Synthesis of sulfonic acid-functionalized MIL-101 for acetalization of aldehydes with diols

Article abstract of DOI:10.1016/j.molcata.2013.12.005

A metal-organic framework (MOF) MIL-101 bearing sulfonic acid groups (SO₃H-MIL-101), was prepared through one-pot hydrothermal synthesis strategy. This MOF was systemically characterized via XRD, N₂ adsorption, acid-base titration, XPS, FT-IR techniques, and elemental analysis, and then used as a heterogeneous catalyst for liquid-phase acetalizations of various aldehydes with diols. The obtained SO₃H-MIL-101 shows high catalytic activity in the acetalizations due to the high utilization efficiency of its functionalized acid sites. Additionally, this catalyst can be easily recovered and reused five times in succession without significant loss of catalytic activity.

Full text of DOI:10.1016/j.molcata.2013.12.005

Journal of Molecular Catalysis A: Chemical 383–384 (2014) 167–171
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis of sulfonic acid-functionalized MIL-101 for acetalization of
aldehydes with diols
Yan Jin, Jing Shi, Fumin Zhang∗, Yijun Zhong, Weidong Zhu
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, 321004 Jinhua,
People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 October 2013
Received in revised form 4 December 2013
Accepted 7 December 2013
Available online 17 December 2013
A metal–organic framework (MOF) MIL-101 bearing sulfonic acid groups (SO3H-MIL-101), was prepared
through one-pot hydrothermal synthesis strategy. This MOF was systemically characterized via XRD,
N2 adsorption, acid–base titration, XPS, FT-IR techniques, and elemental analysis, and then used as a
heterogeneous catalyst for liquid-phase acetalizations of various aldehydes with diols. The obtained
SO3H-MIL-101 shows high catalytic activity in the acetalizations due to the high utilization efficiency of
its functionalized acid sites. Additionally, this catalyst can be easily recovered and reused five times in
succession without significant loss of catalytic activity.
Keywords:
Heterogeneous catalysis
Metal–organic framework
MIL-101
© 2013 Elsevier B.V. All rights reserved.
Sulfonic acid-functionalized MIL-101
Acetalization
1. Introduction
potential application in the fields of gas storage, separation and
catalysis [19–22]. The interest in MOFs arises from their large
Acetalization of aldehydes with diols to form acetals is a useful
method of protecting carbonyl compounds in organic synthesis [1].
Moreover, acetals are extensively used as ingredients or additives
in fragrances, detergents and cosmetics [2]. Traditionally, acetaliza-
tion reactions are conducted in the presence of homogeneous acid
catalysts, such as H2SO4, HCl and p-toluenesulfonic acid. However,
these homogeneous catalysts pose various disadvantages such as
toxicity, corrosiveness and tedious operation procedures. From
industrial and environmental perspectives, the development of
solid acid catalysts is more desirable because they are less corrosive
and can remarkably simplify product separation and catalyst recov-
ery. Consequently, the use of various heterogeneous catalysts, such
as zeolites [3–5], clays and montmorillonites [6–8], mesoporous
materials [9–14], metal phosphates [15], ionic liquids [16,17] and
metal–organic frameworks [18] have been attempted in acetaliza-
tion reactions. However, some of these procedures suffer from high
catalyst/substrate ratio, high reaction temperature and quick deac-
tivation of catalyst. As such, the development of new heterogeneous
catalysts to improve the catalytic activity and reusability in the
acetalizations are urgently required.
porosity, easy tunability of pore size, and shape from the micro-
porous to the mesoporous scale [23–25]. Among known MOFs,
several transition-metal MOFs have high hydrothermal and chem-
ical stabilities. These MOFs include mesoporous chromium (III)
terephthalate (MIL-101) that possesses acceptable resistance to
water, common solvents, and high temperatures up to 300 ◦C
[26,27]. MIL-101 has a rigid zeotype crystal structure consisting
of 2.9 and 3.4 nm mesoporous cages accessible through window
sizes of ca. 1.2 and 1.6 nm, respectively. Due to its high stability,
MIL-101 exhibits no detectable leaching of chromium into aqueous
solutions, ensuring its safe use in different applications [28–35].
