Aanmaak van anorganische materialen met behulp van superkritische CO2

Metal oxides with a large surface area are of major technological importance and can be used in very diverse fields, ranging from selective chemicals separation, petroleum and petrochemical processing, fine chemicals catalysis, optics, and electronics to pharmaceutical formulation, etc. Large surfaces are typically obtained through introduction of porosity or nanosizing of the particles. The synthesis of these porous or nano-materials often involves environmentally harsh conditions, high-energy usage and time consuming processes. Understanding and improving the synthesis of these materials triggers innovation and opens up possibilities towards new applications. The objective of this work is to study and improve the synthesis and properties of two important materials, namely ordered mesoporous silicates and titanium dioxide nanoparticles. Alternative synthesis methods have been developed aiming at a gain of sustainability and functionalization towards needs for applications.Ordered mesoporous silicates are typically used as a carrier material in catalyst preparation or they are functionalized by incorporation of heteroatoms or with organic groups. Emerging applications can also be found outside the world of catalysis such as in drug delivery, electronics, etc. Typically synthesis is performed under hydrothermal conditions, using large quantities of acids or bases and expensive precursors. In this study a new P6m ordered mesoporous silica material coined COK-12 is presented. The COK-12 synthesis procedure is environmentally benign since no heating is required and the reaction is conducted in the absence of mineral acids at near neutral pH utilizing inexpensive sodium silicate. The material can be described as a two dimensional hexagonal stacking of cylindrical pores, with pore diameters of 5 – 6 nm, separated by an amorphous silica wall. The convenient synthesis procedure allows the preparation of ordered mesoporous materials at lab scale and potentially at commercial scale without the need of highly specialized equipment. The very fast formation of only a few minutes allows continuous synthesis of highly ordered solids, which was demonstrated with a peristaltic pump setup. Key to these breakthroughs is the use of a citric acid / sodium citrate buffer at pH 5. The citrate3- and Na+ ions enhance the hydrophobic interaction in the triblock copolymer EO20-PO70-EO20 and the interaction between the surfactant and the silica precursor species. This salting out effect enables synthesis of ordered structures in the absence of electrostatic forces. The formation mechanism of this new material was studied by small angle x-ray scattering and resulted in the proposition of a consistent supramolecular assembly model. Initially, the neutral silicate oligomers tend to concentrate in the outer layer of the surfactant micelles resulting in spherical core shell moieties. Upon further enrichment with silicate, these spherical micelles adapt cylindrical shapes by coagulation. This growth occurs uniform and when all cylinders attain a critical size, in this study 250 nm, cylinders start to organize resulting in a final hexagonal stacking of the coated cylinders. This proposed model is reflected in the particle morphology. The COK-12 particles are very thin hexagonal platelets (250 nm). Furthermore, the cylindrical pores are aligned in the short direction resulting in short path lengths which is very interesting in applications or more fundamental studies in catalysis, adsorption, diffusion studies, replica formation, etc. Ordered mesoporous materials such as SBA-15 have been shown to be advantageous in the formulation of poorly soluble drugs. COK-12 materials should be useful in this type of applications as well. In the second part the synthesis of titanium dioxide semiconductors is studied. The use of semiconductors in photocatalysis is a fast emerging field in the trend towards more sustainable chemistry and processes. Photocatalytic applications range from mimicking photosynthesis reaction to the photodegradation of organic pollutants, e.g. in self-cleaning windows, disinfection of surfaces, air purifiers, water treatment, etc. TiO2 is by far the most studied photocatalyst and is a typical example where nano-sizing is used to increase catalytic activity. These nanoparticles are manufactured using pyrogenic processes at 2000 °C. Here, an original synthesis method was developed to prepare TiO2 nanoparticles dispersed in SiO2 matrices. When titanium alkoxides are processed in supercritical carbon dioxide in the presence of formic acid at relatively low temperature (75°C) dry crystalline material can be recovered after degassing, whereas otherwise only amorphous TiO2 is formed. The crystalline structure can be indexed as anatase, which is considered to be the photocatalytically active phase, without indication of other phases. Furthermore, this finding makes high temperature annealing to induce crystallization superfluous and prevents formation of the mixed phase anatas/rutile at annealing temperatures. The TiO2 particles have a surface area of 180 m2/g, about three times higher than the industrial standard. Composite materials of TiO2 nanoparticles in a SiO2 matrix were prepared by co-condensation of Ti and Si metal alkoxide precursors. These composites have large surface areas up to 600 m²/g and showed excellent activity in different types of photocatalytic degradation of organic pollutants in an aqueous solution and in gas stream. Test reactions were selected based on common problems in purification technology e.g. degradation of phenol, a common pollutant in waste water and degradation of acetaldehyde, a volatile organic carbon causing sick building syndrome symptoms. The turnover numbers of the composite materials were higher compared to the 100 wt% TiO2 counterparts. Moreover, dispersions of nanoparticles in a solid silica matrix help to avoid exposure to toxic nanoparticles and assure lasting high activities by preventing agglomeration. This new synthesis procedure can be extended to the impregnation of a porous material with a Ti alkoxide in scCO2 that is converted to anatase in a next step.status: publishe