Selective hydroisomerization of n-dodecane over platinum supported on zeolites

Article abstract of DOI:10.1007/s11164-011-0386-8

In this study, in order to develop catalysts for the selective isomerization of higher paraffin, the hydroisomerization reaction of n-dodecane was performed as a model reaction. Pt/ZSM-48, Pt/HZSM-5, Pt/HY, and Pt/SAPO-11 were examined for the selective hydroisomerization of n-dodecane. The catalysts were characterized via X-ray powder diffraction, N₂ adsorption, and the temperature-programmed desorption of ammonia. Among the catalysts studied, the Pt/HZSM-48 catalyst exhibited the best isomerization selectivity in the hydroisomerization reaction of n-dodecane, which is attributed to the moderate acid sites and medium-sized pores present in the HZSM-48. The highest iso-dodecane yield was obtained at a reaction temperature of 280 °C in the Pt/HZSM-48 catalyst. The optimal selectivity of the n-dodecane hydroisomerization over the Pt/SAPO-11 catalyst was obtained at approximately 300 °C, which was slightly higher than that of the Pt/HZSM-48 catalyst.

Full text of DOI:10.1007/s11164-011-0386-8

Res Chem Intermed (2011) 37:1215–1223
DOI 10.1007/s11164-011-0386-8
Selective hydroisomerization of n-dodecane
over platinum supported on zeolites
Soyoung Yun
Soon-Yong Jeong
Jong-Ki Jeon
•
Eunok Lee
•
•
Young-Kwon Park
•
•
Jeongsik Han
Byunghun Jeong
•
Received: 31 May 2011 / Accepted: 28 July 2011 / Published online: 15 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract In this study, in order to develop catalysts for the selective isomerization
of higher paraffin, the hydroisomerization reaction of n-dodecane was performed as
a model reaction. Pt/ZSM-48, Pt/HZSM-5, Pt/HY, and Pt/SAPO-11 were examined
for the selective hydroisomerization of n-dodecane. The catalysts were character-
ized via X-ray powder diffraction, N adsorption, and the temperature-programmed
2
desorption of ammonia. Among the catalysts studied, the Pt/HZSM-48 catalyst
exhibited the best isomerization selectivity in the hydroisomerization reaction of
n-dodecane, which is attributed to the moderate acid sites and medium-sized pores
present in the HZSM-48. The highest iso-dodecane yield was obtained at a reaction
temperature of 280 °C in the Pt/HZSM-48 catalyst. The optimal selectivity of the
n-dodecane hydroisomerization over the Pt/SAPO-11 catalyst was obtained at
approximately 300 °C, which was slightly higher than that of the Pt/HZSM-48
catalyst.
Keywords Hydroisomerization ꢀ Dodecane ꢀ Pt ꢀ HZSM-48 ꢀ SAPO-11
S. Yun ꢀ E. Lee ꢀ J.-K. Jeon (&)
Department of Chemical Engineering, Kongju National University, Cheonan 331-717, Korea
e-mail: jkjeon@kongju.ac.kr
Y.-K. Park
School of Environmental Engineering, University of Seoul, Seoul 130-743, Korea
S.-Y. Jeong
Green Chemistry Research Division, Korea Research Institute of Chemical Technology,
Daejeon 305-600, Korea
J. Han ꢀ B. Jeong
Agency for Defense Development, Yuseong, Daejeon 305-152, Korea
123

