Chemical reactions monitored in metal containers

German and Russian scientists have developed a new method of observing chemical reactions, using nuclear magnetic resonance (NMR) spectroscopy with an unusual twist: there is no magnetic field. Their work has been published in the journal Angewandte Chemie International Edition.

As a chemical technique, NMR spectroscopy is used to analyse the composition of substances and to determine their structures. High-field NMR is frequently used, which allows the nondestructive examination of samples. However, this method cannot be used to observe chemical reactions in metal containers because the metal acts as a shield, preventing penetration of the relatively high frequencies. For this reason, NMR sample containers are typically made of glass, quartz, plastic or ceramic. Furthermore, high-field NMR spectra of heterogeneous samples containing more than one component tend to be poor. There are more advanced concepts but these often have the drawback that they do not make in situ monitoring of reactions possible.

Scientists from Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM), working in collaboration with visiting researchers from Russia, thus proposed the use of zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) in order to circumvent the problems. In this case, due to the absence of a strong external magnetic field, a metal container will not have a screening effect. The research group used a titanium test tube and a conventional glass NMR test tube for comparison in their experiments. In each case, para-enriched hydrogen gas was bubbled into a liquid to initiate a reaction between its molecules and the hydrogen.

The team’s results showed that the reaction in the titanium tube could be readily monitored using ZULF NMR. It was possible to observe the kinetics of the ongoing reaction with high spectroscopic resolution while continually bubbling parahydrogen gas.

“This technique has two advantages,” said Professor Dmitry Budker, head of the Mainz-based group. “For a start, we are able to analyse samples in metal containers and, at the same time, we can examine more complex substances made up of different types of components.

The study authors believe their concept could be extremely useful when it comes to practical applications, writing, “We anticipate that ZULF NMR will find application in the field of catalysis for operando and in situ reaction monitoring as well as in the study of chemical reaction mechanisms under realistic conditions.”

Image caption: Chemical reaction monitoring via zero-field nuclear magnetic resonance (NMR). A sequential hydrogenation reaction (A→B→C) is initiated inside a metal reactor inserted into a magnetically shielded enclosure. The NMR spectrum of the heterogeneous (gas/liquid) reaction is recorded with an atomic magnetometer positioned next to the reactor. Analysis of the spectra acquired during the course of the reaction reveals the changing concentrations of compounds B and C. Illustration ©John W. Blanchard.

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