Al-promoted Pt/SO42-/ZrO2 with low sulfate content for n-heptane isomerization

Article abstract of DOI:10.1016/j.apcata.2010.06.010

To reduce the occurrence of cracking reactions and obtain high activity for n-heptane isomerization, we performed this study, aimed at improvements of the Al-promoted Pt/SO₄^2-/ZrO₂ (Pt/SZ) catalyst. The effect of sulfur content was studied and it was found that lowering sulfate content in the Al-promoted Pt/SZ resulted in remarkably enhanced selectivity towards iso-C₇ formation from 25% up to 83% compared with Pt/SZ without a loss of activity. The results of catalyst characterizations revealed that the tetragonal phase of ZrO₂ and its acidity were responsible for the higher activity, and that aluminum helped to stabilize the tetragonal phase in Al-promoted Pt/SZ and hence maintained catalytic activity at low sulfate content, while the low acidity and high Pt dispersion resulted in a high ratio of metal sites to acid sites and hence benefited a higher selectivity for iso-C₇.

Full text of DOI:10.1016/j.apcata.2010.06.010

Applied Catalysis A: General 384 (2010) 94–100
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Al-promoted Pt/SO42−/ZrO with low sulfate content for n-heptane isomerization
2
Ying-Chieh Yanga,b, Hung-Shan Wenga,∗
a
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Refining & Manufacturing Research Institute, CPC Corporation, Taiwan, Chia-Yi 600, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
To reduce the occurrence of cracking reactions and obtain high activity for n-heptane isomerization, we
performed this study, aimed at improvements of the Al-promoted Pt/SO4 /ZrO2 (Pt/SZ) catalyst. The
Received 16 January 2010
Received in revised form 4 June 2010
Accepted 7 June 2010
2−
effect of sulfur content was studied and it was found that lowering sulfate content in the Al-promoted
Pt/SZ resulted in remarkably enhanced selectivity towards iso-C7 formation from 25% up to 83% compared
with Pt/SZ without a loss of activity. The results of catalyst characterizations revealed that the tetragonal
phase of ZrO2 and its acidity were responsible for the higher activity, and that aluminum helped to
stabilize the tetragonal phase in Al-promoted Pt/SZ and hence maintained catalytic activity at low sulfate
content, while the low acidity and high Pt dispersion resulted in a high ratio of metal sites to acid sites
and hence benefited a higher selectivity for iso-C7.
Available online 12 June 2010
Keywords:
Sulfated zirconia
Al- and Pt-promoted
Sulfur content
n-Heptane isomerization
Selectivity
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Sulfated zirconia (SZ) and other sulfated metal oxides demon-
branched carbenium ions or isomer products from Pt/SZ is slower
than that on Pt/WZ, thereby leading to more cracking on Pt/SZ [7].
In addition, the acidity of Pt/SZ is much stronger than that of Pt/WZ
and other zeolite catalysts and hence causes more cracking prod-
ucts [8,9]. Giannetto and co-workers [10] reported that the cracking
reaction prevailed in n-heptane isomerization over PtH/zeolites if
strate strong acidity, which allows them to catalyze butane
isomerization at low temperatures [1]. Pt/SZ is a commercial cata-
lyst used in the isomerization process of C5–C paraffins, exhibiting
6
higher acidity than a zeolite-based catalyst. Thus its operating tem-
perature is lower than that of the zeolite catalyst [2], and the
process gives a higher RON of products because of the benefi-
cial thermodynamic effect on the equilibrium yield [3]. The Pt/SZ
catalyst also has a higher tolerance for water and sulfur con-
tent than Pt on chlorinated alumina [2]. However, it generates
significant cracking in n-heptane isomerization [4], limiting indus-
trial isomerization practice to C –C6 feeds [5]. Pt/WOx–ZrO , Pt or
the nPt/nA ratio (nPt, the available platinum atoms and n , the num-
A
ber of acid sites) was low. In other words, the higher the number of
acid sites, the higher the cracking rate will be, given a constant num-
ber of available platinum atoms. Although the optimum nPt/nA ratio
of Pt/SZ catalysts likely differs from that of PtH/zeolites, increasing
the nPt/nA ratio by reducing the number of acid sites could mitigate
the extent of cracking in n-heptane isomerization.
