Designing hierarchically b-axis shortening for enhanced diffusion and coke accommodation in efficient methanol to olefins

Enhancing the light olefin selectivity and extending the catalytic durability remain critical challenges for ZSM-5 zeolites in the methanol to olefins (MTO) conversion, while the inherent diffusion restriction along the MFI b-axis direction and poor coke accommodation are the main limiting factors. In this work, we developed a hierarchically single-crystalline ZSM-5 sheet architecture that features an interconnected multiscale porosity and a remarkably reduced b-axis thickness (<50 nm), as quantitatively verified by three-dimensional (3D) electron tomography. Real-time confocal laser scanning microscopy (CLSM) tracking demonstrated a significant enhancement in molecular diffusivity compared to conventional micron-sized ZSM-5 counterparts (Micro-ZSM-5). This engineered structure allows abundant aluminum sites to be distributed on the highly accessible diffusion pathways, and displays an enlarged coke accommodation of 16.31 wt% with a coke deposition rate of 0.59 mg g⁻¹ h⁻¹, which was only one third of that in the Micro-ZSM-5. In a continuous MTO process, this novel hierarchical ZSM-5 sheet (Hier-ZSM-5-S) kept an average selectivity to ethylene and propene of 63.5% on stream for 22.2 h (WHSV = 3.6 h⁻¹, T = 480°C), which was 19% higher and 6.5 times longer than those of Micro-ZSM-5, respectively. This hierarchically shortened b-axis structure establishes a generalizable paradigm for enhanced diffusion and coke accommodation in a precisely designed pore system, which is expected to be adjusted and applied for varying reactions. (Figure presented.)