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Communication

Adsorptive Separation of Chlorobenzene and Chlorocyclohexane by Nonporous Adaptive Crystals of Perethylated Pillar[6]arene

by
Sha Wu
1,†,
Yuyue Chi
1,†,
Qian Dong
1 and
Jiong Zhou
1,2,*
1
Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819, China
2
Key Laboratory of Functional Molecular Solids, Ministry of Education, School of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2025, 30(15), 3312; https://doi.org/10.3390/molecules30153312
Submission received: 1 June 2025 / Revised: 28 July 2025 / Accepted: 5 August 2025 / Published: 7 August 2025
(This article belongs to the Special Issue Recent Advances in Supramolecular Chemistry)
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Abstract

The separation of chlorobenzene (CB) and chlorocyclohexane (CCH) using traditional industrial separation technologies (distillation, fractionation, and rectification) is a great challenge due to their close boiling points. Here, we report an innovative method for the separation of the mixture of CB and CCH by nonporous adaptive crystals (NACs) of perethylated pillar[6]arene (EtP6). NACs of EtP6 (EtP6α) can selectively adsorb CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1) with a purity of 99.5%. Furthermore, EtP6α can be recycled for five times without a significant loss of performance.

Graphical Abstract

1. Introduction

The separation of aromatic compounds from their cyclic aliphatic counterparts, such as benzene/cyclohexane, toluene/methylcyclohexane, and chlorobenzene/chlorocyclohexane, is a critical industrial challenge. These cyclic aliphatics are predominantly produced via the catalytic hydrogenation of their corresponding aromatics [1,2,3,4,5,6]. Focusing on CB, this chlorinated aromatic is widely employed as a solvent, reagent, and intermediate in the chemical industry. However, CB poses significant environmental and health risks due to its persistence, bioaccumulation potential, and acute toxicity [7,8,9]. Consequently, converting CB into less harmful or reusable products like CCH through hydrogenation or electrochemical degradation is essential prior to disposal [10,11,12]. A major obstacle in this conversion process is the separation of unreacted CB from the product CCH. Their similar boiling points (CB: 132 °C; CCH: 142 °C) render conventional separation techniques like distillation highly energy-intensive for obtaining high-purity CCH [13,14,15,16]. Thus, it is essential to explore innovative and energy-saving separation strategies for aromatic and cyclic aliphatic compounds.
In recent decades, the use of porous materials like zeolites, metal–organic frameworks, and covalent organic frameworks for adsorptive separation has attracted much attention as an alternative strategy, which provides new inspiration and guidance for the development and innovation of traditional industrial separation technologies [17,18,19,20,21,22,23]. Although porous materials are commonly utilized as adsorbents for adsorptive separation, the instability in the recycling process has limited their further development [24,25,26,27,28]. Therefore, it is necessary to explore efficient and stable adsorptive separation materials.
Nonporous adaptive crystals (NACs) represent a novel category of supramolecular solid adsorbents first proposed and defined by Huang and co-workers [29]. In contrast to traditional porous materials with the Brunauer–Emmett–Teller (BET) specific surface area greater than 100 m2/g, NACs are nonporous in the un-adsorbed state with a BET-specific surface area of less than 10 m2/g. However, the intrinsic pores of NACs can be induced by specific guest molecules, which gives NACs great potential in adsorptive separation [30,31,32,33,34,35,36,37,38]. Moreover, NACs have the advantages of a simple preparation process, good chemical and thermal stability, and high recyclability [39,40,41,42,43,44,45,46,47]. At present, NACs based on various macrocycles, including pillararenes, biphenarenes, hybridarenes, and geminiarenes, have been applied in the field of adsorptive separation [48,49,50,51,52,53,54,55]. However, to the best of our knowledge, pillararenes-based NACs for the separation of CB and CCH have not been studied.
Herein, we investigated the possibility of separating CB and CCH using perethylated pillar[6]arene (EtP6)-based NACs (Figure 1). NACs of EtP6 (EtP6α) could selectively adsorb CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1) with a purity of 99.5%. The experimental and computational results showed that the excellent selectivity of EtP6α to CCH vapor was due to the thermodynamic stability of the host–guest complex formed by EtP6 and CCH. Additionally, the guest molecule loaded in EtP6 could be removed by heating, transforming the crystal structure to its original guest-free state, thus enabling the reuse of EtP6α.

