Postsynthetic Tuning of Metal-Organic Frameworks for Targeted Applications

Morphology- and size-controlled synthesis of a metal-organic framework under ultrasound irradiation: An efficient carrier for pH responsive release of anti-cancer drugs and their applicability for adsorption of amoxicillin from aqueous solution

In this study, we have reported a biocompatible metal-organic framework (MOF) with ultra-high surface area, which we have shown to have uses as both a cancer treatment delivery system and for environmental applications. Using a sonochemical approach, highly flexible organic H₃BTCTB and ditopic 4,4'-BPDC ligands, along with modulators of acetic acid and pyridine were combined to prepare a Zn(II)-based metal-organic framework, DUT-32, [Zn₄O(BPDC)(BTCTB)_4/3(DEF)_39.7(H₂O)_11.3]. Powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FE-SEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize, the particle size, shape, and structure of the DUT-32. To show the effects of shape and size of DUT-32 micro/nano-structures on doxorubicin (DOX) drug release and amoxicillin (AMX) adsorption, time of sonication, initial reagent concentrations, irradiation frequency, and acetic acid to pyridine molar ratios were optimized. The drug-loaded DUT-32 was soaked in simulated body fluid (SBF) and the drug release ratio was monitored through release time to perform in vitro drug release test. A slow and sustained release was observed for DUT-32 micro/nano-structures, having a considerable drug loading capacity. At the pH values 7.4-4.5, various profiles of pH-responsive release were achieved. Also, the prepared DUT-32 micro/nano-structures are found to be biocompatible with PC3 (prostate cancer) and HeLa (cervical cancer) cell lines, when tested by MTT assay. Moreover, DUT-32 micro/nano-structures were studied to show AMX adsorption from aqueous solution. Finally, kinetic studies indicated that AMX adsorption and drug release of DOX via this MOF are of first-order kinetics.

Increased Electrical Conductivity in a Mesoporous Metal-Organic Framework Featuring Metallacarboranes Guests

Nickel(IV) bis(dicarbollide) is incorporated in a zirconium-based metal-organic framework (MOF), NU-1000, to create an electrically conductive MOF with mesoporosity. All the nickel bis(dicarbollide) units are located as guest molecules in the microporous channels of NU-1000, which permits the further incorporation of other active species in the remaining mesopores. For demonstration, manganese oxide is installed on the nodes of the electrically conductive MOF. The electrochemically addressable fraction and specific capacitance of the manganese oxide in the conductive framework are more than 10 times higher than those of the manganese oxide in the parent MOF.

Direct evidence of charge separation in a metal-organic framework: efficient and selective photocatalytic oxidative coupling of amines via charge and energy transfer

For the first time, the photoexcited charge separation in a metal–organic framework has been evidenced with clear ESR signals, based on efficient and selective photocatalytic oxidative coupling of amines.

The selective aerobic oxidative coupling of amines under mild conditions is an important laboratory and commercial procedure yet a great challenge. In this work, a porphyrinic metal–organic framework, PCN-222, was employed to catalyze the reaction. Upon visible light irradiation, the semiconductor-like behavior of PCN-222 initiates charge separation, evidently generating oxygen-centered active sites in Zr-oxo clusters indicated by enhanced porphyrin π-cation radical signals. The photogenerated electrons and holes further activate oxygen and amines, respectively, to give the corresponding redox products, both of which have been detected for the first time. The porphyrin motifs generate singlet oxygen based on energy transfer to further promote the reaction. As a result, PCN-222 exhibits excellent photocatalytic activity, selectivity and recyclability, far superior to its organic counterpart, for the reaction under ambient conditions via combined energy and charge transfer.

