Abstract

Amine-functionalised metal-organic frameworks (MOFs), specifically mmen-Mg2(dobpdc), are premier candidates for Direct Air Capture (DAC) due to their exceptional CO2 capacity and fast kinetics. While it is well- established that humidity does not significantly inhibit CO2 capacity in these materials, the co-adsorption of water is frequently neglected in process modelling. This omission leads to significant errors in mass and energy balances, as the thermal energy required to recover co-adsorbed water during regeneration poses a substantial technoeconomic penalty. This study addresses this gap by identifying the optimal isotherm parameters for two advanced mathematical frameworks: the Weighted Dual-Site Langmuir (W-DSL) model and the Klotz method.

Parameters were determined using non-linear regression to minimise the Sum of Squared Residuals (SSR). Results indicate that the W-DSL model provides a superior fit (R2 = 0.9988; SSR = 0.4017) compared to the Klotz method (R2 = 0.9981; SSR = 0.6204), demonstrating a high sensitivity to the vertical transition zone. The Klotz model further provided physical insight into water clustering with a calculated m value of 4.649. These validated correlations provide a robust foundation for the techno-economic assessment and design of mmen-Mg2(dobpdc)- based DAC systems, ensuring humidity effects are accurately represented in large-scale process simulations.