Dongjie Bai

A Chemically Cross-Linked NbO-Type Metal–Organic Framework: Cage or Window Partition?

By using a presynthetically cross-linked octacarboxylate ligand, a chemically cross-linked version of the NbO-type metal-organic framework (MOF) NOTT-101 (ZJNU-80) was prepared. Single-crystal X-ray structure analysis showed that ZJNU-80 adopts the same topology as the parent compound NOTT-101, and the tethering groups take part in the window partition, not the cage partition. The gas adsorption studies showed that, despite the lower porosity, ZJNU-80a exhibits low-pressure gas adsorption behavior similar to that of the parent MOF NOTT-101a toward CO2, CH4, and N2 at ambient temperature because of the fact that the window partition as a result of chemical cross-linking does not almost alter the pore-size distributions. However, different adsorption behaviors toward 1-butene, a molecule with even larger kinetic diameter than that of the aforementioned adsorbates, were observed because the window partition alters the efficiency with which 1-butene molecules pack within ZJNU-80a and NOTT-101a at conditions close to saturation. This work provides a fundamental understanding on the effect of chemical cross-linking on the MOFs structure and gas adsorption properties.

A porous lanthanide metal–organic framework based on a flexible cyclotriphosphazene-functionalized hexacarboxylate exhibiting selective gas adsorption

Design and synthesis of robust porous lanthanide-based metal–organic frameworks (Ln-MOFs) from flexible organic ligands is currently a formidable task to chemists. In this work, a porous Ln-MOF based on a flexible cyclotriphosphazene-functionalized organic ligand, hexakis(4-carboxylatephenoxy) cyclotriphosphazene, has been solvothermally synthesized. Single-crystal X-ray diffraction analyses show that the compound exhibits a three-dimensional structure built from rod-shaped secondary building units which are linked to each other through the organic ligands to form open frameworks with rectangular channels along the crystallographic a direction. Remarkably, although the flexible ligand was used, the Ln-MOF material after desolvation exhibited permanent porosity which has been established by various gas adsorption isotherms, displaying selective adsorption of C2 hydrocarbons over CH4 at room temperature. This work presents a rare example of a permanently porous Ln-MOF based on a flexible ligand exhibiting selective gas adsorption behaviours.

A family of ssa-type copper-based MOFs constructed from unsymmetrical diisophthalates: synthesis, characterization and selective gas adsorption

In this work, three porous MOFs (ZJNU-74, ZJNU-75 and ZJNU-76) were constructed via solvothermal reactions of unsymmetrical naphthalene-derived diisophthalate ligands and Cu2+ ions. As revealed by single-crystal X-ray diffraction analyses, they consist of dicopper [Cu2(COO)4] paddlewheels connected by the organic ligands to form three-dimensional networks with the same ssa topology, thus representing a MOF platform which allows understanding of the structure–property relationship. The gas adsorption properties of the three MOF compounds with respect to C2H2, CO2 and CH4 have been systematically investigated. They exhibit very high C2H2 uptakes varying from 179.2 to 188.6 cm3 (STP) g−1 and CO2 uptakes varying from 88.1 to 99.2 cm3 (STP) g−1 at 298 K and 1 atm. The predicted IAST selectivities range from 24.9 to 29.7 for an equimolar C2H2/CH4 gas mixture and from 4.8 to 5.4 for an equimolar CO2/CH4 gas mixture at 298 K and 1 atm. These results reveal that they possess very promising potential for C2H2/CH4 and CO2/CH4 separations. In particular, compared to the unmodified parent MOF ZJNU-74, the alkoxy group-functionalized MOFs ZJNU-75 and ZJNU-76 exhibit comparable and even higher C2H2 and CO2 uptake capacities as well as better C2H2/CH4 and CO2/CH4 adsorption selectivities under ambient conditions, which might be attributed to the optimized pore sizes as a result of ligand modification.

