Síntese e caracterização de CeO2, MoO3 e CeO2-MoO3 obtidos via o método Pechini, empregando glicerol como poliálcool, para aplicação na conversão da frutose

Abstract

The development of heterogeneous catalysts, which are efficient and easy to recover, is one of the main obstacles to be overcome to achieve a significant advance (economic and environmental) in biomass conversion technology. Thus, in this work, cerium (Ce100) and molybdenum (Mo100) oxides, as well as a mixed cerium oxide with 25% molybdenum (CeMo25), were synthesized through the Pechini method using glycerol as polyol, for application in the conversion reaction of fructose to high commercial value chemical products. In order to elucidate the structural and textural properties, the catalysts were characterized by infrared absorption spectroscopy (FTIR), nitrogen physisorption analysis at 77 K using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, Thermogravimetric analysis (TG/DTG), X-ray diffraction (XRD), FTIR using pyridine (FTIR-PY), Ultraviolet-visible Diffuse Reflectance Spectroscopy (DRS UV-Vis) and the monitoring of pH as a function of time. The results obtained through TG/DTG indicated good thermal stability for Ce100. In the case of Mo100, sublimation of MoO3 and polymolybdate species was verified. CeMo25 exhibited a mass loss of 19% and may possibly be associated with sublimation of MoO3 and polymolybdate species. The FTIR analyses showed bands corresponding to metal-oxygen bonding. Ce100 exhibited IV-type isotherms with H3-type hysteresis (characteristic of mesoporous materials). For Mo100, due to the low surface area and limitation of the method, it was not possible to draw definite conclusions about the textural characteristics. Furthermore for CeMo25 it was observed that the introduction of the molybdenum species changed the surface area and pore volume. The X-ray diffractograms showed that Ce100 had a face-centered cubic structure fluorite and Mo100 the orthorhombic phase. In the case of CeMo25 no diffractions referring to the individual oxides were identified, only those corresponding to the formation of the cerium molybdate Ce2(MoO4)3. The UV-VIS spectrum of CeMo25 allowed the observation that the addition of molybdenum to cerium caused a change in the electronic structure of the material, promoting a reduction in the band gap energy. pH monitoring over time indicated the presence of both acidic and basic sites on the surface of the catalysts. However, the nature of the acid sites was investigated through FTIR-PY, evidencing the existence of Bronsted and Lewis centers in the oxides, also showing that the addition of molybdenum to Ce100 was an efficient strategy to increase the acidity of the material. Regarding the catalytic activity, the reactions were conducted at 150 °C with time variation from 0.5 to 6 hours. Ce100 exhibited low conversions and high selectivity to 5-HMF. On the other hand, the employment of Mo100 resulted in high conversions but with large formation of insoluble materials. For CeMo25 was also observed high activity in the conversion of fructose, but with less formation of insoluble materials and selectivity to products of the retro-aldolic route (pyruvaldehyde, glyceraldehyde, lactic acid and acetic acid) and 5-HMF. These results showed that the main factor influencing fructose conversion are the acid sites present in the catalysts. Therefore, all the observed characteristics suggest that the synthesized oxides are promising for biomass conversion into valuable chemicals. However, for CeMo25, in addition to high activity, increased thermal stability and an adequate combination of the acid sites could be observed.