Research teams such as Beijing University of Technology used MOFs to remove trace amounts of benzene vapor

On April 28, 2022, Beijing time, researchers from Beijing University of Technology, Nankai University and The University of Limerick in Ireland collaborated to publish a paper entitled “Trace removal of benzene vapor using double-walled metal-dipyrazolate frameworks” in Nature Materials.

In this study, by regulating the pore size and pore environment, the authors generated strong binding sites and effective limits of benzene molecules, enabling metal-organic framework (MOFs) adsorbents to capture trace amounts of benzene vapor, which is expected to be used for the purification of benzene polluted air and environmental remediation.

The corresponding authors are Li Jianrong, Nie Zuoren and Professor Michael J. Zaworotko; the first authors are Dr. He Tao and Kong Xiangjing.

Volatile organic compounds (VOCs), including benzene, are a class of toxic pollutants that can cause indoor and outdoor air pollution and can cause serious environmental and health problems even at trace concentrations. Current methods of removing benzene from the air include plasma oxidation, photocatalytic oxidation, activated carbon adsorption, etc. Due to the ease of use of adsorbents and the low energy consumption of recycling, physical adsorption is a good choice for removing VOCs from polluted air and industrial exhaust gases. However, the non-covalent forces driving physical adsorption tend to be weak, making it challenging to design adsorbents that can efficiently adsorb benzene at low concentrations.

Metal-organic framework (MOFs) is a class of new molecularly-based porous materials with high specific surface area and easy adjustment of structure/function, which has broad application prospects in the fields of gas adsorption storage/separation, and has been used for VOCs removal. There is usually only a weak force between MOFs and aromatic VOCs (such as benzene), and the adsorption capacity of VOCs under low pressure conditions is weak. Although some MOFs have high adsorption of VOCs at relatively high pressures (above 10 kPa), this is not very suitable for evaluating the adsorption performance of MOFs on trace VOCs in the air. Some MOFs that only have high VOCs adsorption at relatively high pressures do not perform well in actual air purification. On the other hand, in addition to good adsorption capacity, high stability is a prerequisite for THE application of MOFs in many fields, but the water and thermal stability of many MOFs is poor, and it is difficult to maintain stable structure and adsorption performance for a long time. In addition, the co-adsorption of water vapor and VOCs under wet conditions will also reduce their adsorption performance.

Research results of the relevant research team of Beijing University of Technology in the construction of stable MOFs and the adsorption/removal of pollutants (Nature Commun. 2013, 4:1538; J. Am. Chem. Soc. 2016, 138, 6204; Chem 2018, 4, 1911; Acc. Chem. Res. 2021, 54, 3083; On the basis of Engineering 2021, 7, 1115), in collaboration with the research team of the University of Limerick and Nankai University in Ireland, a series of double-walled pyrazole MOFs materials (BUT-53 ~ BUT-58) have been constructed, which have a variety of pore sizes and pore environments, which can achieve efficient capture of ppm concentration of benzene vapor in the air, and show good application potential in the fields of benzene polluted air purification and environmental remediation (Figure 1-3).

Figure 1: Molecular chemical engineering strategy was used to construct BUT-53 ~ BUT-58.

Figure 2: Benzene adsorption performance.

The results of benzene vapor adsorption experiments show that but-53 ~ BUT-58 have a high adsorption of benzene at pressures below 298 K and 10 Pa (> 2 mmol g−1). To assess the material’s ability to capture trace amounts of benzene from ambient air, the authors conducted dynamic gas breakthrough experiments, including multiple control experiments of 50% relative humidity and 80% under dry conditions. The results show that compared with the dry conditions, BUT-53, BUT-54, BUT-56 and BUT-57 have reduced their breakthrough performance of 50% and 80% relative humidity, while the humidity has less effect on the performance of BUT-55 and BUT-58, and both can maintain a high benzene adsorption capacity. In particular, the PENETRATION time of THE BUT-55 can reach 8000 h g−1 under the conditions of dryness and relative humidity of 50%.

Figure 3: SCXRD structure and DFT calculation for C6H6@BUT-55.

In order to study the adsorption mechanism of BUT-53 ~ BUT-58 efficient capture of benzene vapor, the authors analyzed the single crystal structure of multiple guest loading phases, and the results showed that there was no significant change in the frame structure before and after the capture of benzene molecules, and the adsorbed benzene molecules had two binding sites: one (yellow) was located in the cavity between two adjacent pairs of BDP2− ligands and two Co(pz)2 units; the other (red) was located on top of pairs of BDP2− ligands. Frame-object and object-object between through multiple C-H… The X interaction achieves strong adsorption of benzene in the framework. To further understand the adsorption behavior of the material, the authors performed DFT calculations, which showed that the benzene molecule interacted more strongly with the I site. In addition, the single crystal structure shows that water molecules are also preferentially adsorbed at the I site; however, when benzene and water coexist, benzene molecules are preferentially adsorbed due to stronger interactions. Therefore, the presence of water molecules has less effect on the adsorption performance of BUT-55.

Through multiple non-covalent interactions between the object molecule and the framework (including CH… X interaction), this study achieves the selective capture of trace amounts of benzene from the air using MOFs adsorbents, and has the advantages of high adsorption and long breakout time. Even in humid conditions, the BUT-55 has the ability to basically completely remove trace amounts of benzene from the air, which is expected to be used for air purification and environmental remediation. (Source: Science Network)

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