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REVIEW ARTICLE
Cite this: DOI: 10.1039/c5cs00837a
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Zr-based metal–organic frameworks: design,
synthesis, structure, and applications
Yan Bai,a Yibo Dou,a Lin-Hua Xie,a William Rutledge,b Jian-Rong Li*a and
Hong-Cai Zhoub
Among the large family of metal–organic frameworks (MOFs), Zr-based MOFs, which exhibit rich structure
types, outstanding stability, intriguing properties and functions, are foreseen as one of the most promising
MOF materials for practical applications. Although this specific type of MOF is still in its early stage of
development, significant progress has been made in recent years. Herein, advances in Zr-MOFs since 2008
are summarized and reviewed from three aspects: design and synthesis, structure, and applications. Four
synthesis strategies implemented in building and/or modifying Zr-MOFs as well as their scale-up preparation
under green and industrially feasible conditions are illustrated first. Zr-MOFs with various structural types are
Received 6th November 2015
then classified and discussed in terms of different Zr-based secondary building units and organic ligands.
DOI: 10.1039/c5cs00837a
Finally, applications of Zr-MOFs in catalysis, molecule adsorption and separation, drug delivery, and
fluorescence sensing, and as porous carriers are highlighted. Such a review based on a specific type of MOF
www.rsc.org/chemsocrev
is expected to provide guidance for the in-depth investigation of MOFs towards practical applications.
1. Introduction
As an emerging class of highly ordered crystalline porous materials,
metal–organic frameworks (MOFs) with various potential
a
Beijing Key Laboratory for Green Catalysis and Separation and Department of
Chemistry and Chemical Engineering, College of Environmental and Energy
Engineering, Beijing University of Technology, Beijing 100124, P. R. China.
E-mail: jrli@bjut.edu.cn
b
Department of Chemistry, Texas A&M University, College Station,
Texas 77842-3012, USA
applications have become a new research hotspot in chemistry
and materials science during the last few decades, which is
mainly attributed to their exceptionally high surface area,
tunable pores, as well as intriguing functionalities.1–5 Many
studies have demonstrated that MOFs perform excellently in
various applications when compared to traditional porous solid
materials, such as zeolites and carbon-based porous materials.
However, most of these results are believed to be conceptual,
because performances under ideal conditions and lack of
economical evaluation do not guarantee the applicability of
the materials. Unlike many industrially applied materials with
Yan Bai obtained her BS degree in
2013 from the Shandong University of Technology. She is currently
a graduate student at the Beijing
University of Technology and
pursues her study under the supervision of Prof. J.-R. Li. Her research
focuses on the design, synthesis,
and application of stable MOFs.
Yan Bai
This journal is © The Royal Society of Chemistry 2016
Yibo Dou
Yibo Dou obtained his BS degree
from the Beijing University of
Chemical Technology (BUCT) in
2009, and then continued his
postgraduate study under the supervision of Prof. X. Duan at BUCT. In
2013, as a visiting student, he
studied at the University of Oxford
under the supervision of Prof. D.
O’Hare. After obtaining his PhD
degree in 2015, he worked at the
Beijing University of Technology.
Currently, his main research topic
is the design and fabrication of
MOF-based composite functional
materials.
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Review Article
Chem Soc Rev
uniform covalent bonds, MOFs are constructed by the coordination bonding of metal ions and organic ligands. The poor stability
of MOFs derived from the reversible nature of coordination bonds
is commonly regarded as the major drawback for their practical
applications in most research fields.6–10
Much effort has been devoted to constructing MOFs with
improved stability.11–22 Among them, post-treatments or postsynthetic modifications have proven effective.23–26 For example,
IRMOF-1 (MOF-5) showed enhanced stability against moisture
after thermal modification and formation of an amorphous
carbon coating on the MOF particle surfaces, or after coating
the MOF surface with a thin hydrophobic polydimethysiloxane
(PDMS) layer via the thermal vapor deposition technique.25,26
However, the improved stability of the MOFs after posttreatment is always at the price of reduction of their micropore
surface areas or functionality. In addition, most experimental
steps or physical techniques in post-treatments are labor and
resource expensive. On the other hand, much research has
been invested to directly construct MOFs with inherent stability
in structure and composition. A remarkable step forward to
this end was the discovery of Zr6(m3-O)4(m3-OH)4(BDC)6 (UiO-66,
UiO stands for the University of Oslo) with 12-coordinated
Zr6(m3-O)4(m3-OH)4(CO2)12 clusters by Cavka et al.27 The structure
of UiO-66 exhibits unprecedented stability, especially hydrothermal
stability beyond most reported MOFs.28–31 From then on, the
discovery of MOFs based on Zr(IV) ions (Zr-MOFs), mostly Zr(IV)
carboxylates, ushered in new structural types being reported,
diverse strategies being adopted to modify their structures
and properties, and various functions and applications being
explored (Fig. 1).
It has been proposed that the stability of MOFs is governed
by multiple factors, including the pKa of ligands, the oxidation
state, reduction potential and ionic radius of metal ions, metalligand coordination geometry, hydrophobicity of the pore
Lin-Hua Xie obtained his PhD in
2010 from Sun Yat-Sen University
under the supervision of Prof.
X.-M. Chen. After postdoctoral
research at the Seoul National
University in Prof. M. P. Suh’s
group from 2010 to 2012, and at
the King Abdullah University of
Science and Technology in Prof.
Z. Lai’s group from 2012 to 2015,
he joined the Beijing University
of Technology as an associate
professor. His research interests
include the functionalization and
separation application of new
porous materials.
William Rutledge obtained his BS
degree in 2014 from Millersville
University, where he worked on
iridium N-heterocyclic carbenebased transfer hydrogenation
catalysts. In 2015, he joined Dr
Hong-Cai Zhou’s group as a PhD
student at Texas A&M University.
His research interest is focused on
the
development
of
MOF
materials for use in clean energy
technologies.
Lin-Hua Xie
Jian-Rong ‘‘Jeff’’ Li obtained his
PhD in 2005 from Nankai
University under the supervision
of X.-H. Bu. Until 2007, he was an
assistant professor at the same
University. From 2008 to 2009,
he was a postdoctoral research
associate,
first
at
Miami
University and later at Texas
A&M University in Prof. H.-C.
Zhou’s group; since 2010 he has
been an assistant research
scientist at the same university.
Jian-Rong Li
Since 2011, he has been a full
professor at the Beijing University of Technology. His research
interest focuses on porous materials for chemical engineering,
energy and environmental science.
Chem. Soc. Rev.
William Rutledge
Hong-Cai ‘‘Joe’’ Zhou obtained his
PhD in 2000 from Texas A&M
University under the supervision
of F. A. Cotton. After a postdoctoral stint at Harvard University with R. H. Holm, he joined
the faculty of Miami University,
Oxford, in 2002. He moved back
to Texas A&M University and
became a full professor in 2008.
He was promoted to a Davidson
Professor of Science in 2014 and a
Robert A. Welch Chair in
Hong-Cai Zhou
Chemistry in 2015. He was
recognized as a Thomson Reuters ‘‘Highly Cited Researcher’’ in both
2014 and 2015. His research focuses on the discovery of synthetic
methods to obtain robust framework materials with unique catalytic
activities or desirable properties for clean-energy-related applications.
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Fig. 1 The year-by-year increase of reported Zr-MOFs in the last eight
years (by SciFinder).
Review Article
determination, as well as scale-up preparation under green and
industrially feasible conditions are illustrated. Then, Zr-MOFs
with various compositions and structural types are classified and
discussed in terms of different Zr-based clusters and organic
ligands. Finally, the applications of Zr-MOFs in catalysis, molecule
adsorption and separation, drug delivery, and fluorescence sensing,
and as porous carriers are highlighted. Most of the reported
Zr-MOFs and their general information are summarized in
Table 1. We have tried our best to present a comprehensive
and up-to-date generalization; however, this topic is highly
active and extensively investigated. Therefore, some studies may
be overlooked and some papers will emerge in the final stages
of manuscript preparation/submission. Here we apologize in
advance to the authors whose studies are related but missed in
the review process.
2. Design and synthesis
surface, etc.11,14,32,33 In particular, metal–ligand bond strength is
believed to be crucial for determining the hydrothermal stability
of MOFs, and it is determined by the nature of both the metal
ion and the ligand. Thus, comparing the bonding strength of
MOFs with different metals and ligands is not a straightforward
affair. Nevertheless, it is well established that metal–ligand bond
interactions in MOFs with a given ligand are greatly affected by
the oxidation state and the ionic radius, or the charge density of
the metal ions. One key feature of Zr-MOFs is the high oxidation
state of Zr(IV) compared with M(I), M(II), and M(III)-based MOFs
(M stands for metal elements). Due to high charge density and
bond polarization, there is a strong affinity between Zr(IV) and
carboxylate O atoms in most carboxylate-based Zr-MOFs.34 This
is in line with Pearson’s hard/soft acid/base concept.35 Zr(IV) ions
and carboxylate ligands are considered hard acid and hard base,
respectively, and their coordination bonds are strong. As a result,
most Zr-MOFs are stable in organic solvents and water, and even
tolerable to acidic aqueous solution. However, it should be
mentioned that reported Zr-MOFs are still less stable in basic
aqueous solution, which can be well explained by natural bond
orbital (NBO) theory.33,36 The NBO charge of O atoms in OH
(1.403) is significantly larger than that of O atoms in the
carboxylate group (0.74), which implies that OH can form a
stronger bond with Zr(IV) than a carboxylate O atom, leading to
the decomposition of Zr-MOFs under basic conditions.
The improvement in the stability of MOFs can expand their
applicable fields, and make the research in some topics such as
catalysis, adsorption and separation, and biomedicine more
reliable and meaningful.28,37–43 Zirconium is widely distributed
in nature and is found in all biological systems. The rich
content and low toxicity of Zr further favor the development
and application of Zr-MOFs.44 As related studies have been
increasing rapidly, a need to review recent research advances of
Zr-MOFs has arisen. In this contribution, advances in Zr-MOFs
since 2008 are summarized based on three aspects: design
and synthesis, structure, and application. First, four synthesis
strategies used in building and/or modifying Zr-MOFs, particularly for growing single crystals for X-ray diffraction structure
This journal is © The Royal Society of Chemistry 2016
Due to the in situ formation of Zr-clusters and Zr–O coordination
bonds, the structural predication of Zr-MOFs faces a tremendous
challenge. Even so, some design strategies and synthetic methods
have been developed, which speed up the discovery of new
Zr-MOFs. In the following section, we have highlighted four
effective synthetic strategies, which are frequently implemented to construct Zr-MOFs, including modulated synthesis,
isoreticular expansion, topology-guided design, and postsynthetic modification.
2.1
Modulated synthesis
Despite being a focus of attention, a significant synthetic issue
existed in the early stages of Zr-MOF development. The crystallinity of synthesized Zr-MOFs was poor, and microcrystalline
powder samples were commonly obtained before introducing
the modulated synthesis strategy. This can be explained by the
high charge density of Zr(IV) which polarizes the Zr–O bond to
present covalent character, slowing down the ligand exchange
reaction between Zr clusters and carboxylate ligands.34 In this
case, it is unfavorable for defect repair during the crystallization process to form high quality crystals. In order to obtain
big phase-pure single crystals, the modulated synthesis strategy
was employed in Zr-MOF synthesis.
Modulated synthesis refers to regulating the coordination
equilibrium by introducing modulators with similar chemical
functionality as the organic ligands used to hinder the coordination
interaction between metal ions and the organic ligands.45 As a
result, the competitive reaction can modulate the rate of nucleation and crystal growth. Simultaneously the selective regulation
of coordination interactions in one of the coordination modes
makes it possible to control the morphology of resulting crystals,
leading to the anisotropic growth of MOF crystals.46 Moreover, it
has been proven that the coordination modulation method can
greatly improve the reproducibility of synthesis procedures and
tune crystal features such as size, morphology, and crystallinity.
In particular, it plays an important role in dominating the degree
of agglomeration/aggregation of crystals.
Chem. Soc. Rev.
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Review Article
Table 1
Summary of the most reported Zr-MOFs
Zr-MOF
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Chem Soc Rev
UiO-66
UiO-67
UiO-68
NU-1000
NU-1100
NU-1101
NU-1102
NU-1103
NU-1104
MOF-801
MOF-802
MOF-804
MOF-805
MOF-806
MOF-808
MOF-812
MOF-841
MOF-525
MOF-535
MOF-545
MOF-867
PCN-56
PCN-57
PCN-58
PCN-59
PCN-94
PCN-221
PCN-222
PCN-223
PCN-224
PCN-225
PCN-228
PCN-229
PCN-230
PCN-521
PCN-700
PCN-777
DUT-51
DUT-52
DUT-84
DUT-67
DUT-68
DUT-69
BUT-10
BUT-11
BUT-30
MIL-140A
MIL-140B
MIL-140C
MIL-140D
MIL-153
MIL-154
MIL-163
CPM-99
UMCM-309a
Zr-TTMC
Zr-BTB
Zr-ABDC
PIZOF
BPV-MOF
mBPV-MOF
mPT-MOF
ZrBTBP
UPG-1
UiO-66(AN)
UiO-66-1,4-Naph
Zr-ADC
Zr-DTDC
Zr-TCPS
Zr-BTDC
Chem. Soc. Rev.
Liganda
2
BDC
BPDC2
TPDC2
TBAPy4
PTBA4
Py-XP4
Por-PP4
Py-PTP4
Por-PTP4
FUM2
PZDC2
BDC-2OH2
NDC-2OH2
BPDC-2OH2
BTC3
MTB4
MTB4
TCPP4
XF4
TCPP4
BPYDC2
TPDC-2CH32
TPDC-4CH32
TPDC-2CH2N32
TPDC-4CH2N32
ETTC4
TCPP4
TCPP4
TCPP4
TCPP4
TCPP4
TCP-14
TCP-24
TCP-34
MTBC4
Me2BPDC2
TATB3
DTTDC2
2,6-NDC2
2,6-NDC2
TDC2
TDC2
TDC2
FDCA2
DTDAO2
EDDB2
BDC2
2,6-NDC2
BPDC2
Cl2ABDC2
pgal3
Hgal3, Hsal
TzGal6
TCBPP4
BTB3
TTMC2
BTB3
ABDC2
PEDC2
BPV2
BPHV2, BPV2
PT2, TPHN2
H3BTBP3
H4TTBMP2
AN2
1,4-NDC2
ADC2
DTDC2
TCPS4
BTDC2-
Zr cluster/core
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-OH)8
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)8
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr8(m4-O)6
Zr6(m3-OH)8
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-OH)8
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)6(m3-OH)2
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)8
Zr6(m3-O)6(m3-OH)2
Zr6(m3-O)6(m3-OH)2
Zr6(m3-O)4(m3-OH)42
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr(m3-O)3O4
Zr(m3-O)3O4
Zr(m3-O)3O4
Zr(m3-O)3O4
ZrO8
ZrO8
ZrO8
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
ZrO6
ZrO6
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Topology
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
csq, (4,8)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
fcu, 12-connected
bct, 10-connected,
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
spn, (3,6)-connected
ith, (4,12)-connected
flu, (4,8)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
csq, (4,8)-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
csq, (4,8)-connected
shp, (4,12)-connected
she, (4,6)-connected
sqc, (4,8)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
ftw, (4,12)-connected
flu, (4,8)-connected
bcu, 8-connected
spn, (3,6)-connected
reo, 8-connected
fcu, 12-connected
(4,4)IIb, 6-connected
reo, 8-connected
8-connected
bct, 10-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
—
—
—
—
—
—
—
ftw, (4,12)-connected
kgd, (3,6)-connected
fcu, 12-connected
kgd, (3,6)-connectedc
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
—
—
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
fcu, 12-connected
flu, (4,8)-connected
fcu, 12-connected
BET surface area (m2 g1)
b
1187
3000b
4170b
2320
4020
4422
4712
5646
5290
990
o20
1145
1230
2220
2060
2335
1390
2620
1120
2260
2646
3741
2572
2185
1279
3377
1936
2223
1600
2600
1902
4510
4619
4455
3411
1807
2008
2335
1399
637
1064
891
560
1848
1310
3940.6
415
460
670
701
—
—
90–170
1030
810
705
613
3000
2080
373
1207
3834
o10
410
627
—
2776
1200
1039
2207
Ref.
27
27
27
121
83
63
63
63
63
155
155
155
155
155
155
155
155
159
159
159
282 and 154
97
97
97
97
163
135
144
143
146
145
65
65
65
34
89
147
49
149
149
50
50
50
205
205
214
139
139
139
139
141
141
142
161
51
54
157
72
73
100
100
100
136
137
150
148
151
152
36
153
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Table 1
Zr-MOF
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Zr-BTBA
Zr-PTBA
MMPF-6
Zr-AP-1
Zr-AP-2
Zr-AP-3
Review Article
(continued)
Liganda
4
BTBA
PTBA4
TCPP4
AP-CH32
AP2
AP2
Zr cluster/core
Topology
BET surface area (m2 g1)
Ref.
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)4(m3-OH)4
Zr6(m3-O)8
Zr6(m3-OH)8
Zr6(m3-OH)8
Zr6(m3-OH)8
ftw, (4,12)-connected
ftw, (4,12)-connected
csq, (4,8)-connected
bcu, 8-connected
dia, 4-connected
bcu, 8-connected
4342
4116
2100
—
—
—
162
162
160
156
156
156
a
Ligands (see their structures in Scheme 1) are abbreviated as: BDC2 = terephthalate; BPDC2 = biphenyl-4,4 0 -dicarboxylate; TPDC2 = [1,1 0 :4 0 ,100 terphenyl]-4,400 -dicarboxylate; TBAPy4 = 1,3,6,8-tetrakis(p-benzoate)pyrene; Py-XP4 = 4 0 ,4 0 0 0 ,4 0 0 0 0 0 ,4 0 0 0 0 0 0 0 -(pyrene-1,3,6,8-tetrayl) tetrakis(2 0 ,5 0 Por-PP4
=
meso-tetrakis-(4-carboxylatebiphenyl)-porphyrin;
PTBA4
=
4-[2-[3,6,8-tris[2-(4dimethyl-[1,1 0 -biphenyl]-4-carboxylate);
carboxylatephenyl)-ethynyl]-pyren-1-yl]ethynyl]-benzoate; Py-PTP4 = 4,4 0 ,400 ,4 0 0 0 -((pyrene-1,3,6,8-tetrayltetrakis(benzene-4,1-diyl))tetrakis(ethyne4
2
2
= meso-tetrakis-(4-((phenyl)ethynyl)benzoate)porphyrin; FUM
= fumarate; PZDC
= 1H-pyrazole-3,52,1-diyl))tetrabenzoate; Por-PTP
dicarboxylate; BTC3 = benzene-1,3,5-tricarboxylate; MTB4 = 4,4 0 ,400 ,4 0 0 0 -methanetetrayltetrabenzoate; TCPP4 = meso-tetrakis(4-carboxylatephenyl)porphyrin; XF4 = 4,4 0 -((1E,10 E)-(2,5-bis((4-carboxylatephenyl)ethynyl)-1,4-phenylene)bis(ethene-2,1-diyl))dibenzoate; BPYDC2 = 2,20 -bipyridine5,5 0 -dicarboxylate; ABDC2 = 4,4-azobenzenedicarboxylate; TCBPP4 = tetrakis(4-carboxylatebiphenyl)porphyrin; ETTC4 = 40 ,400 ,40 0 0 ,40 0 0 0 -(ethene-1,1,2,2tetrayl)tetrabiphenyl-4-carboxylate; TDC2 = 2,5-thiophenedicarboxylate; MTBC4 = 4 0 ,400 ,40 0 0 ,4 0 0 0 0 -methanetetrayltetrabiphenyl-4-carboxylate; TATB3 =
4,4 0 ,400 -s-triazine-2,4,6-triyl-tribenzoate; DTTDC2 = dithieno[3,2-b;2 0 ,30 -d]-thiophene-2,6-dicarboxylate; 2,6-NDC2 = naphthalene-2,6-dicarboxylate;
FDCA2 = 9-fluorenone-2,7-dicarboxylate; DTDAO2 = dibenzo[b,d]thiophene-3,7-dicarboxylate 5,5-dioxide; EDDB2 = 4,4 0 -(ethyne-1,2-diyl)dibenzoate;
H3pgal = pyrogallol; H4gal = gallic acid; H2sal = salicylic acid; H6TzGal = 5,5 0 -(1,2,4,5-tetrazine-3,6-diyl)bis(benzene-1,2,3-triol); BTB3 = 5 0 -(4carboxyphenyl)[1,1 0 :300 ,100 -terphenyl]-4,400 -dicarboxylate; TTMC2 = (2E,4E)-hexa-2,4-dienedioate; ABDC2 = azobenzenedicarboxylate; H6BTBP =
1,3,5-tris(4-phosphonophenyl)benzene; H6TTBMP = 2,4,6-tris(4-(phosphonomethyl)phenyl)-1,3,5-triazine; PEDC2 = 4,4 0 -(1,4-phenylenebis(ethyne-2,1-diyl))dibenzoate; BPV2 = 5,5 0 -bis(carboxylateethenyl)-2,2 0 -bipyridine; BPHV2 = 4,4 0 -bis(carboxylateethenyl)-1,1 0 -biphenyl; TPHN2 =
4,4 0 -bis(carboxylatephenyl)-2-nitro-1,1 0 -biphenyl; ADC2 = 9,10-anthacenyl bis(benzoate); DTDC2 = 3,4-dimethylthieno[2,3-b]thiophene-2,5dicarboxylate; TCPS4 = tetrakis(4-carboxyphenyl) silane; BTBA4 = 4,4 0 ,400 ,4 0 0 0 -(biphenyl-3,3 0 ,5,5 0 -tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoate;
AP2 = 1,6-adipate. b Langmuir surface area. c The structure of Zr-BTB is constructed from two-dimensional (2D) - three-dimensional (3D)
interpenetration based on a 3,6-connected kgd net.
