Tuning the Catalytic Activity of UiO-66 via Modulated Synthesis: Esterification of Levulinic Acid as a Test Reaction

Article abstract of DOI:10.1002/ejic.202000031

Due to their high stabilities and tunable defects, zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) have been promising catalysts in many catalytic reactions. The catalytic activity of UiO-66 MOFs is significantly affected by the density of defects and hydroxyl groups bonded to Zr₆ nodes, which can be tuned by modulators. In this work, a series of defective UiO-66 were synthesized via tuning the ratio of H₂BDC/Zr and adding monocarboxylic acids with different chain length as modulators. The synthesized UiO-66 MOFs were characterized by powder X-ray diffraction, N₂ physisorption, IR and ¹H NMR spectroscopy, scanning and transmission electron microscopy. Esterification of levulinic acid with ethanol was selected to reveal the influence of modulators, defects and hydroxyl groups. The results showed that UiO-66 with 0.5 of H₂BDC/Zr ratio and lauric acid as the modulator owned excellent catalytic activity. The cooperative effects, between the Lewis acid sites offered by unsaturated Zr₆ nodes and Br?nsted acid sites supplied by OH, has been revealed to have a significant influence on the catalytic activity.

Full text of DOI:10.1002/ejic.202000031

A Journal of
Accepted Article
Title: Tuning the Catalytic Activity of UiO-66 via Modulated
Synthesis:Esterification of Levulinic Acid as A Test Reaction
Authors: Ruiping Wei, Jingdeng Fan, Xumin Qu, Lijing Gao, Yuanfeng
Wu, Zhongqi Zhang, Feng Hu, and Guomin Xiao
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000031
Link to VoR: http://dx.doi.org/10.1002/ejic.202000031

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
Tuning the Catalytic Activity of UiO-66 via Modulated Synthesis：
Esterification of Levulinic Acid as A Test Reaction
Ruiping Wei*[a], Jingdeng Fan[a], Xumin Qu[a], Lijing Gao[a], Yuanfeng Wu[a], Zhongqi Zhang[a], Feng
Hu[a], Guomin Xiao[a]
Abstract: Due to its high stabilities and tunable defects, Zirconium
terephthalate UiO-66 type metal organic frameworks (MOFs) has
been a promising catalyst in many catalytic reactions. The catalytic
activity of UiO-66 MOFs is significantly affected by the density of
defects and hydroxyl groups bonded to Zr6 nodes, which can be tuned
by modulators. In this work, a series of defective UiO-66 were
synthesized via tuning the ratio of H2BDC/Zr and adding
monocarboxylic acids with different chain length as modulators. The
synthesized UiO-66 MOFs were characterized by X-ray diffraction, N2
physisorption, IR and 1H NMR spectroscopies, scanning and
transmission electron microscopy. Esterification of levulinic acid with
ethanol was selected to reveal the influence of modulators, defects
and hydroxyl groups. The results showed that UiO-66 with 0.5 of
H2BDC/Zr ratio and lauric acid as the modulator owned excellent
catalytic activity. The cooperative effects, between the Lewis acid
sites offered by unsaturated Zr6 nodes and Brønsted acid sites
supplied by OH, has been revealed to have a significant influence on
the catalytic activity.
including modulated synthesis. To form defect sites, HCl[8], TFA[9]
or a monocarboxylic acid[10] is added to reaction solution as a
modulator, which competes with H2BDC linkers for bonding to the
nodes to satisfy modulated synthesis. Whilst, the correlation
between catalytic activities and defects has been studied by many
researchers. Ma et al.[11] systematically investigated the influence
of structural defects in Pt@UiO‐66‐NH2 on photocatalytic
properties and proved that structural defects could switch on
photocatalysis. Vermoortele et al.[12] synthesized defective UiO-
66 with trifluoroacetic acid and HCl as modulators, and further
probed the relationship between catalytic activity and numbers of
defect in cyclization of citronellal. Liu et al.[13] quantified the
numbers of defects by a potentiometric acid-base titrations and
established the quantitative correlations between defects and
catalytic activity by an epoxide ring-opening reaction. Catalytic
activity is not only affected by defects such as missing linkers and
nodes, but also related to structure and properties of ligands
bonded to Zr6O8 in UiO-66. Recently, the hydroxyl groups on
nodes as Brønsted acid sites were evaluated by some reports[14]
.
Klet et al.[15] considered that the hydroxyl groups on Zr6O8 are μ3-
OH, Zr-OH2 and Zr-OH. And their Brønsted acidity values were
evaluated quantitatively by Potentiometric acid–base titration.
Yang et al.[16] determined the ligands on the node surfaces by IR
and 1H NMR and used ethanol dehydration to probe the
correlation between the ligands on the Zr6O8 surfaces and
catalytic properties.
Introduction
Metal-organic frameworks (MOFs) are crystalline porous
structures synthesized with organic linker molecules and
inorganic metal clusters[1]. Due to their large internal surface
areas and high degrees of defect tunability, MOFs have been
widely investigated in separation[2], storage[3], sensor[4]. Notably,
the high active site densities provided by unsaturated metal nodes,
tailorable pore structures and ease of functionalization commend
MOFs as a potentially materials in catalysis[5]. With incorporating
Zr6O8 clusters as nodes, Zr-Based MOFs (UiO-66) has been one
of the most promising and investigated MOFs as practical
catalysts owing to its high stability under harsh conditions and
appropriate acid sites by tuning structural defects[6].
