σ-Type ethane adsorption complexes with Cu+ ions in Cu(i)-ZSM-5 zeolite. Combined DRIFTS and DFT study

Abstract

A DRIFTS study of ethane adsorption by the univalent copper cations in Cu(I)-ZSM-5 zeolite prepared by chemical reaction of the corresponding hydrogen form (Si/Al = 150) with CuCl vapour at 573 K reveals an unusual spectrum of adsorbed C₂H₆. In addition to the weakly perturbed narrow bands in the region of C–H stretching vibrations, the spectrum exhibits two broad IR bands whose frequencies are lowered more than by 300 cm⁻¹ compared to the C–H stretching vibrations of gaseous or physically adsorbed ethane. This indicates unusually strong perturbation of adsorbed C₂H₆ molecules by Cu⁺ ions, which has never been observed before for ethane adsorption on oxides or on any other cationic form of zeolites. Despite such a strong perturbation, the adsorbed ethane molecules are not involved in heterolytic dissociative adsorption at elevated temperatures. DFT cluster modeling of ethane interaction with Cu⁺ stabilized at α-sites (the six-membered ring composed of two five-membered rings from the straight wall of the ZSM-5 zeolite) as possible adsorption sites indicated that C₂H₆ interaction with Cu⁺ results in formation of rather strong σ-type adsorption complexes. The results obtained indicated that the most preferential adsorption geometry is when the alkane bound in an end-on fashion through two hydrogen atoms of one of the methyl groups. The nature of such bonding can be reasonably described by the synergetic combination of the ligand-to-metal donation from the σ(C–H) orbitals of ethane to the low-occupied Cu⁺ s-orbital with the metal-to-ligand back donation from the copper d_π orbital to the C–H σ*-orbitals. Both these interactions result in a very strong weakening of the C–H bonds and, therefore, in a very large bathochromic shift of the C–H stretching frequencies of methyl group contacting with Cu⁺ ion.