu-ZSM-5-H₂ > 10% Cu-ZSM-5-N₂ > 10% Cu-ZSM-5–air, namely the low copper chemical valence ratio of the sample, was the highest after hydrogen atmosphere treatment.

Following this, electron paramagnetic resonance (EPR) analysis was employed to further clarify the nature of Cu species present. As depicted in Supplementary Figure S4, in contrast to the H-ZSM-5 sample, the 10% Cu-ZSM-5-H₂ sample exhibited two distinct EPR signals, with four sharp signals superimposed on a broader peak. These observations, attributed to the hyperfine interactions resulting from the partial delocalization of electronic and nuclear spins, indicated that the Cu species were localized as isolated entities, likely interacting with the zeolite support surface [36,37].

To delve deeper into the electronic state of Cu ions within the catalyst, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was conducted using CO as a molecular probe. As shown in Figure 3b, the in situ DRIFTS spectra of the CO probe experiment revealed that H-ZSM-5 exhibited negligible peaks in the 2200–2100 cm⁻¹ range. In contrast, the 10% Cu-ZSM-5 catalysts displayed a pronounced vibrational band at 2157 cm⁻¹ post-CO adsorption, which is indicative of the vibrational peak of Cu⁺ (CO) species. For the catalysts treated in N₂ and air, a shoulder peak around 2140 cm⁻¹ was observed in conjunction with the principal peak at 2157 cm⁻¹, which is attributed to CO adsorbed on (Cu⁺-CO-Cu⁺) sites, also reflecting the response signal of [Cu]⁺ ions [38].

The temperature-programmed desorption of CH₄ (CH₄-TPD) test was carried out to deeply analyze the adsorption activation process before CH₄ combustion. As can seen in Supplementary Figure S5, the CH₄ desorption peak intensity of the Cu-loaded H-ZSM-5 was stronger than that of pure H-ZSM-5. In addition, 10% Cu-ZSM-5-H₂ exhibited the strongest peak signal and the lowest desorption temperature, conveying the optimal CH₄ adsorption ability.

2.2. Catalytic Activity and Selectivity for CH4 Conversion

The direct oxidation of CH₄ reaction was conducted in a batch reactor by using H₂O₂ as the oxidant. The effects of the electronic state caused by calcination atmosphere and the Cu content of the H-ZSM-5 on CH₃OH production are shown in Figure 4. From Figure 4a, the yield of 0.57 mmol/(g_cat·h) of CH₃OH was calculated over H-ZSM-5 upon 60 °C with 3 MPa of CH₄ and 0.5 M H₂O₂. The Cu addition significantly facilitated the oxygenates’ production.

Notably, the 10% Cu-ZSM-5-H₂ showed optimal CH₄ conversion performance compared with 10% Cu-ZSM-5–air and 10% Cu-ZSM-5-N₂ (Figure 4a), presenting the maximum CH₃OH yield of 6.85 mmol/(g_cat·h) with a selectivity of 91% within 1 h. Concurrently, the catalyst’s performance under various reaction conditions was evaluated. As the reaction temperature increased, the relationship between CH₃OH productivity and reaction time exhibited a volcano-shaped curve. For instance, the 10% Cu-ZSM-5-H₂ exhibited the best performance of 8.08 mmol/(g_cat·h) CH₃OH and 0.59 mmol/(g_cat·h) CH₃OOH at 70 °C (Figure 4b) at the same time that other catalysts inversely showed lower productivity and selectivity. The yield of CO₂ increased evidently when the temperature came to 80 °C and 90 °C, which revealed the overoxidation trend as the temperature rose. Therefore, in the subsequent experiments, the reaction temperature was set to 70 °C on the basis of other experimental conditions being unchanged.

For the cycling test, the productivity and selectivity to CH₃OH of 10% Cu-ZSM-5-H₂ showed minimal decline after four cycles, as depicted in Figure 5a, suggesting the astonishing stability of 10% Cu-ZSM-5-H₂. Whereafter, the experiment presented that the yield of CH₃OH increased with the rising H₂O₂ concentration, when carried out at a gradually increasing H₂O₂ concentration (Figure 5b). It might be ascribed to the increase in the concentration of H₂O₂ as the concentration of ·OH increased. Furthermore, the direct oxidation of CH₄ over 10% Cu-ZSM-5-H₂ was conducted under different pressures (Figure 5c), stirring speeds (Figure 5d), and reaction times (Figure 5e). These results revealed that the productivity of CH₃OH could be enhanced gradually with the increase in pressure, and increasing the stirring rate within a certain range helped to increase the yield.

