Robust Giant Octahedral 6+8 Porous Organic Cages for Efficient Ethylene/Ethane Separation

Open AccessCCS ChemistryRESEARCH ARTICLES20 Feb 2024Robust Giant Octahedral 6+8 Porous Organic Cages for Efficient Ethylene/Ethane Separation Lijuan Feng, Yan-Xi Tan, El-Sayed M. El-Sayed, Fenglei Qiu, Wenjing Wang, Kongzhao Su and Daqiang Yuan Lijuan Feng State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 College of Chemistry, Fuzhou University, Fuzhou 350116 , Yan-Xi Tan State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , El-Sayed M. El-Sayed Chemical Refining Laboratory, Refining Department Egyptian Petroleum Research Institute, Nasr City, Cairo 11727 , Fenglei Qiu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 College of Chemistry, Fuzhou University, Fuzhou 350116 , Wenjing Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Kongzhao Su *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 and Daqiang Yuan *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108 https://doi.org/10.31635/ccschem.024.202303625 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The exploration of ethane (C2H6)-selective porous materials for the direct production of polymer-grade ethylene (C2H4) from a C2H6/C2H4 mixture in a single energy-saving adsorption step is of utmost importance but remains a significant challenge. Thus, developing robust C2H6-selective adsorbents with high C2H6 capacity and C2H6/C2H4 selectivity is urgently needed for industrial applications. In this study, we have successfully designed and synthesized two novel calix4resorcinarene-based porous organic cages (POCs) named CPOC-501 and CPOC-502. The POCs were formed via a Schiff-base reaction involving face-directed 6+8 condensation between a bowl-shaped tetratopic tetraformylcalix4resorcinarene and triangular tritopic amine synthons. Analysis using single crystal X-ray crystallography revealed that both cages possess large truncated octahedral cavities with a volume of approximately 6500 Å3 and 12 accessible rhombic windows with a side length of approximately 10.5 Å. Furthermore, the cages exhibited excellent chemical stability under neutral, acidic, and basic conditions and high Brunauer–Emmett–Teller specific surface areas of up to 2175 m2 g−1 after desolvation. Both POCs demonstrated superior adsorption capabilities for C2H6 over C2H4. Notably, CPOC-502 exhibited a C2H6 capacity and C2H6/C2H4 selectivity of 83 cm3 g−1 and 2.83, respectively, surpassing most of the best-performing C2H6-selective porous organic materials reported to date. Moreover, breakthrough experiments confirmed that both cages efficiently produced polymer-grade C2H4 (>99.9%) directly from the C2H6/C2H4 mixture, highlighting their outstanding recyclability. Download figure Download PowerPoint Introduction Ethylene (C2H4), the primary feedstock in the petrochemical industry, is predominantly produced through steam cracking of petroleum-based feedstocks, resulting in ethane (C2H6) as the major byproduct.1–3 In industrial settings, the traditional method for removing C2H6 from the C2H4/C2H6 mixture involves large distillation towers operating under low-temperature and high-pressure conditions.4 This process consumes a significant amount of energy, making exploring alternative and more efficient C2H4 purification methods imperative. Among the various technologies available, adsorption and separation utilizing porous adsorbents have shown promise due to their low energy consumption. Two types of porous adsorbents are used for C2H6/C2H4 separation: C2H4-selective and C2H6-selective porous materials.5–7 The former relies on introducing open metal sites or highly polar groups into the pores for effective separation,8–10 while the latter involves creating nonpolar/inert-pore surfaces with aromatic or aliphatic moieties.11–13 However, C2H6-selective porous materials often suffer from low capacity and poor selectivity compared to C2H4-selective materials due to their lack of suitable binding sites. Nevertheless, C2H6-selective