The power of Metal-Organic Frameworks

Collaborative research at Harwell Campus paves the way for innovation

From using Metal-Organic Frameworks (MOFs) to trap nuclear waste to evidencing the first liquid MOF, pioneering research involving Diamond Light Source and ISIS Neutron and Muon Source is advancing our understanding of these remarkable materials.

Since their discovery over 20 years ago there has been an explosion of research interest into MOFs, with scientists synthesising tens of thousands of unique structures. MOFs are made by linking organic and inorganic units with strong bonds, resulting in a structure with exceptional porosity. By synthesising structures of differing pore sizes MOFs can be used to filter, trap, or transport molecules.

This flurry of research into MOFs is driven by their wealth of potential applications in hydrogen storage, catalysis, drug delivery, carbon capture and more.   Diamond and ISIS Neutron Source provide vital insights into these fascinating structures, including advancing real world applications, finding an astonishing new type of liquid structure, and deepening our fundamental understanding of these notable materials.

     

Trapping nuclear waste at the molecular level

Nuclear power currently supplies just over 10% of the world’s electricity. However one factor hindering its wider implementation is the confinement of dangerous substances produced during the nuclear waste disposal process. One such bi-product of the disposal process is airborne radioactive iodine that, if ingested, poses a significant health risk to humans.  The need for a high capacity, stable iodine store that has a minimised system volume is apparent – and this collaborative research project may have found a solution.

Researchers have successfully used ultra-stable MOFs to confine large amounts of iodine to an exceptionally dense area. A number of complementary experimental techniques, including measurements taken at Diamond Light Source and ISIS Neutron and Muon Source, were coupled with theoretical modelling to understand the interaction of iodine within the MOF pores at the molecular level. 

High resolution x-ray powder diffraction (PXRD) data were collected at Diamond’s I11 beamline. The stability and evolution of the MOF pore was monitored as the iodine was loaded into the structure. Comparison of the loaded and empty samples revealed the framework not only adsorbed but retained the iodine within its structure.

Terahertz spectra were also taken at Diamond’s MIRIAM: IR Microspectroscopy beamline (B22). The spectra indicated that the binding interaction between the iodine molecules and the MOF host was not sufficient to distort the symmetry of the iodine molecules – suggesting fully reversible iodineuptake.

Inelastic neutron scattering (INS) measurements were taken at ISIS Neutron and Muon Source on the TOSCA instrument. INS measures the change in the energy of a neutron as it scatters from a sample and is an ideal technique for studying atomic and molecular motions. Upon loading of iodine the INS data, coupled with density functional theory measurements, confirmed the structural model of the interaction of the MOF with the iodine.

The findings here demonstrate that iodine can be reversibly stored in an ultra-stable MOF material. These results have implications for the capture and storage of radioactive iodine, which could remove one barrier limiting wider adoption of nuclear power. 


Figure 1: Airborne radioactive iodine is one of the bi-products of the nuclear waste disposal process. A recent study involving Diamond Light Source and ISIS Neutron and Muon Source showed how MOFs can capture and store iodine which may have implications for the future confinement of these hazardous substances.


For more information on trapping nuclear waste at the molecular level…
Read the full research publication:
X. Zhang el al. “Confinement of iodine molecules into triple-helical chains within robust metal–organic frameworks” J. Am. Chem. Soc., 2017 139 (45), 16289-16296 DOI: 10.1021/jacs.7b08748
To learn out more about the I11 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Prof Chiu Tang: chiu.tang@diamond.ac.uk. For the B22 beamline, the Principal Beamline Scientist is Dr Gianfelice Cinque: gianfelice.cinque@diamond.ac.uk.
To learn more on ISIS Neutron and Muon Source’s instrument TOSCA visit this page.
Link to the full research highlight

The world’s first liquid Metal-Organic Framework

Although MOFs have a wide range of potential industrial uses they currently take the form of a powder, which can make them difficult to use and store.  A liquid MOF would allow the shaping and use of these materials in a much more efficient way than could be achieved in powder form, opening up the possibility of a number of new industrial applications.  

If the concept of a tiny structure able to filter, trap or transport single molecules is hard to envision the idea of a liquid structure which could fulfil the same purpose is even more bewildering.  But a ground-breaking study, selected for the cover of Nature Materials, has evidenced just that - a MOF able to retain its porous properties in both the liquid and glass states.

