(442b) Dendrimer-Derived Bimetallic Ir-Based/γ-Al2O3 Catalysts for Liquid-Phase Hydrogenation Reactions

Bimetallic catalysts prepared by conventional synthetic methods often result in wide particle size distributions and non-uniform materials that can be difficult to characterize on a fundamental level. Dendrimer-metal nanocomposites (DMNs) provide novel synthetic routes that can be applied to produce heterogeneous catalysts. The use of DMN precursors for supported catalysts has demonstrated control of metal nanoparticle size and composition even after thermal removal of the dendrimer ?shell?. Supported Ir catalysts possess unique properties that can enhance activity and selectivity for a variety of reactions. In this work we are reporting for the first time the preparation and characterization of bimetallic supported Ir-based catalysts via the DMN approach. These catalysts have been characterized using a range of techniques such as HRTEM, FTIR spectroscopy, EXAFS, and TPR/TPO. The results show that DMN-derived bimetallic precursors can form different nanoparticle sizes, distributions, and compositions than for catalysts prepared by conventional methods such as incipient wetness or wet impregnation. These catalysts have been evaluated for liquid-phase hydrogenation reactions with notable differences observed depending on the preparation method. For example, benzonitrile hydrogenation kinetic studies show differences in activity and selectivity for monometallic and bimetallic DMN-derived catalysts. The activity of Pd catalysts (regardless of preparation method) is 3-4 times higher than for Ir, with secondary amine (i.e., dibenzylamine) favored for Ir/Al₂O₃, and primary amine (i.e., benzylamine) favored for Pd/Al₂O₃. However, the DMN and conventionally-derived bimetallic catalysts show different product selectivity, with the former selectively producing the secondary amine (dibenzylamine) and the latter forming the primary amine (benzylamine). The future prospects and hurdles of the DMN approach for producing novel bimetallic catalysts will be discussed.