As observed in Figure 5e, the production of CH₃OH initially increased with time but subsequently declined, while the generation of CO₂ showed a steady increase. This trend may be attributed to the formation of by-products and overoxidation due to the excess oxidant present in the solution during the reaction. After a 10 h reaction period, the production of CH₃OH remained relatively high, possibly because CH₃OH formation was favored in this system, and the reduction in oxidation potential prevented further oxidation of CH₃OH in the aqueous solution. The overall decline in the productivity of CH₃OOH can be attributed to its inherent instability. Figure 5f compares the methane conversion performances using different oxidants, highlighting that the 10% Cu-ZSM-5-H₂ catalyst demonstrated significantly higher productivity and selectivity for CH₃OH compared to other studies reported previously.

Assessing the H₂O₂ consumption efficiency in the direct oxidation of CH4 is crucial. The quantity of H₂O₂ consumed during the reaction was determined using visible spectroscopy, employing the conventional titanium potassium oxalate method. The gain factor, a metric commonly utilized to evaluate and compare catalyst utilization, is defined as follows [39,40]:

$$\mathrm{Gain} \mathrm{factor}={\displaystyle \frac{\mathrm{C}1 \mathrm{products} (\mathrm{mmol})}{{\mathrm{H}}_2{\mathrm{O}}_2 \mathrm{consumption} (\mathrm{mmol})}}$$

(1)

Figure 5. Productivity and selectivity of CH₄ oxidation on (a) 10% Cu-ZSM-5-H₂ with different reaction cycles, (b) with different H₂O₂ concentrations, (c) with different pressures, (d) with different stirring speeds, and (e) with different reaction times. Reaction conditions: 25 mg catalysts dispersed in 20 mL of 0.5 M H₂O₂ aqueous solution, 3 MPa CH₄ at 70 °C for 1 h. (f) Productivity to CH₃OH versus CH₃OH selectivity by catalyst type and oxidant [2,19,41,42,43,44,45,46,47]. Data are derived from the publications listed in Supplementary Table S3. Annotated data points from refs. [2,19,41,42,43,44,45,46,47].

Figure 5. Productivity and selectivity of CH₄ oxidation on (a) 10% Cu-ZSM-5-H₂ with different reaction cycles, (b) with different H₂O₂ concentrations, (c) with different pressures, (d) with different stirring speeds, and (e) with different reaction times. Reaction conditions: 25 mg catalysts dispersed in 20 mL of 0.5 M H₂O₂ aqueous solution, 3 MPa CH₄ at 70 °C for 1 h. (f) Productivity to CH₃OH versus CH₃OH selectivity by catalyst type and oxidant [2,19,41,42,43,44,45,46,47]. Data are derived from the publications listed in Supplementary Table S3. Annotated data points from refs. [2,19,41,42,43,44,45,46,47].

As illustrated in Supplementary Figure S6, the pristine H-ZSM-5 exhibited the lowest gain factor among the catalysts. Although the catalyst calcined in air demonstrated the highest gain factor, its productivity was significantly lower compared to the 10% Cu-ZSM-5-H₂ catalyst. The 10% Cu-ZSM-5 catalyst showed good activity, yet its gain factor was lower than that of H-ZSM-5. This could be attributed to the inability of copper species to form stable chemical bonds with the zeolite in the absence of calcination, leading to the detachment of copper species and promoting the decomposition of H₂O₂ rather than the desired reaction.

Copper-loading zeolite is a research hotspot for the high yield coefficient of C1 oxygenates and high selectivity of CH₃OH. Woertink et al. [48] proposed that the bent mono-(μ-oxo)dicupric core in Cu-ZSM-5 is the active site to break the C-H chemical bond. Vaneldere et al. [49] found that there are two distinct [Cu-O-Cu]²⁺ sites in the Cu-MOR. And Grundner et al. [50] found that the trinuclear copper-oxo clusters plays a key role to break the carbon–hydrogen bonds in CH₄. Taking into account the influence of the electronic state and copper loading, catalysts were evaluated under the impregnation of three distinct gases with varying copper percentages. To ascertain the pivotal role of copper in the reaction, a series of catalysts with a gradient of copper masses were synthesized, and their performance was assessed (Supplementary Figures S7–S9). The results indicated that catalysts with a higher proportion of Cu⁺ ions could more effectively utilize even small amounts of copper, as evidenced by the volcano-type curve observed for the 10% Cu-ZSM-5-H₂ catalyst’s activity in relation to copper loading. In contrast, the activities of other catalysts generally exhibited a monotonic increase, highlighting the enhancement in catalyst productivity with optimal copper loading.

