Unusual coordination behavior of Cr3+ in microporous aluminophosphates

被引:36
作者
Beale, AM
Grandjean, D
Kornatowski, J
Glatzel, P
de Groot, FMF
Weckhuysen, BM
机构
[1] Univ Utrecht, Dept Inorgan Chem & Catalysis, Debye Inst, NL-3584 CA Utrecht, Netherlands
[2] Max Planck Inst Kohlenforsch, D-45470 Mulheim, Germany
[3] Nicholas Copernicus Univ, Fac Chem, PL-87100 Torun, Poland
关键词
D O I
10.1021/jp0531006
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
A CrAPO-5 molecular sieve has been investigated with X-ray absorption spectroscopy (EXAFS-XANES) as dehydrated material and after loading with water and ammonia to unravel the coordination geometries of Cr3+ in the framework of a microporous crystalline aluminophosphate, more particularly of the AFI-type. A comparison of the XANES data, a preedge analysis with crystal field multiplet calculations and EXAFS data, pointed toward the presence of framework Cr3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d(3) configuration of Cr3+, this unusual tetrahedral coordination environment strongly tends to transform into the more stable 6-fold coordination geometry by binding two extraframework water molecules during hydration. In the presence of ammonia, tetrahedral Cr3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr3+, to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis.
引用
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页码:716 / 722
页数:7
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