Three-Dimensional Covalent Organic Frameworks Constructed from Irregular Polyhedral Building Blocks

doi:10.21203/rs.3.rs-3333012/v1

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Three-Dimensional Covalent Organic Frameworks Constructed from Irregular Polyhedral Building Blocks

https://doi.org/10.21203/rs.3.rs-3333012/v1

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Developing three-dimensional (3D) covalent organic frameworks (COFs) has paramount significance across numerous applications. However, the conventional design approach that relies on regular building blocks significantly restricts the structural diversity of COFs. In this study, we successfully designed and synthesized two 3D COFs, named JUC-643 and JUC-644, employing a novel strategy based on irregular 8-connected (8-c) building blocks. By using a continuous rotation electron diffraction technique combined with powder X-ray diffraction patterns, their structures were solved and revealed a unique linkage with double helical structure, a phenomenon previously unreported in COFs. In order to precisely describe the topology, these structures should be deconstructed into the unprecedented [4+3(+2)]-c nets instead of the traditional [8(+2)]-c or [6(+2)]-c net. Furthermore, one of the materials (JUC-644) has demonstrated exceptional adsorption capability towards C3H8 and n-C4H10 (11.28 and 10.45 mmol g-1 at 298 K and 1 bar respectively), surpassing the adsorption performance of all known porous materials, and breakthrough experiments have also highlighted the remarkable C3H8/C2H6 and n-C4H10/C2H6 selectivity. This pioneering concept of incorporating irregular building blocks in 3D COFs introduces a promising avenue for designing intricate architectures while enhancing their potential application in the recovery of C2H6 from natural gas liquids.

Physical sciences/Chemistry/Materials chemistry/Metal–organic frameworks

Physical sciences/Chemistry/Polymer chemistry/Conjugated polymers

covalent organic frameworks

irregular building blocks

double links

structural diversity

adsorption and separation

Covalent organic frameworks (COFs) represent a class of crystalline porous materials composed of covalently bonded organic building blocks, which assemble into two-dimensional (2D) or three-dimensional (3D) reticular structures. In the context of topological analysis, these building blocks can be likened to vertices and edges, as documented in early studies^1-5. 2D COFs are characterized by covalent networks extending within 2D layers, resulting in a limited number of predictable and analyzable topologies that are relatively easier to investigate^6-11. On the other hand, 3D COFs offer an extensive range of topological possibilities due to the diverse methods of connection they involve, leading to the formation of pores with varying types and sizes. The pioneering work conducted by Yaghi's research group in 2007¹² initiated the construction of 3D COFs exhibiting various topological features, a milestone that has been followed by the accomplishment of numerous successful efforts^13-16. The distinct porous structure of 3D COFs determines their primary applications, with their respective topologies playing a crucial role in this regard.

As the topological structure of 3D COFs depends on the coordination number and geometrical symmetry of their organic building blocks, rational design of these building blocks is an effective way to create 3D COFs with a particular or novel topology. So far, traditional tetrahedral nodes have been the most prevalent choice for constructing 3D networks (Scheme 1a; see Supplementary Fig. 1 for details)^12,17,18. The success in obtaining 3D COFs with zeolite crb topology (ZOF) and newfound ljh topology are typical cases of designing new topology via adjusting bond angles or steric hindrance of conventional building blocks composed of single organic molecules^19,20. Another more sophisticated method to enrich the connective environment of topological vertices in COFs is to combine several building subunits into a building block. According to this method, building blocks are created by connecting each vertex of a central planar (triangular, rectangular or square) building subunit to an outer triangular building subunit perpendicular to its plane, forming a prismatic building block in D_nh (n=2, 3, 4) or O_h point groups. Up till now, 6-connected (6-c) triangular prismatic building blocks and 8-connected (8-c) cubic or cuboid building blocks have been constructed in this way, forming highly connected 3D COFs with new topologies. However, these new topologies are still limited in types and relatively predictable because all of the current building blocks of COFs are based on planar polygons, regular polyhedrons or triangular prisms with relatively rigid structures and low degrees of configurational freedom (Scheme 1b, 1c and see Supplementary Fig. 1 for details). It is worth mentioning that for such COFs, new topologies are discovered mainly by adopting linkers with different configurations and simply changing building blocks with the same connective polyhedron can hardly produce new topologies. For instance, all of the reported [8(+2)]-c COFs are in bcu or its derived nets^21-24, and all of the reported [8+4]-c COFs are in flu, scu or tty topologies^25-28.

