Experimental and computational investigations of zirconium-based metal-organic frameworks for bilirubin adsorption

Metal-organic frameworks (MOFs) have been proven to be ideal bilirubin adsorbents due to their high specific surface areas and large porosities. A detailed analysis of the structure-activity relationship is necessary for developing MOFs-based adsorbents. This paper synthesized zirconium-based UiO66 and UiO66-NH2, comparing their mechanisms of bilirubin adsorption using instrumental and computer simulation methods. BET, SEM, and XRD results indicated that bilirubin effectively occupied the internal pores of UiO. Adsorption kinetic and isotherm studies revealed that the larger average pore size corresponds with the larger diffusion rate, and UiO66/ UiO66-NH2 adsorbed bilirubin mainly via physiochemical adsorption mechanism. Additionally, both processes were spontaneous exothermic reactions; however, UiO66-NH2 had a significantly higher Delta H value (-169.71 kJ mol-1) compared to UiO66 (-94.96 kJ mol-1), resulting in a larger bilirubin adsorption capacity (1614.24 mg g- 1 vs 1450.82 mg g- 1). Notably, UiO66-NH2 broke the record for the highest reported bilirubin adsorption capacity among freshly-synthesized MOFs so far. FT-IR, XPS, and density functional theory (DFT) results explained that the higher adsorption capacity of UiO66-NH2 was contributed by the stronger H-bonding/pi-pi interaction forces and structure matching effect caused by the introduction NH2.