Isomerization of n-butenes on A-type zeolites studied by infrared spectroscopy. Part 1.—n-butene adsorption on zeolites containing alkali and alkaline earth cations

Abstract

Adsorption of n-butenes on highly activated type A-zeolites containing alkali and alkaline earth cations is studied by infrared spectroscopy. The rate of adsorption is determined by the butenezeolite interaction, depending on the dipole moment and the steric requirements of the butene molecules, and the occupancy of the cation sites.The n-butenes are strongly physisorbed without fragmentation with preservation of the C=C bond. The adsorption is accompanied by the appearance of forbidden i.r. fundamentals, small frequency shifts, a change in band intensities and a decrease in band widths. Narrow bands point to a largely restricted rotational freedom. The frequency shift of the C=C band is cation sensitive and shows a direct polarizing power dependence. Under formation of a v-complex the butenes are attached to the cations in a " side on " mode. In the case of the alkali and alkaline earth forms isomerization is not affected. In the far infrared region phonon induced absorptions are observable.

Generation, Characterization, and Transformations of Unsaturated Carbenium Ions in Zeolites

“…The same intermediates, which are formed from olefins as the result of alcohol dehydration, have also been proposed for alcohol adsorption. The frequency of the CC mode of olefins adsorbed on nonframework cations of zeolites was found to be dependent on the nature of the cations . Since on acidic zeolites the adsorption of olefins is accompanied by fast oligomerization, their CC stretching fundamental can hardly be observed.…”

“…160 Reactions of butenes over zeolites have been exhaustively investigated by many research groups. 137,[161][162][163][164][165][166][167] Due to the different surface complexes formed during adsorption on basic and acidic catalysts, the cis/trans ratio, obtained from isomerization of 1-butene to cis-and trans-2-butenes, is used to characterize the acid-base properties of solid catalysts. On acidic zeolites, fast oligomerization of butenes takes place even below room temperature.…”

“…The frequency of the CdC mode of olefins adsorbed on nonframework cations of zeolites was found to be dependent on the nature of the cations. 167 Since on acidic zeolites the adsorption of olefins is accompanied by fast oligomerization, their CdC stretching fundamental can hardly be observed. Bands appearing between 1613 and 1650 cm -1 are assigned to terminal CdC double bonds of olefins interacting with Brønsted acid sites.…”

Use of FTIR spectroscopy in the investigation of catalytic reactions in zeolites?Possibilities and limitations

“…The first example concerns n-butene isomerization on alkali and alkaline earth ion-exchanged zeolites A, for the study of which FTIR spectroscopy proves to be well-suited in order to both, elucidate the details of interaction with the active sites and monitor the reaction itself [1,2]. The starting n-butene transforms to an equilibrium mixture of cis-butene, transbutene and but-l-ene, while isobutene is missing due to the shape-selective property of the zeolite.…”