Screening, preparation, and prototyping of metal–organic frameworks for adsorptive carbon capture under humid conditions

Adsorption‐based carbon capture has been recognized as an attractive method for mitigating global warming. Metal–organic frameworks (MOFs) are promising candidate adsorbents for this purpose due to their high adsorption uptake and selectivity for carbon dioxide. However, in real‐world applications, such as direct air capture, the presence of moisture in the feed gas may pose a grand challenge for CO2 adsorption in MOFs. This paper aims to address the issue of water–CO2 co‐adsorption in MOFs and present screening criteria for selecting MOFs that preferentially adsorb CO2 under humid conditions. First, we uncover a comprehensive overview of CO2–water co‐adsorption characteristics of various MOFs. Then, the high‐throughput screening methods are summarized. Both computational and experimental efforts have been dedicated to identify the promising MOFs for humid CO2 capture. According to the screening results and adsorption mechanism, the optimal preparation strategies are proposed to modulate the effect of water on CO2 uptake in MOFs. Finally, current MOF‐based CO2 capture prototypes are presented to evaluate their practical feasibility and performance. This work could offer valuable guidance for the development and application of MOFs for CO2 capture in the presence of water and inspire further research in this field.

in the Goddard Institute for Space Studies analysis and constantly reached new records. 3Carbon dioxide removal (CDR) method plays an important role in mitigating global warming.
Carbon capture and storage is regarded as a promising CDR approach to reduce CO 2 concentrations from large industrial plants or discrete emission sources of atmosphere.It can be achieved by various methods, for example, absorption, adsorption, cryogenics, membranes, and microbial/algal systems. 4Adsorption using solid sorbents takes the advantage of less regeneration energy over absorption via liquid sorbents. 5The energy penalty dramatically decreases by reducing evaporation heat of water and the sensible and latent heat required for the desorption process.Metal-organic frameworks (MOFs) is an emerging and favorable branch of solid sorbents among these porous sorbents.It is a network of solids that are composed of metal ions or metal cluster vertices and organic linkers. 6,7Due to the modular and crystalline characteristics, pore sizes, pore shapes, and surface chemical potentials of MOFs can be easily adjusted for CO 2 capture through the modifications of their metal clusters and organic ligands.Due to the bridged/intertwined porous structures with a high surface area, MOFs are well-defined hosts for gas molecules. 8,91][12][13][14] In addition, some MOFs are desirable under low CO 2 concentrations and are emerging as the right adsorbents for direct air capture (DAC). 15,16However, other component gases in gas source may have an impact on their adsorption characteristics of CO 2 , even the stability of original structures.
Water is a ubiquitous component in air and most large industrial CO 2 sources and cannot be completely eliminated through a pretreatment stage.8][19] Due to these hydrophilic structures with polar bonds, CO 2 adsorption capacities have a dramatic drop. 202][23] Some exceptions are described as follows.For MIL-101(Cr), 24 ZIF-8, 25 ZIF-Cl, 26 SALEM-2, 26 SGU-29, 27 etc., a given amount of water might only have a negligible impact on CO 2 adsorption.Moreover, moisture-enhanced CO 2 adsorption can be observed in HKUST-1, 28 MIL-100(Fe), 29 InOF-1, 30 UiO-66, 31 PCN-250(Fe 3 ), and PCN-250(Fe 2 Co). 32Therefore, based on the properties of sorbents and particular moisture contents, CO 2 adsorption of MOFs performs differently under humid conditions.Despite the remarkable poten-tial of MOFs to adsorb CO 2 in the aqueous environment, there is still a lack of full understanding about adsorption analysis, mechanism clarification, and the optimal design of MOFs in such a hot research field.Therefore, further research is required to fill these gaps and provide efficient solutions to mitigating CO 2 emissions in the presence of water.
In order to describe the modification strategies for enhancing CO 2 adsorption of MOFs in a moist environment thoroughly and systematically, it is desirable to seek high-throughput screening methods and analyze the characteristics of CO 2 adsorption in the presence of water.The common traits of the screened MOFs would provide the insights for developing MOFs for the target applications.This review aims to conduct a comprehensive discussion on CO 2 adsorption of MOFs under the humid conditions, and the concerning general schematic is shown in Figure 1.First, the high-throughput screening of MOFs for CO 2 adsorption is revealed in Section 2, which includes preliminary and further processes.Second, modification strategies for the improvement of capture performance based on adsorption mechanisms are summarized in terms of physical and chemical factors in Section 3. Finally, lab-scale carbon capture prototypes using MOFs are presented and verified with regard to CO 2 -water co-adsorption processes in Section 4. In real CDR plants, more water-related issues are involved.Based on research achievements, future challenges and perspectives of MOFs for CO 2 -water coadsorption are revealed in the end.This work would contribute to better CO 2 adsorption performance in wet gases by elaborating contemporary exemplary MOFs in terms of adsorption properties and mechanisms.

SCREENING OF MOFS FOR CO ADSORPTION
CO 2 adsorption by MOFs is based on van der Waals forces, chemical bonds, or Coulombic forces from atoms or ions of sorbents.The driving force is determined by the surface properties of the adsorbates/adsorbents and environmental factors, for example, working temperature and pressure.However, the presence of other impurities in feed gas may change adsorption sites or adsorption affinity, thereby having synergistic or competitive effects on CO 2 adsorption.Table 1 lists the major gas constituents from the post-combustion process, the pre-combustion process, and the atmosphere.This indicates that CO 2 /N 2 adsorption selectivity is generally higher due to the lower quadrupole moment of N 2 , and its kinetic diameter is large enough to separate by tailoring pore size.But water has a smaller kinetic diameter and a higher dipole moment when compared to other two gases.Therefore, pores in the 1 The general schematic of the main parts: high-throughput screening of metal-organic frameworks (MOFs), modification strategies to enhance CO 2 adsorption in the presence of water and the lab-scale prototype demonstration.[37]   adsorbent have a stronger affinity toward water.Due to the smaller kinetic diameter of water, "molecular sieve effect" by decreasing pore size is invalid to improve CO 2 /H 2 O selectivity.Meanwhile, water molecules linked to CUS reduce binding energy between CO 2 and framework of adsorbents. 21Water adsorption hinders CO 2 capacity and selectivity of gas mixtures in most MOFs, especially when humidity is high. 33,34Yu and Balbuena 21 investigated the effect of water on CO 2 adsorption of Mg-MOF-74.As confirmed by molecular simulation, coordinated water abates CO 2 interaction with framework of adsorbents, and CO 2 uptake decreases as water content increases.It is still challenging to achieve selective CO 2 adsorption under humid conditions.

Composition
The CO 2 uptake of various MOFs, as well as molecular sieve, carbon, and zeolite, is compiled under different humidities, as shown in Figure 2. The larger symbols are not limited to areas with low adsorption amounts.Some types of MOFs have the better adsorption performance under high humidity condition.They consist of both physical adsorbents, such as dptz-CuTiF 6 , 38 PCN-250(Fe 2 Co), 32 and CO 2 (dobdc), 21 and chemical adsorbents such as dmpn-Mg 2 (dobpdc) 39 and mmen-Mg 2 (dobpdc). 40n contrast, the hydrophilic nature of zeolite 13X renders it essentially incapable of absorbing CO 2 in the presence of water.Mg-MOF-74, MIL-96(Al), and other compounds exhibit similar reductions in CO 2 adsorption uptake.More attention should be paid to adsorbents that exhibit a slight 0.00 0.10 0.12 0.14 0.16 0.18 0.20 0.22 F I G U R E 2 A summary of CO 2 adsorption capacity of metal-organic framework (MOF) materials and several other types of adsorbents under different humidity conditions.The humidity is shown by the symbol's size.The adsorption temperature is in the range of 298-313 K. 21,24,25,29,32,33,[38][39][40][41][42][43][44][45][46][47][48] reduction in CO 2 uptake under the conditions of an elevated humidity or even perform better when exposed to water molecules.It is also critical to identify the mechanism responsible for the abnormal rise in CO 2 uptake.
CO 2 -water co-adsorption characteristics vary greatly among different materials.Thus, it is essential to identify preferable MOFs under humid conditions.Highthroughput screening via molecular simulation is an effective method for screening MOFs with high potential.The evaluation criteria and material screening results are expounded.

