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Nuclear magnetic resonance studies on the acidity of zeolites and related catalysts

1985; Elsevier BV; Volume: 5; Issue: 5 Linguagem: Inglês

10.1016/0144-2449(85)90158-7

1873-3115

Harry Pfeifer, D. Freude, Michael Hunger,

Zeolite Catalysis and Synthesis

A review is given on the possibilities of the various nuclear magnetic resonance techniques which have been applied up till now to study quantitatively the acidity of zeolites and related catalysts. From the intensity of the wide line proton magnetic resonance signal of unloaded catalysts, the total concentration of protons can be determined. A loading of the samples with (deuterated) pyridine reduces drastically the mean residence time of the acidic protons at the oxygen atoms of the surface through formation and thermally activated motion of pyridinium ions. From the proton magnetic relaxation of the pyridine molecules and pyridinium ions a microdynamical model for the pyridine loaded H—Y zeolites could be derived. The rate constant for the decomposition of the pyridinium-ion-surface-complex which can be determined in this way is used as a measure for the strength of acidity of the OH groups involved. Through magic angle spinning of thermally activated samples (contained in sealed glass ampoules to prevent adsorption of water) it became possible for the first time to determine quantitatively the concentration of non-acidic OH groups, of OH groups having different strength of acidity, and of residual ammonium ions. The results are related to the catalytic activity (cumene cracking) of the amorphous silica-aluminas, H-mordenites and zeolites H—Y studied in the present paper. In contrast to Brønsted acidity, the current application of n.m.r, techniques to study Lewis acidity is more complicated because experimental difficulties, mainly due to fast exchange of adsorbed bases between different kinds of adsorption sites, arise.