The bonding and structure of this innovative MOF was first studied at increasing temperatures as it changed from a solid to a liquid using x-ray diffraction at the Advanced Photon Source in Chicago. After naturally cooling to room temperature the structure of the MOF was studied using the GEM diffractometer at ISIS Neutron and Muon Source using neutron total scattering. Neutron total scattering utilises a unique combination of both Bragg and diffuse scattering to provide information on both the average structure of a sample and short-range fluctuations in a single experiment.

A small amount of the sample used in the GEM diffractometer was then taken to Diamond Light Source for room temperature x-ray total scattering measurements using the XPDF: X-ray Pair Distribution Function beamline (I15-1).  This beamline allows researchers to study the local structure of crystalline, semi-crystalline and amorphous solids and liquids.  

These experimental results were combined with first-principles molecular dynamics simulations to study the melting phenomena and infer the structure of this liquid MOF. Researchers found that the MOF’s chemical configuration, coordinative bonding and most remarkably porosity were maintained even in the liquid state.

This high impact study has the potential to open up a whole new area of research into these novel “liquid MOFs” which may have many industrial applications in areas such as catalysis, ion transport, liquid phase separations and more. 

Figure 2: This study coins the term “liquid MOF” which describes the astonishing case of a porous liquid able to maintain its porosity even in liquid form.  The image above shows the structure of the MOF (ZIF-4 material) at room temperature.  Credit: Copyright F.-X. Coudert / CNRSClose


For more information on the world’s first liquid Metal-Organic Framework…
Read the full research publication: R. Gaillac et al. “Liquid metal–organic frameworks” Nature Materials 16, 1149–1154 (2017). DOI:10.1038/nmat4998
To learn more about the XPDF (I15-1) beamline, or to discuss potential applications, please contact Senior Beamline Scientist Dr Phil Chater: philip.chater@diamond.ac.uk
To learn more on ISIS Neutron and Muon Source’s instrument GEM visit this page.
Link to the full research highlight


Using vibrations to deepen our understanding of Metal-Organic Frameworks

Our limited understanding of the complex physical mechanisms that control the core functions of MOFs may be hindering the potential applications of these remarkable materials. In order to fulfil the vast potential of MOFs their detailed dynamics must first be understood. 

Researchers found that the vibrational modes in the low-energy terahertz region hold a large amount of information regarding the flexibility of MOF materials. The group’s hidden discoveries pave the way for future research to search for unknown relationships within MOFs.

Low frequency terahertz vibrations are pivotal to our understanding of MOFs. The spectra and terahertz vibrational modes of the MOF were experimentally determined using high-resolution inelastic neutron scattering (INS) on the TOSCA and OSIRIS ​instruments, and infra-red spectroscopy on the MIRIAM: IR Microspectroscopy beamline (B22) at Diamond Light Source.

The spectra obtained using inelastic neutron scattering was stronger in the terahertz region however, it gave a complex signal. Whilst the spectra obtained at Diamond using infra-red spectroscopy was less complex, the signal in the THz region was much weaker.

Using these two complementary techniques, alongside density functional theory calculations, researchers were able to pinpoint a number of terahertz nodes at the nanoscale confirming notable physical phenomena in MOFs. The techniques in the paper have opened the door to further research into unexplained physical dynamics of MOFs. 

Figure 3: (left) Symmetric rotor dynamics at 1.48 THz; (right) asymmetric rotor dynamics at 1.34 THz. Reprinted figure with permission from Matthew R. Ryder et al. Phys. Rev. Lett. 118, 255502, 2017. Copyright 2017 by the American Physical Society. DOI: 10.1103/PhysRevLett.118.255502.
 

For more information on using vibrations to deepen our understanding of Metal-Organic Frameworks…
Read the full research publication:  M. R. Ryder et al. “Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework” Phys. Rev. Lett. 118, 255502 (2017). DOI: 10.1103/PhysRevLett.118.255502
To learn out more about the B22 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Dr Gianfelice Cinque: gianfelice.cinque@diamond.ac.uk.
Learn more on ISIS Neutron and Muon Source’s instruments TOSCA and OSIRIS .
Link to the full research highlight


Neutrons, X-rays, muons, THz and UV as complementary probes

It is not just the fact that Diamond Light Source and ISIS Neutron and Muon Source lie less than 500 metres away from one another at Harwell Campus that makes research using both facilities so popular. The complementary techniques at the facilities mean it is common for samples to be investigated at both facilities as highlighted above.

Researchers seeking beamtime at both facilities need not submit two separate proposals. It is possible to submit just one research proposal that will be reviewed by a single review panel. Further details can be found here