Effects of calcination temperature and water-washing treatment on n-hexane hydroisomerization behavior of Pt-promoted sulfated zirconia based catalysts

Article abstract of DOI:10.1016/j.cattod.2012.07.024

Synthesis of Pt-promoted sulfated zirconia catalyst Pt/SO₄ ^2-/ZrO₂ (designated as PSZ), Pt/SO₄ ^2-/ZrO₂-Al₂O₃ (designated as PSZA), and Pt/SO₄^2-/ZrO₂-Al₂O ₃-Y₂O₃ (designated as PSZAY) was reported. These catalysts were characterized by N₂ adsorption-desorption, thermal gravimetric (TG) analysis, NH₃-temperature programmed desorption (NH₃-TPD), and H₂-temperature programmed reduction (H₂-TPR) techniques. Effects of calcination temperature of the catalysts on the n-hexane hydroisomerization behavior were investigated in details. The experiment results revealed that the addition of Al and Y or Al promoter alone could improve the thermal stability of the catalysts. PSZA and PSZAY exhibited higher isomerization activity than unmodified PSZ as the calcination temperature of sulfated Zr(OH)₄ was relatively high, for example, between 650 C and 750 C. For the first time, it was found that water-washing treatment of PSZA and PSZAY at different preparation stages had considerably different impacts on their catalytic activity. Water-washing after calcination of the sulfated Zr(OH)₄, the catalytic activity of PSZA and PSZAY was high, as indicated by approximately 80% of n-hexane conversion. However, water-washing before calcination of sulfated Zr(OH)₄ led to a significant decrease in the catalytic activity of PSZA and PSZAY, that is, the n-hexane conversion diminished to around 35%. It was proposed that the presence of excessive sulfur species over sample was favorable for keeping the active sulfur species in the catalyst surface.

Full text of DOI:10.1016/j.cattod.2012.07.024

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Effects of calcination temperature and water-washing treatment on n-hexane
hydroisomerization behavior of Pt-promoted sulfated zirconia based catalysts
Yueqin Songa,∗, Jing Tiana, Yurong Yea, Yaqing Jina, Xiaolong Zhoua,∗, Jin-An Wangb, Longya Xuc
a Petroleum Processing Research Center, East China University of Science and Technology, 200237 Shanghai, PR China
b Laboratorio de Catálisis y Materiales, ESIQIE, Instituto Politécnico Nacional, Col. Zacatenco, 07738 México, D.F., Mexico
c Dalian National Lab of Clean Energy, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian, Liaoning 116023, PR China
a r t i c l e i n f o
a b s t r a c t
Synthesis of Pt-promoted sulfated zirconia catalyst Pt/SO42−/ZrO2 (designated as PSZ), Pt/SO42−/ZrO2-
Al2O3 (designated as PSZA), and Pt/SO42−/ZrO2-Al2O3-Y2O3 (designated as PSZAY) was reported. These
catalysts were characterized by N2 adsorption–desorption, thermal gravimetric (TG) analysis, NH3-
temperature programmed desorption (NH3-TPD), and H2-temperature programmed reduction (H2-TPR)
techniques. Effects of calcination temperature of the catalysts on the n-hexane hydroisomerization behav-
ior were investigated in details. The experiment results revealed that the addition of Al and Y or Al
promoter alone could improve the thermal stability of the catalysts. PSZA and PSZAY exhibited higher
isomerization activity than unmodified PSZ as the calcination temperature of sulfated Zr(OH)4 was rela-
tively high, for example, between 650 ◦C and 750 ◦C. For the first time, it was found that water-washing
treatment of PSZA and PSZAY at different preparation stages had considerably different impacts on their
catalytic activity. Water-washing after calcination of the sulfated Zr(OH)4, the catalytic activity of PSZA
and PSZAY was high, as indicated by approximately 80% of n-hexane conversion. However, water-washing
before calcination of sulfated Zr(OH)4 led to a significant decrease in the catalytic activity of PSZA and
PSZAY, that is, the n-hexane conversion diminished to around 35%. It was proposed that the presence of
excessive sulfur species over sample was favorable for keeping the active sulfur species in the catalyst
surface.