However, the use of MOFs in applications like catalysis and
separation is still largely hindered by the lack of functional and
selective sites in most ultrastable MOFs. In an attempt to func-
tionalize MOFs, various active sites, including heteropoly acid
and sulfonic acid moiety, have been encapsulated (or loaded)
into MIL-101 through one-pot synthesis, post-grafting, or pre-
modification of organic linker with functionalized ligand [36–43].
In our previous work, we synthesized a sulfonic acid-functionalized
MIL-101 catalyst through one-pot synthesis strategy, and found
the obtained catalyst showed superior catalytic properties in a
series of esterification reactions [41]. However, the synthesis of
the sulfonic acid-functionalized MIL-101 took a relatively long
time (6 d) [40,41]. In the current work, we synthesized the sul-
fonic acid-functionalized MIL-101 through one-pot hydrothermal
reaction of 2-sulfoterephthalic acid monosodium salt, CrO3, and
Metal–organic frameworks (MOFs), an important family of crys-
talline materials, are currently eliciting much attention for their
∗
Corresponding author. Tel.: +86 579 82288919; fax: +86 579 82282234.
E-mail address: zhangfumin@zjnu.edu.cn (F. Zhang).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.12.005

168
Y. Jin et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 167–171
H2O with an appropriate amount of HF. As HF is a mineraliz-
ing agent that can improve crystallinity and promote the crystal
growth of the final product, the crystallization time of the sul-
fonic acid-functionalized MIL-101 was significantly reduced. Thus,
the sulfonic acid-functionalized MIL-101 could be obtained by
hydrothermal reaction of the above mixtures at 180 ◦C only for 1 d.
As a solid acid catalyst, the obtained MOF displayed high catalytic
activity, high selectivity, and excellent reusability in the acetaliza-
tion of various aldehydes with diols.
SO3H-MIL-101
MIL-101
2. Experimental
Simulated MIL-101
2.1. Chemicals
Terephthalic acid (98%) and 2-sulfoterephthalic acid
monosodium salt were purchased from J&K Chemical Ltd. An
ultrastable Y zeolite (USY, Si/Al = 6) was supplied by Zhoucun
Catalyst Factory. Amberlyst-15 was purchased from Fisher Scien-
tific Worldwide Co., Ltd. (Shanghai). Other chemicals (AR grade)
were commercially available and used without further purifi-
cation. Deionized water with a resistance larger than 18.2 Mꢀ
was obtained from Millipore Milli-Q ultrapure water purification
system.
3
4
5
6
7
8
9
10
2 Theta / degree
Fig. 1. XRD partners of various catalysts.
(a)
(b)
1200
900
600
300
0
MIL-101
2.2. Catalyst preparation
SO3H-MIL-101
The one-pot synthesis of sulfonic acid-functionalized MIL-101
was based on the following procedures: 2.5 g of CrO3, 6.7 g of
2-sulfoterephthalic acid monosodium salt, and 16 mmol of HF
(40 wt.%) were added into 100 ml of deionized water and stirred
for 10 min at room temperature. The solution was then transferred
into a teflon-lined stainless steel autoclave. The synthesis was con-
ducted without agitation in an oven at 180 ◦C for 1 d. The produced
solid was filtered and washed with deionized water and then with
methanol three times. Afterward, the solid was dried in a vacuum
desiccator at 150 ◦C for 6 h prior to further analysis or use. The
resulting sulfonic acid-functionalized MIL-101 was referred to as
SO3H-MIL-101. Pure MIL-101 was synthesized following the pro-
cedure reported in the literature [26].
MIL-101
SO3H-MIL-101
0.0 0.2 0.4 0.6 0.8 1.0 1
2
3
4
5
Pore diameter / nm
P/P0
Fig. 2. N2 isotherms (a) and DFT pore size plots (b) of various catalysts.
2.4. Acetalization of aldehyde and alcohol
The liquid-phase acetalization was conducted in a three-necked
round-bottom flask connected with a reflux condenser and a ther-
mometer. The typical procedures for acetalization were as follows:
benzaldehyde (7.42 g, 70 mmol), glycol (7.81 g, 126 mmol), cata-
lyst (0.12 g), and cyclohexane (8 mL, as a water-carrying agent)
were charged successively into the flask and heated at 80 ◦C under
stirring. The reaction mixtures were sampled periodically and ana-
lyzed using a GC (GC-2014) equipped with an FID detector and a
capillary column (DB-5, 30 m × 0.45 mm × 0.42 ␮m). After the reac-
tion, the catalyst was separated from the reaction medium by
centrifugation, washed with acetone (3–4 times) and treated at
150 ◦C for 180 min for activation, and then reused in the next run.
