A Modulator-Induced Defect-Formation Strategy to Hierarchically Porous Metal–Organic Frameworks with High Stability

Article abstract of DOI:10.1002/anie.201610914

The pore size enlargement and structural stability have been recognized as two crucial targets, which are rarely achieved together, in the development of metal–organic frameworks (MOFs). Herein, we have developed a versatile modulator-induced defect-formation strategy, in the presence of monocarboxylic acid as a modulator and an insufficient amount of organic ligand, successfully realizing the controllable synthesis of hierarchically porous MOFs (HP-MOFs) with high stability and tailorable pore characters. Remarkably, the integration of high stability and large mesoporous property enables these HP-MOFs to be important porous platforms for applications involving large molecules, especially in catalysis.

Full text of DOI:10.1002/anie.201610914

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610914
German Edition: DOI: 10.1002/ange.201610914
Hierarchically Porous MOFs
A Modulator-Induced Defect-Formation Strategy to Hierarchically
Porous Metal–Organic Frameworks with High Stability
Guorui Cai and Hai-Long Jiang*
Abstract: The pore size enlargement and structural stability
have been recognized as two crucial targets, which are rarely
achieved together, in the development of metal–organic frame-
works (MOFs). Herein, we have developed a versatile mod-
ulator-induced defect-formation strategy, in the presence of
monocarboxylic acid as a modulator and an insufficient
amount of organic ligand, successfully realizing the control-
lable synthesis of hierarchically porous MOFs (HP-MOFs)
with high stability and tailorable pore characters. Remarkably,
the integration of high stability and large mesoporous property
enables these HP-MOFs to be important porous platforms for
applications involving large molecules, especially in catalysis.
sizes and high stability are of great importance toward
practical applications of MOFs, unfortunately, the two
features are usually contradictory and hardly achieved
together in a single MOF. A very limited number of MOFs,
such as, MIL-100, MIL-101, PCN-22X series, enlarged UiO
structures, which meet both requirements, have received
exceptional attention and been intensively studied. However,
the tailoring of their pore sizes and porosity is difficult. In this
context, the development of reliable methods to obtain stable
HP-MOFs with tailored pore structures and tunable proper-
ties is imperative but remains a grand challenge.
Herein, we have developed a simple and versatile
modulator-induced defect-formation strategy for the rational
design and controllable synthesis of various HP-MOFs with
exceptional stability, in which pore sizes and pore volumes
can be facilely tuned. Different from the traditional synthetic
methods, the current HP-MOFs are prepared via reactions
between metal precursors and insufficient amounts of ligand,
in the presence of monocarboxylic acid as a modulator. The
modulator plays a dual role: the carboxylic acid coordinates
to the metal ion for the formation of metal–oxo clusters, while
the alkyl chain creates structural defects and additional pore
space (Scheme 1a–c), as previously reported for example, for
[
9]
M
etal–organic frameworks (MOFs) as a relatively new
class of porous crystalline materials have attracted intense
[
1]
research interest in recent two decades. The diversity,
tailorability, and highly porous character of their structures
gives MOFs great potential in multifunctional applica-
tions.
functions of MOFs. Although a few mesoporous MOFs have
[
1–4]
The porosity plays a key role in the structures and
[
5]
been reported, the pore sizes of MOFs are mainly tunable in
a microporous regime, which is actually unfavorable as the
small micropore slows down diffusion and restricts the
accessibility of large molecules. Hierarchically porous
MOFs (HP-MOFs), in which micropores contribute to the
high surface area while mesopores/macropores across the
micro-porous matrix provide the required accessibility to
large molecules for quick diffusion, are highly desired to
achieve diverse ends. In addition to the classical methods of
extending the length of ligands and/or appropriate assembly
[
5a,c–f]
between metal clusters and ligands,
recently HP-MOFs
have been developed via different approaches, such as soft-/
hard-template or template-free method, stepwise ligand
exchange, metal–ligand-fragment co-assembly, imperfect
[
6–8]
crystallization.
Unfortunately, most of these approaches
are limited to respective synthetic systems that are adaptable
to several particular MOFs only and/or they focus on some
unstable MOFs.
Scheme 1. Schematic illustration of the synthesis of HP-MOFs with
In fact, most MOFs suffer from framework collapse
adjustable porosity using UiO-66 as an example, see text for details.
