A multifunctional phosphonium triazolate ionic liquid is capable of reversibly absorbing CO₂ at high capacity while decreasing viscosity. The addition of ZIF-8 leads to a porous ionic liquid with enhanced physical absorption, however, an unexpected loss of porosity occurs in the presence of CO₂.

The challenge of CO₂ separation and management in biogas upgrading processes is addressed, which remains a critical bottleneck when considering biomethane as a competitive and sustainable alternative to natural gas. Ionic liquids offer a promising alternative to existing sorbents due to their negligible volatility and their tunable properties. Herein, a multifunctional phosphonium triazolate ionic liquid capable of reacting reversibly with CO₂ without loss of fluidity through both cation and anion is presented. Using a combination of experiments and reaction models the interplay of different absorption mechanisms is demonstrated at varying temperatures and pressures, which lead to high capacity for CO₂ absorption and excellent selectivity for CO₂ over CH₄. The multifunctional phosphonium triazolate can be used to prepare a porous ionic liquid with enhanced physical gas absorption by dispersing up to 10% w/w of ZIF-8. The stability and porosity are maintained after CH₄ absorption but are lost upon prolonged exposure to CO₂ due to dissolution of the porous solid. These findings provide crucial insights for the development and modeling of ionic liquid-based absorbents, paving the way for biogas upgrading technologies with reduced carbon footprint.