PEI (polyethylenimine)

Polyethylenimine is a polymer that is available as solid (linear polymer) or liquid (branched polymer), and soluble in hot water or dispersable in methanol. It has been used for carbon dioxide adsorption as it has high affinity for CO2 and can be readily applied to permeable materials to create a large gas-interface surface area. It has been reported to readily desorb carbon dioxide at 100C. It may also be possible to desorb CO2 with vacuum. It is often supported on substrates of fabric, silica gel, and granular carbon.

PEI is employed as the sorbent in the OpenAir project THURSDAY.

A review of PEI applications for CO2 adsorption is found at https://onlinelibrary.wiley.com/doi/10.1002/ente.201600694 Where it is reported that PEI adsorption capacity for atmospheric CO2 is on the order of 50mg/g, and regeneration heat demand is on the order of 3.2 GJ/ton of CO2.

The structure of PEI is saturated branched chains of two carbons and one nitrogen. Branch points are at nitrogens. All the nitrogen atoms have an unbonded electron pair which attracts the electron-deficient carbon in carbon dioxide molecules. In amine adsorption of carbon dioxide, the CO2 may react with the terminal amines (as in MEA) to form carbamates - desorption of CO2 from PEI requires less energy, suggesting carbamates are not forming.