English

• 3-MP is a highly important intermediate in industrial production, which is extensively utilised in the production of insecticides, herbicides, pharmaceuticals, feed additives, dyes, polycarbonate resins and textiles, rubber products, piperidines, adhesives, and so on [1]. Currently, the main method for the global production of 3-MP include the aldehyde (ketone) ammonia process. However this method of producing 3-MP would have been accompanied by the generation of Py [2, 3]. Similarly, Py is also an essential chemical, pharmaceutical and pesticide raw material with a wide range of applications [4]. In addition to being a solvent, Py can also be used in industry as a denaturant, an accelerant, a raw material or catalyst utilised in the synthesis of a diverse array of products including pharmaceuticals, disinfectants, dyestuffs, food flavouring agents, and adhesives. Py and 3-MP have excellent mutual solubility. In industrial production, separation and purification of these two substances requires distillation, but the distillation process is complicated and energy intensive. More importantly, the purity of the product is not high enough. Therefore, developing straightforward and energy-efficient techniques for the purification of 3-MP and the separation of Py is essential [5, 6].

  Energy-efficient separation of significant petrochemical products and feedstocks is enabled by various materials, including porous zeolites, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) [7-9]. These materials, however, do have certain limitations that cannot be ignored. For instance, MOFs exhibit relatively weak chemical stability, and their structures are easily damaged under high temperatures or acidic conditions [10]. Recently, Huang's group developed macrocyclic non-porous adaptive crystals (NACs) as novel adsorption and separation materials, demonstrating exceptional performance [11-14]. In recent years, NACs have advanced significantly in adsorptive separations. Despite this progress, many NACs materials tend to dissolve when separating target substances, confining their use predominantly to solid–vapor adsorptive separation processes [15-23]. Thus, it is essential to create new materials that possess adequate chemical and thermal stability for the separation of Py/3-MP. Moreover, the development of innovative materials for adsorption and separation applications remains a substantial challenge within contemporary materials science and engineering.

  Cucurbit[n]uril (Q[n] n = 5–8, 10, and 13–15) are a class of caged, highly symmetric macrocyclic compounds in supramolecular chemistry after crown ethers, cyclodextrins, and calixarenes [24-26]. Q[n] features a rigid cavity and dual ports with numerous carbonyl functional groups, making it an excellent receptor for various hydrophobic guests of chemical and biological relevance, including neutral and cationic ones [27-29]. Over the past two decades, its strong affinity for binding guest molecules has led to a broad array of applications in sensing, adsorption separation, catalysis and detection, etc. [30-40]. Recently, Huang's group reported that Q[6] NACs, obtained by recrystallising Q[6] in 4 mol/L HCl solution, can selectively separate the low-boiling azeotropes toluene and pyridine [41]. Xiao's group demonstrated that nor-seco-Q[10] can efficiently and rapidly isolate pyridine from a mixture of toluene, benzene, and pyridine [42]. Among the Q[n] family, Q[6] is simple to synthesise, exhibits excellent chemical and thermal stability, has prefabricated cavities, and an intrinsically rigid channel structure for guest adsorption. Its selectivity is pronounced in relation to the guest's size, volume, and charge distribution, thus enabling effective separation [43-46].

  Hence, leveraging the aforementioned characteristics, Q[6] emerges as a macrocyclic host exhibiting favorable adsorption and separation properties, offering a viable approach for 3-MP purification (Scheme 1). The experimental results showed that both solid–vapor mixtures and solid–liquid mixtures of Py and 3-MP tended to adsorb Py as a target when introduced into Q[6], and the purity of Py was close to 100%. In simulated industrial-scale experiments, the Py content in the mixed solution of Py and 3-MP decreased from 80.19% to 0.25% and the 3-MP content in the solution increased from 19.81% to 99.75% after filling the column with Q[6]. Detailed investigation indicated that the separation resulted from complexation between Py and Q[6], leading to a more stable structure upon Py vapor adsorption in its crystalline form. Additionally, The results of the isothermal titration calorimetry (ITC) and density functional theory (DFT) calculations further confirm the selectivity of Q[6] for Py. Moreover, Py could be removed by heating, allowing Q[6] to revert to its original structure, demonstrating excellent recoverability.

  Scheme 1

  

  Scheme 1.  The schematic illustrating the separation of Py and 3-MP using Q[6].

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  Q[6] synthesised according to published literature, followed by a pretreatment to obtain guest-free Q[6] (Fig. S1 in Supporting information) [47]. Powder X-ray diffraction (PXRD) analysis confirmed that the activated Q[6] retained a crystalline structure [41], consistent with previous findings (Fig. S2 in Supporting information). Our group's research suggested that the Q[6] crystals exhibited a lack of porosity, possibly attributed to their compact packing configurations.

