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Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40 Page 17 of 35

addition, the functionality of the metal sites may also influence the adsorbent properties . Rieth et al.
[153]
modulated the inflection pressure of water vapor adsorption isotherm of MOFs through cation exchange
and obtained a series of MOFs with different psaAWH performances . For some macroporous adsorbents,
[87]
such as hydrogels, the introduction of hydrophilic groups can enhance their water adsorption capacity at
low relative humidity [129,148,154] . Zhao et al. synthesized a hydrogel crosslinked with a hygroscopic polymer
that demonstrated enhanced water uptake (0.7 g g ) under 30% RH . In addition, the combination of
-1
[116]
hygroscopic salts and the introduction of functional polymers can impart functionality and modulate the
adsorbent psaAWH performance.

Generally, for the functional groups in the pore, the main influence is the applied RH range of psaAWH
[Figure 17]. According to the climatic conditions of the area where psaAWH is to be implemented, the
hydrophilicity of the adsorbent pores can be regulated to meet the requirements. In arid areas, the
hydrophilicity of the pores can be increased. In contrast, in areas with high RH, we can reduce the
hydrophilicity of the pores so that the inflection point (P ) is exactly the atmospheric RH [Figure 1B].
2
Therefore, the inflection point (P ) can be reached with minimal energy consumption, which significantly
1
reduces energy consumption [Figure 16A]. In addition, we should pay attention to the change in water
uptake caused by the functional groups in the pore [Figure 16B]. When modifying a pore with a functional
group, significantly reducing the hydrophilicity of the channel should be avoided, which will lead to a
significant decrease in water uptake.

Effect of pore size
The adsorbent pore size has a considerable influence on the adsorbent psaAWH performance. First, it
shows obvious effects on the water molecule transport kinetics. Intuitively, a larger pore size would improve
the mobility of water molecules within the material; this hypothesis is considered in detail in the next
section. However, a larger pore size does not necessarily result in a better adsorbent. For some crystalline
materials such as MOFs, large pore sizes tend to result in poor water and structural stability, as rationalized
by the theory of reticular chemistry, rendering them unsuitable for psaAWH . In addition, for some
[155]
macroporous adsorbents that rely on adsorption sites for water uptake, the number of active sites per unit
volume is lower for larger pore sizes. Accordingly, they exhibited slower water adsorption rates than the
corresponding microporous materials. This phenomenon has also been observed in some macroporous
hydrogels .
[2]

The adsorbent pore size may also influence the applicable RH range for the psaAWH by shaping the water
vapor adsorption isotherm. For example, Abtab et al. synthesized a large-pore MOF (pore diameter = 1.7
nm), and the inflection point of its S-type water vapor adsorption isotherm was 60% RH [Figure 18A],
which is higher than that of some small-pore MOFs (MOF-801, UiO-66, etc.) . Therefore, it can only be
[85]
used in an environment with humidity above 60%. Other materials with large pores, such as MIL-101 ,
[83]
PIZOF-2 , MOF-806 , and COF-670-hydrazine , also demonstrate this behavior. Although the effects
[110]
[19]
[19]
of pore size on the water adsorption curves of these 3D materials are apparent, the complexity of the
interconnected channels of 3D porous materials complicates the mechanistic evaluation. In contrast, for 2D
porous materials, the effect of the pore size on the water vapor adsorption isotherm is clear.
Gilmanova et al. synthesized two types of isoreticular 2D COFs (DUT-175 and DUT-176) and studied the
effect of the pore size on water adsorption . Figure 18B shows that both COFs possess one-dimensional
[107]
channels with different pore sizes (16.4 25.7 and 21.9 31.8 Å, respectively). The larger pore size of DUT-176
resulted in a water vapor adsorption isotherm with a higher pressure inflection point (60% RH) relative to
DUT-175. Similarly, two 2D COFs synthesized by Nguyen et al. also demonstrated this phenomenon .
[110]