Visualizing SO2 gas via selective adsorption–desorption by an α,β-unsaturated iminium salt

Abstract

Aerial SO₂ has been identified as one of the most harmful air pollutants, and its atmospheric levels are rising. We herein propose a facile route for visualizing gaseous SO₂ through reversible adsorption–desorption by a functional material derived from an α,β-unsaturated iminium salt (HLI). Upon surface fabrication of a mesoporous organic silica (MOS) with a hemicyanine derivative, a robust silica nanoparticle, LI@MOS, has been developed as a low-to-no-cost material that can efficiently separate SO₂ gas up to 20 mmol g⁻¹ at room temperature over other gases. The material turns colorless from pink as a result of SO₂ adsorption. The reusability of LI@MOS has been tested and the material can be recovered through aerial exposure or gentle heating. A [4 + 1] cycloaddition reaction between the α,β-unsaturated iminium moiety of LI@MOS and SO₂ has been recognized, resulting in a five-membered N-substituted fused bicyclic sulfonamide, LI@MOS-SO₂, commonly known as a sultam. A concordance between the experimental and simulated UV-vis spectra confirmed the formation of cyclic sulfonamide. Bader's quantum theory of atoms-in-molecules describes the binding features of LI@MOS and SO₂. Because of its durability, ease of recycling, and effective separation capability, LI@MOS appears to be a promising material for SO₂ capture and its recovery from flue gas.