Our previous work[17] has investigated the effects of node groups
on the properties of Zr6O8 and presented that the properties of
ligands could be indicated by μ3-OH. To our best knowledge,
tuning systematically the numbers of defect sites and the hydroxyl
groups on Zr6O8 in defective UiO-66 with long chain
monocarboxylic acids was yet to be explored. Due to efforts have
been devoted to reveal the effects of ligands and groups on the
Zr6O8 node, catalytic activity has not been focused on, especially
about the hydroxyl groups. Therefore, in this work, a series of
defective UiO-66 were synthesized by adjusting the ratio of
H2BDC/Zr and adding monocarboxylic acids with different chain
length as a modulator. Esterification of levulinic acid with ethanol,
which is mainly dominated by Brønsted acid sites was chosen as
a test reaction to well understand the correlation between defects
and properties of ligands on Zr6O8 provided by the hydroxyl
groups such as μ3-OH on Zr6O8 nodes (Figure 1).
Defective engineering in UiO-66 has drawn considerable attention
because defects in structure could be exploited towards altering
physical–chemical properties of the MOFs[7]. Ideal UiO-66,
consisting of 12-coordinated hexanuclear [Zr6(μ3O)4(μ3-OH)4]12+
metal clusters connected to H2BDC linkers, commonly has weak
catalytic activity due to fewer Lewis acid sites provided by
saturated Zr atom. In order to improve catalytic activity, defects
were systematically introduced into UiO-66 via different methods
[a]
Prof. R. Wei, M. J. Fan, M. X. Qu, Prof. L. Gao, Dr. Y. Wu, Dr. Z.
Zhang, M. F. Hu, Prof. G. Xiao
School of Chemistry and Chemical Engineering
Southeast University, Nanjing, Jiangsu, China
E-mail: weiruip@seu.edu.cn
http://publons.com/researcher/3364312/ruiping-wei/
Figure 1. Schematic representation of UiO-66 and defective UiO-66
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
pore volume of UiO-66-0.5(OA) were up to 1266 cm2·g-1 and 0.93
cm3·g-1, respectively. Moreover, the largest mesopore volume
was obtained in UiO-66-0.5(OA) sample compared to other
synthesized ones explained the existence of hysteresis loop in the
N2 adsorption–desorption isotherms of UiO-66-0.5(OA). The
differences in BET area and porosity features of UiO-66 might be
attributed to the number of missing-node defects and missing-
linker defects, which were tuned by the ratio of precursor. In other
words, the reduced amount of H2BDC meant that more
modulators pre-occupied the coordination sites could be retained
but not replaced by linker, leading to larger pore volume and
surface area of the prepared materials. Nevertheless, excessively
insufficient H2BDC was also be detrimental to the formation of
defects and mesoporous structures owing to the coordination
between Zr-oxo clusters and H2BDC was hampered by an excess
of modulators.
Results and Discussion
Properties of UiO-66 with different precursor ratios
The effects of H2BDC/Zr on the crystallinity and structural purity
of as-prepared materials were analyzed by XRD (Figure 2). The
characteristic diffraction peaks of all synthesized UiO-66 MOFs
were consistent with previous report[18], attributing to the (111)
reflection (2θ=7.3°), (002) reflection (2θ=8.5°) and (115) reflection
(2θ=25.7°) and proving the successful preparation of UiO-66
modulated synthesis. As shown in Figure 2, the intensity of
characteristic peaks of UiO-66 increased slightly with the ratio of
H2BDC/Zr from 0.3 to 1.0, and maintained certain intensity with
increasing the ratio of H2BDC/Zr from 1.0 to 2.0. The results
showed that the crystallinity of samples was affected by the ratio
of H2BDC/Zr to some extent.
Figure 2. XRD patterns of UiO-66-X(OA)
Figure 3. N2 adsorption–desorption isotherms of UiO-66-X(OA)
Table 1 Surface area and pore structure of UiO-66-X(OA)
The porosities and surface area of UiO-66 MOFs via modulated
synthesis with different mole ratio of H2BDC/Zr were analyzed by
N2 adsorption–desorption isotherms. The influence of precursor
ratios was clearly reflected in Figure 3. The obviously increase of
adsorption amount under relative pressure (P/P0) less than 0.1
revealed the pores were mainly microporous. Meanwhile, there
was a tendency that the volume adsorbed increased from 400
cm3·g-1 to 590 cm3·g-1 with the decrease of the ratio of H2BDC/Zr
when the ratio was more than 0.5. With a further decrease of the
ratio to 0.3, the N2 adsorption amount of UiO-66 decreased to 510
cm3·g-1 from 590 cm3·g-1. In addition, an obvious hysteresis loop
was found in the N2 adsorption–desorption isotherms of UiO-66-
0.5(OA), which indicated the existence of mesopores in the
material.