2. Results

The guest-free EtP6 (EtP6α) was prepared according to the previously reported method [56,57,58,59,60]. 1H nuclear magnetic resonance (1H NMR) and thermogravimetric analysis (TGA) showed that the solvent was completely removed in EtP6α (Figures S1 and S2). The powder X-ray diffraction (PXRD) experiment showed that EtP6α was crystalline (Figure S3).
The adsorption properties of EtP6α toward CB vapor and CCH vapor were studied by 1H NMR, TGA, and PXRD. Both single-component CB vapor and CCH vapor could be adsorbed by EtP6α, and the proton signal peaks of CB or CCH appeared in 1H NMR of EtP6α after the adsorption of CB vapor or CCH vapor, while the proton signal peaks of EtP6α did not show significant change (Figures S6 and S7). Notably, the adsorption amounts of EtP6α for CB vapor and CCH vapor both increased with time, reaching saturation points at 24 h and 3 h, respectively (Figure 2a,b). Moreover, the adsorption amount was one CB molecule per EtP6 molecule or one CCH molecule per EtP6 molecule calculated by the 1H NMR spectra (Figures S4–S7). The adsorption amounts of CB and CCH by EtP6α were further confirmed by TGA. The weight loss of EtP6α after the adsorption of CB vapor and CCH vapor was 12.5% and 12.0%, respectively, indicating that one EtP6 molecule could adsorb one CB molecule or one CCH molecule (Figure 2c, Figures S8 and S9). These results revealed that EtP6α had good capacity to adsorb CB vapor and CCH vapor. Furthermore, the PXRD patterns of EtP6α after the adsorption of CB vapor or CCH vapor changed significantly, while they were different from each other (Figure 2d, Figures S10 and S11). These results proved that EtP6α could adsorb CB vapor or CCH vapor with crystalline transitions from EtP6α to CB-loaded EtP6 (CB@EtP6) or CCH-loaded EtP6 (CCH@EtP6).
The selective adsorption capacity of EtP6α for the vapor mixture of CB and CCH (v:v = 1:1) was studied in light of its adsorption capacity for CB vapor and CCH vapor. According to the time-dependent adsorption plots of EtP6α for the vapor mixture of CB and CCH (v:v = 1:1), the adsorption amount of CCH by EtP6α increased with time, reaching the saturation point after 3 h (Figure 3a). In contrast, the adsorption amount of CB by EtP6α was negligible. The adsorption amount was one CCH molecule per EtP6 molecule calculated by 1H NMR and TGA (Figures S12 and S13). Moreover, the PXRD pattern of EtP6α after the adsorption of the vapor mixture of CB and CCH (v:v = 1:1) changed and was consistent with that of EtP6α after the adsorption of CCH vapor alone (Figure 3b and Figure S14). These results indicated that EtP6α selectively adsorbed CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1), and the crystal structure of EtP6α was transformed into CCH@EtP6.
