The electrochemical CO₂/CO reduction reaction (eCO₂RR/eCORR) to value-added chemicals and fuels offers one potential pathway to lower greenhouse gas emission. Substantial progress has recently been made in the performance of the eCO₂RR/eCORR system for hydrocarbons below C₃. To date, though, longer, valuable, hydrocarbons, C₄₊, have so far been limited by low selectivity and limited production rates. The factors imply the need for more costly product separation to meet high industrial purity standards for provision of a single C₄ isomer.

We developed a cascade route that could exploit recent progress in the electrified synthesis of low-carbon-intensity ethylene (C₂H₄) from C₁ (CO₂/CO) all the way to C₄ hydrocarbons. We pursued a route to feed the outlet of the ethylene system into the dimerization reactor without purification (Figure a), our goal to save both energy and cost.

Cascade systems have been explored that permute electrochemical, thermochemical and biochemical reactions in CO₂/CO upgrade to multi-carbon products. However, these systems have often relied on harsh reaction conditions (high temperature and pressure) and generated by-products that necessitate costly separation.

We sought instead a way to selectively produce C₄ hydrocarbon under ambient conditions. We pursued the coupled CO₂/CO electrolysis with C₂H₄ dimerization to produce C₄ hydrocarbons with high selectivity and C₂H₄ conversion efficiency under ambient conditions. A direct feed between reactors enabled to reduce separation costs in the system.

To upgrade directly a mixed gaseous stream (CO₂, CO, H₂ and C₂H₄) from the eCO₂RR stage as input into the C₄-producing dimerization reactor, we first investigated the impact of CO₂ and CO on modulating C₂H₄ dimerization to producing C₄ hydrocarbons. We found that CO promotes selective hydrogenation during the ensuing C₂H₄ dimerization unlike CO₂. By controlling the CO concentration, we could dramatically improve the butane (C₄H₁₀) selectivity to 95% (Figure b).

We performed a density-functional-theory calculation to understand this interesting phenomenon: the relationship between the CO environment and C₄ product selectivity. We found that higher CO coverage further destabilizes the *C₄H₉ intermediate, a factor in initiating the hydrogenation mechanism and steering selectivity from the C₄H₈ pathway to the C₄H₁₀ pathway (Figure c).

Finally, we connected the CO electrolyzer directly to a liquid phase C₂H₄ dimerization reactor without any purification. As a result, the cascade eCO-to-C₄H₁₀ system achieved a cascade selectivity of 43%, C₄H₁₀ concentration of 30 wt% and cascade production rate of 24 mM hour-1 to C₄H₁₀ under ambient conditions (Figure d).

This example of a combined electrochemical-thermochemical system offers a renewable-electricity-powered pathway for the selective production of C₄H₁₀ at ambient conditions while avoiding separation costs between two reactors. The engineered presence of CO may offer a useful degree of freedom in influencing electrochemical and thermochemical reactions to produce higher-molecular-weight hydrocarbons.

If you are interested in our work, you may find the full paper here:

https://www.nature.com/articles/s41929-023-00937-0