SBET
SMicro
SMeso
VTotal
VMicro
VMeso
Sample
X=0.3
(m2/g)
(m2/g)
(m2/g)
(cm3/g) (cm3/g) (cm3/g)
960
940
20
0.61
0.48
0.13
X=0.5
X=1.0
X=1.5
X=2.0
1266
1113
1013
988
1181
1049
946
85
64
67
50
0.93
0.79
0.74
0.64
0.61
0.55
0.51
0.50
0.32
0.24
0.23
0.14
938
The morphology of UiO-66 samples was measured by SEM.
Figure 4 showed the differences in cubic morphology and particle
size of UiO-66 synthesized with various ratios of H2BDC/Zr. The
evident typical octahedral cubic crystal was observed in Figure
4c, which indicated that the relative crystallinity of the material
was much high when the ratio of H2BDC/Zr was 1.0. However, the
octahedral cubic morphology was gradually transformed into
spheroid, whether increasing or decreasing the ratio of H2BDC/Zr.
The result revealed that the morphology and crystallinity of UiO-
66 samples could be tuned by altering the ratio of H2BDC/Zr,
which was consistent with the result of XRD patterns. Similarly,
To further compare the BET surface area and characteristics of
porosity of materials synthesized under different ratios, specific
data of them were listed in Table 1. The overall trend in BET area
and porosity features was consistent with that of the N2 adsorption
amount of the synthesized UiO-66 samples. From the data in
Table 1, the conclusion could be drawn more intuitively that the
pores of materials were mainly microporous and partly
mesoporous. It was worth noting that the BET surface area and
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
gradually to 40% when the ratio was further increased to 2.0. In
short, the preferential order to EL was found to be: UiO-66-
0.5(OA) > UiO-66-1.0(OA) > UiO-66-1.5(OA) > UiO-66-2.0(OA) >
UiO-66-0.3(OA). That meaned the highest EL yield (77%) was
achieved via tuning the ratio of H2BDC/Zr to 0.5 under constant
catalytic conditions. The variation trend of catalytic activity was
coincided with that of surface area, pore volume and number of
defects. Based on the characterization of materials, especially the
analysis of BET and H1NMR, it was not difficult to explain the
catalytic result. The observed high activity of UiO-66-0.5(OA) was
probably ascribed to large surface area and uniform crystal size
providing the probability of contact between the reactant and
active sites, and enough pore volume promoting the transfer of
products from active sites. Moreover, the more defects in
structure provided more active sites conducive to the
esterification of levulinic acid with ethanol. The interaction of
several factors endowed the UiO-66-0.5(OA) samples excellent
catalytic activity.
the particle size of UiO-66 materials was regulated via changing
the precursor ratio. When the ratio of H2BDC/Zr was less than 1.0,
the average particle size of the UiO-66 samples increased with
the increase of the ratio of H2BDC/Zr. However, when the ratio of
H2BDC/Zr was more than 1.0, an obvious decrease in the average
particle size of the UiO-66 could be seen with increasing the ratio
of H2BDC/Zr. Besides, the UiO-66 samples with uniform size and
narrow size distribution could be obtained with tuning the ratio of
H2BDC/Zr to 0.5.
Figure 4. SEM of UiO-66-X(OA) (a) X=0.3, (b) X=0.5, (c) X=1.0, (d) X=1.5 and
(e) X=2.0
The data of defect site density per node on UiO-66 were
estimated by liquid 1H NMR spectra, which could determine
accurately the relative concentrations of the molecular
components bonded per node[16,17]. As shown in Table 2, the
overall trend of data were consistent with that of BET surface area
and pore volume. The number of defects in UiO-66 increased first
and then decreased with increasing the ratio of H2BDC to Zr. The
number of defects on the MOFs nodes could be up to 1.61 by
tuning the ratio of H2BDC/Zr to 0.5.
Figure 5. The correclation between ratio of H2BDC/Zr, number of defects and
Table 2 Influence of ratio of H2BDC/Zr on the number of defects per node
yield of EL
determined from 1H NMR spectra
Properties of UiO-66 with different modulators
number of
formate
ligands
number of
DMF
ligands
number of
octanoate
ligands
UiO-66-
X(OA)
number of
defects
In addition to changing the precursor ratio, many reports
confirmed that monocarboxylic acid could be introduced into
X=0.3
X=0.5
0.28
0.17
0.26
0.19
0.25
1.25
0.79
1.61
system to tune defect site density and thus bring out high
12,
activity[7a,
19]. Therefore, the influence of chain length in
modulators on properties of UiO-66 was further investigated with
the precursor ratio constant at 0.5 of H2BDC/Zr, an optimal one
for synthesis of UiO-66 MOFs.
X=1.0
X=1.5
X=2.0
0.62
0.42
0.28
0.58
0.43
0.29
0.29
0.06
0.09
1.49
0.91
0.66
Figure 6 exhibited the crystallinity of sysnthesed materials using
monocarboxylic acids with different chain length as modulators.
