del Arco M, Gutiérrez S, Martín C, Rives V
Departamento de Química Inorgánica, Universidad de Salamanca, Salamanca, 37008 Spain.
Inorg Chem. 2003 Jun 30;42(13):4232-40. doi: 10.1021/ic034034u.
A Mg,Al layered double hydroxide (LDH) with Cr(C(2)O(4))(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 degrees C, 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 A. 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 degrees C, but the layered structure is stable up to 330 degrees C, probably because of the presence of interlayer oxalate anions formed during decomposition of the complex. Calcination leads to oxidation of Cr(3+) ions to the six-valent state, which reverts to Cr(3+) when the calcination temperature is further increased.
一种层间含有[Cr(C₂O₄)₃]³⁻阴离子的镁铝层状双氢氧化物（LDH）通过两种不同途径合成：由轻度煅烧的碳酸镁铝LDH进行重构，以及由硝酸镁铝LDH进行阴离子交换。所制备的固体通过元素化学分析、粉末X射线衍射、傅里叶变换红外光谱（FT-IR）和紫外可见/漫反射（UV-vis/DR）光谱、热分析方法、-196℃下的氮气吸附以及吡啶吸附的FT-IR监测进行了表征。所得结果表明，最合适的方法是阴离子交换，可得到层间距为10 Å的结晶良好的LDH。然而，由于重构方法中溶液的高pH值（>8），得到了一个多相体系，其中除了具有LDH结构的相之外，由于配合物的水解还形成了无定形草酸镁和铬的羟基氧化物。层间配合物在高达200℃时稳定，但层状结构在高达330℃时稳定，这可能是由于配合物分解过程中形成的层间草酸根阴离子的存在。煅烧导致Cr³⁺离子氧化为六价态，当煅烧温度进一步升高时又恢复为Cr³⁺。
