Abstract
Copolyurea networks (co-UNs) were synthesized via crosslinking polymerization of a mixture of tetrakis(4-aminophenyl)methane (TAPM) and melamine with hexamethylene diisocyanate (HDI) using the organic sol-gel polymerization method. The subsequent thermal treatment of between 200 and 400 °C induced the sintering of the powdery polyurea networks to form porous frameworks via urea bond rearrangement and the removal of volatile hexamethylene moieties. Incorporating melamine into the networks resulted in a higher nitrogen content and micropore ratio, whereas the overall porosity decreased with the melamine composition. The rearranged network composed of the tetraamine/melamine units in an 80:20 ratio showed the highest carbon dioxide adsorption quantity at room temperature. The results show that optimizing the chemical structure and porosity of polyurea-based networks can lead to carbon dioxide adsorbents working at elevated temperatures.
Highlights
Porous organic networks have been studied for use as gas adsorbents [1,2,3,4,5,6,7], owing to their high surface area and microporosity [8,9], the tuneability of their organic chemical structures and microstructures for high adsorption efficiency, and selectivity to a targeted gas [1,10,11,12]
In a typical run for the synthesis of T80M20, TAPM (0.82 mmol, 0.31 g) was dissolved in dimethyl formamide (DMF) (7.84 mL), the melamine (0.21 mmol, 0.026 g) was dissolved in dimethyl sulfoxide (DMSO) (0.65 mL), the hexamethylene diisocyanate (HDI) (1.96 mmol, 0.33 g) was dissolved in DMF (8.23 mL), the TAPM solution was added into the HDI solution, and the melamine solution was added to the resulting TAPM/HDI mixture drop-wise at room temperature under a nitrogen atmosphere
Copolyurea networks were prepared by reacting TAPM (T), melamine (M), and HDI in a mixed solvent of DMF and DMSO (Figure 1a)
Summary
Porous organic networks have been studied for use as gas adsorbents [1,2,3,4,5,6,7], owing to their high surface area and microporosity [8,9], the tuneability of their organic chemical structures and microstructures for high adsorption efficiency, and selectivity to a targeted gas [1,10,11,12]. To increase the weight fraction of an aliphatic moiety, one must use aromatic amines with a smaller molar mass than TAPM In the meantime, it would be better for the residual aromatic units to contain more nitrogen atoms in their structure. It would be better for the residual aromatic units to contain more nitrogen atoms in their structure These considerations led us to choose melamine as the amine monomer to obtain high-microporosity, nitrogen-rich RUNs. Melamine consists of a tri-amino pseudo-aromatic ring with a molar mass of about one-third of the TAPM, containing six nitrogens among 15 non-hydrogen elements per molecule. We synthesized copolyurea networks by polymerizing a mixture of TAPM and melamine with HDI through the organic sol–gel method, and studied the effect of the melamine composition on the porosity and CO2 adsorptivity of the resultant thermally rearranged microporous network. Further optimization of the melamine composition in the precursor network resulted in an RUN with a large specific surface area with the highest CO2 adsorption selectivity at room temperature
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