According to the head space gas chromatography (HS-GC) experiment, 99.5% of CCH was loaded in EtP6α, whereas 0.5% of CB was loaded in EtP6α (Figure 3c and Figure S15). This result revealed that EtP6α could separate 99.5% of pure CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1), with a higher separation purity than previously reported for geminiarene (97.5%) (Table S3). Furthermore, the recyclability of EtP6α was investigated by cyclic adsorption experiments. The CCH-loaded EtP6α could release CCH to reactivate upon heating at 120 °C under vacuum. The reactivated EtP6α could be reused to selectively adsorb CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1). Notably, EtP6α showed no obvious loss of performance after recycling five times (Figure 3d and Figure S16).
In order to study the adsorption mechanism of EtP6α for CB and CCH, we tried to grow single crystals of CB@EtP6 and CCH@EtP6. Fortunately, the single crystal of CB@EtP6 was obtained by slowly evaporating the CB solution of EtP6 and characterized by single-crystal X-ray diffraction analysis. One CB molecule was located in the cavity of EtP6 to form a 1:1 host–guest complex, as demonstrated by the single-crystal structure of CB@EtP6 (Figure 4a). The C–H···O interaction between an O atom on the ethoxy group of EtP6 and an H atom of CB stabilized CB@EtP6 (Figure S17) [61,62,63]. Additionally, hexagonal EtP6 molecules were able to pack into window-to-window patterns, which promoted the formation of honeycomb-like infinite edge-to-edge 1D channels (Figure 4b). However, the single-crystal structure of CCH@EtP6 could not be obtained by various methods, so we establish and optimize the structural model of CCH@EtP6. Similarly, CCH was also located in the center of the cavity of EtP6, forming a 1:1 host–guest complex (Figure S18). By measuring the interatomic distances, we estimated that CCH@EtP6 was stabilized by the C–H···Cl interaction between an H atom on the ethoxy group of EtP6 and a Cl atom of CCH, C–H···π interactions between benzene rings of EtP6 and H atoms of CCH, and by C–H···O interactions between O atoms on the ethoxy group of EtP6 and H atoms of CCH (Figure S19). These results suggested that the main driving force for the adsorption process came from non-covalent interactions (C–H···Cl, C–H···π, C–H···O) between EtP6 and guests.
In addition, the thermodynamics (Gibbs free energies and binding energies) of the formation of host–guest complexes (CB@EtP6 and CCH@EtP6) were calculated using density functional theory (DFT) [64,65,66,67], and the mechanism of the selective adsorption of CCH vapor by EtP6α was explained (Table 1 and Table S2). The Gibbs free energies and binding energies were calculated using the following equations:
ΔG = Ghost–guest − Ghost − Gguest, EBE = Ehost–guest − Ehost − Eguest
The Gibbs free energies of CB@EtP6 and CCH@EtP6 were calculated to be −56.27 kJ/mol and −82.84 kJ/mol, respectively. Compared with CB@EtP6, CCH@EtP6 had a lower Gibbs free energy. These results revealed that the adsorption of EtP6α for both CB vapor and CCH vapor was spontaneous, but the adsorption process of EtP6α for CCH vapor was more prone to occur. In addition, the binding energy of CCH@EtP6 was −142.00 kJ/mol, which was lower than that of CB@EtP6 (−113.17 kJ/mol), indicating that CCH@EtP6 was more stable than CB@EtP6.

