Water-stable metal–organic frameworks (MOFs): rational construction and carbon dioxide capture

Abstract

Metal–organic frameworks (MOFs) are considered to be a promising porous material due to their excellent porosity and chemical tailorability. However, due to the relatively weak strength of coordination bonds, the stability (e.g., water stability) of MOFs is usually poor, which severely inhibits their practical applications. To prepare water-stable MOFs, several important strategies such as increasing the bonding strength of building units and introducing hydrophobic units have been proposed, and many MOFs with excellent water stability have been prepared. Carbon dioxide not only causes a range of climate and health problems but also is a by-product of some important chemicals (e.g., natural gas). Due to their excellent adsorption performances, MOFs are considered as a promising adsorbent that can capture carbon dioxide efficiently and energetically, and many water-stable MOFs have been used to capture carbon dioxide in various scenarios, including flue gas decarbonization, direct air capture, and purified crude natural gas. In this review, we first introduce the design and synthesis of water-stable MOFs and then describe their applications in carbon dioxide capture, and finally provide some personal comments on the challenges facing these areas.

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1. Introduction

Metal–organic frameworks (MOFs) are a kind of organic–inorganic hybrid material with intramolecular pores formed by coordination self-assembly of organic ligands and metal ions/clusters.¹ Compared with traditional porous materials (such as porous carbons and zeolite molecular sieves), the pore size and pore surface of MOFs can be precisely regulated by reasonable design of building elements and modification of pores.^2–17 In addition, the structure of MOFs is long-range ordered, which enables the structure–activity relationships of MOFs easy to be revealed by single crystal X-ray diffraction (SCXRD).^18–21 In this context, MOFs have been widely used in various fields, such as gas adsorption and separation,^22–37 pollutant capture,^38–40 electrical/ionic conduction,^41–43 selective recognition,^44–46 and heterogeneous catalysis.^47–50 At the initial stage, scientists mainly focused on the synthesis of MOFs with novel structures and the exploration of new functions of MOFs. However, if MOFs are to be used in the real world as practical materials rather than just as potential materials, they must meet the requirements of practical applications, such as economy, processability and stability.⁵¹ Given the ubiquity of water vapor in the real world, water stability is the first hurdle to be overcome in the practical applications of MOFs. Over the past decade, hundreds of water-stable MOFs have been prepared (Scheme 1 and Table 1), such as famous MILs, UiOs, PCNs, BUTs, ZIFs, and MAFs. With the successful synthesis of more and more water-stable MOFs, the understanding of the water stability of MOFs has also been deepened.^52–62 For instance, the selection of suitable metals and ligands with matching softness/hardness can effectively construct water-stable MOFs, and the introduction of hydrophobic groups into the pores or surfaces of MOFs can further improve their water stability. Recently, a series of water-stable MOFs have been successfully constructed, and a timely summary can provide a guide for the controllable construction of more stable MOFs that can be used in a harsher environment.

Scheme 1 Water stable MOFs based on different strategies. (a) Hard–Soft-Acid–Base engineering (direct preparation of water stable MOFs from hardness matching metals and ligands). (b) Pore engineering (improving the water stability of MOFs through pore modification, including metal metathesis, pore space partition, ligand optimization, and framework interlocking). (c) Hydrophobic engineering (improving the water stability of MOFs through introducing hydrophobic units, including pore hydrophobization and surface hydrophobization).

Table 1 Summary of the water stability of some representative MOFs^a

MOFs	Metal	Ligands	Testing condition	Time	Ref.
a Linkers are abbreviated as: BDC = terephthalate; BDC-NH2 = 2-aminoterephthalate; BTC = benzene-1,3,5-tricarboxylate; H2NDC(SO3H)2 = 4,8-disulfonaphthalene-2,6-dicarboxylatlate; H4tcpp = 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine; BPDC = biphenyl-4,4′-dicarboxylate; BCQD = 5′,5′′′-bis(4-carboxyphenyl)-[1,1′:3′,1′′:4′′,1′′′:3′′′,1′′′′-quinquephenyl]-4,4′′′′-dicarboxylate, PBPTT = 4,4′,4′′,4′′′-(4,4′-(1,4-phenylene)bis(pyridine-6,4,2-triyl))tetrabenzoate, H4BTEB = 4,4′,4′′,4′′′-(benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl)) tetrabenzoic acid, H4CPTTA = 5′-(4-carboxyphenyl)-[1,1′:3′,1′′-terphenyl]-3,4′′,5-tricarboxylic acid, TPDC-2CH3 = 2′,5′-dimethyl-terphenyl-4,4′′-dicarboxylic acid, TPDC-4CH3 = 2′,3′,5′,6′-tetramethyl-terphenyl-4,4′′-dicarboxylic acid, TPDC-4CH2N3- = 2′,3′,5′,6′-tetra(azidomethyl)-[1,1′:4′,1′′-terphenyl]- 4,4′′-dicarboxylate, H2dmcapz = 3,5-dimethyl-4-carboxypyrazole, H2TCPP = tetrakis(4-carboxyphenyl)porphyrin, H3TZBPDC = 4′-(1H-tetrazol-5-yl)-[1,1′-biphenyl]-3,5-dicarboxylic acid, Hetz = 3,5-diethyl-1,2,4-triazole, H2btm = bis(5-methyl-1H-1,2,4-triazol-3-yl)methane; btz = 1,4-bis(4H-1,2,4-triazol-4-yl)benzene; H3BTN = 1,3,5-tri(6-hydroxycarbonylnaphthalen-2-yl)benzene); H3BTB = 1,3,5-tris(4-carboxyphenyl)benzene); H2imPim = 2-(1H-imidzol-2-yl)-3H-imidazo[4,5-c]pyridine; 1,4-BDP = 1,4-benzenedi(4′-pyrazolyl); 1,3-BDP = 1,3-benzenedi(4′-pyrazolyl); H2bdpb = 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]benzene; H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene; H3TPTA = 2,4,6-tris(4-(pyrazolate-4-yl)phenyl)-1,3,5-triazine; H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene; H2fbdim = 2,6-ditrifluoromethyl-benzodiimidazole; H2DTBDA = 3′,5′-di(1H-1,2,4-triazol1-yl)-[1,1′-biphenyl]-3,5-dicarboxylic acid; H3CTTA = dimethyl-5′-(4-(methoxycarbonyl)phenyl)-2′,4′,6′-trimethyl-[1,1′:3′,1′′- terphenyl]-4,4′′-dicarboxylic acid; BPDC-4Me = 3,3′,5,5′-tetramethyl-1,1′-biphenyl; TPDC-2CH2N3 = 2′,5′-bis(azidomethyl)-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylic acid; H3TTNA = 6,6′,6′′-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(2-naphthoic acid)); H2BDB = [1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid; H4TCPB-Br2 = (1,4-dibromo-2,3,5,6-tetrakis(4-carboxyphenyl)benzene; BTBA = biphenyl-3,4,5-tricarboxylic acid; H2FTZB = 2-fluoro-4-(1H-tetrazol-5-yl)benzoic acid; 1,4-H2NDC = 1,4-naphthalenedicarboxylate; MeIM = 2-methyl-1H-imidazole; bIM = 1H-benzo[d]imidazole; IM = 2H-imidazole; nIM = 2-nitro-1H-imidazole; cbIM = 6-chloro-1H-benzo[d]imidazole; ICA = imidazolate-2-carboxyaldehyde; H2bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole); TPMA = tris(pyridin4-ylmethyl)amine; pyz-NH2 = 2-aminopyrazine; H4TPP = 5,10,15,20-tetra(1H-pyrazol-4-yl)porphyrin; H4TPPP = 5,10,15,20-tetra(pyrazolate-4-yl)porphyrin; TBPZ = 3,3′,5,5′-tetraethyl-4,4′-bipyrazolate; o-H2mpba = 2-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid; 1,2,4-TAZT = 1,2,4-triazolate; H4DH3PhDC = 2′,5′-dimethyl-3,3′′-dihydroxy-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylic acid; dtp = 3,6-di(4-pyridyl)-1,2,4,5-tetrazine; pyr = pyrazine; HQc = quinoline-5-carboxylic acid; dobpdc = 4,4′-dioxido-3,3′-biphenyldicarboxylate; TABZ = 3,3′,5,5′-tetramethyl-1H,1′H-4,4′-bipyrazole; F-pymo = 5-fluoropyrimidin-2-olate, HINO = isonicotinic acid N-oxide; H4TBAPy-2 = 1,2,7,8-tetrakis(p-benzoic-acid)pyrene]; L4H = 4-[2-[3,6,8-tris[2-(4-carboxyphenyl) ethynyl]-pyren-1-yl]ethynyl]-benzoic acid; 5-Aa-IPA = 5-acetamidoisophthalate.
MIL-101(Cr)	Cr(III)	BDC	Aqueous solution (pH = 0–12)	2 months	76
MIL-101(Fe)	Fe(III)	BDC	Aqueous solution (pH = 1–12)	1 day	77
MIL-53(Cr)	Cr(III)	BDC	Water	2 days	78
			0.07 M HCl aqueous solution
			0.07 M NaOH aqueous solution
MIL-53(Al)	Al(III)	BDC	Water	2 months	78
			Aqueous solution (pH = 4–12)	3 days
In2S3@MIL-101	Cr(III)	BDC	Water	—	79
			Aqueous solution (pH = 3–8)
NH2-MIL-53(Al)	Al(III)	BDC-NH2	Water	2 months	80
			Aqueous solution (pH = 4, 12)
MIL-101-Fe-NH2	Fe(III)	BDC-NH2	Aqueous solution (pH = 1–12)	1 day	81
MIL-100(Cr)	Cr(III)	BTC	Water	12 months	82
MIL-100(Fe)	Fe(III)	BTC	Water	—	83
MIL-100(Al)	Al(III)	BTC	Water	—	84
BUT-18	Al(III)	H3CTTA	Water, boiling water	24 h	85
			Aqueous solution (pH = 1)
			Aqueous solution (pH = 10)
BUT-8(Cr)	Cr(III)	H2NDC(SO3H)2	10 M H2SO4 aqueous solution	1 week	86
			Concentrated HCl solution
UiO-66	Zr(IV)	BDC	Aqueous solution (pH = 0–12)	2 months	87
NH2-UiO-66	Zr(IV)	BDC	Water	2 months	80
			12 M HCl and 16 M HNO3
UiO-67	Zr(IV)	BPDC	Water	2 months	87
			Aqueous solution (pH = 4)	3 days
			Aqueous solution (pH = 12)	2 months
UiO-67-4Me	Zr(IV)	BPDC-4Me	Water	14 days	88
			Aqueous solution (pH = 2, 12) at 80 °C
UiO-67-4Me–NH2-38%	Zr(IV)	H2DABPDC	Water	14 days	88
			Aqueous solution (pH = 2, 12) at 80 °C
PCN-56(Zr)	Zr(IV)	TPDC-2CH3	Aqueous solution (pH = 2)	2 days	89
			Aqueous solution (pH = 11)	1 day
PCN-57(Zr)	Zr(IV)	TPDC-4CH3	Aqueous solution (pH = 2)	4.5 days	89
			Aqueous solution (pH = 11)	2 days
PCN-58(Zr)	Zr(IV)	TPDC-2CH2N3	Water	24 h	89
			Aqueous solution (pH = 2)	24 h
			Aqueous solution (pH = 11)	15 h
PCN-59(Zr)	Zr(IV)	TPDC-4CH2N3	Water	72 h	89
			Aqueous solution (pH = 2)	20 h
			Aqueous solution (pH = 11)	24 h
PCN-223	Zr(IV)	H2TCPP	Aqueous solution (pH = 0–10)	24 h	90
PCN-225(Zr)	Zr(IV)	H2TCPP	Boiling water	12 h	146
			Aqueous solution (pH = 0–12)
PCN-228(Zr)	Zr(IV)	H4TCP-1	Water	1 day	91
			Aqueous solution (pH = 0)
			Aqueous solution (pH = 12)
PCN-229(Zr)	Zr(IV)	H4TCP-2	Water	1 day	91
			Aqueous solution (pH = 0)
			Aqueous solution (pH = 12)
PCN-230(Zr)	Zr(IV)	H4TCP-2	Aqueous solution (pH = 0–12)	1 day	91
OPA-PCN-222(Zr)	Zr(IV)	H2TCPP	Water	1 month	92
			Aqueous solution (pH = 11)	7 days
BUT-12	Zr(IV)	H3CTTA	Boiling water	24 h	93
			HCl solutions (2 M, 6 M, and concentrated HCl)
			Aqueous solution (pH = 10)
BUT-13	Zr(IV)	H3TTNA	Boiling water	24 h	93
			HCl solutions (2 M, 6 M, and concentrated HCl)
			Aqueous solution (pH = 10)
BUT-14	Zr(IV)	BCQD	Boiling water	48 h	94
			Concentrated HCl aqueous solution
			Aqueous solution (pH = 10)
BUT-15	Zr(IV)	PBPTT	Boiling water	48 h	94
			Concentrated HCl aqueous solution
			Aqueous solution (pH = 10)
BUT-17	Zr(IV)	H4CPTTA	Boiling water	—	95
			Concentrated HCl aqueous solution
			Aqueous solution (pH = 12)
BUT-44	Zr(IV)	H4BTEB	Water	72 h	96
			Aqueous solution (pH = 10)	1 day
			Aqueous solutions (1 M HCl)	1 day
BUT-66	Zr(IV)	H2BDB	Water	1 month	97
			Aqueous solution (pH = 1–10)
BUT-67	Zr(IV)	H2NDB	Water	48 h	97
			Aqueous solution (pH = 1–10)
MOF-808	Zr(IV)	BTC	Boiling water	24 h	98
NU-1100	Zr(IV)	L4H	Water	24 h	99
		H2BPDC
ISO-NU-1000	Zr(IV)	H4TBAPy-2	Water	24 h	100
MIP-207	Ti(IV)	BTC	Water	3 days	101
MIP-208	Ti(IV)	5-Aa-IPA	Boiling water	8 h	102
			Aqueous solution (pH = 0–10)
BUT-172	In(III)	H3CTTA	Boiling water	1 day	103
			Aqueous solution (pH = 3, 10)
BUT-173	In(III)	H3CTTA	Water	1 day	103
			Aqueous solution (pH = 3, 10)
In(tcpp)	In(III)	H4TCPP	Aqueous solution (pH = 2, 12)	24 h	104
			Water
USTC-8(In)	In(III)	H2TCPP	Aqueous solution (pH = 0–11)	12 h	105
Y-BTC	Y(III)	BTC	Water	—	106
Y-pek-MOF-1	Y(III)	BTBA	Water at 85 °C	24 h	107
[(CH3)2NH2]2[Tb6(μ3-OH)8(FTZB)6(H2O)6]·(H2O)22	Y(III)	H2FTZB	Water	24 h	108
Tb-BTC	Tb(III)	H2FTZB	Water	—	106
Tb-pek-MOF-1	Tb(III)	BTBA	Water at 85 °C	24 h	107
[(CH3)2NH2]2[Y6(μ3-OH)8(FTZB)6(H2O)6]·(H2O)52	Tb(III)	H2FTZB	Water	24 h	108
La-BTN	La(III)	H3BTN	Water	1 day	109
			Aqueous solution (pH = 2) at 100 °C
			Aqueous solution (pH = 12)
La-BTB	La(III)	H3BTB	Water at 60 °C and 100 °C	3 days	110
			Aqueous solution (pH = 2–14)
Eu-1,4-NDC-fcu-MOF	Eu(III)	1,4-H2NDC	Boiling water	24 h	111
			Aqueous solution (pH = 3.5–10)
PCN-202	Ni(II)	1,4-H2NDC	Aqueous solution (pH = 12)	1 day	112
PCN-600(Fe)	Fe(III)	1,4-H2NDC	Aqueous solution (pH = 2–11)	24 h	113
ZIF-8/MAF-4	Zn(II)	MeIM	Boiling water	7 days	114 and 115
MAF-23	Zn(II)	H2btm	Water	7 days	116
MAF-stu-1	Zn(II)	H2imPim	Boiling water	9 days	117
			Aqueous solution (pH = 2, 11)
Zn(1,4-BDP)	Zn(II)	1,4-BDP	Water at 40 °C	—	118
Zn(1,3-BDP)	Zn(II)	1,3-BDP	Boiling water	3 days	118
ZIF-11	Zn(II)	bIM	Water	7 days	119
ZIF-61	Zn(II)	IM, MeIM	Boiling water	7 days	120
ZIF-68	Zn(II)	bIm, nIM	Boiling water	7 days	120
ZIF-69	Zn(II)	cbIM, nIM	Boiling water	7 days	120
ZIF-70	Zn(II)	IM, nIM	Boiling water	7 days	120
ZIF-90	Zn(II)	ICA	Water	7 days	120
ZIF-67	Co(II)	MeIM	Boiling water	7 days	120
MFU-1	Co(II)	H2bdpb	Water	6 months	121
MAF-X27-Cl	Co(II)	H2bbta	Acidic solution (0.001 M HCl)	1 week	122
			Alkaline solution (1.0 M KOH)
FJI-H30	Co(II)	TPMA	Water	5 days	123
			Aqueous solution (pH = 2–12)
ZJU-75	Co(II)	pyz-NH2	Water	3 days	124
			Aqueous solution (pH = 1–12)
Co-btz-ht	Co(II)	btz	Aqueous solution (pH = 1–12)	20 h	125
PCN-601	Ni(II)	H4TPP	Saturated NaOH solution at 100 °C	24 h	126
			0.1 mM HCl aqueous solution
PCN-602(Ni)	Ni(II)	H4TPPP	0.1 mM HCl aqueous solution	24 h	127
			1 M NaOH aqueous solution
Ni3(BTP)2	Ni(II)	H3BTP	Boiling water	2 weeks	128
			Boiling aqueous solution (pH = 2–14)
Ni-btz-ht	Ni(II)	btz	Aqueous solution (pH = 1–12)	20 h	125
BUT-32	Ni4 clusters	H3TPTA	Boiling water	24 h	129
			Aqueous solution (pH = 3)
			NaOH aqueous solution (4 M)
BUT-33	Ni4 clusters	H3TPTA	Boiling water	24 h	129
			Aqueous solution (pH = 3)
			NaOH aqueous solution (4 M)
Cu-BTTri	Cu(II)	H3BTTri	Boiling water	3 days	130
			Aqueous solution (pH = 3)	1 day
MAF-2	Cu(I)	Hetz	Water	—	131
MAF-41	Cu(I)	H2fbdim	Water	One week	132
			Boiling water	One week
			Aqueous solution (pH = 3–14)	3 days
Cu2TBPZ	Cu(I)	TBPZ	Water	24 h	133
			Aqueous solution (pH = 3, 12)
[Ag2(o-Hmpba)2(o-H2mpba)2]	Ag(I)	o-H2mpba	Water	8 months	135
			Aqueous solution (pH = 2–13)	1 week
FJI-H14	Cu(II)	H2BTTA	Aqueous solution (pH = 2–12) at 100 °C	24 h	136
FJI-H25Fe	Fe(III)	H2BTTA	Aqueous solution (pH = 8–12)	—	137
FJI-H29	Zn(II)	H2DTBDA	Water	2 days	138
			Aqueous solution (pH = 2–13)
[Zn4(μ4-O)-(μ4-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)3]	Zn(II)	H2dmcapz	Water	—	139
			0.001 M HNO3 aqueous solution
USTC-7	Zn(II)	H3TZBPDC	Aqueous solution (pH = 2–12)	12 h	140
FJI-H36	Ni(II)	H2DTBDA	Water	—	141
			Aqueous solution (pH = 2–12)

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The continued increase of atmospheric carbon dioxide (CO₂) has led to a number of serious climate problems, including rising sea levels and rising temperatures. In addition to causing climate change, high levels of atmospheric CO₂ can also cause a range of health problems for humans in confined spaces (e.g., submarines and airplanes). Furthermore, as an undesired concomitant of many important fine chemicals (e.g., natural gas), CO₂ has greatly affected their production and transportation. Therefore, there is an urgent need to develop efficient and economical carbon capture technologies to trap unwanted CO₂ from air and fine chemicals. Due to their high adsorption capacity and relatively low recovery energy, MOFs have proven to be a kind of promising adsorbent for carbon capture and have been widely used in a variety of scenarios, including flue gas decarbonization, direct air capture, and purified natural gas.^63–75

Considering that water vapor is widely present in flue gas, air, and crude natural gas, a practical MOF-based CO₂ adsorbent should first have good water stability. To date, many water-stable MOFs that can realize efficient CO₂ capture have been reported; and a timely collation of these advances can help scientists design and synthesize MOFs that can be used to capture CO₂ in industry.

