Metal-organic frameworks (MOFs) are built by connecting metal-containing secondary building units (SBUs) with organic ligands1. In general, the inorganic SBUs is treated as vertex and the organic ligands is treated as linkers in simplifying the MOF structures2-6. However, the high-connected organic linkers have made the line between such structural roles of inorganic components and organic components blurred and sometimes the roles were even reversed7-9. For example, MOF-688 had a dia topology with 4-connected tetrahedral tetrakis(4-formylphenyl)methane organic building units as vertices and 2-connected polyoxometalate cluster as linkers10.
Compared to MOFs built on inorganic vertices, a potential advantage of MOFs with part or all organic vertices is the lightweight framework, which is beneficial for high guest uptake in adsorption applications. An excellent example is the series of NU-1500 structures with an acs topology, which is built on trivalent trinuclear metal (Fe3+, Cr3+, Al3+ and Sc3+) clusters and trigonal prismatic ligands11,12. A main difference of NU-1500 from prototypical acs (MOF-235/MIL-88) is half of the 6-connected inorganic node is substituted by 6-connected organic vertices, resulting a rigid framework13-15. NU-1500-Cr showed an impressive water uptake and NU-1501-Al with high porosity and surface area yielded a methane storage capacity over U.S. Department of Energy target 11,12. With the lightweight frameworks, however, the pore size of NU-1500 structures is too large to enable a strong host-guest interaction to confine the small molecules for adsorption/separation applications at ambient conditions.
Pore-space-partition (PSP) strategy, which refers to the division of large pores into small segments, is a fruitful approach to increase the density of binding sites and has achieved great experimental success16-20. Of potential relevance to NU-1500 structures is that PSP has been proved to be incredibly successful in acs topology by introducing a symmetry-matched pore-partitioning agent into the flexible acs framework13,14. The partitioned acs (pacs) platform rank among the best MOFs in a range of properties such as high gas up-take and high chemical stability21. We are thus intrigued by the prospect that the implementation of PSP on lightweight NU-1500 structures may set new adsorption record. The aromatic guest would be of particular interest due to introduction of π-conjugated partitioning ligand.
The application of the PSP strategy on NU-1500 frameworks faces an extra hurdle because the directional nature of covalent bond in organic vertex has led to a rigid acs framework (Fig. 1)11. This is different from the case in previously reported flexible acs phases based on ditopic carboxylate ligands. Due to the nondirectional nature of metal-carboxylate bonds, they showed large size tolerance toward pore-partitioning agent20. As a consequence, in addition to symmetry match, strict size match between the partitioning agent and the framework is required for PSP on NU-1500 frameworks.
With the above considerations in mind, in this work, we were able to make eight MOFs with four ligand pairs and two trivalent metals (Fe3+, Cr3+). The NNM MOFs (NNM stands for Nanjing Normal University Materials) reported here integrated ultralow metal density, high porosity, and high structural stability. Cr based NNMs show ultrastability which can withstand extreme pH conditions from concentrated hydrochloric acid to 1 M NaOH solution. NNMs with high porosity exhibit top-level adsorption performance toward volatile organic compound of benzene due to the introduction of conjugated partitioning ligand. Especially for NNM-750 structures, which show record capture capacity at ambient temperature and a wide range of relative pressure.
PSP on rigid acs and structural analysis. A main issue for PSP in NU-1500 frameworks based on the trigonal prismatic linkers is the rigidity of the framework inherited from rigid organic nodes. A closer look at the structure showed that the motion of metal carboxylate bond at the cluster is restricted by the directional C-C bonding at the linkers, leading to a rigid acs framework (Fig. 1). As such, the implementation of PSP strategy here requires an extra size match between the partitioning ligand (P) and trigonal prismatic ligand (L) in addition to symmetry match in prototypical flexible acs. (Supplementary Fig. 1).
