Microwave-Driven Nonoxidative and Selective Conversion of Methane to Ethylene over Mn-Based Catalysts

Abstract

Recent advancements in microwave-driven nonoxidative catalytic synthesis of C₂H₄ from CH₄ coupling offer a promising, energy-efficient, and eco-friendly alternative to conventional methods, where selective heating under microwave irradiation enables comparable conversions at substantially lower bulk temperatures and shorter reaction times. This study explores the performance of an MnO_X-based catalyst supported on CeO₂ and HY zeolite (silica-to-alumina ratio = 5.1) for the nonoxidative coupling of CH₄ (NOCM) under microwave irradiation. Inspired by the well-established efficacy of MnO_X catalyst in oxidative CH₄ coupling (OCM), their application in NOCM has also shown significant performance. The catalytic system achieved 15% CH₄ conversion and 99% selectivity toward C₂ hydrocarbons and maintained 64% selectivity toward C₂H₄ surpassing the yields reported in the literature even at higher temperatures (700–1000 °C). Catalyst performance was correlated with measurements by in situ Raman spectroscopy, and additional characterizations were performed using H₂–temperature programmed reduction , NH₃–temperature programmed desorption, and BET surface area analysis to understand structural changes during the reactions. These findings suggested that Mn functions as active sites for CH₄ activation in nonoxidative environments while also promoting efficient C–C coupling under microwave irradiation.

1. Introduction

Ethylene is a fundamental petrochemical feedstock utilized in the production of polymers, chemicals, and materials gripping the global chemical industry. The worldwide production capacity of C₂H₄ has increased from 214.3 million metric tons (MMT) in 2021 to 227.6 MMT in 2023 and is projected to reach around 287 MMT by 2030. Economically, the C₂H₄ market is expected to grow at a compound annual growth rate (CAGR) of ∼7%, reaching a valuation of approximately $161 billion by 2026. Conventionally, C₂H₄ is produced through steam cracking, a highly energy-intensive endothermic process that involves the pyrolysis of naphtha derived from a petroleum refinery. , There are concerns over the depletion of non-renewable fossil resources. There is an urgent need to develop alternative, energy-efficient, and sustainable pathways for C₂H₄ production. Conventional technologies available to produce C₂H₄ include (i) steam cracking of hydrocarbon feedstocks, (ii) dehydrogenation and oxidative dehydrogenation of alkenes, and (iii) dehydration of ethanol. In these processes, hydrocarbon feedstocks are thermally converted via gas-phase radical reactions into C₂H₄ and other light olefins, as well as H₂ coproducts, at elevated temperatures exceeding 750 °C. The high endothermicity of these cracking reactions, along with the complex downstream cryogenic separation steps, renders such processes both energy- and carbon-intensive, with an associated emission of approximately 1–2 tons of CO₂ per ton of C₂H₄ produced, depending on the feedstock.

While steam cracking remains the dominant commercial route owing to its ability to coproduce valuable olefins such as propylene and butadiene, it also yields undesired byproducts such as CH₄ and fuel oil. Alternative routes like catalytic pyrolysis, ethanol dehydration, and membrane-assisted C₂H₆ dehydrogenation have been explored to enhance the efficiency and sustainability. Catalytic pyrolysis, for instance, targets higher C₂H₄ yields by using low surface-area alumina supports but is constrained by challenges such as coking, water–gas shift activity, and limited long-term olefin selectivity. − To overcome these challenges, a range of alternative C₂H₄ production methods has emerged. Among the various routes explored for direct CH₄ to C₂H₄ conversion, oxidative coupling of CH₄ (OCM) and nonoxidative coupling of CH₄ (NOCM) have received considerable attention. OCM and NOCM are among the most extensively used routes for C₂H₄ production. While OCM involves the reaction of CH₄ with O₂ to produce C₂H₄ and H₂O, benefiting from favorable thermodynamics, it faces inherent challenges such as overoxidation, leading to undesirable CO and CO₂ byproduct formation. In contrast, NOCM enables the formation of C₂H₄ and H₂ in the absence of oxidants, effectively circumventing CO₂ emissions. Despite requiring very high operating temperatures and facing issues related to coke formation and equilibrium limitations, NOCM offers a compelling advantage in terms of carbon efficiency and selectivity because it directly transforms CH₄ into value-added C₂ hydrocarbons, such as C₂H₄ and C₂H₆, without compromising the carbon balance of the system. ,−

In both OCM and NOCM routes, CH₄ is activated over catalytic metallic sites to generate methyl radicals (*CH₃), which desorb into the gas phase and undergo homogeneous coupling reactions to form C₂ hydrocarbons. Initially, two *CH₃ combine to form C₂H₆, which then dehydrogenates to C₂H₄ under high-temperature conditions (typically >1000 K). These radical-mediated pathways avoid external oxidants, reducing the risk of overoxidation and CO₂ formation. Moreover, suppressing coke formation, which is a common challenge in hydrocarbon upgrading, is addressed by optimizing the catalyst structure to minimize surface C–C coupling and enhance gas-phase interactions. , For instance, Sot et al., reported the formation of higher hydrocarbons (C₂H₆, C₂H₄, C₂H₂, and C₆H₆) with a total selectivity of ∼20% with only 3% CH₄ conversion at 1080 °C. Guo et al., achieved up to 48% CH₄ conversion and 48.4% selectivity toward C₂H₄ at 1090 °C, highlighting the feasibility of significant conversion under optimized conditions. Catalysts such as Mo₂C/ZSM-5, Mo–Fe-/ZSM, and GaN/SBA15–5, have also been explored for NOCM, typically yielding 2% CH₄ conversion below 1000 °C. While Pt-based catalysts have been extensively studied and shown favorable selectivity, their high cost and low CH₄ conversion under NOCM conditions limit scalability. To overcome these thermal and efficiency limitations, alternative energy delivery methods such as microwave heating have gained attention over time.