In this review, we begin with a brief introduction to the importance of water-stable MOFs and the urgency of carbon capture. Then, the definition of stability (including thermodynamic stability and kinetic stability), synthetic strategies (from universal strategies to specific approaches), and important progress (e.g., classic examples and recent advances) of water-stable MOFs are introduced in detail. In addition, the applications of water-stable MOFs in different scenarios (e.g., flue gas decarbonization, direct air capture, and purified natural gas) are systematically discussed. Finally, some personal comments on the challenges facing these areas are presented. Despite our best efforts, we have only been able to cover a small fraction of the many studies on water-stable MOFs. However, we also hope that our review will provide a useful reference for researchers interested in designing water-stable MOFs for CO₂ capture.

2. Design and synthesis of water-stable MOFs

2.1. The definition and characterization of water stability of MOFs

Water stability is used to describe the extent to which a substance is affected by water, also known as water resistance. With regard to MOFs, their water stability refers to the ability to maintain their structure when exposed to water. For a given MOF, if it is exposed to water for a long time, its ordered structure will not be damaged, and its porous properties can be maintained, and it can be considered water-stable. In this context, the water stability of MOFs is usually evaluated by powder X-ray diffraction (PXRD) tests and gas adsorption experiments; in which PXRD analysis is used to determine whether the ordered structure of MOFs is destroyed, and a gas adsorption experiment is used to evaluate whether the porosity of MOFs is effectively maintained. It must be pointed out that the characterization of the water stability of MOFs is directly related to their application scenarios. For example, the use of MOFs in aqueous solutions may lead to the dissolution of partial MOFs. At this time, other tests (e.g., nuclear magnetic resonance (NMR) and inductive coupled plasma emission spectrometer (ICP)) are needed to accurately evaluate their water stability.

2.2. The thermodynamic stability of MOFs

The hydrolysis of MOFs is a complex chemical transformation, in which the nucleophilic H₂O will attack the electrophilic metal ions, and the incompletely dissociated organic ligands will in turn continue to bond with the metal ions; only after a series of complex reactions, the hydrolytic products which are favourable both in kinetics and thermodynamics will be formed. For a given MOF, its thermodynamic stability to water refers to the ΔG of the hydrolysis reaction (Scheme 2). If the ΔG is greater than 0, then its hydrolysis cannot occur spontaneously and it can be considered water-stable. According to the Gibbs equation ΔG = ΔH − TΔS, if ΔG is to be greater than 0, it must be ensured that ΔH is much greater than 0, because hydrolysis of MOFs is a process of entropy increasing (ΔS > 0). This means that the metal–ligand interactions of a water-stable MOF should be greater than the corresponding metal–oxygen interactions formed by hydrolysis. Thus, only the coordination elements with strong enough interaction (generally greater than the corresponding metal–water interaction) can be used to synthesize water-stable MOFs.

Scheme 2 The definition and optimization of the thermodynamic stability of MOFs to water. Black line represents the thermodynamic stability of a given MOF to water, which refers to the ΔG¹ of the hydrolysis reaction. The red line represents the optimized thermodynamic stability of the MOF through enhancing its bonding strength (ΔG² > ΔG¹).

To optimize the thermodynamic stability of MOFs in water, it is necessary to increase the bonding strength of metal ions and organic ligands as much as possible. For any MOF, the organic ligand acts as a Lewis base that provides electrons, while the metal ion acts as a Lewis acid that accepts electrons. According to Hard–Soft-Acid–Base (HSAB) theory, Lewis acids/bases can be divided into three categories, including hard acids/bases, soft acids/bases, and borderline acids/bases. Hard acids/bases refer to particles with a high charge density and small radius, which have low polarizability but high polarity. Soft acids/bases are those particles with a low charge density and large radius, which have high polarizability but low polarity. Thermodynamically stable complexes are preferentially constructed from hard acids and hard bases or from soft acids and soft bases. In this regard, water-stable MOFs can be directly constructed from hard metals (e.g., Al³⁺, Zr⁴⁺, In³⁺, Ti⁴⁺, Hf⁴⁺, Fe³⁺, and Cr³⁺)/ligands (carboxylic acid ligands or other ligands that contain free oxygen atoms) and soft metals (e.g., Zn²⁺ and Cu⁺)/ligands (e.g., imidazolyl derivatives, triazolyl derivatives, and tetrazolyl derivatives). In addition, post-synthesis modification (PSM) can also be used to improve the water stability of the as-prepared MOFs.

2.3. The kinetic stability of MOFs

The thermodynamics of a given reaction can only reveal whether it can occur spontaneously. Whether the reaction can proceed smoothly also depends on its kinetics. For a thermodynamically unstable MOF, the hydrolysis reaction is also difficult to occur if the activation energy of hydrolysis is very high (Scheme 3). Therefore, the kinetic stability of a given MOF to water is related to its hydrolytic activation energy.

Scheme 3 The definition and optimization of the kinetic stability of MOFs to water. Black line represents the kinetic stability of a given MOF to water, which refers to the ΔEa¹ (activation energy) of its hydrolysis reaction. The red line represents the optimized kinetic stability of the MOF through enhancing its activation energy (ΔEa² > ΔEa¹).

To improve the kinetic stability of a MOF to water, the activation energy of hydrolysis should be increased as much as possible. The hydrolysis of the MOF is so complex that its activation energy is difficult to calculate.^142,143 In this case, inhibition of the hydrolysis kinetics of a MOF can only be qualitatively analyzed by reaction rate theory. According to such theory, the hydrolysis reaction of MOFs can be inhibited by preventing the effective collision between H₂O molecules and the metal ions of MOFs. In this regard, introducing enough hydrophobic groups on the surface of a MOF can effectively prevent the contact between H₂O and the MOF. In addition, the introduction of appropriate hydrophobic groups into the pores of MOFs can effectively prevent the diffusion of water in the pores.

2.4. Reported water-stable MOFs

2.4.1. Water-stable MOFs based on hard metal ions/ligands. As described above, water-stable MOFs can be reasonably constructed from hard metal ions and hard ligands, and many water-stable MOFs based on this strategy have been reported. In 2005, Férey and co-workers first reported a water-stable MOF (MIL-101(Cr)) based on μ₃-oxocentered tri-nuclear chromium (Fig. 1g).⁷⁶ Due to the strong bond energy of the Cr(III)–O bond, it could remain stable for two months in both strong acid solutions (pH = 0) and strong base solutions (pH = 12). Isomorphic MIL-101(Fe) and MIL-101-Fe–NH₂ based on the same inert Fe–O motifs also showed excellent water/acid/base resistance.^77,81 Then a series of water-stable MOFs (e.g., MIL-53, MIL-100, BUT-18, and BUT-8) based on various hard metal ions (Cr³⁺, Fe³⁺, and Al³⁺) and different carboxylate ligands were synthesized (Fig. 1).^78,82–85 Notably, the framework of BUT-8(Cr) could remain stable even in concentrated H₂SO₄, meaning that BUT-8(Cr) is one of the reported MOFs with the best acid resistance.⁸⁶ It could be found that the formation of multiple M–O bonds was beneficial in improving the water stability of MOFs. For MOFs with the same framework, the introduction of substituents such as amine groups on the ligands had little effect on their water stability, but the introduction of In₂S₃ nanoparticles would significantly reduce the chemical stability of the framework. This might be because the polar In₂S₃ facilitated the diffusion of water into the pores of MOFs.⁷⁹

Fig. 1 The structures of the MIL series and the used ligands. (a) The H₂BDC ligand. (b) The H₂BDC-NH₂ ligand. (c) The 1,3,5-BTC ligand. (d) MIL-53. (e) MIL-88. (f) MIL-100. (g) MIL-101. Reproduced from ref. 144 with permission from Wiley, Copyright 2018.

The Zr–O unit with very strong bond energy is another building block that is widely used in the synthesis of water-stable MOFs. UiO-66, as a representative example, constructed from 12-coordinated Zr₆O₄(OH)₄(CO₂)₁₂ clusters and linear terephthalic acid (H₂BDC) ligands, was reported by Lillerud and co-workers in 2008 (Fig. 2).^87,145 Due to the extremely strong interactions between Zr⁴⁺ ions and O anions, UiO-66 not only displayed excellent water/acid/base resistance but also resisted various common organic solvents. Then other water-stable UiO series (e.g., UiO-67 and UiO-68) based on the same strategy were prepared by using other linear carboxylate ligands. Very recently, through the modification of ligands and the mixing of modified ligands, Su's group has achieved the controllable construction of UiO-67-4Me-R₂-x% with excellent acid–base stability and adjustable hydrophilicity and hydrophobicity.⁸⁸

Fig. 2 The structures of the UiO series and the used ligands. (a) The H₂BDC ligand. (b) The H₂BPDC ligand. (c) UiO-66. Reproduced from ref. 87 with permission from the American Chemical Society, Copyright 2008. (d) UiO-67. Reproduced from ref. 87 with permission from the American Chemical Society, Copyright 2008. (e) UiO-68. Reproduced from ref. 87 with permission from the American Chemical Society, Copyright 2008.

Based on suitable Zr–O clusters and different carboxylate ligands, Zhou's group also reported a series of water-stable MOFs (PCN series) (Fig. 3). They first used single or mixed linear dicarboxylic acids bearing methyl or azide groups (Fig. 3a–c) to prepare three isoreticular MOFs, PCN-56, PCN-57 and PCN-58 with different functional groups.⁸⁹ Notably, such as-prepared functionalized MOFs could keep their framework and crystallinity well for a long time in diluted HCl solution (pH = 2) and NaOH solution (pH = 11). Then they synthesized a series of more stable MOFs (PCN-225, PCN-228, PCN-229 and PCN-230) using porphyrin-derived tetracarboxylate ligands and zirconium salts. These newly prepared materials had better acid tolerance and their framework could be stabilized in 1 M HCl solution. This may be because porphyrin groups can also react with hydrogen ions.^91,146,147

Fig. 3 The structures of the PCN series and the used ligands. (a) The TPDC-2CH₃ ligand. (b) The TPDC-4CH₃ ligand. (c) The TPDC-2CH₂N₂ ligand. (d) The H₂TCPP ligand. (e) The H₄TCP-1 ligand. (f) The H₄TCP-2 ligand. (g) The H₄TCP-3 ligand. (h) PCN-56. Reproduced from ref. 89 with permission from the American Chemical Society, Copyright 2012. (i) PCN-57. Reproduced from ref. 89 with permission from the American Chemical Society, Copyright 2012. (j) PCN-58. Reproduced from ref. 89 with permission from the American Chemical Society, Copyright 2012. (k) PCN-225. Reproduced from ref. 146 with permission from the American Chemical Society, Copyright 2013. (l) PCN-228. Reproduced from ref. 91 with permission from the American Chemical Society, Copyright 2015. (m) PCN-229. Reproduced from ref. 91 with permission from the American Chemical Society, Copyright 2015. (n) PCN-230. Reproduced from ref. 91 with permission from the American Chemical Society, Copyright 2015.

Li's group also prepared a series of water-stable Zr-based MOFs based on suitable polycarboxylate ligands (Fig. 4). All these reported porous materials possessed excellent chemical stability. For example, BUT-17 not only displayed strong stability in boiling water but also resisted concentrated HCl solution and NaOH aqueous solution (pH = 12).⁹⁵ Notably, several of them (e.g., BUT 12–15 and 17) could resist concentrated HCl, which makes them one of the most acid-resistant MOFs.^93,96,97

Fig. 4 The structures of the BUT series and the used ligands. (a) The H₃CTTA ligand. (b) The H₃TTNA ligand. (c) The BCQD ligand. (d) The H₄CPTTA ligand. (e) BUT-12. Reproduced from ref. 93 with permission from the American Chemical Society, Copyright 2016. (f) BUT-13. Reproduced from ref. 93 with permission from the American Chemical Society, Copyright 2016. (g) BUT-14. Reproduced from ref. 94 with permission from the American Chemical Society, Copyright 2017. (h) BUT-17. Reproduced from ref. 95 with permission from Springer Nature, Copyright 2019.

In addition to Zr⁴⁺, Ti⁴⁺ ions can also be used to construct water-stable Ti-based MOFs. Since the ionic radius of Ti⁴⁺ is smaller than that of Zr⁴⁺, it can react with various oxygen-containing ligands more vigorously, making the preparation of crystalline Ti-based MOFs more challenging. Recently, the Serre team used pre-prepared titanium-oxygen clusters (Ti₈AF) to self-assemble with H₃BTC ligands to achieve the controlled synthesis of Ti-MOF (MIP-207) (Fig. 5a and c), which could be stable in water for more than 3 days.¹⁰¹ By reacting a two-arm carboxylate ligand 5-acetamidoisophthalate (5-Aa-IPA) with Ti₈AF clusters, they synthesized a more stable MOF (MIP-208) (Fig. 5b and d), which could be stable in boiling water and aqueous solutions with pH 0–10, representing the most stable Ti-MOFs reported to date.¹⁰² The enhanced stability was thought to come from the infinite titanium–oxygen chains in its structure.

Fig. 5 The structures of Ti-MOFs and the used ligands. (a) The 1,3,5-BTC ligand. (b) 5-Aa-IPA ligand. (c) The structures of MIP-207. Reproduced from ref. 101 with permission from Elsevier, Copyright 2020. (d) The structures of MIP-208. Reproduced from ref. 102 with permission from Elsevier, Copyright 2020.

Besides tetravalent Zr⁴⁺/Ti⁴⁺, trivalent In³⁺ can also be used to construct water-stable MOFs (Fig. 6).^103–105 Based on tritopic carboxylate ligands, Li et al. successfully obtained two isomeric In(III)-MOFs, BUT-172 and BUT-173, which had slightly different three-dimensional (3D) open framework structures but the same secondary building units (SBUs) (Fig. 6a, d and e). These two In(III)-MOFs were highly stable in water at room temperature due to the relatively strong In–O bonds. However, only BUT-172 could maintain its structural integrity in aqueous solutions with pH between 3 and 10 at ambient temperature or in boiling water. Such unusual stability discrepancy may result from their distinct structural symmetries. Using a π-conjugated ligand, tetrakis (4-carboxyphenyl)porphyrin (TCPP), an indium-based porphyrinic MOF, USTC-8, was successfully synthesized by Jiang's group (Fig. 6c and g).¹⁰⁵ As anticipated, USTC-8 proved its exceptional water stability by maintaining the framework integrity in pH = 1–12 aqueous solutions for 12 h. To detect the pollutant in the aqueous solution, Li's group reported a novel luminous MOF (In(tcpp)) by combining the chromophore ligand 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine (H₄tcpp) and In(III) salts (Fig. 6b and f).¹⁰⁴ The as-prepared sample could remain stable in water, acidic solution (pH = 2), and alkaline solution (pH = 12), confirming its practicability for detecting pollutants in water.

Fig. 6 The structures of representative In(III)-MOFs and the used ligands. (a) The H₃CTTA ligand. (b) The H₄TCPP ligand. (c) The H₂TCPP ligand. (d) BUT-172. Reproduced from ref. 103 with permission from the Royal Society of Chemistry, Copyright 2020. (e) BUT-173. Reproduced from ref. 103 with permission from the Royal Society of Chemistry, Copyright 2020. (f) In(tcpp). Reproduced from ref. 104 with permission from the Royal Society of Chemistry, Copyright 2021. (g) USTC-8. Reproduced from ref. 105 with permission from the American Chemical Society, Copyright 2018.

Due to the high coordination numbers and the strong metal–oxygen interaction, rare-earth (RE) salts are another class of metal ions that are widely used for the synthesis of water-stable MOFs.¹⁴⁸ By employing 1,3,5-benzenetricarboxylate (BTC) as the organic linker, a series of microporous lanthanide-organic framework enantiomers (Y-BTC and Tb-BTC) were prepared by Jiang and co-workers (Fig. 7a and g).¹⁰⁶ The as-synthesized Ln-BTC not only could retain its framework in water but also exhibit excellent thermal stability (450 °C). Kitagawa's group synthesized a La(III)-framework (La-BTB) based on the H₃BTB (H₃BTB = 1,3,5-tris(4-carboxyphenyl)benzene) ligand (Fig. 7e and i), and it could maintain its framework integrity even after being soaked in boiling water, in hot (60 °C) HCl solution (pH = 2) and NaOH solution (pH = 14) for three days.¹¹⁰ Based on the 1,3,5-tri(6-hydroxycarbonylnaphthalen-2-yl)benzene) (H₃BTN) ligand, another La(III)-MOF(La-BTN) was further reported by Kitagawa et al. (Fig. 7f and h).¹⁰⁹ La-BTN was able to retain its structural integrity not only in a boiling acidic solution (pH = 2) but also in a strongly alkaline solution (pH = 12) for over 1 day. Eddaoudi et al. prepared a series of RE-fcu-MOFs (RE = Y(III) or Tb(III)) with fcu topology based on the 2-fluoro-4-(1H-tetrazol-5-yl)benzoic acid (H₂-FTZB) ligand (Fig. 7c and l),¹⁰⁸ in which an unusual metal cluster [RE₆(μ₃-OH)₈(O₂C–)₆(N₄C–)₆]²⁻ was in situ generated and served as a 12-connected metal node for MOFs' construction. Both isomeric MOFs show favourable water and thermal stability. After that, they reported another two La(III)-MOFs (pek-MOF-1 and 2) that were constructed from biphenyl-3,4,5-tricarboxylic acid (Fig. 7d) and 5-(4-carboxybenzyloxy)isophthalic acid, in which two different metal nodes (Fig. 7k), including a 12-connected nonnuclear and an 8-connected hexanuclear RE(III)-cluster, were also in situ formed.¹⁰⁷ Both of them exhibited good water resistance and thermal stability. Furthermore, they used commercial 1,4-naphthalenedicarboxylic acid to prepare a series of isostructural fcu-MOFs (Fig. 7b and j).¹¹¹ Owing to the 12-connected [RE₆(μ₃-OH)₈(O₂C–)₁₂] node, Eu-1,4-NDC-fcu-MOF could maintain its structural integrity in boiling water for at least 24 h.

