燃料化学学报（中英文）

Catalyst	Oxygen and manganese species
Uncalcined	530.7	14.08	0.526
800 ℃ calcined	530.5	21.36	0.263

Catalyst	Oxygen and manganese species
Uncalcined	530.7	14.08	0.526
800 ℃ calcined	530.5	21.36	0.263

Model	CH₄ conversion/%	C₂–C₃ selectivity/%	C₂H₄ selectivity/%	C₂–C₃ yield/%
MCRM, 700 °C	26.6	76.5	44.5	20.4
CRM, 700 °C	12.3	61.9	26.7	7.5
MCRM, 750 °C	27.9	74.9	46.6	20.9
CRM, 750 °C	25	74.1	44.8	18.5
MCRM, 800 °C	27.5	72	49	19.8
CRM, 800 °C	23.5	69.6	46	16.3

Model	CH₄ conversion/%	C₂–C₃ selectivity/%	C₂H₄ selectivity/%	C₂–C₃ yield/%
MCRM, 700 °C	26.6	76.5	44.5	20.4
CRM, 700 °C	12.3	61.9	26.7	7.5
MCRM, 750 °C	27.9	74.9	46.6	20.9
CRM, 750 °C	25	74.1	44.8	18.5
MCRM, 800 °C	27.5	72	49	19.8
CRM, 800 °C	23.5	69.6	46	16.3

Mode	Catalyst	E_a' / (kJ·mol⁻¹)	Ref.
The apparent activation energy (E_a') was calculated based on the Arrhenius equation.
MCRM, 575−650 °C	Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC	27.5	this work
CRM, 575−650 °C	Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC	173	this work
CRM, 680−860 °C	NaWMn/SiO₂	192	[45]

Mode	Catalyst	E_a' / (kJ·mol⁻¹)	Ref.
The apparent activation energy (E_a') was calculated based on the Arrhenius equation.
MCRM, 575−650 °C	Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC	27.5	this work
CRM, 575−650 °C	Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC	173	this work
CRM, 680−860 °C	NaWMn/SiO₂	192	[45]

Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC催化剂上微波催化高效甲烷氧化偶联反应

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孙瑞泽 ¹ , 李冉 ¹ , 周继承 ¹^,² , 徐文涛 ¹^,²^,^*

燃料化学学报（中英文） | 2026,54(5): 20250270-868-20250270-880

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燃料化学学报（中英文） 2026 , 54 (5) : 20250270-868 -20250270-880

Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC催化剂上微波催化高效甲烷氧化偶联反应

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孙瑞泽¹, 李冉¹, 周继承¹^,², 徐文涛¹^,²^,^*

作者信息

¹湘潭大学化工学院绿色催化与反应工程湖南省高校重点实验室, 湖南湘潭 411105

²湘潭大学化工过程模拟与强化国家地方联合工程中心, 湖南湘潭 411105

通讯作者:

徐文涛

Robust microwave catalytic oxidative coupling of methane over Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC

Ruize SUN¹, Ran LI¹, Jicheng ZHOU¹^,², Wentao XU¹^,²^,^*

Affiliations

¹Key Laboratory of Green Catalysis and Chemical Reaction Engineering of Hunan Province, School of Chemical Engineering, Xiangtan University, Xiangtan 411105, China

²National and Local United Engineering Research Center for Chemical Process Simulation and Intensification, Xiangtan University, Xiangtan 411105, China

Corresponding Author:

Tel: 13973218371, E-mail: wentaoxu@xtu.edu.cn

出版时间: 2026-05-10 doi: 10.1016/S1872-5813(25)60623-2

文章导航

摘要

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甲烷氧化偶联（OCM）是甲烷高值化利用的重要途径，但传统热催化反应过程存在转化率和选择性不足等问题。本工作提出微波催化OCM新方法，并研制了Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC微波催化剂。结果表明，经煅烧处理的Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC催化剂的性能优于未煅烧样品，且在微波催化反应模式（MCRM）下的催化性能显著优于常规反应模式（CRM）下。在700 ℃，MCRM模式下甲烷转化率为26.6%，C₂–C₃烃类选择性为76.5%，C₂–C₃收率为20.4%。而在CRM模式下，甲烷转化率为12.3%，C₂–C₃选择性为61.9%，C₂–C₃收率仅为7.5%。这说明MCRM能显著提高C₂–C₃收率至常规模式的约3倍，表明微波不仅可以降低反应温度，而且可以提高产物的收率。此外，该Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC催化剂具有良好的稳定性，在700 ℃微波条件下连续进行OCM反应20 h，仍保持25%的甲烷转化率和76%的C₂–C₃烃类选择性。进一步发现，微波作用使催化剂表观活化能由常规模式下的173 kJ/mol显著降低至27.5 kJ/mol，通过选择性耦合促进自由基偶联并抑制深度氧化。这些结果为实现CH₄低温高效转化及绿色化工可持续发展提供了新思路。