The use of microwave reactors offers a promising strategy to enhance the efficiency of highly endothermic processes such as NOCM. Microwaves, which are electromagnetic radiation with wavelengths in the range of 1 mm to 1 m (300 GHz–300 MHz), can interact with dielectric materials to induce rapid and localized heating. In contrast to conventional heating methods based on conduction, convection, and radiation, microwave irradiation enables direct and volumetric energy transfer to the catalyst bed, either through intrinsic absorption by the catalyst or via a microwave-absorbing susceptor. This localized heating is particularly beneficial for CH₄ conversion reactions, as metal species exhibit higher microwave absorption on catalyst surfaces due to their greater dielectric loss or electrical conductivity compared to inert supports like SiO₂, Al₂O₃. As a result, microwave irradiation preferentially energizes the active metal sites, which enhances the reaction rates and product selectivity. ,

Additionally, microwave heating mechanisms such as dipolar rotation and Debye relaxation induce energy transfer at the molecular level, where microwave energy is selectively delivered to the catalyst bed without significantly heating the surroundings. , Hence, this selective heating at the interface between the active metal sites and reaction intermediates can improve the yield of products, promote the C–H bond activation, and limit coke formation, thereby making microwave-assisted NOCM a highly attractive strategy for direct conversion of CH₄.

This work reports, for the first time, the performance evaluation of Mn-based catalysts in methane coupling reactions under microwave irradiation. A novel investigation of such catalysts was carried out using in situ Raman spectroscopy. In addition, comprehensive characterizations were conducted toward the catalyst, such as XRD, BET, TGA, and NH₃-TPD, helping reveal the mechanism of the CH₄ coupling reaction. Notably, Keller demonstrated the superior performance of MnO_X-based catalysts in OCM, highlighting their potential for CH₄ activation. Building upon these findings, the present study investigated MnO_X individually supported on CeO₂ and HY zeolite (silica to alumina ratio (SAR) = 5.1) for the NOCM under microwave irradiation. CeO₂ was selected as a support for its redox flexibility and ability to generate oxygen vacancies, which promote CH₄ activation. HY zeolite, on the other hand, was chosen for its high surface area and strong acidity, which enhance Mn dispersion and C₂ selectivity. Furthermore, both CeO₂ and HY are microwave-susceptible materials capable of efficient dielectric heating, which aids in localized energy adsorption and localized energy absorption and electron transfer during the reaction. The strategy aims to exploit the redox activity of Mn species, the O₂ vacancy formation and chemisorption capacity of CeO₂, and the high surface area of HY zeolite for enhancing CH₄ activation and C₂H₄ selectivity. By integrating these tailored catalytic properties with microwave heating, this work establishes a selective and efficient pathway for CH₄ conversion to value-added C₂ hydrocarbons.

2. Experimental Section

2.1. Chemicals

Manganese­(II) nitrate tetrahydrate [Mn (NO₃)₂·4H₂O,98%], was purchased from Thermo Scientific and used as a metal precursor. Cerium­(IV) oxide (CeO₂) support was procured from Sigma-Aldrich. The HY Zeolite support, with SiO₂/Al₂O₃ molar ratio (SAR) of 5.1, was supplied by Zeolyst International.

2.2. Catalyst Preparation

Both catalysts were synthesized via the incipient wetness impregnation method, following the procedure discussed by Keller et al., using a metal loading of 5 wt % Mn. The required amount of manganese­(II) nitrate tetrahydrate [Mn (NO₃)₂·4H₂O,98%] was dissolved in distilled water and impregnated onto the cerium­(IV) oxide (CeO₂) and HY zeolite (SAR = 5.1) supports. The impregnated samples were first dried at 60 °C for 30 min, followed by further drying at 110 °C for 2–3 h. The dried materials were then calcined in air at 550 °C for 4 h to obtain the active catalysts. Prior to impregnation, the NH₄ ⁺-form HY zeolite was converted to its protonated form by calcination in air at 550 °C for 4 h in a muffle furnace.

2.3. Catalytic Reaction Process

The performance of the prepared catalysts was evaluated in a 3 kW, 2.45 GHz magnetron microwave reactor equipped with a short-wave infrared (SWIR) system. The experiments were conducted with a 2.45 GHz Sairem 3 kW magnetron microwave interfaced with an automatic 4 stub-impedance tuner, sliding short circuit, and a Monomode high-temperature cavity operating in the TE₁₀ mode. The maximum allowable power applied during the experiments was 300 W. To help enhance temperature uniformity, an aluminum insulation package was employed to minimize heat loss and maintain stable operating conditions. Temperature control was managed via PID feedback integrated into the reactor software. The pyrometers were factory-calibrated, and the microwave waveguide was carefully tuned to minimize reflected microwave power, which was maintained as close to zero as possible throughout all experimental runs. The catalyst bed was prepared by loading the catalyst into a quartz tube, with a packed bed height of approximately 1.5–2.0 cm. The catalyst bed was positioned upright to ensure optimal focusing of microwave radiation, as shown in Figure . A quartz rod was inserted above the bed to prevent catalyst movement during gas flow. For experiments using CeO₂-supported catalysts, ∼1.5 g of catalyst was loaded, whereas only ∼ 0.5 g was used for HY-supported catalysts due to their significantly higher BET surface area. Prior to each reaction, the catalyst bed was flushed with N₂ with 30 sccm flow rate to purge the system and provide a stable baseline. During the reaction, the feed flow was set to a total of 30 sccm with a CH₄:N₂ ratio of 80:20 i.e., 24 sccm CH₄ and 6 sccm N₂. The reactions were investigated at 650, 700, and 750 °C temperatures. To further examine the effect of flow rate on CH₄ conversion and product selectivity, additional experiments were conducted with 25% and 50% increases in total flow rate.

1.

Schematic illustration of the microwave reactor system and the proposed reaction mechanism for MnO_X/CeO₂ catalytic system for nonoxidative methane coupling.