Despite the efforts of topological design, subject to the limited configurational and coordinative variety of the building blocks, the topological variety of COFs is significantly lower than that of metal-organic frameworks (MOFs), where the unique topological phenomenon never observed in COFs may appear. A typical example is that building blocks of MOFs may occasionally be connected by multiple links²⁹. For instance, in snf and fqr topologies, every metal cluster is linked to the six nearest clusters by 12 or 10 linkers, which means partial or all of the clusters are “double-linked” ^30-32. In contrast, all reported COFs are synthesized via bottom-up or self-assembly strategies, a single link connects neighboring building blocks. The phenomenon of multiple links connecting adjacent building blocks has not been previously reported. Designing and combining building blocks in various geometric configurations opens new opportunities for generation of novel 3D COFs, but still remains a great challenge.

Here, we present a novel synthetic strategy based on irregular polyhedral building blocks, resulting in two unique 3D COFs designated as JUC-643 and JUC-644 (JUC refers to Jilin University China). Their structures were determined using a combination of continuous rotation electron diffraction (cRED) technique and powder X-ray diffraction (PXRD). Remarkably, the irregular 8-c building blocks in these structures exhibit unexpected coordination of six, leading to the formation of unconventional "double links". This is the first reported occurrence of such a phenomenon in COF structures. Therefore, to describe the topology precisely, the framework should be deconstructed by splitting the 8-c building block into one 4-c vertex and four 3-c vertices, unveiling an unprecedented [4+3(+2)]-c topology denoted as jca (jca stands for Jilin University China-a) instead of the traditional [8(+2)]-c or [6(+2)]-c topology. JUC-644, in particular, demonstrates a remarkable adsorption capacity for C₃H₈ and n-C₄H₁₀, reaching an exceptional value of 11.28 mmol g^-1 and 10.45 mmol g^-1 at 298 K and 1 bar respectively, which surpasses the previous record among all known porous materials. To further explore its applicability, JUC-644 was subjected to breakthrough experiments for C₂H₆ recovery from simulated natural gas liquids (NGLs), where it exhibited excellent selectivity in terms of C₃H₈/C₂H₆ and n-C₄H₁₀/C₂H₆ ratios.

Structural design

In contrast to conventional approaches that rely on regular building blocks, our strategy introduces irregular polyhedral building blocks by combining multiple organic subunits, wherein at least one subunit assumes a 3D coordination environment (Scheme 1d-g). As an initial implementation of this strategy, we assemble four 3-c triangular outer subunits (C_2v symmetry) with a 4-c 3D regular or twisted tetrahedral central subunit (T_d or D_2d symmetry), resulting in an unconventional 8-c building unit with D_2dsymmetry and high degrees of configurational flexibility. Importantly, the topologies constructed from these D_2d building blocks are inherently unpredictable and have likely not been reported in prior literature. In line with this concept, we design two unique building blocks, namely tetra[(3’’,5’’-diformylphenyl)phenyl] methane (TDFPM, Scheme 2a) and 3,3’,5,5’-tetra[(3’’,5’’-diformylphenyl)-bimesitylene (TDFBM, Scheme 2b). In these designs, the sp³-hybridized carbon atoms and the bimesityl groups naturally serve as regular and twisted tetrahedral central subunits respectively, while the (3’,5’-diformylphenyl) groups act as triangular outer subunits. These distinctive 8-c building blocks are subsequently condensed with p-phenylenediamine (PDA, Scheme 2c) as a linear linker to construct two novel 3D COFs, namely JUC-643 and JUC-644 (Scheme 2d-h), possessing unique structural characteristics and topology.

Synthesis and characterization

Typically, these building blocks (PDA + TDFPM/TDFBM) were dispersed in 1,4-dioxane in the presence of acetic acid, and were then heated under different reaction conditions to build JUC-643 and JUC-644, respectively (see Section S2 for details). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the morphology of multichip for JUC-643 and cuboid for JUC-644 (Figure 1a and Supplementary Figs. 2-5). The Fourier transform infrared (FT-IR) spectra showed the appearance of new C=N stretching bands (1626 cm⁻¹ for JUC-643 and 1624 cm⁻¹ for JUC-644) and the disappearance of C=O (1702 cm⁻¹ for TDFPM and 1706 cm⁻¹ for TDFBM) and N−H (3324 cm⁻¹ for PDA) stretching bands, manifesting the successful polymerization of aldehyde and amine (Supplementary Figs. 6 and 7). In addition, the peaks at 161 ppm for JUC-643 and 158 ppm for JUC-644 in the solid-state ¹³C cross-polarization magic-angle-spinning (CP/MAS) nuclear magnetic resonance (NMR) showed the presence of carbons from the imine groups (Supplementary Figs. 8 and 9). The thermogravimetric analysis (TGA, Supplementary Figs. 10 and 11) and powder X-ray diffraction (PXRD) patterns (Supplementary Figs. 12-15) revealed that both 3D COFs were stable under high temperatures (>400 °C) and in various organic solvents and acid/base aqueous solutions. The PXRD patterns of JUC-643 and JUC-644 indicate that a new crystalline phase has been formed (Supplementary Figs. 16 and 18).

