Dual-Function Precious-Metal-Free Metal-Organic Framework for Photocatalytic Conversion and Chemical Fixation of Carbon Dioxide.

Highly efficient transformation of carbon dioxide (CO2) into value-added chemicals is considered a promising route for clean production and future energy sustainability, which is crucial for realizing a carbon-neutral economy. It remains a great challenge to develop highly stable and active catalysts with low-cost, environmentally friendly, and nontoxic materials for catalytic conversion of CO2. Herein, a precious-metal-free and heterogeneous MOF (LTG-FeZr) catalyst, composed of bis(terpyridine)iron(II) complexes and zirconium(IV) ions, was designed and prepared via a metalloligand approach. LTG-FeZr, with a robust framework and regular 1D channels not only can achieve the photocatalytic reduction of CO2 to HCOOH with a high conversion rate (up to 265 µmol·g-1·h-1) under visible-light irradiation but also exhibits exceptional catalytic activities toward the synthesis of cyclic carbonates via cycloaddition reactions of various epoxides and CO2 in the absence of light. Possible mechanisms for two different conversion processes of CO2 catalyzed by LTG-FeZr have been proposed. LTG-FeZr represents an ideal dual-function MOF platform for the catalytic conversion and utilization of CO2 in all weather conditions.

Mesoporous Mixed-Metal-Organic Framework Incorporating a [Ru(Phen)3]2+ Photosensitizer for Highly Efficient Aerobic Photocatalytic Oxidative Coupling of Amines.

[Ru(Phen)3]2+ (phen = phenanthroline) as a very classical photosensitizer possesses strong absorption in the visible range and facilitates photoinduced electron transfer, which plays a vital role in regulating photochemical reactions. However, it remains a significant challenge to utilize more adequately and exploit more efficiently the ruthenium-based materials due to the uniqueness, scarcity, and nonrenewal of the noble metal. Here, we integrate the intrinsic advantages of the ruthenium-based photosensitizer and mesoporous metal-organic frameworks (meso-MOFs) into a [Ru(Phen)3]2+ photosensitizer-embedded heterometallic Ni(II)/Ru(II) meso-MOF (LTG-NiRu) via the metalloligand approach. LTG-NiRu, with an extremely robust framework and a large one-dimensional (1D) channel, not only makes ruthenium photosensitizer units anchored in the inner wall of meso-MOF tubes to circumvent the problem of product/catalyst separation and recycling of catalysts in heterogeneous systems but also exhibits exceptional activities for the aerobic photocatalytic oxidative coupling of amine derivatives as a general photocatalyst. The conversion of the light-induced oxidative coupling reaction for various benzylamines is ∼100% in 1 h, and more than 20 chemical products generated by photocatalytic oxidative cycloaddition of N-substituted maleimides and N,N-dimethylaniline can be synthesized easily in the presence of LTG-NiRu upon visible light irradiation. Moreover, recycling experiments demonstrate that LTG-NiRu is an excellent heterogeneous photocatalyst with high stability and excellent reusability. LTG-NiRu represents a great potential photosensitizer-based meso-MOF platform with an efficient aerobic photocatalytic oxidation function that is convenient for gram-scale synthesis.

A Discrete 3d-4f Metallacage as an Efficient Catalytic Nanoreactor for a Three-Component Aza-Darzens Reaction.

The exploration and development of coordination nanocages can provide an approach to control chemical reactions beyond the bounds of the flask, which has aroused great interest due to their significant applications in the field of molecular recognition, supramolecular catalysis, and molecular self-assembly. Herein, we take the advantage of a semirigid and nonsymmetric bridging ligand (H5L) with rich metal-chelating sites to construct an unusual and discrete 3d-4f metallacage, [Zn2Er4(H2L)4(NO3)Cl2(H2O)]·NO3·xCH3OH·yH2O (Zn2Er4). The 3d-4f Zn2Er4 cage possesses a quadruple-stranded structure, and all of the ligands wrap around an open spherical cavity within the core. The self-assembly of the unique cage not only ensures the structural stability of the Zn2Er4 cage as a nanoreactor in solution but also makes the bimetallic lanthanide cluster units active sites that are exposed in the medium-sized cavity. It is important to note that the Zn2Er4 cage as a homogeneous catalyst has been successfully applied to catalyze three-component aza-Darzens reactions of formaldehyde, anilines, and α-diazo esters without another additive under mild conditions, displaying better catalytic activity, higher specificity, short reaction time, and low catalyst loadings. A possible mechanism for this three-component aza-Darzens reaction catalyzed by the Zn2Er4 cage has been proposed. These experimental results have demonstrated the great potential of the discrete 3d-4f metallacage as a host nanoreactor for the development of supramolecular or molecular catalysis.

