5 research outputs found
Mesopore etching under supercritical conditions â A shortcut to hierarchically porous silica monoliths
Hierarchically porous silica monoliths are obtained in the two-step Nakanishi process, where formation of a macro microporous silica gel is followed by widening micropores to mesopores through surface etching. The latter step is carried out through hydrothermal treatment of the gel in alkaline solution and necessitates a lengthy solvent exchange of the aqueous pore fluid before the ripened gel can be dried and calcined into a mechanically stable macro mesoporous monolith. We show that using an ethanol water (95.6/4.4, v/v) azeotrope as supercritical fluid for mesopore etching eliminates the solvent exchange, ripening, and drying steps of the classic route and delivers silica monoliths that can withstand fast heating rates for calcination. The proposed shortcut decreases the overall preparation time from ca. one week to ca. one day. Porosity data show that the alkaline conditions for mesopore etching are crucial to obtain crack-free samples with a narrow mesopore size distribution. Physical reconstruction of selected samples by confocal laser scanning microscopy and subsequent morphological analysis confirms that monoliths prepared via the proposed shortcut possess the high homogeneity of silica skeleton and macropore space that is desirable in adsorbents for flow-through applications
Morphological Analysis of Disordered MacroporousâMesoporous Solids Based on Physical Reconstruction by Nanoscale Tomography
Solids with a hierarchically structured,
disordered pore space,
such as macroporousâmesoporous silica monoliths, are used as
fixed beds in separation and catalysis. Targeted optimization of their
functional properties requires a knowledge of the relation among their
synthesis, morphology, and mass transport properties. However, an
accurate and comprehensive morphological description has not been
available for macroporousâmesoporous silica monoliths. Here
we offer a solution to this problem based on the physical reconstruction
of the hierarchically structured pore space by nanoscale tomography.
Relying exclusively on image analysis, we deliver a concise, accurate,
and model-free description of the void volume distribution and pore
coordination inside the silica monolith. Structural features are connected
to key transport properties (effective diffusion, hydrodynamic dispersion)
of macropore and mesopore space. The presented approach is applicable
to other fixed-bed formats of disordered macroporousâmesoporous
solids, such as packings of mesoporous particles and organic-polymer
monoliths