2.3. Mechanism Investigate

The EPR of aqueous solution was performed after the reaction. Firstly, as shown in Figure 6a, the spectrum of H-ZSM-5 rarely showed identical signal intensity of hydroxyl radical (·OH) or methyl radical (·CH₃), but the Cu-impregnated ZSM-5 exhibited evident signal intensity and is matched with the ·OH and ·CH₃ [51]. This discovery demonstrates that introducing copper greatly improves H₂O₂ activation, producing ·OH radicals that break the C-H bond, forming ·CH₃ [52] Notably, the untreated Cu-ZSM-5 displayed a more intense ·OH radical signal compared to those calcined in the three different atmospheres. However, this was accompanied by a decrease in CH₃OH yield, suggesting that an excess of ·OH radicals can lead to overoxidation of CH₃OH [36]. Subsequently, the aqueous solution post-reaction from the 10% Cu-ZSM-5-H₂ catalyst at various time intervals was analyzed. As depicted in Figure 6b, the peak intensities of the ·OH and ·CH₃ radicals diminished over time. The sample collected after a 0.5 h reaction exhibited the most pronounced peaks for these free radicals. The attenuation in peak intensity was attributed to overoxidation processes that consumed both ·OH and ·CH₃ radicals to produce other products. Aligning with the productivity tests of the 10% Cu-ZSM-5-H₂ catalyst over time, the yield of C1 products decreased as the reaction duration increased. The ·OH radicals generated from H₂O₂ were likely involved in the oxidation of CH₃OH and, potentially, other undetected products. The generation of ·OH and ·CH₃ radicals can be summarized as follows [52]:

H₂O₂ → ·OOH + *H

(2)

H₂O₂ → ·OH + *OH

(3)

CH₄ + ·OH → ·CH₃ + H₂O

(4)

·CH₃ + ·OH→CH₃OH

(5)

·CH₃ + ·OOH→CH₃OOH

(6)

Copper-loading zeolite is a research hotspot for the high yield coefficient of C1 oxygenates and high selectivity of CH₃OH. Woertink et al. [48] proposed that the bent mono-(μ-oxo)dicupric core in Cu-ZSM-5 is the active site to break the C-H chemical bond. Vaneldere et al. [49] found that there are two distinct [Cu-O-Cu]²⁺ sites in the Cu-MOR. And Grundner et al. [50] found that the trinuclear copper–oxo clusters plays a key role to break the carbon–hydrogen bonds in CH₄. Taking into account the influence of the electronic state and copper loading, catalysts were evaluated under the impregnation of three distinct gases with varying copper percentages. To ascertain the pivotal role of copper in the reaction, a series of catalysts with a gradient of copper masses were synthesized, and their performance was assessed (Supplementary Figures S7–S9). The results indicated that catalysts with a higher proportion of Cu⁺ ions could more effectively utilize even small amounts of copper, as evidenced by the volcano-type curve observed for the 10% Cu-ZSM-5-H₂ catalyst’s activity in relation to copper loading. In contrast, the activities of other catalysts generally exhibited a monotonic increase, highlighting the enhancement in catalyst productivity with optimal copper loading.

