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Page 6 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

molecules cannot be completely released from the molecular sieve. To address this drawback and regulate
the water adsorption process, Krajnc et al. synthesized a microporous LTA-type zeolite (AlPO -LTA) that
4
shows an S-type water vapor adsorption isotherm, owing to its accessible pore volume, indicates the
potential applicability of psaAWH [Figure 5] . Although the regeneration temperature of zeolites has been
[47]
redused by modification, it is still high compared to the current advanced porous materials.

Clays and organosilicas are two classic porous materials with water adsorption capacity. Clays have a
layered structure composed of silica tetrahedron and alumina octahedron as the basic constituent units, and
the interlayer is connected by counter-cations with a positive charge through non-covalent bond
interactions. Therefore, similar to zeolites, they exhibit good hydrophilicity. However, significant interlayer
swelling is easily caused after water absorption, resulting in a loss of water adsorption capacity after
regeneration . Usually, this problem can be solved by interlayer crosslinking or the formation of interlayer
[41]
metal oxide pillared clays . By constructing oxides of different metal cations (Na , Ca and Mg ) between
+
2+
2+
[48]
layers, Zhu et al. improved the stability and water adsorption performance of clay at low RH . Although
[49]
clay has good water affinity, its pores are primarily derived from interlayer stacking and therefore have a
low water adsorption capacity, which is not suitable for psaAWH. Organosilicas refer to a class of materials
whose main chain contains silicon atoms and organic groups are directly connected to silicon atoms. Most
organosilicas are hydrophobic materials; however, modification of the polymer chains can improve their
hydrophilic properties and enable them to possess water adsorption capacity . Mietner et al. studied the
[50]
effect of different functional group modifications on the water adsorption performance of organosilicas .
[51]
The results indicate that organosilicas with an S-type water vapor adsorption isotherm can be obtained by
surface modification and structural design, and the inflection point of the water vapor adsorption isotherm
can be regulated by different hydrophilic groups. Currently, organosilicas show potential application value
in psaAWH; however, the preparation of organosilicas with a homogeneous micropore structure is still a
challenge, which makes it unsuitable for application to psaAWH at low RH.

Metal-organic frameworks
Metal-organic frameworks (MOFs) are a structurally diverse class of classical porous materials. They are
composed of metal clusters joined by organic linkers and have relatively controllable and uniform pore
sizes [52,53] . Although previously designed MOFs exhibit poor stability , recent MOF designs offer water
[23]
stability and synthetic flexibility [54-61] .

Various MOFs with high water stability have been studied for their water-adsorption properties [62-68] . The
first strategy for building a water-stable MOF is to strengthen the coordination bonds between the metal
clusters and organic linkers. Accordingly, Choi et al. synthesized two types of zinc(II)-pyrazolate MOFs
using a Brønsted base (pyrazole) to replace traditional carboxylic acids as organic linkers . Because the
[69]
stronger coordination interaction between pyrazole and the metal clusters effectively prevents the
degradation of coordination bonds by water molecules, the two zinc(II)-pyrazolate MOFs exhibit excellent
water stability. Subsequently, zeolitic imidazole frameworks (such as ZIF-8 and ZIF-68) have also
demonstrated exceptional water stability [70,71] . Another effective strategy to improve the water stability of
MOFs is to increase the connectivity of secondary building units (SBUs), thereby increasing the steric
hindrance of the metal contact with water molecules. For this reason, Tian et al. synthesized a hetero-triple-
walled MOF that possesses a high-connectivity SBU and shows excellent water stability . In addition, high-
[72]
connectivity Zr-based MOFs exhibit outstanding water stability . Finally, the use of inert cations is an
[19]
effective strategy for preparing water-stable MOFs [73,74] . For example, the inert Cr O clusters in Cr-MIL-101
+
3
7
significantly slow down the degradation of metal clusters by water molecules, allowing them to maintain
crystallinity for months in moist air .
[75]