A novel strategy to improve gas capture performance of metal-free azo-bridged porphyrin porous organic polymers: The design of traps

Porous organic polymers (POPs) have potential applications in gas capture and storage to solve the challenges including decreasing the emission of greenhouse gases and storing green energies. However, they mainly rely on the surface of pores to adsorb gas molecules, and the inner space of the pores is underutilized. Herein, we synthesized two azo-bridged porphyrin POPs with a linear (PPAPP) and bent (PDAPP) backbone, respectively. The Fourier transform infrared (FT-IR) spectra and the element content analysis shows that the extent of reaction of PPAPP is higher than that of PDAPP. The Brunauer-Emmett-Teller surface area of PPAPP is almost 4 times higher than that of PDAPP. Nevertheless, the CH4 and CO2 uptake capacity of PDAPP is equal to or higher than that of PPAPP. The reason for this is that PDAPP contains many traps. Traps with a large inner space and small sized doors could act as mazes for gas molecules. Gas molecules could enter the traps, but it is hard to get out of them. As a result, traps could act as containers for gas molecules. Our research suggests that POPs with traps are potential high-efficiency gas uptake materials because the inner space of the pores could be utilized.