Key Takeaways & Executive Findings
- •• • Microwave catalytic reaction mode (MCRM) at 700 °C achieves CH4 conversion of 26.6%, C2–C3 selectivity of 76.5%, and yield of 20.4%, versus 12.3%, 61.9%, and 7.5% in conventional mode (CRM) – a ~2.7-fold yield enhancement, critical for industrial OCM economics. • • Apparent activation energy is drastically reduced from 173 kJ/mol (CRM) to 27.5 kJ/mol under microwave irradiation, enabling lower operating temperatures and significant energy savings in methane conversion processes. • • The Mn2O3-TiO2-Na2WO4/SiO2+SiC catalyst demonstrates robust stability for 20 h at 700 °C in MCRM, with CH4 conversion >25% and C2–C3 selectivity >76%, indicating commercial viability for continuous operation. • • Calcination treatment enhances Mn–Ti interaction, increasing peroxy species (O2^2-) and lattice oxygen (Oγ), which are key to improving OCM reactivity and selectivity, as confirmed by XRD, XPS, O2-TPD, and Raman analyses.
Abstract
Oxidative coupling of methane (OCM) is a promising route for direct conversion of methane to C2–C3 hydrocarbons, but conventional thermal catalysis suffers from insufficient conversion and selectivity. Here, we report a novel Mn2O3-TiO2-Na2WO4/SiO2+SiC microwave catalyst prepared by ball-milling, which enables efficient OCM under microwave irradiation. At 700 °C, the microwave catalytic reaction mode (MCRM) achieves a CH4 conversion of 26.6%, a C2–C3 selectivity of 76.5%, and a C2–C3 yield of 20.4%, significantly outperforming the conventional reaction mode (CRM) under identical conditions (12.3%, 61.9%, and 7.5%, respectively). The catalyst exhibits stable performance for 20 h in MCRM, maintaining CH4 conversion above 25% and C2–C3 selectivity above 76%. Characterization (XRD, XPS, O2-TPD, Raman) reveals that calcination promotes Mn–Ti interaction, increasing peroxy species and lattice oxygen (Oγ), which enhance reactivity and selectivity. Notably, microwave irradiation reduces the apparent activation energy from 173 kJ/mol (CRM) to 27.5 kJ/mol, facilitating free radical coupling and suppressing deep oxidation. These findings provide a low-temperature, energy-efficient strategy for methane valorization, contributing to sustainable chemical manufacturing.
1. Introduction
Direct conversion of methane to C2–C3 hydrocarbons via oxidative coupling (OCM) has long been pursued as a single-step alternative to the energy-intensive syngas route. However, conventional thermal catalysis faces a fundamental trade-off: high temperatures (>800 °C) are required to activate the stable C–H bond, yet they promote deep oxidation to CO2, limiting C2–C3 yields to economically unattractive levels. This bottleneck has hindered industrial deployment despite decades of research on catalyst formulations such as Mn/Na2WO4/SiO2.
This work addresses the challenge by employing microwave irradiation to create localized hot spots and non-thermal effects that enhance radical coupling while suppressing over-oxidation. The novel Mn2O3-TiO2-Na2WO4/SiO2+SiC catalyst, prepared via ball-milling, leverages the dielectric properties of SiC to absorb microwaves efficiently. The results demonstrate a dramatic reduction in apparent activation energy (from 173 to 27.5 kJ/mol) and a threefold increase in C2–C3 yield at 700 °C compared to conventional heating, offering a promising pathway for low-temperature, energy-efficient methane valorization.
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SUN Ruize, LI Ran, ZHOU Jicheng, XU Wentao (2026). Robust Microwave Catalytic Oxidative Coupling of Methane over Mn2O3-TiO2-Na2WO4/SiO2+SiC. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(25)60623-2
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Frequently Asked Questions
What is the long-term stability of the Mn2O3-TiO2-Na2WO4/SiO2+SiC catalyst under microwave irradiation at 700 °C?
The catalyst maintains CH4 conversion above 25% and C2–C3 selectivity above 76% for 20 h at 700 °C in MCRM, with no evident deactivation, indicating robust stability for extended operation.
How does microwave irradiation reduce the apparent activation energy compared to conventional heating?
Microwave irradiation lowers the apparent activation energy from 173 kJ/mol (CRM) to 27.5 kJ/mol, likely due to non-thermal effects that enhance radical formation and coupling, as well as localized heating that reduces mass transport limitations.
What is the role of calcination in improving catalyst performance?
Calcination promotes Mn–Ti interaction, increasing the amount of peroxy species (O2^2-) and lattice oxygen (Oγ), which are active sites for OCM. This leads to higher CH4 conversion and C2–C3 selectivity compared to the uncalcined catalyst.
How does the microwave catalytic reaction mode (MCRM) compare to conventional mode (CRM) in terms of C2–C3 yield?
At 700 °C, MCRM achieves a C2–C3 yield of 20.4%, which is approximately three times higher than the 7.5% yield obtained in CRM under identical conditions, demonstrating the significant advantage of microwave irradiation.
What are the key characterization techniques used to understand the catalyst structure and reactivity?
XRD, XPS, O2-TPD, and Raman spectroscopy were employed. These techniques revealed that calcination enhances Mn–Ti interaction and increases peroxy species and lattice oxygen, correlating with improved OCM performance.
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