Shape-Selective Particle Synthesis

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Isomorphous substitution of T element in a molecular sieve material is very interesting in order to modify its acidic or redox catalytic and shape selective properties. Different ways to perform such a substitution are now well established either during synthesis or post synthesis in( luding solid-solid reaction between the zeolite and another oxide. The substituted eliiment may be strongly or weakly bound to the framework i.e. may remain stable or may give rise to well dispersed metallic oxide particles entrapped in the cavities. This results in different catalytic properties and may even lead to bifunctional catalysis as for Ga-ZSM-5 material. 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

In summary, an ideal ceramic starting powder should a small size, e.g., <1 pm or at the scale of run, a narrow particle or grain size distrihution, i.e., monodisperse or near monodisperse, a spherical or near-spheiical particle/grain shape, no agglomeration or only soft agglomeration, completed chemical reaction, single phase and high purity. To achieve this, it is important to select the synthesis methods. 

Lee, L, Morales, R., Albiter, M. A., and Zaera, F. 2008. Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity. 

In the wide field of possible applications for MTS, the use of their properties of adsorption and steric selectivity is still to be explored. However, such applications require well-defined particles, especially spherical particles in the micrometric range. The synthesis of MSU-X silica that exhibits these shapes allowed us to test their properties in adsorption HPLC. Non polar solvent such as hexane are suitable to allow a significant separation. Further analyses and testing for size exclusion separation processes are under progress. 

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