In a high-throughput screening study (Chanut et al., 2017), Chanut et al. investigated the effect of pre-equilibrated water on CO₂ uptake in 45 MOFs using thermogravimetric analysis. The MOFs were grouped into various categories based on the extent to which pre-adsorbed H₂O influenced CO₂ uptake. One category, which included MIL-110(Al), MIL-163(Zr), Cu-HKUST-1 and UiO-66(Zr) (Table 1), exhibited a slight increase in CO₂ uptake with a certain amount of pre-adsorbed water. For example, Cu-HKUST-1 showed an approximately 5 wt% increase in CO₂ uptake in the presence of 2–4% relative humidity (RH). This observation is consistent with Yazaydin’s report that CO₂ uptake and its selectivity over N₂ increased in 4 wt% hydrated Cu-HKUST-1 due to the presence of water molecules coordinated to the framework open-metal sites (Yazaydın et al., 2009). This enhancement was initially predicted through molecular simulations and later validated by experiments. Detailed examination of interaction energies using grand canonical Monte Carlo simulations suggested that Coulombic interactions are responsible for the increased CO₂ adsorption–specifically interactions between the quadrupole moment of CO₂ and the electric field generated by water molecules bound to open metal sites. The LeVan group reported similar findings for Cu-HKUST-1 through volumetric measurements (Liu et al., 2010). Collectively, these results suggest an unexpected approach for enhancing CO₂ capture in the presence of water. However, at high humidity levels, Cu-HKUST-1 undergoes structural degradation, which likely explains why enhanced CO₂ uptake was not observed under conditions of high RH. Yu et al. (2016) investigated the effect of water on CO₂ capture in an isostructural series of M-HKUST-1 frameworks (M = Zn, Co, Ni, and Mg) through simulation studies evaluating water coordination within the MOFs. Water-coordination enhanced CO₂ uptake, similar to that observed in Cu-HKUST-1, was found for the Zn-, Co-, and Ni-based analogues. However, for Mg-HKUST-1, water coordination reduced CO₂ adsorption at higher pressures.

Another water-stable MOF, UiO-66, was reported by Hossain et al. to show a similar result, exhibiting a slight enhancement in CO₂ adsorption at low water loading (1.5 mol/kg) under low CO₂ partial pressure (below 5 kPa) at 25°C. This observation was based on experimental binary adsorption isotherms measured volumetrically using a mass balance approach (Hossain et al., 2019). However, increasing the co-adsorbed water loadings to 4.2 and 12 mol/kg led to reduced CO₂ uptake (Figure 1 Left). Molecular simulations supported these findings and further revealed that the effect depends on the type of defect sites within the MOF: missing linker defects promoted the enhancement (Figure 1 Right), whereas missing cluster defects didn’t show this behavior. Expanding on this work, Hernandez et al. conducted computational studies on three amine-linker UiO-66 materials and found that water molecules bridge between metal-oxide clusters by occupying missing linker positions (Hernandez et al., 2021). These water bridges reduce the pore size in defect-laden MOFs and enhance CO₂ adsorption in the presence of co-adsorbed water. Experimental binary isotherm data were consistent with these predictions. These studies underscore the importance of considering defect sites when evaluating CO₂ capture performance in humid conditions.

MIL-100(Fe) was evaluated by Soubeyrand-Lenoir et al., who reported a five-fold increase in CO₂ uptake (105 mg/g) at low pressure (200 mbar) under moderate humidity (40% RH) (Soubeyrand-Lenoir et al., 2012). They hypothesized that water molecules coordinate to the Lewis-acidic metal sites, forming water channels, while CO₂ adsorption occurs in the center of these channels without carbonate formation. In addition to the water stability of the materials, its mesoporosity was highlighted as a key factor contributing to the observed enhancement, as it allows for the formation of microporous water pockets that can subsequently be filled with CO₂ (see Figure 2).

