Concentration and acid strength of hydroxyl groups in zeolites La,Na-X and La,Na-Y with different lanthanum exchange degrees studied by solid-state NMR spectroscopy

摘要

~1H MAS NMR spectroscopy was applied for a quantitative investigation of the concentration of hydroxyl groups on lanthanum-exchanged zeolites X and Y as a function of the dehydration temperature.A maximum concentration of Bransted acidic bridging OH groups (SiOHAl) was reached upon dehydration of zeolites La,Na-X and La,Na-Y at 423 K.Further raise of the dehydration temperature leads to the recombination of the above-mentioned hydroxyl groups,while the dealumination of the framework is negligible.The maximum number of bridging OH groups formed on zeolites La,Na-X and La,Na-Y correlates well with the lanthanum exchange degree.By adsorption of deuterated acetonitrile as a probe molecule,an increase of the acid strength of bridging OH groups in lanthanum-exchanged zeolites X and Y was found.This finding is explained by the polarizing effect of multivalent lanthanum cations,which leads to a withdrawal of electrons from the O-H bonds of Bronsted acid sites in lanthanum-exchanged zeolites with a high concentration of these extra-framework cations.