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INORGANIC CHEMISTRY
FRONTIERS
REVIEW
Cite this: Inorg. Chem. Front., 2021,
8,898
Received 16th November 2020,
Accepted 13th December 2020
DOI: 10.1039/d0qi01362e
rsc.li/frontiers-inorganic
Porous frameworks for effective water adsorption:
from 3D bulk to 2D nanosheets
Lan Zhang,†Wen-Xia Fang,†Cong Wang,†Hui Dong, Shu-Hua Ma and
Yang-Hui Luo *
The generation of freshwater from ubiquitous atmospheric moisture via using appropriate water adsor-
bents in atmospheric water generators has the potential to serve as a powerful strategy to effectively
address global water shortages that are threatening the lives of humans. In this regard, the preparation
and selection of water adsorbents are the essential premise. In this review, we summarize the latest pro-
gress in the development of porous frameworks for water harvesting. First, we introduce systems engin-
eering for hygroscopic salts and the fabrication of nano-porous super-hygroscopic hydrogels, followed
by the design of nanomaterials with controlled morphologies and a structural design strategy for metal–
organic frameworks (MOFs). Porous adsorbents with new forms ( porous organic polymers (POPs),
covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and two-dimen-
sional (2D) materials) are then summarized in detail. Finally, future challenges and directions relating to
this emerging field are discussed.
1. Introduction
As it is the world’s most important molecule, water is the
driving force for all nature.
1
Water is everywhere, in the sky, on
the ground, as well as under the ground, throughout life-sus-
taining biological, chemical, and geological processes.
2,3
Hence, the manipulation of water molecules, in multifarious
dimensions, for various applications, is of fundamental impor-
tance. Among these, water adsorption by porous frameworks,
for the development of water-related industrial processes and
water-production technologies, has occupied the central posi-
tion, attributed to the enormous demand for effective removal
of trace amounts of water for industrial gas transport and
sequestration,
4–6
as well as the urgent need to relieve fresh-
water scarcity through facile and energy-saving methods.
7–10
It
should be noted that, for both of the purposes, the key aspects
lie in the design and fabrication of absorbent porous frame-
works, as well as the related device, by comprehensively balan-
cing the hydrolytic stability, water adsorption capacity and
energy-demand for water release.
11–14
To achieve an ideal water adsorption performance, such as
a high water uptake, low energy demand for water release, fast
water capture/release, high cycling stability, and low cost, sig-
nificant efforts have then been made to rationally design
materials and structures for use as water adsorbents.
15–18
Among them, the most commonly used materials are porous
frameworks such as silica gels, zeolites, zeo-type inorganic
crystalline materials, composites with hygroscopic salts in sup-
porting matrices, and metal–organic frameworks (MOFs).
19–22
Currently, investigations are centered on the following three
aspects: (i) rational functionalization of the existing adsor-
bents with enhanced water affinity, the large surface area and
high porosity; (ii) precise fabrication of devices for practical
water adsorption via structural designs and systems engineer-
ing; and (iii) searching for new adsorbents for the construction
of next generation water adsorption devices with an ideal
performance.
Yang-Hui Luo
Yang-Hui Luo is an associate
professor at Southeast University
at present. He was born in 1988
in Hunan Province, P. R. China.
He received his PhD degree from
Southeast University in 2016.
His research interests are related
to the controlled preparation
and application of two-dimen-
sional (2D) metal–organic frame-
work (MOF) nanosheets and
hydrogen-bonded organic frame-
works (HOFs).
†The authors are contributed equally.
School of Chemistry and Chemical Engineering, Southeast University, Nanjing,
211189, PR. China. E-mail: luoyh2016@seu.edu.cn
898 |Inorg. Chem. Front.,2021,8,898–913 This journal is © the Partner Organisations 2021
With respect to practical applications, as well as to the
development of next-generation water adsorption devices, a
comprehensive understanding of the co-relationship between
the fundamental principles of material and structural designs
and water adsorption performance is necessary. There are a lot
of review articles that have summarized the structural and
functional aspects of MOFs as water adsorbents,
16–22
however,
no summaries for other kinds of porous adsorbents have been
published. Therefore, this review will focus on the current
strategies for the design of water adsorption materials with
promising applications. The representative strategies for struc-
tural designs and systems engineering of the commonly
involved adsorbents (hygroscopic salts, functional nano-com-
posites, hygroscopic hydrogels, as well as MOFs, Fig. 1) within
the last 3 years will be discussed in the next section. The fol-
lowing section deals with newly developed porous adsorbents
species, such as porous organic polymers (POPs), covalent
organic frameworks (COFs), hydrogen-bonded organic frame-
works (HOFs) and two-dimensional (2D) materials (Table 1). In
the final section, the outlook for water adsorbents has been
highlighted.
2. Functionalization of porous
adsorbents
A desirable adsorbent for effective water adsorption relies on
its excellent hydrolytic stability under operational conditions,
a high water capacity that can be maintained even after a mul-
titude of water uptake and release cycles, and low energy-input
for water release, as well as ideal water sorption properties.
Rational materials design and systems engineering can
provide the expected water adsorption performance for adsor- bents, by introduction of functional components that can
promote spontaneous vapor sorption.
2.1 Systems engineering for hygroscopic salts
Hygroscopic salts, such as LiCl, CaCl
2
or CuCl
2
, are classical
water adsorbents, attributed to their outstanding ability to
take up water into their bulk phase at a low relative humidity
(RH). However, the spill-over of desiccant and subsequent cor-
rosion of the device, as well as particle agglomeration attribu-
ted to the deliquesce of salt, have blocked their widespread
applications.
23
Furthermore, the concentrated solutions of
hygroscopic salts can also be used as adsorbents, unfortu-
nately, they suffer from the drawbacks of complicated appar-
atus, high capital costs and diligent management.
