Determining the Thermoresponsive and Sorption Behaviour of Fluoride containing Metal-Organic Frameworks (MOFs) by in situ X-ray Diffraction

Abstract

Understanding structure-property relationships of members belonging to the metal-organic framework (MOF) class of porous materials is essential for the development of these materials towards selective gas adsorption and separation applications. Fluorination of MOFs is known to impart several desirable properties such as improved chemical and thermal stability and increased selectivity towards environmentally important gases such as CO2. Therefore, this thesis is focused on investigating the efficacy of several routes of fluoride incorporation into the inorganic clusters of MIL-53 and rare earth (RE) fcu MOFs. The influence of fluoride incorporation on the thermoresponsive or sorption behaviour of these MOFs has been investigated using X-ray diffraction and spectroscopic methodologies. Three members of the gallium MIL-53 series [Ga(OH)1-xFx(bdc)] (bdc = 1,4-benzenedicarboxylate, x = 0.00, 0.15, 0.20) were produced using reported synthetic routes for x = 0.00 and 0.15, and a novel route utilising GaF3 as the Ga source and DEF as the solvent for x = 0.20. A gradual reduction in the flexibility of these compounds was observed with increased x, in the form of an increase in the relative stability of the large pore (lp) phase over a greater temperature range. By employing the same synthesis methodology as for [Ga(OH)0.8F0.2(bdc)], the novel iron MIL-53 [DEA]+0.5 [FeIII0.5, FeII0.5 (F)(bdc)]0.5- 0.5 (H2O) was synthesised and its redox conversion to iron MIL-53 [FeIIIF(bdc)]x solvent and framework compound [(FeII)2FeIIIF3(C5H5N)4(bdc)2]C5H5N was subsequently explored. The influence of fluorination of [FeIIIF(bdc)] was investigated by in situ CO2 adsorption studies. The partial incorporation of inorganic fluoride via a known synthetic route, has been quantified for the RE fcu frameworks, Y-fum (fum =fumarate) and Y-ndc (ndc =1,4-napthalenedicarboxylate). A novel form of fumarate linker disorder involving the binding mode of the carboxylate groups in Y-fum and ndc linker disorder involving positional disorder of the naphthalene groups in Y-ndc were successfully determined. Differing mu3-F- distribution is observed in the two frameworks. In Y-fum, this results in one of two tetrahedral pores being lined with almost exclusively mu3-F-, while the other is lined with a mixture of mu3-(OH)-/F- sites. A random distribution mu3-F- sites is observed in Y-ndc. Studies of different sorbates in the two MOFs reveal that most bind via the open metal Y site except for CS2 which was located in the OH/F lined cages of Y-fum only, indicating that the distribution of mu3-F- also influences the binding of CS2. Overall, this shows the intricate effects of fluoride incorporation in MOFs, and the importance of expansive studies that reveal novel synthetic routes that could allow for better functionalised classes of MOFs.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Martin Attfield (Supervisor), George Whitehead (Supervisor) & Inigo Vitorica-Yrezabal (Supervisor)