Selective Conversion of n-Butene to Isobutylene at Extremely High Space Velocities on ZSM-23 Zeolites

Abstract

n-Butene has been isomerized to isobutylene on zeolite ZSM-23 catalysts at extremely high space velocities from 171 to 342 WHSV. The zeolite catalysts were prepared with hydrothermal methods by using pyrrolidine as a structure-directing template. The prepared materials have been characterized by SEM-EDX, XRD, FTIR, AA-ICP, TPD, BET surface area/pore size distributions, and pyridine chemisorption. Selectivities to isobutylene ranged from 85 to 95% and yields of isobutylene from 30 to 20%, depending on the space velocity of but-1-ene. Good stability in the catalytic activity for n-butene skeletal isomerization is an important characteristic of such ZSM-23 zeolites. Isobutylene is believed to be formed from n-butene via a methyl cyclopropane carbenium intermediate and this is a reversible process. Dimerization of butene molecules is a primary side reaction for n-butene skeletal isomerization. The dimerized products (octenes) are further cracked into propylene and pentenes via β-scission of carbenium intermediates. Propylene, a product of the secondary reaction, is then dimerized to form hexenes or codimerized with butene to form heptenes. Conversion of but-1-ene to cis/trans-but-2-enes is greater than one predicts from thermodynamic equilibrium data. cis-But-2-ene is observed to be the preferential product for but-1-ene double bond migration. The preferential formation of cis-but-2-ene is due to a steric interaction of the methyl group in the secondary butyl carbenium intermediate with the pore wall of the small pore zeolite, ZSM-23. Zeolite ZSM-23 also shows shape selectivity for adsorption of ammonia, but-l-ene, and isobutylene. The shape selectivities of these materials are further improved after aging of catalysts used in but-1-ene skeletal isomerization.