A colloidal route for delamination of layered solids: Novel porous-clay nanocomposites

Abstract

Under soft conditions, it is possible to cause the irreversible delamination of organoclays (long-chain alkylammonium cation-exchanged smectites, and vermiculite-layered silicate derivatives) via a sol-gel process that involves alkoxysilanes (e.g., tetraethoxysilane) and that finally gives silica-clay heteromaterials. These intermediate silica-organoclay nanocomposites facilitate the diffusion of the alkoxides which, in the presence of water, are hydrolyzed and subsequently polymerized. This process is a heterocoagulation that gives homogeneous gels in which the order in the layer stacking of clays is partially or completely lost, depending on the nature of the layered silicate. After calcination to eliminate the organic moiety, that is, the alkylammonium chains, the gel is irreversibly transformed into a silica-clay material in which the silicate layers are fully separated by the silica network generated by the alkoxide. The resulting solids are inorganic-inorganic nanocomposites which could be compared to polymer-clay nanocomposites, but in the present case the inorganic silica network is the continuous phase and the individual layers the corresponding disperse phase of the nanocomposite. These materials are solids of high specific surface area (> 400 m2 g-1), which exhibit micro- and mesoporosity, and also have properties inherent to both components, the pristine clay (e.g., a cation-exchange capacity) and the silica network (e.g., an ability to be functionalized).