Direct Synthesis of Hierarchically Porous Metal-Organic Frameworks with High Stability and Strong Br?nsted Acidity: The Decisive Role of Hafnium in Efficient and Selective Fructose Dehydration

Article abstract of DOI:10.1021/acs.chemmater.6b00139

The direct synthesis of metal-organic frameworks (MOFs) with strong Br?nsted acidity is challenging because the functional groups exhibiting Br?nsted acidity (e.g., sulfonic acid groups) often jeopardize the framework integrity. Herein, we report the direct synthesis of two hierarchically porous MOFs named NUS-6 composed of either zirconium (Zr) or hafnium (Hf) clusters with high stability and strong Br?nsted acidity. Via the modulated hydrothermal (MHT) synthesis, these two MOFs can be easily synthesized at a low temperature (80 °C) with high throughput. They exhibit BET surface areas of 550 and 530 m² g^-1 for Zr and Hf one, respectively, and a unique hierarchically porous structure of coexisting micropores (～0.5, ～0.7, and ～1.4 nm) and mesopores (～4.0 nm) with dangling sulfonic acid groups. Structural analysis reveals that the hierarchical porosity of NUS-6 is a result of missing linkers and clusters of the parental UiO-66 framework. These unique features make NUS-6 highly efficient and selective solid acid catalysts for dehydration of fructose to 5-hydroxymethylfurfural (HMF), in which NUS-6(Hf) demonstrates a superior performance versus that of NUS-6(Zr) because of the stronger Br?nsted acidity contributed from Hf-μ₃-OH groups as well as smaller pore sizes suitable for the restriction of unwanted side reactions. Our results have demonstrated for the first time the unique attributes of Hf-MOFs featured by superior stability and Br?nsted acidity that can be applied as heterogeneous catalysts in biobased chemical synthesis.

Full text of DOI:10.1021/acs.chemmater.6b00139

Article
pubs.acs.org/cm
Direct Synthesis of Hierarchically Porous Metal−Organic Frameworks
with High Stability and Strong Brønsted Acidity: The Decisive Role of
Hafnium in Efficient and Selective Fructose Dehydration
†
†
†
†
†
‡
Zhigang Hu, Yongwu Peng, Yongjun Gao, Yuhong Qian, Shaoming Ying, Daqiang Yuan,
§
§
∥
†
†
,†
Satoshi Horike, Naoki Ogiwara, Ravichandar Babarao, Yuxiang Wang, Ning Yan, and Dan Zhao*
†Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore
‡State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, 350002 Fujian, China
§
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
∥
CSIRO Manufacturing, Clayton, Victoria 3168, Australia
*
S Supporting Information
ABSTRACT: The direct synthesis of metal−organic frame-
works (MOFs) with strong Brønsted acidity is challenging
because the functional groups exhibiting Brønsted acidity (e.g.,
sulfonic acid groups) often jeopardize the framework integrity.
Herein, we report the direct synthesis of two hierarchically
porous MOFs named NUS-6 composed of either zirconium
(
Zr) or hafnium (Hf) clusters with high stability and strong
Brønsted acidity. Via the modulated hydrothermal (MHT)
synthesis, these two MOFs can be easily synthesized at a low
temperature (80 °C) with high throughput. They exhibit BET
2
−1
surface areas of 550 and 530 m g for Zr and Hf one,
respectively, and a unique hierarchically porous structure of
coexisting micropores (∼0.5, ∼0.7, and ∼1.4 nm) and mesopores (∼4.0 nm) with dangling sulfonic acid groups. Structural
analysis reveals that the hierarchical porosity of NUS-6 is a result of missing linkers and clusters of the parental UiO-66
framework. These unique features make NUS-6 highly efficient and selective solid acid catalysts for dehydration of fructose to 5-
hydroxymethylfurfural (HMF), in which NUS-6(Hf) demonstrates a superior performance versus that of NUS-6(Zr) because of
the stronger Brønsted acidity contributed from Hf-μ -OH groups as well as smaller pore sizes suitable for the restriction of
3
unwanted side reactions. Our results have demonstrated for the first time the unique attributes of Hf-MOFs featured by superior
stability and Brønsted acidity that can be applied as heterogeneous catalysts in biobased chemical synthesis.
INTRODUCTION
Therefore, most researchers adopted the postsynthetic
■
26
modification (PSM) route to introduce sulfonic acid groups
Metal−organic frameworks (MOFs) as emerging porous
crystalline materials consisting of metal ions/clusters and
organic ligands have demonstrated strong potential as
heterogeneous catalysts.
Lewis acid catalysts (unsaturated metal sites) was reported for
2
7
into MOFs for applications such as heterogeneous catalysis
and proton conductivity.
28,29
However, the controlled sulfona-
1
−6
tion, compromised crystallinity, and restrained catalytic
efficiency remain the major issues to be further resolved.
Direct synthesis of sulfonated MOFs may solve most of the
issues mentioned above but relies largely on the superior
stability of the MOF scaffolds. One of the very few successful
The first example of MOFs as
7
the cyanosilylation reaction of aldehydes. Since then,
numerous MOFs have been used as Lewis acid catalysts for
8
−11
various reactions, such as cyanosilylation,
ring opening
1
1,12
9
examples is the Cr-based MOF MIL-101-SO H, which was
reaction,
Mukaiyama-Aldol reaction, Knoevenagel con-
redox reaction,
3
1
3,14
15−18
19
directly synthesized from sulfonated ligand [monosodium 2-
densation,
Pechmann reaction, CO2
2
0
21
fixation, dehydrogenation, etc. Compared to the relatively
comprehensive work that has been conducted on the Lewis
acidity of MOFs, exploring Brønsted acidity of MOFs is more
sulfoterephthalic acid (BDC-SO Na)] and was applied as an
3
30
efficient solid acid catalyst in cellulose hydrolysis and
22,23
challenging and remains less explored,
the weakened framework stability caused by the introduction of
Brønsted acidity typically through sulfonic acid groups.
mainly because of
Received: January 12, 2016
Revised: March 25, 2016
24,25
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
31
esterification of n-butanol. However, the stringent synthetic
conditions and toxic inorganic reagents of Cr MOFs greatly
limit their applications.