Structural determination

According to PXRD patterns and electronic micrographs taken by SEM and TEM, JUC-644 exhibits significantly higher crystallinity than JUC-643. Therefore, the structure of JUC-644 was subjected to a detailed analysis. We used cRED technique to determine the structure of JUC-644. Sixteen individual datasets with a resolution up to 1.5 Å were collected at 99 K and the 3D reciprocal space was preliminarily reconstructed using the software REDp³³. Unit cell of JUC-644 was firstly determined by cRED data (Figure 1c) and the reflection conditions were determined as: 0kl: l=2n; h0l: l=2n; hk0: h=2n from the 2D slices cut from the 3D reciprocal lattice, supporting the inference that space group should be Pcca. The unit cell (a = 31.669(6) Å, b = 17.376(3) Å, c = 25.285(4) Å, α = β = γ = 90°) and space group was confirmed by Pawley Refinement of PXRD data with software TOPAS 5³⁴ with R_wp of 5.71% and R_p of 4.13%. The cRED datasets were then processed using X-ray Detector Software (XDS)³⁵ and six sets of them were merged by the python program package edtools³⁶ to improve the data completeness. Finally, we obtained a *.hkl peaks list and fractional coordinates of all non-hydrogen atoms were determined with software SHELXT³⁷. This is the lowest reported resolution of cRED data for successful structural determination using the ab initio methods. The final crystal structure model was obtained by the Rietveld refinement with TOPAS 5, with R_wp of 9.26% and R_p of 7.20% (Figure 1b and Supplementary Fig. 17, see Supplementary Tables 1-2 for details). The topological structure of JUC-644 is very characteristic with a 2-fold interpenetrated net, which will be discussed in detail below.

On the other hand, because of the relatively flexible structure of the 8-c building block TDFPM, the crystallinity of JUC-643 is not high enough to collect solvable X-ray or electron diffraction data. Since TDFPM and TDFBM are similar in structure, we infer that JUC-643 has the same topology with JUC-644. A non-interpenetrated net in space group Pba2 (No.32) optimized by the Material Studio³⁸ software package matches well with the PXRD pattern (Supplementary Fig. 19). Pawley refinement was applied to PXRD data and showed low residual factors (R_p = 2.00%, R_wp = 2.90%, Supplementary Fig. 20, see Supplementary Table 3 for details). In addition, we also tried alternative structures, such as non-interpenetrated bcu, dmf, and xux net for JUC-643 and their simulated PXRD patterns did not match the experimental result (Supplementary Figs. 21-23, see Supplementary Tables 4-6 for details). All of the evidence above strongly confirms our inference that the structure of JUC-643 is the non-interpenetrated net with the same topology as JUC-644.

Notably, both 3D COFs displayed mesoscopic double helical structures, an extremely uncommon occurrence in COF structures³⁹. The helices of JUC-643 are generated by condensation of the adjacent 8-c building blocks with two PDA linkers along b-axis. Two spirals are linked at the sp³ hybridized carbon atom of the geometric center of TDFPM and they further propagate along [010] direction (Figure 2a, 2c and Supplementary Fig. 24). Similarly, the dual helices of JUC-644 are generated by the alignment of adjacent 8-c building blocks with two PDA linkers along the b-axis. The two helices are connected by C-C bonds at the geometric center of the TDFBM motif and further propagate along [010] direction (Figure 2b, 2d and Supplementary Fig. 25). Both COFs are in achiral space groups because the helices appear in pairs in both structures and the whole frameworks are racemic.