Article history:
Received 13 March 2012
Received in revised form 11 July 2012
Accepted 22 July 2012
Available online xxx
Keywords:
Hydroisomerization
Calcination temperature
Sulfated zirconia
Promoter
Water washing
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
[1], it attracted much attention in acid-catalyzed field, especially
in n-alkane isomerization reaction [2–6]. The isomerization activ-
The transformation of n-alkanes into iso-alkanes through
hydroisomerization process is of practical importance in petro-
chemical industry. For example, n-butane can be converted into
iso-butane and the latter can be alkylated with butene to pro-
duce iso-octane, which is an environmentally friendly high-octane
gasoline booster. In addition, n-pentane and n-hexane can also be
isomerized into their isomers and directly acted as blending com-
ponents of clean high-octane gasoline. The n-hexane isomerization
process was commercialized; however, the used conventional bi-
functional catalyst presented some shortcomings, such as small
amount of high octane number components in the products and
low catalytic activity at reaction temperature below 280 ◦C. Since
sulfated zirconia (SZ) was found to possess super or strong acidity
ity of SZ or Pt-promoted SZ (PSZ) was greatly dependent on
the preparation step or method. It is generally believed that the
high isomerization activity of SZ and PSZ could be obtained only
when amorphous zirconium hydroxide acted as the precursor of
sulfation. In the preparing process of SZ and PSZ, the high calci-
nation temperature was considered to be indispensable [7] and
it strongly affected the catalytic activity of SZ and PSZ to a great
extent [8–13]. The results from Arata et al. [12] and Sun et al.
[13] indicated that the calcination temperature effect on the isom-
erization activity of SZ depended on the preparation method to
some extent. Comelli et al. [8] examined the effect of calcination
temperature on n-hexane isomerization catalytic performance of
Pt-promoted SZ, and these authors considered that the isomeriza-
tion activity reached maximum at the calcination temperature of
550–600 ◦C.
The influence of calcination temperature is usually related
with different promoters. Hua and Sommer [9] once investigated
the calcination temperature of sulfated Zr(OH)4–Al(OH)3 on n-
butane isomerization activity over Pt/SO42−/ZrO2-Al2O3 (PSZA)
and found that the optimized calcination temperature was 650 ◦C,
∗
Corresponding authors. Tel.: +86 21 64252041; fax: +86 21 64252041.
E-mail addresses: songyueqin@ecust.edu.cn, yqsong@mail.tsinghua.edu.cn
(Y. Song), Tianjian@163.com (J. Tian), zjscyyr@126.com (Y. Ye), yqjin@ecust.edu.cn
(Y. Jin), xiaolong@ecust.edu.cn (X. Zhou), jwang@ipn.mx (J.-A. Wang),
lyxu@dicp.ac.cn (L. Xu).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.07.024
Please cite this article in press as: Y. Song, et al., Effects of calcination temperature and water-washing treatment on n-hexane hydroisomerization
behavior of Pt-promoted sulfated zirconia based catalysts, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.024

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in agreement with the previous investigation results from our
groups [10]. Signoretto et al. [11] also reported the optimal calcina-
tion temperature of sulfated Zr(OH)4 modified with Ga2O3 which
depended on the content of Ga2O3 promoter. Our previous results
confirmed the important role of rare earth Y promoter in PSZA in
the isomerization activity and stability [14]. The investigation from
Xia et al. [15] demonstrated that the introduction of Al to PSZ could
not improve the isomerization activity, but was helpful to catalyst
shaping. In fact, the effect of the calcination temperature of sul-
fated Zr(OH)4 before and after the addition of Al or other promoters
may be different, but there existed seldom systematic investigation
about this aspect.
On the other hand, water-washing treatment usually plays
an important role in the SZ catalyst preparation; it can partially
remove sulfur species over catalyst, thus modify the surface acidity
and catalytic activity [16]. However, for Pt-promoted SZ and SZA,
update it is unclear whether or not only water soluble sulfur species
over PSZ and PSZA are the catalytic active centers. According to the
new isomerization reaction mechanism over PSZ suggested by Ebi-
tani et al. and Manoli et al. [17,18] that the new strong acid sites
could be created in the presence of gaseous hydrogen and acted
as the catalytic active sites, after water washing, PSZ or PSZA may
show certain catalytic activity in the hydroisomerization reaction.
