Comparative analysis of functional group effects on CO₂, CH₄, CO, and N₂ adsorption in UiO-66 and its derivatives

This work focuses on the effect of organic linker functionality on the adsorption characteristics of CO₂, CH₄, CO, and N₂ over UiO-66, UiO-66-NH₂, UiO-66-NO₂, UiO-66-COOH, and UiO-66-(COOH)₂. Introducing polar and complex functional groups such as – NH₂, –NO₂, –COOH, and –(COOH)₂ improved the adsorption capacity but also reduced the free volume, negatively impacting adsorption. In the low-pressure region, functionality played a dominant role, whereas at saturation, the pore volume of Metal-Organic Frameworks contributed more to adsorption uptake. For all studied gases, a Type-I isotherm was observed. The isotherms were modeled using the virial equation, and model parameters were used to calculate the enthalpies of adsorption. Ideal Adsorbed Solution Theory was used to predict the selectivity of binary mixtures. The selectivity of CO₂ over CH₄, CO, and N₂ showed that introducing larger and more complex functional groups caused a sudden decrease in selectivity with increasing temperature; this difference was more significant at lower temperatures and negligible at higher temperatures.