Web Date: August 5, 2016
New liquid-liquid approach allows catalytic reactions to go with the flow
It’s an inevitable trade-off in catalysis: Homogeneous catalysts are highly active and selective, while heterogeneous catalysts are easily recoverable and reusable. In the last few decades, biphasic catalysts—where the catalyst is held in a liquid immiscible with the reaction medium—have been explored for their potential to achieve the best of both worlds.
But this approach is not an easy sell. “In most large-scale processes, biphasic systems are avoided,” says Susannah L. Scott, a catalysis chemist at the University of California, Santa Barbara. Biphasic systems are difficult to operate on a large scale because vigorous stirring in batch reactors is needed to keep the two immiscible phases in contact to catalyze the reaction. Also, reaction rates are typically low, and catalyst recovery via phase separation adds a tedious step.
Now, a team of researchers from Shanxi University report a way to operate biphasic catalysis systems as a continuous flow process, instead of in batches, which could dramatically increase opportunities for applying these systems in industry (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b04265).
The scientists used Pickering emulsions, where superfine solid particles surround and stabilize micrometer-sized droplets of a liquid dispersed throughout another immiscible liquid. “The particles form something like a skin,” says Hengquan Yang, a chemist at Shanxi University who led the study.
To make the emulsion, the researchers stirred two liquids—an aqueous solution containing a catalyst like sulfuric acid or an enzyme, and an organic liquid that is either n-octane or toluene—with silica nanoparticles for about five minutes. The aqueous solution formed millions of droplets, each roughly 50 to 100 µm across, within the organic phase, Yang explains.
Then, the scientists filled a glass column with the emulsion. A filter at the bottom retained the coated aqueous droplets containing the catalyst, effectively immobilizing it, while allowing the organic phase to flow through. When reactants are incorporated into the organic phase, catalysis takes place at the surface of the droplets, and the hydrophobic reaction products flow out the bottom of the column with the organic solvent.
“This is a very flexible platform,” says Scott, who was not involved in the study. “You can incorporate just about any homogeneous catalyst in the Pickering emulsion.”
Yang and colleagues tested the system with three types of catalytic reactions to demonstrate its generality: a simple acid-catalyzed alcohol protection, an acid-catalyzed epoxide ring-opening reaction, and an enzyme-catalyzed kinetic resolution of racemic esters. In all three examples, the biphasic flow system was 10 to 20 times as efficient—generating more product per hour per unit of catalyst—as batch systems.
The catalysts remained emulsified and active even after 1,000 hours of continuous reaction. “I think that’s very, very impressive,” says David Cole-Hamilton, a chemist at the University of St. Andrews.
The next step is to try combinations of other liquid phases such as supercritical fluids and ionic liquids, Yang says.
The system could benefit from increased flow rates, Scott says, and the Pickering emulsions’ stability at different temperatures should be studied. Scott speculates that this system could also enable sequential reactions without stopping to isolate intermediates, by stacking two catalyst-filled Pickering emulsions one after the other in a column. “This is something that people have been trying to do for a long time,” she says.
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