Desilication of ZSM-5 and ZSM-12 Zeolites with Different Crystal Sizes: Effect on Acidity and Mesoporous Initiation

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.