Quantifying Humidity-Driven Regeneration Heat Penalties in Indoor DAC with Amine-Functionalized Sorbents

Direct air capture (DAC) is gaining prominence in carbon management and carbon dioxide removal (CDR) discussions, yet its feasibility depends strongly on deployment context and regeneration energy. Indoor environments provide a practical deployment context where elevated CO₂ can impair comfort, work efficiency, and health; however, indoor air is typically humid, and water–CO₂ co-adsorption can reshape both capture capacity and energy demand. Here we quantify these trade-offs for five representative amine-functionalized sorbents—PEI-SBA-15, TEPA-SBA-15, PEI-HP-20, een-Mg₂(dobpdc)-boc, and Lewatit—operated via thermal swing adsorption (TSA, regenerated at 90 °C) under indoor-relevant conditions. The materials span impregnated amines on porous supports (PEI/TEPA-SBA-15, PEI-HP-20), a commercial amine-functionalized ion-exchange resin (Lewatit), and an amine-appended MOF (een-Mg₂(dobpdc)-boc), enabling cross-class comparison relevant to indoor deployment.

Experiments were conducted at 25 °C under a CO₂ concentration of 2000 ppm with relative humidity (RH) spanning 20–80%. CO₂ capture capacities range from 1.05–3.24 mmol g⁻¹ at 20% RH to 1.22–3.71 mmol g⁻¹ at 50% RH and 1.68–3.62 mmol g⁻¹ at 80% RH (material-dependent). The highest capacity is achieved by TEPA-SBA-15 (3.71 mmol g⁻¹ at 50% RH), whereas other sorbents exhibit either near-saturation at intermediate RH (e.g., PEI-SBA-15) or continued capacity gains toward high RH (e.g., PEI-HP-20, Lewatit, and een-Mg₂(dobpdc)-boc). Comparative kinetics at 25 °C and 50% RH, fitted with the Avrami model, show half-times of 34.9–109.0 min with k = 0.0156–0.0278 min⁻¹ and n = 1.092–1.706. The humidity-related capacity enhancement is accompanied by a pronounced regeneration-energy penalty due to coupled water uptake. Across the five sorbents, the total specific regeneration heat (kJ mol⁻¹ CO₂; including sorbent and H₂O sensible heating and CO₂/H₂O desorption) is 186.66–522.33 at 20% RH and increases to 233.32–772.59 with increasing humidity; at 80% RH it is 1.25–2.23 times higher than at 20% RH, consistent with the sharply increasing contribution of water desorption to the total regeneration heat. Cycling tests at 25 °C, 50% RH, and 2000 ppm further reveal durability differences: PEI-SBA-15 and PEI-HP-20 show negligible capacity loss after 10 cycles, Lewatit shows ≤1.6% loss after 10 cycles, TEPA-SBA-15 shows ≤11.6% loss after 10 cycles, whereas een-Mg₂(dobpdc)-boc loses 28.2% after only 3 cycles.

Overall, the results identify humidity-driven regeneration-energy penalties and material-dependent durability as key feasibility limits for indoor DAC via TSA, providing quantitative guidance for sorbent selection and RH operation to balance capacity, kinetics, stability, and regeneration energy in indoor deployment.