The first example of applying the modulated synthesis
strategy to prepare Zr-MOFs was reported by Schaate et al. in
2011.47 In this work they studied the effects of modulator
benzoic acid (HBC), acetic acid (AcOH) and water on the
formation of Zr-BDC (UiO-66), Zr-BDC-NH2, Zr-BPDC (UiO-67),
and Zr-TPDC-NH2 (UiO-68-NH2) crystals. It was found that the
synthesis of UiO-66 could be modulated by varying the amount
of HBC. As the concentration of modulator increased, intergrown
products of UiO-66 turned into individual and bigger crystals.
Similarly, the addition of HBC could also tune the crystal size
and morphology of Zr-BPDC and make the products highly reproducible. In addition, Schaate et al. also proved that water was
crucial for the formation of Zr-BDC-NH2 crystals. By this synthetic
control, they obtained large single crystals of Zr-TPDC-NH2 and
determined its structure by the single-crystal X-ray diffraction
technique, which represented the first single-crystal structure of
a Zr-MOF. Following this, Zr6(m3-O)4(m3-OH)4(FUM)6 (Zr-FUM)
was prepared by Wißmann et al. with formic acid as a modulator
and fumarate as a linker.48 Different from the modulators in
the previous work which slowed the crystallization rate of the
Zr-MOFs, it was found that formic acid accelerated the formation
of Zr-FUM crystals. A possible reason suggested by the authors
is that formic acid is a direct product of the decomposition
of the solvent N,N 0 -dimethylformamide (DMF) with water, and
the presence of formic acid affects the reaction equilibrium and
water content in the reaction system.
In situ formation of coordination complexes between the Zr(IV)
cation and monotopic carboxylic acid modulators was proposed as
the reason why bigger crystals can be obtained with an increasing
amount of modulator. The structures of these complexes should
be similar to secondary building units (SBUs) in Zr-MOFs.47 These
complexes could thus serve as intermediates in the construction of
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final Zr-MOFs, which are formed through an exchange reaction
between the modulators and used bridging ligands. So an increasing
number of modulators would decrease the chances of dicarboxylic
acid bridging with the intermediate and consequently inhibit the
formation of nuclei and nuclei growth, leading to the formation
of bigger MOF crystals.
Encouraged by previous studies, Bon et al. used bent-type
dicarboxylate organic linkers to synthesize a Zr-MOF in the presence
of HBC as a modulator.49 A new structure Zr6(m3-O)6(m3-OH)2(DTTDC)4(BC)2(DMF)6 (DUT-51) with large pores was obtained.
In this work the modulator also behaved as a structure-directing
agent and decreased the connectivity of SBUs from twelve to
eight in the final MOF. Later, with AcOH as a modulator, they
synthesized three new Zr-MOFs, Zr6(m3-O)6(m3-OH)2(TDC)4(AcO)2
(DUT-67), Zr6(m3-O)6(m3-OH)2(TDC)4.5(AcO) (DUT-68), and Zr6(m3-O)4(m3-OH)4(TDC)5(AcO)2 (DUT-69).50
Recently, Ma et al. reported a 6-connected Zr-MOF, Zr6(m3-O)4(m3-OH)4(BTB)2(OH)6(H2O)6 (UMCM-309a), with H3BTB as the
linker and HCl as a modulator.51 UMCM-309a features a stable
two-dimensional (2D) layered network and the structure remained
intact for over 4 months in aqueous HCl (1 M). Similarly, with
HBC and biphenyl-4-carboxylic acid as modulators, UMCM-309b
and UMCM-309c were synthesized with layered structures as in
UMCM-309a but had different inter-layer distances, because these
monocarboxylic acids substituted free hydroxyl sites in the Zr6
clusters and pointed to the space between layers. Consequently,
when the longer modulator biphenyl-4-carboxylic acid was used, the
layer space (14.8 Å) in UMCM-309c was greatly stretched compared
with that (7.04 Å) in UMCM-309a. Moreover, it was found that after
the removal of the modulator, the parent UMCM-309a was obtained.
The modulators thus played another role of tuning the whole
crystal structure of resulting Zr-MOFs.
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Early MOFs synthesized from the direct self-assembly of
discrete metal ions and organic ligands were often unstable
after the removal of guest molecules. Metal clusters were then
introduced into MOF synthesis to improve the stability of resulting
frameworks.52 Some pre-built polynuclear coordination complexes,
having identical structures and functions with SBUs of target
MOFs, were also utilized as so-called ‘‘precursors’’ to construct
MOFs.53 Guillerm et al. used a known complex Zr6(m3-O)4(m3-OH)4(OMc)12 (OMc = methacrylate) as a precursor to react
with dicarboxylic acid H2TTMC and synthesized Zr6(m3-O)4(m3-OH)4(TTMC)6 with the same topology of UiO-66 (Fig. 2).54
During the reaction process, the OMc ligands were substituted
by the dicarboxylate ligands, which may be also regarded as a
modulated synthesis.
Rationally selecting modulators could also control the number
and nature of defect sites of Zr-MOFs, which have been applied to
tune the pore properties of Zr-MOFs for adsorption applications,
as well as achieving open metal sites for catalysis. By varying
the concentration of modulator AcOH, Wu et al. demonstrated
that linker vacancies could be tuned systematically to achieve
pore volume modulation ranging from 0.44 to 1.0 cm3 g1, and
Brunauer–Emmett–Teller (BET) surface areas changing from
1000 to 1600 cm3 g1 of the obtained UiO-66 with different
concentrations of linker vacancies.55 On the other hand, Vermoortele
et al. used trifluoroacetic acid (TFA) as a modulator and HCl as
a crystallizing agent for the synthesis of UiO-66 with ligand
defects and open Zr sites.56 The resulting UiO-66 showed high
catalytic activity for Lewis acid catalytic reactions. It was also
suggested that HCl in this system displayed a double duty of
slowing down the hydrolysis of ZrCl4 and counteracting the
deprotonation of involved carboxylic acids. Consequently,
the presence of HCl favored the incorporation of TFA in the
final sample of UiO-66. Through thermal activation, a defected
UiO-66 with a large number of open Zr sites was thus generated
after the removal of TFA.
2.2
Isoreticular expansion
Two of the most desirable characteristics of MOFs are their
permanent porosity and ultrahigh surface areas. The realization of
MOFs with such characteristics, in turn, can significantly advance
the development of their applications such as in gas sorption and
storage.57–59 Isoreticular expansion is a strategy for enlarging pore
size, increasing surface areas, and/or tuning pore surface functionality for MOFs by propagating the structure of a prototypical
MOF structure. This particular method takes advantage of
experimental conditions of the original MOF to synthesize new
isoreticular frameworks, however with desired pore surface,
larger porosity and/or higher surface areas than the prototypical
structure through modifying the linkers. In this manner, predesigned building blocks are purposefully incorporated into
extended networks with targeted specific features.60–62
One of the most successful examples using the isoreticular
expansion strategy is the synthesis of a series of Zr-MOFs,
Zr6(m3-O)4(m3-OH)4(L1–4)3(NU-1101–1104, L1 = Py-XP4, L2 =
Por-PP4, L3 = Py-PTP4 and L4 = PorPTP4) by Wang et al.
(Fig. 3).63 Among these, Py-PTP4 and Por-PTP4 were obtained
Chem. Soc. Rev.
Chem Soc Rev
Fig. 2 Schematic representation of the modulated synthesis of Zr6(m3O)4(m3-OH)4(TTMC)6 (right) starting from the precursor Zr6(m3-O)4(m3OH)4(OMc)12, in which the OMc ligands were replaced by TTMC2 (left).
Reprinted with permission from ref. 54. Copyright 2010 The Royal Society
of Chemistry.
through inserting a triple bond in the arms of original tetratopic ligands Py-XP4 and Por-PP4, respectively. The geometric
surface areas of NU-1101 to NU-1104 calculated by using a
rolling probe method independent of BET theory64 are 4422,
4712, 5646, and 5290 m2 g1, respectively. It is worth mentioning that NU-1103 with Py-PTP4 as the linker has a pore volume
of 2.91 cm3 g1 and a BET surface area of 6650 m2 g1, being
the highest value reported to date for Zr-MOFs. Such high
surface areas render them attractive for energy gas storage,
such as H2 and CH4. Similarly, Liu et al. inserted a benzene
ring, and one and two triple-bond spacers in the arms of the
ligand to synthesize a series of extended porphyrinic linkers,
H4TCP-1, H4TCP-2, and H4TCP-3.65 Using them, a series of
Zr6(m3-O)4(m3-OH)4(TCP)4 from PCN-228 to -230 were obtained,
which have BET surface areas of 4510, 4619, and 4455 m2 g1,
and pore sizes of 25, 28, and 38 Å, respectively.
Note that although the surface area of MOFs can be increased
with the extension of the ligand length, the framework stability is
usually inversely correlated. Some MOFs synthesized by isoreticular
expansion with increased surface areas became unstable.66–70
For example, the Langmuir surface areas of UiO-66 and -67 are
1187 and 3000 m2 g1, respectively. However, the stability
became worse for UiO-67.29
Another phenomenon often encountered in isoreticular
expansion is network interpenetration, especially when using
a long ligand with small steric hindrance.71 Interpenetration of
networks always leads to a large reduction of surface area and
pore volume of the MOFs, which is not favorable for some
applications, such as gas storage, separation and catalysis
where large molecules are involved. For example, Peter Behrens’s
group synthesized three Zr-MOFs with three different ligands in
length. Among them, Zr-FUM was isostructural to UiO-66 with a
BET surface area of 856 m2 g1.48 Due to the short length of the
FUM2 ligand, it is not surprising that the surface area is smaller
than that of UiO-66 (1069 m2 g1). Later, they reported another
isostructural Zr-MOF, Zr6(m3-O)4(m3-OH)4(ABDC)6 (Zr-ABDC), with
a high surface area of 3000 m2 g1 and a large pore volume of
1.41 cm3 g1.72 With an even more elongated triphenylethynyl
dicarboxylate ligand PEDC-2OMe2, Zr6(m3-O)4(m3-OH)4(PEDC2OMe)4 (PIZOF-2) with a 2-fold interpenetrated structure of
the same topology was obtained.73 As a result, PIZOF-2 showed
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Fig. 3 Tetratopic organic linkers of Py-XP4, Por-PP4, Py-PTP4, and Por-PTP4 used for constructing NU-1101–1104 (top). The increasing dimensions
of the organic linkers result in the isoreticular expansion of the MOF structures, with surface areas of 4422, 4712, 5646, and 5290 m2 g1, respectively
(bottom). Reprinted with permission from ref. 63. Copyright 2015 American Chemical Society.
a BET surface area of about 1250 m2 g1, significantly lower than
Zr-ABDC (3000 m2 g1) regardless of its much longer ligand.
2.3
Topology-guided design
In order to avoid interpenetration in MOFs, some methods
such as changing synthesis conditions and increasing steric
hindrance of ligands have been adopted.71,74–77 However, the
precise control of these synthetic factors is difficult. Take into
consideration that interpenetration probability is closely
related to the framework topology. As a result, much attention
has been directed on MOF topological studies.78–81 Topologyguided design strategies were thus applied in the design and
synthesis of MOFs, usually based on the ‘‘non-interpenetrated
topological structure’’ as the ‘‘template’’.61 By this method,
Zr-MOFs with preconceived structures and desirable properties
have been designed with high synthetic accessibility. Here, for
clarity, some representative topological networks appearing in
Zr-MOFs are shown in Fig. 4.
Apparently, the topology-guided design of MOFs depends on
the judicious selection of metal-containing building units and
organic ligands. At the same time, the symmetrical complementary
is applied as the selection criteria for the synthesis of the desired
network as well. In this regard, Gomez-Gualdron et al. demonstrated that Zr-MOFs with csq, scu, or ftw topologies can be
constructed with a square-shaped tetratopic ligand and a Zr6
cluster, and the ftw topology Zr-MOF exhibits the highest surface
area and has the lowest propensity for structure catenation as
well.82 An ideal ftw Zr-MOF crystallizes in the Pm3% m space group
of the cubic crystal system, where the cube vertices are occupied
by 12-connected [Zr6(m3-O)4(m3-OH)4]12+ clusters, which are connected by planar tetratopic linkers across faces of the cubic unit
cell. It has been found that in order to achieve such a MOF,
two design points should be considered in the selection of
This journal is © The Royal Society of Chemistry 2016
organic linkers: (1) the ligand should display planar geometry
and (2) the relative dimensions of the linker should meet the
requirement of four carboxylate groups being in a rectangular
shape (close to a square).83 Hereafter, based on these selection
criteria Gutov et al. designed and incorporated a pyrene-based
elongated tetratopic ligand PTBA4 into a desired ftw net to
obtain Zr6(m3-O)4(m3-OH)4(PTBA)3 (NU-1100).83 This MOF
features two types of pores: (1) large pores located in the center
of the cubic unit cell and (2) small ones located at the edges of
the cubic unit cell.
It is clear that a fully occupied 12-connected Zr6(m3-O)4(m3-OH)4(CO2)12 cluster has the Oh symmetry and TCPP4 is a
4-connected linker with the D4h symmetry. These two nodes are
connected to each other and thus theoretically should lead to
an ftw network. However, the topology-guided design strategy
failed to realize the target for this system. The reason is that the
spatial orientation of the ligand is as important as its connectivity
and symmetry for construction of MOFs with specific topologies.
In fact, when the 12-connected Zr6 clusters and TCPP4 ligands
substitute corresponding nodes in the ftw network, the peripheral
phenyl rings have no choice but to revolve to the same plane with
the porphyrin center. However, this conformation is energically
unfavorable due to steric hindrance and thus was difficult to
realize. To solve this problem, Liu et al. introduced triple-bond
spacers between adjacent phenyl rings and carboxylate groups
to realize the controllable conformation.65 As a result, a series of
Zr-MOFs, from PCN-228 to -230, were successfully prepared by
using these ligands. The triple-bond spacers in this system not
only elongate the ligands but also alleviate the steric constrain,
which permits the free rotation of peripheral benzoates to adopt a
compatible direction for the Zr6 clusters in the ftw net.
Besides the ftw topology, the default and high symmetry
edge transitive network of the fluorite (flu) topology84–86 is also
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Fig. 5 (a) Schematic representation of the fluorite structure where the
F anions fill all the tetrahedral interstitial cavities; (b) the unoccupied
octahedral interstitial cavities (turquoise) in the fluorite structure; (c) the
structure of PCN-521 with the flu topology in which all the tetrahedral
interstitial cavities are occupied by the MTBC4 ligands; and (d) the
representation of an unoccupied octahedral cavity in PCN-521 with the
size of 20.5 20.5 37.4 Å. Reprinted with permission from ref. 34.
Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
In addition, the combination of a 12-connected Zr6 cluster and
a linear linker usually gives rise to the fcu topology. However, if
the two carboxylate groups of the linear ditopic linker are noncoplanar, it is possible that other topological nets form, such as a
bcu net. Following this topology-guided design strategy, Yuan
et al. introduced two methyl groups into the 2- and 2 0 -positions
of the BPDC2 ligand, leading to two carboxylate groups of the
resulting Me2BPDC2 ligand with an off-plane state.89 With this
ligand, a new Zr-MOF, Zr6(m3-O)4(m3-OH)4(OH)4(H2O)4(Me2-BPDC)4
(PCN-700), with a bcu network structure was obtained.
2.4
Fig. 4 Representative network topologies in reported Zr-MOFs.
intriguing for constructing Zr-MOFs with high permanent
porosity. The flu structure is conceived as cubic close packing of
Ca(II), in which the tetrahedral cavities are filled with F anions,
leaving the octahedral cavities unoccupied. For the flu topology, it
is impossible to translate in all directions without overlapping with
itself, which is favorable for avoiding interpenetration.87,88 Based
on these considerations, Zhang et al. designed and synthesized
Zr6(m3-OH)8(OH)8(MTBC)2 (PCN-521) by using a tetrahedral ligand
MTBC4.34 The structure of PCN-521 consists of cubic close packing of 8-connected Zr6(m3-OH)8 clusters, in which all the tetrahedral
interstitial cavities are occupied by the MTBC4 ligands (Fig. 5).
PCN-521 displayed a BET surface area of 3411 m2 g1, a pore size of
20.5 20.5 37.4 Å, and a void volume of 78.5%.
Chem. Soc. Rev.
Postsynthetic functionalization
Similar to conventional linker pre-functionalization, the functional groups of organic linkers and metal clusters in Zr-MOFs
are also tailorable based on postsynthetic functionalization. It
has been proved that the postsynthetic method is a versatile
tool to prepare topologically identical Zr-MOFs with diverse
functionalities and uncompromised structural stability.90,91
Covalent modification is the most common route of postsynthetic functionalization that endows MOFs with tailor-made
internal surfaces to meet the requirements for specific applications.92–96 Jiang et al. reported the preparation and covalent
modifications of a series of highly stable isoreticular Zr-MOFs,
Zr3O2(OH)2(TPDC-R)3 (PCN-56, R = 2CH3, PCN-57, R = 4CH3;
PCN-58, R = 2CH2N3, and PCN-59, R = 4CH2N3).97 In PCN-58
and -59 the accessible and reactive azide groups enable the
obtained MOFs to undergo a quantitative click reaction98 with
alkynes to form various triazole groups containing substituents
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Fig. 6 Schematic representation of the click reaction for the pore surface
engineering of PCN-58–59 with precise control over the composition,
density, and functionality by two steps. Reprinted with permission from
ref. 97. Copyright 2012 American Chemical Society.
on their pore surfaces (Fig. 6). It was revealed that the loading
of azide groups can be accurately tuned by varying the ratio of
ligands with and without azide groups during synthesis. Thus,
a variety of functional groups were anchored onto the pore
walls of obtained MOFs with precise control over loading,
density, and functionality. Therefore, the resulting materials
showed tunable CO2 selective adsorption ability over N2, being
potentially useful in CO2 capture.
Besides covalent modification, coordination modification of
ligands in postsynthetic metalation (PSM) was also adopted.
