A novel solid acid catalyst synthesized from ZnAl2O4 spinel and its application in the esterification of acetic acid and n-butyl alcohol

Article abstract of DOI:10.1039/c4ra15941a

A new spinel-style S₂O₈^2-/ZnAl₂O₄ solid acid catalyst was prepared by sol-gel method. The catalytic performance was evaluated by the esterification of n-butyl acetate. The structure, morphology and acidity of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM), N₂ physisorption, X-ray photoelectron spectroscopy (XPS) and NH₃ temperature programmed desorption (NH₃-TPD). The results showed that the catalytic performances were significantly affected by the calcination temperature. S₂O₈^2-/ZnAl₂O₄-600 exhibited the highest catalytic activity of 91.7% under the optimum reaction conditions. The characterization showed that the mesoporous structure S₂O₈^2-/ZnAl₂O₄-600 maintained the well crystallinity of spinel as well as the spinel-style carrier in the sulfidation and recycling. This journal is

Full text of DOI:10.1039/c4ra15941a

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A novel solid acid catalyst synthesized from
ZnAl2O4 spinel and its application in the
Cite this: RSC Adv., 2015, 5, 19652
esterification of acetic acid and n-butyl alcohol
*
An-Qi Wang, Xiu-Ling Wu, Jun-Xia Wang, Hui Pan, Xiao-Yun Tian and Yu-Lin Xing
A new spinel-style S2O82ꢀ/ZnAl2O4 solid acid catalyst was prepared by sol–gel method. The catalytic
performance was evaluated by the esterification of n-butyl acetate. The structure, morphology and
acidity of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared
spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM), N2 physisorption, X-ray
photoelectron spectroscopy (XPS) and NH3 temperature programmed desorption (NH3-TPD). The results
showed that the catalytic performances were significantly affected by the calcination temperature.
S2O82ꢀ/ZnAl2O4-600 exhibited the highest catalytic activity of 91.7% under the optimum reaction
conditions. The characterization showed that the mesoporous structure S2O82ꢀ/ZnAl2O4-600
maintained the well crystallinity of spinel as well as the spinel-style carrier in the sulfidation and recycling.
Received 8th December 2014
Accepted 12th February 2015
DOI: 10.1039/c4ra15941a
www.rsc.org/advances
stability.12–14 Therefore, the ZnAl2O4 spinel may be a promising
carrier for solid acid.
1. Introduction
In this work, a series of S2O82ꢀ/ZnAl2O4-T were prepared and
The esterication of carboxylic acids with alcohols is an
evaluated by the esterication of n-butyl acetate. The inuences
of synthesis conditions on the catalytic activities were studied.
In addition, the reaction conditions and catalyst recyclability
were also performed in detail. Furthermore, the structure,
morphology and acidity of the catalyst were characterized by
XRD, FE-SEM, N2 physisorption, FT-IR, NH3-TPD and XPS
techniques. Some valuable evaluations for the S2O82ꢀ/ZnAl2O4
in this paper have some referential value in the design of a novel
ZnAl2O4 carrier for SO42ꢀ/MxOy solid acid synthesis.
essential reaction in organic synthesis, pharmaceuticals, food
2ꢀ
preservatives, biofuels, etc. Two representative catalysts, SO4
/
ZrO2 and SO42ꢀ/TiO2, are widely used in these esterications,
due to their strong acidity, non-toxicity and high catalytic
activity.1,2 However, they have a similar problem in the crystal
structure transformation in the process of synthesis and
modication.3,4 It is well known that the crystal structure of
metal oxide has a great inuence on the catalytic perfor-
mances.5 Therefore, it is desirable for researchers to look for a
stable carrier to synthesize solid acid.
