Highly Selective Epoxidation of Cycloaliphatic Alkenes with Aqueous Hydrogen Peroxide Catalyzed by [PO4{WO(O2)2}4]3−/Imidazole

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

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 [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.