The object of the present work is to further investigate the
effect of calcination temperature of sulfated Zr(OH)4 on the n-
hexane isomerization activity of the Al or Al and Y promoted
Pt/SO42−/ZrO2 catalysts. The role of water washing treatment at
different preparing stages in the catalytic activities is also studied
in detail.
2.2. Catalytic activity measurement
The n-hexane hydroisomerization reaction was carried out in
a continuous flow-type fixed-bed stainless reactor (i.d. = 5 mm)
loaded with 2.0 g of catalyst. The prepared catalyst was crushed
and sieved to 40–60 mesh. Prior to the reaction, the catalyst sam-
ple was pretreated in flowing dry air (20 ml/min) at 400 ◦C for 3 h.
The system was cooled to 250 ◦C and the catalyst was reduced with
flowing hydrogen for 3 h at this temperature. After the reduction,
the temperature of catalyst bed decreased to a set temperature.
Hydrogen gas and n-hexane were simultaneously introduced into
the reactor and the reaction pressure was controlled by hydrogen
gas. The standard reaction conditions were as follows: the reaction
total pressure = 2.0 MPa, n-hexane weight hourly space velocity
(WHSV) = 1.0 h−1, hydrogen/n-hexane molar ratio = 5, and the reac-
tion temperature = 165 ◦C. Normal hexane feedstock was sent to
the reactor by a double plunger micro pump and the hydrogen gas
flow rate was controlled by a mass flow meter. The products were
analyzed by on-line GC-920, equipped with an FID and an OV-101
capillary column. Since weight percentage of all kinds of isomers
and the cracking products could be obtained by area normalization
method in GC peaks, the conversion of n-hexane and the selectivity
of iso-hexane were calculated according to the following formulas
(1) and (2), respectively:
X
(n-hexane conversion) = 100% − wt% of n-hexane in product
(1)
wt% of total iso-hexanes in products
S (iso-hexane selectivity) =
X
(2)
2. Experimental
2.1. Catalyst preparation
2.3. Catalyst characterization
2.3.1. X-ray diffraction (XRD)
XRD patterns were obtained with a Philips MagiX X-ray diffrac-
tometer, using Cu K␣1 radiation and instrumental settings of 40 kV
and 40 mA. The scanning was within a range of 2ꢀ from 10◦ to 70◦
at a scanning rate of 6◦/min.
A prepared 0.2 mol/L ZrO(NO3)2 solution was dropwise added
into 5% ammonia solution with stirring. After precipitation, the
slurry was stirred for additional 0.5 h and then aged statically at
room temperature for 10 h. The obtained Zr(OH)4 white precipi-
tate was repeatedly washed with deionized water by evacuation
filtration till the pH value of the filtrate was ca. 7 and then dried
at 110 ◦C for 20 h. The dried sample was crushed to very fine pow-
der. Part of the dry powder was mixed with boehmite powder. The
mixture powder was extruded with dilute HNO3 and dried again at
110 ◦C. Sulfating procedure was performed by impregnating sep-
arately the dry Zr(OH)4 powder and the extrudate with 0.5 mol/L
of H2SO4 solution (15 ml/g) at room temperature for 6 h. The sul-
fated samples were filtered without washing and dried overnight
at 110 ◦C, denoted as SZ and SZA. Part of SZA sample was impreg-
nated with yttrium nitrate solution to load Y and dried at 110 ◦C for
13 h, denoted as SZAY. Subsequently, the three dry sulfated solid
samples were calcined at certain temperature for 3 h. Finally, the
calcined samples were respectively impregnated in H2PtCl6 solu-
tion to load with 0.5 wt% Pt, dried at 110 ◦C and calcined at 500 ◦C
for 3 h. Thus, the prepared Pt/SO42−/ZrO2, Pt/SO42−/ZrO2-Al2O3 and
Pt/SO42−/ZrO2-Al2O3-Y2O3 catalysts were designated as PSZ, PSZA
and PSZAY. The weight percentage of Al2O3 and Y2O3 in the support
was 3%.
