RESUMEN
This study reports a simple, reusable, and recoverable niobium-based heterogeneous catalysts for Biginelli multicomponent reactions. Different methods of catalysts preparation were investigated. For this purpose, HY-340 (Nb2O5·nH2O) and Nb2O5 were chemically and/or thermally treated and investigated as catalysts for dihydropyrimidinones (DHPMs) production. The catalysts were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, temperature-programmed desorption of NH3, adsorption/desorption of N2 at -196 °C, and thermogravimetric and differential thermal analysis. The characterization results showed that niobium oxides have the potential to be used as catalysts because of high crystallinity and large surface area. Among the tested catalysts, Nb2O5 chemically treated (Nb2O5/T) showed the best catalytic performance. In the absence of solvents, 94% yield of DHPMs was achieved. Also, Nb2O5/T can be reused three times without a significant yield decrease. Additionally, a feasible reaction pathway was suggested based on the Knoevenagel mechanism for DHPM synthesis using niobium-based catalysts.
RESUMEN
It is proposed a new approach to evaluate the performance of ultraviolet photoreactions by integrating UV-LED and a UV-Vis cuvette as a mini-reactor for kinetic monitoring in a spectrophotometer not influenced by external light. This system uses only 3.0â¯mL of solutions in a rectangular quartz cuvette with a mini-bar magnetic stirrer in a cell holder and a UV-LED of 5â¯W with λmax at 370â¯nm was positioned on the top of the cuvette and maintained at 25.0 oC. The effectiveness of this photoreactor was demonstrated by measuring the real-time degradation of two model compounds, salicylic acid and methylene blue, in homogeneous and heterogenous systems. Photolysis of MB with H2O2 results in increasing of rate constants as [H2O2] increased. Heterogeneous photocatalysis of MB and SA was fastest achieved in ZnO dosage of 0,20â¯g.L-1. This mini-photoreactor allows monitoring the real-time kinetic performance collecting almost a thousand points in each experiment, leading to accurate rate constants. Moreover, this system presented positive environmental aspects such as: lower reactants and catalyst amounts, lower cost and waste amounts, use of the UV-LED radiation and labor time saving. This is a novel approach to determine the photoreaction effectiveness and it can be applied to systematic studies especially for the kinetic parameter determinations.