1H magic angle spinning (MAS) NMR and FTIR spectroscopy with deuterated acetonitrile and pyridine as probe molecules, respectively, were used to study the strength of Brønsted acid sites in Pt- and Ir-modified zeolites Y and Beta in comparison with the materials in their H,Na forms. For both types of zeolites, the following sequence of the acid strengths was found: zeolites in their H,Na form>Pt-modified zeolites>Ir-modified zeolites. As the 1H MAS NMR signals of the bridging OH groups (Si(OH)Al) in the noble-metal-modified zeolites have the same spectroscopic properties as those observed in the materials in the H,Na forms, no direct influence of the noble metals on the nature of the hydroxyl protons or on the local structure of Si(OH)Al groups is expected. However, it is suggested that noble metals in these bifunctional zeolite catalysts are involved in charge transfer with neighboring framework atoms, which affects the mean framework electronegativity of the zeolites under study. This effect causes a variation of the acid strength of the Si(OH)Al groups similar to that observed for zeolites with different framework Al contents.
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