The intensity of main diffraction peaks of samples using acetic
acid and octanoic acid with shorter chain length as modulators
were relatively high. Further extending the length of the alkyl chain,
a significant decrease in intensity of peaks was observed. Notably,
a peak broadening was seen in the XRD spectrum of UiO-66 with
myristic acid as a modulator, which clearly revealed that the
Figure 5 showed the correclation between ratio of H2BDC/Zr,
number of defects and the catalytic activity of UiO-66 in the
esterification of levulinic acid with ethanol. In all cases ethyl
levulinate (EL) selectivity was 100 %. There was an obvious
correlation between yield of EL and ratio of H2BDC/Zr. The yield
of EL was improved from 40% to 77% with the increase of ratio of
H2BDC/Zr from 0.3 to 0.5. However, the yield of EL decreased
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
formation of UiO-66 could be disturbed if monocarboxylic acids
with too long chain as modulators. The main reason for the result
was that the extended chain of the modulator blocked the
the mesoporous volume in UiO-66-0.5(OA) was also the largest
compared to that in samples synthesized with other modulators.
When myristic acid was used a modulator, the surface area of
UiO-66 decreased to 346 cm2·g-1 from 1409 cm2·g-1 and the pore
volume decreased to 0.63 cm3·g-1 from 1.25 cm3·g-1, due to
collapse of structure.
coordination between ZrIV and H2BDC[20]
.
Combined with the analysis results of XRD and BET, we found
that moderate chain length in modulators could increase the
adsorption amount, surface area and pore volume while ensure
the crystallinity of UiO-66 materials. Nevertheless, excessively
long alkyl chain disturbed the formation of UiO-66, thus leading to
the decreases of surface area and pore volume
Figure 6. XRD patterns of UiO-66-0.5(M)
Figure 7 showed the N2 adsorption–desorption isotherms of
samples synthesized with different monocarboxylic acids as
modulators respectively. When the ratio of H2BDC/Zr was
determined, the adsorbed quantity of UiO-66 samples depended
mainly on the alkyl chain length of monocarboxylic acids. As the
number of carbon atoms of the alkyl chain increased from 2 to 12,
the amount of N2 adsorption over the synthesized UiO-66 samples
was up to 800 cm3·g-1. Unexpectedly, further increasing the chain
length of modulators, the N2 adsorption amount of the UiO-66
decreased dramatically from 800 cm3·g-1 to 400 cm3·g-1.
Meanwhile, a small hysteresis loop was observed in the N2
adsorption–desorption isotherms of UiO-66-0.5(AA) and UiO-66-
0.5(OA). Further increasing the chain length, a more pronounced
hysteresis loop was found in UiO-66-0.5(LA), which indictatd the
long length modulators were crucial in the formation of
mesoporous.
Figure 7. N2 adsorption–desorption isotherms of UiO-66-0.5(M)
The length of alkyl chain in modulators clearly affected the
morphology and the particle size of UiO-66, which was confirmed
by the SEM (Figure 8). As shown in Figure 8a, the octahedral
cubic structure was possessed by the UiO-66 with acetic acid as
a modulator and the average particle size was about 350 nm. With
the increase of alkyl chain length from 2 to 12 carbon atoms, the
octahedral cubic morphology was gradually transformed into
smooth spheroid and the particle size decreased from 350 nm to
220 nm. However, there were existences of the particle
aggregation and the collapse of crystal morphology in Figure 8d,
the SEM of UiO-66-0.5(MA) synthesized with myristic acid (14
atoms in the alkyl chain) as a modulator. Combined with SEM and
XRD results, a conclusion could be deduced that the number of
carbon atoms in the alkyl chain of modulators was about 12 at
most, to ensure the morphology and crystallinity.
Table 3 Surface area and pore structure of UiO-66-0.5(M)
SBET
(m2/g)
SMicro
(m2/g)
SMeso
(m2/g)
VTotal
VMicro
VMeso
Sample
(cm3/g) (cm3/g) (cm3/g)
M=AA
M=OA
1148
1266
1409
1000
346
1102
1181
1314
912
46
85
95
88
87
0.84
0.93
1.25
1.01
0.63
0.59
0.61
0.69
0.49
0.17
0.25
0.32
0.56
0.52
0.46
To further reveal the influence of the alkyl chain length in
modulators on micropore and mesopore structure, UiO-66-0.5(M)
(M=AA, OA and LA) with high relative crystallinity were chosen to
measure by TEM. Compared with SEM, the TEM showed more
clearly the transformation of crystalline structures in UiO-66
materials with the elongation of alkyl chain in modulators. There
was no obvious structure of mesopore observed in the UiO-66-
0.5(AA) sample (Figure 9d). Instead, the worm-like mesopores
were found in the UiO-66-0.5(OA) (Figure 9e) and UiO-66-
0.5(LA) (Figure 9f), which demonstrated that the formation of
M=LA
Used (LA)
M=MA
259
The surface area and pore volume of samples with different
modulators were listed in Table 3. The moderate elongation of
alkyl chain increased significantly the surface area and porous
volume. The surface area and pore volume of UiO-66-0.5(LA)
were up to 1409 cm2·g-1 and 1.23 cm3·g-1 respectively. In addition,
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
mesopores could be tuned by suitable alkyl chain length of
modulators. The result was in accordance with Table 3 and further
indicated the defects and pore structure could be also controlled
by the modulators with different alkyl chain length.
shift of μ3-OH stretching frequencies should be that the number of
vacancies on per Zr6 node increased with extending the alkyl
chain length of modulators[16]. To further explain why catalytic
activity of activated UiO-66 catalysts reduced compared to UiO-
66 without activation, the change of hydroxyl groups bonded to
nodes between UiO-66-0.5(LA) and UiO-66-0.5(LA) activated at
353 k for 12 h under vacuum was investigated (Figure 10b). The
OH stretching region almost disappeared after activation, which
confirmed that the IR band at 3647 cm-1, 3647 cm-1 and 3633 cm-
1
in UiO-66-0.5(AA), UiO-66-0.5(OA) and UiO-66-0.5(LA)
respectively were assigned to a new OH stretching introduced by
H2O capping the vacant site. The result was consistent with
previous report[22]
.