关键词

微波催化 / 低温高效 / Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC / 甲烷氧化偶联

Abstract

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Oxidative coupling of methane (OCM) is an important way for the high-value utilization of methane (CH₄), but there are still some problems such as insufficient conversion and selectivity under traditional thermal catalysis. Herein, a new Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC microwave catalyst was developed for the oxidative coupling of methane under microwave irradiation. The results indicate that the calcined Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst is superior to the uncalcined counterpart in performance and the microwave catalytic reaction mode (MCRM) is significantly advantageous to the conventional reaction mode (CRM) for the OCM. For OCM at 700 ℃ in MCRM, the conversion of CH₄, the selectivity to C₂–C₃ hydrocarbons, and the yield of C₂–C₃ are 26.6%, 76.5% and 20.4%, respectively, much superior to the values of 12.3%, 61.9% and 7.5% in CRM under identical reaction conditions. That is, microwave irradiation can significantly enhance the yield of C₂–C₃ for OCM at a lower reaction temperature. The Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst shows favorable stability at 700 °C during the OCM reaction in MCRM for 20 h, with a methane conversion of above 25% and a selectivity of above 76% to C₂–C₃ hydrocarbons. Further study illustrates that the microwave irradiation can significantly reduce the apparent activation energy to 27.5 kJ/mol, compared with the value of 173 kJ/mol in the conventional mode, promote the free radical coupling and inhibit deep oxidation of OCM products. These results provide a new idea for the efficient conversion of CH₄ at a low temperature and for the sustainable development of green chemical industry.

Key words

microwave catalysis / low temperature and high efficiency / Mn₂O₃-Na₂WO₄/SiO₂+SiC / oxidative coupling of methane

引用本文

孙瑞泽, 李冉, 周继承, 徐文涛. Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC催化剂上微波催化高效甲烷氧化偶联反应. 燃料化学学报（中英文）, 2026 , 54 (5) : 20250270-868 -20250270-880 . DOI: 10.1016/S1872-5813(25)60623-2

Ruize SUN, Ran LI, Jicheng ZHOU, Wentao XU. Robust microwave catalytic oxidative coupling of methane over Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC[J]. Journal of Fuel Chemistry and Technology, 2026 , 54 (5) : 20250270-868 -20250270-880 . DOI: 10.1016/S1872-5813(25)60623-2

正文

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C₂–C₃ hydrocarbons are essential raw materials in the chemical industry, conventionally derived from the high-energy processing of crude oil^[1]. In recent years, gas conversion aimed at developing sustainable alternatives has received increasing attention^[2−3]. Although traditional methods predominantly proceed through the indirect syngas route, the direct conversion of methane (CH₄) to C₂–C₃ hydrocarbons is considered a more attractive approach^[4−5]. Methane can be transformed into high-value fuels and chemicals through either non-oxidative or oxidative catalytic routes^[6−8]; among them, the non-oxidative pathway demonstrates high C₂–C₃ selectivity and atomic efficiency. However, it requires endothermic operation at a temperature up to 1100 ℃, resulting in significant energy consumption and CO₂ emission^[2]. In contrast, the methane oxidative coupling (OCM) reaction directly converts methane into C₂–C₃ hydrocarbons in a single step^[9], offering the advantages of a shortened pathway with high energy efficiency, which is considered a cutting-edge technology for the high-value utilization of natural gas^[10−11]. However, the OCM reaction still faces challenges including the difficulty in methane activation^[12] and the severe deep oxidation of products^[13], which hinder the balance between conversion and selectivity. This imbalance has long been recognized as a persistent challenge in this field. Therefore, the high-efficient implementation of OCM is not only the core issue in the high-value utilization of natural gas, but also an important direction for advancing the clean conversion of CH₄ resources, alleviating energy pressures, and promoting the sustainable development of the green chemical industry^[14].

