Different Brønsted acidity of H-ZSM-5 and D-ZSM-5 zeolites revealed by the FTIR spectra of adsorbed CD3CN

Abstract

The acidity of the bridging OH/OD groups in high-silica H-ZSM-5 and D-ZSM-5 zeolites was measured using CD 3 CN as an IR probe molecule. We found that the OH groups are more acidic because the ν(CN) modes of the CD 3 CN···HO complexes formed with bridging hydroxyls appeared at frequencies higher by 1.1–1.2 cm − 1 than the respective modes of CD 3 CN···DO complexes. This difference corresponds to ca. 0.2 units of the Hammett acidity function. Similar but less pronounced effect was established with the AlOH/AlOD groups (difference of 0.7 cm − 1 ), i.e., contrary to current opinions, the isotopic effect increases with the acidity strength. Adsorption of CD 3 CN clearly reveals the difference in acidity of the OH and OD groups in ZSM-5 zeolite. • Bridging OH groups in ZSM-5 are more acidic than the corresponding OD groups • The difference in acidity corresponds to 0.2 units of the Hammett acidity function • The isotopic effect decreases with the decrease of the acidity of the OH groups.