Adsorption Equilibria of n-Hexane on Metal–Organic Frameworks in Supercritical CO2: Measurement and Thermodynamic Modeling with a Potential Theory

The adsorption process using supercritical CO₂ is highly effective for separating volatile organic compounds (VOCs) from mixtures, significantly aiding solvent recovery and purification of desired substances. This study explores the adsorption equilibria of n-hexane on metal–organic frameworks (MOFs), namely MOF-177 and ZIF-8, in supercritical carbon dioxide (CO₂). Adsorption measurements were obtained through a fixed-bed method across various temperatures (313–353 K) and pressures (10.0–20.0 MPa). The analysis of adsorption behavior utilized the Dubinin–Astakhov (DA) equation, which provides thermodynamic insights into the adsorption mechanisms under supercritical conditions. The results indicate that adsorption capacity decreases with increasing pressure and increases with higher temperature, mainly due to changes in CO₂ density and n-hexane fugacity. Additionally, MOF-177 demonstrates greater adsorption capacity than ZIF-8, a difference linked to its increased pore volume and surface area. The DA model effectively explained the data, and the derived parameters (characteristic adsorption energy (E_VOC) and maximum adsorption capacity (W_0,VOC)) quantified the influences of CO₂ density and adsorbent characteristics. This research offers significant insights for designing and optimizing adsorption-based separation processes in supercritical CO₂ environments.