Hydrosilation reactions catalyzed by supported bimetallic colloids

The preparation and catalytic properties of bimetallic colloids consisting of a ligand-stabilized Pt shell on Au or Pd cores supported by alumina are described. The aim of this work is to study the influence of the electronegativity of the colloidal core metals on the activity and selectivity of the surface Pt in hydrosilation reactions. Au is more electronegative and Pd is more electropositive than Pt. Results are compared to controls in which the hydrosilation reaction is catalyzed by colloids made of pure Pt which are also stabilized with the same ligands and supported by alumina. The reaction of HMTS with 1-octene to form bis(trimethylsiloxy)octamethylsilane (BTOMS) was selected to serve as an example of a typical hydrosilation reaction. The synthesis of the bimetallic colloids was accomplished by the seed-growth mechanism. The inner core consisting of Au (diameter = 18 nm) or Pd (diameter = 20 nm) was generated by the reduction of HAuCl4 or H2PdCl4 respectively, by sodium citrate. The Pt outer shell was then grown on the core metals by the reduction of H2PtCl6 with hydroxylammonium chloride. The thickness of the Pt shell depended on the molar concentration of H2PtCl6. Colloidal Pt of undefined morphology is used in industrial processes which require temperatures as high as 120 degrees C and a 25% excess of the olefin in order to obtain quantitative yields of BTOMS. The hydrosilation process however became more efficient if Pt colloids of 9 nm diameter (Pt9) or bimetallic Au/Pt18/27 colloids (diameter Au core 18 nm, thickness of Pt shell = 4.5 nm) were used. The first number indicates the diameter of the inner core metal and the second number indicates the total diameter of the bimetallic particle. Both types of colloids were stabilized by p-H2NC6H4SO3Na (sodium sulfanilate) ligands. These colloid catalysts operated best at 60 degrees C and without a need for excess 1-octene. The yield of BTOMS is 92-93% after 24 h reaction time. The hydrosilation process was improved remarkably if bimetallic colloids made of Pd/Pt20/27 catalysts are used. A BTOMS yield of 90% was reached after only 4-6 h and became greater than 96% after a 24 h reaction period. In addition, ca. 1% of byproducts were formed and only 0.2% of the HMTS was left unreacted. Lifetime testing of the heterogenized colloids, especially of the optimal Pd/Pt system, indicated that catalytic activity was constant over at least six cycles. The results presented in this paper clearly prove that catalysts consisting of layered metals also demonstrate significantly enhanced activity resulting from mutual interaction of the component metals.