Fig. 7 The structures of representative rare-earth MOFs and the used ligands. (a) The 1,3,5-BTC ligand. (b) The 1,4-H₂NDC ligand. (c) The H₂FTZB ligand. (d) The BTBA ligand. (e) The BTB ligand. (f) The BTN ligand. (g) Y-BTC. Reproduced from ref. 106 with permission from the American Chemical Society, Copyright 2010. (h) La-BTN. Reproduced from ref. 109 with permission from the Royal Society of Chemistry, Copyright 2014. (i) La-BTB. Reproduced from ref. 110 with permission from Wiley, Copyright 2013. (j) Eu-1,4-NDC-fcu-MOF. Reproduced from ref. 111 with permission from the American Chemical Society, Copyright 2015. (k) pek-MOF-1. Reproduced from ref. 107 with permission from the American Chemical Society, Copyright 2015. (l) [(CH₃)₂NH₂]₂[Tb₆(μ₃-OH)₈(FTZB)₆(H₂O)₆]·(H₂O)₂₂. Reproduced from ref. 108 with permission from the American Chemical Society, Copyright 2013.

In short, many water-stable MOFs were successfully prepared from various hard metal ions and carboxylate-based ligands, including famous MILs, UIOs, PCNs, and BUTs. Due to the very strong metal–oxygen interaction, many of them also exhibit excellent acid/base resistance and thermal stability. It is noteworthy that BUT-8(Cr) even can retain its structural integrity in concentrated H₂SO₄.

2.4.2. Water-stable MOFs based on soft metal ions/ligands. According to HSAB theory, thermodynamically stable MOFs also can be rationally prepared from soft metal ions and soft ligands, and many water-stable MOFs based on this strategy have been developed. For example, the famous zeolitic imidazolate framework (ZIF-8) (also called MAF-4), assembled from soft Zn(II) ions and soft imidazolate ligands, could maintain its structure in boiling water for 7 days (Fig. 8a and l).^114,115 Besides ZIF-8, Yaghi and co-workers also synthesized a series of water-stable ZIFs (e.g., ZiF-11, ZIF-68, ZIF-69, and ZIF-70) (Fig. 8), and all of them have high thermal stability and excellent water-resistance.¹²⁰ Recently, another water-stable Zn(II)-MOF (MAF-stu-1) based on the 2-(1H-imidzol-2-yl)-3H-imidazo[4,5-c]pyridine (H₂imPim) ligand was reported.¹¹⁷ MAF-stu-1 maintained its full crystallinity following 9 days of submerging the bulk samples in boiling water, acidic/basic solutions (pH = 2, 13), or various organic solvents. Moreover, it displayed record thermal stability (up to 680 °C). These results confirm that combinations of Zn(II) ions and suitable N-containing ligands can lead to MOFs with excellent chemical stability (e.g. base stability and thermal stability).

Fig. 8 The structures of representative Zn(II)-MOFs and the used ligands. (a) The IM ligand. (b) The nIM ligand. (c) The MeIM ligand. (d) The PhIM ligand. (e) The cbIM ligand. (f) The H₂imPim ligand. (g) MAF-stu-1. Reproduced from ref. 117 with permission from Wiley, Copyright 2019. (h) ZIF-8 (MAF-4). Reproduced from ref. 111 with permission from the National Academy of Sciences of the USA, Copyright 2006. (i) ZIF-11. Reproduced from ref. 111 with permission from the National Academy of Sciences of the USA, Copyright 2006. (j) ZIF-61. (k) ZIF-68. (l) ZIF-69. (m) ZIF-70. Reproduced from ref. 120 with permission from The American Association for the Advancement of Science, Copyright 2008.

Not limited to Zn²⁺, other low-valent metal ions such as Co²⁺, Ni²⁺, Cu²⁺, and Ag⁺ can also be regarded as soft acids to prepare water-stable MOFs with appropriate N-containing linkers (soft bases). For instance, Yaghi et al. prepared a water-stable Co(II)-MOF (ZIF-67) based on Co(II) salt and the 2-methylimidazolate (MeIM) ligand (Fig. 9a and g).¹²⁰ ZIF-67 could retain its structure after being soaked in water for 7 days. Based on Co(II) salts and 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]benzene (H₂bdpb), Volkmer et al. developed a novel MOF (MFU-1). Due to the inert [CoON₃] units, MFU-1 did not decompose even when exposed to humid air for more than six months (Fig. 9b and h).¹²¹ In 2016, Chen et al. found that MAF-X27-Cl, composed of 1H,5H-benzo-(1,2-d:4,5-d)bistriazole and Co(II) (Fig. 9d and i), showed exceptional chemical stability and could retain its original crystallinity in acidic (0.001 M HCl) or strongly alkaline (1.0 M KOH) solution for at least 1 week.¹²² Recently, we prepared a flexible framework (FJIH-30) (Fig. 9e and j) through combining the tris(pyridin4-ylmethyl)amine (TPMA) ligand with Co(SCN)₂. Its framework could be retained not only in water for 5 days but also in aqueous solution with a wide pH range (from 2 to 12).¹²³ Very recently, Li's group developed a microporous framework (ZJU-75) by using the pyz-NH₂ ligand, Co(NO₃)₂·6H₂O and K₂[Ni(CN)₄]·nH₂O (Fig. 9c and f).¹²⁴ ZJU-75 also displayed high chemical stability; it could maintain its structural integrity after being submerged in water and aqueous solutions of pH 1 and 12 for three days or exposed to air for three months.

Fig. 9 The structures of representative Co(II)-MOFs and the used ligands. (a) The MeIM ligand. (b) The H₂bdpb ligand. (c) The pyz-NH₂ ligand. (d) The H₂bbta ligand. (e) The TPMA ligand. (f) ZJU-75. Reproduced from ref. 124 with permission from Wiley, Copyright 2023. (g) ZIF-67. Reproduced from ref. 120 with permission from The American Association for the Advancement of Science, Copyright 2008. (h) MFU-1. Reproduced from ref. 121 with permission from Wiley, Copyright 2009. (i) MAF-X27-Cl. Reproduced from ref. 122 with permission from the American Chemical Society, Copyright 2016. (j) FJI-H30. Reproduced from ref. 123 with permission from the American Chemical Society, Copyright 2020.

The self-assembly based on Ni²⁺ and N-containing linkers can also lead to water-stable MOFs. In 2011, Long and co-workers used 1,3,5-tris(1H-pyrazol-4-yl)benzene (H₃BTP) and Ni(II) salt to prepare a microporous pyrazolate-bridged MOF (Ni₃(BTP)₂) (Fig. 10c and i).¹²⁸ Remarkably, the framework of Ni₃(BTP)₂ was stable in boiling water, boiling HCl or HNO₃ aqueous solutions at pH 2, and a boiling NaOH aqueous solution at pH 14, and maintained both its crystallinity and porosity after 14 days of uninterrupted tests. By isoreticular expansion, another two water-stable Ni(II)-pyrazolate MOFs (BUT-32 and BUT-33) were synthesized from a conformation-matched elongated pyrazolate ligand, 2,4,6-tris(4-(pyrazolate-4-yl)phenyl)-1,3,5-triazine (H₃TPTA) (Fig. 10d, g and h) by Li's group in 2020.¹²⁹ Given the network interpenetration in BUT-32, the two MOFs possessed the same topology but had different pore sizes. Both the thermodynamic product BUT-32 and the kinetic product BUT-33 could retain their structure in boiling water for 24 hours, a HCl aqueous solution (pH = 3), and NaOH aqueous solution (4 M) at room temperature, respectively. Guided by a top-down topological analysis, Zhou's group rationally synthesized a water-stable Ni(II)-MOF (PCN-601) from a pyrazolate-based 5,10,15,20-tetra(1H-pyrazol-4-yl)porphyrin ligand (H₄TPP) (Fig. 10a and e).¹²⁶ PXRD and N₂ adsorption further revealed its extraordinary base resistance; PCN-601 was the first identified MOF that could retain its crystallinity and porosity in saturated NaOH solution (∼20 mol L⁻¹) both at room temperature and 100 °C. Such excellent chemical stability was believed to come from the high connectivity numbers between the porphyrinic ligands and ([Ni₈(OH)₄(H₂O)₂Pz₁₂], Pz = pyrazolate) clusters, robust Ni(II)–N bonds and a relatively short length of the ligand. Based on a longer 5,10,15,20-tetrakis(4-(pyrazolate-4-yl)phenyl)porphyrin (H₄TPPP) ligand and the same 12-connected [Ni₈(OH)₄(H₂O)₂Pz₁₂] cluster, Zhou's group synthesized another water-stable Ni(II)-MOF (PCN-602) (Fig. 10b and f).¹²⁷ Although PCN-602 has the same secondary building units and topology as PCN-601, its base stability was significantly reduced and it could only remain stable in 1 M NaOH solution. This may be because its pore size is larger than that of PCN-601, which is conducive to the diffusion of hydroxide anions.

Fig. 10 The structures of representative Ni(II)-MOFs and the used ligands. (a) The H₄TPP ligand. (b) The H₄TPPP ligand. (c) The H₃BTP ligand. (d) The H₃TPTA ligand. (e) PCN-601. Reproduced from ref. 126 with permission from the American Chemical Society, Copyright 2016. (f) PCN-602. Reproduced from ref. 127 with permission from the American Chemical Society, Copyright 2017. (g) BUT-32. Reproduced from ref. 129 with permission from the American Chemical Society, Copyright 2020. (h) BUT-33. Reproduced from ref. 129 with permission from the American Chemical Society, Copyright 2020. (i) Ni₃(BTP)₂. Reproduced from ref. 128 with permission from the Royal Society of Chemistry, Copyright 2011.

Additionally, there are also many water-stable MOFs constructed from Cu(I), Cu(II), and Ag(I) ions and N-containing ligands. For example, based on an azolate ligand Hetz (Hetz = 3,5-diethyl-1,2,4-triazole), Chen's group reported a NbO type Cu(I)-framework (MAF-2), which showed good water vapor tolerance (Fig. 11c and i).¹³¹ Another water-stable Cu(I)-MOF, namely Cu₂TBPZ (TBPZ = 3,3′,5,5′-tetraethyl-4,4′-bipyrazolate) (Fig. 11b and h), was synthesized through a stepwise method by Li's group.¹³³ Cu₂TBPZ could maintain its structural integrity after being soaked in various boiling solvents (e.g., THF, toluene, hexane, and DMSO), water, and even in acidic (0.001 M HCl) or basic (0.001 M NaOH) aqueous solutions for 24 h. In 2019, Zhang and co-workers synthesized a new Cu(I)-MOF(MAF-41) based on a linear ligand, 2,6-ditrifluoromethyl-benzodiimidazole (H2fbdim) (Fig. 11d and j).¹³² After complete guest removal below 150 °C, MAF-41 displayed good chemical stability in boiling water for at least one week, in aqueous solutions with a wide range of pH (3–14) at room temperature for at least 3 days. In addition, the structural integrity of MAF-41 could be maintained for at least 6 months when exposed to humid air (∼70 RH%). Their team also developed a water-stable Ag(I)-MOF[Ag₂(o-Hmpba)₂(o-H₂mpba)₂] by using the 2-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (o-H₂mpba) ligand (Fig. 11e and f).¹³⁵ Due to its hydrophobicity, the as-prepared Ag(I)-MOF could remain intact and keep its color unchanged in the air for at least 8 months, and also in aqueous solution with a wide range of pH (2–13) for at least 1 week. Based on the Cu(II) ion and triazolate ligand 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene (H₃BTTri), Long's group prepared a water-stable MOF (Cu-BTTri) (Fig. 11a and g),¹³⁰ which also exhibited strong resistance for both acidic solutions (pH = 3) and boiling water.

Fig. 11 The structures of representative Cu/Ag-MOFs and the used ligands. (a) The H₃BTTri ligand. (b) The TBPZ ligand (TBPZ = 3,3′,5,5′-tetraethyl-4,4′-bipyrazolate). (c) The Hetz ligand. (d) The H₂fbdim ligand. (e) The o-H₂mpba ligand. (f) [Ag₂(o-Hmpba)₂(o-H₂mpba)₂]. Reproduced from ref. 135 with permission from Wiley, Copyright 2020. (g) Cu-BTTri. Reproduced from ref. 130 with permission from the American Chemical Society, Copyright 2009. (h) Cu₂TBPZ. Reproduced from ref. 133 with permission from Wiley, Copyright 2014. (i) MAF-2. Reproduced from ref. 131 with permission from the American Chemical Society, Copyright 2008. (j) MAF-41. Reproduced from ref. 132 with permission from Springer Nature, Copyright 2019.

As previously indicated, chemically stable MOFs can be rationally prepared from soft low-valent metal ions and soft N-containing ligands. However, due to the very strong coordination interaction, the assembly between these soft coordination units usually leads to powder (e.g., microcrystals and amorphous solids) unsuitable for SCXRD analysis. Due to the reversibility of coordination bonds, high-quality single crystals of MOFs can be self-assembled by selecting coordination units with weak coordination (e.g., selecting soft acids and hard bases), but MOFs constructed from weak coordination units are usually less stable. By self-assembling the ligands containing both soft and hard coordination atoms with soft metal ions, both the crystallinity and stability of MOFs can be balanced well. In this context, many water-stable MOFs based on ligands containing both N and O atoms were prepared. For example, Navarro and co-workers prepared a water-stable MOF [Zn₄(μ₄-O)-(μ₄-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)₃] based on a N-containing carboxylate ligand 3,5-dimethyl-4-carboxypyrazole (H₂dmcapz) ligand and Zn(II) ion, which could retain its framework integrity in water and boiling MeOH/benzene for 24 hours (Fig. 12a and e).¹³⁹ Then Jiang's group synthesized a water-stable MOF (USTC-7) by using the N-containing carboxylate ligand 4′-(1H-tetrazol-5-yl)-[1,10-biphenyl]-3,5dicarboxylic acid (H₃TZBPDC) and Zn(II) salt (Fig. 12b and f).¹⁴⁰ USTC-7 exhibited its exceptional stability not only in various boiling solvents but also in pH = 2–12 aqueous solutions for 12 h. Based on N-containing carboxylate ligands, our group designed and synthesized a series of water-stable MOFs with good crystallinity, such as FJI-H14, FJI-H25Fe, FJI-H29 and FJI-H36.^136,141 Through the reaction of 2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid (H₂BTTA) with Cu(NO₃)₂, a water-stable MOF (FJI-H14) was first prepared (Fig. 12c and g). FJI-H14 not only could retain its framework in the pH = 2–12 aqueous solutions at 373 K for 24 hours but also displayed excellent thermal stability (503 K). Structural analyses demonstrate that such excellent stability results from its CuN₂O₃ units with strong coordination and abundant free N atoms that can react with hydrogen ions. Combining the same ligand (H₂BTTA) with other metal nodes, [Fe₃(μ₃-O)(CH₃COO)₆] clusters and Co-doped [Fe₂Co(μ₃O)(CH₃COO)₆] clusters, two isomeric MOFs (FJI-H25Fe and FJI-H25FeCo) with distinct water stability were further prepared (Fig. 12c and h).¹³⁷ The Fe-MOF could retain its framework even in a pH = 12 aqueous solution while FeCo-MOF would quickly hydrolyse in the pH = 10 aqueous solution. The base stability of FeCo-MOF is lower than that of Fe-MOF due to partial substitution of Co; since the weaker Co–O units made the Co substituted framework more vulnerable to be deconstructed by OH⁻ anions. Based on a longer 3′,5′-di(1H-1,2,4-triazol1-yl)-[1,1′-biphenyl]-3,5-dicarboxylic acid (H₂DTBDA) ligand, another two water-stable MOFs (FJI-H29 and FJI-H36) were further synthesized (Fig. 12d, i and j).¹³⁸ Both of them displayed excellent acid/base resistance and good thermal stability. FJI-H29 exhibited better base stability (pH = 13) but less thermal stability (423 K), while FJI-H36 showed better thermal stability (473 K) but less base resistance (pH = 12). This means that the chemical stability (e.g., base resistance and thermal stability) of MOFs can be controllably regulated by the selection of metal ions and ligands with different hardness.

Fig. 12 The structures of representative MOFs based on N-containing carboxylate ligands and the used ligands. (a) The H₂dmcapz ligand. (b) The H₃TZBPDC ligand. (c) The H₂BTTA ligand. (d) The H₂DTBDA ligand. (e) [Zn₄(μ₄-O)-(μ₄-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)₃]. Reproduced from ref. 139 with permission from the American Chemical Society, Copyright 2011. (f) USTC-7. Reproduced from ref. 140 with permission from the Royal Society of Chemistry, Copyright 2016. (g) FJI-H14. Reproduced from ref. 136 with permission from Springer Nature, Copyright 2017. (h) FJI-H25Fe. Reproduced from ref. 137 with permission from Wiley, Copyright 2020. (i) FJI-H29. Reproduced from ref. 138 with permission from the American Chemical Society, Copyright 2020. (j) FJI-H36. Reproduced from ref. 141 with permission from Wiley, Copyright 2023.

In a word, many water-stable MOFs based on various soft metal ions and N-containing ligands were reported, including ZIFs, MAFs, PCNs, BUTs, and FJI-Hs. Due to the strong metal–nitrogen interaction, most of the reported MOFs also displayed good acid/base stability, especially base stability.

Notably, PCN-601 could retain its structural integrity in saturated NaOH solution (about 20 mol L⁻¹). This means that MOFs can be used normally even in a strongly alkaline environment.

2.4.3. Enhancing the thermodynamic stability of MOFs through pore engineering. In addition to directly using suitable coordination units, subsequent modification of pores of MOFs also can enhance their thermodynamic stability. Thus, many different strategies such as interlocking of the framework, increasing the rigidity of the framework, changing the distribution of charge, and pore space partition have been developed, and the water stability of many metastable MOFs has been successfully enhanced.
2.4.3.1. Enhancing stability through interlocking of frameworks. In addition to the weak coordination nodes, the relatively large pores of MOFs can also lead to their instability; this is because large pores facilitate the diffusion of small molecules that can break down their framework. For a specific MOF, the interlocking of its independent framework can not only enhance the interactions between interlocked frameworks but also reduce its pore size, so the interlocking of the framework can be used to enhance the water stability of MOFs. For example, Walton's group used the interpenetration of the framework to enhance the water stability of MOFs.¹⁴⁹ They prepared two isostructural MOFs DMOF [Zn(BDC)(DABCO)_0.5] and MOF-508 [Zn(BDC)(BPY)_0.5] by using different pillared ligands. The DMOF was a non-catenated pillared layer structure while MOF-508 was a two-fold interpenetrated pillared layer structure. Even though the pK_a of DABCO is significantly higher than that of BPY, the interlocked MOF-508 also exhibited better water stability than the non-interlocked DMOF. This enhanced stability is thought to be due to the higher thermodynamic stability and lower water absorption of the interlocked structure. The same enhanced effect can also be found in other interlocked MOFs (e.g., Zn-BTTB-DMBPY and Co-BTTB-DMBPY).¹⁵⁰

Interestingly, the water stability of MOFs can also be influenced by the direction of interpenetration. Chen and co-workers synthesized three isomeric MOFs([Zn(Hmpba)₂]) through a solvothermal reaction of 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H₂mpba) and Zn(NO₃)₂.¹⁵¹ Although three isomers had 4-fold interpenetrated structures, they exhibited entirely different water stability due to the different directions of interpenetration. Here, two nanoporous metastable isomers would spontaneously convert into the most stable nonporous isomer. This not only confirms that the interpenetration of the framework can improve the thermodynamic stability of MOFs but also indicates that the interpenetration with small pores is more stable than that with large pores.

