Efficient separation of xylene isomers by a guest-responsive metal-organic framework with rotational anionic sites

Abstract

The separation of xylene isomers (para-, meta-, orth-) remains a great challenge in the petrochemical industry due to their similar molecular structure and physical properties. Porous materials with sensitive nanospace and selective binding sites for discriminating the subtle structural difference of isomers are urgently needed. Here, we demonstrate the adaptively molecular discrimination of xylene isomers by employing a NbOF₅^2--pillared metal-organic framework (NbOFFIVE-bpy-Ni, also referred to as ZU-61) with rotational anionic sites. Single crystal X-ray diffraction studies indicate that ZU-61 with guest-responsive nanospace/sites can adapt the shape of specific isomers through geometric deformation and/or the rotation of fluorine atoms in anionic sites, thereby enabling ZU-61 to effectively differentiate xylene isomers through multiple C-H···F interactions. ZU-61 exhibited both high meta-xylene uptake capacity (3.4 mmol g^-1) and meta-xylene/para-xylene separation selectivity (2.9, obtained from breakthrough curves), as well as a favorable separation sequence as confirmed by breakthrough experiments: para-xylene elute first with high-purity (≥99.9%), then meta-xylene, and orth-xylene. Such a remarkable performance of ZU-61 can be attributed to the type anionic binding sites together with its guest-response properties.

Fig. 1. Scheme and structure of xylene isomers and ZU-61.

a Electron density (e/au³) of each xylene isomers. b Schematic illustration of the porous adsorbent with “Lewis basic-binding sites and rotational flexibility”. c Structure of ZU-61 with rotational ligand of NbOF₅²⁻ anion and bipyridine (d). e Thermogravimetric analysis results of ZU-61, SIFSIX-1-Cu, and ZU-61-Cu.

Fig. 2. Experimental column breakthrough results of ZU-61.

Breakthrough curves for 1:1:1 pX/mX/oX separations with ZU-61 at 398 K (a), 333 K (b), and recycle performance at 398 K (c).

Fig. 3. Single-component adsorption properties of ZU-61 and Zeolite NaY.

Adsorption isotherms of pX and mX on ZU-61, SIFSIX-2-Cu-i, and zeolite NaY at 333 K in two pressure regions, 0–7 mbar (a) and 0–0.06 mbar (b). The comparison of pX, mX, oX, and EB adsorption behavior on ZU-61 (c) and cycle adsorption of pX on ZU-61 (d) at 333 K.

Fig. 4. Single-crystal X-ray diffraction resolved structure of ZU-61·oX.

Crystal structure of ZU-61 before (a) and after adsorption of o-xylene ZU-61·oX (b). Color code: C, gray 50%; H, gray 10%; Nb, teal; Ni, violet; O, green; N, sky blue; F, red. Note: for the structure of ZU-61·oX, the single-crystal diffraction data are used to resolve the structure of ZU-61 and locate the position of benzene ring of oX, additional DFT-D calculations are employed to locate the position of methyl groups of oX.

Fig. 5. Single-crystal X-ray diffraction resolved structure of ZU-61·pX and ZU-61·mX.

Two adsorption positions of pX molecules in the unite cell of ZU-61 (a). ZU-61·pX with pX molecule in Position I in different directions (b, c). Location of mX in the pores of ZU-61 (d). Color code: C, gray 50%; H, gray 10%; Nb, teal; Ni, violet; O, green; N, sky blue; F, red. Note: the single-crystal diffraction data are not of sufficient quality to locate the precise positions of methyl groups of mX in ZU-61·mX. Therefore, we just give the adsorption positions of benzene ring, additional DFT-D calculations are employed to locate the position of methyl groups of mX.