Hierarchically tetramodal-porous zeolite ZSM-5 monoliths with template-free-derived intracrystalline mesopores

Abstract

In this work, we report a unique synthesis of hierarchical zeolite ZSM-5 monoliths using polyurethane foam (PUF) as a rigid scaffold through a simple steam-assisted crystallization (SAC) method. The resultant monolithic ZSM-5 with a well crystalline structure possesses unique tetra-modal porosity (macropore/macropore/mesopore/micropore). The first modes of macroporosity (pore size of ∼33 μm) is formed after the removal of the PUF scaffolds. The second set of macropores with a size of 0.2–1.7 μm originate from the aggregation of ZSM-5 nanocrystals (crystal size of ∼500 nm) inside the macropores of the PUF scaffolds. The third level of porosity stems from the intracrystalline mesopores (∼53 nm) in each ZSM-5 crystal, along with the fourth intrinsic microporosity of zeolites. The mesopores are derived from the voids formed in the initial dry precursors, which are transformed and preserved as intracrystalline mesopores in each zeolite crystal. This type of ZSM-5 monolith possesses a high macroporosity (75.2%) and mechanical stability (1.2 MPa). The total surface area of the monolithic ZSM-5 is ∼226 m² g⁻¹ and the total pore volume is ∼0.21 cm³ g⁻¹. The ZSM-5 monolith as a structured catalyst for methanol to propylene (MTP) conversion shows excellent catalytic performance with high methanol conversion (above 95%) and propylene selectivity (above 40%) at a high weight hourly space velocity (WHSV, 3.6 h⁻¹). Importantly, the diffusion efficiency of catalyst is remarkably improved. After reaction for 5 h, the selectivity of propylene reaches a steady state.