Síntese e avaliação de microesferas ocas de sílica para adsorção de vapor de água

Abstract

The water adsorption has important applications, whether in the context of using less harmful energy sources, which is the case of natural gas drying, or facing the water scarcity through water atmospheric harvesting. In both scenarios, the study of alternative materials is essential, since the materials commonly used and which have high water adsorption capacities at low relative pressures (zeolites) need higher temperatures to be regenerated, increasing the process energy consumption. Thus, the goal of this work is to evaluate the application of hollow microspheres of silica synthesized with different ethanol/water (EtOH/H2O) ratios (0.4; 1.0; 2.0 and 6.0) in water adsorption, correlating the water adsorption capacity of the materials with its textural and physical-chemical properties. The samples were characterized by N2 adsorption/desorption isotherms at -196 °C, CO2 adsorption isotherms at 0 °C, scanning electron microscopy (SEM), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis. In addition, the water adsorption capacity of the samples was evaluated by water adsorption isotherms at 25, 40 e 55 °C. In general, increasing the EtOH/H2O ratio resulted in a moderate reduction in the textural properties and contributed to the samples morphological development, favoring the formation of spheres. Moreover, the FTIR indicated that all samples have characteristic bands of basic structural units of silica. Regarding the water adsorption capacity at 25 °C, the samples present capacities comparable to those of commercial adsorbents at high relative pressures, however, at low relative pressures, the zeolites still present higher capacities. On the other hand, water isotherms at 55 °C demonstrated that an increase of 30 °C causes a significant reduction (≈ 80%) in the adsorption capacity, suggesting these samples can be regenerated at milder temperatures. In addition, water adsorption/desorption cycles indicated that the samples can be considered for applications involving cyclic processes, as their capacities were not affected throughout the cycles performed. Finally, the synthesized samples are not recommended for natural gas drying, however, they may be suitable for capturing water from the atmospheric air, mainly considering the linear behavior of the water isotherms at low relative pressures and easy regeneration.