Preparation of sulfated alumina supported on mesoporous MCM-41 silica and its application in esterification

Article abstract of DOI:10.1016/j.catcom.2013.02.007

New types of mesoporous SA/MCM-41 solid acid catalysts were prepared by loading sulfated alumina (SA) on MCM-41. The prepared catalysts were characterized by XRD, IR, N₂ physisorption, elemental analysis, FT-IR of adsorbed pyridine and NH₃-TPD. The esterification of acetic acid with n-butanol and citric acid with n-butanol were used as model reactions to test the catalytic activities and reusability of the SA/MCM-41 solid acid catalysts. Compared with SA catalyst, SA/MCM-41 catalysts exhibited higher catalytic performances, which were attributed to their high BET surface area and large pore volume. Moreover, 20SA/MCM-41 solid acid catalyst showed excellent reusability in both esterifications.

Full text of DOI:10.1016/j.catcom.2013.02.007

Catalysis Communications 35 (2013) 27–31
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of sulfated alumina supported on mesoporous MCM-41
silica and its application in esterification
⁎
⁎
Hui Pan, Junxia Wang , Long Chen, Guanghui Su, Jiameng Cui, Dawei Meng , Xiuling Wu
Faculty of Materials Science and Chemistry, Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
New types of mesoporous SA/MCM-41 solid acid catalysts were prepared by loading sulfated alumina (SA) on
MCM-41. The prepared catalysts were characterized by XRD, IR, N2 physisorption, elemental analysis, FT-IR of
adsorbed pyridine and NH3-TPD. The esterification of acetic acid with n-butanol and citric acid with n-butanol
were used as model reactions to test the catalytic activities and reusability of the SA/MCM-41 solid acid catalysts.
Compared with SA catalyst, SA/MCM-41 catalysts exhibited higher catalytic performances, which were attribut-
ed to their high BET surface area and large pore volume. Moreover, 20SA/MCM-41 solid acid catalyst showed
excellent reusability in both esterifications.
Received 6 December 2012
Received in revised form 30 January 2013
Accepted 1 February 2013
Available online 14 February 2013
Keywords:
Mesoporous
Sulfated alumina
MCM-41
© 2013 Elsevier B.V. All rights reserved.
Esterification
Reusability
1. Introduction
of SA loading on the structure, surface properties and catalytic activities
of SA/MCM-41 mesoporous solid acid catalysts were studied in detail.
In recent years, SO42−/MxOy solid acid catalysts have been paid much
attention owing to their strong acidity, high catalytic activity, easy separa-
tion, low waste generation and reduced environmental impact, etc. [1,2].
Among them, sulfated zirconia (SZ) and sulfated titanium oxide (ST) have
been widely investigated because of their good catalytic performances in
many reactions [3–7]. However, conventional SZ and ST have disadvan-
tages of small surface area and wide pore size distribution, which may
limit their potential applications [8,9]. To overcome these problems,
many research groups have prepared SZ and ST mesoporous solid acid
catalysts by supporting SZ and ST on mesoporous materials, such as
MCM-41 [10–12], SBA-15 [9,13] and HMS [14]. Moreover, many reports
have demonstrated that SZ and ST mesoporous solid acid catalysts have
advantages of high specific surface area and narrow pore size distribution,
accordingly, they exhibit higher catalytic performances than conventional
SZ and ST [10–13]. Sulfated alumina (SA) has been considered as an un-
promising solid acid due to its lower activity in comparison with SZ and
ST. However, SA has the advantages of wide availability, heat stability
and low price, which make it an attractive catalyst [15–17]. In addition,
SA overcomes the phase transition which is one reason of deactivation
for SZ during its preparation [15,16]. Nevertheless, little attention has
been paid to the study on SA/MCM-41 mesoporous solid acid catalyst
by supporting sulfated alumina (SA) on MCM-41.
2. Experimental
2.1. Synthesis of catalysts
The support MCM-41 was synthesized according to the procedure
described in the literature [18]. SA/MCM-41 solid acid catalysts were
prepared as follows: 1 g MCM-41 was added to 25 mL aqueous solution
of 0.2 mol/L Al(NO3)3·9H2O, followed by the dropwise addition of 25%
aqueous ammonia with stirring until the final pH of the solution reached
to 6–7. The precipitated product was filtered, washed and dried at
120 °C. Then the precipitate was immersed in 1.25 mol/L (NH4)2S2O8
solution (10 mL/g). Having been filtered and dried, the obtained samples
were calcined at 550 °C for 5 h. The alumina content was taken to be 20%
and 70%, which is based on the total mass of theoretical alumina and
MCM-41, and the prepared samples were designated as 20SA/MCM-41
and 70SA/MCM-41.
2.2. Characterization of catalysts
XRD measurements were performed on a Dmax-3β diffractometer.
IR spectra of the catalysts were recorded by a Nicolet 6700 spectrometer
using KBr pellets. N2 adsorption analysis was carried out using
micromeritics (ASAP-2020) at liquid nitrogen temperature (77 K).
The sulfur content was determined from a VarioElcube elemental analyz-
er. FT-IR of adsorbed pyridine spectra were recorded on a Frontier FT-IR
spectrometer. The NH3 temperature programmed desorption (NH3-TPD)
In this paper, new types of SA/MCM-41 solid acid catalysts were
prepared by supporting sulfated alumina (SA) on MCM-41. The effects
⁎
Corresponding authors. Tel./fax: +86 27 67884596.
E-mail addresses: wjx76@sina.com (J. Wang), dwmeng@cug.edu.cn (D. Meng).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.02.007

