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 (TCPP^4-) 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.

Fig. 1. Infrared spectra of H₄TCPP, Al₁₃–SO₄ and Al₁₃–TCPP (left). Images of H₄TCPP (a) and Al₁₃–TCPP (b) samples (right).

Fig. 2. UV absorption spectra of degradation of CEES by Al₁₃–TCPP at different reaction times (a) and corresponding degradation rate (b). Degradation conditions: 500 W xenon lamp (λ > 400 nm), reaction time 180 min, solvent: 5 mL methanol, catalyst dosage: 5 mg Al₁₃–TCPP, 5 μL CEES.

Fig. 3. The UV absorbance (a) and comparison of the performance of Al₁₃–TCPP, H₄TCPP and Al₁₃–SO₄ in the degradation of CEES in methanol (b). Degradation conditions: xenon lamp (λ > 400 nm) at 500 W, reaction time 180 min, solvent: 5 mL methanol, catalyst dosage: Al₁₃–TCPP (5 mg, 1.36 μmol), H₄TCPP (1.2 mg, 1.36 μmol) and Al₁₃–SO₄ (4.3 mg, 3.62 μmol), 5 μL CEES.

Fig. 4. The UV absorbance (a) and degradation rate (b) of CEES by Al₁₃–TCPP at different reaction times. Degradation conditions: 500 W xenon lamp (λ > 400 nm), reaction time 90 min, reaction solvent: 5 mL methanol and water–methanol solvent mixture (v/v = 1 : 1), catalyst dosage: 5 mg Al₁₃–TCPP, 5 μL CEES.

Fig. 5. The UV absorbance (a) and comparison of the performance (b) of Al₁₃–TCPP, H₄TCPP and Al₁₃–SO₄ in the degradation of CEES in methanol–water solvent mixtures. Degradation conditions: 500 W xenon lamp (λ > 400 nm), reaction time 90 min, catalyst dosage: Al₁₃–TCPP (5 mg, 1.36 μmol), H₄TCPP (1.2 mg, 1.36 μmol) and Al₁₃–SO₄ (4.3 mg, 3.62 μmol), 5 μL CEES.

Fig. 6. Kinetic curve of the degradation of CEES by Al₁₃–TCPP in methanol and methanol–water solvent mixture. Degradation conditions: 500 W xenon lamp (λ > 400 nm), reaction time: 180 min in methanol, 90 min in methanol–water solvent mixture, catalyst dosage: 5 mg Al₁₃–TCPP, 5 μL CEES.

Fig. 7. GC spectra of the products after degradation of CEES in the different solvents (methanol (a and b); methanol–water solvent mixture (c and d)) without (a and c) and with (b and d) Al₁₃–TCPP. Degradation conditions: 500 W xenon lamp (λ > 400 nm), reaction time 180 min, solvent: 5 mL methanol, catalyst dosage: 5 mg Al₁₃–TCPP, 5 μL CEES.

Fig. 8. UV-vis absorption changes of DPA in the presence of Blank (a), H4TCPP (b) and Al₁₃–TCPP (c) under light irradiation with a 500 W xenon lamp (λ > 400 nm), and the corresponding linear relationship of the absorbance of DPA at the wavelength of λ_max = 371 nm with reaction time (d).

Fig. 9. UV-vis absorption changes of NBT in the presence of Blank (a), H₄TCPP (b) and Al₁₃–TCPP (c) under light irradiation with a 500 W xenon lamp (λ > 400 nm), and the corresponding linear relationship of the absorbance of NBT at the wavelength of λ_max = 371 nm with reaction time (d).