The OCM reaction represents a promising and practical pathway for the chemical utilization of methane^[15−17]. Various catalysts have been reported for the OCM reaction, among which Mn-Na₂WO₄/SiO₂ is regarded as the most promising one^[15−20]. These catalysts typically exhibit good stability and C₂–C₃ selectivity, with CH₄ conversion ranging from 20% to 30% at 800–900 ℃^[21]. Thousands of catalysts have been explored, but no significant progress has been made. In 2017, Wang et al.^[22] developed a TiO₂-doped Mn₂O₃-Na₂WO₄/SiO₂ catalyst that exhibited excellent catalytic activity at low temperatures. At 720 ℃, the CH₄ conversion reached 26%, with a C₂–C₃ selectivity of 76%. To date, this remains one of the few catalysts that demonstrate high catalytic activity in the OCM reaction. Guan et al.^[23] proposed a CO₂-assisted chemical-looping oxidative coupling of methane (CLOCM) method. Using an MnO-Na₂WO₄/SiO₂ catalyst at 750 ℃, a CH₄ conversion of 14.29%, C₂–C₃ selectivity of 79%, and CO₂ conversion of 3.01% were achieved. It was confirmed that CO₂ could exchange oxygen with the oxygen vacancies on the catalyst surface, effectively replenishing lattice oxygen, maintaining a high concentration of active oxygen, inhibiting excessive oxidation of CH₄, and promoting the generation of C₂–C₃ hydrocarbons. Lu et al.^[24] constructed a Na₂WO₄-Mn₃O₄/m-SiO₂ nanoreactor, which achieved a CH₄ conversion of 39% and selectivity of 42.3% to C₂H₂ and C₂H₄ under plasma conditions. It was revealed that the hollow-mesoporous structure inhibited excessive dehydrogenation through the Debye shielding effect and enabled cascade catalysis involving Na₂WO₄ site dehydrogenation and Mn₃O₄ site coupling. Xie et al.^[25] developed a MnWO_x-NaWSiO_x dual-phase composite catalyst, which exhibited excellent catalytic performance in the OCM reaction. At 775 ℃, the CH₄ conversion reached 32%, with a selectivity of 79% to C₂–C₃. It was revealed that the MnWO_x phase could efficiently activate O₂ to generate reactive oxygen species, which subsequently migrated to the surface of NaWSiO_x, thereby enabling a dual-phase synergistic promotion of the OCM reaction. This synergy not only improves the yield of C₂–C₃ hydrocarbons but also significantly lowers the required reaction temperature and enhances catalytic stability, making the composite catalyst one of the most effective reported to date. Although extensive research has been conducted on the OCM reaction, reducing the reaction temperature while improving the selectivity and yield of target products remains a major challenge and research hotspot. Therefore, it is imperative to develop new methodologies for the OCM reaction.

In recent years, microwave-assisted reactions have attracted increasing attention for their remarkable capability to accelerate reaction rate and enhance selectivity under relatively mild conditions. Compared with conventional thermal heating, microwave irradiation not only accelerates the reaction rate but also effectively reduces overall energy consumption, thereby serving as an energy-efficient and sustainable heating approach^[26−28]. Studies have shown that microwave heating can significantly reduce energy consumption in OCM^[29−30]. Furthermore, Chen et al.^[31] and Roussy et al.^[32−33] demonstrated that microwave irradiation inhibits the deep oxidation of methyl radicals through rapid cooling, thereby improving the C₂ selectivity. Therefore, microwave heating can significantly enhance the OCM performance by improving temperature distribution and reaction path selectivity, which are crucial for the design and optimization of efficient microwave OCM catalysts.

Herein, we reported a novel method for OCM by microwave catalysis, for which a Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC microwave catalyst is developed via ball milling. This method can significantly reduce the reaction temperature while simultaneously enhance the product yield through the selective heating effect of microwaves, thereby enabling an efficient OCM reaction at a lower temperature. The effects of calcination treatment and reaction mode on the catalytic performance were systematically investigated by using the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O₂-TPD), and Raman characterization techniques. This study provides certain new insights for the design of efficient and stable OCM catalysts.

1　Experimental

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1.1　Chemicals

Gas-phase silica (SiO₂, 250 m²/g) and manganese nitrate aqueous solution (Mn(NO₃)₂, 50%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Sodium tungstate dihydrate (Na₂WO₄·2H₂O, AR) was obtained from Tianjin Chemical Reagents Factory IV. Manganese trioxide (Mn₂O₃, AR) and titanium dioxide (TiO₂, AR) were supplied by Shanghai Aladdin Reagent Co., Ltd. Tetrabutyl titanate (C₁₆H₃₆O₄Ti, ≥98%) was purchased from Guoyao Group Chemical Reagents Co., Ltd. Deionized water (H₂O, secondary) was prepared in the laboratory. All chemicals were used as received, without further purification.

1.2　Catalyst preparation

The catalyst was prepared using the ball milling technique. Briefly, 3.875 g of SiO₂, 0.561 g of Na₂WO₄·2H₂O, 0.31 g of Mn₂O₃ and 0.32 g of TiO₂ were mixed and placed into a ball milling tank. The mixture was milled for 2 h at a ball-to-material mass ratio of 10∶1 and a rotation speed of 300 r/min. The catalyst composition, consisting of 6.2% Mn₂O₃, 6.2% TiO₂ and 10% Na₂WO₄/SiO₂, was obtained and doped with SiC to prepare the microwave catalyst. One portion was directly used in the reaction, while the other was calcined at 800 ℃ for 5 h. For convenience, the catalyst is hereafter referred to Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC. In the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst system, 1 g of Mn₂O₃-TiO₂-Na₂WO₄/SiO₂ was physically mixed with 5 g of SiC. The activity data of SiC alone, showing its limited catalytic performance, is presented in Figure 2(a) and is further discussed in subsequent sections. SiC can effectively absorb microwave energy, thus improving the heating efficiency, but exhibits almost no catalytic activity in the temperature range of 550–750 ℃.

