Hierarchically Structured Silica Monoliths
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HIERARCHICALLY STRUCTURED SILICA MONOLITHS Nicola Hüsing, Christina Raab, Viktoria Torma Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9 A-1060 Vienna, Austria, [email protected]
ABSTRACT Large monoliths with periodic mesopores in a macroporous web-like network structure are prepared using a diol-modified silane as silica precursor and lyotropic liquid crystalline phases of surfactants in water as structure-directing agents. The surfactant concentration is varied from 5 to 40wt%. Highly porous materials are synthesized applying two different drying procedures: supercritical drying with CO2 or ethanol, in addition to ambient pressure drying including surfactant expulsion via silanisation treatments with trimethylchlorosilane or hexamethyl disilazane. The monolithic materials show a very unique macro- and mesostructure which is characterized by N2-sorption measurements, small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
INTRODUCTION Single molecules such as quaternary amines are used as structure-directing agents in the preparation of zeolites; for larger pores supramolecular arrays of molecules such as liquid crystal-like phases of surfactants are of interest, e.g. for templating mesoporous materials.[1] Through the choice of the surfactant dissolved in water, the pore sizes and the pore size distribution of the materials can be exactly tailored.[2] For plenty of applications, such as sensors, catalytic materials and separation media, not only the periodic build-up of the network but also the macroscopic morphology of the material is of crucial importance, often demanding monoliths instead of powders or films. In addition, a hierarchical built-up of the network structure is often required e.g. a well-defined combination of mesopores and macropores.[3] The synthesis of monoliths without randomly distributed mesopores such as aerogels is described in detail in several review articles, only few authors report on the preparation of periodically ordered M41S-type monoliths and even less describe the combination of surfactant templated monoliths with different pore sizes.[4-7] Because a direct templating approach of a liquidcrystalline surfactant phase with an inorganic precursor is typically applied in the preparation of mesostructured monoliths, one of the major problems observed in this process is the sensitivity of liquid-crystal phases towards alcohol which is released upon hydrolysis of the inorganic silica precursor, mostly tetraethoxy- or tetramethoxysilane. Hoffmann et al.[8] reported a novel glycol-based silica precursor, tetrakis(2-hydroxyethyl) orthosilicate which is water-soluble and compatible with lyotropic surfactant phases. We extended this research to the synthesis of low-density silica monoliths with an extraordinary network structure, applying the ethylene glycol modified silane (EGMS) in the presence of surfactants. In addition, we varied the drying procedure from the traditional
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