Heat capacity and thermodynamic functions of partially dehydrated cation-exchanged (Na+, Cs+, Cd2+, Li+, and NH4+) RHO zeolites

Abstract

Synthetic zeolites have a myriad of applications in industry due to their porous frameworks, potential to exhibit flexibility, and specific interactions with guest molecules. One topology of zeolites, RHO, is known to be flexible and have strong interactions with both H2O and CO2. We have performed heat capacity measurements on three partially dehydrated zeolite RHO samples containing extra-framework cations Na+ and Cs+, Cd2+ and Cs+, and Li+ and NH4+ to understand the energetics of these materials. Based on fits of the heat capacity data, we report smooth thermodynamic functions of Cp,m, ΔT0Sm ͦ, ΔT0Hm ͦ, and Φmͦ for these samples. The standard Sm ͦ at 298.15 K are 76.3 ± 0.8, 72.1 ± 0.8, and 68.8 ± 0.7 J∙K−1∙mol−1 for the Na,Cs RHO, Cd,Cs RHO, and Li,NH4 RHO samples, respectively, and the standard Hm ͦ at 298.15 K are 12.1 ± 0.1, 11.4 ± 0.1, and 11.4 ± 0.1 kJ∙mol−1. Our measurements also show a transition in the heat capacity of Na,Cs RHO, the sample with the highest water content, between 180 and 300 K that is not clearly observed in the other two samples. We attribute this transition to labile water and cations in the framework. This movement could also be coupled with a temperature-induced lattice expansion. Future work will include heat capacity measurements on fully dehydrated and fully hydrated zeolite RHO in order to separate these two possible phenomena.