Spirobifluorene copolymers incorporating phenanthroline and phenothiazine for selective CO₂ adsorption and hydrogen uptake

Addressing the critical need for efficient materials for greenhouse gas mitigation and clean energy storage. Two innovative porous organic polymers, referred to as OTC-PN and OTC-PZ, were developed through microwave-assisted Friedel-Crafts alkylation, employing 1,10-phenanthroline and phenothiazine as nitrogen- and sulfur-containing aromatic monomers. At the same time, 9,9-spirobifluorene served as a rigid linker. The materials were characterized using solid-state ¹³C NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis, and powder X-ray diffraction, which confirmed their successful synthesis, remarkable thermal stability (with decomposition temperatures exceeding 400 °C), and amorphous nature. Nitrogen sorption isotherms at 77 K revealed Brunauer-Emmett-Teller (BET) surface areas of 1001 m²/g for OTC-PN and 134 m²/g for OTC-PZ, indicating their microporous characteristics with pore size distributions predominantly below 2 nm, as determined by NLDFT. The evaluation of CO₂ adsorption performance at 273 K and 298 K demonstrated uptakes of 2.40 mmol/g and 1.34 mmol/g for OTC-PN and 1.16 mmol/g and 0.71 mmol/g for OTC-PZ, respectively. The isosteric heat of adsorption (Qst) for CO₂ at zero loading was determined to be 37 kJ/mol for OTC-PN and 43 kJ/mol for OTC-PZ, suggesting a physisorption mechanism. Ideal Adsorbed Solution Theory (IAST) calculations indicated significant CO₂/N₂ selectivities of 35.5 and 97.1 at 273 K for OTC-PN and OTC-PZ, respectively, in a CO₂/N₂ mixture of 15:85. Additionally, hydrogen adsorption evaluations at 77 K and 760 mmHg revealed capacities of 5.1 mmol/g and 1.86 mmol/g for OTC-PN and OTC-PZ, respectively, emphasizing their potential for gas storage applications.