For many applications of industrial relevance, solids providing enhanced porosity such as activated carbons or zeolites have been the key drivers of progress in the past century. Albeit these materials marked an entire era of research, scientists have contributed immense effort to mimic porosity in an artificial way. A rich field to address this challenge is polymer chemistry. Especially covalent triazine frameworks (CTFs), which are exclusively built up from organic matter connected by covalent bonds, have proliferated in the last 10 years and provide remarkable chemical and thermal stability.
Within this thesis, a salt templating method for the synthesis of mesoporous CTF materials was developed that applies binary salt mixtures of ZnCl2 (the conventional reaction medium) in combination with alkali halides. In contrast to existing synthetic concepts that induce mesoporosity via high temperature treatment (up to 700 °C), salt templating was conducted at moderate temperatures (300 – 450 °C) and significantly avoided carbonisation as well as nitrogen loss. By applying this new method, salt templated materials with a four-fold increased total pore volume (CTF 1_LiCl: 2.1 cm3 g-1 vs. conventional CTF-1: 0.5 cm3 g-1) and an almost complete retention of the specific surface area (1320 m2 g-1 vs. 1440 m2 g-1) could be synthesised.
Another aspect of this thesis dealt with a novel approach to generate CTF materials in a solvent-free, time-efficient and scalable manner. To this end, a mechanochemical synthesis route was developed that makes use of the Friedel-Crafts alkylation to generate CTF materials from cyanuric chloride, serving as triazine node, and electron-rich aromatic compounds as linker molecules. By this method, permanently porous materials (up to 570 m2 g-1) could be synthesised from various monomers with different length and geometry. The syntheses could be conducted within two hours and on a gram scale, thus significantly exceeding known methods in terms of time-efficiency and scalability.
Besides these synthetic concepts, three other chapters covered the area of potential applications for CTF materials. To this end, novel CTF materials were synthesised and assessed towards their suitability for use in energy storage systems such as lithium sulfur battery or supercapacitor.
In analogy to SPAN, a sulfur containing conductive poly(acrylonitrile) polymer, CTFs containing covalently bound sulfur (S@CTF) were anticipated as promising cathode material in the lithium sulphur battery. Following the synthesis of a variety of different materials, a particular focus was set on determining the impact of sulfur attachment on the porosity and on illustrating the bonding situation of sulfur within the porous host matrix. Elemental analysis revealed that the highest sulfur loadings (33 w%) were obtained for the CTF samples obtained at the lowest synthesis temperature (500 °C). These findings were in agreement with nitrogen adsorption experiments that showed a reduced porosity after sulfur attachment for each material and the largest percental drop of the total pore volume for those samples with the highest sulfur loadings. XPS investigations suggested the presence of C-S species in the sulfur treated materials and supported the formation of covalently bound sulfur. Whereas the synthesis of S@CTF materials was successful, the electrochemical characterisation in a carbonate-based electrolyte revealed a substantial capacity loss after only a few cycles, which was probably due to a loss of active material and underlined that confinement of sulfur might be the key to obtain cathodes with increased cycling stability.
In this thesis, a novel pyridine-based CTF material was synthesised, which showed beneficial nitrogen doping and a tuneable porosity by careful choice of the reaction temperature (Scheme 3b). An in-depth characterisation by means of argon physisorption, X-ray photoelectron and Raman spectroscopy was conducted. Thereby, the structural changes upon thermal treatment were carefully investigated and interpreted. The non-purified CTFs – still containing large amounts of ZnCl2 – were directly processed into supercapacitor electrodes. Herein, ZnCl2 was serving two purposes: it acted as a porogen during the CTF synthesis (surface areas up to 3100 m2 g-1 were obtained) and as a precursor for an in situ generated aqueos electrolyte. It was demonstrated that this methodology bypasses extensive washing and more importantly, the findings gained from the electrochemical characterisation matched the structural indications from the XPS experiments. Thus, without purifying the material in advance, this method allowed for estimating the materials’ properties based on its behaviour as supercapacitor.
In the last part, a purely CTF-based organocatalyst that benefits from a monomer bearing a catalytically active functionality was synthesised by introducing a charged cationic imidazolium moiety into a microporous covalent triazine framework. A finely adjusted synthetic protocol enabled the structural retention of the thermally labile imidazolium motif, whose successful integration was proven by an in-depth structural characterisation, applying solid-state 1H MAS NMR, XPS and FT-IR spectroscopy. If applied as heterogeneous organocatalyst, the imidazolium-based CTF was active in the carbene-catalysed Umpolung reaction, thus providing clear evidence of an intact structure.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:32350 |
| Date | 05 December 2018 |
| Creators | Troschke, Erik |
| Contributors | Kaskel, Stefan, Thomas, Arne, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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