1.3　Catalysts characterization

The XRD analysis was performed on a Rigaku D/MAX-2500/P X-ray polycrystalline powder diffractometer (Rigaku, Japan), with Cu Kα as the radiation source, at a scanning range from 5° to 90°, a scanning speed of 10(°)/min, and a step size of 0.02°; the tube voltage and current were set to 40 kV and 250 mA, respectively. The XPS analysis was carried out on a Thermo Fisher Scientific ESCALAB 250 xi instrument (Thermo Fisher Scientific, USA), at approximately 2×10⁻⁷ mbar, with a monochromatic Al Kα radiation source (1486.6 eV, 6 mA×12 kV); the beam spot size was 400 μm, using the CAE scanning mode. Raman spectroscopy was carried out on a Renishaw inVia Raman spectrometer (Renishaw, UK); a semiconductor laser was used as the excitation source, with a detection wavelength of 532 nm and the scanning range of 80–1000 cm⁻¹.

The O₂-TPD test was carried out using an automatic chemical adsorption system. The samples were pretreated at 50 ℃ for 1 h under a He atmosphere (30 mL/min) and 50–100 mg of sample was placed in a reaction tube. The temperature was then increased to 500 ℃ at a rate of 10 ℃/min, followed by drying and purging for 1 h. After cooling to 50 ℃, an O₂ mixture (30 mL/min) was introduced for adsorption until saturation, and then the system was switched to a He atmosphere (30 mL/min) for purging for 1 h. Finally, the temperature was increased to 900 ℃ at 10 ℃/min under a He atmosphere, and the desorbed gas was detected using a thermal conductivity detector (TCD).

1.4　Catalytic activity evaluation

The OCM reaction test was carried out using a vertical multi-atmosphere microwave catalytic reactor (Hunan Changyi Microwave Co., Ltd.). A schematic of the device is provided in Figure 1. A quartz tube was used as the reactor in the experiment. This reactor can quickly, efficiently and uniformly heat the absorbing materials and is commonly used in the gas-solid phase catalytic reactions. The catalyst was placed in a vertical tube and an improved thermocouple was positioned within the catalyst bed to minimize the interference from the microwave radiation. At each reaction temperature, the temperature fluctuation range was only 2–3 ℃ under steady state condition. The desired reaction temperature, such as 700 ℃, is achieved by precisely regulating the microwave power output.

A microcomputer integrated in the microwave catalytic reactor is used to control the power output, thereby enabling precise regulation of the catalyst bed temperature. Conventional experiments for comparison were conducted in a tube furnace, with the catalyst placed in a horizontal quartz tube. During the evaluation, 1 g of catalyst (40–60 mesh) was mixed with 5 g of SiC and placed in a quartz reaction tube. The reaction atmosphere consisted of a mixture of CH₄, O₂ and Ar (CH₄∶O₂∶Ar = 5∶1∶4). The total space velocity was 8400 mL/(g·h) and the reaction temperature was monitored in real time using a thermocouple. The products were analyzed by gas chromatography (Agilent GC 7890A). H₂, O₂, CO and CO₂ were detected using a thermal conductivity detector (TCD) coupled with a TDX-01 packed column. CH₄, C₂H₄, C₂H₆, C₃H₆ and C₃H₈ were analyzed using a hydrogen flame ionization detector (FID) coupled with a PoraPlot Q capillary column. The gas composition was quantified using the external standard method. By establishing the standard curves in advance, the content of each component in the exhaust gas was calculated using the fitted formula. Relevant data were collected after 10 min of reaction. For the catalyst characterization after the microwave reaction, SiC was first removed using a sieve to ensure that all analyzed samples were free of SiC.

The conversion of CH₄ (

$ {{x}}_{{\text{CH}}_{\text{4}}} $

) and selectivity to C₂H₄, C₂H₆, C₃H₆, C₃H₈ (

$ {{s}}_{{\text{C}}_{\text{2}}-{\text{C}}_{\text{3}}} $

) were calculated using the standard normalization method based on carbon atom conservation, following the formulae:

$\begin{split} & {{x}}_{{\text{CH}}_{\text{4}}}\left({\%}\right)=\\& \frac{\sum \text{converted reactants}}{\sum \left(\text{converted reactants}\right)+\sum \text{(}\text{remaining reactants})}{\times 100}\% \end{split}$

(1)

$ \begin{split} &{{s}}_{{\text{C}}_{\text{2}}-{\text{C}}_{\text{3}}}\left({\%}\right)=\\& \frac{\text{moles of carbon atom in product N}}{\sum \left(\text{moles of carbon atom in reaction products}\right)}{\times 100}\%\end{split} $

(2)

