Characterization of external surface acid sites and their modification following hierarchization of zeolites with unidirectional pores

Hierarchical zeolites, namely ZSM-12 (MTW), ZSM-23 (MTT), and ZSM-48 (RFE), were prepared through alkaline desilication and acidic dealumination performed in different sequences, and their structural and external surface acidity responses were examined. X-ray diffraction (XRD) and nitrogen (N 2 ) physisorption confirmed the preserved crystallinity and the development of mesoporosity, while solid-state 27 Al and 29 Si nuclear magnetic resonance spectroscopy (NMR) revealed framework modification but few extra-framework Al (EFAL) formation. Total and external surface acidity, quantified using pyridine- and 2,6-Di-tert-butylpyridine (DTBP)-Fourier Transform Infrared (FTIR) spectroscopy, showed that desilication followed by dealumination increases the concentration of accessible Brønsted and Lewis sites, whereas the reverse sequence largely suppresses DTBP-detectable external surface acidity despite similar total acid sites. The results demonstrate that performing dealumination prior to desilication drastically diminishes the concentration of accessible surface acid sites.