adsorbents offer the advantage of directly providing C2H4 in a single adsorption step, eliminating the need for an additional energy-intensive desorption step. This simplifies the separation process and can save nearly 40% of energy consumption.14 Thus, there is a strong justification for developing C2H6-selective adsorbents with exceptional C2H6/C2H4 separation performance (high C2H6 capacity and selectivity) and long-lasting durability. Metal–organic frameworks (MOFs) have been extensively investigated for one-step C2H6/C2H4 separation.15–20 However, research on purely porous organic materials (POMs) with a metal-free composition and native nonpolar/inert pore surfaces, which are potential candidates as C2H6-selective adsorbents, is still in its early stages.21–23 Porous organic cages (POCs) are an emerging class of POMs that are constructed by covalently linking purely organic synthons into zero-dimensional (0D) discrete macromolecules with permanent intrinsic cavities.24–27 With their 0D nature, POCs possess inherent advantages such as solution processing, easy regeneration, and precise modification.28–30 In 2009, Cooper et al.31 published the first investigation on tetrahedral 4+4 imine-linked (C=N) POCs for gas sorption, achieving a remarkable Brunauer–Emmett–Teller (BET) surface area of up to 624 m2 g−1. Since then, the design and synthesis of POCs with different condensation modes, shapes, sizes, and functions have seen substantial growth.32–43 However, the number of POCs reported is still much fewer than that of MOFs and covalent organic framework materials.44–49 Despite over a decade of continuous efforts, the reported POCs mainly exhibit BET values of less than 1000 m2 g−1 for the limiting cage size, with the exception of cuboctahedral 8+12 boronic ester-linked (B–O) POC reported by Mastalerz and coworkers,50 which reached a BET value exceeding 3000 m2 g−1. However, increasing the size of organic cages often results in porosity loss or reduction during desolvation, leading to structure collapse and/or improper packing.51,52 In addition, most reported POCs are assembled through dynamic and reversible imine and boronic ester bonds,53–55 which are susceptible to hydrolysis under humid conditions, resulting in skeleton collapse. These challenges significantly limit the scope of POCs' research, highlighting the urgent need to explore large, robust POCs to expand their applications. Calix4resorcinarene (C4RA), which belongs to the calixarene family, possesses an intrinsic electron-rich π cavity and eight polar phenolic groups on its upper rim. Through hydrogen bonding, π···π interactions, and van der Waals forces, C4RA can effectively interact with various guest molecules encompassing both small gases and large organic compounds.56–58 Furthermore, the upper rims of C4RA are easily functionalized with different groups like amine (–NH2), cyano (–CN), carboxyl (–COOH), and aldehyde (–CHO).59 Recently, our research team successfully utilized tetraformyl-functionalized C4RA (C4RACHO) in combination with different diamine/dihydrazide synthons to synthesize 2+4 lanterns, 3+6 triangular prisms, 4+8 square prisms, and impressive 6+12 octahedrons.60,61 These assemblies have shown promising potential in applications ranging from pollutant removal to energy storage and gas separation.62–67 Notably, the aforementioned octahedrons were constructed through edge-directed 6+12 condensation utilizing tetratopic and ditopic linear synthons (Scheme 1a). However, another directional-bonding approach, face-directed 6+8 condensation employing tetratopic and tritopic synthons (Scheme 1b), can also be employed to construct octahedral cages.68,69 With this in mind, we employed 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) tritopic synthons in combination with C4RACHO to synthesize face-directed 6+8 octahedral POCs, namely CPOC-501 and CPOC-502 (Scheme 1c). Both POCs possess a large truncated octahedral cavity with 12 guest-accessible windows and exhibit excellent stability