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 hyd roxyl groups in evacuated samples is a topic of basic interest since D elta 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], t he deprotonation energy Delta E(DP) of free surface hydroxyl groups wh ich are responsible for the Bronsted acidity of catalysts can be deter mined quantitatively from the H-1 NMR chemical shift delta(H). As a fi rst application of this result, the upper limit of the distribution wi dth delta E(DP) of the deprotonation energy of bridging hydroxyl group s in zeolites has been calculated from the residual linewidth of their H-1 MAS NMR signals.