$ {{Y}}_{{\text{C}}_{\text{2}}-{\text{C}}_{\text{3}}}\left({\%}\right)={{x}}_{{\text{CH}}_{\text{4}}}{\times }{{s}}_{{\text{C}}_{\text{2}}{-}{\text{C}}_{\text{3}}}{\times 100}\% $

(3)

2　Results and discussion

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2.1　Effect of SiC on the catalytic performance and microwave absorption

As the microwave catalytic reactor is not electrically heated, the desired temperature of catalyst bed is highly depended on the microwave absorbing properties of the catalyst bed materials themselves. To clarify the role of SiC in the reaction system, the OCM performance test was conducted separately, as shown in Figure 2(a). Apparently, SiC exhibits almost no catalytic activity in the temperature range of 550–750 ℃, with only a very low conversion of approximately 2% at 800–850 ℃, suggesting that SiC itself does not directly participate in the reaction below 750 ℃. Further tests on the heating behavior of different catalysts shown in Figure 2(b) reveal that the equilibrium temperature of Mn₂O₃-TiO₂-Na₂WO₄/SiO₂ is only about 115 ℃ at 800 W microwave power; however, this temperature significantly increases to 760 ℃ when the catalyst is mixed with SiC and reaches 815 ℃ when SiC is used alone. That is, the introduction of SiC can significantly enhance the microwave absorption capacity of the catalyst bed, enabling the reaction bed to be heated up rapidly at a lower power microwave irradiation and providing the necessary temperatures for the OCM reaction.

2.2　Effect of calcination treatment on the catalyst performance

To investigate the effect of calcination treatment on the catalyst performance, the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂ catalyst was calcined at 800 ℃ for 5 h in a muffle furnace and compared with the uncalcined counterpart. After adding SiC, their performances were further compared under microwave irradiation at different temperatures.

As shown in Figure 3, the uncalcined catalyst displays pretty good performance at 700 ℃, but the calcined counterpart is even better. At 700 ℃, the uncalcined sample displays a CH₄ conversion of 24.2% and selectivity of 64.8% to C₂–C₃, with a C₂–C₃ yield of 14.5%. In contrast, the calcined counterpart achieves a CH₄ conversion of 26.6% and a selectivity of 76.5% to C₂–C₃, with a C₂–C₃ yield of 20.4%. Overall, the C₂–C₃ selectivity over the calcined catalyst can be stably maintained at approximately 75% across different reaction temperatures, which is significantly higher than those over the uncalcined sample. In addition, both catalysts show minimal variation in the activity within the temperature range of 700–800 ℃, indicating that the temperature has a limited effect on the catalyst performance. This suggests that the calcination treatment is a key factor in enhancing the activity of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂ catalyst, which is further elucidated in the subsequent work.

2.3　Catalyst characterization

Figure 4 presents the XRD patterns of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC microwave catalyst before and after calcination and reaction. All samples exhibit α-cristobalite peaks at 21.8°, 26.6° and 32.0°, indicating that SiO₂ transforms into a stable phase at high temperature. Additionally, the characteristic peaks of Na₂WO₄ are observed near 24.0° and 29.0°, while the peaks of Mn₂O₃ appear at 28.3° and 33.0°^[34]. Notably, after calcination at 800 ℃, a new diffraction peak emerges at 35.0°, which is attributed to MnTiO₃, while the intensity of the Mn₂O₃ peak is correspondingly weakened. This indicates that part of Mn₂O₃ reacts with TiO₂ to form the MnTiO₃ active phase, which is considered a key factor in promoting the conversion of CH₄ to C₂–C₃ in the OCM reaction^[22].

The surface element composition and valence state of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalysts were analyzed by XPS, as shown in Figure 5. The characteristic peaks of Na, Mn, W, Ti, O and Si are observed throughout the spectrum, indicating that Mn₂O₃, TiO₂ and Na₂WO₄ have been successfully loaded onto the SiO₂ support. To further elucidate the role of the surface-active species in the catalyst, the O 1s and Mn 2p high-resolution spectra of the catalyst before and after calcination were fitted and analyzed.

Figure 5(b) shows that the O 1s spectrum can be decomposed into three peaks: 530.5 eV corresponding to the peroxy species

${\mathrm{O}}_2^{2-} $

; 532.4 eV to

${\mathrm{CO}}_3^{2-} $

; and 533.2 eV to the surface-adsorbed oxygen species

${\mathrm{O}}_2^{-} $

^[35−37]. As shown in Table 1, the content of

${\mathrm{O}}_2^{2-} $

in the calcined sample is significantly higher than that in the uncalcined sample, which is consistent with their sequence in the OCM activity, indicating that

${\mathrm{O}}_2^{2-} $

plays a key role in the reaction. The higher the content of

${\mathrm{O}}_2^{2-} $

, the higher C₂–C₃ selectivity and yield the catalyst shows.

