π-π stack driven competitive /complementary adsorption of aromatic compounds on MIL-53(Al).

In this study, the selective adsorption of aromatic compounds on mesoporous MIL-53(Al) was investigated, and followed the order: Biphenyl (Biph) > Triclosan (TCS) > Bisphenol A (BPA) > Pyrogallol (Pyro) > Catechol (Cate) > Phenol (Phen), and exhibited high selectivity toward TCS in binary compounds. In addition to hydrophobicity and hydrogen bonding, π-π interaction/stacking predominated, and more evidently with double benzene rings. TCS-containing halogens could increase π interaction on the benzene rings via forming Cl-π stacking with MIL-53(Al). Moreover, site energy distribution confirmed that complementary adsorption mainly occurred in the Phen/TCS system, as evidenced by ΔQpri (the decreased solid-phase TCS concentration of the primary adsorbate) < Qsec (the solid-phase concentrations of the competitor (Phen)). In contrast, competitive sorption occurred in the BPA/TCS and Biph/TCS systems within 30 min due to ΔQpri = Qsec, followed by substitution adsorption in the BPA/TCS system, but not for the Biph/TCS system, likely attributed to the magnitude of energy gaps (Eg) and bond energy of TCS (1.80 eV, 362 kJ/mol) fallen between BPA (1.74 eV, 332 kJ/mol) and Biph (1.99 eV, 518 kJ/mol) according to the density-functional theory of Gaussian models. Biph with a more stable electronic homeostasis than TCS lead to the occurrence of substitution adsorption in the TCS/BPA system, but not in the TCS/Biph system. This study provides insight into the mechanisms of different aromatic compounds on MIL-53(Al).