Enhancing the quantum yield of singlet oxygen: photocatalytic degradation of mustard gas simulant 2-chloroethyl ethyl sulfide catalyzed by a hybrid of polyhydroxyl aluminum cations and porphyrin anions

Abstract

By combining the anionic salt meso-tetra(4-carboxyphenyl)porphyrin (TCPP⁴⁻) and the Keggin polyoxometalate cation cluster [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ via a simple ion-exchange method, a hybrid (C₄₈H₂₆N₄O₈)[Al₁₃O₄(OH)₂₄(H₂O)₁₂]₂(OH)₁₀·18H₂O (Al₁₃–TCPP) was prepared and thoroughly characterized as a prototype of polyoxometalate–porphyrin hybrids for the photocatalytic degradation of the mustard gas simulant 2-chloroethyl ethyl sulfide (CEES). The experimental results showed that the catalytic degradation rate of CEES in the presence of Al₁₃–TCPP reached 96.16 and 99.01% in 180 and 90 min in methanol and methanol–water solvent mixture (v/v = 1 : 1), respectively. The reaction followed first-order reaction kinetics, and the half-life and kinetic constant in methanol and solvent mixture were 39.8 min, −0.017 min⁻¹ and 14.7 min, −0.047 min⁻¹. Mechanism analysis indicated that under visible light irradiation in air, CEES was degraded through a combination of oxidation and alcoholysis/hydrolysis in methanol and the methanol–water solvent mixture. The superoxide radical (O₂˙⁻) and singlet molecular oxygen (¹O₂) generated by Al₁₃–TCPP selectively oxidized CEES into a non-toxic sulfoxide. The singlet oxygen capture experiments showed that Al₁₃–TCPP (Φ = 0.236) had a higher quantum yield of singlet oxygen generation than H₄TCPP (Φ = 0.135) under visible light irradiation in air. The material Al₁₃–TCPP has good reusability, and the degradation rate of CEES can still reach 98.37% after being recycled five times.