The Co-promotion effect of Mo and Nd on the activity and stability of sulfated zirconia-based solid acids in esterification

Article abstract of DOI:10.1016/j.apcata.2010.08.062

SO₄^2-/ZrO₂-MoO₃ (SZM), SO ₄^2-/ZrO₂-Nd₂O₃ (SZN) and SO₄^2-/ZrO₂-MoO₃-Nd₂O ₃ (SZMN) solid acids catalysts were prepared and characterized by XRD, NH₃-FTIR, NH₃-TPD and TG-DTG. The activities and stabilities of the catalysts for the esterification of fatty acids were investigated. Experimental and characterization results show that the excellent activity and stability of SZMN are attributed to the firm combination of sulfur species with t-ZrO₂. The co-addition of Mo and Nd increases the dispersion of t-ZrO₂ and stabilizes the structure of small-crystallite t-ZrO₂, which is favorable to increasing the acid sites amount, strengthening the acidity, and then enhancing the activity and stability of the catalyst.

Full text of DOI:10.1016/j.apcata.2010.08.062

Applied Catalysis A: General 389 (2010) 46–51
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
The Co-promotion effect of Mo and Nd on the activity and stability of sulfated
zirconia-based solid acids in esterification
Kanghua Jiang, Dongmei Tong, Jinqiang Tang, Ruili Song, Changwei Hu∗
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu,
Sichuan 610064, PR China
a r t i c l e i n f o
a b s t r a c t
SO42−/ZrO2–MoO3 (SZM), SO42−/ZrO2–Nd2O3 (SZN) and SO42−/ZrO2–MoO3–Nd2O3 (SZMN) solid acids
catalysts were prepared and characterized by XRD, NH3-FTIR, NH3-TPD and TG–DTG. The activities
and stabilities of the catalysts for the esterification of fatty acids were investigated. Experimental and
characterization results show that the excellent activity and stability of SZMN are attributed to the
firm combination of sulfur species with t-ZrO2. The co-addition of Mo and Nd increases the disper-
sion of t-ZrO2 and stabilizes the structure of small-crystallite t-ZrO2, which is favorable to increasing
the acid sites amount, strengthening the acidity, and then enhancing the activity and stability of the
catalyst.
Article history:
Received 19 June 2010
Received in revised form 28 August 2010
Accepted 31 August 2010
Available online 21 September 2010
Keywords:
Sulfated zirconia-based solid acids
Fatty acids
© 2010 Elsevier B.V. All rights reserved.
Co-promotion
Esterification
1. Introduction
is usually considered as an effective method. Considerable litera-
ture reported that mixed oxides often showed enhanced acidity and
thermal stability than their constituent single oxide [13]. Huang
et al. reported higher catalytic activity of SO42−/MoO3–ZrO2 com-
pared with SO42−/ZrO2 for the esterification reaction of n-butanol
with acetic acid, and the optimal MoO3 loading was 10 wt.% [14].
The addition of Mo into ZrO2 appeared to retard the transforma-
tion of metastable tetragonal ZrO2 phase, which exhibited higher
activity for the esterification, into more stable monoclinic phase
with low activity. SO42−/ZrO2–TiO2/La3+ catalyst also performed
much better than SO42−/ZrO2–TiO2 in terms of activity and sta-
bility for the esterification of free fatty acids [15]. A rare earth
modified solid super acid catalyst SO42−/ZrO2–Nd2O3 prepared
by precipitation-co-impregnation method displayed an excellent
activity in esterification [16]. Yu et al. reported that doping both
Yb2O3 and Al2O3 on SO42−/ZrO2 not only boosted the esterification
activity but also alleviated catalyst deactivation resulted from the
surface sulfur loss by leaching [17]. From the literature mentioned
above, it could be seen that the addition of rare earth element and
other subgroup element could enhance the activity and stability
of sulfated zirconia-based solid acid. However, the dependence of
the acidity and activity of the catalyst on its texture property needs
more research.
Biodiesel, an environmentally beneficial and nontoxic fuel with
relatively low emission profiles [1], is usually synthesized by trans-
esterification of vegetable oils and animal fats with alcohol [2], or
esterification of free fatty acids (FFAs) with alcohol via acid or base
catalysis [3]. Especially for raw materials with high acid value, the
esterification of free fatty acids is an essential step in biodiesel pro-
duction [4]. It is well known that esterification is mainly catalyzed
by homogeneous acid, mineral acid, and organic sulfonic acid [5].