2.4.3.2. Enhancing stability through pore space partition. As mentioned previously, for a metastable MOF containing large pores, its stability can be improved by the interlocking of the framework. This is because interlocking can effectively improve its thermodynamic stability and reduce its pore size. In fact, matched ligands also can be used to enhance the thermodynamic stability of a metastable MOF through pore space partition (PSP).⁷ For example, Du's group prepared two water-stable MOFs (1 and 2) with comparable structural properties but distinct water stability based on an MIL-88 type Co(II)-MOF through the PSP strategy.¹⁵² After introducing two size-matching triangular blocks [Co₂(ina)₃(H₂O)₂]⁺ and 2,4,6-tri(4-pyridyl)-1,3,5-triazine (tpt) into the pores, the moisture stability of the MOFs with tpt substitution rose dramatically. Recently, Bu and colleagues prepared a family of acid/base-resistant Cr-MOFs by dividing the large pore of the original Cr-MOFs into interconnected small pores employing triangle ligands (Fig. 13).¹⁵³ The newly formed Cr-MOFs (CPM-243(F)) could maintain their crystallinity even under very severe conditions such as HCl aqueous solution and 10 M NaOH aqueous solution. Such exceptional stability was due to the presence of the multi-binding pore-partitioning ligand, framework rigidity, and highly connected Cr₃O nodes.

Fig. 13 The structural components, together with the framework viewed along the c axis. NH₂bdc²⁻ = 2-aminoterephthalate, bpdc²⁻ = biphenyl-4,4′-dicarboxylate), tpt = 2,4,6-tri(4-pyridyl)-1,3,5triazine, tpbz = 1,3,5-tri(4-pyridyl)-benzene, and tppy = 2,4,6-tris(4-pyridyl)pyridine. Reproduced from ref. 153 with permission from the American Chemical Society, Copyright 2021.

Chen, Xiang, and co-workers also used the PSP strategy to realize the improvement of MOFs' stability.¹⁵⁴ As shown in Fig. 14, by a triangular ligand (Tripp = 2,4,6-tris(4-pyridyl)pyridine), every metal cluster in the original FJU-88 increased three coordination bonds. Due to the increased connection numbers of metal clusters, the thermal stability of the newly generated MOF (FJU-90) (up to 300 °C) was noticeably higher than that of the initial FJU-88 (durable only below 50 °C). Moreover, FJU-90 could maintain its structure even after being submerged in water for a day or exposed to air for a month, but the mother MOF (FJU-88) could not. The same improved effect also can be found in [Cu₂(obb)₂(bpy)_0.5(DMF)]·2DMF and [Co₂(INT)₃(H₂O)₂].^155,156

Fig. 14 Pore space partition (PSP) through symmetry- and size-matching-regulated ligand insertion. Reproduced from ref. 154 with permission from the American Chemical Society, Copyright 2019.

2.4.3.3. Enhancing stability through increasing the rigidity of the ligand. In some situations, the rigidity of organic linkers tends to reduce the structural diversity but is beneficial to construct chemically stable MOFs. For instance, Zhou, Li, and co-workers prepared 13 Zr-MOFs by combining three ligands with the same topology but different rigidity with Zr₆O₄(OH₄)(CO₂)_n units (n = 8 or 12) (Fig. 15).¹⁵⁷ The MOFs constructed from the less rigid ligand L_B¹ were labile while those with rigid ligands L_B² and L_B³ were chemical stable, which could maintain intact in water, acidic solution (pH = 1), or weakly basic (pH = 11) aqueous solution. The enhancement of stability may be due to the fact that rigid ligands have higher deformation energy barriers than flexible ligands and tend to maintain their conformation in the framework, which significantly restricts the dissociation of M-L and improves its repair. The same improvements also could be found in other isostructural MOFs (e.g., BUT-109(Zr)/BUT-110(Zr), and CoBTTBBPY/CoBTTBAZPY, and ZnBTTBBPY/ZnBTTBAZPY).^158,159

Fig. 15 Construction of a series of Zr-MOFs by using naphthoate-based tetracarboxylate ligands. (a) 12-Connected Zr₆ clusters; (b) 8-connected Zr₆ clusters; (c) different ligands LA¹, LA², and LA³; (d) configuration of ligands; different topologies (e) and structures (f) of LA²-Zr₆⁸-csq (5), LA²-Zr₆¹²-shp (4), LA¹-Zr₆¹²-shp (3), LA¹-Zr₆⁸-csq (2), LA¹-Zr₆⁸-flu (1), and LA³-Zr₆⁸-flu (6) MOFs, respectively. Reproduced from ref. 157 with permission from the American Chemical Society, Copyright 2019.

2.4.3.4. Enhancing stability through ligand charge-separation. In addition to the strategies given above, the charge density of organic ligands also plays a significant role in framework stability. Bu's group developed a ligand charge-separation (LCS) strategy for the stabilization of MOFs with high-charge-density ligands (Fig. 16).¹⁶⁰ By breaking down the ligand 2,5-dioxido-1,4-benzenedicarboxylate (dobdc⁴⁻) of MOF-74-Zn [Zn₂(obpdc)] into OH⁻ and obdc³⁻ (H₃obdc = 2-hydroxyterephthalic acid), two new MOFs CPM-74 [Zn₂(OH)(obpdc)] and CPM-75 [Zn₂(OH)(obpdc)] could be obtained. Compared with parent MOF-74-Zn, the two newly prepared MOFs had the same density of metal sites and similar Zn–O chains, so they could be considered quasi-MOF-74-Zn isomers. Although MOF-74-Zn was sensitive to water, both CPM-74 and CPM-75 could stably exist in aqueous solutions with pH from 3 to 10, exceeding most reported Zn(II)-MOFs based on carboxylate ligands. This significant enhancement may be because the separated low-charge-density ligands can coordinate with Zn(II) ions more flexibly, resulting in more stable coordination configurations.

Fig. 16 (a) ChemDraw representation of [Zn₂(OH)]³⁺ rod and ball-and-stick view of inter-rod connection. (b) Polyhedral view of the rod and the framework of CPM-74. (c) Comparison between the chains in CPM-74 and MOF-74. (d) Building scheme of CPM-75. Zn1, pink; Zn2, blue; Zn3, green; C, gray; O, red. Reproduced from ref. 160 with permission from the American Chemical Society, Copyright 2019.

2.4.3.5. Enhancing stability through metal metathesis. Theoretically, replacing the metal ions in a labile MOF with suitable metal ions that can form stronger coordination bonds and lead to an isomeric but more stable MOF. This means that the metal metathesis can effectively improve the stability of a metastable MOF without changing its framework and pore. Through metal metathesis and oxidation, Zhou and co-workers realized the enhancement of stability of a labile Mg-based MOF (PCN-426-Mg) (Fig. 17).¹⁶¹ After PSM, the as-prepared isostructural PCN-426-Cr possessed the stronger Cr(III)–O units. Although PCN-426-Mg was particularly vulnerable to humidity, the newly formed PCN-426-Cr could maintain its crystallinity even in an extremely acidic solution (4 M HCl) or a pH = 12 aqueous solution. Furthermore, Zhang and Wang developed a mild and efficient method for one-step synthesizing Cr(III)-MOFs from Fe(III)-MOFs through metal metathesis, namely solvent-assisted metal metathesis. They found that acetone could effectively improve the efficiency of metal exchange by coordinating with the Fe(III) ions of MOFs. Assisted by acetone, a series of isostructural Cr(III)-MOFs could be directly prepared from their Fe(III)-based precursors through the exchange of metal ions, and all the modified MOFs displayed enhanced stability.¹⁶²

Fig. 17 (a) The ligand and the structure of PCN-426-Mg; (b) the metal exchange routes for PCN-426. Reproduced from ref. 161 with permission from the American Chemical Society, Copyright 2014.

2.4.4. Enhancing the kinetic stability of MOFs through hydrophobic engineering. One approach to constructing MOFs with long-term water stability is increasing the strength of coordination bonds between metals and ligands (improving thermodynamic stability); the other spotlight can be turned on inhibiting the hydrolysis of MOFs (enhancing kinetic stability). The hydrolysis of MOFs in water could be considered as a competitive reaction between the ligand and H₂O toward metal clusters. To minimize the contact and affinity between MOFs and water, many different strategies, including directly using hydrophobic ligands, bonding hydrophobic groups, and forming hybrid composites have been developed.
2.4.4.1. Enhancing the kinetic stability by directly using hydrophobic ligands. Directly using hydrophobic ligands is the most commonly used method to improve the kinetic stability of MOFs in water. For example, employing hydrophobic polymer ligands and pyridine linkers, Cohen's group synthesized a series of polymer–metal–organic frameworks (polyMOFs) (Fig. 18).¹⁶³ In contrast to the parent MOF without hydrophobic units, polyMOFs could retain their crystallinity after being exposed to boiling water for 24 hours due to the cross-linking of polymer chains in the pores of polyMOFs. Apart from the polymer ligands, other hydrophobic units such as nonpolar alkyl chains and fluorine-containing groups can be introduced into organic linkers to prepare water-resistant MOFs. For instance, with the help of methyl-functionalized ligands, Li and co-workers could realize the controllable construction of a water-stable MOF based on water-unstable Cu(II) paddle wheel units (Fig. 19).¹⁶⁴ Through combining a ligand (H₈tdhb) functionalized with six methyl groups with Cu(II) ions, they prepared a microporous MOF (BUT-155a) with Cu(II) paddle wheel units. Even though the Cu(II) paddle wheel units are highly sensitive to water, the as-prepared BUT-155a also displayed excellent water stability due to the introduction of hydrophobic methyl groups. It could maintain structural integrity even in boiling water for 24 hours and the aqueous solution with a wide pH range (4s–10).

Fig. 18 Packing diagram of MOF 1 along the c-axis direction (top); design concept for creating a polyMOF analogue of MOF 1 via replacing dangling groups by polymer chains (bottom). Reproduced from ref. 163 with permission from the American Chemical Society, copyright 2016.

Fig. 19 (a) Structure of the ligand H₈tdhb, (b) crystal structure of BUT-155a, and (c) octahedral cage (cage A) and (d) cuboctahedral cage (cage B) in BUT-155a (color code: C, black; O, red; Cu, blue; H atoms on ligands are omitted for clarity). Reproduced from ref. 164 with permission from the American Chemical Society, copyright 2017.

Another water-stable Cu(II)-MOF (USTC-6) was prepared by Jiang's group through using a fluorine-containing ligand (4,4′-(per-fluoropropane-2,2-diyl)diphthalic acid). Due to the strong hydrophobicity of the CF₃ groups, USTC-6 demonstrated extraordinary endurance to water and aqueous solutions (pH = 2–10), despite the fact that the majority of Cu–O coordination bonds in MOFs are susceptible to hydrolysis.¹⁶⁵ Similarly, Li and co-workers realized the construction of a water-stable Zn(II) paddle wheel-based MOF by using fluorine-containing ligands. Through separately reacting the ligand 2-(trifluoromethyl) terephthalic acid (BDC-CF₃) and 2, 5-bis(trifluoromethyl) terephthalic acid (BDC-(CF₃)₂) with Zn(II) salt, they constructed two fluorinated Zn-DMOFs (named FDMOF-1 and FDMOF-2) (Fig. 20).¹⁶⁶ FDMOF-1 and FDMOF-2 exhibited ultrahigh structural stability due to the introduction of fluorinated functional groups into the confined pore space. Even at 70% relative humidity, they could maintain their structural integrity well for a period of six months.

Fig. 20 Fluorinated functional group introduction into the confined pore space. Reproduced from ref. 166 with permission from Wiley, Copyright 2023.

In order to explore the effect of the length of hydrophobic chains on the water stability of MOFs, a series of isostructural NbO-type MOFs were synthesized by Zhou's group.¹⁶⁷ They found that the water stability of the resulting Cu(II)-MOFs increased significantly while their thermal stability reduced as the length of the hydrophobic chains increased. Besides the length of hydrophobic groups, the distance between metal nodes and hydrophobic groups is another key parameter for influencing the chemical stability of MOFs. A reasonable comparison between MOF-508 and its two analogues (SCUTC-18 and SCUTC-19) was made by Ma's group.¹⁶⁸ In contrast to the unsubstituted bipyridine ligands of MOF-508, two methyl groups were introduced into the ortho- and meso-sites of the bipyridine ligands in SCUTC-18 and SCUTC-19, respectively. Only SCUTC-18 protected by the closer methyl groups could retain its porous structure after being exposed to humid air for 30 days.

2.4.4.2. Enhancing the kinetic stability by bonding hydrophobic groups. As mentioned above, since hydrophobic groups can effectively block the contact between water and metal nodes, kinetically stable MOFs can be directly constructed by using hydrophobic ligands. Besides directly using hydrophobic ligands, introducing suitable hydrophobic groups through PSM also can be used to stabilize water-sensitive MOFs. In 2010, Cohen's team successfully proved that the hydrophobicity of MOFs could be introduced by PSM.¹⁷¹ According to the PSM strategy, they prepared two types of isoreticular MOFs (IRMOF series and MIL series). For IRMOFs, their water contact angles could increase to 116° after being modified with long alkyl substituents, and this value could be compatible with common hydrophobic materials. After being modified with different alkyl substituents, the water contact angles of MILs could even reach 150°, demonstrating that their hydrophobicity can be comparable with that of superhydrophobic materials. Based on the PSM method, Bradshaw and co-workers also realized the enhancement of the structural stability of UiO-66-NH₂.¹⁷² They used the diazotization reaction of UiO-66-NH₂ to prepare a more stable MOF (UiO-66-N=N-ind). The modified UiO-66-N=N-ind could retain its structural integrity in a pH = 12 aqueous solution, while the unmodified UiO-66-NH₂ only resisted a weaker basic solution (pH = 9).

In addition to organic ligands, the hydrophobic groups also can be introduced into the metal nodes of MOFs through the PSM strategy. For example, different hydrophobic groups were introduced into the Zr₆-oxo nodes of NU-1000 through the solvent-assisted ligand incorporation (SALI) method by Farha, Hupp, and co-workers.^173,174 The water resistance of Zr₆-oxo nodes could be efficiently improved after being modified with organic carboxylates. Notably, the perfluorodecanoic acid functionalized NU-1000 exhibited remarkable water stability; it could maintain the crystallinity and porosity even after 20 cycles of water vapor adsorption. By introducing hydrophobic phenylsilane into the metal-oxo nodes of NH₂-UiO-66, Kim's group realized the controllable construction of a super-hydrophobic MOF NH₂-UiO-66(Zr)-shp (Fig. 21).¹⁶⁹ NH₂-UiO-66(Zr)-shp not only retained the acid resistance of NH₂-UiO-66(Zr) (e.g., resisted 12 M HCl and 16 M HNO₃) but also exhibited highly improved base stability (resisted 0.1 M NaOH solution at least for 5 hours).

Fig. 21 Preparation of super-hydrophobic NH₂-UiO-66(Zr)-shp. Reproduced from ref. 169 with permission from Wiley, Copyright 2019.

2.4.4.3. Enhancing the kinetic stability by forming hybrid composites. Introducing hydrophobic groups into the pores of MOFs indeed can significantly improve their kinetic stability toward water. However, additional hydrophobic groups in the pores usually reduce the porosity of MOFs. Direct compositing of metastable MOFs and hydrophobic materials can also effectively improve the water stability of MOFs but can maintain their pore structure as much as possible. Many water-stable hybrid composites based on water-instable MOFs have been reported. For example, by coating a thin hydrophobic polydimethylsiloxane (PDMS) layer on the surface of initial MOFs, Yu, Jiang, and colleagues developed a universal approach to improve the water-resistance of MOFs. In addition to improving their water resistance, the fragile MOFs (MOF-5, HKUST-1, and ZnBT) coated with PDMS also inherited their original crystalline structure and pore properties.¹⁷⁵ In addition, Queen's team constructed a series of extremely hydrophobic and stable MOF composites utilizing a post-synthetic polymerization strategy (Fig. 22).¹⁷⁰ Here, polydopamine was first coated on the surface of MOFs and hydrophobic molecules were introduced by Michael addition. Notably, a range of MOFs (e.g., HKUST-1 (Cu), Cu-TDPAT (Cu), and Mg-MOF-74 (Mg)) after modification demonstrated outstanding stability even under strongly acidic or basic conditions. In 2018, Cohen's group successfully incorporated UiO-66-NH₂ into nylon (PA-66) polymer fibers to give a hybrid composite by an interfacial polymerization method, which displayed improved stability.¹⁷⁶ Through interfacial polymerization, the UiO-66-NH₂ MOF was copolymerized with polyamide fiber (PA-66) to produce a hybrid composite with 29% MOF (mass fraction). Compared to MOF particles that were merely physically confined in the polymer, the covalently linked hybrid material exhibited better stability.

Fig. 22 The modification process used to prepare hydrophobic MOF@PDA-SF composites. Reproduced from ref. 170 with permission from the Royal Society of Chemistry, Copyright 2019.

Recently, the Pd/UiO-66 nanocomposite surface was successfully covered with a thin layer of hydrophobic polydimethylsiloxane (PDMS) using a chemical vapor deposition (CVD) method by Jiang's group (Fig. 23).¹⁷⁷ The enhanced stability and retained porosity of Pd/UiO-66@PDMS were further confirmed by PXRD and N₂ sorption analysis. Moreover, the modified catalyst (Pd/UiO-66@PDMS) exhibited significantly enhanced surface wettability and recycling stability, paving a new way for developing new catalysts with high activity and stability. Similarly, by combining hydrophobic polymer polystyrene-block-polybutadiene-block-polystyrene (SBS) with epn-functionalized Mg₂(dobpdc) (epn-MOF) and various supports (e.g., Ti mesh, HEPA filter, and granular AC), Hong and co-workers prepared a series of epn-MOF@SBS composites on different supports, including Ti mesh@epn-MOF@SBS, Filter@epn-MOF@SBS, and AC@epn-MOF@SBS (Fig. 24).¹⁷⁸ The as-prepared hybrid materials could maintain their stability and activity even after a long time of exposure to 60% RH.

Fig. 23 Preparative route for Pd/UiO-66@PDMS. Reproduced from ref. 177 with permission from Wiley, Copyright 2016.

Fig. 24 Fabrication of epn-MOF@SBS, Ti mesh@epn-MOF@SBS, Filter@epn-MOF@SBS, and AC@epn-MOF@SBS composites. Reproduced from ref. 178 with permission from Elsevier, Copyright 2022.

To encapsulate HKUST-1 within polystyrene, Maspoch et al. developed a facile spray-drying technique, which could produce water-resistant HKUST-1@polystyrene spheres.¹⁷⁹ Although HKUST-1 is very sensitive to water, the as-prepared hybrid composite (HKUST-1@polystyrene) could also maintain its water uptake capability even after three consecutive cycles. Very recently, Zhao's group realized the controllable construction of a super hydrophobic MOF membrane on polymer substrates through controlling the growth of ZIF-8 and using the surface coating technique.¹⁸⁰ In comparison to its pristine, the as-prepared hybrid composite (PDMS/MOF-NS/PVDF) displayed significantly enhanced hydrophobicity.

