Flexible Co(II) Metal-Organic Frameworks with Mixed Ligands under Controlled Pressure

Sikiti, Phumile (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: A number of flexible metal-organic frameworks (MOFs) are shown to undergo phase changes under CO2 gas pressure and, in two cases, the mechanisms of these changes have been elucidated using computational simulation and in situ variable-pressure single-crystal diffraction (VP-SCD). The primary objective of this work was to utilise suitable ligands to synthesise flexible cobalt-based MOFs that undergo phase changes under gas loading at 298 K.Where possible, the phase change mechanisms were visualised crystallographically at themolecular level by employing an environmental gas cell. The first section describes a new non-interpenetrated flexible MOF, {[Co2(OBA)2(BPMP)]·1.5(DMF)}n (COB-DMF), where OBA = 4,4'-oxybis(benzoic acid), BPMP = 4-bis(pyridin-4-ylmethyl)piperazine, with a new network topology. COB possesses minimal porosity and activation yields a framework with discrete voids and substantial reduction in guest-accessible volume. In the present study it is shown by means of in-situ VP-SCD that COB exhibits structural flexibility under CO2 gas loading at 298 K in a single-crystal to single-crystal manner. The mode of flexibility combines two separate mechanisms, which is highly unusual. The results are supported by in-situ powder X-ray diffraction. The second section describes a different MOF ({[Co2(OBA)2(BPMP)]·2.5(DMF)}n, COB1) prepared using the same components that were used to prepare COB. Although the stoichiometry was the same, the synthesis temperature was different and the framework systems are entirely different. Activation of COB1 yields a narrow-pore framework from a wide-pore phase. The framework breathes and switches between the narrow-pore and wide-pore phases at a specific CO2 loading pressure at 298 K. The proposed mechanism for flexibility is well supported by pressure-gradient differential scanning calorimetry and in situ VP-SCD and was further validated by means of molecular modelling. The final section describes a new interdigitated two dimensional PCP {[Co2(OBA)2(BPY)2]·2(DMF)}n. The material exhibits flexibility at a specific CO2 pressure at 298 K, with large hysteresis upon desorption, the single crystals did not survive CO2 uptake, and the flexibility was validated using VP-PXRD. Gas sorption analysis implies that the host exhibits shape memory upon complete desorption, but closer inspection of the desorption isotherm in the low-pressure range shows that the material reverts to its activated form upon complete guest removal. This constitutes approximately 26 bar of hysteresis in the reversal of the gas-induced phase transition.

AFRIKAANSE OPSOMMING: Dit word getoon dat ‘n aantal buigbare metaal-organiese raamwerke (MOFs) fase verwisselinge ondergaan onder CO2 lading en in twee gevalle is die meganismes van die fase veranderinge uitgelê deur rekenaargebaseerde simulasie en in situ veranderlike-druk enkel-kristal diffraksie (VP-SCD). Die hoof doelwit van hierdie werk was om gepaste ligande te gebruik vir die sintese van buigbare kobalt-gebaseerde MOFs wat fase verwisselinge weens gas lading by 298 K ondergaan. Die meganisme van die fase veranderinge was, waar moontlik, kristallografies gevisualiseer deur van ‘n gassel gebruik te maak. Die eerste afdeling beskryf ‘n nuwe nie-geïnterpenitreerde buigbare MOF, {[Co2(OBA)2(BPMP)]·1.5(DMF)}n (COB-DMF), met ‘n nuwe network topologie. COB beskik oor beperkte poreusheid en aktivering lewer ‘n raamwerk met aparte leemtes en ‘n aansienlike verlaging in gas-toeganklike volume. In die huidige studie word deur middel van in situ VP-SCD gewys dat COB strukturele buigbaarheid toon onder CO2 gas lading by 298 K in ‘n enkel-kristal na enkel-kristal wyse. Die modus van buigbaarheid kombineer twee meganismes, wat uitsonderlik is. Die resultate word ondersteun deur in situ poeier X-straal diffraksie (PXRD). Die tweede afdeling beskryf ‘n ander MOF ({[Co2(OBA)2(BPMP)]·2.5(DMF)}n, COB1) wat verkry is vanaf dieselfde komponente as wat vir die voorbereiding van COB gebruik is. Alhoewel die stoïgiometrie dieselfde is, is die temperatuur van sintese anders en die raamwerk sisteme uiteenlopend anders. Die aktivering van COB1 lewer ‘n nou-porie raamwerk vanaf ‘n wye-porie fase. Die raamwerk ondergaan ‘n asemhaling en skakel tussen die nou-porie en wye-porie fase by ‘n spesifieke CO2 druk by 298 K. Die voorgestelde meganisme vir buigbaarheid word goed ondersteun deur druk-gradiënt differensiële skanderings kalorimetrie, in situ VP-SCD asook molekulêre modellering. Die laaste afdeling beskryf ‘n nuwe gedigiteerde twee dimensionele MOF {[Co2(OBA)2(BPY)2]·2(DMF)}n. Die materiaal vertoon buigbaarheid by ‘n spesifieke CO2 druk by 298 K met groot desorpsie histerese. Die kristal oorleef egter nie en die buigbaarheid is bevestig deur middel van VP-PXRD. Gas sorpsie analise impliseer dat die gasheer vorm-geheue tentoonstel, maar met nouer ondersoek van die desorpsie isoterm is gevind dat die materiaal terugkeer na die geaktiveerde vorm na volledige verwyderig van die gas. Hierdie maak histerese van ongeveer 26 bar uit en ‘n omkeer van die gas-geïnduseerde fase verandering vind plaas.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107068
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