Figure 8. SEM of UiO-66-0.5(M) (a) M=AA, (b) M=OA, (c) M=LA, and (d)
Figure 10. IR spectra in νOH region characterizing (a) UiO-66-0.5(M); (b) UiO-
M=MA
66-0.5(LA) and the same sample activated at 353 k for 12 h under vacuum
1
The data in Table 4 were estimated by liquid H NMR spectra,
which could unravel the effects of modulators in the MOF
synthese. Table 4 showed that the variation of the alkyl chain
length of the modulator in the synthesis allowed the tuning of the
defect numbers. As shown in Table 4, the number of ligands
derived from modulator were the main contributors to more
defects on the MOF nodes. The number of defects per node in
UiO-66-0.5(LA) could be up to 2.1, a relatively high defect site
density. The defect site density was also affected by the pKa of
modulators. Generally, more defects would be formed with
increase of acidity of modulators[16]. But, maybe the effect of the
chain length on the defects was greater than that of the acid
strength in present case, which promoted more defects generated
in UiO-66-0.5(OA).
Figure 9. TEM of UiO-66-0.5(M) (a,d) M=AA, (b,e) M=OA and (c,f) M=LA
Table 4 Influence of modulator on the number of defects per node
determined from 1H NMR spectra
IR spectra could identify the hydroxyl groups incorporated on Zr6
nodes[14b, 17, 21], which acted as Brønsted acid sites in catalytic
reactions. Therefore, we studied the influence of different
modulators on proton topology of Zr6 node vacancies in UiO-66
samples by IR, which was shown in Figure 10a. The peak
observed at 3671 cm-1, 3670 cm-1 and 3667 cm-1 possessed by
the UiO-66 were assigned to the μ3-OH stretching frequency
bonded to Zr6 nodes[17, 22]. The μ3-OH band shifted slightly to low
frequencies at 3667 cm-1 from 3671 cm-1 when lauric acid with
long alkyl chain was used as a modulator. The reason for the red
number
of
number
of
number of
acetate,
number
of
defects
UiO-66-
0.5(M)
formate
ligands
DMF
ligands
octanoate or
laurate ligands
M=AA
0.27
0.28
0.77( acetate)
1.32
M=OA
M=LA
0.17
0.21
0.26
0.19
0.20
0.27
1.25( octanoate)
1.69( laurate)
1.61
2.10
1.54
Activated(LA)
1.01(( laurate))
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
The correclation between modulators, number of defects and
catalytic activity of UiO-66 was further evaluated under same
reaction conditions (Figure 11). To confirm the role of synthesized
UiO-66 on the esterification of levulinic acid with ethanol, the
blank reaction was conducted as a supplementary experiment,
the EL yield of which was only 11% (Figure 11). The catalytic
activity increased initially and then decreased with the elongation
of alkyl chain in modulators. The yield of EL could reach 84% over
the UiO-66 catalysts with lauric acid as a modulator. The yield of
EL decreased from 84% to 33% when UiO-66-0.5(MA) was used
as catalyst, because the excessively long alkyl chain in
modulators disturbed the structure and decreased sharply the
surface area and pore volume of UiO-66, which was confirmed by
XRD, SEM and BET. The increased activity was primarily
attributed to the excellent properties that UiO-66-0.5(LA)
possessed. The UiO-66-0.5(LA) catalyst with high surface area
could offer enough acid sites to activate reagent molecules and
promote the reactivity. In addition, the more mesoporous inside
UiO-66-0.5(LA) benefited the inner diffusion of bulkier reactant
molecules and thus improved the catalytic activity. Meanwhile, the
defects in UiO-66-0.5(LA) acted as Lewis acid sites was a factor
to bring out high activity. The influence of hydroxyl groups on Zr6
nodes on catalytic activity of UiO-66 was also studied. Compared
with UiO-66-0.5(LA), the activity of activated UiO-66-0.5(LA)
material decreased as a result of the decrease in number of
defects and the almost disappearance of hydroxyl group.
Generally, higher catalytic activity could be obtained with more
defects in UiO-66 catalysts. However, though the number of
defects in activated UiO-66-0.5(LA) was more than that in UiO-
66-0.5(AA), the yield of EL with AA-0.5 as a catalyst was higher
than that with UiO-66-0.5(LA) as a catalyst, which indicated that -
OH and μ3-OH on Zr6 nodes acted as Brønsted acid sites was a
crucial factor in the esterification of levulinic acid with ethanol.
esterification of levulinic acid with ethanol under same
experimental conditions of reaction temperature of 351 K, LA to
ethanol molar ratio of 1:15 and reaction time of 8 h. After the
reaction, the catalyst was recovered by centrifugation, washing
with methanol, and then drying at room temperature overnight.
