Control of framework Al distribution in ZSM-5 zeolite via post-synthetic TiCl4 treatment

Evaluating and controlling the distribution of framework Al species in ZSM-5 zeolite, which consists of an orthorhombic MFI framework with 12 tetrahedral sites, are key issues for improving the acid-catalytic properties. The vapor-phase TiCl4 treatment of ZSM-5 at 600 °C led to a drastic change in the framework Al distribution, as confirmed by 27Al magic-angle spinning nuclear magnetic resonance measurements and density functional theory calculations. The maximum of the 27Al NMR peak attributed to framework Al shifted drastically from 55 to 52 ppm following TiCl4 treatment, suggesting that this treatment leads to selective dealumination of the framework Al species corresponding to the peak at 55 ppm. Ti-modified ZSM-5 showed moderate acidity in catalytic reactions such as the ethylene-to-propylene reaction and various cracking reactions. It is suggested that TiCl4 treatment results in the selective release of strong Brønsted acid sites from the MFI framework. Thus, the proposed post-synthetic treatment method provides an effective approach for tuning the catalyti c properties of ZSM-5 zeolite.