Mass-Transfer Enhancement of Heavy Oil Molecules in Hierarchical FCC Catalysts: an in situ Visualization Study Via Fluorescence Imaging

Abstract

This study employed advanced in situ visualization techniques to investigate the mass-transfer enhancement mechanisms of heavy oil components in fluid catalytic cracking (FCC) catalysts. Three FCC catalysts with different matrix compositions were selected: CAT-1 (kaolin), CAT-2 (kaolin/APM-7), and CAT-3 (kaolin/APM-9). Their pore structures were systematically characterized using SEM and N₂ adsorption–desorption. In situ visualization of the diffusion of Rhodamine B-tagged heavy oil components (probe size: 2–5 nm) within the catalyst pores was achieved by laser confocal fluorescence microscopy at a resolution of 200 nm. The results demonstrated that CAT-3, modified with APM-9, exhibited an optimal mesoporous structure (with 35% of pores in the 3–5 nm range) and achieved an effective diffusion coefficient of 1.84 × 10^–13 m²/s, which is 7.3 times higher than that of the conventional kaolin-based CAT-1. Analysis using the Yoon–Nelson model further confirmed a 68% reduction in the adsorption rate constant for CAT-3, revealing the synergistic enhancement effect between hierarchical pore channels and acid sites. This study established a quantitative structure–performance relationship between pore architecture and mass transfer in FCC catalysts, facilitated by the development of an in situ fluorescence visualization technique for probing diffusion within porous materials. This study not only provides a quantitative "pore structure–mass transfer" relationship model for the rational design of FCC catalysts, but also offers a novel characterization method with high spatiotemporal resolution for investigating the mass-transfer mechanism of porous materials.