Formation of the open-cell foam structures in tetraethoxysilane-based gelling systems.

The phase separation process in silica-based gelling systems in the presence of a polymer (polyethylene oxide, PEO) is shown to result in the formation of porous materials with the open-cell foam structure. The PEO concentration range in the reaction mixture where these foam-like structures are observed is determined. It is shown that phase separation begins with the formation of discrete droplets of solvent in the gel phase similar to the phase separation in some polymerizing organic systems. Further, liquid droplets grow and coalesce that leads to the formation of a system of interconnected macropores with a shape close to the spherical one. The macropore sizes are shown to depend on the PEO content, but no significant difference in the mesoporosity and surface area of the monoliths is observed. Dried and calcined porous silica monoliths are obtained with macropore sizes from 0.8 to 41 μm. The maximal compressive strength is 2.2 MPa, porosity is 85%, permeability coefficient is 3.5·10⁻¹² m², and the specific surface area is 210 m² g⁻¹. The materials can be used as supports in flow-through catalytic systems. Highlights: Macroporous open-cell foams are formed in silica-based gelling systems. In the presence of PEO, phase separation in silica gel occurs as in organic polymers. PEO macromolecules are connected with the silica oligomers before and after phase separation. The macropore sizes are determined by the polymer content in the reaction mixture. [ABSTRACT FROM AUTHOR]