Robust Giant Octahedral [6+8] Porous Organic Cages for Efficient Ethylene/Ethane Separation
2024; Chinese Chemical Society; Volume: 6; Issue: 9 Linguagem: Inglês
10.31635/ccschem.024.202303625
ISSN2096-5745
AutoresLijuan Feng, Yan‐Xi Tan, El-Sayed M. El-Sayed, Fenglei Qiu, Wenjing Wang, Kongzhao Su, Daqiang Yuan,
Tópico(s)Gas Sensing Nanomaterials and Sensors
ResumoOpen 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 calix[4]resorcinarene-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 tetraformylcalix[4]resorcinarene 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. Calix[4]resorcinarene (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 deeper insights into the preferential adsorption of C2H6 over C2H4, we conducted modeling studies using the GFN1-xTB semiempirical extended tight-binding quantum chemistry methods implemented in the CP2K program.94,95 These studies aimed to uncover the specific details of C2H6 and C2H4 adsorption on CPOC-501 and CPOC-502. Initially, the cage hosts were generated from single crystal data and optimized, after which C2H6 and C2H4 were loaded into the cavities for further optimization. The resulting gas-loaded structures of CPOC-501 and CPOC-502, along with the lowest-energy binding configurations calculated, are shown in Figure 4e,f and Supporting Information Figure S27. For clarity, only one adsorption site was added, as the remaining five sites within CPOC-501 and CPOC-502 are identical. Our findings indicate that the calculated primary adsorption sites for both C2H6 and C2H4 gases were located at the CR4A cavities, consistent with our previous study.62 Regarding the static binding energies, we observed values of −37.4 and −33.4 kJ mol−1 for C2H6, and −36.7 and −32.5 kJ mol−1 for C2H4, in CPOC-501 and CPOC-502, respectively, as calculated by CP2K. These binding energy results suggest stronger host–guest interactions between C2H6 and CR4A compared to C2H4, which aligns well with our experimental observations. This can be attributed, in part, to the nonplanar nature of C2H6, which possesses more hydrogen atoms and, thus, better matches the CR4A cavities in a steric sense than the planar C2H4 molecule. Specifically, we identified 11 C–H···π hydrogen bonds for C2H6, whereas only eight were present for C2H4. Consequently, the C–H···π interactions between C2H6 guests and CR4A hosts are stronger than those involving C2H4 guests. The breakthrough experiments were conducted in activated CPOC-501 and CPOC-502 solids-packed columns using a home-built setup equipped with a mass spectrometer to evaluate their separation abilities. Before the breakthrough experiments, the samples were vacuumed using a vacuum pump for 3 h, followed by placement into a stainless-steel tube and further activation at 100 °C for 12 h to ensure complete removal of guest solvent molecules. As shown in Figure 5a, the breakthrough results demonstrate the effective separation of an equimolar C2H6/C2H4 mixture at 298 K with a total flow rate of 2.0 mL min−1 using both cages. Specifically, for CPOC-501, the C2H4 gas was first eluted, reaching a retention time of 24.8 min, resulting in an outflow of pure gas with a desirable purity exceeding 99.9%. In contrast, the breakthrough of C2H6 occurred at 37.6 min due to stronger host–guest interactions between C2H6 and the cage molecules. Figure 5 | (a) Experimental breakthrough curves for equimolar mixture of C2H6/C2H4 at 298 K and one bar for CPOC-501 and CPOC-502; (b) the recyclability of CPOC-502 under multiple mixed gas column breakthrough tests. Download figure Download PowerPoint Similarly, CPOC-502 exhibited similar separation behavior, with a retention time of 38.1 min for C2H4 and 57.1 min for C2H6. The calculated breakthrough time for CPOC-502 is 19.0 min, which is higher than that of CPOC-501 (12.8 min). The actual 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 data. This also highlights the significant role played by the solid-state packing of cages in determining gas separation performance. In order to examine the separation performance of CPOC-501 and CPOC-502 