CRYSTAL GROWTH AND BREATHING BEHAVIOUR OF MIL-53 METAL-ORGANIC FRAMEWORK STUDIES USING ATOMIC FORCE MICROSCOPY

Abstract

The aim of this study is 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 diamond-shaped one-dimensional channels that exhibit framework flexibility when exposed to external stimuli. The gallium terephthalate was solvothermally synthesized in water under mild condition for few days to generate the single crystal MIL-53 [Ga(OH)(BDC)]·0.67H2BDC characterized by powder XRD , single-crystal XRD , AFM and SEM. The intercalation of an aromatic guest (pyridine) by single crystal to single crystal solvothermal guest exchange of gallium terephthalate was investigated to generate two MIL-53 gallium pyridine structures, where one of the structures was found to contain one per cent pyridine in the framework channel [Ga(OH)(BDC)]·C5H5N, and the other structure showed 83% occupancy of the framework channel by pyridine [Ga(OH)(BDC)]·0.83C5H5N. The 83%- occupied example was studied further by AFM and SEM. These two structures showed only one orientation for the pyridine guest present in the channel of the MIL-53 [Ga(OH)(BDC)]. Upon removal of the pyridine guest from the channel at 220 °C, an empty large pore structure was observed, [Ga(OH)(BDC)] large pore. At room temperature, the empty large-pore structure collapsed and adsorbed one molecule of water within the channels to give [Ga(OH)(BDC)]·H2O. The dehydrated form of the MIL-53 [Ga(OH)(BDC)]·H2O was obtained upon heating the material at 80 °C and cooling to -123 °C to reveal the [Ga(OH)(BDC)] narrow-pore structure. The thermal behaviour of these crystals was observed during a thermal cycling process. This showed crystals with some crystal defects for as-synthesized MIL-53 [Ga(OH)(BDC)]·0.67H2BDC. During guest exchange with pyridine, some crystal breakage was observed and when the pyridine molecule was extracted from the pores of the MIL-53 [Ga(OH)(BDC)] at 220 °C and 320 °C respectively, more breakages were observed on the crystal surface. As more stress was applied to the crystals upon heating and cooling, the crystals damaged further resulting in an almost powdered sample with small blocks of single crystals.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Anderson (Supervisor) & Martin Attfield (Supervisor)