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(1)-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 alpha-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 sigma-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 sigma(C-H) orbitals of ethane to the low-occupied Cu+ s-orbital with the metal-to-ligand back donation from the copper d(pi) orbital to the C-H sigma*-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.