Product gas analysis was performed using a two-column Micro Gas Chromatography (Micro-GC) system (Agilent Technologies). Nitrogen was used as an internal standard for all calculations of conversion and selectivity and also served as the balance gas owing to its chemical inertness under the reaction conditions, with no nitrogen containing byproducts such as HCN detected in the effluent stream. The following equations (i–iii) were used:

i. CH₄ conversion:

$$C_{\mathrm{Methane}}=\frac{\mathrm{Methane}\mathrm{Input}-\mathrm{Methane}\mathrm{Output}}{\mathrm{Methane}\mathrm{Input}}\times 100$$

ii. Selectivity of C₂H₄ products:

$$S_{\mathrm{Ethylene}}=\frac{\mathrm{Product}(\mathrm{mol}\%)\times \mathrm{Product}\mathrm{carbon}}{\sum \mathrm{Product}(\mathrm{mol}\%)\times \mathrm{Product}\mathrm{carbon}}\times 100\%$$

iii. Yield of C₂H₄:

$$Y_{\mathrm{Ethylene}}=S_{\mathrm{Ethylene}}\times \mathrm{C}{\mathrm{H}}_4\mathrm{conversion}\times 100\%$$

$$\mathrm{Methane}\mathrm{Output}=\mathrm{Total}\mathrm{floerate}\times \mathrm{Methane}\mathrm{Concentration}(\mathrm{from}\mathrm{G}\mathrm{C})$$

$$\mathrm{Total}\mathrm{flowrate}=\frac{\mathrm{Input}\mathrm{flowrate}\mathrm{of}\mathrm{Nitrogen}}{\mathrm{Nitrogen}\mathrm{Concentration}(\mathrm{from}\mathrm{G}\mathrm{C})}$$

where C _Methane = Conversion of CH₄, S _Ethylene = Selectivity of C₂H₄, Y _Ethylene = Yield of C₂H₄

2.4. Catalyst Characterization

Powder X-ray diffraction (PXRD) was carried out using a PANalytical X’Pert Pro diffractometer equipped with Cu Kα radiation, operating at 45 kV and 40 mA. The diffraction patterns were recorded over a 2θ range of 5° to 110° using an X’celerator solid-state detector, with a scan rate of 4.8° per minute. This analysis was used to confirm the crystallinity of the catalyst framework and identify any structural changes postmetal loading.

Hydrogen temperature-programmed reduction (H₂-TPR) was carried out using a Micromeritics Autochem 2950 system to investigate the redox behavior of the catalysts. Approximately 200 mg of catalyst was pretreated in a Helium flow (50 mL/min) at 150 °C for 1 h, followed by cooling to 100 °C. The gas flow was then switched from He to 10%H₂/Ar, and the system was allowed to stabilize for 20 min. The reduction profile was then recorded by heating the sample from 100 to 900 °C at a rate of 10 °C/min under a 10% H₂/He gas mixture flowing at 50 mL/min.

NH₃-temperature-programmed desorption (NH₃-TPD) was performed using a Micromeritics AutoChem 2950 system to investigate the surface acidity of the catalysts. Approximately 200 mg of catalyst was first pretreated under He flow (50 mL/min) at 250 °C for 5 min, followed by cooling to 100 °C. After allowing the temperature to stabilize for 10 min, the gas flow was switched to a 15% NH₃/He mixture, and the sample was exposed to this flow for 30 min to ensure thorough adsorption. The system was then held until a stable baseline was achieved. The TPD profile was subsequently recorded by heating the sample from 100 to 750 °C at a rate of 10 °C/min under a continuous flow of 15% NH₃/He (50 mL/min).

N₂-physisorption measurements were performed using a Micromeritics ASAP 2020 Plus instrument to evaluate the textural properties of the catalysts along with its supports. Prior to analysis, approximately 1 g of CeO₂ support was degassed under vacuum at 300 °C for 1h, while 200 mg of the HY zeolite support was degassed at 300 °C for 4 h to eliminate moisture and surface impurities, ensuring clean surfaces for accurate adsorption measurement. This pretreatment ensured that the sample surfaces were clean and free of contaminants that could interfere with accurate adsorption measurements. Adsorption–desorption isotherms were recorded at −196 °C using high-purity nitrogen gas. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method, and micropore volume and external surface area were determined from the t-plot method. The pore size distribution was calculated from the desorption branch of the isotherms.

Thermogravimetric analysis was conducted using a STD-Q650 instrument (TA Instruments, USA). The samples were initially heated from room temperature to 100 °C at a rate of 10 °C/min and held isothermally for 30 min to remove physiosorbed moisture. Subsequently, the temperature was increased from 100 °C to 900 °C at the same heating rate under a continuous flow of air, followed by an isothermal hold at 900 °C to evaluate thermal stability and decomposition behavior.

In-situ Raman spectroscopy was used to investigate the evolution of Mn species under temperature- programmed and simulated NOCM conditions. The experiments were performed using a Renishaw InVia Raman spectrometer equipped with a 532 nm green laser as the excitation source. In-situ measurements were carried out at 700 °C under a continuous CH₄ and N₂ flow using a specialized high-temperature Raman cell. The temperature was increased from room temperature to 700 °C at a controlled heating rate of 25 °C per minute, allowing real-time observation of structural changes in the catalyst. The Raman shift was calibrated using the silicon reference peak at 520.7 cm^–1 to ensure spectral accuracy.

3. Results and Discussion

3.1. Physicochemical Properties of the Prepared Catalysts

3.1.1. XRD of the Catalysts

Figure a presents the XRD patterns of blank CeO₂, fresh MnO_X/CeO₂, and spent MnO_X/CeO₂ catalysts. The prominent diffraction peaks observed at 2θ = 28.5°, 33.1°, 47.4°, and 56.3° are attributed to the (1 1 1), (0 0 2), (0 2 2), and (1 1 3) crystal planes of CeO₂, consistent with the fluorite-type cubic phase (JCPDS 98–002–8753, space group F_m3m). , These sharp and intense peaks indicate high crystallinity of the CeO₂ support. Upon Mn doping, no additional diffraction peaks corresponding to MnO_X species were detected, suggesting either a high dispersion of MnO_X on the CeO₂ surface or the successful incorporation of Mn ions into the CeO₂ lattice. The slight broadening of diffraction peaks and decrease in intensity in the fresh and spent MnO_X/CeO₂ samples suggest a reduction in crystallite size and a partial loss of crystallinity, likely due to lattice distortion induced by Mn incorporation. Additionally, a very low-intensity peak around 26.9° in the spent catalyst corresponds to disordered carbon, indicating limited coke formation.