However, these acid catalysts have some disadvantages, such as
pollution, separation and recycling difficulties [6]. Therefore, the
development of environmentally benign acid catalysts with high
activity is expected.
Great efforts have been devoted to search for solid acid cata-
lysts because they can reduce production costs of biodiesel through
easier operations and elimination of wastes [7]. Sulfated zirconia
solid acid (SO42−/ZrO2) is a familiar catalyst widely used in ester-
ification and other reactions such as dehydration, isomerization,
alkylation, and so on, due to their strong acidity and high cat-
alytic activity [8–11]. However, SO42−/ZrO2 catalyst exhibits no
satisfactory stability due to the leaching of surface sulfur species
[12]. To overcome the limitation, the modification of SO42−/ZrO2
In the present work, we reported the preparation and charac-
terization of sulfated zirconia-based solid acids promoted by Mo
and Nd, and their application in the esterification of fatty acids. The
effect of co-addition of Mo and Nd on the activity, structure and
acidity of the catalysts was investigated as well.
∗
Corresponding author. Tel.: +86 28 85411105; fax: +86 28 85411105.
E-mail addresses: chwehu@mail.sc.cninfo.net, gchem@scu.edu.cn (C. Hu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.062

K. Jiang et al. / Applied Catalysis A: General 389 (2010) 46–51
47
2. Experimental
Thermo gravimetry (TG) was carried out under pure N2 atmo-
sphere (100 mL/min) with SDT Q600 thermo-analyzer (TA Co.). The
catalyst powder was heated at a rate of 20 ◦C/min from 50 to 900 ◦C.
The weight loss of the sample was recorded.
2.1. Materials
Methyl ester fatty acids (methyl oleate, methyl palmate, methyl
hexanoate, etc.) used for calibration were obtained from Aldrich
(99% purity). Other reagents and chemicals obtained from Kelong
Chemical Company (Chengdu, China) were all of analytical grade
and used without further purification.
2.4. Activity tests and analytical procedures
The activities of the catalysts for the esterification of fatty acids
with methanol were determined at 65 ◦C in a 50 mL two-neck flask
equipped with a reflex condenser under continuous stirring. Vari-
ous parameters, such as the molar ratio of methanol to fatty acid,
amount of catalyst and reaction time were varied to optimize the
reaction conditions. After the reaction, catalyst was separated by
centrifuging. The conversion of fatty acid was determined by GC
fitted with a SE-54 capillary column (30 m × 0.25 mm), a hydro-
gen flame-ionization detector (FID), and analyzed with normalized
percentage method.
2.2. Catalyst preparation
SZM: Zr(NO3)4·5H2O was dissolved in de-ionized water to pre-
pare a 17 wt.% Zr ion containing solution into which 28 wt.%
aqueous ammonia was added to adjust the pH value to 9–10.
The precipitate was aged for 24 h followed by filtration, wash-
ing, and then drying at 110 ◦C for 12 h to form the precipitate
Zr(OH)4. Ammonium heptamolybdate (NH4)6Mo7O24 was dis-
solved in 0.5 mol/L H2SO4 to prepare a Mo ion containing solution.
Then the precipitate Zr(OH)4 was impregnated in the above solu-
tion for 12 h. The treated solid was evaporated at 80 ◦C until being
dried to powder, and then calcined in a muffle furnace at 600 ◦C for
4 h to obtain catalyst SZM. The MoO3 amount was 10 wt.% of the
catalyst.
2.5. Stability measurement
The reusability of catalyst was studied using the used catalyst
in the next consecutive reaction cycles after being washed with
acetone and calcined in a muffle furnace at 600 ◦C for 2 h.
SZN: Zr ion containing solution was prepared by the method
described above. An appropriate amount of Nd2O3 was dissolved
in 2 mol/L H2SO4 to prepare a Nd ion containing solution. The
two solutions were mixed according to Zr/Nd molar ratio of 75/1.
Adding aqueous ammonia into the mixed solution at a pH of 9–10 to
get Zr–Nd binary hydroxide, which was followed by drying, aging
and washing with the method mentioned above. After that the
precipitate was impregnated with 0.5 mol/L H2SO4 for 12 h, and
evaporated and calcined by the same way in SZM preparation to
obtain catalyst SZN.
