Hydrophobic CHA-ZIFs with a junctional trap between cha and d6r cages for adsorption of 2,3-butanediol in aqueous solution
The adsorption and separation of diols from dilute aqueous solution using hydrophobic materials is very challenging due to the strong diol–water hydrogen-bonding interactions. Herein, we screened hydrophobic zeolitic imidazolate frameworks (ZIFs) with chabazite (CHA) topology for separation of 2,3-butanediol (2,3-BDO) and 1,3-propanediol (1,3-PDO), which had junctional and hydrophobic traps matching the two end methyl groups of the 2,3-BDO molecule. Based on CHA-ZIFs with the same small-sized ligand 2-methylimidazole (mIm) and different large-sized ligand benzimidazole derivatives (RbIm), CHA-ZIFs with larger surface areas were obtained by the addition of excess small-sized ligand mIm in the synthesis process. We showed that all of the hydrophobic CHA-ZIFs preferentially adsorbed 2,3-BDO over 1,3-PDO by static batch adsorption and dynamic column adsorption experiments. But ZIF-301 and ZIF-300 with halogen groups exhibited better adsorptive separation performance for 2,3-BDO/1,3-PDO than ZIF-302 with methyl groups. For a typical ZIF-301, its adsorption capacity for 2,3-BDO was 116.4 mg·g –1 and selectivity for 2,3-BDO/1,3-PDO was 3.8 in dynamic column adsorption of the binary-component system (2,3-BDO/1,3-PDO: 50/50 g·L –1 ). Computational simulations revealed that 2,3-BDO preferentially adsorbed in a trap at the junction between the cha and d6r cages of CHA-ZIFs, meaning the strong host–guest interactions. Therefore, the hydrophobic CHA-ZIFs with a junctional trap were promising candidate materials for adsorbing 2,3-BDO, which also provided a new perspective for separating diols in dilute aqueous solutions.
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