The nature of active sites in the isomerization of 1-hexene on cracking catalysts

The molar ratio of cis/trans 2-hexene isomers formed from 1-hexene on NaY zeolites exchanged to various levels with protons or lanthanum ions can be explained by assuming that two types of sites are active for the 1–2 isomerization reaction. The first gives both isomers, while the second gives only the cis isomer. In HY zeolite, active sites for the isomerization reaction can be produced by substitution of the most easily replaced sodium ions, either by simple washing with water, or by calcination of a partially exchanged ammonium form. The catalyst activity and the observed ratio of cis/trans isomers formed on a partially exchanged LaY zeolite, has been found to depend on the degree of hydration of the catalyst. Ratios of cis/trans 2-hexene are closer to their equilibrium value when the reaction is carried out on ZSM-5, mordenite and amorphous silica-aluminas rather than on Y zeolite. This is explained in terms of the proportion of each type of site present, although diffusional limitations within the narrower pore structures of ZSM-5 and mordenite may also play a role.