SZMN: The Zr–Nd binary hydroxide (the molar ratio of Zr/Nd var-
ied from 20/1 to 100/1) prepared by the method described above
was impregnated with Mo containing solution for 12 h, then evap-
orated and calcined by the same way in SZM preparation to obtain
catalyst SZMN. The samples with different Zr/Nd ratio were cal-
cined at 600 ◦C to test the effect of Zr/Nd ratio on the catalysts. To
observe the effects of calcination temperature, the samples with
the Zr/Nd ratio of 75/1 were calcined at 500–700 ◦C. If otherwise
specified, SZMN is used to denote the sample with a Zr/Nd ratio of
75/1 calcined at 600 ◦C in the present work.
3. Results and discussion
3.1. Effects of preparation and reaction conditions on catalytic
performance
3.1.1. Effect of the addition of Mo and Nd
The catalytic activity of SZMN, SZM, and SZN for the ester-
ification of oleic acid (OA) was examined and shown in Fig. 1.
The conversion of OA was very low when the esterification was
carried out without catalyst. The three catalysts exhibited good
catalytic activity for the esterification and the conversion of OA
increased gradually with reaction time. The relative activity order
was SZM < SZMN < SZN. After 4 h, the maximum value (98.5%) over
SZMN was obtained. It could be seen that the addition of Nd into
SZM is beneficial to enhancing the activity for esterification. Similar
phenomenon was observed over SO42−/ZrO2–SnO2–Nd2O3 sam-
ple for the esterification of acetic acid with terpineol [18]. It was
reported that the conversion of acetic acid in methyl esterification
over SO42−/ZrO2–MoO3 calcined at 550 ◦C was up to nearly 98%
2.3. Catalyst characterization
X-ray diffraction (XRD) patterns of the catalysts were recorded
on a DX-1000 diffractometer, operating at 40 kV and 25 mA and
using nickel-filtered Cu K␣ radiation. The scanning speed was
3.6◦/min and scanning range was over 10–70◦.
Fourier transform infrared spectroscopy of adsorbed NH3 (NH3-
FTIR) on the catalysts were recorded on a NEXUS 670 FTIR
spectrometer. Before measurement, the catalyst sheets were evac-
uated at 300 ◦C for 2 h under vacuum of 10−2 Pa and then cooled to
ambient temperature. After NH3 adsorption for 30 min and evacu-
ation at 100 ◦C for 1 h, IR spectra were recorded.
NH3-temperature-programmed desorption (NH3-TPD) was per-
formed in a fixed-bed reactor at atmosphere pressure. The catalysts
were swept by N2 flow (50 mL/min) at 300 ◦C for 1 h and then cooled
to 80 ◦C. NH3 gas with a flow rate of 20 mL/min was introduced
about 20 min till the samples achieved saturated adsorption. Then
He flow (50 mL/min) was intook into the reactor at 100 ◦C for 2 h to
remove physisorbed NH3. The samples were heated from 100 ◦C to
700 ◦C at a rate of 10 ◦C/min. The desorbed NH3 was analyzed by gas
chromatogram (GC) with a thermal conductivity detector (TCD).
Fig. 1. Effect of reaction time on the activity of the catalysts in esterification of
OA. Reaction conditions: 10 wt.% of catalyst based on fatty acid, molar ratio of
methanol/OA of 9/1, and Zr/Nd molar ratio of 75/1.

48
K. Jiang et al. / Applied Catalysis A: General 389 (2010) 46–51
Table 1
Effects of Zr/ Nd molar ratio and calcination temperature on the activity of SZMN
catalysts.a
Zr/Nd molar
ratiob
OA conversion
(%)
Calcination
temperature
(◦C)c
OA conversion
(%)
20/1
30/1
50/1
75/1
100/1
94.6
95.5
96.7
98.5
96.8
500
550
600
650
700
91.9
96.1
98.5
85.8
68.5
a
Reaction conditions: 10 wt.% of catalyst based on fatty acid, molar ratio of
methanol/OA of 9/1 and reaction time of 4 h.
b
The samples were calcined at 600 ◦C.
The Zr/Nd ratio for the samples was of 75/1.
c
[14]. Comparing with the literature, the OA conversion over SZM
catalyst in this work was 92.6%. This insufficiency on the SZM cat-
alyst might be due to the higher calcination temperature of the
catalyst and the difference in reactants.
