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FIHM Group

Functional Inorganics and Hybrid Materials

Studying at Cambridge

 

Dr Thomas D. Bennett

Dr Thomas D. Bennett

Research Fellow, Trinity Hall, University of Cambridge


Office Phone: 01223 334342

Biography:

Born in South Shields, (1986), I read the Natural Sciences Tripos at the University of Cambridge from 2004-2008, receiving an MSci (Hons) in Chemistry. Afterwards, I moved to the Department of Materials Science and Metallurgy, to conduct my PhD research under Professor Anthony Cheetham. The thesis, titled "Thermo-mechanical Properties of Zeolitic Imidazolate Frameworks", completed in January 2012, focused on the rich diversity in physical properties in Metal-Organic Frameworks (MOFs). Notably, structural collapse of hybrid frameworks under pressure, and temperature was observed, and the products characterised by an array of techniques. Much of the foundations for the fundamental structure-property relationships in MOFs were also laid down at this time and I was elected to an honorary fellowship at the Ras Al Khaimah Center for Advanced Materials.

I performed a short stint as an EPSRC 'PhD +' Fellow, continuing my work on utilising the 'soft' mechanical properties of hybrid frameworks, before taking some time out to lecture Advanced Chemistry. In 2013, I was elected to a research fellowship at Trinity Hall, Cambridge, where I was fortunate enough to be awarded the PanAlytical award for an outstanding contribution to X-ray diffraction.

My recent work focuses on the interplay of defects and structural collapse, along with the structure and properties of metal-organic framework glasses.

Research Interests

 Porosity in Metal-Organic Framework Glasses:

 

Metal-Organic Framework Liquids and Glasses:

The first example of a MOF known to undergo melting is reported, alongside structural characterization of the porosity in the glass phase, and relationships between melting temperature and starting crystalline MOF composition. The work provides new pathways to functional hybrid glasses, alongside opportunities in shaping crystalline MOFs into the thin films and fibres necessary for application.  

 Next Generation Hybrid Frameworks:

Hybrid frameworks include both porous coordination polymers (PCPs) and metal-organic frameworks (MOFs).  The latter are network solids in which inorganic nodes (clusters or metal ions) are linked via organic ligands in an infinite array. The synthesis of novel MOF materials has been the subject of intense research and debate over the last decade, mainly because of their potential for application in gas storage and separation, catalysis and chemical sensing. 

However, despite the overwhelming attention given to both for their gas sorption properties, work on their physical properties has stagnated.  This is extremely surprising, given their proposed applications require them to retain structural integrity under temperature, pressure and stress. Specifically, hybrid frameworks are generally produced as microcrystalline powders, though these need to be converted or shaped into thin films, layers and bulk products suitable for coatings, before they can be used in 'real-life' applications.  Their poor mechanical properties cause structural collapse under such processing conditions, and thus solutions need to be found.

I concentrate on moving the field forwards with innovative solutions and research into next generation hybrid frameworks, which include:

 

  • Creating 'designer' hybrid glasses, through tailoring the chemical properties of MOFs before melting them

 

  • Developing 'super-strong' frameworks with enhanced mechanical properties capable of withstanding the harsh post-processing conditions necessary for production of thin films and bulk products

 

  • Researching the in-situ shaping of hybrid frameworks, so that instead of the polycrystalline powders produced, thin films, spheres or coatings are the products of chemical synthesis.

 

 Utilizing Poor Mechanical Properties as Virtues:


The 'softer' mechanical properties of MOFs need not be seen as detrimental, but can be utilized in processes such as reversible and irreversible structural collapse, from porous to non-porous, or sealed, species. Such transitions would be of use in H2 storage in vehicles, drug delivery applications and radioactive isotope storage (inorganic porous zeolites were employed in the aftermath of the Chernobyl and Fukushima nuclear incidents).

 

Amorphous Metal-Organic Frameworks:

A small but growing number of cases of MOFs which lack the long-range order characteristic of crystalline structures are steadily capturing scientific interest.  Most of this work has been done on a sub-family of MOFs called Zeolitic Imidazolate Frameworks (ZIFs).