2.3.2. NH3-temperature programmed desorption (NH3-TPD)
NH3-TPD was carried out at a homemade equipment. The sam-
ple (0.14 g) was loaded into a stainless steel U-shaped microreactor
(i.d. = 5 mm) and pretreated at 600 ◦C for 0.5 h in flowing He.
After the pretreatment, the sample was cooled to 150 ◦C and was
exposed to NH3 atmosphere. As the catalyst was saturated with the
adsorbed NH3, helium was used as carrier to remove NH3 phys-
ically adsorbed until the baseline was stable. NH3-TPD was then
carried out in a constant flow of He (20 ml/min) from 150 ◦C to
650 ◦C at a heating rate of 18 ◦C/min. The concentration of ammonia
in the exit gas was monitored continuously by a gas chromato-
graph (Shimazu 8A) equipped with a thermal conductivity detector
(TCD).
2.3.3. H2-temperature programmed reduction (TPR)
TPR experiments were performed in a domestic apparatus.
The sample (0.1 g) was loaded in a quartz tube and pretreated
in helium for 0.5 h. After cooled to room temperature, a 5%
H2/He mixed gas passed through the sample with a flow rate of
30 ml/min. The sample began to be heated at a heating rate of
10 ◦C/min from room temperature to 800 ◦C. The hydrogen con-
sumption was measured by using a thermal conductivity detector
(TCD).
In order to investigate the water washing treatment, part of dry
sulfated Zr(OH)4 was washed with water three times (30 ml/g),
denoted as WSZ. Part of calcined sulfated Zr(OH)4 at 650 ◦C was
washed with water three times (30 ml/g), denoted as SWZ. Then
these samples were mixed with boehmite powder. The following
preparation steps were the same as the above. The final catalysts
were named as PWSZA and PSWZA.
Please cite this article in press as: Y. Song, et al., Effects of calcination temperature and water-washing treatment on n-hexane hydroisomerization
behavior of Pt-promoted sulfated zirconia based catalysts, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.024

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3
T
T
PSZA
PSZ
T
T
T
T
M
M
PSZA600
PSZA650
M
PSZ600
PSZ650
PSZ700
PSZ750
PSZA700
PSZA750
20
30
40
50
60
70
20
30
40
50
60
70
2Theta (degree)
2Theta (degree)
T
PSZAY
T
T
PSZAY600
PSZAY650
M
PSZAY700
PSZAY750
20
30
40
50
60
70
2Theta (degree)
Fig. 1. XRD patterns of the samples at different calcinations temperatures (T – tetragonal zirconia; M – monoclinic zirconia).
2.3.4. Thermal gravimetric analysis (TGA)
some monoclinic zirconia phase. The above phenomena allowed us
to conclude that the crystalline structure of Pt-promoted SZ-based
catalyst was related not only with the calcination temperature, but
also with promoters, in agreement with the report from Signoretto
et al. [11]. On the other hand, the effect of calcination temperature
on the crystalline structure was complicated, because it also would
influence sulfur content of the resultant catalyst [19]. Thus we will
report the sulfur content in our catalyst in the following section.
TGA was carried out on SDT-Q600 TG analysis equipment (TA,
USA). The catalyst sample (0.02 g) was heated from room tempera-
ture to 1000 ◦C in an air stream at a heating rate of 10 ◦C/min. In the
recorded profiles, the weight loss before 600 ◦C was attributed to
desorption of water. The decrease of weight from 600 ◦C to 1000 ◦C
was caused by the removal of sulfur on the catalyst. Therefore,
2−
SO4 content in the sample could be estimated according to the
following formula:
3.1.2. Effect on the sulfur content
M1 − M2
2−
SO4
amount (g/g cat) =
(3)
The variation of sulfur content over PSZ, PSZA and PSZAY with
the calcination temperature is shown in Table 1. It could be seen
that at the same calcination temperature, the sulfur content over
PSZA and PSZAY was obviously higher than that over the unmodi-
fied PSZ sample. Even at higher calcination temperature, the Al and
Y modified PSZ sample still contained relatively higher sulfur con-
tent than the unmodified PSZ sample. For example, at 750 ◦C, the
sulfur content increased from 1.43% over PSZ, to 2.23% over PSZA,
and to PSZAY over 2.59%. Obviously, the introduction of Al or rare
M1
where M1 represents the weight percent of catalyst after water
desorption and M2 represents the weight percent of the catalyst
after the removal of sulfur.