Multifunctional Aromatic Carboxylic Acids as Versatile Building Blocks for Hydrothermal Design of Coordination Polymers

Selected recent examples of coordination polymers (CPs) or metal-organic frameworks (MOFs) constructed from different multifunctional carboxylic acids with phenyl-pyridine or biphenyl cores have been discussed. Despite being still little explored in crystal engineering research, such types of semi-rigid, thermally stable, multifunctional and versatile carboxylic acid building blocks have become very promising toward the hydrothermal synthesis of metal-organic architectures possessing distinct structural features, topologies, and functional properties. Thus, the main aim of this mini-review has been to motivate further research toward the synthesis and application of coordination polymers assembled from polycarboxylic acids with phenyl-pyridine or biphenyl cores. The importance of different reaction parameters and hydrothermal conditions on the generation and structural types of CPs or MOFs has also been highlighted. The influence of the type of main di- or tricarboxylate ligand, nature of metal node, stoichiometry and molar ratio of reagents, temperature, and presence of auxiliary ligands or templates has been showcased. Selected examples of highly porous or luminescent CPs, compounds with unusual magnetic properties, and frameworks for selective sensing applications have been described.

Single Pt Atoms Confined into a Metal-Organic Framework for Efficient Photocatalysis

It is highly desirable yet remains challenging to improve the dispersion and usage of noble metal cocatalysts, beneficial to charge transfer in photocatalysis. Herein, for the first time, single Pt atoms are successfully confined into a metal-organic framework (MOF), in which electrons transfer from the MOF photosensitizer to the Pt acceptor for hydrogen production by water splitting under visible-light irradiation. Remarkably, the single Pt atoms exhibit a superb activity, giving a turnover frequency of 35 h^-1 , ≈30 times that of Pt nanoparticles stabilized by the same MOF. Ultrafast transient absorption spectroscopy further unveils that the single Pt atoms confined into the MOF provide highly efficient electron transfer channels and density functional theory calculations indicate that the introduction of single Pt atoms into the MOF improves the hydrogen binding energy, thus greatly boosting the photocatalytic H₂ production activity.

Computational Design of Functionalized Metal-Organic Framework Nodes for Catalysis

Recent progress in the synthesis and characterization of metal-organic frameworks (MOFs) has opened the door to an increasing number of possible catalytic applications. The great versatility of MOFs creates a large chemical space, whose thorough experimental examination becomes practically impossible. Therefore, computational modeling is a key tool to support, rationalize, and guide experimental efforts. In this outlook we survey the main methodologies employed to model MOFs for catalysis, and we review selected recent studies on the functionalization of their nodes. We pay special attention to catalytic applications involving natural gas conversion.

Effect of Redox "Non-Innocent" Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal-Organic Frameworks

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu(II) precursors triggers the reduction of Cu(II) by the phenyl-catechol groups to Cu(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal-support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports.

Green and rapid mechanosynthesis of high-porosity NU- and UiO-type metal–organic frameworks

Dodecanuclear zirconium precursor enables a rapid and efficient mechanochemical synthesis of advanced MOFs NU-901 and UiO-67 with surface areas of up to 2250 m² g⁻¹.

Tunability of fluorescent metal–organic frameworks through dynamic spacer installation with multivariate fluorophores

Through dynamic spacer installation, five fluorescent metal–organic frameworks (MOFs) have been constructed based on a proto-MOF LIFM-28 and multivariate fluorophores.

Covalent post-assembly modification in metallosupramolecular chemistry

This review examines the growing variety of covalent reactions used to achieve the post-assembly modification of self-assembled metallosupramolecular complexes.

Revisiting the structural homogeneity of NU-1000, a Zr-based metal–organic framework

Synthesis and activation of phase-pure and defect-free metal–organic frameworks (MOFs) are essential for establishing accurate structure–property relationships.

Operando study of palladium nanoparticles inside UiO-67 MOF for catalytic hydrogenation of hydrocarbons

Formation of Pd nanoparticles inside UiO-67 MOF was monitored by in situ X-ray absorption and diffraction.

Synthesis of homochiral zeolitic imidazolate frameworks via solvent-assisted linker exchange for enantioselective sensing and separation

Two homochiral zeolitic imidazolate frameworks (S- and R-ZIF-78h) of GME topology are synthesized via solvent-assisted linker exchange (SALE) of ZIF-78, which exhibit permanent porosity and enantioselective sensing and separation of proline.

Multi-component hybrid films based on covalent post-synthetic functionalization of silicon chip using both ZIF-90 and lanthanide complexes for luminescence tuning

Multi-component hybrid films (Si-APTES-ZIF-90, Phen-Ln-MAPTMS-Si-APTES-ZIF-90 (Ln = Eu, Tb, Sm, Dy)) are prepared by a covalent functionalization strategy, and the photoluminescence tuning is studied in detail.