  Despite its nonporous nature, we explored the adsorption capacities of Q[6]. First, we investigated the vapor adsorption capacity of Q[6] crystals for a single component by solid–vapor adsorption experiments at 298 K over time (Fig. 1a). The results indicate that the adsorption of Py reached saturation after roughly 24 h, and that of single-component 3-MP vapors also took 24 h to reach saturation. ¹H NMR analysis further confirmed the adsorption and retention of 3-MP and Py within Q[6] (Figs. S3 and S4 in Supporting information). According to the ¹H NMR spectrum, the stoichiometry of Py and Q[6] in the complex is 1:1. Similarly, equilibrium calculations indicated that each Q[6] molecule can uptake one molecule of 3-MP. Additionally, PXRD analyses were employed to assess structural changes (Fig. 1b). The PXRD patterns of Q[6] altered significantly upon absorbing Py and 3-MP vapors, suggesting structural transformations within Q[6] upon their capture.

  Figure 1

  

  Figure 1.  (a) Time-dependent solid–vapor adsorption isotherms for Q[6] with single-component vapors of Py and 3-MP. (b) PXRD patterns of (Ⅰ) original Q[6]; (Ⅱ) post-Py vapor adsorption; (Ⅲ) post-3-MP vapor adsorption.

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  To investigate the adsorption mechanism, we obtained guest-loaded single crystals by gradually evaporation of solutions of Q[6] containing either Py or 3-MP. The X-ray crystallography has shown that the cavity of Q[6] contains a Py molecule, which participates in a 1:1 host–guest complex formation. In the Py-loaded Q[6] crystal structure (Py@Q[6], Fig. 2a and Fig. S5 in Supporting information), one Py molecule is encapsulated in the Q[6] cavity and stabilised by multiple C−H···O hydrogen bonds to stabilise it. Q[6] forms honeycomb-like infinite 1D channels through edge-to-edge assembly. In the crystal structure of 3-MP-loaded Q[6] (3-MP@Q[6], Fig. 2b and Fig. S6 in Supporting information), one 3-MP molecule is found in the Q[6] cavity, and the methyl group in the 3-MP lecule may interact with the carbonyl of Q[6] with weaker interaction forces. The interaction between the 3-MP molecule and Q[6] is likely to be realised mainly through weaker interaction forces such as van der Waals forces. The comparison of PXRD patterns derived from these crystal structures with experimental data (Figs. S7 and S8 in Supporting information) verified the transformation of Q[6] into Py-loaded and 3-MP-loaded Q[6] upon vapor uptake.

  Figure 2

  

  Figure 2.  Single crystal structures: (a) Py@Q[6]; (b) 3-MP@Q[6].

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  Based on the adsorption capacity and the distinct crystal structures formed after guest vapor adsorption, we examined the potential of Q[6] to separate Py and 3-MP mixtures. A solid–vapor sorption experiment was conducted with Q[6] over time on a Py/3-MP mixture at equal volumes in a 1:1 ratio. The ¹H NMR spectrum showed only proton-related peaks of Py were observed (Fig. S11 in Supporting information), indicating selective adsorb of Py. Furthermore, the PXRD pattern of Q[6] after the equimolar mixture adsorption was identical to the one after the sole adsorption of Py (Fig. 3b). Additionally, the absorption process was monitored by GC. The results indicated a notably higher Py adsorption compared to 3-MP in the vapor mixture (1:1 molar ratio), with Py selectivity exceeding 99.9% (Fig. 3c and Fig. S14 in Supporting information). The PXRD pattern matched well with the crystal structure simulated from Py@Q[6], indicating a structural transition from Q[6] to Py@Q[6].

  Figure 3

  

  Figure 3.  (a) Time-dependent solid–vapor adsorption plots of Q[6] for a mixed vapor of Py and 3-MP (v/v = 1:1). (b) PXRD patterns of Q[6]: (Ⅰ) original Q[6]; (Ⅱ) post-Py vapor adsorption; (Ⅲ) post-3-MP vapor adsorption; (Ⅳ) post-adsorption of Py and 3-MP vapor mixtures. (c) Relative amount of Py and 3-MP adsorbed by Q[6] over 24 h. (d) Relative absorption levels of Py and 3-MP by Q[6] over 24 h after the recycling of Q[6] for five consecutive cycles.