A pair of polymorphous metal-organic frameworks based on an angular diisophthalate linker: synthesis, characterization and gas adsorption properties.

The combination of an angular diisophthalate ligand, 5,5'-(naphthyl-2,7-yl)diisophthalate (H4L), and copper ions under different solvothermal conditions afforded two polymorphous metal-organic frameworks (ZJNU-77 and ZJNU-78) with the same framework composition of [Cu2(L)(H2O)2], providing a platform to investigate the relationship between MOF polymorphism and gas adsorption properties. As determined by single-crystal X-ray diffraction, ZJNU-77 and ZJNU-78 exhibited three-dimensional networks crystallizing in different space groups. Their structural differences were mainly manifested by the ligand's conformation, the level of framework interpenetration and the network's topology. Interestingly, gas adsorption studies showed that the two compounds after desolvation displayed comparable gas adsorption properties with respect to C2H2, CO2 and CH4, despite their different surface areas and pore volumes. The C2H2, CO2, and CH4 uptake capacities at 298 K and 1 atm are 120.2, 78.1, and 18.4 cm3 (STP) g-1 for ZJNU-77, and 122.0, 82.0, and 18.9 cm3 (STP) g-1 for ZJNU-78, respectively. The IAST adsorption selectivities for the equimolar C2H2/CH4 and CO2/CH4 mixtures are 28.6 and 5.7 for ZJNU-77, and 28.4 and 5.9 for ZJNU-78 at 298 K and 1 atm. These results indicate that besides the surface area, the pore size also plays a crucial role in gas adsorption. This work not only represents an intriguing example of MOF polymorphism achieved by controlling solvothermal conditions, but also provides an insight into the correlation between MOF polymorphism and gas adsorption properties.

A comparative study of C2H2 adsorption properties in five isomeric copper-based MOFs based on naphthalene-derived diisophthalates.

In this work, five positional isomeric ligands consisting of two peripheral isophthalate moieties attached to the central naphthyl core in different ways, namely, 5,5'-(naphthyl-1,3-diyl) diisophthalate (H4L1), 5,5'-(naphthyl-1,4-diyl) diisophthalate (H4L2), 5,5'-(naphthyl-1,5-diyl) diisophthalate (H4L3), 5,5'-(naphthyl-1,6-diyl) diisophthalate (H4L4) and 5,5'-(naphthyl-2,6-diyl) diisophthalate (H4L5), have been used to generate five copper-based MOF isomers. As revealed by single-crystal X-ray diffraction studies, they adopted two different types of topologies depending on the organic ligands: ssa topology for the MOFs ZJNU-71 and ZJNU-74 based on the ligands H4L1 and H4L4, respectively, and nbo topology for the MOFs ZJNU-72, ZJNU-73 and NOTT-103 derived from the ligands H4L2, H4L3 and H4L5, respectively. Furthermore, their C2H2 adsorption properties were systematically investigated, revealing that their different C2H2 uptake capacities can be mainly related to their different pore sizes since they possess the same chemical compositions and gravimetric densities of open metal sites. In particular, among these five MOF compounds investigated, ZJNU-71 exhibits the highest gravimetric C2H2 uptake of 208.1 cm3 (STP) g-1 at 295 K and 1 atm. The value is also among the highest reported for MOF compounds under the same conditions. This work provides a fundamental understanding of the impact of the positional isomerism of the organic ligands on the structures as well as gas adsorption properties of the resulting MOFs.

A porous metal-organic framework based on an asymmetric angular diisophthalate for selective adsorption of C2H2 and CO2 over CH4.

