Highly efficient supported catalysts based on Zr-containing polyoxometalates and multiwalled carbon nanotubes for selective oxidation of thioethers with H2O2†

Abstract

Zirconium-containing polyoxometalates (Zr–POM) with Lindqvist, Keggin, and Wells–Dawson structures have been irreversibly attached to the surface of nitrogen-free and nitrogen-doped multi-walled carbon nanotubes (CNT and N-CNT) using H⁺ or Zn²⁺ ions as immobilization agents. The supported catalysts were characterized using low temperature N₂ adsorption, thermogravimetric analysis, FT-IR spectroscopy, XPS, HRTEM and HAADF-STEM. Catalytic activity was assessed for the selective oxidation of thioethers to sulfoxides using methyl phenyl sulfide as model substrate and aqueous 30% H₂O₂ as green oxidant, and the relationships between the catalyst composition and its activity and selectivity have been established. Among the catalysts tested, Zr–POM of the Keggin structure [{PW₁₁O₃₉Zr(μ-OH)}₂]⁸⁻ (PW₁₁Zr) immobilized on N-CNT (1.8 at% N) using Zn²⁺ (18 wt% PW₁₁Zr/Zn–N-CNT) exhibited superior performance in terms of turnover frequency (TOF 10 200 h⁻¹ in acetonitrile at room temperature) and H₂O₂ utilization efficiency (90%). Sulfoxide selectivity depends on the solvent nature and varies from 88% (acetonitrile) to 98% (ethanol) at 80–83% substrate conversion attained with only one equiv. of H₂O₂ and 0.1 mol% of PW₁₁Zr. The reaction can proceed successfully even with 0.01 mol% of the catalyst, achieving turnover number of 9360 with retention of the Zr–POM structure. Hot filtration tests confirmed the heterogeneous nature of the catalysis. The catalyst showed excellent reusability over, at least, five cycles without degradation and was resistant to Zr–POM and Zn leaching, even when using the highly polar protic solvent ethanol. The catalyst 18 wt% PW₁₁Zr/Zn–N-CNT was able to accomplish sulfoxidation of the practically important omeprazole sulfide with 98% selectivity at 95% conversion.