Abstract
In the present work, the desilication of the H-ZSM-5 and H-ZSM-12 zeolites with Si/AL ratios of 28 and 56 and crystal sizes of around 200 and 600 nm, respectively, was performed. Through electron transmission microscopy analysis, it was possible to verify the influence of aluminium gradients on smaller crystals in addition to the exact moment at which the formation of mesoporous is initiated inside the zeolitic framework. Moreover, the crystal size exerts a significant influence on the total amount of acid sites remaining after alkaline treatment, as verified by means of temperature-programmed desorption of ammonia. This is because the presence of larger crystals during desilication may increase the likelihood of reinsertion of aluminium atoms in the zeolite framework and, consequently, maintain the same total amount of acid sites in the zeolite as before alkaline treatment. Meanwhile, aluminium atoms tend to be removed from smaller crystals, thereby decreasing the total amount of acid sites.
Highlights
In catalytic reactions with bulky molecules, the unique presence of micropores in zeolites can limit catalytic performance by restricting molecular transport within its crystals
Comparing the zeolite diffractogram of H-ZSM-5 shown in Figure 1a with other standards available in the literature, 22,23 it is possible to state that the material corresponds to an H-ZSM-5 zeolite with high crystallinity and without the presence of undesirable phases
The same conclusion can be reached on observing the zeolite diffractograms of H-ZSM-12, shown in Figure 1b . 17,24-27
Summary
In catalytic reactions with bulky molecules, the unique presence of micropores in zeolites can limit catalytic performance by restricting molecular transport within its crystals. Possible solutions to this problem would be to decrease the zeolite crystal size or to develop materials with larger pores. Zeolitic nanocrystals tend to aggregate and often present different properties when compared with traditional materials, such as lower stability and loss of crystallinity 1. The development of materials with additional porosity combines the advantages of the conventional microporous zeolites, which possess acidic catalytic functions and shape-selective features and are stable at high temperatures because of the crystalline structures, and the mesoporous materials with efficient mass transport 2.
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