3. Results and discussion
3.1. Effect of calcination temperature
3.1.1. Effect on the crystalline phases
Table 1
The effects of calcination temperature of sulfated Zr(OH)4 on
the phase composition of PSZ, PSZA and PSZAY are shown in Fig. 1.
It could be seen from Fig. 1a that in the temperature range 600
and 700 ◦C, PSZ mainly contained tetragonal zirconia with only
few percentage of monoclinic zirconia. Further increasing the tem-
perature led to monoclinic phase entirely disappearance. As for
PSZA (Fig. 1b), when the calcination temperature was 750 ◦C, PSZA
only contained tetragonal phase. In the Y promoted sample, PSZAY
(Fig. 1c), even if the temperature was up to 750 ◦C, it still contained
Content of sulfated species over the samples calcined at different temperatures.
Sample Sulfur content Sample
(wt%)
Sulfur content Sample
(wt%)
Sulfur content
(wt%)
PSZ600 3.21
PSZ650 2.87
PSZ700 2.15
PSZ750 1.43
PSZA600 4.68
PSZAY600 4.77
PSZA650 3.46
PSZA700 3.10
PSZA750 2.23
PSZAY650 4.64
PSZAY700 3.28
PSZAY750 2.59
“600” in PSZ600 represented the calcination temperature of sulfated Zr(OH)4.
Please cite this article in press as: Y. Song, et al., Effects of calcination temperature and water-washing treatment on n-hexane hydroisomerization
behavior of Pt-promoted sulfated zirconia based catalysts, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.024

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Table 2
T
Textural structures of PSZA catalysts calcined at different temperatures.
a
Sample
SBET (m2/g)
Pore diameter (nm)
Total volume (cm3/g)
PSZA600
PSZA650
PSZA700
PSZA750
135.05
118.29
108.13
101.75
3.821
5.585
6.504
4.325
0.142
0.152
0.147
0.132
T
T
M
earth promoter could increase the sulfur content over PSZ and the
thermal stability of sulfur species.
PSZA
PWSZA
PSWZA
3.1.3. Effect on the textural property
Besides, the specific surface area also changed with the calcina-
tion temperature, as shown in Table 2. As for PSZA, the BET surface
area of PSZA600 was up to ca. 135 m2/g. And the increase in the
calcination temperature only led to a slight decrease in the area. As
the calcination temperature increased up to 750 ◦C, the surface area
decreased to 101 m2/g. The pore diameter and pore volume reached
maximum (ca. 6 nm and 0.15 m3/g) as the calcination temperature
was intermediate, i.e. 650–700 ◦C.
20
30
40
50
60
70
2Theta (degree)
T
b
3.1.4. Effect on the catalytic behavior
T
The dependence of isomerization activity of PSZ, PSZA and
PSZAY on the calcination temperature is shown in Fig. 2. When
the calcination temperature was relatively low, i.e. 600 ◦C, the con-
version of n-hexane on PSZ600 was about 63%, which was higher
than that over PSZA600 and PSZAY600. As the calcination temper-
ature increased to 650 ◦C, the conversion over the three samples
greatly increased, for example, n-hexane conversion over PSZAY
catalyst almost doubly increased from approximately 36% to 74%.
Further increasing the calcination temperature to 700 ◦C resulted
in a great decrease in n-hexane conversion over PSZ, and in a slight
decrease over PSZA. While over PSZAY, n-hexane conversion was
almost unchanged at 700 ◦C. When the calcination temperature
reached 750 ◦C, the n-hexane conversion over PSZ750 significantly
reduced to ca. 5%, which was much lower than that was achieved
over PSZA and PSZAY. At this temperature, the n-hexane conver-
sion over PSZAY750 was the highest, i.e. 40%, which was similar
to the conversion over PSZ700. Obviously, the calcination at low
temperature was favorable to the isomerization reaction over PSZ;
while, high calcination temperature was favorable to the reaction
over PSZA and PSZAY.