24
To solve
these thorny aspects, one promising strategy is to integrate the
hygroscopic salts into a stable porous matrix. Wang et al. have
made anhydrous salts (copper chloride (CuCl
2
), copper sulfate
(CuSO
4
), and magnesium sulfate (MgSO
4
)) into bilayer water
collection devices,
25
with a silica fibrous filter substrate as the
bottom layer for salt-loading, and carbon nanotubes (CNTs) as
the top layer for photothermal assisted water release (Fig. 2).
This design strategy has enabled these devices to be capable of
capturing water from air, even at a low RH (down to 15%), and
Fig. 1 A summary of currently used water adsorbents.
Table 1 The classification of the water adsorbents listed in this review
Category Ref.
Functionalization
of porous
adsorbents
Systems
engineering of
hygroscopic salts
25–30
Fabrication of
nano-porous
super-
hygroscopic
hydrogels
34–36
Nanomaterials
with controlled
morphologies
Nanorods 39
Nanofibers 48–50
Bioinspired
nanostructures 53–57
Surface
functionalization 61–63
Structural design
of metal–organic
frameworks
Improve the
stability of MOFs 71–75
Pursuit of S-shaped
water isotherms 76–82
Functionalization
of MOFs 83–92
Incorporation of
MOFs with
functional
materials
93–96
Porous adsorbents
with new forms Porous organic
polymers (POPs) 99
Covalent organic
frameworks
(COFs)
100
Hydrogen-
bonded organic
frameworks
(HOFs)
101
Two-dimensional
(2D) materials 103
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the release of the adsorbed water can be triggered by regular
and even weakened sunlight (i.e., 0.7 kW m
−2
).
Apart from a stable porous matrix, a polymer matrix can
also act as a versatile support for hygroscopic salts. Moreover,
a polymer matrix usually features excellent properties for
various applications in terms of macroscopic shaping. Wang
et al.
26
have further rationally fabricated a PAM-CNT-CaCl
2
hydrogel, by using CaCl
2
, acrylamide (AM) and CNTs (Fig. 3a).
Among the composite hydrogels, the AM-based platform can
maintain the solid form of CaCl
2
even after adsorbing a large
amount of water. As a consequence, a superior water sorption
capacity even in low humidity air, and easy release of the har-
vested water under regular sunlight via the photothermal
effect, have been achieved (Fig. 3b and c), providing a cheap
and affordable strategy for water harvesting and delivery.
Based on these results, Fröba et al. have incorporated CaCl
2
into a sodium alginic acid-based polymer matrix to obtain algi-
nate-based hydrogels,
27
the latter can further generate compo-
site beads with a size of around 2 mm via a facile ionotropic
gelation method (Fig. 4a), suggesting a cheap, non-toxic, and
easily accessible strategy for the preparation of the salt hydro-
gel. It should be noted that this kind of composite bead can
adsorb 100% of its own weight in water from air in arid
climate zones (at 10 mbar water vapor pressure and 28 °C),
and 90% of the adsorbed water can be released at 100 °C, pro-
viding a potentially solar-driven atmospheric water harvesting
strategy.
More recently, Wang et al. have modified sodium alginate
by using lithium chloride (LiCl) and calcium chloride (CaCl
2
)
to give a binary hydrophilic polymeric salt for atmospheric
water harvesting for the first time,
28
the functionalized carbon
nanotubes (FCNTs) are embedded in the hydrogel structure to
enable the solar-driven process (Fig. 4b). As a consequence, an
adsorption capacity of about 5.6 g g
−1
(desiccant) has been
achieved, a value that is almost three times higher than that of
the individual salts (Fig. 4c). This research team further pre-
pared a binary salt composite by using hydrophilic salts LiCl
and MgSO
4
, supported by an activated carbon fiber (ACF).
29
This obtained composite can be fabricated into a prototype for
atmospheric water harvesting, with an adsorption capacity of
0.92 g g
−1
in an arid climate powered by solar energy.
All of the afore-mentioned strategies deal with the hygro-
scopic salts at the macro-scale, which may result in the draw-
back of local particle agglomeration. Hence, Wang et al.
30
incorporated LiCl into a nano carbon hollow capsule (Fig. 5a),
generating a novel nano vapor sorbent with an improved water
adsorption capacity of over 100% of its own weight. In
addition, embedding of the resulting nano sorbent to the
fibrous silica substrate created a batch-mode atmospheric
water harvesting device, which showed a water adsorption
Fig. 2 The fabrication process for bilayer water collection devices.
Reproduced with permission from ref. 25. Copyright 2018, American
Chemical Society.
Fig. 3 (a) A schematic diagram of the PAM-CNT-CaCl
2
hydrogel syn-
thesis process. (b) and (c) water vapor sorption curves of PAM-CaCl
2
and PAM-CNT-CaCl
2
. Reproduced with permission from ref. 26.
Copyright 2018, American Chemical Society.
Fig. 4 (a) A schematic diagram of the production of composite beads.
Reproduced with permission from ref. 27. Copyright 2018, Springer
Nature. (b) A schematic diagram of the sorption-desorption mechanism
of Bina/FCNT. (c) A comparison between the water sorption capacity
and dynamic behavior of samples at 25 °C with RH = 70%. Reproduced
with permission from ref. 28. Copyright 2020, American Chemical
Society.
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900 |Inorg. Chem. Front.,2021,8,898–913 This journal is © the Partner Organisations 2021
capacity of 1.6 kg
water
kg
sorbent
−1
in outdoor conditions
(Fig. 5b–d), hinting at a possible method of large-scale deploy-
ment of atmospheric water harvesting for practical purposes.
2.2 Fabrication of nano-porous super-hygroscopic hydrogel
It is well-known that super water-absorbent gels are capable of
absorbing ten times its own weight in liquid water, therefore
the design of special nano-porous moisture-absorbent gels
with an ultrahigh moisture-absorbing capacity and water-
storing ability, is expected. In addition, the enormous surface
area of nano-porous gels promotes fast moisture absorption
from the atmosphere, and the binding forces between water
and the material are usually dominated by physisorption
rather than hydroxylation, which thus contributes to water
release in an energy-saving manner.