2.

XRD pattern of blank supports, fresh and spent catalysts of (a) MnO_X/CeO₂ and (b) MnO_X/HY.

Figure b shows the XRD patterns of blank HY zeolite, fresh MnO_X/HY, and spent MnO_X/HY catalysts. The blank HY sample exhibits characteristic peaks at 2θ = 6.29°, 10.28°, and 15.87°, corresponding to the (1 1 1), (0 2 2), and (1 3 3) planes (JCPDS 98–002–4869), confirming the typical crystalline structure of HY zeolite. The Mn-doped HY sample retains most of the structural features, with minor shifts observed at 14.1°, 15.83°, and 23.9°, corresponding to the (1 1 1), (1 3 3), and (3 3 5) planes (JCPDS 98–003–6206), indicating that the structural integrity of the zeolite is largely preserved upon Mn incorporation. However, the spent MnO_X/HY catalyst exhibits notable changes in the diffraction pattern, with additional peaks appearing at 5.74°, 10.41°, 25.0°, 25.4°, 28.1°, 28.8°, 31.1°, 35.7°, 36.5°, and 40.6°, corresponding to various planes (1 0 2), (1 0 1), (2 1 0), (2 0 3), (3 0 1), (3 0 2), (0 2 0), (1 0 4), (2 2 1),(2 2 3) identified with JCPDS 98–003–0870. These changes indicate the formation of new crystalline phases, partial dealumination of the zeolite, or the accumulation of MnO_X phases and carbonaceous residues during the reaction.

3.1.2. H₂-TPR Analysis of MnO_X Catalysts Supported on CeO₂ and HY: Influence of Support on Redox Behavior

Figure shows the H₂-TPR profiles of the blank supports and their Mn-loaded counterparts. The H₂-TPR profile of pure CeO₂ is characterized by three distinct reduction peaks. Two prominent peaks appear in the range of 400–600 °C, corresponding to the reduction of surface oxygen species. The first peak, observed near 400 °C, is attributed to the reduction of stoichiometric Ce⁴⁺–O-Ce⁴⁺ surface sites. The second peak, centered around 510 °C, is associated with the reduction of nonstoichiometric oxygen species, specifically Ce³⁺–O–Ce⁴⁺ surface linkages, which represent loosely bound surface-capping oxygen. These features highlight the redox flexibility of ceria, which readily transitions between Ce⁴⁺ and Ce³⁺ oxidation states, enabling its well-known oxygen storage and release capacity. A third, broader reduction peak emerges above 800 °C, corresponding to the reduction of bulk CeO₂, indicating deeper lattice oxygen involvement in the redox process.

3.

H₂ -TPR profiles of blank CeO₂ and HY supports and MnO_X/CeO₂ and MnO_X/HY catalysts.

Upon Mn incorporation, a substantial modification of the CeO₂ reduction behavior is observed. The H₂-TPR profile of MnO_X/CeO₂ exhibits a notable shift toward lower reduction temperatures, indicating enhanced reducibility resulting from strong interactions between Mn species and the CeO₂ support. Distinct new reduction peaks emerge around 290 and 350 °C, corresponding to the reduction of amorphous MnO₂ and Mn₂O₃, respectively, along with the simultaneous formation of oxygen vacancies in the CeO₂ lattice. , Concurrently, the diminished intensity of original surface-related CeO₂ peaks suggests partial substitution or modification of surface oxygen environments by Mn species. This behavior underscores the improved low-temperature redox capability of the Mn-loaded catalyst.

In contrast, the blank HY support exhibited a nearly featureless H₂-TPR profile, consistent with its nonreducible aluminosilicate framework. However, after Mn impregnation, two new peaks emerged at approximately 310 and 390 °C, indicating the formation of Mn species that are weakly interacting with the HY surface but still contribute to hydrogen uptake at moderate temperatures. These results show that metal incorporation into both CeO₂ and HY significantly enhances the redox behavior of the system.

3.1.3. NH₃-TPD Analysis of Mn-Based Catalysts: Influence of Support on Surface Acidity

The surface acidity of the catalysts was examined using NH₃-temperature-programmed desorption (NH₃-TPD) as shown in Figure . The position and area of the desorption peaks reflect the strength and density of acid sites, respectively.

4.

NH₃-TPD profiles of blank CeO₂ and HY supports and MnO_X/CeO₂ and MnO_X/HY catalysts.

The NH₃-TPD profile of HY (SAR = 5.1) zeolite displayed a major desorption peak around 250 °C, with a shoulder near 400 °C. The low-temperature peak corresponds to weak Lewis and/or Brønsted acid sites, while the shoulder at higher temperature indicates strong Brønsted acidity.

In contrast, Mn-doped CeO₂ showed three distinct desorption peaks assigned to weak (100–250 °C), medium (350–450 °C), and strong (>450 °C) acid sites, indicating the presence of both Lewis and Brønsted acidic functionalities. The midrange peak (100–300 °C) is attributed to NH₃ desorption from Lewis acid sites, whereas high-temperature peaks (400–550 °C) correspond to Brønsted acid sites. Blank CeO₂ exhibited negligible NH₃ desorption, confirming its inherently weak Lewis acidity. Although the total number of acid sites on MnO_X/CeO₂ (represented by the area under the peaks) is less than MnO_X/HY catalyst, the strength of acid sites (represented by the temperature of the peaks) is much higher.

