Deca-dodecasil 3R

DDR Deca-dodecasil 3R [Sii20O240]- R. R = 1 -aminoadamantane Sigma-1 (Si02/Al203 60)... 

In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems Table 6). 

Fig.3 The three kinds of cage found in deca-dodecasil 3R[l2].

As one would have expected based on its structure, aluminum-free deca-dodecasil 3R readily adsorbs small molecules like CO2, N2O and NH3 [20]. On the other hand, alkanes and alkenes with three or four carbon atoms show only retarded adsorption at 296 K, while 1,3-butadiene is readily adsorbed under these conditions. This peculiar behavior has been attributed to the more elliptical cross section of the diolefin due to the presence of sp -hybridized carbon atoms. The former obviously has a better fit to the elliptical windows in the DDR structure than the circular cross section of saturated hydrocarbons [20]. In the following, the synthesis of materials with the DDR framework topology will be described in more detail. 

So far, essentially two templates have been reported to promote the crystallization of materials with the DDR framework topology, viz. 1-aminoadamantane (ADA) and methyltropiniumiodide. Their structures are depicted in Fig. 1. Both are relatively bulky molecules which, after hydrothermal synthesis, are encapsulated inside the large cavities of the DDR structure and have to be removed by calcination in an oxygen-containing atmosphere at temperatures of at least 813 K. Materials which have been prepared using 1-aminoadamantane are denoted as deca-dodecasil 3R [20,22] or Sigma-1 [18], while zeolite ZSM-58 is prepared using methyltropiniumiodide [16,17,19]. 

The synthesis of the pure silica form of deca-dodecasil 3R was first described by Gies [22,24] and later adapted by den Exter et al. [20]. A typical molar composition of the synthesis gel is 47 ADA 100 SiO2 404 EN 11240 H2O. Tetra-methylorthosilicate is used as silica source and hydrolyzed through the addition of the base ethylenediamine (EN). The crystallization occurs in glass tubes [22,... 

Membranes of various zeolites, such as deca-dodecasil 3R (DDR) type, 15 16 19... 

The only porous crystalline silica (porosil) found naturally is melanophlogite, but, including melanophlogite [55], a number of porosils have now been made (silicalites I and II, dodecasils IH [55a] and 3C [39], silica-sodalite [54], nonasil [56], and deca-dodecasil 3R [57]). Category 5 of Table III thus includes various porosils with topologies not observed in nature. There seems no theoretical reason why, in numbers and structural diversity, porosils could not eventually compare with zeolites. 

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