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Department of Chemical Engineering and Biotechnology

 
Magnet Hall at CEB

Researchers from our department’s magnetic resonance research centre (MRRC) show new insight provided by ultra-low-field techniques.

By detecting nuclear magnetic resonance (NMR) in magnetic fields that are approximately 100 times weaker than the Earth’s field, the researchers are able to obtain information that more widely used NMR methods cannot achieve. 

The findings, which have been published in the journal Applied Physics Letters, include details on the behaviour of liquids imbibed inside porous catalytic materials and bulk liquids that are completely sealed in metal containers.

The work was carried out by Dr Michael Tayler and PhD student Jordan Ward-Williams, working in our department under the supervision of Professor Lynn Gladden. Dr Tayler is now a research fellow at the Institute of Photonic Sciences in Barcelona, Spain.

NMR is a radio-frequency analytical technique that is widely used to quantify chemical composition, dynamics, and reactivity of fluids and fluid mixtures. A key feature of NMR is its ability to probe inside structures that are opaque to UV-vis-IR frequencies, including many heterogeneous catalyst reaction systems, which are a central object of study at the MRRC.

Bigger is not always better  

Traditionally, NMR spectroscopy involves the use of strong (10 to 20 teslas) and highly uniform (field gradients <10 parts per billion) superconducting electromagnets. Samples are placed into the field of the magnet and a spectrum of signals, with peaks specific to different chemical compounds in the fluid phase, is recorded. The resolution of each chemical species in the sample allows its mole fraction, as well as other properties relating to inter-molecular interactions and dynamics, to be determined.  

In these traditional methods, the signal strength increases quadratically with magnetic field strength, while resolution increases linearly. On both counts, the stronger the magnet, the clearer your spectrum.

However, the technique struggles to study liquids inside metallic containers – including metal pipes – or materials. The high conductivity of metals reduces the depth to which NMR signals can penetrate the material, and in this case, the depth scales inversely with the strength of magnetic field, so the stronger the field, the smaller the distance the signal can penetrate. 

Metals, as well as other materials, can cause further problems due to magnetic fields produced by the materials themselves, which may result in signal broadening. Peaks in the spectrum can blur together, making a given chemical component in a mixture hard to distinguish from the rest. 

How low can you go?

Tayler’s interest in using ultra-low-field magnetic resonance to solve these challenges began back in 2014, with a European Commission (Marie Curie) funded project that enabled him to visit the US to learn about the technique. The final year of this project saw him come to apply this new knowledge in Professor Lynn Gladden’s laboratory. A follow-up year to further explore the industry potential of the technique was supported by Royal Dutch Shell, a long-standing collaborator of the MRRC.

Image caption: Table-top ultralow-field NMR instrument. The experiment takes place inside the magnetically shielded cylinder.

Applications for screening and materials study

Using ultra-low field strengths of around 100 times weaker than the Earth’s magnetic field, or 1 million times weaker than those used in high-field NMR, the researchers could detect NMR through an aluminium container whose walls were several millimetres thick. Moreover, the researchers were able to measure the subtle magnetic field produced by the container itself.

“Normally we don't think of aluminium as being a ferromagnetic material”, says Tayler. “If you hold a permanent magnet close to aluminium, it doesn't stick. But we weren't using pure aluminium, we were using an aluminium alloy, containing 4-5% of other elements including manganese and chromium, which in their pure form are ferromagnetic.

“These alloying elements were causing some residual but extremely weak magnetism in the alloy, and we could measure the magnitude of that field (about 50 nT) and its orientation. By elaborating the technique, we should be able to obtain a 3D map of the magnetic field inside this container.”

This technique could be used to investigate the structure of materials, enabling scientists to identify locations of magnetic impurities, and obtain a clearer picture of a container’s chemical composition.

“Being able to directly measure the magnetic field inside a closed container is valuable" says Tayler. “While you might say that some other sensor could be used to measure the magnetic field surrounding the container, on the outside, it is not a trivial task to convert that information into a field map inside the container”.

“All we had to do was fill the container with water and put it next to our NMR sensor; it's fairly general. If the container has different geometry or convoluted parts that are difficult to get a sensor inside, you just flood it with water and the technique works.”

Image caption: Subtle magnetic behaviour, including a remanent ferromagnetic moment of this aluminium container, can be detected using the low-field NMR technique.