Based on the geometry of the framework, a linear equation was derived to determine the relation between the size of trigonal prismatic linkers and the partitioning ligands. dL is defined as the length of L projected onto c plane where the length of L is the length from the center of L to the center of two terminal carboxylate oxygens. The P-related size (dP) is the distance from the center to terminal nitrogen atom on pyridine. The relation between dL and dP is finally determined to be dP = dL – 1.8 Å (Supplementary Fig. 2). It could be deduced that four pairs of ligands match well for the implementation of PSP strategy (L1-P1 for NNM-750, L2-P2 for NNM-751, L2-P3 for NNM-752, and L3-P1 for NNM-753, Fig. 2a and Supplementary Fig. 3).
The crystal size of all the Fe MOFs were suitable for single crystal X-ray diffraction (SCXRD) measurement (Supplementary Fig. 4). The detailed crystallographic information is shown in Supplementary Table 1-4. The experimental PXRD patterns of Fe phases matched well with simulated ones from single-crystal data, suggesting the phase purity of as-synthesized samples (Supplementary Fig. 5). SCXRD analysis suggests that the NNMs have lower symmetry compared with their parent structures. In particular, the partitioned NU-1500 (NNM-750) has a lowest space group of P3, in comparison with P-6m2 of NU-1500. Detailed structural analysis shows that the distances from 6-c ligand to the trimers in NNM-750 are not consistent, with three shorter ones on one side and three longer ones on the other (Supplementary Fig. 6). Such a distortion is probably due to the subtle size mismatch between the framework and the ligands. The extra-framework volume for the NNMs were calculated to be 66.0 % (NNM-750), 77.1 % (NNM-751), 76.9 % (NNM-752) and 70.8 % (NNM-753) by using PLATON program, lower than their corresponding nonpartitioned structures due to the insertion of P ligands.
An evidence for the size match is that the unit cells have remained essentially unchanged after PSP. For example, the a-axis for NU-1500-Fe and NU-1501-Fe are 19.57 Å and 24.96 Å, while for PSP resulted NNM-750-Fe and NNM-751-Fe are 19.31 Å and 24.69 Å respectively, with the size difference smaller than 3 percent (Supplementary Fig. 7). The deviation is attributed to slight bending of the long branch of L ligands, and the size tolerance for partitioning ligand is expected to marginally increase with the length of the branch. Further controlled experiments were carried out to certify the rigorous size match for PSP. We have tried to use tripyridyl ligands with different size to partition the rigid NU-1500 and NU-1501, e.g., smaller P ligand of tpt (dP = 5.55 Å) or bigger P ligand of tpbtc (dP = 7.88 Å) to partition NU-1500; a slight bigger P of tpapa (dP = 10.60 Å) to partition NU-1501, but only got non-partitioned acs structures. For reference, dp for P1, P2, and P3 is 6.93 Å, 10.02 Å, 9.91 Å respectively.
Compared with NNM-750-752 based on trigonal prismatic ligands with stereo iptycene core22-24, the discovery of NNM-753 is special because it is predicted and synthesized by only considering the size relation of P1 and L3 with no parent acs structure reported before (Supplementary Fig. 8 and Fig. 9). In L3, the introduction of three steric methyl groups on the centered benzene twists the ligand conformation from planar to trigonal prism25,26. In fact, we have tried but failed to obtain its corresponding non-partitioned acs structure, indicative of the power of PSP strategy to direct the assembly of the partitioned framework.
Another interesting point here is the use of anionic P1 ligand in NNM-750 and NNM-753. The anionic feature of P1 ligand comes from the acidic central N-rich hexaazaphenalene ring with the presence of six nitrogens to resonance-stabilize the anionic site27-30. The large aromatic plane enables a strong π–π interaction and the six nitrogens in the hexaazaphenalene core can be involved in multiple H-bonds, which could be beneficial for the adsorption of aromatic guest such as benzene as discussed below. In addition, the combination of cationic metal trimers of [M3O(COOR)6]+ and anionic ligand P1- in NNM-750 and -753 with 1 : 1 ratio leads to a neutral framework. In comparison, NNM-751 and -752 are cationic.