Zr MOFs such as UiO-66 have received a great deal of
attention because of their facile synthesis, high chemical
32,33
stability, and wide choice of ligands.
attempted the direct solvothermal synthesis of sulfonated UiO-
6 (UiO-66-SO H), which however collapsed upon activation
Foo et al. initially
6
3
and was attributed to the anionic charge or protons from the
sulfonate groups interacting with the 12-connected Zr
clusters. A modified solvothermal synthesis was adopted by
6
3
4
Biswas et al. in synthesizing a more stable UiO-66-SO H that
3
2
survived after activation with a Langmuir surface area of 769 m
−1
35
g . The increased stability was attributed to different
synthetic conditions as well as different guest molecules
̃
encapsulated within the pores. Juan-Alcaniz et al. used BDC-
SO K to synthesize sulfonated UiO-66 with pure water as the
3
solvent, affording HSO -Zr-MOF with a new framework
3
topology containing 8-connected Zr clusters that is different
6
31
from the UiO-66 type topology. This MOF exhibited high
activity and full reusability in the esterification of n-butanol with
acetic acid, but with limited thermal stability. Recently, Taylor
et al. modified the hydrothermal synthesis of HSO -Zr-MOF by
3
using BDC-SO H as the ligand with various amounts of acids
3
36
(
acetic acid or sulfoacetic acid) as additives. The fully
determined crystal structure of this MOF revealed a
combination of micropores and mesopores in this framework
caused by missing ligands and Zr clusters. However, this MOF
also suffered from limited thermal stability and started to
decompose upon activation at higher temperatures (≥70 °C).
Besides organic ligands, the inorganic metal clusters of MOFs
might be another source of intrinsic Brønsted acidity.
Belonging to the same elemental group, Hf and Zr share
similar properties. Therefore, Hf MOFs should possess equally
high thermal and chemical stability, which has been proven
3
7
previously. In addition, the higher dissociation energy of the
−
1
Hf−O bond (802 kJ mol ) versus that of the Zr−O bond (776
Figure 1. Rietveld refinements of (a) NUS-6(Zr) and (b) NUS-6(Hf)
based on synchrotron PXRD data.
−1
kJ mol ) could possibly turn the μ -OH groups of Hf clusters
into Brønsted acid sites, as suggested by Beyzavi et al.
Therefore, sulfonated Hf MOFs containing both organic and
inorganic Brønsted acid sites should be excellent solid acid
catalysts, which however have not been reported so far. In our
previous studies, we have reported a modulated hydrothermal
3
20
36
Taylor et al., but with slightly different unit cell sizes (Table
S1). Taking NUS-6(Hf), for example, we can best describe this
crystal structure by partially replacing the 12-connected Hf6
clusters in the parental UiO-66 type framework (Figure 2a)
(
MHT) approach that can be used to synthesize a series of
highly stable Zr and Hf MOFs in a green and scalable
with 9-connected Hf clusters yielding periodic missing of
6
ligands and Hf clusters (Figure 2b). As a result, squashed
6
3
3,38,39
way.
In this study, the MHT approach is used to prepare
mesopores (∼2.6 nm × 3.6 nm, measured from Hf to Hf
vertexes) that are interconnected with microporous tetrahedral
and octahedral cavities from UiO-66 type topology affording a
hierarchical porous structure can be generated (Figure 2c).
Such a hierarchically porous structure should be highly
beneficial for heterogeneous catalysis because of the dense
catalytic sites packed in micropores and facile mass transfer
through mesopores, as has been suggested in MOFs and
two sulfonated and hierarchically porous MOFs containing Zr
and Hf clusters, respectively. By acid−base titration, we have
for the first time proven the Brønsted acidity of Hf MOFs is
stronger than that of Zr MOFs. In addition, the Hf MOFs
exhibited catalytic performance better than that of their Zr
counterparts in catalyzing the dehydration of fructose to 5-
hydroxymethylfurfural (HMF).
11,40,41
zeolites.
On the basis of the isostructural analysis of the
RESULTS AND DISCUSSION
■
calculated crystal model, a molecular formula of
Powder crystalline MOF products named NUS-6 were easily
obtained by a typical MHT process in which mixtures of BDC-
SO Na with either ZrCl or HfCl were mildly heated at 80 °C
C120O291S Zr H {[Zr O (OH) L] ·xH O, MW = 8873,
15
21 386
6
4
8
3.5
2
Zr/S molar ratio of 1.40} was further obtained for NUS-6 (Zr),
while NUS-6 (Hf) shared an identical formula with Zr being
replaced by Hf (Table S1).
3
4
4
in aqueous solutions with the addition of acetic acid as the
modulator (see the Supporting Information for details).
Rietveld refinements of the synchrotron powder X-ray
diffraction (PXRD) data (Figure 1) of both NUS-6(Zr) and
NUS-6(Hf) reveal isostructures identical to the one reported by
Besides the structural features mentioned above, the rich and
versatile acid sites also make NUS-6, especially NUS-6(Hf),
attractive as heterogeneous catalysts. As revealed by one
fragment of the NUS-6(Hf) framework containing one 12-
B
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Figure 2. (a) A 12-connected Hf cluster. (b) A 9-connected Hf cluster. (c) A 2 × 2 × 2 super unit cell of UiO-66(Hf) with a = 20.7006(3) Å. (d) A
6
6
single unit cell of NUS-6(Hf) with a = 41.4718(2) Å. (e) A 1 × 2 × 2 super unit cell of NUS-6(Hf) with mesopores indicated by yellow spheres. (f)
A fragment of NUS-6(Hf) with potential Brønsted acid sites labeled by red clouds and Lewis acid sites labeled by cyan clouds.
connected Hf cluster and one 9-connected Hf cluster linked
6
6
by one sulfonated ligand (Figure 2f), two sources of Brønsted
acidity can be identified, one from the sulfonated ligand and the
other from μ -OH groups of Hf clusters. In addition, the
3
coordinatively unsaturated Hf sites in the 9-connected Hf6
cluster may potentially serve as Lewis acid sites. Compared to
NUS-6(Hf), NUS-6(Zr) may lack the source of Brønsted
acidity from Zr clusters, but with a possibly stronger Lewis
6
acidity from the coordinatively unsaturated Zr sites due to its
42
smaller cation radius. These features make NUS-6 very special
solid acids containing both Brønsted acidity and Lewis acidity
43
that have rarely been reported in MOFs.
