Influence of the lanthanum exchange degree on the concentration and acid strength of bridging hydroxyl groups in zeolites La,Na-X
1H MAS NMR spectroscopy was applied for the quantitative investigation of the concentration of hydroxyl groups in zeolites La,Na-X with different lanthanum-exchange degrees as a function of the dehydration temperature. A maximum concentration of Br鴑sted acidic bridging OH groups (SiOHAl) corresponding to ca. 1 OH per lanthanum cation was reached upon dehydration of zeolites La,Na-X at 423 to 473 K. Further raise of the dehydration temperature led to a dehydroxylation of the above-mentioned zeolites, while the dealumination of the framework was negligible. By adsorption of deuterated acetonitrile as probe molecule, a significant increase of the acid strength of bridging OH groups was found with increasing lanthanum exchange degree. This finding is explained by the polarizing effect of the multivalent lanthanum cations, which leads to a withdrawal of electrons from the O-H bonds of Br鴑sted acid sites in zeolites La,Na-X possessing a high concentration of these multivalent extra-framework cations.
J. Huang Y. Jiang M. Hunger
Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany
国际会议
北京
英文
2007-08-12（万方平台首次上网日期，不代表论文的发表时间）