3.1.4. BET Surface Area and Pore Size Analysis of Mn-Based Catalysts

The nitrogen adsorption–desorption analysis revealed notable differences in BET surface area and porosity between the CeO₂ and HY supported catalysts. As shown in Table , the BET surface area of the blank CeO₂ support was relatively low at 25.19 m²/g, which is consistent with previously reported values for pure Ceria, typically ranging from 20 to 65 m²/g depending on its synthesis conditions and morphology. Upon incorporation of 5 wt % Mn, the surface area further decreased to 15.23 m²/g, indicating possible pore blockage or surface coverage by MnO_X species during the impregnation process. In contrast, the HY zeolite support has shown a high BET surface area of 606.72 m²/g, which has decreased to 491.23 m2/g, possibly due to the pore blockage attributed to the loading of Mn metal to the zeolite channel. , Additional information regarding the isotherm and pore size distribution are provided in the Figures S1 and S2.

1. BET Surface Area of Blank Supports and Mn-Doped Supports.

Sample	S BET / (m 2 /g)	V t / (cm 3 /g)	D /(nm)	V Micro / (cm 3 /g)
Blank CeO2	25.1947	0.014638	2.3240	0.000209
Blank HY (SAR = 5.1)	606.7187	0.340767	2.2466	0.300695
5%Mn/CeO2	15.2319	0.008859	2.3265	0.000459
5%Mn/HY	491.2250	0.275434	2.2428	0.242152

^aBET (Brunauer–Emmett–Teller) specific surface area.

^bTotal pore volume, measured at P/P ⁰= 0.4304.

^cAverage pore diameter of samples, calculated as 4 V _t/S _BET.

^dMicropore volume derived from t-plot.

3.1.5. Thermogravimetric Analysis (TGA)

The thermogravimetric profiles of CeO₂ and HY-supported catalysts are presented in Figure . The TGA experiments were conducted in air, and the observed weight loss was attributed to structural changes and oxygen release/uptake processes. The blank CeO₂ support and fresh MnO _x /CeO₂ catalyst, as in Figure a, exhibited an initial weight loss below 200 °C, primarily associated with the desorption of physically adsorbed water and surface moisture. However, the fresh MnO_X/CeO₂ catalyst has shown two major weight loss events in the temperature range 550–650 °C and 650–800 °C, which can be attributed to the stepwise reduction of MnO₂ to Mn₂O₃ and subsequently to MnO, where Mn reduces from Mn⁴⁺ → Mn³⁺ →Mn²⁺. These changes highlight the redox activity induced by Mn incorporation and the oxygen storage/release behavior promoted by the CeO₂ support under elevated temperatures. The weight gain observed around 200 °C in the spent MnO_X/CeO₂ sample is attributed to reoxidation of reduced Mn species (Mn⁰ or MnO) and possible Mn carbides (MnC_X) formed under reaction conditions, along with oxidation of surface carbon deposits. This low-temperature reoxidation step, commonly reported for Mn phases, indicates that Mn participates actively in redox cycling, complementing CeO₂’s oxygen storage capacity.The reoxidation of the M-carbide and oxygen deficient Mn (due to the reductive atmosphere) is the reason for the uptake. However,the spent MnO_X/CeO₂ catalyst displayed weight loss of 0.6% in the 450–500 °C range, corresponding to the release of surface lattice oxygen, suggesting the retention of redox activity even after the reaction. This retention of redox functionality in the spent catalyst indicates that MnO_X/CeO₂ maintains its structural integrity and capacity for oxygen mobility even after prolonged reaction cycles. Such stability is crucial for sustained CH₄ activation and C–C coupling under NOCM conditions, as the reversible redox transitions of Mn species (Mn⁴⁺ → Mn³⁺ → Mn²⁺) coupled with the oxygen storage and release properties of CeO₂ ensure continuous regeneration of active sites. The observed weight loss pattern thus not only confirms the reducibility of Mn species but also demonstrates the strong metal–support interactions that contribute to the catalyst’s durability and performance under microwave irradiation. ,

5.

TGA of (a) blank CeO₂ and MnO_X/CeO₂ fresh and spent catalysts, (b) blank HY support, and MnO_X/HY fresh and spent catalysts.

In comparison, the TGA curves for the HY-supported systems, as in Figure b, indicate high thermal stability across all samples. The blank HY support and fresh MnO _x /HY catalyst showed similar profiles, with minor weight losses of 3.96% and 2.12%, respectively, in the 100–300 °C region primarily due to dehydration. , However, the weight loss in the spent catalysts occurred in three distinct steps: 1.74% at 600 °C, 0.89% at 695 °C, and approximately 2.69% at 900 °C. The spent MnO _x /HY catalyst showed a noticeably higher total weight loss of 5.324% compared to the fresh samples, likely due to the removal of carbon or coke buildup accumulated during the reaction. Notably, the progressive weight loss above 600 °C in both the spent catalyst is attributed to the combustion of coke and possible deposition of residual impurities. These observations confirm that while some degree of carbon deposition occurs during the reaction, both MnO_X/CeO₂ and MnO_X/HY catalysts exhibit excellent thermal stability and structural integrity postreaction. The DTA and DSC plots of MnOX/CeO2 and MnOX/HY are shown in the Figures S2–S4 in the Supporting Information.

3.1.6. In-Situ Raman Spectroscopy of MnO_X/CeO₂

Ex-situ and in situ Raman spectroscopy were employed to investigate the evolution of Mn oxidation states and carbonaceous deposits during the (OCM) over MnO_X/CeO₂ catalysts. The ex-situ spectra (Figure a and b) provide information on structural and electronic changes after reaction, while the in situ spectra (Figure c and d) capture the dynamic transformations under CH₄/N₂ flow at elevated temperatures.

6.

Ex-situ Raman measurement for (a) fresh MnO_X/CeO₂ (b) spent MnO_X/CeO₂ (c) In-situ Raman measurement for fresh MnO_X/CeO₂ as a function of temperature (d) as a function of reaction time at 700 °C.

