(371a) Gas Solubility Measurements in 17 [Tf2 N]–-Based Ionic Liquids and Development of an Improved Process-Relevant Index for CO2/CH4 Separation

This research explores the intricate relationship between cation identity and ionic liquid (IL) properties, with a focus on their potential for CO₂/CH₄ separation. ILs are a unique class of molten salts that remain liquid below 100 °C, exhibiting tunable physicochemical properties, negligible volatility, and high thermal stability, making them attractive for industrial gas separation processes. Their structural versatility allows for the fine-tuning of key parameters such as viscosity, density, and gas solubility, offering an alternative to conventional solvents for CO₂ capture. Among various IL families, those incorporating the bis(trifluoromethylsulfonyl)imide ([Tf₂N]^–) anion have garnered significant attention due to their low viscosity, hydrophobic nature, and hydrolytic stability, which enhance their suitability for gas absorption applications. Despite this, systematic investigations into the impact of cation structure on IL performance in CO₂/CH₄ separation remain limited.

In this study, CO₂ and CH₄ solubilities are measured in 17 room-temperature [Tf₂N]^–-based ILs at 60 °C, alongside key physicochemical characterizations including density, viscosity, and thermal stability. To bridge the gap between molecular-level properties and process-scale performance, an improvised Process-Relevant Absorption and Performance (PRAP) index is introduced as a novel screening tool for evaluating ILs. This index integrates gas solubility data with diffusion resistance and normalizes performance to Selexol, providing a comprehensive and industrially relevant metric that extends beyond traditional solubility-based assessments. A key takeaway from this work is that the conventional approach of designing ILs with ultra-low viscosity—akin to Selexol and other physical solvents—or selecting ILs solely based on molar CO₂ solubility is often overemphasized. Instead, a systematic assessment of both physicochemical properties and process-relevant parameters is necessary to determine the most suitable solvent for a given application. The PRAP index is a step in that direction, offering a more coherent framework for understanding process-structure-property relationships and enabling the rational screening of ILs for industrial CO₂ separation.