Effect of LiNO3 Modification on UiO-66-nLiNO3 for CO2, CH4, and N2 Adsorption Performance from Low to High Pressures

Abstract

This study systematically investigates the influence of lithium nitrate (LiNO₃) modification on the gas adsorption characteristics of UiO-66. The adsorption isotherms for carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂) were acquired within a pressure range of 0 to 20 bar. The results from the cyclic testing indicate that the UiO-66–0.05LiNO₃ variant exhibits commendable stability. Notably, the incorporation of LiNO₃ enhances the CO₂ adsorption capacity by 29.48%, reaching 2.24 mmol·g^–1 at a pressure of 1 bar. This is attributed to the introduction of lithium ions, which enhance the electrostatic interaction between CO₂ and the MOF. Analysis of the high-pressure data reveals that the Langmuir–Freundlich model provides the most robust fit, whereas the Langmuir and Dual-site Langmuir models demonstrate increased discrepancies as pressure escalates. According to calculations based on the Ideal Adsorption Solution Theory (IAST), the maximum selectivity of the material for the CO₂/CH₄ and CO₂/N₂ mixtures is 11.5 and 56.0, respectively. These results suggest that LiNO₃ modification significantly enhances both the adsorption capacity and the selectivity for CO₂ separation.