[2d]
toward even water/moisture.
Although both large pore
[
8f,h]
[*] G. Cai, Prof. Dr. H.-L. Jiang
creating spng- and pmg-MOF-5.
The pore diameter can be
Hefei National Laboratory for Physical Sciences at the Microscale,
CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innova-
tion Center of Suzhou Nano Science and Technology, Department of
Chemistry, University of Science and Technology of China
Hefei, Anhui 230026 (P.R. China)
E-mail: jianglab@ustc.edu.cn
Homepage: http://staff.ustc.edu.cn/~jianglab/
systematically tuned via altering the length and concentration
of the modulator (Scheme 1d–f). To demonstrate the general-
ity of this approach, in addition to the detailed investigations
on UiO-66 and its derivatives, different types of HP-MOFs
based on stable MIL-53, DUT-5, MOF-808, have also been
prepared. It is worth stressing that the stability of these MOFs
well remains upon being converted into their hierarchically
porous forms. Compared to the pristine MOFs, the resultant
[
10]
The ORCID identification number(s) for the author(s) of this article
can be found under http://dx.doi.org/10.1002/anie.201610914.
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stable HP-MOFs manifest much improved catalytic perfor-
mance in large-molecule reactions.
dominant micropores in the original structure. Further
increasing of the ratio to 1:0.5, a pronounced hysteresis loop
revealed the typical mesoporous character, in agreement with
the pore size distribution of 1–8.6 nm with the peak at 5.5 nm.
However, the very high ratio of 1:0.3 led to the small
hysteresis and the pore size range was reduced to 5.9 nm with
the peak at 3.4 nm. The results clearly show that, the
formation of structural defects and mesopores as well as
lower surface area is caused by the reduced amount of BDC
ligand (optimal Zr/BDC = 1:0.5), a crucial factor to guarantee
the generation of the hierarchical pores.
To clarify the length influence of the alkyl chain in the
modulator, different monocarboxylic acids with varying
lengths were introduced, while Zr/BDC ratio was kept to
the optimized 1:0.5. Figure 1c,d show the representative N2
sorption isotherms and pore size distributions of the resultant
UiO-66 and HP-UiO-66 in the presence of acetic acid,
octanoic acid, dodecanoic acid and palmitic acid, respectively.
When the alkyl chain length is very short, such as acetic acid,
the mesopore is hardly generated and thus no obvious
The well-established MOF, microporous UiO-66 (Zr O -
6
4
(
OH) (BDC) , BDC = 1,4-benzenedicarboxylate), with high
4 6
chemical/thermal stability, was firstly investigated, which can
be obtained by solvothermal reaction of ZrCl and H BDC in
4
2
[
10a]
DMF.
If monocarboxylic acid has been introduced into the
reaction system to pre-coordinate to Zr-oxo clusters, a car-
boxylate ligand (such as H BDC) with lower pKa than the
2
monocarboyxlic acid modulator readily replaces the mono-
carboyxlic acid, and an incomplete exchange would cause
[11]
structural defects. Inspired by this, we envision that it might
be possible to create more and larger structural defects by
introducing excess modulator to pre-occupy the coordination
sites but an insufficient amount of H BDC to only partially
2
replace the modulator. Then the modulator can be removed
to release the large pore space to afford hierarchically porous
UiO-66 (HP-UiO-66) (Scheme 1a–c).
The influence of the amount of BDC ligand was examined
by using an excess of modulator with Zr/dodecanoic acid of
1
1
:35 and the varying ratios of Zr/BDC from 1:1, 1:0.8, 1:0.5 to
:0.3 (Figure 1a,b). When Zr/BDC was 1:1, a ratio for perfect
mesoporous character can be found in N sorption curves.
