Effects of ZSM-5 Zeolite Confinement on Reaction Intermediates during Dioxygen Activation by Enclosed Dicopper Cations

Abstract

We investigate how nanospaces surrounded by a 10-membered ring of ZSM-5 zeolite affect the reaction intermediates formed during dioxygen activation by enclosed dicopper cations. Two types of dioxygen intermediates are considered: one is an O(2)...Cu(2) complex, where dioxygen binds to the two Cu cations, and the other is a bis(mu-oxo)dicopper complex converted from an O(2)...Cu(2) complex by the cleavage of the O-O bond. We employ large-scale density functional theory (DFT) calculations with the B3LYP functional to examine the energetics of the two dioxygen intermediates inside a 10-membered ring of ZSM-5 with double Si --> Al substitutions at variable locations. The properties of the O(2)...Cu(2) complexes, such as the dioxygen bridging modes and dioxygen activation, are strongly affected by the locations of the two Al atoms within the 10-membered ring. In particular, the O(2)...Cu(2) complexes have either end-on or side-on bridging modes depending on the substituted Al positions. On the other hand, the steric hindrances of a ZSM-5 cavity play crucial roles in determining the properties of the bis(mu-oxo)dicopper complexes containing a diamond Cu(2)O(2) core. By restricting its Cu(2)O(2) core to a 10-membered ring of ZSM-5 in which the two Al atoms are second-nearest neighbors, each Cu cation is tetrahedral four-coordinate. On the other hand, the Cu cations have almost square planar coordination inside a ZSM-5 where the Al atoms are fourth-nearest neighbors. The different Cu coordination environments are responsible for the different levels of stability; the planar diamond Cu(2)O(2) core is 30.7 kcal/mol more stable relative to the tetrahedral case. Since the ZSM-5 nanospaces directly influence the stability of the bis(mu-oxo)dicopper complexes by changing the Cu coordination environments, zeolite confinement effects on the bis(mu-oxo)dicopper complexes are more noticeable than those in the O(2)...Cu(2) cases. The DFT findings are important in terms of catalytic functions, because the spatial constraint from the ZSM-5 should significantly contribute to the stability of the reaction intermediates formed during the dioxygen activation.

The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study

“…When a transition metal atom is embedded in a nanometer-scale cavity of a host, the interactions with a guest unsaturated hydrocarbon can be further affected by host confinement. The confinement effects on the inner bond formation have been well discussed in our recent theoretical studies [11,12,13,14,15,16]. In particular, we found various types of binding of a guest molecule into copper cations enclosed in the restricted environment of a ZSM-5 zeolite [14,15,16].…”

Synthesis of an unexpected [Zn2]2+ species utilizing an MFI-type zeolite as a nano-reaction pot and its manipulation with light and heat

“…Previously, our studies confirmed that this B3LYP-optimized MFI zeolite model comprised of 10-MR in cavities reproduced well the experimentally observed data. [12][13][14]22,[24][25][26][27][28][29][30][31] Taking our previous studies into consideration, we used the M7 site for the Al substitution position; 32 and we defined the Al configuration as the [M7-S2] arrangement. [12][13][14]30 To optimize fully the larger MFI geometry with a 10-MR cavity, we used the 6-311G(d) basis set for the zinc atom and the 6-31G (d) basis set for the Al atom, as well as four O atoms that were connected to the substituted Al atom.…”

Transition-Metal Ions in Zeolites: Coordination and Activation of Oxygen

“…From the calculations of Yumura et al . both side-on peroxo and bis(μ-oxo) dicopper cores were found to be energetically favorable in the 10MR of ZSM-5 91. In the work of Goodman et al .…”