In SZ and Pt/SZ catalysts, total acidity and Brønsted-acid sites
decrease with decreasing sulfur content [11–13]. Föttinger et al.
[12] and Katada et al. [13] observed that Lewis-acid sites are pre-
dominant in low sulfur content SZ and that the monoclinic ZrO2
phase coexists with the tetragonal structure; hence SZ has low
activity or is inactive for n-alkane isomerization. F a˘ rca s¸ iu et al. [14]
studied the effect of sulfur content in SZ catalysts and found that
SZ with optimal sulfur content, ca. 3 wt%, contained the maximum
amount of sulfate near the surface and hence had high isomer-
4
2
Pd/zeolite and Pd-H SiW12O40/SiO2 [4] have good iso-C7 selectiv-
4
ity and lower extent of cracking reactions in n-C7 isomerization
compared with Pt/SZ. Therefore, research on n-C7 isomerization
reaction has focused on these kinds of catalysts. There have been
few reports on modifying the properties of Pt/SZ to reduce cracking
in n-C7 isomerization. Most studies have concentrated on skeletal
isomerization of C –C6 over the Pt/SZ catalyst and on the kinetics
4
of n-C7 isomerization [6].
◦
During the isomerization of n-heptane or higher alkanes,
cracking reactions occur mainly by secondary reactions of
branched carbenium ions and isomer products rather than by the
dimerization-cracking mechanism [5]. However the desorption of
ization activity for methylcyclopentane at 65 C. Laizet et al. [15]
reported that tetragonal zirconia with a sulfate density of ca. 2.5
−
2
SO4 groups nm was required for high activity and selectivity for
n-C6 isomerization. As mentioned above, optimal sulfur content is
necessary for the Pt/SZ catalyst to obtain high catalytic activity for
the isomerization reaction, and a Pt/SZ catalyst with low sulfur con-
tent has low activity or is even inactive. However, adding a small
amount of Al2O3 or Ga2O3 to SZ improves catalytic activity and sta-
∗ Corresponding author. Tel.: +886 6 2757575x62673; fax: +886 6 2344496.
E-mail address: hsweng@mail.ncku.edu.tw (H.-S. Weng).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.06.010

Y.-C. Yang, H.-S. Weng / Applied Catalysis A: General 384 (2010) 94–100
95
bility for n-C and n-C isomerization [16–18]. Adding Al O retards
a flow apparatus with an on-line automatic pH titrator to deter-
mine catalyst acidity. A sample of 0.1 g was first activated in air
4
6
2
3
the transformation of the metastable tetragonal zirconia phase to
the monoclinic phase and helps retain more sulfur content on the
catalyst surface [18,19].
◦
◦
at 400 C for 1 h and then reduced in hydrogen at 250 C for 1 h;
◦
◦
the system was then cooled under helium to 100 C. At 100 C,
NH3 was adsorbed until saturation. The sample was then heated
Some researchers intended to improve the activity of Al-
promoted Pt/SZ (Pt/SZA) catalyst for n-C6 isomerization by trying
various preparation methods with the aid of catalyst character-
ization. For instance, Guevara-Franco et al. [20] tried to find a
◦
◦
to 800 C at a heating rate of 7 C/min to desorb NH . The evolved
3
ammonia was trapped in a solution of boric acid and NH Cl, and
4
titrated by sulfamic acid with an on-line automatic pH titrator
[26]. Aluminum, platinum and sulfur contents were determined
by inductively coupled plasma atomic-emission spectrometry (ICP-
AES) using a PerkinElmer ICP 4300 instrument.
2−
proper Al O -to-ZrO ratio for mixed Al O -ZrO /SO
catalysts,
2
3
2
2
3
2
4
and Zhou et al. [21] searched for an optimal calcination temperature
for Pt/SZA catalyst. Volkova et al. [22] claimed that the rate of n-C6
isomerization was mainly determined by the density of Lewis-acid
X-ray diffraction (XRD) measurements were performed on a
Rigaku instrument using Cu K␣ radiation in an operating mode of
2−
sites in Pt/SO4 /Al O /ZrO . Yu et al. [23] reported that the Pt/SZA
2
3
2
◦
◦
catalyst prepared with a new Al-promoted method has a better
crystallization and Pt promotion effect and thus has a higher n-C6
isomerization activity. However, as above-mentioned, the catalyst
with high activity and selectivity for n-C6 isomerization usually has
the problem of pronounced cracking reactions when being used for
n-C7 isomerization [24,25] because their reaction mechanisms are
different.
