Chemical and Physical Absorption of SO2 by N-Functionalized Imidazoles: Experimental Results and Molecular-level Insight

Sulfur dioxide (SO2) removal is a key component of many industrial processes, especially coal-fired power generation. Controlling SO2 emissions is vital to maintaining environmental quality, as SO2 is a contributor to acid rain, but has value as a chemical feedstock. Although a number of novel solvents/materials including ionic liquids (ILs) have recently been proposed for alternatives to limestone scrubbing for SO2 capture/removal from point sources, the imidazole architecture presents a convenient, inexpensive and efficient class of low volatility and low viscosity solvents to accomplish this goal. On the basis of our prior work with imidazoles for CO2 capture, we have extended our interests toward understanding the relationship between imidazole structure and SO2 absorption. Using a series of imidazole compounds with various substituents at the 1, 2 and/or 4(5) positions of the five-membered ring, SO2 absorption via both chemical and physical mechanisms was observed. The chemical absorption product is a relatively stable 1:1 SO(2)imidazole complex, while physical absorption of SO2 is dependent on pressure and temperature. Because imidazoles are relatively small molecules, they are an efficient absorption medium for SO2 and can form adducts wherein the mass fraction of bound SO2 is >40 wt %. The SO(2)imidazole complexes were also observed to produce distinct color and/or phase changes that are associated with the nature of the substituents present. The chemically bound SO2 can be released under vacuum at moderate temperature (similar to 100 degrees C) and vacuum, yielding the original neat solvent, while the physically dissolved SO2 can be readily removed at ambient temperature under vacuum. This behavior corresponds to a much smaller enthalpy of absorption for physical dissolution (-4 to -13 kJ/mol) as determined via thermodynamic relationships compared to the binding energies of chemical complexation (-35 to -42 kJ/mol) as determined via density functional theory calculations. Increasing chemical complexation energies are correlated with increased substitution on the imidazole ring. Simulations were also employed to gain insight into the structures of the SO2-imidazole complexes, illustrating changes in partial charge distribution before and after complexation as well as confirming a charge transfer complex is formed based on the NS bond length.