Effect of Hydrothermal Treatment on Adsorption Properties of Silica Gel

Abstract

It has been proven by coupling near infared spectroscopy to second derivative processing of the spectra that the free and hydrogen bonded silanol groups attached to water molecules absorb at 5314 and 5270 cm−1, respectively. The same method was employed to probe the surface of the hydrothermally treated silica gel samples after pretreatment and evacuation at three different temperatures. The chemical and physical properties of silica gel depend on the nature of the surface silanol groups. Free silanol groups and hydrogen bonded silanol groups play an important role in adsorption of water molecules. Any change or variation in the silanol groups is expected to change the adsorption properties of any silica gel. Two silica gel samples with varying surface characteristics were used in this study. After heating and evacuating at 200, 750, and 1100 °C, the samples were hydrothermally treated in steel bombs and their surface functionalities were probed using near-infrared spectroscopy. The same samples were also dehydroxylated at 450 and 650 °C and analyzed by near-infrared spectroscopy. Furthermore, effectiveness of the treated samples in adsorbing water molecules was followed by monitoring the mass of water adsorbed by silica gel. These investigations form the basis for understanding the relationship between adsorption effeciveness and types of silanol groups on silica gel surface. The results clearly indicate that the hydrothermal treatment of silica gel pretreated at temperatures up to 750 °C increases the concentration of hydrogen bonded silanol groups on the silica gel surface. The sample treated at 1100 °C did not undergo any change under hydrothermal treatment. The concentration of free silanol groups increases in the dehydroxylated samples. The results also show that the adsorption rates of water molecules on the surface decrease significantly after pretreatment of these samples at a particular humidity.