High-throughput screening indicators
A large number of MOFs with the adjustable pores and functional groups have been discovered.In the Cambridge Structural Database, crystallographic information files of over 10 000 non-disordered MOFs are indexed. 49t is convinced that variable pore sizes, functional group diversities, and structural differences all have a significant influence on CO 2 adsorption properties. 50In general, the experimental conditions are challenging to complete largescale material screening, especially when water effect needs to be counted.Thus, with the rapid advancement of computer technology, more researchers have conducted molecular simulations to screen ideal CO 2 adsorption materials under wet conditions using a high-throughput method that is validated by the experimental results.
High-throughput screening studies on CO 2 adsorption materials are summarized by considering water effect.
First, its screening methods and performance indicators are discussed.The common evaluation indicators are shown in Table 2.Among these indicators, Henry constant (K H ), Henry selectivity (S H ), and adsorption enthalpy (∆H ads ) are used for the preliminary screening.The working capacity (∆N) and regenerability (R%) are the basic indicators that only focus on CO 2 adsorption of materials, regardless of other components.The additional parameters include selectivity (S), percent uptake loss (P UL ), and separation potential (∆Q), which are related to the performance of multicomponent adsorbates' adsorption or separation.The adsorbent performance score (APS) is a comprehensive evaluation indicator.

Indicators of preliminary screening
Although GCMC is an ideal method, the computational cost of the simulation for adsorption with the participation of water molecules will significantly increase because acceptance probabilities of insertion and deletion of water molecules are very low.In addition, the accuracy of simulation strongly depends on H 2 O model.The TIP4P model represents each water molecule with four atoms, which is widely used. 51Zhao et al. 52 performed 1 × 10 7 working cycles on water adsorption, but others only conducted 5 × 10 5 cycles.With regard to computer performance and screening efficiency, more convenient indicators should be adopted for the preliminary screening instead of wet adsorption isotherms of materials.The Henry constant is the most commonly used screening index for the preliminary high-throughput screening TA B L E 2 Evaluation indicators for material screening.

Evaluation indicators Formula Components
Henry's constant a  H = The relationship between CO 2 capacity loss and the K H of H 2 O. Reproduced with permission. 55Copyright 2017, Wiley.
of the materials for CO 2 adsorption when considering the influence of water.The relationship between P UL and K H of water is indicated in Figure 3, where each point represents a kind of MOF.Except for a few MOFs, the loss of CO 2 capacity of most types shows a positive linear correlation with the logarithm of  H,H2O .Several common calculation methods can be used to obtain K H of water.One method is to calculate the slope of the straight line region of adsorption isotherm (ratio of gas adsorption capacity to bulk phase concentration), but it is not suitable for highthroughput screening.Besides, K H can also be calculated by Widom insertion method 53 or deduced from the infinite dilution enthalpy, 54 which could correspond to Formulas (a) and (b) in Table 2.
In the actual screening process, S H for CO 2 to H 2 O is often used as an indicator of the preliminary screening.Li et al. 56 screened out top 15 structures with the largest S H from CoRE MOF database, which has 5109 MOF structures. 57Figure 4A shows that S H is greater than that calculated by GCMC simulation in most cases.A reasonable explanation is that K H only considers adsorption properties at low pressure but ignores the changes at high pressure.It is obvious that interaction between adsorbate molecules increases at high pressure, which is almost ignored at low pressure.Especially for water molecules, a hydrogen bonding network formed at high pressure greatly enhances the interaction between water molecules.Furthermore, their team also investigated the influence of different charge assignment methods on K H , as shown in Figure 4B.K H for CO 2 and N 2 are slightly influenced by the method of charge calculation.Particularly in the case of a large K H of H 2 O, the extended charge equilibration (EQeq) method tends to underestimate its value, which also leads to a larger S H than GCMC selectivity.
Besides, Li et al. 58 studied the effect of charge assignment methods on K H . Except for EQeq method, the density-derived electrostatic and chemical methods are adopted to assign the partial charges.A similar conclusion Comparison between K H calculated by extended charge equilibration (EQeq) method and REPEAT method.Reproduced with permission. 56opyright 2016, ACS.
can be yield as follows.There is little difference between K H of CO 2 and N 2 calculated by different charge methods, but a great difference between the K H of H 2 O can be found.To explain the difference, van der Waals force and Coulomb force of three molecules are calculated, as shown in Figure 5. Compared with CO 2 and N 2 , energy composition of water adsorption is significantly different, in which the Coulomb force has a large proportion.This explains why charge assignment method has a large influence on the estimation of K H of H 2 O.In addition to K H , ∆H ads can also be used for the preliminary screening, which partially reflects the strength of the interaction between adsorbate and adsorbent.An effective screening strategy is to discard structures, in which the enthalpy of water adsorption is greater than evaporation heat. 56

Indicators of further screening
After a preliminary screening of desirable adsorbents for CO 2 adsorption under humid conditions based on the parameters of K H and ∆H ads , more accurate charge assignment methods will be used.Also more detailed GCMC simulations involving the multicomponent adsorption isotherm and higher adsorption pressure are considered to further explore performance differences and identify the top-performing materials.Various indicators can be computed and analyzed to identify adsorption performance.A comparison of the adsorbate-host interaction is required to achieve high adsorption capacity and selectivity.Figure 6A-C shows S, ∆N, APS, and R% of CO 2 of the selected MOFs from the mixed CO 2 /N 2 and CO 2 /N 2 /H 2 O.This indicates that water exerts a non-negligible impact on various performance indicators. 59 four grades: A (less than 25%), B (25%-50%), C (50%-75%), and D (more than 75%).For real capture process, it is necessary to pursue a high ΔN  2 while minimizing adsorption uptake of other

High-throughput screening outputs
After considering the indicators for the preliminary and further screening, some MOFs appear to be the candidates for carbon capture under humid conditions.selection or design of adsorbents are revealed by the existing screening results.The features of MOFs can be classified into two types according to their top-performing adsorption characteristics: (i) high adsorption interaction with CO 2 provided by adsorbents with superb chemical structure or co-adsorption adsorbates and (ii) the confinement effect arising from appropriate pore sizes of adsorbents.
Interaction strength between adsorbates and adsorbents can be reflected by reaction heat.Findley and Sholl 61 used adsorption heat to screen MOFs and zeolites using Widom insertion Monte Carlo simulations.It is demonstrated that almost all MOFs from the 2014 CoRE MOF database are deemed to unsuitable for DAC in the presence of water due to the much higher adsorption heat of H 2 O under humid atmospheric conditions.This situation can be improved in the application of higher CO 2 concentrations such as post-combustion carbon capture.Li et al. 56 screened 15 MOFs from the CoRE MOF databases with high CO 2 /H 2 O and CO 2 /N 2 selectivity, which are primarily due to strong van der Waals interactions between CO 2 and adsorbent at 80% relative humidity (RH) for post-combustion cap-ture.Similarly, Rogacka et al. 63 revealed 13 promising MOF structures with high CO 2 /CH 4 selectivity under ambient conditions.Nine of the screened MOFs have ligands with alkaline nitrogen groups that have a stronger chemical interaction with CO 2 .According to Canepa's research, 64 MOF-74 with the noble metals Rh, Pd, Os, Ir, and Pt competes with water due to the redox nature of noble metals.Boyd et al. 62 identified three types of effective CO 2 adsorption sites in the screening process of MOFs for wet CO 2 capture, inspired by traditional drug molecular design strategies. 65,66They are parallel aromatic rings (A1), metal-oxygen bridges (A2), and OMS (A3).As shown in Figure 7A,B, those materials with parallel aromatic rings have a high  H,CO 2 and a low  H,H 2 O .Later, they further studied MOFs with frz topology, which was characterized by tetra-carboxylated organic ligands coordinated to one-dimensional metal-oxygen rods and contained the preferred adsorption sites. 62By choosing appropriate metal element and parallel aromatic ring space, one highly promising MOF material, Al 2 (OH) 2 (H 2 TCPP) (Al-PMOF) H 2 TCPP, meso-tetra(4-carboxyphenyl)porphyrin), was designed and experimentally validated for its good hydrophobicity, water stability, and high CO 2 adsorption uptake.Other studies have reported that MOFs with different proprietary adsorption sites can achieve better CO 2 -water co-adsorption, which is conducive to solve competitive adsorption issues. 55he majority of small pores have a confinement effect on CO 2 adsorption and impede water cluster formation. 56,58,62i et al. 58 summarized characteristics of materials suitable for CO 2 adsorption under wet conditions through screening methods.The results show that the Henry selectivity of CO 2 /H 2 O decreases as the largest cavity diameter (LCD) increases.Materials with the lower accessible surface area and void fraction usually have a higher selectivity, as shown in Figure 7C,D.Therefore, porous materials with smaller pore sizes may be better suited for CO 2 adsorption under wet working conditions, and these small pores can prevent the formation of hydrogen bond networks among water molecules.
In summary, high-throughput screening performance indicators that are simple to calculate, such as K H , are frequently evaluated for the preliminary screening to save the computational resources, and then further simulations are performed to obtain more effective indicators.During the screening process, researchers investigated common characteristics of adsorbents such as pore size, surface area, pore volume, and chemical functionality, and designed materials based on these findings. 67However, few studies of carbon capture have focused on the effect of water based on the high-throughput screening.9][70] Thus, relevant databases of other sorbents need to be extend.
The results of a high-throughput screening for CO 2 adsorption in the presence of water show that preferential CO 2 adsorption is related to adsorbent structures.CO 2 /H 2 O adsorption selectivity is not just a simple function of MOF structure properties, for example, pore size, surface area, etc. 63 The operation conditions for adsorption processes, such as humidity, temperature, and partial pressure should also be taken into consideration.To understand the mechanism of MOFs with high CO 2 /H 2 O selectivity based on existing researches, a comprehensive review of optimization strategies for increasing CO 2 adsorption in wet gases should be conducted, which could provide some insights for material development.