for practical industrial applications, multiple C2H6/C2H4 mixed-gas dynamic breakthrough experiments were conducted. Before each recycling experiment, the samples were regenerated in situ in the column using helium gas, with a total flow rate of 10.0 mL min−1 at 100 °C for 12 h. The cyclic breakthrough experiments revealed that both cages maintained their separation performance well, as evidenced by the almost unchanged breakthrough curves observed over six consecutive cycles (as shown in Figure 5b and Supporting Information Figure S28). These findings suggest that CPOC-501 and CPOC-502 are robust enough to serve as promising adsorbents for C2H6/C2H4 separation. Conclusion In summary, the directional-bonding approach has guided the rational design and synthesis of two novel shape-persistent [6+8] octahedral CR4A-based POCs, namely CPOC-501 and CPOC-502. C4RACHO was utilized as the base-building block, and two different tritopic linkers, TAPB and TAPT, were employed to prepare these cages. Both CPOC-501 and CPOC-502 were characterized by SCXRD analysis, revealing cavity diameters of up to 3.3 nm and cavity volumes of approximately 6500 Å3. These dimensions render them among the largest POCs reported to date. Importantly, their crystal structures represent the first examples of [6+8] POCs determined by X-ray diffraction. The exceptional chemical stability exhibited by these cages in various aqueous, acidic, and basic conditions can be attributed to the multivalences resulting from multiple amine bonds via keto-enamine tautomerization. Notably, CPOC-501 and CPOC-502 display an impressive surface area of up to 2175 m2 g−1 and exhibit a pronounced preference for adsorbing C2H6 over C2H4. The C2H6/C2H4 selectivities observed in these cages reach remarkable values of up to 2.83, which presents one of the highest in reported POMs. Moreover, we have conducted breakthrough experiments to validate the practical performance of these POCs. The results demonstrate the capability of CPOC-501 and CPOC-502 to effectively separate C2H4 from C2H6/C2H4 gas mixtures in a single adsorption step, yielding high-purity C2H4 (>99.9%). Furthermore, these POCs exhibit excellent recyclability, maintaining their separation efficiency for up to six cycles without deterioration. Moreover, the CPOCs can be prepared easily under mild reaction conditions and separated easily in considerably high yields. This suggests that typical yields of CPOCs in grams can be maintained, and they can be easily scaled up for industrial application. Our research efforts are currently directed towards synthesizing POCs using different and functionalized calixarenes as synthons. These modifications are anticipated to yield POCs with enhanced encapsulation capabilities for specific gas guests, thereby further improving their separation performance for industrially important gas mixtures. Supporting Information Supporting Information is available and includes general information, synthesis and characterization, X-ray data collections and structure determinations, additional figures, and gas adsorption measurements. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the National Nature Science Foundation of China (grant nos. 22071244 and 22275191), the Youth Innovation Promotion Association CAS (grant no. 2022305), and the Natural Science Foundation of Fujian Province of China (grant nos. 2022J01503, 2020J05087, and 2022I0037). References 1. Sholl D. S.; Lively R. P.Seven Chemical Separations to Change the World.Nature2016, 532, 435–437. Google Scholar 2. Matar S.; Hatch L. F.Chemistry of Petrochemical Processes, 2nd ed.; Gulf Professional Publishing: Houston, TX, 2001. Google Scholar 3. Corma A.; Corresa E.; Mathieu Y.; Sauvanaud L.; Al-Bogami S.; Al-Ghrami M.; Bourane A.Crude Oil to Chemicals: Light Olefins from Crude Oil.Catal. Sci. Technol.2017, 7, 12–46. Google Scholar 4. Ren T.; Patel M.; Blok K.Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes.Energy2006, 31, 425–451. Google Scholar 5. Barnett B. R.; Gonzalez M. I.; Long J. 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