The ex-situ Raman spectrum of the fresh MnO_X/CeO₂ catalyst exhibited a strong peak at 453 cm^–1, corresponding to the F_2g Raman-active mode of CeO₂. This mode reflects the symmetric breathing vibration of oxygen ions surrounding Ce⁴⁺ cations within the fluorite lattice structure. , In contrast, the ex-situ spectrum of the spent catalyst taken from a sintered chunk of the catalyst bed revealed two distinct peaks at 1346 cm^–1 and 1584 cm^–1 which are the characteristics of disordered and crystalline carbonaceous species. The peak at 1584 cm^–1 is associated with the in-plane stretching vibration of C = C stretching vibrational mode with E_2g symmetry found in well-ordered graphite structures and corresponds to the G band. The 1346 cm^–1 peak, corresponding to the D-band, is attributed to zone-boundary A_1g phonons and arises from the vibrational modes of disordered graphitic domains. This mode becomes Raman active due to lattice symmetry breaking near the boundaries of microcrystalline carbon domains. , This band corresponds to a decrease in symmetry near the boundaries of microcrystalline domains, where lattice distortions reduce the local symmetry from D_6h to C_3v, or even to C_s, enabling the D-band to appear in the Raman spectrum. Together, these features indicate the formation of amorphous and defective carbon structures on the catalyst surface during reaction.

In-situ Raman measurements conducted from room temperature to 800 °C under CH₄/N₂ flow showed notable transformations in both Mn and Ce species. As the temperature reaches 400 °C, a new peak appears at 646 cm^–1, corresponding to MnO₂ with Mn in the +4 oxidation state. This peak intensifies at 500 °C and subsequently shifts to 545 cm^–1 by 800 °C, marking a significant change in Mn speciation. Initially, the ceria peak at 453 cm^–1 appears to be dominant but its intensity decreases gradually over time. Simultaneously, the peaks associated with MnO at 530 cm^–1, Mn₂O₃ at 545 cm^–1, and MnO₂ near 680 cm^–1 are observed. As the reaction proceeds, signals corresponding to Mn₂O₃ and MnO₂ diminish approximately after 30 min, while the MnO peak remains evident in the spent catalyst, which is also supported by H₂-TPR analysis. This progression indicates the gradual reduction of Mn species from +4 and +3 oxidation states to the more stable +2 state under the reaction conditions. ,

Additionally, a shift in the ceria Raman peak from 453 cm^–1 to 440 cm^–1 is observed during reaction. This red-shift can be attributed to the inhomogeneous strain broadening caused by variations in the particle size and lattice parameters within the crystalline domains. Such shifts are well explained by the phonon confinement model, which accounts for reduced phonon energies in nanostructured systems. ,

3.2. Temperature Optimization for Non-Oxidative CH₄ Coupling

To determine the optimum reaction temperature for the nonoxidative coupling of CH₄ (NOCM) to C₂H₄, experiments were conducted at 650 °C, 700 °C, and 750 °C (Figure ). Among these, 700 °C was found to be the most favorable temperature, offering a balance between selectivity, conversion, and thermal stability. At 650 °C, the system remained thermally stable but showed a lower C₂H₄ selectivity of approximately 52% (Figure ) and a total C₂ (C₂H₄ + C₂H₆ + C₂H₂) selectivity of around 94% (Figure ), with CH₄ conversion limited to ∼12% (Figure ). When the temperature was raised to 700 °C, C₂H₄ selectivity improved significantly to about 65% as shown in Figure , and total C₂ selectivity reached 99.5% as shown in Figure , with a modest increase in methane conversion to ∼ 15% (Figure ). This temperature also maintained thermal stability over longer durations, making it optimal for continuous operation. ,

7.

Optimization of reaction temperature to maximize C₂H₄ selectivity.

9.

Selectivity of C₂H₂, C₂H₄, and total C₂s at (a) 650 °C, (b) 700 °C, and (c) 750 °C.

8.

%CH₄ conversion at various reaction temperatures.

In contrast, operation at 750 °C led to severe temperature instability, coking, and excessive formation of undesired byproducts such as C₂H₂ and aromatic compounds. Although CH₄ conversion increased substantially to 27% at this higher temperature (Figure ), the total C₂ selectivity dropped sharply to ∼85% (Figure ), compromising overall process efficiency. These results are consistent with findings reported by Julian et al., and Desta et al., who also identified 700 °C as the ideal operating point for maximizing ethylene yield in microwave-assisted NOCM systems.

3.2.1. Catalytic Performance of MnO_X/CeO₂ And MnO_X/HY at 700 °C

To evaluate the catalytic performance of the Mn-loaded supports for CH₄ conversion and selective C₂H₄ production, both catalysts were tested under microwave irradiation, as shown in Figures and . Figure illustrates the CH₄ conversion, and Figure shows the %C₂H₄ selectivity and also the selectivity toward major hydrocarbon products (C₂H₆, C₂H₄, and C₂H₂) for 5 wt % MnO_X supported on CeO₂ and HY zeolite. Both catalysts exhibited excellent selectivity toward C₂H₄ during the early phase of the reaction, known as the induction period, which was observed at approximately 50 min of time on stream (TOS). During this stage, the catalysts achieved their highest C₂H₄ production rates, highlighting a critical phase in which Mn predominantly exists in the +2-oxidation state, as confirmed by in situ Raman spectroscopy. The Raman spectra showed the emergence and stabilization of bands associated with Mn²⁺ species, indicating their active role in CH₄ activation and C–C coupling. This behavior is attributed to the maximum microwave absorption capacity of the catalysts during this period, which generates localized hotspots that enhance catalytic activity. The correlation between the Mn oxidation state transition and catalytic performance, as captured by in situ Raman, strongly supports this interpretation.

10.

%CH₄ conversion of 5 wt % MnO_X/CeO₂ and MnO_X/HY at 700 °C.

11.

% C₂H₄ selectivity and total C₂ selectivity using (a) MnO_X/CeO₂ and (b) MnO_X/HY.

