Alumina Incorporation in Self-Supported Poly(ethylenimine) Sorbents for Direct Air Capture

Self-supported branched poly(ethylenimine) scaffolds with ordered macropores are synthesized with and without Al₂O₃ powder additive by cross-linking poly(ethylenimine) (PEI) with poly(ethylene glycol) diglycidyl ether (PEGDGE) at -196 °C. The scaffolds' CO₂ uptake performance is compared with a conventional sorbent, i.e., PEI impregnated on an Al₂O₃ support. PEI scaffolds with Al₂O₃ additive show narrow pore size distribution and thinner pore walls than alumina-free materials, facilitating higher CO₂ uptake at conditions relevant to direct air capture. The PEI scaffold containing 6.5 wt % Al₂O₃ had the highest CO₂ uptake of 1.23 mmol/g of sorbent under 50% RH 400 ppm of CO₂ conditions. In situ DRIFT spectroscopy and temperature-programmed desorption experiments show a significant CO₂ uptake contribution via physisorption as well as carbamic acid formation, with lower CO₂ binding energies in PEI scaffolds relative to conventional PEI sorbents, likely a result of a lower population of primary amines due to the amine cross-linking reactions during scaffold synthesis. The PEI scaffold containing 6.5 wt % Al₂O₃ is estimated to have the lowest desorption energy penalty under humid conditions, 4.6 GJ/t_CO2, among the sorbents studied.