2.3. Catalyst characterization
The powdered X-ray diffraction (XRD) patterns were performed
on a Philips PW3040/60 diffractometer using Cu K␣ radiation
(ꢁ = 0.1541 nm) in a scanning range of 2–10◦ at 0.5◦/min. The text-
ural properties of the prepared samples were determined by N2
adsorption at −196 ◦C using a Micromeritics ASAP 2020 instrument.
The sample was outgassed in vacuum at 150 ◦C for 10 h prior to the
adsorption measurement. The infrared (IR) spectra were collected
on a Nicolet NEXUS670 Fourier transform IR spectrophotometer in
KBr disks at room temperature. The acid capacities of SO3H-MIL-
101 were determined by acid–base titration using NaCl solution as
an ion-exchange agent. In a typical procedure, 0.5 g of SO3H-MIL-
101 was suspended in 20 g of aqueous NaCl saturated solution. The
resulting suspension was stirred at room temperature for at least
24 h, followed by filtration and washing with 30 ml of deionized
water. The filtrate was then titrated with 0.1 M NaOH solution. The
amounts of the Cr and S species in SO3H-MIL-101 were measured by
an IRIS Intrepid II XSP inductively coupled plasma-atomic emission
spectrometer (ICP-AES) and XRF-1800 (Shimadzu), respectively.
The surface electronic states of the synthesized samples were
investigated by X-ray photoelectron spectroscopy (XPS, Thermo
Scientific EscaLab 250Xi using Al K␣ radiation). The XPS data were
internally calibrated, fixing the binding energy of C 1s at 284.6 eV.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of the synthesized SO3H-MIL-101 and MIL-
101 are shown in Fig. 1. The XRD patterns of SO3H-MIL-101 and
MIL-101 match well with those of the simulated one in the litera-
ture [26], demonstrating that the MOF structure is well preserved
after the sulfonic acid groups were inserted into the framework
of MIL-101. The textural properties of MIL-101 and SO3H-MIL-101
assessed from N2 adsorption at −196 ◦C are presented in Fig. 2a

Y. Jin et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 167–171
169
Table 1
Textural properties and chemical compositions of various catalysts.
a
b
Catalyst
S/Cra
(mol/mol)
SBET (m2/g)
Vtotal
(cm3/g)
Amount of acid
(mmol/g)
MIL-101
0
0.95
–
3933
1754
38
1.89
0.91
0.18
0.33
–
SO3H-MIL-101
Amberlyst-15
USY
1.8c
4.7c
0.95d
b
c
–
595
a
Elemental analysis.
BET method.
Acid–base titration.
1025
1025
b
c
1230
1185
624
624
1080
d
Temperature-programmed desorption of ammonia [39].
1230
1185
1080
(b)
(a)
S 2p scan
S 2p1/2
O 1s
S 2p3/2
1800
1500
1200
900
600
Wavenumber / cm-1
Fig. 4. FT-IR spectra of MIL-101 (a), SO3H-MIL-101 (b), and used SO3H-MIL-101 after
the fifth-run reaction.
C 1s
Cr 2p
S 2p
600
400
200
0 174 172 170 168 166 164
Binding energy /eV
Binding energy /eV
Fig. 3. XPS spectra of SO3H-MIL-101: (a) survey spectrum; (b) high-resolution
image of S spectrum.
and Table 1. The N2 adsorption amount of SO3H-MIL-101 decreases
more than that of the bulk MIL-101, a change attributable to the
blockage of cages of MIL-101 by the introduction of sulfonic acid
moieties in its framework.
Fig. 5. Conversion of benzaldehyde as a function of reaction time in the acetalization
of aldehyde with glycol over different catalysts. Reaction conditions: 0.12 g of cat-
alyst, 70 mmol of benzaldehyde, 126 mmol of glycol and 8 mL of cyclohexane used
as the water removal agent at 80 ◦C.
The DFT pore size distribution curve of MIL-101 displays two
different pore sizes centered at about 2.3 and 2.7 nm (Fig. 2b),
confirming the presence of two types of mesoporous cages in MIL-
101. However, these values are underestimated by the DFT method
compared with the van der Waals diameters calculated from the
crystalline structure (2.9 nm and 3.4 nm, respectively) [26]. Some
investigators have speculated that this difference may be a result
of the residual terephthalic acid species in the pores of MIL-101
[43]. While, in our perspective, the applied methods for calculating
the pore size distribution may also have led to the different results.