under acidic and basic conditions and high porosity after desolvation. Intriguingly, both cages preferred adsorbing C2H6 over C2H4, as confirmed by breakthrough experiments. These cages directly produced polymer-grade C2H4 from a mixture of C2H6 and C2H4 in a single adsorption step while demonstrating exceptional recyclability. Scheme 1 | (a) The synthetic route of 6+12 and (b) 6+8 octahedral cages; (c) the chemical structures of C4RACHO, TAPB, and TAPT. Download figure Download PowerPoint Experimental Methods Synthesis of CPOC-501 C4RACHO (0.05 mmol, 41 mg) and TAPB (0.067 mmol, 21 mg) were added to o-Xylene (3 mL). The mixture was sealed in a 10 mL glass vial, and put in an oven with a temperature of 100 °C for 72 h without stirring, during which time red block crystals were formed. After removing the glass vial from the oven and allowing it to cool to room temperature, red block crystals were then separated by filtration and washed three times with methanol. The separated crystals were further immersed and exchanged six times every 24 h in methanol before activating at 100 °C under a high vacuum for 12 h to afford CPOC-501 with a yield of about 71%. 1H NMR (400 MHz, CDCl3, 298 K): δ 16.47 (m, 1H), 10.64 (s, 1H), 9.18 (s, 1H), 7.77 (s, 1H), 7.74 (s, 1H), 7.47 (s, 1H), 7.39 (s, 1H), 4.71 (t, 1H), 2.15 (t, 2H), 1.26 (m, 1H), 1.08 (d, 6H). Synthesis of CPOC-502 C4RACHO (0.05 mmol, 41 mg) and TAPT (0.067 mmol, 21 mg) were added to DMA (3 mL). The mixture was sealed in a 10 mL glass vial and kept at 2–8 °C for 15 days without stirring, during which time red block crystals were formed. The crystals were then separated by filtration and washed three times with methanol. The separated crystals were further immersed and exchanged six times every 24 h in methanol before activating at 100 °C under a high vacuum for 12 h to afforded CPOC-501 with a yield of about 65%. 1H NMR (400 MHz, CDCl3, 298 K): δ 16.68 (m, 1H), 11.14 (s, 1H), 9.37 (s, 1H), 8.75 (s, 1H), 7.62 (t, 1H), 7.40 (t, 1H), 4.73 (m, 1H), 2.14 (d, 2H). 1.26 (m, 1H), 1.10 (d, 6H). Results and Discussion Red block crystals of CPOC-501 and CPOC-502 were obtained in yields of 71% and 65%, respectively, using a 24-fold Schiff base reaction of C4RACHO (1 equiv) with (TAPB, 1.33 equiv) and (TAPT, 1.33 equiv). Subsequent single crystallographic X-ray determination (SCXRD) revealed that CPOC-501 (CCDC No. 2222407, Figure 1a) crystallized in a triclinic P-1 space group with half of a 6+8 organic cage in its asymmetric unit. In contrast, CPOC-502 (CCDC No. 2222408, Figure 1b) crystallized in a trigonal R-3 space group with one-sixth of the cage ( Supporting Information Table S1). The asymmetric units of both cages incorporated residual electron density as highly disordered solvent molecules, which accounted for approximately 67.8% of the unit cell volume and were subsequently removed by the routine SQUEEZE function of PLATON.70 The characteristic feature of CPOC-501 and CPOC-502 was the concave-shaped C4RACHO as six nodes in an octahedral arrangement, along with tritopic amine linkers on the triangular facets. Notably, this kind of 6+8 octahedral arrangement has only been reported in coordination cage systems71 and has not yet been observed in organic cages in the Cambridge Structural Database.72 CPOC-501 and CPOC-502 had a void volume of approximately 6500 Å3, calculated using Voidoo,73,74 and an inner diameter (din) of around 3.3 nm. This places them among the largest purely organic cages, such as the cubic 8+12 salicylimine cage with a din of 3.3 nm, the 6+12 octahedral CPOC-303 with a din of 3.9 nm, and the 12+24 porphyrin cages with a din of 4.3 nm.60,75,76 There are 12 accessible rhombic windows with a side length of approximately 10.5 Å in both CPOC-501 and CPOC-502, which are the distances between the centers of C4RA's phenyl ring and those of TAPB and TAPT. The pore window to the cavity has a diameter of approximately 7.5 Å, providing sufficient space for gas molecules to enter the cavity. The solid-state