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

Abstract

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.

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

“…The (then porous) glass particles act as building blocks of the resulting hierarchical monolith. Techniques of salt/filler sintering, foam replication, draw-down sintering of borosilicate glass tubes, freeze-casting, and ionotopic gelation can be used to prepare the macroporous preform [87]. However, the textural parameters of the macropore network in these monoliths (macropore size, skeleton thickness) do not address the range to which particular attention is payed here.…”

“…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.…”

“…Starting materials with monomodal pore structure and surface areas < 50 m 2 g -1 can be transformed into hierarchical silica monoliths with specific surface areas 1000 m 2 g -1 . Macropore diameters > 0.15 mm at a glass pore volume of at least 1 cm 3 g -1 are required for complete transformation [87]. The partial pseudomorphic transformation of the walls in macroporous glass monoliths into MCM-41 or MCM-48 type phases (Fig.…”

“…14), but the technology principally allows to prepare monoliths with sub-micrometer macropore size and skeleton thickness (Fig. 15) [87,90]. Pore size in the silica skeleton can be controlled in the meso-and macropore range (2 -200 nm).…”

High-performance monoliths in heterogeneous catalysis with single-phase liquid flow

“…17 Their hierarchical architecture is realized with a continuous block of silica perforated by intersecting networks of larger and smaller pores. 61,62 Macropores (>50 nm) allow fast, advection-dominated transport through the material; micro-and mesopores (<2 nm and 2-50 nm, respectively), accessible only by diffusion, generate a large surface area for adsorption and reaction. The silica-based monolithic columns used in the current work are prepared with tetramethoxysilane and polyethylene oxide as reactants in a sol-gel process accompanied by phase separation.…”