The presence and distribution of sulfonate groups in NUS-6
are confirmed by Fourier transform infrared spectroscopy
(
(
1
FTIR) (Figure S1) and energy-dispersive X-ray spectroscopy
EDS) (Figures S2 and S3). The characteristic FTIR peaks at
−1
230 and 1180 cm can be attributed to the symmetric and
28
asymmetric stretching of OSO, respectively. Because of
the strong acidic reaction conditions, the ligand BDC-SO Na is
3
fully protonated in situ into BDC-SO H in NUS-6 as revealed
3
Figure 3. FE-SEM images of (a) NUS-6(Zr) and (b) NUS-6(Hf)
scale bars represent 1 μm; inset scale bars represent 100 nm) and
HR-TEM images of (c) NUS-6(Zr) and (d) NUS-6(Hf) (scale bars
by the missing Na signal from EDS elemental mapping (Figures
S2 and S3). This finding is in agreement with previous studies
in which BDC-SO Na or BDC-SO K was also protonated in
(
3
3
represent 2 nm).
situ when used for the growth of MOFs under acidic synthetic
3
1,34
conditions.
copy (FE-SEM) images (Figure 3a,b) show that both NUS-
(Zr) and NUS-6(Hf) crystals have a highly symmetric
polyhedral morphology, which is rarely seen in our previous
The field-emission scanning electron micros-
44
6
crystal growth mechanism. The internal fine structures of
NUS-6 crystals were investigated by high-resolution trans-
mission electron microscopy (HR-TEM) (Figure 3c,d), where
highly crystalline domains can be found with lattice spacings of
∼0.29 nm representing the (110) crystal plane of NUS-6
MOFs. These crystalline domains have a small size (∼5 nm)
and are distributed randomly within the crystal, suggesting an
agglomeration of crystalline subparticles within one big quasi-
crystal that also proves the suggested crystal growth
mechanism. It is probably the amphiphilicity of ligand and
aqueous reaction media that leads to fast nucleation. However,
the detailed crystal growth mechanism remains to be further
studied.
33,38
studies of Zr MOFs obtained through the MHT approach.
Closer observation of these crystals reveals agglomerations of
small particles (Figure 3a,b, insets), suggesting a reversed
crystal growth mechanism in which starting materials (ligands
and metal salts) form disordered aggregates that undergo
44
recrystallization from surface to core. The crystal growth
process of NUS-6(Zr) was further monitored by PXRD (Figure
S4), FE-SEM (Figure S5), and dynamic light scattering (DLS)
(Figure S6), confirming the formation of amorphous nano-
particles at first, which agglomerated and grew into crystalline
particles matching well with the aforementioned reversed
C
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Stability is another important factor in evaluating sulfonated
MOFs as heterogeneous catalysts. The previously reported
sulfonated Zr MOFs are more or less limited by their low
thermal stabilities: the frameworks start to decompose upon
Å (Figure 4b). NUS-6(Hf) has a pore size of ∼5.5 Å that is
slightly smaller than that of NUS-6(Zr) (∼6 Å), possibly
because of the larger radius of Hf that helps to reduce the pore
size. Interestingly, the pore size distribution calculated using the
Barrett−Joyner−Halenda (BJH) model suggests the existence
of mesopores with sizes of ∼3.8 and ∼4.2 nm for NUS-6(Zr)
and NUS-6(Hf), respectively (Figure 4b). Such mesopores may
come from either missing ligands/clusters revealed by the
crystal structures or the interstitial voids of crystalline
31,34−36
activation at higher temperatures.
To test the stability of
NUS-6 MOFs, the samples were vigorously activated by solvent
exchange for 3 days and evacuation under a dynamic vacuum at
1
50 °C for 24 h to fully remove trapped guest and solvent
molecules such as ligand, water, acetic acid, etc. To our surprise,
both NUS-6(Zr) and NUS-6(Hf) can survive under such harsh
activation conditions, which is proven by almost unchanged
38
subparticles constituting the NUS-6 quasi-crystals.
Compared to the previously synthesized sulfonated Zr
31,34−36
PXRD patterns (Figure 5) and hybrid type I/IV N sorption
MOFs,
there are mainly three distinctions in our
2
isotherms at 77 K with mild hystereses between adsorption and
desorption branches (Figure 4a), indicating hierarchical micro/
synthetic procedure. (1) BDC-SO Na is directly used as the
3
ligand without preprotonation. (2) Water is used as the solvent
with the addition of acetic acid as the modulator using a
different ratio. (3) The reaction is conducted at a lower
temperature of 80 °C, which actually is the lowest temperature
33
reported so far for the synthesis of Zr or Hf MOFs. We
suspect it is the amphiphilic ligand and mild reaction condition
that lead to molecular-level self-assembly of reagents
accounting for the reversed crystal growth. As a result, NUS-
6
quasi-crystals containing crystalline subparticles, which may
prevent the framework from collapsing under harsh activation
conditions due to the milder capillary force experienced by each
46
subparticle, are formed. To fully evaluate the stability of NUS-
MOFs, PXRD and N sorption tests were performed on a
6
2
series of samples under various conditions (Figure 5 and Figure
Figure 5. PXRD patterns of NUS-6 MOFs under various conditions.
S7). Both NUS-6(Zr) and NUS-6(Hf) can survive under acidic
conditions (pH 1) without major changes in PXRD and N2
uptake. However, their stability under basic conditions is very
Figure 4. (a) N sorption isotherms at 77 K of NUS-6 MOFs with the
inset featuring adsorption−desorption hystereses. (b) Pore size
distribution of NUS-6 MOFs calculated by NLDFT and BJH models
2
(
inset).
poor as shown by the almost completely amorphous structures
after exposure to basic solutions (pH 12). The different stability
of NUS-6 under acidic versus basic conditions is identical to
those of other reported Zr MOFs such as UiO-66. The long-
term stability test of NUS-6 was performed by exposing the
samples to air for 10 months. Although there is no noticeable
change in the PXRD patterns, NUS-6(Zr) exhibits a partial
mesoporous textures that are in accordance with the crystal
structures. NUS-6(Zr) has a Brunauer−Emmett−Teller (BET)
surface area of 550 m g , which is lower than the value of
47
2
−1
2
−1
1
187 m g reported by Taylor et al., indicating either partial
36
framework collapse or the existence of nonporous impurities.
Compared to NUS-6(Zr), NUS-6(Hf) has a relatively lower
degradation as revealed by the reduced rate of N uptake, while
2
2
−1
BET surface area of 530 m g , probably because of the
NUS-6(Hf) remains less affected (Figure S7). The superior
stability of NUS-6(Hf) versus that of NUS-6(Zr) can be
explained by the dissociation energy of the Hf−O bond (802 kJ
45
introduction of heavier Hf cations. The pore size distribution
of NUS-6 calculated using nonlocal density functional theory
−1
(
NLDFT) reveals three distinct pore sizes of ∼6, ∼8, and ∼12
mol ) being higher than that of the Zr−O bond (776 kJ
D
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
−
1
mol ), which is also confirmed by thermogravimetric analysis
TGA) (Figure S8) in which NUS-6(Hf) exhibits a less obvious
The hierarchical porosity, high stability, and hybrid
Brønsted/Lewis acidity suggest that NUS-6 should be efficient
heterogeneous catalysts. Their catalytic performance was
evaluated on the basis of the dehydration of fructose into 5-
hydroxymethylfurfural (HMF), which is an important process
in biomass conversion and is typically catalyzed by Brønsted
(
weight loss (5.65%) compared to that of NUS-6(Zr) (17.88%)
in the temperature range of 200−400 °C. These results confirm
the excellent stability of NUS-6, especially NUS-6(Hf), which
grants their application in various fields such as gas separation
and heterogeneous catalysis.
