A Structural Study of the Nature of Molecular Zirconium Species in Aqueous Solution

Abstract

The structural study of complex molecular ions in solution has often proved difficult but is vital to fully understand how these solutions behave in the reaction pathways of material synthesis. Attempts have been made previously to quantitatively determine the structure of Zr(IV) solutions using a number of techniques. The work presented here aims to determine the optimal method for the study of these solutions and critically compares extended X-ray absorption fine structure (EXAFS), a technique long seen as the logical choice for the study of metal containing complex ions in solution, and X-ray pair distribution function (XPDF).

To initially test the validity of the use of XPDF to study these solutions, structural studies were undertaken on solutions of zirconium-oxychloride, (ZrOCl2·8H2O). This work determined that the tetrameric phase [Zr4(OH)8(OH2)16]8+ previously proposed by Clearfield et al. 1 is present in solutions and that there is a strong coordination between the complex tetrameric ion and the chloride counter ions. Through the iterative method development that was used to study this zirconium-oxychloride solution, an optimal method was derived to be carried forward for subsequent XPDF studies.

The work then attempts to compare the results of studies performed by other groups using EXAFS to study laboratory synthesised solutions prepared from the addition of acetic acid to ZrOCl2·8H2O solutions, with commercially available solutions of zirconium-acetate, [Zr(CH3COO)4]. It was found that the results seen in literature for laboratory synthesised solutions do not match that seen in commercially available materials. The commercial materials showed a tendency towards a tetrameric phase, [Zr4(OH)8(OH2)16]8+, up to acetic acid : zirconium ratios well in excess of 5.0 contrary to that seen in laboratory synthesised material which seem to shower a higher prevalence of the hexameric phase, Zr6(O)4(OH)4(CH3COO)12, at acetic acid : zirconium ratios higher than 0.8.

Alongside this work an ageing study was conducted to determine if the structure or relative composition of tetrameric [Zr4(OH)8(OH2)16]8+ and hexameric, Zr6(O)4(OH)4(CH3COO)12, phases could be altered through ageing or varying the solution concentrations. It was found that while all solutions adopt a two phase system of the hexameric and tetrameric phases, and that the main influence on this ratio was concentration, there are potentially two conformations that the hexameric Zr6(O)4(OH)4(CH3COO)12 can adopt, and the validity of both these models is discussed within with a “fixed-core” model being preferred but not being without chemical inconsistencies in the extended length of some of the chelating Zr-O bonds between the core and the carboxylate ligands. Some of these achieving lengths of over the common literature value of 2.2 ± 0.2 Å.
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Finally, a model compound synthesis study was undertaken to determine if any of the variations observed in the ageing study described above manifested as variation in final products when these solutions were used as zirconium precursors as they so often are in industrial applications. It was determined that the consistency of the model compound, the microporous zirconosilicate umbite K2ZrSi3O9·H2O, is extremely high with a single crystalline phase with consistent orthorhombic lattice parameters (a = 10.2916-10.3131 Å, b = 13.3071-13.3623 Å, c = 7.1845-7.2107 Å) and bond lengths and angles consistent with those reported in literature. The principal variation observed in the zirconosilicate was the quantity of amorphous phase obtained varying from 9.80-88.12% crystallinity. Whilst little seemed to impact the maximum crystallinity that could be achieved for a sample it was noted that higher ratios of acetic acid : zirconium gave a broader range of crystallinity.

It is hoped that the methods developed here can subsequently be applied to alternative solutions and that this is not only limited to zirconium complexes but all metal complex solution species. The information derived on the specific zirconium materials studied here, namely zirconium-oxychloride and zirconium-acetate, can inform their employment in synthesis pathways.