STRATEGIES FOR THE IMPROVED CO 2 ADSORPTION
In order to obtain the materials with good adsorption performance through screening, the internal adsorption mechanisms should be further explored to guide material synthesis.The mechanism is explained by a flow chart, as shown in Figure 8. Physisorption produced by intermolecular interactions is favored by surface affinity and confinement effect.However, chemisorption occurs only through surface affinity due to the action of chemical bonds.The confinement effect arises from the overlap of the host-guest potential energy surface in cavities of adsorbent structure, which leads to CO 2 adsorption in narrow pores. 71Surface affinity is determined by the chemical composition of the solid, which has no bearing on framework topology.According to the existing research, the promoting effect on CO 2 adsorption could occur in some sorbents via thermodynamic (surface affinity) or kinetic (confinement effect) factors.These characteristics that promote CO 2 adsorption under humid conditions can be classified by action mechanism as follows: (i) CO 2 adsorption sites with the suitable polarization interactions [72][73][74][75] ; (ii) adsorption sites with strong chemical interactions with CO 2 [76][77][78][79] ; and (iii) confinement effect of channel structure or the coordinated H 2 O molecules. 28,38,80 4 lists some advanced MOFs with high CO 2 uptake under humid conditions.Their excellent performance is evaluated by using adsorption mechanism.

Polarization interaction
The binding forces for CO 2 adsorption include van der Waals forces, chemical bonds, or Coulombic forces.For physical adsorption, there is no need to consider chemical bond formation.Adsorption enthalpy is only proportional to intermolecular interactions, for example, van der Waals forces and Coulombic forces. 85Physical adsorption forces are related to the polarizability of adsorbents and adsorbates, which is an important fundamental fact for the following content.The polarity of major gas compositions can then be determined.Water with a higher polarizability competes with CO 2 for adsorption sites.Furthermore, the dipole moment of water has an undeniable interaction with the quadrupole moment of CO 2 . 55The interaction changes depending on the magnitude and orientation of the intermolecular force, resulting in different effects.
The key to promoting CO 2 adsorption is to erode water's competitive advantages or increase synergistic effects.To improve CO 2 uptake in wet gases, some approaches based on the affinity of CO 2 and H 2 O to adsorbents have been proposed: (i) increasing the hydrophobicity within MOFs to decrease the polarity of adsorption sites; (ii) building the specific adsorption sites; and (iii) adsorbing an appropriate amount of H 2 O to append CO 2 adsorption sites.

Hydrophobic modification
There are two common methods for MOF modification treatment, that is, pre-functionalization, also called direct synthesis and in situ synthesis and post-synthetic modification.7][88] Yu et al. 72 investigated a series of UiO-66 derivatives decorated by alkyl or perfluoroalkyl groups bound to ligands of different lengths (terephthalate or 4,4′-azobenzenedicarboxylate).This demonstrates that increasing the number of substituents increases hydrophobicity.As a result of fluorination, perfluoro-alkylated groups among the derivatives of UiO-66 exhibit more hydrophobicity than alkyl groups.Although the polarity of pore walls decreases, introduction of functional groups can also increase the confinement effect to promote adsorption.This effect is more apparent in UiO-66D, which possesses the large pores, as shown in Figure 9.
Fluorine-functionalized MOFs have a low surface energy, which could be attributed to fluorine's high electronegativity and relatively low electric polarizability.The treatment of fluorinated building blocks can generate the fabulous hydrophobic porous structures. 73MOFs containing inorganic fluorinated monoanions achieve preferential CO 2 adsorption over the interaction of F atoms and CO 2 .Noro et al. 75 synthesized a flexible and hydrophobic copper porous coordination polymer, which indicates a high selectivity for CO 2 over CH 4 and the high stability under humid conditions.The whole structure is composed of low  Another viable method for post-synthetic exchange that allows for simple modification of existing MOFs is coating the crystals with hydrophobic polymers.Qian et al. 90 fabricated an organosilicon coating on surface of various MOFs via facile solution-immersion process and obtained water-stable MOFs with a high contact angle.Choe et al. 84 synthesized Mg 2 (dobpdc)-alumina composite microbeads (MOF/Al) through spray dry method, and then further functionalized MOF/Al with een and coated with long alkyl chain silanes to obtain splendid MOFs with the high carbon capture capacity under humid conditions.As shown in Figure 10, een-MOF/Al-Si exhibits the best stability and the lowest affinity toward water when compared to MOFs without a hydrophobic silane coating.Nanochannels of MOFs can also be used as the desirable spaces for polymerization, which belongs to in situ synthetic modification. 91Ding et al. 92 developed a new strategy to partition the channels of MOF-5 into the confined compartments by in situ polymerization of aromatic acetylene to improve the stability of the framework to moisture, as shown in Figure 11A,B, respectively.Recently, Yu's group proposed a novel in situ microencapsulated synthesis method to synthesize MOF-based hybrid sorbents. 93s shown in Figure 11C,D Besides, some MOF-derived porous materials are stable in wet gas.Kim et al. 94 studied CO 2 adsorption of porous carbon materials zinc-containing MOFs.Due to the intrinsic hydrophobic feature of porous carbon and lack of metal sites, the breakthrough curves of CO 2 and N 2 are identical under dry and wet conditions.Also, some zeolitic imidazolate frameworks (ZIFs) have been reported to have the high thermal and chemical stability in refluxing organic and aqueous media. 95Nguyen et al. 96 synthesized three hydrophobic ZIFs by incorporating two distinct imidazolate links.These materials exhibit remarkable hydrophobic properties and CO 2 uptakes.Additionally, the series ZIF exhibits significant CO 2 separation performance over three working cycles from a ternary mixture of N 2 and water without degradation of ZIF structure or performance.Hu et al. 26 also reported a series of ZIFs with the same sodalite (SOD) topology with different functional groups.The presence of water is found to have a negligible effect on CO 2 adsorption in terms of ZIF-8, ZIF-Cl, and SALEM-2, which contain no or weakly polar groups.Therefore, ZIFs are the potential candidates for the hydrophobic treatment.