Xian et al. further investigated this idea in MIL-100(Fe) using CO₂ temperature programmed desorption (TPD) and in situ Fourier transform infrared spectroscopy (FTIR) (Xian et al., 2015). TPD measurements revealed two CO₂ desorption peaks in the hydrated sample (50% RH), compared to only one in the dehydrated sample, indicating the creation of an additional adsorption site in the presence of water. The authors proposed that water molecules dissociate to form node hydroxyl groups, which serve as extra adsorption sites for CO₂, thereby leading to an additional adsorption site to enhance the material’s uptake capacity. In situ FTIR results supported this conclusion by revealing faster CO₂ adsorption under humid conditions, as evidenced by the more rapid appearance of CO₂ stretching mode peaks (see peaks at 2,350, 3,600, and 3,700 cm⁻¹ in Figure 3). The water dissociation hypothesis is worthy of further investigation.

For MOFs containing bridging hydroxo ligands as components of nodes, computational studies have predicted that at low water loadings, H₂O molecules can be efficiently packed through strong hydrogen bonding to the–OH groups (Liu et al., 2023; He et al., 2023). These well-ordered water molecules can, in turn, improve CO₂ adsorption by forming favorable hydrogen bonds with CO₂ within the microchannels. In other words, the hydroxo ligands act as directing agents for efficient water arrangement, and the pre-adsorbed water molecules introduce confinement effects that further promote CO₂ uptake.

Building on this principle, the Ibarra group investigated a series of MOFs containing µ₂/µ₃-OH ligands, including NOTT-400 (Gonzalez et al., 2015), NOTT-401 (Lara-García et al., 2015; Sánchez-González et al., 2016), InOF-1 (Peralta et al., 2015), and Mg-CUK-1 (Sagastuy-Breña et al., 2018), and demonstrated that their CO₂ capture capacities were enhanced to varying degrees under moderate humidity (RH < 40%) at 30°C. In a combined experimental and computational study, Breña et al. further reported humidity-enhanced CO₂ adsorption in Mg-CUK-1, a framework featuring one-dimensional microporous channels (Sagastuy-Breña et al., 2018). Using static CO₂ adsorption isotherms and thermogravimetric analysis under a constant CO₂ flow (60 mL/min), they observed a maximum CO₂ uptake of 8.5 wt% at 18% RH, compared to 4.6 wt% under dry conditions. However, beyond 20% RH, a rapid decline in CO₂ adsorption was observed, with almost negligible CO₂ uptake at 25% RH.

Chen et al. (2018) reported unusual moisture-enhanced CO₂ adsorption in PCN-250(Fe₃) and PCN-250(Fe₂Co). These compounds are constructed from trimetallic-oxy clusters, i.e., Fe₃(μ₃-O)(CH₃COO)₆ or Fe₂Co(μ₃-O)(CH₃COO)₆, as nodes, and ABTC^4- units as linkers (H₄ABTC = 3,3′,5,5′-azobenzenetetracarboxylic acid). For PCN-250(Fe₃), the uptake of CO₂ increases by 54% under 50% RH, compared to dry conditions (from 1.18 to 1.82 mmol/g). PCN-250(Fe₂Co) exhibited a 69% increase in CO₂ uptake under the same conditions (from 1.32 to 2.23 mmol/g). Even at 90% RH, significant increases in CO₂ adsorption were observed (44% for PCN-250(Fe₃) and 70% for PCN-250(Fe₂Co)) compared to their respective uptakes under dry conditions. Molecular simulations revealed that node-based, bridging oxo ions (μ₃-O) act as directing agents for H₂O adsorption. These water molecules, in turn, help position CO₂ molecules closer to metal centers on the opposite side of the pore, enhancing CO₂ adsorption via confinement effects. The CO₂/MOF interaction is further strengthened by what the authors term a “plier effect”, where coordinated water molecules appear to “clamp” CO₂ molecules onto open metal sites (see Figure 4), increasing the efficiency of adsorption under unsaturated conditions by maximizing the use of available adsorption sites. The plier effect, involving CO₂, H₂O, and the MOFs, enables effectively use of more of the candidate adsorption sites by CO₂; in turn, the amount of CO₂ adsorbed increases. The mechanism behind the enhanced CO₂ uptake at 90% RH is less clear, as such high humidity would presumably saturate the pores with water, leaving little space for CO₂.