52−54
acids such as sulfonic acids or solid acids.
We initially
Anthropogenic CO emissions have aroused worldwide
concern, and MOFs have been proposed as promising
attempted this reaction using water as the solvent, but with a
negligible yield of HMF (∼5%). Therefore, further attempts
were performed using dimethyl sulfoxide (DMSO) as the
2
4
8
adsorbents for postcombustion CO capture, in which CO2
2
needs to be separated from flue gas containing mainly N
solvent whose positive role for this reaction has been proven
2
4
9
55−57
(
∼75%) and CO (∼15%) saturated with moisture. The ideal
previously.
We started with NUS-6(Zr) using a loading
2
27
adsorbents for this operation should have the features of high
CO working capacity (CO uptake at 0.15 bar and 298 K),
amount of 3.5 mol % similar to the value from the literature.
Via optimization of the reaction temperature (Table 1, entries
2
2
good CO /N selectivity, excellent water stability, and low
2
2
water interference. NUS-6 MOFs were systematically evaluated
Table 1. Catalytic Dehydration of Fructose to HMF
for this application. NUS-6(Hf) has a CO working capacity
2
−1
(
0.30 mmol g ) slightly higher than that of NUS-6(Zr) (0.29
mmol g ) (Figure 6a), but the former has a smaller BET
−1
HMF
catalyst
loading
(mol %)
fructose
conversion
(%)
temp time
selectivity yield
entry
(°C)
(h)
(%)
(%)
a
1
3.5
3.5
3.5
5
80
100
120
100
100
100
100
100
100
100
100
1
<5
>99
>99
>99
77
nd
84
72
69
51
72
71
nd
62
98
nd
67
<5
84
72
69
39
54
71
nd
62
98
nd
8
a
2
1
a
3
1
a
4
1
a
5
1.75
3.5
3.5
−
3.5
3.5
3.5
1
a
6
0.5
1.5
1
75
a
7
>99
5
b
8
c
9
1
>99
>99
5
d
1
1
1
0
1
2
1
e
f
1
Figure 6. (a) CO and N uptake of NUS-6 MOFs at 298 K. (b) Q of
2
2
st
3.5
100
1
12
CO in NUS-6 MOFs. (c) IAST CO /N selectivity of NUS-6 MOFs.
2
2
2
a
b
c
d
e
f
NUS-6(Zr). No catalyst. DMST. NUS-6(Hf). UiO-66(Zr). UiO-
6(Hf).
(
d) Water vapor uptake of NUS-6 MOFs at 273 K.
6
surface area. This is probably because of the smaller pore size of
1−3), NUS-6(Zr) exhibits an almost quantitative conversion of
fructose (>99%) with a high chemoselectivity (84%) and yield
(84%) of HMF at 100 °C. The HMF yield is comparable to
that of postsynthetically sulfonated microporous UiO-66(Zr)
(85%), while the latter reaction was performed at a higher
NUS-6(Hf) as well as the polarized μ -OH groups of Hf
3
clusters, which strengthen the interactions between CO and
2
4
5,50
MOFs.
Such strengthened CO -MOF interactions are
2
quantified by the isosteric heat of adsorption (Q ) of CO2
st
27
(
Figure 6b), whereas the zero-coverage Q of CO in NUS-
temperature of 120 °C. By increasing the reaction temper-
ature from 100 to 120 °C, we observed that the HMF
selectivity decreased from 84 to 72%. This is different from the
continuous increase in HMF yield with the increase in
st
2
−1
6
(Hf) (27.7 kJ mol ) is indeed higher than that in NUS-6(Zr)
−1
(
24.8 kJ mol ). The CO /N selectivity of NUS-6 was further
2 2
evaluated using ideal adsorption solution theory (IAST), in
which the single gas isotherms were used to predict the
27
temperature from 100 to 140 °C reported by Chen et al.
51
selectivity of the mixed gas. Similar to the trend of Q , NUS-
Meanwhile, NUS-6(Hf) also exhibits a similar trend of reduced
HMF yield at higher temperatures from 98% to 89%. This can
be attributed to the stronger acidity of NUS-6 MOFs (higher
st
6
(Hf) also has an IAST CO /N selectivity higher than that of
2 2
NUS-6(Zr) [30.2 vs 21.7 (Figure 6c)], confirming the
importance of CO −MOFs interactions. The high density of
concentration of -SO H groups) that further convert HMF to
2
3
the polar sulfonic acid group in NUS-6 suggests a hydroscopic
property, which is proven by the quasi-type I water vapor
sorption isotherms where large amounts of water uptake are
other byproducts (e.g., alcohols) at high temperatures as HMF
55,58
is inherently not stable, especially at high temperatures.
Increasing the loading of NUS-6(Zr) to 5 mol % results in a
reduced HMF yield (69%), possibly because of the intensified
side reactions (Table 1, entry 4), while decreasing NUS-6(Zr)
loading to 1.75 mol % leads to a lower fructose conversion
(77%) and HMF yield (39%) caused by insufficient catalysis
(Table 1, entry 5). The reaction time was further optimized
observed at a P/P of <0.3 (Figure 6d). Such a hydroscopic
0
property of NUS-6 may suggest high water interference for
CO capture. Therefore, dehydration of flue gas should be
2
processed beforehand were these materials to be used as
39
adsorbents for postcombustion CO capture.
2
E
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
with fixed reaction temperature (100 °C) and NUS-6(Zr)
loading (3.5 mol %), proving that a reaction duration of 1 h
gives the best results under such conditions (Table 1, entries 6
and 7). A control experiment without any catalyst afforded <5%
fructose conversion with a negligible HMF yield (Table 1, entry
(Figure 7). When DMST was used as the catalyst, the reaction
had a fast kinetics with quantitative fructose conversion
obtained after ∼10 min, while the desired HMF yield was
moderate (75%) and started to decline to 62% in the end
because of uncontrollable side reactions caused by homoge-
57
8), confirming the critical role of NUS-6(Zr) as the catalyst. In
neous catalysts. On the other hand, both NUS-6(Hf) and
NUS-6(Zr) displayed remarkably enhanced catalytic activity
and chemoselectivity, with slower but steady increases in both
fructose conversion and HMF yield within 1 h. The slower
reaction kinetics of NUS-6 MOFs can be explained by the
restricted mass transfer within porous media, which on the
other hand might restrain the unwanted side reactions leading
to higher HMF yields. Compared to those of NUS-6(Zr),
NUS-6(Hf) demonstrated both faster reaction kinetics and a
higher HMF yield. Given the similar crystal structure between
these two MOFs, the difference in catalytic performance can be
attributed to the extra Brønsted acidity in NUS-6(Hf) caused
comparison, dimethyl 2-sulfoterephthalate (DMST) was used
as a homogeneous catalyst for this reaction under a similar
reaction condition (3.5 mol % loading, 100 °C, 1 h), resulting
in a quantitative fructose conversion but worse chemoselectivity
(
62%) and yield (62%) of HMF (Table 1, entry 9), suggesting
rampant side reactions that can be mitigated using NUS-6(Zr)
as the heterogeneous catalyst.
