Highly selective removal of heterocyclic impurities from toluene by nonporous adaptive crystals of perethylated pillar[6]arene

Weijie Zhu, Errui Li, Jiong Zhou, Yujuan Zhou, Xinru Sheng and Feihe Huang*
State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China. E-mail: fhuang@zju.edu.cn; Fax: +86-571-8795-3189; Tel: +86-571-8795-3189

Received 19th May 2020 , Accepted 15th June 2020

First published on 16th June 2020


The removal of heterocyclic impurities from toluene in the petrochemical industry is necessary but challenging. Herein, we provide a convenient and environmentally friendly adsorptive separation strategy using nonporous adaptive crystals of perethylated pillar[6]arene (EtP6). These crystals show a highly selective preference for heterocyclic compounds, and are capable of selectively adsorbing them from a mixture of toluene and heterocyclic compounds, ultimately improving the purity of toluene from 96.78% to 99.00%. Single crystal structures indicate that the selectivity derives from the differences in the structural stability of the guest-loaded EtP6 crystals.


Introduction

Aromatic hydrocarbons including benzene, toluene, ethylbenzene and xylene isomers (referred to as BTEX) are the most common organic chemical raw materials in the petrochemical industry, and are widely used in many fields such as pesticides, plastics and fibers.1 As an important member of BTEX, toluene (Tol) can not only be used as a solvent, fuel additive and dealkylation reactant to produce benzene, but can also be used to synthesize important chemical products like dyes, flavors, explosives and pharmaceutical intermediates.2 In industry, toluene is obtained mainly from coal and petroleum through catalytic reforming, hydrocarbon pyrolysis and coking.3 However, there are always small amounts of heterocyclic compounds remaining, including pyridine (Py), 2-methylthiophene (2-MTP), and 3-methylthiophene (3-MTP), which seriously affect the odour and acid wash color of toluene. Therefore, the removal of these heterocyclic compounds to obtain high-purity toluene is very important.

These heterocyclic compounds have very similar boiling points to toluene (Table S1, ESI), making it difficult to separate them by conventional distillation. The common methods of removing heterocyclic compounds in industry are hydrogenation and extractive distillation.4 However, such methods have some disadvantages that cannot be ignored, like large investment, high energy consumption, environmental damage, and the inability to recover high-value-added pyridine and methylthiophene (MTP) compounds. The adsorption of toluene and heterocyclic compounds by porous materials is a potential separation strategy. Although the purification of toluene has been achieved by zeolite molecular sieves,5 there are still some defects that deserve attention in practical applications. For example, the selectivity and cycling performance of molecular sieves for separation are unsatisfactory, which limits their practical application.6 Hence, there is an urgent need, but still a significant challenge, to develop an adsorbent with high selectivity and good cycling performance for purifying toluene.

As a new class of supramolecular hosts, pillararenes have been vigorously researched over the past decade.7 Owing to their highly symmetrical shapes and electron-rich cavities, they have been applied in a variety of fields, such as molecular recognition, fluorescent sensors, drug delivery and cancer therapy.7,8 Recently, our group put forward the concept of nonporous adaptive crystals (NACs).9 These crystals have been demonstrated as a new kind of highly efficient adsorption and separation material for hydrocarbons.9,10 In this article, for the first time, we used pillararene-based NACs to achieve the purification of toluene by removing trace heterocyclic compounds. It was found that NACs of perethylated pillar[6]arene (EtP6) were capable of isolating trace heterocyclic compounds (Py, 2-MTP and 3-MTP) from toluene with high selectivity (Fig. 1). Structural analyses of single crystals demonstrated that the selectivity was derived from the different structural stabilities of EtP6 after the uptake of various guests. Interestingly, after removing the guests, the structure of EtP6 transformed back to the original guest-free state, indicating that EtP6 crystals can be recycled without degradation.


image file: d0qm00334d-f1.tif
Fig. 1 Chemical structures and cartoon representations: (a) EtP6; (b) Tol, Py, 2-MTP, and 3-MTP.