Fig. 2. Conversion of OA as a function of thecatalyst reuse cycle. Reaction conditions:
10 wt.% of catalyst based on fatty acid, molar ratio of methanol/OA of 9/1 and reaction
time of 4 h.
cycle of the catalysts. After the sixth time of reuse, the OA con-
version over SZN catalyst reduced to 30.9%, which was less than
one-third of the initial conversion (98.9%) achieved on the fresh
catalyst. Over SZM catalyst, the OA conversion gradually decreased
to 55.5% after the sixth reuse. Comparing with SZN, although SZM
catalyst could not give the highest activity, it showed relatively
better stability. A desired result was observed over SZMN that the
conversion of OA still maintained its high value (more than 96%)
after the sixth cycle, which was differed from the other two cata-
lysts. Based on the above results, it can be suggested that adding
Nd into sulfated zirconia-based solid acid is favorable to obtaining
the high activity, while adding Mo is more favorable to keeping
the stability. This indicates that the co-addition of Mo and Nd is
of great importance in enhancing not only the activity but also the
stability.
3.1.2. Effects of Zr/Nd molar ratio and calcination temperature of
SZMN catalyst
As mentioned above, SZMN exhibited equivalent activity to SZN
for the esterification of OA. In order to study the catalytic per-
formance of SZMN in detail, the effects of Zr/Nd molar ratio and
calcination temperature on the activity of SZMN catalyst were
examined. The results were shown in Table 1. The conversion of
OA increased slightly with the Zr/Nd molar ratio, and was up to the
maximum value of 98.5% at the Zr/Nd molar ratio of 75/1. However,
the activity of SZMN decreased with further increase of Zr/Nd molar
ratio up to 100/1. It could be suggested that too much Nd added into
SZMN is disadvantageous for the activity of catalyst while moderate
amount of Nd is able to enhance the activity. Deng et al. reported a
close Zr/Nd molar ratio of SO42−/ZrO2–Nd2O3 for the esterification
of lactic acid with n-butanol, and the butyl lactate yield reached
98.8%, but its activity was not stable [16].
The calcination temperature had a great influence on the cat-
alytic activity of SZMN in esterification of OA, as shown in Table 1.
The OA conversion increased with calcination temperature, and
reached a maximum value of 98.5% when the catalyst was calcined
at 600 ◦C. Then, the conversion descended sharply when SZMN was
calcined at 650 ◦C and higher temperature. The decreasing activity
might be attributed to the transformation of metastable tetrago-
nal ZrO2 (t-ZrO2) exhibiting higher activity for the esterification
into monoclinic ZrO2 (m-ZrO2) [14], and the possible decomposi-
tion of surface sulfur species leading to the loss of sulfur, which
resulted from the calcinations at higher temperature. This is prob-
ably why the calcination temperature of 600 ◦C was widely used in
the preparation of the Nd doped solid acid catalysts [16,18].
3.1.4. Catalytic activity of SZMN for the esterification of fatty
acids
The catalytic activity of SZMN in the esterification of different
fatty acids (C2–C18) was also tested in the present work, as shown
in Table 2. The conversions of different fatty acids were very similar.
This illuminates that SZMN is an excellent catalyst for the esterifi-
cation of fatty acids in spite of the chain length, and there is almost
no fatty acid chain-length-dependence of activity. The result was
also reported over SO42−/ZrO2–SiO2 catalyst by Chen et al. [19]. It
is suggested that SZMN catalyst prepared in the present work is
practical for the esterification of different fatty acids.
3.2. Characterization of the catalysts
3.1.3. The stability of SZM, SZN and SZMN
3.2.1. XRD
The stability of the three catalysts was studied by the above
described process. It was found from Fig. 2 that the OA conversion
over SZN and SZM catalysts markedly diminished with the reuse
As shown in Fig. 3(a), no independent lines due to MoO3 or
any compound formed between MoO3 and ZrO2 were observed,
and no diffraction patterns of Nd compound existed in all sam-
Table 2
Activity of SZMN catalyst in esterification of fatty acids.a
Methanol/fatty
acid molar ratio
Conversion of fatty acids (%)
Amount of SZMN catalyst/wt.% fatty acid
Acetic acid (C2)
Caproic acid (C6)
Lauric acid (C12)
Palmitic acid (C16)
Oleic acid (C18)
7
10
7
10
7
10
7
10
7
10
6/1
9/1
93.9
99.8
99.8
99.9
92.4
95.4
95.9
97.5
93.2
95.9
95.4
97.3
95.5
98.7
96.7
98.4
96.1
96.8
97.2
98.5
a
Reaction time: 4 h.

