Nature of the Cu+-NO bond in the gas phase and at different types of Cu+ sites in zeolite catalysts

The activation of Cu+ ions for NO binding by the zeolite ligand is analyzed. Density functional theory (DFT) calculations show that the interaction of NO with isolated Cu+ ions and with Cu+ ions in zeolites is of different character. In gas-phase Cu+-NO complexes interactions of the a' singly occupied orbital (SOMO) on NO with the unoccupied 4s and the occupied 3d orbitals on Cu+ depopulate the antibonding SOMO and this strengthens the NO bond. In the Cu/zeolite system electrostatic attraction increases the Pauli repulsion and pushes up the (occupied) 3d orbitals, which can interact with the a LUMO of NO. This interaction populates the antibonding LUMO of NO and, hence, weakens the NO bond. The binding of NO onto Cu+ sites in high-silica zeolites NIFI and FER is investigated by the combined quantum mechanics/interatomic potential function approach that describes the particular framework topology. Two types of Cu+ sites are studied, on the channel wall (coordination to 3-4 framework O atoms) and on the channel intersection (coordination to 2 O atoms). Upon the interaction with NO the Cu+ ion stays coordinated to only two framework oxygen atoms belonging to the AlO4 tetrahedron and the structures of all ON-Cu/zeolite adsorption complexes become very similar. EPR hyperfine coupling constants (HFCC were calculated and compared with experiment. The isotropic component of HFCC strongly depends on CuNO angle, but due to small variations of this angle, it will be difficult to resolve EPR signals (or other spectroscopic signals) for different sites. The interaction of NO with the Cu+ sites on the channel intersection is significantly stronger (29.5-27.1 kcal/mol) than the interaction with the Cu+ sites on the channel wall (22.6-15.0 kcal/mol) because framework deformation into a state prepared for bonding NO costs (deformation) energy.