Triptycene-based microporous poly(diaminophosphazene)

Abstract

Porous materials have recently drawn much attention owing to their potential applications in gas storage, separations, and heterogeneous catalysis. As a D3h-symmetric rigid structure, triptycene and its derivatives can be used as suitable building blocks to prepare porous materials with high porosities. Based on the air-stable hexaammoniumtriptycene hexachloride and hexachlorocyclotriphosphazene, porous polymers (TrpPOP-1 and TrpPOP-2) were prepared via one-step polymerization through N-P linkage. The two polymers were characterized at the molecular level by 13C NMR and 31P NMR as well as IR. The two polymers possess type I nitrogen gas sorption isotherm according to the IUPAC classification. Both TrpPOP-1 and TrpPOP-2 show permanent microporous nature with the Brunauer-Emmett-Teller specific surface area of 790 and 640 m2·g-1 and exhibit narrow pore size distribution, with dominant pore size locating at 0.59 and 0.63 nm, respectively. Porous polymers with a narrow pore distribution may interact attractively with small gas molecules through improved molecular interaction. Their gas (hydrogen and carbon dioxide) adsorption capacities were measured based on the obtained gas physisorption isotherms. The hydrogen uptake of TrpPOP-1 is 1.30 wt% at 77 K and 1.0 bar, and the carbon dioxide uptake is up to 16.2 wt% at 273 K and 1.0 bar. The higher carbon dioxide loading capacity of TrpPOP-1 may be attributed to its higher charge density at the nitrogen sites of networks that can facilitate local-dipole/quadrupole interactions with carbon dioxide. Meanwhile, the adsorption capacity of the obtained materials for poisonous and harmful organic vapors such as formaldehyde was also investigated. TrpPOP-2 shows a better formaldehyde uptake, which is 5.5 mg·g-1 at 298 K. Formaldehyde, as a volatile organic compound, is a major air pollutant indoor, therefore, the uptake performance of TrpPOP-2 for formaldehyde would be very promising to remove harmful indoor air pollutant in the environment.