Projet de recherche PX/7/LP/30 (Action de recherche PX)
Zeolites are microporous crystalline silicate materials. The project ‘Crystallization of Zeolites’ covers a number of microgravity experiments which all had as goal the improvement of scientific insight intothe mechanisms of silica-structuring with the help of an organic template to enable finally the designand synthesis of tailor made zeolites. Given the proliferation of zeolite applications in a sustainablechemical industry, the project is motivated by the continuous need for new porous materials withtailored properties.
The project departed from the discovery of a self-organization process which results in the formation of MFI-type zeolites. The template molecules (Tetrapropylammonium - TPA) convert a silica source (Tetraethylorthosilicate - TEOS) into specific precursor species which at room temperature aggregate successively into nanoscopic zeolite building units of defined size and shape. These nanoslabs are stable in suspension. Further aggregation needs to be triggered either by heating or by changing the colloidal properties in the suspension.
During two ballistic rocket missions (MAXUS4 and -5) the kinetics of the individual aggregation steps of the nanoslabs during heating were analysed. For this a 30 mini-autoclave unit with individual programmed heating and a collective quenching was used. The evolution of the particle size evolution was determined by X-ray scattering of the retrieved samples. It was shown that microgravity significantly slowed aggregation. Surprisingly, the aggregation of the smallest entities (smaller than 5 nm diameter) was most retarded by microgravity. To confirm this observation at lower reaction temperatures and longer experiment duration, the experiment NANOSLAB was performed inside the MSG facility of the ISS. NANOSLAB contained of a sample tray with 30 cartridges of clear solutions which were individually heated and simultaneously quenched on the MSG cold-plate. Sample analysis was performed as before. This experiment fully confirmed the observation that the aggregation of the nanoscopic building units strongly is affected under reduced flow and shear rates. Convection usually applies to micrometer domains. The nanometre size regime is expected to be governed by Brownian motion only. A concise research effort with Dynamic Light Scattering and Viscosity measurements on ground resulted in the discovery of physical nanoslab-collectives in the liquid phase. Nanoslabs are strongly negatively charged, anisotropic, and covered with a layer of positively charges template molecules which results in the formation of these Ordered Liquid Phases (OLPs). Those, reaching micrometer size regime, are affected by gravity phenomena and are responsible for the observed microgravity effect. The current model assumes that OLPs formed under microgravity are better ordered and stronger but form more slowly than on earth. Further studies are necessary to observe and study OLPs under microgravity in situ.
The ZEOGRID experiment performed aboard of the ISS supported this OLP model. Aggregation of nanoslabs was triggered by addition of secondary structure giving agent. The resulting materials are characterised by their hierarchical structure, combining microporous zeolite walls which form a mesoporous lattice. ZEOGRID contained 20 cartridges with two fluids (clear solutions on one side and secondary structure giving agent on the other) that can be manually mixed by turning a handle.Throughout it was observed that the hierarchical material particles obtained under microgravity conditions were of larger size, monolithic character, and better ordered internally. The most intriguing result of ZEOGRID was the formation of a solid material at room temperature in the absence of a secondary template. Instead of retrieving a concentrated suspension of nanometre-sized particles, millimetre sized, solid particles were obtained. Close analysis showed these particles to consist of nanoslabs being embedded layer-like into a matrix of TPA-cations. Obviously this was the first hard evidence for the favoured self-organization of nanoslabs into strongly cohesive OLPs under microgravity conditions.
The discovery of OLPs has strong impact on the view of zeolite synthesis. Further study is necessary to exploit this phenomenon for the tailoring of porous silica materials.
Selection of publications related to the project:
Microgravity Effect on the Self-Organization of Silicalite-1 Nanoslabs.
Kremer S.P.B., Theunissen E., Kirschhock C., Jacobs P.A., Martens J.A., and Herfs W.
Adv. Space Res., 32 (2), 259-263, 2003
European facilities for the study of zeolite formation on the international space station
Kirschhock, C., Kremer, S., Jacobs, P., Pletser, V., Minster, O., Kassel, R., Preudhomme, F., Martens J.
Proceedings of the 14th IZC, Cape Town, South Africa, 2004, Studies in Surface Science and Catalysis, 154, E. van Steen, L.H. Callanan and M. Claeys, eds.
Invited Concept paper: Design and Synthesis of Hierarchical Materials from Ordered Zeolitic Building Units.
Christine E. A. Kirschhock, Sebastien P. B. Kremer, Jan Vermant, Gustaaf Van Tendeloo, Pierre A. Jacobs, Johan A. Martens
Chemistry - A European Journal, 11, (2005), 4306-4313
Unexpected Microgravity Effect During the Self-Organization of Silicalite-1 Nanoslabs
C.E.A. Kirschhock, S.P.B. Kremer, E. Theunissen, P.A. Jacobs, J. Vermant, B. Pauwels, O. I. Lebedev, G. Van Tendeloo, J.A. Martens
Microgravity sci. Technol. XVI-I (2005)
Satellite(s) or flight opportunity(ies):
- International Space Station
Field of research:
Physical Science: Fluid Dynamics and Surface Tension, Multi-Component Systems; Materials Designed from Fluids