In this study we developed a new Pt/SZ catalyst that reduces
cracking reactions and has high activity for n-heptane isomeriza-
tion. Pt/SZ catalysts with low sulfur contents were synthesized and
further promoted with Al to overcome the disadvantages of the
Pt/SZ catalysts described above. The catalytic activity and selec-
tivity of Al-promoted and un-promoted Pt/SZ were measured, and
the effects of catalyst characteristics, including acidity, Pt disper-
sion and the ratio of available platinum to acid sites, on activity and
selectivity were also examined.
40 kV and 30 mA. The range from 20 to 80 was scanned at a speed
of 4.0 /min.
◦
Chemisorption of H2 on all catalysts was measured with a
Micromeritics ASAP 2020 instrument. Before each measurement,
◦
the sample was degassed at 250 C for 6 h. In the measurement sta-
◦
tion, the sample was first reduced in flowing hydrogen at 250 C for
120 min. After the sample was evacuated, H2 chemisorption was
◦
measured at 35 C.
H2 temperature-programmed reduction (H -TPR) of different
2
catalysts was carried out using a Thermo TPD/R/O 1100 instrument
with a thermal conductivity detector (TCD). About 0.2 g of sample
◦
was pretreated in a flowing air at 400 C for 1 h before cooled down
to room temperature in N . Subsequently the sample was heated
2
◦
under a 20 cc/min flowing gas mixture of 10% H in N up to 700 C.
The heating rate was 10 C/min.
2
2
◦
Considering that a lower acidity might benefit the iso-C7 selec-
tivity, we used ammonium sulfate in the catalyst preparation as the
source of sulfate instead of sulfuric acid which is usually employed
by most of researchers for sulfating zirconia catalysts.
2
.3. Catalytic reaction
The n-heptane isomerization experiments were carried out
using a vertical stainless-steel tube reactor with an inner diameter
of 7.0 mm placed in an electric furnace. Prior to the isomerization
test, the catalyst (1.0 g) was pretreated in situ with air at atmo-
spheric pressure at 450 C for 3 h and then in hydrogen for 1.5 h at
250 C. The reaction conditions for n-C7 isomerization were set to a
2
. Experimental
◦
◦
2.1. Catalyst preparation
−
1
weight hourly space velocity (WHSV) of 2.32 h , a H /hydrocarbon
2
Ten grams of ZrOCl ·8H O (98+%, Sigma–Aldrich) and the
mole ratio of 6.1, a total pressure of 2.1 MPa and a temperature
ranging from 170 to 400 C. The stainless-steel lines between the
2
2
◦
required amount of Al(NO ) ·9H O (J.T. Baker) corresponding to
3
3
2
5
nominal mol% Al O were dissolved in 100 mL of distilled water.
reactor exit and the gas chromatograph were covered with heating
tapes to maintain a gaseous state of the reaction product during
transfer. The products were analyzed on-line using a gas chro-
matograph (Agilent Technologies 6850 series II) equipped with
a flame-ionization detector and an HP-1 column (40 m × 0.1 mm,
0.2 ␮m).
2
3
Ammonium hydroxide (NH3 (aq.), 25%) was added dropwise with
stirring to a final solution pH of 9. The precipitate of Al-containing
zirconium hydroxide was washed with distilled water to complete
◦
chlorine elimination and was dried at 160 C for 16 h. Subse-
quently, the dry powder was impregnated with an (NH ) SO
4
2
4
solution (not H SO4 solution) by the incipient-wetness impregna-
2
2−
tion method to obtain 0–15.0 wt% sulfate content (SO4 ) (based
on the dry powder weight). The impregnated sample was then
3
3
3
. Results and discussion
◦
dried at 100 C overnight and further ground to a fine powder and
.1. Catalyst characterization
◦
calcined at 650 C for 5 h in static air to obtain the Al-containing
sulfated zirconia, designed xSZA, where x represents the corre-
sponding impregnated sulfate content (wt%). The xSZ catalyst was
.1.1. BET surface area and pore-size distribution
Table 1 shows the characterization results with various sulfate
prepared in the same way as xSZA starting from the ZrOCl ·8H O
2
2
2−
(SO4 ) contents in the Pt/SZ and Pt/SZA catalysts. BET surface
precursor.
area increased with increased amounts of sulfate impregnated in
the Pt/SZ, in agreement with the literature [13]. The increased
surface area was due to crystal-structure alteration [14]. The rela-
tion between crystallite size and sulfate content in Pt/SZ or Pt/SZA
will be discussed in the following section on XRD measurement.