As mentioned above, non-penetrated JUC-643 is in non-centrosymmetric Pba2 while 2-fold interpenetrated JUC-644 is in centrosymmetric Pcca. Pba2 is a translationengleiche subgroup of Pcca in mm2 Laue group by removing the c-glide plane along [010] direction, which means the interpenetration of JUC-644 is completed by 180° rotation around the a- or c-axis rather than simple translation to form the center of symmetry in the structure (Figure 2e and 2f). This is similar to the interpenetration mode in the ZOF structure, in which two crb nets are interpenetrated by 90° rotation¹⁹.

Topological analysis

Although the structures of both COFs have been determined, their topologies cannot be defined easily. According to traditional topological analysis methods, the whole building block in COFs is always regarded as a vertex. Under this deconstruction method, the structures are surprisingly in a [6(+2)]-c pcu-like topology: every 8-c building block is connected with 6 neighbors along 3 mutually orthogonal directions and there are 2 PDA linkers between neighboring building blocks along the direction parallel to the helices and 1 PDA along the other directions. This implies that adjacent building blocks are connected by multiple links, forming the double helical structure mentioned above, which is unreported in all known 3D COF structures (Scheme 1e and 1f). Keeping considering the topology as a [6(+2)]-c pcu net is unreasonable because the “double links” parts are oversimplified to 1D and unable to accurately describe the porous and helical structures generated by the multiple links. This is also a subversive counter-example for the traditional topological analysis method of COFs, indicating that the entire building block cannot always be taken as a single topology vertex.

To precisely describe the topology of JUC-643 and JUC-644, we divided the 8-c building block into a 4-c vertex located in the center of the tetrahedron and four 3-c vertices in the center of the benzene ring of 3',5'-diformylphenyl group (Scheme 3a). In other words, the structure is deconstructed to a [4+3(+2)]-c net according to the position of the subunits rather than a [6(+2)]-c net according to the whole building block (Scheme 1g and 3b-d). The deconstruction process was completed using the software Material Studio manually and using software ToposPro⁴⁰ by automatically clustering, and both resulting topologies were analyzed and proven to be identical based on vertex symbols and td10 values (see Supplementary Table S7 for details). The result topology was further analyzed by program Systre⁴¹, showing the highest space group symmetry of the topology is in Pban (No.50). The topology is different from all the contents in the RCSR database⁴², which means it is a completely unprecedented topology.

Another intriguing aspect of the new topology is that the projection patterns along and directions are identical (Supplementary Fig. 26), suggesting the structure has a good chance to be related to tetragonal symmetry. The cell parameter was set to be a=b to make a tetragonal cell, and 3-c vertices' fractional coordinates were set to be x=y to make them on the diagonal of the xOy plane. The resulting vertex-only structure is obviously in tetragonal symmetry, but the helical edges along the c axis break the C4 axis of rotation (Supplementary Fig. 27a and b). A tetragonal [4+4]-c topology in space group P4/nbm (No.125, a minimal translationengleiche supergroup of Pban) can be accordingly defined by completing the edges along the c axis, which is also an unprecedented topology. The relationship between mentioned [4+4]-c and [4+3]-c topologies is similar to that between pcu and cds (Supplementary Fig. 27c and d). Deleting part of edges along specific directions would reduce the topological symmetry.

Derived topologies in 4-c structures can be defined by dividing a 4-c vertex into two 3-c vertices. For instance, dmd, dmg, dmh, sur, tfi and tfk are derived from pts by splitting tetragonal or square vertices along different directions. The twisted tetrahedral vertex in JUC-644 can be split into two triangular vertices defined in the center of the mesitylene ring, forming a novel [3+3]-c topology. This is the third new topology discovered in this work. The highest space group of this [3+3]-c topology is also Pban. In descending order of symmetry, the new-found [4+3]-c, [4+4]-c, and [3+3]-c topologies are respectively named as jca, jcb (jcb stands for Jilin University China-b) and jcc (jcc stands for Jilin University China-c, Scheme 3e and Supplementary Fig. 28, see Supplementary Tables 7-9 for details). Comparison of jca, jcb, and jcc with other similar topologies in RCSR database⁴² is detailed in Supplementary Table 10 and Figs. 29 and 30.

