Efficient and reversible capture of SO2 by pyridinium-based ionic liquids

A series of thermally stable pyridinium-based ionic liquids (ILs), including [C4Py][BF4], [C6Py][BF4], [C8Py][BF4], [C43MPy][BF4], [C63MPy][BF4], [C83MPy][BF4], [C4Py][SCN] and [C4Py][Tf2N], were firstly applied as new absorbents for SO2 capture. It was found that among the investigated ILs [C4Py][SCN] has the highest absorption capacity of 0.841 gSO2 gIL−1 under ambient conditions, which is much higher than that of the most reported imidazolium-based ILs. The selectivity for SO2/CO2, SO2/N2 and SO2/O2 was also studied and the higher selectivity for SO2 to other gases using the [C4Py][SCN] was achieved. Moreover, how the water content affects the absorption capacity of SO2 was further investigated. The absorption mechanism was studied using FT-IR and NMR spectroscopy, as well as Quantum Chemical calculation and Molecular Dynamic (MD) simulation. It was demonstrated that the physical absorption occurs in pyridinium-based ILs for SO2 capture. Comparing with cation of IL, anion plays a dominant role in SO2 absorption, which was proved both by the interaction enthalpy of IL-SO2 using Quantum Chemical calculation and the experimental results. MD simulation results further confirmed that the higher absorption capacity of SO2 in [C4Py][SCN] is mostly attributed to the stronger electrostatic interaction between the anion and SO2. In addition, the [C4Py][SCN] can still keep the stable absorption performance after five cycles of absorption and desorption, implying the pyridinium-based ILs show great potentials as cost effective and green absorbents for SO2 capture applications.