3.1.2
Pores with the specific adsorption sites In most cases, water has an influence on CO 2 adsorption due to the higher affinity toward adsorption sites.Recently, some researchers discovered MOFs with CO 2 -specific sites, such as amine groups, which perform exceptionally well under humid conditions. 74There is less interaction between the adsorption of gases in MOFs with distinct adsorption sites due to the varied pore sizes or bifunctionality.Benoit et al. 33  uptake significantly decreases in moisture, CO 2 adsorption enthalpy displays a maximum value at 10% RH with regard to the higher confinement effect and interaction of pre-adsorbed molecules.Moreover, MIL-96(Al) contains more hydrophilic groups, that is, Al(1)-H 2 O terminal , Al(3′)-OH terminal , and Al(3)-H 2 O terminal , which leads to the adsorption of CO 2 molecules at the remaining sites with the lower polarity, as shown in Figure 12.
Similarly, Datta et al. 27 reported a highly stable microporous coppersilicate that has the square bipyramidal crystal morphology (SGU-29).From Fourier transform infrared spectra of SGU-29 in Figure 13, the growth rates, peak shapes, and intensities of adsorbed CO 2 peaks in wet gases could be comparable to those observed in the absence of H 2 O, which only shows a broad peak of the adsorbed water.The results verify that there are CO 2 -specific and H 2 O-specific sites but no sharing sites; thus, moisture does not influence CO 2 capture ability of SGU-29.This kind of sorbent can be seriously considered as a promising candidate for binary adsorption.The exploration of CO 2specific sites is the key to filling existing research gaps.
Moreover, the mechanism of specific sites requires more research.

3.1.3
New sites generated by pre-adsorbed water The adsorbed water molecules exhibit the antagonistic effects to some degree, which block intrinsic CO 2 adsorption sites while also having the potential to become new CO 2 adsorption sites.Several recent studies have shown that pre-adsorption of a spot of water preferentially interacts with hydroxyl groups within the pore wall of MOFs, resulting in a significant increase in CO 2 adsorption enthalpy due to an increase in co-adsorbate interactions. 33,81As shown in Figure 14, it is verified by the molecular simulation that CO 2 can interact with H 2 O, but the interaction is relatively weak and has only a tiny promoting effect.
In 2009, Yazaydın et al. 98 initially discovered that water molecules can promote CO 2 uptake due to the increased interactions between the quadruple moment of CO 2 and the electric field of the hydrated framework.In Figure 15A, both non-Coulombic energy and Coulomb energy increase by introducing the coordinated water molecules to MOF structure, and the latter plays a leading role in energy rise.The simulation results support a greater interaction between CO 2 and adsorbent, in which water interacts with the coordinatively unsaturated Cu sites.Besides, the coordinated water blocks the pores for nonpolar CH 4 /N 2 , and a breathing effect takes place, which could open the pores in favor of CO 2 .As a result, CO 2 selectivity rises, as depicted in Figure 15B.Jajko et al. 31 revealed that the CO 2 loading is enhanced by pre-adsorption of water in UiO-66.To understand adsorption mechanism of performance enhancement in CO 2 adsorption, the intermolecular adsorption energy is calculated, which includes CO 2 -CO 2 , CO 2 -H 2 O, and CO 2 -UiO-66.It is indicated that adsorption energy of CO 2 -H 2 O is greater than that of CO 2 -CO 2 at pressures of 0-1 bar.This demonstrates that the interaction of pre-adsorbed water increases CO 2 adsorption capacity.The average occupational density profiles are also calculated, and the results show that water molecules adsorb in the corners of octahedrons, which is explained by entropy impact of the size and shape of water molecules.CO 2 molecules begin to fill the octahedral cages after water adsorption, which is different from pure gas adsorption.It is concluded that water transforms unfavorable sites into favorable sites for CO 2 adsorption.Thus, water promotes the interaction of CO 2 adsorption.Similarly, the hydrated MIL-101 exhibits a higher CO 2 adsorption capacity than that of the dehydrated MIL-101 at low pressure because the terminal water molecules act as extra CO 2 adsorption sites, which increases the interaction between quadrupole moment of CO 2 and dipole moment of adsorbents. 97t is worth noting that pre-adsorption may not accurately reflect the real situation of carbon capture from humid streams.After water saturation, the moisture content does not remain stable during adsorption process, which will cause a slight underestimate of CO 2 uptake, particularly when only using gravimetric method to monitor CO 2water co-adsorption.Other methods for measurement, for example, monitoring outlet gas concentration by using gas chromatography, are more reliable to obtain a reasonable CO 2 adsorption capacity.F I G U R E 1 5 (A) Interaction energies between CO 2 and dry Cu-BTC and hydrated Cu-BTC (4 wt%) from Grand Canonical Monte Carlo (GCMC) simulations at 298 K. To convert molecules per unit cell to milligrams of gas adsorbed per gram of sorbent, multiply by 4.3557.(B) CO 2 selectivity from the simulations of the equimolar mixtures of CO 2 /CH 4 and CO 2 /N 2 at 298 K. Reproduced with permission. 98Copyright 2009, ACS.BTC, benzene-1,3,5-tricarboxylate.

Chemical interaction
To overcome the drawbacks of physisorption, amine functional solid adsorbents have attracted considerable interest due to their exceptional chemisorption properties, especially under humid conditions, which are also considered in MOF functionalization. 99,100Aminefunctionalized MOFs are prepared in three general routes, that is, in situ synthesis, post-modification, and physical mixing of unfunctionalized MOFs and polyamines. 101A common method for in situ synthesis is to use ligands with amine.There are myriad pathways to decorate MOFs according to the type of amine functional groups, the position, the possibility of coordination to node, and the environmental conditions. 102he stability, polarity, alkalinity, and pore structure, can be influenced by introducing functional groups to pores.Free electrons located on amino nitrogen result in a remarkably high affinity toward CO 2 due to Lewis acid-base interactions and non-covalent interactions. 102thiraj et al. 76 synthesized a series of the mixed-ligand UiO-66 through two different methods.The volumetric CO 2 isotherms show a higher CO 2 capacity in terms of lowtemperature synthesis since the larger surface area could be obtained.The results also demonstrate that the affinity toward CO 2 increases with the increase in NH 2 content in samples.The heteroaromatic amines that have nitrogen atoms in aromatic rings also exhibit the high CO 2 adsorption capacity.Cui et al. 77 synthesized a zeolite-like microporous tetrazole-based MOF, which is composed of flexible tetrazole derivative ligands and Zn 2+ ions.It has a high CO 2 adsorption capacity of up to 8.09 mmol g −1 at 273 K and ambient pressure.This can be attributed to multipoint interactions of framework toward CO 2 , which includes π-quadrupole system of aromatic rings and CO 2 , potentially exposed nitrogen atoms and C atom of CO 2 , and partial C-H bonds and O atoms of CO 2 .Besides, post-synthesis can prepare amine-functionalized MOFs.Bahamon et al. 78 investigated six amine models of different chain lengths and substitutions grafted onto the unsaturated metal sites of M 2 (dobpdc) by using molecular simulation.The results show that mmen/Mg/dobpdc has a higher CO 2 adsorption capacity and cycling performance than MOFs with other structures.However, due to an increase in steric hindrances, additional amine functional groups or full functionalization of metal centers do not show the improvement in CO 2 separation capabilities.Hence, functionalizing MOFs by amino groups could occupy the partial pore sites and result in the decrease in void space and surface area, which is not conducive to CO 2 adsorption.This is the reason why no enhancement of adsorption capacity is found in MIL-101(Cr) -NH 2 . 103owever, MIL-101(Fe)-NH 2 displays 1.4-fold increase in CO 2 adsorption capacity than MIL-101(Fe) at 4 MPa and is more stable against water and ethanol. 104Mahdipoor et al. 104 also deduced the mechanism of CO 2 adsorption by amino functional groups based on three factors: electric field generated by different charges, hydrogen bonding between CO 2 and NH 2 functional groups, and dipolar ion (zwitterion) formation by the acidic CO 2 and basic NH 2 groups.
6][107] Lee et al. 106 prepared a diaminefunctionalized MOF by grafting the primary amine to open coordination sites of Mg 2 (dobpdc).The material has a significant CO 2 uptake (2.83 mmol g −1 ) at ambient CO 2 pressure.Notably, it can retain CO 2 adsorption capacity after exposure to humidity.However, this structure is not stable enough in which H 2 O and CO 2 usually compete with metal sites and therefore it loses original grafted amine.To avoid this, Lyu et al. 42 used a robust MOF-808 system to append amino acids through carboxylates to Zr centers, which forms a strong MOF backbone and prevents water or CO 2 from breaking the bonds between metal atoms and the carboxylates with amino acids.Both CO 2 adsorption isotherm and dynamic CO 2 breakthrough curve reveal a significant increase in CO 2 uptake in MOF-808-Gly upon humidification of stream, as shown in Figure 16A,B.Later, they used solid-state nuclear magnetic resonance to explore the mechanism behind the binary adsorption.As demonstrated in Figure 16C, bicarbonate formation under humid conditions only needs an amine to adsorb each CO 2 molecule from natural gas flue emissions, which is less than what is needed when the conditions are dry.Moreover, it is noted that bicarbonate formation can be removed by vacuum process, which is desirable for vacuum swing adsorption (VSA).Siegelman et al. 79 reported a diamine-appended Mg 2 (dobpdc) MOF with a high CO 2 capacity under humid conditions.DFT calculations show that the pre-adsorbed water increases CO 2 binding energy by 18 kJ mol −1 , as shown in Figure 17.This is because ammonium carbamate chains can form easily due to the hydron bonding interaction provided by water molecules and the carbamate nitrogen atoms of nearby ammonium carbamate chains.
Introduction of the basic amine functional groups onto the surface of a solid material can increase the interaction between CO 2 and solid surface by physisorption and chemisorption.For CO 2 -water co-adsorption, the increase in CO 2 adsorption occurs mainly with the increase in amine efficiency.The presence of water also changes adsorption heat, which affects the affinity toward CO 2 .Furthermore, grafting amine functional groups reduces pore sites and surface area.Water molecules as impurities can clog some adsorption sites and decrease CO 2 adsorption capacity.A balance is considered by tuning up the amount of amine grafted.Besides, the long-term stability of amine-functionalized adsorbents is necessary to avoid the loss of amine.

