Benzothiazole- and benzoxazole-linked porous polymers for carbon dioxide storage and separation†
Incorporation of CO2-philic heteroatoms (i.e. N, S, and O) into porous organic polymers has been instrumental in achieving selective CO2 capture. Here, we report the synthesis of porous benzothiazole and benzoxazole linked polymers which have sulfur and oxygen atoms, respectively, in addition to the nitrogen functionality. Their structural properties have been analyzed and compared to their analogous benzimidazole linked polymers which have only nitrogen heteroatoms. The polymers exhibit high surface areas (SABET = 698–1011 m2 g−1), high physicochemical stability, and considerable CO2 storage capacity. Low pressure gas uptake experiments were used to calculate the binding affinity of small gas molecules and revealed that the polymers have high heats of adsorption (Qst) for CO2 (28.7–33.6 kJ mol−1). Comparison of CO2 uptakes and Qst values of benzothiazole-, benzoxazole- and benzimidazole-linked polymers demonstrated that smaller pores facilitate CO2 adsorption with higher Qst values and the total CO2 uptake capacity mainly depends on the surface areas provided that the pore sizes are significantly small in lower micropore regions. The reported polymers also show moderate to high adsorption selectivity for CO2/N2 (40–78) and CO2/CH4 (5.7–7.8) as determined from the Ideal Adsorbed Solution Theory (IAST) calculation using pure gas isotherms at 298 K.