Synthetic porous materials : a study of adsorption selectivity and structure-property relationships

De Villiers, Dawie (2015-12)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The aim of this thesis was to study structure-property relationships in porous materials using various adapted analytical techniques and in-house instruments. The thesis is divided into two sections, and the first section of work constitutes the majority of the thesis. The first section of work deals with the theoretical versus experimental classification of sorption selectivity in porous compounds. A transiently porous metallocycle that can adsorb acetylene and carbon dioxide served as a model host for this experiment. A volumetric sorption instrument had to be constructed to carry out sorption with acetylene. Even though the metallocycle should theoretically be selective for acetylene over carbon dioxide based on single-gas sorption isotherms, this was not the case during the sorption of a mixture of the two gases. Furthermore, high-pressure single-crystal diffraction was carried out utilising an in-house environmental gas cell, and structural elucidation indicated that both acetylene and carbon dioxide coexist in a single cavity of the host. Additional complementary techniques are discussed that were used to confirm that both gases are present in a single host cavity. The techniques included infrared spectroscopy as well as high-pressure florescence and Raman spectroscopy, which had to be conducted with a specially designed pressure vessel and with adapted instrumentation. Finally, density functional theory calculations were employed to explain how host-guest and guest-guest interactions lead to the change in adsorption selectivity. It is concluded that researchers need to show experimentally that a compound is selective for the adsorption of a specific gas, because theoretical models are not always accurate. The second part of this work focuses on a fundamental study of the structure-property relationships in a porous hydrogen-bonded organic framework. The section starts off by exploring the activation conditions and thermal stability of the framework. This is followed by an exploration of a possible phase transformation or thermal expansion in the framework, but neither of these occurred. Thereafter, an extended study of the framework’s sorption behaviour with various gases is discussed. Then, a structural study of its solvated phase is used to explain the framework’s stability. Finally, a novel analytical method is introduced, and two examples are used to demonstrate why the instrument is useful in the field of supramolecular chemistry. The chapter is concluded by stating the importance these fundamental studies, as well the development of new analytical techniques.

AFRIKAANSE OPSOMMING: Die doel van hierdie tesis was om die struktuur-afhanklike eienskappe van poreuse materiale te ondersoek. Die studie het gebruik gemaak van verskeie aangepasde analitiese metodes asook instrumente wat spesifiek vir die studie gebou was. Die werk word in twee dele verdeel, en die meerderheid van die tesis word in die eerste deel bevat. In die eerste deel van die tesis word die validiteit van teoretiese- teen eksperimentele adsorpsie selektiwiteit opgeweeg. ʼn Gasheer wat bestaan uit ringvormige koördinasie-verbindings en wat asetileen asook koolstof dioksied kan adsorbeer, dien as ʼn model gasheer vir die studie. ʼn Volumetriese sorpsie instrument was spesiaal vir die studie gebou sodat asetileen sorpsie gedoen kon word. Volgens asetileen en koolstof dioksied se enkel-gas adsorpsie isoterme moet asetileen teoreties met voorkeur geadsorbeer word gedurende ʼn adsorpsie eksperiment waarin beide gasse teenwoordig is, maar eksperimenteel was dit bepaal dat dit nie so is nie, dus is daar ʼn verandering in die gasheer se adsorpsie selektiwiteit. Hierna word strukturele data van die gasheer, onder ʼn hoë druk van die gas mengsel, versamel deur gebruik te maak van enkel-kristal diffraksie en ʼn spesiaal-gemaakde gas sel. Die strukturele data toon dat beide asetileen en koolstof dioksied teenwoordig is binne elke porie van die gasheer. Daar word dan van addisionele analitiese metodes gebruik te maak om die observasie te bevestig. Die analitiese metodes sluit in infrarooi spektroskopie asook hoë-druk fluoressensie en Raman spektroskopie wat geëis het dat ʼn spesiale druk-bestande monster houer gebou moes word en dat analitiese instrumente gemodifiseer moet word. Ten slotte was daar van “density functional theory” gebruik gemaak om te verduidelik dat die interaksie tussen die gasheer en gas sowel as die interaksie tussen twee gasse lei tot die verandering in adsorpsie selektiwiteit. Uit hierdie bevinding word die gevolgtrekking gemaak dat navorsers met meer eksperimentele data vorendag sal moet kom voordat ʼn gevolgtrekking gemaak kan word dat ʼn raamwerk selektief een gas adsorbeer. Die tweede afdeling van die werk fokus op ʼn fundamentele studie van die struktuur-afhanklike eienskappe van ʼn poreuse waterstof-verbinde organies raamwerk. Die afdeling begin deur ʼn ondersoek van die aktivering kondisies sowel as die temperatuur-afhanklike stabiliteit van die raamwerk. Dit word gevolg deur te soek na moontlike fase veranderings of temperatuur-afhanklike uitsetting van die raamwerk, maar nie een van die twee eienskappe word waargeneem nie. Daarna word die deeglike ondersoek van die raamwerk se adsorpsie vermoë met verskeie gasse bespreek. Dit word gevolg deur ʼn strukturele studie van die solvaat van die raamwerk, wat dan gebruik word om die stabiliteit van die raamwerk te verduidelik. Ten slotte word ʼn analitiese metode bekend gestel, en twee voorbeelde word gebruik om te wys hoe nuttig die metode is om ʼn kombinasie van resultate te bekom. Die hoofstuk word saamgevat deur te verduidelik hoekom dit belangrik is om hierdie tipe fundamentele studies te doen asook waarom nuwe analitiese metodes ontwerp moet word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97839
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