ZSM-5 zeolite is a typical representative of shape-selective catalyst and widely used in catalytic reaction, such as methanol-to-propylene (MTP) reaction, isomerization, alkylation and aromatization. Generally, the reaction products distribution can be modulated by adjusting the ratio of two channels of ZSM-5 zeolite, which consist of a sinusoidal channel (5.1 × 5.5 Å) in the a-c plane and a straight channel (5.3 × 5.6 Å) in the b direction.Therefore, knowledge of the mass transport in straight and sinusoidal channels can contribute to the understanding of product distribution in the catalytic reaction and the guidance of catalyst synthesis with specific orientation.

Recently, a research group led by Prof. Xushutao, Prof. Weiyingxu and Liuzhongmin from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences made a new progress on studying of intracrystalline diffusion of ZSM-5 zeolite, and successfully differentiatedthe diffusivity in different channels of ZSM-5 Zeolite by Pulsed Field Gradient (PFG) NMR.

Although theoretical method such as molecular dynamics simulations can obtain the diffusivities of MFI structure on three coordinates, differentiating diffusivities in sinusoidal channeland straight channel is still an experimentalchallenge.In this work, the diffusivity of n-butane in ZSM-5 zeolite were described as a tensor (D_xxdenotes the diffusion tensor alongsinusoidal channel, D_yydenotes the diffusion tensor alongstraight channel), and the information about the diffusion tensor is contained in the shape of the spin echo attenuation, which was obtained through the¹H PFG NMR experiments. By preciselyfitting of the spin-echo attenuation curves, the diffusion coefficient of 2.0×10-⁹m²/s and 2.6×10-¹⁰m²/s for n-butane in the straight channel and sinusoidal channel are extractedwith the loading of 4 molecules per unit cell at 333 K.This method is of great importance in differentiating the diverse diffusion in different directions of complex catalyst channels, contributing to a better understanding of the mass transfer phenomena in catalytic reactions and a guidance of the catalyst synthesis with specific orientation.

This work was published inChemCatChem, and supported by National Science Foundation of China. (Text by Shu Zeng)

Differentiating Diffusivity in Different Channels of ZSM‐5 Zeolite by Pulsed Field Gradient (PFG) NMR. Shu Zeng, Shutao Xu*, Shushu Gao, Mingbin Gao, Wenna Zhang, Yingxu Wei*, Zhongmin Liu*, ChemCatChem, 12(2):463-468, 2020.