Direct dehydrogenation of n-butane over Pt/Sn/Zn-K/Al2O3 catalyst: Effect of hydrogen in the feed

Article abstract of DOI:10.1166/jnn.2016.10985

Al₂O₃ was prepared by a sol-gel method for use as a support. Pt/Sn/Zn-K/Al₂O₃ catalyst was then prepared by a sequential impregnation method, and it was applied to the direct dehydrogenation of n-butane to n-butenes and 1,3-butadiene. Physicochemical properties of Pt/Sn/Zn-K/Al₂O₃ catalyst were examined by X-ray diffraction (XRD), nitrogen adsorption-desorption isotherm, inductively coupled plasma atomic emission spectroscopy (ICP-AES), temperature-programmed reduction (TPR), CO chemisorption, and temperature-programmed oxidation (TPO) measurements. In order to improve the catalyst stability, the effect of hydrogen in the feed on the catalytic performance in the direct dehydrogenation of n-butane was studied. The catalyst stability and reusability in the direct dehydrogenation of n-butane was also investigated. Experimental results revealed that the addition of hydrogen in the feed decreased conversion of n-butane and yield for total dehydrogenation products but improved the stability of the catalyst. The catalytic activity and stability of regenerated Pt/Sn/Zn-K/Al₂O₃ catalyst in the presence of hydrogen slightly decreased compared to those of fresh Pt/Sn/Zn-K/Al₂O₃ catalyst due to the slight sintering of platinum particles.

Full text of DOI:10.1166/jnn.2016.10985

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 4580–4586, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Direct Dehydrogenation of n-Butane Over
Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of
Hydrogen in the Feed
Jong Kwon Lee1, Hyun Seo1, Jeong Kwon Kim1, Hanuk Seo2, Hye-Ran Cho2,
Jinsuk Lee2, Hosik Chang2, and In Kyu Song1ꢀ∗
1School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University,
Shinlim-Dong, Kwanak-Ku, Seoul 151-744, South Korea
2Samsung Total Petrochemicals Corporation, Daesan 356-711, South Korea
Al2O3 was prepared by a sol–gel method for use as a support. Pt/Sn/Zn–K/Al2O3 catalyst was
then prepared by a sequential impregnation method, and it was applied to the direct dehydrogena-
tion of n-butane to n-butenes and 1,3-butadiene. Physicochemical properties of Pt/Sn/Zn–K/Al2O3
catalyst were examined by X-ray diffraction (XRD), nitrogen adsorption-desorption isotherm, induc-
tively coupled plasma atomic emission spectroscopy (ICP-AES), temperature-programmed reduc-
tion (TPR), CO chemisorption, and temperature-programmed oxidation (TPO) measurements. In
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order to improve the catalyst stability, the effect of hydrogen in the feed on the catalytic perfor-
IP: 37.230.212.122 On: Sun, 05 Jun 2016 23:00:11
mance in the direct dehydrogenation of n-butane was studied. The catalyst stability and reusability
Copyright: American Scientific Publishers
in the direct dehydrogenation of n-butane was also investigated. Experimental results revealed that
the addition of hydrogen in the feed decreased conversion of n-butane and yield for total dehydro-
genation products but improved the stability of the catalyst. The catalytic activity and stability of
regenerated Pt/Sn/Zn–K/Al2O3 catalyst in the presence of hydrogen slightly decreased compared
to those of fresh Pt/Sn/Zn–K/Al2O3 catalyst due to the slight sintering of platinum particles.
Keywords: Direct Dehydrogenation of n-Butane, Pt/Sn/Zn–K/Al2O3 Catalyst, H2 Addition,
Recycle Test.