Fig. 1(a) shows the pore-size distribution analyses on Pt/SZ. This
work observed a unimodal BJH pore-size distribution for Pt/0.0SZ,
which does not contain sulfur and has the largest pore diameter
among the Pt/SZ catalysts. However, when sulfur was added, the
current study observed bimodal pore-size distributions when the
impregnated sulfate was lower than 3 wt%. Impregnating a larger
Pt/xSZA and Pt/xSZ catalysts were prepared by incipient-
wetness impregnation of xSZA and xSZ with an aqueous solution
◦
of H PtCl , followed by drying at 100 C and calcination in static air
2
6
◦
at 500 C for 3 h. The Pt content for all catalysts was 0.3 wt%.
2.2. Characterization
The surface areas and pore-size distributions of the catalysts
◦
were determined by nitrogen desorption at −196 C with a Mir-
comeritics TriStar 3000 instrument. NH -TPD was performed in
3

9
6
Y.-C. Yang, H.-S. Weng / Applied Catalysis A: General 384 (2010) 94–100
Fig. 1. BJH pore-size distributions of Pt/SZ (a) and Pt/SZA (b) catalysts.
amount of sulfate obtained unimodal pore-size distributions again.
These phenomena are explained as follows:
that on the Pt/SZ catalysts when the calcination temperature was
650 C.
◦
Adding sulfate resulted in the formation of Zr(SO4)2 with small
pores. Aside from being a structural promoter, the sulfate group
The total acidity of both Pt/SZ and Al-promoted Pt/SZ increased
with increasing sulfate content. The acidity of Al-promoted Pt/SZ
was lower than that of Pt/SZ at similar sulfate content after calci-
nation. Al- or Ga-promoted SZ catalysts have higher acidity than
that of SZ [11,28]; however, in a study on the acid properties of
sulfated zirconia catalysts by 31P MAS NMR, Chen et al. [30] found
that the total acidity of Al/SZ was lower than that of SZ catalysts.
Zalewski et al. [11] observed that the number of acid sites with
intermediate strength in Pt/SZ could be enhanced by adding alu-
minum. Wang and Mou [18] further showed that these acid sites
were weak Brønsted-acid sites and could improve catalytic activity.
is also a textural promoter for ZrO ; ZrO2 crystallite sintering can
2
be suppressed by sulfate groups [27]. Increased sulfate content
increases the amount of small pores formed by Zr(SO ) . The corre-
4
2
sponding peak (the peak at 3.5 nm) became larger, while the large
pores formed by the ZrO2 crystallite shrank due to sulfate-group
suppression, causing the corresponding major pore peak to shift
to a smaller size. As the sulfate content continued to increase, the
major pore peak covered the small peak and the pore-size distri-
bution became unimodal.
The BET surface areas of all the Pt/SZ catalysts increased after
being promoted with aluminum [28] and the surface area of Pt/SZA
displayed the same trend as Pt/SZ for different sulfate contents.
Fig. 1(b) shows the unimodal pore-size distributions for all the
Pt/SZA catalysts, with a major peak at 3–4 nm. The formation of
Zr–O–Al bonds over the Al/SZ [29] probably helped stabilize sulfate
species at the oxide surface.
