MCM-22 type zeolites are a class of aluminosilicates suitable for exploring the fundamental aspects of the binding of rare earth elements (REEs). The aim of the present study is to explore the RE nitrate binding to the silanol and Bronsted-Acid Sites (BAS) of MCM-22 and compare the binding efficiency of the sites based on the calculated binding energy values. A cluster model of MCM-22 was constructed to represent the external and internal surfaces having twenty-four and thirty-seven Si sites (denoted as T sites), respectively (Figure 1). To generate the BAS in both external and internal surfaces, suitable Si atoms are replaced with Al and H was added to the O atom that bridges the Al with an adjacent Si. The choice of the Si atoms for the replacement to generate BAS is performed based on the accessibility of the RE nitrates for binding - T1, T2 and T3 sites. The binding energy for the bonding between RE nitrate and silanol/BAS is calculated using the following equation:
Cluster-model + Nd(NO₃)₃à Cluster-Nd(NO₃)₂+ HNO₃
BE = E[Cluster-model] + E[Nd(NO₃)₃] – E[Cluster-Nd(NO₃)₂] – E[HNO₃]
As the presence of Na⁺ in the MCM-22 structures was noticed experimentally, we have removed the H atom from our -Si-O(H)-Al- cluster models and placed a Na⁺in the vicinity leading to the formation of -Si-O(Na)-Al- clusters.
For the cluster models with Na⁺, the binding energies are calculated as follows:
Cluster-model-Na + Nd(NO₃)₃à Cluster-Nd(NO₃)₂+ NaNO₃
BE = E[Cluster-model-Na] + E[Nd(NO₃)₃] – E[Cluster-Nd(NO₃)₂] – E[NaNO₃]
The calculated results reveal stronger binding at BAS sites for both external and internal BAS sites, but much weaker binding at silanol sites. This is consistent with the picture deduced from recent experiments. Moreover, it is also evident that the -Si-O(H)-Al- shows stronger binding with Nd(NO₃)₃ than -Si-O(Na)-Al- clusters.

<b>Figure 1.</b> MCM-22 surface and bulk cluster models.

Figure 1. MCM-22 surface and bulk cluster models.

Tamalika Ash

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