When the alkyl chain length gradually increases from 8 to 12
2
UiO-66, the introduced modulator did not disturb the
formation of intact UiO-66, inferring that BDC was able to
replace all pre-coordinated modulators. When the ratio was
carbon atoms, N sorption isotherms of the obtained HP-UiO-
2
66 exhibit accordingly enlarged hysteresis loops. Amongst
them, the largest hysteresis loop and pore size up to 8.6 nm
(peak at 5.5 nm) can be obtained with dodecanoic acid. The
results indicate that the elongated alkyl chain of the
modulator could occupy a large space and cause steric
increased to 1:0.8, N sorption isotherm gave a small hyste-
2
resis with pore size range extending to 6.8 nm, compared to
IV
hindrance that hampers the coordination between Zr and
BDC around the long modulator, thus leading to large pores
upon the removal of the modulator. However, upon adding
the very long palmitic acid with too high pK , the pore
a
diameter shrinks instead, possibly owing to the acid’s low
IV
solubility and weaker coordination with Zr .
Knowing that dodecanoic acid is the most suitable
modulator, its optimized amount was further examined by
altering the feed ratio of Zr/dodecanoic acid. With gradually
increased X value in the molar ratio of Zr/BDC/dodecanoic
acid = 1/0.5/X, the generated mesopores reached a maximum
value at X = 35, followed by a slight decrease (Figure 1e,f).
Note, UiO-66 features micropores only and a low surface area
was obtained without modulator, suggesting that the modu-
lator is necessary not only for creating structural defects and
additional pore space but also for improving MOF crystal-
linity and surface area. In other words, the insufficient ligand
amount and the presence of suitable modulator are the two
pre-requisites for the formation of HP-MOFs. It is proposed
that the reaction begins with a rapid formation of the discrete
modulator-capped Zr-oxo clusters, which then assembles into
the final 3D structure via reversible ligand exchange with the
ditopic BDC linker. Owing to the insufficient amount of
BDC, the presence of residual modulators makes the
structure contain the defects of missing linkers. The defect
concentration of the missing linkers is controlled by the
amount of BDC and modulator, as well as the alkyl chain
length in the modulator. Upon the final removal the
modulator by activation, large pores can be generated in the
HP-UiO-66.
Figure 1. a),c),e) N sorption isotherms and b),d),f) DFT pore size
distributions for HP-UiO-66 prepared under different reaction condi-
tions: a),b) molar ratio of Zr/BDC/dodecanoic acid (1/X/35, X=1.0,
2
0
.8, 0.5, 0.3); c),d) in the presence of various modulators (x acid:
x=2: acetic acid, x=8: octanoic acid, x=12: dodecanoic acid, and
x=16: palmitic acid) with a fixed Zr/BDC/modulator ratio of 1/0.5/35;
e),f) molar ratio of Zr/BDC/dodecanoic acid (1/0.5/X, X=0, 20, 35,
5
0).
2
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Based on the above parameters, ideal HP-UiO-66 can be
induced approach, all their corresponding hierarchically
obtained with the optimal recipe of Zr/BDC/dodecanoic
acid = 1/0.5/35, the default parameter for HP-UiO-66 here-
after unless otherwise mentioned. Transmission electron
microscopy (TEM) observation for HP-UiO-66 clearly
shows the worm-like mesopores (Figure S1 in the Supporting
Information). The observed smaller pore sizes than those
from pore size distribution analysis could be due to the
porous structures can be prepared (Figures S11–S25). Similar
to the UiO series of MOFs, all these HP-MOFs inherit the
very high chemical stability of their parent MOFs (Figur-
es S19–S26). For comparison the porous structural features
and stability of the HP-MOFs obtained and their correspond-
ing parent MOFs have been summarized in Table 1 and
Table S1.
[6b]
shading/overlay of pore walls in the images and/or common
[12]
instability of MOFs exposure to electron beam. It is worth
noting that, the emergence of new mesopores in HP-UiO-66
does not significantly affect its intrinsic micropores, as
Table 1: Surface area, pore features, and stability for HP-MOFs.