Toyao et al. reported the coordination modification in free
chelating sites of 2,2 0 -bipyridine in Zr6(m3-O)4(m3-OH)4(BPYDC)6
(Zr-MOF-BPYDC) with CuBr2.99 It was found that after loading
B3.3 wt% of CuBr2 to the MOF, 20.6% of BPYDC units could be
metalated with Cu(II). Another typical example was reported by
Manna et al. In their work, a series of bipyridyl- and phenanthrylbased Zr-MOFs, Zr6(m3-O)4(m3-OH)4(BPHV)6 (BPHV-MOF), Zr6(m3-O)4(m3-OH)4(BPHV)4(BPV)2 (mBPV-MOF), and Zr6(m3-O)4(m3-OH)4(TPHN)4(PT)2 (mPT-MOF), were prepared.100 Treatment of these
Zr-MOFs with different equivalences of [Ir(COD)(OMe)]2 in tetrahydrofuran (THF) afforded Ir-functionalized Zr-MOFs, BPV-MOF-Ir,
mBPV-MOF-Ir, and mPT-MOF-Ir. Inductively coupled plasma-mass
spectroscopy (ICP-MS) analyses of the Ir/Zr ratio revealed that the Ir
loadings could reach 65%, 16%, and 20% for the three MOFs,
respectively. Significantly, these Ir-functionalized Zr-MOFs showed
high catalytic activity for tandem hydrosilylation of aryl ketones and
aldehydes. Additionally, based on the postsynthetic metalation,
Manna et al. also treated Zr6(m3-O)4(m3-OH)4(BPYDC)6 (BPY-UiO)
with [Ir(COD)(OMe)]2 in THF to obtain BPY-UiO-Ir, and with
[Pd(CH3CN)4][BF4]2 in dimethyl sulfoxide (DMSO) to obtain BPYUiO-Pd, in which Zr/Ir in BPY-UiO-Ir and Zr/Pd ratios in BPYUiO-Pd reached 30 and 24%, respectively.101
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Amine-tagged MOFs can also be used to graft functional
groups.102–104 After the covalent modifications of amine groups, the
resulting MOFs are also well suited for the coordination modification. Rasero-Almansa et al. treated UiO-66-NH2 with aldehyde
in dichloromethane (CH2Cl2), followed by another treatment
with [IrCl(COD)]2, leading to quantitative conversion of chelating
Ir-functionalized Zr-MOF materials.105 Obviously, the presence
of functional group ‘‘tags’’ in parent frameworks is essential for
the subsequent covalent and coordination modifications.
It should be noted that not all Zr-MOFs contain functional
group ‘‘tags’’, or are endurable for covalent and coordination
modifications. In such cases, postsynthetic ligand exchange
(PSE) of parent MOFs is exploited as an alternative means to
introduce functionalized ligands into MOFs under mild conditions.
Han et al. have reviewed such substitution/exchange reactions
occurring in MOFs and metal–organic polyhedra (MOPs) in
detail.106 In this review, a strategy combining PSE and PSM is
highlighted. Based on this strategy, the linkers without functional groups can be exchanged by those with functional groups
in Zr-MOFs, and then the resulting MOFs are metallated. As a
result, various combinations of chelators and metal ions can be
introduced into the MOFs.
Catechol is one of the most common metal chelators applied
in synthesizing coordination complexes. Fei et al. obtained
Zr6(m3-O)4(m3-OH)4(CAT-BDC)6 (UiO-66-CAT, CAT-H2BDC =
2,3-dihydroxyterephthalic acid) via the PSE by exposing UiO66 into DMF/water solution of CAT-H2BDC.107,108 The highest
loading value of CAT-BDC could be up to 90% after two rounds
of the PSE process. UiO-66-FeCAT and UiO-66-CrCAT were then
obtained by metalizing UiO-66-CAT with Fe(ClO4)3 and K2CrO4,
respectively. The atomic ratios confirmed by energy-dispersed
X-ray spectroscopy (EDX) were 1.00 : 0.21 (Zr : Fe) and 1.00 : 0.04
(Zr : Cr) for UiO-66-FeCAT and UiO-66-CrCAT, respectively. In
addition, they also replaced the ligand in UiO-66 with thiocatecholate, which thus provided the metal-chelating motif for
PSM. As a result, UiO-66-PdTCAT (TCAT-H2BDC = 2,3-dimercaptoterephthalic acid) was obtained after the PSE and PSM processes.108 This functionalized Zr-MOF was demonstrated to be
a recyclable catalyst for the regioselective functionalization
of sp2 C–H bonds.
Incorporating multiple functionalities with synergistic effects
within one MOF is particularly interesting for its unique properties and specific applications.97,109–112 Different from the conventional one-pot synthesis approach with mixed linkers, Yuan et al.
developed sequential ligand installation (SLI) as a controllable
strategy to construct multivariate MOFs (MTV-MOFs), which could
precisely arrange the positions of functional groups (Fig. 7).89 In
this study, a prototype Zr-MOF, PCN-700, built from 8-connected
Zr6(m3-O)4(m3-OH)4(OH)4(H2O)4 clusters served as the platform.
After removing terminal OH/H2O ligands on the adjacent Zr6
clusters, two types of natural ‘‘pockets’’ with different sizes were
formed, 16.4 Å for pocket A and 7.0 Å for pocket B, which allowed
for sequential installation of two types of linkers with different
lengths. Then, BDC2 and TPDC-Me22 ligands were selected
based on the pocket size. Next, the SLI process was performed:
PCN-700 crystals were exposed to solutions of H2BDC and
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Fig. 8 Schematic representation of Zr(IV) substitution in UiO-66 by Ti(IV)
and Hf(IV) for the synthesis of UiO-66(Zr/Ti) and UiO-66(Zr/Hf).117
Fig. 7 Schematic representation for synthesis of PCN-701–703 based on
the sequential ligand installation in PCN-700.89
H2TPDC-Me2 in DMF at enhanced temperature, respectively,
resulting in mixed-ligand Zr-MOFs, Zr6(m3-O)4(m3-OH)2(OH)4(BPDC-Me2)4(BDC) (PCN-701) and Zr6(m3-O)4(m3-OH)2(OH)4(BPDC-Me2)4(TPDC-Me2) (PCN-702). Then the resultant PCN-701
was soaked in a DMF solution of H2TPDC-Me2 sequentially to
obtain Zr6(m3-O)5(m3-OH)3(OH)2(BPDC-Me2)4(BDC)(TPDC-Me2)0.5
(PCN-703). Note that after the incorporation of TPDC-Me22, the
pocket is elongated to 8.2 Å which is slightly longer than the
size of H2BDC (6.9 Å). Thus PCN-703 could not be obtained from
PCN-702. Similarly, based on this synthetic strategy, functionalized
linkers of 2-amino-1,4-benzenedicarboxylate (BDC-NH22) and
2 0 ,5 0 -dimethoxyterphenyl-4,400 -dicarboxylate
(TPDC-(CH3O)22)
were also introduced successfully into MTV-MOFs to obtain
Zr6(m3-O)5(m3-OH)3(OH)2 (BPDC-Me2)4(BDC-NH2)(TPDC-(CH3O)2)0.5
(PCN-704).
Apart from the ligand substitution/exchange, the metal nodes
in Zr-MOFs can be exchanged through post-synthetic modifications. Ti(IV) is considered as an excellent candidate for MOF
construction because of its low toxicity, high redox activity, and
extraordinary photocatalytic properties. However, due to a strong
tendency toward the hydrolysis of Ti(IV) ions, directly synthesizing
Ti-based MOFs is quite challenging, and only a few examples
have been reported so far.113–116 Alternately, Kim et al. obtained
Ti-containing UiO-66 MOFs through the central metal ion
substitution approach (Fig. 8).117 Experimentally, UiO-66 samples
were soaked in DMF solutions of three different Ti(IV) salts,
TiCp2Cl2, TiCl4(THF)2, and TiBr4 for 5 days at 85 1C, respectively.
The Ti-substituted MOF UiO-66(Zr/Ti) was obtained with different
exchange degrees, which relied on the used Ti(IV) salts. Among
them, TiCl4(THF)2 showed the highest exchange level, with about
38 wt% of Zr(IV) ions being substituted by Ti(IV) ions. Using a
similar experimental procedure, UiO-66(Zr/Hf) was obtained
Chem. Soc. Rev.
after UiO-66 was treated with HfCl4. But, only about 20% of Hf(IV)
ions were introduced into UiO-66 even at an enhanced temperature.
On the other hand, substitution of coordinated guest molecules represents another method for the pore modification of
MOFs. Deria et al. used mesoporous Zr6(m3-OH)8(OH)8(TBAPy)2
(NU-1000) as the platform to prepare a series of functionalized
Zr-MOFs (Fig. 9). NU-1000 contains octahedral [Zr6(m3-OH)8(OH)8]8+
nodes, where eight of twelve octahedral edges are connected
with carboxylate groups of TBAPy4 ligands, while the remaining
coordination sites of Zr(IV) ions are occupied by eight terminal
–OH groups. First, based on a solvent-assisted ligand incorporation (SALI) method, these –OH groups were substituted by a
series of perfluoroalkyl carboxylate entities with varying chain
length by exposing a NU-1000 sample into a DMF solution of
respective fluoroalkyl carboxylic acids.118 It was found that about
3.4–4.0 perfluoroalkyl carboxylates per Zr6 node could be incorporated within NU-1000. As a result, these modified MOFs showed
enhanced CO2 capture capacities due to the presence of C–F dipoles.
Then, they also incorporated other carboxylate-based alkyl and
aromatic functional groups into NU-1000 by a similar procedure.119
Chemical reactions such as click chemistry, imine condensation,
and pyridine quaternization were then performed for the further
modification of these materials.
Similarly, NU-1000 was modified by phenylphosphonate
(PPA) through the SALI method.120 It was found that the
phosphonate displaced OH/H2O ligands and meanwhile chelated
with Zr(IV) atoms of the MOF through phosphonate O atoms.
Moreover, they also demonstrated that the incorporation of
PPA rendered parent NU-1000 more renitent to hydroxide
attack and framework dissolution. Except for the above SALI
method, reactive incorporation of metal ions via atomic layer
deposition in MOFs (AIM) was proposed by the same group as
well.121 They exposed the NU-1000 microcrystalline sample into
diethylzinc (ZnEt2) or trimethylaluminum (AlMe3) vapors, and
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Fig. 9 Molecular representations of NU-1000 with TBAPy4 as a linker
(top) and the schematic representation of the SALI (bottom). R refers to
the phenylphosphonate, carboxylic functional groups, perfluoroalkyl
carboxylate, respectively. Reprinted with permission from ref. 119. Copyright 2014 The Royal Society of Chemistry.
new materials, Zn-AIM and Al-AIM, were prepared, respectively. On
average, 0.5 Zn or 1.4 Al atoms per Zr atom were observed in the
resulting materials. In-depth studies demonstrated that multiple
metals, binary metal merge, and metal oxide clusters could also
be incorporated into NU-1000 based on the AIM process.
In addition, some acidic or basic active sites can also be
introduced into Zr-MOFs by PSM. For example, Ameloot et al.
functionalized m3-OH groups of Zr6 clusters in UiO-66 with
basic lithium t-butoxide (LiOtBu).122 After the dehydration of
UiO-66 at high temperature, the exposed coordinatively unsaturated Zr sites in the Zr6O6 units were brought subsequently in
contact with LiOtBu in a m3-capping fashion. Assuming that per
cluster possessed two possible grafting sites, approximately 25%
of available sites were occupied by tBuO anions. The obtained
LiOtBu grafted UiO-66 was also demonstrated to be a superior
solid ionic conductor. After that, López-Maya et al. reported the
synthesis of UiO-66@LiOtBu using the above method.123 Further,
they prepared missing-linker-defect Zr6(m3-O)4(AcO)6(BDC)5
(UiO-66@AcO) through the incorporation of AcO. By PSM,
an acidic HSO4 group was further introduced into this MOF by
soaking activated UiO-66@AcO in a KHSO4 aqueous solution to
obtain functionalized Zr6(m3-O)4(AcO)4(BDC)5(HSO4)2 (UiO-66@
HSO4) with Brønsted acidic sites.
To improve the CO2 trapping capacity, Li et al. also successfully grafted ethanolamine (Hea) onto the pore surface of UiO-66
by a two-step PSM.124 First, UiO-66 was dehydrated to form
Zr6(m3-O)6(BDC)6 with a distorted Zr6(m3-O)6(COO)12 cluster, which
then reacted with anhydrous Hea to form Zr6(m3-O)4(m3-OH)2(EA)2(BDC)6 (UiO-66-EA). Due to the charge-assisted coordination
bond between m3-OCH2CH2NH2 and Zr(IV) as well as the steric
hindrance protection effect, the –NH2 groups could be firmly
immobilized even under thermal and humid conditions. As
predicted, UiO-66-EA exhibited higher CO2 adsorption enthalpy
and selectivity of CO2/N2 than UiO-66.
It should be pointed out that although great efforts have been
devoted to synthesizing Zr-MOFs, most reported preparations
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were carried out at high temperature and high pressure with
flammable toxic and expensive organic solvents. As a result,
feasible approaches for scale-up production of Zr-MOFs are still
lacking. It is highly desirable to develop facile and industrially
applicable synthetic methods to prepare Zr-MOFs. In 2013,
Yang et al. put forward a feasible water-reflux method to synthesize Zr6(m3-O)4(m3-OH)4(BDC-(COOH)2)6 (UiO-66-(COOH)2) with a
relatively high-scale production.125 In this synthesis, water
instead of DMF was used, being important in consideration of
cost and regeneration issues. Interestingly, due to the introduction of polar free carboxylic groups into the pores, the obtained
UiO-66-(COOH)2 showed strong interaction with CO2 and high
selectivity of CO2/N2. Following this work, Reinsch et al. successfully
prepared Zr6(m3-O)2(m3-OH)6(BDC-F4)6(SO4) (H2BDC-F4 = tetrafluoroterephthalic acid) and Zr6(m3-OH)8(OH)2.8(SO4)3.6(BDC-NH2)3(H2O)7.4
(H2BDC-NH2 = 2-aminoterephthalic acid) using Zr(SO4)2 as the metal
salt and water as the solvent under mild conditions.126 Hu et al. also
developed a modulated hydrothermal (MHT) approach for the facile,
green (aqueous solutions), and scale-up synthesis of UiO-66-F4,
UiO-66-(OCH2CH3)2, and UiO-66-(COOH)4 with Zr(NO3)4 as the
metal salt, which could not be isolated by common solvothermal
methods.127 In addition, Taddei et al. reported the microwave
assisted synthetic method for the scale-up preparation of UiO-66,
with high quality.128 Although it is still a challenge to develop
industrially applicable synthetic approaches, the significant
achievements mentioned above delineated an explicit blueprint
for the scale-up production and commercialization of Zr-MOFs
in the future.
3. Structures
Zr-MOFs are constructed by interconnection of polyatomic
inorganic Zr-containing clusters and polytopic organic ligands.
The structural diversity of the two components and various
ways the two components connect contribute to the variety of
structures in Zr-MOFs. The structures of reported Zr-MOFs are
discussed in this section, in terms of assorted Zr-based clusters
generated and organic ligands utilized.
3.1
Zr-based cluster/core
The progression from simple metal ions to complicated SBUs
proved to be a milestone in the development of MOFs.12,61,129,130
As of this writing, Zr3, Zr4, Zr5, Zr6, Zr8, Zr10, and Zr18 discrete
clusters have been synthesized and characterized in cluster
chemistry.131–134 However, only two types of discrete Zr-based
clusters have been observed in reported Zr-MOFs: Zr6O8 clusters
with a variety of coordination environments and Zr8O6 clusters
as a 12-connected node only observed in PCN-221.135 In addition, there are some other forms of Zr cores in Zr-MOFs, such as
single Zr(IV) ions and chain structures formed from Zr(IV) ions
and ligands. Here, we classify these structures according to
coordination geometries of the Zr(IV) ions, including ZrO6 in
phosphate-based Zr-MOFs,136–138 ZrO7 in MIL-140 series,139,140
and ZrO8 in phenolic Zr-MOFs141,142 (Fig. 10).
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Fig. 10 Observed Zr-based clusters/cores in Zr-MOFs: (a) Zr6O8 cluster,
(b) Zr8O6 cluster, (c) ZrO6, (d) ZrO7, and (e) ZrO8.
3.1.1 Zr6O8 cluster. The Zr6(m3-O)4(m3-OH)4 octahedral cluster
is the most commonly observed in Zr-SBUs, in which the six
vertices of the octahedron are occupied by Zr(IV) centers and
eight triangular faces are alternatively capped by four m3-OH and
four m3-O groups. Each Zr(IV) is eight-coordinated by O atoms in a
square-antiprismatic coordination geometry. One square face of
the square-antiprism is formed by four O atoms supplied by
carboxylate groups while the other one consists of four O atoms
from two m3-O and two m3-OH groups. It should be pointed out
that upon activation at high temperature, the Zr6(m3-O)4(m3-OH)4
cluster was able to lose two water molecules, leading to a distorted
Zr6(m3-O)6 cluster with the Zr–O coordination numbers reduced
to seven (Fig. 11). The symmetry of the cluster thus changes
from Td to D3d. These two forms are commonly referred to
as hydroxylated and dehydroxylated phases, respectively. When
exposed to water vapor, the dehydroxylated form readily converts
back to the hydroxylated form.27,31
It is well known that the connectivity and symmetry of a SBU
are the most important factors affecting the net topology of
associated MOFs. When the Zr6(m3-O)4(m3-OH)4 cluster is fully
Fig. 11 (a) The dehydroxylation of the Zr6(m3-O)4(m3-OH)4 cluster upon
thermal treatment at 300 1C in a vacuum, leading to a distorted Zr6(m3-O)6
cluster, (b) stick and ball representation of the perfect Zr6 octahedron, and
(c) stick and ball representation of a squeezed Zr6 octahedron. Reprinted
with permission from ref. 31. Copyright 2011 American Chemical Society.
Chem. Soc. Rev.
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coordinated by twelve carboxylate groups, a Zr6(m3-O)4(m3-OH)4(CO2)12 SBU is generated with the Oh-symmetry, which is
observed in most of the reported Zr-MOFs (Fig. 12a). However,
in Zr6(m3-O)4(m3-OH)4(TCPP)3 (PCN-223) reported by Feng et al.,
an unprecedented 12-connected D6h symmetric Zr6 cluster
was found.143 For the D6h Zr6 cluster, each Zr(IV) located in
the equatorial plane of the Zr6 octahedron is coordinated by
eight O atoms, four O atoms from two m3-O and two m3-OH
groups, and four O atoms only supplied by three carboxylates.
So, only eight carboxylates are bridging two adjacent Zr atoms,
and the other four in the equatorial plane are chelating single
Zr atoms, respectively (Fig. 12b). The cluster was initially regarded
as a 12-connected Zr18 cluster in this MOF structure, but additional
experiments confirmed that it was a 12-connected Zr6 cluster
instead of Zr18. The Zr18 cluster was actually a crystallographic
disordered model where three Zr6 clusters oriented in different
directions.
The Oh symmetry includes many symmetry subgroups. From
a topological point of view, there is a large possibility for
construction of three-dimensional (3D) frameworks from the
highly symmetric Zr6(m3-O)4(m3-OH)4(CO2)12 SBU and organic
ligands by reducing the symmetry and/or connectivity. For
example, in DUT-69, only ten edges of the Zr6 octahedral cluster
are occupied by carboxylate groups while the remaining four
coordination sites in the equatorial plane are occupied by two
AcO and two water molecules, leading to the formation of a
10-connected [Zr6(m3-O)4(m3-OH)4(CO2)10]2+ (Fig. 12c).50
In addition, Feng et al. also reported Zr6(m3-OH)8(OH)8(TCPP)2
(PCN-222), in which only eight edges of the Zr6 octahedral
cluster are occupied by carboxylate groups while the remaining
eight coordination sites of Zr(IV) centers in the equatorial plane
are occupied by terminal OH groups.144 Consequently, the
SBU becomes 8-connected Zr6(m3-OH)8(OH)8(CO2)8 with the
symmetry being reduced from Oh to its subgroup D4h (Fig. 12d).
In addition to PCN-222, another 8-connected SBU was observed
in Zr6(m3-O)4(m3-OH)4(OH)4(H2O)4(TCPP)2 (PCN-225) reported
by Jiang et al.145 Interestingly, in this MOF six Zr atoms formed
an exact octahedron, while eight m3-O capped on the triangular
faces formed a highly distorted polyhedron instead of an ideal
one as observed in PCN-222. As a result, the cluster symmetry of
D4h in PCN-222 reduces to D2d in PCN-225, leading to lower
symmetry of the latter (Fig. 12e).
Furthermore, a 6-connected Zr6 octahedron SBU was observed
in Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(TCPP)1.5 (PCN-224).146 Only six
edges of the Zr6 octahedron are occupied by carboxylate groups in
the structure, leading to the formation of a hexagonal planar SBU
with the D3d symmetry (Fig. 12f). In addition, Feng et al. also
synthesized another Zr-MOF, Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(TATB)2
(PCN-777), in which another 6-connected trigonal prismatic
Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(CO2)6 SBU was observed (Fig. 12g).147
It was suggested that switching positions of carboxylate groups
and terminal OH/H2O groups in the Zr6 cluster could realize the
cluster interconversion between PCN-777 and PCN-224.