Figure 5(c) shows the high-resolution Mn 2p spectrum, which can be decomposed into two sub-bands: 2p_3/2 and 2p_1/2. The peaks at 641.6 and 652.8 eV correspond to Mn³⁺, while those at 644.2 and 655.4 eV correspond to Mn^4+[38]. As shown in Table 1, the Mn⁴⁺/Mn³⁺ ratio for the uncalcined sample is 0.526, much higher than the value of 0.263 for the calcined sample, indicating the conversion of part Mn³⁺ to Mn⁴⁺ during the calcination process. It can be observed that the oxidation process of Mn³⁺ to Mn⁴⁺ readily induces lattice defects, enhances the surface oxygen migration ability, and improves the redox performance, thereby promoting the OCM reaction^[39]. Therefore, the high

${\mathrm{O}}_2^{2-} $

content and Mn⁴⁺/Mn³⁺ ratio are key factors that contribute to the excellent microwave reactivity of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst.

To explore the oxygen species on the catalyst surface, O₂-TPD tests were performed, as shown in Figure 6. The desorption peak below 300 ℃ corresponds to weakly physically adsorbed oxygen O_α^[40−41], which is not directly related to CH₄ conversion and is only detected in the O₂-TPD profiles of uncalcined catalyst.

The desorption peak above 700 ℃ is attributed to the lattice oxygen species in Mn₂O₃. The desorption peaks in the range of 300–700 ℃ are further divided into two regions: the peak at 300–450 ℃ corresponds to the surface reactive oxygen species O_β, which can activate CH₄ but is easily overoxidized to CO₂ at relatively low temperatures^[42]; in contrast, the peak at 450–700 ℃ is attributed to the surface lattice oxygen O_γ, which can effectively promote the conversion of CH₄ to C₂H₄^[41]. It is worth noting that the calcined catalyst exhibits distinct desorption peaks in the ranges of 450–700 ℃ and 750 ℃, indicating the presence of more reactive oxygen species that can participate in the OCM reaction; accordingly, the overall catalytic performance of the calcined catalyst is improved.

Figure 7 presents the Raman spectra of the spent Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalysts after the reaction. Both calcined and uncalcined catalysts display the characteristic signal of MnTiO₃ after the reaction^[43], though the related signal of the calcined one is much stronger. Meanwhile, the Raman peak for Mn₂O₃ is not detected, indicating that Mn₂O₃ has been largely consumed during the reaction and converted to MnTiO₃ both in the bulk and on the surface of the catalyst. It can be confirmed that the formation of MnTiO₃ plays a crucial role in enhancing the performance of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst in the OCM reaction.

2.4　Comparison in the catalytic performance between microwave and conventional reaction modes

To further investigate the effect of reaction mode on the catalyst activity in the OCM reaction, the calcined Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst was compared under both microwave and conventional conditions, as shown in Figure 8 and Table 2. At an initial temperature of 600 ℃, both modes show insignificant activity. As the temperature increases, the catalyst activity is gradually improved, but the performance in microwave mode consistently outperforms that in the conventional mode. At 700 ℃, the CH₄ conversion, selectivity to C₂–C₃ hydrocarbons and C₂–C₃ yield in MCRM are 26.6%, 76.5% and 20.4%, respectively, much superior to the corresponding values of 12.3%, 61.9% and 7.5% under CRM. That is, the catalyst under MCRM exhibits higher CH₄ conversion and C₂–C₃ selectivity, with its C₂–C₃ yield nearly three times higher than that under conventional conditions. Combined with the characterization results, it can be observed that the calcination treatment promoted the interaction between Mn and Ti, leading to the formation of the active MnTiO₃ phase. In addition, the calcined catalyst displays a higher portion of O_γ species as measured by O₂-TPD, which was beneficial to the formation of C₂ hydrocarbons, creating a synergistic effect with the enhanced activity in the microwave field. In summary, microwave heating not only significantly enhances catalyst activity at lower temperatures but also gives higher product yields under optimal reaction conditions. The microwave enables more uniform and efficient heating of the catalyst bed, which promotes the function of the active sites at lower temperatures. The rapid and selective microwave-matter interactions create a homogeneous reaction environment, reduce thermal gradients, and enhance catalyst stability. Consequently, the overall catalytic performance and product yield are significantly improved under microwave heating compared with the conventional heating that suffers from slow and uneven heat conduction^[44].

2.5　Stability evaluation of catalyst in microwave mode

To investigate the catalyst stability, the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC microwave catalyst calcined at 800 ℃ was subjected to a continuous 20 h reaction test at 700 ℃ under microwave reaction mode. As shown in Figure 9, the catalyst maintains highly active even after prolonged reaction times. The CH₄ conversion and C₂–C₃ selectivity remain stable at approximately 25% and 76%, respectively, while the C₂–C₃ yield is maintained at around 20% without significant deactivation. That is, the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst exhibits excellent stability and long-term application potential in the OCM reaction.