The discovery of three new topologies indicates the success of our synthetic strategy. Notably, the number of vertices in jca, jcb and jcc topologies is not inversely proportional to their connected numbers, but is determined by the design of 8-c building blocks. This means the quantity ratio of topological vertices in COF structures can be defined manually by controlling the central and outer subunits ratio. For the D_2d 8-c building block itself reported in this article, more COFs in different topologies may be designed by adjusting subunits' orientation and steric hindrances, and by connecting the building blocks to linkers with various configurations and coordination numbers. Furthermore, new topologies can also be designed and discovered by altering the geometry of central or outer subunits to tetrahedrons, pyramids, prisms or more complicated shapes. Building blocks may be even composed of multiple central subunits and different types of outer subunits, producing more possibilities for new topologies. The core requirement of this strategy is at least one subunit should be in 3D coordination environment, making the building block in irregular polyhedrons with high degrees of configurational freedom.

Porosity analysis

The nitrogen (N₂) adsorption and desorption isotherms were performed to determine the porosity of both COFs at 77 K. As illustrated in Supplementary Fig. 31 and Figure 1d, JUC-643 and JUC-644 exhibit a sharp gas uptake at low pressure (P/P₀ < 0.1) demonstrating their microporous nature. The inclination of isotherms in the pressure (P/P₀) range of 0.8 to 1.0 and slight desorption hysteresis for JUC-643 can be attributed to the presence of textural mesopores from the agglomeration of COF crystals. These two COFs both show microporous cavities and the measured pore sizes are 1.2 nm, 1.9 nm, and 1.5 nm along a, b, and c axis for JUC-643 and are 0.7 nm, 1.0 nm, and 1.0 nm along a, b, and c axis for JUC-644 (Supplementary Figs. 32-37). Their theoretical pore size distributions calculated by the nonlocal density functional theory (NLDFT) also revealed micropores with the pore sizes of 1.21, 1.81, and 1.43 nm for JUC-643, and 0.71 and 0.98 nm for JUC-644 along different axis (Supplementary Fig. 38 and Figure 1e), perfectly fitting on the proposed models. The pore size data provided additional support for the non-interpenetrated structure of JUC-643, establishing its topological congruence with that of JUC-644. Respectively, the Brunauer-Emmett-Teller (BET) surface area for JUC-643 and JUC-644 is 1464 m² g^-1 and 2771 m² g^-1 respectively (Supplementary Figs. 39 and 40).

Gas adsorption and separation

The novel porous structure and high surface area of JUC-644 provide promising potential for gas adsorption and separation. In this regard, we investigated the adsorption characteristics of JUC-644 towards various light hydrocarbon molecules. Single-component adsorption isotherms of C₂H₆, C₃H₈, and n-C₄H₁₀ on JUC-644 were measured at three different temperatures: 283 K, 298 K, and 313 K (Figure 3a, 3b, and Supplementary Figs. 41-43). Our results reveal that JUC-644 exhibits higher adsorption capacities for C₂H₆, C₃H₈, and n-C₄H₁₀ compared to JUC-643 at 298 K (4.83, 11.28, 10.45 mmol g⁻¹ in JUC-644 and 1.23, 2.47, 3.32 mmol g⁻¹ in JUC-643 respectively), demonstrating the highest C₃H₈ and n-C₄H₁₀ adsorption uptake among all porous materials, surpassing values reported for C₃H₈ in BSF-1 (1.94 mmol g⁻¹)⁴³, PAF-40-Fe (2.58 mmol g⁻¹)⁴⁴, and g-C₃N₄@Zr–BPDC (8.90 mmol g⁻¹, Figure 3c and Supplementary Table 11)⁴⁵, as well as outperforming values reported for n-C₄H₁₀ in NaX zeolite (1.56 mmol g⁻¹)⁴⁶, Zn-BTM (2.01 mmol g⁻¹)⁴⁷, and Zn-ZIF-8 (4.69 mmol g⁻¹, Figure 3d and Supplementary Table 12)⁴⁶. The n-C₄H₁₀ isotherms of JUC-644 exhibited a slightly steeper increase compared to C₃H₈, and both were considerably steeper than the C₂H₆ isotherm. This indicates that the framework affinity of JUC-644 for guest molecules follows the order of n-C₄H₁₀ > C₃H₈ > C₂H₆.