packing of CPOC-501 suggests that each single cage is closely surrounded by six neighboring cages, forming an efficient packing. The cage cavities are connected through the windows, creating open channels along the a, b, and c axes (Figure 1c and Supporting Information Figures S12 and S13). In contrast, a different packing style was observed for CPOC-502 compared to CPOC-501, as they crystallized in different space groups. Specifically, the solid-state packing of CPOC-502 (calculated density = 0.5254 g cm−3) indicated that each single cage was closely surrounded by eight neighboring cages, resulting in a denser packing compared to CPOC-501 (calculated density = 0.5051 g cm−3). Additionally, CPOC-502 exhibited only obvious one-dimensional channels, with the openings being the cage windows in the c direction (Figure 1d and Supporting Information Figures S14 and S15). Figure 1 | X-ray single-crystal structures of (a) CPOC-501 and (b) CPOC-502; solid-state packing of (c) CPOC-501 and (d) CPOC-502 viewed from c direction; hydrogen atoms are omitted for clarity. Carbon is gold, oxygen red, nitrogen blue, and hydrogen lavender. Download figure Download PowerPoint The powder X-ray diffraction patterns of desolvated CPOC-501 and CPOC-502 were found to be consistent with the simulated patterns generated from their single crystal data ( Supporting Information Figures S10 and S11), thus indicating the maintenance of structural shape persistence and framework rigidity. This observation is notable as it is uncommon for large POCs, which often experience a crystallinity loss or collapse upon desolvation.75,77 Interestingly, both CPOCs exhibited the keto-enol tautomerization of imine bonds (C=N) to more chemically stable amine (C–NH) bonds, which was confirmed through Fourier transform infrared (FT-IR) and 1H NMR spectra analyses ( Supporting Information Figures S1–S5). This discovery prompted us to thoroughly investigate the chemical stability of these compounds, as it has yet to be explored in the previously reported C4RA-based POCs. Remarkably, both CPOC-501 and CPOC-502 displayed structural integrity in water, as well as under acid (pH = 1.5) and base (pH = 13.5) conditions, as evidenced through 1H NMR (Figure 2a,b) and FT-IR ( Supporting Information Figures S6 and S7) analyses. These findings align with the exceptional chemical stability observed in 2+3 lantern-shaped POCs assembled from 1,3,5-triformylphloroglucinol (TP) with various analogs of alkanediamine,78 as well as 4+6 octahedral POCs assembled from TP and binaphthylenediamine.79 Notably, all these POCs demonstrated remarkable resistance to degradation in water, acids, and bases. However, it is worth mentioning that their sizes regarding din were all less than 1.4 nm, considerably smaller than that of CPOC-501 and CPOC-502. This observation suggests that the multivalences resulting from the tautomeric transformation of multiple amine bonds provide robust support for the structural integrity of larger POCs. In addition to their chemical stability, CPOC-501 and CPOC-502 exhibited high thermal stability, displaying remarkable structural resilience up to 300 °C under N2 conditions ( Supporting Information Figures S8 and S9). Figure 2 | 1H NMR spectra measured after treatment of (a) CPOC-501 and (b) CPOC-502 under different conditions. Download figure Download PowerPoint The permanent porosity of desolvated CPOC-501 and CPOC-502 was confirmed through N2 gas sorption experiments conducted at 77 K. As shown in Figure 3, the isotherms of both cages exhibit the typical type I adsorption behavior. According to the BET model ( Supporting Information Figures S16 and S17), the surface area of CPOC-501 is 1832 m2 g−1, while that of CPOC-502 is 2175 m2 g−1. Similarly, the Langmuir model ( Supporting Information Figures S18 and S19) estimates the surface areas to be 2221 m2 g−1 for CPOC-501 and 2747 m2 g−1 for CPOC-502. These results demonstrate the substantial influence of the solid-state packing style on the cage surface