Kinetic Characterization of Multimetallic Cluster-Derived Catalysts Used for Selective Hydrogenation of Citral
Karen J. Uffalussy1, Burjor Captain2, Richard D. Adams2, Ana B. Hungria3, John R. Monnier1, and Michael D. Amiridis1. (1) Chemical Engineering, University of South Carolina, 627 Canon Gate Dr., Cary, NC 27518, (2) Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St., Columbia, SC 29208, (3) Material Science and Metallurgy, University of Cambridge, Cambridge
Cluster-derived multimetallic supported catalysts have been investigated using an α,β-unsaturated aldehyde hydrogenation reaction (i.e., the selective hydrogenation of citral) to help identify the role each metal plays for increasing the selectivity to the less thermodynamically probable α,β-unsaturated alcohol. The catalysts examined included supported multimetallic clusters containing combinations of Pt, Ru, and Sn, as well as samples of identical compositions prepared by impregnation from individual salt precursors on various oxide supports. Kinetic data were collected at 70°C in a stirred autoclave reactor under a hydrogen pressure of 460psig. Samples were analyzed by GC to determine the conversion and selectivity to the α,β-unsaturated alcohols (i.e., geraniol and nerol). The results indicate that the overall citral hydrogenation reaction is first order with respect to citral and zero order with respect to hydrogen. Notably, the trimetallic cluster-derived catalyst, PtRu5Sn, showed enhanced activity and selectivity over bimetallic cluster-derived and metal salt-impregnated catalysts. Presumably the addition of Sn to the Pt-Ru cluster-derived system stabilizes the resulting supported moieties and modifies the catalytic properties of Pt and Ru, directly affecting overall activity and selectivity to the α,β-unsaturated alcohols. Conversely, the addition of Sn to the coimpregnated Pt-Ru samples does not substantially change their catalytic performance. The Pt-Ru bimetallic cluster impregnated with Sn salt precursor was also studied to determine the effect that Sn coordination has on the conversion and selectivity. Additionally, TEM was performed on selected samples, and particle size distributions show that the cluster-derived particle size is smaller (1.0 – 1.2nm) than the corresponding particle size in the metal salt-derived catalysts (1.0 – 5.4nm). EDX spot analysis done on these samples shows that the PtRu5Sn cluster-derived catalyst particles have a consistent composition analogous to the cluster precursor, yet the Pt-Ru-Sn metal salt-derived catalyst particles varied greatly in composition. The decomposition of the trimetallic and bimetallic clusters under He and H2 flow using FTIR and TPD was also studied. When comparing the kinetic data and TEM/EDX results, the presence of these metals in close proximity appears to greatly enhance selectivity and activity for the organometallic cluster-derived catalysts. Therefore, the use of bimetallic and trimetallic organometallic cluster precursors can yield better defined and more uniform catalysts with distinct advantages over their analogous metal salt precursors.