Additionally, in situ DRIFTS was employed to examine the surface species on the 10% Cu-ZSM-5-H₂ catalyst following exposure to CH₄, H₂O₂, and H₂O at 70 °C. This analysis aimed to elucidate the mechanism of free radical oxidation of methane under mild conditions. As depicted in Figure 6c, the distinctive peaks for CH₄ were observed at 1304 cm⁻¹ and 1343 cm⁻¹. Peaks at 1415 cm⁻¹ and 1374 cm⁻¹ were attributed to the *CH₃ species, and the peaks at 1463 cm⁻¹ correspond to the scissoring vibration of *CH₃. These observations provided insights into the intermediate species formed during the catalytic oxidation process [13,42,53]. Despite the influence of H₂O₂ solution introduction via bubbling, the reaction’s inherent production of H₂O resulted in a more pronounced peak compared to the system devoid of H₂O₂. This indicated that the water generated during the reaction contributed significantly to the observed spectral intensity. The surface species on the 10% Cu-ZSM-5-H₂ catalyst were analyzed over time, as illustrated in Figure 6d. The accumulation of *CH₃ peaks with time, concurrent with the decline in CH₄ peaks, suggested the progressive decomposition of CH₄. This observation aligned with the intermediate formation and subsequent conversion processes occurring on the catalyst surface during the oxidation of CH₄.

Building on the preceding discussion, the direct oxidation mechanism of methane was delineated in Figure 7. The C-H bond cleavage was proposed to be homolytic, as evidenced by the detection of the ·CH₃ radical in the EPR spectrum [40]. Based on the catalyst characterization and reactivity test outcomes, the oxidation pathway of CH₄ over Cu-ZSM-5 was hypothesized, as depicted in Figure 7. H₂O₂ generated two types of free radicals, ·OOH and ·OH. In the presence of H-ZSM-5 in the heterogeneous catalytic system, H2O2 preferentially produced ·OOH over ·OH, leading to the combination of ·OOH with the ·CH₃ radical derived from CH₄ to form CH₃OOH, which was produced in higher yields compared to CH₃OH. Under identical reaction conditions, the incorporation of copper into the catalyst markedly enhanced the yield of CH₃OH and relatively diminished that of CH₃OOH. This suggested that copper had the capacity to facilitate the transition of H₂O₂ toward ·OH formation within this heterogeneous catalytic system.

3. Materials and Methods

3.1. Materials and Reagents

Copper nitrate trihydrate (Cu(NO₃)₂·3H₂O, ≥99%) was purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). H-ZSM-5 was bought from Nankai University Catalyst Co., Ltd. (Tianjin, China). The gas used in the reaction was obtained from Messer Industrial Gas, and 30 wt% H₂O₂ was purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). All the materials and reagents were used directly, without purification.

3.2. Catalysts Preparation

A series of Cu-ZSM-5 catalysts with varied Cu loading from 5 to 30 wt% were synthesized by a wetness impregnation method [42]. Typically, 4.0 g of H-ZSM-5 dispersed in 100 mL of deionized water and a known amount of Cu(NO₃)₂·3H₂O were mixed in a flat-bottom beaker at room temperature with vigorous agitation. After a 12 h standing period, the resultant sample was subjected to centrifugation, followed by overnight drying in an oven at 100 °C. These samples were designated as x% Cu-ZSM-5, where x corresponded to 5, 10, 15, and 30. Then, the samples were calcined at 300 °C for 1 h in a tubular furnace under air, nitrogen, and hydrogen, respectively, with a ramp rate of 5 °C/min. The catalysts used were denoted as x% Cu-ZSM-5-Y, where Y represented air, N₂, and H₂.

3.3. Characterizations

XRD was conducted through an Empyrean from Malvern Panalytical to identify the structural and crystal size information. The measurement was scanned from 5° to 60°, with a scan rate of 0.1°/s. The morphology of the catalysts was obtained with SEM, which was achieved on a Merlin from Zeiss. TEM images and the corresponding elemental mapping EDX images were recorded by Talos F200x from Thermo Fisher (Waltham, MA, USA) at an acceleration voltage of 200 KV. The specific surface area, pore size distribution, and total pore volume of the catalysts were measured by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods on ASAP 2460, using the desorption branches of N₂ isotherm at −196 °C. The liquid products were quantitatively analyzed by Nuclear Magnetic Resonance (NMR, BRUKER AscendTM 400). TGA was used on a TGA 8000 from PerkinElmer (Waltham, MA, USA) to evaluate the thermal stability of samples. The oxidation states of the surface elements were analyzed through XPS on a ESCALAB Xi⁺ with C 1s of 284.8 eV as calibration. EPR spectra were obtained on Bruker E500 to detect the valence state of the transition metal, Cu, in the samples.