areas, given the closely matched cage volumes of both CPOCs. The pore-size distribution (PSD) was determined using the density functional theory model. According to Zeo++ software,80 the PSD values for CPOC-501 (∼1.85 nm) and CPOC-502 (∼1.94 nm) correlate with the calculated largest pore diameters of 1.94 and 1.99 nm, respectively. Figure 3 | N2 gas sorption isotherms at 77 K for CPOC-501 and CPOC-502, inset: the calculated PSD of CPOC-501 (red line) and CPOC-502 (blue line). Download figure Download PowerPoint To investigate the C2H6/C2H4 separation performance of the two CPOCs, single-component adsorption isotherms were measured at both 298 and 273 K (Figure 4a and Supporting Information Figures S20 and S21). At 100 kPa, CPOC-501 exhibited C2H6 and C2H4 loadings of 67 and 51 cm3 g−1 at 298 K, and 103 and 84 cm3 g−1 at 273 K. On the other hand, CPOC-502 showed C2H6 and C2H4 loadings of 83 and 61 cm3 g−1 at 298 K, and 121 and 93 cm3 g−1 at 273 K. It is evident that both cages exhibited higher C2H6 capacities compared to C2H4 at both temperatures, indicating a stronger affinity of the C2H6 guest with the cage hosts. This observation was further confirmed by calculating the isosteric heat of adsorption (Qst) using a virial equation based on the adsorption isotherms at different temperatures (Figure 4b and Supporting Information Figures S22–S25). The calculated Qst values for C2H6 in CPOC-501 and CPOC-502 were found to be 28.0–19.7 and 29.2–21.6 kJ mol−1, respectively, which were higher than those of C2H4 in CPOC-501 (25.8–19.3 kJ mol−1) and CPOC-502 (27.4–21.0 kJ mol−1). These Qst results indicate that CPOC-501 and CPOC-502 have stronger interactions with C2H6 compared to C2H4. The selectivities of CPOC-501 and CPOC-502 were evaluated using the ideal adsorbed solution theory (IAST).81 The results show that the C2H6/C2H4 selectivities at a 1:1 ratio were as high as 2.68 for CPOC-501 and 2.83 for CPOC-502 at 298 K and zero coverage, which are among the highest in reported POMs (Figure 4c,d),22,23,62,82–88 although still somewhat lower than benchmark MOFs such as Fe(O2)(dobdc),15 NKMOF-8,89 IRMOF-8,90 and CPM-733,91 which have selectivities exceeding 4.0. Notably, both C2H6/C2H4 selectivity and C2H6 uptake capacity are important factors in C2H6-selective materials. CPOC-502 displayed an exceptional balance of very high C2H6/C2H4 selectivity and C2H6 adsorption from C2H6/C2H4 mixtures, making it one of the top-performing C2H6-selective materials based on these criteria (Figure 4d).22,92 Overall, these results indicate that CPOC-502 is among the best-performing C2H6-selective POMs. Furthermore, the higher C2H6 capacity and C2H6/C2H4 selectivity of CPOC-502 make it more effective in separating C2H6/C2H4 than CPOC-501. Therefore, the separation potentials (Δq),93 a comprehensive index that combines capacity and selectivity, were used to evaluate the separation effectiveness of the adsorbents. The calculated Δq values for CPOC-501 and CPOC-502 were determined to be 0.57 and 0.78 mmol g−1, respectively, which align well with our expectations ( Supporting Information Figure S26). Figure 4 | (a) C2H6 and C2H4 sorption isotherms at 298 K; (b) Qst values of C2H6 and C2H4; (c) C2H6/C2H4 IAST selectivity; (d) comparison of C2H6 uptakes and C2H6/C2H4 selectivity with different POMs at 298 K and zero coverage; comparison of preferential (e) C2H6 and (f) C2H4 adsorption sites in CPOC-502. Carbon is gold, oxygen red, and hydrogen lavender. Dashed bonds highlight C–H··· π interactions. Download figure Download PowerPoint To gain into the preferential adsorption of C2H6 over C2H4, we conducted using the methods in the These to the specific of C2H6 and C2H4 adsorption on CPOC-501 and CPOC-502. the cage were generated from single crystal data and after which C2H6 and C2H4 were into the cavities for further The resulting structures of CPOC-501 and CPOC-502, along with the binding are shown in Figure and Supporting Information Figure only one adsorption was as the sites CPOC-501 and CPOC-502 