When NUS-6(Hf) was used as the catalyst under a similar
reaction condition, a quantitative conversion of fructose
(>99%) was obtained along with extremely high chemo-
selectivity (98%) and yield (98%) of HMF (Table 1, entry 10).
This result is better than those of NUS-6(Zr) (84%
chemoselectivity and 84% yield) and other postsynthetically
sulfonated MOFs such as MIL-53(Al) (80% chemoselectivity
and ∼79% yield), UiO-66(Zr) (∼86% chemoselectivity and
20
by polarized μ -OH groups in Hf clusters. In a control
3
experiment, pristine UiO-66(Zr) and UiO-66(Hf) without any
sulfonic acid groups were used as the catalyst for the reaction
described above. As a result, UiO-66(Zr) was almost
completely feckless (Table 1, entry 11) while UiO-66(Hf)
exhibited a measurable catalytic activity (Table 1, entry 12),
partially confirming the Brønsted acidity in Hf-containing
MOFs. Acid−base titration experiments were conducted to
quantify the Brønsted acid concentration. Before titration, an
aqueous NaCl solution was used as an ion-exchange reagent to
release the acidic protons from MOFs into the solution, which
∼
85% yield), and MIL-101(Cr) (91% chemoselectivity and
27
∼90% yield). To the best of our knowledge, NUS-6(Hf)
represents the most efficient and selective solid acid catalyst for
54
this reaction so far. We also examined the heterogeneity and
recyclability of NUS-6(Hf). The supernatant of the reaction
media obtained by using a regular filter during the dehydration
reaction did not yield any additional product, indicating the role
of the heterogeneous catalyst played by NUS-6(Hf) (Figure 7).
30
was later titrated after the removal of MOFs. Compared to
direct titration of MOF suspensions, this method can avoid
MOF decomposition during titration with more accurate
reading. However, it is based on the kinetic equilibrium of
+
cation exchange and can detect only those Na accessible acidic
protons with low pK values (large dissociation constants).
a
UiO-66(Zr) was titrated as a blank compare, which surprisingly
exhibited a mild Brønsted acidity of 0.01 mol mol− (mole of
acidic proton per mole of MOFs based on molecular formula)
1
(
Figure 8). Because formic acid was used as the modulator
during the synthesis of UiO-66(Zr), the mild Brønsted acidity
may come from the residual formic acid trapped inside the
framework. Compared to UiO-66(Zr), UiO-66(Hf) has 5 times
−1
more acidic protons (0.06 mol mol ), which strongly suggests
its higher Brønsted acidity is contributed by the μ -OH groups,
3
which also explains its catalytic activity in fructose dehydration.
Figure 7. Kinetic study of dehydration of fructose into HMF via the
catalysis of various catalysts.
Upon completion of the reaction, NUS-6(Hf) could be
recovered in quantitative yield and used repeatedly without a
significant loss of catalytic activity for the three subsequent runs
(
quantitative fructose conversion with 94, 92, and 90% yields of
HMF for one, two, and three additional runs, respectively).
Recycled NUS-6(Hf) retains its high crystallinity (Figure S9)
but with compromised porosity possibly due to the trapping of
reagents in the framework that are hard to remove during
regeneration (Figure S10).
To further study the superior catalytic activity of NUS-6(Hf)
over NUS-6(Zr) and their homogeneous counterpart DMST,
we performed the kinetic study of this reaction using 3.5 mol %
loading of catalysts at 100 °C for a continuous period of 1 h
Figure 8. Concentrations of acidic protons in various MOFs obtained
by acid−base titration.
F
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Because of the sulfonic acid groups, the acidic proton
concentration of NUS-6(Zr) is much higher (12.9 mol
MOFs featuring superior stability and Brønsted acidity that can
be applied as solid acid catalysts in biobased chemical synthesis.
−1
mol ), while NUS-6(Hf) exhibited an even higher acidic
−1
proton concentration (13.9 mol mol ), which might due to
ASSOCIATED CONTENT
* Supporting Information
■
the extra Brønsted acidity contributed by μ -OH groups as well
S
3
as the easier dissociation of acidic protons caused by oxophilic
Hf cations. However, the measured acidic proton concen-
trations of NUS-6 MOFs are still lower than the theoretical
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.6b00139.
−1
value of 15 mol mol contributed solely by the sulfonic acid
groups, suggesting that not all the acidic protons can be
Experimental procedures, calculations, EDS elemental
mapping, gas sorption isotherms, etc. (PDF)
+
replaced by Na possibly because of (1) insufficient equilibrium
time, (2) different dissociation constants of available acidic
protons, (3) steric hindrance caused by partial framework
collapse, and (4) trapping of protons by defect-containing Zr or
AUTHOR INFORMATION
Corresponding Author
*E-mail: chezhao@nus.edu.sg.
■
3
6
Hf clusters.
Besides the stronger Brønsted acidity, a more suitable pore
size might be another factor in determining the superior
catalytic activity of NUS-6(Hf) versus that of NUS-6(Zr) for
dehydration of fructose into HMF. It is widely accepted that
Notes
The authors declare the following competing financial
interest(s): A U.S. provisional patent (62/132,608) has been
filed on March 13, 2015, based on the presented result.
59
this reaction starts from the fructofuranosyl form of fructose.
Many Brønsted acids are able to promote this trans-
ACKNOWLEDGMENTS
This work is supported by the National University of Singapore
CENGas R-261-508-001-646) and the Singapore Ministry of
■
(
5
7,60−62
formation,
but selectivity for HMF could seldom reach
9
8% as in NUS-6(Hf) mainly because of the severe side
63
reaction of humin formation via aldol condensation. We have
noticed that the smallest pore of NUS-6(Hf) has a diameter of
Education (MOE AcRF Tier 1 R-279-000-410-112, AcRF Tier
R-279-000-429-112). R.B. is the recipient of Australian
2
∼
5.5 Å (Figure 4b), which is only slightly larger than the
Research Council DECRA fellowship.
