The zero-length column technique was performed on four hydrotreating catalyst pellets with different pore size distributions at a temperature range from 303 to 423 K. In order to analyze the experimental curves of alumina-supported catalysts with meso/macroporous structures, normalized desorption curves were corrected with blank data, and the model expansion method was applied to process data and determine diffusivities at the middle region of the curves, in which the diffusion of the probe molecule was assumed as a desorption-diffusion process. The results showed that diffusional time constants D e /R 2 and pore diffusivities εD p of these catalyst pellets increased in the presence of macropores. Moreover, a typical mathematical formula for restricted diffusion,, was used to obtain restricted factors (1 − λ) z and tortuosity τ by combining the pore diffusion coefficients and bulk diffusion coefficients. The fitting parameters confirmed the assumption used in the analysis of the desorption curves, which indicated that additional macropores in catalyst pellets enhanced the pore diffusivities by shortening the diffusional path length and decreasing the restricted factor.

Hierarchical ZSM-5 zeolites have been receiving increasing attention from both fundamental research and industrial applications. From the chemical engineering viewpoint, the introduction of building block of ZSM-5 could give consideration to both external surface acidities and diffusion properties which are important in parallel sequence reaction for the final product distribution. In this work, hierarchical ZSM-5 with different sizes of building blocks were successfully prepared by tuning the water/silica ratio during the synthesis. With varying the size of building blocks, the diffusion property and external surface acidity were finely regulated, and it significantly influences the performances of ZSM-5 during catalytic cracking reaction. The catalyst with proper size of building blocks exhibited the optimized yield of olefins and lowest carbon deposition during 72 h reaction in catalytic cracking of i-butane. The strategy proposed herein could be helpful for the engineering design of hierarchical zeolites for industrially important catalytic reactions.

A series of core‐shell HZSM‐5@mesoSiO2 with tunable shell thickness from 10 to 70 nm was prepared and studied for n‐butane catalytic cracking. With introducing of SiO2 shell, the catalytic performance of HZSM‐5@mesoSiO2 was largely enhanced, and n‐butane conversion rate per Al site reached to 2.43 min−1 over HZSM‐5@mesoSiO2(1:4) at 675°C which is nearly twice to that of HZSM‐5 (1.34 min−1). The diffusion property of n‐butane over as‐prepared sample was quantified by measuring the diffusional time constants using zero length column chromatography technique (ZLC). Combining with chemical reaction kinetic analysis, the quantitative relationship between diffusion property and catalytic performance was effectively established for the first time in n‐butane catalytic cracking. Positive linear correlation between diffusional time constant (D/R2) and n‐butane conversion rate per Al site could be found, which confirms that diffusion enhancement by hierarchical structure is an effective strategy to improve the activity of HZSM‐5 in catalytic cracking.

Diffusivities of a series of hydrocarbons with different configurations in two zeolites (ZSM-5 and SBA-15) and a hydrotreating catalyst (NiMo/Al 2 O 3 ) were determined by the zero-length column technique. The results revealed that the molecular configuration of hydrocarbons has a significant effect on the diffusion property in zeolites. In addition, the aromatics show surface diffusion effect in the mesoporous SBA-15 and NiMo/ Al 2 O 3 . The diffusion characteristic of 1,3,5-triisopropylbenzene (TiPB) was applied to evaluate the change of diffusion properties of a mesoporous-scale hydrotreating catalyst with 2000 h of deactivation. Arrhenius plots of the diffusional time constants show two distinct linear fitting regions for both the fresh and aged catalysts. The reduction degree in diffusivity over the aged catalyst shows an increasing tendency at low temperatures but gradually decreases at high temperatures. A correlation between pore size and diffusivities was established to reveal the influence of structure on its diffusion property.

No abstract