Confinement effect
Adsorption sites are classified into two types: binding sites via electrostatic interaction that dominates CO 2 adsorption and pore sites through confinement effect. 108The excellent adsorption capacity of pores with confinement effect can promote CO 2 uptake in some MOFs with optimal pore sizes.The confinement effect, termed as nest effect, was initially proposed by Derouane, 109 which represents that a molecule and its environment tend to reciprocally optimize their intermolecular force.This effect is related to crystal topology and the auxiliary effects of atom dispersive interactions on cavity walls. 71The confinement impact of the channel construction and the adsorbed H 2 O molecules should be considered in the binary adsorption of CO 2 and H 2 O.

Confinement effect of framework topology
Some researchers have dedicated their efforts to determining the ideal pore size for gas separation.Gomez et al. 71 discovered new adsorption regions around inorganic building units, resulting in the large H 2 uptake in PCN-12, NOTT-103, NOTT-112, and HKUST-1 due to the confinement effect caused by small pore size (D ≤ 8.5 Å) or narrow windows in cavities.For MOF-5 and MOF177 with larger pore sizes (10.8 and 11.2 Å), no adsorption site is ascribed to confinement effect, which is confirmed by first-principle calculations.It can be inferred that confinement effect could happen in MOFs with the appropriate pore sizes.Apparently, the pore size must be larger than dynamic diameter of adsorbate.1][112][113][114] With an appropriate channel size, zincbased Calgary Framework 20 (CALF-20) may selectively physisorb CO 2 . 80The CO 2 binding sites are in the middle of the pores, which may prevent the creation of networks of hydrogen bonds between water molecules.Liang et al. 38 used an interpenetration approach to develop a fluorinated MOF with ideal pore aperture and channel sizes (4.5-6 Å).It allows for favorable CO 2 adsorption versus N 2 and reduces adsorption heat because of the strong interaction via confinement effect.Besides, it indicates the extraordinary resistance to moisture, as shown in Figure 18, which is presumably due to the nucleophilic nature of TiF 6 2− pillars.The sieving effect is a useful technique for gas separation that involves changing the pore window and size Reproduced with permission. 117Copyright 2022, Springer Nature.
to better match the size of the gas molecules. 115Pore topology plays a crucial role in the capacity and selectivity of adsorbents for multicomponent adsorption.
Tuning the pore size can be achieved by controlling the size of metal ions or clusters and adjusting the lengths of ligands. 116Wang et al. 117 displayed a fine pore engineering by altering anionic linkers and metal nodes in [Zn(dps) 2 (SiF 6 )] to explore appropriate pore sizes and the specific binding sites for C 2 H 2 /CO 2 separation, as shown in Figure 19.This reveals that [Cu(dps) 2 (SiF 6 )] has the highest C 2 H 2 uptake and selectivity for C 2 H 2 /CO 2 .Similarly, Cui et al. 118 reported several "SIFSIX" materials with different pore structures, which were tuned by changing organic linkers, metal nodes, or framework interpenetration.The best sorbent could be found with the ideal hybrid pore chemistry and the optimal pore size to form gas clusters.Li et al. designated this method as "singlemolecule trap."This group also successfully synthesized a cage-type structure with the desired size and strong electrostatic interactions with CO 2 molecules.Similar techniques may likely be applied to separate additional gases.Nevertheless, studies on tuning pore size for better separation of CO 2 and H 2 O are rarely reported.It can be explained by the fact that H 2 O has a smaller kinetic diameter (2.65 Å) than CO 2 (3.30Å), and consequently, the sieving effect for CO 2 capture in wet conditions is invalid.However, most high-performing MOFs with good CO 2 /H 2 O selectivity have narrow pores, which result in the confinement effect and prevent the formation of water clusters via hydrogen binding networks.According to the study from Li's group, 58 pore size of ∼5 Å can attain the highest CO 2 /H 2 O selectivity in most MOFs based on 1627 MOFs in CoRE MOF database.Thus, further research on pore topology is required to focus on this range of pore diameters.