MOF
S
[m g
DMeso
Stability range
acid/base
BET
2
À1 [a]
[b]
2
À1
]
[nm]
confirmed by the similar surface areas (980 vs. 1204 m g ,
respectively for HP-UiO-66 and UiO-66). Intuitively, the
abundant defects in the structure would be expected to lead to
a weak MOF structure as a result of a lack of ligand support
for Zr–oxo clusters making the structure fragile. Unexpect-
edly, HP-UiO-66 is chemically viable in solutions from
concentrated HCl to alkaline solution (pH 12), similar to
the pristine UiO-66, as evidenced by powder X-ray diffraction
HP-UiO-66
HP-UiO-66-NH
HP-UiO-66-NO2
HP-UiO-67
HP-MIL-53
HP-MIL-53-NH2
HP-DUT-5
HP-MOF-808
980
789
755
1948
889
605
1337
453
5.5
4.5
4.3
2-12
5-50
2.5-50
4-50
5-50
conc.HCl/pH 12
conc.HCl/pH 12
conc.HCl/pH 12
pH 1/pH 12
pH 2/pH 10
pH 2/pH 10
2
pH 2/pH 11
conc.HCl/pH 11
(
PXRD) data (Figure 2). The results are encouraging as the
[a] BET surface area. [b] Mesopore diameter determined by the density
MOFs featuring hierarchical pores and high stability are
highly sought-after for practical applications in diverse
domains.
functional theory (DFT) method by N adsorption branch at 77 K.
2
With highly stable HP-MOFs in our hands, as a proof-of-
concept study, we compared the ability of HP-UiO-66 and its
parent microporous UiO-66, toward applications involving
large molecules. For the encapsulation of functional guest
molecules, such as dyes, polyoxometalates, and metallopor-
phyrins, which are usually larger in sizes than the micropores
of MOFs such as UiO-66, HP-UiO-66 unambiguously dis-
plays its advantages. As visual evidence, the dye uptake
experiments were conducted to confirm that the large pores
are involved in uptake by HP-UiO-66. Upon soaking HP-
UiO-66 in a DMF solution of coomassie brilliant blue R250
(R250, ca. 2.7 ꢀ 1.7 ꢀ 0.9 nm in size), the solution gradually
faded and the white HP-UiO-66 accordingly turned to
brilliant blue. In addition to the large pores (ca. 5.5 nm) in
HP-UiO-66, the defect generation might cause the framework
to be positively charged, facilitating the trapping of anionic
Figure 2. PXRD patterns showing chemical stability of a) UiO-66 and
b) HP-UiO-66.
The presence of functional groups on UiO-66 has been
demonstrated to be well tolerated in the formation of HP-
MOFs. As representatives, H BDC-NH and H BDC-NO
2
[13]
dyes such as R250. In stark contrast, the pristine UiO-66
(window size: ca. 0.6 nm) cannot uptake such a large molecule
and its color remains unchanged after even longer soaking
time (Figure 3a).
2
2
2
employed in the synthesis have successfully produced HP-
UiO-66-NH and HP-UiO-66-NO , respectively, evidenced
2
2
by the hysteresis loops in the N2 sorption curves and
corresponding pore size distributions up to 6 nm (Figures S2
and S3). It is well established that the swapping the BDC
ligand in UiO-66 for the longer 4,4’-biphenyldicarboxylic acid
The incorporation of catalytically active moieties into
MOFs is an effective solution to protect them from aggrega-
tion and facilitates their dispersion. The in situ incorporation
of foreign molecules usually impedes the MOF formation and
the post-incorporation after MOF formation poses a pre-
requisite of large pore sizes. In this case, HP-MOFs create
a straightforward approach for the impregnation of large
active species into its mesopores while its micropores would
merit capillary force. Taking metalloporphyrin, such as Fe-
TCPP, as an example of biomimetic catalytic probe, although
it almost cannot be adsorbed by the pristine UiO-66 from
DMF solution, the HP-UiO-66 with approximately 5.5 nm
mesopores was able to efficiently capture the molecule and
exhibited fairly good catalytic activity and recyclability
(Figure 3b, and Figures S27, S28).
(
BPDC) gives UiO-67. Likewise, HP-UiO-67 can be obtained
by such a modulator-induced approach, featuring large
hysteresis loop in its N sorption curves and wide pore size
2
range of 1–12 nm (Figure S4). To our delight, all the
functionalized HP-UiO-66 and HP-UiO-67 exhibit high
chemical stabilities that are almost identical to their corre-
sponding pristine structures, making them applicable to real-
life conditions (Figures S5–S10).