The disordered nature of amorphous MOFs (aMOFs) is probed using total scattering experiments, which gives information on both Bragg and diffuse scattering.  After suitable data treatment, the Fourier transform of S(Q) yields the pair distribution function (PDF), G(r), which is in effect a map of the distances between atom pairs.  The PDF can then be used to provide insight into structural behavior, though accurate structural modeling based on the PDF using programs such as RMC Profile.  Recently, we discovered that amorphous MOFs can be synthesized from the application of stress (pressure, heat, electrical discharge or ball-milling) on existing frameworks, and exhibit substantially stronger mechanical properties than their crystalline counterparts.  In many cases, their structures were found to be much like that of conventional silica glass.

The mechanical properties of aMOFs have been found to be superior to those of their crystalline counterparts.  I have been exploring the design and use of MOFs to selectively adsorb different harmful molecules (e.g. I2 – one of the major radioactive isotopes released during the Fukushima nuclear incident in 2011), followed by subsequent collapse of the frameworks to irreversibly trap the harmful molecule within the porous interior.  I have also looked at possibility for tailored long term delivery of drugs inside the human body by trapping them inside porous MOF frameworks, whilst super strong hybrid glasses have also been synthesized by melting frameworks, raising possibilities for the manufacture of electroluminescent and optically active glasses.

 

Recent Research Highlights

Porosity in metal–organic framework glasses

porousglass

 The porosity of a glass formed by melt-quenching a metal–organic framework, has been characterized by positron annihilation lifetime spectroscopy. The results reveal porosity intermediate between the related open and dense crystalline frameworks ZIF-4 and ZIF-zni. A structural model for the glass was constructed using an amorphous polymerization algorithm, providing additional insight into the gas-inaccessible nature of porosity and the possible applications of hybrid glasses.

Melt-Quenched Glasses of Metal-Organic Frameworks

Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of three-dimensional MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic non-metallic, organic and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to starting chemical composition. The transfer of functionality from crystal to glass points towards new routes to tunable, functional hybrid glasses.

 

Hybrid glasses from strong and fragile metal-organic framework liquids

 glases

Hybrid glasses connect the emerging field of metal-organic frameworks (MOFs) with the glass formation, amorphization and melting processes of these chemically versatile systems. Though inorganic zeolites collapse around the glass transition and melt at higher temperatures, the relationship between amorphization and melting has so far not been investigated. Here we show how heating MOFs of zeolitic topology first results in a low density ‘perfect’ glass, similar to those formed in ice, silicon and disaccharides. This order–order transition leads to a super-strong liquid of low fragility that dynamically controls collapse, before a subsequent order–disorder transition, which creates a more fragile high-density liquid. After crystallization to a dense phase, which can be remelted, subsequent quenching results in a bulk glass, virtually identical to the high-density phase. We provide evidence that the wide-ranging melting temperatures of zeolitic MOFs are related to their network topologies and opens up the possibility of ‘melt-casting’ MOF glasses.

 

Connecting defects and amorphization in UiO-66 and MIL-140 metal–organic frameworks: a combined experimental and computational study

The mechanism and products of the structural collapse of the metal–organic frameworks (MOFs)UiO-66MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal–ligand bonding in each case. The amorphous products contain inorganic–organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.

Improving the mechanical stability of zirconium-based metal–organic frameworks by incorporation of acidic modulators

The ability to retain structural integrity under processing conditions which involve mechanical stress, is essential if metal–organic frameworks (MOFs) are to fulfil their potential as serious candidates for use in gas sorption, separation, catalysis and energy conversion applications. A series of zirconium dicarboxylates, predicted to be amongst the more mechanically robust MOFs, have been found to undergo rapid collapse upon ball-milling, resulting in catastrophic losses of porosity. An inverse relationship between collapse time and framework porosity has been found. Addition of acidic modulator ligands (e.g. trifluoroacetic acid) to UiO-66 provided a striking increase in mechanical robustness, the degree of which is inversely related to modulator pKa. This effect, caused by an increased strength of the zirconium–carboxylate bond, provides an important concept to design microporous hybrid frameworks capable of retaining their structure under harsh processing conditions.

Amorphous Metal-Organic Frameworks

 

 

In this Account, we describe the preparation of aMOFs by introduction of disorder into their parent crystalline frameworks through heating, pressure (both hydrostatic and nonhydrostatic), and ball-milling. The main method of characterizing these amorphous materials (analysis of the pair distribution function) is summarized, alongside complementary techniques such as Raman spectroscopy. Detailed investigations into their properties (both chemical and mechanical) are compiled and compared with those of crystalline MOFs, while the impact of the field on the processing techniques used for crystalline MOF powders is also assessed. Crucially, the benefits amorphization may bring to existing proposed MOF applications are detailed, alongside the possibilities and research directions afforded by the combination of the unique properties of the amorphous domain with the versatility of MOF chemistry.

