Synthesis and characterization of bare and silica -coated group IB cluster catalysts for partial oxidation of propylene

Catalysts consisting of pure Group IB metal and metal–SiO 2 compound clusters a few nanometers in diameter dispersed on SiO 2 and A12O3 supports are synthesized and characterized. The compound clusters are synthesized by two gas phase methods. One method is to decompose tetraethylorthosilicate in the presence of pre-formed metal clusters in the gas phase in order to deposit SiO2 on the surface of the metal. A second method is to co-evaporate metal and silicon from a carbon crucible using a distributed DC-arc in flowing argon and subsequently to oxidize the clusters. Bare and silica-coated Ag and Cu clusters are examined for their behavior as catalysts for partial oxidation of propylene with oxygen. For copper clusters supported on alumina, the presence of a SiO2 coat increases the selectivity toward acrolein formation and decreases the selectivity toward complete combustion. The SiO2 coat also increases the activation energy for combustion from 13.8 kcal/mol for bare Cu clusters supported on α-alumina to 23.8 kcal/mol for Cu–SiO2 clusters supported on α-alumina. The presence of a SiO2 coat on Ag clusters supported on α-alumina has a similar effect. The SiO2 coat increases the selectivity toward acrolein and decreases the selectivity toward complete combustion. The highest selectivities toward propylene oxide were observed using Cu/SiO2 catalysts. A Cu/SiO2 catalyst prepared using the gas phase cluster synthesis gave 36.5% selectivity toward propylene oxide at 250°C. A catalyst prepared using an ion-exchange technique gave 43.0% selectivity toward propylene oxide at the same conditions. Reaction rates were normalized to the number of exposed surface metal atoms as determined by titration with nitrous oxide. For the catalyst prepared using the gas phase cluster synthesis, the turnover frequency of propylene to propylene oxide at 250°C was 0.00068/sec. For the catalyst prepared using the ion-exchange technique, the turnover frequency was 0.019/sec.