T
M
PSZAY
PWSZAY
PSWZAY
20
30
40
50
60
70
2Theta (degree)
Fig. 3. XRD patterns of the catalyst samples washed with water at different steps (T
– tetragonal zirconia; M – monoclinic zirconia).
3.2. Effect of water washing treatment
3.2.1. Effect on the phase composition
Li et al. [16] found that some water-soluble sulfur species existed
over the SZ sample, which was considered to be catalytic active
centers for the isomerization reaction of n-alkane. Similar results
were also reported by Manoilova et al. [20]. In the present work,
since both PSZA and PSZAY catalysts at 650 ◦C show better ther-
mal stability, as evidenced by the above experimental results, in
the following section, the two samples PSZA650 and PSZAY650
were chosen for investigating the influence of water-washing on
the crystalline structure and hydroisomerization activity.
The effect of water-washing on the crystalline phase of PSZA650
is shown in Fig. 3. It could be clearly seen from Fig. 3a that PWSZA
sample which corresponds to water washing treatment before the
calcination, contained only tetragonal phase; while the crystalline
structure of PSWZA which corresponds to water-washing after the
calcination, was similar to that of the unwashed sample PSZA as it
contained a large amount of tetragonal phase with very few mono-
clinic phase. This indicates that water-washing treatment before
calcination led to the formation of complete tetragonal phase in
PSZA, and after calcination, the water-washing treatment could not
alter the crystalline structure.
PSZ
90
75
60
45
30
15
0
PSZA
PSZAY
600
650
700
750
However, for PZAY sample, no matter the water-washing treat-
ment was operated before or after calcination, both the resulted
PWZAY and PZWAY catalysts contain pure tetragonal phase, as
shown in Fig. 3b. This result indicates that water-washing treat-
ment is insensitive to the Y modified catalyst. Therefore, the
Calcination temperature ( oC)
Fig. 2. n-Hexane conversion over PSZ and modified PSZ catalysts calcined at
different temperatures (WHSV = 1.0 h−1, P = 2.0 MPa, n(H2)/n(n-C6) = 5:1, reaction
temperature = 165 ◦C, TOS = 3 h).
Please cite this article in press as: Y. Song, et al., Effects of calcination temperature and water-washing treatment on n-hexane hydroisomerization
behavior of Pt-promoted sulfated zirconia based catalysts, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.024

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5
100
98
96
94
92
a
615
PSZA
813
PSWZA
PWSZA
PSWZA
PWSZA
PSZA
0
200
400
600
800
1000
1200
Temperature (oC)
100
200
300
400
500
600
700
800
Temperature (oC)
100
98
96
94
92
90
88
Fig. 5. NH3-TPD profiles of PSZA catalysts before and after water-washing.
b
630
805
water-soluble sulfur species over the calcined catalyst was thermal
instable ones.
3.2.3. Effect on the surface acidity
The effect of water-washing on the surface acidity of PSZA
before and after water-washing is characterized by NH3-TPD, as
shown in Fig. 5. Compared with PSZA, the water-washed samples
(PSWZA and PWSZA), no matter after or before calcination, the
NH3 desorption peak area recorded at high temperature greatly
decreased; while the low temperature peak area only exhibited
slightly decrease. This implies that the water-washing treatment
led to a great decrease in the strong acid sites, which should be
attributed to the drop of sulfur content, in line with the suggestion
from Manoilova et al. [20].
PSWZAY
PWSZAY
PSZAY
0
200
400
600
800
1000
1200
Temperature (oC)
Fig. 4. TG curves of the (a) PSZA and (b) PSZAY samples before and after water-
washing.
3.2.4. Effect on the H2 reduction behavior
The reduction property of PSZA, PWSZA and PSWZA character-
ized by H2-TPR is shown in Fig. 6. There appeared two reduction
peaks for all the three samples. In the TPR profile of the PSZA sample,
one high temperature peak was centered at 550 ◦C and another low
temperature peak at 450 ◦C. After water-washing treatment, the
peak area of the samples (PWSZA and PSWZA) obviously lowered.
influence of water washing treatment on the phase composition
is also affected by the different promoters.
