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Page 8 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

Figure 6. Crystal structures of the four sufficiently crystalline patterns showing the successive steps in dehydration (top) and changes in
the local molecular environment (bottom). Reproduced with permission [77] . Copyright John Wiley and Sons.

hydroxyl groups through strong hydrogen-bond interactions. Subsequently, water molecules continue to
grow at these nucleation sites to form water clusters and fill the pores. In fact, the adsorption mechanism of
most MOFs during psaAWH is cluster adsorption, and water far away from the nucleation center is
preferentially removed during desorption [Figure 7B].

Capillary condensation, which often occurs in MOFs with large pore sizes, is the final mechanism of water
vapor adsorption by MOFs. According to previous reports, when the pore size of MOFs exceeds 2 nm,
capillary condensation occurs easily in the pores at room temperature (~25 °C) . Capillary condensation is
[76]
a physical adsorption process that has weaker binding energy than chemisorption at open metal sites and
can be regenerated at lower temperatures. However, note that when capillary condensation occurs, a
hysteresis loop usually exists between the desorption and adsorption curves of MOFs, which is unfavorable
in psaAWH because it will lead to a wider operating RH. For example, MIL-101 (Cr) has two pores with
diameters of 2.9 and 3.4 nm, respectively, which results in an obvious hysteresis loop in its water vapor
desorption curve [Figure 7C] . Another mesoporous MOF (NU-1000) also exhibited this behavior .
[80]
[81]
With the emergence of increasingly water-stable MOFs, Furukawa et al. first proposed utilizing MOFs as
adsorbent materials for AWH in 2014 . In 2017, freshwater was successfully obtained from the atmosphere
[19]
using MOF (MOF-801) as an adsorbent material . Since this breakthrough, various MOFs have been
[34]
successfully used as adsorbents for psaAWH [82-88] . Subsequently, practical psaAWH using an MOF adsorbent
was performed in a desert and freshwater (0.175 kg over 24 h) was successfully harvested without providing
any external energy . However, although it is gratifying to successfully obtain fresh water in arid zones
[26]
using MOF, such production is insufficient for sustaining human life. Incorporating hygroscopic salts (such
as LiCl and CaCl ) into porous MOFs is an effective method for increasing water production [89,90] . For
2
example, Xu et al. prepared a LiCl@MIL-101(Cr) composite material that displays a multi-step water
sorption process, leading to excellent water production (0.7 L kg adsorbent -1 d ) at relatively low humidity when
-1
combined with a light-absorbing material (carbon black; Figure 8A) .
[83]