Efficient separation of xylene isomers by nonporous adaptive crystals of hybrid[3]arene in both vapor and liquid phases

Abstract

The separation of xylene isomers is a major challenge in the petrochemical industry. However, the traditional distillation method is an energy-intensive process for the separation of xylene isomers. Herein, we develop nonporous adaptive crystals based on hybrid[3]arene H (Hα) for the efficient separation of xylene isomers. Hα shows high selectivity for ortho-xylene from the mixture of xylene isomers in both vapor and liquid phases, with a purity of 90.22% and 99.48%, respectively. The single crystal structure analysis suggests that the selectivity is derived from multiple C–H⋯O and C–H⋯π interactions between H and the preferred guest molecule, ortho-xylene, which is also confirmed by visual study of weak intermolecular interactions and electrostatic potential maps between H and xylene isomers. Besides, the Gibbs free energies of Hα for xylene isomers show that the adsorption energy of Hα for ortho-xylene is lower than that of meta-xylene or para-xylene, further confirming the preferred adsorption of Hα for ortho-xylene. Moreover, Hα is highly recyclable due to the reversible transformation between guest-free and guest-contained structures. This work will afford a new strategy for the separation of other important aromatic isomers and provide inspiration for the use of supramolecular host-based nonporous adaptive crystals in other energy-intensive separation methods.

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Introduction

The separation of xylene isomers has been a major challenge in the past few decades and is considered as one of “seven chemical separations to change the world” due to its significant impact on the chemical industry.^1–3 Xylene isomers are extracted from crude oil during the refining process, and are widely used as solvents and chemical feedstocks for synthetic polymers.^4–7 Among these xylene isomers, ortho-xylene (oX) can be converted to phthalic anhydride, which produces plasticizers and alkyd resins.⁸Meta-Xylene (mX) is used as a co-monomer in the production of high-value resins.⁹Para-Xylene (pX) is an important raw material for polyester and polyethylene terephthalate.¹⁰ Three xylene isomers serve as prized feedstocks for various chemical operations. But xylene isomers usually come in mixtures. Hence, it is imperative to separate and purify xylene isomers.

The industrial separation of xylene isomers is achieved by distillation and crystallization.^11–14 However, these methods are energy-intensive, accounting for a large proportion of global energy consumption.^15,16 Moreover, the separation effect is not satisfied due to the close boiling points of xylene isomers (Table S1, ESI†). To address these issues, porous adsorbents such as zeolites, organic cages and metal–organic frameworks (MOFs), have been explored for the separation of hydrocarbons in the last few decades.^17–26 A particular advantage of porous adsorbents is the diversity of their structures, which can result in fascinating adsorptive capabilities.^27–33 However, porous adsorbents have some inherent drawbacks that limit their practical applications.^34,35 For example, zeolites have weak adsorption ability for hydrocarbons, making it difficult to achieve the precise separation of hydrocarbons using them. The chemical stability of MOFs is relatively weak, and the structures are likely to be destroyed under high temperatures or acidic conditions. Thus, it is of great significance to explore novel materials for the separation of xylene isomers.

Recently, nonporous adaptive crystals (NACs) show remarkable potential as adsorption and separation materials.^36–56 Different from traditional porous materials, NACs are nonporous in their original states. However, when suitable guest molecules are introduced, NACs can form internal and external pores, which result in excellent adsorption performances.⁵⁷ Although great progress has been made in the separation of xylene isomers by supramolecular host-based NACs, the separation of oX from the ternary mixture of xylene isomers in both vapor and liquid phases has not been reported.^58–60

Hybrid macrocycles belong to the relatively novel class of macrocyclic hosts that offer abundant reaction sites for further modification.⁶¹ These macrocycles possess unique structural characteristics and multifunctional properties, which make them ideal materials for various applications.⁶² Recently, our group has successfully synthesized hybrid[3]arene, and constructed the first NACs based on hybrid[3]arene.^63,64 While conducting research on NACs based on hybrid[3]arene for adsorption and separation, it was discovered that these materials exhibited high sieving capabilities in the separation of benzene and cyclohexane, which could not be achieved by NACs of pillar[n]arenes or other traditional supramolecular hosts.^65–67 Besides, NACs of hybrid[3]arene showed high stability and recyclability for the separation of benzene and cyclohexane. This finding has inspired further exploration into the use of NACs based on hybrid[3]arene for more challenging but crucial separation of isomers.

