The synthesis and studies of type II porous liquids

  • Francesca Alexander

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Porous solids, such as metal-organic frameworks and zeolites are widely recognised for their applications in catalysis and molecular separation, whilst the area of Porous Liquids (PLs) is still a relatively new concept. PLs, like porous solids, are designed to exhibit permanent microporosity but PLs have the added advantage of fluidity. As such, they can act as enhanced, selective solvents and may ultimately find applications which are not possible for porous solids, such as continuous flow separation processes. Type II PLs consist of empty molecular hosts dissolved in size-excluded solvents and to date have mainly been based on hosts that have limited chemical and thermal stability. In chapter 3 of this thesis, Noria, a rigid cyclic oligomer has been identified as a new host for the synthesis of more robust Type II PLs. Although the structure of Noria is well-documented (X-ray crystal structures of the cyclic oligomer have been reported), it has been found that literature has overlooked the true composition of bulk Noria samples. From the work conducted in chapter 2, it has been realised that bulk samples (as obtained by the standard literature procedure) typically consist of Noria (approximately 40%), a Noria isomer, specifically a resorcinarene trimer, “R3” (approximately 30%) and other unidentified oligomers (approximately 30%). Moreover, Noria has been characterised crystallographically as a diethyl ether solvate and its 1H NMR spectrum fully interpreted and assigned for the first time. The previously postulated but unreported R3 has also been characterised crystallographically as a dimethyl sulfoxide solvate, which confirms its alternative connectivity to Noria. Noria and R3 have low solubility which precludes their use in Type II PLs, however, the partially ethylated derivative Noria-OEt dissolves in the size-excluded solvent 15-crown-5 to give a new Type II PL. This PL exhibits enhanced uptake of methane (CH4) gas supporting the presence of empty pores in the liquid. Detailed molecular dynamics simulations support the existence of pores in the liquid and show that occupation of the pores by CH4 is favoured. Additionally, chapter 3 also looks at investigating the gas adsorption properties of another potential Type II PL synthesised from the pillar[5]arene derivative, triethylene oxide pillar[5]arene (TEOP[5]). CH4 gas solubility experiments revealed that dissolution in TEOP[5] in 15-crown-5 was endothermic and therefore showed interesting “inverse” temperature dependent gas uptake behaviour i.e., gas solubility was greater at higher temperatures.
Finally, the work discussed in chapter 4 focuses on the cyclophane cyclobis (paraquat-p-phenylene), more commonly known as blue box (BB4+). Despite improvement towards the synthetic accessibility of BB4+ in recent years, the synthetic routes remain stepwise and are often low yielding unless harsh reaction conditions are used. Therefore, chapter 4 investigates an alternative synthetic route to BB4+ using mechanochemistry, specifically ball milling. Interestingly, it has been found that the mechanochemical synthesis does not form the desired BB4+ molecule but instead most likely a polymer, specifically a xylene-bridged polyviologen. Nonetheless, the work here highlights how mechanochemistry can offer an alternative and efficient route to certain products that are normally synthesised in solution.

Thesis embargoed until 31 July 2025.
Date of AwardJul 2022
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy
SupervisorStuart James (Supervisor) & Amilra De Silva (Supervisor)

Keywords

  • Porous Liquids
  • Type II
  • Noria
  • mechanochemistry

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