(217fl) 1-n-Alkylimidazoles for Acid Gas Removal: A Study of SO2:N-Functionalized Imidazole Complexes and Absorption Properties

Recent strives in environmental research have proposed several novel solvent technologies for the removal of CO₂ and SO₂ as fugitive, contaminant emissions from such sources as coal-fired power plants.  The most employed technique, aqueous amine solvents, undergo a reversible chemical reaction with acid gas , however, are still energy intensive during solvent regeneration due to evaporative water losses and lead to corrosiveness and volatility issues.  Ionic liquids (ILs) lack sufficient capacity for efficiently capturing CO₂ and other acid gases, including H₂S, SO₂, etc., to replace aqueous amine technology that has been industrially used for over 8 decades to date.  ILs being physical solvents by nature pose to be better for contaminant acid gas removal at higher partial pressures, as in natural gas "sweetening".  Recent breakthroughs with reactive and reversible IL platforms, such as Imidazolium-based, amine-funtionalized, "task-specific" ILs (TSILs), show improvements in acid gas separations by forming reversible complexes with CO₂ and SO₂.  However, these materials can still be costly, highly viscous, and lead to equipment fouling issues as in current amine technology.

Recently, our group has explored the application of imidazoles, starting materials in the synthesis process for ILs, as well as imidazole-amine hybrid solvents for acid gas removal as highly-tunable, organic bases with low volatility and lower viscosities than imidazolium-based ILs.  With the addition of amines to imidazoles required to observe CO₂ reactions, this study investigates the direct reactivity and reversible complexing of imidazoles with SO₂.  Current SO₂ scrubbing technologies utilize the oxidation of CaSO₃ to CaSO₄, which is commonly sold as “dry-wall”.  However, direct reactivity of SO₂ in a solvent-based process eliminates this solids handling issue and also provides the capability for the production of sulfur-based chemicals (i.e. H₂SO₄). 

Initial results with 1-hexylimidazole show 1.17 mol-SO₂/mol 1-hexylimidazole absorbed under low pressure (~1 psig) conditions at room temperature.  With both physical and chemical absorption present, the gel-complexed sample was then swept with N2 for several hours until the equilibrated value of ~0.5 mol-SO₂/mol 1-hexylimidazole was observed.  This implies the molar reactivity of SO₂ with alkylimidazoles to be 1:2, and the chemically-bound SO₂ can be simply released via moderate heating with N₂ sweep with no chemical degradation present.  Our most recent experimental studies have led to investigating the effects of more novel functional groups compared to simple 1-n-alkylimidazoles, including derivatives of 1-ethylimidazole (1-ethyl-4-methyl; 1,2-diethyl-4-methyl; etc.) and imidazoles with oligo(ethylene glycol) substituents (i.e. PEG_n-imidazoles).  SO₂ absorption results show that all solvents approach ~3:1 ratio at room temperature.  The derivatives of 1-ethylimidazole were vacuumed down to approximately  ~1.5:1, implying a different complex formation than proposed earlier and having higher SO₂ reactivity and loading capacity.  PEG_n-imidazoles also approached the 3:1 ratio during low pressure loading but were vacuumed down to ~1:1, with the reactive nitrogen site still only binding to one equivalent of SO₂.  Heating of these complexed functional imidazoles could regenerate these to neat solvents, thus, yielding higher loading capacity than simple 1-n-alkylimdazoles.