Synthesis and environmental applications of defective metal organic frameworks

Abstract

Metal organic frameworks (MOFs) are attracting increasing attention in the field of pollution control due to their inherent characteristics, such as high porosity, large specific surface areas, and tunable structure and composition. Though more than 20000 kinds of MOFs structures have been reported so far, the majority of them are regular and composed of different kinds of building units. Defect engineering in MOFs is an exciting concept for tailoring material properties, which opens up novel opportunities in practical application. Defects are usually present in MOFs, which would alter the performance of MOFs by changing surface properties, pore structure, and the number of active sites. Since defects can enhance the performance of MOFs in adsorption and catalytic degradation of pollutants, various methods for designing and generating defects have been employed. Based on selected reports spanning the last decades, this review focuses on the most recent and significant developments in the classification of defects in MOFs and methods of introducing defects into MOFs. According to the composition of MOFs, defects can be divided into missing-linkers defect and missing-clusters defect. The main methods of introducing defects into MOFs include de novo synthesis and post-synthetic treatment. De novo synthesis method includes mixed linker approach, modulation approach and fast crystal growth approach. Post-synthetic treatment method involves acid/base treatment approach, harsh activation approach, solvent-assisted linker exchange and mechanical treatment approach. The use of defective MOFs as adsorbent and catalyst in pollutant removal is also highlighted. Because single physical-chemical characterization has its limitations, researchers adopt various techniques to characterize the existence and concentration of defects, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), etc. Reasonable defects could be controlled by adjusting the synthesis conditions in an attempt to obtain MOFs with excellent performance in adsorption and catalysis. At the hand of adsorption, the adsorption of pollutants in water and gases onto defective MOFs is mainly affected by pore distribution, specific surface area and surface charge. The introduction of missing-linkers defect or missing-clusters defect would make the regular pore structure into locally disordered structure, and create new micropores, mesopores even macropores. Thereby, the pore volume and specific surface area of MOFs are increased, and then enhancing the adsorption performance. In view of photocatalysis, introducing defects into photo-responsive MOFs can not only promote the transfer of photo-generated electrons, but also inhibit electron-hole recombination, and even optimize the energy band structure, which synergistically results in the improvement of photocatalytic performance. From the perspective of chemical catalysis, the pores formed by missing-linkers defect or missing-clusters defect in MOFs would expose the interior surface, which creates additional catalytic sites for pollutants reaching, and then enhances the catalysis performance of MOFs. In short, defective MOFs have a wide range of applications in the fields of adsorption and catalysis. After summarizing the recent progress of the field, the advantages and shortcomings of present defective MOFs in pollution control are presented. On the basis of this, it is apparent that further deepening the basic theoretical study of defective MOFs will be helpful for its wide applications. The defect engineering of MOFs is beneficial for further industrialization. Finally, some potential research topics in construction of defects in MOFs for environmental application have also been proposed in the review.