2.6　Microwave catalytic effect

2.6.1　Microwave direct catalytic effect

To investigate the effect of microwave irradiation on the kinetics and activation energy of the OCM reaction, the apparent activation energy was determined, with the CH₄ conversion rate in the temperature range of 575–650 ℃ over the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC microwave catalyst. By adjusting the inlet flow rate and catalyst loading, the methane conversion was maintained at a low level. As shown in Figure 10, the CH₄ conversion under MCRM is consistently higher than that under CRM, indicating that microwave irradiation benefits the OCM process and can lower the reaction temperature.

To consider the difference in OCM reaction kinetics between microwave and conventional reaction modes, the Arrhenius equation is used as follows:

$ {k}={A}\text{exp}\left(-\frac{E'_{\text{a}}}{{RT}}\right) $

(4)

where k is the rate constant, R is the molar gas constant, T is the thermodynamic temperature,

$ E'_{\text{a}} $

is the apparent activation energy, and A is the pre-exponential factor.

A linear regression fitting on 1/RT and ln(k) is made, as shown in Figure S1 of the Supporting Information and Table 3. The apparent activation energy of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst in the conventional reaction mode is 173 kJ/mol, in good agreement with the value of approximately 192 kJ/mol reported in literature^[45]. In contrast, the apparent activation energy of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst in the microwave reaction mode is decreased to 27.5 kJ/mol, indicating that microwave irradiation can significantly reduce the activation energy of OCM, thereby accelerating the reaction process.

2.6.2　Microwave selective catalytic effect

To further explore the effect of microwave irradiation on the product selectivity of the OCM reaction, the product distribution of the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst under different reaction modes was compared.

As shown in Figure 11, the selectivity to C₂−C₃ and C₂H₄ under microwave conditions is higher than that under conventional mode, particularly in the temperature range of 650–700 ℃, while the selectivity of both is similar at 750 ℃. Meanwhile, the proportion of by-product CO_x in microwave mode is significantly reduced, with only 49.8% at 600 ℃, much lower than the value of 100% observed in the conventional mode. These results indicate that microwave irradiation not only promotes the coupling reaction of methyl radicals but also effectively inhibits the deep oxidation process, thereby achieving a higher selectivity to the C₂–C₃ hydrocarbons.

The mechanism for the acceleration of chemical reactions by microwave irradiation remains a subject of debate. The traditional view is that microwave creates hot spots within the catalytic bed, with a temperature about 100–200 ℃ higher than the overall temperature, thereby accelerating the reaction rate^[46−47]. However, this study and other existing proofs demonstrate that, even if hot spots exist, the conversion and selectivity in the conventional reaction mode still do not reach the levels observed in the microwave reaction mode. Therefore, although the hot spot effect may contribute to the reaction under microwave irradiation in part, it is still insufficient to explain the enhanced performance under microwave conditions that significantly exceeds the thermodynamic predictions^[48].

An increasing number of studies support the selective catalytic effect of microwave irradiation. In the decomposition of H₂S, Chen et al.^[48–49] found that microwave irradiation can break the equilibrium limit and increase the conversion rate. Ramirez et al.^[27,50] demonstrated that microwave irradiation enables rapid selective heating of catalyst particles, while the gas phase maintains at a lower overall temperature, which helps to reduce the side reactions and improves the selectivity to the target product. These findings suggest that microwave irradiation not only heats up the system but also alters the reaction path and rate-determining steps by selectively coupling with polar or high-loss species.

Such an effect is particularly pronounced in the OCM system. The CH₄ conversion involves five parallel reactions, as shown in Figure 12. CH₄ likely generates methyl radicals under the influence of surface-active oxygen, while C₂H₆ is possibly formed by coupling in the gas phase, which may be further converted into C₂H₄. Simultaneously, the OH group likely facilitates the desorption of water and the regeneration of active sites. At a high temperature, some free radicals may react with gaseous O₂ or surface oxygen to form CO_x, which can potentially reduce the yield of the target product^[51−52]. Therefore, the reaction selectivity becomes a key factor limiting the efficiency of the OCM process. The selective catalytic effect of microwave irradiation is mainly reflected in two aspects. First, the promoting effect of microwave irradiation itself is selective and varies across different reaction systems^[44]. In the OCM reaction system, the promoting effect of microwave irradiation on side reactions is weaker than that on the main reaction, thereby the MCRM mode can greatly reduce the formation of by-products and improve the selectivity to target products. Second, microwave irradiation enables rapid heating of solid-phase particles, while the gas-phase temperature remains relatively lower, resulting in a gas-solid temperature difference^[27], which further suppresses the gas-phase chain reactions and surface deep oxidation. The results of Pak et al.^[53] also indicate that the complete oxidation of CH₄ occurs predominantly in the gas phase, whereas the oxidation of C₂H₄ mainly takes place on the catalyst surface, consistent with the findings of this study.

