Location of Zn2+ ions in Zn-Y zeolites and the SO2-induced activities in the isomerization of cis-but-2-ene

Abstract

The SO₂-induced isomerizations of cis-but-2-ene, the cis-trans conversion and the double-bond shift, have been studied over Zn-Y zeolites as functions of the degree of zinc-exchange of Na-Y zeolite. The location of Zn²⁺ in the zeolites has been examined by volumetric measurements of CO and CO₂ adsorptions and by infrared absorption measurement of CO adsorption. The Zn²⁺ introduced may locate preferentially in the small cages (the prisms and the sodalite cages) for zeolites with degree of exchange less than ca. 60%. Above this level of exchange the divalent cations can locate in the supercages. These characteristic locations of Zn²⁺ have also been reported for the X-type zeolites. The Zn²⁺ in the supercages are the most effective for causing the SO₂-induced cis–trans isomerization for both the X- and Y-type zeolites. The Zn²⁺ located in the small cages of Y zeolites can both initiate the SO₂-induced cis–trans isomerization and generate the Brönsted acid sites necessary for the carbenium ion type isomerization. However, the Zn²⁺ in the small cages of X zeolites did not show such ability. The different results observed between the X-and Y-type zeolites have been ascribed to the difference in the distributions of SiO^4–₄ and AlO^5–₄ in the six-membered rings for the two types of zeolite.