The pursuit of framework zinc affirmation in zincosilicate zeolites: Clues from conventional and spectroscopic characterization

Fine-tuning of active sites in zeolites makes it possible to achieve targeted catalytic activity for many industrial processes. Incorporation of zinc into aluminum-free zeolites grants Lewis acidity while simultaneously creating divalent cation exchange positions. Unfortunately, verification and quantification of zinc sites in zeolites is particularly arduous because metal insertion is accompanied by the formation of extra-framework zinc (hydr) oxide phases, which does not offer the desired properties but also carry some Lewis acidity, leading to difficulty in distinguishing between the two. Here, we present a rigorous characterization of (supposedly) Zn-substituted MFI zeolites. For this, two series of Zn-MFI zeolites were prepared, one via classic batch and the other via the recently invented Electro-Assisted Synthesis (EAS) approach, and these were further benchmarked against Znimpregnated samples. A suite of analytical techniques was used, among which divalent cation capacity (DCC), infrared spectroscopy (FT-IR), and X-ray absorption spectroscopy (XAS) were proven to be the most efficient. Particularly, DCC highlights the framework Zn stability and ability towards exchange of divalent ions. FT-IR spectroscopy demonstrates the healing of silanol nests upon Zn incorporation into the framework. Additionally, a consistent band at around 525 cm􀀀 1 was assigned to framework Zn-MFI vibrations. Furthermore, pyridine adsorption followed by FT-IR revealed a shift in the position of Lewis acid sites peak, when comparing Zn- and Al-MFI, while confirming the absence of Lewis acidity for zinc (hydr)oxide phases. Eventually, XAS complemented our findings by distinguishing two types of ZnII species among prepared zeolites.