Investigation of the carbon dioxide sorption properties of selected organic macrocycles

Basson, Jeanice Letitia (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: The inclusion of guest molecules within host compounds has been an ever-growing interest in supramolecular chemistry. Host-guest chemistry has attracted much attention due to potential applications in drug delivery, catalysis and the separation and storage of natural gases. As a result, a large variety of porous systems such as porous coordination polymers (PCPs), porous molecular systems, covalent organic frameworks (COFs) and supramolecular organic frameworks (SOFs) have been developed. In this study, three well-known organic macrocycles, namely p-tert-butylcalix[4]arene (TBC4), cucurbit[6]uril (CB[6]) and cucurbit[8]uril (CB[8]) were investigated as hosts for gaseous carbon dioxide (CO2). The aim of this study was to gain insight into the properties of these hosts and gain a better understanding of the inclusion of CO2 in these materials. The first section discusses TBC4 as a host for CO2. The guest-free low density polymorph of TBC4 contains discrete pockets (zero-dimensional porosity) and captures CO2 in a stepwise fashion. The system undergoes a gas-induced single-crystal to single-crystal (SC-SC) transformation to a more porous phase with an increased CO2 loading capacity. The CO2 inclusion compounds were investigated in-situ using single-crystal X-ray diffraction (SCXRD). The gas-induced phase transformation was further investigated using variable-pressure powder X-ray diffraction (VP-PXRD) and pressure-ramped differential scanning calorimetry (P-DSC). Interestingly, this porous phase is preserved when the CO2 molecules are removed. However, the original guest-free phase can be regenerated under mild activation conditions. The study was extended by investigating CO2 uptake by the high density polymorph of TBC4. Despite being a close-packed structure, CO2 molecules are able to diffuse through the host to instigate a gas-induced transformation at high pressure, and a possible mechanism is discussed. The second section describes the porosity of the well-known host, CB[6]. This host undergoes various phase transformations in order to produce a framework with permanent one-dimensional porosity. The various phases were subjected to thermal and structural analysis, where possible. This host, like most molecular organic hosts, produces a more close-packed phase when activated, and it expands upon CO2 loading. The CB[6] host framework displays a large affinity for CO2 and does not undergo a structural change at high CO2 pressure. In-situ SCXRD was used to investigate the CO2 inclusion compounds at various gas pressures to study the host-guest interactions. The final section discusses CB[8] as a CO2 adsorbent. Due to the high CO2 affinity demonstrated by CB[6], CB[8] was studied as a solid-state host. This host undergoes a phase transformation during desolvation to produce a more stable polycrystalline phase of CB[8]. Although the host could not be characterized structurally, it was evident from the CO2 sorption analysis that the host framework is porous. CB[8] displays stepwise uptake and release of CO2 molecules and has a larger affinity for CO2 as compared to methane and nitrogen gas. VP-PXRD was used to investigate the dynamic nature of the host with respect to CO2 uptake and release. Several attempts at preserving the single crystallinity of CB[8] during desolvation are also discussed.

AFRIKAANSE OPSOMMING: Die insluiting van gas molekules in gasheerverbindings is van belang in die area van supramolekulêre chemie weens verskeie potensiële toepassings. Hierdie toepassings sluit dwelm aflewering, katalise en die skeiding en berging van natuurlike gasse in. 'n Groot verskeidenheid poreuse sisteme soos poreuse koördinasie polimere (PCPs), poreuse molekulêre stelsels, kovalente organiese raamwerke (COFs) en supramolekulêre organiese raamwerke (SOFs) was ontwikkel om hierdie toepassings te bewerkstellig. In hierdie studie was drie bekende organiese makrosiklusse, naamlik p-tert-butielcalix[4]areen (TBC4), cucurbit[6]uril (CB[6]) en cucurbit[8]uril (CB[8]), ondersoek as gashere vir koolstofdioksied (CO2) gas. Die doel van hierdie studie was om eienskappe van hierdie gashere in die vaste toestand te ondersoek en om die insluiting van CO2 in hierdie materiale beter te verstaan. In die eerste afdeling word TBC4 as 'n gasheer vir CO2 bespreek. Die gasvry lae digtheid polimorf van TBC4 bevat diskrete porieë (nul-dimensionele porositeit) en absorbeer CO2 stapsgewys. Die stelsel ondergaan 'n gas-geïnduseerde enkel-kristal (SC-SC) transformasie na 'n meer poreuse fase van TBC4 wat 'n verhoogde CO2 laai kapasiteit het. Die CO2-ingeslote verbindings was ondersoek in-situ deur gebruik te maak van enkel-kristal X-straal diffraksie (SCXRD) analise. Die gas-geïnduseerde fase transformasie was verder ondersoek deur gebruik te maak van drukversnelling poeier X-straal diffraksie (VP-PXRD) analise en drukversnelling differensiële skanderings kalorimetrie (P-DSC). Hierdie meer poreuse fase van TBC4 word behou selfs wanneer al die CO2 molekules verwyder word. Die oorspronklike gasvrye fase kan egter onder ligte aktiveringstoestande geregenereer word. Die studie was uitgebrei deur ook die opname van CO2 deur die hoë digtheid polimorf van TBC4 te ondersoek. Ten spyte van die feit dat dié polimorf geen porieë bevat nie, kan CO2 molekules steeds deur die gasheer diffundeer om 'n gas-geïnduseerde transformasie by hoë druk in werk te stel. Moontlike meganismes was die opname van CO2 deur die twee TBC4 polimorfe word bespreek. Die tweede afdeling beskryf die porositeit van die bekende gasheer, CB[6]. Hierdie gasheer ondergaan verskeie fase transformasies om 'n raamwerk te skep met permanente een-dimensionele porositeit. Die enkel-kristal struktuur en termiese stabiliteit van die verskeie fases was, waar moontlik, geanaliseer. Hierdie gasheer, soos meeste molekulêre organiese gashere, produseer 'n meer diggepakte fase wanneer dit geaktiveer word en brei weer uit wanneer dit met CO2 gelaai word. Die CB[6] raamwerk toon 'n groot affiniteit vir CO2 en ondergaan geen strukturele verandering onder hoë CO2 druk nie. In-situ SCXRD analise was gebruik om die CO2-ingeslote verbindings by verskeie gasdruk te ondersoek om die gas-gasheer interaksies te bestudeer. Die laaste afdeling bespreek CB[8] as 'n CO2 adsorbens. As gevolg van die hoë CO2 affiniteit wat deur CB[6] getoon word, het ons besluit om CB[8] as 'n potensiële gasheer vir CO2 te ondersoek. Hierdie gasheer ondergaan 'n fase transformasie tydens die aktiveringsproses om 'n meer stabiele polikristallyne fase van die CB[8] te lewer. Alhoewel die gasheer nie struktureel gekenmerk kon word nie, was dit duidelik vanaf die CO2 sorpsie analise dat die gasheerraamwerk poreus is. CB[8] vertoon 'n stapsgewyse opname en vrylating van CO2 molekules en het 'n hoër affiniteit vir CO2 gas in vergelyking met metaan gas en stikstof gas. VP-PXRD was gebruik om die dinamiese aard van die gasheerraamwerk te ondersoek onder verskillende drukke van CO2. Die verskeie pogings om die kristalliniteit van CB[8] te bewaar tydens die aktiveringsproses word ook bespreek.

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