31–33
Tan et al. have
designed an amorphous super-hygroscopic hydrogel, which is
a non-stoichiometric oxide of zinc derived from zinc acetate in
the presence of glycol ether, amino-alcohol and water
(Fig. 6a).
34,35
This hydrogel features nano porous and fringed
contours (Fig. 6b–d), which provide an enormous surface area
for moisture absorption from the atmosphere, accompanied
by weak binding forces with water molecules. As a conse-
quence, this hydrogel is capable of absorbing water from
highly humid atmospheres (above the surface of the sea) of
over 420% of its own weight (Fig. 6e), and the desorption
process can be triggered by natural sunlight (at around 55 °C),
providing an external energy-less water harvesting approach
for the air above sea water (Fig. 6f).
Apart from the randomly synthesized amorphous super-
hygroscopic hydrogel, the precise design of a hydrogel with a
controlled hydrophilicity has also attracted attention. Yu
et al.
36
have constructed a super moisture-absorbent gel by
using hygroscopic poly-pyrrole chloride (PPy-Cl) penetrated
into a hydrophilicity-switchable polymeric network of poly
N-isopropylacrylamide (poly-NIPAM) (Fig. 7a and b). It was
interesting that the different components within the gel have
shown a clear division in functionality: the PPy-Cl was respon-
sible for moisture absorption and liquefaction, the network of
poly-NIPAM was responsible for water storage, while the hydro-
philicity switching of poly-NIPAM was responsible for rapid
water release. As a result, in situ water liquefaction, high-
density water storage and fast water release under different
weather conditions, have been achieved (Fig. 7c), providing a
novel design strategy to improve atmospheric water harvesting,
as well as other water management systems for environmental
cooling, surficial moisturizing and beyond.
2.3 Nanomaterials with controlled morphologies
It is well-known that nanomaterials with controlled mor-
phologies, such as nanowires, nanofibers and nanorods, often
feature interesting properties.
37,38
For water adsorption, the
application of nanomaterials with controlled morphologies
may produce unusual water adsorption–desorption mecha-
nisms, as well as an unprecedented water adsorption
performance.
2.3.1 Nanorods. Nune and Heldebrant et al.
39
have pre-
pared carbon-based rods by using 1-hexanol, 1,8-diazabicy-
cloundec-7-ene and CS2, in the presence of FeCl
3
(Fig. 8a).
Water adsorption experiments have suggested that the confine-
Fig. 5 (a) A schematic illustration of the fabrication process for a nano-
sorbent and its related water harvesting device. (b)–(d) Static RH tests of
LiCl, HCS, and HCS-LiCl under different RH conditions. Reproduced
with permission from ref. 30. Copyright 2019, Elsevier.
Fig. 6 (a) The simulated stable structure of Zn : O with the ratio 1 : 1.1
(blue balls represent zinc atoms and yellow balls represent oxygen
atoms). (b)–(d) Scanning electron microscopy (SEM) images of the
hydrogel showing the porous network and nano-fringes on the surface.
(e) Absorption rates for different surface area to mass (SAW) ratio hydro-
gels. (f ) The prototype device floating on the sea surface. Reproduced
with permission from ref. 34. Copyright 2019, Wiley.
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ment-mediated solvent cavitation mechanism, the adsorption
of water at low RH beginning with the formation of a mono-
layer of water on the surfaces of rods, which is followed by the
condensation of water in the confined space between adjacent
rods, can be attributed to the interfacial forces between the
confined rod surfaces. Finally, with the increase of the RH, the
surface-induced evaporation phenomenon occurs (Fig. 8b). It
should be noted that, this process (the adsorption of water at
low RH and release at high RH) is reversible, suggesting that
this kind of rod possesses a promising potential for use in
unconventional water separations, as well as for humidity-
responsive applications.
2.3.2 Nanofibers. Among the technologies used for atmos-
pheric water capture and release, the fabrication of functional
nanofibers by using several synthetic materials with adjustable
geometries have attracted considerable attention.
40–42
The
corresponding nanofibers are capable of controlling their
surface wettability to change their hydrophilicity and hydro-
phobicity, which are inspired by the surface structures of
natural systems such as spider webs and cacti.
43–47
Although
among the afore-mentioned functional nanofibers, the temp-
erature-responsive properties are of particular interest, as they
can undergo a phase transition at the lower critical solution
temperature (LCST) as a response to the external temperature,
which contributes to variation of the hydrophilic–hydrophobic
domains.
47
Choi et al. have designed a temperature-responsive hydrogel
nanofiber with a core–shell structure, by using poly(VCL-co-AA)
(VCL = N-vinylcaprolactam; AA = acrylic acid) as the shell and
PAN (polyacrylonitile) as the core (Fig. 9a).
48
The core part fea-
tures a high mechanical, thermal, and chemical resistance,
while the shell part acts as a temperature-sensitive system with
a self-cross-linking capacity and increased hydrophilicity,
which thus contributes to the switchable swelling capacity for
the hydrogel nanofiber. As a consequence, a higher water
uptake (≈234%) has been observed under high humidity air at
a low temperature (Fig. 9b and c), which can be attribute to
the diffusion of water molecules in the shell layers that are
present between the polymer chains, providing a versatile
model for the design of temperature-responsive nano-struc-
tures with controllable hydrophilic–hydrophobic properties in
response to small temperature changes.
Apart from the functionalization of synthetic materials, the
appropriate matching of hydrophobic and hydrophilic fibers
can also enable the controllable wetting properties of nano-
fibers. Stachewicz et al.
49
prepared optimal meshes for harvest-
ing water from fog, by combining hydrophobic polystyrene
(PS) and hydrophilic polyamide 6 (PA6) with a two-nozzle
Fig. 7 (a) A schematic illustration of the skeleton, porous structure, and
interpenetrating network of poly-NIPAM and PPy-Cl clusters. (b) SEM
images of a dried gel at different magnifications, showing the porous
structure. (c) Water production from 24 h of atmospheric water harvest-
ing (AWH) at different RH levels. Reproduced with permission from ref.