3. Materials, Theoretical Calculations, and Methods

3.1. Materials

All chemicals, including chlorobenzene (CB) and chlorocyclohexane (CCH), were purchased and used as received. Perethylated pillar[6]arene (EtP6) was synthesized as described previously [32]. Activated crystalline EtP6 (EtP6α) was recrystallized from acetone and dried under vacuum at 120 °C overnight.

3.2. Theoretical Calculations

All-electron DFT calculations were carried out by Gaussian G09. For geometry optimization and frequency calculations, the BLYP functional and def2-SVP basis set were used, and the optimal geometry for each compound was determined. The singlet point energy calculations were performed with B3LYP functional and a larger def2-TZVP basis set. The weak interaction was corrected by the DFT-D3 dispersion correction with BJ-damping, which improved the calculation accuracy.

3.3. Methods

All experimental methods can be found in the Supplementary Materials.

4. Conclusions

In summary, we investigate the ability of nonporous adaptive crystals of perethylated pillar[6]arene (EtP6α) to separate CCH from the mixture of CB and CCH. EtP6α can separate 99.5% of pure CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1). DFT calculations show that the ability of EtP6α to selectively adsorb CCH vapor from the vapor mixture of CB and CCH (v:v = 1:1) is due to the higher thermodynamic stability of CCH-loaded EtP6 (CCH@EtP6) than CB-loaded EtP6 (CB@EtP6). Additionally, EtP6α has good recycling performance due to the reversible transformation between the guest-loaded structure and the guest-free structure. Therefore, EtP6α is a promising material for the adsorptive separation of aromatic hydrocarbons and cyclic aliphatic compounds. Future research will focus on how to increase the adsorption amount of EtP6α to achieve more efficient separation progress; for example, optimizing the crystal structures, adjusting the adsorption conditions, and combining with other porous adsorbents.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules30153312/s1, Figure S1: 1H NMR spectrum (600 MHz, CDCl3, 293 K) of EtP6α; Figure S2: TGA of EtP6α; Figure S3: PXRD pattern of EtP6α; Figure S4: 1H NMR spectrum (600 MHz, CDCl3, 293 K) of EtP6α after adsorption of CB vapor for 24 h; Figure S5: 1H NMR spectrum (600 MHz, CDCl3, 293 K) of EtP6α after adsorption of CCH vapor for 3 h; Figure S6: 1H NMR spectrum (600 MHz, CDCl3, 293 K) of (I) EtP6α and (II) EtP6α after adsorption of CB vapor; Figure S7: 1H NMR spectrum (600 MHz, CDCl3, 293 K) of (I) EtP6α and (II) EtP6α after adsorption of CCH vapor; Figure S8: TGA of EtP6α after adsorption of CB vapor for 24 h; Figure S9: TGA of EtP6α after adsorption of CCH vapor for 3 h; Figure S10: PXRD patterns of (I) original EtP6α and (II) EtP6α after adsorption of CB vapor; Figure S11: PXRD patterns of (I) original EtP6α and (II) EtP6α after adsorption of CCH vapor; Figure S12: 1H NMR spectrum (600 MHz, CDCl3, 293 K) of EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for 3 h; Figure S13: TGA of EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for 3 h; Figure S14: PXRD patterns of (I) original EtP6α, (II) EtP6α after adsorption of CCH vapor, and (III) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1); Figure S15: Relative uptakes of CCH and CB vapors adsorbed by EtP6α for 3 h using HS-GC; Figure S16: 1H NMR spectra (600 MHz, CDCl3, 293 K) of EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for five cycles: (a) original EtP6α; (b) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for the first cycle; (c) EtP6α after desorption of CCH for the first cycle; (d) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for the second cycle; (e) EtP6α after desorption of CCH for the second cycle; (f) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for the third cycle; (g) EtP6α after desorption of CCH for the third cycle; (h) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for the fourth cycle; (i) EtP6α after desorption of CCH for the fourth cycle; (j) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1) for the fifth cycle; Figure S17: Single crystal of CB@EtP6: illustration of C–H···O interaction between EtP6 and CB; Figure S18: The optimized structure of CCH@EtP6: (a) top view; (b) side view; Figure S19: The optimized structure of CCH@EtP6: illustration of C–H···Cl, C–H···π, and C–H···O interactions between EtP6 and CCH; Table S1: Experimental single-crystal X-ray data for CB@EtP6; Table S2: The optimized structures, Gibbs free energies, and binding energies of EtP6, CB, CCH, CB@EtP6, and CCH@EtP6; Table S3: Comparison of the efficiency of separating CCH from CB for EtP6α with other reported adsorbents. Refs. [68,69,70] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, S.W. and J.Z.; methodology, S.W.; software, S.W. and Y.C.; validation, S.W., Q.D. and J.Z.; formal analysis, S.W.; investigation, S.W.; resources, J.Z.; data curation, S.W.; writing—original draft preparation, S.W.; writing—review and editing, Q.D. and Y.C.; supervision, J.Z.; project administration, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22101043), the Fundamental Research Funds for the Central Universities (N25LPY027, N2205013, N232410019), the Natural Science Foundation of Liaoning Province (2023-MSBA-068), the Opening Fund of State Key Laboratory of Heavy Oil Processing (SKLHOP202203006), the Key Laboratory of Functional Molecular Solids, Ministry of Education (FMS2023005) and Northeastern University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The author declares no conflicts of interest.