Spinel-type oxide has drawn great attention for catalytic
reactions because of its desirable properties, such as single
2. Experimental
2.1 Catalysts preparation
crystal shape, high thermal stability and well diffusion, etc.6–9
A
S2O82ꢀ/ZnAl2O4 solid acid catalysts were prepared by the
following steps: 0.04 mol Al(NO3)3$9H2O and 0.02 mol
Zn(NO3)2$6H2O were dissolved in 20 mL ethyl alcohol, followed
by adding 5 wt% (the total weight of nitrates) polyethylene
glycol (PEG-2000) into the ethyl alcohol solution with magnetic
stirring for 4 h at room temperature. Aerward, the obtained
mixture was evaporated for 1 h at 60 ꢁC to obtain the sol. The sol
was dried and then grounded into a ne powder. The ne
nanosized sulfated ZnFe2O4 catalyst with highly ordered mes-
oporous was found to be highly active towards commercially
important ne chemicals.10 The high specic surface area
SO42ꢀ/CoFe2O4 solid acid with the spinel as carrier displayed
the excellent activity in the synthesis of ethyl acetate.11 Never-
2ꢀ
theless, the literature on the catalytic applications of S2O8
/
ZnAl2O4 catalyst is scanty. In our previous studies, the
composite catalysts of Al-based catalysts have been developed
and studied in terms of the catalytic activity and reusability,
which performed the high catalytic activity and excellent
ꢁ
powder was calcined at 400 C, 500 ꢁC, 550 ꢁC, 600 ꢁC and 650
ꢁC for 5 h in air to obtain the precursors. The obtained
precursors with different calcination temperature were desig-
nated as ZnAl2O4-T. The ZnAl2O4-T were then sulfated for 12 h
by impregnating with 0.75 mol Lꢀ1, 1.00 mol Lꢀ1, 1.25 mol Lꢀ1
,
Faculty of Materials Science and Chemistry, Engineering Research Center of
Nano-Geomaterials of Ministry of Education, China University of Geosciences,
Wuhan 430074, China. E-mail: jxwyqh@sina.com; Fax: +86-27-87407079; Tel: +86-
27-87407079
1.50 mol Lꢀ1, 1.75 mol Lꢀ1 and 2.00 mol Lꢀ1 (NH4)2S2O8 solu-
tion. The suldation was done by impregnating 1 g of ZnAl2O4
in 10 mL of (NH4)2S2O8 solution for 12 hours. Aer having been
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ꢁ
ltered and dried, the solids were calcined at 550 C for 5 h in
air to obtain the S2O82ꢀ/ZnAl2O4-T catalysts. The S2O82ꢀ/ZnO-
600 and S2O82ꢀ/Al2O3-600 catalysts were synthesized by the
same steps.
2.2 Catalysts characterization
The X-ray diffraction (XRD) was conducted on a Bruker AXS D8-
Focus X diffractometer, using CuKa radiation at 40 kV and 40
mA; The Fourier transform infrared spectroscopy (FT-IR)
spectra of the catalysts were recorded in the range of 400–
4000 cmꢀ1 by a Nicolet 6700 IR spectrometer using the KBr
pellet technique; the morphology was examined by using Field
emission scanning electron microscopy (FE-SEM) on a SU 8010
electron microscope; the X-ray photoelectron spectroscopy
(XPS) was performed on a VG Multilab 2000; the NH3 temper-
ature programmed desorption (NH3-TPD) experiment was
carried out using a Micromeritics AutoChem II 2920 quipped
with a TCD detector; N2 adsorption analysis was carried out
using micromeritics (ASAP-2020) at liquid nitrogen temperature
(77 K). The surface area was calculated by the BET method and
the pore size distribution was obtained from the adsorption
isotherm by the BJH method.
2.3 Catalytic activity test
The esterication efficiency of n-butyl acetate was evaluated in a
250 mL three-necked ask equipped with a magnetic stirrer, a
thermometer and a reuxing condenser. The mixture was stir-
red at about 118 ꢁC under atmospheric pressure. Various
parameters of the reaction conditions, including the molar ratio
of n-butanol to acetic acid, catalyst amount (percentage content
of the reaction mixture) and reaction time, were varied to the
optimum. The initial acid and residual acid were detected by
means of titration with 0.10 mol Lꢀ1 NaOH solution. The
esterication efficiency of acetic acid can be calculated using
the following equation (by the method of GB1668-81):
Fig. 1 XRD patterns of ZnAl2O4-T and S2O82ꢀ/ZnAl2O4-T: (a) different
calcination temperature of carriers with 0.75 mol Lꢀ1 (NH4)2S2O8
impregnating solution; (b) different concentration of (NH4)2S2O8
solution.
M0 ꢀ M1
Esterification efficiency of acetic acid ð%Þ ¼
ꢂ 100
M0
where M0 is the acid value before reaction and M1 is the acid
value aer reaction.
excellent advantage of structure stability in suldation.
2ꢀ
Compared with the carriers, the crystallite size of S2O8
/
ZnAl2O4-T is larger than ZnAl2O4-T as shown in Table 1. This
result may be that the high calcination temperature in the
process of suldation causes crystalline grain further to grow.
