Controlling the morphology of mesostructured silicas by pseudomorphic transformation: A route towards applications

Micelle-templated silicas (MTSs) such as MCM-41 and MCM-48 feature unique textural properties owing to their uniform distribution of mesopores with tunable sizes. MTS synthesis is relevant to unique self-assembly processes between surfactants and inorganic matter. The properties of MTSs have been explored in view of applications in fields as diverse as catalysis, chromatography, sensing, photonics, optics, drug delivery, etc. The aim of this contribution is to review, and to highlight by new results, a synthesis strategy we have developed since 2002 to control the particle morphology of MTSs at the micro- to millimeter scale, a key step for transferring these materials from the status of beautiful artworks to applicable products. It is based on the concept of pseudomorphic synthesis. Pseudomorphism is well known in the mineral world. It allows Preparation of a mineral with a morphology that is not related to its crystallographic symmetry group. The resulting mineral assumes the outward crystal habit of a different mineral. This principle occurs at a nonconstant matter content by using a mineralization solution that exchanges anions (or cations) with an existing (Preshaped) solid body, and allows the new structure to precipitate while maintaining the existing morphology. The concept of pseudomorphic transformation is now applied to amorphous preshaped silica particles to produce MTSs with the same morphology, using an alkaline solution to dissolve the silica and reprecipitate it around surfactant micelles into the ordered MTS structures. MTSs with hexagonal and cubic symmetry, different pore sizes, and controlled morphology have been synthesized. The new pseudomorphs have been successfully used as supports in chromatography, a very demanding application in terms of particle size and morphology.