28
H. Pan et al. / Catalysis Communications 35 (2013) 27–31
experiments were carried out using a XianQuan TP-5079 catalytic
surfaces analyzer quipped with a TCD detector.
high-angle XRD patterns show that Al2(SO4)3 crystalline phases exist in
the SA and SA/MCM-41 samples, which might be attributed to the
long-time immersion and the dissolution of aluminum hydroxide in
(NH4)2S2O8 solution (Fig. 1). The similar phenomenon was also observed
in the other SO42−/MxOy systems [16,21–23]. However, the Al2(SO4)3
crystalline phase disappears in 20SA/MCM-41 sample after being used
for six times, which may be due to the dispersion of Al2(SO4)3 in the
reaction mixture. No diffraction peaks of crystalline alumina are
observed in all samples, indicating that alumina may be present in an
amorphous state or homogeneously mixed with MCM-41.
2.3. Catalytic activities test
The esterification of acetic acid with n-butanol was carried out in
a three-necked flask equipped with a magnetic stirrer, a thermometer
and a refluxing condenser tube. The reaction conditions were as
follows: the range of reaction temperatures was 115–118 °C, the
molar ratio of n-butanol to acetic acid was 3:1; the reaction time was
3 h; the catalyst amount was 0.38% to 1.55% (percentage content of
the reaction mixture). The residual acid was determined by means of
titration. Namely, 0.50 mL reaction mixture was added in 20.00 mL
absolute alcohol and titrated by 0.10 mol/L NaOH solution using
phenolphthalein as an indicator. Then, the conversion of acid was mea-
sured as follows [19,20]:
Fig. 2 shows the IR spectra of MCM-41, SA, SA/MCM-41 samples and
20SA/MCM-41 sample after being used for six times. Spectrum of the SA
sample exhibits the special bands in the range of 900–1400 cm−1, which
are associated with the acid structures of the catalyst [24–27]. Among
them, the bands at 1121 cm−1 and 1182 cm−1 are assigned to the
stretching vibration of S\O, and the band at 1203 cm−1 is assigned to
the stretching vibration of S_O, indicating that a chelate with the
bidentate structure has been formed between S2O82− ion and alumina
[25]. For the 20SA/MCM-41 sample, there is a broad band in the range
of 1085–1235 cm−1 because the special bands of acid structures easily
overlap with framework bands of MCM-41 in this range. However,
owing to the high SA content, the special bands of acid structures become
obvious for 70SA/MCM-41 sample. A band at 1321 cm−1 assigned to
asymmetric stretching frequency of the covalent S_O is found in the
SA, 20SA/MCM-41 and 70SA/MCM-41 samples. The suction-induced
complex S_O promotes the electron-accepting ability for the metal
atoms, making the samples possess super acidic sites [26,27]. In the IR
spectrum of 20SA/MCM-41 sample after being used for six times,
bands at 1384 cm−1 (CH3, symmetric deformations) and 1463 cm−1
(CH2, deformations) are obtained, which may be due to the absorption
of reaction mixture in 20SA/MCM-41 sample during the reaction.
The nitrogen adsorption–desorption isotherms and pore size distribu-
tion curves of samples are depicted in Fig. 3. The isotherms of MCM-41
support and 20SA/MCM-41 sample show typical type IV features and
have H1-type hysteresis loop, which are characteristic of mesoporous ma-
terials with tubular pores [11,12]. It is found that the SA sample displays a
wider pore size distribution. Accordingly, the SA sample has a small BET
surface area of 88 m2/g and shows a low pore volume of 0.044 cm3/g
(Table 1). By supporting SA on MCM-41, the pore size distribution of
20SA/MCM-41 sample becomes narrower, and the BET surface area
and pore volume increase to 324 m2/g and 0.412 cm3/g, respectively
(Table 1). Therefore, it is clear that a new mesoporous acidic catalyst
system is successfully obtained by supporting SA on MCM-41, which
has advantages of narrow pore size distribution, high BET surface area
and large pore volume. However, the pore size distribution of 70SA/
M0 − M1
The conversion of acidð%Þ
¼
ꢀ
100
M0
where M0 was the dissipative volume of NaOH solution before reaction
and M1 was the dissipative volume of NaOH solution after reaction.
The reaction conditions for esterification of citric acid with n-butanol
were as follows: the range of reaction temperatures was 115–120 °C,
the molar ratio of n-butanol to citric acid was 5.5:1; the reaction time
was 3 h; and the catalyst amount was 1.13% to 2.23%.
In order to test the catalyst lifetime, the samples were used for a
batch reaction process repeatedly. When each catalytic evaluation
experiment finished, the catalyst was filtered and dried at 60 °C for
12 h in air, then reused in the next evaluation.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of MCM-41, SA, SA/MCM-41 samples and 20SA/
MCM-41 sample after being used for six times are shown in Fig. 1.
Three peaks corresponding to the diffraction of (100), (110) and (200)
planes are observed in MCM-41 support, which are characteristic of the
long-range ordered hexagonal mesostructures of MCM-41 [1,4]. Similar
peaks are also observed in 20SA/MCM-41 sample and 20SA/MCM-41
sample after being used for six times, indicating that the ordered
mesostructures are still maintained in the two samples. However, the
three peaks in 70SA/MCM-41 sample are not detected, which may be
due to high content of SA in the channels of MCM-41 support [4]. The
Fig. 1. XRD patterns of MCM-41, SA, SA/MCM-41 samples and 20SA/MCM-41 sample after
Fig. 2. IR spectra of MCM-41, SA, SA/MCM-41 samples and 20SA/MCM-41 sample after
being used for six times.
being used for six times.