3. Results and discussion
3.1 Characterization of catalysts
2ꢀ
Fig. 1(a) shows the XRD patterns of ZnAl2O4-T and S2O8
/
ZnAl2O4-T. It reveals that the calcination temperatures cause
apparently changes in the crystalline structure of the carriers.
The ZnAl2O4-400, ZnAl2O4-500 and ZnAl2O4-550 are mainly
consisted of amorphous phase. When the calcinations
temperature further increases, the amorphous gradually trans-
forms into spinel. The characteristic peaks at 2q ¼ 18.9ꢁ, 31.4ꢁ,
36.90ꢁ, 44.2ꢁ, 49.2ꢁ, 55.8ꢁ, 59.5ꢁ and 65.4ꢁ are observed in
ZnAl2O4-600 and ZnAl2O4-650, which are corresponding to the
Table 1 Crystallite sizes of ZnAl2O4-T and S2O82ꢀ/ZnAl2O4-T
Crystallite sizea (nm)
400
500
550
600
650
ZnAl2O4-T
—
5.83
—
8.69
7.23
19.28
17.99
20.72
25.82
27.62
S2O82ꢀ/ZnAl2O4-Tb
a
From Debye–Scherrer equation using the width (at half maximum) of
crystal planes of the spinel structure (JCPDS File no. 05-0669).
b
[3 1 1] line. The synthesis condition: calcination temperaturꢀe1 was
2ꢀ
Similar peaks are observed in S2O82ꢀ/ZnAl2O4-600 and S2O8
/
600 ꢁC; the concentration of (NH4)2S2O8 solution was 1.50 mol L
.
ZnAl2O4-650, indicating that the spinel carriers have the
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Meanwhile, it can be found that the crystallite size gradually
increases with increasing temperature. Besides, ZnSO4$H2O
and Al2(SO4)3 are observed in sulfated catalyst, which may be
due to the long-time impregnation and the interaction between
2ꢀ
excess S2O8 and the metal ions.15,16 Correlating the crystalli-
zation with the following catalytic activities, the result shows
that the spinel is an available carrier for solid acid.
Fig. 1(b) clearly shows the XRD patterns of S2O82ꢀ/ZnAl2O4-
600 with different impregnation concentration. It is noteworthy
that all the catalysts appear the strong diffraction peaks of
spinel (JCPDS File no. 05-0669). The well crystallization indi-
cates that the spinel carrier perform the advantages of single
crystal shape and stable structure with different impregnation
2ꢀ
concentration. Besides, the XRD patterns of used S2O8
/
Fig. 3 N2 adsorption–desorption isotherms and pore-size distribu-
tions of S2O82ꢀ/ZnAl2O4-600 (impregnated with 1.50 mol Lꢀ1
(NH4)2S2O8 solution).
ZnAl2O4-600 in Fig. 1(b) show no change in the crystallinity by
comparing to the fresh catalyst, indicating the stable structure
in recycling test.
FE-SEM images in Fig. 2 display the S2O82ꢀ/ZnAl2O4-T with
specic surface area, mean pore diameter and pore volumes are
investigated by N2 adsorption analysis in Fig. 3. The N2
adsorption/desorption isotherms correspond to the IV isotherm
with a hysteresis loop in the low relative (P/P0) range of 0.4–1,
which are typical for mesoporous materials. The surface area
and average pore diameter are 18.776 m2 gꢀ1 and 8.864 nm,
respectively. Similar porous structures can be observed in high
different calcination temperatures. The irregular agglomera-
2ꢀ
tions are apparently appeared on the surfaces of S2O8
/
ZnAl2O4-400, S2O82ꢀ/ZnAl2O4-500 and S2O82ꢀ/ZnAl2O4-650,
which may be one reason of their low catalytic activities. The
S2O82ꢀ/ZnAl2O4-550 and S2O82ꢀ/ZnAl2O4-600 show the
morphology with at blocks. By comparison, S2O82ꢀ/ZnAl2O4-
600 is composed of porous structures, which is assembled by
numerous nanoparticles. Similarly, the fresh S2O82ꢀ/ZnAl2O4-
600 (Fig. 2(f)), impregnated with 1.50 mol Lꢀ1 (NH4)2S2O8
solution, performs the porous structures. Correspondingly, the
2ꢀ
2ꢀ
active SO4 /TiO2 catalysts.5 These porous structures of S2O8
/
ZnAl2O4-600 may be benecial to its high catalytic activity.
