Abstract
The sol–gel process accompanied by phase separation is one of the methods to prepare hierarchically porous monoliths, hierarchically porous monolith, which is applicable not only to oxides but also to various materials compositions such as metal phosphates, organic-polymers/carbons, metal-organic frameworks. It is not until recently, however, that progress has been made in the preparation of low-valence metal oxide HPMs, such as those of magnesium, manganese, cobalt, nickel, etc. Due to the difficulty of divalent metal precursors to form homogeneous gels, different approaches from those established for trivalent and tetravalent counterparts have been attempted. This short review introduces the methods and trials in the preparation of metal oxide HPMs from divalent metal salts.
Highlights
Sol–Gel ProcessSol–gel process, in general, is a method for producing solid materials with controllable particles sizes at ambient temperature from small molecular precursors in a liquidus medium
Spinodal decomposition is a mode of phase separation distinct from nucleation/growth, in which any small compositional fluctuation grows without an activation barrier
When the spinodal decomposition is combined with a sol–gel transition, a hierarchically porous monolith (HPM) can be obtained, in which larger pores are formed by phase separation and smaller pores by network formation via sol–gel process
Summary
Sol–gel process, in general, is a method for producing solid materials with controllable particles sizes at ambient temperature from small molecular precursors in a liquidus medium. Upon hydrolysis or any other activation, the precursor molecules polymerize, forming chemical bonds with each other to result dimers, trimers, oligomers, and polymers. When such polymerized species (particle size: d 1–1,000 nm) are dissolved/dispersed homogeneously in the system, the system is termed sol. Since the isoelectric point (IEP) of silica/siloxane spices lies pH 2,3, the reaction is regarded to take place under acid-catalyzed conditions when pH < 2; and under basecatalyzed conditions when pH > 3. Water- or alcohol-producing condensation is catalyzed by acid so that the gelation time shortens as pH decreases below pH 2. The nucleophilic attack silicon atom governs both hydrolysis and condensation
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