11 research outputs found
Novel Synthetic Pathways for Tailored Covalent Triazine Frameworks with Catalytic and Electrochemical Applications
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 â 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 â still containing large amounts of ZnCl2 â 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 materialsâ 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
Application of Thermal Response Measurements to Investigate Enhanced Water Adsorption Kinetics in Ball-Milled C2N-Type Materials
Sorption-based water capture is an attractive solution to provide potable water in arid regions. Heteroatom-decorated microporous carbons with hydrophilic character are promising candidates for water adsorption at low humidity, but the strong affinity between the polar carbon pore walls and water molecules can hinder the water transport within the narrow pore system. To reduce the limitations of mass transfer, C2N-type carbon materials obtained from the thermal condensation of a molecular hexaazatriphenylene-hexacarbonitrile (HAT-CN) precursor were treated mechanochemically via ball milling. Scanning electron microscopy as well as static light scattering reveal that large pristine C2N-type particles were split up to a smaller size after ball milling, thus increasing the pore accessibility which consequently leads to faster occupation of the water vapor adsorption sites. The major aim of this work is to demonstrate the applicability of thermal response measurements to track these enhanced kinetics of water adsorption. The adsorption rate constant of a C2N material condensed at 700 °C remarkably increased from 0.026 sâ1 to 0.036 sâ1 upon ball milling, while maintaining remarkably high water vapor capacity. This work confirms the advantages of small particle sizes in ultramicroporous materials on their vapor adsorption kinetics. It is demonstrated that thermal response measurements are a valuable and time-saving method to investigate water adsorption kinetics, capacities, and cycling stability
Topochemical conversion of an imine-into a thiazole-linked covalent organic framework enabling real structure analysis
Stabilization of covalent organic frameworks (COFs) by post-synthetic locking strategies is a powerful tool to push the limits of COF utilization, which are imposed by the reversible COF linkage. Here we introduce a sulfur-assisted chemical conversion of a two-dimensional imine-linked COF into a thiazole-linked COF, with full retention of crystallinity and porosity. This post-synthetic modification entails significantly enhanced chemical and electron beam stability, enabling investigation of the real framework structure at a high level of detail. An in-depth study by electron diffraction and transmission electron microscopy reveals a myriad of previously unknown or unverified structural features such as grain boundaries and edge dislocations, which are likely generic to the in-plane structure of 2D COFs. The visualization of such real structural features is key to understand, design and control structure-property relationships in COFs, which can have major implications for adsorption, catalytic, and transport properties of such crystalline porous polymers
Novel Synthetic Pathways for Tailored Covalent Triazine Frameworks with Catalytic and Electrochemical Applications
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 â 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 â still containing large amounts of ZnCl2 â 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 materialsâ 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
Schiffâbases for sustainable battery and supercapacitor electrodes
Abstract The quest for more efficient ways to store electrical energy prompted the development of different storage devices over the last decades. This includes but is not limited to different battery concepts and supercapacitors. However, modern batteries rely on electrochemical principles that often involve transition metals which can for instance suffer from toxicity or limited availability. More sustainable alternatives are needed. This sparked the search for organic electrode materials. Nevertheless, compared to their inorganic counterparts, organic electrode materials remain less intensely investigated. Besides the often more complicated electrochemical principles, one likely reason for that are the complex synthetic skills required to develop novel organic materials. Here we review materials synthesized by an old and comparably simple reaction from the field of organic chemistry, namely Schiffâbase formation. This reaction can often yield materials under relatively mild conditions, making them especially interesting for the formation of sustainable electrodes. The main weakness of Schiffâbase materials, susceptibility to hydrolysis, is of limited concern in most of the battery systems as they mostly anyways require waterâfree conditions. This review gives an overview of some selected nanomaterials obtained from Schiffâbase formation as well as their carbonized derivatives which are of interest for energy storage. Firstly, the general chemistry of Schiffâbases is introduced, followed by an inâdepth survey of the most important breakthroughs in the formation of organic battery electrodes that involve materials based on Schiffâbase reaction. Lastly, an outlook considering the main hurdles as well as future perspectives of this research area is given
Tailoring the adsorption of ACE-inhibiting peptides by nitrogen functionalization of porous carbons
