Abstract
The amount and location of acid sites (strong Brønsted and weak silanols) in zeolites are crucial for their applications. In this work we revealed the enigma of the complex H-bonded silanol networks in pure silica MFI-type zeolites using 1H solid-state NMR and IR spectroscopy combined with DFT calculations. The silanol sites have a determining role in phase selectivity during the zeolite synthesis and also play an imperative role in setting their acidity, stability, lifetime and hydrophobicity. The spectral signatures of silanol defects in pure silica zeolites are disclosed. Based on the experimental and theoretical observations we identified four types of silanols in pure silica zeolites which varied depending on the crystallite size: (i) isolated (free) silanols, not participating in hydrogen bonds either as proton-donors nor as proton-acceptors, (ii) proton-acceptor silanols, participating in hydrogen bonds only as proton-acceptors, (iii) proton-donor silanols, participating in weak hydrogen bonds and (iv) medium and strong hydrogen-bonding silanols, participating as proton-donors or simultaneously as proton-donors and proton-acceptors. The main factor determining the strength of the hydrogen bond of a specific silanol group is the possibility of its proton to approach closer to the oxygen center of the proton accepting silanol. This suggested that the flexibility of the zeolite fragment at which the proton-donating and proton-accepting silanols are bound is the main factor determining the formation and strength of hydrogen bonds and their corresponding spectral features.