On the measurement of the deprotonation energy of surface hydroxyl groups by H-1 MAS NMR spectroscopy

The measurement of the deprotonation energy Delta E(DP) of surface hydroxyl groups in evacuated samples is a topic of basic interest since Delta E(DP) is a measure for the strength of gas phase acidity of these hydroxyl groups. H-1 MAS NMR spectroscopy allows a separate detection of the different types of surface hydroxyl groups in zeolites. Using the results of recent ab initio quantum chemical calculations [1-3], the deprotonation energy Delta E(DP) of free surface hydroxyl groups which are responsible for the Bronsted acidity of catalysts can be determined quantitatively from the H-1 NMR chemical shift delta(H). As a first application of this result, the upper limit of the distribution width delta E(DP) of the deprotonation energy of bridging hydroxyl groups in zeolites has been calculated from the residual linewidth of their H-1 MAS NMR signals.