SO2 capture by ionic liquid and spectroscopic speciation of sulfur(iv) therein

Abstract

The absorption of equimolar sulfur dioxide (SO₂) by tributyloctylphosphonium bicarbonate ([P₄₄₄₈]HCO₃) resulted in the formation of a corresponding bisulfite ionic liquid ([P₄₄₄₈][bisulfite]) accompanied by carbon dioxide (CO₂) release. The liquid formed absorbed an additional 0.6 equivalents of SO₂. The speciation of sulfur(IV) in the SO₂-loaded ionic liquid was performed using Raman and NMR spectroscopies. The two known isomeric forms of bisulfite ion in aqueous systems were identified while the condensation of bisulfite anion was suppressed in [P₄₄₄₈][bisulfite]. The isomer with the proton bonded to the sulfur atom (HSO₃⁻) was more abundant than the one with the proton bonded to the oxygen atom (HOSO₂⁻). The isomeric exchange rate was much slower in the IL than in water as distinguished by ¹H NMR. When excess SO₂ was absorbed by [P₄₄₄₈][bisulfite], the presence of molecular SO₂ and HS₂O₅⁻ were suggested by Raman bands as an indication of concerted physisorption and chemisorption.