Direct Air Capture of CO2 with Amine-Functionalized Solid Sorbents: Material Performance, Contactor Design, and Engineering Scale-Up

Amine-functionalized solid sorbents are a leading low-temperature route for direct air capture (DAC). Material-level studies have established their ability to capture CO2 at atmospheric partial pressures. However, once these sorbents are incorporated into practical contactors, cyclic performance becomes sensitive to mass-transfer path length, active-phase loading, support heat capacity, and air-side pressure drop. Long-term operation introduces further constraints associated with water co-adsorption, the oxidative degradation of amines, structural stability, and batch-to-batch reproducibility. This review follows these scale-dependent effects across low-partial-pressure adsorption and kinetics, structured sorbent fabrication, temperature swing adsorption (TSA), temperature–vacuum swing adsorption (TVSA), steam-assisted regeneration, and the progression from kg-scale test units to outdoor trials and early commercial systems. Across the reported studies, equilibrium adsorption capacity alone does not predict engineering performance. Dynamic mass transfer, pressure drop, regeneration demand, and sorbent degradation jointly affect contactor sizing, energy consumption, and annual CO2 capture. Comparisons among reported systems remain difficult because humidity, face velocity, vacuum conditions, cycle configuration, and system boundaries are not reported on a consistent basis, while long-duration field data remain limited. Accordingly, the available evidence is organized by development stage, with particular attention to the gaps that directly affect sorbent screening, contactor design, and process scale-up. Distinct from reviews organized mainly by sorbent class or individual process step, this review uses a scale-linked engineering framework to connect material performance with shaping, cyclic operation, integrated testing, and field deployment.