Generating TON zeolites with reduced [0 0 1] length through combined mechanochemical bead-milling and porogen-directed recrystallization with enhanced catalytic property in hydroisomerization

Abstract

Mechanochemical bead-milling of micron-meter sized zeolites to nanometers provides an effective route to enhance their diffusion and catalytic properties. This post-synthetic method is particularly important to reduce crystals of unidimensional zeolites preferentially along their channel direction, as direct synthesis is difficult to achieve since they habitually grow into rod-like crystals. The milling process is often escorted by surface amorphorization and structure degradation, hence, a subsequent recrystallization is often entailed to restore crystallinity. Herein, taken ZSM-22 as an example, it is demonstrated that secondary crystal growth and coalescence during recrystallization, a process that often offset the effect of milling, could be suppressed by the introduction of porogens in the post-milling recrystallization process. The combined bead-milling and porogen-directed recrystallization method allows the preparation of highly crystalline TON crystals with an axis length of largely 100–300 nm, which is record short in axis [001] channel length. The Si/Al ratio could be manipulated between 40 and 300, permitting the influence of acidity and diffusion to be evaluated. The catalytic assessments in n-heptane hydroisomerization demonstrate that isomer yields can be enhanced from 56% for parent material to up to 74% for an optimized catalyst with Si/Al of 200. Correlating of zeolite structure, kinetic studies and catalytic behavior discloses that low acid site density and improved diffusion property are critical factors to decrease cracking probability and improve isomerization selectivity. This study opens new ways to tailor diffusion properties of unidimensional zeolites, and are potentially extendable to other zeolites.