Glycol-Modified Silanes in the Synthesis of Mesoscopically Organized Silica Monoliths with Hierarchical Porosity

Abstract

Silica monoliths exhibiting a unique hierarchical network structure with a bimodal pore size distribution and high surface areas were prepared from three different glycol-modified silanes by sol−gel processing. Tetrakis(2-hydroxyethyl)-, tetrakis(2-hydroxypropyl)-, and tetrakis(2,3-dihydroxypropyl)orthosilicate were obtained by transesterification reaction from tetraethylorthosilicate and the corresponding alcohols. The present work shows that, for ethylene glycol- and propane-1,2-diol-modified silanes, simply the release of the corresponding diols during sol−gel processing in the presence of block copolymeric surfactants such as Pluronic P123 results in phase separation on different levels. In addition to an extraordinary cellular network structure with interconnected macropores of several hundreds of nanometers in diameter, the material exhibits a well-ordered mesostructure with periodically arranged mesopores of about 6−7 nm in diameter. Interestingly, the application of glycerol-modified silanes at the given synthesis conditions results in the formation of a disordered silica mesostructure. The architectural properties and the morphology of the gel network cannot only be controlled by the choice of the glycol but also by the amount of acid catalyst in the starting composition.

Relationship Between Pore Structure and Sorption-Induced Deformation in Hierarchical Silica-Based Monoliths

“…The vertical dashed lines indicate the calculated peak positions for a 2D hexagonal mesopore lattice with a lattice parameter of = 13.05 nm, confirming the hexagonal pore lattice expected for this type of materials. The value of is considerably larger than the value derived for similar samples, which were calcined (typically less than 12 nm, see [31]). This confirms the above statement that calcination leads to more shrinkage and consequently smaller mesopore size.…”

“…The monolithic silica-based sample with hierarchical pore structure was synthesized by the method reported in detail in [30,31]. In brief, the block copolymer Pluronic ® P123 (molar mass = 5800 g/mol) and trimethylchlorosilane were purchased from Sigma-Aldrich and used without further purification.…”

“…Ethylene glycol was dried over anhydrous sodium sulfate and distilled from magnesium. The synthesis of tetrakis(2-hydroxyethoxy)silane (EGMS) was performed according to Mehrotra and Narain with some modifications in the transesterification reaction [31,35]. The block copolymer P123 was dissolved in hydrochloric acid in a ratio of P123/(1 molar HCl) = 30/70 wt %.…”

“…From Figure 1c the pore to pore distance is roughly estimated to be between 10 and 15 nm. TEM on similar samples [31] revealed that the struts in these monolithic samples indeed comprise cylindrical mesopores on a 2D hexagonal lattice, with a pore to pore distance of roughly 11-13 nm, depending on the details of sample preparation. The bulk density of the sample and the density of the solid phase are given in Table 1.…”

“…SBA-15 and comparable materials, were only available as powders or thin films and therefore not accessible to macroscopic investigations. More recently, the synthesis of monolithic materials with hierarchical porosity has been reported [30][31][32][33]. These silica based materials consist of a macroporous network of struts, each strut containing cylindrical mesopores on a 2D hexagonal lattice with similar pore characteristics as SBA-15.…”