Besides, the used S2O82ꢀ/ZnAl2O4-600 catalyst in Fig. 2(g)
maintains the agglomerate nanoparticles as the fresh catalyst.
Fig. 4 records the FT-IR spectra of the ZnAl2O4-600 carrier,
S2O82ꢀ/ZnAl2O4-600 solid acid catalyst and recycling catalyst. It
can discern that three bands (around 672, 556 and 493 cmꢀ1
)
appeared in all the samples are consistent with characteristics
of Al–O stretching vibrations, Zn–O stretching vibrations and
Al–O bending vibrations in the crystal structure of ZnAl2O4
spinel, respectively.17 The result conrms that the spinel
Fig. 2 FE-SEM images of S2O82ꢀ/ZnAl2O4-T: (a) S2O82ꢀ/ZnAl2O4-
2ꢀ
400; (b) S2O82ꢀ/ZnAl2O4-500; (c) S2O82ꢀ/ZnAl2O4-550; (d) S2O8
/
ZnAl2O4-600; (e) S2O82ꢀ/ZnAl2O4-650; (f) S2O82ꢀ/ZnAl2O4-600 (g) Fig. 4 FT-IR spectra of ZnAl2O4-600 carrier, fresh and used S2O8
/
2ꢀ
S2O82ꢀ/ZnAl2O4-600 for three recycling times. ((a–e): 0.75 mol Lꢀ1 ZnAl2O4-600 catalysts (impregnated with 1.50 mol Lꢀ1 (NH4)2S2O8
(NH4)2S2O8 solution; (f) and (g): 1.50 mol Lꢀ1 (NH4)2S2O8 solution.).
solution).
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structure is formed in all the prepared samples, which is in acidic site of weak and medium strength, respectively.20 The
agreement with the XRD analysis. Compared with ZnAl2O4-600 peak above 500 ꢁC assigns to strong acidic site.21 The area of the
carrier, there are distinguishing absorption peaks between 900 NH3 desorption curve shown in Fig. 5(b) is in proportion to the
and 1400 cmꢀ1 in sulfated catalyst and recycling catalyst. The content of the corresponding acid sites. It is clearly found that
additional absorption peak characterizes the sulfate groups on the number of acid sits decreases in the used catalysts, which
the surface which are related to catalytic activity. The three may be the one of reasons for its deactivation in recycling test.
bands at 982, 1103 and 1145 cmꢀ1 are assigned to the stretching
The near-surface chemical valence states are investigated by
vibration of S–O. The bands at 1220 and 1398 cmꢀ1 are assigned XPS within a range of binding energies of 0–1100 eV. Fig. 6
to the stretching vibration of S]O.18 The existences of S]O and shows the whole XPS spectra of S2O82ꢀ/ZnAl2O4-600, revealing
S–O bonds contribute to the coordination of the inorganic the strong peaks of Zn, O, C, S, and Al elements.6 Among them,
chelating bidentate sulfate ion with surface metal cation.19 The the observed C element is due to carbon tape from XPS
recycling catalyst also displays the similar peaks belonged to instrument itself. The binding energy of Al 2p at 74.9 eV
suggests the presence of Al3+ cation. XPS spectrum of the Zn
2ꢀ
spinel structure and sulfate groups, indicating that the S2O8
/
ZnAl2O4-600 catalyst has the advantage of structure stability in shows two outstanding peaks are located at 1021.8 eV and
recycling test. This result is in agreement with the XRD analysis. 1045.2 eV, corresponding to Zn 2p3/2 and Zn 2p, respectively.22
In addition, the band around 1627 cmꢀ1 is assigned to the In addition, the S 2p3/2 peak at 168.95 eV is ascribed to the
bending modes of the –OH group.
sulfur oxidation state of +6 which plays an essential role on the
As shown in Fig. 5(a), the acid strength distributions of formation of the acidity structure.23,24 The suction-induced
S2O82ꢀ/ZnAl2O4-600 and reused catalysts are characterized by complex S]O improves the electron-accepting ability of the
the NH3-TPD. The higher of desorption peak temperature metal atoms, contributing to the formation of supper acid. This
2ꢀ
represents the stronger of the acid strength. The fresh S2O8
/
result is consistent with the special bands in the range of 900–
ZnAl2O4-600 catalyst shows prominent broad peaks in the range 1400 cmꢀ1 in IR analysis. The surface sulphur content is 5.97%
of 100 ꢁC to 700 ꢁC, indicating a wide distribution of acidic sites. (atomic ratio), which is the quantitative estimated by means of
The two peaks around 190 ꢁC and 350 ꢁC correspond to the XPS.