Results and discussion

Single-component adsorption experiments of EtP6β

To use EtP6 as an adsorptive separation material, we first prepared guest-free EtP6 crystals (EtP6β) according to previous reports,9e which were proved to be crystalline and nonporous due to their densely packed arrangement (Fig. S1–S3, ESI). In spite of this nonporous nature, we tested the vapor adsorption possibility of EtP6β. As shown in Fig. 2a, the single-component time-dependent solid–vapor sorption experiments of Tol, Py, 2-MTP and 3-MTP demonstrated their adsorption capacity. After 2 hours, the adsorbed Tol and Py vapors reached the saturation points, while it took about 3 hours for 2-MTP and 3-MTP. As calculated from the 1H NMR spectra, the amount of toluene adsorbed by EtP6β was 1.0 mol Tol/mol EtP6 (Fig. S4, ESI). Interestingly, the amounts of the three heterocyclic compounds adsorbed by EtP6β were 2-fold the amount of toluene adsorbed (Fig. S5–S7, ESI). These adsorption amounts were confirmed by thermogravimetric analysis (TGA) (Fig. S8–S11, ESI). To better understand the adsorption process, powder X-ray diffraction (PXRD) experiments were performed. As can be seen in Fig. 2b, the PXRD patterns of EtP6β after adsorbing four components separately were different from that of the original EtP6β, which indicated the formation of new guest-loaded crystalline structures. It is worthwhile mentioning that the PXRD patterns after adsorption of 2-MTP and 3-MTP (Fig. 2b, IV and V) were similar.
image file: d0qm00334d-f2.tif
Fig. 2 (a) Time-dependent EtP6β solid–vapor sorption plot for single-component vapors; Tol (orange squares), Py (blue circles), 2-MTP (magenta triangles) and 3-MTP (olive diamonds). (b) PXRD patterns of EtP6: (I) original EtP6β; (II) after adsorption of Tol vapor; (III) after adsorption of Py vapor; (IV) after adsorption of 2-MTP vapor; (V) after adsorption of 3-MTP vapor.

Structural analysis of EtP6 loaded with toluene and heterocyclic compounds

Several reports concluded that hydrogen bonds, CH⋯π interactions and π–π stacking interactions are the main driving forces that stabilize the hydrocarbon guests in EtP6.9a–m In order to reveal the new structures and explore the adsorption mechanism, single crystals of EtP6 loaded with heterocyclic compounds were obtained and characterized by X-ray crystallography. For the Py-loaded EtP6 single crystal structure (Fig. 3a), two Py molecules occupy the cavity and are parallel to each other (Fig. 3a, middle), stabilized by π–π stacking interactions between the two guests (Fig. S12, ESI). The EtP6 molecules assemble into honeycomb-like infinite edge-to-edge 1D channels due to the hexagonal structure of EtP6 and the window-to-window packing mode (Fig. 3a, right). Besides, the PXRD pattern of EtP6β upon adsorption of Py resembled that simulated from (Py)2@EtP6, which indicated the crystal structure transformation from EtP6β to (Py)2@EtP6 upon capturing the Py guest (Fig. S13, ESI). However, in the crystal structures of 2-MTP-loaded EtP6 ((2-MTP)3@EtP6) and 3-MTP-loaded EtP6 ((3-MTP)3@EtP6) (Fig. 3b–e), two MTP molecules are located in the cavity (Fig. 3b and c, middle) and the main driving forces are hydrogen bonds and CH⋯π interactions (Fig. S15 and S18, ESI). Meanwhile another MTP molecule lies in the channel formed by two adjacent EtP6 molecules (Fig. 3d, e and Fig. S14, S17, ESI), having negligible interaction with the hosts. However, in the solid–vapor sorption experiments, the results of 1H NMR spectra and TGA showed that 1 mol EtP6 could only adsorb 2 mol MTP (Fig. S6, S7, S10 and S11, ESI). The reason for the different results is that prior to NMR and TGA analyses we removed surface-physically adsorbed species by heating (see the details in the ESI). As a consequence, the MTP molecules located in the channels were easily desorbed, but the crystal structures did not change. Similarly, EtP6 molecules formed honeycomb-like edge-to-edge 1D channels (Fig. 3b and c, right) and the PXRD results of EtP6β after absorption of MTP were consistent with the calculated patterns of their corresponding single crystal structures (Fig. S16 and S19, ESI). It also indicated that the uptake of 2-MTP or 3-MTP vapor transformed EtP6β to (2-MTP)3@EtP6 or (3-MTP)3@EtP6, respectively. According to a previous report,9g in the Tol-loaded EtP6 single crystal structure (Fig. 3f), the cavity of EtP6 was occupied by a Tol molecule and π–π stacking interactions was the main driving force. Meanwhile, infinite angle-to-angle 1D channels in Tol@EtP6 resulted from the window-to-window packing of EtP6 molecules (Fig. 3f, right).
image file: d0qm00334d-f3.tif
Fig. 3 Single crystal structures: (a) (Py)2@EtP6; (b and d) (2-MTP)3@EtP6; (c and e) (3-MTP)3@EtP6; (f) Tol@EtP6.9g