Besides hydrophobic polymers, carbon nanotubes could also be incorporated into MOFs, affording a hybrid composite with significantly improved water stability. The growth of Hf₁₂-porphyrin metal–organic frameworks on carbon nanotubes (CNTs) was reported by Lin's group in 2018.¹⁸¹ The as-synthesized Hf₁₂-CoDBP/CNT hybrid composite could remain inert even in a pH = 1 aqueous solution. The same improved effects could also be achieved in other hybrid composites (e.g., MOFMC, C-CNT, IM-CNT, and LM-CNT) based on MOFs and carbon nanotubes.^182,183

3. Water-stable MOFs for carbon dioxide capture

3.1. Background of MOFs for carbon capture

Due to the rapid increase of population and the acceleration of urbanization, human's demand for fossil energy has increased dramatically, which leads to excessive emissions of CO₂. It is reported that the concentration of CO₂ in the air has increased from 310 ppm in 1960 to 415 ppm in 2021. Excess CO₂ in the atmosphere leads to a series of serious environmental problems, such as global warming, sea level rise, ocean acidification, and extreme weather. More than half of global CO₂ emissions come from the flue gas of power plants; thus, CO₂ emissions should be greatly reduced if unwanted CO₂ can be effectively captured from flue gas. At present, organic amine solution is mainly used in power plants to capture CO₂ from flue gas, but the desorption of CO₂ from organic amine requires a lot of energy; in addition, organic amine can cause serious corrosion of equipment. Therefore, there is an urgent need to develop more energy-efficient and green technologies for capturing CO₂ from flue gas.

Although CO₂ from flue gas can be captured by decarbonizing devices installed in power plants, about 27% of global CO₂ emissions are difficult to capture effectively. So, it is necessary to develop technologies that can be used to capture trace CO₂ from the air. Direct air capture (DAC) is a negative emission technology in which chemical processes are used to capture CO₂ from ambient air, which can be used to not only reduce the excessive CO₂ (>415 ppm) in the atmosphere to the optimal level below 350 ppm, but also compensate for the CO₂ emissions from economic activities that are difficult to decarbonize. Currently, there are a few absorbents available for DAC, including alkaline aqueous solutions (such as NaOH and KOH) and porous solids loaded with amines. These technologies also have the problem of a high recovery energy, and more energy-efficient DAC technologies should be developed.

Natural gas has been considered as a promising clean energy due to its high energy-conversion efficiency and low environmental impact. Unfortunately, crude natural gas usually contains a large amount of CO₂, which not only sharply reduces the burning heat of methane, but also causes serious corrosion to the transmission pipelines. In order to capture CO₂ from crude natural gas, alkali adsorption technology has been widely used in industry, which also requires a high recovery energy. Thus, more energy-efficient technologies for purifying natural gas are urgently needed.

Alkali solution can indeed efficiently capture unwanted CO₂, but it requires a large amount of energy to recover and tends to corrode equipment. Using suitable porous materials to capture CO₂ can overcome these challenges and achieve more energy-efficient carbon capture. Compared with traditional porous materials such as activated carbon, mesoporous silica, and zeolite, the pore size and pore environment of MOFs can be precisely regulated, which enables them to capture CO₂ more efficiently and selectively. To achieve efficient carbon capture of MOFs in various scenarios (e.g., flue gas decarbonization, direct air capture, and natural gas upgrading), many MOFs with different active sites and pore sizes have been designed and synthesized. Ideally, a practical carbon capture adsorbent should have not only excellent adsorption performance, such as high adsorption capacity, high adsorption selectivity, and low adsorption enthalpy; but also excellent chemical stability, recyclability, and high cost-effectiveness. However, developing such a MOF remains a huge challenge, as it is very difficult to achieve these functions simultaneously through one MOF.

3.2. Physical and chemical properties of carbon dioxide

To achieve efficient and selective capture of CO₂ by MOFs, it is necessary to fully understand the physical and chemical properties of CO₂ and its competing gases (e.g., N₂ and CH₄) (Scheme 4). CO₂ is a linear molecule with strong polarity, composed of one sp hybrid carbon atom and two sp² hybrid oxygen atoms. The as-formed carbon–oxygen bond has the characteristics of both carbon–oxygen double bond and carbon–oxygen triple bond. The carbon atom in the middle is electrically deficient, while the oxygen atoms at both ends are electrically rich. The size and kinetic diameter of CO₂ are 3.19 × 3.34 × 5.39 ų and 3.3 Å respectively. Due to its specific structure, CO₂ can not only coordinate with metal ions providing empty orbitals, but also generate dipole interactions with various polar atoms (e.g., H, N, and O) as well as π–π interactions with conjugated molecules (e.g., benzene ring). In this regard, by introducing suitable active sites that can interact with CO₂ into a MOF, its CO₂ adsorption performance can be improved thermodynamically, and the construction of pores matched with CO₂ (about 3.3 Å) is kinetically conducive to CO₂ capture. N₂, the main component of flue gas and air, may compete with CO₂ in the process of flue gas decarbonization and direct air capture; therefore, inhibiting the competitive adsorption of N₂ is conducive to improving the efficiency of CO₂ capture. N₂ is a spherical, non-polar molecule with a size and kinetic diameter of 3.6 × 3.6 × 3.6 ų and 3.64 Å, respectively. As the main component of natural gas, CH₄ is a spherical molecule with weak polarity, whose size and kinetic diameter are 3.7 × 3.7 × 3.7 ų and 3.758 Å, respectively. By inhibiting the competitive adsorption of CH₄, the purification efficiency of natural gas can be significantly improved. Therefore, by increasing the pore polarity of MOFs and controlling its pore size (smaller than the kinetic diameter of N₂/CH₄), the competitive adsorption of N₂/CH₄ can be significantly inhibited.

Scheme 4 Physical and chemical properties of CO₂ (a), N₂ (b), and CH₄ (c).

3.3. The adsorption mechanisms and characterization of MOFs for carbon dioxide

When a fluid comes into contact with a porous solid, its components accumulate on the surface of the solid, which is called adsorption. If the interactions between the adsorbate and the adsorbent are weak intermolecular interactions (e.g., van der Waals forces, hydrophobic interactions, electrostatic forces, and hydrogen bonding), this process can be defined as physical adsorption, also known as physisorption, which usually exhibits low adsorption heat but fast adsorption and high desorption kinetics. When an adsorption process involves one or more chemical reactions, it can be defined as chemical adsorption (chemisorption), which is an irreversible adsorption process with a high adsorption enthalpy. If the interaction between a specific MOF and CO₂ is relatively weak (e.g., the adsorption enthalpy is less than 40 kJ mol⁻¹), then its adsorption of CO₂ is undoubtedly physical adsorption. In contrast, the adsorption between CO₂ and a MOF that can react with CO₂ while producing new species should be a chemical adsorption process. Despite having a strong interaction with CO₂ (e.g., the adsorption enthalpy is greater than 50 kJ mol⁻¹), a particular MOF does not react with the adsorbed CO₂ or generate new species. We still discuss it according to physical adsorption, even though the real situation may be the coexistence of physical and chemical adsorption.

When CO₂ enters the pores of MOFs and interacts with the active sites, not only will the charge distribution or strength of carbon–oxygen bonds be changed, but potential new species may also be formed. All of these can be detected by NMR and infrared absorption spectroscopy (IR). In addition, when the adsorbed CO₂ molecules are orderly distributed in the pores of MOFs, their adsorption sites can be accurately located through SCXRD, PXRD, and neutron powder diffraction (NPD) analyses. Therefore, the adsorption character of MOFs for CO₂ can be analyzed in detail. In addition, due to the determinacy of the structure of MOFs, molecular simulation (MS) is also a useful supplement to understanding the adsorption mechanism of MOFs for CO₂. Next, we briefly introduce the reported adsorption mechanisms and characterization of MOFs for CO₂, hoping to provide a useful guidance for designing MOF-based CO₂ adsorbents with better performance.

3.3.1. Physical adsorption of carbon dioxide in MOFs. Due to the potential of coordination between metal ions and CO₂, various open metal sites (OMSs) were widely introduced into MOFs for improving CO₂ adsorption, with many different types of metal–CO₂ interactions observed. For example, through the coordination between Ni(II) ions and CO₂, Dietzel and co-authors used SCXRD to observe the adsorption sites of CO₂ in MOFs for the first time, where CO₂ was directly attached to the Ni(II) site of CPO-27-Ni (known as Ni-MOF-74) with a Ni–O distance of 2.29 Å (Fig. 25a).¹⁸⁴ In isomorphic MOFs containing harder Co(II) ions, Long and collaborators used in situ SCXRD to confirm that open Co(II) ions were the preferential adsorption sites for CO₂ while finding other CO₂ adsorption sites inside the pores (Fig. 25b).¹⁸⁵ For the isomerized MOF (UTSA-74) containing softer Zn(II) ions, Chen's group found that the adsorbed CO₂ molecules could be stabilized by two neighbouring Zn(II) ions synergically (Fig. 25c).¹⁸⁶ In addition, we proved that the Cu(II) ions with planar four-coordination were highly efficient adsorption sites for CO₂ through in situ IR, GCMC simulation and DFT optimization (Fig. 25d),¹³⁷ which have been widely introduced into MOFs for CO₂ capture.

Fig. 25 Carbon dioxide adsorption on open metal ions through coordination interactions. (a) The Ni(II)–CO₂ interactions in CPO-27-Ni. Reproduced from ref. 184 with permission from the Royal Society of Chemistry, Copyright 2008. (b) The Co(II)–CO₂ interactions in Co-MOF-74. Reproduced from ref. 185 with permission from the Royal Society of Chemistry, Copyright 2017. (c) The Zn(II)–CO₂ interactions in UTSA-74. Reproduced from ref. 186 with permission from the American Chemical Society, Copyright 2016. (d) The Cu(II)–CO₂ interactions in FJI-H14. Reproduced from ref. 136 with permission from Springer Nature, Copyright 2017.

In addition to various open metal sites, polar atoms that can form strong dipole interactions with CO₂ are another type of adsorption site for CO₂, which have also been widely introduced into MOFs for carbon capture, and a variety of polar groups are proved to stabilize the adsorbed CO₂ molecules. For instance, in the MOF (Zn₂(Atz)₂(ox), Atz = 3-amino-1,2,4-triazole, ox = oxalate) containing aromatic amine groups, Shimizu and co-workers observed the bonding of adsorbed CO₂ and aromatic amine groups for the first time, where the electrically deficient C atoms could be stabilized by the electrically rich N atoms. The distance between them was about 3.152 Å, less than the sum of van der Waals radii of N and C atoms (Fig. 26a).¹⁸⁷ Based on the uncoordinated triazole groups, Chen and Zhang also observed strong N⋯C dipole interactions (Fig. 26b),¹⁸⁸ where the adsorbed CO₂ molecules were synergistically stabilized by two adjacent N atoms (C15–N8, 3.178(8) Å; C15–N2, 3.011(8) Å; C16–N11, 3.15(1) Å; C16–N5, 3.26(1) Å), resulting in a high adsorption enthalpy. Polar oxygen atoms can also be served as adsorption sites for CO₂ capture. For example, Schröder and Yang used nitro groups to achieve an efficient promotion of CO₂ adsorption. Through NPD analyses, they further found that there were strong C⋯O dipole interactions between the adsorbed CO₂ and nitro groups with a distance of 2.88 Å and 3.21 Å, respectively (Fig. 26c).¹⁸⁹ Recently, our group proved that the carboxylic acid group could well adsorb and stabilize CO₂ through SCXRD. In the structure of CO₂@FJI-H38, the adsorbed CO₂ is simultaneously stabilized by two O atoms of the carboxylic acid group, with the shortest C–O distance of only 2.78 Å, far below the sum of van der Waals radii of C and O atoms (Fig. 26d).¹⁹⁰ Like N and O atoms, the more electronegative fluorine atoms can also adsorb CO₂ effectively. As confirmed by Eddaoudi's group, the CO₂ adsorbed in the pores of NbOFFIVE-1-Ni can be efficiently stabilized by four different (NbOF₅)²⁻ with a C⋯F distance of 3.050(8) Å, lower than the sum of the van der Waals radii of C and F atoms (Fig. 26e).¹⁹¹ In another isomorphic MOF (dptz-CuTiF₆), the CO₂ located in the pore was also stabilized by the surrounding TiF₆²⁻ ions, where the distance between the F and C atoms was only 2.512 Å, indicating that a stronger C⋯F dipole interaction is formed (Fig. 26f).¹⁹²

Fig. 26 Carbon dioxide adsorption on electrically rich atoms through dipole interactions. (a) The N⋯CO₂ dipole interactions in Zn₂(Atz)₂(ox). Reproduced from ref. 187 with permission from the American Association for the Advancement of Science, Copyright 2010. (b) The N⋯CO₂ dipole interactions in [Zn₂(btm)₂]. Reproduced from ref. 188 with permission from the American Chemical Society, Copyright 2012. (c) The O⋯CO₂ dipole interactions in MFM-102-NO₂. Reproduced from ref. 189 with permission from the Royal Society of Chemistry, Copyright 2020. (d) The O⋯CO₂ dipole interactions in FJI-H38. Reproduced from ref. 190 with permission from Wiley, Copyright 2023. (e) The F⋯CO₂ dipole interactions in NbOFFIVE-1-Ni. Reproduced from ref. 191 with permission from the American Chemical Society, Copyright 2016. (f) The F⋯CO₂ dipole interactions in dptz-CuTiF₆. Reproduced from ref. 192 with permission from Elsevier, Copyright 2019.

Since electron-rich oxygen atoms easily form hydrogen bonding with electron-deficient hydrogen atoms, active hydrogen atoms can also serve as adsorption sites for CO₂. In 2012, Chen's group found that a MOF (UTSA-16) without OMS could also efficiently capture CO₂. Through SCXRD analysis, they found that the coordinated H₂O on the framework was the adsorption site of CO₂ (O3w–H⋯O72 = 2.971(1) Å and O3w–H⋯O71 = 3.067(1) Å) (Fig. 27a).¹⁹³ In the same year, Schröder and Yang confirmed that the hydroxyl group was also an efficient adsorption site for CO₂ through PXRD. In the structure of CO₂@NOTT-300, the adsorbed CO₂ and hydroxyl group formed a stronger hydrogen-bonding interaction than that between CO₂ and UTSA-16, with a distance of only 2.298 Å between H and O atoms (Fig. 27b).¹⁹⁴ In 2017, they confirmed that the active hydrogen on the amide group and phenyl group could also form strong hydrogen-bonding interactions with adsorbed CO₂ through NPD experiments. The distance of the adsorbed CO₂ from the H atoms of the amide group and the phenyl group was 2.22 and 1.97 Å, respectively (Fig. 27c).¹⁹⁵ In the structure of CO₂@MFM-102-NO₂, they further found that the adsorbed CO₂ was only 1.91 and 1.74 Å away from the H atoms of the phenyl group, implying the formation of stronger hydrogen-bonding based on CH and CO₂ (Fig. 27d).¹⁸⁹

Fig. 27 Carbon dioxide adsorption on electron-deficient H atoms through hydrogen-bonding interactions. (a) The OH⋯O hydrogen-bonding interactions in UTSA-16. Reproduced from ref. 193 with permission from Springer Nature, Copyright 2012. (b) The OH⋯O hydrogen-bonding interactions in NOTT-300. Reproduced from ref. 194 with permission from Springer Nature, Copyright 2012. (c) The NH/CH⋯O hydrogen-bonding interactions in MFM-188a. Reproduced from ref. 195 with permission from Springer Nature, Copyright 2017. (d) The CH⋯O hydrogen-bonding interactions in MFM-102-NO₂. Reproduced from ref. 189 with permission from the Royal Society of Chemistry, Copyright 2020.

Due to the presence of π bonds and p electrons, π–π or p–π interactions can be formed between CO₂ and suitable conjugated molecules. Therefore, molecules containing conjugated groups (e.g., phenyl groups) can also serve as adsorption sites of CO₂. Both Walton and Smit's group used MS to confirm that the phenyl rings could indeed serve as an effective adsorption site of CO₂ (Fig. 28a and b).^196,197 However, due to its weak strength and the lack of clear directionality, it is difficult to directly characterize the π–π interactions between adsorbed CO₂ and MOFs. So far, only very limited examples that can directly reveal the π–π interactions between CO₂ and MOFs have been reported. In 2013, Li and Paris first observed the π–π/p–π interactions between adsorbed CO₂ and phenyl groups through SXCRD analysis, where the adsorbed CO₂ was located between two centroids of the phenyl groups, with an average distance of 3.81 Å (Fig. 28c).¹⁹⁸ Later, Schröder and Yang discovered another type of interaction between adsorbed CO₂ and phenyl groups through NPD analysis. In the structure of the CO₂-adsorbed MFM-300(V^IV), CO₂ bound with the two adjacent phenyl rings in a ‘sandwich’ form, and the distance between the O atom of CO₂ and the phenyl group was 3.069(2) and 3.146(3) Å respectively (Fig. 28d).¹⁹⁹

Fig. 28 Carbon dioxide adsorption on phenyl groups through π–π interaction. (a) and (b) The potential π–π interaction between adsorbed CO₂ and phenyl rings based on MS. Reproduced from ref. 196 with permission from the American Chemical Society, Copyright 2013. Reproduced from ref. 197 with permission from Springer Nature, Copyright 2019. (c) The π–π/p–π interactions between adsorbed CO₂ and phenyl groups in CaSDB. Reproduced from ref. 198 with permission from Wiley, Copyright 2013. (d) The π–π/p–π interactions between adsorbed CO₂ and phenyl groups in MFM-300(V^IV). Reproduced from ref. 199 with permission from Springer Nature, Copyright 2017.

In short, OMSs that can provide empty orbitals for CO₂, polar atoms (e.g., N, O, and F) that can generate dipole interactions with CO₂, H atoms that can form hydrogen-bonding with CO₂, and conjugated molecules can all serve as effective sites for the physical adsorption of CO₂. It should also be pointed out that multiple CO₂ adsorption sites can exist simultaneously in MOFs, and adsorbed CO₂ molecules can be synergistically stabilized by multiple adsorption sites.