64
hydrodynamic radius of monosaccharides (∼4 Å). Therefore,
the guest fructose molecules can be well-confined within the
smallest pores of NUS-6(Hf), which might prevent isomer-
ization of fructose from its naturally predominant furanosyl
form to the linear form, leading to an increase in the reaction
rate. More importantly, the fructofuranosyl form disfavors
byproduct formation as it does not contain a carbonyl group,
which is responsible for the undesired aldol condensation. To
the best of our knowledge, the utilization of the pore
confinement effect in MOFs to simultaneously enhance
reaction rate as well as inhibit side reactions for biomass
conversion as demonstrated in this study is unprecedented.
REFERENCES
■
(
1) Wang, Z.; Chen, G.; Ding, K. L. Self-supported catalysts. Chem.
Rev. 2009, 109, 322−359.
2) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.;
(
Hupp, J. T. Metal-organic framework materials as catalysts. Chem. Soc.
Rev. 2009, 38, 1450−1459.
(
́
3) Corma, A.; Garcia, H.; Llabres i Xamena, F. X. Engineering metal
organic frameworks for heterogeneous catalysis. Chem. Rev. 2010, 110,
606−4655.
4) Liu, Y.; Xuan, W. M.; Cui, Y. Engineering homochiral metal-
4
(
organic frameworks for heterogeneous asymmetric catalysis and
enantioselective separation. Adv. Mater. 2010, 22, 4112−4135.
5) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. Commercial metal-
(
CONCLUSION
organic frameworks as heterogeneous catalysts. Chem. Commun. 2012,
8, 11275−11288.
6) Liu, J. W.; Chen, L. F.; Cui, H.; Zhang, J. Y.; Zhang, L.; Su, C. Y.
■
4
(
In summary, we have directly synthesized two sulfonated
MOFs named NUS-6(Zr) and NUS-6(Hf) via the modulated
hydrothermal (MHT) approach at a low reaction temperature
of 80 °C. Rietveld refinements of synchrotron PXRD data
reveal hierarchically porous structures stemming from a
parental UiO-66 topology with missing ligands and clusters.
Unlike the previously reported sulfonated Zr MOFs, NUS-
(Zr) and NUS-6(Hf) exhibit excellent stability and can retain
their porosity under vigorous activation at 150 °C, possibly
because of the stable crystalline subparticles caused by reversed
crystal growth. These MOFs exhibited decent CO capture
Applications of metal-organic frameworks in heterogeneous supra-
molecular catalysis. Chem. Soc. Rev. 2014, 43, 6011−6061.
(7) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. Preparation,
clathration ability, and catalysis of a two-dimensional square network
material composed of cadmium(II) and 4,4′-bipyridine. J. Am. Chem.
Soc. 1994, 116, 1151−1152.
(8) Schlichte, K.; Kratzke, T.; Kaskel, S. Improved synthesis, thermal
6
stability and catalytic properties of the metal-organic framework
compound Cu BTC . Microporous Mesoporous Mater. 2004, 73, 81−
3
2
8
8.
2
(9) Horike, S.; Dinca, M.; Tamaki, K.; Long, J. R. Size-selective Lewis
̆
performance and were used as solid acid catalysts for
dehydration of fructose to 5-hydroxymethylfurfural (HMF),
in which NUS-6(Hf) exhibited better reaction kinetics and
chemoselectivity with quantitative fructose conversion and a
HMF yield of 98% that make it one of the best heterogeneous
catalysts for such conversion. The superior catalytic activity of
NUS-6(Hf) was attributed to its stronger Brønsted acidity as
well as more suitable pore size that can inhibit side reactions.
Acid−base titration experiments were used to experimentally
prove the superior Brønsted acidity of Hf-MOFs. Our results
have demonstrated for the first time the uniqueness of Hf-
acid catalysis in a microporous metal-organic framework with exposed
Mn2 coordination sites. J. Am. Chem. Soc. 2008, 130, 5854−5855.
(10) Ladrak, T.; Smulders, S.; Roubeau, O.; Teat, S. J.; Gamez, P.;
Reedijk, J. Manganese-based metal-organic frameworks as heteroge-
neous catalysts for the cyanosilylation of acetaldehyde. Eur. J. Inorg.
Chem. 2010, 2010, 3804−3812.
+
(
11) Hu, Z. G.; Peng, Y. W.; Tan, K. M.; Zhao, D. Enhanced catalytic
activity of a hierarchical porous metal-organic framework CuBTC.
CrystEngComm 2015, 17, 7124−7129.
(12) Wee, L. H.; Wiktor, C.; Turner, S.; Vanderlinden, W.; Janssens,
N.; Bajpe, S. R.; Houthoofd, K.; Van Tendeloo, G.; De Feyter, S.;
Kirschhock, C. E. A.; Martens, J. A. Copper benzene tricarboxylate
G
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
metal-organic framework with wide permanent mesopores stabilized
(30) Akiyama, G.; Matsuda, R.; Sato, H.; Takata, M.; Kitagawa, S.
Cellulose hydrolysis by a new porous coordination polymer decorated
with sulfonic acid functional groups. Adv. Mater. 2011, 23, 3294−3297.
by Keggin polyoxometallate ions. J. Am. Chem. Soc. 2012, 134, 10911−
10919.
(
13) Hwang, Y. K.; Hong, D. Y.; Chang, J. S.; Jhung, S. H.; Seo, Y. K.;
̃
(31) Juan-Alcaniz, J.; Gielisse, R.; Lago, A. B.; Ramos-Fernandez, E.
Kim, J.; Vimont, A.; Daturi, M.; Serre, C.; Ferey, G. Amine grafting on
coordinatively unsaturated metal centers of MOFs: consequences for
catalysis and metal encapsulation. Angew. Chem., Int. Ed. 2008, 47,
V.; Serra-Crespo, P.; Devic, T.; Guillou, N.; Serre, C.; Kapteijn, F.;
Gascon, J. Towards acid MOFs - catalytic performance of sulfonic acid
functionalized architectures. Catal. Sci. Technol. 2013, 3, 2311−2318.
(32) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;
4
(
144−4148.
14) Opanasenko, M.; Dhakshinamoorthy, A.; Shamzhy, M.;
Bordiga, S.; Lillerud, K. P. A new zirconium inorganic building brick
forming metal organic frameworks with exceptional stability. J. Am.
Chem. Soc. 2008, 130, 13850−13851.
̌
́ ̌
Nachtigall, P.; Horacek, M.; Garcia, H.; Cejka, J. Comparison of the
catalytic activity of MOFs and zeolites in Knoevenagel condensation.
(33) Hu, Z. G.; Zhao, D. De facto methodologies toward the synthesis
Catal. Sci. Technol. 2013, 3, 500−507.
and scale-up production of UiO-66-type metal-organic frameworks
(
15) Marx, S.; Kleist, W.; Baiker, A. Synthesis, structural properties,
and membrane materials. Dalton Trans. 2015, 44, 19018−19040.
and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in
(34) Lin Foo, M.; Horike, S.; Fukushima, T.; Hijikata, Y.; Kubota, Y.;
the oxidation of benzene derivatives. J. Catal. 2011, 281, 76−87.
