Relationships between the Hydrogenation and Dehydrogenation Properties of Rh-, Ir-, Pd-, and Pt-Containing Zeolites Y Studied by In Situ MAS NMR Spectroscopy and Conventional Heterogeneous Catalysis

The intrinsic hydrogenation activities of homologous series of noble-metal-containing zeolites Y were studied by in situ solid-state NMR spectroscopy under semibatch conditions. For the hydrogenation of acrylonitrile, reaction rates in the sequence Pd/H,Na-Y > Rh/H,Na-Y > Pt/H,Na Y > Ir/H,Na-Y were determined. The dehydrogenation of propane at these zeolites gave a sequence of the turnover frequencies of Ir/H,Na-Y > Rh/H,Na-Y > Pd/H,Na-Y, while Pt/H,Na-Y zeolites showed significantly higher activities. The temperature-programmed desorption of hydrogen (H-2-TPD) was utilized for studying the strength of H-2/metal interactions. The positions of the high temperature peaks were arranged according to 2.8Pd/H,Na-Y (723 K) > 2.3Rh/H,Na-Y (713 K) > 4.7Ir/H,Na-Y (663 K). Comparison of these data indicates that strong H-2/metal interactions are accompanied by a preferred formation of surface hydrogen atoms, which are the reason for the high hydrogenation activity of Pd/H,Na-Y zeolites compared with Rh/H,Na-Y and Ir/H,Na-Y zeolites. In the case of the propane dehydrogenation, the strong H-2/Pd interactions in Pd/H,Na-Y zeolites hinder the desorption of the reaction product H-2, explaining the lower dehydrogenation activity of these zeolites compared with Rh/H,Na-Y and Ir/H,Na-Y zeolites. For the high catalytic activities of the Pt/H,Na-Y zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt clusters is discussed.