Cooperative lattice theory for CO2 adsorption in diamine-appended metal-organic framework at humid direct air capture conditions

The effect of humidity on the cooperative adsorption of CO2 from the air on amine-appended metal–organic frameworks is studied both experimentally and theoretically. Breakthrough experiments show that, at low relative humidities, there is an anomalous induction effect, where the kinetics at short times are slower than kinetics at long times. The induction effect gradually vanishes as relative humidity is increased, corresponding to an increase in CO2 adsorption rate. A new theory is proposed based on the lattice kinetic theory (LKT), which explains these experimental results. LKT can accurately represent the measured data over the full range of humidities by postulating that the presence of adsorbed water shifts the equilibrium clusters from cooperatively bound chains to non-cooperatively bound CO2. A consequence of this transition is that CO2 exhibits step adsorption isotherm in dry air and a standard Langmuir adsorption isotherm in high humidity air.