Takata, M.; Kitagawa, S. Ligand-based solid solution approach to
stabilisation of sulphonic acid groups in porous coordination polymer
(
16) Lykourinou, V.; Chen, Y.; Wang, X. S.; Meng, L.; Hoang, T.;
Ming, L. J.; Musselman, R. L.; Ma, S. Q. Immobilization of MP-11 into
a mesoporous metal-organic framework, MP-11@mesoMOF: a new
platform for enzymatic catalysis. J. Am. Chem. Soc. 2011, 133, 10382−
Zr O (OH) (BDC) (UiO-66). Dalton Trans. 2012, 41, 13791−
6
4
4
6
1
(
3794.
35) Biswas, S.; Zhang, J.; Li, Z. B.; Liu, Y. Y.; Grzywa, M.; Sun, L. X.;
Volkmer, D.; Van der Voort, P. Enhanced selectivity of CO over CH
4
10385.
(
17) Feng, D. W.; Gu, Z. Y.; Li, J. R.; Jiang, H. L.; Wei, Z. W.; Zhou,
2
in sulphonate-, carboxylate- and iodo-functionalized UiO-66 frame-
H. C. Zirconium-metalloporphyrin PCN-222: mesoporous metal-
organic frameworks with ultrahigh stability as biomimetic catalysts.
Angew. Chem., Int. Ed. 2012, 51, 10307−10310.
works. Dalton Trans. 2013, 42, 4730−4737.
(36) Taylor, J. M.; Komatsu, T.; Dekura, S.; Otsubo, K.; Takata, M.;
Kitagawa, H. The role of a three dimensionally ordered defect
sublattice on the acidity of a sulfonated metal-organic framework. J.
Am. Chem. Soc. 2015, 137, 11498−11506.
(
18) Johnson, J. A.; Luo, J.; Zhang, X.; Chen, Y. S.; Morton, M. D.;
Echeverria, E.; Torres, F. E.; Zhang, J. Porphyrin-metalation-mediated
tuning of photoredox catalytic properties in metal-organic frameworks.
ACS Catal. 2015, 5, 5283−5291.
(37) Jakobsen, S.; Gianolio, D.; Wragg, D. S.; Nilsen, M. H.; Emerich,
H.; Bordiga, S.; Lamberti, C.; Olsbye, U.; Tilset, M.; Lillerud, K. P.
Structural determination of a highly stable metal-organic framework
with possible application to interim radioactive waste scavenging: Hf-
UiO-66. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 125429.
̌
19) Opanasenko, M.; Shamzhy, M.; Cejka, J. Solid acid catalysts for
(
coumarin synthesis by the Pechmann reaction: MOFs versus zeolites.
ChemCatChem 2013, 5, 1024−1031.
(
20) Beyzavi, M. H.; Klet, R. C.; Tussupbayev, S.; Borycz, J.;
(38) Hu, Z. G.; Peng, Y. W.; Kang, Z. X.; Qian, Y. H.; Zhao, D. A
Vermeulen, N. A.; Cramer, C. J.; Stoddart, J. F.; Hupp, J. T.; Farha, O.
K. A hafnium-based metal organic framework as an efficient and
modulated hydrothermal (MHT) approach for the facile synthesis of
UiO-66-type MOFs. Inorg. Chem. 2015, 54, 4862−4868.
multifunctional catalyst for facile CO fixation and regioselective and
2
(39) Hu, Z. G.; Nalaparaju, A.; Peng, Y. W.; Jiang, J. W.; Zhao, D.
enantioretentive epoxide activation. J. Am. Chem. Soc. 2014, 136,
Modulated hydrothermal synthesis of UiO-66(Hf)-type metal-organic
frameworks for optimal carbon dioxide separation. Inorg. Chem. 2016,
1
(
5861−15864.
21) Nguyen, H. G. T.; Schweitzer, N. M.; Chang, C. Y.; Drake, T.
5
5, 1134−1141.
40) Rolison, D. R. Catalytic nanoarchitectures - the importance of
nothing and the unimportance of periodicity. Science 2003, 299, 1698−
701.
41) Mitchell, S.; Pinar, A. B.; Kenvin, J.; Crivelli, P.; Kar
Perez-Ramírez, J. Structural analysis of hierarchically organized
zeolites. Nat. Commun. 2015, 6, 8633.
42) Moon, S. Y.; Liu, Y. Y.; Hupp, J. T.; Farha, O. K. Instantaneous
L.; So, M. C.; Stair, P. C.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T.
Vanadium-node-functionalized UiO-66: a thermally stable MOF-
supported catalyst for the gas-phase oxidative dehydrogenation of
cyclohexene. ACS Catal. 2014, 4, 2496−2500.
(
1
(
̈
ger, J.;
(
22) Jiang, J. C.; Gan
Klemperer, W. G. Superacidity in sulfated metal-organic framework-
08. J. Am. Chem. Soc. 2014, 136, 12844−12847.
23) Jiang, J. C.; Yaghi, O. M. Brønsted acidity in metal-organic
frameworks. Chem. Rev. 2015, 115, 6966−6997.
24) Burtch, N. C.; Jasuja, H.; Walton, K. S. Water stability and
adsorption in metal-organic frameworks. Chem. Rev. 2014, 114,
0575−10612.
25) Canivet, J.; Fateeva, A.; Guo, Y. M.; Coasne, B.; Farrusseng, D.
́
dara, F.; Zhang, Y. B.; Na, K.; Yaghi, O. M.;
́
8
(
(
hydrolysis of nerve-agent simulants with a six-connected zirconium-
based metal-organic framework. Angew. Chem., Int. Ed. 2015, 54,
(
6
(
795−6799.
43) Li, B. Y.; Leng, K. Y.; Zhang, Y. M.; Dynes, J. J.; Wang, J.; Hu, Y.
1
(
F.; Ma, D. X.; Shi, Z.; Zhu, L. K.; Zhang, D. L.; Sun, Y. Y.;
Chrzanowski, M.; Ma, S. Q. Metal-organic framework based upon the
synergy of a Brønsted acid framework and Lewis acid centers as a
highly efficient heterogeneous catalyst for fixed-bed reactions. J. Am.
Chem. Soc. 2015, 137, 4243−4248.
Water adsorption in MOFs: fundamentals and applications. Chem. Soc.
Rev. 2014, 43, 5594−5617.
(
26) Cohen, S. M. Postsynthetic methods for the functionalization of
metal-organic frameworks. Chem. Rev. 2012, 112, 970−1000.
