Diversity of monomeric dioxo chromium species in Cr/silicalite-2 catalysts: A hybrid density functional study

The present paper deals with the theoretical characterization of the dioxo Cr species exchanged into the silicalite-2 structure via hybrid density functional theory with cluster models to represent the active site. The molecular models range from a simple mono silicon atom ring to more complex Cr-siloxane rings of 7T atoms. The common B3LYP and M06 density functionals were used for the calculations. The results indicate that the chemical hardness increases from a complex ring to a simple geminate type. Overall, the QTAIM calculations point to intermediate ionic-covalent nature of the Cr-oxygen bonds. An excellent correlation is found between the positive eigenvalue of the Hessian at the bond critical point with the Cr-O bond distance. Whereas the enthalpy change on the exchange reaction alters from -20.3 to 56.1 kcal/mol at the M06/6-311+G* level, the binding energy lies in a shorter range of 907.5-926.6 kcal/mol. As concluded from the Gibbs free energy changes on the exchange reaction with both of the methods, only the clusters of 4T size or larger would be formed spontaneously. However, M06 predicts similar to 13% stronger bindings for the Cr ion. Overall, the electronic properties depend upon both the Si-O-Cr-O-Si aperture size and the local configuration of the matrix. Furthermore, a fivefold coordination of Cr is accompanied with a lower ionization potential, a longer CrAO distance, and a less negative charge on the terminal O atoms. (C) 2016 Elsevier B.V. All rights reserved.