3
.1.3. XRD measurement
Fig. 2 displays the XRD patterns of catalysts. Pt/15SZ (i.e., with
5 wt% of the impregnating sulfate) primarily showed tetragonal
1
ZrO2 reflections. The tetragonal phase decreased with decreasing
sulfate content, and was eliminated at low sulfate content, leav-
ing only the monoclinic phase. The crystal-phase types of sulfated
zirconia depend on many preparation conditions, including aging
time for the precursor [31], calcination temperature and sulfate
concentration [11,32]. Mercera et al. [33] proposed that the ZrO2
phase is affected by crystallite size. Crystallites smaller than the
critical size result in tetragonal zirconia; however, transformation
from tetragonal to monoclinic crystallites occurs as crystallite size
becomes larger than the critical size. They suggested that the criti-
3.1.2. Sulfate content and acidity measurement
The percentages of sulfate retained in Pt/SZ and Pt/SZA after
calcination are presented in Table 1. Calcination eliminates a frac-
tion of sulfate species in the Pt/SZ, so the actual sulfate content
is lower after calcination than the nominal loading amount in
impregnation [13,15]. Observations have shown a high extent of
sulfate elimination for a high content of impregnated sulfate. How-
ever, in this study, more sulfate was retained after Pt/SZ was
promoted by aluminum. Table 1 shows that the sulfate contents
of the Pt/SZA catalysts after calcination were higher than those of
◦
cal crystallite size after calcination at 650 C is within the range of
9
0–123 Å. The presence of sulfate enhances the stability of both the
surface area and the tetragonal phase due to a retarded crystalliza-
tion process [32]. Regardless of the amount of sulfate content, all
Al-promoted Pt/SZ catalysts show a pure tetragonal phase. Many
previous studies have reported that adding alumina increased the
tetragonal ratio [18,28]. Alumina may help prevent large crystal-
lites from forming, stabilizing the metastable tetragonal phase [11].
◦
the Pt/SZ catalysts. Most Zr(SO4)2 decomposes around 650–700 C
[
19]; however, the decomposition of Al-promoted sulfated zir-
◦
conia occurred at relatively higher temperatures, ca. 800 C. The
amount of sulfur retained on the Pt/SZA catalysts was higher than
Table 1
Physical and surface properties of Pt/SZ and Pt/SZA catalysts.
SO42 (wt%)
−
Pt (wt%)
Al (wt%)
BET area (m /g)
2
Total aciditya
␮mol/gcat.)
Pt dispersionb (%)
Sulfur concentration
(atoms/nm )
nPt/nA ratioc
Sample
2
(
Pt/1.5SZ
Pt/3.0SZ
Pt/9.0SZ
Pt/15.0SZ
Pt/1.5SZA
Pt/3.0SZA
Pt/9.0SZA
1.71
2.58
3.18
3.90
1.62
3.21
4.32
0.31
0.32
0.29
0.28
0.32
0.30
0.28
–
–
–
–
2.50
2.42
2.43
53.4
74.8
84.2
94.9
74.4
80.5
97.3
141.6
177.7
207.2
226.0
126.4
171.9
210.3
33.8
20.5
15.2
13.6
55.4
29.0
5.9
2.0
2.2
2.4
2.6
1.4
2.5
2.8
0.038
0.019
0.011
0.009
0.072
0.026
0.004
a
Total acidity measured by TPD of NH3.
From H2 chemisorption measurement.
nPt: the number of accessible Pt atoms calculated from the Pt content and dispersion. nA: the number of acid sites.
b
c

Y.-C. Yang, H.-S. Weng / Applied Catalysis A: General 384 (2010) 94–100
97
Fig. 4. The n-heptane conversion vs. the reaction temperature over different Pt/SZ
and Pt/SZA catalysts.
atoms were wrapped around the support, and that metallic Pt par-
ticles exhibited mainly Pt–O interactions. Yori et al. [34] reported
that the Pt did not have metallic properties on Pt/SZ due to a strong
interaction with the support. Those different situations or Pt states
may hinder hydrogen or CO chemisorption [5,35] and may result
in lower Pt dispersion. Based on the above facts, we conclude that
lowering the sulfate content reduces the interaction between Pt
and sulfate, increasing Pt dispersion. Adding aluminum to a cata-
lyst with low sulfate content may prevent platinum from sintering
due to interaction between Pt and the aluminum-added catalyst as
explained in the above, so that the Al-promoted Pt/SZ would have
a higher Pt dispersion.
Fig. 2. XRD patterns of Pt/SZ and Pt/SZA catalysts with different impregnated sulfate
contents.
