Tailored Porous Materials for Sustainable Catalysis

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.

The aim of this work is focused on the study of a series of non-traditional catalytic nanomaterials to transform greenhouse CO₂ gas into added-value products. We found encouraging results of CO₂ hydrogenation activity over sodium titanates with different morphologies. The yield to methanol increases with the increase in the Na incorporated in the nanostructures confirming the proposed mechanism. Samples were prepared at different times of hydrothermal treatment (HTT) with NaOH solutions, and these materials were labeled as Ti-nR-x with x as the hours on the HTT. HRTEM initially showed sphere-shaped nanoparticles in the TiO₂ commercial nanopowder, increasing the HTT resulted in morphological changes in which the structures passed from nanosheets and finally to nanorods after 30 h. The X-ray diffraction and Raman spectroscopy results indicated the formation of sodium titanates such as Na₂Ti₃O₇ with short Ti-O bonds and that Na begins to be incorporated into the distorted TiO₆ crystalline structure after 5 h of HTT (until 12 wt%). The crystalline and shape transformation resulted in a significant modification on the textural properties passing from 51 m².g⁻¹ to 150 m².g⁻¹ and from a pore volume of 0.12 cm³.g⁻¹ to 1.03 cm³.g⁻¹ for Ti-ref and Ti-nR-30 respectively. We also observed differences in the electronic properties as the bandgap presented a blue shift from 3.16 eV on the TiO₂ reference nano-powder to 3.44 eV for the Ti-nR-30 calcined sample. This fact coincides with the presence of a more electron-rich state of the Ti⁴⁺ and the formation of negative charge layer induced by the presence of Na⁺ interlayer cations detected by XPS analysis, at the same this helped us to explain the catalytic activity results.