Development of task specific ionic liquids incorporated porous sorbents for post-combustion CO2 capture
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Abstract
Amino functionalized ionic liquids (AAILs), also known as task-specific ionic liquids (TSILs), have demonstrated CO2 capture ability similar to amines while maintaining ionic liquid properties such as low regeneration energy, volatility, and thermal stability. However, high synthesis costs and viscosity prevent their broad usage in CO2 capture technologies. Recently discovered porous materials like metal-organic frameworks (MOFs) and ordered mesoporous silica have stimulated scientists’ interest in CO2 capture applications. However, these materials have limited CO2 absorption and poor CO2/N2 selectivity, particularly at post-combustion CO2 capture conditions (0.15 bar). Immobilizing TSILs in solid pores to boost CO2 capture is an innovative way to address the drawbacks of both TSILs and porous materials. This study incorporated 1-ethyl-3-methylimidazolium [Emim] cations with Glycine [Gly] and Alanine [Ala] as reactive Amino Acid (AA) anion, resulting in [Emim][Gly] and [Emim] [Ala]. Three porous solid supports were used, metal-organic-framework (MOF-177), zeolitic imidazolate framework (ZIF-8), and ordered mesoporous silica (MCM-48) leading to TSILs@MOF/ZIF/MCM composites. TGA and XRD measurements were performed to determine the composites’ thermal and structural stability. The specific surface area and the pore volume distribution were determined by using N2 adsorption-desorption isotherms at 77 K. CO2 adsorption isotherms were measured using an intelligent gravimetric analyzer (IGA) at three temperatures (303, 313, and 323 K), and N2 adsorption isotherms were measured at 313 K for a pressure range of 0.1 to 10 bar, for all composites and pristine solids. The CO2/N2 selectivities were computed using the CO2 and N2 adsorption isotherms. Adsorption isotherms were modeled by the Dual-Site Langmuir (DSL) model, and the isosteric enthalpy of adsorption was computed. [Emim][Gly]@ZIF-8 composites demonstrated excellent improvements in CO2 uptake and CO2/N2 selectivity at 30 wt. % loading. CO2 uptake was 10 times higher than in pure ZIF-8 at 0.1 bar and 303 K, and selectivity improved to 28 from 5 at 0.1 bar and 313 K. At 20 wt. % loading, AAILs-encapsulated composites surpassed pure MOF-177 in CO2 uptake by a factor of 3. The ideal AAIL loading was 20 wt. % and increasing the loading to 30 wt.% did not increase CO2 uptake for the AAILs@MOF177 composite. [Emim][Gly]@MCM-48 and [Emim][Ala]@MCM-48 composites enhanced CO2 uptake 10-fold and CO2/N2 selectivity to 17 from 2 at 0.1 bar for 40 wt. % loading. The improved CO2 capacity and selectivity can be attributed to the formation of C-N bonds between CO2 and the -NH2 functional group, as suggested by the isosteric enthalpy of adsorption. In addition, blended systems of amine (PZ) with 1-ethyl-3-methylimidazolium acetate [Bmim][Ac] have the potential for high CO2 capture capabilities like TSILs without inheriting TSIL limitations such as high synthesis cost and viscosity. CO2 absorption was unaffected by 30 wt. % IL in the aqueous PZ, while 60 wt. % IL greatly increased it. Furthermore, this aqueous blended system (PZ+IL+H2O) and a second non-aqueous system of ethylene glycol (EG) mixed with MEA were examined as slurry systems in which porous solid ZIF-8 was suspended. Nonaqueous slurry systems outperformed aqueous slurry systems, which could be attributed to the collapse and/or pore filling of ZIF-8 in aqueous systems as evident from the TGA and XRD analysis of the recovered ZIF-8. These research results can be used to build sorbents with superior qualities to address environmental concerns since they shed light on the synthesis, structure, and sorption capacity of these innovative composite materials.