Ultramicroporous polyureas synthesized with amines and 1,1′-Carbonyldiimidazole and their CO2 adsorption

Abstract

Microporous polyureas can be highly stable, but isocyanates or phosgene are normally used for the synthesis. Here, it was postulated and demonstrated that 1,1′-carbonyl diimidazole (CDI) could be used for the synthesis. By reacting tetrakis(4-aminophenyl)methane with CDI, a series of new polyureas with ultramicropores (pores <0.7 nm) were synthesized. To tailor thermal properties and porosity, the ratio of tetraamine-to-CDI and the reaction temperature were varied. The CO2 adsorption capacities (with values up to 0.74 mmol/g at 0.15 bar/273 K and 1.91 mmol/g at 1 bar/273 K) were ascribed to the ultramicroporosity. The CO2-based Dubinin-Radushkevich surface areas reached 395 m2/g (at 273 K), while the N2-based BET surface areas (at 77 K) were small. The apparent CO2-over-N2 selectivity was also high for the polymers at 273 K with estimated values of 31–92 for 15/85 v/v mixtures of CO2 and N2. This high selectivity was ascribed to the kinetic hindrance of N2 diffusion. It was noted that one of the polymers changed color irreversibly upon heating. In conclusion, it was shown that CDI and amines could be used to synthesize ultramicroporous polyureas, and that these polymers can exhibit irreversible thermochromism. This thermal effect was attributed to the electron-rich urea moieties, aromatic units, and conjugation.