The Clausius−Clapeyron equation⁴⁸ is utilized to calculate the isosteric heat of adsorption (Q_st) values for n-C₄H₁₀, C₃H₈, and C₂H₆ at near-zero loading, yielding values of 56, 41, and 29 kJ mol⁻¹, respectively (Figure 3e, see Supplementary Tables 13-15 for details). This outcome aligns with the observed lower uptake and more gradual isotherms for C₂H₆, indicating a high adsorption selectivity for n-C₄H₁₀/C₂H₆ and C₃H₈/C₂H₆. The ideal adsorbed solution theory (IAST) selectivity⁴⁹ parameter is paramount in assessing materials' adsorption and separation efficiency. Within the 0-100 kPa range, the n-C₄H₁₀/C₂H₆ selectivity of JUC-644 exhibits an increasing trend, while the C₃H₈/C₂H₆ selectivity initially decreases and then increases. The maximum values of the selectivity of n-C₄H₁₀/C₂H₆ and C₃H₈/C₂H₆ at 100 kPa are 79.2 and 10.4, respectively (Figure 3f and Supplementary Figs. 44-52, see Supplementary Tables 16-19 for details).

Density-functional theory (DFT) calculations

In order to investigate the interaction between gas molecules and the channels, ab initio molecular dynamics (AIMD) simulations were carried out using the CP2K software, employing DFT as the computational method^50-51. We initially positioned the gas molecules at the center of the channels and simulated their trajectory changes at 298.15 K. As is shown in Supplementary Fig. 53 and the GIF images in attaching files, during the process of simulation, the color of the guest molecules transitioned from blue to white, and then to red (the blue-to-white transition represents the adsorption process of gas molecules from the center of the channels to the channel walls, while the white-to-red transition represents the thermal motion trajectory of the molecules after adsorption within the channels). It can be observed that the white-to-red trajectories of C₂H₆ and C₃H₈ show clear separation, indicating weaker restrictive effects of the COF channels on these two gas molecules. The thermal motion within the channels is relatively facile for these molecules, resulting in faster diffusion. Furthermore, examining the positions of the white-to-red trajectories of C₂H₆ and C₃H₈, it is evident that the white-to-red trajectory of C₂H₆ is closer to the center of the COF channels, indicating weaker interaction between C₂H₆ and the channels, thus facilitating easier diffusion within the channels (Supplementary Fig. 53a-f). In contrast, the white-to-red trajectory of n-C₄H₁₀ largely overlaps, and even partially overlaps with some blue trajectories, suggesting stronger restrictive effects of the COF channels on this molecule (Supplementary Fig. 53g-i). Consequently, the thermal motion and diffusion of n-C₄H₁₀ within the channels are relatively restricted, leading to slower diffusion.

Breakthrough experiment

The substantial disparity in adsorption capacities between C₂H₆ and longer alkanes suggests the promising potential of JUC-644 for recovering C₂H₆ from NGLs. The recovery of C₂H₆ from NGLs is a significant source of industrial C₂H₆, which is a primary raw material for C₂H₄ production^52,53. Nevertheless, conventional cryogenic distillation and solvent absorption methods employed for gas separation are characterized by their high energy consumption and detrimental environmental impact. In light of these inherent shortcomings, adsorption separation has emerged as a promising and sustainable alternative, offering energy efficiency and environmental friendliness^54,55. A key aspect of this approach lies in developing porous materials possessing exceptional adsorption capacity and separation selectivity^56,57. Although MOFs have gained considerable attention in the realm of light hydrocarbon separation^58-60, their practical utilization in this context has been impeded by their relatively lower adsorption capacities for C₃H₈ and n-C₄H₁₀ at ambient temperatures, as well as their inadequate stability under humid conditions. Addressing these limitations, JUC-644 stands out as an exceptional porous adsorbent, perfectly suited for ethane recovery from NGLs.

We initially tested the separation performance of JUC-644 in C₂H₆/C₃H₈ and C₂H₆/n-C₄H₁₀ (1:1, Tfr: 2 mL min^-1) mixtures. The results revealed that in the C₂H₆/C₃H₈ mixture, the retention times for C₂H₆ and C₃H₈ were 73.8 min g⁻¹ and 183.6 min g⁻¹ (Figure 4a and Supplementary Figs. 54 and 55). In the C₂H₆/n-C₄H₁₀ mixture, the retention times for C₂H₆ and n-C₄H₁₀ were 61.9 min g⁻¹ and 220.5 min g⁻¹, respectively (Figure 4b and Supplementary Figs. 56 and 57). To validate the practical application of JUC-644 in C₂H₆ recovery from NGL, we also conducted separation tests on a C₂H₆/C₃H₈/n-C₄H₁₀ (46:34:20, v/v/v, Tfr: 5 mL min^-1) mixture at 298 K. The results demonstrated that C₂H₆ rapidly passed through JUC-644 with a retention time of 29.2 min g⁻¹, while C₃H₈ and n-C₄H₁₀ exhibited retention times of 73.8 min g⁻¹ and 162 3 min·g⁻¹, respectively (Figure 4c and Supplementary Figs. 58 and 59). The yield of C₂H₆ (purity >99.99%) was 4.73 mmol g⁻¹. Furthermore, breakthrough experiments were performed under moist conditions (~50% relative humidity). The retention times of C₃H₈ and n-C₄H₁₀ remained relatively unchanged (73.1 min g⁻¹ and 157.2 min g⁻¹, respectively) in the presence of water (Supplementary Figs. 60 and 61). Additionally, the performance of JUC-644 was verified through five consecutive separation cycles, demonstrating its sustained separation efficiency (Figure 4d). These results confirm that JUC-644 is a promising material for efficient and environment-friendly recovery of C₂H₆ from NGLs.

