(137f) Methane-to-Ethylene Conversion Via Integrated Post-Plasma Catalysis: A Novel Pathway to Sustainability

The chemical industry is continually evolving, making the development of novel and sustainable synthesis pathways increasingly critical. Electrified chemical processes are paving the way to a net-zero world, attracting widespread attention from academic research to industrial innovation. One promising approach in this landscape involves the electrified conversion of hydrocarbon streams to valuable C_2-3 olefins through a combination of plasma technology and catalysis science [1-4], aligning with the goals of olefin producers and policymakers through green electricity use. We hereby present a cutting-edge reactor concept where selective reaction of methane (CH₄) to ethylene (C₂H₄) occurs through a two-stage reactor in which a nanosecond-pulsed discharge (NPD) is placed in direct thermal contact with a catalytically coated copper structure (i.e. post-plasma catalyst). [5] A nanosecond pulsed power supply ignites the discharge and controls its frequency by a waveform generator, where the controlled, repeated contact between the discharge and the ground electrode induces heating of the latter. The residual heat of the ground electrode is channelled downstream through thermal contact with the coated catalytic structure [5,6]. In this instance, we demonstrate the use of a bimetallic Pd-Ag/Al₂O₃ catalyst deposited via washcoating on a thermally conductive, 3-D printed copper structure, promoting selective hydrogenation of the C₂H₂-rich stream produced by the CH₄/H₂-fed NPD discharge and attaining the highest C₂H₄ yield (34.4% mol.) to date in a reactor of this kind. [6]

Additionally, we present an extensive set of intrinsic kinetic data on C₂H₂ hydrogenation on Pd collected in an isothermal reactor, bridging the gap between typical industrial-scale hydrogenation conditions (inlet C₂H₄/C₂H₂ = 50-100 [7, 8]) and post-plasma catalytic hydrogenation conditions (inlet C₂H₄/C₂H₂ = 0-0.5 [3, 4]). Both a typical industrial tail-end 0.05 wt. % Pd/Al₂O₃ catalyst and a post-plasma 1 wt. % Pd/Al₂O₃ catalyst are characterized, focusing on the transition zone between tail-end hydrogenation and post-plasma stream compositions. The fitness of previous kinetic models throughout this zone is shown to be insufficient [9, 10], requiring the formulation of new kinetic hypotheses. A modified Langmuir Hinshelwood Hougen Watson (LHHW) model is proposed with low computational cost and good performance in post-plasma scenarios where high thermal sensitivity is expected due to high C₂H₂ amounts.

These results show promise in adjuvating the future realization of novel, integrated methane (CH₄)-to-ethylene(C₂H₄) plasma-based processes. The demonstration of a high-yield process on a small-scale, combined with the quantitative prediction of C₂H₄ and byproduct C₂H₆ production rates via accurate kinetic models, represent critical steps to meet the demands of industrial olefin producers. The tailoring of post-plasma 3-D catalyst geometry via higher-order models remains an essential step in the upcoming development of the technology. The flexibility of such a reactor in utilizing variable ratio CH₄/H₂ streams (e.g. from the demethanizer of a steam cracking process) and its modularity, allowing for valorization of stranded sources of natural gas, all play a role in its potential role within the value chain of the chemical industry.

Bibliography

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