Herein, for the first time, NACs based on hybrid[3]arene H (Hα) selectively adsorbed oX from the mixture of xylene isomers in both vapor and liquid phases (Fig. 1). It was found that Hα could selectively adsorb oX vapor from the binary or ternary vapor mixture of xylene isomers. Specifically, Hα could adsorb oX vapor with a purity of 97.49% in the vapor mixture of oX and mX, and a purity of 97.61% in the vapor mixture of oX and pX. For the ternary vapor mixture of xylene isomers, Hα showed a purity of 90.22% for oX vapor. Besides, liquid–solid adsorption experiments also showed that Hα had excellent adsorption and separation performances for oX liquid. In the ternary liquid mixture of xylene isomers (oX : mX : pX, v : v : v = 1 : 1 : 1), the adsorptive effect of Hα for oX liquid was close to 100%. The selectivity was owing to the stability of crystal structures loaded with different xylene isomers. There were multiple non-covalent interactions between H and oX molecules, including C–H⋯O and C–H⋯π interactions, which ultimately led to the formation of a stable host–guest complex oX@H. Besides, the Gibbs free energies of Hα after adsorbing xylene isomers showed that the adsorption energy of Hα for oX was the lowest, further confirming the selectivity of Hα for oX. Importantly, Hα could return to the original guest-free state after releasing the adsorbed oX molecule under heating. The selectivity of Hα did not decrease significantly after 5 cycles, demonstrating its outstanding recycling performance and application prospects.

Fig. 1 Schematic illustration of vapor–solid and liquid–solid adsorption of Hα for xylene isomers.

Results and discussion

Preparation and characterization of NACs

The synthesis and activation of NACs based on hybrid[3]arene H (Hα) were performed using a previous method.⁶³Hα was characterized by ¹H NMR, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) (Fig. S1–S3, ESI†). The PXRD pattern demonstrated that Hα was crystalline. The N₂ adsorption experiment showed that Hα was nonporous, with a Brunauer–Emmett–Teller surface area of 0.900 m² g⁻¹.

Single-component adsorption experiments

The adsorption capabilities of Hα to xylene isomers vapors were investigated through single-component vapor–solid adsorption experiments. It was notable that the adsorption amount of oX or mX vapor by Hα increased with time, while the adsorption amount of pX vapor by Hα remained low (Fig. 2a). It took about 10 h to reach the saturation point after adsorption of oX or mX vapor by Hα. Moreover, ¹H NMR further verified the adsorption capabilities of Hα for single-component xylene isomers (Fig. S4–S9, ESI†). The results showed that the adsorption amounts of oX and mX vapors were 0.66 mol/Hα and 0.56 mol/Hα, respectively, but the adsorption amount of pX vapor was less than 0.1 mol/Hα.

Fig. 2 Characterization of Hα after adsorbing single-component xylene isomers. (a) Time-dependent vapor–solid adsorption plots of Hα for single-component xylene isomers vapors. (b) PXRD patterns of: (I) origin Hα; (II) Hα after adsorption of oX vapor; (III) Hα after adsorption of mX vapor; (IV) Hα after adsorption of pX vapor.

