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. Auch an index is based on the zeolite breakdown temps. 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. (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. 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. Te max. vol. contraction of the zeolite unit cell appears to be controlled by the weighted ionic potential. Among different factors related to the framework topol., the expected relationship between the framework d. and the stability index was not found.