Immobilized Ti-calixarenes are active and selective
complexes onto silica produces effective epoxidation catalysts,
according to a study from the University of California, Berkeley.
Chemical engineering professors
Alexander Katz and Enrique
Iglesia and graduate student Justin
M. Notestein have shown that the
immobilized form of the catalyst (shown) is more than 20 times more
active and far more selective than the solution-phase compound in
olefin epoxidation reactions using organic hydroperoxides as oxidizing
agents [J. Am. Chem. Soc., published online Nov. 24, http://dx.doi.org/10.1021/ja0470259].
The researchers propose that the multidentate and bulky structure
of the calixarene ligand keeps the metal centers isolated from one
another during reaction with alkenes. Separating the titanium centers
prevents oligomerization, which would lead to formation of unreactive
and unselective TiOTi structures. The team notes that
the catalysts are robust and reusable and exhibit long-term stability
under ambient storage conditions.
& Engineering News