Toluene (Tol) and alcohol (methanol [MeOH] and ethanol [EtOH]) are significant organic solvents extensively utilized in the organic chemical, synthetic pharmaceutical, and fine chemical industries [1–3]. Under atmospheric pressure, Tol forms azeotropic mixtures with MeOH or EtOH in specific proportions. Separating these mixtures is crucial for efficiently producing high-purity chemicals and recovering value-added substances [4,5]. However, the small relative volatility differences of the azeotropic components make obtaining high-purity toluene and alcohol through traditional distillation methods is challenging, complex, and energy-intensive [6,7]. Additionally, Tol, a toxic volatile organic pollutant, often causes personal hazard and environmental contamination during the separation of toluene-alcohol azeotropes, necessitating adsorption treatment. Therefore, there is a pressing need to develop a simpler, more operationally efficient, and energy-saving method for the separation of toluene-alcohol azeotropes.

In recent years, various innovative energy-saving strategies for the separation of benzene series/alcohol azeotropes have been proposed, focusing on the differences in molecular size and geometric shape between benzene series and alcohol [8–10]. One effective strategy involves the use of selective molecular sieving membranes, such as include metal-organic framework membranes [11], silicalite-embedded chitosan membranes [12]. However, these membranes often exhibit instability and lack recyclability, making their practical application challenging. Another approach employs macrocyclic arene-based crystalline materials for selective adsorption and separation [13]. Despite their potential, most of these materials are nonporous, resulting in time-consuming adsorbate diffusion. Consequently, there is an urgent need for the development of a rapid adsorptive separation material for toluene-alcohol azeotropes that offers high selectivity.

With the advancement of supramolecular chemistry [14–22], an increasing number of supramolecular macrocyclic host compounds with unique structures and functions have been synthesized and extensively utilized [23–34]. Recently, our team reported the synthesis of a novel macrocyclic aromatic receptor, fluorinated leaning pillar[6]arene (FLP6) (Fig. 1a) [35], which has showed extensive applications in molecular recognition and selective molecular sieving [36]. Notably, Khashab and colleagues have shown that fast adsorption kinetics can be attained using porous molecular sieving platforms, activated FLP6 to exhibit 20:1 benzene selectivity over cyclohexane. Furthermore, as a macrocyclic arene-based porous crystalline material, the Brunauer-Emmett-Teller (BET) surface area of activated FLP6 has been measured to reach approximately 200 m²/g [37].

The FLP6 was synthesized through a fragment cyclization method according to our previous work [35,38]. High-quality single crystals of FLP6 suitable for X-ray single crystal diffraction were yielded in a DCM/hexane mixture solution. Unlike most previously reported macrocyclic arene-based crystalline materials [39–46], the porosity of FLP6 arises from its unique crystal stacking structure. In this structure, opposite phenyl groups on the FLP6 macrocyclic skeleton are parallel, presenting a regular structure with central symmetry. Interestingly, FLP6 molecules crystallizes into tilt-aligned one-dimensional sub-nanotubes, facilitated by C-H···O and C-H···F interactions (Figs. 2b and c, Fig. S1 in Supporting information). These sub-nanotubes further self-assemble into a three-dimensional porous framework through C-H···F and C-H···π contacts between adjacent sub-nanotubes in the solid state (Fig. 2d and Fig. S2 in Supporting information).

An in-depth analysis of the crystals, including the cavity size and porosity of FLP6, was conducted. Given the structural differences between toluene and alcohol (Fig. 1b), we became interested in preparing FLP6-based porous crystalline materials to separate toluene-alcohol azeotropes. To achieve this, activated FLP6 crystals (referred to as FLP6α) were prepared by recrystallization of FLP6 from DCM/hexane mixture solution and dried at 70 ℃ for 8 h [37]. The complete removal of solvent molecules from these hosts was confirmed through ¹H nuclear magnetic resonance (¹H NMR) spectra and thermogravimetric analysis (TGA) (Figs. S3 and S4 in Supporting information). Furthermore, powder X-ray diffraction (PXRD) experiments demonstrated that the powders remained crystalline in the solid state and matched the crystal structure simulation (Fig. S5 in Supporting information).

First, we investigated the adsorption capacity of FLP6α for toluene-alcohol azeotropes. To evaluate the potential of FLP6α for Tol/alcohol separation, we conducted time-dependent solid vapor adsorption experiments for single-component Tol, MeOH, and EtOH. As shown in Fig. S6 (Supporting information), FLP6α reaches equilibrium saturation in just 70 min. At the saturation point, the adsorption amount of Tol was approximately one Tol molecule per FLP6α molecule, as determined from the ¹H NMR spectra and weight change after adsorption. In contrast, no adsorption was observed for MeOH and EtOH. TGA of FLP6α showed a weight loss of 10.25% at 310 ℃ after adsorption of Tol vapor for 2 h, confirming that one FLP6α molecule contained one Tol molecule. However, there was nearly no weight loss of FLP6α before 380 ℃ after adsorption of MeOH and EtOH vapor for 2 h. These results indicate that FLP6α captured Tol, but not alcohol (Figs. S7–S12 in Supporting information). Additionally, the PXRD patterns of activated FLP6α and Tol-loaded FLP6α were significantly different, indicating crystal structural changes during the adsorption process (Fig. S14 in Supporting information). These findings suggest that FLP6α exhibit selective adsorption for Tol.

Moreover, the complexation between FLP6 and Tol molecules in solution was investigated by ¹H NMR experiments. As shown in Fig. S13, in the ¹H NMR spectrum of FLP6@Tol, the NMR signals of the protons on Tol molecules showed down field shifts. These results indicated the host–guest complexation between FLP6 and Tol molecules in solution, which further confirmed the Tol vapors capture capacity of FLP6α.