K. Jiang et al. / Applied Catalysis A: General 389 (2010) 46–51
49
Fig. 4. NH3-FTIR spectra of SZM, SZN and SZMN solid acid catalysts.
from the formation of t-ZrO2 minicrystal with smaller grain size.
The stabilization of t-ZrO2 by the addition of promoters such as
Al2O3, La2O3 V2O5 and Yb2O3 was also reported in the literature
[17,24,29].
The crystallite phase of ZrO2 in sulfated zirconia-based catalyst
was strongly influenced by calcination temperature, as shown in
Fig. 3(b). The catalysts calcined below 550 ◦C were amorphous, for
no diffraction peaks in XRD patterns were detected. When the sam-
ple was calcined at 600 ◦C, t-ZrO2 formed in the SZMN catalyst. The
transformation from amorphous structure to t-ZrO2 was attributed
to the loss of water from the amorphous hydrous ZrO2 [30]. While
above 650 ◦C, the presence of m-ZrO2 was detected, and the grain
size increased with the calcination temperature according to the
intensified diffraction peaks for ZrO2 shown in Fig. 3(b). The litera-
ture reported that m-ZrO2 over SO42−/ZrO2 usually formed at lower
temperature (550 ◦C, even 450 ◦C) than 650 ◦C [25,31]. It was sug-
gested that Mo addition likely inhibited the phase transition from
t-ZrO2 to m-ZrO2 and stabilized the tetragonal phase of ZrO2 [32].
As mentioned above, the high activity of SZMN may be related to
the formation of t-ZrO2 with smaller grain size. When SZMN was
calcined at 650 ◦C or higher temperature, the reduction of OA con-
version was possibly attributed to the formation of m-ZrO2 with
low activity and the decomposition of surface sulfur species [33].
This result indicates that calcination temperature plays a key role
in the crystallite formation and the interaction of the active sulfur
species with ZrO2, which will affect the catalyst activity.
Fig. 3. XRD patterns of zirconia-based solid acid catalysts. (a) SZM, SZN and SZMN,
(b) SZMN catalysts calcined at different temperatures.
ples. This indicates that Mo and Nd are highly dispersed on the
catalysts [20]. The XRD patterns showed that only t-ZrO2 formed
in SZM catalyst, while t-ZrO2 and Zr(SO4)2·4H2O co-existed in
SZN catalyst. The formation of Zr(SO4)2·4H2O might be due to
the long-time immersion and the dissolution of ZrO2·nH2O in
H2SO4 [21], furthermore, the addition of Nd could weaken the
crystallinity of ZrO2 and strengthen the interaction of the sulfur
species with ZrO2 in the solid acid [18]. Combining with the activity
result, it could be speculated that zirconium sulfate in SZN catalyst
is able to provide the excellent catalytic activity, but the easy-
leaching of active sulfur species in the form of zirconium sulfate
from the catalyst results in the worst stability of SZN aforemen-
tioned [8]. The XRD patterns of SZMN sample shown in Fig. 3(a)
revealed that only t-ZrO2 formed in the catalyst, and the same was
observed on SZM. It was reported that both t-ZrO2 and m-ZrO2 pre-
sented in the sulfated zirconia-based catalysts calcined at 550 ◦C
[22–25]. While t-ZrO2 existed stably in SZM and SZMN catalysts
calcined at 600 ◦C in the present work, which exhibits higher activ-
ity than m-ZrO2. This indicates that the addition of Mo retards the
transformation of metastable t-ZrO2 phase into more stable mon-
oclinic phase [26–28]. The grain size of t-ZrO2 in SZMN sample
(4.3 nm) was smaller than that of t-ZrO2 in SZM (8.7 nm) (calculated
with Scherrer formula by MDI Jade 5 software). Combining with
the activity result, the better performance of SZMN might result
3.2.2. NH3-FTIR
Infrared spectroscopic studies of ammonia adsorbed (NH3-FTIR)
on solid surfaces have made it possible to distinguish between
Brönsted acid and Lewis acid sites. As seen from Fig. 4, for the cat-
alysts SZM, SZN, SZMN, the IR bands between 480 and 700 cm−1
were assigned to crystalline ZrO2 [23]. The ꢀS–O or ꢀS
of inor-
O
2−
ganic bidentate SO4 ions coordinated to ZrO2 was detectable at
800–1350 cm−1 [30]. The presence of the band at 1400–1401 cm−1
was associated with NH3 adsorbed on Brönsted acid sites, and that
at 1631–1632 cm−1 was characteristic of NH3 adsorbed on Lewis
acid sites [6]. It is shown that Brönsted acid and Lewis acid sites
co-exists on the surface of the catalysts. The relative intensity of
Brönsted acid/Lewis acid sites over SZMN sample (B/L = 2.76) was
higher than that over SZN (B/L = 2.18) and SZM (B/L = 1.89). Brön-
sted acid sites play a prior effect on catalytic activity. It was also
reported that the catalytic activity of sulfated zirconia-based solid
acid correlated with the amount of Brönsted acid sites [34].

