Heat capacity and thermodynamic functions of transition metal ion (Cu2+, Fe2+, Mn2+) exchanged, partially dehydrated zeolite A (LTA)

We have measured the heat capacity from 1.8 to 300 K of partially dehydrated zeolite A (LTA), fully exchanged with Cu2+, Fe2+ and Mn2+ ions. The samples have a broad excess heat capacity contribution centered around 4 K, which we attribute to local electric fields splitting the magnetic moments of the cations. The excess heat capacity is modelled using a sum of several Schottky anomalies. From these models, we conclude that the cations in the Cu2+ zeolite reside in at least four distinct coordination environments, and that some of the coordination environments in all three zeolites are highly asymmetric. We also report theoretical fits of the heat capacity data, and values of the standard thermodynamic functions C-p,C-m, Delta S-T(ok)m degrees, Delta H-T(ok)m degrees and Phi m degrees, at smooth temperatures. The standard molar entropies at 298.15 K are 71.8 J.K-1.mol(-1) for Cu-zeolite A (Cu0.22Al0.49Si0.51O2 center dot 1.04 H2O), 71.1 J.K-1.mol(-1) for Fe-zeolite A (Na0.01Fe0.23Al0.50Si0.51O2 center dot 0.77 H2O), and 66.0 J.K-1.mol(-1) for Mn-zeolite A (Mn-0.2 Al0.49Si0.50O2 center dot 0.53 H2O). (C) 2021 Elsevier Ltd.

Automated summary

The study reveals that cations in Cu2+ zeolite are in at least four distinct coordination environments, with some coordination environments in all three zeolites being highly asymmetric. The standard molar entropies are 71.8 J.K-1.mol-1 for Cu-zeolite A, 71.1 J.K-1.mol-1 for Fe-zeolite A, and 66.0 J.K-1.mol-1 for Mn-zeolite A at 298.15 K.