3.2 Catalytic performance
Fig. 7 displays the effects of the calcination temperature and
impregnating concentration of the carrier on catalytic activities.
These results show that the calcination temperature has a
signicant inuence on catalytic activity. It can be observed that
the S2O82ꢀ/ZnAl2O4-600 performs the highest catalytic activity
with the maximum 80.0% esterication efficiency. Moreover,
the results of XRD and FE-SEM show that the S2O82ꢀ/ZnAl2O4-
600 possesses mesoporous structure and well crystallinity of
spinel. These properties may contribute to the high catalytic
activity.
The inuence of impregnating concentration indicates
that the catalytic activities are enhanced with increasing the
2ꢀ
Fig.
5
NH3–TPD curves and areas for fresh and used S2O8
/
Fig. 6 XPS spectra of the S2O82ꢀ/ZnAl2O4-600 (impregnated with
ZnAl2O4-600 (impregnated with 1.50 mol Lꢀ1 (NH4)2S2O8 solution).
1.50 mol Lꢀ1 (NH4)2S2O8 solution).
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Table 2 The effect of reaction conditions on catalytic activities of
S2O82ꢀ/ZnAl2O4-600
Esterication
Catalyst
Amountb Time (h) a : nc ratio efficiency (%)
S2O82ꢀ/ZnAl2O4-600a 1.55
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
1.5
2.0
2.5
3.5
4.0
3.0
1 : 1
1 : 2
1 : 3
1 : 4
1 : 5
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
56.1
79.5
91.7
91.6
93.0
38.0
76.0
79.1
85.4
91.4
91.7
77.9
81.3
86.9
91.0
91.4
45.2
1.55
1.55
1.55
1.55
0.00
0.37
0.73
1.14
1.85
2.21
1.55
1.55
1.55
1.55
1.55
ZnAl2O4-600
1.55
S2O82ꢀ/ZnAl2O4-600a catalyst recycling experiments
Run 1
Run 2
Run 3
Run 4
Run 5
1.55
1.55
1.55
1.55
1.55
3.0
3.0
3.0
3.0
3.0
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
91.7
82.6
72.3
65.7
58.9
a
The synthesis condition: calcination temperature was 600 ꢁC; the
concentration of (NH4)2S2O8 solution was 1.50 mol Lꢀ1
was calculated on the basis of total weight of the reactants. The
.
The amount
b
c
Fig. 7 The effects of the calcination temperature and impregnating
concentration of the carrier on catalytic activities of S2O82ꢀ/ZnAl2O4
(the reaction condition: the molar ratio of acetic acid to n-butanol was
1 : 3; catalyst amount was 1.55 wt%; reaction time was 3 h).
molar ratio of acetic acid to n-butanol.
esterication efficiency is 38% when the reaction is carried
out without catalyst. The esterication efficiency shows an
apparently upward trend with the catalyst amount increasing
from 0 to 1.55 wt%. Aerward, the reaction experiences
stable esterication efficiency about 91% when the catalyst
amount exceeds 1.55 wt%. The reason may be that the excess
acid amount might promote the reverse reaction at the mean
time.27 Thus, 1.55 wt% is sufficient for the esterication
reaction.
impregnation concentration, reaching a maximum value of
91.7% at 1.5 mol Lꢀ1. However, the catalytic activities
descend sharply when the concentration further increases.
The reason may be that excess sulfate groups covering the
surface make the acidity of solid acid decrease.25 Thus an
appropriate concentration of 1.5 mol Lꢀ1 is favorable for high
catalytic activity.
Table 2 shows the effects of reaction conditions on cata-
lytic activities, including the molar ratio, catalyst amount
and reaction time. The molar ratio of acetic acid to n-butanol
has a signicant inuence on the esterication efficiency.
Stoichiometrically, the molar ratio of acetic acid to n-butanol
is 1 : 1. Nevertheless, the excess n-butanol is used to promote
the equilibrium to the direction of producing the ester.