Throughout the reaction, the CH₄ conversion remained relatively stable, ranging from 12% to 15% for both catalysts. Notably, both MnO_X/CeO₂ and MnO_X/HY catalysts demonstrated C₂H₄ selectivity significantly higher than those reported in the literature , for similar systems, underscoring the effectiveness of Mn species in promoting C–H bond activation and facilitating C–C coupling reactions. This superior performance can be correlated with the H₂-TPR results, which revealed enhanced low-temperature reducibility and stronger Mn-support interactions, providing abundant redox-active sites that directly contribute to the observed improvement in C₂ product selectivity. These dual functionalities of Mn, both in bond cleavage and in coupling pathways, appear to play a vital role in driving C₂H₄ production.

C₂H₄ emerged as the dominant product across both catalyst systems, with C₂H₆ also produced in appreciable quantities. Minor amounts of C₂H₂ and aromatic species (BTX: benzene, toluene, xylene) were detected, although their selectivity remained low. Importantly, the overall C₂ ⁺ hydrocarbon selectivity exceeded 95% for both catalysts. Between the two systems, MnO_X/CeO₂ exhibited slightly superior performance, achieving higher C₂H₄ selectivity and total C₂ ⁺ selectivity compared to the HY-supported catalyst. This difference may be attributed to the stronger metal–support interactions in the CeO₂ system, together with the higher oxygen mobility of the CeO₂ lattice, which facilitate C–H bond activation and C–C coupling, thereby enhancing C₂ product yields, as also evidenced by the H₂-TPR results.

3.2.2. Catalytic Performance of MnO_X/CeO₂ and MnO_X/HY at 700 °C under Varied Gas Hourly Space Velocity

To evaluate the effect of total flow rate on CH₄ conversion and product selectivity, a series of experiments were conducted at three different CH₄:N₂ flow conditions using both MnO_X/CeO₂ and MnO_X/HY (SAR = 5.1) catalysts. Figures and represent the results, showing CH₄ conversion and total C₂ hydrocarbon selectivity (C₂H₆, C₂H₄, and C₂H₂) as a function of time on stream (TOS). Additionally, CH₄ conversion data under varied flow rates is presented in S5.

12.

Influence of gas hourly space velocity (GHSV) on the performance of MnO_X/CeO₂ catalyst at 700 °C.

13.

Influence of gas hourly space velocity (GHSV) on the performance of MnO_X/HY catalyst at 700 °C.

For the MnO_X/CeO₂ catalyst, under the baseline flow rate of 24 mL/min CH₄ and 6 mL/min N₂, having a total flow of 2830 h^–1 GHSV as in Figure , CH₄ conversion initially reached ∼24% and gradually declined to stabilize around 15%. Concurrently, total C₂ selectivity increased, peaking at nearly 92% before stabilizing at approximately 90%, with C₂H₄ selectivity reaching a maximum of 55%. Increasing the total flow rate by 25% to 3537.7 h^–1 GHSV maintained CH₄ conversion at comparable levels while significantly improving product selectivity where total C₂ selectivity rising to 99.6%, and C₂H₄ selectivity peaked at 65%. At an even higher flow rate of 4245.3 h^–1 GHSV (50% increase), CH₄ conversion remained stable, C₂H₄ selectivity reached 62%, and total C₂ selectivity initially peaked at 98%, followed by a slight decline to 90%. These trends indicate that higher flow rates favor the formation of C₂H₄ and C₂H₆, while suppressing C₂H₂ formation, likely due to reduced secondary reactions.

Similar behavior was observed for the MnO_X/HY catalyst as shown in Figure . At a baseline flow rate of 30 mL/min, having a total flow of 2830 h^–1, GHSV, CH₄ conversion initially reached ∼26%, then gradually declined to ∼12% over time. During this period, total C₂ selectivity remained high (∼95%), with C₂H₄ selectivity around 57%. Increasing the total flow rate by 25% led to further enhancement of C₂H₄ selectivity (∼61%) and peak total C₂ selectivity of ∼99%, again reflecting a shift toward olefin production with reduced C₂H₂ generation. However, a further 50% increase in flow rate to 4245.3 h^–1 GHSV resulted in a decrease in both total C₂ and C₂H₄ selectivity, likely due to reduced residence time, limiting effective CH₄ conversion and optimal product distribution.

The observed trends suggest that increasing flow rate helps minimize side reactions and promotes the desired C₂ products by reducing the extent of secondary hydrogenation or overcracking reactions. Higher flow rates also reduce the opportunity for C₂H₄ to further react toward heavier or undesired products. , On the other hand, excessively high flow rates may lower effective residence time and catalyst-contact efficiency, thereby limiting CH₄ conversion. The superior performance of the Mn-promoted catalysts across these experiments can be attributed to their enhanced redox properties and surface acidity, as supported by H₂-TPR and NH₃-TPD results.

Figure a and b illustrate the detailed product distributions for MnO_X/CeO₂ and MnO_X/HY catalysts, respectively, under varying total flow rate conditions during NOCM at 700 °C. In both catalysts, C₂H₄ consistently emerged as the dominant product, followed by C₂H₆ and smaller amounts of C₂H₂. For MnO_X/CeO₂ (Figure ), increasing the total flow rate led to a noticeable enhancement in C₂H₄ selectivity, rising from 58.49% at the baseline flow rate to 63.22% at a 25% higher flow rate. C₂H₆ selectivity also increased slightly, while C₂H₂ selectivity decreased, indicating that higher flow rates help suppress secondary pathways responsible for acetylene formation. ,

14.

Product distribution under varying flow rate conditions for 5 wt % MnO_X incorporated (a) CeO₂ and (b) HY-supported catalysts during NOCM at 700 °C. The effect of flow rate on C₂H₄, C₂H₆, and C₂H₂ selectivity is shown for each catalyst system.

A similar pattern was observed for the MnO_X/HY catalyst (Figure ), where C₂H₄ selectivity improved from 56.95% to 61.12% with increasing flow rate, while the proportion of C₂H₂ was reduced. This shift in product distribution can be attributed to the shortened residence time at higher flow rates, which reduces the likelihood of secondary hydrogen abstraction and undesired polymerization reactions. The decrease in C₂H₂ formation suggests that at elevated flow rates, reactive intermediates such as CH_X and C₂H_X species are more effectively stabilized, promoting the formation of more desirable C₂ olefins.

