MOFs: NMR reveals inner secrets

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  • Published: Sep 15, 2012
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: MOFs: NMR reveals inner secrets

Think zinc

A Canadian research team have shown for the first time that NMR spectroscopy could be as useful in analysing the zinc atom centres in a type of porous, synthetic mineral known as a metal-organic framework (MOF) as its aluminium-based counterpart is in studying the more widely known natural porous minerals zeolites. Credit: Chem - A Eur Journal/Wiley/Terskikh  

A large international research team from Canada and China has shown for the first time that zinc-67 solid-state NMR spectroscopy could be as useful in analysing the zinc atom centres in a new type of synthetic porous materials known as metal-organic frameworks (MOFs) as its aluminium-based counterpart, aluminium-27 NMR is in studying the more widely known natural porous zeolite minerals.

MOFs have been the focus of increasingly urgent research in recent years as their porosity endows them with very large internal surface areas while their synthetic nature means they could be designed to have particular physical and chemical characteristics tailored to specific applications. MOFs are already being investigated as designer alternatives to zeolites, aluminosilicate minerals. They might one day be used as even more powerful catalysts, molecular sieves and high-density gas storage media than their natural counterparts.

Now, Yining Huang from the University of Western Ontario (London, Ontario) and his colleagues have turned to zinc-67 solid-state magic angle spinning (MAS) and static NMR spectroscopy to investigate the metal centres in a particular class of MOFs. They point out that normally the low natural abundance and the quadrupolar properties (spin 5/2) of the zinc-67 nucleus does not lend itself to NMR sensitivity but with an ultrahigh magnetic field strength of 21.1 Tesla it is possible to circumvent these problems and dig into the centre of the pores of zinc-based MOFs nevertheless.

"Zinc-67 is a quadrupolar nucleus, which makes it very sensitive to local symmetry, as say compared with carbon-13 NMR of the organic linkers," explains NMR spectroscopy team member Victor Terskikh from the NRC Canada (Ottawa). "The reason researchers have not previously turned to zinc-67 NMR for MOFs is due to the low-gamma and low natural-abundance, and until now it was limited mostly to high-symmetry environments and to bulk chemicals with high zinc content." Terskikh told SpectroscopyNOW how important it is that researchers get to grips with the chemistry and internal structure of MOFs. "This new class of porous materials has attracted a lot of attention due to their unique sorption properties," he told us. "These materials are the latest hot topic in materials science." Fundamentally, MOFs are built from metal centres and organic linkers connecting them. "The molecular level characterization is of the most importance yet challenging, because the signal crystals suitable for high-quality X-ray are hard to grow," adds Terskikh. The onus is thus on NMR spectroscopists to tease out the details inaccessible to crystallographers.

In particular the team examined ZIF-8 and ZIF-14, two MOF materials themselves related to zeolites, zeolitic imidazolate frameworks. These two materials were investigated first as they have only one zinc centre in each reducing the potential complexity of the spectrum. Other ZIF hybrid materials have several different types of zinc centre in the framework.

The team has not only been able to identify different zinc metal sites in various types of MOFs, they could actually see how the framework structure changes upon introduction of sorbent molecules. "This was a very unexpected experimental result," adds Terskikh, and which was later confirmed by molecular dynamics simulations carried out by Tom Woo and his group from the University of Ottawa. Other collaborators on this project include materials scientists Qi Shi, Zhengwei Song and Jinxiang Dong from China's Taiyuan University of Technology, and San Yuan Ding and Wei Wang from the State Key Laboratory of Applied Organic Chemistry, Lanzhou University.

The team has shown, for the first time that even with natural abundance zinc-67 it is possible to obtain solid-state NMR and moreover that the spectra are very straightforward to acquire. "These spectra are very informative," adds Terskikh. "Given that high-field solid-state NMR instruments are no longer a rarity we hope our research will encourage other people to experiment with the technique more."

Crystal complement

The team concludes in their paper that they have established zinc-67 solid-state NMR as a tool complementary to X-ray diffraction for solving structural problems in known MOFs that will allow the determination of novel MOFs yet to be prepared.

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Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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