New liquid-liquid approach allows catalytic reactions to go with the flow | Chemical & Engineering News
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Web Date: August 5, 2016

New liquid-liquid approach allows catalytic reactions to go with the flow

Emulsions immobilized within columns turn biphasic catalysis into a continuous process
Department: Science & Technology
News Channels: JACS In C&EN, Materials SCENE, Nano SCENE, Organic SCENE
Keywords: catalysis, homogeneous catalysis, continuous process, flow catalysis, reactor, biphasic catalyst
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Using solid particles to stabilize an emulsion of immiscible liquids allows researchers to carry out homogeneous catalysis in a continuous flow process.
Credit: J. Am. Chem. Soc.
Schematic of continuous flow process for biphasic catalysis.
 
Using solid particles to stabilize an emulsion of immiscible liquids allows researchers to carry out homogeneous catalysis in a continuous flow process.
Credit: J. Am. Chem. Soc.

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.

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This fluorescence confocal microscopy image shows aqueous droplets emulsified in an organic phase, stabilized by silica particles at the droplet surface. The silica particles are dyed with a fluorescein isothiocyanate isomer.
Credit: J. Am. Chem. Soc.
Fluorescence image of aqueous droplets stabilized by silica particles in an organic liquid.
 
This fluorescence confocal microscopy image shows aqueous droplets emulsified in an organic phase, stabilized by silica particles at the droplet surface. The silica particles are dyed with a fluorescein isothiocyanate isomer.
Credit: J. Am. Chem. Soc.

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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Comments
David Dodds (Wed Aug 10 16:45:36 EDT 2016)
This reminds me of the work by the late Bert Holland of Brock University in Ontario, who encapsulated aqueous droplets containing biocatalysts (inclding spores) using interfacial polymerization of adipoyl chloride and HMDA to give a nylon membrane around the droplet. As I recall, the membrane was only about one micron thick, and substate molecules could diffuse across it easily. The encapsulated droplets were stable in organic phases, and had sufficient physical strength to be filtered, etc.

Encapsulating a multiple enzyme system inside the droplet with the ability to generate NADH by using a sacrificail substrate (say, isopropanol) delivered in the organic phase, would be useful.
Wolfgang Brill (Thu Aug 11 02:47:38 EDT 2016)
Dear Editor
The concept of a liquid catalyst being immobilized onto a solid matrix is indeed very effective. It is the principle used in the air- oxidation of SO2 to SO3 during the sulfuric acid synthesis. Here, droplets of certain vanadates bound imbedded onto a support are the catalyst.
Dr Yogesh Ghalsasi (Thu Aug 11 04:27:46 EDT 2016)
This is great achievement in the field of Catalysis. Frequently typical homogeneous catalysts although selective, pose a problem for sample recovery. The flexibility offered by liquid- liquid systems with immobilized catalyst offer selectivity of homogeneous catalysis with easy recovery and regeneration of the catalyst. This approach of using emulsions for stabilizing the catalyst requires to be tested for heterogeneous systems as well as on the large scale operations.However, the idea is novel and offers greater possibilities with super critical fluids or other liquid systems.

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