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Dr Thomas Douglas Bennett

Research Fellow in the Sciences, Trinity Hall

Thomas Bennett is available for consultancy.


Office Phone: 01223334342

Research Interests

Metal-organic frameworks (MOFs) 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.  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).

 

Synthesis and Structure of Amorphous Metal-Organic Frameworks:

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.

 

Properties and Applications of Amorphous Metal-Organic Frameworks:

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

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

ZIFs were used to first sorb molecular Iodine, before amorphization by ball-milling.  It was observed that the amorphous frameworks trapped the Iodine (of interest due to its release in large quantities during the Fukushima disaster in 2011).  It is thought that this may be a general method to trap harmful species, using amorphization.

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.


Keywords

  • Particle sizing
  • Carbon Capture and Storage
  • Materials
  • Emissions
  • Sensors
  • Chemistry
  • Pollution

Key Publications

1. T. D. Bennett, A. L. Goodwin, D. A. Keen, et al., Structure and Properties of an Amorphous Metal-organic FrameworkPhys Rev Lett2010104, 115503.

2. T. D. Bennettet al., Facile Mechanosynthesis of Amorphous Zeolitic Imidazolate FrameworksJ Am Chem Soc 2011133, 14546-14549.

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

4.  T. D. Bennett, P. Simoncic, S. A. Moggach, et al.Reversible Pressure-induced Amorphization of the Zeolitic Imidazolate Framework ZIF-4, Chem Commun 201147, 7983-7985.

5.  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 Edit 200948, 7087-7089.