In addition, TGA experiments were implemented to investigate the weight loss of Hα after adsorption of single-component xylene isomers. The result showed that the weight loss of Hα after adsorption of oX vapor was 13.92% at about 150 °C, indicating that each Hα molecule adsorbed 0.9 oX molecule (Fig. S10, ESI†). Similarly, after adsorption of mX vapor by Hα, there was a weight loss of 11.75% at 150 °C, showing that 0.8 mX molecule was adsorbed by each Hα molecule (Fig. S11, ESI†). However, there was almost no weight loss of Hα after adsorption of pX vapor, demonstrating that Hα hardly adsorbed pX vapor (Fig. S12, ESI†). These results indicated that Hα could capture oX and mX vapors of single-component xylene isomers.

Then PXRD experiments were performed to explore the structural changes of Hα after adsorption of xylene isomers vapors (Fig. 2b). The PXRD pattern of Hα adsorbed oX or mX vapor was different from that of the original Hα, indicating the formation of new crystalline structures. However, when Hα was exposed to pX vapor, the PXRD pattern of Hα did not change significantly. These results were consistent with ¹H NMR and TGA, demonstrating that Hα could adsorb single-component oX or mX vapor from xylene isomers.

Single crystal structural analysis of host–guest complexes

To reveal the interaction between H and xylene isomers, single crystals of H loaded with xylene isomers were obtained by slow evaporation method. In the crystal structure of oX@H, the adsorbed oX molecule was located, not in the cavity of H, but in the channel formed by H, which was stabilized by the C–H⋯O interaction between hydrogen atoms on the methyl groups of oX and the oxygen atom of H (distance: C–H⋯O: 2.855 Å) (Fig. 3c). Besides, driven by the C–H⋯π interaction, the hydrogen atom on the methyl group of H was perpendicular to the benzene ring of oX (distance: C–H⋯π: 2.284 Å) (Fig. S13, ESI†). The PXRD pattern of Hα after adsorption of oX matched well with the simulated pattern based on the single crystal of oX@H, indicating that the structural transformation from Hα to oX@H (Fig. S17, ESI†). In the crystal structure of mX@H, H became distorted and formed a 1 : 1 host–guest complex with the mX molecule. Driven by C–H⋯O and C–H⋯π interactions, the mX molecule was located in the channel and sandwiched between H molecules (distances: C–H⋯O: 2.659 Å, C–H⋯π: 2.828 Å) (Fig. 3f). The PXRD pattern of Hα after adsorption of mX was consistent with that simulated from single crystal of mX@H, demonstrating a structural transformation from Hα to mX@H (Fig. S18, ESI†). The structure of pX@H was optimized with density functional theory (DFT) calculations (Fig. S19, ESI†). Similar to the structures of oX@H and mX@H, the pX molecule was sandwiched by two H molecules, forming a host–guest complex.

Fig. 3 Single crystal structure of oX@H at different views. (a) Along the a axis. (b) Along the b axis. (c) Illustration of C–H⋯O and C–H⋯π interactions between H and oX (A = 2.855 Å, B = 2.284 Å). Single crystal structure of mX@H at different views. (d) Along the a axis; (e) along the b axis. (f) Illustration of C–H⋯O and C–H⋯π interactions between H and mX (C = 2.659 Å, D = 2.828 Å).

According to DFT calculations, the visual study of weak intermolecular interactions of oX@H and mX@H showed that these host–guest complexes were stabilized by strong C–H⋯π and C–H⋯O interactions, further indicating the great adsorption capabilities of Hα for oX and mX (Fig. 4a and b). Electrostatic potential maps of oX@H and mX@H showed that the electron-deficient methyl groups of H pointed to the electron-rich benzene rings of oX and mX, driven by C–H⋯π interactions (Fig. 4c and d). The results confirmed that the great capabilities of Hα not only arose from the stability of the newly formed crystal structures after adsorption of guest molecules, but also came from the strong intermolecular interactions between H and oX or mX.

Fig. 4 Visual study of weak intermolecular interactions and electrostatic potential maps between H and xylene isomers. (a) Visual study of weak intermolecular interactions of oX@H. (b) Visual study of weak intermolecular interactions of mX@H. (c) Electrostatic potential maps of oX@H. (d) Electrostatic potential maps of mX@H.