The obtained results from recycling experiments were shown in
Figure 12. No significant decrease of catalytic activity was seen,
which signified that the synthesized catalyst possessed excellent
reusability under this reaction conditions.
Figure 12. Reusability of UiO-66-0.5(LA) catalyst
To further check the stability of UiO-66-0.5(LA) catalyst, the used
sample was characterized by XRD (Figure 13) and N2
physisorption (Figure 14). No noticeable change in crystal
structure was observed for the used catalyst, demonstrating that
UiO-66-0.5(LA) had long-term stability. As seen in Figure 13 and
Table 3, compared to fresh sample, the surface area of the used
UiO-66-0.5(LA) decreased to 1000 cm2·g-1 from 1409 cm2·g-1 and
the pore volume the used UiO-66-0.5(LA) decreased to 1.01
cm3·g-1 from 1.25 cm3·g-1, hence leading to the slight decrease in
catalytic activiity.
Figure 11. The correclation between modulators, number of defects and yield
of EL
Figure 13. XRD patterns of fresh UiO-66-0.5(LA) and used UiO-66-0.5(LA)
The reusability of UiO-66-0.5(LA) catalyst was studied in
after five times
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis
synthesis of UiO-66 with different precursor ratios. ZrCl4 (360
mg,1.55 mmol) and 8.7 ml octanoic acid (54.25 mmol, 35
equivalents with respect to Zr4+) were dissolved in 60 ml DMF in
a 100 ml Teflon liner with ultrasound for five minutes at room
temperature. After that, different amounts of terephthalic acid (77
mg, 129 mg, 258 mg, 387 mg, and 516 mg, about 0.3, 0.5, 1.0,
1.5 and 2.0 equivalents with respect to Zr4+ respectively) were
separately added into the mixed solution and dissolved for 15
minutes with ultrasound. The resulting solution was then
transferred into a preheated oven at 393 K for 48 h without stirring.
After centrifugation, the white crystalline powders were washed
with fresh DMF two times to remove unreacted precursors and
soaked in methanol 24 h (centrifugation every 8 h). After dried at
room temperature, the resulting catalysts were denoted as UiO-
66-X(OA), where X stands for the mole ratio of H2BDC and Zr.
Synthesis of UiO-66 with different Modulators. ZrCl4 (360
mg,1.55 mmol) and 54.25 mmol different monocarboxylic acids
(acetic acid, octanoic acid, lauric acid and myristic acid, 35
equivalents with respect to Zr4+) were dissolved in 60 ml DMF in
a 100 ml Teflon liner with ultrasound for five minutes at room
temperature. Terephthalic acid (linker, 129 mg, 0.775 mmol) was
added into the above mixed solution and dissolved for 15 minutes
with ultrasound. The resulting solution was then transferred into a
preheated oven at 393 K for 48 h without stirring. After
centrifugation, the white crystalline powders were washed with
fresh DMF two times to remove unreacted precursors and soaked
in methanol 24 h (centrifugation every 8 h). After dried at room
temperature, the resulting catalysts were denoted as UiO-66-
0.5(M), where M stands for the modulator (AA, OA, LA and MA).
Characterization
Figure 14. N2 adsorption–desorption isotherms of fresh UiO-66-0.5(LA) and
used UiO-66-0.5(LA) after five times
Conclusions
A series of defective UiO-66 with different defect densities were
successfully synthesized by tuning the ratio of H2BDC/Zr and
adding monocarboxylic acids with different chain length as
modulators. The UiO-66 MOFs synthesized with 0.5 of H2BDC/Zr
possessed higher surface area, pore volume, uniform particle size
and higher defect density than that synthesized with other ratios.
Meanwhile, the number of carbon atoms in modulator was 12 at
most for ensuring the crystallinity and structure. Thus, the most
active UiO-66 was synthesized with largest surface area, pore
volume, uniform particle size and the highest defect density (2.1
per cluster) when the ratio of H2BDC/Zr was 1:0.5 and Lauric acid
was used as a modulator. In addition, the comparative experiment
between UiO-66-0.5(LA) and activated UiO-66-0.5(LA) confirmed
that the μ3-OH and OH introduced by H2O acted as Brønsted acid
sites in catalytic reactions. The yield of EL was up to 84% at 351
K for 8 h catalyzed by the cooperative effect between Lewis acid
sites and Brønsted acid sites. In addition, the synthesized
defective UiO-66 also had excellent stability. This research
provided a strategy to facilely tune the defect density and hydroxyl
groups on nodes by introducing monocarboxylic with optimum
alkyl chain length. The confirmation of cooperative effect between
defects and hydroxyl groups can be considered when designing
active UiO-66 for Brønsted acid catalyzed reactions.
Powder X-ray Diffraction. Power X-ray diffraction (PXRD)
patterns of UiO-66 were obtained using a Rigaku D/max-A
diffractometer. The scanning range (2θ) was 5–50°, with a
scanning rate of 10°·min−1 and a step function of 0.02°.
BET Surface Area Measurements. All N2 adsorption–desorption
isotherms were collected with a Beishide 3H-2000 analyzer by
static N2 physisorption at 77 K after samples were degassed at
393 K for 12 h under vacuum. The surface area of the UiO-66 was
calculated based on multipoint Brunauer–Emmett–Teller (BET)
method.