Confinement effect of the coordinated H 2 O molecules
The confinement of small amounts of polar molecules such as water and alcohols or non-polar molecules such as hydrocarbons within MOFs can also increase CO 2 uptake, which proposes a novel method to address the issues of CO 2 -water co-adsorption. 43It occurs inside the pores, which leads to the diffusional limitations and structural constraints on the transition state. 108Also, pre-adsorbed molecules can interact with CO 2 . 120Numerous investigations have found a trend of increased CO 2 uptake in co-adsorption when compared to that under dry conditions.
It is noted that CO 2 adsorption capacities of some MOFs increase first and then decrease with the increase in the humidity, such as HKUST-1, 28 MIL-101(Cr), 97 MIL-100(Fe), 29 Mg-CUK-1, 81 InOF-1, 30 and UiO-66. 31Liu et al. 28 studied the equilibrium of CO 2 /H 2 O binary adsorption of two types of MOFs: HKUST-1 (Cu-BTC) and Ni/dobdc (CPO-27-Ni or Ni/MOF-74).Water adsorption progress on HKUST-1 is reversible.A small amount of the adsorbed water does not inhibit CO 2 adsorption but may promote adsorption capacity of HKUST-1, which contributes to the addition of Coulombic interactions between CO 2 and H 2 O molecules.However, there is no enhancement of CO 2 adsorption capacity in Ni/dobdc after introducing H 2 O molecules into feed gases.Zhao et al. 121 reached a similar conclusion.HKUST-1 slightly degrades, and Ni/dobdc exhibits a better stability after several working cycles, which could be ascribed to higher transition state energies of Ni/dobdc for hydration. 122Soubeyrand-Lenoir et al. 29 investigated adsorption uptake and enthalpy of three topical porous MOFs.The results show that water is not conducive to CO 2 adsorption capacity of UiO-66(Zr) under different RHs, which is inconsistent with the finding from the research by Jajko et al. 31 MIL-100(Fe) exhibits a striking fivefold improvement in CO 2 uptake with the increase in RH.Given the poor MIL-100(Fe) test results, this conclusion is dubious.Álvarez et al. 45 demonstrated NOTT-400 that pre-adsorbed water could enhance CO 2 uptake via confinement effect.The CO 2 uptake presents an approximate 2.5-fold increase after exposure to 20% RH conditions.Adsorbed water molecules form strong hydrogen bonds with the bridging hydroxo functional groups (μ 2 -OH) in NOTT-400, which leads to the augmented CO 2 sequestration.Likewise, PCN-250(Fe 3 ) and PCN-250(Fe 2 Co) represent moisture-enhanced features. 32ater can have strong interactions with the hydroxo functional groups (μ 3 -O) in their framework.The coordinated H 2 O molecule acts as a plier that clamps CO 2 on accessible adsorption site.Furthermore, it is demonstrated that CO 2 moves closer to OMS when hydroxo functional groups (μ 3 -O) are coordinated with H 2 O by GCMC.González-Zamora and Ibarra 44 concluded that water molecules can strongly interact with hydroxo functional groups via hydrogen bonding, which leads to a confinement effect allowing for more efficient CO 2 packing.
Water loading amount is critical to the effectiveness of the confinement effect.The directing effect of hydroxo functional groups is greatly reduced at high RH due to an increase in water disorder caused by thermal agitation. 44t is essential to determine the optimal water loading of confinement effect on CO 2 .Sánchez-González et al. 41 explored the structural transformation of Mg-CUK-1 at different H 2 O loadings by using powder X-ray diffraction and refined it by the Le Bail strategy in Figure 20.With eight hydrogen-bonded H 2 O molecules per unit cell, the window dimension was reduced to 2.56 Å inducing non-accessible for CO 2 .A window reduction is also resulted by four or two H 2 O molecules per unit cell and thereby the diffusion of CO 2 is restrained.CO 2 has enough free space to interact with adsorbent and adsorbate only when there is one H 2 O molecule per unit cell, which results in "bottleneck effect."Indeed, Mg-CUK-1 exhibits a 1.8-fold increase in CO 2 uptake at 18% RH (one hydrogen-bonded H 2 O molecule per unit cell) due to pore windows partially constrained by water.Soubeyrand-Lenoir et al. 29 elucidated the mechanism of the confinement effect in a mesoporous architecture.Pre-equilibrated water initially fills microporous pockets in MIL-100(Fe), as shown in Figure 21A.CO 2 molecules are then adsorbed at the accessible sites (Figure 21B) and even displace some water (Figure 21C), which would not happen under anhydrous condition.Nevertheless, some MOFs have no accessible microporosity after water absorption (Figure 21D).As a result, there is no increase in CO 2 adsorption uptake.
High-throughput screening results indicate that a small pore size (LCD of around 5 Å) is better for CO 2 /H 2 O separation, which satisfies CO 2 diffusion and obstructs water cluster formation.Moreover, the pre-installing water of MOFs can also cause a confinement effect, thus changing the interaction between CO 2 and sorbent.The confinement effect is definitely a novel approach to develop MOFs with a suitable framework for carbon capture in practical humid working conditions.
It is noted that not all water loadings give rise to more efficient CO 2 molecule accommodation by using the above methods.For the methods of changing the polarization interaction of surface or creating confinement effects can only be used in low humidity ranges because adsorption type is still physisorption.The presence of extra water can occupy partial adsorption sites, which leads to a reduction in adsorption amount.Relatively, the chemical method can be employed for larger water contents because the chemisorption force is stronger.However, as Figure 2 shows, there are also some exceptions.Thus, we herein cannot offer a quantitative description.

Hydrolytic stability
Along with the aforementioned techniques to improve CO 2 adsorption selectivity, it is also important for MOFs to have the high stability and resistance to irreversible reactions with water in moisture gases. 123If the MOF structure collapses under humid conditions, its durability and application will be limited.The hydrolytic reactivity and initial decomposition pathway of MOFs depend on the specific type of ligand and second building unit (SBU). 124OFs with various metal ions and ligands have been investigated, which involves water-labile and water-stable MOFs.122,[125][126][127] For water-stable MOFs, the stronger bonds between metal clusters and organic linkers can be found to possess a structure with greater stability in moisture.28 Water can react with MOFs via ligand displacement and/or hydrolysis.122 As shown in Equation ( 1), ligand displacement inserts an H 2 O molecule into a metal-ligand bond.The hydrolysis reaction involves water dissociation into a hydroxide ion and a hydrion.Bond formation between a metal with hydroxide ion and a ligand with hydrion is shown in Equation (2).
Many hydrolytic stability factors, such as metal composition and coordination, chemical functionality of organic ligands, framework dimensionality, and interpenetration, have an influence MOF stability from an aspect of thermodynamic or kinetics. 127To explore the stabilities of various MOFs, Low et al. 122 conducted an experimental test together with molecular modeling.A steam stability map is shown in Figure 22.It is found that MOFs bridged by carboxylates need less activation and reaction energy since the metal-ligand bonds are weaker than those of imidazolates, which is linked to imidazolates' greater basicity. 128Hence, MOFs bridged by imidazolates such as ZIF-8 tend to be more hydrolytically stable than those bridged by carboxylates.Besides, it is significant to regard the stability of SBUs in the MOF structure.The higher oxidation of metals or metal ions, the better stability toward hydrolysis becomes.In general, metals with the higher metal valence are discovered to be more resistant to hydration.For instance, the relative positive charge per Zn of Zn 4 O 6+ cluster in MOF-5 is 1.5, which is less than metal valence in HKUST-1 and MIL-101.It is suggested that HKUST-1 and MIL-101 could form stronger metallinker bonds than MOF-5, which can prevent the collapse structures. 122Although a small amount of water (0.5 mole equivalent) has no impact on the network structure stability of HKUST-1, higher water adsorption may cause the F I G U R E 2 2 Map of steam stability for four metal-organic frameworks (MOFs).Position of the structure for an MOF represents its maximum structural stability by X-ray diffraction (XRD) measurement, while activation energy for ligand displacement by a water molecule determined by molecular modeling is represented by a magenta number (in kcal mol −1 ).Reproduced with permission. 122Copyright 2009, ACS.network structure to be partially broken at Cu-O coordination sites. 129Besides, chemical stability to water can be increased with the increased inertness of central metal ions. 130For example, the chemical stability of isotypic MOFs such as Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr), and V-BDC (MIL-47-V) decreases in the order, Cr-BDC > Al-BDC > V-BDC. 130hus, it is possible to enhance water stability of MOFs by strengthening metal-ligand interactions from a thermodynamic perspective, or enhancing hydropho-bicity or steric hindrance from kinetic perspective.From a thermodynamic aspect, the enhanced strength of metal-ligand bonds can be achieved by replacing metal ions with the higher valence cations or more inert metal ions since cation-ligand bonds are inclined to hydrolyze for low-valent metals. 72Alternatively, changing the ligand can improve the bond energy.Colombo et al. 131 applied polyazolate-bridging ligands in MOFs, which leads to strong metal-nitrogen bonds when compared to carboxylate-based counterparts.This indicates that strength of M-N bonds is related to required for the deprotonation of N-H bonds.Thus, frameworks generated from organic ligands functionalized with 1,2,3triazole (pK a = 13.9) have greater thermal stability than analogs based upon tetrazole (pK a = 4.9). 132or the kinetic aspect, it is effective to increase water stability by synthesizing highly hydrophobic surfaces or coating MOFs with appropriate polymers to increase structure steric hindrance and hydrophobicity. 133,134For instance, some MOFs with water-repellent groups such as methyl groups incorporated into organic ligands exhibit excellent stability under humid conditions.Wu et al. 135 reported an MOF by strategically with non-polar functional groups, trifluoromethoxy, in the frameworks.Trifluoromethoxy groups in MOFs prevent water intrusion into framework structures, which improves the stability in the humid air.Linkers modified by either polar or non-polar groups enhance the overall stability of the lattice by drawing water toward new adsorption sites, thus hindering water from reaching the metal center. 136Besides, steric hindrance is an undeniable factor in reducing hydrolysis.MOFs with high coordination numbers, such as UiO-66(Zr) 137 and MIL-125(Ti), 138 have high connectivity and provide steric shielding against water for SBUs.This factor can also be confirmed in research by Tang et al. 139 Ca-based MOFs are stable up to 98% RH due to ligand distortion, which causes more steric hindrance effect of benzene rings to protect Ca 2+ ions from water attack.Moreover, contracting or rigidifying the ligands can also enhance the rigidity of framework and consequently water stability. 140,141Lv et al. 142 constructed 13 Zr-MOFs consisting of three classes of ligands.The results imply that rigid linkers with lower degree of rotational freedom have a self-repair mechanism after water substitution, and this effect is more effective with the higher connectivity.Moreover, shorter linkers can have greater deformation energy barrier due to the larger angle during framework destruction. 143