CH₄ adsorption capacity of the catalysts was determined by CH₄-TPD on a Micromeritics AutoChem II 2920 device with a thermal conductivity detector (TCD). In a typical procedure, 50 mg of sample was placed in the reaction tube. Before analysis, the sample was pretreated at 300 °C under a helium (He) steam (50 mL/min) for 1 h to remove the adsorbed gases and surface moisture. After that, the sample cooled down to 50 °C. The 10 vol % CH₄/He gas mixture was introduced for the catalyst sample for 1 h until saturation. Then, the gas was switched to He flow (50 mL/min) for 1 h to remove the weak physical absorption of CH₄ on the surface. Finally, the desorption temperature was raised from 50 to 800 °C under a He atmosphere at a heating rate of 10 °C/min, and the desorption gas signals were monitored by TCD.

In situ DRIFTS experiments were measured on a Nicolet iS50 spectrometer using a mercury cadmium telluride (MCT) detector by averaging 64 sequentially collected scans at a resolution of 4 cm⁻¹. In the CO-DRIFTS sample chamber, the sample to be tested was pretreated with Ar (50 mL/min) at 250 °C for 30 min and then cooled down to 50 °C. After that, 1% CO/He (50 mL/min) was introduced and adsorbed to the saturated coverage, and then the gas phase CO was purged with Ar (50 mL/min). The infrared spectrum curve was collected to analyze the species of Cu. For CH₄-DRIFTS, 1% CH₄/He was introduced and adsorbed.

3.4. Evaluation of Catalytic Performance

In a standard experimental procedure, the direct oxidation of CH₄ was conducted within a 100 mL Teflon-lined stainless-steel autoclave reactor. Typically, 25 mg of catalyst and 20 mL of a 0.5 M H₂O₂ aqueous solution were introduced into the reactor, followed by purging with methane to remove any residual air within the autoclave. The reactor was then pressurized to 3 MPa. Subsequently, the system was heated to the desired reaction temperature, ranging from 30 to 90 °C, for a duration of 30 min to 10 h, while maintaining a stirring speed of 800 rpm to ensure homogeneous mixing and efficient catalytic activity.

After the reaction ended, the reactor was rapidly cooled down by ice bags to ensure the products without using vaporization. Then, the gaseous product was detected by a gas chromatograph (GC, Fuli Instrument GC9720Plus) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The liquid-phase oxygenates were collected via centrifugation and analyzed using GC-FID and ¹H NMR. Specifically, FID was employed to detect CH₃OH, while ¹H NMR was used for the quantitative analysis of CH₃OOH.

The yields of products and the oxygenates’ selectivity were calculated using Equations (1) and (2):

$$\mathrm{Productivity} (\mathrm{mmol}·/({\mathrm{g}}_{\mathrm{cat}}·\mathrm{h}))={\displaystyle \frac{\mathrm{Products} (\mathrm{mmol})}{\mathrm{Catalysts} (\mathrm{g})·\mathrm{Reaction} \mathrm{time} (\mathrm{h})}}$$

(7)

$$\mathrm{Selectivity} \mathrm{of} {\mathrm{CH}}_3\mathrm{OH} (\%)={\displaystyle \frac{{\mathrm{CH}}_3\mathrm{OH} (\mathrm{mmol})}{\mathrm{Total} \mathrm{products} (\mathrm{mmol})\times 100}}$$

(8)

4. Conclusions

In this study, it was demonstrated that precise control of Cu valence states in Cu-ZSM-5 catalysts via annealing under varied atmospheres (H₂, N₂, and air) profoundly influenced methane oxidation pathways under mild conditions. The H₂-annealed 10% Cu-ZSM-5 catalyst, with optimized Cu⁺/Cu²⁺ ratios, achieved exceptional methanol productivity of 8.08 mmol/(g_cat·h) and 91% selectivity at 70 °C and 3 MPa, using H₂O₂ as an oxidant. Comprehensive characterization (XPS, EPR, and CH₄-TPD) revealed that Cu⁺ species enhanced CH₄ adsorption and facilitated H₂O₂ activation to generate ·OH and ·CH₃ radicals, which synergistically drove selective C-H bond cleavage while suppressing overoxidation. In situ DRIFTS and radical trapping experiments further corroborated the pivotal role of Cu⁺ in stabilizing methyl intermediates and steering the reaction toward methanol formation. This work not only establishes a robust valence engineering strategy for designing efficient zeolite-based catalysts but also provides mechanistic insights into radical-mediated methane activation.