Hybridization of metal–organic framework and monodisperse spherical silica for chromatographic separation of xylene isomers

Core-shell metal-organic framework/silica hybrid with tunable shell structure as stationary phase for high performance liquid chromatography

Metal-organic framework/silica composite (SSU) were prepared by growing UiO-66 on the amino-functionalized SiO2 core-shell spheres (SiO2@dSiO2) via a simple one-pot synthesis approach. By controlling the concentration of Zr4+, the obtained SSU have two different morphologies: spheres-on-sphere and layer-on-sphere. The spheres-on-sphere structure is formed by the aggregation of UiO-66 nanocrystals on the surface of SiO2@dSiO2 spheres. SSU-5 and SSU-20, which contain spheres-on-sphere composites have mesopores with a pore size of about 45 nm in addition to the characteristic micropores of UiO-66 with a pore size of 1 nm. In addition, UiO-66 nanocrystals were grown both inside and outside the pores of SiO2@dSiO2, resulting in a 27% loading of UiO-66 in the SSU. The layer-on-sphere is the surface of SiO2@dSiO2 covered with a layer of UiO-66 nanocrystals. SSU with this structure has only a characteristic pore size of about 1 nm belonging to UiO-66 and is therefore not suitable as a packed stationary phase for high performance liquid chromatography. The SSU spheres were packed into columns and tested for the separation of xylene isomers, aromatics, biomolecules, acidic and basic analytes. With both micropores and mesopores, SSU with spheres-on-sphere structure achieved baseline separation of both small and large molecules. Efficiencies up to 48,150, 50,452 and 41,318 plates m - 1 were achieved for m-xylene, p-xylene and o-xylene, respectively. The relative standard deviations of the retention times of anilines for run-to-run, day-to-day and column-to-column were all less than 6.1%. The results show that the SSU with spheres-on-sphere structure has great potential for high performance chromatographic separation.

The Current Status of Metal-Organic Frameworks (MOFs) for Use in Liquid Chromatography

Metal-organic frameworks (MOFs) are self-assembled combinations of metals and inorganic ligands that result in a relatively young class of highly ordered, porous materials. Because of the number of structural and chemical possibilities, high surface area, controlled pore volume, and favorable thermal properties, MOFs are being investigated in several fields, including chromatography. Because of the enormous interest in a 2018 “Column Watch” article on the subject and the high level of research in the field, this article explores recent (2019–present) activity specifically toward the application of MOFs for liquid chromatography (LC).

Uniform Core-Shell Microspheres of SiO2@MOF for CO2 Cycloaddition Reactions

A SiO₂@MOF core-shell microsphere for environmentally friendly applications was introduced in this study. Several types of metal-organic framework core-shell microspheres were successfully synthesized. To achieve high stability and favorable catalytic performance, modification and coating methods were necessary for optimization. The improved SiO₂@MOF core-shell microspheres were used in the cycloaddition reaction of carbon dioxide and propylene oxide. Dispersion ability was enhanced by the addition of core-shell microspheres, which also produced high catalytic activity. Accompanied with tetrabutylammonium bromide as a co-catalyst, SiO₂@ZIF-67 had a maximum conversion of 97%, and the results revealed that SiO₂@ZIF-67 could be used for 5 reaction cycles while maintaining high catalytic performance. This recycling catalyst was also reacted with a series of terminal epoxides to form corresponding cyclic carbonates with high conversion rates, indicating that SiO₂@MOF core-shell microspheres exhibit promise in the field of catalysis.