PROTOTYPE VERIFICATION
To address the large energy penalty of common aqueous monoethanolamine solution scrubbing technology for post-combustion carbon capture, adsorbents and process design need to be more effective and energy intensive.Porous solid sorbents offer the advantage of lower regeneration heat when compared to aqueous amine solvents.When desorption heat is assumed to be consistent, a higher sorbent operating capacity can lead to a lower thermal energy input. 144Several promising adsorbents have been proposed for carbon capture, which have high CO 2 selective adsorption capacity under wet conditions.Besides, the capture process of sorbent in the reactor should also be evaluated when considering real heat and mass transfer processes.In order to evaluate adsorbent features that more closely resemble actual use, this section aims to present simplified prototype testbeds using MOFs for carbon capture.Adsorption CO 2 removal can be achieved by temperature swing adsorption (TSA), pressure swing adsorption (PSA), VSA, or other hybrid working processes.For CO 2 and H 2 O binary adsorption, VSA and TSA are more commonly used to achieve higher CO 2 purities than PSA. 145nder the circumstances of simulated coal flue gas, Lyu et al. 42 investigated MOF-808-Gly's performance of carbon capture with a specially designed VSA apparatus, as shown in Figure 23.The inlet of sorption reactor is connected to flue gases, and the outlet is used to expel CO 2lean stream.The inlet transforms into an outlet CO 2 -rich stream during the regeneration process.To avoid water condensation on the gas sensor and to increase the purity of product gas, a membrane drier is installed along the regeneration branch.The simulated flue gases are balanced in N 2 at 20 • C-23 • C, which contain 15% CO 2 at an RH of 20%.The regeneration process of the system starts once CO 2 concentration reaches more than 2%.A glycine-filled MOF-808-Gly-functionalized sorption reactor displays good cycle stability at 80 cycles with an average CO 2 uptake of 0.42 mmol g −1 h −1 .Likewise, Hossain et al. 146 investigated similar volumetric system to test multicomponent isotherms of UiO-66 with three distinct water loadings (low, pore filling, and saturation).The results indicate that CO 2 uptake slightly increases at low water loading (1.5 mol kg −1 ), which is consistent with the results from Jajko et al.'s study, 31 and then gradually decreases as water loading increases.The cooperative effect of low adsorbed water can be attributed to the confinement effect and affinity toward CO 2 .When water loading approaches saturation, CO 2 uptake will decrease by up to 60% from 0 to 5 kPa.However, the flue gases will be nearly saturated with water when dropping to ambient temperature since water content in flue gases is almost 7%, which has been ignored in most researches.Thus, the simulated flue gases with saturation water loadings at a moderate temperature require additional attention.In this work, we are just concerned with the adsorption properties of capacity and selectivity.Other aspects such as adsorption kinetics, thermal conductivity, and adsorption heat should also be considered for process metrics such as productivity, specific energy, and purity. 147he zero length column (ZLC) technique, which is effective in obtaining transient desorption curves, has gained application in the determination of the effect of water.In a ZLC experiment, the exterior mass and heat transfer resistance of sorption bed can be ignored.Hu's team 148 assessed the behavior of M/dobdc (M = Co, Ni, Mg) and commercial 13X zeolite pellets in ZLC with 1% (v/v) water.The water content was purposefully adjusted to a low value, presuming that flue gas had already been dried for physical sorbents.Co/dobdc and Mg/dobdc lack stability to moisture, but Ni/dobdc is less sensitive to water.However, when exposed to wet flue gas conditions, adsorption capacity of Ni/dobdc declines as the duration of the exposure increases.Co/dobdc appears to require a longer time for the regeneration process; that is, it only has a slight loss in CO 2 uptake after exposure to wet flue gas.This is in line with Kizzie's work, which finds that Co/dobdc had the highest CO 2 capacity (85% of that of pristine samples) at 70% RH among M/dobdc. 149Also, Mangano et al. 150 employed ZLC to investigate water stability.For water stability of Mg-CPO-27 and Ni-CPO-27, Mg-CPO-27 is deactivated in the presence of water.However, Ni-CPO-27 passes water-stable test and remains at a reduced and steady CO 2 capacity in the moist flue gas.The performance of MOFs in the volumetric system is basically comparable to that of gravimetric apparatus.One issue that needs to be pointed out in the volumetric experiment is the transformation of the morphology of MOF.MOFs in pellets have the less uptake and slower adsorption kinetics when compared to pure powder.This issue cannot be avoided if real carbon capture progress proceeds.More CO 2 -water co-adsorption factors should be considered in a practical CDR facility, as shown in Figure 24.For example, after the nitrogen oxides, particulates, and sulfur oxides have been removed, the flue gas will be cooled using cooling water from power plant's cooling tower.The temperature of the flue gas will fall below 40 • C, and its water content will be reduced by at least 75% until it reaches saturation.Therefore, we should consider CO 2 -water co-adsorption and energy consumption of regeneration.Meanwhile, a water zone may be formed in the adsorption bed and alter thermal profiles. 151More heat or vacuum degrees are required to desorb the water zone.The water that comes along with the CO 2 -rich stream is collected in the vacuum pump or other drying devices.This system is only appropriate for MOFs on which the saturated water has a favorable or insignificant effect on CO 2 adsorption ability.For other MOFs with a dramatic decline in CO 2 adsorption capacity under humid conditions, multiple stages might be a viable option to pre-adsorbed water vapor in flue gases.

CONCLUSIONS AND FUTURE PERSPECTIVES
Nanoporous MOFs have great potential for carbon capture because of their customizable and adjustable pore surface, high porosity, structure diversity, etc.To find MOFs with excellent adsorption performance under humid conditions, it is feasible and convenient to perform numerical screenings of MOFs based on their structures for selective adsorption.The Henry constant, Henry selectivity, and adsorption enthalpy are three more practical indicators that can be used in the initial screening.Then, depending on the specific capture objectives, more targeted indicators such as separation potential, percent uptake loss, and selectivity should be analyzed for top-performing MOFs.
The screening results of CO 2 adsorption in the presence of water show that high surface affinity toward CO 2 provided by adsorbents with superb chemical structures or co-adsorption adsorbates and suitable pore sizes results in good CO 2 /H 2 O selectivity.CO 2 uptake or selectivity can be improved through these methods.The higher polarization interaction for CO 2 than H 2 O can come from hydrophobic modification, pores with specific-CO 2 adsorption sites, or new adsorption sites generated by pre-adsorbed water.However, the extremely strong hydrophobicity leads to poor CO 2 adsorption capacity.In addition, amine functional sorbents with chemisorption ability have drawn wide attention due to their remarkable CO 2 capacity, especially in humid conditions, which could be attributed to the increase in amine efficiency and promotion of ammonium carbamate chain formation via hydrogen bonding interactions.However, the desorption process also faces larger energy barriers.The confinement effect is another condition that can increase CO 2 uptake in wet gases.This is caused by water pre-adsorption and leads to diffusional restriction and structural constraints.Due to physisorption, they also have a low desorption heat, but their CO 2 adsorption capacity at relatively low partial pressure is not very impressive.Researches on changing the pore window and size to improve the confinement effect of cavities in MOFs for CO 2 adsorption and prevent water cluster formation are still needed for further exploration.Moreover, the structure of MOFs should be robust enough to avoid structure collapse in the presence of water.The water stability can be improved by increasing the strength of the metal-linker bond by replacing more stable metal ions or ligands from a thermodynamic aspect, or it can be refined by hydrophobic treatment or/and enhancing the steric hindrance with respect to adsorption kinetics.
For the simplified prototype testbeds, working capacity of MOFs is also related to heat and mass transfer rate and actual operating conditions.It is not involved in this review, which is only concerned with humidity impact.It should be noted that the change in humidity level depends on the application circumstance.If flue gases are pretreated to remove moisture, sorbents with confinement effect are suitable under the condition of low humidity since water vapor cannot be completely removed.Without pretreatment or in DAC, the porous amine-functionalized MOFs are emphasized as having a higher binding strength to collect CO 2 in conditions of high RH or/and the extremely low CO 2 partial pressure.After water-CO 2 co-adsorption, the regeneration energy of water and drying of product gas should both be considered.
In summary, various modification methods have been applied for carbon capture in the presence of water.The intrinsic properties of adsorbents are very important to enhance the affinity of CO 2 , which can be tuned by altering the constituent elements and topologies of MOFs.It is also crucial to strike a balance between a low affinity for H 2 O and a strong affinity for CO 2 .This work undoubtedly offers practical and useful guidelines to choose and design MOFs in the near future.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.
U R E 4 (A) Comparison between CO 2 selectivity calculated by K H and Grand Canonical Monte Carlo (GCMC) simulation.(B)

