Isomorphous substitution in ZSM-22 zeolite. The role of zeolite acidity and crystal size during the skeletal isomerization of n-butene

The influence of acid-site density, acid strength and crystal size on the skeletal isomerization of 1-butene using Al-, Ga-, and Fe-ZSM-22 zeolites with different Si/T ratios has been investigated. The highest isobutene selectivity (90% selectivity at 40% conversion) was found with the Fe-ZSM-22 which has milder acid sites. The fact that some iron comes out from the zeolite framework during the treatment of the catalyst, makes that the activity and selectivity of the ironsilicates is similar regardless of the starting Si/Fe ratio. This was not the case for the aluminosilicates, for which the isobutene selectivity increases when decreasing the acid-site density, i.e. when increasing the Si/Al ratio. The zeolite synthesis procedure (static or rotating conditions) produces samples with different crystal sizes. This has important implications on zeolite stability, activity and selectivity. All samples display a stable behavior with time-on-stream, and the kinetic results suggest that isobutene is formed by a monomolecular reaction pathway.