Using MRI to study gas reactions

Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory and the University of California, Berkeley, have successfully studied gas-phase reactions on the microscale using magnetic resonance imaging (MRI).

The application is seen as a significant step towards improving the design of future catalysts and catalytic reactors, especially for microfluidic lab-on-a-chip devices.

The scientists have developed a technique whereby a parahydrogen-polarised gas is used to make an MRI signal strong enough to provide direct visualisation of the gas-phase flow of active catalysts in packed-bed microreactors. This means that the MRI can be used to track gases and liquids in microfluidic devices as well as in the void spaces of a tightly packed catalyst reactor bed.

‘This is the first time hyperpolarised gas has been used to directly study catalytic reaction products on such a small scale and without the use of tracer particles or gas,’ said Louis Bouchard, one of the chemists who carried out the research.

Prior to this project, conventional MRI techniques were considered to have too low a sensitivity for microscale catalysis research, but parahydrogen helps to overcome this problem.

MRI signals are made possible by a property found in the atomic nuclei of most molecules, called ‘spin’, which makes the nuclei act like bar magnets with a north and south pole. To obtain an MRI signal, there needs to be an excess of nuclei in a sample with spins pointing in one direction or the other.

In parahydrogen, the spins of the two protons in the nuclei point in opposite directions. By increasing the fraction of parahydrogen in the gas mixture there is net excess in the para spin state even at room temperature and in the absence of a magnetic field.

The researchers have found that using parahydrogen enhanced gas in combination with propylene gas and a heterogenised catalyst resulted in a strong MRI signal from samples in the gas-phase, something that has only been done before using hyperpolarised noble gases and expensive polarisation equipment.

‘The enhanced MRI sensitivity provided by parahydrogen induced polarisation allows us to overcome the inherent problem of low sensitivity in thermally polarised gas-phase MRI.

‘This is the reason we are able to get such high-spatial resolution MRI images in the gas phase,’ said Bouchard.

A mixture of propylene and parahydrogen enriched gas is sent through a reactor cell containing a catalyst that has been immobilised on a modified silica gel. As the gas passes over the catalyst, hydrogenation occurs, which produces spin polarised propane gas that is transferred to an MRI magnet. The catalyst-free hyperpolarised propane gas can then be used to enhance MRI signals.

According to the researchers, the technique is now ready for use in the study of hydrogenation reactions.

‘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,’ said Bouchard.

Scott Burt, another chemist who also worked on the project, added: ‘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.’