Transformation of AlPO-53 to JDF-2: Reversible Dehydration of a Templated Aluminophosphate Studied by MAS NMR and Diffraction
Transformation of AlPO-53 to JDF-2: Reversible Dehydration of a Templated Aluminophosphate Studied by MAS NMR and Diffraction
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AlPO-53 向 JDF-2 的转化：通过 MAS NMR 和衍射研究模板化铝磷酸盐的可逆脱水
DOI：
10.1021/jp902074s
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
Ashbrook S
中科院分区：
文献类型：
--
作者：
Ashbrook S
我们描述了磷酸铝AlPO-53的详细研究，在其作为制造和煅烧的形式。在AlPO-53（A）的制造状态下，该材料由甲基铵阳离子模板化，含有封闭的水分子和氢氧离子，这些氢氧离子桥接铝原子对，使其配位数增加到5。固态NMR实验证实了粉末X射线衍射的先前结构模型中提出的铝和磷原子的局部环境。31 P NMR显示存在强度比为1：1：2：1的四个不同的共振。27 Al三量子MAS NMR解析了五个铝峰，两个具有四配位Al的NMR参数特征，三个具有五配位Al的NMR参数特征。这与来自两个不同晶体学Al位点的两个重叠信号的存在一致。NMR参数的第一性原理计算提供了一个完整的光谱分配，并确认我们的解释未解决的光谱。发现AlPO-53（A）在适度加热下（例如在NMR转子中长时间旋转时）容易转化为第二结晶相，并且变温粉末X射线实验与TGA一起表明这是产生第二铝磷酸盐JDF-2的脱水过程。这一点用~（31）P和~（27）Al NMR得到了证实，第一性原理计算支持了JDF-2的光谱归属。在300 °C下煅烧AlPO-53（A）或脱水材料JDF-2产生微孔开放骨架材料AlPO-53（B），其为具有三个Al和三个P位点的四面体网络，如通过NMR和第一原理计算所证实。除了证明NMR，第一原理计算和衍射的详细结构研究的组合使用的力量，我们表明，在所做的AlPO-53和类似系统的可逆脱水的可能性是一个重要的考虑因素，在结构研究，并提供证据表明，公布的结构模型AlPO-53（A）可能是不完整的。
We describe a detailed study of the aluminum phosphate AlPO-53 in both its as-made and calcined forms. In its as-made state, AlPO-53(A), the material is templated by methylammonium cations and contains occluded water molecules and also hydroxide ions that bridge pairs of aluminum atoms, increasing their coordination number to 5. Solid-state NMR experiments confirm the local environment of the aluminum and phosphorus atoms proposed in a previous structural model from powder X-ray diffraction.31P NMR shows the presence of four distinct resonances with an intensity ratio of 1:1:2:2, consistent with the expected six crystallographic P sites.27Al triple-quantum MAS NMR resolves five aluminum peaks, two with NMR parameters characteristic of four-coordinate Al and three of five-coordinate Al. One of these latter signals has a greater intensity than that of the others, consistent with the presence of two overlapping signals from two distinct crystallographic Al sites. First-principles calculations of NMR parameters provide a complete spectral assignment and confirm our interpretation of unresolved spectra. AlPO-53(A) is found to convert easily into a second crystalline phase on moderate heating (upon spinning in the NMR rotor for an extended period, for example), and variable-temperature powder X-ray experiments, together with TGA, suggest that this is a dehydration process yielding a second aluminophosphate, JDF-2. This is confirmed using both31P and27Al NMR, with the spectral assignment of JDF-2 supported by first-principles calculations. Calcination of AlPO-53(A) or of the dehydrated material, JDF-2, at 300 °C yields the microporous open-framework material AlPO-53(B), a tetrahedral network with three Al and three P sites, as confirmed by NMR and first-principles calculations. In addition to demonstrating the power of the combined use of NMR, first-principles calculations, and diffraction for detailed structural investigations, we show that the possibility of a reversible dehydration in as-made AlPO-53 and similar systems is an important consideration in structural studies and provides evidence that the published structural model for AlPO-53(A) may be incomplete.
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作者：
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DOI：
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发表时间：
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