F I G U R E 5
Figure 6D compares P UL of CO 2 in the presence of water, which can be divided into Adsorption reaction enthalpy of (A) CO 2 , (B) H 2 O, and (C) N 2 within the selected metal-organic frameworks (MOFs) from density-derived electrostatic and chemical (DDEC)-based screening.Reproduced with permission. 58Copyright 2017, Wiley.
against those of CO 2 .The colors represent three different adsorbaphores in the top-performing structures: A1, parallel aromatic rings; A2, metal-oxygen bridges; A3, open metal sites.Some materials have both A1 and A2 sites.(B) Adsorbaphore containing parallel aromatic rings (A1) (adapted from Ref. 62).(C) CO 2 /H 2 O selectivity based on the ratio of Henry's constants against the largest cavity diameter (LCD).Data points are colored based on accessible surface area (ASA) and (D) void fraction (VF).Reproduced with permission. 58Copyright 2017, Wiley.

F I G U R E 8
Explanation of adsorption mechanism by using a flow chart.
, three fluids flow through glass double-capillary devices: precursors of MOFs (inner fluid), hydrophobic ultraviolet photopolymerizable silicone (shell fluid), and an aqueous surfactant-containing carrier fluid, which are used to form MOF-based hybrid adsorbents with excellent hydrophobicity (outer fluid).It is demonstrated that CO 2 adsorption capacity of the encapsulated HKUST-1 was slightly lower than that of HKUST-1, but the encapsulated HKUST-1 showed the good recyclability and stability after 10 working cycles.
also found a similar principle through the experiments and GCMC simulations for MIL-96(Al) to investigate its adsorption behavior.Although CO 2 een-MOF/Al een-MOF/Al-Si F I G U R E 1 0 (A) Adsorption CO 2 isotherms for een-MOF/Al-Si at various temperatures.(B) Water vapor isotherms of een-MOF, een-MOF/Al, and een-MOF/Al-Si.(C) CO 2 adsorption capacity of een-MOF, een-MOF/Al, and een-MOF/Al-Si after exposure to ambient air.(D) CO 2 adsorption capacity of een-MOF, een-MOF/Al, and een-MOF/Al-Si after exposure to 10% H 2 O and 90% CO 2 at 140 • C. The flow rate is 50 cm 3 min −1 .Reproduced with permission.84Copyright 2021, Springer Nature.MOF, metal-organic framework.

F
I G U R E 1 1 (A) Illustration of the competitive adsorption of CO 2 against H 2 O at the surface and edge of polynaphthylene (PN).(B) Polymerization of 1,2-diethynylbenzene (DEB) in metal-organic frameworks (MOFs).Reproduced with permission. 92Copyright 2016, ACS.(C) Schematic of the microfluidic system for in situ encapsulated synthesis of MOFs.(D) The overall CO 2 sorption and desorption steps by the encapsulated MOFs.Reproduced with permission. 93Copyright 2021, KeAi.F I G U R E 1 2 Maps of the occupied positions of (A) CO 2 (orange), (B) N 2 (blue), and (C) H 2 O (cyan) in 1000 equilibrated frames extracted from the Grand Canonical Monte Carlo (GCMC) simulations for co-adsorption of CO 2 /N 2 (20/80) in MIL-96(Al) at 303 K and 1.0 bar in the presence of humidity, relative humidity (RH) = 8.5% (adapted from Ref. 33).

3
The effect of adsorbed water on the adsorbed CO 2 .Progressive change in the Fourier transform infrared spectra of dried SGU-29 with time (as indicated) under the flow of (A) the dry flue gas and (B) the humid flue gas.Dry flue gas consists of CO 2 , O 2 , and N 2 (100/190/723 mbar, respectively), and humid flue gas consists of H 2 O, CO 2 , O 2 , and N 2 (29/97/185/702 mbar, respectively) at 298 K.The flow rate is equal to 5 mL min −1 .Reproduced with permission. 27Copyright 2015, AAAS.F I G U R E 1 4 (A) The radial distribution functions calculated by Grand Canonical Monte Carlo (GCMC) simulations at 1 bar and 303 K for the pair OCO 2 -H w of hydrated Mg-CUK-1.(B) The representative snapshot showing the interactions between CO 2 and the atoms of metal-organic framework (MOF) pore wall.The dashed lines in the snapshots denote interactions between CO 2 molecules and the surrounding atoms at distances below 3 Å (adapted from Ref. 81).

F I G U R E 1 7
Projections along pore axis and first coordination spheres of Mg(II) sites for van der Waal (vdW)-corrected, density functional theory (DFT) calculated structures of evacuated L-2-ampd-R-Mg 2 (dobpdc) (top left) and the framework following CO 2 insertion (bottom left), water adsorption (top right), and co-adsorption of CO 2 and H 2 O (bottom right).The vdW-corrected, DFT-calculated binding energies (ΔE) are provided for each adsorption process, and the available experimental differential binding enthalpies (ΔH ads ) are included in parentheses.Green, blue, gray, red, and white spheres represent Mg, N, C, O, and H atoms, respectively.Reproduced with permission.79Copyright 2019, ACS.

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I G U R E 1 8 (A) Dynamic column breakthrough tests for dptz-CuTiF 6 with a gas mixture of 10% CO 2 and 90% N 2 (8 cm 3 min −1 flow rate) at 298 or 328 K and 1 bar under dry and humid conditions.Direct visualization of CO 2 molecules inside the crystal structure of dptz-CuTiF 6 , as viewed along the (B) c-axis and (C) b-axis.This highlights the optimal arrangement of the CO 2 molecules within the square-shaped channels.Reproduced with permission.38Copyright 2019, Elsevier.F I G U R E 1 9 (A) Synthesis procedure of three isoreticular metal-organic frameworks (MOFs).(B) Views of site I and (C) site II in activated NbOFFIVE-dps-Cu, GeFSIX-dps-Cu, SIFSIX-dps-Cu, and UTSA-300a with varied pore aperture sizes.Color code: Cu, green; F, light green; S, bright yellow; N, light blue; C, gray; Si, orange; Ge, navy blue; Nb, wine red; solvent molecules are omitted for the clarity.

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I G U R E 2 0 structures of Mg-CUK-1 with different H 2 O molecule loadings, from top to bottom, 0, 1, 2, 4, and 8 water molecules per unit cell.(A) View through the a-axis showing the hydrogen-bonded H 2 O molecules to the hydroxo functional groups (H 2 O⋅⋅⋅OH-μ3), and (B) side view of the channel (through the c-axis) and accessible surface, yellow circles represent CO 2 kinetic diameter.Reproduced with permission. 41Copyright 2020, PMC.

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I G U R E 1 (A-D) Schematic diagram of the possible mechanisms of CO 2 adsorption in the presence of water.Reproduced with permission.29Copyright 2012, ACS.
This research was supported by the National Key Research and Development Program of China (Nos.2022YFB4101700 and 2022YFE0128600), the National Natural Science Foundation of China (Nos.52276022, 22278365, and 22225802), and the Basic Research Funds for the Central Government 'Innovative Team of Zhejiang University' under contract number 2022FZZX01-09.Long Jiang and Jinyuan Yong contributed equally to this work.

TA B L E 3
High-throughput screening studies of metal-organic frameworks (MOFs) for CO 2 capture from the wet gases.

Table
The structure, CO 2 uptake, and features of advanced metal-organic frameworks (MOFs).
TA B L E 4 2 O and CO 2 .