The pore size distributions of SO3H-MIL-101 decreases slightly in
size compared with those of the parent MIL-101, indicating that the
introduction of SO3H groups can block the cages of MIL-101.
Fig. 3a shows the survey XPS data and indicates that SO3H-MIL-
101 contained four elements Cr, O, C, and S. The S 2p spectrum of
SO3H-MIL-101 shows two different peaks (Fig. 3b), which can be
attributed to the S O (168.7 eV) and S O bonds (169.9 eV) at an
intensity ratio of 1:2 with a peak separation of 1.2 eV [44,45]. These
two bonds further indicate that the S in SO3H-MIL-101 is mainly in
the form of SO3H groups bonded to the MIL-101 framework. The
FT-IR bands related to the sulfonic acid groups are well observed
in bands ranging from 600 cm−1 and 1400 cm−1 for SO3H-MIL-101
(Fig. 4) [40,41]. Furthermore, the concentration of the detectable
acid groups is 1.8 mmol/g for SO3H-MIL-101 and the S/Cr molar
ratio measured by the elemental analysis is 0.95 (Table 1), The
measured S/Cr molar ratio is in accordance with the results of its
counterpart reported in the literature [40,41]. The combination of
the results of XRD, N2 adsorption, acid–base titration, FT-IR and
XPS characterizations, and elemental analysis leads to the conclu-
sion that the sulfonic acid-functionalized MIL-101 was successfully
prepared through the strategy reported in the current work. How-
ever, the handling of HF is difficult because of its harmful nature,
although it has an important function in the synthesis of SO3H-
MIL-101. Thus, the use of HF hinders the mass production of MOFs.
Therefore, further studies are still needed to successfully obtain
SO3H-MIL-101 with comparable short synthesis time under HF-
free conditions.
3.2. Catalytic acetalization
The liquid-phase acetalization of benzaldehyde with glycol was
conducted over MIL-101, SO3H-MIL-101, and USY, as well as in the
absence of a catalyst. The conversions of benzaldehyde in these
different cases as a function of reaction time were compared, as
shown in Fig. 5. Benzaldehyde glycol acetal was observed as the
only product of the reaction. No other by-product was detected
by GC under the applied reaction conditions. A rather low conver-
sion of benzaldehyde (26.6%) was obtained in the blank experiment,
suggesting that a proper catalyst is required in this reaction. There-
fore, the conversion of benzaldehyde increased smoothly with an
increase in reaction time, and the conversion reached 97.3% at
80 ◦C for 180 min over SO3H-MIL-101. The high catalytic activity

170
Y. Jin et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 167–171
Table 2
Acetalization of different aldehydes and alcohols catalyzed by SO3H-MIL-101 and Amberlyst-15.a
Entry
Aldehyde
Alcohol
Time (min)
Yield of acetalf (%)
TOFg (mol/(molacid min))
1b
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Furfural
Glycol
180
240
300
300
180
180
180
240
240
300
180
180
180
97.3
93.2
89.5
68.7
93.1
87.8
69.2
93.1
88.7
41.6
99.9
70.1
55.0
12.7
11.9
10.8
9.9
12.1
11.2
10.3
9.4
8.3
7.2
7.8
4.9
2b
1,2-Propanediol
1,4-Butanediol
1,5-Pentadiol
Glycol
1,2-Propanediol
1,5-Pentadiol
Glycol
1,4-Butanediol
1,5-Pentadiol
Glycol
3b
4b
5b
6b
Furfural
Furfural
7b
8b
Phenylacetaldehyde
Phenylacetaldehyde
Phenylacetaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
9b
10b
11c
12d
13e
Glycol
Glycol
4.1
a
Reaction conditions: aldehyde (70 mmol), diol (126 mmol), catalyst (0.12 g), cyclohexane (8 mL), 80 ◦C.
Over SO3H-MIL-101.
Over the fresh Amberlyst-15.
Over the used Amberlyst-15 after the first-run reaction.
Over the used Amberlyst-15 after the second-run reaction.