The PSP on prototypical flexible acs and rigid acs frameworks share some common features. The connectivity of metal trimer increases from 6 to 9, accompanied with the annihilation of all the open metal after incorporating the tritopic P ligands (Fig. 2b and Supplementary Fig. 10). The consecutive hexagonal channel along c direction is partitioned into infinite number of small segments (Fig. 2c,d).
The differences between rigid acs and prototypical flexible acs framework set off a chain reaction in the partitioned frameworks. Compared with prototypical acs, the loss of half metal trimers in NU-1500 structures has led to the loss of half partitioned ligands in NNM MOFs. As such, the distance between adjacent partitioned ligands is just equal to cell length of c axis in NNM MOFs while it is equal to c/2 in partitioned flexible acs. The stacking of the partition ligands also changed from ABAB stacking with a rotation degree of 60o to an eclipsed stacking fashion, which results in a more open channel along c direction (Fig. 2c and Supplementary Fig. 11). In addition, the original pacs structures has a (3, 9)-c nia-d topology, while NNM MOFs here have a (3, 6, 9)-c 3-nodal net with a new topology (detailed topological analysis in Page 45 of SI).
All the Cr phases were prepared in the polycrystalline form with the size around hundreds of nanometers with in-situ one-pot reactions (Supplementary Fig. 12-15). The isomorphic structures of Cr MOFs and Fe MOFs were identified by comparing their powder diffraction patterns and FTIR spectra (Supplementary Fig. 16 and Fig. 17). In particular, the NNM MOFs exhibit a strong diffraction peak that belongs to (001) lattice plane caused by the partitioning ligand, dramatically different from those non-partitioned structures (Supplementary Fig. 18). Rietveld refinements were also carried out on Cr phase. The resulted refined patterns fit well with experimental ones with low residual values, further verified their structures (Supplementary Fig. 19 and Table 5-8).
The use of trigonal prismatic ligands as organic vertices in NU-1500 MOFs has led to a low metal density and the introduction of P ligands here made it even lower. Supplementary Table 9 shows the theoretical metal-site densities of some highly porous MOFs31. Highly porous DUT-6 has a metal density of 3.73 mmol g-1. NU-1500(Cr) has a low metal density of 2.52 mmol g-1. NNM MOFs here has much lower metal density, from 1.39 to 1.94 mmol g-1. In particular, NNM-752-Fe has a metal density of 1.39 mmol g-1, resulting in a near-organic backbone with metal mass fraction as low as 5%. Such lightweight frameworks of NNM-MOFs are beneficial for high guest uptake.
Porosity and chemical stability. The permanent porosity of eight NNMs, after thermal activation from dichloromethane-exchanged samples, was analyzed through N2 adsorption at 77 K. Among them, NNM-750 and NNM-753 show reversible type-I isotherms, while the isotherms for NNM-751 and NNM-752 are slightly different due to the existence of larger pores. The pore-size distribution based on a density functional theory (DFT) model reveal that NNM-750 have pore size ranging from 0.5 nm to 1.2 nm, NNM-751/752 ranging from 0.5 nm to 2.0 nm, and NNM-753 ranging from 0.6 nm to 1.2 nm (Supplementary Fig. 20). The Brunauer-Emmett-Teller (BET) surface areas of NNM-750~753(Fe) were calculated to be 2387, 3514, 4084 and 2845 m2 g-1, while NNM-750~753(Cr) are 2388, 2083, 2714 and 2097 m2 g-1, after satisfying all four BET consistency criteria with BETSI software (Fig. 3a and Supplementary Fig. 21-28)32,33.
