Templated mesoporous materials
Periodic micro- and mesoporous materials have gained world-wide attention for their potential applications in industrial catalysis, electrochemistry, adsorption and separation technology, and so forth. Mesoporous materials are those that have pore diameters between 20 and 500 Å. The discovery that mesoporus materials with a well defined arrangement of pore and a small size distribution can be synthesized using self assembled organic aggregates (like micelles) as template therefore opened new possibilities in materials science. The synthesis mixture contains inorganic precursors (mostly, but not only silica-based), organic template molecules, a solvent and acid (or base) to catalyze the hydrolysis and silica polymerization. The product is a mesostructure with the organic aggregates trapped within the inorganic solid, with structures that highly resemble those of lyotropic liquid crystals. Removal of the organics yields materials with ordered arrays of pores of hexagonal or cubic symmetries, and disordered silica walls. Recently this method has been used to obtain mesoporous carbon.
Our goal is to resolve the formation mechanism(s) of templated mesoporous materials on both the molecular and mesoscale levels, and most importantly their correlation. Such understanding will help resolve the relation between the reaction conditions and the properties of the final material, and therefore lead to a more efficient, rational design of mesoporous materials.
Using a variety of spin probes we follow the evolution of the reaction in-situ. The spin probes are either incorporated into the organic molecules assemblies, or copolymerize with the silica or wall. The EPR techniques we use are continuous wave EPR, which is highly sensitive to the molecular motion and the polarity of the region of the spin probe. In addition we used freeze quench pulse EPR techniques such as ESEEM (electron-spin echo envelope modulation) techniques to examine compositional changes during the reaction and DEER (double electron-electron resonance) to follow changes in aggregate shapes and aggregate separation. This molecular level information is then correlated with the meso- scale events, namely the evolution of the micellar structure in solution obtained by cryogenic transmission electron microscopy (cryo-TEM), which is an excellent tool for direct imaging of liquid or semi-liquid specimens. This is done in collaboration with Ishi Talmon, Chemical Engineering, Technion.
