Development of methods for determining the intrinsic kinetics and for clarifying the mechanisms of heterogeneously catalyzed hydrogenation reactions by in situ MAS NMR under flow conditions

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Methods of in situ MAS NMR spectroscopy under flow conditions will be developed, which can be utilized for the (i) determination of the intrinsic hydrogenation activity of metal-modified solid catalysts and for (ii) elucidating the mechanisms of these reactions. The (i) determination of the intrinsic hydrogenation activity will be performed via semibatch experiments consisting at first in the adsorption of the reactants to be hydrogenated on the activated and reduced solid catalyst and, subsequently, in the study of the hydrogenation kinetics under flowing hydrogen by in situ MAS NMR spectroscopy. The advantage of this approach is the decoupling of the time-dependence of the hydrogenation reaction from transport limitations of the reactants and reaction products as well as the time-dependent catalyst deactivation. After optimizing the experimental parameters and the reaction conditions as well as the selection and investigation of suitable reactant molecules, comparative studies of the intrinsic hydrogenation activity of different metal-modified zeolite catalysts and novel solid catalysts, such as metal-modified FSP-aluminosilicate nanoparticles (FSP: flame spray pyrolysis), in microemulsions prepared metal-nanoparticles on mesoporous supports, and metal-organic frameworks, will be performed. By this way, new findings on the effects of the nature, composition, and size of metal atoms and clusters as well as different support and host materials on the intrinsic hydrogenation activities of the above-mentioned catalyst systems are expected. Preparing the studies of the (ii) mechanisms of heterogeneously catalyzed hydrogenation reactions, experiments on the use of parahydrogen-induced polarization (PHIP) are planned. In the case of a pairwise incorporation of the two hydrogen atoms of a parahydrogen molecule into a reactant, such as by hydrogenation of olefins, the initial spin order is converted into a large nonequilibrium spin polarization (hyperpolarization). The corresponding NMR signals, which are strongly enhanced in their intensities, have characteristic antiphase shapes. Metal-modified solid catalysts with high intrinsic hydrogenation activities will be applied for hydrogenating olefins of different sizes and adsorption properties with parahydrogen, and their PHIP effect will be studied by in situ MAS NMR spectroscopy under flow conditions. Furthermore, experiments focusing on the transfer of hyperpolarization from 1H nuclei to hetero-nuclei in product molecules of hydrogenation reactions and in the local structure of adsorption sites are planned. These investigations focus on elucidating the mechanisms of heterogeneously catalyzed hydrogenation reactions, which should be, inter alia, discussed in relation to mechanisms suggested in literature.