Synthesis, characterization and applications of surface-modified and molecularly-imprinted silica aerogels -

Abstract

We report herein the synthesis, characterization and application of surface-functionalized and molecularly-imprinted silica gel materials. Surface-functionalized silica aerogels and alcogels prepared via a two-step sol-gel process through the combination of different silicon precursors were used in the adsorption of methylene blue from aqueous media. The effect on adsorption in batch reactors of the precursors nature, the solvent used in the adsorbents synthesis, and the pH of the dye solution was monitored. Phenyl-functionalized silica materials revealed the highest adsorption capacity. Two phenyl-modified silica aerogels were widely tested in adsorption under various experimental conditions where the effect of pH, temperature, particle size, contact time, initial dye concentration, and adsorbent dose were investigated. The synthesis solvent was found to have a clear effect on the behavior of the adsorbent whose particle size affects crucially its adsorptive capacity. pH 8 and 9 were revealed optimal and the adsorption kinetics follow a pseudo-second order representative of a co-existence of chemisorption and physisorption processes. The adsorption data fits the Sips isotherm and reveals a maximum adsorption capacity for the best aerogel 49.2 ± 0.1 mg of dye per gram of adsorbent. The thermodynamic study revealed the adsorption to be an exothermic and ordered adsorption process. Molecularly-imprinted silica alcogels and aerogels with caffeine and 2-naphthol were used in the specific targeted adsorption of theophylline and PAHs from aqueous media. The effect on adsorption in batch reactors of the precursors nature, template molar ratio, water to silica ratio, light and adsorbate nature was investigated. Phenyl functionalized silica aerogels were revealed to be the best materials, imprinted with 0.075 molar ratio of template. The water to silica ratio was found to play a crucial effect on adsorption, where a ratio that provides a balance between adsorptive capacity and selectivity is closer to be f