H. Pan et al. / Catalysis Communications 35 (2013) 27–31
29
Fig. 3. The nitrogen adsorption–desorption isotherms and pore size distribution curves
of MCM-41, SA and SA/MCM-41 samples.
MCM-41 sample becomes wider again. Accordingly, the BET surface area
and pore volume reduce to 149 m2/g and 0.148 cm3/g, respectively,
because SA partly blocks the mesoporous of MCM-41 support, which is
consistent with the XRD results. As can be observed from Table 1, the
atomic ratio of S:Al in the samples shows a descending tendency: 20SA/
MCM-41>70SA/MCM-41>SA, indicating that the highly exposed alumi-
na in SA/MCM-41 samples adsorb more S2O82− anions than bulk crystal-
line alumina in SA sample. However, in the 20SA/MCM-41 sample after
being used for six times, the S content decreases from 6.9% to 4.3%. It
may cause the decrease of catalytic activities in esterifications [21].
The acidity of SA and SA/MCM-41 samples was measured by FT-IR
using pyridine as a probe molecule (Fig. 4(a)). The peaks at 1450 cm−1
and 1540 cm−1 are observed in the spectra of samples, indicating that
both Lewis and Brönsted acid sites exist in all three samples [10,12,28].
The NH3-TPD measurement was performed to evaluate the strength
and the amount of acidic sites on the samples (Fig. 4(b)). It is found that
all the samples show broad desorption peaks in the range of 100 °C to
650 °C, suggesting a broad heterogeneity of acidic sites [24]. The peaks
below 400 °C represent the acidic sites of weak and medium strength,
and the peaks above 550 °C correspond to strong acidic sites [29]. It is
observed that the number of acidic sites based on per gram of the
samples can be placed in the following ascending order: 20SA/
MCM-41bSAb70SA/MCM-41.
Fig. 4. The pyridine adsorption FT-IR spectra and NH3-TPD spectra of SA and SA/
MCM-41 samples.
adding catalysts, and the conversion of acids increases with increasing
catalyst amounts to a value and then remains almost unchanged. The
70SA/MCM-41 sample exhibits the maximum conversion of acetic
acid with 99.3% when the catalyst amounts are 1.55% (Fig. 5(a)). The
result is in accordance with the number of acidic sites tested by
NH3-TPD. As can be seen in Fig. 5(a), the 20SA/MCM-41 shows higher
conversion of acetic acid compared to SA, which is not proportional to
the total number of acidic sites. The reason is maybe that the esterifica-
tion of acetic acid with n-butanol is catalyzed by the weak and medium
strength acidic sites which are roughly the same in the two samples
[30]. Moreover, the high BET surface area and large pore volume of
20SA/MCM-41 are beneficial to enhancing its catalytic activities. It is
interesting to note that 20SA/MCM-41 sample gives the maximum con-
version of citric acid with 98.3% which is higher than 70SA/MCM-41
sample (Fig. 5(b)). Although the number of acidic sites present on
70SA/MCM-41 sample is more than 20SA/MCM-41 sample, but the
high amount of alumina in 70SA/MCM-41 sample results in blockage
of mesoporous and aggregation of alumina on its surface, thus impeding
the access of large molecules like citric acid to the acidic sites. However,
the acidic sites are accessible to small molecules like ammonia.
3.2. Catalytic activities
Fig. 5(a) and (b) shows the catalytic activities of SA and SA/
MCM-41 samples in esterifications with different catalyst amounts.
It is found that the conversion of acids is very low in the absence of
catalyst. However, the reactions are significantly accelerated by
Table 1
Physico-chemical properties of MCM-41, SA, SA/MCM-41 samples and 20SA/MCM-41
sample after being used for six times.
Samples
BET surface Pore
Pore
Sulfur
S:Al
area (m2/g) volume diameter content (atomic ratio)
(cm3/g) (nm)
(wt.%)
MCM-41
681
324
–
0.769
0.412
–
2.8
2.7
–
–
–
3.3. Reusability
20SA/MCM-41
20SA/MCM-41 (used)
70SA/MCM-41
SA
6.9
4.3
9.4
9.8
1:1.61
–
The main advantage of SO42−/MxOy solid acid catalysts is reusability.
However, many SO42−/MxOy solid acid catalysts commonly suffer from
rapid deactivation owing to the loss of sulfur species as well as carbon
149
88
0.148
0.044
–
1:3.87
1:5.39
–
The atomic ratio of S:Al was calculated from elemental analysis.

