Systematic study of Cr3+ substitution into octahedra-based microporous aluminoborates.
Systematic study of Cr3+ substitution into octahedra-based microporous aluminoborates.
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DOI：
10.1021/ic500352g
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发表时间：
2014-05
影响因子：
4.6
通讯作者：
Qiaoqi Li;Cong Lin;Zhengyang Zhou;Jing Ju;Guobao Li;Jianhua Lin;Wenliang Gao;Rihong Cong;
中科院分区：
文献类型：
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作者：
Qiaoqi Li;Cong Lin;Zhengyang Zhou;Jing Ju;Guobao Li;Jianhua Lin;Wenliang Gao;Rihong Cong;
合成了纯的PKU-1（Al_3B_6O_（11）（OH）_5·nH_2O）晶体，并用X射线衍射重新确定了晶体结构。（27）Al NMR结果表明，R3结构模型中存在三个独立的Al原子，Al 3处于畸变的八面体环境中。Al 3 O 6八面体的这种变形释放了主骨架的强静态应力，并导致对称性从先前报道的R3降低到目前报道的R3。我们采用预处理制备Al（3+）/Cr（3+）水溶液;作为结果，可以在PKU-1中实现非常高的Cr（3+）到Al（3+）取代含量（约50原子%），这对于催化目的来说远远超过。此外，在PKU-1：0.32Cr（3+）的粉末X射线数据的Rietveld精修中观察到Cr（3+）在Al 1和Al 2位点的优先取代。通过热重-差示扫描量热法、真空原位高温XRD和炉内退火实验，系统研究了PKU-1：xCr（3+）（0 ≤ x ≤ 0.50）的热行为。在1100 °C真空条件下，Cr（3+）取代的PKU-1的主要骨架部分得以保留。当0.04 ≤ x ≤ 0.20时，在750 °C空气中退火5 h，PKU-1：xCr（3+）转变为PKU-5：xCr（3+）（Al 4 B6 O 15：xCr（3+））结构。进一步升高温度导致分解成莫来石相，Al 4 B2 O 9：xCr（3+）。在PKU-1：xCr（3+）中，当x > 0.20时，热处理导致Cr（3+）取代的PKU-5和Cr2 O3的复合物，因此PKU-5结构中Cr（3+）的掺杂上限约为20atom%。
Single crystals of pure aluminoborate PKU-1 (Al3B6O11(OH)5·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 (27)Al 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 R3 to the presently reported R3. We applied a pretreatment to prepare Al(3+)/Cr(3+) aqueous solutions; as a consecquence, a very high Cr(3+)-to-Al(3+) substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr(3+) substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr(3+). We also systematically investigated the thermal behaviors of PKU-1:xCr(3+) (0 ≤ x ≤ 0.50) by thermogravimetric-differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr(3+)-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ x ≤ 0.20, PKU-1:xCr(3+) transferred to the PKU-5:xCr(3+) (Al4B6O15:xCr(3+)) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al4B2O9:xCr(3+). For x > 0.20 in PKU-1:xCr(3+), the heat treatment led to a composite of Cr(3+)-substituted PKU-5 and Cr2O3, so the doping upper limit of Cr(3+) in PKU-5 structure is around 20 atom %.