 

Crystallography of Metal-Organic Frameworks

 

Metal-Organic frameworks (MOFs) are one of the most intensely studied materials in recent times. Their networks, resulting by the formation of strong bonds between inorganic and organic building units, offer unparalled chemical diversity and pore environments of growing complexity. Therefore advances in single crystal X-ray diffraction equipment and techniques are required to characterize materials with increasingly larger surface areas, and more complex linkers. In addition, whilst structure solution from powder diffraction data is possible, the area is much less populated and we detail the current efforts going on here. We also review the growing number of reports on diffraction under non-ambient conditions, including the response of MOF structures to very high pressures.  Such experiments are important due to the expected presence of stresses in proposed applications of MOFs – evidence suggesting rich and complex behavior. Given the entwined and inseparable nature of their structure, properties and applications, it is essential that the field of structural elucidation is able to continue growing and advancing, so as not to provide a rate limiting step on characterization of their properties and incorporation into devices and applications.  

 

Ball-Milling Induced Amorphization of Zeolitic Imidazolate Frameworks (ZIFs) for the Irreversible Trapping of Iodine 

 

The I2-sorption and -retention properties of several existing zeolitic imidazolate frameworks (ZIF-4, -8, -69) and a novel framework, ZIF-mnIm ([Zn(mnIm)2 ]; mnIm=4-methyl-5-nitroimidazolate), have been characterised using microanalysis, thermogravimetric analysis and X-ray diffraction. The topologically identical ZIF-8 ([Zn(mIm)2]; mIm=2-methylimidazolate) and ZIF-mnIm display similar sorption abilities, though strikingly different guest-retention behaviour upon heating. We discover that this guest retention is greatly enhanced upon facile amorphisation by ball milling, particularly in the case of ZIF-mnIm, for which I2 loss is retarded by as much as 200 °C. It is anticipated that this general approach should be applicable to the wide range of available metal-organic framework-type materials for the permanent storage of harmful guest species.

Facile Mechanosynthesis of Amorphous Zeolitic Imidazolate Frameworks

A fast and efficient mechanosynthesis (ballmilling) method of preparing amorphous zeolitic imidazolate frameworks (ZIFs) from different starting materials is discussed. Using X-ray total scattering, N2 sorption analysis, and gas pycnometry, these frameworks are indistinguishable from one another and from temperature-amorphized ZIFs. Gas sorption analysis also confirms that they are nonporous once formed, in contrast to activated ZIF-4, which displays interesting gate-opening behavior. Nanoparticles of a prototypical nanoporous substituted ZIF, ZIF-8, were also prepared and shown to undergo amorphization.

 

Thermal Amorphization of Zeolitic Imidazolate Frameworks

A stable, recoverable, amorphous phase (see topology model) was produced by heating each of four
different zeolitic imidazolate frameworks ZIF-1, -3, -4, and Co-ZIF-4. By comparing nanoindentation results, density measurements,
and X-ray total scattering results, it is concluded that the structure of the amorphous phase is equivalent in
each case. Amorphization was only observed in ZIFs encompassing unsubstituted
imidazolate ligands.

 

Structure and Properties of an Amorphous Metal-Organic Framework

ZIF-4, a metal-organic framework (MOF) with a zeolitic structure, undergoes a crystal–amorphous transition on heating to 300 C. The amorphous form, which we term a-ZIF, is recoverable to ambient conditions or may be converted to a dense crystalline phase of the same composition by heating to 400 C. Neutron and x-ray total scattering data collected during the amorphization process are used as a basis for reverse Monte Carlo refinement of an atomistic model of the structure of a-ZIF. The structure is best understood in terms of a continuous random network analogous to that of a-SiO2. Optical microscopy, electron diffraction and nanoindentation measurements reveal a-ZIF to be an isotropic glasslike phase capable of plastic flow on its formation. Our results suggest an avenue for designing broad new families of amorphous and glasslike materials that exploit the chemical and structural diversity of MOFs.

Other Professional Activities

Key Publications

Please see the following link for a full list of publications.

