Technology : A quick rinse, and caged catalyst can be used again

Technology 2 August 1996

THEY may be invaluable in speeding up reactions but catalysts have to be
separated from the reaction mixture once they have finished the job. This
can be
an expensive and wasteful business.

The problem is that for some catalysts, particularly those used in the
pharmaceuticals industry, the process can be complicated, involving several
steps. And the chiral catalysts often used in the industry can cost up to
$350 per gram to replace when they cannot be completely recovered.

Now Belgian researchers think they have solved the problem. Ivo Vankelecom
and his colleagues at the Catholic University of Leuven have developed a
membrane that acts as a support for expensive homogeneous catalysts that would
normally have to be dissolved in the reaction mixture.

Made of polydimethylsiloxane, the elastic membrane traps the catalyst in the
tiny spaces between the polymer chains. The catalyst can then be easily lifted
out of the flask or tank once the reaction is finished and used again
after only
a quick rinse (Angewandte Chemie, vol 35, p 1346).


Various attempts have been made to get homogeneous catalysts to work in a
solid form, rather than mixing them into the reaction solution. But these
experiments have generally relied on chemical or electrostatic forces to hold
the molecules of the catalyst in place, and this seems to adversely affect the
efficiency of the reaction. The Belgian membrane, which holds the molecules of
the catalyst in place by physically caging them, apparently leaves the
of the catalyst unchanged.

Vankelecom and his colleagues carried out two test reactions—an alkene
epoxidation and a hydrogenation reaction—which are common intermediate
processes in the production of many drug precursors. The team found that the
catalyst supported on the elastic membrane was selective for making one chiral
form of the product and, equally important, it could be regenerated almost
completely, claims Vankelecom. “A catalytically active membrane that could be
regenerated provided essentially the same results as the homogeneous catalyst
alone,” he says.

The team is now investigating the effect on the activity of the catalysts by
varying the thickness of the membrane and the number of cross-links between the
polymer chains.