Further characterization results confirm the synergistic effect of MnTiO₃-Na₂WO₄ under microwave irradiation. The XRD and Raman results reveal the formation of MnTiO₃, while the XPS and O₂-TPD results demonstrate that the catalyst is rich in active oxygen species capable of participating in the redox cycle. In addition, Na₂WO₄ inhibits side reactions and stabilizes the selectivity to target products. Under microwave irradiation, MnTiO₃ and Na₂WO₄ act synergistically, leading to simultaneous improvements in both activity and selectivity. Similarly, Kaneda et al.^[54] reported that a SiC@MgO core-shell catalyst significantly enhanced the C₂ yield by improving the temperature distribution and avoiding hot spots and deep oxidation.

From a kinetic perspective, the apparent activation energy of the OCM reaction over the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst under MCRM is only 27.5 kJ/mol, much lower than the value of 173 kJ/mol under CRM that is consistent with the conventional value of approximately 200 kJ/mol reported in the literature. Similarly, in the decomposition of H₂S, the apparent activation energy over Mo₂C@CeO₂ decreased from 495.62 to 21.6 kJ/mol under microwave irradiation, suggesting the universality of microwave irradiation in significantly reducing the reaction energy barrier^[55]. Hunt et al.^[56] also found that the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation for the Boudouard reaction between carbon and carbon dioxide.

As a new type of heating method, microwave irradiation can significantly accelerate the reaction process and improve the reaction selectivity. In the reaction process, microwave irradiation plays two key roles. On the one hand, as an external field, microwave irradiation can regulate the reaction system and enhance reaction selectivity, often yielding better product selectivity than traditional thermocatalytic reactions. On the other hand, the energy provided by microwave irradiation not only heats the reaction system and increases the catalyst’s temperature but also directly reduces the activation energy of the reaction, thereby accelerating the reaction process, which was also reported in one of our earlier works^[44]. Therefore, microwave radiation is not only an innovation of heating method, but also provides a new power source for reaction, which challenges the traditional heating method and has great application potential.

In summary, the promoting effect of microwave irradiation on the OCM process cannot be simply attributed to the hot spot effect but rather to the selective coupling among microwave, catalyst, and reactants. By rearranging the relative rates of the main and side reactions and regulating the gas-solid temperature field, microwave irradiation significantly reduces the activation energy and suppresses the formation of by-products. Combined with the synergistic effect of MnTiO₃-Na₂WO₄, it can be concluded that the selective catalytic effect of microwave irradiation is the key for the high-efficiency and low-temperature OCM reaction, which provides a new perspective for the future optimization and design of catalyst structures.

3　Conclusions

收起

In this work, an Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC microwave catalyst was prepared using the ball-milling method and a new approach of microwave-catalytic OCM was proposed to achieve efficient conversion of methane to C₂–C₃ hydrocarbons at lower temperatures. The XRD, XPS, O₂-TPD and Raman characterization results reveal that the calcination treatment can promote the interaction between Mn and Ti, increase the amount of peroxy species

${\mathrm{O}}_2^{2-} $

and lattice oxygen O_γ, and thereby enhance both OCM reactivity and selectivity to target products. In addition, the calcined catalyst is superior to the uncalcined counterpart and MCRM is significantly better than CRM for the OCM reaction. For the OCM reaction at 700 ℃ in MCRM, the conversion of CH₄, the selectivity to C₂–C₃ hydrocarbons, and the yield of C₂–C₃ are 26.6%, 76.5% and 20.4%, respectively, much superior to the values of 12.3%, 61.9% and 7.5% in CRM under identical reaction conditions. That is, the C₂–C₃ yield under microwave irradiation is about three times higher than that under the conventional mode. In addition, the Mn₂O₃-TiO₂-Na₂WO₄/SiO₂+SiC catalyst shows favorable stability at 700 °C during the OCM reaction in the MCRM for 20 h, with a methane conversion of above 25% and a selectivity of above 76% to C₂–C₃ hydrocarbons, with no evident deactivation observed. Further study illustrates that MnTiO₃ formation and microwave irradiation are the key to enhancing the OCM reactivity; the apparent activation energy under microwave irradiation is significantly reduced to 27.5 kJ/mol, compared with the value of 173 kJ/mol in the conventional mode. These results provide new insights into the development of efficient and stable OCM catalysts.

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doi: 10.1016/S1872-5813(25)60623-2

接收时间：2025-10-03
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¹湘潭大学化工学院绿色催化与反应工程湖南省高校重点实验室, 湖南湘潭 411105

²湘潭大学化工过程模拟与强化国家地方联合工程中心, 湖南湘潭 411105

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科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
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红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
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				红菇属 Russula	17	8.13
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Catalyst	Oxygen and manganese species
	${\mathrm{O}}_2^{2-} $		Mn⁴⁺/Mn³⁺
	binding energy/eV	atomic fraction/%	Mn⁴⁺/Mn³⁺
Uncalcined	530.7	14.08	0.526
800 ℃ calcined	530.5	21.36	0.263