Hydrophobic CHA-ZIFs with a junctional trap between cha and d6r cages for adsorption of 2,3-butanediol in aqueous solution

Abstract

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 g·L−1/50 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.

1. Introduction

Effective development and utilization of biomass is extremely important to the realization of China's “carbon peaking” and “carbon neutrality” goals. The 1,3-propanediol (1,3-PDO) and 2,3-butanediol (2,3-BDO) are vital bio-based platform compounds, which act as polymer monomers to synthesize high-performance polyester materials [1,2]. Typically, bio-based 1,3-PDO is produced through glycerol fermentation [3–5]. The corresponding fermentation broth is a low concentration and multi-component aqueous solution of 80%–90% water, 30–100 g·L−1 1,3-PDO and 10–70 g·L−1 2,3-BDO [6–9]. However, 1,3-PDO and 2,3-BDO are diols with strong hydrophilicities and high boiling points, which further complicates the separation and purification process.

At present, several technologies have been studied for the separation of 1,3-PDO and 2,3-BDO from the fermentation broth, such as distillation [10,11], extraction [12–14] and adsorption [15–22]. Compared to other separation methods, adsorption has advantages of low energy consumption, environmental protection and less investment in equipment. Several adsorbent materials have been studied to adsorb 1,3-PDO or 2,3-BDO, including zeolites [15–17], metal–organic frameworks (MOFs) [18,19] and resins [20–22]. Hydrophilic adsorbents preferentially adsorbed 1,3-PDO, whereas hydrophobic adsorbents preferentially adsorbed 2,3-BDO. Typically, Jin et al. [18] reported that hydrophilic SIM-1 was useful for enriching 1,3-PDO. Li et al. [16] showed that hydrophobic F-ZSM-5 was suitable for adsorbing 2,3-BDO, with the adsorption capacity of 59 mg·g−1. Our recent work demonstrated that introducing adsorption sites on hydrophobic ZIF-71 improved the adsorption capacity and mass transfer of 2,3-BDO [19]. Based on 2,3-BDO with methyl groups at both ends, we designed adsorbent materials with hydrophobic and confined pore structures, which facilitated the adsorptive separation of 2,3-BDO from dilute aqueous solutions.