Crystal Size–Acid Sites Relationship Study of Nano- and Micrometer-Sized Zeolite Crystals

H/D isotope exchange was employed to quantify the total number of Brønsted acid sites present in zeolites of different crystal size. The latter method was complemented by XRD, SEM, FT-IR, multinuclear MAS NMR, and N2 adsorption measurements to obtain an entire picture of zeolite characteristics. The acidities of two zeolite-type materials, MFI and BEA, having crystal sizes in the range 50 nm to 20 μm (ZSM-5) and 50 nm to 2 μm (BEA) and close Si/Al ratios were investigated in detail. The number of Brønsted acid sites in the series of ZSM-5 zeolite crystals does not depend on the crystal size. In contrast, BEA nanocrystals exhibit a higher amount of OH groups than do micrometer-sized crystals, which was attributed to the intergrowth of two polymorphs and the contribution of the external surface of zeolite nanocrystallites as demonstrated by solid-state NMR analysis.