1. INTRODUCTION
the development of rubber industry for motor vehicle
tires in the developing countries. At present, butenes and
butadiene in the petrochemical industry are mainly pro-
duced from naphtha cracking process. However, naphtha
cracking process operated at relatively high temperature
(>800 ꢀC) involves many problems in both marketing and
energy management, because this process produces not
only butenes and butadiene but also ethylene, propylene,
and isobutene.4ꢀ5
Dehydrogenation of n-butane and catalytic conversion
of ethanol have been recognized as alternative processes
for producing butenes and butadiene because of the above-
mentioned disadvantages of naphtha cracking process. Cat-
alytic conversion of ethanol is an industrially well proven
process for the production of butadiene. This process has
an advantage in a sense that it uses a renewable resource
(ethanol) as a feedstock. Furthermore, investment cost
for this process is low. However, this process has some
Butenes and butadiene are important raw materials for
synthetic resins, plastics, rubbers, and other petrochem-
ical products.1 Butenes are used as alkylate, monomer
for polybutene, and monomer for synthetic resins such
as low density polyethylene (LDPE) and high density
polyethylene (HDPE).2 Butadiene is an important material
for manufacturing synthetic rubbers such as acrylonitrile-
butadiene-styrene (ABS), polybutadiene rubber (BR), and
styrene-butadiene rubber (SBR).3
The shale gas revolution has decreased the produc-
tion of C4 petrochemicals (butenes and butadiene) avail-
able for conventional chemical processes because of the
switch to lighter feedstocks. Furthermore, the demand
for C4 petrochemicals has continuously increased due to
∗Author to whom correspondence should be addressed.
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Lee et al.
Direct Dehydrogenation of n-Butane Over Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of Hydrogen in the Feed
disadvantages such as high operating cost and formation of
many by-products. Although dehydrogenation of n-butane
is more energy intensive than ethanol conversion process,
the former process is more efficient for selective produc-
tion of C4 dehydrogenation products. In particular, direct
dehydrogenation of n-butane produces large amount of
butenes rather than butadiene, which is also of great impor-
tance for butene application.
Dehydrogenation of n-butane can be operated at lower
temperature than naphtha cracking process and is inde-
pendent of naphtha cracking process in the production
of butenes and butadiene. n-Butane, which is used as
a reactant in the dehydrogenation reaction, has vari-
ous advantages such as wide availability, low price, and
environmentally friendly nature in its inert state. In the
direct dehydrogenation of n-butane, hydrogen is produced
together with butenes and butadiene. Hydrogen, which is
obtained from the dehydrogenation of n-butane, has been
considered as an ideal energy carrier to support sustain-
able energy development. Therefore, many recent studies
have been conducted on the dehydrogenation of n-butane
for producing butenes and butadiene.6–10
A known amount of aluminum precursor (Aluminum
nitrate nonahydrate, Sigma-Aldrich) was dissolved in
ethanol with vigorous stirring for hydration of Al3+ ions.
Propylene oxide as a gelation agent was then added into
the aluminum precursor solution to make hydroxyl group
on the hydrated ions and to induce polycondensation reac-
tion between Al3+ ions. Molar ratio of aluminum precur-
sor:propylene oxide was fixed at 1:10. After maintaining
the resulting solution for several minutes, a white opaque
alumina gel was formed. The gel was aged for 2 days,
ꢀ
and then it was ꢀdried at 80 C. The resulting powder was
calcined at 800 C for 4 h to yield the Al2O3 support.
Zn–K/Al2O3 sample was prepared by an impregnation
method using an excess aqueous solution of Zn precursor
(Zinc nitrate hexahydrate, Sigma-Aldrich) and K precur-
sor (Potassium nitrate, Sigma-Aldrich). After impregna-
tion, it was dried at 80 ꢀC overnight and calcined at 600 ꢀC
for 4 h. Sn precursor (Tin (II) chloride dihydrate, Sigma-
Aldrich) was then impregnated on the Zn–K/Al2O3 sample
ꢀ
using hydrochloric acid solution. It was dried at 80 C
ꢀ
overnight and calcined at 600 C in air for 4 h. Pt pre-
cursor (Chloroplatinic acid hexahydrate, Sigma-Aldrich)
was finally impregnated on the Sn/Zn–K/Al2O3 sample,
Dehydrogenation of n-butane has been investigated over
a number of metal catalysts (Cr, Pt, Pd, Ga, and V) sup-
ported on various metal oxides (Al2O3, SiO2, MgAl2O4,
and ZnAl2O4ꢁ.11–15 Among these catalysts, Pt catalyst sup-
ported on Al2O3 has been widely employed for dehydro-
ꢀ
followed by drying at 80 C overnight and calcination at
ꢀ
550 C in air for 4 h. The contents of Pt, Sn, Zn, and
K in the Pt/Sn/Zn–K/Al2O3 catalyst were fixed at 1 wt%,
1 wt%, 0.5 wt%, and 0.5 wt%, respectively.