(
44) Self, K.; Telfer, M.; Greer, H. F.; Zhou, W. Z. Reversed crystal
growth of RHO zeolitic imidazolate framework (ZIF). Chem. - Eur. J.
015, 21, 19090−19095.
(
27) Chen, J. Z.; Li, K. G.; Chen, L. M.; Liu, R. L.; Huang, X.; Ye, D.
Q. Conversion of fructose into 5-hydroxymethylfurfural catalyzed by
recyclable sulfonic acid-functionalized metal-organic frameworks.
Green Chem. 2014, 16, 2490−2499.
2
(45) Hu, Z. G.; Zhang, K.; Zhang, M.; Guo, Z. G.; Jiang, J. W.; Zhao,
D. A combinatorial approach towards water-stable metal-organic
frameworks for highly efficient carbon dioxide separation. Chem-
SusChem 2014, 7, 2791−2795.
(46) Nelson, A. P.; Farha, O. K.; Mulfort, K. L.; Hupp, J. T.
Supercritical processing as a route to high internal surface areas and
permanent microporosity in metal-organic framework materials. J. Am.
Chem. Soc. 2009, 131, 458−460.
(47) Kandiah, M.; Nilsen, M. H.; Usseglio, S.; Jakobsen, S.; Olsbye,
U.; Tilset, M.; Larabi, C.; Quadrelli, E. A.; Bonino, F.; Lillerud, K. P.
(
̃
28) Goesten, M. G.; Juan-Alcaniz, J.; Ramos-Fernandez, E. V.; Sai
Sankar Gupta, K. B.; Stavitski, E.; van Bekkum, H.; Gascon, J.;
Kapteijn, F. Sulfation of metal-organic frameworks: opportunities for
acid catalysis and proton conductivity. J. Catal. 2011, 281, 177−187.
(
29) Phang, W. J.; Jo, H.; Lee, W. R.; Song, J. H.; Yoo, K.; Kim, B.;
Hong, C. S. Superprotonic conductivity of a UiO-66 framework
functionalized with sulfonic acid groups by facile postsynthetic
oxidation. Angew. Chem., Int. Ed. 2015, 54, 5142−5146.
H
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Synthesis and stability of tagged UiO-66 Zr-MOFs. Chem. Mater.
2
(
010, 22, 6632−6640.
48) Haszeldine, R. S. Carbon capture and storage: how green can
black be? Science 2009, 325, 1647−1652.
49) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.;
(
Bloch, E. D.; Herm, Z. R.; Bae, T. H.; Long, J. R. Carbon dioxide
capture in metal-organic frameworks. Chem. Rev. 2012, 112, 724−781.
(
50) Hu, Z. G.; Faucher, S.; Zhuo, Y. Y.; Sun, Y.; Wang, S. N.; Zhao,
D. Combination of optimization and metalated-ligand exchange: an
effective approach to functionalize UiO-66(Zr) MOFs for CO2
separation. Chem. - Eur. J. 2015, 21, 17246−17255.
(
51) Myers, A. L.; Prausnitz, J. M. Thermodynamics of mixed-gas
adsorption. AIChE J. 1965, 11, 121−127.
52) van Putten, R. J.; van der Waal, J. C.; de Jong, E.; Rasrendra, C.
(
B.; Heeres, H. J.; de Vries, J. G. Hydroxymethylfurfural, a versatile
platform chemical made from renewable resources. Chem. Rev. 2013,
1
13, 1499−1597.
53) Wang, T. F.; Nolte, M. W.; Shanks, B. H. Catalytic dehydration
of C6 carbohydrates for the production of hydroxymethylfurfural
HMF) as a versatile platform chemical. Green Chem. 2014, 16, 548−
72.
54) Wang, L.; Xiao, F. S. Nanoporous catalysts for biomass
conversion. Green Chem. 2015, 17, 24−39.
55) Laugel, C.; Estrine, B.; Le Bras, J.; Hoffmann, N.; Marinkovic,
(
(
5
(
(
S.; Muzart, J. NaBr/DMSO-induced synthesis of 2,5-diformylfuran
from fructose or 5-(hydroxymethyl)furfural. ChemCatChem 2014, 6,
1
(
195−1198.
56) Tsilomelekis, G.; Josephson, T. R.; Nikolakis, V.; Caratzoulas, S.
Origin of 5-hydroxymethylfurfural stability in water/dimethyl sulfoxide
mixtures. ChemSusChem 2014, 7, 117−126.
(
57) Peng, Y. W.; Hu, Z. G.; Gao, Y. J.; Yuan, D. Q.; Kang, Z. X.;
Qian, Y. H.; Yan, N.; Zhao, D. Synthesis of a sulfonated two-
dimensional covalent organic framework as an efficient solid acid
catalyst for biobased chemical conversion. ChemSusChem 2015, 8,
3
(
208−3212.
58) Besson, M.; Gallezot, P.; Pinel, C. Conversion of biomass into
chemicals over metal catalysts. Chem. Rev. 2014, 114, 1827−1870.
59) Zhang, J.; Weitz, E. An in situ NMR study of the mechanism for
(
the catalytic conversion of fructose to 5-hydroxymethylfurfural and
then to levulinic acid using 13C labeled D-fructose. ACS Catal. 2012, 2,
1
(
211−1218.
60) Tong, X. L.; Li, M. R.; Yan, N.; Ma, Y.; Dyson, P. J.; Li, Y. D.
Defunctionalization of fructose and sucrose: iron-catalyzed production
of 5-hydroxymethylfurfural from fructose and sucrose. Catal. Today
2
(
011, 175, 524−527.
61) Xie, H. B.; Zhao, Z. K.; Wang, Q. Catalytic conversion of inulin
and fructose into 5-hydroxymethylfurfural by lignosulfonic acid in
ionic liquids. ChemSusChem 2012, 5, 901−905.
(
62) Ngee, E. L. S.; Gao, Y. J.; Chen, X.; Lee, T. M.; Hu, Z. G.; Zhao,
D.; Yan, N. Sulfated mesoporous niobium oxide catalyzed 5-
hydroxymethylfurfural formation from sugars. Ind. Eng. Chem. Res.
2
(
014, 53, 14225−14233.
63) Siankevich, S. Conversion of Biomass into Value-Added
Chemicals. Ph.D. Thesis, EPFL, Lausanne, Switzerland, 2015.
64) Schultz, S. G.; Solomon, A. K. Determination of the effective
hydrodynamic radii of small molecules by viscometry. J. Gen. Physiol.
961, 44, 1189−1199.
(
1
I
DOI: 10.1021/acs.chemmater.6b00139
Chem. Mater. XXXX, XXX, XXX−XXX