New nanoparticle catalysts made of cobalt-graphene have potential for producing vital chemicals using metal-organic frameworks (MOFs). These nano-catalysts proved efficient in hydrogenation, a process that has various applications in the food, petrochemical and pharmaceutical industries. The catalysts could also be used up to five times without losing efficiency.
Researchers are exploring the benefits of fabricating nanoparticles using MOFs – structures comprising metal ions with organic linkers, which can be built in various tailorable ways, and have proven very useful for separating and storing gases.
A team including Ahmad Alshammari at KACST, with researchers from Universität Rostock in Germany and King Fahd University of Petroleum and Minerals in Dhahran, recently demonstrated a way of using MOFs to fabricate cobalt nanoparticles, which proved to be excellent catalysts for amination (the addition of nitrogen-based amine groups to molecules). Now, the team has adapted their method, using a different MOF, to produce new cobalt-based nanoparticles better suited to hydrogenation.
Hydrogenation — the addition of hydrogen atoms to other molecules — is vital for synthesizing many fuels and chemicals for industry and life sciences applications. Generally, hydrogenation reactions require expensive precious-metal-based catalysts. Cheaper nickel-based catalysts have been developed, but are not always sensitive enough to be efficient.
For these reasons, researchers have realised the potential of changing not just the metal used for catalysts, but also the size of the particles. Metal nanoparticles have very large surface areas to provide active reaction sites, and they bring the possibility of controlling reaction rates by changing particles sizee. While it would be better to tailor the shapes of nanoparticles to catalyse specific complex molecules, this is difficult to achieve with existing methods.
The Saudi-German team used transmission electron microscopy to show that their MOF process resulted in a mix of cobalt oxide and pure cobalt nanoparticles that were between 5 and 30 nanometres, encapsulated in graphene shells.
The researchers demonstrated the effectiveness of their nanoparticles by using them to hydrogenate several useful compounds. These included halogenated benzylic amines, vital building blocks for many industrial chemicals and drugs, and hexamethylenediamine, a central precursor for making nylon. Crucially, the researchers reused the nanoparticles five times without significant loss of catalytic activity, although some particles changed structure and moved out of their graphene shells.
In their paper, published in Chemical Science, the researchers highlight another benefit of their work: “Compared to other recently developed nano-catalysts, this [cobalt-graphene] catalyst does not require nitrogen-doping for activity, which opens new avenues for catalyst design.”
Murugesan, K., Senthamarai, T., Sohail, M., Alshammari, A.S., Pohl, M-M., Beller, M., & Jagadeesh, R.V. Cobalt-based nanoparticles prepared from MOF-carbon templates as efficient hydrogenation catalysts. Chemical Science 9, 8553 (2018) | article