Single crystal to single crystal transformations of MIL-53 Metal Organic Frameworks

Abstract

The aim of the first leg of this study was to investigate the single crystal to single crystal breathing behaviour of one member of the MIL-53 family [Ga(OH)(BDC)]. The structure of this MIL-53 is built from infinite chains of trans corner-sharing GaO4(OH)2 octahedra linked to each other through the terephthalic acid, resulting in the formation of a large rhombic-shaped one-dimensional channels that exhibit framework flexibility when exposed to external stimuli. The MOF was hydrothermally synthesized in water under mild condition for 3 days to generate the single crystal MIL-53 [Ga(OH)(BDC)]·0.74H2BDC. A solvent exchange of the guest molecule from terephthalic acid to pyridine was studied using in-situ single crystal X-ray diffraction data collections. A crystal structure for [Ga(OH)(BDC)]·Pyridine was shown to have only one orientation of pyridine guest molecules within its pore. Eight data collections and sequential crystal structures make up an isobar, identified for the [Ga(OH)(BDC)] under 1.5 bar N2 from 473 K to 300 K. A decrease in unit cell volume was observed from a very large pore at 473 K to a large pore at 300 K, which highlights the MOFs flexibility. At 300 K N2 gas had a low enough thermal energy to be crystographically identified in the structure. A series of in-situ single crystal X-ray diffraction data collections were made under varying temperatures and pressures of CO2. CO2 molecules were modelled as rigid body’s and show how the gas interacts with the MOF. Extrapolated from this is the importance the OH linker has on CO2 absorption onto the MOF. Large changes in unit cell size and pore size highlight the flexibility that [Ga(OH)(BDC)] has. The next section of this study was to investigate the single crystal to single crystal breathing behaviour of another member of the MIL-53 family [Ga(OH, F)(BDC)]. The MOF has an extremely similar structure to [Ga(OH)(BDC)] but has an 80:20 ratio of F linking the gallium centres. The MOF was synthesised similarly to [Ga(OH)(BDC)] but with the addition of HF to give [Ga(OH, F)(BDC)]·0.74H2BDC. A solvent exchange was completed to get [Ga(OH, F)(BDC)].pyridine. Finally, a solvent exchange of the guest molecule from pyridine to H2O was studied using in-situ single crystal X-ray diffraction data collections which lead to the presentation of a newly reported closed pore structure.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Martin Attfield (Supervisor), George Whitehead (Supervisor) & Inigo Vitorica-Yrezabal (Supervisor)