Carbon Nanotubes Modified by Venturello Complex as Highly Efficient Catalysts for Alkene and Thioethers Oxidation...

In this work, we elaborated heterogeneous catalysts on the basis of the Venturello complex [PO₄{WO(O₂)₂}₄]^3- (PW₄) and nitrogen-free or nitrogen-doped carbon nanotubes (CNTs or N-CNTs) for epoxidation of alkenes and sulfoxidation of thioethers with aqueous hydrogen peroxide. Catalysts PW₄/CNTs and PW₄/N-CNTs (1.8 at. % N) containing 5-15 wt. % of PW₄ and differing in acidity have been prepared and characterized by elemental analysis, N₂ adsorption, IR spectroscopy, HR-TEM, and HAADF-STEM. Studies by STEM in HAADF mode revealed a quasi-molecular dispersion of PW₄ on the surface of CNTs. The addition of acid during the immobilization is not obligatory to ensure site isolation and strong binding of PW₄ on the surface of CNTs, but it allows one to increase the PW₄ loading and affects both catalytic activity and product selectivity. Catalytic performance of the supported PW₄ catalysts was evaluated in H₂O₂-based oxidation of two model substrates, cyclooctene and methyl phenyl sulfide, under mild conditions (25-50°C). The best results in terms of activity and selectivity were obtained using PW₄ immobilized on N-free CNTs in acetonitrile or dimethyl carbonate as solvents. Catalysts PW₄/CNTs can be applied for selective oxidation of a wide range of alkenes and thioethers provided a balance between activity and selectivity of the catalyst is tuned by a careful control of the amount of acid added during the immobilization of PW₄. Selectivity, conversion, and turnover frequencies achieved in epoxidations over PW₄/CNTs catalysts are close to those reported in the literature for homogeneous systems based on PW₄. IR spectroscopy confirmed the retention of the Venturello structure after use in the catalytic reactions. The elaborated catalysts are stable to metal leaching, show a truly heterogeneous nature of the catalysis, can be easily recovered by filtration, regenerated by washing and evacuation, and then reused several times without loss of the catalytic performance.