In order to reveal the adsorption mechanism of FLP6α more thoroughly, we obtained FLP6@Tol single crystals by dissolving FLP6α in Tol solution and allowing it to evaporate slowly. The resulting crystals clearly showed a 1:1 host–guest complex between FLP6 and Tol in the solid state (Figs. 3a and b). Three pairs of C-H···π interactions were observed between the hydrogen atoms of FLP6 and the aromatic rings of Tol, with the distances of 2.877, 2.756, and 2.830 Å. Additionally, a C-H···π interaction was identified between the hydrogen atoms of the aromatic rings of Tol and FLP6 (Fig. S15–S17 in Supporting information). The electrostatic potential surfaces further reveal that FLP6 is electronegative around the cavity skeleton, while the edges of Tol are positively charged [47]. This charge contrast facilitates multiple intermolecular interactions between FLP6 and Tol (Fig. S18 in Supporting information). Consequently, the high selectivity of FLP6 towards Tol is attributed to the formation of a stable crystal structure upon adsorption of Tol molecules.

Further insights into host–guest interactions were obtained through independent gradient model (IGM) analysis [48]. This analysis revealed that the noncovalent interactions between FLP6 and Tol are primarily located on the outer side of the macrocycle skeleton, rather than being concentrated in the center of the FLP6 cavities (Figs. 3c and d). This spatial distribution alters the original stacking pattern of FLP6α, as Tol molecules occupy the space between adjacent FLP6 molecules. FLP6α is soluble in Tol but not in methanol or ethanol. In the presence of Tol/alcohol vapor mixtures, FLP6α selectively adsorbs Tol and undergoes recrystallization, forming the FLP6@Tol crystal (Figs. 3e and f). Additionally, the PXRD pattern of FLP6 after trapping Tol closely resembles the crystal structure simulation (Fig. S19 in Supporting information).

After elucidating the adsorption properties and mechanism of Tol in FLP6α, we investigated the ability of FLP6α to separate Tol/alcohol mixture. We conducted time-dependent solid-vapor adsorption experiments using an equal volume Tol/MeOH/EtOH mixture. As shown in Fig. 4a, the adsorption of Tol in FLP6α increased with time, reaching saturation within just 70 min. This rate is stronger than that of most reported nonporous adaptive crystals. Based on the ¹H NMR results, at the saturation point, one FLP6α molecule adsorbed one Tol molecule, while the adsorption of MeOH and EtOH was negligible. These results indicate that FLP6α adsorbs Tol rapidly and with high selectivity, whereas it is ineffective for MeOH and EtOH adsorption. This finding is consistent with previous single-component adsorption experiments involving FLP6α and vapors of Tol, MeOH, and EtOH. Moreover, the PXRD patterns of FLP6α after the adsorption of toluene Tol closely resembled those observed in single-component adsorption experiments with Tol. In contrast, the PXRD spectra remained unchanged when FLP6α was exposed to MeOH and EtOH vapor environments, indicating that the original structure of FLP6α was preserved. These findings suggest that FLP6α exhibits selective adsorption properties for Tol (Fig. 4b). The gas chromatography (GC) results further confirmed that the adsorption of Tol in FLP6α was as high as 100% (Fig. 4c and Fig. S20 in Supporting information) in an equal volume Tol/MeOH/EtOH mixture.

In practical industrial production, recyclability and stability are essential parameters for evaluating adsorbents. Heating FLP6α adsorbed with Tol at 70 ℃ for 4 h effectively removes the adsorbed Tol molecules. This process regenerates the original FLP6α structure, as confirmed by PXRD (Fig. S21 in Supporting information). Additionally, the performance loss was negligible over at least five consecutive recovery tests (Fig. 4d and Figs. S22–S26 in Supporting information).

In summary, we successfully achieved the selective separation of Tol from a mixture of toluene-alcohol azeotropes using activated FLP6 crystals. Initially, we verified that FLP6 can spontaneously form three-dimensional porous framework via self-assembly in the solid state. This novel porous crystalline material demonstrated high selectivity for separating an equal volume Tol/MeOH/EtOH mixture. The selectivity of FLP6α was determined using ¹H NMR and GC techniques. Remarkably, the FLP6α crystals can be restored to their initial crystalline state upon the removal of the guest molecules. FLP6α presents several advantages, including simple synthesis, a rapid adsorption rate, high separation efficiency, and excellent recyclability. These characteristics make it an appealing green alternative to traditional energy-intensive separation technologies. In particular, FLP6α effectively separates benzene series and alcohol azeotropes, showing significant potential for applications in the petrochemical industry.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Jingxiong Jiang: Writing – original draft, Formal analysis, Data curation. Yao Dong: Formal analysis, Data curation. Yuchun Wang: Formal analysis. Lijuan Qi: Methodology. Zhen-Yu Li: Formal analysis, Data curation. Tai-Bao Wei: Writing – review & editing, Funding acquisition. Wen-Juan Qu: Writing – review & editing, Formal analysis. Qi Lin: Writing – review & editing, Supervision, Funding acquisition, Formal analysis, Conceptualization. Bingbing Shi: Writing – review & editing, Writing – original draft, Supervision, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 22001214, 21662031, 21661028 and 22061039), the Science Fund for Distinguished Young Scholars of Gansu Province (No. 22JR5RA131), the Longyuan Innovation and Entrepreneurship Talent Project of Gansu Province, the Major Project Cultivation Program of Northwest Normal University, the Top Leading Talents Project of Gansu Province, the Key R & D program of Gansu Province (No. 21YF5GA066), College Industry Support Plan Project of Gansu Province (No. 2022CYZC-18).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110759.