Visualizing SO2 Gas Via Selective Adsorption/Desorption by an α,β-Unsaturated Iminium Salt

Abstract

The rising levels of SO2 gas emissions have been identified as one of the most harmful air pollutants. 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 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 areal 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 , trivially known as 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 comprehends 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.