Bioactive peptides, such as isoleucyl-tryptophan (IW), exhibit a high potential to inhibit the angiotensin-converting enzyme (ACE). Adsorption on carbon materials provides a beneficial method to extract these specific molecules from the complex mixture of an alpha-lactalbumin hydrolysate. This study focuses on the impact of nitrogen functionalization of porous carbon adsorbents, either via pre- or post-treatment, on the adsorption behavior of the ACE-inhibiting peptide IW and the essential amino acid tryptophan (W). The commercially activated carbon Norit ROX 0.8 is compared with pre- and postsynthetically functionalized N-doped carbon in terms of surface area, pore size, and surface functionality. For prefunctionalization, a covalent triazine framework was synthesized by trimerization of an aromatic nitrile under ionothermal conditions. For the postsynthetic approach, the activated carbon ROX 0.8 was functionalized with the nitrogen-rich molecule melamine. The batch adsorption results using model mixtures containing the single components IW and W could be transferred to a more complex mixture of an alpha-lactalbumin hydrolysate containing a huge number of various peptides. For this purpose, reverse-phase high-pressure liquid chromatography with fluorescence detection was used for identification and quantification. The treatment with the three different carbon materials leads to an increase in the ACE-inhibiting effect in vitro. The modified surface structure of the carbon via pre- or post-treatment allows separation of IW and W due to the certain selectivity for either the amino acid or the dipeptide
On the mechanistic role of nitrogen-doped carbon cathodes in lithium-sulfur batteries with low electrolyte weight portion
The lithium-sulfur (LiâS) battery is a promising alternative to overcome capacity and specific energy limitations of common lithium-ion batteries. Highly porous, nitrogen-doped carbons as conductive host structures for sulfur/lithium sulfide deposition are shown herein to play a critical role in reversible cycling at low electrolyte/sulfur ratio. The pore geometry is precisely controlled by an efficient, scalable ZnO hard templating process. By using an electrolyte volume as low as 4 ÎŒL mg , the beneficial nitrogen functionality leads to a twofold increased cell lifetime turning our findings highly favorable for real applications. Stable cycling of up to 156 cycles (59 cycles with undoped carbon) with high sulfur loadings of 3 mg cm is achieved. Operando X-ray diffraction measurements during cycling show the transformation pathway of the sulfur â polysulfide â LiS species. The observed intermediates critically depend on the nitrogen doping in the cathode carbon matrix. Nitrogen-doped carbons facilitate polysulfide adsorption promoting the nucleation of crystalline LiS. These results provide new insights into the significant role of heteroatom doping for carbons in Li-S batteries with high specific energy
Pore Nanoarchitectonics of Carbon Nitrides for the ExcitedâState Deactivation of Confined Methylene Blue
Abstract The confinement of organic chromophores within mesoporous material architectures can exert a considerable alteration on their physicoâchemical properties. This study presents a detailed spectroscopic investigation of methylene blue (MB) entrapped in mesoporous polymeric carbon nitrides (mPCNs) with different pore architecturesusing transient absorption spectroscopy (TAS). The spatial confinement of MB molecules results in a prominent change in absorption spectra, characterized by both redshifts and the appearance of additional shoulder peaks, arising from the formation of MB dimers (MB2) concomitant with a distortion of the MB structure. Upon photoexcitation, entrapped MB molecules exhibit a notable altered excitedâstate absorption feature, along with a drastic excitedâstate quenching within 2â
ns compared to molecues in bulk solutions. In contrast to the ultrafast quenching of sole MB2 with a lifetime of ~3.6â
ps in highly concentrated solutions, the concentrationâdependent quenching behavior of MB aggregates in confined environments suggests the effect is caused by excimers formed in close proximity. The findings of this work highlight the impact of constrained environments and intermolecular interactions on the relaxation pathways of the excited states in photoactive molecules
Towards general network architecture design criteria for negative gas adsorption transitions in ultraporous frameworks
International audienceSwitchable metal-organic frameworks (MOFs) have been proposed for various energy-related storage and separation applications, but the mechanistic understanding of adsorption-induced switching transitions is still at an early stage. Here we report critical design criteria for negative gas adsorption (NGA), a counterintuitive feature of pressure amplifying materials, hitherto uniquely observed in a highly porous framework compound (DUT-49). These criteria are derived by analysing the physical effects of micromechanics, pore size, interpenetration, adsorption enthalpies, and the pore filling mechanism using advanced in situ X-ray and neutron diffraction, NMR spectroscopy, and calorimetric techniques parallelised to adsorption for a series of six isoreticular networks. Aided by computational modelling, we identify DUT-50 as a new pressure amplifying material featuring distinct NGA transitions upon methane and argon adsorption. In situ neutron diffraction analysis of the methane (CD4) adsorption sites at 111âK supported by grand canonical Monte Carlo simulations reveals a sudden population of the largest mesopore to be the critical filling step initiating structural contraction and NGA. In contrast, interpenetration leads to framework stiffening and specific pore volume reduction, both factors effectively suppressing NGA transitions
Towards General Network Architecture Design Criteria for Negative Gas Adsorption Transitions in Ultraporous Frameworks
Critical design criteria for negative gas
adsorption (NGA), a counterintuitive feature of pressure amplifying materials,
hitherto uniquely observed in a highly porous framework compound (DUT-49), are
derived by analysing the physical effects of micromechanics, pore size,
interpenetration, adsorption enthalpies, and the pore filling mechanism using
advanced in situ X-ray and neutron diffraction, NMR spectroscopy, and
calorimetric techniques parallelized to adsorption for a series of six
isoreticular networks. Aided by computational
modelling, we identify DUT-50 as a new pressure amplifying material featuring
distinct NGA transitions upon methane and argon adsorption. In situ neutron
diffraction analysis of the methane (CD4) adsorption sites at 111 K supported
by grand canonical Monte Carlo simulations reveals a sudden population of the
largest mesopore to be the critical filling step initiating structural contraction
and NGA. In contrast, interpenetration leads to framework stiffening and
specific pore volume reduction, both factors effectively suppressing NGA
transitions.</p