title: Functional Triazine-Based Porous Organic Networks
Creators: name: Liebl, Mario
Publisher: name: University of Bayreuth
Year: 2018
Subjects: subject: carbon dioxide capture
subject: porous organic polymers
subject: gas separation
subject: 500
subject: 540

description: The aim of the present thesis was the synthesis and characterization of triazine-based porous organic networks. The pore structure of the networks and their gas sorption properties were investigated. The overall synthesis strategy is based on two key concepts. First, the synthesis of porous permanently stable network structures with high specific surface areas and high gas sorption capacities. Therefore, the choice of suitable monomer connector and linker molecules was essential. The second concept focuses on the introduction of functional groups into the porous network structures to influence the interaction between adsorbed gas molecules and sorbent surface. The synthesized porous networks aim at a high affinity and sorption selectivity towards gaseous CO2, which was achieved by implementation of polar functionalities, e.g. triazine, imide, carbonyl or sulfonyl moieties, into the network structure. The energetically beneficial interaction with CO2 results from strong dipole-quadrupole and quadrupole-quadrupole interactions between sorptive and sorbent. The synthesized porous imide-based network materials were investigated for their potential as gas storage and gas separation materials for simulated CO2/N2 gas mixtures. The triazine nodes which are characteristic for the organic polymer networks in this thesis were synthesized by cyclotrimerization of aromatic nitrile building blocks under different reaction conditions. In a straightforward approach, the introduction of methyl groups in a structure similar to CTF-1 by cyclotrimerization of 2-methylterephthalonitrile in molten zinc(II) chloride was investigated. Infrared and 13C MAS NMR spectroscopy of the resulting non-porous polymer confirmed successful triazine formation. The presence of considerable amounts of unreacted nitrile groups was ascribed to incomplete cyclotrimerization due to sterical hindrance effects caused by methyl groups in ortho-position to the nitrile groups. An increase of reaction temperature and time decreased the amount of methyl groups and overall nitrogen content. To avoid an undesired sterical hindrance effect of ortho-methyl groups on nitrile cyclotrimerization, a similar reaction of 2,2’-dimethyl-4,4’-dicyanobiphenyl in molten zinc(II) chloride was conducted. The obtained porous material showed a high specific surface area of almost 1600 m2 g-1, however, only minor amounts of methyl and triazine groups were found in the synthesized material most likely due to thermal decomposition. A network material synthesized by cyclotrimerization of 4,4’-dicyanobiphenyl in molten zinc(II) chloride yielded corresponding results. To avoid two different routes to synthesize porous triazine networks were performed by cyclotrimerization of 1,4-dicyanobenzene under autoclave conditions using Y(OTf)3, La(OTf)3 and Zn(OTf)2 in combination with either piperidine or ammonium chloride, respectively, as catalyst systems. Infrared and 13C MAS NMR spectroscopy confirmed successful triazine formation in all synthesized materials. For both catalyst systems the obtained materials did not show any considerable gas uptakes, however, contrary to the amorphous ATP-P polymers, powder XRD measurements detected long-range order in the three ATP-A materials. The materials SMP-1 and SMP-2 were synthesized by nitrile cyclotrimerization of 1,4-dicyanobenzene in two different eutectic salt mixtures, SM-1 and SM-2. The lower melting points of the eutectic salt mixtures compared to neat zinc(II) chloride enabled nitrile cyclotrimerization at lower reaction temperatures. Again, 13C MAS NMR and infrared spectroscopy confirmed successful triazine formation in SMP-1 and SMP-2. Both materials were found to be non-porous and powder XRD measurements revealed long-range order within both structures. The syntheses of porous triazine-based polyimides (TPIs) were conducted in a facile one-pot polycondensation reaction of 2,4,6 tris(p-aminophenyl)-1,3,5-triazine (TAPT) with various aromatic tetracarboxylic acid dianhydride monomers. Solid state NMR and infrared spectroscopy measurements confirmed the formation of cross-linked polyimide networks and the successful introduction of functional groups for all TPI polymers. All investigated TPIs were insoluble in common organic solvents and exhibited high thermal stability up to 450 °C under air. Argon sorption measurements showed specific BET equivalent surface areas up to 809 m2 g-1. The pore structure of TPIs was characterized as mainly microporous with pore diameters dominating in the range between 0.4 and 3 nm. The highest CO2 uptakes at 273 K and 1 bar were reported at 2.45 mmol g-1. The isosteric enthalpies of adsorption ranged from 29 kJ mol-1 to 34 kJ mol-1. The highest gas sorption selectivity for CO2 over N2 at 298 K and 1 bar yielded a value of 56 calculated from initial slope calculations. It was shown that the nature of the applied functionalities have a considerable influence on the gas sorption selectivities of the respective polymer networks. The crystal structures of 2,4,6-tris(p-bromophenyl)-1,3,5-triazine (TBPT) and TAPT were solved from powder X-ray diffraction data. The synthesis of TBPT yielded a white, microcrystalline material. The crystal structure of TBPT was determined by X-ray diffraction analysis (orthorhombic, P2₁2₁2₁, a = 4.5359(4) Å, b = 19.7809(11) Å, c = 21.6744(11) Å, V = 1944.70(24) Å3). The material 2,4,6-tris(p-aminophenyl)-1,3,5-triazine was obtained as a yellow, microcrystalline material. The crystal structure of TAPT was determined by X-ray diffraction analysis (orthorhombic, Pbcn, a = 17.7386(6) Å, b = 12.6142(4) Å, c = 7.7503(2) Å, V = 1734.18(9) Å3).