Zeolites upon heating: Factors governing their thermal stability and structural changes

This-paper attempts to review and rationalize the results concerning the zeolite structural changes due to heating treatments. A new parameter, namely the Stability Index, was introduced to quantify the thermal stability of zeolites. Such an index is based on the zeolite breakdown temperatures from X-ray diffraction studies. The correlation between the stability index and the Si/Al ratio confirmed the importance of the latter ratio in controlling thermal stability of zeolites. It was observed that: (i) zeolites with Si/Al >= 3.80 are very stable; (ii) zeolites with Si/Al <= 1.28 are quite unstable; and (iii) zeolite stability in the intermediate Si/Al range cannot directly be predicted from the Si/Al ratio. In the present study, the inverse relationship between thermal stability and the ionic potential, (Z/r)(wt), was also confirmed: (i) zeolites with (Z/r)(wt) < 0.072 are very stable; (ii) the ones with (Z/r)(wt) > 0.187 are unstable, but (iii) the (Z/r)(wt) ratio does not allow a discrimination in the intermediate region. The maximum volume contraction of the zeolite unit cell appears to be controlled by the weighted ionic potential. Among different factors related to the framework topology, the expected relationship between the framework density and the Stability Index was not found. (c) 2006 Elsevier Ltd. All rights reserved.