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 Al 2 O 3 powder additive by cross-linking poly(ethylenimine) (PEI) with poly(ethylene glycol) diglycidyl ether (PEGDGE) at -196 °C. The scaffolds' CO 2 uptake performance is compared with a conventional sorbent, i.e., PEI impregnated on an Al 2 O 3 support. PEI scaffolds with Al 2 O 3 additive show narrow pore size distribution and thinner pore walls than alumina-free materials, facilitating higher CO 2 uptake at conditions relevant to direct air capture. The PEI scaffold containing 6.5 wt % Al 2 O 3 had the highest CO 2 uptake of 1.23 mmol/g of sorbent under 50% RH 400 ppm of CO 2 conditions. In situ DRIFT spectroscopy and temperature-programmed desorption experiments show a significant CO 2 uptake contribution via physisorption as well as carbamic acid formation, with lower CO 2 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 % Al 2 O 3 is estimated to have the lowest desorption energy penalty under humid conditions, 4.6 GJ/t CO2 , among the sorbents studied.