Accordingly, it is clearly seen that the esterication efficiency
of S2O82ꢀ/ZnAl2O4-600 increases with increasing the molar
ratio of acetic acid to n-butanol from 1 : 1 to 1 : 3. However, a
mass of n-butanol may dilute the concentration of acidity and
slow down the reaction.26 Therefore, it witnesses a slight
downward trend in the esterication efficiency when the
molar ratio exceeds 1 : 3. Therefore, the molar ratio of 1 : 3 is
optimum to achieve a high yield.
The effect of reaction time on the esterication efficiency is
shown in Table 2. On the initial stage of 3 h, the esterication
efficiency grows rapidly with prolongation of the reaction time.
Within 3 h, 91.7% esterication efficiency is achieved. Then, the
esterication efficiency is noticed to be stable at around 91%
because of the attainment of equilibrium. The optimum reac-
tion time is considered to be 3 h, with the maximum esteri-
cation efficiency of 91.7%.
The catalytic activity of ZnAl2O4-600 is performed as the
blank experiment. By comparison, it is found that the ZnAl2O4-
600 shows lower catalytic activity with 45.16% efficiency.
However, the S2O82ꢀ/ZnAl2O4-600 catalyst exhibits high catalytic
activity with above 90% efficiency.
For the recycling test, the catalyst was ltered aer
completion of reaction and dried at room temperature
without further treatment. Then, the dried catalyst is used for
the next recycling experiment with fresh reaction. As shown
The results of catalytic activities with the various catalyst
amounts from 0 to 2.21% are demonstrated in Table 2. The
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Table 3 Comparison of catalytic activity of the S2O82ꢀ/ZnAl2O4-600 catalyst with other reported solid acid catalysts
Reaction conditions
Catalyst
a : na ratio
Amountb (wt%)
Reaction time (h)
Esterication efficiency (%)
Ref.
S2O82ꢀ/ZnAl2O4-600
S2O82ꢀ/ZnO-600
S2O82ꢀ/Al2O3-600
S2O82ꢀ/ZrO2
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 1.2
1 : 3
1.55
1.55
1.55
12
12
12.09
2
3
3
3
2
91.7
52.9
79.3
91.8
96.6
92.2
60.1
97.1
85
Present work
Present work
Present work
28
28
29
30
30
31
32
32
S2O82ꢀ/ZrO2–CeO2
Sulfated TiO2
2
2.5
0.75
0.75
0.5
2
SO42ꢀ–TiO2
SO42ꢀ–Ce0.02/TiO2
SO42ꢀ/TiO2–Zr–La
Ti(SO4)2
1 : 3
2
1 : 1.56
1 : 0.69
1 : 0.69
4.88
5.15
5.15
90.22
87.62
S-TiO2/MCM-41
2
a
The molar ratio of acetic acid to n-butanol. b Catalyst amount is calculated on the basis of total weight of the reactants.
in Table 2, the catalyst shows an inevitable decrease in
catalytic activity aer 5 times recycling. The deactivation
Acknowledgements
presumably is owing to the adsorption of organic groups on
the surface as well as the reduction of the catalyst amount.
Besides, the XRD and FT-IR analyses of the reused catalyst
demonstrate that S2O82ꢀ/ZnAl2O4-600 has the advantage of
structure stability in recycling test.
The authors gratefully acknowledge the nancial support from
NSFC (nos 21203170 and 41472042) and the National College
Students' Innovative Training Program (nos 201410491024
and 201310491019), the Fundamental Research Founds for
National University (no. 1410491B04) and TLOFCUG (no. SKJ
2013008).
Table 3 shows the comparison of S2O82ꢀ/ZnAl2O4-600 with
S2O82ꢀ/ZnO-600, S2O82ꢀ/Al2O3-600 and other reported solid acid
catalysts. The S2O82ꢀ/ZnAl2O4-600 catalyst perform the higher
catalytic activity than the S2O82ꢀ/ZnO-600 and S2O82ꢀ/Al2O3-600
catalyst with simple oxide carrier. The catalytic activity of
S2O82ꢀ/ZnAl2O4-600 is comparable with the series of SO42ꢀ/TiO2
and SO42ꢀ/ZrO2 catalysts in esterication reactions.28–32
However, crystal structure transformation was commonly
observed in the synthesis and modication of SO42ꢀ/TiO2 and
Notes and references
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2ꢀ
SO4 /ZrO2, which affected their catalytic performances.3,4
Notably, ZnAl2O4 as a new carrier of solid acid exhibits the
advantages of the simpler crystal and excellent crystal structure
stability in present work.
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A
series of S2O82ꢀ/ZnAl2O4-T with spinel structure was
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