Abstract: n-Hexane was employed as a probe molecule, and a series of ZSM-5 molecular sieve samples with different levels of trace coke deposition were prepared by controlling the number of reactant injections. The effect of coke species evolution on the mass transfer properties of molecular sieve was systematically investigated. A combination of X-ray diffraction, chemisorption, in-situ infrared spectroscopy, among others was used to elucidate the morphology, distribution and chemical nature of coke species. Meanwhile, the diffusion kinetics and adsorption thermodynamics of n-hexane were quantitatively evaluated using the zero-length column technique and an intelligent gravimetric analyzer, respectively. The results demonstrate that the introduction of trace amounts of coke does not damage the crystalline framework structure of the ZSM-5molecular sieve, with coke species preferentially accumulating at the micropore mouth and the adjacent outer surface areas. At the early stage of coke formation, low-ring aromatic coke covers Bronsted acid sites at the pore mouth, weakens the strong adsorption interactions between n-hexane molecules and pore-mouth hydroxyl groups, and thereby reduces the diffusion barrier, leading to enhanced intracrystalline diffusion. As the reaction proceeds further, higher-ring polycyclic aromatic hydrocarbons (e.g., anthracene and phenanthrene) and graphitized coke gradually accumulate, forming a continuous deposit at the pore mouths and external surfaces. This results in partial pore blockage and a substantial increase in diffusion resistance, significantly suppressing the mass transfer and diffusion rate of n-hexane. Overall, this study clarifies the dual regulatory effect of trace coke deposition on n-hexane diffusion behavior, providing new insights into coke-induced deactivation and pore-mouth diffusion regulation in molecular sieve catalysts.

Key words: trace coke deposition, n-hexane, ZSM-5, molecular sieve, diffusion resistance of pore mouth, coke species