Although heteropolyacids are excellent homogeneous polyoxometalate catalysts, recovering these molecules at the end of a reaction is often tricky and can have an impact on their application. By inserting the polyoxometalate into the cavity of a metal-organic framework, scientists in Belgium have developed a way of releasing and re-trapping the catalyst at will.
The key to the system is to take advantage of the solvent-dependent solubility of the metal-organic framework. The team, led by Johan Martens from the University of Leuven, dissolved the caged catalyst, triggering the collapse of the metal-organic framework and releasing the heteropolyacid. They then used hexane to re-assemble the catalyst and metal-organic framework, producing a blue solid that could be removed easily by centrifugation.
'At the beginning, we struggled a lot with finding the right solvent. We were very excited when we finally managed to close the cycle,' says Martens.
Until now, the easiest way to recover these catalysts was to permanently bind them to solid supports such as zeolites, mesoporous silica or metal-organic frameworks. However, this often lowered the catalytic performance and could introduce new problems, such as stability. By using a metal-organic framework that can easily disassemble and re-assemble, the heteropolyacid is no longer permanently bound and can preserve its high catalytic performance, even when repeatedly re-used.
'I think this work addresses one of the most important issues of liquid-phase catalysis and offers an interesting solution for the common problem of homogeneous catalyst recovery and reuse,' says Nataliya Maksimchuk, an expert in polyoxometalate catalysts at the Boreskov Institute of Catalysis, Novosibirsk, Russia.
This easy recovery method opens new doors for heteropolyacid catalysts and their use in industry. 'We have already been approached by colleagues working in the field of catalysis with heteropolyacids to collaborate and explore the potential of this new method in their reactions,' says Martens.
- N Janssens et al, Chem. Sci., 2012, DOI: 10.1039/c2sc01102f