Particulate matters (PMs) are the major source of air pollution, particularly in developing countries. Long-term exposure to PM could cause respiratory problems. PM emitted from power plants and refineries can be toxic. Metal-organic framework-based membranes are found to be effective to control air pollution. The presence of various ions and water vapor makes PM highly polar. The unbalanced metal ions on the surface of metal-organic framework and the defects present in it could impart positive charges. Hence, the electrostatic interactions between metal-organic framework and PM could be exploited for the removal PM using metal-organic framework based membranes/filters. In this perspective, researchers at Beijing Key Laboratory of Photoelectronic/ Electrophotonic Conversion Materials, Beijing Institute of Technology, China have developed a roll-to-roll hot pressing method for the preparation of MO Filters (MOF) for the removal of particulate matter (PM).
The roll-to-roll hot pressing method enables mass production of MO filters (Fig 1). Three different zeolite imidazolate framework, viz., ZIF-8, ZIF-67, and Ni-ZIF-8 were used to develop the filters on substrates such as plastic mesh, glass cloth, metal mesh, nonwoven fabric, and melamine foam. ZIF-8 is also coated on the plastic mesh by a layer-by-layer fashion. ZIF-8@plastic mesh was prepared by covering the plastic mesh (thickness: 300 μm; width: 10 cm) with ZIF-8 precursors (Zn(OAc)2·2H2O, 2-methylimidazole, and polyethylene glycol-200) and rolled between two rollers ~80 °C at 15 rpm. With repeated cycles of operation, ZIF-8@Plastic mesh with one coating layer (ZIF-8@Plastic mesh-1st) to seven coating layers (ZIF-8@Plastic mesh-7th) were prepared. The SEM images and photographs of representative MO filters are shown in Fig. 2.
Fig. 1 Schematic representation of the roll-to-roll production of various MOF-based filters (MO Filters) for the removal of PM.
Fig. 2 SEM images (a, c, e, g, i) and photographs (b, d, f, h, j) of different MO filters: (a, b) ZIF-8@Plastic mesh-1st; (c, d) ZIF-8@Melamine foam-3rd; (e, f) ZIF-8@Nonwoven fabric-3rd; (g, h) ZIF-8@Glass cloth-3rd; and (i, j) ZIF-8@Metal mesh-3rd.
The MO filters are highly robust and offer excellent PM removal efficiency. ZIF-8@Plastic mesh-7th filter maintained its crystallinity and morphology after several cycles. Similarly, ZIF-8@Melamine foam-3rd tolerated 1000 cycles of bending and twisting with negligible weight loss.
For ZIF-8@Melamine foam-3rd, the removal efficiency for PM2.5 and PM10 is 99.5% ± 1.7%, and 99.3% ± 1.2%, respectively (PM2.5 and PM10 refer to PM with an aerodynamic diameter < 2.5 and 10 μm). When tested for its long-term efficiency using a simulated pipe system with a large amount of PM (PM2.5 > 800 μg/m3 and PM10 > 1000 μg/m3), ZIF-8@Melamine foam-3rd retained >95.4% efficiency after 12 h. Most importantly, the tested ZIF-8@Melamine foam-3rd can be easily cleaned using tap water and ethanol, and dried at 60 °C for 3 h. ZIF-8@Melamine foam-3rd is promising for pipe filtration systems for fine PM removal.
ZIF-8@Glass cloth and ZIF-8@Metal mesh are found to be suitable for removal of PM at high temperatures. When tested at 200 °C, both of them exhibit a good efficiency for PM removal (ZIF-8@Glass cloth-3rd, PM2.5: 96.8% ± 1.3%, PM10: 95.8% ± 1.4% and ZIF-8@Metal mesh-3rd, PM2.5: 91.6% ± 1.3%, PM10: 90.7% ± 1.1%). ZIF-8@Glass cloth and ZIF-8@Metal mesh are suitable for baghouse dust collectors, pipe filters, and inlet barrier filters and exhaust pipes filters for vehicle or aircraft engine systems.
ZIF-8@Plastic mesh-7th offered a reasonably good efficiency (PM2.5: 56.3% ± 1.6%, PM10: 58.4% ± 2.1%) for PM removal in simulated living environment. Long-term testing indicates that ZIF-8@Plastic mesh-7th could retain >90% of PM removal after a month. The used ZIF-8@Plastic mesh-7th can be easily recycled by brush cleaning using water and ethanol. The excellent long-term stability and reusability make ZIF-8@Plastic mesh-7th as a promising filter for the removal of PM in residential environments.
The easy scalability of the roll-to-roll hot pressing method for mass production, the efficiency, robustness, stability, long-term performance and reusability of MO filters are promising and they are suitable for the removal of PM from residential as well as industrial environments.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Yifa Chen et al., Roll-to-Roll Production of Metal-Organic Framework Coatings for Particulate Matter Removal, Adv. Mater. 2017, 1606221, DOI: 10.1002/adma.201606221
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