Green catalyst: Hybrid approach

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  • Published: Mar 1, 2018
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Green catalyst: Hybrid approach

Noble approach

Diagram of the microsphere-packed, tubular reactor used in the new

The noble metal palladium is used to drive catalytic processes in the manufacture of almost three-quarters of pharmaceuticals. Now, a more environmentally benign approach has been developed by US chemists who have used various techniques to study their "green" catalyst.

The team has made palladium-loaded poly-hydromethylsiloxane (PHMS) microparticles, which are of tuneable size and elasticity to pack a capillary-based coaxial flow-focusing microfluidic device built with off-the-shelf components that promises to improve efficiency and cut costs by reducing processing time for a wide range of reactions used in the synthesis of pharmaceuticals.

The team at North Carolina State University explains that their system will be used to drive the carbon bond-making reactions used to convert simpler starting materials into the often complicated structures of larger organic molecules. The approach sidesteps the limitations of both homogeneous processes, wherein palladium salts are dissolved allowing maximum exposure to the reagents, but leading to a lot of waste and requiring separation processes and heterogeneous process in which the solid catalyst is held on a substrate, which makes it less efficient and slower.

Hybrid catalysis

"We've created and tested a new process called pseudo-homogeneous catalysis, which combines the best of both worlds: it is nearly as fast as homogeneous catalysis, while it preserves virtually all of the palladium," explains NC's Milad Abolhasani. The new technique relies on novel, elastic silicone-chemistry based microspheres developed by the research team and using microfluidics.

"We used a microfluidic strategy to make elastomeric microspheres with a narrow size distribution to make them loadable into a tubular reactor without clogging," Abolhasani adds. "That was essential, because conventional batch scale polymerization techniques result in elastomeric microspheres with a large size distribution that would clog the reactor when loaded." Each silicone microsphere is then loaded with palladium metal. The reactants are passed through the microspheres and interact with the palladium. The products emerge from the other end, leaving the microspheres behind without loss of palladium but with speed and efficiency close to homogeneous catalysts.


"The flexible spheres allow the palladium catalyst to settle inside the microreactor environment," explains team member Jan Genzer. "The flexibility of the silicone sphere allows the palladium catalyst to adopt very many configurations during the reaction - as is the case in homogeneous processes. The palladium catalyst is retained for further use - as is the case in heterogeneous processes." The system requires no volatile organic solvents, other than ethanol in water.

The team has already demonstrated proof of principle with this approach and shown it to be much faster than heterogeneous catalysis and only marginally slower than conventional homogeneous catalysis. "We're currently working on optimizing the properties of our elastic microspheres to improve the reaction yield," adds Abolhasani. "It is important to demonstrate that green chemistry approaches can be used to make a process that is, in all, more efficient than existing techniques," he says. "You do not have to trade safety for cost-effectiveness."

The team used 3D focused ion beam-scanning electron microscopy (FIB-SEM) in combination with transmission electron microscopy (TEM) to confirm the presence of reduced Pd within the PHMS microparticles. Energy dispersive x-ray spectroscopy (EDS) and Raman spectroscopy were also used in the study.

Related Links

AlChE J 2018, online: "Microfluidic Synthesis of Elastomeric Microparticles: A Case Study in Catalysis of Palladium-Mediated Cross-Coupling"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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