50
K. Jiang et al. / Applied Catalysis A: General 389 (2010) 46–51
Fig. 6. DTG curves of SZM, SZN and SZMN solid acid catalysts.
phase exhibit super acidic nature, and t-ZrO2 with smaller grain
size is more beneficial to enhancing the acidity of solid acid than
large-grain t-ZrO2.
Calcination temperature plays a key role in the crystallite for-
mation and the interaction of the active sulfur species with ZrO2,
leading to the difference of surface acidity. ZrO2-based catalysts
calcined at different temperatures had different acid strength dis-
tributions [31]. The shift of the peak temperature of ammonia
desorption peak from 540 ◦C to 580 ◦C was observed with cal-
cination temperature below 600 ◦C, as seen from Fig. 5(b). This
observation indicated that the acid strength increased with calci-
nation temperature below 600 ◦C, and the variation of the amount
of acid sites possessed the same tendency. Above 600 ◦C, the acid
strength decreased as the presence of only medium strong acid
sites on SZMN (c-650 ◦C) and weak acid on SZMN (c-700 ◦C), and
the amount of acid sites also drastically declined, which resulted
in the reduction of activity of SZMN catalysts calcined at higher
temperatures than 600 ◦C. Combining with the results of XRD, the
declining acid strength on SZMN catalysts may be related to the
formation of m-ZrO2 phase and the increase of grain size of t-ZrO2.
Furthermore, the calcination of solid acid at higher temperature
than 600 ◦C may cause the decomposition of sulfur species, which
could decrease the amount and strength of acid sites.
Fig. 5. NH3-TPD profiles of zirconia-based solid acid catalysts. (a) SZM, SZN and
SZMN (b) SZMN catalysts calcined at different temperatures.
3.2.3. NH3-TPD
The amount of acid sites and acid strength distribution obtained
from the NH3-TPD spectra was listed in Table 3 and the NH3-TPD
profiles were shown in Fig. 5. Among the catalysts, the amounts of
acid sites on SZMN (6.63 mmol/g) and on SZN (6.12 mmol/g) were
higher than that on SZM (5.74 mmol/g). This result was consistent
well with the higher activity of SZMN and SZN than that of SZM
for the esterification of OA. As shown in Fig. 5(a), a broad des-
orption profile of ammonia over the range from 150 ◦C to 650 ◦C
suggested a broad distribution of acid strength from weak, medium,
and strong to super acidity. Similar ammonia desorption peaks on
SZMN and SZN from 560 ◦C to 650 ◦C indicated their super acidic
nature. The acid strength of SZM was lower than that of SZMN and
SZN for the lower desorption temperature of ammonia from 460 ◦C
to 560 ◦C. It is suggested that the addition of Nd into zirconia-based
support results in the shift of the peak temperature of ammonia
desorption from 530 ◦C to 600 ◦C. In all cases the amount of acid
sites was enhanced after the co-addition of Mo and Nd. Combining
with the XRD results, it is suggested that t-ZrO2 and Zr(SO4)2·4H2O
3.2.4. TG–DTG
The fresh catalysts SZM, SZN, SZMN were subjected to TG–DTG
analysis to study the relationship between the activity and the ther-
mal stability. As shown in Fig. 6, the weight loss at high temperature
(above 580 ◦C) is due to the decomposition of surface sulfur species
[24]. The shift in the weight loss peaks for SZMN catalyst towards
higher temperature over the range from 680 ◦C to 780 ◦C than other
samples indicated an increase in the thermal stability of surface
sulfur species over SZMN sample. The enhanced thermal stability
of sulfur species by mixed oxides over SO42−/ZrO2–Al2O3 was also
reported by Reddy et al. [27]. The thermal stability characterizes the
combination strength of sulfur species with t-ZrO2, which is related
to the acidity and activity. According to the above results, the ther-
Table 3
Amounts of acid sites on zirconium-based solid acid catalysts.