Yield of acetal was based on aldehyde.
b
c
d
e
f
g
TOF was based on acetal produced per mole of the acid determined by the acid–base titration and the 10 min reaction.
100
80
60
40
20
0
1
2
3
4
5
Run
Fig. 6. Reusability of SO3H-MIL-101 in the acetalization of benzaldehyde with gly-
col. Reaction conditions: 0.12 g of catalyst, 70 mmol of benzaldehyde, 126 mmol of
glycol, and 8 mL of cyclohexane used as the water removal agent at 80 ◦C for 180 min.
of SO3H-MIL-101 is probably due to the highly dispersed SO3H
groups in the framework of MIL-101 [40,41]. By contrast, a mod-
erate conversion (53.1%) is achieved over pure MIL-101 under
the same reaction conditions. The limited catalytic activity of
pure MIL-101 is due to the Lewis acidity of its unsaturated metal
sites [16]. For comparison, the USY zeolite shows low catalytic
activity in acetalization (50.3%).
Furthermore, SO3H-MIL-101 is readily separated from the reac-
tion system by simple filtration, a unique feature of heterogeneous
catalysts that allows the study of the catalyst’s recyclability. Indeed,
SO3H-MIL-101 can be reused in five cycles of benzaldehyde acetal-
ization with glycol without significant loss of catalytic activity
(Fig. 6), demonstrating its excellent reusability in catalytic acetal-
ization. Additionally, the result of the FT-IR characterization for the
used SO3H-MIL-101 catalyst after the fifth-run reaction is almost
identical to those of the fresh one (Fig. 4), confirming the stability
of the prepared SO3H-MIL-101.
Scheme 1. Mechanism of acetalization over SO3H-MIL-101.
activities. Good to excellent yield of acetals ranging from 87.8% to
99.9% were obtained in the reactions under investigation, suggest-
ing the application potential of SO3H-MIL-101. For comparison,
Amberlyst-15, a commercial resin catalyst often used in acid-
catalyzed reactions [46–48], was also evaluated in the acetalization
of benzaldehyde with glycol. The results are also shown in Table 2.
The fresh Amberlyst-15 displays higher activity on a weight basis
compared with SO3H-MIL-101. Evidently, the difference in activ-
ity between the commercial resin and the MOF-based catalyst can
be explained by the number of acid sites: the amount of H+ in
Amberlyst-15 is 4.7 mmol/g, a value higher than that of SO3H-
MIL-101, which is 1.8 mmol/g (Table 1). However, in terms of TOF,
the catalytic activity of Amberlyst-15 is much lower than that of
SO3H-MIL-101. Additionally, the catalytic activity of Amberlyst-
15 decreases substantially from the first- to third-run reactions
(entries 11–13) as a result of its swelling [49].
3.3. Catalytic acetalization of different aldehydes and diols
To investigate the scope and limitation of SO3H-MIL-101 as acid
catalyst for acetalization, different aldehydes and diols as reac-
tants were also tested. The results were summarized in Table 2. In
these acetalization reactions, SO3H-MIL-101 showed high catalytic
3.4. Mechanism for acetalization
Based on the work of Miao and coworker [50], the possible
reaction mechanism is illustrated in Scheme 1. In the first step,

Y. Jin et al. / Journal of Molecular Catalysis A: Chemical 383–384 (2014) 167–171
171
the carbonyl group of aldehyde 1 took a proton from SO3H-MIL-
101. Then, the protonated carbonyl group 2 was activated for the
nucleophilic addition of O atom of the alcohol. After deprotonation,
hemiacetal 4 was formed. Afterwards, the hemiacetal 4 was pro-
tonated again. After the loss of water, oxonium ion 6 was formed.
It accepted a second alcohol hydroxyl group to form 7, and subse-
quent deprotonation gave the acetal 8. The catalyst SO3H-MIL-101
possesses a considerable number of acid sites, which are vital to the
reaction. As a result, high catalytic activity occurred in the acetal-
ization.
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A sulfonic acid-functionalized MIL-101 catalyst was prepared
by the one-pot synthesis strategy. The obtained catalyst shows
some excellent catalytic properties in the acetalization of various
aldehydes and diols, and is superior to the traditional zeolite and
commercial resin catalysts. This heterogenized catalyst combines
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and is therefore an easily recyclable and reusable solid catalyst
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This project was supported by Zhejiang Provincial Natural Sci-
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