In conclusion, we have successfully designed and synthesized two novel 3D COFs, JUC-643 and JUC-644, by connecting two 8-c organic building blocks in D_2d symmetry via linear PDA linkers. Both COFs were determined using a combination of cRED technique and PXRD. The datasets with a resolution up to 1.5 Å were collected for JUC-644, which is the lowest reported resolution of cRED data for successful structural determination using the ab initio methods. Notably, these COFs exhibit intricate structures featuring interpenetration through 180° rotations, resulting in a center of symmetry and rare mesoscopic double helical linkages, representing a significant advancement in COF structures. To precisely describe their topologies, we develop a new strategy for topological analysis. The framework structures can be deconstructed by splitting the 8-c building block into one 4-c vertex and four 3-c vertices, and identified as an unprecedented [4+3(+2)]-c net (jca) instead of the traditional [8(+2)]-c or [6(+2)]-c net. Additionally, through manual symmetry improvement to the tetragonal system, we defined a [4+4]-c topology (jcb) in the P4/nbm space group and a [3+3]-c topology (jcc) in the Pban space group, both of which are not also found in the RSCR or ToposPro database. This research reveals the complexity of COF structures and highlights the importance of cRED for accurate structural determination during COF topology analysis. Furthermore, JUC-644 exhibits exceptional adsorption capacities for C₃H₈ and n-C₄H₁₀, reaching 11.28 and 10.45 mmol g^-1 at 298 K and 1 bar respectively, which surpasses the adsorption performance of all known porous materials. Breakthrough experiments also demonstrate the excellent C₃H₈/C₂H₆ and n-C₄H₁₀/C₂H₆ selectivity. Therefore, this new strategy for incorporating irregular building blocks in 3D COFs offers a promising path for designing complex COF structures. At the same time, the success of these materials in gas separation underscores the potential for C₂H₆ recovery from NGLs.

Synthesis of JUC-643. TDFPM (0.02 mmol, 17.0 mg) and PDA (0.08 mmol, 8.6 mg) were loaded into a Pyrex tube, 1.0 mL 1,4-dioxane and 0.1 mL of acetic acid (6 M) were added. The liquid nitrogen bath was used to freeze the Pyrex tube, which was evacuated to an interior pressure of ca. 19.0 mbar and flame-sealed, decreasing the whole by ca. 10.0 cm. The tube was placed in an oven at 100 °C for 3 d after the temperature increased to room temperature. The resulting precipitate was filtered and exhaustively washed with acetone 5 times. The obtained powder was immersed in anhydrous n-hexane, and the solvent was exchanged with fresh n-hexane several times. The sample was then transferred to a vacuum chamber and evacuated to 20 mTorr under 65 °C. Yield: 18.7 mg, 73.0%. Anal. Cald: C: 85.56; H: 4.58; N: 9.86; F: 5.99. Found: C: 85.98; H: 4.22; N: 9.35.

Synthesis of JUC-644. TDFBM (0.02 mmol, 15.3 mg) and PDA (0.08 mmol, 8.6 mg) were loaded into a Pyrex tube, 1.0 mL 1,4-dioxane, 0.1 mL aniline, and 0.1 mL of acetic acid (6 M) were added. The liquid nitrogen bath was used to freeze the Pyrex tube, which was evacuated to an interior pressure of ca. 19.0 mbar and flame-sealed, decreasing the length by ca. 10.0 cm. The tube was placed in an oven at 120 °C for 7 d after the temperature increased to room temperature. The resulting precipitate was filtered and exhaustively washed with acetone 5 times. The obtained powder was immersed in anhydrous n-hexane, and the solvent was exchanged with fresh n-hexane several times. The sample was then transferred to a vacuum chamber and evacuated to 20 mTorr under 65 °C. Yield: 18.1 mg, 75.7%. Anal. Cald: C: 85.38; H: 3.85; N: 10.75. Found: C: 85.86; H: 3.51; N: 10.63.

