Reduction of superintegrable systems: the anisotropic harmonic oscillator

We introduce a new 2N--parametric family of maximally superintegrable systems in N dimensions, obtained as a reduction of an anisotropic harmonic oscillator in a 2N--dimensional configuration space. These systems possess closed bounded orbits and integrals of motion which are polynomial in the momenta. They generalize known examples of superintegrable models in the Euclidean plane.Comment: 6 pages. Version accepted in Physical Review

Selective Photocatalytic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxaldehyde by Polymeric C3N4-H2O2 Adduct

Selective Photocatalytic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxaldehyde by Polymeric C3N4-H2O2 Adduc

Valorisation of microalga Chlorella sp. into furans in the presence of Nb2O5 catalysts

Despite the interest in niobia-based catalysts and the importance of biomass valorisation, studies on these catalysts typically utilize model substrates like simple sugars. In this study, a series of niobium oxide-based catalysts was prepared for the application in aqueous phase catalytic conversion of sugars extracted from Chlorella sp. microalga into value-added furans. The solid catalysts were firstly characterized by various techniques including X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Raman and X-ray photoelectron (XPS) spectroscopy as well as low-temperature N2 physisorption. Moreover, the acidity of the catalysts was assessed by using the temperature-programmed NH3 desorption (NH3-TPD), by titration of water suspended catalyst with NaOH solution, and by P-bearing molecular probes loaded catalysts through 31P and 1H solid-state nuclear magnetic resonance (NMR) techniques. Herein, we focused on the catalytic transformation of Chlorella sp. and glucose solution as model molecule into furans. The best Nb2O5 catalysts for valorizing Chlorella sp. into furans exhibited a larger number of Brønsted acid sites, achieving conversion yields to 5-HMF and furfural of ca. 20–22 % with respect to the extracted sugars from algae. The results showed a discernible dependence of the conversion yields to 5-HMF and furfural on catalyst acidity, specific surface area, and the presence of the Brønsted acid sites. Conversely, when using the glucose solution as substrate is concerning, the highest yield to 5-HMF was reached by using a catalyst that showed also the presence of Lewis acid sites. A systematic investigation of the structure–activity relationships in niobium oxide application for aqueous phase dehydration using real biomass substrates to obtain furanic derivatives has not been documented thus far. Therefore, the current research is significant as it demonstrates the feasibility of transforming the carbohydrate content in microalgal biomass into furans by identifying the best catalyst to use

Selective photocatalytic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxaldehyde by polymeric carbon nitride-hydrogen peroxide adduct

Polymeric carbon nitride-hydrogen peroxide adduct (PCN-H2O2) has been prepared, thoroughly characterised and its application for selective photocatalytic conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxaldehyde (FDC) in aqueous suspension has been studied. The PCN-H2O2adduct is stable in aqueous suspension under UV and solar irradiation up to 100 \uc2\ub0C. It is also stable up to 200 \uc2\ub0C if heated in air, while at temperatures close to 300 \uc2\ub0C its decomposition takes place. Based on the obtained characterisation data it has been proposed that H2O2attaches to the non-polymerised carbon nitride species and to the heptazine nitrogen atoms, thus producing strong hydrogen bonding within the PCN-H2O2adduct. The blockage of the surface amino-groups in PCN-H2O2by H2O2hinders the interaction of HMF with these sites, which are responsible for unselective substrate conversion. PCN-H2O2, although being less active, possesses a superior selectivity in natural solar light assisted oxidation of HMF to FDC reaching 80% with respect to its thermally etched PCN counterpart, which gives rise to a 40\ue2\u80\u9350% selectivity. We believe that the exceptional performance of the applied photocatalyst in the selective photocatalytic conversion of HMF to a high added value FDC in a green solvent under natural illumination makes a significant contribution to the development of environmentally friendly technologies for biomass valorisation