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genation of n-butane due to its high catalytic activity and
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high selectivity for butenes and butadiene.14ꢀ15 It has been
Copyright: American Scientific Publishers
2.2. Characterization
reported that platinum is an active metal and tin serves
Crystalline phases of the reduced catalyst were investi-
gated by XRD measurements (Rigaku, D-MAX2500-PC)
using Cu-Kꢂ radiation (ꢃ = 1ꢄ541 Å) operated at 50 kV
and 100 mA. Platinum, tin, zinc, and potassium contents
in the prepared catalyst were determined by ICP-AES
(Shimadz, ICP-1000IV) analyses. Brunauer-Emmett-Teller
(BET) surface area, pore volume, and average pore
size of Pt/Sn/Zn–K/Al2O3 catalyst were measured by N2
adsorption-desorption measurements using a BELSORP-
mini II (BEL Japan) instrument. In order to examine the
reduction behavior of metal species of Pt/Sn/Zn–K/Al2O3
catalyst, temperature-programmed reduction (TPR) mea-
surement was carried out in a conventional flow system
with a moisture trap connected to a thermal conductivity
detector (TCD) at temperatures ranging from room tem-
as an efficient activity modifier for platinum in the direct
dehydrogenation of n-butane.11 Thus, major studies on the
direct dehydrogenation of n-butane have been focused on
the platinum-tin catalyst supported on Al2O3.14–16 It is also
known that the addition of Zn to Pt/Sn/Al2O3 catalyst
resulted in high activity by changing electronic properties
of Pt active sites of the catalyst. K has been used as a pro-
moter to suppress the acidity of the catalyst and to enhance
the selectivity for dehydrogenation products.17ꢀ18
In this work, Al2O3 was prepared by a sol–gel method
for use as a support. Pt/Sn/Zn–K/Al2O3 catalyst was then
prepared by a sequential impregnation method, and it
was applied to the direct dehydrogenation of n-butane to
n-butenes and 1,3-butadiene. To improve the catalyst sta-
bility, the effect of hydrogen in the feed on the catalytic
performance of Pt/Sn/Zn–K/Al2O3 catalyst in the direct
dehydrogenation of n-butane was investigated. In order to
examine the catalytic stability and reusability of Pt/Sn/Zn–
K/Al2O3 catalyst, the catalytic performance of regenerated
Pt/Sn/Zn–K/Al2O3 catalyst was also evaluated.
ꢀ
ꢀ
perature to 700 C with a ramping rate of 5 C/min. For
the TPR measurement, a mixed stream of H2 (2 ml/min)
and N2 (20 ml/min) was used for 0.1 g of catalyst sample.
Carbon monoxide chemisorption experiments (BELCAT-
B, BEL Japan) were conducted to measure the platinum
dispersion and platinum surface area of Pt/Sn/Zn–K/Al2O3
catalysts. Prior to the chemisorption measurements, 50 mg
of each catalyst was reduced with a mixed stream of
2. EXPERIMENTAL DETAILS
ꢀ
2.1. Preparation of Pt/Sn/Zn–K/Al2O3 Catalyst
Al2O3 support was prepared by a sol–gel method, accord-
ing to the similar methods reported in the literatures.19ꢀ20
hydrogen (2.5 ml/min) and argon (47.5 ml/min) at 570 C
for 3 h, and subsequently, it was purged with pure heliꢀum
ꢀ
(50 ml/min) at 570 C for 10 min and cooled to 50 C.
