Isomerization of cis-but-2-ene enhanced by SO2 adsorbed over decationated-Y and NaY zeolites

Abstract

The promoting action of SO₂ on the isomerization of cis-but-2-ene over decationated-Y and NaY zeolites has been characterized. SO₂ over decationated zeolite enhanced cis–trans isomerization when the zeolite was pre-activated in a vacuum at temperatures < 450°C. For zeolites activated at higher temperatures, SO₂ promoted two types of the isomerizations, i.e., cis—trans conversion and double-bond shift. The generation of electronegative sites by dehydroxylation of the zeolite at high temperatures is thought to be essential for the favourable action of SO₂ on the double-bond isomerization. SO₂ adsorbed on the site may enhance the acid strength of Brönsted sites by attracting negative charge from the catalyst. SO₂ over NaY zeolite induced cis—trans isomerization alone. Selective cis—trans isomerizations over NaY and over the decationated zeolite activated at low temperatures ( < 450°C) were interpreted by the polysulphone- or co-oligomer-accompanying mechanism proposed previously. The presence of Na⁺ ions in the zeolites is essential for the selective isomerization.