New NMR technique keeps tabs on micro-scale reactions

By Huw Kidwell

- Last updated on GMT

Related tags: Catalysis, Hydrogen

NMR (nuclear magnetic resonance)/MRI (magnetic resonance imaging)
is a fairly mature technology but has limited sensitivity at the
micro-scale, holding back its usefulness for analysing catalytic
reactors and reactions in microfluidic devices.

Now with the new research from a group based at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley for the first time NMR is being used to study gas phase reactions in microfluidic catalysis, on a micro-scale. The work is expected to aid the future design of catalysts and catalyst systems for microfluidic (lab-on-a-chip) devices. The technique works using parahydrogen polarised gas to amplify the signal and produce a strong enough gas phase NMR/MRI response to observe and study the gas phase processes during the use of catalysts in a packed-bed microreactor. Glen Bouchard, one of the researchers who developed the technique at the University of California Berkeley commented: "This is the first time hyperpolarized gas has been used to directly study catalytic reaction products on such a small scale and without the use of tracer particles or gas... It opens the door for future studies of heterogeneous catalysis in which all the unique benefits of MRI, such as velocimetry and spatially dependent quantities, are available​." At the moment the technique has only been used for the study of catalytic hydrogenation reactions, but it may well be developed in the future to study a range of catalytic reactions and aid in research to develop more efficient catalyst systems. Scott Burt another key researcher involved in the project said: "our results indicate that our approach to using parahydrogen can be extended to other chemical reactions beyond hydrogenation, which significantly broadens the impact and potential use of our technique​." NMR is inherently not a very sensitive technique and sometimes requires the use of techniques to boost the signal. In the case of the work at Berkeley the amplified MRI/NMR signal from samples in the gas-phase come from using a mixture of propylene and hydrogen gas, which is enriched with about 40 per cent parahydrogen (the 'para' of hydrogen form has nuclei with the required 'spin' properties to give a strong signal). During the experiment the gas mixture is flowed through the reactor cell that contains the well known hydrogenation catalyst, Wilkinson's catalyst (supported on deactivated silica gel). The hydrogenation takes place over the catalyst producing an excess of the spin polarised from of propane gas (the reaction and processes can be followed by NMR). The hyperpolarization technique can be used to boost the strength of MRI/NMR signals from other nuclei by several orders of magnitude. Burt commented: "Our MRI/NMR technique provides the ability to directly measure the spatial dependence of conversion and allows one to do a reality check on any simulations or assumptions used to design a catalytic reactor. Design can therefore become an iterative process that converges on an optimal performance... This would be very exciting as there are few existing techniques that provide such information apart from simulations. And for microreactors, there is simply no competing method for studying such gas-phase reactions at this level of detail and spatial resolution.​" The research has definite applications for the future study of chemical processes., not only the development of new catalysts but also other approaches to 'green' chemistry, boosting yields and reducing wastage and costs. Greg Bouchard commented: "We also have new ideas on how to get high-resolution temperature and pressure maps of the catalyst bed that will convey information about the energetics of the chemical reaction and mechanics of fluid transport during the reaction.​"

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