J. Nanosci. Nanotechnol. 16, 4580–4586, 2016
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Direct Dehydrogenation of n-Butane Over Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of Hydrogen in the Feed
Lee et al.
ꢀ
The amount of carbon monoxide uptake was measured by
periodically injecting diluted carbon monoxide gas (5%
carbon monoxide and 95% helium) into the reduced cata-
lyst using an on-line sampling valve. Platinum dispersion
and platinum surface area were calculated by assuming
that one carbon monoxide atom occupies one surface plat-
inum atom. In order to elucidate the coke deposition on the
catalysts, temperature-programmed oxidation (TPO) mea-
surements were carried out in a conventional flow sys-
tem. 0.05 g of each catalyst was loaded into the U-shaped
quartz reactor. Afꢀter purging the reactor with He flow
(20 ml/min) at 50 C for 30 min, temperature of the reac-
(iv) reduction with H2 at 570 C for 3 h, and ꢀ
(v) n-butane dehydrogenation reaction at 550 C for 15 h
(n-butane:hydrogen= 1:1).
moles of n-butane reacted
Conversion of n-butane =
moles of n-butane supplied
Selectivity for TDP
number of n-butenes formed+moles of 1,3-butadiene formed
=
moles of n-butane reacted
Yield for TDP = ꢅConversion of n-butaneꢁ
×ꢅSelectivity for TDPꢁ
ꢀ
tor was increased from room temperature to 700 C at a
heating rate of 10 ꢀC/min under a flow of He (20 ml/min).
O2 flow (2 ml/min) was also introduced to the reactor
during the TPO measurement for complete desorption of
deposited carbon species. Desorbed species were detected
using a GC-MSD (6890N GC-5975MSD, Agilent).
3. RESULTS AND DISCUSSION
3.1. Characterization
In order to determine the physicochemical proper-
ties of Pt/Sn/Zn–K/Al2O3 catalyst, the catalyst was
characterized by X-ray diffraction (XRD), nitrogen
adsorption-desorption isotherm, inductively coupled
plasma atomic emission spectroscopy (ICP-AES),
temperature-programmed reduction (TPR) measurements.
Figure 1 shows the XRD pattern of reduced Pt/Sn/Zn–
K/Al2O3 catalyst. Pt/Sn/Zn–K/Al2O3 catalyst retained
a mixed phase of ꢆ-Al2O3 and ꢇ-Al2O3, indicating
that Al2O3 support of Pt/Sn/Zn–K/Al2O3 catalyst was
2.3. Direct Dehydrogenation of n-Butane
Direct dehydrogenation of n-butane with nitrogen and
direct dehydrogenation with hydrogen over Pt/Sn/Zn–
K/Al2O3 catalyst were caꢀrried out in a continuous flow
fixed-bed reactor at 550 C under atmosphere pressure.
Each catalyst (0.25 g) was preliminarily reduced with a
mixed stream of H2 (150 ml/h) and N2 (150 ml/h) at
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in the transition state between ꢆ-Al2O3 and ꢇ-Al2O3.
Any diffraction peaks for Pt, Sn, Zn, and K species
IP: 37.230.212.122 On: Sun, 05 Jun 2016 23:00:11
ꢀ
570 C for 3 h. For the reaction, n-butane was contin-
Copyright: American Scientific Publishers
were not observed in the XRD pattern of reduced
Pt/Sn/Zn–K/Al2O3 catalyst, indicating that Pt, Sn, Zn,
and K species were finely dispersed on the support or
less crystallized.21ꢀ22 Pt, Sn, Zn, and K contents in the
Pt/Sn/Zn–K/Al2O3 catalyst are summarized in Table I.
