n-Butene skeletal isomerization to isobutylene on shape-selective zeolites: Ferrierite, ZSM-23, \lbrack B\rbrack -ZSM-5, and \lbrack B\rbrack -ZSM-11

The strengths of acid sites and the pore sizes of zeolites have been successfully controlled for shape-selective isomerization of n-butene to isobutylene. Medium pore (B) -ZSM-5 and (B) -ZSM-11 and small pore ferrierite/ZSM-35 and ZSM-23 zeolites have been synthesized using hydrothermal methods. Crystallinity, morphology, acidity, porosity, thermal stability, bonding properties, and surface properties of these zeolites have been extensively characterized by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, scanning Auger microscopy, temperature-programmed desorption, microbalance, BET surface area measurements, pore size distributions, and inductively coupled plasma analysis. Different kinds of acid sites responsible for skeletal isomerization and dimerization reactions have been revealed. Weakening of strength of acid sites of medium pore zeolites ZSM-5 and ZSM-11 by boron substitution leads to a limited success in improvements of yields and selectivities to isobutylene. Use of small pore zeolites ferrierite/ZSM-35 and ZSM-23 yields a shape selective isomerization of n-butene to isobutylene. Coke deposition enhances shape selectivity by partially blocking the zeolitic channels so that the space around acid sites is not large enough for dimerization reactions. It also poisons some strong acid sites responsible for dimerization and subsequent cracking reactions. Mechanisms of n-butene skeletal isomerization and accompanying side reactions have also been carefully elucidated. Methods for modification of non-template synthesized ferrierite/ZSM-35 materials are proposed and successfully established. Effective removal of non-shape selective acid sites and decreasing of the density of acid sites significantly improve catalytic performances of non-template synthesized materials to the level similar to template-synthesized materials of higher crystallinity. At the present time, this process is on the way to commercialization by Texaco, Inc. for an alternative source of isobutylene to produce methyl t-butyl ether for boosting the octane number of gasoline and reducing the automobile emission of volatile organic compounds.