Ethylene (C₂H₄) is the most in-demand building block in the chemical industry with a global annual production of 192 Mt (2019).¹ C₂H₄ is currently produced through energy-intensive steam cracking of long-chain hydrocarbons, a high temperature pyrolysis in the presence of steam, which produces 2-3 tCO₂/tC₂H₄.^{2, 3} Increasing concerns over global warming as a result of excessive CO₂ emissions has led to investigation into sustainable resources in the chemical industry, the largest consumer of energy which will have a 20% share of total energy consumption by 2040. Electrification of chemicals production can suppress CO₂ emission by switching from fossil fuels to renewable electrical energy whilst also reducing the production costs.⁴ In addition, electrosynthesis approaches utilizing biomass-derived feedstock provides a potentially sustainable synthesis method, with decreased carbon footprint compared to conventional thermochemical methods, creating a carbon-neutral cycle.

With global production of more than 100 billion litres per annum, bioethanol is produced via fermentation of sugars, starch and cellulose by microorganisms such as yeast.⁵ While 65% of bioethanol is currently being used directly as fuel, transition to electric transportation over the next two decades will result in a large ethanol surplus. Ethylene can be produced by catalytic dehydration of ethanol at high temperatures of 300-500°C.⁶ However, economic evaluation of the thermo-catalytic process indicates that this process would not be profitable due to the feed cost of pure ethanol. Electrocatalysis, on the other hand, can use lower purity ethanol streams as a feed, providing source flexibility. In this presentation we will describe a novel technology based on electrocatalytic conversion of ethanol to ethylene, targeting current densities > 150 mA/cm² at < 3 V using a low concentration (~15%) ethanol feed.

Electrochemical reduction of alcohols is rarely reported, and available mechanistic studies are limited. Electroreduction of alcohols in aqueous electrolytes generally involves a dehydration reaction to produce alkenes followed by hydrogenation to the corresponding alkanes. Both propene and propane have been reported as the product of electrochemical reduction of propanol at platinized Pt surface in acidic electrolytes.^{7, 8} Precise tuning of the electrode/electrolyte is required to control the selectivity and activity of the electroreduction process. We present rational design of catalyst based on mechanistic understanding of the underlying electrochemistry to steer the reaction pathway toward ethylene production by supressing hydrogenation of alkenes to alkanes. We adopt a membrane electrode assembly (MEA) cell configuration to enable electrochemical reduction of ethanol at industrially relevant current densities via enhancing the mass transfer kinetics. Our results provide a novel path toward emission-free ethylene production from biomass and renewable electricity.

Fig. 1: Schematic illustrating of electrochemical conversion of bioethanol to ethylene.

References

1. Global Ethylene Industry Outlook to 2024 - Capacity and Capital Expenditure Forecasts with Details of All Active and Planned Plants.
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4. Shi, R.; Wang, Z. P.; Zhao, Y. X.; Waterhouse, G. I. N.; Li, Z. H.; Zhang, B. K.; Sun, Z. M.; Xia, C. A.; Wang, H. T.; Zhang, T. R., Room-temperature electrochemical acetylene reduction to ethylene with high conversion and selectivity. Nature Catalysis 2021, 4 (7), 565-574.
5. Mohsenzadeh, A.; Zamani, A.; Taherzadeh, M. J., Bioethylene Production from Ethanol: A Review and Techno-economical Evaluation. ChemBioEng Reviews 2017, 4 (2), 75-91.
6. Oliveira, C. C. N.; Rochedo, P. R. R.; Bhardwaj, R.; Worrell, E.; Szklo, A., Bio‐ethylene from sugarcane as a competitiveness strategy for the Brazilian chemical industry. Biofuels, Bioproducts and Biorefining 2019, 14 (2), 286-300.
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8. Shukun, H.; Youqun, S.; Jindong, Z.; Jian, S., Mechanism of electroreduction of allyl alcohol at platinized platinum electrode in acidic aqueous solution. J. Org. Chem. 2001, 66 (13), 4487-93.