Framework structural changes

The zeolites that are useful as molecular sieves do not demonstrate an appreciable change in the basic framework structure on dehydration although the cations move to positions of lower coordination. After dehydration, zeolite A and others are remarkably stable to heating and do not decompose below about 700°C. The cavities in dehydrated zeolite A amount to about 50% of the volume. 

Investigations performed by Minchev et al (215) indicated that the framework of crystalline silicoaluminophosphates can be damaged upon the rehydration of the template-free material. In the case of rehydrated template-free H-SAPO-5 and H-SAPO-34, for example, a strong loss of the crystallinity occurs in the presence of water. However, the crystallinity can be completely restored after an additional dehydration at 823 K. Hydration of H-SAPO-37 at room temperature causes irreversible structural changes and leads to a material that is totally amorphous to X-ray diffraction (216). At temperatures of more than 345 K, template-free H-SAPO-37 exhibits a high stability toward hydration (216). 

Apparent in the foregoing review is the lack of any systematic attempt to investigate hydration states and structural modifications of the analcite framework under a broad range of compositions at elevated temperatures. Accordingly, this study was done to determine compositional limits, dehydration behavior, and associated structural changes in the analcite group. 

It is interesting that similar low-temperature weight loss peaks on the DTG curves were also observed for lamellar silicates templated by CTMA+, but the overall shape of the DTG curves was somewhat different [22]. It is not fully clear why these lamellar materials exhibited surfactant decomposition at such low temperatures. This behavior may be related to either properties of their silicate surface or the structural changes in their framework upon heat treatment, or both of these factors, which would promote the Hoffmann elimination at lower temperature [22]. The second of these possible factors may be related to the stress in... 

The choice of model often depends on the experiments involved. Workers in the area of, say, the effects of structural changes on the oxygen affinity and Hill constant for hemoglobin prefer the MWC model because it is essentially a structural theory. It provides a simple framework for the prediction and interpretation of experiments. Application of the theory to the Hill constant and other equilibrium measurements gives very acceptable results. Kineticists prefer the KNF model, since the kinetic measurements are more sensitive to the presence of intermediates. There are more variables in the KNF theory and there is more flexibility in fitting data. 

Some zeolites undergo a discontinuous change in the framework structure during ion exchange. A characteristic example is analcite [24] ... 

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