Isomerization reactions of n-butenes over isomorphously substituted B/A1-ZSM-11 zeolites

Abstract

Low levels of boron have been isomorphously substituted into pentasil zeolites and are active in butene isomerization reactions. The acidity of B/A1-ZSM-11 zeolites, measured by NH 3 adsorption, is significantly lower than the acidity of B 3+-free Al/ZSM-11 zeolites. These materials exhibit both Brønsted and Lewis acidities as determined by pyridine adsorption studies. Adsorption/desorption experiments with 1-C 4H 8 and i-C 4H 8 were studied to determine reactant and product shape selectivities. Complete isomorphous substitution of framework Al 3+ by B 3+ in ZSM-11 zeolites results in inactivity. Our data show that the reaction is first order in 1-C 4H 8 with slow deactivation via coke formation. The effect of flow rates on conversion, yields, selectivities, and product distributions has been studied on B/A1-ZSM-11 materials. Isobutene yields of 22% and selectivities of 47% have been obtained under optimal operating conditions using B/Al-ZSM-11 catalysts. Lower yields of i-C 4H 8 were obtained using Pt/B/Al-ZSM-11 zeolites with n-C 4H 10 as a feed gas. A temperature of 523°C was used in isomerization reactions while a temperature of 580°C was required in dehydrogenation/isomerization reactions. Propene is the major byproduct of this reaction, believed to form as a result of cracking of dimerized (and polymerized) olefins. This side reaction is the primary cause for deactivation and coke formation on the catalyst.