3.2.2. Effect on the sulfur content
The variations in the sulfur content over PSZA and PSZAY before
and after water-washing were determined by TG technology, as
shown in Fig. 4 and Table 3. It could be clearly seen that the water-
washing treatment before and after calcination of sulfated Zr(OH)4
removed partial sulfur species over PSZA and PSZAY, as evidenced
by the decrease in the sulfur content (in Table 3). This suggests that
there exists some water-soluble sulfur species over the PSZA and
PSZAY samples even after calcination, in agreement with the sug-
gestion from Li et al. [16]. Moreover, the water-washing at different
stages resulted in similar sulfur content for PSZA or PSZAY.
In TG curves, the weight loss occurring above 600 ◦C is gener-
ally attributed to the loss of sulfur species. It could be seen that
the weight decrease of PSWZA and PSWZAY in TG curves due to
the sulfur loss (in Fig. 4) occurred at temperature above ca. 800 ◦C,
which was higher than that for other samples, demonstrating that
after calcination, the water-washing treatment mainly removed
the sulfur species with thermal instability. In another word, the
3.2.5. Effect on the textural property
Table 4 listed the textural data of the PSZA catalyst before
and after water-washing. Water-washing of the calcined SWZA650
PSZA
PSWZA
PWSZA
Table 3
Sulfur contents of PSZA sample before and after water-washing.
Sample
Sulfur content (wt%)
Sample
Sulfur content (wt%)
100
200
300
400
500
600
700
800
PSZA
PWSZA
PSWZA
3.41
2.60
2.67
PSZAY
PWSZAY
PSWZAY
4.77
3.11
2.97
Temperature /
Fig. 6. H2-TPR profiles of PSZA catalysts before and after water-washing.
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Table 4
25
The textural data of PSZA sample before and after water-washing.
a
Sample
SBET (m2/g)
Pore diameter (nm)
Total volume (cm3/g)
20
15
10
5
PSZA
PWSZA
PSWAZ
118
94
116
5.59
5.59
5.58
0.152
0.136
0.158
PSZA
PWSZA
PSWZA
sample had no influence on the surface area, pore diameter and
pore volume. However, washing uncalcined sample, WSZA650, led
to an appreciable decrease in the surface area and pore volume. It
was inferred that after the calcination at high temperature, the rel-
atively stable textural structures of the catalyst were formed and
water-washing the calcined sample could not change these prop-
erties. On the contrast, water-washing sulfated Zr(OH)4 caused the
removal of partial sulfur species, and during calcination the sin-
tering phenomenon may occur due to the decrease in the sulfur
content, and thus, the textural structure of the resulting sample
would change to some extent.
0
0
50
100
150
200
250
300
350
Time on-stream (minute)
20
15
10
5
b
3.2.6. Effect on the catalytic activity
The effects of water-washing before and after calcination on
the isomerization performances of PSZA650 are shown in Fig. 7.
The n-hexane conversion over PSZA650 was similar to that over
PSWZA650, varied between 80% and 85%, depending on the time
on stream. It was much higher than the value of ca. 37% achieved
PSZAY
PWSZAY
PSWZAY
100
a
0
0
50
100
150
200
250
300
350
80
Time on-stream (minute)
PSZA
Fig. 8. Comparison in DMB yield over the catalysts before and after water-washing
(DMB – dimethyl butanes).
60
40
20
PWSZA
PSWZA
over PWSZA 650 (in Fig. 7). Similar results were achieved on the
PZAY650 sample.
The influence of water-washing on the isomers distribution was
similar to that on the isomerization activity. The yield of di-methyl
butanes (DMB) over PSZA and PSZAY was 15–20%, very close to
the value over PSWZA and PSWZAY; while the yield over PWSZA
and PWSZAY was extremely low, below 5%, as shown in Fig. 8. This
indicates that after calcination of sulfated Zr(OH)4, water-washing
treatment does not affect on the isomerization behavior of PSZA
and PSZAY. However, before calcination, the water-washing treat-
ment seriously deteriorates the isomerization performance of the
catalysts, by lowering the activity and altering the isomers distri-
bution.