36. Copyright 2019, Wiley.
Fig. 8 (a) SEM images of a carbon rod synthesized at 230 °C. (b) A
schematic illustration of the proposed water expulsion mechanism.
Reproduced with permission from ref. 39. Copyright 2016, Springer
Nature.
Fig. 9 (a) An illustration of the preparation process of thermo-respon-
sive P(VCL-co-AA)/PAN core–shell nanofibers and thermo-triggered
water capture and release. (b) Water uptake kinetics of core–shell
nanofibers at 25 °C and 80% RH. (c) Water sorption isotherms of the
CS-5 nanofibers measured at 15, 25, 40, and 50 °C. Reproduced with
permission from ref. 48. Copyright 2019 American Chemical Society.
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902 |Inorg. Chem. Front.,2021,8,898–913 This journal is © the Partner Organisations 2021
electrospinning setup. Without any chemical modifications,
this kind of hierarchical composite can be used for practical
water harvesting from fog, attributed to the water condensing
ability of the hydrophobic microfibers and water delivery
ability of the hydrophilic nanofibers (Fig. 10).
In addition to the combination of different synthetic
materials or different kinds of fibers, modification of fibers
with a gradient roughness can also act as a versatile alternative
to achieve controllable hydrophilic–hydrophobic properties.
Zheng and Hou et al.
50
fabricated heterostructure rough
spindle-knot microfibers (HRSFs) via a flexible parallel-nozzle
microfluidic method (Fig. 11), which featured a roughness gra-
dient between the spindle-knots and joints. During the water
collection process, the joint part acts as the transmission
channel that coalesces the tiny droplets of moisture and trans-
ports them to the spindle-knot sections. It should be noted
that, the water collection efficiency was dominated by the
surface morphology of spindle-knots, the higher the roughness
gradient, the higher the water collection efficiency, suggesting
it has a high potential to be used in large-scale water
harvesting.
2.3.3 Bioinspired nanostructures. During the process of
the design of microstructures to facilitate water harvesting,
natural environments have provided us with abundant ideas,
especially from organisms that are native to arid environ-
ments, such as cacti spines, desert moss, lizards spider webs,
and Namib desert beetles.
51,52
The multiscale surface mor-
phology of which plays a key role in the water collection
efficiency, attributed to the variation of Laplace pressures that
are determined by their conical or pointed morphologies and
the chemical composition of materials. Xue and co-workers
have proposed the idea of a cascading effect for atmospheric
water harvesting,
53
by controlling the Laplace gradient, the
water droplets can coalesce with the adjacent one from a
similar structure (Fig. 12a), which thus has a sufficient volume
to roll downward and assimilate all the small water droplets to
be condensed on the flat surface of the harvester, effectively
clearing the PWHR ( passive water harvesting region) columns
with a considerable water harvesting ability (Fig. 12b). Inspired
by leaf veins with hierarchical micro-nanostructures for water
deposition, Sariola et al.
54
fabricated water-harvesting func-
tional surfaces consisting of high-density copper oxide nano-
needles (Fig. 12c), the later featured a high wettability that
could be enhanced by the hydrophobic coating, providing valu-
able insights into the design of water harvesters in arid or
semi-arid environmental conditions (Fig. 12d).
Despite the control of the Laplace gradient, the design of
nano-materials with similar structures to native organisms has
attracted significant attention. Cao and co-workers
55
have pre-
pared a wax-infused kirigami with an anisotropic 2D triangle,
which shows similar spines to the 3D cone of cactus kirigami
Fig. 10 An illustration of the combined mechanism for the hierarchical
composite and corresponding water harvesting performance.
Reproduced with permission from ref. 49. Copyright 2019 American
Chemical Society.
Fig. 11 (a) An illustration of the fabrication process of bioinspired
microfibers via the microfluidic method. (b) and (c) Optical images of
collected dehydrated microfibers in large quantities. (d) and (e)
Microscopic images of the microfiber before (d) and after (e) dehydra-
tion. Reproduced with permission from ref. 50. Copyright 2019 Wiley.
Fig. 12 (a) A schematic representation of the mechanism of water har-
vesting that results in a cascading effect that clears the PHWR of tiny
water droplets. (b) A comparison between the water harvested per area
of different water harvesting arrays (WHAs). Reproduced with permission
from ref. 53. Copyright 2019 American Chemical Society. (c) The guided
transportation of water via hydrophilic milled channels in uncoated CuO
nanoneedles (left) and coated CuO nanoneedles (right). (d) Fog harvest-
ing dynamics as the volume of water collected per square meter over a
surface area of 22 500 mm
2
for all surfaces. Reproduced with permission
from ref. 54. Copyright 2019 Royal Society of Chemistry.
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(Fig. 13a). This kind of 2D triangle can reproduce the function
of the 3D cone, and can capture fog droplets effectively and
rapidly refresh the collecting interface through directional
droplet self-propulsion (Fig. 13b), providing a rational design
for advanced fog harvesters. More recently, inspired by the
directional transportation of water droplets to the apex on
both the beaks of shore birds and wheat awns, Zheng and co-
workers
56
have constructed a topological alloy net with a
V-shaped asymmetric geometry in its mesh for fog collection
(Fig. 13c), which not only improved the water-collection rate
owing to the efficient drainage along the designated pathways,
but also resolved the issue of mesh clogging, thus providing a
useful insight into the development of novel fog-collecting
materials with excellent performances along all directions
(Fig. 13d). Inspired by the microchannels of natural woods
that serve as pathways to pump and transport water from the
ground via transpiration, Ding and co-workers
57
have fabri-
cated a moisture pump with multilayer wood-like cellular net-
works and interconnected open channels (Fig. 13e), by using a
facile and scalable two-step electrospinning and impregnation
method. In the pump, both a desiccant layer and a photother-
mal layer are presented, contributing to the unprecedented
moisture absorption capacity of 3.01 g g
−1
at 90% RH triggered
by solar-irradiation (Fig. 13f and g).
