Dhara Beatriz de Amorim Pryston
SÍNTESE E CARACTERIZAÇÃO DE ÓXIDOS MISTOS À BASE DE CÉRIO E NIÓBIO PARA APLICAÇÃO NA CONVERSÃO DE BIOMASSA
Resumo
One of the main challenges in biomass conversion is the development of efficient and easily recoverable heterogeneous catalysts. Therefore, in this study, Ce and/or Nb-based oxides were synthesized, varying the niobium content at 5, 15, and 25%, using two synthesis methodologies: Pechini modified with glycerol and metal-chitosan complexation. In order to elucidate the structural, textural, and electronic properties, the catalysts were extensively characterized by mid-infrared absorption spectroscopy (FTIR), nitrogen physisorption analysis (by BET and BJH methods), thermogravimetric analysis (TG/DTG), X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance spectroscopy (DRS), Raman spectroscopy, scanning electron microscopy (SEM), energydispersive X-ray fluorescence spectrometry (EDX), temperature-programmed desorption (TPD-NH3 and CO2), and FTIR with pyridine adsorption. The results obtained by TG revealed good thermal stability up to 1000 °C, and the infrared spectra showed bands corresponding to the different metal-oxygen (M-O) bonds. Raman profiles revealed a progressive decrease in the intensity of the mixed oxide band around 468 cm⁻¹, relative to CeO₂, which can be attributed to the presence of defects due to the addition of Nb. XRD analyses showed that CeO₂ has a fluorite-type structure, while niobium oxide predominantly exhibits the orthorhombic phase. Regarding the mixed oxides, peaks were identified that correspond solely to the fluorite structure of CeO₂, indicating high dispersion or low crystallinity of the Nb-containing species. Rietveld refinement showed that doping caused small changes in lattice parameters induced by the incorporation of niobium. Nitrogen physisorption analyses yielded type IV isothermal profiles with H3- type hysteresisloops for all catalysts, also showing modifications in specific surface areas due to the insertion of niobium, regardless of the synthesis method. SEM revealed that the mixed oxides exhibit a morphology close to an aggregate of non-uniform spheres, tending towards greater homogeneity when compared to pure Ce. UV-Vis DRS analysis showed that the addition of different niobium contents to cerium caused alterations in the material's electronic structure, resulting in a progressive reduction of the band gap energy. EDX analysis allowed for the estimation of the niobium content in the mixed oxides. Finally, NH3 and CO2 TPD showed that the addition of Nb positively modified the acid-base properties, and Py FTIR showed the predominant presence of Brønsted sites in all materials. The catalytic activity was evaluated in the conversion of fructose. Ce (Pec and Qui) exhibited the lowest conversions, however, the mixed oxides (CeNbX) showed progressively higher conversion values, with a maximum of ~80% for CeNb25 (Pec and Qui) at 6 h, suggesting that the incorporation of Nb plays a relevant role in the conversion of fructose. Furthermore, these CeNb systems also showed selectivity to 5-HMF, with a maximum of 80% at 4 h for CeNb25 Pec, and less formation of insoluble materials compared to pure Nb. Reuse and leaching tests indicated the stability of the systems for four cycles, maintaining conversion and selectivity to 5-HMF, reinforcing the robustness of the systems in aqueous solution. Therefore, the results indicate the effectiveness of using renewable materials, glycerol and chitosan, in strategic synthesis routes more aligned with the principles of green chemistry, demonstrating how the addition of niobium promotes improvements in the transformation of biomass into valuable chemical products.