Zeolite morphology and catalyst performance: conversion of methanol to hydrocarbons over offretite

The synthesis, characterization and catalytic performance of offretite with four distinct crystal morphologies (oval, hexagonal, broccoli-like, and spherical) are presented. As a member of the ABC-6 family of zeolite structures, offretite is likely to form intergrowths which can affect its shape-selective properties. Herein, a combination of experiments (SC-XRD, PXRD, benzene uptake and the methanol-to-hydrocarbon conversion) allows the determination of the mechanism and fraction of twinning. It is demonstrated that the catalysts with the same crystal aspect ratio (AR = 3) yield product spectra dominated by C-2-C-4 aliphatics. Such product selectivity is associated with zeolites with an 8-ring channel system. Interestingly, a significant production of bulky aromatics is seen for the catalyst with the spherical morphology (AS = 1.1) which is a manifestation of the 12-ring pore system of the OFF framework topology. This constitutes the first report of this behavior. The particular display of product shape selectivity can be ascribed to the catalyst particle morphology.