2.3.4 Functionalization of the surface. Apart from the con-
struction of nanomaterials with controlled morphologies, the
functionalization of surfaces to generate a hydrophobic-hydro-
philic gradient has also served as a versatile alternative for
water adsorption.
58,59
Inspired by the lubrication effect of
Nepenthes pitcher plants, Wang and co-workers have prepared
slippery liquid-infused porous surfaces by synergistically con-
structing a regular micro-pincushion and nanoparticles, which
display the well maintained dropwise coalescence of water
from fog and an efficient water harvesting performance.
60
Similarly, Chen et al. have fabricated a flexible functional
surface that features a superhydrophobic region and hydro-
phobic region by using an infused lubricating oil, which has
successfully achieved directional water collection.
61
It should be highlighted that, although MOFs themselves
are versatile water adsorbents (which will be discussed later),
they can also be used for surface functionalization to provide
an improved water adsorption capacity or water transport per-
formance.
62
Song et al. performed selective hydrophobic modi-
fication on the surface of a soy protein, using the construction
of hierarchical micro-/nano-crystals of ZIF-8 (Fig. 14a). This
strategy was inspired by the Stenocara beetle that can collect
water from moist air, and have generated controllably hydro-
Fig. 13 (a) The concept of simplifying 3D into 2D for the design of
cactus kirigami. (b) The fog-collecting performance on the kirigami
spines. Reproduced with permission from ref. 55. Copyright 2020 Royal
Society of Chemistry. (c) The topological alloy net with V-shaped asym-
metric geometry. (d) Water-collection rates with different orientations
compared to an equivalent conventional mesh. Reproduced with per-
mission from ref. 56. Copyright 2019 American Chemical Society. (e) A
schematic diagram showing the 3D self-assembly mechanism of the
moisture pump with a multilayer wood-like cellular network structure.
(f ) and (g) The moisture absorption kinetics of PAN/MIL@LiCl NFM at
25 °C and at various humidities. Reproduced with permission from ref.
57. Copyright 2020, Springer Nature.
Fig. 14 (a) A proposed mechanism illustrating the interaction between
ZIF-8 and SPI (soy protein isolate). (b) The water collection mechanism
of SA-ZIF-8@SPI films. Reproduced with permission from ref. 62.
Copyright 2019 Elsevier.
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philic-superhydrophobic patterns on their protein film surface
for efficient water harvesting from fog (Fig. 14b), with the
maximum water collection efficiency being as high as
917.6 mg cm
−2
h
−1
.
63
2.4 Structural design for metal–organic frameworks
Among the current investigated water adsorbents, MOFs
occupy a special position. Attributed to the modular construc-
tion from molecular building blocks, MOFs feature a large
diversity of frameworks that allow for the chemical and geo-
metrical optimization necessary to achieve the desired water
sorption properties, which thus endow MOFs with the merits
of a high chemical stability to water, tailorable hydrophilicity,
as well as an adjustable pore diameter to fine tune the adsorp-
tion profile and modulate the sorption kinetics, meeting the
high requirements for next generation adsorbents.
64–70
Plenty
of reviews have summarized the progress of MOFs for water
adsorption.
4–6,19–22
Therefore, this part will focus on the repre-
sentative strategies for MOFs-based water adsorbents within
the last three years.
2.4.1 Improving the stability of MOFs. The desired water
stability of MOFs is a precondition for their application as
water adsorbents. Generally, two strategies can be employed to
construct MOFs with a special topology and improve the water
stability, one is the incorporation of hydrophobic groups, the
other is the use of high oxidation state metals.
71–75
Li et al.
71
have constructed MOFs with the-a topology, by using ZrCl
4
with fluorescent ligand acids, 5′-(4-carboxyphenyl)-2′,4′,6′-tri-
methyl-[1,1′:3′,1″-terphenyl]-4,4″-di carboxylic acid (H3CTTA)
(Fig. 15b) and 6,6′,6″-(2,4,6-trimethylbenzene -1,3,5-triyl)tris(2-
naphthoic acid) (H3TTNA) (Fig. 15b). This kind of topology
endows these MOFs with an excellent stability in water, HCl
solutions and NaOH solutions. Later, guided by this topologi-
cal design approach, base-resistant,
72,73
acid-resistant,
74
as
well as stability adjustable MOFs,
75
have all been constructed
by Li and co-workers. The results have provided effective hints
for the development of MOFs water adsorbents.
2.4.2 Pursuit of S-shaped water isotherms. S-shaped water
isotherms, which mean that large water uptakes and releases
can be achieved through relatively small temperature or
pressure gradients, are desired for use in adsorption heat
transformation systems for water harvesting, because they are
a necessary prerequisite for energy efficient water release from
the sorbent.
4–6,19–22,76–78
MOFs usually tend to exhibit
S-shaped isotherm profiles, which can be attributed to the for-
mation of ordered, hydrogen-bonded water molecule networks
within the crystalline metal organic framework (MOF) during
the water nucleation and pore-filling process. To achieve the
desired S-shaped water isotherms, hydrophilicity, and pore
diameter are of critical importance. Ideally, the pore hydrophi-
licity must be sufficient to allow for water nucleation and pore
filling below approximately 30% RH, while the pore size must
be below the critical diameter (D
c
) of the working fluid, to
eliminate undesirable hysteresis upon water desorption. It
should be noted that, for water at 25 °C, the D
c
is 20.76 Å,
demonstrating that a pore diameter of approximately 20 Å for
porous adsorbents is optimal for water adsorption with irre-
versible capillary condensation having been perfectly
avoided,
79
hence the structural design of MOFs is of signifi-
cant importance. Dinca et al.
79
demonstrated a series of MOFs
with the formula M
2
Cl
2
(BTDD) (M = Mn, Co, Ni; BTDD = bis
(1H-1,2,3-triazolo [4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) (Fig. 16a)
that are desirable materials for practical water adsorption.
These MOFs featured large mesoporous channels with a dia-
meter of 22 Å, as a consequence, S-shaped water isotherms
under simulated desert conditions (daytime 45 °C and 5% RH
and nighttime 25 °C and 35% RH) have been achieved,
accompanied by a water adsorption capacity of over 82% of its
own weight below 30% RH (Fig. 16b). Later on, Farha et al.