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S. Yun et al.
Introduction
The Fischer-Tropsch (F-T) synthesis is a reaction that introduces a synthesis gas
CO/H ) into a catalyst-charged reactor and generates a higher hydrocarbon [1, 2].
(
2
The higher hydrocarbon generated from the F-T synthesis is primarily wax, and has
a low pour point. In order to improve the cold-flow characteristics (viscosity, pour
point, and freezing point) of the F-T synthetic oil, a higher n-paraffin in the F-T
oil should be converted to a higher iso-paraffin through hydroisomerization.
The hydroisomerization reaction of n-paraffin is accompanied by a hydrocracking
reaction and increases the yield of low molecular products. While multi-branched
alkane is more prone to cracking, mono-branched paraffin is not. Thus, it is clear
that multi-branching should be limited in order to reduce the cracking reactions.
The hydroisomerization reaction usually occurs in the dual functional catalysts,
in which the hydrogenation/dehydrogenation and skeletal isomerization through
carbenium ions occur at a metal active component and acid site, respectively. There
has been research reported on the acid supports used in hydroisomerization catalysts
as amorphous oxides and oxide compounds (HF-processed Al O , silica-alumina,
2
3
2
-
and ZrO /SO ), zeolite (Y, beta, modernite, ZSM-5, and ZSM-22), and
2
4
mesoporous materials (MCM-41, AlMCM-41) [3–14]. Noble metals (Pd and Pt)
and dual-metal systems (Ni/Co, Ni/W, Ni/Mo, and W/Mo in its sulfided form) have
been used in the hydrogenation/dehydrogenation [3, 5, 10–12, 14].
In this study, platinum catalysts impregnated on different types of zeolites were
synthesized, and the hydroisomerization reaction over the catalysts was investigated
using n-dodecane as a model reactant. The effects of the pore size and acid
characteristics of the zeolite on the catalytic performance in the n-dodecane
hydroisomerization were analyzed. The effects of the impregnated amount of Pt on
the acid characteristics and n-dodecane hydroisomerization over the SAPO-11 were
also investigated.
Experimental
HZSM-5 and HY were purchased from Zeolyst. SAPO-11 was purchased from
Tianjin Chemist Scientific Ltd. Pt/zeolite catalysts were obtained via the impreg-
nation of the zeolite with an aqueous H PtCl solution. After the impregnation,
2
6
the materials were dried at 120 °C for 4 h, and then calcined at 480 °C for 4 h.
Pt/HZSM-48 catalyst was provided from Korea Research Institute of Chemical
Technology. Prior to the reaction, the catalysts were reduced in situ in hydrogen at
4
00 °C for 4 h.
The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku
˚
D/max-2500 X-ray diffractometer fitted with Cu Ka radiation (k = 1.5404 A). The
BET surface area was determined using a Micromeritics ASAP-2010 physical
analyzer using N adsorption at 196 °C. The NH –TPD was performed in helium
2
3
(
40 mL/min), and the ammonia was adsorbed at 100 °C [15]. The desorption
process was monitored with a TCD at a temperature ramp from 100 to 600 °C with a
1
23

Selective hydroisomerization of n-dodecane
1217
heating rate of 20 °C/min. The distribution of the acid sites on the zeolites was
deconvolved by simulating the TPD plots through Gaussian curves.
The hydroisomerization of the n-dodecane as a model reaction was undertaken at
atmospheric pressure in a continuous-flow, fixed-bed stainless-steel reactor with an
-
H /n-C molar ratio of 15 and WHSV of 2 h . The amount of catalyst was 0.24 g.
1
2
12
The products were analyzed using an online gas chromatograph (YL 6100 GC)
equipped with FID and a HP-5 (capillary 30 m 9 0.32 mm).
Results and discussion
Pt/zeolite catalysts
Figure 1 shows the NH -TPD results of the zeolite supports and the 0.5 wt%
3
platinum catalysts impregnated on HY, HZSM-5, and HZSM-48. Peaks were
observed in the NH -TPD of HZSM-5 catalysts at 250 and 430 °C, which
3
correspond to the weak and strong acid sites, respectively. The NH -TPD pattern
3
of the Pt/HZSM-5 catalyst was similar to that of the HZSM-5 catalyst with only
a difference in terms of the reduced amount of the acid sites. There were two peaks
in the NH -TPD of HY catalysts at 230 and 350 °C, which correspond to the weak
3
and medium strength acid sites, respectively [16]. The amount of acid sites slightly
decreased in the Pt/HY catalysts when compared with the HY catalysts. There were
two peaks in the NH -TPD of Pt/HZSM-48 catalyst at 220 and 360 °C, which
3
is similar to the NH -TPD of Pt/HY catalyst. There were fewer acid sites of the
3
Pt/HZSM-48 catalyst than in the Pt/HZSM-5 and Pt/HY catalysts. The acid site
characteristics of the Pt/HZSM-48 catalyst, however, were more developed at the
medium strength acid site of 350 °C than at the weak acid site of 220 °C.
Figure 2 shows the n-dodecane conversion, selectivity of the iso-dodecane,
and yield of the iso-dodecane in the n-dodecane hydroisomerization reaction under
8
6
4
2
00
00
00
00
0
HY
HZSM-5
Pt/HY
Pt/HZSM-5
Pt/HZSM-48
100
200
300
400
500
600
700
°
Temperature( C)
3
Fig. 1 NH -TPD curves of the catalysts
123

