Preparação e caracterização de sílicas híbridas com surfactantes catiônicos

Abstract

The silica with structure MCM-41 has a hexagonal mesoporous arrangement with one dimensional pore system. This silica as synthesized contains cations (usually the CTA+) in your pores and catalytic sites with basic character due to the presence of anions siloxy (≡SiO-) associated with these cations. In this work, the synthesis route of MCM 41 silica was modified using a cationic surfactant different from the usual CTABr (cetyltrimethylammonium bromide), variation in the synthesis composition and sources of hydroxyl ion. Among a large number of cationic surfactants, the CADMABr (cetilalildimetilamônio bromide), CPyBr (cetylpyridinium bromide) and CnTABr (alkyltrimethylammonium bromide) were used. The CADMABr differs from CTABr by the substitution of a methyl group with hydrophilic head for an allyl group, the CPyBr has a pyridine group in place of the trimethylamine, while the CnTABr has a change in the number of carbon atoms in the hydrophobic tail. The hybrid silicas were synthesized using different molar ratios of surfactant and sources of hydroxyl ion, keeping the temperature at 30 °C and synthesis time in 2 hours. The surfactants were characterized by electric conductivity, in order to determine the critical micelle concentration (CMC) and their aqueous dispersions at different concentrations by small-angle X-ray scattering (SAXS) to verify the micellar behavior. The hybrid silicas were characterized using the X-ray diffraction (XRD) for phase identification, together with thermogravimetry (TGA), nitrogen physisorption and scanning electron microscopy (SEM). In addition, the X-ray photoelectron spectroscopy (XPS) was measured to verify the basicity of the siloxy sites (≡SiO-) with the modification of the cations in the hybrid silicas synthesis. The catalytic properties of these silicas were evaluated using the transesterification model reaction of monoester and the transesterification reaction of vegetable oil to biodiesel manufacture, both under mild conditions. The results show improvement in catalytic stability with increasing basicity and the molar ratio of Surfactant: SiO2 of the silicas synthesis mixture. In addition, the hybrid silicas obtained by the method of Araújo et al. (2013) modified, with higher molar ratio Surfactant: SiO2, had higher values of IR (%) and smaller particle size.