Selectivity analysis of NACs for xylene isomers

Inspired by the great adsorption capabilities of Hα for oX and mX vapors during single-component adsorption experiments, we conducted an investigation into the selective adsorption capabilities of Hα in the binary vapor mixture (oX : mX or oX : pX or mX : pX, v : v = 1 : 1). For the vapor mixture of oX and mX, time-dependent vapor–solid adsorption experiments showed that the adsorption amount of oX vapor by Hα increased with time and reached the saturated point after 10 h (Fig. 5a). The adsorption amount of oX vapor was nearly 0.7 mol/Hα, while the adsorption amount of mX vapor was less than 0.1 mol/Hα, showing that Hα had selectivity for oX vapor from the vapor mixture of oX and mX (Fig. S20 and S21, ESI†). The PXRD pattern of Hα after adsorption of the vapor mixture of oX and mX was consistent with the pattern after adsorption of single-component oX vapor and the simulated pattern based on single crystal of oX@H (Fig. 5b and Fig. S22, ESI†). These results indicated that Hα could selectively adsorb oX vapor from the vapor mixture of oX and mX. Head space gas chromatography revealed that the purity of oX vapor adsorbed by Hα was 97.49%, proving the excellent selectivity of Hα towards oX vapor (Fig. 5c and Fig. S23, ESI†).

Fig. 5 Characterization of Hα after adsorbing vapor binary mixture of xylene isomers (oX : mX or oX : pX, v : v = 1 : 1). (a) Time-dependent vapor–solid adsorption plots of Hα for the vapor mixture of oX and mX. (b) PXRD patterns of: (I) original Hα; (II) Hα after adsorption of oX vapor; (III) Hα after adsorption of mX vapor; (IV) Hα after adsorption of the vapor mixture of oX and mX. (c) Relative uptakes of oX and mX vapors by Hα after 12 h as measured by head space gas chromatography. (d) Time-dependent vapor–solid adsorption plots of Hα for the vapor mixture of oX and pX. (e) PXRD patterns of: (I) original Hα; (II) Hα after adsorption of oX vapor; (III) Hα after adsorption of pX vapor; (IV) Hα after adsorption of the vapor mixture of oX and pX. (f) Relative uptakes of oX and pX vapors by Hα after 12 h as measured by head space gas chromatography.

Similarly, after adsorption of the vapor mixture of oX and pX, it was observed that the adsorption amount of oX vapor by Hα increased over time and eventually reached the saturation point after 12 h. But the adsorption amount of pX vapor by Hα was less than 0.1 mol/Hα, confirming that Hα exhibited selectivity for oX vapor from the vapor mixture of oX and pX (Fig. 5d and Fig. S24, S25, ESI†). The PXRD pattern of Hα after adsorbing the vapor mixture of oX and pX matched well with the pattern after adsorption of the single-component oX vapor and the simulated pattern based on single crystal of oX@H (Fig. 5e and Fig. S26, ESI†). Furthermore, head space gas chromatography showed that the purity of oX vapor adsorbed by Hα was 97.61% (Fig. 5f and Fig. S27, ESI†). The results indicated that Hα could serve as an adsorbent to selectively adsorb oX vapor from the binary vapor mixture of oX and pX. Besides, the selectivity of Hα for the vapor mixture of mX and pX was investigated. The adsorption amounts of mX and pX vapors were 0.13 mol/Hα and 0.1 mol/Hα, respectively (Fig. S28, ESI†). The results suggested that Hα exhibited insignificant adsorption capability and selectivity towards the vapor mixture of mX and pX.

