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Web Date: January 22, 2009

Phase-Switching Catalysis

CO2-regulated solubility adds a new twist to catalyst recycling
Department: Science & Technology
SWITCHPHOS
Bubbling CO2 and then N2 into the reaction tube modifies the rhodium catalyst's triphenylphosphine ligands, switching the catalyst from the organic reaction phase (yellow, left), to the aqueous phase (yellow, center) while the organic product is removed, and then back to a fresh organic phase (yellow, right) for another reaction cycle.
Credit: Angew. Chem. Int. Ed.
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SWITCHPHOS
Bubbling CO2 and then N2 into the reaction tube modifies the rhodium catalyst's triphenylphosphine ligands, switching the catalyst from the organic reaction phase (yellow, left), to the aqueous phase (yellow, center) while the organic product is removed, and then back to a fresh organic phase (yellow, right) for another reaction cycle.
Credit: Angew. Chem. Int. Ed.

A pair of chemists in Scotland has come up with a neat trick for reversibly shuttling a homogeneous catalyst between the organic and aqueous phases in a biphasic solvent system by simply adding or removing carbon dioxide (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200804729).

The phase-switchable catalyst designed by Simon L. Desset and David J. Cole-Hamilton of the University of St. Andrews allows reactions to take place in either the organic or aqueous phase. This chemical trickery adds flexibility to the often complicated techniques required to isolate products and recycle catalysts during homogeneous reactions. The versatile new system could lead to simpler and greener industrial chemical processes.

The secret to the switchability is a weakly basic amidine group,

–N=C(CH3)N(CH3)2, that the researchers added to the phenyl rings of triphenylphosphine. The rhodium catalyst made with the modified triphenylphosphine ligand, which the team has named switchphos, is soluble in organic solvent.

On bubbling CO2 into an aqueous-organic reaction system containing the catalyst, the CO2 reacts with water to form carbonic acid (H2CO3). The acid protonates the amidine groups and renders the catalyst water-soluble. Subsequently bubbling N2 into the biphasic system drives off the CO2 and shifts the equilibrium of the catalyst-carbonic acid complex, leading the catalyst to deprotonate and making it water-insoluble again.

After a reaction is completed in either organic solvent or in water, the researchers separate the product and catalyst into different phases, remove the product, and then shuttle the catalyst back into the original phase for the next reaction cycle.

Desset and Cole-Hamilton demonstrated the capabilities of rhodium switchphos by carrying out the hydroformylation of octene in toluene to make nonanal and 2-methyloctanal and by separately carrying out the hydroformylation of allyl alcohol in water to make 2-hydroxyfuran.

The new catalyst system builds on the switchable chemistry concept originally developed by chemistry professor Philip G. Jessop of Queens University, in Kingston, Ontario, and coworkers. Those researchers have reported using amidine groups to imbue surfactants and solvents with CO2-controlled switchability, and Jessop's group is currently developing other types of switchable reagents (Green Chem., DOI: 10.1039/b821239b).

Building switchability into basic chemicals facilitates cleaner reactions and less-energy-intensive separations, Jessop notes. The phase-shuttling catalyst "is an imaginative development of this switchable chemistry," he says. "In particular, I like that the Cole-Hamilton team has shown applications in both organic- and aqueous-phase reactions."

Some engineering would be required to speed up the switching and prevent cross-contamination of the aqueous and organic phases for industrial scale-up, notes chemical engineering professor Eric J. Beckman of the University of Pittsburgh. "I'm a big fan of benign reversible triggers in chemical processes," he says, and this phase-switching process "is truly a clever way to handle catalyst cycling."

 
Chemical & Engineering News
ISSN 0009-2347
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