These trends emphasize that optimizing flow rate is critical for maximizing C₂H₄ production while minimizing byproducts. Higher flow rates improve C₂ selectivity by reducing secondary reactions and limiting further conversion of C₂H₄ into heavier species or coke precursors. However, excessively high flow rates may also lower CH₄ conversion, due to reduced catalyst-reactant contact time. The superior performance of Mn-promoted catalysts under these varying conditions can be linked to their enhanced redox properties and surface acidity, as confirmed by H₂-TPR and NH₃-TPD results. In addition, BET surface area analysis indicated that the textural properties of the catalysts, particularly surface area and porosity, play a key role in providing sufficient active sites and improving accessibility, thereby supporting the observed enhancement in C₂H₄ selectivity.

These findings highlight that careful tuning of both catalyst properties and reaction parameters, such as flow rate, can significantly enhance the efficiency of microwave-assisted NOCM for C₂H₄ production. Moreover, the ability of MnO_X/CeO₂ to sustain high C₂H₄ selectivity across different flow rates reflects the beneficial role of strong metal–support interactions and oxygen mobility in stabilizing reaction intermediates, offering practical insights for designing more effective NOCM processes.

3.2.3. Performance Comparison between This Work and the Literature

A literature review for the methane conversion and product selectivity studies in the methane coupling studies has been presented in Table . As indicated below, many systems reported in the literature typically need high reaction temperatures over 900 °C to obtain a significant CH₄ conversion level, but with relatively undesirable C₂ selectivity and high CO₂ and aromatics formation. On the other hand, the MnO_X-supported catalysts prepared in this study exhibited significantly improved performance, achieving CH₄ conversions of 15% with MnO_X/CeO₂ and 12% with MnO_X/HY with exceptionally high C₂ selectivity of 97.16% and 99.07%, respectively. In comparison, CO₂ and aromatic formation remained below 0.3% across both systems. These results highlight the potential of MnO_X-based catalysts for achieving efficient and selective CH₄ coupling at relatively lower temperatures, demonstrating their potential for further development in NOCM processes.

2. Comparison of the Performance of Different Catalytic Systems under NOCM Conditions.

					% Product Selectivity
Reaction	Temp (°C)	Catalyst	Feed Composition	%CH 4 Conversion	C 2	CO 2	Aromatics	Method	Refs
OCM	800	Metal oxides/alpha-Al2O3	CH4/O2 50:50	11	60	6	N/A	Fixed Bed Quartz/Stainless
NOCM	1090	Fe©SiO2	100% CH4	48.1	48.4	-	Negligible	Fixed Bed Quartz
OCM via dielectric heating	400–1000	La2O3/Al2O3, CeO2/Al2O3	CH4/O2 75:25	40	Yield ∼ 3.3	High at low temperature	Low at high temperature	Packed Bed Microwave and Conventional
OCM	500–800	Li/MgO and BaBiO3‑x	CH4/O2/He 13.33–6.7–80 vol %	Li/MgO = 30 BaBiO3‑x = 15	60	N/A	N/A	Fixed Bed Microwave
NOCOM	∼1000	Ni	CH4/He 25:75	6.1	83	10	10	Microwave
		Fe		10	68	35	12
		AC		31.2	72	0	28
NOCOM	∼1250	Ni/Fe powder AC	CH4/He - 25:75	45	85	5	33	Microwave
NOCM	550	H-(Fe)-ZSM-5	CH4/N2 - 50:50	40	50	N/A	N/A	Fixed Bed Microwave
NOCM	700	SiC monolith 4%Mo/H-ZSM5	CH4/N2 - 75:25	11.7	35	N/A	28	Microwave
NOCM	700	1Cs-3Mo/CeO2	CH4/N2 – 83–17	22	90	N/A	4	Microwave
NOCM	700	5%MnO/CeO2	CH4/N2 −80–20	15	97.16	<0.3	<0.2	Microwave	Current work
		5%MnO/HY		12	99.07	<0.1	<0.1

4. Conclusions

NOCM was investigated over MnO_X-supported CeO₂ and HY (SAR = 5.1) catalysts under microwave irradiation at 700 °C. The catalytic performance was evaluated in terms of CH₄ conversion and selectivity toward C₂H₄ and C₂ hydrocarbons. Both catalysts exhibited superior activity, achieving up to 15% CH₄ conversion, 99% overall C₂ selectivity, and 64% C₂H₄ selectivity, which surpasses the values reported in the literature at comparable or higher reaction temperatures. Characterization via H₂-TPR and in situ Raman spectroscopy revealed that Mn incorporation significantly enhanced the redox behavior and surface oxygen mobility, promoting CH₄ activation and stabilizing intermediate oxidation states. Raman analysis further confirmed the evolution of Mn species and the suppression of coke formation under microwave-assisted reaction conditions. The localized and selective heating facilitated by microwave irradiation has enhanced C–H bond cleavage while minimizing energy losses. These findings highlight the potential of Mn-based catalysts for energy-efficient and selective CH₄ coupling to C₂ hydrocarbons. Future studies should explore the reaction kinetics and long-term stability of Mn-based systems under continuous microwave operation to support scale-up for industrial applications.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.iecr.5c02894.

• Figure S1: isotherm and pore size distribution plot of (a) MnO_X/CeO₂ and (b) MnO_X/HY; Figure S2: DSC plot of MnO_X/CeO₂; Figure S3: DSC plot of MnO_X/HY; Figure S4. TGA and DTG plots of (a) MnO_X/CeO₂ and (b) MnO_X/HY; and Figure S5: % CH₄ conversion achieved using (a)­MnO_X/CeO₂ and (b) MnO_X/HY at 700°C temperature under varied flowrates (PDF)

The authors declare no competing financial interest.