Toward consistent thermodynamic modeling of CO2 adsorption on Lewatit VPOC 1065 under dry conditions: isotherm variability, data gaps, and model fitting

Accurate thermodynamic characterization of CO₂ adsorption on solid amine-functionalized sorbents is essential for modeling and optimizing direct air capture (DAC) processes. This study presents a systematic compilation and comprehensive analysis of available adsorption isotherm data for Lewatit® VPOC 1065, one of the most studied benchmark sorbents for DAC applications. Six independent literature datasets were critically evaluated, revealing significant discrepancies in reported adsorption capacities and inconsistencies across the temperature and partial pressure ranges, particularly within the low-pressure regime relevant for DAC. Global fitting of a temperature-dependent Toth model was performed to investigate the capability of this widely used single-mechanism approach to capture experimental trends across the entire dataset. The Toth model demonstrated substantial limitations, particularly at low partial pressures, highlighting inadequacies in representing the complex adsorption behavior of the sorbent. Moreover, comparative analyses indicated that these limitations stem partially from inter-author variability, experimental uncertainties at ultra-low pressures, and potential unknown adsorption mechanisms, for example, a physisorption — chemisorption dual site mode. Based on these insights, future research directions were identified.