3.1.2 Zr8O6 cluster. The appearance of the Zr8(m4-O)6 cubic
cluster (Fig. 10b) in Zr8(m4-O)6(OH)8(TCPP)3 (PCN-221) reported by
Feng et al. further enriched the structural diversity of Zr-MOFs.135
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Fig. 12 The observed SBU based Zr6(m3-O)4(m3-OH)4 cluster in reported Zr-MOFs: (a) 12-connected Zr6(m3-O)4(m3-OH)4(CO2)12 in UiO-66; (b) 12-connected
Zr6(m3-O)4(m3-OH)4(CO2)12 in PCN-223; (c) 10-connected Zr6(m3-O)4(m3-OH)4(CO2)10(AcO)2 in DUT-69; (d) 8-connected Zr6(m3-OH)8(OH)4(CO2)8 in PCN-222;
(e) distorted 8-connected Zr6(m3-O)4(m3-OH)4(OH)4(H2O)4(CO2)8 in PCN-225; (f) 6-connected Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(CO2)6 in PCN-224; and
(g) 6-connected Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(CO2)6 in PCN-777. Atom color scheme: C, black; O, red; and Zr, blue.
In the Zr8(m4-O)6 cluster, each Zr atom coordinates with three
O atoms from carboxylates and three m4-O atoms, forming a
distorted octahedral coordination environment. Eight Zr atoms
connecting six m4-O atoms lead to the formation of a cubic
cluster based on the Zr arrangement, in which eight vertices are
occupied by Zr atoms and six faces are capped by m4-O atoms. In
the structure of PCN-221, each edge of the Zr8O6 cube cluster is
bridged by a carboxylate group from a TCPP4 ligand to yield a
Zr8(m4-O)6(OH)8(CO2)12 SBU with Oh symmetry. Comparing the
structures of Zr8 and Zr6 clusters, it is interesting to find that the
exchange of the positions of Zr and m4-O atoms in a Zr8(m4-O)6
cluster results in a cluster similar to Zr6(m4-O)4(m4-OH)4
discussed above.
3.1.3 ZrO6 core. A ZrO6 octahedron coordination core was
observed in Zr-phosphonate MOFs, in which each Zr atom is
coordinated by six O atoms from six different phosphonate
groups.136–138 With various phosphonate ligands, the ZrO6 core
presented different connection modes, leading to the formation
of various SBUs. For instance, in dehydrated Zr3(H3BTBP)4
(ZrBTBP) the SBU is composed of only three ZrO6 cores (see
Fig. 22 of Section 3.2.4).136 In this SBU, the center ZrO6 core
connects with two other crystallographically equivalent terminal
ZrO6 via six bidentate PO3C and six monodentate PO3C groups.
In addition, in another Zr-phosphonate MOF, Zr(H4TTBMP)2
(UPG-1), the SBU exhibits a one-dimensional (1D) chain structure
extending along the c-axis.137 In such a catenulate SBU, every ZrO6
octahedron is coordinated by six PO3C groups. However, different
from that in Zr3(H3BTBP)4, only two of the six PO3C connecting
adjacent ones are bidentate, while the other four are monodentate.
3.1.4 ZrO7 core. A ZrO7 core was observed in an infinite chain
type SBU of the MOF ZrO(O2C-R-CO2) (R = C6H4 (MIL-140A), C10H6
(MIL-140B), C12H8 (MIL-140C), and C12N2H6Cl2 (MIL-140D))
reported by Guillerm et al. (Fig. 13).139,140 Every Zr atom coordinates with three m3-oxo groups and four carboxylate O atoms from
the ligands in the ZrO7 core. The chain SBU can be considered
as the linkage of two parallel corner-sharing or edge-sharing
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dimers of the Zr(m3-O)3O4 polyhedra. In MIL-140, each chain SBU
connects with six other chains via the dicarboxylate linkers,
delimiting triangular channels in its structure.139
3.1.5 ZrO8 core. Zr(IV) with a ZrO8 coordination geometry
was found in the chain SBU series of phenolic Zr-MOFs recently
reported by Devic et al. (see Fig. 23 of Section 3.2.4).141,142
The Zr(IV) ions in these Zr-MOFs are eight coordinated with
eight O atoms, but there are some slight differences in the
coordination environment for different compounds. For example,
in Zr(Hgal)(gal)0.5(DMF)0.5(DMAH2) (MIL-153, DMAH2 = dimethylammonium), Zr(IV) is coordinated by eight O atoms of four
pyrogallates in a chelated way and each ligand is chelated to
two Zr(IV) atoms.141 While, in Zr(Hgal)(Hsal)(DMF) (MIL-154),
the Zr(IV) atom is coordinated by five phenolic O atoms of three
gallate ligands, two carboxylate O atoms of salicylate ligands,
and one DMF O atom.142 Similarly, both SBUs in the two MOFs
are composed of edge-sharing ZrO8 polyhedra. As discussed
above, rich structures of the Zr-based clusters and the single
Zr coordination core lead to diverse Zr-MOFs. In particular,
different ways that certain clusters and ligands combine further
increase the unpredictability of resulting MOFs, such as those
observed in the Zr6(m3-O)4(m3-OH)4-TCPP system. Meanwhile,
there is infinite opportunity to develop new Zr-MOFs with
various properties and functions.
3.2
Diverse structures with different ligands
The combination of various ligands with above five types of
Zr-clusters/cores has led to dozens of reported Zr-MOFs. Among
them, aromatic polycarboxylates are the most commonly used
ligands due to their rich coordination modes and feasible
tailorability. Simultaneously, different coordination ways of the
carboxylate group have led to the generation of different SBUs.
Besides carboxylate ligands, several other ligands have also been
used to construct Zr-MOFs. In this section, Zr-MOFs constructed
from the following five types of ligands are discussed: (1) ditopic
carboxylate ligands, (2) tritopic carboxylate ligands, (3) tetratopic
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Fig. 13 (a) Structures of a series of MIL-140 viewed along the c axis and
(b) the chain SBU based on Zr(m3-O)3O4 cores in MIL-140. Reprinted with
permission from ref. 139. Copyright 2012 Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim.
carboxylate ligands, (4) phosphate ligands, and (5) phenolate
ligands. Most of the used ligands are summarized briefly
in Scheme 1.
3.2.1 Ditopic carboxylate ligands. Linear dicarboxylate
ligands have been widely used to build Zr-MOFs. A representative one is terephthalate (BDC2) which is used for the construction of UiO-66, the first reported Zr-MOF.27 In UiO-66, all
twelve edges of the octahedral [Zr6(m3-O)4(m3-OH)4]12+ cluster are
occupied by twelve carboxylates from twelve BDC2 ligands to yield
the Zr6(m3-O)4(m3-OH)4(CO2)12 SBU. Each BDC2 links two SBUs to
form a 3D open framework with tetrahedral and octahedral cages,
where each octahedral cage is connected to eight tetrahedral cages
through sharing triangular windows (Fig. 14). With increasing
length of used ligands, several isoreticular Zr-MOFs including
UiO-67 and UiO-68 were also reported early.
With H2PEDC-2OMe, Schaate et al. later reported the synthesis of PIZOF-2 (Fig. 15).73 The structure and connectivity of the
Zr6(m3-O)4(m3-OH)4(CO2)12 SBU and the ligand in PIZOF-2 are
the same as those in UiO series. However, PIZOF-2 has a twofold interpenetrating network structure. In each single net,
each SBU is connected to twelve other SBUs by the ligands to
form a 3D cubic framework with octahedral and tetrahedral
cages. It is interesting that there exist both convex and concave
tetrahedral cavities of 19 and 14 Å in diameter since the ligand
is slightly bent. The concave tetrahedral voids accommodate
Zr6(m3-O)4(m3-OH)4(CO2)12 SBUs of the other identical net, while
the convex ones are empty. It is clear that the triangular
windows formed by the extended linkers are big enough to let
other linkers pass, thus leading to the framework interpenetration. Furthermore, using ditopic H2FUM and H2ABDC ligands
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the same group also synthesized Zr-FUM and Zr-ABDC MOFs,
which have 12-connected fcu network structures as in UiO-66.48,72
All of the above Zr-MOFs constructed with linear ditopic
carboxylate ligands have a fcu topological network, where the
Zr-based SBUs are all 12-connected. In contrast, PCN-700 with
bcu topology was constructed from 8-connected Zr clusters and
linear ditopic ligand Me2-BPDC2.89 In PCN-700, each Zr6 cluster
is linked by eight Me2-BPDC2 ligands: four above and four
below the equatorial plane of the Zr6 octahedron. Eight terminal
OH/H2O groups occupy the equatorial plane. Interestingly,
after the SLI process, 10-connected frameworks of PCN-701
and PCN-702, and 11-connected frameworks of PCN-703 and
PCN-704 were obtained.
Apart from benzene-based ligands, naphthalene and anthracenebased linear ditopic carboxylate ligands have also been used in the
synthesis of Zr-MOFs. In 2010, Garibay et al. reported the isoreticular
synthesis of UiO-66-1,4–Naph with 1,4-NDC2 as the ligand.148 Later,
Zr6(m3-O)4(m3-OH)4(NDC)6 (DUT-52) and Zr6(m3-O)8(NDC)3(AcO)2
(DUT-84) with 2,6-NDC2 as the ligand were reported by Bon
et al.149 DUT-52 is isoreticular to UiO-66. However, DUT-84
crystallizes in the orthorhombic space group of Cmma and has
a different topology from that of UiO-66. In DUT-84, each Zr6
SBU is connected with only six ligands. Four ligands link the Zr6
clusters forming a layer, and the other two ligands connect two
such layers, leading to a double-layer structure of DUT-84. This
is the first reported Zr-MOF with a 2D structure. Though the
structure is 2D, pores exist with an inner diameter of 6.5 Å.
Meanwhile, Pu et al. used H2AN and synthesized UiO-66 analog
Zr6(m3-O)4(m3-OH)4(AN)6 (UiO-66(AN)).150 Furthermore, Wang et al.
obtained an ‘‘X-ray scintillating’’ Zr-MOF, Zr6(m3-O)4(m3-OH)4(ADC)6
through using anthracene-based emitter compound 9,10anthracenyl-bis(benzoate) (ADC2) as the bridging ligand.151
In addition, linear heterocyclic dicarboxylate ligands were
also introduced into the design and preparation of Zr-MOFs.
Wang et al. used S-heterocyclic dimethyl substituted thiophene
DTDC2 as the ligand to synthesize Zr6(m3-O)4(m3-OH)4(DTDC)6
(Zr-DTDC) with the same structural topology of UiO-66.152 Due
to the joint action of the electronic effect and steric hindrance
from methyl groups as well as strong Zr–O bonds, Zr-DTDC shows
ultrahigh hydrothermal stability. Yoon et al. adopted BTDC2
to construct a UiO-66 type structure Zr6(m3-O)4(m3-OH)4(BTDC)6
(Zr-BTDC).153 The sizes of the octahedral and tetrahedral cages in
Zr-BTDC are about 12 and 8 Å, respectively. In addition, in order
to incorporate Lewis basic sites into the pore surfaces of Zr-MOFs,
Li et al. exploited N-heterocyclic ligand BPYDC2 and also obtained
Zr6(m3-O)4(m3-OH)4(BPYDC)6 (UiO(BPYDC)) with fcu topology.154
Aside from the above-discussed linear ligands, angular
dicarboxylate ligands have also been explored to construct
Zr-MOFs. For example, DUT-51 reported by Bon et al. was built
using the DTTDC2 ligand with a bend angle of 148.611 and
a [Zr6(m3-O)6(m3-OH)2]10+ octahedral cluster (Fig. 16).49 In its
structure, each Zr-based cluster is 8-linked by DTTDC2 ligands
through the carboxylate coordination on eight edges of the Zr6
octahedral cluster. The remaining coordination sites in the
equatorial plane of the Zr6 cluster are occupied by six DMF
and two BC ligands. As a result, the assembly between the
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Scheme 1
Some ligands used in the construction of Zr-MOFs.
[Zr6(m3-O)6(m3-OH)2]10+ cluster and the bent ligand generates a
3D framework of DUT-51 with the reo topology. The framework
contains two types of cages: octahedral and cubo-octahedral
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ones with diameters of 15.6 and 18.8 Å, respectively. The cubooctahedral cage possesses six square and eight triangular windows
with the size of 9 and 4.2 Å, respectively. Each triangular
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Fig. 14 Structure of UiO-66: (a) tetrahedral cage; (b) octahedral cage; and
(c) packing of the two types of cages.27
Fig. 16 Structure of DUT-51: (a) [Zr6(m3-O)6(m3-OH)2]10+ cluster based
SBU; (b) the remaining four coordination sites occupied by DMF and
benzoic acid molecules in the equatorial plane; (c) octahedral and cubooctahedral pores; and (d) polyhedral packing. Reprinted with permission
from ref. 49. Copyright 2012 The Royal Society of Chemistry.
Fig. 15 (a) One of the two frameworks of PIZOF-2 showing the arrangement of concave and convex tetrahedral cavities; (b) topological representation of the interpenetrated structure in PIZOF-2; (c) convex cavity;
(d) concave cavity; and (e) the whole structure of PIZOF-2. Reprinted with
permission from ref. 73. Copyright 2011 Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim.
window is shared with one octahedral cage, and each square
window is shared with one cubo-octahedral cage.
Following this work, the same group synthesized a series of
Zr-MOFs, DUT-67–69, by using the bent TDC2 ligand under
different reaction conditions (Fig. 17).50 Carboxylate groups of
TDC2 with a bridging angle of 147.91 are spaced at a distance of
5.31 Å, which can be viewed as a shortened version of DTTDC2.
The structure of DUT-67 has a reo topological net, similar to
that of DUT-51, containing 8-connected [Zr6(m3-O)6(m3-OH)2]10+
octahedral clusters and cuboctahedral and octahedral cage pores.
DUT-68 crystallizes in the Im3% m space group and has two different
crystallographically independent 8-connected [Zr6(m3-O)6(m3-OH)2]10+
clusters with a binodal net. The structure of DUT-68 represents
Chem. Soc. Rev.
a complicated hierarchical pore system containing four types of
cage pores of the rhombicuboctahedral mesoporous cage, the
square antiprismatic cage, the cuboctahedral cage, and the
smallest octahedral cage. Their inner diameters are 27.7, 13.9,
12.5, and 8 Å, respectively. The largest rhombicuboctahedral
cage shares its square windows with six cuboctahedral cages,
while the remaining windows are shared by twelve square
antiprismatic cages. The octahedral cage is formed as a result
of stacking six square antiprismatic cages and two rhombicuboctahedral cages. DUT-69 with the bct topology is the first
example of a Zr-MOF containing a uninodal 10-connected
Zr6(m3-O)4(m3-OH)4(CO2)10(AcO)2 SBU. The structure of DUT-69
involves octahedral cages with a diameter of 5 Å and rectangular
channels of 9.15 2.66 Å in size. Similar to the structure of DUT-69,
Furukawa et al. utilized PZDC2 to synthesize Zr6(m3-O)4(m3-OH)4(PZDC)5(HCO2)2(H2O)2 (MOF-802), in which each SBU is
10-connected with PZDC2 linkers, while formate and DMF ligands
occupy the remaining coordination sites.155
As discussed above, using rigid aromatic ditopic carboxylates
has resulted in a variety of Zr-MOFs. In contrast, few Zr-MOFs
based on flexible aliphatic ditopic carboxylates have been reported
until now. Compared with the framework of rigid aromatic
counterparts, a skeleton of a Zr-MOF with flexible aliphatic
carboxylate is highly possible to exhibit dynamic behavior and
unique properties, which is worth exploring.
For the first time, using flexible AP2 and AP-CH32 ligands
Reinsch et al. synthesized Zr6(m3-OH)8(OH)8(AP-CH3)4, Zr6(m3-OH)8(OH)4(SO4)4(AP)2 and Zr6(m3-OH)8(OH)2(Cr2O7)3(AP)4, respectively
(Fig. 18).156 For all the three Zr-MOFs, the inorganic nodes
are based on the well-known Zr6O8 cluster. In the structure of
Zr6(m3-OH)8(OH)8(AP-CH3)4, eight carboxylate groups coordinate
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Fig. 18 (a) The SBU and framework connectivity in Zr6(m3-OH)8(OH)8(AP-CH3)4; (b) core architecture and framework structure of Zr6(m3OH)8(OH)4(SO4)4(AP)2; and (c) core architecture and framework structure
of Zr6(m3-OH)8(OH)2(Cr2O7)3(AP)4. Reprinted with permission from ref. 156.
Copyright 2015 The Royal Society of Chemistry.
Fig. 17 (a) Linker orientation and local environment of Zr clusters in
DUT-67–69; (b) the framework structure of DUT-67 showing two types
of cages; (c) the framework structure of DUT-68 showing four types of
cages; and (d) the framework structure of DUT-69 showing one type
of cage. Reprinted with permission from ref. 50. Copyright 2013 American
Chemical Society.
with a Zr6(m3-OH)8(OH)8 cluster, resulting in a uninodal net with
the bcu topology. Different from those in Zr6(m3-OH)8(OH)8(AP-CH3)4, four SO42 ions in Zr6(m3-OH)8(OH)4(SO4)4(AP)2 coordinate to the cluster all in a bidentate fashion. With these
binding modes, the substructure with the dia topology is
formed by hydrogen bonding of Zr-sulfate clusters, while
the Zr(IV) atoms in the equatorial plane of the cluster are
coordinated by the organic linkers, leading to a square grid
layer. In Zr6(m3-OH)8(OH)2(Cr2O7)3(AP)4 the Cr2O72 ions served
as connectors between Zr-based clusters in a monodentate
coordination fashion. The remaining coordination sites in the
Zr clusters are occupied by eight carboxylate groups of organic
ligands, which extend the structure to form a bcu framework. It
is clear that these inorganic anions played an important role in
tuning the connectivity of the Zr-based clusters in this system,
thereby forming different network structures of Zr-MOFs.
3.2.2 Tritopic carboxylate ligands. Among tritopic ligands,
BTC3 was first used in Zr-MOF construction. Zr6(m3-O)4(m3-OH)4(BTC)2(HCO2)6 (MOF-808) reported by Furukawa et al. crystallizes
in the cubic space group of Fd3% m, in which each Zr6(m3-O)4(m3-OH)4(CO2)12 SBU is connected to six BTC3 ligands and
each BTC3 ligand coordinates with three SBUs, leading to
the formation of a (3,6)-connected 3D framework with spn
topology.155 HCO2 ions act as the terminal ligand which
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coordinate with the [Zr6(m3-O)4(m3-OH)4]12+ cluster. The framework
of MOF-808 features tetrahedral cages with the internal pore
diameter of 4.8 Å, where the Zr6(m3-O)4(m3-OH)4(CO2)12 SBUs are
at the vertices and the BTC3 ligands at the faces of the tetrahedron. Furthermore, these tetrahedral cages share their vertices
to form a large adamantine-like cage with the internal pore
diameter of 18.4 Å. Then, using another tritopic ligand TATB3
Feng et al. synthesized zeotype PCN-777 with a b-cristobalite-type
structure similar to that of MOF-808 (Fig. 19).147 However, because
the size of organic linker TATB3 is almost twice that of BTC3,
the internal pore diameter of the adamantine-like cage reaches
up to 38 Å, which ranks PCN-777 among mesoporous MOFs.
Soon after, Wang et al. synthesized Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(BTB)2 (Zr-BTB) by using BTB3 as the ligand.157
Fig. 19 (a) The Zr6(m3-O)4(m3-OH)4(CO2)12 SBU and trigonal-planar
organic linker TATB3 in constructing PCN-777; (b) structure of supertetrahedra in PCN-777; and (c) the overall framework structure of PCN-777
with a large mesoporous cage.147
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Zr-BTB crystallizes in the space group of I41/amd. In its structure, the Zr6 clusters as 6-connected nodes were connected
by BTB3 ligands as the 3-connected linkers to give rise to a
(3,6)-connected 2D network with the kgd topology. The 2D
framework possesses elliptical windows with the size of
7.16 12.61 Å. Moreover, the parallel 2D layers of one set
interpenetrate perpendicularly with another set to generate a
3D open framework with 1D square channels (14.37 14.37 Å).
Zr-BTB thus represented the first Zr-MOF with a 2D - 3D
interpenetration structure.