Considering the favorable selectivity of Hα towards oX vapor in the binary vapor mixture, we further investigated the selectivity of Hα for the ternary vapor mixture (oX : mX : pX, v : v : v = 1 : 1 : 1). The adsorption of oX vapor by Hα reached the saturation point after 12 h, with the adsorption amount of 0.65 mol/Hα, as demonstrated by time-dependent vapor–solid experiments (Fig. 6a and Fig. S29, ESI†). The adsorption amounts of mX and pX vapors were less than 0.1 mol/Hα, which could be negligible in comparison to that of oX vapor. These results indicated that Hα exhibited great selectivity for oX vapor from the ternary vapor mixture. The PXRD pattern of Hα changed completely after adsorption of the ternary vapor mixture, which was in good agreement with that of adsorption of oX vapor (Fig. 6b and Fig. S30, ESI†). Besides, the PXRD pattern after adsorbing the ternary vapor mixture was similar to the simulated pattern based on the single crystal of oX@H. Moreover, it was found that the purity of oX vapor adsorbed by Hα was 90.9% (Fig. S32, ESI†). These results confirmed that Hα showed remarkable selectivity for oX vapor from the ternary vapor mixture.

Fig. 6 Visual study of weak intermolecular interactions and electrostatic potential maps between H and xylene isomers. (a) Time-dependent vapor–solid adsorption plots of Hα for the ternary vapor mixture (oX : mX : pX, v : v : v = 1 : 1 : 1). (b) PXRD patterns of: (I) original Hα; (II) Hα after adsorption of the ternary vapor mixture of xylene isomers; (III) Hα after adsorption of single-component oX vapor; (IV) simulated from the single crystal structure of oX@H. (c) Relative uptakes of oX, mX and pX vapors by Hα after 5 cycles. (d) PXRD patterns of: (I) original Hα; (II) Hα after adsorption of the ternary vapor mixture of xylene isomers; (III) Hα after 5 adsorption–desorption cycles for the ternary mixture of xylene isomers. (e) Illustration of the desorption process of oX@H using ethanol to dissolve oX.

Cycle performance

Recyclability is a critical standard in industrial practice.⁶⁸ It was found that Hα could be recovered to its original state by adding oX@H into ethanol and dissolving oX molecules. The newly formed Hα retained its selectivity for oX vapor from the vapor mixture of xylene isomers, demonstrating great performance without loss of selectivity after 5 cycles (Fig. 6c). Besides, the PXRD pattern of Hα after 5 adsorption–desorption cycles was consistent with the pattern of original Hα, indicating that the structure of Hα was the same as that of the original Hα (Fig. 6d).

Liquid–solid adsorption experiments

The vapor–solid adsorption of Hα exhibited remarkable efficacy in separating xylene isomers, which led us to inquire whether Hα could also be utilized to separate oX liquid from the liquid mixture. Time-dependent liquid–solid adsorption experiments of Hα for single-component xylene isomers liquids showed that the adsorption amounts of three isomers all increased with time and reached the saturated points after 60 min (Fig. 7a). However, Hα exhibited differences in adsorption amounts for three xylene isomers. The adsorption amounts of oX, mX, and pX liquids by Hα were 0.9, 0.8, and 0.4 mol/Hα, respectively (Fig. S37–S39, ESI†). Single-component liquid–solid adsorption experiments indicated that Hα could effectively adsorb all three xylene isomers.

Fig. 7 Characterization studies of Hα after adsorbing liquid single component and ternary mixture of xylene isomers. (a) Time-dependent liquid–solid adsorption plots of Hα for single-component xylene isomer liquids. (b) Time-dependent liquid–solid adsorption plots of Hα for the ternary liquid mixture (oX/mX/pX, v : v : v = 1 : 1 : 1). (c) PXRD patterns of: (I) original Hα, (II) Hα after adsorption of oX liquid; (III) Hα after adsorption of mX liquid; (IV) Hα after adsorption of pX liquid. (V) Hα after adsorption of the ternary liquid mixture; (d) relative uptakes of oX, mX and pX liquids adsorbed by Hα after 150 min as measured by head space gas chromatography.