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S. Yun et al.
1
00
Pt/HZSM-48
Pt/HZSM-5
Pt/HY(12)
Pt/HY(60)
8
6
4
2
0
0
0
0
0
(a)
1
00
80
60
40
20
0
(b)
1
00
8
6
4
2
0
0
0
0
(
c)
0
1
00
150
200
250
300
350
400
Temperature(°C)
Fig. 2 a Conversion of n-dodecane, b selectivity to iso-dodecane, and c yield of iso-dodecane over the
-1
2
catalysts (reaction condition: H /n-dodecane = 2, reaction pressure 1 atm, and WHSV 40 h )
conditions of a H /n-dodecane of 2, a reaction pressure of 1 atm, and a WHSV of
2
-
0 h . The n-dodecane conversion increased in all catalyst systems according to
1
4
the rising temperature regardless of the zeolite carriers. The n-dodecane conversion
occurred in the order of Pt/HZSM-48 [ Pt/HY(12) [ Pt/HY(60) [ Pt/HZSM-5.
The findings of the effects of the reaction temperature on the selectivity to iso-
1
23

Selective hydroisomerization of n-dodecane
Table 1 Effect of platinum
1219
2
3
S
BET (m /g)
V
total (cm /g)
2
loading on the N adsorption
analysis results of the samples
SAPO-11
121
82
0.15
0.13
0.13
0.11
SAPO-11 Pt 0.2 (wt%)
SAPO-11 Pt 0.5 (wt%)
SAPO-11 Pt 0.8 (wt%)
92
43
SAPO-11 Pt 0.8 wt%
SAPO-11 Pt 0.5 wt%
SAPO-11 Pt 0.2 wt%
SAPO 11
10
20
30
40
50
60
70
80
2θ
Fig. 3 Effect of platinum loading on the XRD of the SAPO-11
dodecane indicate that the Pt/HZSM-48 and Pt/HY catalysts had a temperature
range within the optimal selectivity. In particular, the Pt/HZSM-48 catalysts
recorded a very high selectivity of 80% or more in the range of 230–280 °C.
As a result, the highest yield of the iso-dodecane (60 wt%) was obtained with the
Pt/HZSM-48 catalyst at 280 °C.
Efficient catalysts for n-dodecane hydroisomerization are only possible through
the minimization of the cracking reactions. A cracking reaction lowers the
selectivity of the iso-dodecane that decomposes the n-dodecane into light products.
Selectivity in the iso-dodecane usually depends on the balance between the metal
and acidity of the catalyst: the appropriate balance plays a decisive role in
determining the reactivity and selectivity of dual functional catalysts [3]. The
catalysts of high hydrogenation ability and low acidity can be optimal for maximum
hydroisomerization reactions. Because it contains strong acid sites, HZSM-5 is
more prone to cracking reactions than the isomerization reaction of n-dodecane.
The HZSM-48 catalyst has fewer acid sites than other zeolites and contains only
weak and medium strength acid sites without strong acid sites, thus being rarely
subject to cracking reactions when compared with other zeolites. Although the
Pt/HY catalysts have similar acid site strengths to those of the Pt/HZSM-48 catalyst,
the iso-dodecane selectivity was much lower than that of the Pt/HZSM-48 catalyst.
123

