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CO2-based Electrosynthesis of ethylene oXIDE

Periodic Reporting for period 2 - CO2EXIDE (CO2-based Electrosynthesis of ethylene oXIDE)

Reporting period: 2019-07-01 to 2021-06-30

In order to enable a truly sustainable and circular economy, utilization of carbon dioxide (CO2) as raw material (Carbon Capture and Use, CCU) is an important technical approach.

In this context, the central objective of the CO2EXIDE project was the development of a sustainable alternative production process for the bulk chemical ethylene oxide (EO). The proposed technology consists of the combination of cathodic and anodic electrochemical processes (“simultaneous electrochemical factories”) driven by renewable energy and utilizing water and CO2 as only raw materials.

Specifically, the CO2EXIDE process chain (Figure 1) comprises: i) the supply of raw materials, in particular CO2 from biogenic sources; ii) the simultaneous electrocatalytic conversion of CO2 to ethylene (cathode) and of water to hydrogen peroxide, H2O2 (anode); iii) the separation and purification of the generated ethylene; and iv) the chemical reaction of the purified ethylene with the anodically generated H2O2 to the platform chemical ethylene oxide.

In conclusion of the work performed in the project, it was demonstrated that the CO2EXIDE process enables the conversion of CO2 into the platform chemical ethylene oxide through combination of an electrochemical and chemical reaction step. Process steps were individually developed and then connected to form the entire process chain. Remaining challenges on the way towards industrial application are the selectivity and stability of CO2 reduction (particularly when using technical CO2 from real point sources), the achievement of sufficiently high H2O2 concentrations and the realization of fast and effective epoxidation of ethylene in the presence of carbonate salts (electrolyte).
Biogenic CO2 capture and purification
Biogenic CO2 has been successfully captured from a biogas plant. 99 % (v/v) pure biogenic CO2 was filled in gas cylinders and shipped to partners as feedstock for the synthesis of ethylene oxide.

Catalyst development
Catalyst development for CO2 reduction to ethylene and water oxidation to H2O2 has been an important research focus in CO2EXIDE. For the CO2 reduction to ethylene, selectivities beyond the state of the art have been achieved at current densities of 200 mA cm-2 and even above. Most important challenge remains the still limited stability of the catalyst layer. For the anodic water oxidation to H2O2, substantial progress beyond the state of the art has been achieved by applying boron-doped diamond (BDD) electrodes, facilitating high selectivities and H2O2 concentrations at high current densities and under stable operating conditions.

Electrocatalytic reactor unit (ERU)
The electrocatalytic reactions were investigated and developed at different scales, ranging from about 10 cm2 electrodes up to 300 cm2. Electrochemical reactions were typically operated at current densities of 150-200 mA cm-2 and a CO2 flow rate of 1500 cm3 min-1. Electrodes with Cu sputter-deposited on carbon-based gas-diffusion layers were used as cathodes, while BDD electrodes served as anodes. Aqueous KHCO3 solutions were used as electrolytes for both half-cell reactions.

Ethylene enrichment unit (EEU)
Product gas stream from the ERU contained the target product ethylene, but also other components, such as unreacted CO2, methane and hydrogen. To increase the concentration of ethylene in the gas flow for further chemical conversion, an ethylene enrichment unit (EEU) was developed, based on membrane technology. The EEU facilitates ethylene concentrations of more than 30% at ethylene recovery rates of >90%.

Chemical conversion: Epoxidation of ethylene to ethylene oxide
The products of the electrochemical process step, ethylene and H2O2, were chemically converted into ethylene oxide (EO) under mild reaction conditions (50 °C). Process development was conducted in small-scale batch reactor systems. At the end of the reaction, temperature was increased to complete the hydrolysis of the generated ethylene oxide to ethylene glycol. The developed process was then transferred into a large-scale (2 L) autoclave system for the CO2EXIDE demonstrator unit.

CO2EXIDE demonstrator
In the final phase of CO2EXIDE, the entire process chain was implemented in an integrated demonstrator. Electocatalytic reactor unit (ERU) and Ethylene enrichment unit (EEU) were physically connected and tested, using both, bottled and technical-grade CO2 sourced from a biogas plant. In combination with the Ethylene epoxidation unit (EOU), the tests successfully demonstrated that the CO2EXIDE process enables the conversion of technical-grade CO2 sourced from a biogas upgrading plant into the platform chemicals ethylene oxide and ethylene glycol.

System-level assessment of environmental and economic performance potentials
Lifecycle and techno-economic assessments showed advantageous performance potentials. Provided that the required electric energy is generated from renewable sources, synthesis of ethylene oxide could evolve to a net-zero CO2 emission technology. Costs of production of ethylene oxide are mainly driven by the overall energy efficiency, electricity prices and investment costs. Cost-competitive production, compared to established conventional (fossil-based) processes, is only conceivable under very favourable boundary conditions – a cost premium is likely to remain for the renewable CO2EXIDE process. Regulatory measures, such as carbon pricing, are required to facilitate economic competitiveness. Another important issue for the environmental and economic performance is the utilization of side products, such as methane and hydrogen.
As one of the most versatile chemical intermediates, the industrial production of EO is based on the direct oxidation of fossil-based ethylene, which is globally produced at a scale of above 150 million tons/yr. The CO2EXIDE technology represents an environmentally friendly alternative route to produce the important bulk chemicals ethylene, H2O2, ethylene oxide and ethylene glycol only from CO2, water and renewable electric energy.

The central achievement of CO2EXIDE beyond the state of the art is the demonstration of the entire process chain. For the first time, EO was produced directly from CO2, water and electric energy, i.e. in an entirely renewable way.

With respect to process components, electrocatalysts for CO2 reduction to ethylene and for water oxidation to H2O2 were substantially developed beyond the state of the art, particularly the electrocatalysts for water oxidation. A scaled-up Electrocatalytic reactor unit (ERU) with 300 cm2 electrode surface was designed, constructed and operated. For the downstream gas conditioning, a novel process unit for the enrichment of ethylene (Ethylene enrichment unit, EEU) was developed, tested and operated. This unit was successfully connected to the Electrocatalytic reactor unit (ERU) and the outgoing gas stream enriched in ethylene for further conversion in the Ethylene epoxidation Unit (EEU). The experimental work was accompanied by system-level assessment of environmental and economic performance potentials, clearly showing the way forward towards an industrially viable process, ready for implementation.
Figure 2: The hardware components to demonstrate the CO2EXIDE process chain.
Figure 1: The CO2EXIDE process chain.