Intensities of IR Stretching Bands as a Criterion of Polarization and Initial Chemical Activation of Adsorbed Molecules in Acid Catalysis. Ethane Adsorption and Dehydrogenation by Zinc Ions in ZnZSM-5 Zeolite

Abstract

A DRIFT study of ethane adsorbed by zinc cations in ZnZSM-5 prepared by chemical reaction of the hydrogen form of the zeolite with zinc vapor at 770 K, or by wet ion exchange, reveals unusual spectra of adsorbed C2H6 species. In addition to the weakly perturbed narrow bands in the region of C-H stretching vibrations, these spectra exhibit a very intense broad IR band with a frequency that is more than 200 cm(-1) lower than those of the C-H stretching vibrations of gaseous or physically adsorbed ethane. The very high relative intensity of this band indicates a very strong polarizability of the corresponding vibrational mode. It is concluded that these strongly polarized vibrations are closely connected with the subsequent heterolytic dissociation of ethane at moderately elevated temperatures, resulting in the formation of acidic hydroxyl groups and zinc ethyl fragments. At higher temperatures, the zinc ethyl fragments decomposed, resulting in the formation of zinc hydrides and ethylene. The unusual DRIFT spectrum of the molecular form of ethane adsorption by zinc cations represents a first example of anisotropy of polarizability of adsorbed molecules, which may provide a new explanation for selectivity of the acid-catalyzed reactions. In this connection, it is suggested to use the relative intensities of IR bands of adsorbed molecules as a reactivity index that is directly connected with chemical activation of adsorbed molecules via their polarization by the active sites.

Unusual Infrared Spectrum of Ethane Adsorbed by Gallium Oxide

“…Obviously, the perturbation and polarization of the other C-H bonds are much weaker. These results are similar to those earlier reported in our study [2] on ethane adsorption by Zn +2 ions in ZnZSM-5 zeolite, where it was also shown that polarization of adsorbed molecules was followed by heterolytic dissociative adsorption at elevated temperatures.…”

“…[1][2][3] such new spectral approach was supported by our experimental results on adsorption of methane and ethane by ZSM-5 zeolite modified with Zn +2 cations. In these studies we demonstrated that, at elevated temperatures, the most intense C-H vibrations which correspond to the stretching of the highly polarized C-H bonds of molecules adsorbed by the coordinatively unsaturated zinc cations are involved into subsequent heterolytic dissociative adsorption via abstraction of protons by the adjacent basic oxygen atoms.…”

Intensities of combination IR bands as an indication of the concerted mechanism of proton transfer from acidic hydroxyl groups in zeolites to the ethylene hydrogen-bonded by protons

“…In contrast, unlike adsorption of light paraffins on the Lewis sites that has been previously studied by us in Refs [2,3], formation of the π-complexes has practically no influence on intensities of the fundamental bands from C-H stretching vibrations. This indicates the absence of polarization of C-H bonds in the π-bonded species which are not involved into reaction coordinate of proton transfer to adsorbed ethylene.…”

“…Bearing this in mind, in previous work [2,3] we proposed using intensities of C-H IR stretching bands as measures of the polarization of adsorbed light paraffins by the low-coordinated cations in zeolites. The obvious advantage of this new spectral approach lies in the closer connection of the intensities of IR bands with polarization of C-H chemical bonds on stretching via vibrational excitation.…”

Self-organization of extraframework cations in zeolites

“…For example, by reacting bridging hydroxyl groups in high-silica zeolites with vapours of metallic Zn (Kazansky & Serykh 2004;Kazansky & Pidko 2005a;Pidko & Kazanskii 2005) and Cd (Serykh 2005) at a high temperature, their quantitative replacement by bare Zn 2+ and Cd 2+ cations at the zeolitic exchange sites was achieved. However, owing to the inhomogeneous distribution of pairs of framework aluminium, a substantial variation in the chemical reactivity of the introduced cationic species was observed.…”