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Sun et al. Microstructures 2023;3:2023032 https://dx.doi.org/10.20517/microstructures.2023.32 Page 7 of 21

The ability of MOFs to capture CO from varying gas mixtures depends on their inherent properties and the
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attributes of the gas mixtures. Over the past two decades, diverse topological structures and function-
oriented MOFs have been synthesized, leading to the formation of various branches of materials, including
iso-reticular MOFs (IRMOFs), zeolitic imidazolate frameworks (ZIFs), materials of institute Lavoisier
frameworks (MILs), and porous coordination networks (PCNs), etc. [86-89] . Different branches possess specific
characteristics to cater to distinct applications. Currently, several relatively mature strategies [Figure 2] have
been applied to synthesize CO capture-oriented MOFs [90-92] .
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Functionalized modification strategy
The -NH functional group is widely utilized in various adsorbent materials due to its strong attraction of
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CO to the amine group, which gives the amine molecule higher adsorption and selectivity for CO .
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Moreover, numerous polar functional groups, including halogen atoms, hydroxyl, carboxyl, cyano, and
nitro, have been demonstrated to influence the adsorption ability of CO in MOFs [93-95] .
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Diamine-functionalized MOFs in the form of diamine-Mg (dobpdc) (dobpdc = 4,4′-dioxidobiphenyl-3,3′-
4-
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dicarboxylate) offer the most potential for carbon capture applications due to their adjustable, stair-like
profiles for CO adsorption. In view of this, Dinakar et al. reported that MOFs containing dmen-Mg 2
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(dobpdc) (dobpdc = 1,2-diamino-2-methylpropane) composition [Figure 3] can capture CO from coal-
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fired flue gas at moderate pressure . Further, using Mg (pc-dobpdc) (pc-dobpdc = 3,3′-dioxobiphenyl-4,4′-
[96]
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dicarboxylate) with higher structural symmetry to avoid sub-stability during CO adsorption, dmen-Mg 2
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(pc-dobpdc) demonstrates carbon capture capabilities under similar conditions in a simulated coal-fired
power plant, achieving a complete CO adsorption effect.
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It has been shown that some specific sites of MOFs can effectively capture CO , including unsaturated metal
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sites (UMSs) and Lewis base sites (LBSs), such as amines, pyridines, sulfones, and amides. UMSs are capable
of establishing potent electrostatic interactions with CO and have a high CO capture capacity. However,
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the ubiquitous water molecules tend to coordinate with UMSs in a competitive manner, resulting in a
significant reduction in the ability of CO adsorption. Regarding LBSs, compared with -NH , the amide
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functional group exhibits a robust attraction towards CO due to the existence of two binding sites, carbonyl
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(CO-) and amine (NH-), leading to superior CO adsorption and selectivity [97-99] . In addition, MOFs
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modified by amide functional groups tend to be more stable.
Fe-dbai (dbai = 5-(3,5-Dicarboxybenzoylamino) isophthalic acid) combines two specific functional sites,
UMSs and amide functional groups. Its CO adsorption capacity is measured at 6.4 mmol g , while its
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[100]
CO /N selectivity is 64 (298 K, 1 bar), surpassing multiple other reported MOFs . Importantly, in the
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breakthrough experiments, the CO adsorption capacity of Fe-dbai at 60% RH (Relative Humidity: the
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percentage of water vapor pressure in air to the saturated water vapor pressure at the same temperature)
was able to maintain 94% of its capacity under dry conditions. Molecular simulation results showed that the
amide CO-group, with its electronegative properties, exhibits a strong affinity towards CO and enhances
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the interaction between Fe-UMS and CO [Figure 4]. The outstanding CO capture efficiency of Fe-dbai
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suggests its potential suitability for real-world implementation of CO capture.
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Amino acid (AA)-modified MOFs also show great potential for CO capture applications. Modification of
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MOF-808 with 11 different AAs resulted in a series of MOF-808-AA structures [Figure 5]. Under fume
conditions, MOF-808 functionalized with glycine and DL-lysine (MOF-808-Gly and MOF-808-DL-Lys) was
observed to exhibit the greatest CO adsorption capacity. The increased CO capture efficiency in the
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presence of water was detected and analyzed by single-component adsorption isotherms, CO /H O
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