Síntese, caracterização e avaliação de compostos de Nióbio como catalisadores para o tratamento de águas e reaproveitamento do glicerol

Abstract

The largest amount of Niobium reserves of the planet is available at Brazil, which is estimated at approximately 98%. Despite that, the interest of the government in exploiting Niobium is limited, what caused a reduction of the awareness of new environmental and technological applications of niobium-based compounds. Thus, studies focusing on niobium compounds are necessary. Therefore, this research aimed to evaluate the applicability potential of niobium compounds in water treatment and recovery of co-product from biodiesel production (glycerol). The results obtained were divided into two different chapters in the format of an article. Regarding water treatment, the use of niobium oxyhydroxide (NbO2OH) was applied as a catalytic agent in the discoloration of methyl orange using solar photocatalysis. The NbO2OH was characterized by X-ray diffractometer (XRD), N2 adsorption and desorption, Fouriertransform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM), ultravioletvisible diffuse reflectance spectroscopy (UV DRS) and point of zero charge (pzc). The characterizations showed that NbO2OH is a material with amorphous structure, composed of Nb-OH bonds and presents a band-gap of 3.6 eV, which fosters the application of Niobium in photocatalysis. The catalytic efficiency of NbO2OH was assessed in experiments with different concentrations of methyl orange, H2O2, pH and NbO2OH. The best treatment conditions occurred with the use of 15 mg L-1 of methyl orange, 0.1 M of H2O2, pH = 4.0 and 1 g L-1 of NbO2OH, resulting in the complete discoloration of the coloring after 10 minutes of treatment. The NbO2OH was reused and, after five cycles of treatment, the efficiency remained unchanged, which confirms the stability of the niobium compound. Subsequently, Niobium catalysts supported in sodium vermiculite (Nb2O5/V0) and in vermiculite treated with 3 mol L1 of hydrochloric acid (Nb2O5/V3) were prepared using incipient impregnation method. Also, Nb2O5 was synthesized to comparison. All catalysts were characterized by N2 adsorption and desorption, X-ray dispersive spectroscopy (EDS), X-ray diffractometer (XRD), spectroscopy of diffuse reflectance in the visible ultraviolet region (UV DRS) and Raman spectroscopy. The techniques of characterization evidenced the existence of Nb2O5 in the TT crystal phase at the surface of V0 support and in the amorphous phase at the surface of V3 support. The best dispersion of niobium oxide occurred in V3 support (SBET = 464.5 m2 g -1 ) due to its bigger specific area compared to V0 (SBET = 0.7 m2 g -1 ). The glycerol fast pyrolysis using Nb2O5/V0, Nb2O5/V3 and Nb2O5 catalysts was investigated with Py-GC-MS. The results indicated high glycerol decomposition using Nb2O5 (82.2%), presenting higher yield for oxygenated compounds such as acids (25.1%), alcohols (19.8%) and esters (23.7%). In comparison, a higher deoxygenation occurred in the presence of the catalyst Nb2O5/V3 (CO2 = 6.4%), observing the presence of acids (15.9%), alcohols (19.7%) and aldehydes (10.8%), while esters were not identified. The Nb2O5/V0 showed the least influence over all the observed products similarly to the thermal pyrolysis. The distinct behaviors of the three catalysts can be attributed to different levels of acidity and textural properties. The Niobium compounds analyzed are potentially indicated catalysts for the applications addressed in this study, stimulating the development of technologies that use Niobium in Brazil.