Systematic Study of Cr3+ Substitution into Octahedra-Based Microporous Aluminoborates

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• 作者：Qiaoqi Li ; Cong Lin ; Zhengyang Zhou ; Jing Ju ; Guobao Li ; Jianhua Lin ; Wenliang Gao ; Rihong Cong ; Tao Yang
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：June 2, 2014
• 年：2014
• 卷：53
• 期：11
• 页码：5600-5608
• 全文大小：753K
• 年卷期：v.53,no.11(June 2, 2014)
• ISSN：1520-510X

文摘

Single crystals of pure aluminoborate PKU-1 (Al₃B₆O₁₁(OH)₅路nH₂O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the R3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by ²⁷Al NMR results. This distortion of Al3O₆ octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported R3̅ to the presently reported R3. We applied a pretreatment to prepare Al³⁺/Cr³⁺ aqueous solutions; as a consecquence, a very high Cr³⁺-to-Al³⁺ substitution content (50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr³⁺ substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr³⁺. We also systematically investigated the thermal behaviors of PKU-1:xCr³⁺ (0 鈮?x 鈮?0.50) by thermogravimetric鈥揹ifferential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr³⁺-substituted PKU-1 could be partially retained at 1100 掳C in vacuum. When 0.04 鈮?x 鈮?0.20, PKU-1:xCr³⁺ transferred to the PKU-5:xCr³⁺ (Al₄B₆O₁₅:xCr³⁺) structure at 750 掳C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al₄B₂O₉:xCr³⁺. For x > 0.20 in PKU-1:xCr³⁺, the heat treatment led to a composite of Cr³⁺-substituted PKU-5 and Cr₂O₃, so the doping upper limit of Cr³⁺ in PKU-5 structure is around 20 atom %.