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

Abstract

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 CP,m, Δ0KTSm°, Δ0KTHm°, andΦm° 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⋅1.04 H2O), 71.1 J·K−1·mol−1 for Fe-zeolite A (Na0.01Fe0.23Al0.50Si0.51O2⋅0.77 H2O), and 66.0 J·K−1·mol−1 for Mn-zeolite A (Mn0.26Al0.49Si0.50O2⋅0.53 H2O).