3.3.2. Chemical adsorption of carbon dioxide in MOFs. As a weak acidic gas, CO₂ can react with aliphatic amines and their derivatives to form ammonium carbamates/ammonium bicarbonates, which can be reversibly decomposed into CO₂ and free amines under the induction of suitable external stimuli (e.g. temperature). Therefore, amino groups that can reversibly react with CO₂ can be introduced into MOFs as chemisorption sites for CO₂ capture. Although Long's group had already achieved the chemisorption of CO₂ using an ethylenediamine functionalized MOF as early as 2009,²⁰⁸ it was not until 2015 that he and collaborators directly observed the interactions between adsorbed CO₂ and aliphatic amines using high-resolution PXRD. In the adsorbed CO₂ MOF (mmen-Mn₂(dobpdc) (mmen = N,N′-dimethylethylenediamine, dobpdc⁴⁻ = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate)), the reaction between adsorbed CO₂ and the coordinated mmen can be promoted by the adjacent mmen, thus achieving the rapid formation of ammonium carbamate at a low pressure. When CO₂ adsorption was saturated, these newly formed ammonium carbamate would form an infinite ammonium carbamate chain (Fig. 29a).²⁰⁰ Through ¹³C solid-state NMR analyses, Yaghi's group found that the carbon capture product of primary alkylamine functionalized MOFs (IRMOF-74-III-(CH₂NH₂)₂) under dry conditions was carbamic acid (the signal at 160.3 ppm) rather than ammonium carbamate, which is the major product of other primary amine functionalized MOFs. However, in the presence of water vapor, the as-formed carbamic acid could effectively convert into ammonium carbamate (the signal at 164.0 ppm) (Fig. 29b).²⁰¹ Through the regulation of metal ions and ligands, Long and co-workers realized the regulation of CO₂ chemisorbed products in diamine-M₂(dobpdc) MOFs. For example, the chemisorbed products of a Zn-based MOF (dmpn-Zn₂(dobpdc), dmpn = 2,2-dimethyl-1,3-diaminopropane) could be ammonium carbamate and carbamate pairs (the signal at 161.1 ppm) (Fig. 29c),²⁰² which was confirmed using ¹³C solid-state NMR and SCXRD analyses. For an isomorphic Mg-based MOF (dmpn-Mg₂(dobpdc)), an unusual mixture of carbamic acid and ammonium carbamate at a ratio of 1 : 1 would be formed (Fig. 29d).²⁰³

Fig. 29 Carbon dioxide adsorption on nitrogen atoms through the formation of ammonium carbamate/carbamic acid. (a) The ammonium carbamate chain in mmen-Mn₂(dobpdc). Reproduced from ref. 200 with permission from Springer Nature, Copyright 2015. (b) The carbamic acid and ammonium carbamate in IRMOF-74-III-(CH₂NH₂)₂. Reproduced from ref. 201 with permission from the American Chemical Society, Copyright 2017. (c) The carbamate pairs in dmpn-Zn₂(dobpdc). Reproduced from ref. 202 with permission from the American Chemical Society, Copyright 2017. (d) The mixture of carbamic acid and ammonium carbamate in dmpn-Mg₂(dobpdc). Reproduced from ref. 203 with permission from the American Chemical Society, Copyright 2018.

In addition to aliphatic amines, O-containing alkaline groups (e.g., hydroxyl anions and alcohol anions) can also react reversibly with CO₂ to produce carbonates. Thus, suitable O-containing groups that can react reversibly with CO₂ can also be introduced into MOFs as chemisorption sites of CO₂. Due to the strong alkalinity of O-containing alkaline groups, only MOFs with an excellent base tolerance can introduce them. So only very limited examples of CO₂ adsorption based on O-containing alkaline groups have been reported. In 2011, Stoddart and Yaghi used CD-MOF-2 to achieve the chemisorption of CO₂ in MOFs for the first time, where free primary alcohol groups could react with adsorbed CO₂ to form alkyl carbonates, which can be monitored through ¹³C solid-state NMR (the signal at 158 ppm) (Fig. 30a).²⁰⁴ In 2015, Zhang and Chen achieved another mode of CO₂ chemisorption using a MOF (MAF-X25ox, [Mn^IIMn^III(OH)Cl₂(bbta)], H₂bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) containing monodentate hydroxide. The newly formed bicarbonate could be determined by IR spectroscopy, where multiple new adsorption peaks (the peaks at 3682/1224/1050 cm⁻¹) attributed to bicarbonate were generated simultaneously (Fig. 30c).²⁰⁶ By mimicking the active centre of carbonic anhydrase, both Wade and Dincǎ’s groups synthesized different MOFs containing active ZnN₃OH units by introducing hydroxide anions into Zn(II) metal nodes, separately realizing their chemical adsorption of CO₂.²⁰⁹ Here, the Zn–OH group also reacts with the adsorbed CO₂ to produce bicarbonate, which can be confirmed by in situ IR and diffuse reflectance infrared Fourier transform spectroscopy respectively (Fig. 30d).²⁰⁷ Recently, Milner and collaborators also achieved the chemical adsorption of CO₂ using a MOF (KOH CD-MOF), which was isomorphic to CD-MOF-2 but less alkaline. Here, the adsorbed CO₂ reacted with the hydroxyl anions in the pores of the MOF to form bicarbonate, which could be confirmed using solid ¹³C NMR (the signal at 159.6 ppm) and IR spectra (the peak at 1663 cm⁻¹) simultaneously (Fig. 30b).²⁰⁵

Fig. 30 Carbon dioxide adsorption on oxygen atoms through the formation of carbonate. (a) The alkyl carbonates in CD-MOF-2. Reproduced from ref. 204 with permission from the American Chemical Society, Copyright 2011. (b) The bicarbonate in KOH CD-MOF. Reproduced from ref. 205 with permission from Wiley, Copyright 2022. (c) The bicarbonate in MAF-X25ox. Reproduced from ref. 206 with permission from the Royal Society of Chemistry, Copyright 2015. (d) The bicarbonate in [Zn(ZnOH)₄(bibta)₃]. Reproduced from ref. 207 with permission from the American Chemical Society, Copyright 2018.

In short, suitable alkaline groups (e.g., aliphatic amines, hydroxyl anions, and alcohol anions) that can react reversibly with CO₂ can be used as chemisorption sites of CO₂, and the newly formed species based on adsorbed CO₂ can be characterized through solid-state NMR and IR spectroscopy. It is worth mentioning that physical adsorption can also coexist even if an MOF has a high density of chemisorption sites.

3.4. Carbon capture based on water stable MOFs

As mentioned previously, since the pore size and pore environment of MOFs can be precisely controlled through structural design, they have better prospects in the field of carbon capture than traditional porous materials such as mesoporous silicon and zeolite. Indeed, the carbon capture performance of some MOFs has surpassed that of commercial zeolite materials. Considering that water vapor is widely present in flue gas, air, and crude natural gas, a practical carbon adsorbent should first have a good water vapor resistance. Although there have been many reviews on the progress of MOFs in the field of carbon capture, these articles are mainly focused on carbon capture performance and mechanisms. Here, we focus on the progress of water-stable (at least moisture stable) MOFs in flue gas decarbonization, direct air capture, and natural gas purification, hoping to provide a useful reference for the development of practical and excellent carbon adsorbents.

3.4.1. Carbon dioxide capture from flue gas. The total pressure of flue gas produced by a coal-fired power plant is about 1 atm, which contains 73–77% N₂, 15–16% CO₂, 5–7% H₂O, 3–4% O₂. This means that a practical MOF should have a high adsorption capacity and CO₂/N₂ adsorption selectivity, as well as a good water stability. In order to achieve flue gas decarbonization, many MOFs with various CO₂ active sites or matching CO₂ pores have been reported, and in this section, only MOFs that are at least wet resistant are discussed (Table 2).

Table 2 The performances of some representative MOFs for CO₂ capture from flue gas

MOFs	Stability	CO2 adsorption amount at 1 bar (mmol g^−1)	CO2 adsorption amount at 0.15 bar (mmol g^−1)	IAST selectivity at 1 bar (15/85) CO2/N2	Q st (kJ mol^−1)	Temperature (K)	Ref.
a 1.2 bar. b At 40 mbar. c At 20 bar. d 0.06 bar. e Calculated from Henry's law constant. f 800 torr. g Dry air. h RH 10%. i 0.1 bar.
UiO-66	pH = 0–12	1.79	0.37	19.4	24.5	298K	234
UiO-66-NH2	Water-stable	3	∼0.11	∼57	28	298	235
UiO-66-(OPr)2	pH = 1–11	∼2.6	1.19	39.1	∼34.5	298	236
UiO-66-(OH)2	Water-stable	∼4.6	1.92	66	34.5	298	236
UiO-66-SO3H-0.15	pH = 1–12	2.23	0.51	16.3	26.6	298	234
UiO-66-SO3Li-0.15	pH = 1–12	3.28	0.93	39.7	25.4	298	234
UiO-66-(COOLi)4-EX	Water-stable	2.34	0.7	26.62	27.5	298	237
UiO-66(Zr)-(COOLi)2	Water-stable	1.36	0.44	50.8	34.7	298	238
UiO-66-EA	Water-stable	1.7	1.05	365	66	298	239
UiO-66(Hf)-(OH)2	pH = 1–10	4.06	1.81	93	28.4	298	240
NOTT-300	Water-stable	7.0	2.64	815^e	27–30	273	194
Al(HCOO)3	(HCl 12M-pH 14)	∼4	2.7	368	47.9	320	210
MIP-207	Water-stable	2.11	0.89	52.8	—	298	101
CPM-74	pH = 3–10	∼2.91	—	72–77 (10 : 90)	33	198	160
NOTT-140	Water-stable	20.72^c	—	—	24.7	283	246
La-BTN	pH = 2–12	3.904	—	93	28.5	273	109
NKMOF-3-Eu	Moisture-stable (90% RH)	1.9	—	∼160	47	298	212
NJU-Bai53	pH = 2–11	5.01	—	—	88.3	298	233
ZIF-301	Moisture-stable (80% RH)	1.47^f	—	19^e	∼30	298	247
ZIF-300	Moisture-stable (80% RH)	1.32^f	—	22^e	∼28	298	247
ZIF-1001	pH = 3–14	∼0.67	0.328	19.7	26.4	298	248
ZIF-1002	pH = 3–14	∼0.50	0.268	21.5	26.9	298	248
ZIF-1003	pH = 3–14	∼0.49	0.265	21.6	28.4	298	248
ZIF-1004	pH = 3–14	∼0.65	0.348	24.4	28.1	298	248
ZnF(daTZ)	pH = 1–12	∼1.8	0.96	120	33	298	249
ZnF(aTZ)	pH = 2–12	∼1.85	0.56	46	32.3	298	249
ZnF(TZ)	pH = 3–12	∼1.63	0.27	13	23.6	298	249
SIFSIX-3-Zn	Moisture-stable (74% RH)	2.55	—	1818	45	298	214
SIFSIX-2-Cu-i	Moisture-stable (95% RH)	5.41	—	140	31.9	298	213
UTSA-120	Moisture-stable (80% RH)	5.0	3.56	∼600	27–31	296	215
dptz-CuTiF6	Moisture-stable	4.52	2.93^i	—	38.2	298	192
Cu-BTTri	Water-stable	3.24	0.277^d	21	21	298	130
SIFSIX-3-Cu	Moisture-stable (74% RH)	∼2.50	—	—	54	298	214
Qc-5-Cu-sql-β	Water-stable	2.16	—	40 000	36	293	216
Cu–F-pymo	pH = 1–14	1.19	—	>10^7	29.1	298	250
TIFSIX-Cu-TPA	Water-stable	2.1	—	910	39.2	298	251
SIFSIX-18-Ni-β	Moisture-stable (90% RH)	∼3	∼2.8	—	52	298	230
SIFSIX-3-Ni	Moisture-stable (75% RH)	2.7	∼2.5^i	—	50.9	298	231
TIFSIX-3-Ni	Moisture-stable (75% RH)	∼2.42	2.39	—	50	298	229
NbOFFIVE-1-Ni	Moisture-stable (95% RH)	2.2	—	—	54	298	191
CALF-20	Water-stable	4.07^a	—	230 (10 : 90)	39	293	217
FJI-H14	pH = 2–12	6.52	2.3	51	26.6	298	136
FJI-H29	pH = 2–13	1.79	0.92	146	34	298	138
ZU-301	pH = 2–12	2.44	—	∼850	38	298	252
PCN-200	pH = 2–12	∼1.56	∼1.69	60	38.5	273	253
MOF-808-DL-Lys	Water-stable	—	1.04	—	80	298	211
MOF-808-Gly	Water-stable	—	0.54^g	—	46	298	211
			0.69^h
een-Mg2(dobpdc)	Moisture-stable (100% RH)	5.05	4.04	—	70–74	313	226
en-Mg2(dobpdc)	Moisture-stable (100% RH)	4.57	3.62	230	49–51	298	219
ipen-Mg2(dobpdc)	Moisture-stable (100% RH)	4.05	3.47	—	76–85	313	226 and 242
een-MOF/Al–Si–C17-200	Moisture-stable	∼3.47	2.82	342(15 : 75)	52−66	313	220
Mg2(dobpdc)(3-4-3)	Moisture-stable	∼3.4^b	—	—	99	363	243
en-CuBTTri	Water-stable	1.27	0.366^d	25	90	298	130
MAF-X25ox	Water-stable	7.14	4.08	250	120	298	206
MAF-X27ox	Water-stable	6.70	4.06	262	124	298	206
KHCO3 CD-MOF	Water-stable	2.50	0.63	81	49.5 ± 1.8	313	205
PN@MOF-5	Moisture-stable (40% RH)	3.48	—	212(14 : 86)	29	273	222

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As mentioned above, water-stable MOFs can be directly constructed from hard metal ions and carboxylate ligands. Then, using this strategy, many water-stable MOFs have been synthesized and used for carbon capture. For example, based on the cheap Al(III) ions and formate ligands, Cheetham, Zhao, and co-workers prepared a chemically stable MOF (ALF), which could retain its framework in both strong base solutions (1 M NaOH) and acidic solutions (12 M HCl) (Fig. 31a).²¹⁰ Due to its polar ultramicro-pores, ALF could selectively capture CO₂ from N₂ with a high CO₂/N₂ (85 : 15) selectivity of 368 (Fig. 31b). Notably, ALF could be used to effectively extract CO₂ from simulated flue gas (15/85 CO₂/N₂ binary gas at 323 K) (Fig. 31c) with a high adsorption capacity (0.80 mmol g⁻¹) and adsorption selectivity (75) (Fig. 31c), and such excellent separation performance could remain for at least 130 cycles (Fig. 31d). Furthermore, ALF could be prepared on a large scale, making it a potential carbon capture reagent for flue gas decarbonization.

Fig. 31 The structure and CO₂ adsorption performance of ALF. Reproduced from ref. 210 with permission from the Advancement of Science, Copyright 2022. (a) The structure of ALF. (b) The CO₂/N₂ sorption isotherms of ALF at 298 K. (c) The breakthrough curves of ALF at 323 K. (d) 130 cycles of adsorption and desorption.

Due to the excellent water stability of Zr–O clusters, many functionalized Zr-MOFs have been synthesized for carbon capture.^234–240 For example, Yaghi's group prepared a series of amino acid (AA) functionalized MOFs (MOF-808-AA) based on a highly stable MOF-808 [Zr₆O₄(OH)₄(BTC)₂(HCOO)₆], which could effectively capture CO₂ from simulated flue gas in the presence of water.²¹¹ The CO₂ uptake performances of MOF-808AAs varied from 0.277 to 1.040 mmol g⁻¹ at 15 kPa (15% CO₂ in 1 atm gas mixture, relevant to flue gas). According to the binary CO₂/H₂O adsorption isotherms, the CO₂ uptake of MOF-808-Gly in the presence of water was significantly higher than that under dry conditions. This might be because the amine residues in the pores enabled MOF-808-AA to have enough affinity for CO₂ to form bicarbonate. Moreover, the adsorbed CO₂ could be released via vacuum swing adsorption instead of heating. By introducing ethanolamine (EA) into water-stable UiO-66, Zhang and co-workers also realized an efficient CO₂ capture under humid conditions.²³⁹ Due to the strong interactions between amino groups and CO₂, the newly formed UiO-66-EA displayed higher adsorption enthalpy (66 kJ mol⁻¹) and adsorption selectivity (356) than the original UiO-66 (30 kJ mol⁻¹ and 15), and such excellent CO₂ adsorption performance could be completely maintained even at 82% RH.

Water-stable lanthanide MOFs can also serve as efficient CO₂ adsorbents. For example, Zhang's team synthesised a series of robust ultramicroporous Ln-MOFs (NKMOF-3-Ln) (Fig. 32a), which could maintain their framework intact after being exposed to humid air with 90% RH for 7 days.²¹² Due to its enzyme-like channels, NKMOF-3-Ln could be used to realize molecular sieving of CO₂ from N₂ (Fig. 32b and c), in which the CO₂ adsorption capacity and adsorption enthalpy would be enhanced gradually with the contraction of the ionic radius of lanthanide elements (Fig. 32d).

Fig. 32 (a) The structure of NKMOF-3-Ln. (b)–(d) The CO₂/N₂ adsorption isotherms, CO₂ adsorption enthalpy, and breakthrough curves of CO₂/N₂ (15/85) mixtures at 298 K. Reproduced from ref. 212 with permission from the American Chemical Society, Copyright 2019.

As described previously, thermodynamically stable MOFs could also be constructed using soft metal ions and soft N-containing ligands. Many water/moisture-stable MOFs based on this strategy have been used for carbon capture. For example, Zaworotko, Eddaoudi and co-workers used moisture-stable ultramicroporous frameworks (SIFSIX-2-Cu-i and SIFSIX-3-Zn) to realize an efficient and selective CO₂ capture (Fig. 33a). Due to the matched pore size (5.15 Å and 3.84 Å) and strong polar F⁻ anions, both of them could be used to effectively and selectively capture CO₂ at a relatively low CO₂ partial pressure (0.1 bar) (Fig. 33c and d).^213,214 Notably, SIFSIX-2-Cu-i could maintain its CO₂ adsorption capacity in the presence of water vapour (Fig. 33b). Such excellent CO₂ adsorption performance makes SIFSIX-2-Cu-i a promising absorbent for capturing CO₂ from flue gas.

Fig. 33 (a) The structure of SIFSIX-2-Cu-i and SIFSIX-3-Zn. (b) PXRD patterns of SIFSIX-2-Cu-i after breakthrough experiments, high-pressure absorption, and water adsorption. (c) and (d) The CO₂ sorption isotherms of SIFSIX-3-Zn (c) and SIFSIX-2-Cu-i at different temperatures. Reproduced from ref. 213 and 214 with permission from Springer Nature, Copyright 2015.

To further enhance CO₂ capture, Chen et al. synthesized another ultramicroporous MOF (UTSA-120) using 3, 6-di(4-pyridyl)-1,2,4,5-tetrazine ligand, which had a similar pore to SIFSIX-2-Cu-i but with abundant N atoms.²¹⁵ Compared with SIFSIX-2-Cu-i, UTSA-120 not only displayed a better CO₂ sorption capacity (3.56 mmol g⁻¹ at 296 K) at a low pressure (0.15 bar), but also exhibited a higher CO₂/N₂ adsorption selectivity. Notably, the IAST selectivity of UTSA-120 for simulated flue gas was 4 times higher than that of SIFSIX-2-Cu-i. Such an enhanced CO₂ sorption capacity at a low pressure was thought to have resulted from the high density of nitrogen sites. Even after an exposure of the UTSA-120 sample to air for 1 month, its carbon capture performance could be completely maintained.

As mentioned previously, the crystallinity and stability of MOFs can be optimized using self-assembling ligands containing both soft and hard coordination atoms with soft metal ions. Based on this strategy, many water-stable MOFs have been developed and used for CO₂ capture. For instance, Zaworotko, Zhang and co-workers prepared a water-stable MOF (Qc-5-Cu-sql-β) based on quinoline-5-carboxylic acid (HQc) and Cu(II) salts,²¹⁶ which had a very small pore that could match with CO₂ very well (3.3 Å). Notably, Qc-5-Cu-sql-β could maintain its structural integrity not only in water for 21 days but also in air with 75% RH for 14 days. Due to the well-matched pores, Qc-5-Cu-sql-β could be used to realize the molecular sieving of CO₂ with a very high IAST selectivity of 40 000. Dynamic breakthrough experiments further demonstrated that the CO₂ separation capacity of Qc-5-Cu-sql-β could also be maintained even at 75% RH.