The coordinating saturation of metal sites in combination with the rigid backbone contributed to high structural stability. Thermogravimetric analysis (TGA) curves for NNMs show only slight weight losses (< 8%, corresponding to the release of adsorbed solvents) before 400 oC, suggesting their good thermostability (Fig. 3b). Due to the highest inertness of Cr (III) among all the metal cations used in MOFs, the chemical stability of Cr based NNMs were studied34,35. NNM-750-Cr and NNM-752-Cr were chosen to evaluate the chemical stability. After immersing into boiling water, concentrated hydrochloric acid (12 M), and 1 M sodium hydroxide solution for 24 h, both of two structures retained high crystallinity, as suggested by PXRD patterns (Fig. 3c,e). N2 adsorption isotherms further confirmed the high stability under harsh conditions (Fig. 3d,f). Improved N2 uptake after boiling water or acid treatments is likely due to the additional activation effect. A small decrease of N2 uptake can be observed with the treatment of 1 M NaOH, probably due to the partial decomposition of the structure. The resistance to strong bases by Cr MOFs here is notable, considering that most prior Cr-MOFs have a basic resistance lower than pH 1236. It could be concluded that the incorporation of partitioning ligands endows the Cr based highly porous NNMs with ultrastability, among the best stable MOFs (Supplementary Table 10)36-42.
Benzene adsorption performance. NNMs with excellent structural stability, highly conjugated aromatic backbone are suitable for aromatic VOCs adsorption related applications, such as trace removal of benzene. Single-component benzene adsorption measurements on NNMs were conducted at 298 K. For comparison, benzene adsorption isotherms for non-partitioned structures MIL-88-Fe, NU-1500-Fe and NU-1501-Fe were also collected. As shown in Figure 4a and Figure S29, all the samples exhibit sharply benzene uptake at low pressure, suggesting the strong interaction between host framework and benzene. The saturated benzene capture capacity for most Fe based NUs and NNMs is positively correlated with the extra-framework volume and is determined to be 11.03 mmol/g for NU-1500-Fe, 10.35 mmol/g for NNM-750-Fe, 15.31 mmol/g for NNM-751-Fe, 17.57 mmol/g for NNM-752-Fe, 8.41 mmol/g for NNM-753-Fe. The adsorption capacity of NU-1501-Fe is only 3.86 mmol/g, which is probably due to the partial collapse of the structure, as suggested by PXRD patterns (Supplementary Fig. 30). The uptake for Cr based NNM-750-753 is determined to be 8.43, 9.48, 13.65 and 5.46 mmol/g, respectively. While the flexible MIL-88-Fe showed negligible benzene adsorption at low pressure (Supplementary Fig. 31), it is worth noting that the partitioned structures exhibit enhanced capture capacity at low vapor pressure (P/P0 < 0.01) compared to non-partitioned structures. For example, at P/P0 = 0.01, the benzene uptake for NNM-750-Fe and NU-1500-Fe is almost the same. However, when P/P0 is equal to 0.005, the benzene uptake for NNM-750-Fe is about 7.92 mmol/g, almost three times of NU-1500-Fe’s (2.74 mmol/g). At P/P0 = 0.002, the benzene uptake for NNM-750-Fe still reaches up to 6.02 mmol/g but NU-1500-Fe could only adsorb 0.84 mmol/g (Fig. 4b).
NNM-750-Fe set a new benchmark for trace benzene capture which shows the highest benzene capture capacity at a wide pressure range and far exceeds previous records. At 298 K and P/P0 = 0.01, the benzene adsorption capacity for NNM-750-Fe is 8.97 mmol g-1, which outperform other previously reported benzene adsorbents such as MOF-74-Mn (6.30 mmol g-1), ZJU-520-Al (5.98 mmol g-1), BUT-54-Co (4.31 mmol g-1), UiO-66-Cu(II) (3.92 mmol g-1), ZJU-620-Al (3.80 mmol g-1), and PAF-1 (3.66 mmol g-1) (Fig. 4c, Supplementary Table 11)43-49. The record trace benzene capture capacity is identified at a wide range of relative pressure from 0.003 to 0.01 (Fig. 4d). The volumetric benzene capture for NNM-750-Fe is only second to Mn-MOF-74 at P/P0 = 0.01 but is significantly higher than Mn-MOF-74 and other samples at lower pressure (Fig. 4c,d). We also carried out multiple benzene adsorption and desorption experiments on NNM-750 at 298 K to evaluate their durability and recyclability (Fig. 4e). There is no obvious decrease of benzene uptake capability after 3 adsorption-desorption cycles and PXRD patterns also showed that NNM-750 still had a good crystallinity after benzene adsorption, indicating the high structural robustness and excellent regeneration ability (Supplementary Fig. 32).