80
designed acs-MOFs by using trivalent trinuclear metal clusters
and a rigid trigonal prismatic ligand (Fig. 16c), which resulted
in a 6-c acs topology with a pore size as large as 1.4 nm
(Fig. 16d), which also featured S-shaped water isotherms with
a high water uptake of 1.09 g g
−1
(Fig. 16e) and a considerable
cycling ability (Fig. 16f), providing a practical design strategy
for the construction of highly stable functional porous MOFs
with a targeted pore size and geometry for next-generation
porous water vapor sorbents.
As one of the smallest organic linkers, formate (HCO
2
−
) can
be used to build porous MOFs for effective water adsorption,
attributed to its various coordination modes thanks to the
smallest possible side group on the carboxylate carbon. In
addition, formate can be conveniently derived in situ from the
hydrolysis of amide-based solvents such as DMF or DEF
during the solvothermal process, which thus can contribute to
a simple, straightforward, and highly effective one-pot syn-
thesis. Lah et al.
81
synthesized Zr-based MOFs by using in situ
Fig. 15 Structures of (a) H3CTTA and (b) H3TTNA, as well as the crystal
structures of their related MOFs with D
4h
8-connected Zr6 clusters.
Reproduced with permission from ref. 71. Copyright 2016 American
Chemical Society.
Inorganic Chemistry Frontiers Review
This journal is © the Partner Organisations 2021 Inorg. Chem. Front.,2021,8,898–913 | 905
generated formate from the hydrolysis of DMF, and produced
a macrocyclic [Zr
6
]
6
and super-cage-like {[Zr
6
]
6
}
8
building unit
(Fig. 17a). These MOFs featured a highly polar surface and flex-
ible crystal packing, which thus contributes to the high heat of
water adsorption and unrestricted uptake under high humid-
ity. In addition to the direct construction of MOFs with excel-
lent water adsorption capacities, formic acid can also be used
to regulate MOFs with improved water adsorption perform-
ances. Chakraborty et al.
82
have found that the addition of
formic acid can promote, to a certain extent, the increment in
water adsorption for Al Fum (aluminum fumarate) MOFs (up
to 12.5%, Fig. 17b). Additionally, a novel facile synthetic strat-
egy for MOFs could also improve the water adsorption per-
formance to some extent.
83
2.4.3 Functionalization of MOFs. To obtain the desired
water adsorption performance for MOFs, functionalization is
the most commonly used strategy. In most cases, the
functionalization of MOFs is focused on the metal sites, the
free anions, structural defects, or the surfaces of particles or
pores. For metal sites, grafting of active groups on coordina-
tively unsaturated metal centers is an effective strategy.
84
Meanwhile, the employment of high-valence or rare-earth
metals can contribute to improvement of the water adsorption
performance for MOFs,
85,86
thanks to the stronger MOF–H
2
O
interactions provided by the terminal active groups bound to
the rare earth sites to give a high water affinity (Fig. 18a).
87
Furthermore, functionalization of the metal clusters can also
precisely tune the water adsorption performances of MOFs.
88
For free anions, Lin et al.
89
calculated that the terminal anions
(e.g.,F
−
, Cl
−
, or OH
−
) play a key role in the water adsorption
performance for MOF MIL-100(Fe), they found that MIL-100
(Fe)_F was the best owing to the strongest interaction between
the terminal F
−
anion and water molecules. The conclusion
was confirmed by Dinca et al.,
90
who exchanged the Cl
−
anions in Ni
2
Cl
2
BTDD to F
−
, Br
−
and OH
−
anions (Fig. 18b),
Fig. 16 (a) The crystal structure of Co
2
Cl
2
(BTDD) projected along the c
axis: Co, purple; C, gray; N, blue; O, red; and Cl, green. Hydrogen atoms
are omitted for clarity. (b) Water isotherms of Co
2
Cl
2
(BTDD) under simu-
lated desert conditions. Reproduced with permission from ref. 79.
Copyright 2017 American Chemical Society. (c) A schematic representa-
tion of the design and synthesis of the acs-a net. (d) An illustration of the
hexagonal channels inside the acs-a net. (e) A comparison of the water
sorption isotherms of the acs-a net between the first cycle and after 20
cycles of pressure swing between 20% RH and 70% RH. (f ) The cycling
testing for the acs-a net. Reproduced with permission from ref. 80.
Copyright 2019 American Chemical Society.
Fig. 17 (a) The crystal structure of the {[Zr
6
]
6
}
8
building unit. Color
code: green, Zr; red, O; gray, C; white, H; and orange, Cl. Reproduced
with permission from ref. 81. Copyright 2018 American Chemical
Society. (b) The adsorption–desorption behaviors of parent and FAM
(formic acid modulated) Al Fum MOFs at 25 °C. Reproduced with per-
mission from ref. 82. Copyright 2018 Elsevier.
Fig. 18 (a) The crystal structure of the high-valence MOF [Er
(dcbp)
3/2
(DMF)(H
2
O)
2
]·2H
2
O (H
2
dcbp = 4,4’-dicarboxy-2,2’-bipyridine).
Left: The binding style of the active groups with the metal sites. Right: A
view parallel to the aaxis. Reproduced with permission from ref. 87. (b)
A view of anion-exchanged secondary building units perpendicular to
the caxis and synthetic pathways for crystal Ni
2
X
2
BTDD (X = Cl, F, Br,
OH). (c) Water vapor adsorption (closed symbols) and desorption (open
symbols) isotherms measured at 25 °C. Reproduced with permission
from ref. 90. Copyright 2019 American Chemical Society.
Review Inorganic Chemistry Frontiers
906 |Inorg. Chem. Front.,2021,8,898–913 This journal is © the Partner Organisations 2021
and found that the F
−
anions can improve the water adsorp-
tion performance (Fig. 18c), attributed to the increase in the
pore size and strength of the MOF-water hydrogen bonding
interactions.