1
1H NMR Spectroscopy. Liquid H NMR spectra were recorded
with a Bruker Avance DPX-500 NMR spectrometer (500 MHz).
Samples were prepared by adding 15 mg UiO-66 into a 2 ml vial
with 1 ml solution consisting of 1 M NaOH in D2O, which could
only dissolved the organic portion of the MOF (linker, solvent,
components formed from the modulator, and any other such
components)[16,17]. After digesting 24 h, the clear solution was
transferred into an NMR tube.
Experimental Section
Infrared Spectroscopy. The transmission IR spectra of MOF
powder samples was collected by
a Bruker IFS 66v/S
Materials
spectrometer with a spectral resolution of 2 cm−1.
ZrCl4 (99.9%) and levulinic acid (99%) were purchased from
Aladdin. Ethanol (99.9%), Methanol (99%), acetic acid (AA, 99%),
octanoic acid (OA, 99%), lauric acid (LA, 98%), myristic acid (MA,
98%), D2O (99%), NaOH (96%) and DMF (99.5%) were
purchased from Sinopharm Chemical Regent co. Ltd. All reagents
were used without further purification.
Scanning Electron Microscopy. After the gold sputtering
treated three times, the morphology of UiO-66 samples was
measured by a FEI Inspect F50 SEM instrument.
Transmission Electron Microscopy. The samples measured
were dispersed in Ethanol by ultrasound for 30 minutes. Then,
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
[7]
a) H. Wu, Y. S. Chua, V. Krungleviciute, M. Tyagi, P. Chen, T. Yildirim,
W. Zhou, J. Am. Chem. Soc. 2013, 135, 10525-10532; b) G. C. Shearer,
S. Chavan, J. Ethiraj, J. G. Vitillo, S. Svelle, U. Olsbye, C. Lamberti, S.
Bordiga, K. P. Lillerud, Chem. Mat. 2014, 26, 4068-4071.
TEM images were collected with a FEI Tecnai G2 T20 TEM
instrument.
Catalytic Test
Esterification of levulinic acid with ethanol was carried out in a
bath reactor with 1:15 of molar ratio (levulinic acid: ethanol), 1.8
mol% of catalyst amount with respect to levulinic acid, 351 K of
reaction temperature and 8 h of reaction time. The reaction
products were analyzed by gas chromatography (GC-6890)
equipped with SE-54 capillary column (30 m×0.32 mm×0.3 μm)
and an FID detector. Analysis conditions were as follows: 443 K
of column oven temperature, 543 K of injector temperature, and
553 K of detector temperature. Octane was used as the internal
standard.
[8]
[9]
M. Vandichel, J. Hajek, F. Vermoortele, M. Waroquier, D. E. De Vos, V.
Van Speybroeck, Crystengcomm 2015, 17, 395-406.
K. Xuan, Y. F. Pu, F. Li, A. X. Li, J. Luo, L. Li, F. Wang, N. Zhao, F. K.
Xiao, J. CO2 Util. 2018, 27, 272-282.
[10] R. J. Marshall, C. L. Hobday, C. F. Murphie, S. L. Griffin, C. A. Morrison,
S. A. Moggach, R. S. Forgan, J. Mater. Chem. A 2016, 4, 6955-6963.
[11] X. Ma, L. Wang, Q. Zhang, H.-L. Jiang, Angew. Chem. Int. Ed. 2019, 58,
12175-12179.
[12] F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde, M. Vandichel,
K. Houthoofd, A. Vimont, M. Daturi, M. Waroquier, V. Van Speybroeck,
C. Kirschhock, D. E. De Vos, J. Am. Chem. Soc. 2013, 135, 11465-11468.
[13] Y. Liu, R. C. Klet, J. T. Hupp, O. Farha, Chem. Commun. 2016, 52, 7806-
7809.
[14] a) J. C. Jiang, O. M. Yaghi, Chem. Rev. 2015, 115, 6966-6997; b) D.
Yang, V. Bernales, T. Islamoglu, O. K. Farha, J. T. Hupp, C. J. Cramer,
L. Gagliardi, B. C. Gates, J. Am. Chem. Soc. 2016, 138, 15189-15196.
[15] R. C. Klet, Y. Liu, T. C. Wang, J. T. Hupp, O. K. Farha, J. Mater. Chem.
A 2016, 4, 1479-1485.
Acknowledgements
[16] D. Yang, M. A. Ortuno, V. Bernales, C. J. Cramer, L. Gagliardi, B. C.
Gates, J. Am. Chem. Soc. 2018, 140, 3751-3759.
[17] R. Wei, C. A. Gaggioli, G. Li, T. Islamoglu, Z. Zhang, P. Yu, O. K. Farha,
C. J. Cramer, L. Gagliardi, D. Yang, B. C. Gates, Chem. Mat. 2019, 31,
1655-1663.
This work was financially supported by Natural Science
Foundation of Jiangsu Province (No. BK20161415), National
Natural Science Foundation of China (No. 21676054),
Fundamental Research Funds for the Central Universities (No.
3207049413).
