Particulate matters removal by using cotton coated with isomeric metal-organic frameworks (MOFs): Effect of voidage of MOFs on removal

https://doi.org/10.1016/j.jiec.2020.12.035Get rights and content

Highlights

  • Isomeric MOFs were firstly applied to capture particulate matters (PMs) from air.

  • Three isomeric MAFs (MAF-5, -6 and -32) were applied to understand basics of PM removal.

  • Removal efficiency and quality factor of MAFs increased linearly with increasing voidage.

  • MAF-6 was highly effective in PM removal compared with MOFs like ZIF-8 and UiO-66.

  • PM removal might be explained with contribution of small functional groups of PMs.

Abstract

High quality of air is indispensable for our comfortable life and sustainability; however, air contamination with various substances, especially with particulate matters (PMs), is worsening in recent days. In this study, we applied three isomeric metal-organic frameworks (MOFs) such as MAF-5, MAF-6 and MAF-32 (MAF means metal-azolate framework, a sub-class of MOF), after coated on cotton, in removing PMs from air. Isomeric MOFs were applied to identify an important parameter of MOFs in PM capture/removal since those MOFs have different characteristics (even have the very same composition). It was found that removal efficiency and quality factor (QF) of MAFs/cotton linearly increased with increasing the voidage (v/v) of the investigated MAFs, suggesting the importance of voidage of MOFs in PM capture. Moreover, we could explain how MAFs with small pore (compared with the size of PMs) is effective in interaction (for capture) between PM and MAFs. Finally, the MAF-6, the best MAF among the studied MAFs, was much more efficient in PM removal than conventional or widely applied MOFs like ZIF-8 (for example, QF for PM2.5 of cotton, MAF-6/cotton and ZIF-8/cotton are 0.016, 0.060 and 0.030 Pa−1, respectively).

Introduction

Currently, air quality is worsening very rapidly with volatile organic compounds (VOCs), O3, NOx, SOx, and particulate matters (PMs) and so on. Especially, contamination of air with PMs is very severe in big cities and some countries. PMs are derived from sources including industrial pollution, vehicular emission, coal/biomass combustion, soil dust, etc. [1], [2], [3], [4], [5], [6], [7], [8], [9]. Additionally, PMs can be derived from secondary reactions among various environmental pollutions like ammonia, SOx, NOx, and VOCs [7], [8]. Because of several origins and complex reactions, composition of PMs is not simple. Thus far, the main components of PMs are sulfate, nitrate, ammonium (SNA), carbon, and so on [1], [2], [3], [4], [5], [6], [7], [8], [9]. Conventionally, PMs have been divided with the size or aerodynamic diameter rather than with the composition or origin. Generally, PM10 and PM2.5 mean the PM with size less than 10 and 2.5 μm, respectively [1], [2], [3], [4], [5], [6], [7], [8], [9]. PMs have a serious negative impact on visibility, human health, and solar illumination. Notably, our health is endangered (from respiratory disease to mortality) by PMs, especially smaller ones, because of toxicity of PMs.

Considering the bad impact of PMs, not only preventing PMs evolution (from sources) but also PMs removal from the environment is very crucial for our life and sustainability. Removal of PMs from air has been done with air filters (porous or fibrous filters) [10] composed of polymer, oxide, metal, silk, chitosan, carbon and so on [1], [2]. Air filters for PM removal have been developed continuously and have advantages like low cost, facile manufacturing, and versatility in applications (with a wide range of shapes and sizes) [1], [2], [10], [11], [12], [13], [14], [15]. However, further developments in air filters are required for high removal efficiency (RE), high transparency, facile reusability, low cost, low pressure drop, light weight, etc.

