Unusual Coordination Behavior of Cr³⁺ in Microporous Aluminophosphates

Abstract

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 Cr 3+ 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 Cr 3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d 3 configuration of Cr 3+ , 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 Cr 3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr 3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr 3+ , to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis.

Synthesis, characterization and catalysis of (Co, V)-, (Co, Cr)- and (Cr, V)APO-5 molecular sieves

“…Thus, a close vicinity of Cr 3+ ions to the lattice Al sites would lead to a shift of tetrahedral 27 Al line [40]. On the other hand, the possibility for the incorporation of Cr 3+ in the framework could not be excluded [16,21,25,26,31] as the framework incorporation of Cr 3+ in AlPO-5 was recently evidenced by the XAFS technique [32]. In addition, it is also supported by chemical analysis.…”

“…À was an effective co-templating molecule for the framework incorporation of Cr when triethylamine and pseudo-boehmite were used as template and Al source respectively [25]. Incorporation of Cr 3+ ions in the framework of AlPO 4 -5 molecular sieves has recently been confirmed as well by X-ray absorption spectroscopy (EXAFS-XANES) [32], although it required the presence of two extra-framework oxygen atoms to complete its pseudo-octahedral coordination environment. Nevertheless, the general non-unity of the chemical compositions of Cr-containing samples indicates that among the studied transition metal ions, Cr 3+ is the most difficult one to be incorporated in the framework ( Table 1).…”

“…In addition, it is also supported by chemical analysis. The observation of Cr 3+ ion in an octahedral coordination in general originates from its 3d 3 configuration, which strongly makes the distorted tetrahedral coordination state transform into the more stable sixfold coordination environment by bonding two water molecules located in the channel [32].…”

Silica‐Immobilized Chromium Colloids for Cyclohexane Autoxidation

“…[14] For the conventional Cr/ AlPO 4 -5, experimental evidence was found that the Cr is not incorporated in the framework, but rather is present as octahedral Cr III ions at the surface of the zeolite crystal. [15] However, it cannot be excluded that changing the synthesis conditions [16,17] might result in stable, supported, Cr-based autoxidation catalysts. The performance of recently discovered three-dimensional mesoporous chromium oxide in autoxidation reactions remains to be examined.…”

Study on Chromium‐Containing Aluminophosphate CrAPO‐5 via Density Functional Theory

“…Recently, Beale et al found the unusual coordination behavior of Cr 3+ in AFI via X-ray absorption spectroscopy (EXAFS-XANES). 11 They claimed that the Cr 3+ ion could be incorporated into the framework of AFI by replacing the Al 3+ ion and that the coordination geometry of Cr 3+ in dehydrated CrAPO-5 could take on a distorted tetrahedral coordination state. The coordination geometry of Cr 3+ could transform into 5-fold or 6-fold coordination in the presence of ammonia or water molecules.…”