Carbon Nanotubes Modified by Venturello Complex as Highly Efficient Catalysts for Alkene and Thioethers Oxidation With Hydrogen Peroxide

In this work, we elaborated heterogeneous catalysts on the basis of the Venturello complex [PO(4){WO(O(2))(2)}(4)](3−) (PW(4)) 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 PW(4)/CNTs and PW(4)/N-CNTs (1.8 at. % N) containing 5–15 wt. % of PW(4) and differing in acidity have been prepared and characterized by elemental analysis, N(2) adsorption, IR spectroscopy, HR-TEM, and HAADF-STEM. Studies by STEM in HAADF mode revealed a quasi-molecular dispersion of PW(4) on the surface of CNTs. The addition of acid during the immobilization is not obligatory to ensure site isolation and strong binding of PW(4) on the surface of CNTs, but it allows one to increase the PW(4) loading and affects both catalytic activity and product selectivity. Catalytic performance of the supported PW(4) catalysts was evaluated in H(2)O(2)-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 PW(4) immobilized on N-free CNTs in acetonitrile or dimethyl carbonate as solvents. Catalysts PW(4)/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 PW(4). Selectivity, conversion, and turnover frequencies achieved in epoxidations over PW(4)/CNTs catalysts are close to those reported in the literature for homogeneous systems based on PW(4). 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.