New nitrogen-rich azo-bridged porphyrin-conjugated microporous networks for high performance of gas capture and storage

Abstract

A series of new conjugated microporous polymers (Azo-1, Azo-2 and Azo-3) based on a nitrogen-rich porphyrin building unit and an azo bond linkage were synthesized by KOH assisted condensation. These materials were characterized by Fourier transform infrared spectroscopy (FT-IR), solid-state ¹³C NMR, XPS, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and tested for gas (N₂, CO₂ and H₂) adsorption. It was revealed that the azos presented the formation of porous polymer networks affording amorphous particles with rough surfaces and irregular morphology with excellent thermal stability under nitrogen conditions. The Brunauer–Emmett–Teller (BET) model of the N₂ adsorption gave apparent surface area ranges of 520–675 m² g⁻¹. The results from non-local density functional theory (NL-DFT) calculations suggested a pore size distribution between 1.6 and 4.0 nm. The gas (CO₂, H₂) adsorption isotherms demonstrated outstanding CO₂ uptake up to 17.5 wt% (3.98 mmol g⁻¹ for Azo-2) and moderate H₂ storage. The isosteric heats of adsorption (Q_st) are high, with values of 36–37 kJ mol⁻¹ for the azo polymers. Moreover, the azo-polymer networks exhibited excellent selectivity with CO₂/N₂ up to 64.3 for Azo-2 at 273 K/1 bar. It was suggested that the nitrogen-rich active sites of the polymers play an important role for CO₂ capture and storage.