The measured Pt, Sn, Zn, and K contents were in good
agreement with the designed values. BET surface area,
pore volume, and average pore size of the catalyst are also
summarized in Table I. The Pt/Sn/Zn–K/Al2O3 catalyst
uously supplied into the reactor with nitrogen or hydro-
gen. The feed composition of n-butane:nitrogen was 1:1
for direct dehydrogenation of n-butane with nitrogen. The
feed composition of n-butane:hydrogen was 1:1 for direct
dehydrogenation of n-butane with hydrogen. Gas hourly
space velocity (GHSV) was fixed at 600 ml/h · g-cat on
the basis of n-butane. Reaction products were periodically
sampled and analyzed using a gas chromatograph (Var-
ian CP-3380) equipped with a GS-alumina column and a
flame ionization detector (FID). Conversion of n-butane
and selectivity for total dehydrogenation products (TDP,
n-butenes and 1,3-butadiene) were calculated on the basis
of carbon balance as follows. Yield for TDP was calcu-
lated by multiplying conversion of n-butane and selectivity
for TDP.
θ-Al2O3
γ-Al2O3
In order to confirm the catalytic stability and reusability
of the catalyst, the long-term reaction test and recycle test
for Pt/Sn/Zn–K/Al2O3 catalyst were performed. The long-
term reaction test and recycle test involved the following
steps;
ꢀ
(i) H2 reduction at 570 C for 3 h,
(ii) n-butane dehydrogenation reaction at 550 C for 72 h
ꢀ
20
30
40
50
60
70
80
2 Theta (degree)
(n-butane:hydrogen = 1:1),
Figure 1. XRD pattern of Pt/Sn/Zn–K/Al2 O3 catalyst reduced at 570 ꢀC
for 3 h.
(iii) regeneration
with
an
oxidant
mixture
ꢀ
(oxygen:nitrogen = 1:1) at 550 C for 5 h,
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Lee et al.
Direct Dehydrogenation of n-Butane Over Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of Hydrogen in the Feed
Table I. Metal content, BET surface area, pore volume, and average
pore size of Pt/Sn/Zn–K/Al2 O3 catalyst.
(a)
80
70
60
50
40
30
20
10
0
Dehydrogenation with nitrogen
Dehydrogenation with hydrogen
Pt content (wt%)a
0ꢄ9
1ꢄ0
0ꢄ5
Sn content (wt%)a
Zn content (wt%)a
K content (wt%)a
BET surface area (m2/g)b
Pore volume (cm3/g)c
Average pore size (nm)d
0ꢄ5
167
0ꢄ55
14ꢄ0
Notes: aDetermined by ICP-AES analyses; bCalculated by the BET equation’ cBJH
desorption pore volume; dBJH desorption average pore size.
showed high surface area (167 m2/g), large pore volume
(0.55 cm3/g), and mesoporous pore size (14.0 nm). This
indicates that mesoporous Pt/Sn/Zn–K/Al2O3 catalyst was
successfully prepared as attempted in this work.
60 90 120 150 180 210 240 270 300 330 360
Time on stream (min)
(b)
80
70
60
50
40
30
20
10
0
Dehydrogenation with nitrogen
Dehydrogenation with hydrogen
To investigate the reduction behavior of metal species
and interaction between metal species and support in the
Pt/Sn/Zn–K/Al2O3 catalyst, TPR measurement was carried
out. Figure 2 shows the TPR profile of Pt/Sn/Zn–K/Al2O3
catalyst. Pt/Sn/Zn–K/Al2O3 catalyst showed two reduction
ꢀ
bands; the reduction band at 265 C was attributed to the
reduction of Pt oxide and a small fraction of Sn and Zn–K
oxide species, whereas the shoulder band at around 400 ꢀC
represented the partial reduction of Sn4+ to Sn2+ and Sn2+
to Sn0 interacted with other metal species and alumina
support.23–25
90
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Time on stream (min)
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3.2. Effect of Hydrogen Addition on Direct
Copyright: American SFciigeunretif3i.c Publishers
(a) Conversion of n-butane and (b) yield for TDP over
Dehydrogenation of n-Butane Over
Pt/Sn/Zn–K/Al2O3 Catalyst
Pt/Sn/Zn–K/Al2 O3 catalyst with time on stream in the dehydrogenation
with nitrogen and dehydrogenation with hydrogen.