Catalysts
SZM
SZN
SZMN
SZMN
SZMN
SZMN
SZMN
c-500 ◦C
c-550 ◦
C
c-600 ◦
C
c-650 ◦
C
c-700 ◦
C
Amount of acid sites (mmol/g)
Acid strengtha
5.74
M, S&Su
6.12
W, M, S&Su
4.69
W, M, S&Su
4.28
S&Su
6.63
S&Su
0.09
M&S
0.15
W
a
W – weak acid, M – medium strong acid, S – strong acid, Su – super acid.

K. Jiang et al. / Applied Catalysis A: General 389 (2010) 46–51
51
Table 4
largest amount of acid sites on SZMN was observed. The excellent
activity and stability of SZMN are attributed to the firm combination
of sulfur species with small-crystallite t-ZrO2, which hinders the
leaching of sulfur species from the surface. The co-addition of Mo
and Nd stabilizes the structure of small-crystallite t-ZrO2, which is
favorable to increasing the amount of acid sites, strengthening the
acidity, and then enhancing the activity and stability of catalyst.
Sulfur species weight loss of catalysts being fresh and used three, six times.
Catalyst
Weight loss (%)
Fresh
Third
Sixth
SZM
SZN
SZMN
24.38
33.90
25.39
18.14
20.92
20.55
11.90
15.87
20.53
Acknowledgements
mal stability of t-ZrO2 with smaller grain size was higher than that
of one with larger grain size, which implies that the combination
of sulfur species with t-ZrO2 on the smaller one is relatively strong.
So SZMN sample with t-ZrO2 minicrystal showed stronger acidity
and higher activity. Moreover, the DTG results suggested that the
optimum temperature for calcination of SZMN sample was 600 ◦C
where no weight loss of sulfur species could be observed.
This work is financially supported by the National Basic Research
Program of China (973 Program, No. 2007CB210203) and the
Scientific Research Foundation for Young Teachers of Sichuan Uni-
versity (No. 2009SCU11104). The characterization of catalyst from
Analytical and Testing Center of Sichuan University are greatly
appreciated.
The DTG curve of catalyst SZMN calcined at 600 ◦C in Fig. 6 indi-
cated that sulfur species began to decompose from 680 ◦C to 780 ◦C.
Therefore, the decrease in catalytic activity of SZMN calcined at
700 ◦C can be attributed to the phase transformation of t-ZrO2 into
m-ZrO2 and the decomposition of surface sulfur species leading to
the loss of sulfur. While the decreasing activity of SZMN calcined
at 650 ◦C should be mainly ascribed to the phase transformation
because of no decomposition of sulfur species at 650 ◦C.
In order to correlate the content of sulfur species on the catalysts
with their activity and stability, the used catalysts after the third
and the sixth cycle followed by calcination procedure at 600 ◦C were
also subjected to TG–DTG analysis. The weight loss over the range
from 580 to 780 ◦C was shown in Table 4, the value of which may
characterize the content of sulfur species on the catalyst [24]. The
weight loss of sulfur species on fresh SZN got the maximum value
(33.9%) among the three fresh catalyst samples, but it decreased
obviously to 15.87% after the sixth reuse. So the activity of SZN
declined sharply with the reuse cycle. It can be suggested that
the leaching of sulfur species on SZN in the reaction leads to the
decrease of the activity. The decrease of activity also occurred on
SZM catalyst. While in the case of SZMN catalyst, the weight loss
of sulfur species decreased slightly from 25.39% to 20.55% after the
third reuse and maintained a stable value till the sixth reuse. It can
be speculated that the excellent activity and stability of SZMN is
attributed to the firm combination of sulfur species with small-gain
t-ZrO2, which hinders the leaching of active species from catalyst
surface.
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4. Conclusions
SZM, SZN and SZMN solid acids were prepared and characterized
in the present work. Over SZMN catalyst with Zr/Nd molar ratio of
75/1, OA conversion of 98.5% was obtained. The activity of catalyst
could be recovered, and a stable OA conversion of 96.1% could be
obtained after the sixth reuse. Among the investigated catalysts,
the super acid sites existed on SZN and SZMN catalysts, and the