Synthesis and functionalization of phase-pure NU-901 for enhanced CO2adsorption: the influence of a zirconium salt and modulator on the topology and phase purity

Phase-pure NU-901 was functionalized with amines through solvent-assisted linker incorporation resulting in more than double the typical CO₂adsorption capacity.

Photonic functional metal–organic frameworks

The recent progress in photonic MOFs for luminescence sensing, white-light emission, photocatalysis, nonlinear optics, lasing devices, and biomedicine is summarized.

Conducting polymer-coated MIL-101/S composite with scale-like shell structure for improving Li–S batteries

Lithium–sulfur batteries are regarded as a promising energy storage system. However, they are plagued by rapid capacity decay, low coulombic efficiency, a severe shuttle effect and low sulfur loading in cathodes. To address these problems, effective carriers are highly demanded to encapsulate sulfur in order to extend the cycle life. Herein, we introduced a doped-PEDOT:PSS-coated MIL-101/S multi-core–shell structured composite. The unique structure of MIL-101, large specific area and conductive shell ensure high dispersion of sulfur in the composite and minimize the loss of polysulfides to the electrolyte. The doped-PEDOT:PSS-coated sulfur electrodes exhibited an increase in initial capacity and an improvement in rate characteristics. After 192 cycles at the current density of 0.1C, a doped-PEDOT:PSS-coated MIL-101/S electrode maintained a capacity of 606.62 mA h g⁻¹, while the MIL-101/S@PEDOT:PSS electrode delivered a capacity of 456.69 mA h g⁻¹. The EIS measurement revealed that the surface modification with the conducting polymer provided a lower resistance to the sulfur electrode, which resulted in better electrochemical behaviors in Li–S battery applications. Test results indicate that the MIL-101/S@doped-PEDOT:PSS is a promising host material for the sulfur cathode in the lithium–sulfur battery applications.

Lithium–sulfur batteries are regarded as a promising energy storage system.

Lanthanide hybrids of covalently-coordination cooperative post-functionalized metal–organic frameworks for luminescence tuning and highly-selectively sensing of tetrahydrofuran

Lanthanide based MOFs are synthesized through covalently-coordination cooperative post-functionalization, and exhibit multi-color luminescence and highly-selectively sensing of THF.

Efficient extraction of inorganic selenium from water by a Zr metal–organic framework: investigation of volumetric uptake capacity and binding motifs

MOF-808 displays enhanced gravimetric and volumetric uptake capacities for selenium oxyanions and the binding motifs are crystallographically characterized.

New synthetic strategies to prepare metal–organic frameworks

This critical review summarizes the recent developments in the application of new synthetic strategies for preparing MOFs, including the ionothermal method, deep eutectic solvent usage, surfactant-thermal process, and mechanochemistry.

Light-enhanced acid catalysis over a metal–organic framework

A sulfonate group-functionalized metal–organic framework exhibits unprecedented light-enhanced catalytic activity, even higher than H₂SO₄, and excellent recyclability toward acid-engaged reactions under light irradiation.

Ratiometric Luminescent Detection of Organic Amines Due to the Induced Lactam-Lactim Tautomerization of Organic Linker in a Metal-Organic Framework

Here we demonstrate that a fluorescent benzothiadiazole (BTD)-conjugated terphenyldicarboxylate (TPDC) linker (denoted as H₂-ostpdc) has been hybridized by a quinoxaline-2,3-(1H,4H)-dione (QD) moiety possessing lactam-lactim tautomerism, which was further integrated into a robust and porous UiO-68 type zirconium metal-organic framework (MOF UiO-68-osdm) by utilizing the mixed two dicarboxylate struts with the same ligand lengths. The resultant MOF UiO-68-osdm can work as a ratiometric luminescent sensor for visual and selective detection of alkyl amines. Furthermore, it can discriminate secondary alkylamines from other type amine species.