Figure 3 shows the conversion of n-butane and yield for
TDP over Pt/Sn/Zn–K/Al2O3 catalyst with time on stream
in the direct dehydrogenation of n-butane with nitrogen
and dehydrogenation with hydrogen. The catalytic activity
of Pt/Sn/Zn–K/Al2O3 catalyst in the direct dehydrogena-
tion with nitrogen was very high at the initial stage of
reaction, but it experienced a severe catalyst deactivation
during the reaction. On the other hand, the catalytic activ-
ity in the direct dehydrogenation with hydrogen was very
stable during the reaction. Detailed catalytic performance
of Pt/Sn/Zn–K/Al2O3 catalyst in the dehydrogenation with
nitrogen and dehydrogenation with hydrogen obtained
24
1-Butene
cis-2-Butene
trans-2-Butene
265
20
1,3-Butadiene
iso-Butane
iso-Butene
16
Cracking products
12
8
4
0
Dehydrogenation
with nitrogen
Dehydrogenation
with hydrogen
100
200
300
400
500
600
700
Figure 4. Yield for TDP over Pt/Sn/Zn–K/Al2 O3 catalyst after
360 min-reaction in the dehydrogenation with nitrogen and dehydrogena-
tion with hydrogen.
a
Temperature (ºC)
Figure 2. TPR profile of Pt/Sn/Zn–K/Al2 O3 catalyst.
J. Nanosci. Nanotechnol. 16, 4580–4586, 2016
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Direct Dehydrogenation of n-Butane Over Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of Hydrogen in the Feed
Lee et al.
Table II. Carbon monoxide chemisorption results for fresh Pt/Sn/Zn–
K/Al2O3 and regenerated Pt/Sn/Zn–K/Al2 O3 catalysts.
Pt
Pt
n-Butane
n-Butene
n-Butadiene
A, Pt
A
Pt
Pt surface
Pt particle
size (nm)a
Catalyst
Fresh
Pt/Sn/Zn–K/Al2 O3
Regenerated
dispersiona (%)
area (m2/g)a
A, Pt
A
A
A
Cracked products
53.6
35.2
132ꢄ4
86ꢄ9
2.2
3.2
A
A, Pt
Pt
A
Polymers
iso-Butane
iso-Butene
Pt/Sn/Zn–K/Al2 O3
A, Pt
Coke
Note: aCalculated by assuming stoichiometry factor (CO/Pt) = 1.
(Pt = Platinum site, A = Acid site)
3.3. Long-Term Reaction and Recycle Test for
Pt/Sn/Zn–K/Al2O3 Catalyst
Figure 5. Reaction pathways on platinum and acid sites in the direct
dehydrogenation of n-butane.
In the direct dehydrogenation of n-butane, the catalytic
stability and reusability are important subjects. There-
fore, long-term reaction test and recycle test for Pt/Sn/Zn–
K/Al2O3 catalyst were performed in order to investigate
the catalytic stability of fresh Pt/Sn/Zn–K/Al2O3 catalyst
and the reusability of regenerated Pt/Sn/Zn–K/Al2O3 cat-
alyst. Figure 6 shows the result of long-term reaction test
and recycle test in the direct dehydrogenation of n-butane
over Pt/Sn/Zn–K/Al2O3 catalyst with time on stream. In
the long-term reaction test, fresh Pt/Sn/Zn–K/Al2O3 cata-
lyst showed a stable catalytic performance with time on
stream. In the recycle test, the catalytic performance of
regenerated Pt/Sn/Zn–K/Al2O3 catalyst slightly decreased
with time on stream.
