Thermodynamic study of alkali and alkaline-earth cation-exchanged natrolites

A series of synthetic natrolites with different non-framework cations (Li-NAT, Na-NAT, K-NAT, Rb-NAT, Cs-NAT, Ca-NAT, Sr-NAT and Pb-NAT) are investigated by high temperature oxide melt solution calorimetry. The formation enthalpy from the component oxides (ΔHf,ox) becomes less exothermic with increasing ionic potential (Z/r) of the cations. The dehydration behavior of NATs is examined using TG–DSC. The strength of water binding decreases with increasing cation size. Similar to the trend seen in anhydrous zeolites, a linear dependence of ΔHf,ox on Al/(Al + Si) ratio for hydrous zeolites is established.

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► Enthalpies of formation of alkali and alkaline earth cation exchanged natrolites were measured.
► The results show systematic trends with cation type and Al/(Al + Si).
► The energetics of hydration and tendency toward amorphization were studied.
► A model for enthalpies of formation for hydrous aluminosilicate zeoiltes has been established.