Chemicaland Physical Absorption of SO2by N-FunctionalizedImidazoles: ExperimentalResults and Molecular-level Insight.

Sulfur dioxide (SO2) removalis a key component of manyindustrial processes, especially coal-fired power generation. ControllingSO2emissions is vital to maintaining environmental quality,as SO2is a contributor to acid rain, but has value asa chemical feedstock. Although a number of novel solvents/materialsincluding ionic liquids (ILs) have recently been proposed for alternativesto limestone scrubbing for SO2capture/removal from pointsources, the imidazole architecture presents a convenient, inexpensiveand efficient class of low volatility and low viscosity solvents toaccomplish this goal. On the basis of our prior work with imidazolesfor CO2capture, we have extended our interests towardunderstanding the relationship between imidazole structure and SO2absorption. Using a series of imidazole compounds with varioussubstituents at the 1, 2 and/or 4(5) positions of the five-memberedring, SO2absorption via both chemical and physical mechanismswas observed. The chemical absorption product is a relatively stable1:1 SO2–imidazole complex, while physical absorptionof SO2is dependent on pressure and temperature. Becauseimidazoles are relatively small molecules, they are an efficient absorptionmedium for SO2and can form adducts wherein the mass fractionof bound SO2is >40 wt %. The SO2–imidazolecomplexes were also observed to produce distinct color and/or phasechanges that are associated with the nature of the substituents present.The chemically bound SO2can be released under vacuum atmoderate temperature (∼100 °C) and vacuum, yielding theoriginal neat solvent, while the physically dissolved SO2can be readily removed at ambient temperature under vacuum. Thisbehavior corresponds to a much smaller enthalpy of absorption forphysical dissolution (−4 to −13 kJ/mol) as determinedvia thermodynamic relationships compared to the binding energies ofchemical complexation (−35 to −42 kJ/mol) as determinedvia density functional theory calculations. Increasing chemical complexationenergies are correlated with increased substitution on the imidazolering. Simulations were also employed to gain insight into the structuresof the SO2–imidazole complexes, illustrating changesin partial charge distribution before and after complexation as wellas confirming a charge transfer complex is formed based on the N–Sbond length. [ABSTRACT FROM AUTHOR]