Amine-Site Accessibility and Low-Temperature Deep-Vacuum Regeneration of Biomass-Derived Porous Carbons for Direct Air Capture

Direct air capture (DAC) with supported amines depends on reversible CO2–amine interactions within a confined phase, so working capacity is governed by both access to the amine-containing phase and regeneration conditions. In this study, tetraethylenepentamine (TEPA)- and polyethylenimine (PEI)-functionalized biomass-derived porous carbons were evaluated in a fixed-bed system at 500 ppm CO2. TEPA samples showed substantially higher amine-mass-normalized uptake than PEI at comparable loading, indicating more favorable adsorption-transport behavior under the fixed-bed conditions. TEP-30 and TEP-40 reached 0.38–0.39 mmol g−1 under dry atmospheric-pressure conditions. Increasing the adsorption pressure to 0.2 MPa(g) raised the TEP-30 capacity to 0.58 mmol g−1. Regeneration at 110 °C caused rapid capacity loss, whereas vacuum alone at 500 Pa was insufficient. Coupling 80 °C with 500 Pa improved short-term cyclic working-capacity retention under the tested cycles by combining thermal activation of desorption with a low gas-phase CO2 chemical potential. The results establish a reaction–transport framework for interpreting supported-amine DAC and demonstrate the potential of moderate-temperature, deep-vacuum regeneration under the tested fixed-bed cycles.