Physicochemical and Catalytic Properties of Copper Ethylenediamine Complex Encapsulated in Various Zeolites

Abstract

Bis(ethylenediamine)copper(II) [Cu(NH2CH2CH2NH2)2]2+(Cu-en) was encapsulated in NaY, KL, AlMCM-41, Naβ, and Na-ZSM-5. Characterization of the catalyst was done by IR, UV−vis, and electron paramagnetic resonance (EPR), whereas redox properties of neat and encapsulated Cu-en complexes were evaluated via cyclic voltammetry. The difference in UV−vis and EPR parameters between neat and intrazeolite complexes suggests that the metal complex does not have the same geometry as that in the neat complex but does undergo distortion inside the system. Peak broadening and different electrochemical responses are observed on encapsulation of the Cu-en complex in various zeolites, thus indicating that Cu-en complexes have altered values of redox potential in various positions of zeolite. Change of redox potential of Cu-en complex in zeolites is due to a change in the positions of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the metal complexes. Density functional theory is used to calculate the position of the HOMO and LUMO levels of the metal complexes in zeolite cluster models. The HOMO and LUMO levels increase on encapsulation. When a substitution of the heteroatoms (B, Al, Ga) occurs, the HOMO and LUMO levels are stabilized, compared to those in pure silicious zeolites. An electric field acting inside the zeolite matrix is responsible for the change in the position of HOMO and LUMO levels of the metal complexes in various zeolites. Catalytic activity toward the oxidation of dimethyl sulfide by intrazeolite Cu-en complexes exhibits higher catalytic activity, compared to the neat complex.