The synthesis and characterization (spectroscopic and catalytic activity) of a titanosilicate epoxidation catalyst that contains only trinuclear Ti₃ sites is described. Reaction of the trinuclear complex Ti₃O₂Cl₃(O₂C-tolyl)₅ with Si₈O₂₀(SnMe₃)₈ initially produces 3 eq. of ClSnMe₃ with the replacement of the three chloride ligands with Si₈O₂₀ cubes. Heating this product in the solid state results in the formation of approximately 4 eq. of the trimethyl tin ester, Me₃SnO₂C-tolyl. The resulting titanosilicate solid has been characterized via DRUV and EXAFS spectroscopic investigations. Clear evidence of multiple Ti---Ti features in the EXAFS are observed that are consistent with the presence of the trinuclear core in the product titanosilicate. The epoxidation activity of this heterogeneous catalyst has been investigated with cyclohexene. This new trinuclear catalyst is over 50 times faster than the well-known mononuclear epoxidation catalyst, TS-1. Titanosilicate catalysts made by grafting the Ti₃ precursor complex to mesoporous silicate supports such as MCM-41 or Davisil showed very low activity and selectivity for the desired epoxide product. Finally, the catalyst is very robust showing good activity repeatedly when run to completion with limiting amounts of either cyclohexene or peroxide. The addition of fresh substrate or peroxide restarts catalysis with little loss of activity. TONs of greater than 5000 have been measured. Mechanisms of epoxidation involving the participation of bridging oxo ligands in the activation of cumyl peroxide are discussed.

Michael Orick Ph.D.

Appearances