03 Fakultät Chemie

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Open Access
High‐performance carbon fibers prepared by continuous stabilization and carbonization of electron beam‐irradiated textile grade polyacrylonitrile fibers
(2021) König, Simon; Bauch, Volker; Herbert, Christian; Wego, Andreas; Steinmann, Mark; Frank, Erik; Buchmeiser, Michael R.
The manufacturing of high‐performance carbon fibers (CFs) from low‐cost textile grade poly(acrylonitrile) (PAN) homo‐ and copolymers using continuous electron beam (EB) irradiation, stabilization, and carbonization on a kilogram scale is reported. The resulting CFs have tensile strengths of up to 3.1 ± 0.6 GPa and Young's moduli of up to 212 ± 9 GPa, exceeding standard grade CFs such as Toray T300. Additionally, the Weibull strength and modulus, the microstructure, and the morphology of these CFs are determined.
Open Access
Melt-spinning of an intrinsically flame-retardant polyacrylonitrile copolymer
(2020) König, Simon; Kreis, Philipp; Herbert, Christian; Wego, Andreas; Steinmann, Mark; Wang, Dongren; Frank, Erik; Buchmeiser, Michael R.
Poly(acrylonitrile) (PAN) fibers have two essential drawbacks: they are usually processed by solution-spinning, which is inferior to melt spinning in terms of productivity and costs, and they are flammable in air. Here, we report on the synthesis and melt-spinning of an intrinsically flame-retardant PAN-copolymer with phosphorus-containing dimethylphosphonomethyl acrylate (DPA) as primary comonomer. Furthermore, the copolymerization parameters of the aqueous suspension polymerization of acrylonitrile (AN) and DPA were determined applying both the Fineman and Ross and Kelen and Tüdõs methods. For flame retardancy and melt-spinning tests, multiple PAN copolymers with different amounts of DPA and, in some cases, methyl acrylate (MA) have been synthesized. One of the synthesized PAN-copolymers has been melt-spun with propylene carbonate (PC) as plasticizer; the resulting PAN-fibers had a tenacity of 195 ± 40 MPa and a Young’s modulus of 5.2 ± 0.7 GPa. The flame-retardant properties have been determined by Limiting Oxygen Index (LOI) flame tests. The LOI value of the melt-spinnable PAN was 25.1; it therefore meets the flame retardancy criteria for many applications. In short, the reported method shows that the disadvantage of high comonomer content necessary for flame retardation can be turned into an advantage by enabling melt spinning.
Open Access
Macrocyclization of dienes under confinement with cationic tungsten imido/oxo alkylidene N‐heterocyclic carbene complexes
(2023) Ziegler, Felix; Bruckner, Johanna R.; Nowakowski, Michal; Bauer, Matthias; Probst, Patrick; Atwi, Boshra; Buchmeiser, Michael R.
Macrocyclization reactions are still challenging due to competing oligomerization, which requires the use of small substrate concentrations. Here, the cationic tungsten imido and tungsten oxo alkylidene N-heterocyclic carbene complexes [[W(N-2,6-Cl2-C6H3)(CHCMe2Ph(OC6F5)(pivalonitrile)(IMes)+ B(ArF)4-] (W1) and [W(O (CHCMe2Ph(OCMe(CF3)2)(IMes)(CH3CN)+ B(ArF)4-] (W2) (IMes=1,3-dimesitylimidazol-2-ylidene; B(ArF)4-=tetrakis(3,5-bis(trifluoromethyl)phenyl borate) have been immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 3.3 and 6.8 nm, respectively, using a pore-selective immobilization protocol. X-ray absorption spectroscopy of W1@OMS showed that even though the catalyst structure is contracted due to confinement by the mesopores, both the oxidation state and structure of the catalyst stayed intact upon immobilization. Catalytic testing with four differently sized α,ω-dienes revealed a dramatically increased macrocyclization (MC) and Z-selectivity of the supported catalysts compared to the homogenous progenitors, allowing high substrate concentrations of 25 mM. With the supported complexes, a maximum increase in MC-selectivity from 27 to 81 % and in Z-selectivity from 17 to 34 % was achieved. In general, smaller mesopores exhibited a stronger confinement effect. A comparison of the two supported tungsten-based catalysts showed that W1@OMS possesses a higher MC-selectivity, while W2@OMS exhibits a higher Z-selectivity which can be rationalized by the structures of the catalysts.
Open Access
Chromium(VI) bisimido dichloro, bisimido alkylidene, and chromium(V) bisimido iodo N‐heterocyclic carbene complexes
(2020) Panyam, Pradeep K. R.; Stöhr, Laura; Wang, Dongren; Frey, Wolfgang; Buchmeiser, Michael R.
