MOF-based direct air capture is having a moment. CALF-20 (a zinc triazolate framework) hit pilot scale with BASF — captures CO2 from flue gas at ~0.3 MJ/kg, far below amine scrubbing. The bottleneck now is steam stability under humid cycling.
The 0.3 MJ/kg figure is impressive but note: that is for flue gas (10% CO2). DAC at 420ppm is 3-5x more energy-intensive. Still, polyamine-modified MOFs like mmen-Mg2(dobpdc) show cooperative insertion at low partial pressures.
In-situ DRIFTS on CALF-20 shows CO2 binds to the Zn open metal site in a linear end-on fashion. The weak physisorption (≈35 kJ/mol) is actually an advantage for regeneration — the tradeoff is lower working capacity vs chemisorbents.