David Keen

Abstract:

We report the solvent-free synthesis of a crystalline heterometallic imidazolate derivative with formula [Fe₁Zn₂(im)₆(Him)₂], designated MUV-25, incorporating both iron and zinc. The structure imposes strict positional constraints on the metal centres due to the lattice containing distinct geometric coordination sites, tetrahedral and octahedral. As a consequence, each metal is exclusively directed to its specific coordination site, ensuring precise spatial organization within the lattice. Atom locations were meticulously monitored utilizing X-ray diffraction (single crystal and total scattering) and XAS techniques, demonstrating that the tetrahedral sites are occupied exclusively by zinc, and the octahedral sites are occupied by iron. This combination of metal centres results, upon heating, in a structural phase transformation to the zni topology at a very low temperature. Further heating causes the melting of the solid, yielding a heterometallic MOF-derived glass. The methodology lays the groundwork for tailoring crystalline structures to advance the development of novel materials capable of melting and forming glasses upon cooling.

Abstract:

Metal-organic framework (MOF) composites are proposed as solutions to the mechanical instability of pure MOF materials. Here, we present a new compositional series of recently discovered MOF-crystalline inorganic glass composites. In this case, formed by the combination of a photocatalytic titanium MOF (MIL-125-NH₂) and a phosphate-based glass (20%Na₂O-10%Na₂SO₄-70%P₂O₅). This new family of composites has been synthesized and characterized using powder X-ray diffraction, thermal gravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and X-ray total scattering. Through analysis of the pair distribution function extracted from X-ray total scattering data, the atom-atom interactions at the MOF-glass interface are described. Nitrogen and carbon dioxide isotherms demonstrate good surface area values despite the pelletization and mixing of the MOF with a dense inorganic glass. The catalytic activity of these materials was investigated in the photooxidation of amines to imines, showing the retention of the photocatalytic effectiveness of the parent pristine MOF.

Abstract:

In this work, we investigated the response of the metal-organic framework MIL-125-NH₂ to ball-milling. Both localised and bulk analyses revealed prolongued ball-milling results in a complete loss of long-range structural order. Investigation of this disorder revealed partial retention of the local bonding of the secondary building unit, suggesting structure collapse progressed primarily through metal-linker bond breakage. We explored the photocatalytic performance of the materials, and examined the materials' band gap using UV-Vis reflectance spectroscopy.

Abstract:

Crystalline metal-organic frameworks (MOFs) exhibit enormous potential application in gas separation, thanks to their highly porous structures and precise pore size distributions. Nevertheless, the inherent limitations in mechanical stability of crystalline MOFs cause challenges in processing MOF powders into bulky structures, particularly for membrane filtrations. Melt-quenched MOF glasses boast excellent processability due to liquid-like properties. However, the melting process diminishes the inherent porosity, leading to reduced gas adsorption capacities and lower gas diffusion coefficients. In this work, we demonstrated that enhancing the porosity of MOF glasses is achievable through topological engineering on the crystalline precursors. Crystalline zeolitic imidazolate frameworks (ZIFs) with large 12-membered rings pores, including AFI and CAN topology, were synthesized by using both structure-directing agents and mixed organic ligands. The large pores are partially preserved in the melt-quenched glass as evidenced by high-pressure CO₂ absorption at 3000 kPa. The a_gAFI-[Zn(Im)_1.68(bIm)_0.32] glass was then fabricated into self-supported membranes, which shows high gas separation performance, for example, CO₂ permeance of 3.7 × 10⁴ GPU with a CO₂/N₂ selectivity of 14.8.