Flammability and Thermal Kinetic Analysis of UiO-66-Based PMMA Polymer Composites

Metal-organic frameworks (MOFs) are emerging as novel flame retardants for polymers, which, typically, can improve their thermal stability and flame retardancy. However, there is a lack of specific studies on the thermal decomposition kinetics of MOF-based polymer composites, although it is known that they are important for the modeling of flaming ignition, burning, and flame spread over them. The thermal decomposition mechanisms of poly (methyl methacrylate) (PMMA) have been well investigated, which makes PMMA an ideal polymer to evaluate how fillers affect its decomposition process and kinetics. Thus, in this study, UiO-66, a common type of MOF, was embedded into PMMA to form a composite. Based on the results from the microscale combustion calorimeter, the values of the apparent activation energy of PMMA/UiO-66 composites were calculated and compared against those of neat PMMA. Furthermore, under cone calorimeter tests, UiO-66, at only 1.5 wt%, can reduce the maximum burning intensity and average mass loss rate of PMMA by 14.3% and 12.4%, respectively. By combining UiO-66 and SiO₂ to form a composite, it can contribute to forming a more compact protective layer, which shows a synergistic effect on reducing the maximum burning intensity and average mass loss rate of PMMA by 22.0% and 14.7%, respectively.

MOF-74@SiO2 core-shell stationary phase: Preparation and its applications for mixed-mode chromatographic separation

The development of versatile mixed-mode stationary phase materials is of important meanings for solving the increasing demands for real sample analysis. Herein, with 2,5-dihydroxyterephthalic acid as the organic ligand and nickel as the metal centre, MOF-74 nanocrystal materials were facilely grafted on the surface of carboxyl-functionalized silica gel via layer-by-layer assembling technique. The structures of the monodisperse MOF-74@SiO₂ material were proved by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, elemental analysis, thermogravimetric analysis, and Brunauer-Emmett-Teller specific surface area and pore size analyzer, respectively. Because the introduced 2,5-dihydroxyterephthalic acid is of hydrophilic carboxyl and hydroxyl groups, the packed MOF-74@SiO₂ column reveals hydrophilic interaction/reversed-phase mixed-mode retention properties. Compared with commercial C₈ column or silica-based column, the MOF-74@SiO₂ column shows distrinct separation selectivity in short separation time for polycyclic aromatic hydrocarbons, phenolic compounds and polar sulfonamide compounds. The developed MOF-74@SiO₂ column was further successfully applied for the separation and detection of illegal addition of glucocorticoid in children's face cream as well as sulfonamides veterinary drug residues in pure milk. The research provides a simple and convenient approach to prepare multifunctional MOFs-based stationary phase materials.

Functional Metal Organic Framework/SiO2 Nanocomposites: From Versatile Synthesis to Advanced Applications

Metal organic frameworks (MOFs), also called porous coordination polymers, have attracted extensive attention as molecular-level organic-inorganic hybrid supramolecular solid materials bridged by metal ions/clusters and organic ligands. Given their advantages, such as their high specific surface area, high porosity, and open active metal sites, MOFs offer great potential for gas storage, adsorption, catalysis, pollute removal, and biomedicine. However, the relatively weak stability and poor mechanical property of most MOFs have limited the practical application of such materials. Recently, the combination of MOFs with inorganic materials has been found to provide a possible strategy to solve such limitations. Silica, which has excellent chemical stability and mechanical properties, shows great advantages in compounding with MOFs to improve their properties and performance. It not only provides structured support for MOF materials but also improves the stability of materials through hydrophobic interaction or covalent bonding. This review summarizes the fabrication strategy, structural characteristics, and applications of MOF/silica composites, focusing on their application in chromatographic column separation, catalysis, biomedicine, and adsorption. The challenges of the application of MOF/SiO2 composites are addressed, and future developments are prospected.

Metal-Organic Frameworks in Green Analytical Chemistry

Metal-organic frameworks (MOFs) are porous hybrid materials composed of metal ions and organic linkers, characterized by their crystallinity and by the highest known surface areas. MOFs structures present accessible cages, tunnels and modifiable pores, together with adequate mechanical and thermal stability. Their outstanding properties have led to their recognition as revolutionary materials in recent years. Analytical chemistry has also benefited from the potential of MOF applications. MOFs succeed as sorbent materials in extraction and microextraction procedures, as sensors, and as stationary or pseudo-stationary phases in chromatographic systems. To date, around 100 different MOFs form part of those analytical applications. This review intends to give an overview on the use of MOFs in analytical chemistry in recent years (2017–2019) within the framework of green analytical chemistry requirements, with a particular emphasis on possible toxicity issues of neat MOFs and trends to ensure green approaches in their preparation.