after a 360 min-reaction is shown in Figure 4. As shown in
Figure 4, 1-butene, cis-2-butene, trans-2-butene, and 1,3-
butadiene were produced as main products together with
by-products such as iso-butane, iso-butene, and cracking
products (methane, ethane, ethylene, propane, and propy-
lene). It is generally accepted that platinum and acid prop-
erty in the Pt/Sn/Zn–K/Al2O3 catalyst play key roles in
the direct dehydrogenation of n-butane. Figure 5 shows
the possible reactions that take place on platinum (Pt)
and acid (A) sites in the direct dehydrogenation of n-
butane over Pt/Sn/Zn–K/Al2O3 catalyst. According to this
reaction pathway, the improved catalytic stability in the
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presence of hydrogen may be understood by the follow-
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3.4. Pt Dispersion and Pt Surface Area of
Copyright: American Scientific Publishers
ing reason. Addition of hydrogen in the feed leads to the
competitive adsorption between n-butane and hydrogen on
the platinum active sites, which decreases the amount of
adsorbed n-butane.26 It is believed that the use of hydro-
gen decreases the formation of reaction product and coke
precursor, resulting in the formation of small amount of
coke.
Pt/Sn/Zn–K/Al2O3 Catalyst
Carbon monoxide chemisorption measurements were con-
ducted in order to determine the platinum dispersion,
platinum surface area, and average platinum size in
the fresh Pt/Sn/Zn–K/Al2O3 and regenerated Pt/Sn/Zn–
K/Al2O3 catalysts. Carbon monoxide chemisorption results
for fresh Pt/Sn/Zn–K/Al2O3 and regenerated Pt/Sn/Zn–
K/Al2O3 catalysts are listed in Table II. Platinum surface
464
100
(x10)
1 cycle
2 cycle
90
80
70
60
50
40
30
20
10
0
525
1 Cycle (72 hr)
Regeneration
Conversion of n-butane
Selectivity for TDP
Yield for TDP
451
515
2 Cycle (15 hr)
100
0
10
20
30
40
50
60
70
0
10
0
200
300
400
500
600
700
Time on stream (hr)
Temperature (ºC)
Figure 6. Catalytic performance over fresh Pt/Sn/Zn–K/Al2 O3 and
regenerated Pt/Sn/Zn–K/Al2 O3 catalysts in the dehydrogenation with
hydrogen.
Figure 7. TPO profiles of 1 cycle-used, regenerated, and 2 cycle-used
Pt/Sn/Zn–K/Al2 O3 catalysts.
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Lee et al.
Direct Dehydrogenation of n-Butane Over Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of Hydrogen in the Feed
n
n
Acid site
Al2O3 support
Figure 8. Schematic mechanism for direct dehydrogenation of n-butane over Pt/Sn/Zn–K/Al2 O3 catalyst.
area of fresh Pt/Sn/Zn–K/Al2O3 catalyst was higher than
that of regenerated of Pt/Sn/Zn–K/Al2O3 catalyst. Thus,
active platinum species were more finely dispersed in the
fresh Pt/Sn/Zn–K/Al2O3 catalyst than in the regenerated
Pt/Sn/Zn–K/Al2O3 catalyst, resulting in smaller average
platinum size. It is known that sintering process takes place
at high reaction temperatures (>500 ꢀC). It is also reported
that metal sintering occurs relatively rapidly in the pres-
ence of oxygen but occurs relatively slowly in the presence
mechanism for direct dehydrogenation of n-butane over
Pt/Sn/Zn–K/Al2O3 catalyst is proposed in Figure 8. As
shown in Figure 7, any oxidation peaks of coke species
were not observed in the TPO profile of regenerated
Pt/Sn/Zn–K/Al2O3 catalyst, indicating that the 1 cycle-
used Pt/Sn/Zn–K/Al2O3 catalyst was completely regener-
ated under our regeneration conditions.
