Metal-Organic Framework based Filters for the Removal of Particulate Matter

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-1^st) to seven coating layers (ZIF-8@Plastic mesh-7^th) 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-1^st; (c, d) ZIF-8@Melamine foam-3^rd; (e, f) ZIF-8@Nonwoven fabric-3^rd; (g, h) ZIF-8@Glass cloth-3^rd; and (i, j) ZIF-8@Metal mesh-3^rd.

The MO filters are highly robust and offer excellent PM removal efficiency. ZIF-8@Plastic mesh-7^th filter maintained its crystallinity and morphology after several cycles. Similarly, ZIF-8@Melamine foam-3^rd tolerated 1000 cycles of bending and twisting with negligible weight loss.

For ZIF-8@Melamine foam-3^rd, the removal efficiency for PM_2.5 and PM₁₀ is 99.5% ± 1.7%, and 99.3% ± 1.2%, respectively (PM_2.5 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 (PM_2.5 > 800 μg/m³ and PM₁₀ > 1000 μg/m³), ZIF-8@Melamine foam-3^rd retained >95.4% efficiency after 12 h. Most importantly, the tested ZIF-8@Melamine foam-3^rd can be easily cleaned using tap water and ethanol, and dried at 60 °C for 3 h. ZIF-8@Melamine foam-3^rd 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-3^rd, PM_2.5: 96.8% ± 1.3%, PM₁₀: 95.8% ± 1.4% and ZIF-8@Metal mesh-3^rd, PM_2.5: 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-7^th offered a reasonably good efficiency (PM_2.5: 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-7^th could retain >90% of PM removal after a month. The used ZIF-8@Plastic mesh-7^th can be easily recycled by brush cleaning using water and ethanol. The excellent long-term stability and reusability make ZIF-8@Plastic mesh-7^th 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

Capturing CO2 using metal-organic framework (MOF)

The steady increase in concentration of CO₂ in the atmosphere (from 310 ppm to > 380 ppm during the past five decades) and its continuous increasing trend until this moment, is really a matter of concern. Power plants contribute to ~ 60% of the total CO₂ emission worldwide. Hence, development of effective CO₂ capture systems that could selectively remove CO₂ from the exhaust gas is warranted. Porous metal-organic frameworks (MOFs) are promising for CO₂ capture. Nevertheless, development of MOFs for CO₂ capture directly from the exhaust gas of power plants is indeed challenging.

Researchers at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, University of Science and Technology of China, Hefei National Laboratory for Physical Sciences at the Microscale and Wenzhou University, China have designed and synthesized a Cu(II)-MOF (FJI-H14) with high density of open metal sites (OMS) and Lewis basic sites (LBS) in which both OMS and LBS interact synergistically with CO₂ and help to capture it.

A mixture of 2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid (H₂BTTA) (0.05 mM) and Cu(NO₃)₂·3H₂O (0.05 mM) in H₂O (4 ml) in a sealed Teflon vial under hydrothermal conditions at 120 °C for 3 days has lead to the formation of rod-shaped blue crystals of FJI-H14 ([Cu(BTTA)H₂O]_n·6_nH₂O) with 73% yield based on the organic ligand H₂BTTA (Fig. 1).

Fig. 1 Structural illustration of FJI-H14: (a) ligand H₂BTTA; (b) co-ordination environment of Cu(II) ions with BTTA; (c) one-dimensional nano-porous channels; and (d) topology of MOF (Cu atom, cyan; C atom, gray; O atom, red; N atom, blue; H atom, white)

The FJI-H14 is stable in boiling water as well as in acidic and basic environments (pH: 2 to 12) at temperatures as high as 373 K. It is also thermally stable up to 230 °C. The Brunauer–Emmett–Teller (BET) specific surface area of FJIH14 is 904 m²/g and its Langmuir-specific surface area is 1004 m²/g. The total pore volume of FJIH14 estimated from CO₂ isotherm is 0.45 cm³/g. The high porosity and high concentration of open active sites in the framework has lead to an increase in the extent of CO₂ uptake up to 279 cm³/g (Fig. 2(a)). The strong absorption bands at 2,340 cm⁻¹ and 2,328 cm⁻¹ in the IR spectra indicate that the CO₂ molecules tend to stack around the open Cu(II) sites, which is also in line with the theoretical calculations. Besides high adsorption capacity, reusability is an important property for any adsorbent. FJI-H14 maintains 100% adsorption capacity even after five cycles of adsorption, suggesting its suitability as a reusable adsorbent for CO₂ capture (Fig. 2(b)).

Since the flue gas from power plants contains a large amount of N₂ (73–77 %) than CO₂ (15–16 %), CO₂/N₂ selectivity is a crucial parameter in CO₂ capture applications. The CO₂/N₂ selectivity FJI-H14 (for the 15/85 CO₂/N₂ mixture at 298 K and at 1 atm) is 51. The high selectivity for  adsorption of CO₂ over N₂ suggests that the densely populated open active sites in the framework have a positive effect on CO₂ adsorption. The relatively narrow pores in FJIH14 could have easily blocked the relatively large N₂ molecules thus favouring selectivity for CO₂ (Figs. 2(c) and 2(d)). FJI-H14 is also capable of catalyzing chemical transformation of CO₂ into value-added chemicals, such as dimethyl carbonate, cyclic carbonates, N,N’-disubstituted ureas or formic acid.

Fig. 2 Experimental CO₂ adsorption by FJI-H14: (a) CO₂ adsorption isotherm for FJI-H14 at 195 K; (b) Cycles of CO₂ adsorption for FJI-H14 at 298 K; (c) N₂ and CO₂ adsorption isotherms for FJI-H14 at 298 K; and (d) CO₂/N₂ selectivity for 15/85 CO₂/N₂ mixture at 298 K.

FJI-H14 possesses the characteristics of an ideal MOF in terms of high CO₂ uptake at ambient conditions, excellent chemical and thermal stabilities, selectivity for CO₂ over N₂, reusability, direct and smooth conversion of CO₂ into corresponding cyclic carbonates and ease of preparation at large scale.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Liang et al., Carbon dioxide capture and conversion by an acid-base resistant metal-organic framework, Nature Communications, 8 (2017) 1233, DOI: 10.1038/s41467-017-01166-3