3.2.3 Tetratopic carboxylic ligands. Compared with di- and
tricarboxylate ligands, tetracarboxylate ligands have more coordination sites and rich coordination patterns. More importantly,
tetracarboxylic ligands with large space steric hindrance could
effectively avoid framework interpenetration, being favorable for
constructing mesoporous Zr-MOFs. Among all tetracarboxylic
ligands for constructing reported Zr-MOFs, H4TCPP is quite
common. Some related studies can be found in a published
review on metal-metalloporphyrin frameworks reported by Gao
et al.158 The variation of its connecting number and symmetry
provides a large number of topological possibilities to form
various frameworks with different pore sizes and shapes. In
addition, most reported porphyrinic Zr-MOFs exhibited excellent
thermal and chemistry stability, providing good platforms for
exploring the applications of MOFs under harsh conditions.
Currently, eight Zr-MOFs constructed using H4TCPP have been
reported by Zhou’s group from PCN-221 to PCN-225,135,143–146
Yaghi’s group for Zr6(m3-O)4(m3-OH)4(TCPP)3 (MOF-525) and
Zr6(m3-O)8(H2O)8(TCPP)2 (MOF-545),159 and Ma’s group for
Zr6(m3-O)8(H2O)8(CO2)8 (MMPF-6).160 In PCN-221, four peripheral
benzene rings of the ligand are perpendicular to the central
porphyrin ring.135 Each TCPP4 linker connects with four Zr8
clusters in a 4-connected mode, and each Zr8 cluster is linked
by twelve TCPP4 linkers in a 12-connected node, resulting in
a (4,12)-connected ftw topological network. In addition, the
structure features two types of polyhedral cages with distorted
octahedral and cubic shapes. The slightly distorted octahedral
cage, with a cavity diameter of B11 Å, involves two Zr8 clusters at
the vertices along the axial direction and four TCPP4 ligands in
the equatorial plane. The vertices and faces of the cubic cage
with the edge length of B20 Å are delimited by eight Zr8 clusters
and six TCPP4 ligands, respectively. For PCN-223, following the
same topological simplification way of PCN-221, each Zr6 cluster
can be regarded as a 12-connected node and the TCPP4 linker
can be simplified to a 4-connected node.143 The framework
represents the first (4,12)-connected MOF with the shp topology.
In addition, PCN-223 contains uniform triangular 1D channels
of 12 Å in diameter delimited by three SBUs and three ligands.
MOF-545 prepared by Morris et al. crystallizes in the hexagonal
space group P6/mmm.159 In its structure, eight edges of the Zr6
octahedral cluster are bridged by carboxylate groups and the
remaining positions are occupied by terminating water, leading
to the formation of a SBU with a formula of Zr6(m3-O)8(CO2)8(H2O)8.
Combination of these SBUs and the tetratopic linker TCPP4
resulted in a network with the csq topology. The structure of
MOF-545 features triangular and hexagonal 1D channels with
Chem. Soc. Rev.
Chem Soc Rev
Fig. 20 Structures of MOF-525 and -545: (a) Zr6(m3-O)4(m3-OH)4(CO2)12
SBU in MOF-525; (b) TCPP4 linker used in MOF-525 and -545; (c) ftw
topology; (d) the structure of MOF-525; (e) Zr6(m3-O)8(CO2)8(H2O)8 SBU in
MOF-545; (f) csq topology; and (g) the structure of MOF-545. Reprinted
with permission from ref. 159. Copyright 2012 American Chemical Society.
diameters of 8 and 36 Å, respectively. In addition, MOF-525
with the ftw topology was also obtained in the same work. It
crystallizes in the space group Pm3% m and features large cubic
cages with an edge length of 20 Å, which are packed in a
primitive cubic lattice. The cubic cage is comprised of eight
corner-sharing Zr6(m3-O)4(m3-OH)4 units and six face-sharing
porphyrin-based units (Fig. 20). Parallel to this work, PCN-222
and MMPF-6 with nearly the same structure as MOF-545 were
reported at almost the same time.
In PCN-225, the Zr6 cluster and the TCPP4 ligand can also
be viewed as 8- and 4-connected nodes, respectively.145 But
different from the (4,8)-connected csq topology of MOF-545, the
whole network of PCN-225 can be simplified as the (4,8)-connected
sqc net. There are two types of channels in PCN-225 running along
the crystallographic a and b direction, respectively. The small
quadrangle-shaped channel with the dimension of 8 15 Å is
comprised of two Zr6 clusters and two TCPP4 ligands, whereas the
big pear-like one with a size of 9 22 Å is delimited by three Zr6
clusters and three TCPP4 ligands. As for PCN-224, the Zr-based
cluster and the TCPP4 ligand are 6- and 4-connected, respectively.146 The framework can thus be regarded as a (4,6)-connected
she topological net. As the coordination number of the cluster
is low, large 3D open channels of 19 Å are found in PCN-224.
Besides TCPP4, other tetratopic carboxylate ligands used
in building Zr-MOFs can be classified into four categories:
augmented porphyrinic derivatives63,65,161 (in constructing
CPM-99, NU-1102, NU-1104, PCN-228, -229, and -230), planar
pyrene-based derivatives63,83,118–121,162 (in constructing NU-1000,
-1100, -1101, -1103, and Zr-PTBA), flexible planar tetratopic
ligands159,162,163 (in constructing PCN-94, MOF-535, and Zr-BTBA),
and tetrahedral ligands34,155 (in constructing PCN-521, MOF-812,
and MOF-841).
All reported Zr-MOFs constructed by tetratopic augmented
porphyrinic derivatives are the ftw network structures. Lin et al.
reported Zr6(m3-O)4(m3-OH)4(TCBPP)3 (CPM-99) with TCBPP4
as the ligand.161 The CPM-99 framework includes large cubic
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cavities with an edge length as large as 25 Å, which are packed
in a primitive cubic lattice. Each 12-connected Zr6 cluster
resides on one cube vertex while the faces of the cube are
occupied by TCBPP4 ligands. Simultaneously, the distorted
octahedron cage with B11 Å size in diameter is created by two
Zr6 clusters in the axial sites and four TCBPP4 ligands located
in the equatorial plane. With the same ligand, Wang et al.
prepared isostructural NU-1102. Meanwhile, NU-1104 was
prepared with an extended version of the TCBPP4 ligand,
designed by inserting triple-bonds between the phenyl groups
of each arm.63 The diameters of the large cubic cage and
octahedral cage in NU-1104 are 24.2 and 13.5 Å, respectively.
Furthermore, by further increasing the arms’ length, PCN-228
to -230 were synthesized with TCP-14, TCP-24, and TCP-34,
respectively.65 Due to the extra-large size of these three porphyrinic
linkers, the resulting Zr-MOFs are mesoporous with pore sizes
ranging from 25 to 38 Å.
Most reported Zr-MOFs built with pyrene-based ligands, such
as the above-mentioned NU-1100, -1101, and -1103, possess
ftw topological structures as well, with the exceptional case of
NU-1000. NU-1000 crystallizes in the space group of P6/mmm,
similar to PCN-222.118–121 In the structure of NU-1000 eight of
the twelve edges in the octahedral Zr6 cluster are occupied by
eight carboxylate groups of TBAPy4 ligands and the remaining
positions are occupied by terminal –OH groups pointing into
the mesoporous channels. The resulting 3D structure can thus
be viewed as 2D Kagome sheets linked by TBAPy4 ligands.
With rigid PTBA4 as the ligand, Kalidindi et al. prepared
[Zr6(m3-O)4(m3-OH)4(PTBA)2.75(OH)](C7H6O2)0.25(H2O)2
(Zr-PTBA),
which is isostructural to MOF-525 built from TCPP4.162 According
to the CHN elementary analysis and TGA results, it was suggested
that about 8% of the PTBA4 linkers are replaced by a hydroxide
anion per formula in Zr-PTBA. Despite the defects observed,
Zr-PTBA showed permanent porosity, but was not stable in water.
Compared with rigid ligands, the tetratopic ligands with
conformational flexibility should be more compatible with Zr
clusters. Tetrabenzoic acid H4BTBA with a configurationally
flexible biphenyl core was used by the same group to synthesize
Zr6(m3-O)4(m3-OH)4(BTBA)3 (Zr-BTBA).162 It should be noted that
BTBA4 has the same four-fold connectivity being essential
identical metrics, but distinct torsional flexibility compared
with PTBA4. Zr-BTBA crystallizes in the Pm3% m space group.
The edges of the octahedral Zr6(m3-O)4(m3-OH)4(CO)12 SBUs are
interconnected by twelve BTBA4 ligands, which in turn are
bridging four neighboring SBUs, resulting in an ftw framework
as in MOF-525. Zr-BTBA displays a 3D porous system with an
average pore diameter of about 19 Å. Unlike its free conformation, a significant deviation from planarity of the BTBA4 ligand
was observed in Zr-BTBA. Interestingly, it was found that contrary to Zr-PTBA, Zr-BTBA showed excellent water stability,
although its mechanical stability was poor. Combining the
stability features of Zr-PTBA and Zr-BTBA, a solid solution
material Zr6(m3-O)4(m3-OH)4(PTBA)2.6(BTBA)0.4 with a BET surface
area up to 4000 m2 g1 was then prepared using a mixed ligand
strategy (Fig. 21). Remarkably, the synergy of flexible BTBA4
and rigid PTBA4 linkers makes this material hydrolytically
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Review Article
Fig. 21 The designed synthesis of Zr6(m3-O)4(m3-OH)4L1XL23X with the
mixed tetratopic ligand acids H4BTBA (H4L1) and H4PTBA (H4L2), highlighting
its stability. Reprinted with permission from ref. 162. Copyright 2015 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.
and structurally stable, which is difficult for Zr-BTBA and Zr-PTBA,
individually.
Similarly, Wei et al. synthesized Zr6(m3-O)4(m3-OH)4(ETTC)3
(PCN-94) by using the ETTC4 ligand, which is isostructural to
MOF-525.163 The size of the cubic cage in PCN-94 is about 17.5
Å across an edge. Such cages are packed in a primitive cubic
lattice to form additional channels with a window size of 14 14 Å2. Note that there exists a prominent conformational
change for the twisted ETTC4 linker in the Zr-MOF framework
compared with that in its free state. The minimum angle
between adjacent carboxylate groups, the average dihedral
angles between phenyl rings, as well as those between phenyl
rings and the central plane in the ETTC4 ligand are all larger
than those in its free conformation.
With the exception of planar tetratopic ligands as discussed
above, tetrahedral carboxylate-based ligands with the Td symmetry are also quite intriguing for MOF construction.164–172
However, due to inherent 3D directive conformation of this type
of ligand, geometrical symmetry incompatibility emerges when
they combine with SBUs of high symmetry. As a result, reported
Zr-MOFs with tetrahedral ligands are relatively rare. Currently,
only four Zr-MOFs, PCN-521 as discussed above with the
MTBC4 ligand,34 Zr6(m3-O)4(m3-OH)4(H2O)2(MTB)3 (MOF-812),155
Zr6(m3-O)4(m3-OH)4(MTB)2(HCO2)4(H2O)4 (MOF-841) with the MTB4
ligand,155 and Zr6(m3-O)4(m3-OH)4(OH)4(H2O)4(TCPS)2 (Zr-TCPS)
with the TCPS4 ligand,36 have been reported. Except for the
ith type network of MOF-812, the other three Zr-MOFs all are
(4,8)-connected networks with the flu topology.
PCN-521 crystallizes in the tetragonal space group of I4/m
and its framework consists of 8-connected Zr6(m3-O)4(m3-OH)4(OH)4(H2O)4(CO2)8 SBUs linked by tetrahedral MTBC4 linkers.34
Similar to those in PCN-222 and MOF-545, the symmetry of the
Zr6(m3-O)4(m3-OH)4(H2O)4(CO2)8 SBU is D4h being symmetrically
compatible with the tetrahedral MTBC4 with the D2d symmetry.
As a result, a (4,8)-connected network with the flu topology was
generated. The structure of PCN-521 contains interstitial cavities
with the size of 20.5 20.5 37.4 Å. Hereafter, Furukawa et al.
obtained MOF-841 and MOF-812 with the MTB4 ligand.155 The
framework structure of MOF-841 is similar to that of PCN-521,
but with small cavities of 11.6 Å due to its shortened ligand.
Different from that in MOF-841, the SBUs in MOF-812 are
connected to twelve MTB4 ligands, with four monodentate
carboxylate groups. Each SBU in MOF-812 can thus be simplified
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to an icosahedron, being different from the cuboctahedral
12-connected SBUs in fcu Zr-MOFs. Meanwhile, each MTB4
ligand is bridging four SBUs, leading to a 3D ith network with
5.6 Å cavities in diameter. In addition, Fang et al. synthesized
Zr-TCPS by using a silane-cored TCPS4 ligand. Zr-TCPS is
isostructural to PCN-521 with a (4,8)-connected 3D flu network.36
In this MOF, there exist channels with a window size of 10.2 7.98 Å running along the crystallographic a-axis direction, as
well as 13.3 Å octahedron cages in diameter.
Interestingly, it was found that monocarboxylates could also
be used to build Zr-MOFs, which were commonly used as
modulators to control crystal growth during the synthesis
of Zr-MOFs as discussed above. Liang et al. obtained a novel
Zr-MOF, Zr6(m3-O)4(m3-OH)4(FA)12 (ZrFA) with formate as the
ligand.173 ZrFA crystallizes in the orthorhombic space group
of Cmcm. The structure is also constructed by the common
Zr6(m3-O)4(m3-OH)4 clusters. The eight edges up and down the
equatorial plane of the Zr6 octahedron are occupied by eight formate
ions in a chelating way, forming a Zr6(m3-O)4(m3-OH)4(HCO2)8 cluster.
In the ac plane, Zr6(m3-O)4(m3-OH)4(HCO2)8 clusters are bridged by
other inter-cluster formate linkers to give infinite 2D layers which are
further stacked into 3D structure along the b-axis.
3.2.4 Phosphate ligands. Aside from carboxylates, phosphate
ligands have also been used to construct Zr-MOFs. Taddei et al.
synthesized Zr3(H3BTBP)4 with a honeycomb-like layered structure (Fig. 22).136 In its structure, each Zr atom is coordinated with
six phosphate O atoms forming a ZrO6 entity. Three ZrO6
assembled together through sharing O atoms to form a trimeric
SBU, in which the center ZrO6 entity connects with the other two
via six bidentate PO3C and six monodentate PO3C groups. Each
SBU is further connected with the nearest six equivalent SBUs via
H3BTBP3 linkers, leading to a layered structure. The 3D structure
of Zr3(H3BTBP)4 with cavities is formed by these layers stacking
along the crystallographic c-axis direction in an ABAB mode.
However, the generated cavities are capped by the SBUs of
adjacent layers, thus making them inaccessible. In addition,
the ligand H6TTBMP with slight conformational freedom was
applied to synthesize another Zr-phosphonate MOF, UPG-1.137 In
the structure of UPG-1, three phosphate groups of each
H4TTBMP2 ligand display different coordination fashions with
Zr atoms: two are connected to adjacent Zr–O chains while the
third is non-coordinated and protonated. The ligands link Zr–O
Chem Soc Rev
Fig. 23 Structure of MIL-163: (a) view of a single chain SBU showing the
Zr atoms coordinated by 1,2,3-trioxobenzene groups; and (b) view of the
structure along the [001] direction showing square-shaped channels.
Reprinted with permission from ref. 142. Copyright 2015 Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim.
chains to finally form an open framework with two types of
channels with the sizes of about 10 and 5 Å, respectively. In
contrast to Zr3(H3BTBP)4, UPG-1 displays permanent porosity
and strong absorption affinity towards n-butane and CO2.
3.2.5 Phenolate ligands. Recently, it was demonstrated that
phenolate ligands could also be used to construct Zr-MOFs.
These MOFs should be much more stable because of the high
pKa of the phenolate group, which could strengthen the Zr–O
bonds. In early stages, Cooper et al. synthesized two Zr–hydroxycarboxylate compounds, (MIL-153) and (MIL-154) with a phenolate
ligand.141 The two compounds have a 1D inorganic chain structure
composed of edge-sharing ZrO8 entities. On the basis of this work,
with ditopic phenolic acid H6TzGal as the starting ligand resource,
Mouchaham et al. recently successfully obtained a highly stable
porous Zr-MOF, Zr(H2TzGal) (MIL-163).142 Similar to that in
MIL-153, each 1,2,3-trioxobenzene group chelates two Zr atoms
and two pairs of these groups coordinate with one Zr atom
through all O coordination to form a chain SBU (Fig. 23a).
These chains are further connected with each other through
H2TzGal2 linkers to yield a 3D open framework with squareshaped channels of about 12 12 Å in diameter (Fig. 23b).
4. Properties and applications
Due to their diverse structures, large and accessible specific
surface areas, uniform and tunable pore sizes, outstanding
stability, and specific properties, intensive research has been
directed towards the exploration of Zr-MOF applications. In
this section, we discuss the studies of Zr-MOFs in applications
ranging from catalysis, molecule adsorption and separation,
drug delivery, and fluorescence sensing, and as porous carriers.
4.1
Fig. 22 (a) The single layer of ZrBTBP along the c-axis; and (b) the
stacking of layers in ZrBTBP in an ABAB mode. Reprinted with permission
from ref. 136. Copyright 2014 The Royal Society of Chemistry.
Chem. Soc. Rev.
Catalysis
Catalysis is one important application field of MOFs. In this
regard, several comprehensive reviews have been published,
in which state-of-the-art in MOF catalysis was highlighted
and analyzed. In the following section, the catalytic properties
of Zr-MOFs and their functionalized derivatives are discussed
based on different types of catalytic reactions, including Lewis
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Fig. 24 The schematic representation of Meerwein reduction of 4-tertbutylcyclohexanone catalyzed by UiO-66 with unsaturated metal sites.56
acid catalysis, oxidation catalysis, biomimetic catalysis, electrocatalysis, hydrogenation catalysis, and photocatalysis.
4.1.1 Lewis acid catalysis. Zr-MOFs with unsaturated metal
sites were mainly applied in Lewis acid catalysis reactions, in
which the open metal sites act as electron pair acceptors
capable of accelerating the reaction process. Vermoortele
et al. revealed that coordinatively unsaturated metal sites in
UiO-66 can be drastically increased by using specific modulators in the synthesis.56 Thus, the Lewis acid catalytic activity of
the MOF can be improved correspondingly. With UiO-66 as the
catalyst the Meerwein reduction of 4-tert-butylcyclohexanone
(TCH) with isopropanol (IPA) was studied, which requires
activating the alcohol and ketone simultaneously to facilitate
the transfer of hydride (Fig. 24). The results illustrated that the
non-modulated UiO-66 shows nearly no activity, while the
materials modulated with ten equivalents of TFA display
high catalytic activity, leading to appreciable yields of tertbutylcyclohexanol. Moreover, it was also demonstrated that
the space around the Zr cluster in UiO-66-NO2 increased when
HCl and TFA were used as modulators, which could facilitate
the simultaneous activation of reactants. As a result, 93%
conversion of tert-butylcyclohexanol was achieved.
Similarly, Wang et al. successfully removed coordinated water of
Zr6(m3-O)4(m3-OH)4(OH)6(H2O)6(CO2)6 SBUs in Zr-BTB (Fig. 25).157
Fig. 25 The single layer structure of Zr-BTB and its Lewis acid catalyzed
reaction of carbonyl compounds with cyanide.157
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Review Article
Thus, the coordinative unsaturated metal sites were generated,
which are favorable for the Lewis acid catalytic reaction of
carbonyl compounds with cyanide. The resulting material
showed high catalytic activity, especially for the cyanosilylation.
The conversion reached 100% in 24 hours at room temperature,
which was comparable to the performance of other reported
MOF catalysts in the cyanosilylation of benzaldehyde, but much
higher in the cyanosilylation of naphthaldehyde.174–176
It is worth noting that postsynthetic modification can be
used as a method to obtain coordinatively unsaturated metal
sites in MOFs for improving their catalytic activity. Mondloch
et al. used the AIM strategy to obtain metallized NU-1000,
denoted as Zn-AIM and Al-AIM.121 As a proof-of-concept to
verify whether these AIM materials are able to elicit new
catalytic behavior, Knoevenagel condensation reactions between
ethyl cyanoacetate and benzaldehyde were studied. As expected,
the Zr sites in NU-1000 proved inactive towards the Knoevenagel
condensation. Interestingly, Zn-AIM and Al-AIM were active catalysts, due to the presence of Lewis acidic Zn(II) and Al(III) sites.
