Síntese de adsorventes de sílica nano-estruturados por tecnologia sol-gel e tratamento hidrotermal

Abstract

In the present work, silica adsorbents were synthesized by sol-gel technology using Tetraethylorthosilicate (TEOS) as silica precursor, water and three different types of alcoholic solvents (ethanol, isopropanol and tert-butanol), respecting a TEOS:H2O:alcohol molar ratio of 1:4:4 and without using catalyst. The gelation and aging processes, carried out in autoclave, were carried out at different temperatures, ranging from room temperature (approximately 30°C) to 180°C. The synthesis of a 2D/3D set was also investigated, where 3D silica gels were formed on the surface of 2D graphene oxide (GO) sheets. The structures of the gels produced here were characterized by Fourier transform infrared spectroscopy (FTIR), N2 adsorption and desorption, and scanning electron microscopy (SEM). The effect of different synthesis conditions on the properties of the gels and their N2 and water vapor adsorption capacities were compared with the one of a commercial silica in order to establish the best conditions. As a result, it was noticed that with the increase of the autoclave temperature, there was an increase of the pores average diameter, mesoporous specific volume and adsorption capacities, surpassing the commercial silica. This increasing porosity appears to be the result of a greater entrapment of alcohol and water molecules during the formation of the silica network through sol-gel reactions, despite an increasingly important densification within the autoclave. This densification process was also shown to be more significant when using an alcohol with a higher boiling point, drastically reducing the adsorption capacity for the sample synthesized with tert-butanol and gelled at 120ºC. The water vapor adsorption capacities of ground or unground samples were shown to be very close, with the use of unground material being considered preferable due to the ease of operation and advantages in terms of safety and environmental sustainability. Comparing the samples with and without GO, it was observed that there was a strong decrease in the specific volume of pores for the sample with the insertion of graphene oxide associated with the fact that the graphene oxide suspension has an acid pH, causing an expressive reduction in the condensation rate during the formation of the silica network.