Silica monoliths with hierarchical porosity obtained from porous glasses

Abstract

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.

Sol‐Gel and Porous Glass‐Based Silica Monoliths with Hierarchical Pore Structure for Solid‐Liquid Catalysis

“…5). Thus, several routes for the integration of ''hierarchy'' into porous glass monoliths have been developed during the last five years, summarized in two review articles [86,87]. Materials with macropores > 10 mm can be obtained by the sintering of (nonporous) initial glass particles combined with selective leaching after the phase separation.…”

Synthesis and Textural Characterization of Mesoporous and Meso-/Macroporous Silica Monoliths Obtained by Spinodal Decomposition

“…Physical spinodal decomposition is used for decades to produce porous glasses, well known under the names of CPG or Vycor [7]. Porous glasses are formed by mixing at high temperature (1200˝C) two solids as SiO 2 and B 2 O 3 in the presence of alkaline oxides M 2 O (with M = Na, K or Li) to obtain a single thermodynamic alloy alkali borosilicate phase, which is then rapidly cooled to a temperature at which thermodynamic equilibrium favors a silica-rich phase coexisting with a borate-rich phase.…”

Recent advances in the synthesis of hierarchically porous silica materials on the basis of porous glasses

“…Accordingly, for a successful synthesis of hierarchical pore systems by direct pseudomorphic transformation (see Section 2.2.1), controlling the building block diffusion during the dissolution process is a key factor. If a direct pseudomorphic transformation is not 15 In the case of transformation into zeolites, the focus was usually on obtaining granules, beads or discs of MFI-type and BEA structure types and not on preserving the leaching-based pore system, though there was usually a random intraparticle porosity present, which was accessible from the outside (e.g. during mercury intrusion experiments) and thus formed together with the zeolitic micropores a hierarchical pore system within the obtained zeolitic shapes.…”

“…48 The second step is then the pseudomorphic transformation. 15,41,43 In a typical procedure, 46 carbon negatives (precursor: mesophase pitch 49 ) of the initial porous glass beads are prepared and loaded with a silica gel by the classical sol-gel process. Prior to the carbon exotemplate removal, a pseudomorphic transformation of this pore-protected silica gel into MCM-41 materials is carried out.…”

“…13 Further approaches for the modification of the textural properties of porous glasses, but with a special focus on obtaining hierarchical pore systems, 14 were reviewed by us in 2013. 15 Guillot et al showed in a first study the potential of glasses with hierarchical macro-and mesoporosity for applications as supports in catalysis and solid phase extraction processes. 16 The introduction of an additional MCM-41-like mesopore system into a macroporous glass resulted in an increase in catalytic activity and adsorption capacity after subsequent functionalisation with different silanes.…”