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  In practical industry, the recycling performance of an adsorbent is a crucial parameter for evaluating its overall effectiveness and quality. Q[6] crystals has good recyclability and the regeneration process is simple. By heating the Q[6] crystals loaded with Py at 100 ℃ under vacuum for 6 h, the absorbed Py was completely released and produced guest-free crystals similar to those of other NACs materials. The guest-free Q[6] crystals could be reused without degradation after 5 times of recycling without any significant performance degradation as monitored by ¹H NMR and GC (Fig. S23 in Supporting information). Furthermore, considering the more operational recycling application measures in the industry, we wondered if we could try to utilise solvent recovery methods. Because Q[6] has poor solubility in organic solvents, whereas Py and 3-MP are soluble in most organic solvents, we wondered if it was possible to extract Py from the complexes with organic solvents. Then the following experiments were carried out. Immerse approximately 10 mg Py@Q[6] crystal powder in an organic solvent (chloroform, aceton and acetonitrile etc.). After stirring for 10 min, Q[6] was separated from the organic phase by filtration, dried, and reused again. In the ¹H NMR experiments, we chose aceton and acetonitrile as the organic solvent for the extraction. After extraction, only peaks associated with Py were present in the organic phase (Fig. S24 in Supporting information). The above results clearly demonstrate that Q[6] crystals isolated Py from the mixture. Compared with the regeneration process of other NACs materials, Q[6] has better recyclability and the regeneration process is more energy-efficient and simpler.

  To enhance the separation conditions used in industrial settings and meet the requirements for energy-saving and emission reduction, we developed two additional practical scale-up techniques for separating Py and 3-MP. The first method is the solid–liquid adsorption experiment. About 20 mg of Q[6] crystal powder was immersed in a Py and 3-MP mixture (v: v = 80:20) for 24 h. Then, the crystals of the host–guest complex were gathered by filtration and subjected to vacuum drying at 60 ℃ for 10 min to eliminate the guest molecules adhering to the powder surface. GC experiments further confirmed Q[6]'s exceptional selectivity towards Py, demonstrating a 100% selectivity rate (Fig. 4b and Fig. S15 in Supporting information). The PXRD patterns of Q[6] after adsorbing equimolar mixtures were approximately the same as those after adsorption of Py alone (Fig. 4a). Thus, these results suggest that Q[6] preferentially adsorbs Py rather than 3-MP in their mixture, aligning with solid–vapor adsorption experiment findings. The recovered Q[6] sample could be subjected to at least five Py selection experiments without any degradation in performance.

  Figure 4

  

  Figure 4.  (a) PXRD patterns of Q[6]: (Ⅰ) the original Q[6]; (Ⅱ) post-Py liquid adsorption; (Ⅲ) post-3-MP liquid adsorption and (Ⅳ) post-adsorption of Py and 3-MP mixed liquids. (b) Quantities of Py and 3-MP adsorbed by Q[6] over 24 h. (c) Diagram of separation process simulating real industrial samples.

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  For the second method, column chromatography was utilised with a semi-permeable stationary phase (Fig. 4c). Initially, excess solid Q[6] was packed evenly into the glass column equipped with a fritted disc at the bottom (Fig. S16 in Supporting information). Subsequently, the Py and 3-MP, intended for separation, were slowly introduced into the column, and their contents were measured before and after separation using GC. The outcomes revealed a significant enhancement in 3-MP purity, rising from 19.81% to 99.75% following the adsorption and separation process (Figs. S17 and S18 in Supporting information). These findings demonstrate the efficacy of Q[6] as an adsorptive separation material in achieving exceptionally high purity levels of Py and 3-MP.

  To investigate the mechanism of selective adsorption, we used ITC to determine the thermodynamic parameters of the interactions between Q[6] and Py, as well as 3-MP. The binding constants (Ka) for Q[6] with Py and 3-MP were 2.45 × 10⁵ and 1.18 × 10⁵ L/mol (Figs. S9 and S10 in Supporting information), respectively, indicating that the binding strength of Q[6] with Py is greater than that with 3-MP. We further explain the selectivity mechanism through DFT theory calculations. The binding energy of Q[6], single-component Py/3-MP and host–guest complexes were calculated based on VASP code. The results showed that the binding energy of Q[6] for Py was −89 kJ/mol, and that for 3-MP was −67 kJ/mol (Fig. 5). Therefore, the adsorption of Q[6] for Py and 3-MP were spontaneous processes. In addition, compared with the adsorption process of 3-MP, the adsorption process of Py was more prone to occur. These results indicated Q[6]'s strong preference for Py in Py/3-MP mixtures, which is in accordance with the results of solid–vapor, solid–liquid experiments mentioned above.

  Figure 5

  

  Figure 5.  Structures and binding energies of complexes Py@Q[6] and 3-MP@Q[6].

