Direct Air Capture of CO₂ Using Poly(ethyleneimine)-Functionalized Expanded Poly(tetrafluoroethylene)/Silica Composite Structured Sorbents

The rapidly increasing atmospheric CO2 concentration has driven research into the development of cost- and energy-efficient materials and processes for the direct air capture of CO2 (DAC). Solid-supported amine materials can give high CO2 uptakes and acceptable sorption kinetics, but they are generally prepared in powder forms that are likely not practically deployable in large-scale operations due to significant pressure drops associated with packed-bed gas–solid contactors. To this end, the development of effective gas–solid contactors for CO2 capture technologies is important to allow processing high flow rates of gas with low-pressure drops and high mass transfer rates. In this study, we demonstrate new laminate-supported amine CO2 sorbents based on the impregnation of low-molecular-weight, branched poly(ethyleneimine) (PEI) into an expanded poly(tetrafluoroethylene) (ePTFE) sheet matrix containing embedded silica particles to form free-standing sheets amenable to incorporation into structured gas–solid contactors. The free-standing sheets are functionalized with PEI using a highly scalable wet impregnation method. This method allowed controllable PEI distribution and enough porosity retained inside the sheets to enable practical CO2 capacities ranging from 0.4 to 1.6 mmol CO2/gsorbent under dry conditions. Reversible CO2 capacities are achieved under both dry and humid temperature swing cycles, indicating promising material stability. The specific thermal energy requirement for the regeneration based on the measured CO2 and water capacities is 287 kJ/mol CO2, where the molar ratio of water to CO2 of 3.1 is achieved using hydrophobic materials. This is the lowest molar ratio among published DAC sorbents. A larger laminate module is tested under conditions closer to larger-scale operations (linear velocities 0.03, 0.05, and 0.1 m/sec) and demonstrates a stable capacity of 0.80 CO2/gsorbent over five cycles of CO2 adsorption and steam regeneration. The PEI-impregnated ePTFE/silica composite sorbent/contactors demonstrate promising DAC performance derived from the amine-filled silica particles contained in hydrophobic ePTFE domains to reduce water sorption and its concomitant regeneration energy penalty.