Simple process for preparing mesoporous sol-gel silica adsorbents with high water adsorption capacities

Abstract

In this work we present a simple route to prepare sol-gel silica samples for water adsorption applications. These materials were not chemically modified neither heat-treated before performing the water adsorption tests at 30 °C and atmospheric pressure. The materials studied in this work were examined by N2 adsorption and Fourier transform infrared spectroscopy (FTIR). The samples prepared herein showed higher adsorption capacities than a commercial silica commonly used as water adsorbent. It was shown that it is possible to produce materials with high water adsorption capacities controlling the sol pH. Samples obtained from sols with very low pH presented high number of small pores, whereas those prepared from sols without acid displayed larger pore; both cases increase the volume of pores and, consequently, the water uptake of the material. It is an important finding since several works available in the literature deals with complex and multi-step procedures, commonly using pore-formers or pore-expanders agents, which makes them cost ineffective and time consuming. The absence of acidic reactants in the preparation step of the adsorbent with the highest adsorption capacity represents an advantage in terms of safety and environmental sustainability, besides reinforcing the simplicity of the processing route employed herein. The pseudo-second order kinetic model was the best model to fit the water adsorption kinetics of the investigated samples, indicating a high interaction between the silica surface and water molecules. Besides, the water desorption was successfully carried out in relatively short times (<2 h) and at temperatures as low as 90 °C.