4. CONCLUSIONS
Al2O3 support was prepared by a sol–gel method,
of hydrogen. It is inferred that sintering of platinum par-
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ticles occurred during the dehydrogenation reaction and
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and Pt/Sn/Zn–K/Al O3 catalyst was then prepared by a
Copyright: American Scientific Publishers2
regeneration process at high temperature (550 C), leading
ꢀ
sequential impregnation method for use in the direct
dehydrogenation of n-butane. The catalytic activity of
Pt/Sn/Zn–K/Al2O3 catalyst in the direct dehydrogenation
with nitrogen was very high at the initial stage of reaction,
but it experienced a severe catalyst deactivation during the
reaction. On the other hand, the catalytic activity in the
direct dehydrogenation with hydrogen was stable during
the reaction. In order to investigate the catalytic stability
of fresh Pt/Sn/Zn–K/Al2O3 catalyst and the reusability of
regenerated Pt/Sn/Zn–K/Al2O3 catalyst, long-term reaction
test and recycle test for Pt/Sn/Zn–K/Al2O3 catalyst were
performed. It was revealed that fresh Pt/Sn/Zn–K/Al2O3
catalyst in the long-term reaction test showed a stable
catalytic performance with time on stream. However, the
catalytic activity and stability of regenerated Pt/Sn/Zn–
K/Al2O3 catalyst slightly decreased compared to those of
fresh Pt/Sn/Zn–K/Al2O3 catalyst, although the coke on the
1 cycle-used catalyst was completely removed during the
regeneration process. The decreased catalytic performance
of regenerated Pt/Sn/Zn–K/Al2O3 catalyst was due to the
increase of platinum particle size caused during the reac-
tion and regeneration processes (Table II).
to a decrease of platinum surface area and dispersion of
regenerated Pt/Sn/Zn–K/Al2O3 catalyst compared to fresh
Pt/Sn/Zn–K/Al2O3 catalyst.
3.5. TPO Profiles of 1 Cycle-Used, Regenerated, and 2
Cycle-Used Pt/Sn/Zn–K/Al2O3 Catalysts
In order to investigate the coke deposition on the
Pt/Sn/Zn–K/Al2O3 catalysts, TPO measurements were car-
ried out. Figure 7 shows the temperature-programmed oxi-
dation (TPO) profiles of 1 cycle-used, regenerated, and 2
cycle-used Pt/Sn/Zn–K/Al2O3 catalysts. The TPO profiles
of 1 cycle-used and 2 cycle-used Pt/Sn/Zn–K/Al2O3 cat-
ꢀ
alystꢀs showed two oxidation peaks at around 460 C and
520 C, indicating that two kinds of carbon species (soft
coke and hard coke) were formed during the reaction. It
is known that n-butane is initially dissociated on the metal
and coke precursor is formed through dehydrogenation
reaction. The “soft coke” is then generated on the metal
from the coke precursor.27ꢀ28 The coke precursor gener-
ated on the metal also migrates to acid site of the support.
On the support, the coke precursor and the adsorbed n-
butene/butadiene undergo polymerization/oligomerization,
condensation, cyclization and hydride transfer, resulting
in the formation of “hard coke.”27ꢀ28 This indicates that
coke on the metal has a higher hydrogen/carbon ratio than
that on the support. Based on these results, schematic
Acknowledgments: The authors wish to acknowl-
edge support from the Samsung Total Petrochemicals
Corporation.
J. Nanosci. Nanotechnol. 16, 4580–4586, 2016
4585

Direct Dehydrogenation of n-Butane Over Pt/Sn/Zn–K/Al2O3 Catalyst: Effect of Hydrogen in the Feed
Lee et al.
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Received: 9 September 2014. Accepted: 9 October 2014.
Delivered by Ingenta to: West Virginia University/ Health Sci Ctr LIb
IP: 37.230.212.122 On: Sun, 05 Jun 2016 23:00:11
Copyright: American Scientific Publishers
4586
J. Nanosci. Nanotechnol. 16, 4580–4586, 2016