Intercalation of [Cr(C2O4)3]3- complex in Mg,Al layered double hydroxides

A Mg,Al layered double hydroxide (LDH) with [Cr(C2O4)(3)](3-) anions in the interlayer has been synthesized following two different routes: reconstruction from a mildly calcined Mg,Al-carbonate LDH, and anion exchange from a Mg,Al-nitrate LDH. The solids prepared have been characterized by elemental chemical analysis, powder X-ray diffraction, FT-IR and UV-vis/DR (diffuse reflectance) spectroscopies, thermal methods, nitrogen adsorption at -196 degreesC, and FT-IR monitoring of pyridine adsorption. The results obtained indicate that the most appropriate method is anion exchange, leading to a well crystallized LDH with an interlayer spacing of 10 Angstrom. Due to the high pH value (>8) of the solution in the reconstruction method, however, a polyphasic system is obtained, where, in addition to a phase with the LDH structure, amorphous magnesium oxalate and chromium oxohydroxides are also formed due to hydrolysis of the complex. The interlayer complex is stable up to 200 degreesC, but the layered structure is stable up to 330 degreesC, probably because of the presence of interlayer oxalate anions formed during decomposition of the complex. Calcination leads to oxidation of Cr3+ ions to the six-valent state, which reverts to Cr3+ when the calcination temperature is further increased.