Identification and Quantification of Defects in the Cation Ordering in Mg/Al Layered Double Hydroxides
Identification and Quantification of Defects in the Cation Ordering in Mg/Al Layered Double Hydroxides
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DOI：
10.1021/cm200029q
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发表时间：
2011-06-14
影响因子：
8.6
通讯作者：
Massiot, Dominique
中科院分区：
文献类型：
--
作者：
Cadars, Sylvian;Layrac, Geraldine;Massiot, Dominique
阳离子有序被认为对许多物理化学性质具有至关重要的影响，这些物理化学性质使得层状II双氢氧化物（LDH）材料作为药物递送系统、纳米复合材料或催化的主体结构受到相当大的关注。在这里，我们首先明确地证实，固态（1）H NMR在快速（60-65 kHz）魔角旋转（MAS）可以用来区分和量化不同的本地Mg（n）Al（3-n）OH（n = 1，2，和3）的LDH层中的羟基环境。通过结合不同的固态（1）H和（27）Al一维和二维NMR测量与第一性原理计算，我们证明，虽然全球有序，富铝Mg/Al-2 LDHs中的阳离子分布包含可检测到的铝簇。虽然很小，但可以量化相对于完美有序的阳离子排列（其中避免了Al Al对）错位的Al原子的分数。它们的数量被示出以抵消错位的Mg原子的数量为Mg/Al比为2，并强烈降低Al含量。这表明，虽然不受欢迎，但在富铝LDH材料中不排除Al-O-Al键，这一发现将强烈影响我们对这些材料的局部酸性及其广泛利用的阴离子交换和重建特性的理解。
Cation ordering is believed to have crucial effects on many of the physicochemical properties that make layered I I double hydroxides (LDHs) materials of considerable interest as host structures for drug delivery systems, nanocomposite materials, or for catalysis. Here we first unambiguously confirm that solid-state (1)H NMR at fast (60-65 kHz) magic-angle spinning (MAS) can be used to distinguish and quantify the different local Mg(n)Al(3-n)OH (n = 1, 2, and 3) environments of hydroxyl groups in LDH layers. By combining different solid-state (1)H and (27)Al one- and two-dimensional NMR measurements with first-principles calculations, we demonstrate that, although globally ordered, the cation distribution in Al-rich Mg/Al-2 LDHs contains detectable amounts of Al clustering. Though small, the fraction of Al atoms misplaced with respect to the perfectly ordered cation arrangement (where Al Al pairs are avoided) could be quantified. Their number is shown to counterbalance the number of misplaced Mg atoms for a Mg/Al ratio of 2 and to strongly decrease for reduced Al contents. This establishes that, although not favored, Al-O-Al linkages are not excluded in Al-richer LDH materials, a finding that will strongly impact our understanding of the local acidity of these materials and their widely exploited anion exchange and reconstruction properties.