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  Previous experimental results confirmed Q[6]'s significant selectivity towards Py. To evaluate if other members within the Q[n] family demonstrate comparable selectivity, analogous experiments were performed using Q[7, 8] as adsorbents for separation. The GC results from the solid–vapor phase indicated that Q[7] adsorbed 81.42% of Py (Fig. S19 in Supporting information) whereas, in the solid–liquid phase, the adsorption rate was 78.39% (Fig. S20 in Supporting information). Additionally, Q[8] showed a Py adsorption mass percentage of 77.96% in the solid–vapor phase (Fig. S21 in Supporting information), which dropped to 55.76% in the solid–liquid phase (Fig. S22 in Supporting information). The reduced absorption efficacy of Py by Q[7] and Q[8] compared to Q[6] might be ascribed to their larger cavity sizes and insufficient stabilising effects, which lead to a lower adsorption capacity and selectivity.

  In conclusion, we investigated the separation of Py/3-MP mixtures utilising Q[6]. Py was isolated from a 1:1 (v/v) mixture of Py and 3-MP with a resulting purity of 100%. Single-crystal structures, ITC experimental results and DFT calculations indicate that selectivity is determined by the stability of the newly formed structures after guest adsorption. PXRD patterns further demonstrated the transformation of the host crystal structures into host–guest complexes following adsorption. Excellent results were obtained in laboratory scale-up separation experiments with simulated industrial samples. In addition, it is worth mentioning that Q[6] shows excellent recoverability in at least five runs after solvent desorption or vacuum-heated desorption. Moreover, given Q[6]'s high selectivity, straightforward and cost-effective synthesis, excellent thermal and chemical stability, and superior recycling performance, it shows considerable potential as an alternative separation adsorbent in the chemical industry. The use of these simple Q[n] compounds in this study also demonstrates that minor structural variations can lead to significant changes in properties, offering valuable insights for designing adsorbents or substrates with precisely tailored binding sites. And other required homologs, isotopes and chiral compounds for the separation of Q[n] or their derivatives are being carried out in our laboratory.

  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

  Yongqing Zeng: Writing – original draft, Data curation. Caijun Liang: Data curation. Xin Lu: Methodology. Lingxue Zhao: Data curation. Fangting Wu: Validation. Tao Hou: Writing – original draft, Conceptualization. Anting Zhao: Visualization. Menglan Lv: Resources. Zhu Tao: Visualization. Qing Li: Writing – review & editing, Supervision, Funding acquisition.

  Acknowledgments

  This work is supported by the Guizhou Provincial Basic Research Program (Natural Science) Youth Guidance (Nos. [2024]110, [2024]378); Science and Technology Innovation Team of Natural Science Foundation of Guizhou Province (No. CXTD[2023]005); Science and Technology Innovation Team of Higher Education Department of Guizhou Province (No. QJJ[2023]053); Natural Science Special of Guizhou University (No. 202137); Guizhou Provincial Key Laboratory Platform Project (No. ZSYS[2025]008); PhD Foundation of Guizhou University (No. [2021]83).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110807.

  
  
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• 

  Scheme 1  The schematic illustrating the separation of Py and 3-MP using Q[6].

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  Figure 1  (a) Time-dependent solid–vapor adsorption isotherms for Q[6] with single-component vapors of Py and 3-MP. (b) PXRD patterns of (Ⅰ) original Q[6]; (Ⅱ) post-Py vapor adsorption; (Ⅲ) post-3-MP vapor adsorption.

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  Figure 2  Single crystal structures: (a) Py@Q[6]; (b) 3-MP@Q[6].

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  Figure 3  (a) Time-dependent solid–vapor adsorption plots of Q[6] for a mixed vapor of Py and 3-MP (v/v = 1:1). (b) PXRD patterns of Q[6]: (Ⅰ) original Q[6]; (Ⅱ) post-Py vapor adsorption; (Ⅲ) post-3-MP vapor adsorption; (Ⅳ) post-adsorption of Py and 3-MP vapor mixtures. (c) Relative amount of Py and 3-MP adsorbed by Q[6] over 24 h. (d) Relative absorption levels of Py and 3-MP by Q[6] over 24 h after the recycling of Q[6] for five consecutive cycles.

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  Figure 4  (a) PXRD patterns of Q[6]: (Ⅰ) the original Q[6]; (Ⅱ) post-Py liquid adsorption; (Ⅲ) post-3-MP liquid adsorption and (Ⅳ) post-adsorption of Py and 3-MP mixed liquids. (b) Quantities of Py and 3-MP adsorbed by Q[6] over 24 h. (c) Diagram of separation process simulating real industrial samples.

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  Figure 5  Structures and binding energies of complexes Py@Q[6] and 3-MP@Q[6].

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