To further illustrate the universality of this synthetic
strategy, different types of stable MOFs, including MIL-53,
DUT-5, MOF-808, and their functionalized derivatives, have
been investigated. Fortunately, following this modulator-
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forms for encapsulating large active molecules (such as dyes,
polyoxometalates, metalloporphyrins) to give porous func-
tional composites, which primarily manifest their superiority
in catalysis, but also exhibit significantly enhanced activity in
large-molecule-based reactions compared to the parent
microporous MOFs. This versatile synthetic strategy offers
an exciting opportunity and paves the way to the preparation
of diverse stable MOFs with the combination of intrinsic
micropores and additional mesopores or even macropores,
which are highly desirable for various real-life applications.
Acknowledgements
We gratefully thank the anonymous reviewers for their
valuable suggestions. This work is supported by the NSFC
(
21371162, 21673213, 21521001, 51301159), the 973 program
Figure 3. a) Color of the powder samples of UiO-66 and HP-UiO-66
before and after dye adsorption; b) UV/Vis absorption spectra record-
ing the TMB oxidation over different catalysts; c) Time conversion of
the ring opening of styrene oxide with methanol over HPW/HP-UiO-
(2014CB931803), NSF of Anhui Province (1408085MB23),
the Recruitment Program of Global Youth Experts, and the
Fundamental Research Funds for the Central Universities
6
6, HPW/UiO-66, HP-UiO-66, and UiO-66; d) Conversion of [3+3]
(WK2060190026, WK2060190065).
cycloaddition reactions between 1,3-cyclohexanedione and a,b-unsatu-
rated aldehydes over UiO-66 or HP-UiO-66.
Conflict of interest
As another alternative, the heterogenization of polyoxo-
metalates, a class of excellent acid catalysts, into HP-MOFs
was attempted. After soaking UiO-66 and HP-UiO-66 in
The authors declare no conflict of interest.
a
methanol solution of phosphotungstic acid (HPW,
Keywords: heterogeneous catalysis ·
H PW O ·nH O), the catalytic methanolysis of styrene
hierarchically porous materials · large molecules · metal–
organic frameworks · stability
3
12 40
2
oxide was examined. The resultant HPW-impregnated HP-
UiO-66 (HPW/HP-UiO-66) gave significantly higher activity
than HP-UiO-66, UiO-66, and HPW-impregnated UiO-66
(
HPW/UiO-66) because a higher content of active HPW can
be loaded into the HP-UiO-66 (Figure 3c). Moreover, HPW/
HP-UiO-66 not only exhibited superior catalytic performance
to HPW-loaded UiO-66, even based on a similar HPW
loading amount, but also was recyclable without remarkable
activity drop, indicative of good stability (Figure S29–S31). In
addition to the advantage in the incorporation of large active
components, the hierarchical pores facilitate the transport of
large substrates/products. The [3+3] cycloaddition reactions
between 1,3-cyclohexanedione and various a,b-unsaturated
aldehydes of different sizes were investigated (Figure 3d).
UiO-66 and HP-UiO-66 gave similar results for small
substrates, although UiO-66 showed a slightly lower activity
owing to the limited pore size, slow mass diffusion, and even
inaccessible active sites, which became much more pro-
nounced when a larger substrate was involved, again demon-
strating the superiority of HP-MOFs.
In summary, we have developed a modulator-induced
defect-formation strategy to the controllable synthesis of HP-
UiO-66 with tailored pore features. The synthetic approach
can be adapted to not only functionalized and elongated UiO
structures but also a variety of MOFs, suggesting the potential
universality for the preparation of HP-MOFs. Particularly,
these HP-MOFs effectively integrate the sought-after advan-
tages of high stability and hierarchical pore sizes, after the
removal of the modulator (Figure S32–S39). As a result, the
HP-MOFs obtained not only are promising as porous plat-
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Manuscript received: November 8, 2016
7, 7101.
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hierarchically Porous MOFs
G. Cai, H.-L. Jiang*
Cause and defect: Pore size enlargement
and structural stability are two crucial
targets but hardly achieved together in
metal–organic frameworks (MOFs). A
versatile modulator-induced defect-for-
mation strategy has been developed to
provide hierarchically porous MOFs (HP-
MOFs) with high stability and tailorable
pore characters, which are important
porous platforms for large molecule-
involved applications, especially in catal-
ysis.
&&&& — &&&&
A Modulator-Induced Defect-Formation
Strategy to Hierarchically Porous Metal–
Organic Frameworks with High Stability
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!