This review deals with "classical" porous glasses which are prepared by physical phase separation of alkali borosilicate glasses of suitable composition in combination with selective leaching. The resulting materials are characterized by a controllable pore size in the nanometer range, high mechanical, thermal and chemical stability and an adjustable macroscopic shape, which enables manufacturing of glass monoliths with various geometries. As a result of their formation, porous glasses obtained from physical phase separation exhibit a monomodal pore structure. There are only a few examples in the literature for the synthesis of hierarchically porous glasses. This review covers several synthesis strategies for the introduction of hierarchy into "classical" porous glass monoliths, including sintering and fusion of alkali borosilicate initial glasses as well as partial or complete pseudomorphic transformation of porous glasses into zeolites or ordered mesoporous materials.
The thermal phase separation and subsequent leaching of sodium borosilicate glasses is a well established route for the preparation of porous glasses exhibiting adjustable pore sizes in the range of 1 nm up to almost 1 mm as well as a very flexible geometric shape. The combination of this route with a large spectrum of synthesis strategies for the implementation of an additional pore system enables the preparation of hierarchically porous glass-based materials. This review covers a wide range of preparative routes for hierarchically porous silica materials starting from the sodium borosilicate glass with a special emphasis on the very recent developments in this versatile field of materials engineering.
Hierarchically structured, porous materials have been successfully prepared via a combination of sintering and phase separation of a sodium borosilicate glass. The materials were characterized using N2‐adsorption, Hg‐intrusion, and scanning electron microscopy. Some hardness measurements were performed to investigate the mechanical stability. Secondary pore sizes were adjusted by the use of different grain fractions of the filler, different primary pore sizes could be obtained by a variation of the annealing conditions. Thus, both pore systems can be adjusted independently. The resulting monoliths consist of a system of secondary pores between 20 and 150 μm and primary pores within the walls of the open‐pored sintered material between 1 and 70 nm. The hierarchical porous materials exhibit surface areas up to 420 m2/g and total porosities up to 74%.
The following study discusses the synthesis of macroporous glass beads, featuring variable pore sizes, and their application as starting material for a double templating route according to the nanocasting principle. In the first step, the initial porous glass was filled with the carbon precursor, a mesophase pitch, which after the subsequent carbonization and dissolution of the glass matrix results in an inverse macroporous carbon replica. Afterwards, the carbon beads were filled with amorphous silica by a typical sol‐gel‐process. The next step can be divided into two phases. The silica gel phase was first “structured” inside the macropores of the inverse carbon replica by converting it into an ordered mesoporous phase via pseudomorphic transformation. This process demanded alkaline conditions and a surfactant, that finally converts the silica into hierarchically structured beads with the dimension and the pores of the initial glass and MCM‐41 pores inside the walls. The final step comprised the template removal via calcination. The obtained materials were characterized by mercury intrusion, nitrogen adsorption, scanning electron microscopy, x‐ray powder diffraction and particle size analysis. In comparison to the previously reported approaches, the new method allows a higher flexibility in the texture properties of the resulting hierarchically structured materials including a variable ratio between ordered mesopores and additional macropores by parallel control of the total porosity and wall thickness in the starting porous glass.
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