0
50
100
150
200
250
300
350
Time on-stream (minute)
100
b
80
60
40
20
3.3. Discussion
PSZAY
PWSZAY
PSWZAY
The effect of calcination temperature of sulfated Zr(OH)4 or
sulfated Zr(OH)4 modified with Al on catalytic activity in hydroiso-
merization of n-alkane has been reported in previous investigation
[8–10]. However, comparison between the effect of calcination
temperature on the isomerization catalytic activity of PSZ, PSZA
and PSZAY is rarely reported up to now. The previous investigation
from our group showed that the optimized calcination temper-
ature for PSZA was 650 ◦C [10]. The present experiment results
also indicate that PSZ and PSZ modified with Al and rare earth
Y exhibited relatively higher catalytic activity when the calcina-
tion temperature was 650 ◦C. However, the further increase in
the temperature had greater effect on the catalytic activity of PSZ
0
50
100
150
200
250
300
350
Time on-stream (minute)
Fig. 7. Comparison in iso-hexanes yield over the catalysts before and after
water-washing (WHSV = 1.0 h−1, P = 2.0 MPa, n(H2)/n(n-C6) = 5:1, reaction temper-
ature = 165 ◦C).
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7
than that of PSZA and PSZAY. The catalytic activity of the latter
two was much higher than that of the former at higher calcina-
tion temperature. This indicated that the promoted PSZ possessed
better thermal stability. As evidenced by TG in Table 1, PSZ pos-
sessed lower sulfur content than the modified PSZ (i.e. PSAZ and
PSZAY). Especially PSZ750 had very lower sulfur content, which
may be the main factor leading to the very low catalytic activ-
ity.
4. Conclusions
The introduction of promoters (Al or Al and Y) greatly improved
the thermal stability of the Pt/SO42−/ZrO2 catalyst. High calcina-
tion temperature of the sulfated Zr(OH)4 was more favorable to
the isomerization reaction over PSZ modified with Al or Al and Y
in comparison with the unmodified PSZ. For the first time, it was
found that the resultant catalysts showed very different isomeri-
zation behaviors if the water-washing treatment was operated at
different preparation stages. When water-washing was done after
calcination of the sulfated Zr(OH), the PSZA or PSZAY exhibited high
catalytic activity. In contrast, if the water-washing was performed
before calcination of the sulfated Zr(OH)4, the catalytic activity of
the catalysts drastically decreased to approximately 50% of that
achieved on the catalysts with water-washing after calcination. It
was proposed that the presence of the excessive sulfur species over
the catalyst surface was indispensable to prepare PSZA or PSZ with
high catalytic activity.
The effect of water-washing of SZ on the n-alkane hydroiso-
merization activity has been reported by Li et al. [16] and they
found that the water-washing treatment for calcined sulfated
Zr(OH)4 led to an almost complete loss of isomerization activity.
In contrast to their results, in the present work, water-washing
treatment after calcination had no distinct effect on the catalytic
activity. The distinct discrepancy between the catalytic activity of
the water-washed PSWZA in the present investigation and that
of water-washed SZ in the report from Li et al. [16,20] may be
related with the addition of Pt. For SZ or SZA unpromoted with
Pt, the original acidity or/and re-dox played the key role in the
isomerization reaction. Water-washing led to a great decrease in
the strong Brønsted acid sites, as confirmed by Manoilova et al.
[20], which resulted in the significant reduction of catalytic activ-
ity. In the case of Pt-promoted SZ or SZA and in the presence of
hydrogen gas, Pt could dissociate hydrogen molecules into hydro-
gen atoms and hydrogen atoms spilt over SZ to create new strong
Brønsted acid sites [17]. These newly created acid centers may
act as the catalytic active centers. Therefore, even if partial sul-
fur species was removed during the water-washing, the newly
created active centers could still catalyze the isomerization reac-
tion.
Acknowledgements
This work is supported by Young Scientist Fund Project from
the National Natural Science Foundation of China (Grant No.:
21103049) and project of Instituto Politécnico Nacional, Mexico
(Grant No.: IPN-SIP-20120229).
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Please cite this article in press as: Y. Song, et al., Effects of calcination temperature and water-washing treatment on n-hexane hydroisomerization
behavior of Pt-promoted sulfated zirconia based catalysts, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.024