Based on the H₂BTTA ligand containing both N and O atoms, our group also realized the controlled construction of an acid/base-resistant MOF (FJI-H14) (Fig. 12g) and its efficient carbon capture.¹³⁶ FJI-H14 exhibited a very high CO₂ adsorption capacity of 6.52 mmol g⁻¹ while only with a relatively low adsorption enthalpy (the Q_st at a low coverage is 26.6 kJ mol⁻¹) (Fig. 34a and b). Its IAST selectivity for a CO₂/N₂ (15 : 85) mixture was calculated to be 51 at 298 K and 1 bar (Fig. 34c). Even after 5 cycles, FJI-H14 still retained almost 100% adsorption capacity, suggesting its excellent CO₂ tolerance (Fig. 34d). Mechanistic studies indicated that such excellent CO₂ adsorption performance (e.g., very high adsorption capacity and high adsorption selectivity) came from the synergy of the confined pores and high density of open metal sites.

Fig. 34 (a) The CO₂ and N₂ adsorption isotherms for FJI-H14 at 298 K. (b) The isosteric heat of CO₂ adsorption (Q_st) for FJI-H14. (c) The IAST selectivity of FJI-H14 for CO₂/N₂ (15 : 85). (d) 5 Cycles of CO₂ adsorption for FJI-H14 at 298 K. Reproduced from ref. 136 with permission from Springer Nature, Copyright 2017.

Using 1,2,4-triazolate, oxalate, and Zn(II) as coordination units, Shimizu and co-workers prepared another highly stable MOF ([Zn₂(1,2,4-triazolate)₂(oxalate)], CALF-20) (Fig. 35a).²¹⁷ Even after being exposed to steam at 150 °C for a week, CALF-20 could well maintain its structure. The uptake of CALF-20 for CO₂ was up to 4.07 mmol g⁻¹ at 1.2 bar and 293 K, and its IAST selectivity for a CO₂/N₂ mixture (10 : 90) was calculated to be 230 (Fig. 35b). Such excellent CO₂ adsorption capacity could also be retained in the presence of water vapor due to different adsorption sites of CO₂ and H₂O. Furthermore, CALF-20 was resistant to wet acid gases and could be reused (>450 000 cycles) without the loss of adsorption capacity. In addition, the kg-scale of CALF-20 could be mildly prepared. All of these make CAL-20 a potential adsorbent for decarbonization in power plants.

Fig. 35 (a) The structure of CALF-20. (b) The CO₂ and N₂ isotherms of CALF-20 at different temperatures. Reproduced from ref. 217 with permission from the Advancement of Science, Copyright 2021.

Active sites (e.g., amines and hydroxides) that can strongly interact with CO₂ are often also able to coordinate with metal ions, so it is difficult to directly synthesize MOFs containing these active sites. By introducing these functional groups into water-stable MOFs, CO₂ adsorption can be promoted while also maintaining their stability. For example, through introducing ethylenediamine (en) into a water-stable MOF (CuBTTri) constructed from soft N-containing ligands, Long and co-workers successfully prepared an en-functionalized MOF (en-CuBTTri) and realized an enhanced CO₂ capture.¹³⁰ After ethylenediamine grafting, the selectivity of CO₂ over N₂ increased from 21 to 25 at 1 bar; and the adsorption enthalpy of CO₂ significantly rose from 21 kJ mol⁻¹ to 90 kJ mol⁻¹, especially at extremely low loadings. Such enhanced adsorption performance could be attributed to the affinity of alkylamine for carbon dioxide. To further improve CO₂ adsorption capacity, N,N′-dimethylethylenediamine (mmen) was also introduced into the framework of CuBTTri by Long's group.²¹⁸ In comparison to unmodified CuBTTri, modified mmen-CuBTTri was able to adsorb 4.2 mmol g⁻¹ of CO₂ at 298 K and 1 bar, showing an increase of 15% in adsorption capacity. According to ideal adsorbed solution theory, mmen-CuBTTri exhibited a relatively high selectivity of 327 for the CO₂/N₂ gas mixture (15 : 85) at 298 K. The extremely high Q_st for CO₂ (96 kJ mol⁻¹ at zero coverage) was considered to be the main drive for the enhanced adsorption capacity and adsorption selectivity. Furthermore, even after 72 cycles of use, mmen-CuBTTri could also retain its adsorption capacity for CO₂. Besides Long's group, Hong and co-workers also reported several amine-functionalized MOFs, which exhibited enhanced CO₂ adsorption capacity.^219,220

Through introducing a hydroxide group into water-resistant MOFs (MAF-X25/27) based on N-containing ligands,²²¹ Chen's group also realized enhanced CO₂ adsorption by synthesizing two hydroxide-functionalized isostructural frameworks [Mn^IIMn^III(OH)Cl₂(bbta)] (MAF-X25ox, 1′) and [Co^IICo^III(OH)Cl₂(bbta)] (MAF-X27ox, 2′).²⁰⁶ In comparison to unmodified precursors, their CO₂ adsorption capacities at 1 bar and 298 K increased from 5.4 and 4.2 mmol g⁻¹ to 7.1 and 6.7 mmol g⁻¹, respectively. At 0.15 bar and 298 K, the adsorption capacity of 1′ and 2′ was 410% and 540% larger than that of their precursors, reaching 4.08 and 4.06 mmol g⁻¹, respectively. Moreover, the CO₂/N₂ selectivity of 1′ and 2′ at 298 K was calculated to be 250 and 264, 8.6 and 9.2 times higher than that of their precursors, respectively. The significant increase in Q_st (from 38 and 28 kJ mol⁻¹ to 99 and 110 kJ mol⁻¹) demonstrated the ultrahigh CO₂ affinity of 1′ and 2′. Notably, the breakthrough curves of 2′ were nearly the same in both dry and high-humidity environments, while its precursor almost could not capture CO₂ from the wet mixture of CO₂ and N₂. Based on a hydroxide-functionalized MOF (KHCO₃ CD-MOF), Milner and co-workers also realized the enhancement of CO₂ adsorption capacity.²⁰⁵

In addition to enhancing the kinetic water stability of MOFs, hydrophobic engineering can also be used to improve their CO₂ capture performance. For instance, Wang's group successfully realized the improvement of water stability and CO₂ adsorption by introducing hydrophobic polynaphthylene (PN) into MOF-5 (Fig. 36a).²²² In contrast to water-sensitive MOF-5, hydrophobically modified PN@MOF-5 could retain its crystallinity and porosity upon exposure to humid air (40% RH) for 40 h. Due to the smaller pores and the hydrophobic aromatic groups, PN@MOF-5 exhibited significantly enhanced CO₂ capture performance (Fig. 36b). Compared with the original MOF-5, the adsorption capacity was doubled and the adsorption selectivity for CO₂/N₂ (14 : 86) at 1 bar and 273 K increased by 23-fold (Fig. 36c). Such excellent CO₂ adsorption performance could be largely maintained at 65% RH.

Fig. 36 (a) Introducing hydrophobic polynaphthylene (PN) into MOF-5 through the polymerization of DEB. (b) The CO₂ adsorption isotherms of MOF-5 and PN@MOF-5 at different temperatures. (c) The IAST selectivity of MOF-5 and PN@MOF-5 for a CO₂/N₂ mixture (14 : 86) at 273 K. Reproduced from ref. 222 with permission from the American Chemical Society, Copyright 2016.

To achieve efficient decarbonization of flue gas, many water-stable MOFs based on different synthetic strategies were prepared. Water-stable MOFs based on the Hard–Soft-Acid–Base strategy can not only be directly used for CO₂ adsorption (e.g., ALF), but also be used as potential precursors for PSM to achieve more efficient CO₂ capture (e.g., MOF 808 and CuBTTri). In addition, molecular sieving of CO₂ can be achieved by using MOFs with matched pores (e.g., SIFSIX-2-Cu-I and UTSA-120), exhibiting very high adsorption selectivity but relatively low adsorption enthalpy. The self-assembly of ligands containing both N and O atoms with soft metals is beneficial for constructing MOFs with high performance, stability, and economy, such as CLAF-20. It is worth mentioning that CLAF-20 can be directly used for the decarbonization of flue gas and can be also mass-produced by BASF, making it a promising industrial CO₂ adsorbent.

3.4.2. Carbon dioxide capture from air. Compared with flue gas, the CO₂ concentration in the air is much lower. Currently, the concentration of CO₂ in the atmosphere is about 400 ppm while that in some confined spaces may reach 1000–10000 ppm. Therefore, developing MOFs that can capture trace CO₂ is more challenging than MOFs for flue gas. As far as we know, only limited MOFs that have very effective active sites (e.g., amino groups and hydroxide groups) or highly matched pores for CO₂ can be used to capture trace CO₂, and several water-stable MOFs with excellent trace CO₂ capture performance are covered in this session (Table 3).^{191,192,207,214,219,223–232}

Table 3 The performances of some representative MOFs for carbon dioxide capture from ambient air

MOFs	Stability	CO2 uptake at different pressures (mmol g^−1)		Q st (kJ mol^−1)	Temperature (K)	Ref.
		390 ppm	10 000 ppm
a 400 ppm. b 500 ppm. c 1000 ppm.
NJU-Bai51	pH = 1–12	0.13	—	33.3	298	269
NJU-Bai53	pH = 2–11	0.64^a	—	88.3	298	233
[Zn(ZnOH)4(bibta)3]	Moisture-stable	2.2	—	71	298	207
SIFSIX-3-Cu	Moisture-stable (74% RH)	1.24^a	2.34	54	298	214
dptz-CuTiF6	Moisture-stable	—	2.1	38.2	298	192
TIFSIX-3-Co	Moisture-stable	1.05^a	2.63	—	298	228
NbOFFIVE-1-Ni	Moisture-stable (95% RH)	1.30^a	∼2.15	54	298	191
TIFSIX-3-Ni	Moisture-stable (75% RH)	1.12^a	1.75	50.9	298	229
SIFSIX-18-Ni-β	Moisture-stable (90% RH)	0.4^b	2.2	52	298	230
SIFSIX-3-Ni	Moisture-stable (75% RH)	0.35^a	1.9	45	298	231
GeFSIX-3-Ni	Moisture-stable	1.07^a	2.41	55.5	298	270
FJI-H38	pH = 2–12	—	2.33	27	298	190
Mg2(dobdc)(N2H4)1.8	Moisture-stable (82% RH)	3.89	—	118	298	223
en-Mg2(dobpdc)	Moisture-stable (100% RH)	2.83		49–51	298	219
ED-Mg-MOF-74	Moisture-stable	1.5	—	—	298	268
epn-MOF80@SBS	Moisture-stable (60% RH)	—	2.8^c	67	298	178

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Based on hard metals/ligands, Bai's group successfully synthesised two isostructural MOFs, [Fe₃O(TPBTM⁶⁻)(Cl)(H₂O)₂]_n (NJU-Bai52) and [Sc₃O(TPBTM⁶⁻)(OH)(H₂O)₂]_n(NJU-Bai53) (TPBTM = N,N′,N′′-tris(isophthalyl)-1,3,5-benzenetricarboxamide) (Fig. 37a).²³³ Due to the strong Fe–O/Sc–O units, both of them could maintain their structural integrity in H₂O for two months. Notably, NJU-Bai53 could not only preserve its framework in aqueous solutions (pH = 2 to 11) but also maintain its porosity after water treatment (Fig. 37b). Due to the presence of monodentate hydroxide, NJI-Bai53 displayed higher CO₂ adsorption capacity than NJI-Bai52. At 298 K and 0.4 mbar, it could adsorb 0.64 mmol g⁻¹ of CO₂, which was 50 times higher than that of NJU-Bai52 (0.013 mmol g⁻¹) (Fig. 37c). At zero-coverage, the adsorption enthalpy of NJU-Bai53 for CO₂ was up to 88.3 kJ mol⁻¹, demonstrating the presence of strong chemisorption interactions (Fig. 37d). According to the simple selectivity equation S = (q₁/q₂)/(p₁/p₂), the CO₂/N₂ (0.4/800) selectivity of NJU-Bai53 at 298 K was calculated to be 6397. All of these make NJU-Bai53 a potential adsorbent for CO₂ capture from the air.

Fig. 37 (a) The structure of NJU-Bai52 and NJU-Bai53. (b) The N₂ sorption isotherms of NJU-Bai53 (77 K) before and after water treatment. (c) The CO₂ adsorption isotherms of NJU-Bai52 and NJU-Bai53 at 298 K. (d) The adsorption enthalpy of NJU-Bai52 and NJU-Bai53 for CO₂. Reproduced from ref. 233 with permission from the American Chemical Society, Copyright 2019.

Besides hard metals/ligands, soft metals/ligands are also widely used to construct water/moisture MOFs for direct air capture. For instance, Eddaoudi's group prepared a moisture-stable ultramicroporous MOF (SIFSIX-3-Cu) based on the pyrazine ligand and CuSiF₆.²¹⁴ SIFSIX-3-Cu had not only very narrow pores (3.5 Å) suitable for CO₂ but also high-density F⁻ anions that could form strong interactions with CO₂. Due to its polar ultramicroporous pores, SIFSIX-3-Cu displayed not only a very high trace CO₂ adsorption capacity (1.24 mmol g⁻¹ at 0.4 mbar and 298 K) but also extremely high adsorption selectivity (10 500 for CO₂/N₂ (1/999)). Further breakthrough experiments demonstrated that such excellent adsorption performance could be retained under humid conditions (74% RH). By replacing (SiF₆)²⁻ with (NbOF₅)²⁻, Eddaoudi's group synthesized another hydrolytically stable ultramicroporous MOF (NbOFFIVE-1-Ni) with a similar but smaller pore (about 3.2 Å) than SIFSIX-3-Cu (Fig. 38a).¹⁹¹ Due to the more matched pore (pore size and pore environment). NbOFFIVE-1-Ni displayed a higher trace CO₂ adsorption capacity (1.3 mmol g⁻¹ at 0.4 mbar and 298 K) than SIFSIX-3-Cu, meaning that NbOFFIVE-1-Ni was the most efficient physical adsorbent for direct air capture (Fig. 38b). Single-crystal X-ray diffraction studies demonstrated that such excellent adsorption capacity resulted from the good match between the adsorbed CO₂ and the pores of NbOFFIVE-1-Ni (Fig. 38c).

Fig. 38 (a) The structure of NbOFFIVE-1-Ni. (b) The CO₂ sorption isotherms of NbOFFIVE-1-Ni at different temperatures. (c) The CO₂ adsorption sites in NbOFFIVE-1-Ni. (d) The breakthrough curves with the mixed-gas CO₂/N₂ (1/99) at 1 bar and 298 K in both dry and humid environments. Reproduced from ref. 191 with permission from the American Chemical Society, Copyright 2016.

Furthermore, the adsorption capacity of NbOFFIVE-1-Ni for CO₂ could be maintained even at high humidity (Fig. 38d) and the adsorbed CO₂ could be completely desorbed at 328 K under vacuum. All of these make NbOFFIVE-1-Ni a promising adsorbent for capturing CO₂ in the atmosphere.

Using a methylated dipyrazolyl ligand (3,3′,5,5′-tetramethyl-1H,1′H-4,4′-bipyrazole) and NiSiF₆ as coordination units, Zaworotko's group realized the controlled construction of a more hydrophobic ultramicroporous framework (SIFSIX-18Ni-β).²³⁰ Due to its ultramicropores, SIFSIX-18-Ni-β also exhibited high adsorption capacity for trace CO₂ (e.g., 0.4 mmol g⁻¹ at 500 ppm and 2.2 mmol g⁻¹ at 10 000 ppm). More importantly, due to the introduction of hydrophobic methyl groups, SIFSIX-18-Ni-β displayed a very low H₂O uptake (1.6 mmol g⁻¹) even at 95% RH, which allowed SIFSIX-18-Ni-β to preferentially adsorb CO₂ from nitrogen and water in any case. This work not only provides a potential trace CO₂ adsorbent for confined spaces but also offers a useful strategy for developing efficient trace CO₂ adsorbents resisting water vapor.

Very recently, through the reaction of a soft N-containing ligand (3, 5-di(4H-1, 2, 4-triazol-4-yl) benzoic acid, HDTBA) with Co(II) salts, our group reported an acid/base-resistant ultramicroporous MOF (FJI-H38), which could capture trace CO₂ from humid air (Fig. 39a).¹⁹⁰ Due to the stable CoN₄O nodes, FJI-H38 could retain its framework in aqueous solutions with a pH of 2–12. It could capture trace CO₂ from air with the high adsorption capacity (0.57 mmol g⁻¹ at 0.5 mbar and 2.33 mmol g⁻¹ at 0.01 bar) and the very high adsorption selectivity (6469 at 0.5 mbar) but a relatively low adsorption enthalpy (27 kJ mol⁻¹) (Fig. 39b and c). Such outstanding CO₂ adsorption performance could be largely maintained even at high humidity (75 RH%) (Fig. 39d). Mechanistic studies demonstrated that this excellent adsorption performance came from the synergism of flexible frameworks, ultramicropores, and different adsorption sites of H₂O and CO₂. Furthermore, FJI-H38 could be prepared on a large scale and reused without losing adsorption capacity, making it a potential adsorbent for trace CO₂ in industry.

Fig. 39 (a) The structure of FJI-H38. (b) The CO₂ sorption isotherms of FJI-H38 at different temperatures. (c) The IAST selectivity of FJI-H38 for CO₂/N₂ at various partial pressures. (d) The breakthrough curves of FJI-H38 for CO₂/N₂ (1 : 99) under different humid conditions. Reproduced from ref. 190 with permission from Wiley, Copyright 2023.

By introducing amino groups into MOFs through pore engineering not only can their CO₂ capture capacity be improved, but their water resistance is also enhanced. For example, Chen, Zhang and co-workers introduced a very small amine molecule (N₂H₄) into the pores of Mg₂(dobdc), preparing a amine-functionalized MOF (Mg₂(dobdc)(N₂H₄)_1.8) (Fig. 40a).²²³ Due to the extremely high concentration of amine groups (7.08 mmol cm⁻³ or 6.01 mmol g⁻¹), the as-synthesized Mg₂(dobdc)(N₂H₄)_1.8 could capture 3.89 mmol g⁻¹ of CO₂ at 0.4 mbar and 298 K, demonstrating its excellent trace CO₂ adsorption capacity (Fig. 40b). Even at high humidity, such excellent CO₂ adsorption performance could also be retained (Fig. 40c), which might be because the grafted N₂H₄ was not substituted by H₂O. In addition to (Mg₂(dobdc)(N₂H₄)_1.8), there were several amine-functionalized MOFs (e.g., dmpn-Mg₂(dobpdc) and nmen-Mg₂(dobpdc)) that could realize efficient capture of trace CO₂.^202,226

Fig. 40 (a) Introducing N₂H₄ into pores of Mg₂(dobdc). (b) The CO₂ sorption isotherms of (Mg₂(dobdc)(N₂H₄)_1.8) at different temperatures. (c) The breakthrough curves of (Mg₂(dobdc)(N₂H₄)_1.8) for CO₂/N₂ (1 : 999) under different humid conditions. Reproduced from ref. 223 with permission from the Royal Society of Chemistry, Copyright 2016.

Hydrophobic engineering can be used to simultaneously improve the water stability and trace CO₂ adsorption of MOFs. For example, based on water-resistant Mg₂(dobpdc) hybrids after hydrophobic modification (Fig. 24), Hong's group realized enhanced trace CO₂ capture performance.¹⁷⁸ Especially, epn-MOF80@SBS demonstrated an impressive CO₂ adsorption of 2.8 mmol g⁻¹ at 1000 ppm. Under the conditions with 60% RH, it could maintain consistent CO₂ removal performance after over 10 cycles and even 7 days of exposure.