3.1.4. Platinum-dispersion measurement
The amounts of sulfate and aluminum added affect the extent
of Pt dispersion on Pt/SZ. Table 1 shows that the Pt dispersion
increased with decreasing sulfate content. Moreover, adding alu-
minum to Pt/SZ with low sulfate content further improved Pt
dispersion, so the Pt/1.5SZA catalyst had the highest Pt disper-
sion among the Pt/SZ and Pt/SZA catalysts. The improvement on
Table 1 also provides the values of the nPt/nA ratio (metal/acidity
ratio, defined as the ratio of the number of accessible Pt atoms cal-
culated from the Pt content and dispersion to that of the number
of acid sites) of the Pt/SZ and Pt/SZA catalysts. For both the Pt/SZ
and Pt/SZA catalysts, those with lower sulfur contents had a larger
nPt/nA ratio because the catalyst had a higher Pt dispersion and
lower acidity. Moreover, the Pt/SZA with the lowest sulfate con-
tent had the highest nPt/nA ratio among all the Pt/SZ and Pt/SZA
catalysts.
Pt dispersion by adding aluminum is confirmed by H -TPR patterns
2
shown in Fig. 3 which reveal the obvious shift of the reduction peak
due to the interaction between Pt and the aluminum-added cata-
lyst. Most studies to date have obtained low metal dispersion, less
than 5%, over Pt/SZ catalysts [5,34,35]. However, in this work we
obtained much higher Pt dispersion; the maximum Pt dispersion
was 55%, probably due to the lower sulfate contents impregnated
or the use of different preparation methods and procedures than in
other studies [5,34,35].
Ivanov et al. [36] studied Pt/SZ and Pt/ZrO2 catalysts and indi-
cated that the presence of sulfate ions on the Pt/SZ catalyst
suppresses Pt particle reduction. Manoli et al. [37] found that Pt
3.2. Reaction tests
3.2.1. Catalytic performance of Pt/SZ and Pt/SZA catalysts
Fig. 4 depicts the n-heptane (n-C7) conversion vs. the reaction
temperature on Pt/SZ catalysts loaded with different sulfate con-
tents. When the impregnated sulfate was lower than 3.0 wt%, the
maximum n-C7 conversion was only ca. 55%, even at reaction tem-
◦
peratures higher than 390 C. The n-C7 conversion declined with
increasing time due to sulfate loss resulting from reacting with
◦
hydrogen when the reaction temperature was higher than 300 C
[
38]. The higher the reaction temperature was, the more obviously
this phenomenon was observed.
Compared with Pt/SZ, the n-C7 isomerization conversion on
Pt/SZA was much higher than that on Pt/SZ with the same sulfur
content. The temperature required to achieve a given n-C7 con-
◦
version on Pt/SZA was at least 60 C lower than that on Pt/SZ at
the same impregnated sulfate content. Catalytic activity was much
improved as the impregnated sulfate content increased from 1.5
to 3.0 wt%; the temperature for achieving a given n-C7 conversion
◦
was reduced by ca. 55 C, while the temperature was reduced ca.
◦
1
0 C when the impregnated sulfate content increased from 3.0 to
.0 wt%.
9
The relation between reaction temperature required for attain-
ing a preset conversion and catalyst acidity shown in Fig. 5(a) is
Fig. 3. H2-TPR profiles of Pt/1.5SZ and Pt/1.5SZA catalysts.

9
8
Y.-C. Yang, H.-S. Weng / Applied Catalysis A: General 384 (2010) 94–100
Fig. 5. Temperature required for attaining preset conversion vs. acidity (a) and Pt dispersion (b).
plotted by using the data listed in Table 1 and Fig. 4. For either Pt/SZ
or Pt/SZA catalyst, the one with a higher acidity has a higher cat-
alytic activity for n-C7 isomerization, so the reaction temperature
required for attaining the preset conversion is lower.
