SELECTIVE CONVERSION OF N-BUTENE TO ISOBUTYLENE AT EXTREMELY HIGH SPACE VELOCITIES ON ZSM-23 ZEOLITES

n-Butene has been isomerized to isobutylene on zeolite ZSM-23 catalyst s at extremely high space velocities from 171 to 342 WHSV. The zeolite catalysts were prepared with hydrothermal methods by using pyrrolidin e as a structure-directing template. The prepared materials have been characterized by SEM-EDX, XRD, FTIR, AA-ICP, TPD, BET surface area/por e size distributions, and pyridine chemisorption. Selectivities to iso butylene 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 importan t characteristic of such ZSM-23 zeolites. Isobutylene is believed to b e formed from n-butene via a methyl cyclopropane carbenium intermediat e and this is a reversible process. Dimerization of butene molecules i s a primary side reaction for n-butene skeletal isomerization. The dim erized products (octenes) are further cracked into propylene and pente nes via beta-scission of carbenium intermediates. Propylene, a product of the secondary reaction, is then dimerized to form hexenes or codim erized with butene to form heptenes. Conversion of but-1-ene to cis/tr ans-but-2-enes is greater than one predicts from thermodynamic equilib rium data. cis-But-2-ene is observed to be the preferential product fo r but-1-ene double bond migration. The preferential formation of cis-b ut-2-ene is due to a steric interaction of the methyl group in the sec ondary butyl carbenium intermediate with the pore wall of the small po re zeolite, ZSM-23. Zeolite ZSM-23 also shows shape selectivity for ad sorption of ammonia, but-l-ene, and isobutylene. The shape selectiviti es of these materials are further improved after aging of catalysts us ed in but-1-ene skeletal isomerization. (C) 1994 Academic Press, Inc.