Moreover, no Zn or Al elements were determined in the catalytic
reaction systems after removing the functionalized MOFs,
suggesting that Zn- and Al-AIM acted as the stable catalysts.
In addition, Rasero-Almansa et al. prepared multifunctional
heterogeneous catalyst Ir–Zr-MOFs (Zr–[IrL]) using a PSM process
for the hydrogenation of aromatic compounds.105 This reaction
involves the activation of aromatic compounds by Lewis acids
and a sequential hydrogenation step. This bifunctional Zr–[IrL]
catalyst with Zr as the Lewis acid and Ir as the active metal allows
the activation process to work cooperatively. Using aniline as a
model compound, its selective conversion to cyclohexanamine
catalyzed by Zr–[IrL] resulted in a 100% yield in only 10 h,
indicating the high catalytic activity of this MOF based catalyst.
In addition, the Zr–[IrL] catalyst can be reused after the reaction
via a simple centrifugation treatment.
4.1.2 Oxidation catalysis. Currently, oxidation reactions with
Zr-MOFs as catalysts mainly include the oxidation of alkanes,
alkenes, and water, as well as regioselective C–H oxidation of
benzo[h]quinolone. In industry, the selective oxidation of cyclohexane to produce cyclohexanone (K) and cyclohexanol (A) is
conducted over cobalt-based homogeneous catalysts with about
only 4% conversion and 70–85% selectivity for the K/A product
when the oxidant is air. Therefore it is highly desirable to develop
new catalysts which will improve the conversion and selectivity of
this reaction. Due to the existence of accessible active porphyrinic
iron(III) centers, Zr-PCN-221(Fe) was employed by Feng et al.
as the catalyst for the selective oxidation of cyclohexane with
tert-butyl hydroperoxide (TBHP) as the oxidant.135 The results
demonstrated that a quick oxidation reaction proceeded for only
about 11 h. Moreover, the utilization efficiency of the oxidant
TBHP reached nearly 100%. More importantly, Zr-PCN-221(Fe)
as the catalyst displayed high selectivity of cyclohexanone up to
86.9%, and for cyclohexanol selectivity was only 5.4%. This
observed cyclohexanone selectivity of Zr-PCN-221(Fe) was higher
than those of other MOF-based or conventional molecular sieve
catalysts.177–179 On the other hand, the epoxidation of alkenes
is one of the most important reactions in organic synthesis
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Table 2 Results of the selective oxidation of cyclooctene to cyclooctene
oxide with TBHP as the oxidant for 12 h using different catalysts. Reprinted
with permission from ref. 99. Copyright 2015 American Chemical Society
Catalyst
Conversion (%)
Yield (%)
Selectivity (%)
Zr-MOF-BPYDC-CuBr2
BPYDC-CuBr2
Zr-MOF-BPYDC
No catalyst
88.5
28.5
23.0
13.4
84.3
21.2
20.1
8.5
95.3
74.4
87.3
63.2
since the epoxide group is a key intermediate in the industrial
synthesis of polymers and bioactive molecules.108,180,181 However, traditional transition metal compounds such as homogeneous catalysts generally suffer from product contamination
and poor recyclability. In order to solve these problems, Toyao
et al. immobilized CuBr2 into Zr-MOF-BPYDC to obtain
Zr-MOF-BPYDC-CuBr2 through a PSM method, which was then
used as the catalyst for the selective oxidation of cyclooctene
to produce cyclooctene oxide using TBHP as the oxidant.99
Zr-MOF-BPYDC-CuBr2 was found to show higher catalytic activity
and selectivity than Zr-MOF-BPYDC and (BPYDC)CuBr2 in this
reaction. The yield of the cyclooctene oxide after 12 h reaction
period reached 84.3% with 95.3% selectivity (Table 2). Moreover,
Zr-MOF-BPYDC-CuBr2 exhibited an excellent recyclability without a significant loss of catalytic activity over three cycles due to
the highly stable framework of the Zr-MOF. It should be pointed
out that the observed high catalytic activity can be ascribed to the
larger pore size (7 Å) in the MOF compared with the size of the
cyclooctene molecule (5.5 Å), which facilitates the free diffusion
of cyclooctene inside the pores. Furthermore, the formation
of Cu-peroxo species with TBHP might also be favorable for
accelerating the catalytic process.
In addition, regioselective C–H oxidations with Zr-MOF
catalysts were also investigated. Fei et al. used UiO-66PdTCAT with site-isolated Pd to catalyze the C–H oxidation of
benzo[h]quinoline with iodobenzene diacetate [PhI(OAc)2]
as the oxidant (Fig. 26).108 It was found that the metalated
Pd(thiocatecholato) sites exhibited highly efficient, reusable,
and selective catalysis for the oxidation of the aromatic C–H
bond. Nearly 99% yield of methoxy-functionalized benzo[h]quinoline
Fig. 26 Schematic representation of the regioselective C–H oxidation of
benzo[h]quinolone catalyzed by UiO-66-PdTCAT. Reprinted with permission from ref. 108. Copyright 2015 American Chemical Society.
Chem. Soc. Rev.
was achieved using UiO-66-PdTCAT (5 mol% in Pd) as the catalyst.
In contrast, no obvious catalytic activity was observed for both
pristine UiO-66 and UiO-66-TCAT. Moreover, UiO-66-PdTCAT could
be recycled at least five times without a significant decrease in the
product yields (92–99%). In addition, such chelate-directed oxidation
could also be expanded to halogenation of benzo[h]quinoline, in
which N-halosuccinimides served as both oxidants and halogenating
reagents. The yield of monochlorinated benzo[h]quinoline could
reach up to 95%. The high catalytic activity and excellent recyclability
of the UiO-66-PdTCAT catalyst in converting C–H bonds to ethers
and aryl-halides could be attributed to the strong covalent metalthiocatecholato binding.
It is also interesting that Zr-MOF catalysts can be used in water
oxidation reactions. Wang et al. adopted a mix-and-match synthetic strategy to obtain Ir(III) functionalized Zr-MOFs by replacing
the ligands in UiO-67 with Cp*Ir(DCPPY)Cl, [Cp*Ir(BPY)Cl]Cl,
and [Ir(DCPPY)2(H2O)2]OTf (Cp* = pentamethylcyclopentadienyl
and H2DCPPY = 2-phenylpyridine-5,4 0 -dicarboxylic acid).182
The obtained MOFs were used for water oxidation with cerium
ammonium nitrate (CAN) as the oxidant. All of these Zr-MOFs
were proven to be effective for the water oxidation reaction with
high turnover frequencies (TOFs) up to 4.8 h1. In addition,
these Zr-MOF catalysts retain their integrity after reactions and
can be recovered from the reaction systems. However, compared
with the homogeneous catalysts, those Zr-MOFs displayed lower
TOFs because CAN is apparently too large to enter the MOF
pores, where catalytic active sites are located.
4.1.3 Biomimetic catalysis. MOFs have been attracting intense
interest for biomimetic catalysis, where the self-destruction of the
catalyst and the dilution of active site density can be effectively
avoided. However, many MOFs are not stable enough to be applied
in biomimetic catalysis, especially in an aqueous medium. In
addition, most MOFs are microporous, limiting the access of
large substrate molecules to catalytic active sites and slowing
down the catalysis rate. In contrast, ultrahigh stable Zr-MOFs
with large mesopores and rich accessible redox sites are promising for biomimetic catalysis.
PCN-222(Fe) with large 1D open channels (37 Å) prepared by
Feng et al. exhibited exceptional stability, even in concentrated
hydrochloric acid, which is favorable for biomimetic catalysis.144
Three standard oxidation assays with pyrogallol, 3,3,5,5-tetramethylbenzidine, and o-phenylenediamine as substrates were
carried out to check the catalytic performances of PCN-222(Fe)
(Fig. 27). For the pyrogallol oxidation reaction, the derived kcat
(catalytic activity) value using PCN-222(Fe) (16.1) was seven times
higher than that of free hemin (2.4). The derived Km (binding
affinity) value of 0.33 was lower than that of natural horseradish
peroxidase (HRP) enzyme (0.81), which indicated strong affinity
between the substrate and the PCN-222(Fe) framework. For the
other two substrates, PCN-222(Fe) also showed superior catalytic
activity to free hemin. These outstanding catalytic performances
could be attributed to the high density of porphyrin metal active
centers in PCN-222(Fe). Simultaneously, Chen et al. reported
MMPF-6 as a biomimetic catalyst for the oxidation of pyrogallol
(THB) and 2,2 0 -azinodi(3-ethylbenzothiazoline)-6-sulfonate (ABTS)
with oxygen and electron transfer capabilities, respectively.160
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Fig. 27 (a) Structure of PCN-222(Fe); (b) schematic representation of
peroxidase; (c) the reactions catalyzed by the peroxidase-like catalyst
PCN-222(Fe).144
MMPF-6 exhibited a very fast initial rate of 1.39 103 mM s1
for purpurogallin formation and 8.18 104 mM s1 for
ABTS + formation, which are about one-third and one-fourth
as fast as that of free heme protein Mb (oxygen storage protein),
respectively. Thus, it was indicated that MMPF-6 was highly
active for oxidation catalysis both involving oxygen transfer and
electron transfer. Moreover, this Zr-MOF also showed excellent
adaptability for organic solvents and stable reusability.
On the other hand, the catalytic decomposition of phosphate
ester bonds is of great importance for the destruction of chemical
warfare agents (CWAs).183–186 Considering that Zr–OH–Zr bonds
are similar to the bimetallic Zn–OH–Zn active site found in phosphotriesterase enzymes, Katz et al. used UiO-66 as a biomimetic
catalyst for the methanolysis and hydrolysis of phosphatebased nerve agent simulants, dimethyl 4-nitrophenyl phosphate
(DMNP) and p-nitrophenyl diphenyl phosphate (PNPDPP).187 It
was found that UiO-66 had fast half-lives of nearly 45 and 10 min
for the two solvolysis reactions, respectively, benefitting from the
functions of strong Lewis-acidic Zr(IV) and bridging hydroxide
anions on its pore surface. Following this, Mondloch et al. also
used NU-1000 as the catalyst for the catalytic hydrolysis of DMNP
and CWA O-pinacolyl methylphosphonofluoridate (GD).188
77% conversion was achieved over the course of 60 min with
a measured half-life of 15 min. In particular, dehydrated
NU-1000 was found to be remarkably active for the hydrolysis of
DMNP, giving a half-life of only 1.5 min and exhibiting 100%
conversion after approximately 10 min. For the catalytic hydrolysis
of GD, the measured half-life was as low as 3 min in the presence
of N-ethylmorpholine buffer. Extending this work, a series of
Zr-MOFs with different structural features were applied in the
degradation of DMNP by the same group.189 Among them,
MOF-808 showed the highest hydrolysis rates. The observed
TOF was up to 350-fold compared with those of other checked
Zr-MOFs, while the half-life was less than 0.5 min.
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In addition, López-Maya et al. reported the introduction of
lithium alkoxide into the UiO-66 framework (UiO-66@LiOtBu)
to improve the phosphotriesterase activity for the capture and
hydrolytic degradation of CWA analogues.123 The observed halflife and TOF for the hydrolysis of P–F, P–O, and C–Cl bonds
were 5 min and 0.13 min1, 25 min and 0.02 min1, and 3 min
and 0.17 min1, respectively. Inspired by such a boosting of
the phosphotriesterase catalytic activity, this material was
integrated onto textiles as self-detoxifying filters for CWAs.
4.1.4 Electrocatalysis. Due to their high stability, another
emerging application of Zr-MOFs is electrocatalysis for
oxygen reduction reaction (ORR) as precursors. Lin et al. used
Fe-porphyrinic carbon CPM-99X/C (X = H2, Zn, Co, Fe) composites, derived from the carbonization of CPM-99X at 700 1C as
the electrocatalysts for ORR.161 As shown in Fig. 28, among the
four materials, CPM-99Fe/C exhibited the best electrocatalytic
activity, with the ORR peak at the most positive potential of
0.836 V and the onset and half-wave potentials of 0.950 and
0.802 V, respectively, being comparable to the commercial Pt/C
catalyst. Moreover, it exhibited long-term durability and was
methanol-tolerant, superior to Pt/C. The Koutecky–Levich (K–L)
plots from rotating ring electrode (RDE) polarization curves of
CPM-99Fe/C ranging from 0.1 to 0.6 V presented a good linear
feature, with the calculated electron-transfer number of 4.1 (0.1),
indicating the complete reduction of O2. In addition to the
basic medium in use, the CPM-99Fe/C catalyst also exhibited
high activity in O2-saturated acidic medium, illustrating its high
adaptability for solvent medium. The prominent performances of
CPM-99Fe/C could be attributed to the unique structural features
of the parent Zr-MOF framework, including high porosity, thermal
stability, and uniform distribution of FeN4 active sites, as well as
the hard-templating effect of its rigid skeleton.
4.1.5 Hydrogenation catalysis. Hydrogenation of olefins is
one important processing technology in the petrochemical
industry, which can improve the quality of oil products
and the yield of light oil. Currently, industrial hydrogenation
reactions still rely on noble-metal catalysts, which suffer from
high costs and inherent toxicity. Exploring the substitution of
precious metal catalysts with earth-abundant and less toxic
base metals thus occupies the forefront of contemporary molecular catalysis research.190–197
However, base metals in the homogeneous catalytic system
are inclined to deactivation. To avoid this problem, Manna
et al. afforded robust and highly active sal-M-MOFs (M = Fe
or Co) as single-site solid catalysts for olefin hydrogenation.190
Sal-M-MOF materials were designed by the incorporation of an
orthogonal secondary functional group salicylaldimine (sal)
into the UiO-67 backbone along with postsynthetic metalation
with iron and cobalt salts (Fig. 29). The obtained sal-Fe-MOF
displayed excellent catalytic activity in the hydrogenation reaction
of a wide variety of aliphatic and aromatic terminal alkenes. With
only a 0.05–0.01 mol% catalyst loading, styrene and its alkoxy and
halide derivatives could be completely hydrogenated. The yields
were up to 93–100%. Moreover, the Sal-Fe-MOF also displayed
high turnover numbers (TONs) up to 145 000 for 1-octene hydrogenation in 8 h with only 6.5 104 mol% of the catalyst.
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Fig. 28 (a) Structure of CPM-99; (b) cyclic voltammetry (CV) curves and (c) RDE polarization curves of CPM-99X/C and 20% Pt/C in 0.1 M KOH solution;
(d) RDE polarization curves of CPM-99Fe/C at different rotation rates. Inset: K–L plot of J1 vs. o1 at different potentials; (e) RDE polarization curves of
CPM-99X/C and 20% Pt/C in 0.1 M HClO4 solution. Reprinted with permission from ref. 161. Copyright 2015 American Chemical Society.
Fig. 29 The alkene hydrogenation catalyzed by sal-M-MOFs obtained by
the postsynthetic metalation of sal-MOF. Reprinted with permission from
ref. 190. Copyright 2014 American Chemical Society.
In addition, the sal-Fe-MOF could be recycled and reused at
least eight times without the loss of catalytic activity. Interestingly, the catalyst could be regenerated after deactivation and
further reused for another six cycles with a simple treatment of
ten equivalents of NaBEt3H followed by THF washing.
4.1.6 Brønsted acid catalysis. Brønsted acid catalytic
reactions are mainly utilized in hydrocarbon cracking and
restructuring of the petroleum refining industry. The catalysts
used to activate hydrocarbons are mainly superacids (Hammett
acidity functions H0 r 12). Jiang et al. treated MOF-808 with
aqueous sulfuric acid for 1 day to generate a sulfated analogue,
MOF-808-2.5SO4 as a superacid (Fig. 30).198 Based on the
standard catalytic acid/base test reactions of the cyclization of
citronellal to isopulegol and a-pinene isomerization, it was shown
that Brønsted acidity was introduced into MOF-808-2.5SO4,
which was catalytically active in various Brønsted acid-catalyzed
reactions including Friedel–Crafts acylation, esterification, and
isomerization.
Chem. Soc. Rev.
Fig. 30 Schematic representation for constructing superacid MOF-8082.5SO4. Reprinted with permission from ref. 198. Copyright 2014 American
Chemical Society.
4.1.7 Photocatalysis. The harvesting of solar energy and
conversion into renewable energy has attracted increasing
attention due to global energy consumption and increasing
concern with environmental contamination. Numerous photocatalysts have been investigated for harvesting solar energy.
With respect to MOFs, pristine UiO-66 as a photocatalyst was
first applied for hydrogen evolution, which showed poor photocatalytic activity due to the absence of efficient catalytic sites for
substrate activation and inefficient electron transfer between
the catalyst and substrates. In order to enhance photocatalytic
performance, an effective way is to build elaborate hybrid
photocatalytic systems by introducing functional entities.
In 2011, Wang et al. doped [Re(CO)3(5,5 0 -BPYDC)Cl] into the
UiO-67 framework using a mix-and-match synthetic strategy
for photochemical CO2 reduction (Fig. 31).182 The activity of
the resulting Zr-MOF catalyst was examined in CO2-saturated
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degraded by UiO-66(AN). The high performances could be
attributed to the extended visible-light response generated by
the introduction of a greater conjugation system of AN. Moreover, the structural integrity of this MOF after the photocatalytic
reactions was retained, which guarantees its potential practical
application. Meanwhile, Wang et al. used UiO-66(Ti) nanocomposites prepared by a modified post-grafting method in
the removal of methylene blue (MB).201 The removal efficiency
could reach 87.1%, which may be attributed to the synergistic
effect of the adsorption and photo-degradation.
4.2
Fig. 31 Structure model of the doped UiO-67 and the illustration of its
photocatalytic application in CO2 reduction and organic transformations.182
acetonitrile (MeCN). The results showed that this catalyst leads
to the selective reduction of CO2 to CO under light with the
TON of 10.9, which is three times higher than that of complex
Re(CO)3(5,5 0 -BPYDC)Cl. Extending this work, phosphorescent
[IrIII(PPY)2(DCBPY)]Cl (H2L5) and [RuII(BPY)2(BPYDC)]Cl2 (H2L6)
(PPY = 2-phenylpyridine, BPY = 2,2 0 -bipyridine) were also incorporated into the UiO-67 framework for photocatalytic organic
transformations (aza-Henry reaction, aerobic amine coupling,
and aerobic oxidation of thioanisole) (Fig. 31). Both generated
Zr-MOFs, Zr6(m3-O)4(m3-OH)4(L5)6 and Zr6(m3-O)4(m3-OH)4(L6)6,
showed high catalytic efficiency towards aza-Henry reactions
between nitromethane and methoxy-substituted tetrahydroisoquinoline, with conversion as high as 96%. For the aerobic
amine coupling, the catalytic performances of the latter MOF
Zr6(m3-O)4(m3-OH)4(L6)6 were comparable to those of homogeneous molecular catalysts. Furthermore, Zr6(m3-O)4(m3-OH)4(L6)6
also represented excellent selectivity towards the aerobic oxidation of thioanisole, giving a conversion as high as 73% after 22 h.
Meanwhile, Sun et al. used NH2-UiO-66(Zr) as a photocatalyst
for the CO2 reduction, which exhibited high catalytic activity.199
A proposed mechanism was that under visible-light irradiation,
the photoinduced electron from the excited ligand can be transferred to the Zr oxo clusters generating Zr(III), which reduce CO2 to
HCOO with TEOA (triethanolamine) serving as a hydrogen
source. Moreover, Lee et al. prepared mixed metal (Zr/Ti),
mixed ligand UiO-66 derivative Zr4.3Ti1.7O4(OH)4(BDC-NH2)5.17(BDC-(NH2)2)0.83 using the PSE strategy, which could also act as
an effective photocatalyst for CO2 reduction to HCOOH under
visible light irradiation in the presence of TEOA as a sacrificial
base and 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial reductant.200 Such a mixed ligand strategy introduced
new energy levels into the band structure of the MOF and thus
enhanced the photocatalytic activity.
In addition, Zr-MOFs have also served as the photocatalyst
for the degradation of dyes. Pu et al. performed photocatalytic
reactions for the degradation of methyl orange with UiO66(AN).150 In this study, 65% of methyl orange was effectively
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Molecule adsorption and separation
The remarkable properties that distinguish MOFs from
other porous solid materials are their permanent porosity,
ultrahigh specific surface area, as well as feasible and tailorable
functionalization. Combined with unique chemical stability, even
in the presence of water, Zr-MOFs showed promising performances
for molecule adsorption and separation applications.202–204
The porosity and surface area of Zr-MOFs are important
factors for molecule adsorption and separation, which can be
tuned by controlling the framework topology and ligand size.