Besides acidity, the activities of Pt/SZ and Pt/SZA catalysts are
also affected by Pt content [39] and the type of crystalline phase
3.2.2. Isomerization selectivity
Fig. 6 displays the iso-C7 selectivities in the n-C7 conversion over
Pt/SZ and Pt/SZA catalysts. Iso-C7 selectivity declined rapidly with
increasing conversion over the Pt/SZ. Although Pt/1.5SZ provided
the highest iso-C7 selectivity among the various Pt/SZ catalysts, its
isomerization activity was very low. The temperature needed for
◦
of ZrO , tetragonal and monoclinic phases. As above-mentioned,
55% conversion approached 380 C, and conversion did not further
2
the tetragonal phase of ZrO2 on the Pt/SZ or Pt/SZA catalysts ben-
efits catalytic activity. Moreno and Poncelet [28] showed that n-C4
isomerization conversion increased proportionally to the fraction
of the tetragonal phase in SZ catalysts. As we can see from the
XRD patterns, no tetragonal phase was formed in Pt/1.5SZ and
Pt/3.0SZ, while only tetragonal phase existed in all Pt/SZA catalyst;
therefore, the relation between the catalytic activity and the tetrag-
onal/monoclinic ratio cannot be established. However, the catalytic
activities of all Pt/SZA catalysts were much higher than that of Pt/SZ
catalysts at the same acidity. The catalytic activity of the Pt/9.0SZ
was also higher than other Pt/SZ catalysts under the same conver-
sion. Because all Pt/SZA catalysts have only tetragonal phase, their
catalytic activities are not affected by the tetragonal/monoclinic
ratio, but mainly by acidity.
increase at higher temperature.
Fig. 6 also shows that the iso-C7 selectivity greatly improved
over Pt/SZA compared with Pt/SZ, especially for Pt/1.5SZA and
Pt/3.0SZA. Although iso-C7 selectivity on Pt/3.0SZA was a little
lower than that on Pt/1.5SZA, the catalytic activity of the former
was much higher than that of the latter (Fig. 4). The iso-C7 selectiv-
ity on Pt/9.0SZA was much lower than that on Pt/3.0SZA. The iso-C7
selectivities on both Pt/1.5SZA and Pt/3.0SZA were higher than 83%
at 70% n-C7 conversion, while iso-C7 selectivity on Pt/9.0SZA was
lower than 50% and that on Pt/9.0SZ was only 25%.
Fig. 7, plotted by combining the data of Table 1 and Fig. 6, shows
the relation between isomerization selectivity and nPt/nA ratio with
conversion level as a parameter for Pt/SZ and Pt/SZA. The iso-C7
selectivity increases gently and almost linearly with the nPt/nA ratio
for Pt/SZ, while it increases sharply at low nPt/nA ratio but becomes
level when nPt/nA ratio > 0.026 for Pt/SZA. These results indicate
that both the number of accessible Pt atoms and the number of
acid sites affect the iso-C7 selectivity for Pt/SZ catalysts, while the
accessible Pt atoms play an important role when the nPt/nA ratio is
small but do not much affect the selectivity when the nPt/nA ratio
is large. de Lucas et al. [41] also proposed that the isomerization
reaction over the acid sites is the limiting step of n-C8 transforma-
tion, because the iso-C8 yield does not depend on the number of
available platinum atoms when the nPt/nA ratio is large.
Some researchers reported that the activity of Pt/SZ or Pt/SZA
for n-C6 isomerization is mainly determined by the Lewis acidity
[
22,40], Zhou et al. [21] further claimed that the strong Lewis-acid
sites is major factor affecting on the activity. In the present study,
the Brönsted acidities of Pt/1.5SZA, Pt/3.0SZA and Pt/9.0SZA were
quite different (22, 42 and 73 ␮mol/g, respectively) with Lewis
acidities being nearly the same (89, 86and 84 ␮mol/g, respectively),
therefore we can conclude that the activity of Pt/SZA catalyst for
n-C7 isomerization was mainly influenced by Brönsted acidity.
Fig. 5(b), which is plotted by using the data of Table 1 and Fig. 4,
shows the reaction temperatures required for attaining preset con-
version increases with increasing Pt dispersion. In other words,
the catalytic activity of Pt/SZ or Pt/SZA catalyst decreases with Pt
dispersion.
The previous studies proposed that the Pt on Pt/SZ or Pt/SZA
+
catalyst can activate the dissociation of H2 on Pt producing H and
−
+
H
[37] which then migrate to the SZ by spillover. The H , which
displays a catalytic role, can improve catalytic activity by increas-
ing the hydride transfer ability in n-C7 isomerization [5]. Therefore,
increasing the Pt dispersion might benefit the catalytic activity. The
contradictory results might be attributed to the fact that the cat-
alytic activity of n-C7 isomerization is affected by several factors.