Through polymer coating, Zaworotko's group also improved the water resistance of SIFSIX-3-Ni and its adsorption for trace CO₂ adsorption (Fig. 41a). Compared with uncoated SIFSIX-3-Ni, the HPMCP-coated HPMCP/SIFSIX-3-Ni displayed a significantly enhanced water resistance. After coating, the water contact angle increased by 8 times, and the framework could be preserved after being exposed to moisture for a long time. In high humid environments, HPMCP/SIFSIX-3-Ni could also display excellent gas separation performance while SIFSIX-3-Ni could not (Fig. 41b). Such excellent CO₂ separation performance could be retained for at least 5 cycles (Fig. 41c), suggesting that it could be used to control indoor air quality.²⁵⁴

Fig. 41 (a) Coating HPMCP on SIFSIX-3-Ni. (b) The breakthrough curves of SIFSIX-3-Ni and HPMCP/SIFSIX-3 for CO₂/N₂ (1 : 99) under different humid conditions. (c) The HPMCP/SIFSIX-3-Ni recyclability tests for humid 1% CO₂/99% N₂ and 15% CO₂/85% N₂ gas mixtures (regeneration at 80 °C). Reproduced from ref. 254 with permission from the American Chemical Society, Copyright 2020.

Compared to flue gas decarbonization, there are very few water-stable MOFs available for direct air capture. These MOFs either have strong active sites (e.g., NJI-Bai53, and Mg₂(dobdc)(N₂H₄)_1.8) that can chemically adsorb CO₂ or have ultramicropores that match well with CO₂ (e.g., SIFSIX-3-Cu, NbOFFIVE-1-Ni, SIFSIX-18-Ni-β, and FJI-H38). Compared with chemisorption, physical adsorption based on matched pores has lower recovery energy and is more conducive to the recycling of adsorbents.²⁴⁴ It is worth mentioning that the capture capacities of SIFSIX-3-Cu, NbOFFIVE-1-Ni, and FJI-H38 exceed that of commercial zeolite 13X. Among these materials, FJI-H38 has lower adsorption enthalpy and better stability, indicating that it is a more practical adsorbent for trace CO₂ capture.

3.4.3. Carbon dioxide capture from crude natural gas. As mentioned previously, CO₂ from flue gas and the air can be captured efficiently and selectively through reasonably designed MOFs. Compared with CO₂, both N₂ and CH₄ have larger kinetic diameters but less polarity, which means that MOFs efficient at capturing CO₂ from flue gas can also be used to capture CO₂ from crude natural gas. While many MOFs that can selectively capture CO₂ from CH₄ have been reported, only those that have both water/wet stability and excellent CH₄ purification performance are discussed in this section (Table 4).^{189,191,213,214,216,232,241,255–264}

Table 4 The performances of some representative MOFs for purification of crude natural gas^a

MOFs	Stability	CO2 adsorption amount at 1 atm (mmol g^−1)	IAST selectivity at 1 bar (50/50) CO2/CH4	Q st (kJ mol^−1)	Temperature (K)	Ref.
a Calculated from Henry's law constant.
UiO-66-SO3Li-0.15	pH = 1–12	3.28	12.7	25.4	298	234
UiO-66-SO3Rb-1	pH = 1–12	0.54	54.2	35.0	298	234
UiO-66-SO3H-0.64	pH = 1–12	1.54	15.2	33.2	298	234
UiO-66-SO3Na-0.64	pH = 1–12	0.94	46.2	35.1	298	234
UiO-66-SO3K-1	pH = 1–12	0.91	18	31.9	298	234
UiO-66(Hf)	Water-stable	1.50	12	22.8	298	240
UiO-66(Hf)-(OH)2	pH = 1–10	4.06	30	28.4	298	240
UiO-66(Hf)-NH2	Water-stable	2.80	15	25.6	298	240
UiO-66(Hf)-(COOH)2	Water-stable	1.20	13	28.2	298	240
UiO-66(Hf)-(F)4	Water-stable	0.82	16	23.4	298	240
PCN-222	Water-stable	1.16	4.3	18.3	298	134
NJU-Bai52	Water-stable	5.3	13.5	44.2	298	233
NJU-Bai51	pH = 1–12	—	10.7	33.3	298	269
NJU-Bai50	pH = 1–12	—	4.4	26.6	298	269
CPM-231	Moisture-stable (75% RH)	6.77	2.65	20.4	298	258
ee-2-Mg2(dobpdc)	Water-stable	∼4.5	—	−65 ± 1	298	260
UTSA-280	Water-stable	3.0	—	42.9	298	267
[Zn3(OH)2(pzdc)(atz)]	pH = 2–12	2.1	1.3 × 10^7	48	273	263
SIFSIX-3-Zn	Moisture-stable	2.55	231	45	298	214
UTSA-49	Moisture-stable	3.08	33.7	27	298	273
UTSA-120	Moisture-stable	5.0	100	27–31	296	215
UTSA-121	Moisture-stable	2.65	10	21.9	298	274
Qc-5-Cu-sql-β	Water-stable	2.16	3300	36	293	216
SIFSIX-2-Cu-i	Moisture-stable	5.41	33	31.9	298	213
Cu(MTABA)	Water-stable	1.5	9.9	38.4	298	276
Cu–F-pymo	pH = 1–14	1.19	>10^7	29.1	298	250
Co-btz-ht	pH = 1–12	2.16	63	38	273	125
Ni-btz-ht	pH = 1–12	4.24	54	∼38	273	125
GeFSIX-3-Co	Moisture-stable	2.65	160	39.1	298	270
GeFSIX-3-Ni	Moisture-stable	2.61	930	55.5	298	270
WOFOUR-1-Ni	pH = 2–13	2.32	26	65.5	298	277
NbOFFIVE-1-Ni	Moisture-stable (95% RH)	2.2	6528	54	273	191
CSMCRI-9	pH = 4–10	3.34	5.39	28.21	273	278
CSMCRI-3	pH = 5–10	2.4	11.7	27	273	279
MUF-16	Water-stable	2.13	6690	32.3	293	265
MUF-16(Ni)	Water-stable	2.14	1220	37.3	293	265
MUF-16(Mn)	Water-stable	2.25	470	36.6	293	265
IITKGP-6	Water-stable	1.67	5.1	23	295	280
[Co2(4,4′-bpy)(STBZ)]	Water-stable	1.63	9.9	29–31	298	281
FJI-H29	pH = 2–13	1.79	25	34	298	138
ZJNU-27	Water-stable	3.54	5.4	26.7	298	282
ZJNU-28	Water-stable	3.43	9.6	32.1	298	282
ZJNU-26	Water-stable	3.06	4.3	30.4	298	282
SNNU-52	pH = 3–11	2.64	7.3	28.3	298	257
SNNU-53	pH = 3–11	1.96	7.1	34.2	298	257
SNNU-55	pH = 3–11	1.49	10	33.6	298	257
SNNU-56	pH = 3–11	2.49	6.2	32.5	298	257
SNNU-54	Water-stable	2.31	7.5	38	298	257
SNNU-51	Water-stable	2.4	15.2	23.3	298	257
MOF-508b	Water-stable	1.85	3	19.2	303	283
Zn(BDC)(BPY)0.5	Water-stable	0.32	12.3	10.0–37.2	298	284
QI-Cu	Water-stable	4.56	6.1	23.75 ± 0.49	293	271
UTSA-16GO	Water-stable	3.62	114.4	42.1	296	259
MOF-505@GO	Moisture-stable	3.94	8.6	—	298	275

---

Like flue gas decarbonization, many water-stable MOFs based on hard metal/carboxylate ligands have been prepared and used to purify natural gas. For instance, based on the hard Ca(II) ion and squaric acid ligand, Chen's group also achieved the controlled synthesis of a water-stable MOF ([Ca(C₄ O₄)(H₂O)] UTSA-280) and its efficient purification of CH₄ (Fig. 42a).^264,265 Due to the strong Ca–O bonds, the as-synthesized UTSA-280 could remain its framework and porosity even after being immersed in water for 1 month. Due to its matched pore, UTSA-280 exhibited a relatively high CO₂ adsorption capacity (3.0 mmol g⁻¹) but almost no CH₄ adsorption (Fig. 42b). Dynamic breakthrough experiments demonstrated that UTSA-280 could capture 1.918 mmol g⁻¹ of CO₂ from a CO₂/CH₄ (50/50) mixture at 298 K, and such excellent separation performance could be completely retained for over 5 cycles (Fig. 42c and d). All of these make UTSA-280 a potential adsorbent for natural gas purification.

Fig. 42 (a) The structure of UTSA-280. (b) The CO₂/CH₄ sorption isotherms of UTSA-280 at 273 and 298 K. (c) and (d) The breakthrough curves and cycles of UTSA-280 for CO₂/CH₄ (1 : 1). Reproduced from ref. 245 with permission from Wiley, Copyright 2020.

Water-stable MOFs based on soft metals/ligands have also been widely prepared and used to purify crude natural gas. For example, Eddaoudi's group prepared a moisture stable MOF (AlFFIVE-1-Ni, KAUST-8) based on the pyrazine ligand and Ni(NO₃)₂, which could maintain its framework integrity even after being exposed to humid air with 95% RH (Fig. 43a).^266,267 Due to its specific pore environment, AlFFIVE-1-Ni could effectively and equally adsorb CO₂ and H₂S from gas mixtures containing CO₂, H₂S and CH₄ in a wide temperature range. This unusual co-adsorption of CO₂ and H₂S was further proved by temperature-dependent desorption (Fig. 43b). Moreover, such excellent separation performance could be retained not only in the presence of water vapour, but also during multiple cycling tests (Fig. 43c and d). This work represents a simple and efficient method for the simultaneous removal of acid gases (e.g., H₂S and CO₂) from crude natural gas.

Fig. 43 (a) The framework of AlFFIVE-1-Ni. (b) The breakthrough experiments using a CO₂/H₂S/CH₄:5/5/90 gas mixture on AlFFIVE-1-Ni at 298 K and 323 K (1 bar). (c) The breakthrough trials using a fresh sample under the same parameters (298 K) after optimal activation (105 °C) and after five breakthrough cycles. (d) The breakthrough experiments under humid conditions (65% RH). Reproduced from ref. 266 with permission from the American Association for the Advancement of Science, Copyright 2017. Reproduced from ref. 267 with permission from Springer Nature, Copyright 2018.

Based on soft Cu(II) ions and 5-fluoropyrimidin-2-olate ligands, Chen's group also achieved the synthesis and natural gas purification of a water-stable MOF (Cu–F-pymo). Due to the stable CuN₄ nodes, Cu–F-pymo could preserve its framework in aqueous solutions with a pH of 1–14.²⁵⁰ Due to its polar ultramicropores (about 3.3 Å), Cu–F-pymo could exclusively capture CO₂ over CH₄ and N₂. At 298 K and 1 atm, Cu–F-pymo exhibited a relatively high CO₂ adsorption capacity of 1.6 mmol g⁻¹ but very low adsorption capacities for both CH₄ (0.042 mmol g⁻¹) and N₂ (0.0075 mmol g⁻¹). Such unusual size-sieving separation performance could be further confirmed by their IAST selectivity for both CO₂/CH₄ (50 : 50) (>10⁷) and CO₂/N₂ (15 : 85) (>10⁷) mixtures. Such excellent separation performance could be maintained for at least 5 cycling tests.

Through selecting soft N-containing ligands (5-aminoisophthalic acid, H₂aip), Telfer's team prepared a series of water-stable microporous MOFs (M(Haip)₂; M = Co, (MUF-16); M = Ni, MUF-16(Ni); M = Mn, MUF-16(Mn)), all of which could be used to purify natural gas effectively (Fig. 44a).²⁶⁵ They could remain their frameworks after being immersed to water for two weeks. At 1 atm and 293 K, the CO₂ adsorption capacity of isomeric MUF-16, MUF-16(Ni) and MUF-16(Mn) was about 2.13 mmol g⁻¹, 2.13 mmol g⁻¹ and 2.25 mmol g⁻¹, respectively (Fig. 44b). Meanwhile, MUF-16 could only capture a very small amount of CH₄ (0.054 mmol g⁻¹) under the same conditions. Notably, the IAST selectivity of MUF-16 for the mixture of CO₂ and CH₄ (50/50) was up to 6690 at 293 K and 1 bar (Fig. 44c). Mechanistic analysis showed that such excellent selectivity came from the match of its pore and CO₂ (from size to polarity). Moreover, such an excellent CO₂ adsorption capacity could be maintained even after being soaked in water for 24 h and being exposed to humid air (80% RH) for 12 months (Fig. 44d).

Fig. 44 (a) The structure of MUF-16. (b) The CO₂ adsorption isotherms of MUF-16s at 293 K. (c) The IAST selectivity of MUF-16 for CO₂/CH₄ (50 : 50) at 293 K. (d) The CO₂ adsorption isotherms of MUF-16 at 293 K, after 4 consecutive sorption cycles, 12 months of exposure to air with 80% RH, and 24 hours of immersion in water. Reproduced from ref. 265 with permission from Springer Nature, Copyright 2021.

Based on another soft N-containing ligand (3,5-pyrazoledicarboxylic acid, H₃pzdc), Zhang and co-workers also realized the controllable preparation and natural gas purification of a water-stable MOF ([Zn₃(OH)₂(pzdc)(atz)], Hatz = 3-amino-1,2,4-trizole).²⁶³ It could retain its framework in aqueous solutions (pH 3–12) and boiling water. Due to its polar ultramicropores, the activated [Zn₃(OH)₂(pzdc)(atz)] displayed a relatively high CO₂ adsorption capacity of 2.1 mmol g⁻¹ but a very low adsorption capacity for CH₄ (0.056 mmol g⁻¹), leading to a very high single-component selectivity for CO₂/CH₄ (50/50) (1.3 × 10⁷). This means that [Zn₃(OH)₂(pzdc)(atz)] is a very rare adsorbent, which can realize molecular sieving of CO₂ from CO₂/CH₄.

As mentioned above, hydrophobic engineering could enhance the water stability and CO₂ capture performance of MOFs simultaneously. Through introducing hydrophobic groups, Zhao and co-worker also realized the improvement of the water stability and CH₄ purification capacity of NOTT-101.²⁷¹ They prepared a new MOF QI-Cu through incorporating methyl and methylol groups into a famous MOF NOTT-101. Notably, the modified QI-Cu could remain its framework in water for at least 1 month while the framework of NOTT-101 would completely collapse in water within 24 h. Due to the reduced pores and increased groups, QI-Cu exhibited better CO₂ adsorption capacity and higher CO₂/CH₄ adsorption selectivity than NOTT-101.

Compared to direct air capture, there are more water-stable MOFs available for purifying natural gas. Due to the similar polarity and size of CH₄ and N₂, MOFs can be used for decarbonization of flue gas, as well as for purifying natural gas. Purifying natural gas is mainly to remove CO₂ impurities from CH₄, so the adsorption selectivity of MOFs for CO₂/CH₄ (1 : 1) is the most important indicator, and many recently reported MOFs display very high CO₂/CH₄ adsorption selectivity (e.g., [Zn₃(OH)₂(pzdc)(atz)], MUF-16, UTSA-280, and AlFFIVE-1-Ni). Among these MOFs, UTSA-280 and MUF-16 can be prepared from inexpensive building blocks and are expected to be used as potential adsorbents for natural gas purification.

4. Summary and perspective

In order to develop practical MOFs for the real world, increased studies have been carried out on the design and synthesis of water-resistant MOFs, with a series of water-stable MOFs constructed reasonably. In this review, we first briefly introduce the definition of water stability of MOFs and how to characterize their stability. To discuss the water stability of MOFs more fully, we divide it into thermodynamic stability and kinetic stability, and then introduce a method to improve their thermodynamic and kinetic stability. According to Hard–Soft-Acid–Base theory, thermodynamically stable MOFs can be reasonably constructed through directly selecting metal ions and ligands with matched softness/hardness. Therefore, we first discuss the reported water-stable MOFs directly constructed from coordination units with matched hardness, including (1) water-stable MOFs based on hard metal ions and ligands and (2) water-stable MOFs based on soft metal ions and ligands. In addition, other specific strategies that can be used to improve the thermodynamic stability of MOFs (e.g., interlocking of frameworks and improving their rigidity) are also introduced. The kinetic stability of MOFs to water can be improved by reducing the kinetics of their hydrolysis reaction. Therefore, a series of strategies that can inhibit the kinetics of MOF hydrolysis, including the direct use of hydrophobic ligands, the introduction of hydrophobic groups into the pores of MOFs, the coating of hydrophobic films on the surface of MOFs, and the increase of steric hindrance around metal nodes, are discussed in detail.

Although great advance has been made in this area, there are several challenges that still need to be overcome. For example, because the currently used PXRD technique only reflects the state of MOFs before and after exposure to water, the interactions between MOFs and water remain unclear at the microscopic level. If such microscopic interactions between water and MOFs can be revealed by combining various in situ characterization and computational simulations,²⁷² the water-stable MOFs that meet different application requirements (e.g., carbon capture at high humidity and radioactive metal capture under strong acids) can be precisely designed and synthesized. In addition, balancing the water stability and activity of MOFs is another challenge. Metal ions and ligands with matched hardness can be used to directionally construct water-stable MOFs. However, the functions of MOFs are also achieved by introducing specific Lewis acid or base sites. Therefore, it is necessary to not only avoid the competitive active sites interfering with the assembly of MOFs, but also ensure that the active sites can be reasonably introduced into the channels of MOFs, which is a contradiction. The hydrophobic modification of MOFs may also affect the performance of MOFs, because the channels of MOFs can be changed. More importantly, if water-stable MOFs are to be widely used in industry, both their economy and processability need to be solved. For economy, first of all, cheap organic ligands and metal salts should be selected as the raw materials for a target MOF, and then large-scale synthesis should be realized through cheap reaction media (such as water and ethanol). Finally, the recovery of unreacted ligands/metals and reaction media should be achieved. In terms of processability, the controllable preparation of a target MOF with different sizes should be achieved first, and then appropriate devices (e.g., fixed bed, thin film, etc.) should be made according to different application scenarios.

The controllable preparation of water-stable MOFs further expands the application scenarios of MOFs. Due to the necessity and urgency of carbon capture, we only introduce the progress of water-resistant MOFs in carbon capture in this review. We begin with a brief introduction to the background of carbon capture and then discuss the applications of water-stable MOFs to flue gas decarbonization, direct air capture, and the purification of crude natural gas. Considering that the carbon capture capacity of some reported MOFs has been superior to that of commercial molecular sieves, it is very possible to use appropriate MOFs to achieve carbon capture in industry. Before realizing industrialization, the following problems need to be solved. (1) The target MOFs need to be tested and cycled for a long time under real working conditions; (2) it is also necessary to solve the processability and economy of the target MOFs.

In a word, more and more water-stable MOFs are synthesized, and the understanding of the relationship between the water stability and structure of MOFs is getting deeper and deeper, and the use of water-stable MOFs is more and more extensive. Notably, the carbon capture capacity of some reported MOFs has been better than that of commercial molecular sieves. So, we believe that MOFs will eventually be widely used as a type of porous material for carbon capture in the real world. We hope that this review could accelerate the achievement of this goal.

Author contributions

Conceptualization: M. Hong and Q. Chen; writing – original draft: C. Xiao; validation: J. Tian; writing – review & editing: M. Hong and Q. Chen.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We gratefully acknowledge the financial support from the National Key Research and Development Program of China (2021YFA1500400 and 2018YFA0704500) and the Natural Science Foundation of Fujian Province (2023J06046).

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