Data availability.

The authors declare that the data supporting the findings of this study are available within the Article and its Supplementary Information files, or from the corresponding author on reasonable request.

Acknowledgements

This work was supported by National Key R&D Program of China (2022YFB3704900 and 2021YFF0500504), National Natural Science Foundation of China (22025504, 21621001, 22105082, U21A20285, 22125102, and 21527803), the SINOPEC Research Institute of Petroleum Processing, "111" project (BP0719036 and B17020), China Postdoctoral Science Foundation (2020TQ0118 and 2020M681034), Ministry of Science and Technology of the People’s Republic of China (grant No. 2020YFA0210700), and the program for JLU Science and Technology Innovative Research Team. V.V., Q.F. and S.Q. acknowledge the collaboration in the Sino-French International Research Network “Zeolites”.

Author contributions

Q.F., H.L., and J.S. were responsible for the overall project design, direction, and supervision. J.C. and H.Z. conducted the synthesis and characterization of all samples. Z.Z. and J.S. collected and analyzed the cRED data and topology structure. J.C., Z.Z., H.Z. and H.L. finalized the PXRD refinement and structure analyses. J.C., H.Z. and H.L. conducted the alkane sorption tests and breakthrough experiments. J.S. and F.C. performed DFT calculations and assisted with the TGA and FT-IR tests. C.J. assisted with the calculation of Q_st and IAST for alkane sorption. All authors contributed to the discussion of results and manuscript writing.

Additional information

Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to H.L., J.S. and Q.F.

Competing interests

The authors declare no competing interests.

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Schemes 1 to 3 are available in the Supplementary Files section

There is NO Competing Interest.

DFTforC2H6.gif
DFTforC3H8.gif
DFTfornC4H10.gif
JUC644.cif
JUC-644.cif
SI0907.pdf
Scheme1.png
Scheme 1. Schematic illustration of 3D COFs based on different strategies. a-c, The traditional strategy based on regular polyhedrons. The [4(+2)] and [4+3] connectivity patterns using 4-connected 3D-T_d building blocks (a). The [6(+2)] and [6+4] connectivity patterns using 6-connected 3D-D_3h building blocks (b). The [8(+2)] and [8+4] connectivity patterns using 8-connected 3D-O_h(D_4h, D_2h) building blocks (c, and please refer to Supplementary Fig. 1 for additional connectivity patterns in 3D COFs). d-g, A new strategy for the design and synthesis of 3D COFs based on low-symmetry irregular polyhedrons. This strategy is exemplified by the designed and synthesized 8-connected building block with D_2d symmetry (d). An 8-connected 6-coordination system is investigated in this connection, showcasing a distinctive ligand arrangement in contrast to conventional structures (e). Unique ‘‘double links’’ connectivity in the structure when considering the entire building block as a vertex (f). A remarkable [4+3(+2)] connectivity pattern when dividing the building block into a 4-c vertex and four 3-c vertices (g).
Scheme2.png
Scheme 2. Schematic Representation for constructing JUC-643 and JUC-644. a-c, Molecular structures of TDFPM and TDFBM (a and b) as 8-connected D_2dbuilding blocks and PDA (c) as a linear linker. d-g, Two novel 3D COFs, JUC-643 (d and f) and JUC-644 (e and g), were constructed from the condensation reaction of PDA with TDFPM or TDFBM. h, A simplified connectivity of JUC-643 or JUC-644.
Scheme3.png
Scheme 3. Schematic illustration of the discovery of novel topologies. a,b, Simplified 8-connected building blocks (a) and COF framework with linearized double links (b). c, a [6(+2)]-c net based on traditional topological analysis, where the entire building block is considered as a vertex. d, A [4+3(+2)]-c net from the new topology analysis strategy, where the 8-c building block is divided into one 4-c vertex and four 3-c vertices for a precise topological representation. e, Three novel topological structures ranked in descending order of symmetry. By adjusting the symmetry and 3-c vertex coordinates, a [4+4]-c jcb net was discovered. Splitting the 4-c vertex revealed a [3+3]-c jcc net.
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Three-Dimensional Covalent Organic Frameworks Constructed from Irregular Polyhedral Building Blocks

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