Metal-Free Electrocatalytic C2H4 Production from CO2

CO₂ to C₂H₂

Use of the greenhouse gas CO₂ as a chemical raw material would not only reduce emissions, but also the consumption of fossil feedstocks. A novel metal-free organic framework could make it possible to electrocatalytically produce ethylene, a primary chemical raw material, from CO₂. As a team around Chengtao Gong and Fu-Sheng Ke from Wuhan University, China, has reported, nitrogen atoms with a particular electron configuration play a critical role for the catalyst.

Ethylene is an essential starting material for many products, including polyethylene and other plastics. Ethylene is produced industrially by the high-energy cracking and rectification of fossil feedstocks. The electrochemical conversion of CO₂ to ethylene would be a promising route to reducing CO₂ emissions while also saving energy and fossil resources.

Building A Catalyst

CO₂ is very stable, which makes it difficult to induce into reaction. With the use of electricity and catalysts, it is currently possible to convert it into C1 chemicals such as methanol and methane. The additional challenge in producing ethylene is that a bond must be formed between two carbon atoms. This has previously only been achieved with copper catalysts. Metal-free electrocatalysis would be advantageous because metals are a cost factor and can cause environmental problems.

The researchers have developed a metal-free electrocatalyst for the conversion of CO₂ to ethylene. The catalyst is based on a nitrogen-containing covalent organic framework (COF). COFs are a class of porous, crystalline, purely organic materials with defined topology. In contrast to metal-organic frameworks (MOFs), they require no metal ions to hold them together. Their pore sizes and chemical properties can be tuned over a wide range through selection of the building blocks.

Proper Electron Configuration

The new COF contains nitrogen atoms with sp³ hybridization as catalytically active centers. These sp³ nitrogen centers bind the individual building blocks into a framework through an aminal link (two amino groups bound to one carbon atom; pictured above).

In contrast to COFs with a classic imine-linkage (–C=N–), aminal COFs have strict requirements regarding the lengths and angles of the bonds between building blocks, which causes the frameworks to be formed through ring closures. The researchers found a suitable combination by using piperazine and a building block made of three aromatic, six-membered carbon rings.

When used as electrodes, their new COFs demonstrated high selectivity and performance (Faraday efficiency up to 19.1%) for the production of ethylene. Success of the aminal COFs is due to the high density of active sp³-nitrogen centers, which both very effectively capture CO₂ and transfer electrons. This results in a high concentration of excited intermediates that can undergo C–C coupling. In contrast, a variety of imine-linked COFs, which contain sp² nitrogen instead of sp³, were similarly tested and produced no ethylene. This proves the importance of proper electron configuration for the electrochemical reduction of CO₂ to ethylene.