The selectivity studies of EtP6 for binary mixtures

Considering the adsorption capacity and the different packing modes of toluene and heterocyclic compounds loaded in the EtP6 crystal structures, we investigated whether EtP6β had the ability to distinguish the two-component mixtures of toluene/heterocyclic compounds. Time-dependent EtP6β solid–vapor sorption experiments for three groups of binary mixtures (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) were conducted (Fig. 4a, c and e). As expected, the uptakes of Py, 2-MTP, and 3-MTP in EtP6β were much higher than that of Tol, and the times needed to reach the saturation points of three binary mixtures were all within 3 hours. This suggested the apparent selectivity towards the heterocyclic compounds. According to 1H NMR experiments, the uptakes of Py, 2-MTP and 3-MTP were calculated to be nearly 2 mol per mole EtP6 (Fig. S20, S24 and S28, ESI), which coincided with the results of the above discussed single-component adsorption experiment. Gas chromatography (GC) revealed the percentages of heterocyclic compounds adsorbed in EtP6β (97.7% for Py, 92.8% for 2-MTP, 89.9% for 3-MTP), further confirming the high selectivities towards heterocyclic compounds in EtP6β (Fig. S21, S25 and S29, ESI). Besides, the PXRD pattern of EtP6β upon exposure to the toluene/pyridine mixture was in accordance with that of EtP6β capturing Py vapor and that simulated from (Py)2@EtP6 (Fig. 4b, III, IV and VI). These results implied the crystal structure transformation from EtP6β to (Py)2@EtP6. Similar conclusions were drawn from the adsorption experiments of toluene/methylthiophene mixtures in EtP6β (Fig. 4d and f).
image file: d0qm00334d-f4.tif
Fig. 4 (a) Time-dependent EtP6β solid–vapor sorption plot for Tol/Py mixture vapor. (b) PXRD patterns of EtP6: (I) original EtP6β; (II) after adsorption of Tol vapor; (III) after adsorption of Py vapor; (IV) after adsorption of Tol/Py mixture vapor; (V) simulated from the single crystal structure of Tol@EtP6; (VI) simulated from the single crystal structure of (Py)2@EtP6. (c) Time-dependent EtP6β solid–vapor sorption plot for Tol/2-MTP mixture vapor. (d) PXRD patterns of EtP6: (I) original EtP6β; (II) after adsorption of Tol vapor; (III) after adsorption of 2-MTP vapor; (IV) after adsorption of Tol/2-MTP mixture vapor; (V) simulated from the single crystal structure of Tol@EtP6; (VI) simulated from the single crystal structure of (2-MTP)3@EtP6. (e) Time-dependent EtP6β solid–vapor sorption plot for Tol/3-MTP mixture vapor. (f) PXRD patterns of EtP6: (I) original EtP6β; (II) after adsorption of Tol vapor; (III) after adsorption of 3-MTP vapor; (IV) after adsorption of Tol/3-MTP mixture vapor; (V) simulated from the single crystal structure of Tol@EtP6; (VI) simulated from the single crystal structure of (3-MTP)3@EtP6.