30
H. Pan et al. / Catalysis Communications 35 (2013) 27–31
Fig. 5. Catalytic activities and reusability of SA and SA/MCM-41 samples in esterifications: (a) esterification of acetic acid with n-butanol with different catalyst amounts, (b) esterification
of citric acid with n-butanol with different catalyst amounts, (c) reusability in esterifications of acetic acid with n-butanol, and (d) reusability in esterifications of citric acid with n-butanol.
deposition [21,31]. So, it is of great importance to evaluate the catalyst
lifetime. The reusability of SA and SA/MCM-41 samples is summarized
in Fig. 5(c) and (d). Both SA/MCM-41 samples exhibit higher reusability
than SA sample. Among them, 20SA/MCM-41 sample shows the best
reusability, it remains above 91.2% conversion of acetic acid and 88.1%
conversion of citric acid even after being used for six times, respectively.
The results are comparable with SZ and Yb2O3–Al2O3 promoted SZ
catalysts [31,32]. Compared with the 20SA/MCM-41 sample, the reus-
ability of 70SA/MCM-41 decreases, which may be owing to its high SA
content. From the characterization results of XRD, IR and elemental
analysis, it is deduced that the decrease of S content and the blockage
of acidic sites with adsorbing reaction mixture may be the main reasons
for the decrease of catalytic activities in the esterification [18].
Acknowledgments
The authors gratefully acknowledge the financial support from
NSFC (nos. 21203170 and 41172051), SFBSRCC (no. CUGL090227),
ERCNGME (no. CUGNGM 20133) and Self-Determined and Innovative
Research Funds of CUG (no. 2013-09).
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A new mesoporous acidic catalyst system has been successfully
obtained by supporting SA on mesostructured MCM-41, having advan-
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