Studying the surfaces of catalysts

Many important chemical reactions in industry employ metal catalyst particles dispersed on a solid porous matrix with a high surface-area-to-volume ratio, and the reagents and products are flowed past in a liquid or gas phase.  Even after decades of study, little is known about the molecular dynamics taking place inside these systems.  

“It's a basic question, yet we do not have a clear answer,” explains Tayler. “Chemists are often left guessing how a molecule moves when it's at the heterogeneous catalyst surface, and how reactions proceed. The kinetics of molecules near surfaces is not well understood, especially in the tight, confined space of the catalyst pore.”

Image caption: The dynamics of molecules inside porous materials, from macro scale down to the pore surface scale, is key to understanding how reactions proceed as well as how they can be made more efficient

The group also used the ultra-low field NMR technique to look at liquids inside a highly porous cobalt-on-silica material. The porous material system is chemically similar to a Fisher-Tropsch catalyst, which involves a mixture of metal centres (iron, cobalt, ruthenium) to convert carbon monoxide and hydrogen into long-chain, high-value petrochemicals. Professor Gladden’s group already studies this reaction using traditional high-field NMR, with an impressive experimental arrangement that allows them to study the reaction under high temperature-high pressure operando conditions.

“The group has been able to study the flow rate of reactants and products in and out of the reactor rig and determine the spatial distribution of products inside the reactor using MRI,” explains Tayler. “But what they cannot get is information about the molecular dynamics inside the catalyst particles.”

The non-reduced cobalt catalyst is strongly paramagnetic and this causes the signals in a traditional high-field NMR spectrum to broaden, and resolution of each chemical species is lost.

“To get meaningful information you want one NMR signal that comes, say, from the reactant and another signal that comes from the product and you can tell the difference between the two of them,” says Tayler. “That's something that's quite hard to get in these systems because the paramagnetic nature of the catalyst material causes a great broadening of the signals: they all get smeared into one another and it's hard to tell them apart.”

Using the ultra-low field technique, this paramagnetic broadening effect is significantly reduced, to the point that the presence of the cobalt atoms makes no difference to the NMR spectrum. Therefore, in principle, the signals of different chemical components may still be resolved.

Image caption: ultralow magnetic fields avoid broadening of the NMR line due to paramagnetic species on the porous material surface.

In the future, the technique may allow chemists to study exactly what’s going on inside highly paramagnetic catalysts during reactions – something no other analytical technique is currently capable of.

“If you want chemical resolution under operando conditions, magnetic resonance spectroscopy is pretty much the only way to go,” says Tayler.

“You can look at a catalyst surface under a microscope that inspects the structure down to atomic resolution. You can also place molecules on the surface and find out which ones stick best, but these methods do not provide information about the dynamics and the behaviour of the molecules under reaction conditions.”

Growing the field

High-field NMR spectroscopy techniques for chemical analysis have been around since the 1950s but the use of ultra-low-field NMR detection is still infant, having only been used by a handful of research groups over the past 10 to 20 years. When Tayler built the ultralow-field NMR instrument in Cambridge, it was the first of its kind in the UK, although more have since been developed. Tayler hopes the broad scientific applications that the technique offers, complementing high-field NMR, as well as relatively inexpensive apparatus, will encourage further growth in this field of research.

“The frequencies at which we're working enable us to leverage mass-market, low-cost electronics, such as Arduino microcontrollers. We use these to control the whole experimental apparatus and do all the data acquisition and execute NMR pulse sequences, so we're controlling a research grade instrument using only a £10 microcontroller board. In addition to solving these science questions, we're aiming to make the technology more easily shared with other researchers, and for other groups to replicate our apparatus.”

Tayler will continue to explore the applications for this technique in his new position at the Institute of Photonic Sciences, while Professor Gladden’s group will continue to explore how using a range of magnetic field stengths in magnetic resonance measurements, including relaxation time analysis, diffusion measurements and imaging, can give new insights to how molecules behave in confined spaces, with particular focus on catalysis, materials for new energy technologies, and controlled pharmaceutical release.

Read or download the full paper, ‘Ultralow-field nuclear magnetic resonance of liquids confined in ferromagnetic and paramagnetic materials’, published in Applied Physics Letters.

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