Lin further cooperated with Grossman
91
to calculate the
effect of structural defects on the water adsorption perform-
ance of MOF-801, and they found that the high defect density
may be responsible for the hydrophilic adsorptive behaviors,
and that the water adsorption performances are dependent on
the spatial configuration of defects. In addition, the photoche-
mically induced water harvesting in MOF Ni-IRMOF74-III has
been calculated by Kim et al., (Fig. 19a)
92
the results suggest
that the trans-to-cis isomerization of the azopyridine molecules
can lead to a significant enhancement in the water adsorption
performance (Fig. 19b), which can be attributed to the differ-
ence in pore partitioning for trans/cis configurations, as the
trans case possesses a more available surface area than the cis
case. These results have provided a special blueprint for the
design of next generation water harvesting materials.
2.4.4 Incorporation of MOFs with functional materials.
Apart from the smart structural design, the incorporation of
MOFs with functional materials can also serve as an effective
strategy to achieve the desired water adsorption performance.
Via an in situ polymerization strategy, Zhao and Maurin et al.
have constructed MOF/polymer composite materials by using
MIL-101(Cr) and PNIPAM (poly(N-isopropylacrylamide),
(Fig. 20a).
93
Mediated by a LCST, the resulting polymer@por-
ous MOF composite showed an unprecedented water adsorp-
tion capacity of approximately 440 wt% at 96% RH and 25 °C
(Fig. 20b), and the adsorbed water can be released under mild
conditions (40% RH and 40 °C). It should be noted that, these
promising aspects can be attributed to the hydrophilic-to-
hydrophobic phase transition of the PNIPAM component as a
response to the thermal variation, which is expected to shed
light on the development of stimuli-responsive porous adsor-
bent materials.
Additionally, the incorporation of hygroscopic salts into
MOFs,
94,95
as well as the coating of MOFs on stainless-steel
meshes for the construction of underoil super-hydrophilic sur-
faces,
96
can significantly improve the water adsorption per-
formances of MOFs.
3. Porous adsorbents with novel
forms
Apart from the above-mentioned popular porous frameworks,
the other novel forms of porous frameworks, such as POPs,
COFs, HOFs and 2D materials, have also shown potential to be
used in water adsorption, which has extensively extended the
scope of material classes suitable for water adsorption, as well
as bringing great benefits to water adsorption technology.
3.1 POPs
The POPs have attracted significant attention owing to their
high physicochemical stability, high surface areas, and
tunable functionalities, which thus can serve as versatile plat-
forms to provide an optimum binding energy for water mole-
cules, via balancing between the uptake capacity and regener-
ation temperature.
97
In particular, epoxy functional groups can
act as a sound candidate for water adsorption, owing to their
high hydrophilicity and moderate binding enthalpy for water
molecules, as a consequence, a high water uptake capacity and
low-temperature regeneration can be expected.
98
Coskun
et al.
99
have prepared multi-dimensional ep-POPs via a cata-
lyst-free, one pot Diels–Alder cycloaddition polymerization
(Fig. 21a), these were found to be highly microporous and
exhibit specific surface areas up to 852 m
2
g
−1
. As a conse-
quence, high water-uptake capacities of up to 39.2–42.4 wt%
under a wide temperature range of 5–45 °C have been achieved
Fig. 19 (a) A snapshot of H
2
O adsorption in both trans and the cis azo-
pyridine-IRMOF74-III at 2 kPa. (b) The H
2
O adsorption isotherm from
azopyridine-IRMOF74-III at 25 °C. Reproduced with permission from ref.
92. Copyright 2019 American Chemical Society.
Fig. 20 (a) An illustration of the preparation of a MOF/polymer compo-
site and the temperature-triggered water capture and release process.
(b) The water sorption isotherms of various samples at 25 °C.
Reproduced with permission from ref. 93. Copyright 2020 Wiley.
Inorganic Chemistry Frontiers Review
This journal is © the Partner Organisations 2021 Inorg. Chem. Front.,2021,8,898–913 | 907
(Fig. 21b), providing a promising strategy for the design of
POPs materials for desiccant-driven dehumidification and
water capture.
3.2 COFs
Similar to MOFs, COFs are versatile water adsorbents owing to
their exceptional porosity, large diversity of chemical compo-
sitions and accessible topologies, furthermore, their water
sorption properties can be tuned in a large variety of ways.
However, the relatively lower crystallinity of COFs usually pre-
cludes the formation of highly ordered molecular water net-
works within the porous framework, which thus hinders the
practical applications of COFs for water adsorption. Yaghi
et al.
100
have synthesized a COF with a (3,4,4)-c mtf topology,
by using 1,1,2,2-tetrakis(4-aminophenyl) ethene (ETTA) and
1,3,5-triformyl benzene (TFB) (Fig. 22a). This specific topology
has endowed COFs with a high water uptake capacity that is
comparable to most of the MOFs (Fig. 22b), and accompanied
by a considerable cycling stability (Fig. 22c), which has thus
paved the way for future research on COFs as water-harvesting
materials.
3.3 HOFs
Similar to MOFs and COFs, HOFs also feature exceptional
porosity and tunable framework behaviors for a large variety of
applications. However, a high stability and permanent porosity
remain a challenge for HOFs. In our previous work,
101
by
employing the “equidistance effect”, HOF TCPP-1,3-DPP (TCPP
=meso-tetra(carboxy-phenyl)-porphyrin, 1,3-DPP = 1,3-di(4-
pyridyl) propane) (Fig. 23a), with a permanent porosity
(Fig. 23b), has been prepared. This shows a high affinity and
selectivity to water with a maximum adsorption capacity of
about 9.8% (Fig. 23c), providing a practical strategy for the
design of HOFs-based water adsorbents.
3.4 2D materials
It is well-know that birnessite (i.e., a layered structure MnO
2
)
displays a layered structure that is intercalated by cations and
water molecules, which thus shows the potential to be used for
water harvesting.
102
Suib et al.