A comparative study of the effect of functional groups on C2H2 adsorption in NbO-type metal–organic frameworks

Investigation of the effect of functional groups on gas adsorption in a MOF of a given structure is undoubtedly very important because it facilitates targeting porous MOFs with enhanced storage capacities by ligand functionalization. In this work, we chose an NbO-type MOF platform to evaluate the impact of organic functional groups on C2H2 adsorption. Correspondingly, we synthesized five diisophthalate ligands with the same length but different organic functionalities (NH2, CH3, NO2, F, and CF3), and successfully incorporated them into MOFs with underlying NbO topology. C2H2 adsorption experiments reveal that these functional moieties can enhance the affinity towards C2H2, but not all these compounds outperform the unfunctionalized parent compound NOTT-101 in terms of C2H2 uptake. At 298 K and 1 atm, the C2H2 uptake varies from 153.7 to 193.8 cm3 (STP) g−1, depending on the attached organic functional groups. Particularly, the amine group-functionalized compound ZJNU-34(NH2) exhibits the maximum C2H2 uptake of the six compounds evaluated, reaching as high as 203.6 cm3 (STP) g−1 at 295 K and 1 atm. Such a 10% increase of C2H2 uptake compared to the parent compound might be attributed to acid–base and/or hydrogen-bonding interactions between the NH2 groups with adsorbed C2H2 molecules. The fundamental understanding of the impact of functional groups on C2H2 adsorption demonstrated in this work provides valuable information for future designing of porous MOFs with enhanced acetylene-storage capacities.

An anionic metal–organic framework based on angular tetracarboxylic acid and a mononuclear copper ion for selective gas adsorption

An anionic metal–organic framework [CuL]·2(CH3)2NH2·DMF (ZJNU-55, L = 5,5′-(pyrimidin-2-methyl-4,6-diyl)diisophthalate) was synthesized by a solvothermal reaction of a custom-designed angular diisophthalate and CuCl2·2H2O, and structurally characterized by single-crystal X-ray diffraction. Notably, although the ligand bears a close resemblance to the previously reported diisophthalate used to construct PCN-307, the resultant MOFs structure is totally different from that of PCN-307. The framework exhibits a mononuclear Cu(COO)4 inorganic secondary building unit, instead of the typical dinuclear Cu2(COO)4 paddlewheel unit. The combination of Cu(COO)4 with the deprotonated organic ligand results in a three-dimensional network structure featuring helical nanotube channels along the crystallographic c axis. TOPOS software analyses indicate that two alternative simplifications based on the organic linkers can produce a 4-connected binodal net with the Schlafli symbol (4·62·83)(4·64·8) or a (3,4)-connected 3-nodal network with the Schlafli symbol (6·8·9)2(6·83·92), which have been rarely observed in MOF chemistry. Furthermore, its gas adsorptive properties were systematically investigated. The activated ZJNU-55a exhibits highly selective adsorption of C2H2 and CO2 over CH4 with the adsorption selectivities of 39.2–64.9 and 13.1–29.2 at 298 K, respectively. As far as we are aware, this is a rare example of a MOF constructed from mononuclear Cu(COO)4 for selective gas adsorption. This work also demonstrates that subtle ligand modification can drastically affect the structures and properties of the resultant MOF.

A rare Pb9 cluster-organic framework constructed from a flexible cyclotriphosphazene-functionalized hexacarboxylate exhibiting selective gas separation

It is quite challenging to fabricate metal–organic frameworks with permanent porosity using flexible organic ligands owing to their conformation freedom and flexibility making the resultant frameworks very difficult to stabilize. Successful synthesis of such frameworks will enrich the domain of porous compounds for various applications. In this work, we designed a flexible cyclotriphosphazene-functionalized hexacarboxylate, and used it to successfully construct a porous metal–organic framework ZJNU-62 upon solvothermally reacting with Pb2+ ions. Single-crystal X-ray diffraction analyses show that ZJNU-62 is composed of nonanuclear Pb9 clusters connected by organic linkers to form a (4,12)-connected three-dimensional infinite network of Al3La topology with one-dimensional rectangular channels running along the crystallographic c axis. Although the organic ligand is flexible, the resulting MOF compound ZJNU-62 can sustain the desolvation and exhibit permanent porosity, which has been confirmed by various gas adsorption isotherms. In particular, this compound can selectively adsorb C2 hydrocarbons and CO2 from methane under ambient conditions, indicating its promising potential for natural gas purification. The predicted IAST adsorption selectivities are 21.0, 10.9, 13.2 and 5.8 for the equimolar C2H2/CH4, C2H4/CH4, C2H6/CH4 and CO2/CH4 gas mixtures at 298 K and 100 kPa, respectively. This work represents a rare example of a Pb-based MOF constructed from a flexible organic ligand exhibiting selective gas adsorption properties.