Superacidity and spectral signatures of hydroxyl groups in zeolites

Abstract

The hydroxyls, Brønsted acid sites (BAS) and silanols, provide key contributions in the global acidity of zeolites and have significant impact on their properties and applications. In this work, we present the acidity of BAS and silanols in zeolites depending on their configurations in zeolite nanoparticles. The acidity was evaluated based on the deprotonation energy (DPE) calculated by the density functional method and compared to experimental spectra. The calculated DPE and available experimental data for acidity of small molecules in a gas phase allowed us to position the hydroxyl groups in zeolites into the general scale of gas phase acidity for the first time. The simulated deprotonation enthalpies for the bridging hydroxyls are in the range 1113–1187 kJ/mol while for silanols they vary in larger range 1186–1376 kJ/mol. Compared to gas phase acids, these values imply that the Brønsted acid sites fall in the range of superacids while silanols cover wide range from strong acids to superacids. The high gas phase acidy of the zeolite hydroxyls may be explained with the flexibility of the zeolite framework that efficiently accommodates the negative charge of deprotonated center via structural relaxation, electron density redistribution or formation of hydrogen bonds. Nanosized zeolite in proton form (HZSM-5) was used as a model system, and the proximities between bridging hydroxyls and 27Al centers was estimated by 1H{27Al} REAPDOR MAS NMR technique. A linear correlation between the 1H NMR chemical shifts and stretching O–H vibrational frequencies of the BAS was found similar to the silanol groups. However, no correlation between the deprotonation energy and the spectral characteristics of the corresponding hydroxyl (BAS and silanols) was observed. Thus, the acidity of the hydroxyls cannot be estimated based on the spectral characteristics, which accounts mainly for the formation and strength of hydrogen bonds.