103
have demonstrated that the
water molecules can be quickly adsorbed into the interlayers
of birnessite at a lower RH (Fig. 24a), and the adsorbed mole-
cules can form multilayer water–water interactions on the 2D
surfaces, via hydrogen bonding contacts. As a consequence, at
a higher RH, the water molecules can be condensed in situ.
Meanwhile, birnessite features an excellent solar absorptivity
that can convert solar to thermal energy, which thus contrib-
utes to the solar-triggered release of water. More importantly, a
prototype of a water harvesting device based on birnessite has
Fig. 21 (a) The synthesis of 2D and 3D epoxy-functionalized ep-POPs
using Diels–Alder cycloaddition polymerization. (b) Volumetric water-
adsorption–desorption isotherms of 2D and 3D ep-POPs at 298 K.
Reproduced with permission from ref. 97. Copyright 2018 Wiley.
Fig. 22 (a) The reaction of ETTA and TFB to generate (3,4,4)-c mtf
topology (shown in its augmented form). (b) Water sorption analysis
measured at different temperatures. (c) Water cycling stability testing for
300 adsorption–desorption cycles conducted at a constant water vapor
pressure (1.7 kPa). Reproduced with permission from ref. 98. Copyright
2020 American Chemical Society.
Fig. 23 (a) The crystal structure of TCPP-1,3-DPP: the connecting style
of the one-dimensional (1D) porous stripe (left), and the stacking of 1D
stripes to provide 1D oval-shaped channels on the 3D framework. (b)
Low-pressure N
2
sorption isotherms at 77 K. (c) The sorption isotherms
of activated TCPP-1,3-DPP for H
2
O, CO
2
, CH
4
, and H
2
at 25 °C; the solid
symbols represent adsorption, while the open symbols represent de-
sorption. Reproduced with permission from ref. 99. Copyright 2018
Wiley.
Review Inorganic Chemistry Frontiers
908 |Inorg. Chem. Front.,2021,8,898–913 This journal is © the Partner Organisations 2021
been constructed, suggesting its high potential for practical
applications (Fig. 24c). One thing that should be stressed is
that the employed 2D materials are in the bulk phase, if they
were exfoliated into ultra-thin 2D nanosheets, the full
exposure of 2D surfaces could be achieved, then higher water
adsorption capacities could be expected. Considering that
there is a great variety of 2D materials, this work will inspire
further future research on 2D materials as practical water
adsorbents.
4. Outlook for water adsorbents
Water scarcity is increasingly being perceived as a global chal-
lenge threatening the lives of humans, and has attracted sig-
nificant attention, and AWH has could be a promising solu-
tion. AWH technology can generate freshwater regardless of
the geographical and hydrologic conditions, and renewable
energy can be utilized to avoid energy consumption, which has
thus been considered to be widely used as a decentralized
water supply in the near future. For all of the AWH methods
(i.e., water harvesting, dewing, and sorption-based technology),
the key aspect lies in the design of the adsorbents to balance
the requirement of moisture concentration and water release.
As a reflection of all the aforementioned research efforts and
promising prospects, the current challenge for the develop-
ment of ideal water adsorbents can be summarized as follows:
firstly, the search for novel sorbent materials with improved
working capacities under temperature and/or pressure swing
conditions is critically needed; secondly, a comprehensive
understanding of the sorption kinetics, via both experimental
exploration and theoretical prediction, for the in-depth inspec-
tion of the migration and aggregation behaviors of water mole-
cules during the AWH process, is still lacking; and thirdly,
precise device-engineering to render sorbent materials into
freshwater delivery systems remains to be established.
Among the above-mentioned three major tasks, the search
for novel sorbent materials is the basic premise. For all of the
aforementioned materials, the pros and cons are co-existent.
Specifically, for hygroscopic salts, how to fully reach their
potential after being integrated into a stable porous matrix, is
the key issue, and requires an exact match between the salts
and matrix. For the hygroscopic hydrogels, expanding their
working RH range, especially under low RH conditions, is
necessary for their wide-spread application. For nanomaterials
with given morphologies, a clear understanding of the mor-
phology-performance relationship is highly desired. While for
the porous frameworks such as MOFs, POPs, COFs and HOFs,
the water stability, adsorption capacity, and the recyclability
are the bridges that need to be crossed prior to their practical
application.
In our opinion, 2D materials show the greatest potential for
the development of next-generation sorbent materials; the
reasons for this are as listed as follows: (i) the interlayers of 2D
materials can serve as warehouses for the quick adsorption
and condensation of water molecules over a large range of RH
values; (ii) the bulk phases of 2D materials have already shown
high adsorption capacities, let alone exfoliated 2D nanosheets.
The latter features fully exposed 2D surfaces that can promote
the physical adsorption of water molecules via weak inter-
actions, which thus contribute to effective water release at the
expense of a small amount of energy input; and (3) the agile
size and morphology changes of 2D materials are more condu-
cive to device fabrication. Inspired by this, the application of
2D MOF nano-sheets as effective water adsorbents for practical
AWH is currently being carried out in our lab.
Conflicts of interest
There are no conflicts to declare.
Fig. 24 (a) An illustration of water adsorption and condensation
between interlayers, as well as solar-triggered water release. (b) The
water vapor isotherms of different MnO
2
sorbents at 25 °C. (c) The water
sorption isotherm of the MnO
2
-1 sample and simulated curves; the inset
shows a schematic diagram of the water sorption mode. (d) A prototype
of a water harvesting device with images of collected water on the
surface of the condenser. Reproduced with permission from ref. 103.
Copyright 2019 American Chemical Society.
Inorganic Chemistry Frontiers Review
This journal is © the Partner Organisations 2021 Inorg. Chem. Front.,2021,8,898–913 | 909
Acknowledgements
This work was supported by the Natural Science Foundation of
China (Grant No. 21701023), the Natural Science Foundation
of Jiangsu Province (Grant No. BK20170660), the Zhishan
Youth Scholar Program of the SEU and PAPD of Jiangsu
Higher Education Institutions.
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