A new copper-based metal-organic framework [Cu2L(H2O)2]·5DMF·2H2O (ZJNU-56) has been solvothermally synthesized using a custom-designed asymmetric rigid bent diisophthalate ligand, 5,5'-(1-amine-naphthyl-2,4-diyl) diisophthalic acid (H4L), and structurally determined by single-crystal X-ray diffraction. ZJNU-56 features a three-dimensional (3D) open framework incorporating three different types of metal-organic cages and two distinct types of one-dimensional channels. With a moderate BET surface area of 1655 m2 g-1, optimized pore structure, and functional sites (open copper sites and Lewis basic amine groups) on the cage surface, ZJNU-56 after desolvation exhibits highly selectively adsorptive separation of C2H2 and CO2 over CH4 under ambient conditions. At 298 K, the predicted IAST selectivities are 35.7-72.9 for an equimolar C2H2/CH4 gas mixture and 6.8-7.0 for an equimolar CO2/CH4 gas mixture at pressures varying from 1 to 109 kPa, respectively, which are among the highest reported to date for copper-based MOFs.

A Porous Zirconium-Based Metal-Organic Framework with the Potential for the Separation of Butene Isomers.

By using a novel C3 -symmetrical tricarboxylate (4,4',4''-benzene-1,3,5-triyl-1,1',1''-trinaphthoic acid), a novel zirconium-based metal-organic framework ZJNU-30 was solvothermally synthesized and structurally characterized. Single-crystal X-ray structural analyses show that ZJNU-30 consists of Zr6 -based nodes connected by the organic linkers to form a (3,8)-connected network featuring the coexistence of two different polyhedral cages: octahedral and cuboctahedral cages with the dimensions of about 14 and 22 Å, respectively. Remarkably, ZJNU-30 is very stable when exposed to air for one month. More importantly, with a moderately high surface area, hierarchical pore structures, and an aromatic-rich pore surface in the framework, ZJNU-30, after activation, exhibits a promising potential for the selective adsorptive separation of industrially important butene isomers consisting of cis-2-butene, trans-2-butene, 1-butene, and iso-butene at ambient temperature. This separation was established exclusively by gas adsorption isotherms and simulated breakthrough experiments. To the best of our knowledge, this is the first study investigating porous metal-organic frameworks for butene-isomer separation.

An aminopyrimidine-functionalized cage-based metal-organic framework exhibiting highly selective adsorption of C2H2 and CO2 over CH4.

There has been considerable interest in adsorptive separation of C2H2/CH4 and CO2/CH4 gas mixtures due to its industrial significance and scientific challenge. In this work, we have designed and synthesized a bent diisophthalate ligand functionalized with aminopyrimidine groups, and constructed via a solvothermal reaction, a porous copper-based framework. Single-crystal X-ray diffraction studies show that the framework is a three-dimensional network containing three different types of polyhedral nanocages, which are stacked together to form two distinct types of one-dimensional channels along the crystallographic c axis. The compound after activation shows exceptionally high C2H2 and CO2 uptakes of 211 and 120 cm(3) (STP) g(-1) at 295 K and 1 atm, as well as impressive adsorption selectivities towards C2H2 and CO2 over CH4. High C2H2 and CO2 uptake capacities as well as significant adsorption selectivities of C2H2 and CO2 over CH4 imply potential applications in the adsorptive separation and purification of C2H2/CH4 and CO2/CH4 gas mixtures, which have been verified by column breakthrough experiments. Several important binding sites for C2H2 and CO2 in ZJNU-54 were revealed by quantum chemical calculations, demonstrating that the organic linkers in ZJNU-54 form unique structures that facilitate the adsorption of C2H2, while the amine groups and the Lewis basic pyrimidine-ring nitrogen sites in the organic linker improve the adsorption energies for CO2, finally leading to the increase of adsorption capacities for these two gas molecules. This work provides an efficient strategy for incorporating specific functional groups into cage-based MOFs for generating new adsorbents for highly selective gas storage and separation.

High methane storage and working capacities in a NbO-type metal-organic framework.

To improve methane adsorption by pore structure optimization, we developed a new organic linker and used it to construct a NbO-type metal-organic framework ZJNU-53 that, after activation, exhibits exceptionally high methane storage and working capacities of 241 and 190 cm(3) (STP) cm(-3) at 298 K and 65 bar, respectively, if the packing loss is not considered, which are among the highest reported for MOF materials.

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 MOF's structure and gas adsorption properties.