Page 17 - Read Online

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

Figure 8. (A) Schematic of the multi-step water sorption/desorption processes of LiCl@MIL-101(Cr). Reproduced with permission [83] .
Copyright John Wiley and Sons; (B) preparation of the MOF/polymer composite and the temperature-triggered water capture and
release process. Reproduced with permission [88] . Copyright John Wiley and Sons.

Covalent organic frameworks
Covalent organic frameworks (COFs) are crystalline porous materials composed of organic ligands
connected by covalent bonds [94,95] . Hundreds of COFs have been widely used in a variety of fields, as
reported previously [96-98] . The water adsorption performance of COFs has also been evaluated [99-107] . As a
crystalline porous material, the water adsorption/desorption process of COFs in psaAWH is similar to that
of MOFs [Figure 7B]. However, they do not have open metal sites, which usually cannot form strong
chemisorption. Therefore, the cluster adsorption process forms the main mechanism of COFs. In COFs, the
nucleation sites of water clusters generally include Schiff bases or other hydrophilic groups on the
monomers. Ma et al. studied the adsorption of water molecules in TpPa-1 through solid-state NMR,
indicating that water molecules interact with hydrophilic groups in the framework of COF pores . Tan et
[105]
al. further studied the formation of water clusters in COFs . In the initial stage of adsorption, the N chain
[100]
in the COFs pores serves as the nucleation site for water cluster growth, and water molecules gradually fill
the entire pore from this point [Figure 9]. Similarly, when the pore size of the COF is too large, capillary
condensation of water molecules will also occur, resulting in an obvious hysteresis loop between the water
vapor desorption and adsorption curves .
[108]
Given the recent successful applications of MOFs in psaAWH, analogous applications of COFs have drawn
considerable interest, as indicated by a number of recent publications [27,109,110] . For example, in 2020, Nguyen
et al. successfully synthesized a 2D COF (COF-432) that presented an S-type water vapor adsorption
isotherm featuring a steep step in the range of 30%-40% RH [Figure 10] . Owing to the square grid of mtf
[27]
topology, water molecules can fill the pores of COF-432 at 40% RH, reaching a water uptake of
approximately 0.25 g g adsorbent -1 . Moreover, COF-432 possesses ultrahigh water stability and can be
water
regenerated at a low temperature (35 °C). At 30 °C and 40% RH, COF-432 offered an expected water
production per cycle of 0.23 g g adsorbent -1 . However, because COFs are developing porous materials, they
water
are underutilized in psaAWH. In addition, the high cost and harsh synthetic conditions restrict its
commercial application.

Hygroscopic polymers
Hygroscopic polymers are also common porous materials that exhibit high water adsorption capacities
because of their abundant surface and bulk hydrophilic groups . Hygroscopic polymers, particularly
[111]
hydrogels, primarily rely on layer adsorption and capillary condensation to achieve the adsorption of water