are findings indicate that the calculated primary adsorption sites for both C2H6 and C2H4 gases were at the consistent with our the binding we observed values of and kJ for and and kJ for C2H4, in CPOC-501 and CPOC-502, respectively, as calculated by These binding energy results stronger interactions between C2H6 and compared to C2H4, which well with our This can be in to the of which possesses more hydrogen atoms the cavities in a than the C2H4 Specifically, we hydrogen bonds for only eight were for C2H4. the interactions between C2H6 and are stronger than those involving C2H4 The breakthrough experiments were conducted in CPOC-501 and CPOC-502 using a with a to evaluate their separation the breakthrough the were using a vacuum for 3 by into a and further at 100 °C for 12 h to removal of guest solvent As shown in Figure the breakthrough results demonstrate the effective separation of an C2H6/C2H4 mixture at 298 K with a of mL using both Specifically, for CPOC-501, the C2H4 gas was first a time of resulting in an of gas with a exceeding In contrast, the breakthrough of C2H6 at due to stronger interactions between C2H6 and the cage Figure | (a) Experimental breakthrough for mixture of C2H6/C2H4 at 298 K and one for CPOC-501 and CPOC-502; (b) the of CPOC-502 under multiple gas breakthrough Download figure Download PowerPoint Similarly, CPOC-502 exhibited separation with a time of for C2H4 and for The calculated breakthrough time for CPOC-502 is which is higher than that of CPOC-501 The breakthrough experiments indicate that the C2H6/C2H4 separation performance of CPOC-502 is superior to that of CPOC-501, which is consistent with the Δq This also the significant by the solid-state packing of cages in gas separation In to the separation performance of CPOC-501 and CPOC-502 for industrial multiple C2H6/C2H4 dynamic breakthrough experiments were each the were in in the using with a of mL at 100 °C for 12 The breakthrough experiments revealed that both cages their separation performance as evidenced by the breakthrough observed over six shown in Figure and Supporting Information Figure These findings that CPOC-501 and CPOC-502 are robust to as promising adsorbents for C2H6/C2H4 In the directional-bonding has the design and synthesis of two novel 6+8 octahedral POCs, namely CPOC-501 and CPOC-502. C4RACHO was utilized as the and two different tritopic TAPB and were employed to these Both CPOC-501 and CPOC-502 were by cavity diameters of up to 3.3 and cavity volumes of approximately 6500 These them among the largest POCs reported to date. their crystal structures the first of 6+8 POCs determined by X-ray The exceptional chemical stability exhibited by these cages in various acidic, and basic conditions can be to the multivalences resulting from multiple amine bonds via Notably, CPOC-501 and CPOC-502 an impressive surface area of up to 2175 m2 g−1 and exhibit a for adsorbing C2H6 over C2H4. The C2H6/C2H4 selectivities observed in these cages remarkable values of up to 2.83, which one of the highest in reported POMs. Moreover, we have conducted breakthrough experiments to the performance of these POCs. The results demonstrate the of CPOC-501 and CPOC-502 to effectively C2H4 from C2H6/C2H4 gas in a single adsorption step, C2H4 Furthermore, these POCs exhibit excellent their separation for up to six without Moreover, the CPOCs can be easily under reaction conditions and separated easily in considerably high This suggests that typical yields of CPOCs in can be and they can be easily up for industrial research are POCs using different and functionalized as synthons. These are to yield POCs with capabilities for specific gas further their separation performance for important gas Supporting Information Supporting Information is and synthesis and X-ray data and structure additional and gas adsorption of There is of to Information This was by the Science of and the Innovation and the Science of Fujian of and Chemical to the of to from from and in and M. Wang and Synthesis of for