Incorporation of azolate-based ionic liquids into MCM-41 silica for direct air capture of CO2

Direct air capture (DAC) of CO 2 has emerged as a vital technology to mitigate the growing climate crisis, yet it remains constrained by the lack of high-performing sorbents capable of operating under varying ambient conditions. In this study, we report the synthesis, characterization, and performance evaluation of a novel class of hybrid sorbents comprising azolate-based ionic liquids immobilized on mesoporous MCM-41 silica. A library of eight ionic liquids impregnated in MCM-41 was synthesized using four anions - benzimidazolate, imidazolate, pyrazolate, triazolate and two cations - tetrabutylphosphonium and tetrabutylammonium. Comprehensive thermogravimetric and lab-scale DAC testing identified tetrabutylphosphonium benzimidazolate in MCM-41 ([P 4444 ][BenIm]@MCM-41) as the most promising material, demonstrating a CO 2 working capacity of 1.05 wt% under dry, 400 ppm CO 2 in 1 h with 94% desorption in 0.5 h, and a working capacity of 3.70 wt% under humid 400 ppm CO 2 (70% relative humidity) in 1 h with 93% desorption efficiency under the same conditions. Notably, this is among the highest humidity conditions studied for supported ionic liquid sorbents for DAC to date. Additionally, rapid kinetics and excellent cyclic stability were demonstrated under both dry and humid environments while maintaining the promising CO 2 adsorption capacities. The relatively high isosteric heat of adsorption (40–58 kJ/mol) confirms strong yet reversible CO 2 binding, placing [P 4444 ][BenIm]@MCM-41among the most promising IL-based sorbents for practical direct air capture applications. Our findings demonstrate the feasibility of tailoring ionic liquid chemistry to engineer robust, moisture-tolerant sorbents with real-world DAC potential, addressing key limitations of current materials in this field.