Dynamic gas breakthrough experiments at 298 K were carried out on NNM-750-Fe and NU-1500-Fe to further evaluate their ability to capture low-concentration benzene. A gas mixture of benzene vapor (1000 ppm) and nitrogen with the molar ratio of 1 : 4 was passed through a column packed with 50 mg NNM-750 sorbents or NU-1500- Fe at a total gas flow rate of 50 mL min-1. As shown in Figure 4f, the benzene starts to break through the column of NU-1500-Fe after 2.07 h (41.37 h g-1), corresponding to the benzene capture capacity of 1.11 mmol g-1. After the pore-space-partition by P1 ligand, the breakthrough of benzene in the resulting NNM-750-Fe delayed to 4.06 h (81.10 h g-1) corresponding to the benzene capture capacity of 2.17 mmol g-1. This result further demonstrates that PSP in NU-1500 greatly promote low-concentration benzene capture from air.
DFT calculations for benzene adsorption sites. To uncover the underlying reason for the contribution of PSP on the boosted benzene adsorption, DFT calculations were conducted to investigate the benzene adsorption in NU-1500-Fe and NNM-750-Fe. For NU-1500, benzene molecule can form multiple C-H···C interactions and C-H···π interactions with the benzene ring at three corners of the trigonal prismatic ligands (site A), affording a moderately high binding energy of -49.70 kJ mol-1 (Fig. 5a). Another adsorption site (site B) in NU-1500 is the Fe OMS with Fe-C distances being 3.298 and 3.424 Å, respectively (Fig. 5b). However, the binding energy is only -31.15 kJ mol-1. NNM-750 possesses the same benzene adsorption site around the trigonal prismatic ligand (site A) and very similar binding energy (-45.76 kJ mol-1) (Fig. 5d). Although the OMS site of NU-1500 was blocked after the implementation of PSP, but in the meantime the inserted P1 linker provided additional adsorption site (site B’) (Fig. 5e). Notably, the large π-conjugated hexaazaphenalene-based ligand in NNM-750 can interact with coplanar benzene molecule by π···π stacking, with the C-N distances varying from 3.560 to 3.794 Å. Meanwhile, the N atoms of the inserted P1 ligand can also form multiple intermolecular hydrogen bonds with the H atom in benzene, and the N···H distances are 3.413-3.432 Å. The combined π···π stacking and multiple hydrogen bonds interactions endow NNM-750 higher benzene affinity and the binding energy is as high as -79.91 kJ mol-1. Besides, to display the host-guest interaction visually, independent gradient model (IGM) analysis was employed. As shown in Figure 5c and Figure S33, the green isosurfaces indicate that NU-1500 exists Van der Waals interaction with benzene molecule at the corner of triene unit and Fe OMS site. IGM analysis also shows a green isosurface between benzene molecule and P1 motif in NNM-750, which is fatter and larger than that in site B in NU-1500 (Fig. 5f), demonstrating stronger benzene affinity of NNM-750. This is in line with the experimental results that NNM-750 exhibits significantly improved benzene adsorption performances at low pressure compared with NU-1500.
In summary, in this work, we have demonstrated the successful implementation of PSP strategy in a series of rigid and lightweight acs type frameworks based on 6-connected trigonal prismatic linkers as organic vertices. Eight MOFs with four ligand pairs and two metals were made. The introduction of pyridyl ligands with aromatic backbone and the lightweight framework of NNM MOFs has contributed to record-high trace benzene adsorption, which far exceeds previous record at a wide range of pressure. DFT calculations indicated that the use of hexaazaphenalene-based ligand has a much higher binding energy than that of open metal sites. The PSP on rigid acs frameworks here significantly broaden the application of PSP concept and therefore highlight new possibilities for the construction of new pacs MOFs. In addition, the use of large π-conjugated hexaazaphenalene-based ligand in other chemical systems may also have promising applications for the adsorption of aromatic guests