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Molecules 30 03312 g001
Figure 1. Chemical structures of (a) perethylated pillar[6]arene (EtP6) and (b) chlorobenzene (CB) and chlorocyclohexane (CCH).
Figure 1. Chemical structures of (a) perethylated pillar[6]arene (EtP6) and (b) chlorobenzene (CB) and chlorocyclohexane (CCH).
Molecules 30 03312 g001
Molecules 30 03312 g002
Figure 2. Time-dependent solid–vapor adsorption plots of EtP6α after adsorption of (a) CB vapor and (b) CCH vapor. (c) TGA of EtP6α after adsorption of CB vapor for 24 h and CCH vapor for 3 h. (d) PXRD patterns of (I) EtP6α and (II) EtP6α after adsorption of CB vapor; (III) EtP6α after adsorption of CCH vapor.
Figure 2. Time-dependent solid–vapor adsorption plots of EtP6α after adsorption of (a) CB vapor and (b) CCH vapor. (c) TGA of EtP6α after adsorption of CB vapor for 24 h and CCH vapor for 3 h. (d) PXRD patterns of (I) EtP6α and (II) EtP6α after adsorption of CB vapor; (III) EtP6α after adsorption of CCH vapor.
Molecules 30 03312 g002
Molecules 30 03312 g003
Figure 3. (a) Time-dependent solid–vapor adsorption plots of EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1). (b) PXRD patterns of (I) EtP6α and (II) EtP6α after adsorption of CCH vapor; (III) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1). (c) Relative amounts of CB and CCH adsorbed by EtP6α after 3 h as measured by HS-GC. (d) Relative amounts of CB and CCH adsorbed by EtP6α after five cycles.
Figure 3. (a) Time-dependent solid–vapor adsorption plots of EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1). (b) PXRD patterns of (I) EtP6α and (II) EtP6α after adsorption of CCH vapor; (III) EtP6α after adsorption of the vapor mixture of CB and CCH (v:v = 1:1). (c) Relative amounts of CB and CCH adsorbed by EtP6α after 3 h as measured by HS-GC. (d) Relative amounts of CB and CCH adsorbed by EtP6α after five cycles.
Molecules 30 03312 g003
Molecules 30 03312 g004
Figure 4. Single crystal of CB@EtP6: (a) top view and side view; (b) packing mode along the a axis (H atoms were omitted for clarity).
Figure 4. Single crystal of CB@EtP6: (a) top view and side view; (b) packing mode along the a axis (H atoms were omitted for clarity).
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Table 1. The optimized structures, Gibbs free energies, and binding energies of CB@EtP6 and CCH@EtP6.
Table 1. The optimized structures, Gibbs free energies, and binding energies of CB@EtP6 and CCH@EtP6.
SpeciesStructuresΔG (kJ/mol)EBE (kJ/mol)
CB@EtP6Molecules 30 03312 i001−56.27−113.17
CCH@EtP6Molecules 30 03312 i002−82.84−142.00
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Wu, S.; Chi, Y.; Dong, Q.; Zhou, J. Adsorptive Separation of Chlorobenzene and Chlorocyclohexane by Nonporous Adaptive Crystals of Perethylated Pillar[6]arene. Molecules 2025, 30, 3312. https://doi.org/10.3390/molecules30153312

AMA Style

Wu S, Chi Y, Dong Q, Zhou J. Adsorptive Separation of Chlorobenzene and Chlorocyclohexane by Nonporous Adaptive Crystals of Perethylated Pillar[6]arene. Molecules. 2025; 30(15):3312. https://doi.org/10.3390/molecules30153312

Chicago/Turabian Style

Wu, Sha, Yuyue Chi, Qian Dong, and Jiong Zhou. 2025. "Adsorptive Separation of Chlorobenzene and Chlorocyclohexane by Nonporous Adaptive Crystals of Perethylated Pillar[6]arene" Molecules 30, no. 15: 3312. https://doi.org/10.3390/molecules30153312

APA Style

Wu, S., Chi, Y., Dong, Q., & Zhou, J. (2025). Adsorptive Separation of Chlorobenzene and Chlorocyclohexane by Nonporous Adaptive Crystals of Perethylated Pillar[6]arene. Molecules, 30(15), 3312. https://doi.org/10.3390/molecules30153312

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