Based on the topology-guided design strategy, Gutov et al. prepared well-designed NU-1100 with a pore volume of 1.53 cm3 g1
and a BET surface area of 4020 m2 g1.83 No obvious variation of
crystallinity and porosity was observed after soaking in liquid
water for 24 h, indicating its excellent stability against water.
NU-1100 was then checked for the H2 storage application. It was
found that the total volumetric H2 adsorption of NU-1100 was
43 g L1 and gravimetric uptake is 0.092 g g1 at 65 bar and 77 K,
which are comparable with those of most reported MOFs. In
addition, the gravimetric and volumetric CH4 storage capacities
at 65 bar and 298 K of NU-1100 were approximately 0.27 g g1
and 180 vSTP/v, respectively, which ranked this Zr-MOF among
the most promising methane-storage materials.
Functionalization of the pore surface is also an efficient
approach for enhancing molecule adsorption capacity of MOFs.
In order to achieve high guest molecule adsorption of Zr-MOFs,
extensive efforts have been devoted to increasing the affinity
between the framework pore surface and the guest molecules.
Two of the most effective methods are the generation of active
sites (such as open metal sites) by regulating the template agent
and the encapsulation of functional groups on the pore surface.
By varying the concentration of modulator agent, Wu et al.
achieved a high concentration of missing-linker defects in the
UiO-66 framework, yielding a dramatic increase of both pore
volume and surface area up to B150% and B60%, respectively,
which greatly affect gas adsorption behaviors in the resulting
material.55 The modified UiO-66 with most defects showed a
B50% higher uptake of CO2 than that of the one with the
least defects at 35 bar. On the other hand, immobilization of
functional groups to increase adsorption sites appears to be a
promising strategy for enhancing gas capacity. However, the
conventional introduction of functional groups often results in
the reduction of specific surface area due to space occupation
or blockage by functional moieties, which is a negative effect on
adsorption capacity of resulting materials. In this respect, the
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heterocyclic ligands without declining original pore spaces of
parent frameworks are the best choices. Li et al. synthesized
UiO(BPYDC), in which the bipyridyl moieties were incorporated
as free Lewis basic sites in the framework.154 The high density
of basic sites on the pore surface of UiO(BPYDC) endowed this
Zr-MOF with high uptake capacities towards H2, CO2, and CH4.
H2 uptake of this MOF was 5.7 wt% at 77 K and 20 bar, 26.6%
higher than that in UiO-67 (4.5 wt% under the same conditions),
being among the top rank of H2 uptakes in MOF materials. The
CO2 uptakes at 0.15 and 1 bar were 1.5 and 8.0 wt%, respectively.
At 293 K and 20 bar, the adsorbed amount of CH4 was 12.2 wt%
(7.63 mmol g1) in this MOF. Moreover, the predicated CH4
storage capacity (10.52 mmol g1) reached the Department of
Energy (DOE) target of 180 vSTP/v at ambient temperature and
35 bar, making UiO(BPYDC) one of the few MOFs that meet the
DOE target. In addition, Yoon et al. reported Zr-BTDC, being
composed of electronegative sulfur-containing ligands as strong
gas-binding sites.153 In spite of the lower surface area of this
MOF contrasted against UiO-67, it showed higher H2 and
CO2 sorption capacities of 2.3 wt% at 77 K and 4.05 mmol g1
at 273 K, respectively.
Apart from adsorption and storage, the selective adsorption
and separation of molecules is also one of the most attractive
research topics in Zr-MOFs. Recently, selective CO2 capture
from gas mixtures (especially CO2/CH4 and CO2/N2) has been
widely investigated due to its practical significance in halting
the greenhouse effect.203 To improve the selectivity of CO2
adsorption, Wang et al. used carbonyl- and sulfone-functionalized
ligands and isolated two UiO-67 analogues, Zr6(m3-O)4(m3-OH)4(FDCA)6 (BUT-10) and Zr6(m3-O)4(m3-OH)4(DTDAO)6 (BUT-11)
(Fig. 32).205 The adsorption isotherms of pure CO2 of the
Chem Soc Rev
evacuated UiO-67, BUT-10 and BUT-11 demonstrated that the
maximal CO2 uptakes were 22.9, 50.6, and 53.5 cm3 g1 at 298 K
and 1 atm, respectively. It is clear that the adsorption capacities
of CO2 in BUT-10 and -11 were more than double that of parent
UiO-67. The functionalized BUT-10 and -11 presented enhanced
heats of adsorption towards CO2 (21.8 and 25.9 kJ mol1,
respectively), illustrating strong interactions between CO2
and their frameworks. Moreover, the evaluated selectivities
for CO2/CH4 were 2.7–2.9, 5.1–5.2, and 9.0–9.2, while those
for CO2/N2 were 9.4–9.9, 18.6–22.9, and 31.5–43.1 in UiO-67,
BUT-10, and BUT-11, respectively. Clearly, the selectivities of
CO2/CH4 and CO2/N2 were enhanced greatly in BUT-10 and -11
in contrast to those in UiO-67.
Toxic Hg0 vapor is difficult to remove from some systems
because of its insolublility. Based on hard-soft acid–base theory,
the soft base S atom is apt to preferentially interact with soft
metal Hg. The introduction of a thiophene group into Zr-MOFs
can provide the ability to capture and sense Hg0 vapor.206–209
Using a thiophene-based ligand DTDC2, Wang et al. synthesized ultrastable Zr-DTDC, which can remain intact in aqueous
solutions over a wide pH range from 0 to 12.152 The exceptional
stability makes it an excellent adsorbent for Hg0 vapor in harsh
environments. It was found that after the adsorption of Hg0
vapor the color of Zr-DTDC crystals changed from white to
yellow, and about 70% of photoluminescence (PL) intensity of
the MOF was suppressed. Clearly, the remarkable fluorescence
quenching allows Zr-DTDC to function as an optical sensor for
detecting Hg0 vapor.
Although molecule adsorption in MOFs has been widely
studied, water adsorption has been rarely reported because
most MOFs are not sufficiently stable in the presence of water.
Fig. 32 CO2 adsorption isotherms at 298 K and IAST-predicted selectivities toward CO2/N2 in BUT-11, BUT-10, and UiO-67. Reprinted with permission
from ref. 205. Copyright 2014 American Chemical Society.
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However, Zr-MOFs with their remarkable stability can exhibit
high water uptake capacity, being useful in some practical
applications, such as drying and sensing. Furukawa et al.
proposed three criteria for consideration in the design of MOFs
with excellent water absorption performance, involving condensation pressure of water in the pores, uptake capacity,
recyclability and water stability of the material.155 Based on
these criteria, they prepared a series of Zr-MOFs including
MOF-802, Zr6(m3-O)4(m3-OH)4(NDC-(OH)2)6 (MOF-805), Zr6(m3-O)4(m3-OH)4(BPDC-(OH)2)6 (MOF-806), MOF-808, MOF-812, and
MOF-841 by connecting Zr6(m3-O)4(m3-OH)4(CO2)n (n = 6, 8, 10,
or 12) SBUs with different carboxylate ligands. The water
sorption properties of those MOFs along with other reported
Zr-MOFs were evaluated. Among them, MOF-801 and MOF-841
showed excellent performances: the former took up 22.5 wt% of
water at P/P0 = 0.1 and water uptake of the latter was 44 wt% at
P/P0 = 0.3 (Fig. 33). Interestingly, the two Zr-MOFs showed a
step water uptake and maintained stable adsorption after five
consecutive cycles. Thus, it was anticipated that with these
excellent characteristics, MOF-801 might be potentially applied
in advanced thermal batteries and MOF-841 was a good candidate to be used in the capture and release of atmospheric water
in remote desert areas, respectively. Such good performance
could be attributed to the fact that the appropriate pore size of
MOF-801 and MOF-841 can lead to the aggregation of water
molecules resulting from the formation of hydrogen bonds
between water and Zr-SBUs.
Selenium is a naturally occurring element which is essential
for human health. Howarth et al. examined the adsorption
ability of seven porous, water stable Zr-MOFs for the removal of
selenate and selenite anions from aqueous solutions.210 Among
them NU-1000 exhibited the fastest adsorption rate and the
highest gravimetric adsorption capacity. Such high performance can be attributed to the large apertures and substantial
Fig. 33 (a) Structure of MOF-841; (b) recycled performance of water uptake
in MOF-841 at 25 1C; (c) structure of MOF-801; (d) recycled performance of
water uptake in MOF-841 at 25 1C. Reprinted with permission from ref. 155.
Copyright 2014 American Chemical Society.
This journal is © The Royal Society of Chemistry 2016
Review Article
numbers of substitutionally Zr(IV) coordination sites in NU-1000.
Then, NU-1000 was further used for the extraction of SO42 from
water solution.211 The overall maximum adsorption capacity
could reach 56 mg g1, within only one minute. In addition,
NU-1000 displayed high selectivity even in the presence of high
concentration of competitive anions and could be recycled without a decrease of adsorption capacity.
Highly toxic and persistent organic dyes in wastewater have
led to serious environmental pollution problems.212 Recently,
several MOFs have been explored for the adsorption and
capture of pollutants including organic dyes.212,213 Lv et al.
synthesized Zr6(m3-O)4(m3-OH)4(EDDB)6 (BUT-30) with a UiO-66
type structure, and used it to adsorb dyes.214 Interestingly,
through checking thirteen different dyes it was found that BUT-30
was only able to adsorb cationic dyes such as rhodamine B but
exclude others. This adsorption selectivity may be accounted by the
negatively charged framework generated from the deprotonation of
coordinated OH groups in involved [Zr6(m3-O)4(m3-OH)4]12+ clusters,
which are prone to selectively absorb cationic molecules.
Herbicides and pesticides in aqueous solution can also
be adsorptively removed by Zr-MOFs. Zhu et al. reported the
removal of glyphosate (GP) and glufosinate (GF) with UiO-67
based on its abundant Zr–OH groups which served as natural
anchors for GP and GF molecules.215 Due to the strong affinity
towards the phosphoric groups in these guest molecules and
the moderate pore size in UiO-67, the adsorption capacities
were as high as 3.18 mmol (537 mg) g1 for GP and 1.98 mmol
(360 mg) g1 for GF, which outclassed other reported adsorbents.
In addition, Seo et al. used UiO-66 for the adsorptive removal of
methylchlorophenoxypropionic acid (MCPP) from water.216 The
result illustrated that UiO-66 had a fast adsorption rate (nearly
30 times that of activated carbon) and a very high adsorption
capacity, especially at low MCPP concentrations (B7.5 times at
1 ppm MCPP). Moreover, UiO-66 could be reused after washing
with a solvent. Such rapid and high uptakes of MCPP may be
attributed to its strong affinity with the MOF framework generated
by p–p interactions and electrostatic interactions.
Bioaccumulation of these persistent pharmaceuticals and
personal care products (PPCPs) is a threat to aquatic life.
Diclofenac sodium (DCF) is the most frequently detected PPCP,
the aqueous-phase adsorption of which using UiO-66 and
functionalized UiO-66s (with SO3H/NH2) was studied by Hasan
et al.217 Those Zr-MOFs performed well in terms of adsorption
kinetics and capacities. In particular, at low concentration the
adsorption capacity for SO3H-functionalized UiO-66 is nearly
13 times higher than the traditional adsorbent activated carbon,
which has a great significance in practical application.
In addition, nuclear power has been proposed as an alternative
energy source for many years.218 However, the negative effects such
as release of legacy nuclear waste and environmental contamination currently are awaiting a disposal solution. Recently, Zr-MOFs
were investigated to be used in nuclear power-related processes.
Abney et al. treated UiO-66 with NaOH, Na3PO4, and H3PO4 to
remove its BDC2 ligands, leading to ZrOx, ZrOxyPhos, and ZrPhos
(Fig. 34).219 Those resulting porous inorganic materials preserved
the original morphology of the parent UiO-66, with useful
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Fig. 34 Schematic representation of the ligand extraction process in
UiO-66 to form the porous amorphous material, ZrPhos (top) and
an efficient radionuclide sequestration process with ZrPhos (bottom).
Reprinted with permission from ref. 219. Copyright 2014 American
Chemical Society.
surface functionalities (hydroxyl and phosphoryl groups), as
well as high stability and porosity. Leveraging those excellent
properties, these inorganic materials as sorbents were investigated for several nuclear power-related processes, including
decontamination of high-level nuclear waste (HLW), lanthanide
(Ln) extraction, and remediation of radioactive seawater. It was
found that ZrOxyPhos was superior to others in decontaminating
the HLW stimulant. In the Ln separation application, ZrPhos
could extract approximately 99% Ln from a slightly acidic aqueous
solution. Just as expected, the adsorption properties of these new
sorbents in both capacity and kinetics were superior to the current
state-of-the-art materials, which should be attributed to the high
porosity, well-defined morphologies, imparted surface hydroxyl
groups, as well as accessibility of coordinating metal sites in these
porous inorganic materials.
4.3
Drug delivery
In recent years, increasing attention has been paid to the
application of MOFs as drug delivery vehicles.220,221 However,
most MOFs are difficult to meet the prerequisites of this
application due to the limitation of their poor chemical and
thermal stabilities.222–225 In contrast, stable, non-toxic Zr-MOFs
with ease of nanoparticle formation are considered ideal
candidates for drug delivery.47
It has been well documented that zirconia oxides can be
modified by phosphonates relying on the strong coordination
between the Zr atom and the phosphonate O atom.226 As a result,
Zr-MOFs with regularly arrayed Zr–O clusters are expected to be
suitable for the controllable capture and release of alendronate
(AL) molecules. Accordingly, Zhu et al. used UiO-66 nanoparticles
(NPs) as carriers for AL delivery.227 It was found that about 42.7%
of adsorbed AL could be released by UiO-66 NPs at pH = 7.4
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in 60 h, whereas over 59% of the drug was released at pH = 5.5
within the same time duration (Fig. 35a). It was proposed that
the observed pH-responsive release character resulted from the
protonation of phosphates in acidic medium.228 To further
verify that the UiO-66 NPs could be effectively endocytosized
by cancer cells, UiO-66-FMN, made by grafting fluorescent
flavin mononucleotide (FMN) onto UiO-66, was incubated into
HepG2 cells for 4 h. As shown in Fig. 35b, the UiO-66-FMN NPs
were remarkably internalized within a short period and distributed intensively in the cytoplasm. In addition, the therapeutic
effect of AL-UiO-66 was tested through in vitro cytotoxicity
measurements against cancer cells. It was observed that in the
first 24 h of incubation for both cancer cells HepG2 and MCF-7,
the cytotoxicity of AL-UiO-66 was slightly lower than that of free
AL with an equivalent dose. However, after 48 h of incubation,
the AL-UiO-66 NPs caused a higher mortality rate of cancer cells
than free AL (Fig. 35c and d). It was also interesting that the IC50
(a measure of drug effectiveness) values of AL-UiO-66 were far
below those of free AL.
4.4
Fluorescence sensing
Fluorescent materials have riveted significant attention due to
their wide applications, especially as light-emitting diodes
(LEDs) and solid state sensors.229–233 In spite of a wide variety of
organic fluorescent molecules being explored, they often suffer
from self-quenching and consequently low quantum yields.234,235
Recently, researchers conceived that fixture of fluorophores into
MOFs could solve these problems effectively.236–240 Generally, the
fluorescence of MOF materials originates from three aspects: the
organic ligands, lanthanide metal cations in Ln-MOFs, as well as
the charge transfers between metal ions and organic ligands.240–245
As for Zr-based fluorescent MOFs, in most cases fluorescence
originates from organic ligands.
As a well-known fluorophore, tetraphenylethylene (TPE) is
attractive due to its aggregation-induced emission property.246
Wei et al. prepared the TPE-based Zr-MOF, PCN-94, and studied
its fluorescence properties (Fig. 36).163 The twisted ligand
conformation in PCN-94 led to bright blue fluorescence emission at 470 nm, which displayed a large blue shift compared
to the 545 nm yellow emission of the free ligand H4ETTC.
Moreover, the quantum yield of PCN-94 was as high as 99.9 0.5% under Ar, which could be primarily attributed to the
strong coordination between the ETTC4 ligand and Zr atoms. In
addition, the fluorescence lifetime and relative intensity increased
anomalously when heating the solid PCN-94 from cryogenic to
ambient temperatures. It was believed that the change in ligand
conformation had a tremendous influence on the photophysical
properties of the MOFs. This light-emitting Zr-MOF with unusual
photoluminescence properties can be potentially applied in molecular electronics or sensor technologies.
Fluorescent PCN-225 prepared by Jiang et al. exhibited
exceptional chemical stability in aqueous solutions with a remarkably wide pH range.145 Interestingly, the pH-dependent fluorescence of PCN-225 was observed (Fig. 37). The emission intensity
rose along with the increase of pH value. However, it was not a
simple linear relationship. When the pH was below 5, an
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Fig. 35 (a) Free AL release profile from the dialysis bag at pH 7.4 (black) and the cumulative release profiles of AL from UiO-66 NPs in 5 mM PBS at pH
values of 5.5 (red) and 7.4 (green) at 37 1C; (b) the merged confocal image of HepG2 cells after their incubation with 100 mg mL1 of UiO-66-FMN for 4 h
at 37 1C; cell viabilities of (c) HepG2, (d) MCF-7 cells incubated with free AL and AL-UiO-66 at different concentrations for 24 and 48 h. Reprinted with
permission from ref. 227. Copyright 2014 The Royal Society of Chemistry.
increase of pH value led to only a slightly increase of fluorescence intensity. The reason for this unique pH-sensitive fluorescence in PCN-225 was that the protons in acidic solution
combined with pyridine-type N atoms of the porphyrin center,
destroying the p-electron conjugated double-bond system of
porphyrin, ultimately leading to significant fluorescence
quenching. However, when the pH was above 7, the fluorescence intensity increased significantly in close association with
the pH values, which might arise from the deprotonation of the
imino group of porphyrin. Based on these findings, PCN-225 was
regarded with potential applications in pH sensing, especially in
the pH range of 7–10.
Capitalizing on the intrinsic fluorescence of MOFs, researchers
explored extensive sensing applications.247–259 For instance, high
levels of phosphates may lead to the pollution of water and
consequent massive death of creatures.260 As discussed above,
Zr-MOFs presented high affinity towards phosphoric groups due
to the formation of Zr–O–P bonds. Thus, Yang et al. exploited
This journal is © The Royal Society of Chemistry 2016
intrinsically fluorescent UiO-66-NH2 as a probe for the selective
recognition of phosphate anions in aqueous solution, in which
Zr–O clusters played the role of phosphate recognition sites with
the ligands as the signal reporters (Fig. 38).261 The fluorescence
intensity showed a good linear correlation with the concentration
of phosphate. As a result, UiO-66-NH2 as a fluorescent sensor
displayed a wide phosphate detection range of 5–150 mM. Moreover, the detection limit was estimated to be 1.25 mM, which is far
below the detection requirement of phosphate discharge criteria.
Meanwhile, UiO-66-NH2 was also used by Ghosh’s group for the
deamination reaction-based detection of aqueous phase nitric
oxide (NO), which showed high sensory activity and selectivity,
even with potentially interfering species.262 Based on a similar
sensing mechanism, UiO-66-N3 and UiO-66-NO2 served as fluorescence turn-on probes, which can detect H2S rapidly and selectively
even under physiological conditions.263,264 Extending those
studies, Zr6(m3-O)4(m3-OH)4(L)6 (UiO-67@N, H2L = 2-phenylpyridine5,4 0 -dicarboxylic acid) was prepared to selectively detect nitro
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photoelectric effect. The fast electrons then transfer the energy
to anthracene-based emitters by inelastic scattering. Finally,
the excited anthracene-based ligand emits the visible photons
for detection. Through the synergistic functionalization of
Zr-SBUs and anthracene-based bridging ligands, the resultant
MOFs exhibited excellent X-ray-to-light converting capabilities
superior to the two individual components.
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4.5
Fig. 36 (a) ETTC4 linker in PCN-94; (b) PCN-94 framework; (c) solidstate absorption (dashed lines) and emission spectra (solid lines) of PCN94 and H4ETTC at room temperature; (d) photos of PCN-94 and H4ETTC
are shown under ambient light and UV light. Reprinted with permission
from ref. 163. Copyright 2014 American Chemical Society.
explosives in aqueous media.265 Especially for 2,4,6-trinitrophenol (TNP) with concentrations as low as 2.6 mM, fast and
high fluorescence quenching was observed, illustrating low
detection limitation for UiO-67@N. In contrast, all of the other
nitro analytes had minor effects on the fluorescence intensity,
indicative of high selectivity towards TNP. Such unprecedented
selective fluorescence quenching can be attributed to the occurrence of both electron and energy transfer processes between the
MOF and TNP.
