Xiu-Liang Lv

Photocatalytic organic pollutants degradation in metal–organic frameworks

Efficient removal of organic pollutants from wastewater has become a hot research topic due to its ecological and environmental importance. Traditional water treatment methods such as adsorption, coagulation, and membrane separation suffer from high operating costs, and even generate secondary pollutants. Photocatalysis on semiconductor catalysts (TiO2, ZnO, Fe2O3, CdS, GaP, and ZnS) has demonstrated efficiency in degrading a wide range of organic pollutants into biodegradable or less toxic organic compounds, as well as inorganic CO2, H2O, NO3−, PO43−, and halide ions. However, the difficult post-separation, easy agglomeration, and low solar energy conversion efficiency of these inorganic catalysts limit their large scale applications. Exploitation of new catalysts has been attracting great attention in the related research communities. In the past two decades, a class of newly-developed inorganic–organic hybrid porous materials, namely metal–organic frameworks (MOFs) has generated rapid development due to their versatile applications such as in catalysis and separation. Recent research has showed that these materials, acting as catalysts, are quite effective in the photocatalytic degradation of organic pollutants. This review highlights research progress in the application of MOFs in this area. The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the catalytic performance, the involved challenges, and the prospect are discussed and estimated. It is clear that MOFs have a bright future in photocatalysis for pollutant degradation.

Pyrazolate-Based Porphyrinic Metal–Organic Framework with Extraordinary Base-Resistance

Guided by a top-down topological analysis, a metal-organic framework (MOF) constructed by pyrazolate-based porphyrinic ligand, namely, PCN-601, has been rationally designed and synthesized, and it exhibits excellent stability in alkali solutions. It is, to the best of our knowledge, the first identified MOF that can retain its crystallinity and porosity in saturated sodium hydroxide solution (∼ 20 mol/L) at room temperature and 100 °C. This almost pushes base-resistance of porphyrinic MOFs (even if MOFs) to the limit in aqueous media and greatly extends the range of their potential applications. In this work, we also tried to interpret the stability of PCN-601 from both thermodynamic and kinetic perspectives.

A Base-Resistant Metalloporphyrin Metal–Organic Framework for C–H Bond Halogenation

A base-resistant porphyrin metal-organic framework (MOF), namely PCN-602 has been constructed with 12-connected [Ni8(OH)4(H2O)2Pz12] (Pz = pyrazolate) cluster and a newly designed pyrazolate-based porphyrin ligand, 5,10,15,20-tetrakis(4-(pyrazolate-4-yl)phenyl)porphyrin under the guidance of the reticular synthesis strategy. Besides its robustness in hydroxide solution, PCN-602 also shows excellent stability in aqueous solutions of F-, CO32-, and PO43- ions. Interestingly, the Mn3+-porphyrinic PCN-602, as a recyclable MOF catalyst, presents high catalytic activity for the C-H bond halogenation reaction in a basic system, significantly outperforming its homogeneous counterpart. For the first time, a porphyrinic MOF was thus used as an efficient catalyst in a basic solution with coordinating anions, to the best of our knowledge.

A stable porphyrinic metal–organic framework pore-functionalized by high-density carboxylic groups for proton conduction

A new porphyrinic metal–organic framework (MOF), [Co(DCDPP)]·5H2O, was designed and constructed by using a bifunctional ligand, 5,15-di(4-carboxylphenyl)-10,20-di(4-pyridyl)porphyrin (H2DCDPP). This MOF features a three-dimensional framework structure with open channels, surrounded by high-density non-coordinating carboxylic groups, and exhibits a high proton conductivity of 3.9 × 10−2 S cm−1 at 80 °C and 97% relative humidity, higher than most conductive MOFs. Moreover, it also shows excellent stability in boiling water and cyclic proton conduction.

Tuning Water Sorption in Highly Stable Zr(IV)-Metal–Organic Frameworks through Local Functionalization of Metal Clusters

Water adsorption of metal-organic frameworks (MOFs) is attracting intense interest because of their potential applications in atmospheric water harvesting, dehumidification, and adsorption-based heating and cooling. In this work, through using a hexacarboxylate ligand, four new isostructural Zr(IV)-MOFs (BUT-46F, -46A, -46W, and -46B) with rare low-symmetric 9-connected Zr6 clusters were synthesized and structurally characterized. These MOFs are highly stable in water, HCl aqueous solution (pH = 1), and NaOH aqueous solution (pH = 10) at room temperature, as well as in boiling water. Interestingly, the rational modification of the metal clusters in these MOFs with different functional groups (HCOO-, CH3COO-, H2O/OH, and PhCOO-) enables the precise tuning of their water adsorption properties, which is quite important for given application. Furthermore, all four MOFs show excellent regenerability under mild conditions and good cyclic performance in water adsorption.

Flexible metal–organic frameworks for the wavelength-based luminescence sensing of aqueous pH

Luminescence sensing is commonly based on the change of emission intensity, lifetime, and/or quantum yield of luminescent materials. Wavelength-based luminescence sensing has been comparatively rarely documented. Flexible metal–organic frameworks (MOFs) with rich fluorophores are promising sensory materials due to their unique stimuli-responsive luminescence. However, it is still challenging to construct such materials with high hydrolytic stability, which would allow them to be utilized in water systems. Here, we present two isoreticular flexible MOFs synthesized by carbazole-derived ligands, which show high porosity and excellent stability in aqueous solutions with a wide range of pH values. Interestingly, both MOFs show a structural breathing behavior with variation of the guest molecules inside their pores due to the topologically allowed flexibility. With their frameworks breathing, sensitive fluorescence changes in the wavelength of the maximum emission have been observed. Remarkably, the wavelengths of the maximum emission are well dependent on the pH values (1 to 8) of the aqueous solution in a linear relationship, suggesting a new means of accomplishing pH sensing based on emission wavelength instead of intensity. Wavelength-based fluorescence sensing thus offers an alternative way of achieving pH indication, free from some of the drawbacks associated with intensity-based fluorescence sensing, which should be fundamentally important for the development of new pH sensors in environmental and life sciences.

Nanocage containing metal-organic framework constructed from a newly designed low symmetry tetra-pyrazole ligand

Abstract 1368-Tetra(1H-pyrazol-4-yl)-9H-carbazole (H4CTP), a tetra-pyrazole ligand with Cs symmetry, has been synthesized based on a carbazole core. A solvothermal reaction of this ligand with NiCl2·6H2O gave a three-dimensional (3-D) metal-organic framework (MOF), [Ni(H4CTP)Cl2]·nS (BUT-41), which crystallized in the cubic space group Pm-3 in spite of H4CPT with a central carbazole core and four peripheral pyrazole rings has low symmetry. The framework of BUT-41 can be regarded as a four-connected 3-D net with the rhr topology when both the organic ligand and the metal center are considered as four-connected nodes. Nanocages with internal diameter of 2 nm are present in the framework of BUT-41, which are formed by interconnecting 12 H4CTP ligands and 20 Ni(II) ions. Each nanocage connects with six adjacent cages through sharing hexagonal windows with diameter over 7 Å, resulting in 3-D intersecting channels of the MOF. Although the tetra-pyrazole ligand is not deprotonated after coordination with the metal ions, powder X-ray diffraction and N2 adsorption experiments reveal that the framework of BUT-41 is rigid and permanently porous with the Brunauer-Emmett-Teller surface area up to 1551 m2 g−1. Furthermore, gas adsorption experiments show that this MOF selectively adsorbs CO2 over N2 and CH4.