Design and Properties of Confined Nanocatalysts by Atomic Layer Deposition

Governing the process and outcome of chemical reactions is the most important aim of catalytic chemistry. The confinement of active sites inside nanosized spaces provides a powerful strategy to achieve this goal. Reacting molecules (reactants, intermediates, and products of a reaction) and nanomaterials (metal/metal-oxide nanoparticles) confined inside nanoreactors have been observed to exhibit modified behaviors and properties with respect to their unconfined counterparts. Typically, catalysts confined in zeolites, mesoporous materials, metal-organic frameworks, and nanotubes are obtained by traditional liquid-phase methods. However, excess metals or undesired solvents and other reagents must be removed. It is also difficult to precisely regulate the confined nanostructures and assemble multifunctional sites in the confined nanospaces. Atomic layer deposition (ALD) provides a controllable method to fabricate confined catalysts due to its outstanding advantages. In this Account, we describe our progress in the design and properties of confined nanocatalysts by ALD. ALD is an elegant method to directly deposit highly dispersed metal or oxide species into porous materials, including zeolites and mesoporous materials. We deposited Pt nanoclusters in the micropores of a KL zeolite with precisely controlled size by ALD. We also introduced CoO_x nanoclusters into mesoporous SBA-15. We have reported pioneering works on the synthesis of confined nanoparticles with metal-in-nanotube structures by a template-assisted ALD method. Confined Cu nanoparticles were prepared by reducing CuO nanowires coated with Al₂O₃, TiO₂, or alucone layers by ALD. Confined Cu and Au nanoparticles were also prepared starting from the corresponding metal nanowires with the assistance of sacrificial layers produced by ALD. In a more facile strategy, Au nanoparticles confined in Al₂O₃ nanotubes were produced using a sacrificial template by ALD. Furthermore, we synthesized a multiply confined Ni-based nanocatalyst through a template-assisted ALD method. We assembled multiple interfaces (Ni/Al₂O₃ and Pt/TiO₂) in a confined nanospace for tandem reactions by template-assisted ALD. The synergistic effect of two interfaces enhanced the tandem reaction, and the confined nanospace favored the instant transfer of intermediates between the two interfaces. In addition, porous TiO₂ nanotubes with spatially separated Pt and CoO_x cocatalysts were also produced by ALD. The confined catalysts can be further treated by ALD. We used ALD to modify the mesoporous SBA-15 support to precisely tune the active species-support interaction. In addition to the support, the confined metal nanoparticles can also be coated with an ultrathin oxide layer by ALD to further improve their catalytic activities. Moreover, the structure and size of the confined nanospace can be tuned precisely by ALD. Overall, ALD has exhibited noteworthy applications in and will provide new opportunities for the design and synthesis of highly effective confined catalysts.

Rational Design of S-UiO-66@GO Hybrid Nanosheets for Proton Exchange Membranes with Significantly Enhanced Transport Performance

Metal-organic frameworks (MOFs) are being intensively explored as filler materials for polymeric proton exchange membranes (PEMs) due to their potentials for the systematic design and modification of proton-conducting properties. S-UiO-66, a stable MOF with functional groups of -SO₃H in its ligands, was selected here to prepare S-UiO-66@graphene oxide (GO) hybrid nanosheets via a facile in situ growth procedure, and then a series of composite PEMs were prepared by hybridizing S-UiO-66@GO and sulfonated poly(ether ether ketone) (SPEEK). The resultant hybrid nanosheets not only possessed abundant -SO₃H groups derived from the ligands of S-UiO-66 but also yielded a uniform dispersion of S-UiO-66 onto GO nanosheets, thus effectively eliminating the agglomeration of S-UiO-66 in the membrane matrix. Thanks to the well-tailored chemical composition and nanostructure of S-UiO-66@GO, the as-prepared SPEEK/S-UiO-66@GO composite PEMs present a significant increase in their proton conductivity under various conditions. In particular, the proton conductivity of the SPEEK/S-UiO-66@GO-10 membrane was up to 0.268 S·cm^-1 and 16.57 mS·cm^-1 at 70 °C-95% RH and 100 °C-40% RH (2.6 and 6.0 times that of recast SPEEK under the same condition), respectively. Moreover, the mechanical property of composite membranes was substantially strengthened and the methanol penetration was well-suppressed. Our investigation indicates the great potential of S-UiO-66@GO in fabricating composite PEMs and also reveals that the high proton conductivity of MOFs can be fully utilized by means of MOF/polymer composite membranes.