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 (SA_BET = 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 (Q_st) for CO₂ (28.7-33.6 kJ mol^-1). Comparison of CO₂ uptakes and Q_st values of benzothiazole-, benzoxazole- and benzimidazole-linked polymers demonstrated that smaller pores facilitate CO₂ adsorption with higher Q_st 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.