Abstract
Silica monoliths featuring either mesopores or flow-through macropores and mesopores in their skeleton are prepared by combining spinodal phase separation and sol-gel condensation. The macroporous network is first generated by phase separation in acidic medium in the presence of polyethyleneoxides while mesoporosity is engineered in a second step in alkaline medium, possibly in the presence of alkylammonium cations as surfactants. The mesoporous monoliths, also referred as aerogels, are obtained in the presence of alkylpolyethylene oxides in acidic medium without the use of supercritical drying. The impact of the experimental conditions on pore architecture of the monoliths regarding the shape, the ordering, the size and the connectivity of the mesopores is comprehensively discussed based on a critical appraisal of the different models used for textural analysis.
Highlights
Porous silica monoliths with controlled pore sizes and high surface area are of particular interest for process intensification of numerous continuous flow applications in catalysis, adsorption or separation [1] and for applications requiring self-standing bodies [2,3] featuring adjustable and controlled pore sizes such as Li-ion batteries [4] or super thermal insulators [5,6]
Silica monoliths of 6 mm diameter and 10 cm length exhibiting a hierarchical network of macroand mesopores have been synthesized by chemical spinodal decomposition using polyethylene oxides (PEO) of 10 to 100 kDa in acidic (HNO3 ) aqueous medium in the presence of tetraethylorthosilicate as silica source
For silica monoliths with different mesopores diameters, the best fit between capillary condensation pressures and geometrical calculations have been obtained for 6V/S* revealing that the mesopores in silica monolith are of spherical shape with cavities sizes from 4 to 26 nm
Summary
Porous silica monoliths with controlled pore sizes and high surface area are of particular interest for process intensification of numerous continuous flow applications in catalysis, adsorption or separation [1] and for applications requiring self-standing bodies [2,3] featuring adjustable and controlled pore sizes such as Li-ion batteries [4] or super thermal insulators [5,6]. A monolith of these two coexisting homogeneous continuous phases is formed, the borate-rich phase is removed by chemical leaching by mineral acids, water or alcohols at 100 ̋ C leading to a porous silica with a homogeneous interconnected pore network in the mesoporous range from 2 to 1000 nm depending on the synthesis conditions. Chemical spinodal decomposition is a mechanism for the rapid unmixing of a homogeneous mixture of hydrated silica species/polymer/water from a single phase to form two coexisting phases. 40 ̋ C, silica species start condensate into silica oligomers or clusters and polymers start interacting with the newly formed silica surface This is the starting point of the spinodal decomposition leading to phase separation into a silica/polymer-rich phase and a water-rich phase. The relevance of macropore and mesopore architecture networks in controlling pressure drop and mass transfer, respectively, in continuous flow applications will be highlighted
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