Silica-sodalite

The conformation of ethylene glycol occluded in high silica sodalite has been determined [25]. In the category of zeolite-hydrocarbon complexes, we have seen completion of studies at relatively low loading levels of benzene in zeolites L [26,27] and Na-Y [28]. Benzene has also been studied in... 

Synopsis of Nicholas et al. (1991) Molecular Modeling of Zeolite Structure. 2. Structure and Dynamics of Silica Sodalite and Silicate Force Field . 

Figure 3.9 The repeating structural unit (with connections not shown) that makes up silica sodalite. What kinds of terms would be required in a force field designed to model such a system ...

Is required. However, in situations where D4R contain more Si than Al, a TMA template is required the template could act to bring the D4R together in the absence of a strong tendency to do so or the TMA could act to stabilize high silica sodalite units which have recently been shown to be less stable than low silica sodalite units (.22). 

The mechanism of HS formation is less obvious. The stabilizing effect of TMA on high silica sodalite units(22) may have some bearing. The destabilization of D4R and D6R at high Na/Si ratios in solution (high pH ) has been observed(16). HS forms as higher pH than the other zeolites studied here. This may lead to a mechanism Involving SnR. 

The effect the deliberate addition of water on the crystalization of silica sodalite was investigated in a separate series of experiments where increasing amounts of water were added to the system. Addition of water up to water/ethylene glycol ratios of 0.05 had no observable effect, x-ray diffraction showing 100 percent silica sodalite. Above this ratio increasing amounts of meta sodium silicate were formed until this became the only product at water/ethylene glycol ratios above 0.5. 

This is a very significant conclusion because it is widely believed (9) that, in order to synthesize systems with 5-rings, such as ZSM-5, the only requirement is to synthesize systems low in alumina. Both our quantum-chemical and our electrostatic lattice calculations contradict this theory. The calculations show, for example, that sodalite, which contains only 4- and 6-rings and no alumina, is more stable than ZSM-5, in which 5-rings predominate. This agrees with recent experimental work relating to the synthesis of high-silica sodalite (10). 

Fig. 1. Silica sodalites a) The topology of the sodalite fiamework (oxygen atoms omitted) b) position of a 1,3,5-trioxane molecule in the cage of the silica sodalite structure. This corresponds to a snapshot because the trioxane molecule is disordered in the sodalite cage c) disorder cluster of the trioxane molecule in the sodalite cage d) organic molecules known to direct the silica sodalite structure.

To act as an SDA, a molecule must fulfil certain other prerequisites besides stability [5-7]. A critical factor is its solubility in the solvent used (usually water). The potential SDA must possess at least a limited solubility to take part in the reaction. However, if the solubility is too high or if the SDA forms strong hydrogen bonds with water, then its tendency to co-crystallize with the silica will be low and it will prefer to stay in solution. For this reason, alcohols are only weak SDAs, for instance. The fact that ethylene glycol and ethanolamine are grouped with other SDAs for the synthesis of silica sodalite in Fig. Id seems to contradict this conclusion. However, these molecules act as SDAs only in solvothermal synthesis, in which the SDA simultaneously acts as the solvent. 

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