Sorption of tryalkoxysilane in low-cost porous silicates using a supercritical CO2 method

The internal surface of micro and nanoporous substrates can be modified in terms of charge, functionality or even reactivity or stability by means of bifunctional organic molecules able to self-assembly. This work investigates the impregnation with trialkoxysilanes of porous systems using supercritical carbon dioxide (scCO2) as a solvent, which combines several advantages such as liquid-like density and high solvating power with gas-like transport properties. Moreover, scCO2 does not have specific interactions with the substrate and there is no competition for absorption between added reagents and solvent molecules. The work aims at describing a generic liquid-solventless impregnation method applicable to macro (perlite), meso (silica gels and agglomerated nanoparticles) and microporous (zeolite) silica-based substrates using scCO2. The hydrophobic octyltriethoxysilane was used to impregnate the internal surface of the chosen substrates with the objective of obtaining high capacity oil adsorbents. FTIR, TGA and N2 adsorption isotherms were used to determine the interactions between the substrate and the silane deposited monolayer. The acquired hydrophobic character was evaluated using the Karl Fischer method to measure the loss in water adsorption capacity after silane grafting.

Optical pictures showing the modification of the color of silica blue samples from blue (dry) to pink (water saturate) by exposing them to a relative humidity of 64% at 21 °C. Starting from oven-dry samples, the commercial material adsorbed the ambient water and turned pink, while the hydrophobized silane material kept the blue color.Figure optionsDownload as PowerPoint slideHighlights
► Supercritical CO2 used for nanoporous materials silanization eliminated diffusion restrictions.
► Supercritical silanization can be applied efficiently to micro, macro and mesoporous materials.
► Chemisorption is the dominant process found in mesoporous substrates.
► Hydrophobic disordered and cheap mesoporous materials are obtained.
► Water adsorption properties ate significantly modified after supercritical silanization.