Fine Tuning the Diffusion Length in Hierarchical ZSM-5 to Maximize the Yield of Propylene in Catalytic Cracking of Hydrocarbons

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.