Isomerization of n-butenes on a-type zeolites studied by infrared spectroscopy. Part 3.—Adsorption and isomerization of n-butenes on zeolites containing transition metal ions

Abstract

Adsorption and isomerization of n-butenes on A-type zeolites containing zinc, silver and cobalt ions have been studied at room temperature by Fourier transform infrared spectroscopy. In the hydrated samples two kinds of water can be distinguished: unspecifically adsorbed water and water specifically interacting with the cations. After complete dehydration no hydroxyl groups are observed in the spectra.

The penetration rate of the n-butenes, which generally follows the sequence trans-but-2-ene > but-1-ene > cis-but-2-ene, is sensitive to the occupation of the S2 sites.

Upon adsorption, rotation of the butenes becomes restricted. In addition to the donor bond, there is back-donation from the cations into antibonding π*-orbitals of the butenes, as evidenced by the unexpectedly high cation-specific red-shift of the CC stretching band.

While silver-containing A-type zeolites are catalytically inactive due to the small charge: radius ratio of the cations, on zeolites exchanged with zinc ions spontaneous isomerization is observed, which is enhanced on samples containing a non-stoichiometric excess of zinc. Addition of the cocatalyst sulphur dioxide greatly increases the isomerization rate. Also, in the case of ZnA zeolites the reaction is first order in both n-butene and SO₂ and is in accord with a concerted reaction mechanism which has been shown to occur on A-type zeolites exchanged with alkali and alkaline earth ions.