Basic zirconium oxalate is an inorganic compound and coordination polymer with the formula Zr(OH)2(C2O4). It contains zirconium in the +4 oxidation state and consists of a three-dimensional framework built from hydroxide- and oxalate-bridged zirconium centres. A hemihydrate, Zr(OH)2(C2O4)·0.5H2O, is also known and behaves as a microporous host with reversible water sorption.
Preparation The hemihydrate can be prepared by controlled diffusion of zirconyl nitrate and oxalic acid through a silica-gel medium.
Anhydrous basic zirconium oxalate can then be obtained by thermal removal of the guest water from the hemihydrate.
A later preparation produced crystalline Zr(OH)2(C2O4) by slow evaporation of an acidic solution containing basic zirconium nitrate and oxalic acid at about 50 °C.
Properties Structure Both the hemihydrate and anhydrous compound crystallize in the tetragonal crystal system, in space group I4/m (No. 87).
The structure is a three-dimensional coordination framework. In the anhydrous material, it can be described as an interconnection of zirconium hydroxide chains, 1∞[Zr(OH)2]2+, and zirconium oxalate chains, 1∞[Zr(C2O4)]2+.
The oxalate groups act as bridging ligands between zirconium centres, while hydroxide ligands form additional bridges to generate the three-dimensional network.
Porosity and hydration The hemihydrate, Zr(OH)2(C2O4)·0.5H2O, is microporous. Its water molecules occupy cavities in the zirconium–oxalate framework rather than being required for the connectivity of the framework itself.
Heating removes the guest water without destroying the crystalline framework, giving anhydrous Zr(OH)2(C2O4). Exposure of the anhydrous phase to water restores the hydrated phase. The dehydration and rehydration therefore occur by reversible single-crystal-to-single-crystal transformations.
Thermal decomposition After dehydration, further heating causes condensation of hydroxide groups and decomposition of the oxalate ligands.
Thermal analysis indicates formation of an intermediate approximated by Zr2O(CO3)(C2O4) during decomposition of the basic oxalate.
At higher temperature, the organic component is destroyed and zirconium dioxide is formed. Modern thermal studies found that decomposition of alkali-free zirconium oxalates initially produces amorphous zirconia together with residual carbon; oxidation of the carbon at about 540 °C occurs alongside crystallization of tetragonal ZrO2.
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