colloidal aspects of template directed zeolite formation
- Paper number
IAC-08.A2.1.4
- Author
Prof. christine kirschhock, KULeuven, Belgium
- Coauthor
Dr. Alexander Aerts, KULeuven, Belgium
- Coauthor
Mrs. Lana Follens, KULeuven, Belgium
- Coauthor
Prof. Jan Vermant, KULeuven, Belgium
- Coauthor
Prof. Johan Martens, KULeuven, Belgium
- Year
2008
- Abstract
Synthetic zeolites are crystalline porous silicates with invaluable worth in a sustainable industrialised world. Zeolite adsorbents and catalysts are workhorses of hydrocarbon processing. They are used in large volumes in detergents and as drying agents. Applications extend into a wealth of new areas including environmental protection, production of fine chemicals, serving as e.g. pharmaceuticals, nutriceuticals, fragrances, flavours, agrochemicals, and further as sensors, electro-optical devices etc.. The industrial synthesis of zeolites is a slow process involving the lengthy hydrothermal conversion of a silicate-based hydrogel into crystalline material. Depending on the synthesis composition and the presence of structure directing agents (templates) in form of cations or organic molecules, different pore topologies are obtained. How a specific structure is formed and why the crystallisation takes that long is presently poorly understood. Improved insight into the basic molecular mechanisms during zeolitisation is indispensable for the design and synthesis of tailor-made materials for numerous potential applications. Zeolitisation is a complex interplay between silica in solution and suspension with solvent and templates. The synthesis mixtures react very sensitively on changes in composition, pH, and convection. This circumstance makes in situ observation mandatory. The so-called clear solution synthesis, where no gel-formation occurs, is an ideal model system to study the processes leading up to crystalline zeolites. Specifically the formation of Silicalite-1 from tetraethylorthosilicate, tetrapropylammoniumhydroxide and water has been studied in detail by a number of laboratories. Very early in clear solution formation 2-4nm sized particles appear, which by now are accepted to be precursors for the final zeolite structure. They consist of a silica core enclosed by a template shell. Over time silica-template interplay leads to successive structure optimization in these precursors. Suitable building units now start a successive aggregation leading to the crystal [1,2,3]. Such a process is a classical example of self-organisation on different time- and length-scales. Microgravity experiments aboard sounding rockets and the ISS clearly demonstrated that already the 2nm sized precursors are affected by the absence of gravity which is a manifestation of long-range phenomena [4,5]. These results prompted intensive on-ground study focussing on the colloidal interactions which lead to the local enrichment of suitable building units for aggregation in so-called ordered liquid phases (OLPs). Careful analysis of dynamic light scattering (DLS) data of clear solutions provided insights for the relevance of interactions in these concentrated suspensions [6]. DLS revealed two different diffusion processes of the same type of particles: a fast and a slow mode. These fast and slow modes refer to the collective and self- diffusion of the precursors, as confirmed by SAXS and NMR measurements. These two diffusive modes can only be observed if strong, far reaching interactions between slightly polydisperse species occur. The long range interactions of the precursor-structures in zeolite synthesis suspensions most probably give rise to the self-organization process leading to the formation of domains enriched in suitable zeolite building units (OLPs) wherein crystal formation is initiated. With this concept it was possible to re-direct the self-organization by adding a secondary structure directing agent towards the formation of a new class of hierarchical porous materials, the zeotiles [1]. However, even though over the last few years the understanding of zeolite formation has significantly progressed, many open questions remain. Especially the initial organization of unstructured silica into specific precursors and the then following integration into crystals needs attention. Microgravity experiments are necessary to understand in detail the multiscale-aspects of the formation of OLPs and their role in the overall aggregation mechanism. In the future an experiment dedicated to this study to answer these questions is planned. The development of precursors, their optimisation, enrichment and aggregation will be followed in situ aboard the ISS. As suitable diagnostic means recording DLS and monitoring viscosity have been identified. The current model of template directed zeolite growth will be presented and specifically discussed in terms of the expected effect of microgravity on the individual steps. Based on this the planned experiment for in situ study of precursor-formation and –organization will be outlined. [1] Design and Synthesis of Hierarchical Materials from Ordered Zeolitic Building Units. Martens J.A., Kirschhock C., Kremer S.P.B., Vermant J., Van Tendeloo G., and Jacobs P.A. Chem. Eur. J., 11, 4306-4313, 2005 [2] What has become of the Silicalite nanoslab? - Recent insights into key steps of template-directed silicalite-1 formation. C. E. A Kirschhock, A. Aerts, J.A. Martens, in Studies in Surface Science and Catalysis. From Zeolites to Porous MOF Materials - The 40th Anniversary of International Zeolite Conference, Proceedings of the 15th International Zeolite Conference, Volume 170, Part 2 ed.; Ruren Xu, Z. G. J. C. a. W. Y., Ed.; Elsevier: 2007; pp 1473-1478. [3] TEM Observation of Aggregation Steps in Room-Temperature Silicalite-1 Zeolite Formation. D. Liang, L.R.A. Follens. A. Aerts, J.A. Martens, G. VanTendeloo, C.E.A. Kirschhock, J. Phys. Chem. C 2007, 111 (39), 14283-14285. [4] 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 [5] 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 [6] Combined NMR, SAXS, and DLS Study of Concentrated Clear Solutions Used in Silicalite-1 Zeolite Synthesis. A. Aerts, L.R.A. Follens, M. Haouas, T.P. Caremans, M.A. Delsuc, B. Loppinet, J. Vermant, B. Goderis, F. Taulelle, J.A. Martens, C.E.A. Kirschhock, Chem. Mater. 2007, 19 (14), 3448-3454.
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