16. T. D. Bennett*, Y. Yue, P. Lim A. Qiao, H. Tao, G. N. Greaves, T. Richards, G. I. Lampronti, Simon. A. T. Redfern, F. Blanc, O. K. Farha, J. T. Hupp, A. K. Cheetham and D. A. Keen, Melt-Quenched Glasses of Metal-Organic FrameworksJ. Am. Chem. Soc., 2016, Just-accepted

15. A. W. Thornton, K. E. Jelfs, K. Konstas, C. M. Doherty, A. J. Hill, A. K. Cheetham and T. D. Bennett*, Porosity in metal-organic framework glassesChemical Communications, 2016, 52, 3750-2753

14. C. L. Hobday, R. J. Marshall, C. F. Murphie, J. Sotelo, T. Richards, D. Allan, T. Duren, F.X Coudert, R. S. Forgan*, C. A. Morrison*, S. A. Moggach* and T. D. Bennett*, Geometric frustration in UiO frameworks: A computational and experimental approach linking disorder, high-pressure behaviour and mechanical propertiesAngew. Chem. Intl. Ed., 2016, 128, 2447-2451

13. Bennett, T. D.,* Todorova, T. K., Baxter, E. F., Reid, D. G., Gervais, C., Bueken, B., Van De Voorde, B., De Vos, D., Keen, D. A and Mellot-Draznieks, C.,* Connecting defects and amorphization in UiO-66 and MIL-140 metal-organic frameworks: a combined computational and experimental study, Phys. Chem. Chem. Phys., 2016, 18, 2192-2201 

12. Marshall, R. J., Richards, T., Hobday, C. L., Murphie, C. F.,Wilson, C., Moggach, S. A., Bennett, T. D,* and Forgan, R. S.*, Postsynthetic bromination of UiO-66 analogues: Altering linker flexibility and mechanical complianceDalton Trans., 2016, DOI: 10.1039/C5DT03178H

11. Bennett, T. D,* Tan, J. C., Yue, Y. Z., Baxter, E., Ducati, C., Terril, N., Yeung, H. H. M, Zhou, Z., Chen, W., Henke, S., Cheetham, A. K, Greaves, G. N.*, Hybrid glasses from strong and fragile metal-organic framework liquids, Nat. Commun., 20156, 8079

10.  B. Van De Voorde, I. Stassen, B. Bueken, F. Vermootele, D. De Vos, R. Ameloot, Jin-Chong Tan and T. D. Bennett*, Improving the mechanical stability of zirconium-based metal-organic frameworks by incorporation of acidic modulators, J. Mat. Chem. A., 2015, 3, 1737-1742

9.  T. D. Bennett*, J. Sotelo, Jin-Chong Tan and S. A. Moggach, Mechanical properties of zeolitic metal-organic frameworks: mechanically flexible topologies and stabilization against structural collapse, CrystEngComm, 2015, 17, 286-289

8.  T. D. Bennett* and A. K. Cheetham*, Amorphous Metal Organic FrameworksAcc. Chem. Res., 2014, 47, 1555-1562.

7.  T. D. Bennett* and F. Gandara*, Crystallography of Metal Organic Frameworks, International Journal of Crystallography2014, 1, 563-570

6.  T. D. Bennett et al., Ball-Milling Induced Amorphization of Novel and Existing Zeolitic Imidazolate Frameworks (ZIFs) for the Irreversible Trapping of IodineChem. Eur. J., 2013, 19, 7049-7055.

5.  T. D. Bennett, A. L. Goodwin, D. A. Keen, et al., Structure and Properties of an Amorphous Metal-organic Framework, Phys. Rev. Lett., 2010, 104, 115503.

4.  T. D. Bennett, et al., Facile Mechanosynthesis of Amorphous Zeolitic Imidazolate Frameworks, J. Am. Chem. Soc., 2011, 133, 14546-14549.

3.  T. D. Bennett, D. A. Keen, J. C. Tan, et al., Thermal Amorphization of Zeolitic Imidazolate Frameworks, Angew. Chem. Int. Ed., 2011, 50, 3067-3071.

2.  T. D. Bennett, P. Simoncic, S. A. Moggach, et al., Reversible Pressure-induced Amorphization of the Zeolitic Imidazolate Framework ZIF-4, Chem. Commun., 2011, 47, 7983-7985.

1. S. A. Moggach, T. D. Bennett, A. K. Cheetham, The Effect of Pressure on ZIF-8: Increasing Pore Size with Pressure and the Formation of a High-Pressure Phase at 1.47 GPa, Angew. Chem. Int. Ed., 2009, 48, 7087-7089.