Clathrasil, synthesis

The phase transformation behavior of Py-D3C was far simpler than that reported for tetrahydrofuran/N2- and tetrahydrofuran/Xe-D3C in reference 10. We attribute this difference in part to impurities in the samples employed, samples that contained methanol and ethylenediamine according to 13 C CPMAS NMR spectroscopy. We have observed that use of Si(OCH3)4 as a silica source or ethylenediamine as a catalyst in clathrasil synthesis introduces defects that can alter phase transition temperatures by as much as 30 °C and/or introduce new phase transformations. 

Another important factor influencing the produet in clathrasil synthesis is the active concentration of the SDA. For some SDA, high concentrations lead to frameworks with higher densities of framework-speeifie eages. Pyrrolidine, for example, gives siliea sodalite (2.9 eages per 1000 A) at high concentrations of SDA. whereas RUB-10 (2.0 per 1000 A ) and dodecasil 3C (1.1 per 1000 A ) form at lower eoncentrations. 

Up till now, no insitu growth of DD3R on supports or membrane syntheses has been reported. It is to be noted that the synthesis of this clathrasil is marked by a fixed procedure [36], making changes in concentrations or temperature, in order to study the growth on supports, difficult. 

P. Caullet, J.L. Guth, J. Hazm, J.M. Lamblin, and H. Gies, Synthesis, Characterization and Crystal-Structure of the New Clathrasil Phase Octadecasil. Eur. J. Solid State Inorg. Chem., 1991, 28, 345-361. 

G. Vandegoor, C.C. Freyhardt, and P. Behrens, The Cobalticinium Cation [Co-III(ETA(5)-CsHs ] + - a Metal-Organic Complex as a Novel Template for the Synthesis of Clathrasils. Z. Anorg. Allg. Chem., 1995, 621, 311-322. 

G. van de Goor, B. Lindlar, P. Behrens, and J. Felsche, Solvent-free Synthesis of Clathrasils using Metal-organic Complexes as Structure-directing Agents. J. Chem. Soc., Chem. Com-mun., 1995, 2559-2561. 

The porosils have appeared only recently upon the scene [lOj. They include the silica end-products of synthesis of zeolites richer and richer in silica, such as silicalites I and II, and also other species which do not always have a zeolite counterpart. They can be sub-divided into clathrasils in which openings between intracrystalline cavities are too small for molecule migration and zeosils in which these openings are adequate for molecule diffusion. Examples of each sub-division are ... 

TABLE 4. Guest molecules M" which have successfully been used in the synthesis of various clathrasils [29]... 

STUDIES ON CLATHRASILS VII. A NEW CLATHRATE COMPOUND OF SILICA SYNTHESIS, CRYSTALLOGRAPHIC, AND THERMAL PROPERTIES. 

Despite the fact that clathrate hydrate science can be considered to be a mature field, numerous challenges remain. In part, this is because hydrates exist in nature and hence exhibit the complexity of mineral phases, where the structure and composition reflect not only the conditions of synthesis but also subsequent exposure to changing conditions after synthesis. Another part is that the principal component of these materials is water, likely the most important material on earth, and the clathrates offer a possibility to study the diverse ways in which water interacts with other materials. In this respect, it is interesting to note that new structures continue to be found. Structurally related families of materials such as the sUica-based zeosils and clathrasils tend to show a far greater structural diversity. In part, this is because of the covalent nature of the silica lattices that give far more robust frameworks than the water-based ones however, each structure in these classes of materials could be considered as a candidate for clathrate synthesis. 

Table 2a. Selection of structure-directing agents (SDA) that have been used successfully for the synthesis of different clathrasil structure types ...

Table 3. Summary of aliphatic and cyclic molecules that have be used successfully as structuredirecting agents (SDA) for the synthesis of the clathrasil dodecasil IH (DOH) ...

A similar role is played by the fluoride anion in the fluoride synthesis route when the ion pair is actively templating. In the examples given above, the presence of the fluoride anion is decisive for the formation of the particular structure type, and, as has been shown in structure analyses and NMR experiments, the fluoride anion occupies specific small cavities in clathrasil phases (see Table 7). 

Table 8. Influence of the synthesis temperature on the type of clathrasil formed with a given structure-directing agent... 

---