Based on the great adsorption capability of Hα in the single-component liquid–solid experiment, we conducted an investigation into the adsorption capability of Hα for the ternary liquid mixture (oX : mX : pX, v : v : v = 1 : 1 : 1). It was found that the adsorption amount of oX liquid increased with time and reached 0.7 mol/Hα after 150 min, while the adsorption amount of mX or pX liquid was less than 0.1 mol/Hα (Fig. 7b and Fig. S40, ESI†). The results demonstrated that Hα showed selectivity for oX liquid from the ternary liquid mixture. The PXRD pattern of Hα after adsorbing the ternary liquid mixture corresponded to that of Hα after adsorption of oX liquid (Fig. 7c and Fig. S41, ESI†), confirming that Hα exhibited selectivity for oX liquid from the ternary liquid mixture with structural transformation from Hα to oX@H. Head space gas chromatography experiment demonstrated that the purity of oX liquid from the ternary liquid mixture was 99.48% (Fig. 7d and Fig. S42, ESI†). Moreover, it was demonstrated that the performance did not decrease obviously after 5 cycles (Fig. S43, ESI†). These findings indicated that Hα exhibited good performance in the liquid–solid adsorption of the xylene ternary liquid mixture.

Theoretical calculations

To investigate the mechanism of Hα selective adsorption of xylene isomers, host–guest complexes structures were built (Fig. 8a–c, ESI†). The Gibbs free energies of Hα, single-component xylene isomers and host–guest complexes were calculated based on Gaussian 09W (Table S4, ESI†). The adsorption energies of Hα for xylene isomers were calculated according to the following formula:

Fig. 8 The optimized structures by DFT calculations and adsorption energies of host, guests and host–guest complexes. (a) oX@H; (b) mX@H; (c) pX@H. (d) Adsorption energies of Hα for oX, mX and pX, respectively.

The results showed that the adsorption energy of Hα for oX was −0.410 kJ mol⁻¹, and that for mX was −0.097 kJ mol⁻¹ (Fig. 8d, ESI†). Therefore, the adsorption of Hα for oX and mX were spontaneous processes. However, the adsorption energy of Hα for pX was 0.905 kJ mol⁻¹, which could not achieve the vapor–solid adsorption. In addition, compared with the adsorption process of mX, the adsorption process of oX was more prone to occur. Therefore, Hα showed a great selectivity for oX from the mixture of xylene isomers.

Conclusions

In summary, we developed a novel approach for the separation of oX from xylene isomers based on NACs of hybrid[3]arene H (Hα). Hα showed an effective selectivity for oX vapor from the vapor mixture of xylene isomers. The purity of oX adsorbed by Hα was 97.49% for the binary vapor mixture of oX and mX, and 97.61% for the binary vapor mixture of oX and pX. Meanwhile, Hα could also separate oX vapor from the ternary vapor mixture of xylene isomers with a purity of 90.22%. More importantly, it was observed that Hα could selectively adsorb oX liquid from the ternary liquid mixture of xylene isomers, with the purity approaching 100%. The high selectivity of Hα was ascribed to the stability of the newly formed crystal structure after adsorption of the favorable guest molecule, oX. Furthermore, the adsorption performance did not decrease after 5 cycles. The excellent recyclability of Hα was attributed to its reversible structural changes between guest-free and guest-contained states. Based on the distinctive structure and excellent separation performance of Hα, it is anticipated that this research will afford a new strategy for the separation of benzene isomers, such as ethyltoluene isomers, trimethylbenzene isomers and other important aromatic isomers. Furthermore, it will provide inspiration for the use of supramolecular host-based NACs in other energy-intensive separation methods.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22101043), the Fundamental Research Funds for the Central Universities (N2205013 and N232410019), the Opening Fund of State Key Laboratory of Heavy Oil Processing (SKLHOP202203006) and Northeastern University.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2214282 and 2214284. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qm01231j

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