Understanding the unique interaction of amine-containing ionic compounds with SO₂ for high absorption capacity

Control of the interaction between sorbents and acid gases (SO₂ and CO₂) is of prime importance for a fundamental understanding of acid-base chemistry to aid in the development of new absorbents. However, delicated and precise monitoring of the reaction dynamics by conventional one-dimensional (1D) spectroscopy is still challenging due to overlapping bands and restricted static information. Herein, we investigated the unique interactions of an amine-containing ionic compound, [Bztmeda][MeSO₃], and SO₂, which are related to the high absorption capacity and reversibility of the reaction, by principal component analysis (PCA) and two-dimensional infrared correlation spectroscopy (2D IR COS). The physical transition of [Bztmeda][MeSO₃], arising from the hydrogen bonding interactions with SO₂, could not be observed by 1D IR spectra, but was captured by PCA analysis. Synchronous 2D correlation spectra demonstrated that the absorption of SO₂ was derived from the hydrogen bonding interactions between the tertiary amine groups of [Bztmeda] cations and the oxygen atoms of SO₂ and/or between the sulfonate groups of [MeSO ₃] anions (bridged by water molecules) and the oxygen atoms of SO₂. Asynchronous 2D correlation spectra decoupled the new bands of amine and sulfonate groups under the different chemical environments, showing the order of the actual sequence of the SO₂ absorption processes.