Pt function is only one of them.
If the hydride transfer is the limiting step of n-C7 isomeriza-
tion, then Pt function will play important role and increasing the Pt
dispersion will improve the catalytic activity. However our result
indicates that this is not true. Therefore the surface reaction on acid
sites might be the limiting step.
Fig. 6. Iso-C7 selectivity over different Pt/SZ and Pt/SZA catalysts.

Y.-C. Yang, H.-S. Weng / Applied Catalysis A: General 384 (2010) 94–100
99
Fig. 7. Iso-C7 selectivity at various conversion levels vs. nPt/nA.
Pt/SZ catalysts with normal sulfate content, i.e., higher than
.0 wt%, have high catalytic activity for n-C7 isomerization, but the
forms the linear alkane into mono-branched isomers; then part of
the isomers are further transformed into multi-branched ones; and
some of these will decompose to cracking products. de Lucas et al.
[41] studied the effect of metal loading in the hydro-isomerization
of n-octane over ␤-zeolite catalysts, and observed that the I/C and
mono/multi isomer ratios both increased with the metal/acidity
ratio. The Pt/SZ catalysts with different sulfate contents yielded
similar results, but Pt/SZ and Pt/SZA with similar metal/acidity
ratios had greatly differing I/C and mono/multi ratios due to their
different physical and chemical properties.
According to the bifunctional mechanism, when the value of
nPt/nA is very low, the metallic function is not sufficiently active
to hydrogenate iso-olefin intermediates before they encounter
acid sites. Therefore, the amount of multi-branched isomers is
higher than that of mono-branched isomers and cracking reac-
tions prevail and form a significant amount of coke, causing rapid
deactivation [41]. When the Pt/SZA catalyst has an intermediate
3
significant rate of cracking reactions limits their application. The
Pt/SZ with low sulfate content could reduce cracking due to its
high nPt/nA ratio (high Pt dispersion and low acidity) and large pore
size. Unfortunately, its activity was very low due to the elimina-
tion of the tetragonal phase [11,31] and low acidity. Compensating
for a low catalytic activity requires a higher reaction temperature,
but the sulfate group is lost because of reaction with hydrogen at
high temperature and this loss causes a reduction in catalytic activ-
ity [38]. However, the Al-promoted Pt/SZ with low sulfate content
not only had a high iso-C7 selectivity but also retained a high cat-
alytic activity due to having a 100% tetragonal phase even with
the lowest sulfate content, as shown in Fig. 2. In conclusion, alu-
minum can stabilize the tetragonal phase in Al-promoted Pt/SZ
catalysts and help maintain high catalytic conversion at low sulfate
content.
value of nPt/n , olefinic intermediates generated on hydrogenat-
A
ing sites (Pt atoms) contact fewer acid sites and mainly undergo
skeletal rearrangement and then hydrogenation, giving high I/C
and mono/multi ratios. Guisnet et al. [10] studied the effect
of nPt/nA ratios in different zeolite catalysts for n-heptane iso-
merization and observed that different zeolites had different I/C
and mono/multi ratios even at the same nPt/nA ratio. In this
work, the nPt/nA ratio was changed by changing sulfur content
at constant Pt loading, thereby both Pt dispersion and acid-
site concentration were different for different Pt/SZA catalysts,
3
.2.3. The ratio of isomers to cracking products and mono- to
multi-branched isomers
Figs. 8 and 9 provide the formation ratios of isomers to cracking
products (I/C) and mono- to multi-branched isomers (mono/multi)
on Pt/SZA catalysts. The I/C and mono/multi ratios over all the
Pt/SZA catalysts decreased with increasing conversion. The Pt/SZA
with lower sulfur content had both high I/C and mono/multi ratios,
however, the Pt/SZA with high sulfur content gave the opposite
results. These results showed that n-C7 isomerization first trans-
Fig. 8. Isomerization/cracking ratio over Pt/SZA catalysts.
Fig. 9. Mono-branch/multi-branch ratio over Pt/SZA catalysts.

1
00
Y.-C. Yang, H.-S. Weng / Applied Catalysis A: General 384 (2010) 94–100
and the trend of activity related to nPt/nA ratio was not the
same.
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2
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