New insight into seeding process leading to improved zeolitic acidity and surface properties for its catalytic application in propane aromatization

Abstract

Three ZSM-5 samples (Z-0.1, Z-1.5 and Z-5.0) were prepared by the application of Silicalite-1 seed with amount of 0.1 wt.%, 1.5 wt.% and 5.0 wt.% SiO2 in initial gel. As more seeds were added, the particles size of resulting zeolites reduced dramatically from 1.5 μm to 0.1 μm while the mesopore volume increased from 0.099 cm3/g to 0.458 cm3/g. More interestingly, the bulk Brönsted acid sites density decreased and the crystal surface varied from silicon-rich to aluminum-rich in the meantime. Their catalytic performance on propane aromatization was therefore influenced. Z-1.5 and Z-5.0 exhibited better conversion and BTX selectivity than Z-0.1. The H-type ZSM-5 samples were then modified with Zn impregnation method. By the combination of UV–vis, XPS and Py-FT-IR techniques, Zn was discovered to have a better distribution and generated more (ZnOH)+ species on Z-5.0 sample, which had a smaller particle size and an aluminum-rich surface. Despite the similar initial activity, the catalytic stability was greatly improved from Zn/Z-0.1, Zn/Z-1.5 to Zn/Z-5.0, and the BTX selectivity kept at around 55 %. Serious coke was found on spent Zn/Z-0.1 catalyst with nearly half of the coke in the micropore system due to its high Brönsted acid sites density, poor Zn distribution and a long diffusion pathway, while the coke on spent Zn/Z-5.0 was mainly on the external surface. This paper offers a new insight of seeding into tuning not only the morphological properties but more importantly the acidity and the surface properties of zeolitic materials as catalysts and catalyst supports.