Direct Air Capture of CO 2 Using Poly(ethyleneimine)-Functionalized Expanded Poly(tetrafluoroethylene)/Silica Composite Structured Sorbents.

The rapidly increasing atmospheric CO 2 concentration has driven research into the development of cost- and energy-efficient materials and processes for the direct air capture of CO 2 (DAC). Solid-supported amine materials can give high CO 2 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 CO 2 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 CO 2 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 CO 2 capacities ranging from 0.4 to 1.6 mmol CO 2 /g sorbent under dry conditions. Reversible CO 2 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 CO 2 and water capacities is 287 kJ/mol CO 2 , where the molar ratio of water to CO 2 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 CO 2 /g sorbent over five cycles of CO 2 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.