MOFs are also applicable in metal sensing, mainly involving
the detection of Eu(III), Fe(III), Tb(III), Cu(II), and Ag(I).247,252–259 As
for Zr-MOFs, Carboni et al. prepared Zr6(m3-O)4(m3-OH)4(H2O)8(L1)4
(H2L1 = H2TPDC-NH2 derivative) with metal binding sites orthogonal to succinic acid, which possesses a broad emission band
at 390 nm upon excitation at 330 nm.266 The ligand H2L1 was
prepared by treating H2TPDC-NH2 with succinic acid in dry DMSO
at room temperature for 12 h. Thus, the chelation of amide
groups with transition metal cations leads to changes of the
fluorescence intensity. Interestingly, those transition metal ions
with unpaired d-electrons presented a significant quenching
effect even at low metal concentrations, especially for Mn(II).
An exceptionally high KSV value of (0.91 0.04) 106 m1, with
the detection limit of o0.5 ppb for Mn(II), was obtained on
Zr6O8(H2O)8(L1)4, which is three to four orders of magnitude
greater sensitivity than those of previously reported MOFs.
In addition to the fluorescence properties, scintillation is
also an important emissive phenomenon found in luminescent
materials. For example, luminescent organic crystals can work
as radiation scintillators for detecting low-energy b-rays and
neutrons.267,268 X-ray scintillating Zr-MOFs, Zr6(m3-O)4(m3-OH)4(ADC)6
with anthracene-based bridging ligands H2ADC as emitters, were
designed and prepared by Wang et al. (Fig. 39).151 The high
atomic number of Zr in the SBUs serves as the effective X-ray
antenna. Upon X-ray absorption, the outer-shell electrons
of the Zr(IV) ions are converted to fast electrons via the
Chem. Soc. Rev.
Electrochemistry
Recently, there has been rapidly growing interest in electrochemistry applications of MOFs, such as in energy storage and
conversion (supercapacitors) or electrode materials, which have
been proven to be excellent candidates in this field.269–273 It is
mainly because the chemical composition of MOFs can be tuned
at the molecular level. Moreover, the high porosity of MOFs
is favorable for fast mass transportation of related species. The
studies of Zr-MOFs in electrochemistry application mostly
involve three aspects: ionic and proton conduction, as well as
supercapacitors.
Ameloot et al. prepared LiOtBu-grafted UiO-66 upon
dehydration of Zr-based clusters followed by lithium alkoxide
grafting.122 The grafted tBuO anions shielded the negative
charge, leading to a higher mobility of Li(I) ions. As a result, the
ionic conductivity of the LiOtBu-grafted UiO-66 was promoted
to be 1.8 105 S cm1, which is on par with current solid
polymer electrolytes.274,275 The activation energies (Ea) declined
as low as 0.18 eV, which is in the range for superionic
conductors and even slightly lower than that of H+ conduction
in Nafion.276,277 Moreover, the impedance spectra obtained
from a setup prepared by placing a pellet of LiOtBu-grafted
UiO-66 in a Swagelok cell showed that no Li(I) blocking passivation layer was formed. Furthermore, this cell could be cycled
three times and the crystallinity of the MOF was intact after
contacting with the Li electrode for three days, indicating a
high electrode compatibility. With these properties, the solid
electrolytes based on this LiOtBu-grafted UiO-66 are potentially
usable in Li-based batteries.
On the other hand, ligand defects in MOFs have been demonstrated to have important effects on their pore sizes and surface
properties, which subsequently influence their performances,
such as in gas storage and catalysis. Interestingly, Taylor et al.
first showed that defect control could also enhance the proton
conductivity of Zr-MOFs.278 Different from common methods
for improving proton conductivity of MOFs by increasing overall acidity, they controlled the formation of ligand defects by
introducing stearic acid to change the proton mobility within
the UiO-66 framework.279–281 The results demonstrated that the
ligand defects were favorable for improving proton mobility.
As a result, proton conductivity increased by nearly three orders
of magnitude to 6.93 103 S cm1.
In addition, supercapacitors represent an important class of
energy storage devices because of their high power density.270,283
Choi et al. recently prepared twenty-three different nanocrystals
of MOFs (nMOFs) with various structures, geometries and
functionalities, which were applied in the fabrication of MOF
thin films doped with graphene and then incorporated into
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Fig. 37 (a) Structure of PCN-225; (b) pH dependent fluorescence intensity of PCN-225 with pH ranging from 0 to 10.2 measured under excitation of
415 nm; (c) fluorescence emission of PCN-225 at different pH values matching the simulation fluorescence intensity; (d) protonation and deprotonation
processes of porphyrin involved in the PCN-225 framework in experimental acidic and basic media (pH = 0–10.2). Reprinted with permission from
ref. 145. Copyright 2013 American Chemical Society.
Fig. 38 A schematic illustration of the phosphate coordination induced fluorescence enhancement effect in MOFs of UiO-66-NH2 for the selective and
sensitive detection of phosphate with inorganic nodes as natural recognition sites and organic struts as the fluorescent reporter. Reprinted with
permission from ref. 261. Copyright 2015 The Royal Society of Chemistry.
This journal is © The Royal Society of Chemistry 2016
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Fig. 39 (a) Synthesis of Zr6(m3-O)4(m3-OH)4(ADC)6; (b) scheme showing
the X-ray induced generation of fast photoelectrons from heavy metals
followed by the scintillation of the anthracene-based linkers in the visible
spectrum.151
devices to serve as supercapacitors.282 Among them, Zr6(m3-O)4(m3-OH)4(BPYDC)6 (nMOF-867) exhibited the highest capacitance
(Fig. 40). The max stack capacitance (0.644 F cmstack3) and max
areal capacitance (5.085 mF cmareal2) were over six and ten
times those of activated carbon and graphene, respectively. The
gravimetric capacitance, maximum energy, and power densities
of nMOF-867 were estimated to be 726 F gnMOF-867/electrode,
6.04 104 Wh cmstack3, and 1.097 W cmstack3, respectively.
It is worth noting that at the power density of 0.386 W cmstack3,
the energy density (2.86 104 Wh cmstack3) of nMOF-867 was
over three times that of activated carbon. Moreover, nMOF-867
based on this supercapacitor could consistently preserve its excellent performances for at least 10 000 cycles under the conditions of
exposing to the maximum voltage (0–2.7 V).
4.6
Porous carrier
Zr-MOFs are excellent candidates as carriers for various applications because of their porosity and high chemical and thermal
stability. The integration of Zr-MOFs with encapsulated functional entities leads to the formation of new multifunctional
composites, which could exhibit unique properties superior to
the individual components.284,285 Moreover, the multifunctional
nature and the remarkable properties of such composites will
enrich functional applications of MOFs, typically in catalysis,
molecule adsorption, as well as drug delivery.
As a mesoporous Zr-MOF, PCN-777 has high chemical and
thermal stability with the highest pore volume of 2.8 cm3 g1.147
Moreover, in this MOF each Zr6 cluster is coordinated by six
carboxylates, leaving rich terminal OH groups pointing into the
mesoporous cages, which renders PCN-777 a suitable candidate
as a carrier. Feng et al. used 4-tert-butylbenzolate (tBu) and
4-carboxy-1-methylpyridinium iodide (Me–Py) to modify the internal surface of PCN-777. After that, the originally hydrophilic
properties of the internal surface changed into highly hydrophobic.
Three functionalized guest molecules, meso–tetra(4-sulfonatophenyl)porphyrin (TSPP), tris(2,20 -bipyridine)-dichlororuthenium(II)
([Ru(bpy)3]Cl2), and tetra-amido macrocyclic ligand catalytic
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activator (TAML-NaFeB*), were then encapsulated into the
modified PCN-777. Due to the formation of hydrogen bonding
between peripheral sulfonic groups in TSPP and terminal OH/H2O
groups in the Zr-based clusters, the pristine PCN-777 exhibited the
highest loading amount and the slowest leaching towards TSPP
relatively. Moreover, the Me–Py-modified PCN-777 with the lowest
porosity showed the highest volumetric loading amount of anionic
TAML-NaFeB*. As expected, the tBu-modified PCN-777 displayed
the weakest interactions with both TSPP and TAML-NaFeB*
because of the nonpolar feature of the internal pore surface of
the modified PCN-777. In contrast, no preferential [Ru(bpy)3]Cl2
loading in any modified PCN-777 was observed since the functional part [Ru(bpy)3]2+ is cationic.
Phosphorescent Zr-MOFs can serve as supports to encapsulate
molecular photosensitizers for hydrogen evolution reactions
(HERs). Initially, Zhang et al. loaded Pt nanoparticles with different
sizes into two phosphorescent Zr-MOFs, Zr6(m3-O)4(m3-OH)4(BPDC)5.94(L1)0.06 and Zr6(m3-O)4(m3-OH)4(L2)6 via the MOF-mediated
photoreduction of K2PtCl4 (Fig. 41a and b).286 The [Ir(ppy)2(bpy)]Clderived dicarboxylic acids H2L1 and H2L2 were firstly synthesized by
treating [Ir(ppy)2Cl]2 with diethyl (2,20 -bipyridine)-5,50 -dicarboxylate
(Et2L1) and dimethyl (2,2 0 -bipyridine)-5,5 0 -dibenzoate (Me2L2),
respectively, followed by the base-catalyzed hydrolysis. Based
on the synergistic photoexcitation of the MOF frameworks and
electron injection into the Pt nanoparticles, the obtained Pt@MOFs
can serve as effective photocatalysts for hydrogen evolution. It
was found that the two Pt@MOFs gave a total Ir-TON of 3400 and
7000, respectively, which are 1.5 and 4.7 times as high as that of
homogeneous control [Ir(ppy)2(bpy)]Cl/K2PtCl4 (2200 and 1500)
under the same conditions. The enhanced photocatalytic activity
of Pt@MOFs was thought to be mainly attributed to the efficient
electron transfer between [Ir(ppy)2(bpy )] species and Pt NPs.
That is not only favorable for improving hydrogen reduction rates
but also avoiding the decomposition of Ir complexes. Then Wang
et al. developed the charge-assisted self-assembly process to encapsulate [P2W18O62]6 into Zr6(m3-O)4(m3-OH)4(L)6 bearing [Ru(bpy)3]2+
to get the POM@UiO composite (Fig. 41c and d).287 The [Ru(bpy)3]2+derived dicarboxylate ligand (H2L) was prepared by heating the
above Me2L2 followed by base-catalyzed hydrolysis. It was found
that with methanol as the sacrificial electron donor, TONs for the
hydrogen production could reach a maximum value of 79. However,
with triethanolamine as the sacrificial electron donor, the TONs of
the photocatalytic HERs reached 540, which was thirteen times
higher than that of the homogeneous control. Moreover, POM@UiO
could be recycled and reused at least three times with only a
slight loss of catalytic activity and crystallinity of the MOF.
In addition, Choi et al. embedded Pt nanoparticles (NPs) into
the nanocrystalline UiO-66 samples functionalized with –SO3H
(S) and –NH3+ (N) or both of them to form Zr-MOF supported
composites: Pt C nUiO-66-S, Pt C nUiO-66-N, and Pt C nUiO66-SN.288 By treating three samples with NaCl, triethylamine,
and sodium bicarbonate, another three samples Pt C nUiO-66S*, N*, and S*N* embedded with Pt-NPs were obtained. These
six samples with different chemical functionalities were
applied in the catalytic conversion of methylcyclopentane. It
was found that these chemical functionalizations played an
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Fig. 40 (a) Construction for nMOF supercapacitors; (b) the stack capacitance of nMOF-867 and comparison to activated carbon and graphene.
Reprinted with permission from ref. 282. Copyright 2014 American Chemical Society.
Fig. 41 Schematic representation of the synergistic photocatalytic hydrogen evolution process via the photoinjection of electrons from the light harvesting
MOF frameworks into the Pt NPs (a) and POMs (c); (b) time-dependent hydrogen evolution curves of Pt@1 (green), Pt@2 (red), and homogeneous control
[Ir(ppy)2(bpy)]Cl/K2PtCl4 (blue and black for different Pt/Ir ratios) under optimized conditions; (d) time-dependent HER TONs of POM@UiO with methanol as
the sacrificial electron donor. Reprinted with permission from ref. 286 and 287. Copyright 2012 and 2015 American Chemical Society.
important role in the product selectivity and the conversion of
methylcyclopentane (MCP) to acyclic isomers, olefins, cyclohexane,
and benzene. Pt C nUiO-66-S showed the highest selectivity to
cyclohexane and benzene (62.4% and 28.6%, respectively), and
This journal is © The Royal Society of Chemistry 2016
the catalytic activity was twofold compared with the nonfunctionalized Pt C nUiO-66. For Pt C nUiO-66-N, while the
selectivity for C6-cyclic products decreased to o50%, the
acyclic isomer selectivity increased to 38.6%. Interestingly, for
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the Pt C nUiO-66-SN, benzene was the dominant product with
olefins and acyclic isomers as byproducts, while no cyclohexane
was produced. In addition, Xu et al. modified UiO-66 with
predesigned Pt-NPs for the electrocatalysis of H2O2 oxidation.289
The Pt-NPs@UiO-66 composite presented excellent electrocatalytic activity reflected by remarkable anti-interference performance,
the extended linear range (5 mM–14.75 mM), the low detection
limit, as well as good stability and reproducibility. Except commonly used Pt-NPs and UiO-66, Zhuang et al. incorporated Pd-NPs
into DUT-67 to obtain Pd-NP@DUT-67, which was then applied
in a Suzuki coupling reaction.290 It was found that the conversion could reach 99%, while the selectivity was 89%. In addition,
Pd-NPs@DUT-67 also showed good catalytic properties for the
hydrogenation of nitrobenzene, with a conversion of 99% and a
selectivity of 99%.
Note that Zr-MOFs as carriers are not only used in catalysis but
also in the encapsulation of drugs224,291–294 and cosmetics.295–297
D. Cunha et al. chose different functionalized UiO-66-X (X = H,
CH3, 2OH, NH2, NO2, Cl, Br, and 2CF3) MOFs to encapsulate
bioactive molecules, ibuprofen, and caffeine.298 The functional
groups in the MOF played a crucial role in the encapsulation
performance. It was demonstrated that the caffeine entrapping
was enhanced when the functionalized linker had a large octanol–
water partition coefficient and low hydrogen bond accepting
ability. In contrast, the ibuprofen loading was optimized when
the functional groups of the linker had a large solvent surface area
and free volume, as well as low hydrogen bond accepting ability.
Moreover, the solvents used for the biomolecule entrapping also
significantly impacted the encapsulation performances due to
the existence of competitive adsorption between active molecules and the solvents. More importantly, the drug payloads of
these Zr-MOFs outperformed conventional porous solids or
current drug formulations.
Similarly, the studies of Zr-MOFs as carriers for the encapsulation of luminescent molecules have also been performed.
The resulting composites displayed excellent sensing properties. For instance, He et al. reported the use of nanoscale UiO68-NH2 as a carrier of fluorescein isothiocyanate (FITC) for the
intracellular pH sensing in live cells.299 The resulting composite was demonstrated to have high FITC loadings, effective
fluorescence, and remarkable ratiometric pH-sensing properties. The Zr-MOF also remained structurally intact after the
rapid and efficient endocytosis process. In addition, Hao et al.
used UiO-66-(COOH)2 as a carrier to encapsulate Eu(III) cations
to get Eu(III)@UiO-66-(COOH)2, which was applied as a fluorescent probe for Cd(II) detection in aqueous solutions. This
luminescent probe showed excellent sensitivity with the detection limit of 0.06 mM and a fast response speed within B1 min.
The luminescence intensity of Eu(III) was enhanced to about
8-fold, which could be ascribed to the effective energy transfer
from the ligand to Eu(III) generated from the coordination of
Cd(II) to Eu(III)@UiO-66-(COOH)2.
With well-defined porous structures, MOFs can serve as
versatile carriers for the growth of metallic nanostructures
in their pores. Additionally, the high surface area and large
opening windows of MOFs ensure that the surface of metallic
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Fig. 42 Schematic representation of MOF-545 as the carrier for the
growth of gold metallic nanowires. Reprinted with permission from
ref. 300. Copyright 2015 American Chemical Society.
nanostructures is readily accessible for appropriate functions
for specific applications. Volosskiy et al. used a Zr-MOF, MOF545, with a 1D pore as a carrier to grow ultrafine gold metallic
nanowires with a well-controlled shape and size (Fig. 42).300
The intrinsic surface could be preserved because no addition of
bulky and long-chain surfactants was used for the nanowire
growth inside the MOF. Such a strategy was also extended to
the synthesis of a hybrid Pt@MOF. In addition, the synergy of
high surface area, porphyrin active sites as well as highly active
nanowires contributes to distinct properties and thus provides
platforms of hybrid composites for catalytic applications.
5. Conclusion and prospect
Although the class of Zr-MOFs is just a member of the large
MOF family, starting from the seminal discovery of the UiO-66
series, Zr-MOFs have received widespread attention by virtue
of their preeminent chemical and physical properties. In particular, Zr-MOFs inspired great interest of researchers in the
last three years, which can be demonstrated through an everescalating number of structures and function explorations.
From the point of intriguing properties, Zr-MOFs provide an
ideal platform for tapping into the superior applications in the
future. Through summarizing a majority of the reported studies
of Zr-MOFs, this review provides a holistic generalization of
state-of-the-art development of the synthesis, structure, and
applications of Zr-MOF materials. A large number of MOF
materials with excellent properties and potential applications
were thus reviewed.
In terms of the design strategies and synthetic methods for
constructing Zr-MOFs, the examples of modulated synthesis,
isoreticular expansion, topology-guided design, and postsynthetic modification are presented in detail. All of these
methods and strategies are successfully used in building
various Zr-MOFs, which open a new avenue for the rational
design of targeted Zr-MOFs with the enhancement of chemical
compatibility for expanding a wide variety of potential applications. In addition, these methods and strategies also provide good
guidance and reference for the design and synthesis of other
MOFs. However, these strategies are still developed by trial and
error to some extent and in-depth understanding into the mechanism of formation is still lacking. So, further exploration is highly
desirable in this regard. With regard to the structures, five types
of Zr-clusters, Zr6O8, Zr8O6, ZrO6, ZrO7, and ZrO8, as the cores
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of SBUs have appeared in the literature. We are confident that,
with the gradual development of this field, Zr-MOFs based on
other SBUs will spring up in the future years. In particular,
relying on the use of newly selected/designed organic ligands,
the structural types of Zr-MOFs will continuously increase, and
more new members of Zr-MOFs with unique properties and
functions will emerge.
Based on their excellent porosity and structural stability,
significant efforts have been put into versatile potential applications of Zr-MOFs, including catalysis, molecule adsorption and
separation, drug delivery, fluorescence sensing, electrochemistry,
porous carrier and so on. Even if these studies of Zr-MOFs cover a
wide scope of applications, distribution of the existing studies in
different application aspects is not balanced, which are mainly
focused on catalysis right now. Moreover, the overall investigation
on Zr-MOFs in practical applications still lacks a systematic
approach and depth. In comparison to many other MOF
subclasses, such as ZIFs, Cr-MOFs, Al-MOFs, Cu-MOFs, and
Zn-MOFs, Zr-MOFs are still at their early stage. Therefore, more
extensive investigations are highly desired to explore the great
potential applications of various Zr-MOFs.
Notwithstanding there are still great challenges to be overcome in the Zr-MOF field. This class of materials remains
predominant in the MOF family due to their framework stability
and unique properties. Moreover, green and economically viable
methods for the scalable synthesis of some Zr-MOFs with repeatable
quality have been successfully developed. The massive production
of Zr-MOFs with excellent stability implies their unlimited
potential in commercial applications. Once practical applications
are in place, it will be certainly a leap in the development of MOF
materials. With this review, we also hope to inspire researchers
with new and smart ideas to stimulate the emergence of new
concepts and applications in the field of MOFs, with numerous
possibilities offered in this fascinating area.
Acknowledgements
We thank the financial support of the Natural Science Foundation of China (No. 21322601, 21271015, and 21576006) and
the Program for New Century Excellent Talents in University
(No. NCET-13-0647).
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