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S. Yun et al.
SAPO-11 Pt 0.8wt%
SAPO-11 Pt 0.5wt%
SAPO-11 Pt 0.2wt%
SAPO-11
1
00
200
300
Temperature( C)
400
500
°
Fig. 4 Effect of the platinum loading on the NH
3
-TPD of the SAPO-11
The Y zeolite pores, which are formed via a 12-membered ring, have a relatively
˚
˚
large diameter of 7.4 A. The inner cavity has a diameter of 12 A. The ZSM-48
zeolite has a ten-membered ring channels with a one-dimensional pore system and
˚
medium pore size (5.3–5.8 A) [17]. The Pt/HY catalysts have large pore openings
and accordingly produce multi-branched iso-paraffin relatively easily, which is
highly prone to cracking. The pore opening of the Pt/HZSM-48 catalysts is
sufficiently small compared to that of the Pt/HY catalysts, which means that the Pt/
HZSM-48 catalysts deter large iso-paraffins from reacting at acid sites inside the
pores and have superior selectivity in the n-dodecane conversion compared to the
Pt/HY catalysts. Consequently, the Pt/HZSM-48 catalysts have weak acid sites and
small pore sizes, thus being effective for producing iso-dodecane in hydroisomer-
ization reactions.
Pt/SAPO-11 catalysts
Table 1 presents the effect of the platinum loading on the N adsorption analysis
2
results of the SAPO-11, and it can be seen that the BET surface area of the SAPO-
11 decreased with increasing amounts of platinum. The effects of the platinum
loading on the XRD of the SAPO-11 samples are shown in Fig. 3. There are no
1
23

Selective hydroisomerization of n-dodecane
1221
1
00
Pt 0.2wt%
Pt 0.5wt%
Pt 0.8wt%
Pure
8
6
4
2
0
0
0
0
0
(
a)
1
00
8
6
4
2
0
0
0
0
0
(
b)
1
00
8
6
4
2
0
0
0
0
0
(
c)
1
00
150
200
250
300
350
400
450
°
Temperature( C)
Fig. 5 Effect of the platinum loading on a the conversion of n-dodecane, b the selectivity to iso-
dodecane, and c the yield of iso-dodecane over the SAPO-11 (reaction conditions: H /n-dodecane = 2,
2
-
1
)
reaction pressure 1 atm, and WHSV 40 h
significant changes to the XRD patterns of the SAPO-11 until the Pt loading
increases to 0.8 wt%, which indicates that the SAPO-11 structure is not significantly
affected by the Pt loading.
In the results of the NH -TPD analysis shown in Fig. 4, two peaks at weak acid
3
site near 160 °C and a medium strength acid site near 240 °C can be observed over
123

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S. Yun et al.
the Pt/SAPO-11 catalysts. While increasing the Pt loading had little impact on the
peak near 160 °C, the maximum temperature at the medium strength acid site
increased slightly to highest 280 °C for the Pt 0.8 wt%/SAPOI-11 catalyst. These
findings indicate that the increasing Pt loading causes a slight increase in the acid
strength of a medium strength acid site and does not significantly reduce the amount
of acid sites despite the apparently reduced BET surface area.
The n-dodecane conversion in hydroisomerization was very low over the SAPO-
1, as shown in Fig. 5. With an increasing Pt loading, the n-dodecane conversion
1
increased by approximately 40–60% over a catalyst loading of 0.2–0.8 wt% Pt
at a reaction temperature of 300 °C and of C90% at 350 °C. Selectivity to the
iso-dodecane reaches its highest value in the range of 230–300 °C. The results
demonstrate that the iso-dodecane yield achieved its highest value at 300 °C. The
iso-dodecane yield was not significantly influenced by the Pt loading in the range of
0.2–0.8 wt% and reached its highest near 40%. The structure of the SAPO-11
consists of a one-dimensional ten-membered ring channel system with a pore size of
˚
.9 9 6.4 A [18]. The Pt/SAPO-11 catalyst, which did not contain strong acid sites
but had small pore sizes, exhibited excellent activity and selectivity in the
3
hydroisomerization reaction of the n-dodecane.
Conclusions
Among the catalysts that use zeolites as support, the Pt/HZSM-48 catalyst exhibited
the best isomerization selectivity in the hydroisomerization reaction of n-dodecane,
which is attributed to the moderate acid sites and medium-sized pores of the HZSM-
4
8. The highest iso-dodecane yield was obtained at a 280 °C reaction temperature
in the Pt/HZSM-48 catalyst. The Pt/SAPO-11 catalyst showed remarkable perfor-
mance in the n-dodecane hydroisomerization reaction of the n-dodecane due to its
small pore size and lower acidity. The optimal selectivity of the n-dodecane
hydroisomerization over the Pt/SAPO-11 catalyst was obtained at approximately
3
00 °C, which was slightly higher than that of the Pt/HZSM-48 catalyst.
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