posted on 2018-10-08, 00:00authored byXinglong Dong, Sohel Shaikh, Jullian R. Vittenet, Jianjian Wang, Zhaohui Liu, Kushal D. Bhatte, Ola Ali, Wei Xu, Isidoro Osorio, Youssef Saih, Jean-Marie Basset, Shaikh A. Ali, Yu Han
Direct synthesis
of hierarchical zeolites using macromolecules
as soft templates can yield zeolite particles with precisely controlled,
ultrashort diffusion lengths, leading to the ability to tune the product
selectivity in the catalytic conversion of hydrocarbons. In this study,
we prepared hierarchically structured, mesoporous zeolite ZSM-5 via
one-pot synthesis using specially designed polymers as bifunctional
templates. We demonstrated that the mesoporosity and thus the effective
crystallite size (or diffusion length) of hierarchical ZSM-5 can be
finely tuned by varying the density of structure-directing agent groups
in the polymer template. When evaluated in the cracking of hydrocarbons
to light olefins, hierarchical ZSM-5 catalysts with enhanced mesoporosity
exhibited higher activity for feedstocks containing large molecules;
furthermore, their short diffusion length favored the selectivity
of propylene and total light olefins, irrespective of the feedstock
used. While maintaining the Si/Al ratio in ZSM-5 and the reaction
conditions, we maximized the yield of propylene by reducing the apparent
diffusion length in ZSM-5 down to ∼7 nm through our controlled
templating synthesis process.