Cation mobility and structural changes on the water removal in zeolite-like zinc hexacyanometallates (II)

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Abstract

The cation (A+) mobility and structural changes on the water molecules removal in zeolite-like zinc hexacyanometallates series, Zn3A2[Fe(CN)6]2·xH2O with A=Na, K, Rb and Cs, were studied from X-ray diffraction data recorded for hydrated and anhydrous samples at room temperature and at 77 K. The crystal structure for the anhydrous phases were solved and refined and then compared with those corresponding to their hydrated form. On the water molecules removal the charge balancing cation (A+) migrates to favor a stronger interaction with the N ends of the CN bridges where the framework negative charge is located. This cation–framework interaction model is supported by the recorded IR spectra for both hydrated and anhydrous samples. The new cation position induces distortion for both the cavity shape and their windows and also leads to cavity volume reduction. This is relevant for the properties of this family of solids as porous materials and their behavior in adsorption and separation processes, among them for hydrogen storage.

Highlights

► Exchangeable metal mobility. ► Porous framework flexibility. ► Cavity geometry modulation through the exchangeable metal. ► Nanoporous coordination polymer. ► Porous solids with tunable framework.

Introduction

Zeolite-like zinc hexacyanometallates (II), Zn3A2[M(CN)6]2·xH2O with A=Na, K, Rb and Cs, and M=Fe, Ru and Os, in the following Zn3A2M2·xH2O, have received certain attention as prototype of porous solids for hydrogen storage [1], [2], [3], [4]. The porous framework of these solids is formed by ellipsoidal cavities of ca. 12.5×9×8 Å3, which remain communicated by elliptical windows of about 5 Å [5], [6], [7], [8], [9], [10]. The exchangeable metal (A+) is found within these cavities, close to cavity windows. The metal (A+) represents a charge center able to favor the H2 stabilization within the cavity by electrostatic type interactions. The electrostatic interactions are related to the permanent quadrupole moment of the hydrogen molecule and to the polarization of its electron cloud by a charge center [4]. A molecule with quadrupole moment interacts with an electric field gradient. The strength of the polarization interaction depends on the local electric field, and from this fact porous materials with exchangeable metals are particularly attractive for H2 storage. Such a possibility has been considered for H2 storage in zeolites [11], [12]. However in zeolites the ion polarizing power is partially shielded by the framework oxygen atoms electron cloud [12], an effect less pronounced for zeolite-like zinc hexacyanometallates [2]. From the recorded H2 adsorption isotherms in Zn3A2[M(CN)6]2 evidence on the cation mobility within the cavity has been obtained [2] but not studied. In addition the H2 adsorption isotherms are usually recorded on dehydrated samples under cryogenic conditions, usually at 77 K, after the sample degassing by heating under moderate vacuum. All these factors: crystal water molecules removal, cation mobility and low temperature could be contributing to a probable reversible structural transition in that series of porous solids. Such a possibility was evaluated in this study from X-ray diffraction (XRD) powder patterns recorded for hydrated and anhydrous samples of Zn3A2[Fe(CN)6]2·xH2O with A=Na, K, Rb and Cs both at room temperature and at 77 K. As a reference material, Zn3[Co(CN)6]2 was also studied. This last composition, in its rhombohedral modification, is iso-structural to the Zn–Fe series, and it is free of exchangeable metal and of water molecules within the cavities. The XRD data were complemented with IR spectra recorded for both hydrated and anhydrous samples. The samples to be studied were previously characterized from energy-dispersed spectroscopy (EDS), infrared (IR), Mössbauer and thermogravimetric (TG) data. No previous studies on the cation mobility and structural changes on the water molecules removal for this series of porous solids have been reported.

Section snippets

Experimental

The samples to be studied were prepared as already reported [9], [10], in summary: hot aqueous solutions (0.01 M) of zinc chloride and K4[Fe(CN)6]·3H2O (or K3[Co(CN)6]) were mixed, and the resulting precipitate was separated after 2 days of aging within the mother liquor at 60 °C. The obtained solid was washed several times with distilled water in order to remove all the accompanying ions and then dried in air until it had constant weight. For Zn ferrocyanide, the solid precipitate from sodium

Characterization of the samples to be studied

The samples under study correspond to previously characterized materials [9], [10] and herein only a summary of their features is provided. Additional information of their structural and spectroscopic characterization is available from Supplementary materials. This series of porous solids crystallizes with a rhombohedral unit cell in the R-3c space group where the zinc atom has tetrahedral coordination to N ends of CN groups. Related to this coordination mode for the Zn atom, the material has a

Conclusions

When zinc zeolite-like hexacyanometallates (II) are dehydrated the charge balancing cation migrates to a position near N ends of the CN bridges, where the negative charge is concentrated, in order to maximize the strength for the cation–framework electrostatic interaction. The cations migration toward the cavity windows also minimizes the repulsive electrostatic interaction between them. This interaction favors the charge subtraction from the framework through a polarization mechanism, which

Supplementary information

Structural information derived from the crystal structures refinement for the anhydrous samples of the materials under study has been deposited at ICSD Fachinformationszentrum Karlsruhe (FIZ) (E-mail: [email protected]) with ICSD file numbers: 421892: Zn3K2[Fe(CN)6]2; 421893: Zn3Rb2[Fe(CN)6]2; 421894: Zn3Cs2[Fe(CN)6]2.

Acknowledgement

This research was partially supported by the Projects ICyTDF-PIFUTP08-158 and CONACyT 123480. Access to the LNLS synchrotron radiation facility (at Campinas, Brazil) is also acknowledged.

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