[18] M. J. Katz, Z. J. Brown, Y. J. Colon, P. W. Siu, K. A. Scheidt, R. Q. Snurr,
J. T. Hupp, O. K. Farha, Chem. Commun. 2013, 49, 9449-9451.
[19] Z. L. Fang, B. Bueken, D. E. De Vos, R. A. Fischer, Angew. Chem. Int.
Ed. 2015, 54, 7234-7254.
[20] G. Cai, H. L. Jiang, Angew. Chem. Int. Ed. 2017, 56, 563-567.
[21] a) J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S.
Bordiga, K. P. Lillerud, J. Am. Chem. Soc. 2008, 130, 13850-13851; b)
D. Yang, S. O. Odoh, T. C. Wang, O. K. Farha, J. T. Hupp, C. J. Cramer,
L. Gagliardi, B. C. Gates, J. Am. Chem. Soc. 2015, 137, 7391-7396.
[22] N. Planas, J. E. Mondloch, S. Tussupbayev, J. Borycz, L. Gagliardi, J. T.
Hupp, O. K. Farha, C. J. Cramer, J. Phys. Chem. Lett. 2014, 5, 3716-
3723.
Keywords: Metal−organic framework • UiO-66 • Modulators •
Defects • Levulinic acid
[1]
a) S. Yuan, L. Feng, K. C. Wang, J. D. Pang, M. Bosch, C. Lollar, Y. J.
Sun, J. S. Qin, X. Y. Yang, P. Zhang, Q. Wang, L. F. Zou, Y. M. Zhang,
L. L. Zhang, Y. Fang, J. L. Li, H. C. Zhou, Adv. Mater. 2018, 30, 1704303;
b) H. Furukawa, K. E. Cordova, M. O'Keeffe, O. M. Yaghi, Science 2013,
341, 1230444; c) M. T. Zhao, Y. Huang, Y. W. Peng, Z. Q. Huang, Q. L.
Ma, H. Zhang, Chem. Soc. Rev. 2018, 47, 6267-6295; d) Z. Yin, S. Wan,
J. Yang, M. Kurmoo, M. H. Zeng, Coord. Chem. Rev. 2019, 378, 500-
512.
[2]
[3]
a) S. L. Qiu, M. Xue, G. S. Zhu, Chem. Soc. Rev. 2014, 43, 6116-6140;
b) J. R. Li, J. Sculley, H. C. Zhou, Chem. Rev. 2012, 112, 869-932.
a) L. Wang, Y. Z. Han, X. Feng, J. W. Zhou, P. F. Qi, B. Wang, Coord.
Chem. Rev. 2016, 307, 361-381; b) D. X. Xue, Q. Wang, J. F. Bai, Coord.
Chem. Rev. 2019, 378, 2-16.
[4]
[5]
a) Z. C. Hu, B. J. Deibert, J. Li, Chem. Soc. Rev. 2014, 43, 5815-5840;
b) Z. Z. Lu, R. Zhang, Y. Z. Li, Z. J. Guo, H. G. Zheng, J. Am. Chem. Soc.
2011, 133, 4172-4174.
a) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen, J. T. Hupp,
Chem. Soc. Rev. 2009, 38, 1450-1459; b) J. D. Xiao, H. L. Jiang,
Accounts Chem. Res. 2019, 52, 356-366; c) K. Chen, C. D. Wu, Coord.
Chem. Rev. 2019, 378, 445-465; d) Y. B. Huang, J. Liang, X. S. Wang,
R. Cao, Chem. Soc. Rev. 2017, 46, 126-157.
[6]
a) M. Kandiah, M. H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M.
Tilset, C. Larabi, E. A. Quadrelli, F. Bonino, K. P. Lillerud, Chem. Mat.
2010, 22, 6632-6640; b) Y. Bai, Y. B. Dou, L. H. Xie, W. Rutledge, J. R.
Li, H. C. Zhou, Chem. Soc. Rev. 2016, 45, 2327-2367; c) M. Rimoldi, A.
J. Howarth, M. R. DeStefano, L. Lin, S. Goswami, P. Li, J. T. Hupp, O. K.
Farha, ACS Catal. 2017, 7, 997-1014; d) D. Yang, B. C. Gates, ACS
Catal. 2019, 9, 1779-1798.
This article is protected by copyright. All rights reserved.

10.1002/ejic.202000031
European Journal of Inorganic Chemistry
FULL PAPER
Text for Table of Contents
Author(s), Corresponding Author(s)*
Esterification of levulinic acid with
ethanol was catalyzed by the
cooperative effects between the Lewis
acid sites offered by unsaturated Zr6
nodes and Brønsted acid sites
supplied by OH in defective UiO-66
MOFs with high defect density, which
was tuned by precursor ratios and
modulators.
Ruiping Wei*, Jingdeng Fan, Xumin Qu,
Lijing Gao, Yuanfeng Wu, Zhongqi
Zhang, Feng Hu, Guomin Xiao
Page No.1 – Page No.8
Title
Tuning the Catalytic Activity of UiO-66
via
Modulated
Synthesis
：
Esterification of Levulinic Acid as A
Test Reaction
Key Topic for Table of Contents
UiO-66, Catalysis
This article is protected by copyright. All rights reserved.