Reaction of CrCl2(N-tBu)2 with 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl2), 1,3-di(2-propyl)imidazol-2-ylidene (IPr), 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-bis(2,6-(2-Pr)2C6H3)imidazol-2-ylidene (IDipp) yields the corresponding N-heterocyclic carbene (NHC) adducts CrCl2(IMe)(N-tBu)2 (1), CrCl2(IMeCl2)(N-tBu)2 (2), CrCl2(IPr)(N-tBu)2 (3), CrCl2(IMes)(N-tBu)2 (4) and CrCl2(IDipp)(N-tBu)2 (5). Likewise, reaction of CrCl2(N-2,6-(2-Pr)2C6H3)2 and CrCl2(N-adamantyl)2 with IMes yields CrCl2(N-2,6-(2-Pr)2C6H3)2(IMes) (6) and CrCl2(N-adamantyl)2(IMes) (7), respectively. Reaction of CrCl2(N-tBu)2 with the bidentate NHCs 1-R-3-(1-(2-LiO-C6H4))imidazol-2-ylidene yields the corresponding pentacoordinated Cr(VI) complexes CrCl2(1-R-3-(1-(2-O-C6H4))imidazol-2-ylidene)2C6H3)2(IMes) (R = 2,4,6-(CH3)3C6H2, 8), (R = tBu, 9), (R = 2-phenyl-C6H4, 10). Reaction of the chromium(VI) complex Cr(N-2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 with 1,3-dimethylimidazol-2-ylidene·AgI yields the bimetallic silver adduct of the chromium alkylidene complex (11) along with the tetrahedral chromium(V) complex CrI(N-2,6-(2-Pr)2-C6H3)2(1,3-dimethylimidazol-2-ylidene) (12). Compounds 1-4, 7, 9-12 were characterized by single-crystal X-ray analysis. Finally, the chromium(VI) bisimido-amido complexes 13-14 bearing the N-6-(2-(diethylboryl)phenyl)pyridyl-2-yl-motif are reported.
Open Access
High‐performance magnesium‐sulfur batteries based on a sulfurated poly(acrylonitrile) cathode, a borohydride electrolyte, and a high‐surface area magnesium anode
(2020) Wang, Peiwen; Trück, Janina; Niesen, Stefan; Kappler, Julian; Küster, Kathrin; Starke, Ulrich; Ziegler, Felix; Hintennach, Andreas; Buchmeiser, Michael R.
Post‐lithium‐ion battery technology is considered a key element of future energy storage and management. Apart from high gravimetric and volumetric energy densities, economic, ecologic and safety issues become increasingly important. In that regards, both the anode and cathode materials must be easily available, recyclable, non‐toxic and safe, which renders magnesium‐sulfur (Mg-S) batteries a promising choice. Herein, we present Mg-S cells based on a sulfurated poly(acrylonitrile) composite cathode (SPAN), together with a halogen‐free electrolyte containing both Mg[BH4]2 and Li[BH4] in diglyme and a high‐specific surface area magnesium anode based on Rieke magnesium powder. These cells deliver discharge capacities of 1400 and 800 mAh/gsulfur with >99 % Coulombic efficiency at 0.1 C and 0.5 C, respectively, and are stable over at least 300 cycles. Energy densities are 470 and 400 Wh/kgsulfur at 0.1 C and 0.5 C, respectively. Rate tests carried out between 0.1 C and 2 C demonstrate good rate capability of the cells. Detailed mechanistic studies based on X‐ray photoelectron spectroscopy and electric impedance spectroscopy are presented.
Open Access
Sulfurized polypropylene as low‐cost cathode material for high‐capacity lithium‐sulfur batteries
(2022) Du, Qian; Benedikter, Mathis; Küster, Kathrin; Acartürk, Tolga; Starke, Ulrich; Hoslauer, Jean‐Louis; Schleid, Thomas; Buchmeiser, Michael R.
Among ‘beyond lithium ion’ energy storage, lithium sulfur (Li-S) batteries are one of the most promising technologies, as a result of the potential for high theoretical energy capacity at low cost. A key obstacle in exploiting the vast potential of Li-S batteries is the formation of soluble polysulfide species. Here, we report sulfurized polypropylene (S/PP‐500) synthesized in one‐step by reacting polypropylene (PP) with sulfur as a new polysulfide shuttle‐free cathode material for Li-S batteries. It exhibits a reversible capacity as high as 1000 mAh/gsulfur at 0.1 C and a sulfur loading of up to 68 wt%, which in turn allows for high sulfur loadings up to 47 % in the final cathode. The low‐cost starting materials together with the simple synthetic procedure and the good electrochemical performance in combination with a commercially available eslectrolyte make the S/PP‐500 a very promising cathode material for Li‐S batteries.
