Effect of acidity and porosity of hierarchical HBEA zeolite on catalytic stability of α-methylnaphthalene isomerization

Hierarchical HBEA catalysts were fabricated through alkaline treatment and evaluated in a fixed-bed reactor. The relative crystallinity, pore property, Si/Al molar ratio, coke formation and structure of catalysts were investigated by the methods including X-ray diffraction, N₂ physisorption, X-ray fluorescence, thermogravimetric analysis and transmission electron microscope, respectively. The acidity of catalysts was studied by Fourier transform infrared spectrometry using pyridine as the probe molecule. These results demonstrated that pure NaOH treatment improved the mesoporosity of catalyst, while decreased the β-methylnaphthalene selectivity and catalyst stability due to the partially destroyed framework and an increase in the concentration of Lewis acid sites. The Lewis acid sites are responsible for the hydride transfer and subsequent reaction that form coke. In contrast, the presence of tetra-propyl-ammonium hydroxide in the mixed-alkali treatment resulted in regular mesopores development, preserved intrinsic zeolite properties and Lewis sites proportion, leading to the maintained β-MN selectivity and coke deposition rate. The increased mesoporosity and strong Brönsted acid sites amount per number of coke were responsible for the improved coke tolerance of mixed-alkali treated HBEA, and thus its deactivation rate was 41.7% lower than that of HBEA catalyst.