Nanoporous materials [16], [17], [18], [19], [20], [21], which might be actively applied for air filters, have been advanced rapidly because of porous metal-organic frameworks (MOFs) [22], [23], [24], [25], [26] and MOF composites [27], [28], [29], [30], [31]. MOFs are produced by forming a coordination bond between organic ligands and metallic species. MOFs, different from other nanoporous materials like zeolites and carbons, have several strong points such as large porosity, ample porous structures, various compositions and ready modification [32], [33]. Since 2016 [34], [35], [36], MOFs, after loaded onto substrates, have been actively applied in PM removal. As has been reviewed by us [37], various MOFs such as ZIF-8 [34], [35], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], UiO-66 [34], [35], [51], [52], [53], MOF-74 (or, CPO-27) [34], [35], ZIF-67 [54], [55], [56], [57], MIL-53 [58], [59], Cu-BTC [34], [60], and MIL-101 [35], [61] have been introduced onto substrates, for PM removal, via in situ preparation, electrospinning, hot pressing, and covalent bonding.

Even though there were noticeable progresses [37] on (i) increment in removal efficiency or quality factor (QF) (by large quantity) and (ii) perfect removal of PMs with MOFs, further works are required to understand basics on PM removal, especially with MOFs. So far, effective PM removal with MOFs has been explained [37] with several factors like functional groups, high porosity/hierarchical pores, surface charge (or defect), open metal site and so on. However, fundamental understanding on PM removal mechanisms is not very complete and important parameters of MOFs that contribute to effectively remove/capture PMs should be investigated more.

In this work, we applied three isomeric MOFs called metal-azolate frameworks (MAFs, a sub-class of MOFs) such as MAF-5, -6, and -32 [62], [63] (whose structures and characteristics, including voidage, are illustrated as Fig. S1 and Table S1, respectively) in PMs removal from air to understand an important parameter of MOFs since isomeric MOFs have different crystal structures even though those have the very same chemical composition. The MAFs were coated onto cotton fabric since cotton is non-toxic (useful as face masks), bio-degradable and easily available. Via this research, it could be confirmed that not only removal efficiency but also quality factor increased linearly with the voidage of the investigated three MOFs. Therefore, it could be suggested that high voidage of porous materials is very important in PM removal. Moreover, we try to explain how MAFs could remove PMs even though MAFs have much smaller pore than the size of general PMs like PM2.5 and PM10.

Section snippets

Chemicals

The chemicals applied in this study are summarized in Supporting Information (SI).

MOFs synthesis

The isomeric MAFs like MAF-5, -6 and -32 were synthesized hydrothermally to follow the reported protocols [64], [65]. Similarly, other MOFs such as MOF-74, UiO-66 and ZIF-8 were prepared to follow well-known recipes [66], [67], [68].

MOFs coating on cotton

The MOFs including MAFs were coated onto cotton by using a linker, (3-aminopropyl)trimethoxysilane (APTMS), between MOFs and cotton substrate. As shown in Scheme 1, a MOF can be coated

Characterization of MOFs and filters

The XRD patterns of MAFs, both synthesized and coated (onto cotton) ones, are shown in Fig. S2. Based on the similarity of the XRD patterns (of simulated, synthesized and coated MAFs), it could be confirmed that the three MAFs were synthesized and coated well on the cotton. With increasing the coated quantity of MAF-6, the intensity of characteristic XRD patterns from MAF-6 increased accordingly (Fig. 1), confirming the facile coating of MAF-6 with a range of quantities (10–28%). FTIR spectra

Important parameter of MAFs in PM elimination

Thus far, the removal of PMs with MOFs has been explained with (i) functional group [34], [42], [51], [52], [61], (ii) large porosity (or BET surface area) [45], [58], [70]/hierarchical pores [60]/specific structure [74], (iii) surface charge or charge separation [34], [51], [52], (iv) defect or open metal site [40], [42], [75] and so on. Even though there are a few quantitative analyses of each factor by us [52], [70], further study to understand other important parameters of MOF-based filters

Conclusion

Through the research to capture particulate matters from air with various MOFs, including three isomeric MAFs, coated on cotton, following conclusion can be derived. First, not only removal efficiency but also quality factor of air filters linearly increased (R2  0.886) with increasing the voidage (v/v) of the coated MAFs, suggesting the importance of voidage in PM removal. Second, MAF-6 with high voidage was more effective in PM capture than conventional and widely investigated MOFs such as

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of competing interest

The authors declare no competing financial interest.

Acknowledgement

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (grant number: 2017R1A2B2008774).

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