Open Access
Toward sustainable fiber‐reinforced polymer composites
(2024) Elser, Iris; Buchmeiser, Michael R.
Fiber‐reinforced polymer composites (FRPCs) are versatile materials with applications in diverse fields such as transportation, construction, and electronics. With the composites market expected to reach 15.5 Mt by 2026, increasing the sustainability of FRPCs is imperative. The main factors driving the sustainability of FRPCs, namely end‐of‐life management and recyclability, the use of natural, bio‐based, and sustainable materials, as well as biodegradability and product simplification are presented and discussed.
Open Access
Sulfur‐composites derived from poly(acrylonitrile) and poly(vinylacetylene) : a comparative study on the role of pyridinic and thioamidic nitrogen
(2023) Kappler, Julian; Klostermann, Sina V.; Lange, Pia L.; Dyballa, Michael; Veith, Lothar; Schleid, Thomas; Weil, Tanja; Kästner, Johannes; Buchmeiser, Michael R.
Sulfurized poly(acrylonitrile) (SPAN) is a prominent example of a highly cycle stable and rate capable sulfur/polymer composite, which is solely based on covalently bound sulfur. However, so far no in‐depth study on the influence of nitrogen in the carbonaceous backbone, to which sulfur in the form of thioketones and poly(sulfides) is attached, exists. Herein, we investigated the role of nitrogen by comparing sulfur/polymer composites derived from nitrogen‐containing poly(acrylonitrile) (PAN) and nitrogen‐free poly(vinylacetylene) (PVac). Results strongly indicate the importance of a nitrogen‐rich, aromatic carbon backbone to ensure full addressability of the polymer‐bound sulfur and its reversible binding to the aromatic backbone, even at high current rates. This study also presents key structures, which are crucial for highly cycle and rate stable S‐composites.
Open Access
Hydrosilylation of alkynes under continuous flow using polyurethane‐based monolithic supports with tailored mesoporosity
(2022) Acikalin, Hande; Panyam, Pradeep K. R.; Shaikh, Abdul Wasif; Wang, Dongren; Kousik, Shravan R.; Atanasova, Petia; Buchmeiser, Michael R.
Non‐porous polyurethane‐based monoliths are prepared under solvent‐induced phase separation conditions. They possess low specific surface areas of 0.15 m2 g-1, pore volumes of 1 µL g-1, and a non‐permanent, solvent‐induced microporosity with pore dimensions ≤1 nm. Mesoporosity can be introduced by varying the monomers and solvents. A tuning of the average solubility parameter of the solvent mixture by increasing the macroporogen content results in a decrease in the volume fraction of micropores from 70% to 40% and an increase in the volume fraction of pores in the range of 1.7-9.6 nm from 22% to 41% with only minor changes in the volume fraction of larger mesopores in the range of 9.6–50 nm. The polymeric monoliths are functionalized with quaternary ammonium groups, which allowed for the immobilization of an ionic liquid that contained the ionic Rh‐catalyst [1‐(pyrid‐2‐yl)‐3‐mesityl)‐imidazol‐2‐ylidene))(η4‐1,5‐cyclooctadiene)Rh(I) tetrafluoroborate]. The supported catalyst is used in the hydrosilylation of 1‐alkynes with dimethylphenylsilane under continuous flow using methyl‐tert‐butyl ether as second liquid transport phase. E/Z‐selectivity in hydrosilylation is compared to the one of the analogous biphasic reactions. The strong increase in Z‐selectivity is attributed to a confinement effect provided by the small mesopores.
Open Access
Structure evolution in polyethylene‐derived carbon fiber using a combined electron beam‐stabilization‐sulphurization approach
(2021) Frank, Erik; Muks, Erna; Ota, Antje; Herrmann, Thomas; Hunger, Michael; Buchmeiser, Michael R.
A new approach is described for the production of poly(ethylene) (PE) derived carbon fibers (CFs) that entails the melt spinning of PE fibers from a suitable precursor, their cross-linking by electron beam (EB) treatment, and sulphurization with elemental sulphur (S8), followed by pyrolysis and carbonization. Instead of focusing on mechanical properties, analysis of CF structure formation during all process steps is carried out by different techniques comprising solid-state nuclear magnetic resonance spectroscopy, thermogravimetric analysis coupled to mass spectrometry/infrared spectroscopy, elemental analysis, energy dispersive X-ray scattering, scanning electron microscopy, Raman spectroscopy, and wide-angle X-ray diffraction. A key step in structure formation is the conversion of PE into poly(thienothiophene)s during sulphurization; these species are stabile under inert gas up to 700 °C as confirmed by Raman analysis. Above this temperature, they condense into poly(napthathienophene)s, which are then converted into graphite-type structures during pyrolysis.

03 Fakultät Chemie

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