Shi et al. (2020) reported a series of metal-triazolate MOFs, constructed from ZnF rods and 1,2,4 triazolate linkers functionalized with various groups (e.g., -NH₂ and -CH₃), among which the MOF featuring a 3,5-diamino-1,2,4-triazolate linker (ZnF(daTZ)) exhibited a CO₂/N₂ thermodynamic adsorption selectivity of 120 at 298K and 0–101 kPa, and a CO₂/H₂O kinetic adsorption selectivity of 70 at 298K and 33% RH. DFT calculations revealed a 25%–30% increment in the heat of CO₂ adsorption in the presence of co-adsorbed water, indicating stronger CO₂ binding under humid conditions (see Figure 5). This enhancement was attributed to the preferential localization of water and CO₂ molecules within the MOF framework, i.e., water molecules tended to occupy the corner sites, while CO₂ molecules were primarily located at the center of the channels. However, this study does not fully elaborate on how this spatial distribution contributes to the enhanced CO₂ adsorption under humid conditions. A similar spatial preference was reported in amine-functionalized UiO-66, where H₂O and CO₂ adsorbed at different sites (Hernandez et al., 2021). The authors proposed that water molecules formed hydrogen-bonded bridges between metal nodes by occupying missing linker positions, effectively reducing the pore size and enhancing CO₂ adsorption.

Functionalizing MOFs with diamines allows one amine group to coordinate to an open metal site, while the other points into the pore to interact with CO₂ molecules. For instance, diamine-modified MOF-74 materials have displayed selective CO₂ adsorption over water due to the formation of carbamate species, particularly at low CO₂ concentrations (Milner et al., 2018). However, carbamate formation typically requires two amine groups to react with one single CO₂ molecule, which limits the overall CO₂ uptake capacity.

The impact of RH on the performance of amine-appended MOFs remains relatively underexplored, with few studies reporting CO₂ uptake across a broad range humidity levels. Holmes et al. (2023) investigated this effect on (2-ampd)₂Mg₂(dobpdc) MOF (2-ampd is 2-(aminomethyl)piperidine) using both gravimetric and breakthrough adsorption techniques. Their findings identified three distinct RH regions based on the influence of water on CO₂ uptake: competitive adsorption from 0%–20% RH, enhanced adsorption between 20%–40% RH, and hindered adsorption due to pore saturation at RH levels above 40%, see Figure 6. A significant enhancement in CO₂ uptake at 1 bar and 40°C was observed at 30% RH (5.7 ± 0.2 mmol/g), compared to 3.35 mmol/g under dry conditions, see Figure 6. This increase was attributed to a mixed adsorption mechanism, wherein CO₂ binds to both primary and secondary amines in 2-ampd, forming ammonium carbamate and carbamic acid, respectively. This mechanism was supported by TPD data showing co-desorption of water and CO₂, as well as DRIFTS measurements revealing the loss of N-H stretching from secondary amines and the emergence of O-H stretching in humid samples.

Didas et al. (2014) reported that co-adsorption of CO₂ and water on mesoporous silica with low amine surface coverage leads to bicarbonate formation. Recently, Lyu et al. (2022) and Chen et al. (2024) both demonstrated that bicarbonate formation within MOFs can enhance CO₂ uptake. In their studies, they developed amine-functionalized MOF-808 materials: one featuring amino acids coordinated to Zr ions (MOF-808-AAs), first reported by Lyu et al. (2022), and the other incorporating polyamines covalently attached to a chloro-functionalized framework (MOF-808-PAs), reported by Chen et al. (2024), see Figure 7 (top). Both series exhibited improved CO₂ capture performance under humid conditions for direct air capture of CO₂, where the CO₂ concentration is approximately 420 ppm in the atmosphere. At 50% RH, the l-lysine- and tris(3-aminopropyl)amine-functionalized variants exhibited remarkable uptakes of 1.205 and 0.872 mmol/g at 400 ppm CO₂ and 25°C corresponding to 97% and 75% increases compared to the dry uptakes, respectively. The detailed sorption study using solid-state NMR revealed that for the wet conditions, MOF-808-Lys exhibited a single signal at 167 ppm under 50% RH attributed to ammonium bicarbonate formation. The presence of water enhanced the amine utilization efficiency by forming bicarbonate species (see Figure 7 (bottom)), resulting in increased CO₂ uptake compared to the dry conditions.