Based on the above results, the selective adsorption ability of EtP6 towards toluene and heterocyclic compounds may be related to the guest-loaded crystal structures. On one hand, EtP6 has stronger host–guest interactions with heterocyclic compounds than toluene (Fig. S12, S15 and S18, ESI), resulting in the formation of more stable host–guest complex crystals. On the other hand, different guest-loaded crystals adopt different packing modes, which also influence the stability of crystals.8 Combining these two factors, it was concluded that the selectivities came from the differences in the structural stabilities of the guest-loaded EtP6 crystals. As a consequence, upon exposure to toluene/heterocyclic compound mixtures, EtP6 preferred to adsorb the heterocyclic compound vapors.

In addition, we performed desorption experiments under vacuum. What surprised us is that the complete removal of guests in heterocyclic compound-loaded EtP6 crystals produced crystals that were confirmed to be the original EtP6β by PXRD and TGA (Fig. S32, S33, S37, S38, S42, and S43, ESI). Moreover, the reproduced crystals were still capable of selectively separating toluene and heterocyclic compounds after 5 cycles without loss of performance (Fig. S34–S36, S39–S41, and S44–S46, ESI).

Toluene purification experiments

In the petrochemical industry, Py, 2-MTP, and 3-MTP often coexist in the production of Tol, and there is no denying that obtaining high purity toluene in a simple, rapid and low energy consumption method is very important. Thus we explored the possibility of purifying toluene in a multi-component mixture through EtP6β. Considering the percentage of toluene and heterocyclic compounds in the crude product,3 we prepared a mixture containing four components with a 96.5[thin space (1/6-em)]:[thin space (1/6-em)]1.5[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 volume ratio of Tol, Py, 2-MTP and 3-MTP, applied EtP6β as an adsorbent, and tested the purities via GC-MS of toluene before and after adsorption. To our delight, after reaching adsorption saturation, the purity of toluene in the mixture was improved from 96.78% to 99.00%, while the percentage of heterocyclic impurities decreased from 3.22% to 1.00% (Fig. 5 and Fig. S47, S48, ESI). Therefore, EtP6β can be used as an effective adsorbent to remove heterocyclic compounds and obtain toluene with high purity.
image file: d0qm00334d-f5.tif
Fig. 5 Schematic representation of the toluene purification by using EtP6β.

Conclusions

In summary, we demonstrated the highly selective separation of toluene and heterocyclic compound mixtures through nonporous adaptive crystals of EtP6β. For three equal-volume binary mixtures, EtP6β exhibited a preference for heterocyclic compounds and was able to adsorb them with high selectivities (Py 97.7%, 2-MTP 92.8%, 3-MTP 89.9%); the selectivities were attributed to the different stabilities of the newly formed EtP6 crystal structures after the uptake of guests. Therefore, in the multi-component mixture, EtP6β was used as an effective adsorbent to remove trace heterocyclic compounds, affording toluene with a purity of 99.00%. Moreover, the EtP6β crystals revealed good cycling performance; they could be used more than five times without degradation because of the reversible transitions between the guest-free and guest-loaded EtP6 structures. Considering the high selectivity in various organic compounds, NACs may have great potential in removing heterocyclic impurities. Future work will focus on improving the purification of other important fine chemicals.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21434005, 91527301) and the fundamental research funds for the central universities.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Thermogravimetric analysis, 1H NMR, powder X-ray diffraction, gas chromatography and crystal data. CCDC 1958820, 1958824 and 1958825. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qm00334d

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