Oxidative dehydrogenation of n-butane on nano-carbon catalysts having graphitic structures

Article abstract of DOI:10.1007/s11164-011-0380-1

The catalytic activity of well-nanostructured carbon, for example glassy carbon spheres, SWCNT, graphene, and graphite, has been demonstrated in the oxidative dehydrogenation (ODH) of butane to obtain olefins. The catalytic performance of the carbon samples was stable, with prolonged reaction time. The proportion of butenes in the product decreased as the reaction temperature increased, whereas selectivity for ethene and propene increased with increasing temperature. Pd-containing carbon nanofibers (CNF) had superior selectivity for butadiene formation than Pd free CNF catalyst. Carbon with graphitic structures was highly selective for propylene and butenes without severe combustion in ODH of butane.

Full text of DOI:10.1007/s11164-011-0380-1

Res Chem Intermed (2011) 37:1145–1156
DOI 10.1007/s11164-011-0380-1
Oxidative dehydrogenation of n-butane on nano-carbon
catalysts having graphitic structures
•
Da Young Jang Hyung Gyu Jang
•
•
Gye Ryung Kim Geon-Joong Kim
Received: 31 May 2011 / Accepted: 21 July 2011 / Published online: 24 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract The catalytic activity of well-nanostructured carbon, for example glassy
carbon spheres, SWCNT, graphene, and graphite, has been demonstrated in the
oxidative dehydrogenation (ODH) of butane to obtain olefins. The catalytic
performance of the carbon samples was stable, with prolonged reaction time.
The proportion of butenes in the product decreased as the reaction temperature
increased, whereas selectivity for ethene and propene increased with increasing
temperature. Pd-containing carbon nanofibers (CNF) had superior selectivity for
butadiene formation than Pd free CNF catalyst. Carbon with graphitic structures was
highly selective for propylene and butenes without severe combustion in ODH of
butane.
Keywords SWCNT Á Oxidative dehydrogenation Á Butane Á Alkene selectivity
Introduction
Butenes, butadiene, and propylene, which are useful intermediates for the synthesis
of synthetic rubbers, plastics, and a number of industrially important chemicals,
are synthesized traditionally by oxidative dehydrogenation (ODH) of n-butane or
propane over various mixed metal oxides [1–10]. Despite a great deal of research,
alkene selectivity is severely limited in this ODH reaction [11]. One important
reason is that the unsaturated products are much more readily oxidized to CO2 than
is the starting alkane. A high oxygen-to-butane ratio is required to maintain the
activity of such catalysts, which leads to unwanted oxidation of products. Carbon
materials have been reported to catalyze the ODH of saturated hydrocarbon
molecules, for example propane, butane, and ethylbenzene [12–15]. However,
D. Y. Jang Á H. G. Jang Á G. R. Kim Á G.-J. Kim (&)
Department of Chemical Engineering, Inha University, Incheon 402-751, Korea
e-mail: kimgj@inha.ac.kr
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D. Y. Jang et al.
conventional types of carbon, for example particular active carbons, underwent
unavoidable deactivation because of coking or combustion [16–19]. Recently, it was
shown that only well-nanostructured carbons are stable and coke-free catalysts for
these reactions [19]. Su group has shown that carbon nanotubes with modified
surface functionality efficiently catalyze the ODH of less active n-butane to butenes
[12, 14, 15]. Compared with metal oxide-based catalysts, CNTs had enhanced
selectivity for C4 alkenes, especially butadiene. For low O2-to-butane ratios, high
selectivity to alkenes was achieved for periods as long as 100 h. They concluded
that such high selectivity may occur as a result of ketonic C=O groups formed via a
combination of parallel and sequential oxidation steps under the mild conditions.
In their study, addition of a small amount of phosphorus greatly improved the
selectivity by suppressing combustion of hydrocarbons. This progress in ODH leads
to new applications of nano-structured carbon with enormous possibilities.
In this work, we demonstrated the catalytic activity of well-nanostructured
carbon, for example CNTs, graphene, and graphite, in the ODH reaction to obtain
the olefins. The effects of carbon structure, surface properties, and reaction
conditions on the selectivity for the olefins was investigated. Carbon with graphitic
structures had high selectivity for propylene and butenes without severe combustion
of butane during ODH. Furthermore, in this work we synthesized carbon nanofibers
(CNF) in the presence of palladium, and these were used as catalyst in the ODH
reaction to obtain the olefins. Pd-containing CNF had superior selectivity for
butadiene formation than Pd-free CNF catalyst.
Experimental
Single-wall CNT (Nano Solution; 1–1.5 nm single wall), glassy spherical carbon
powder (Sigma–Aldrich; 99%), graphene (Cheap tubes, USA; 99%, 2 lm, surface
area; 100 m2/g), graphite (Timlex KS44; 44 lm) and active carbon (Oriental
Chemical) were used as catalysts without further purification. For oxidation of CNT,
2 g pristine CNT was heated under reflux in 200 ml concentrated HNO3 solution for
2 h. The mixture was filtered, washed with deionized water, and dried at 80 °C in
air, overnight. The resulting solid was then calcined in nitrogen at 450 °C for
30 min. P-modified graphite was prepared by incipient wetness impregnation of
fresh graphite (2 g) with aqueous solutions of (NH4)3PO4 (Aldrich, [99%), to
achieve loadings of 2 wt% P2O5. Impregnated samples were dried in air at 120 °C
overnight for use as a catalyst. The dried solids were further calcined in nitrogen for
30 min. The catalytic tests were performed at atmospheric pressure and 400–600 °C
in a continuous-flow, fixed-bed quartz tube with a total flow rate of 150 ml min-1
.
For each run, 500 mg catalyst was used and the reactant mixture usually comprised
n-butane–air–N2 (n-butane–O2 = 1:1–2:4, with N2 as balance). Gas mixtures were
analyzed on a gas chromatograph (GC; Hewlett–Packard, HP-6890) with FID and
TCD detectors. The morphology of carbon samples was characterized by field
emission transmission electron microscopy operating at 200 kV (FE-TEM, S-4200)
and scanning electron microscopy (SEM) with an Hitachi S-4200 micro-
scope. Elemental analysis of carbon samples was conducted by use of an XPS
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Oxidative dehydrogenation of n-butane on nano-carbon catalysts
1147
(Sigma-Probe (Thermo VG, UK) spectrometer, with MgKa radiation as excitation
source (hm = 1253.6 eV)).
Results and discussion
For a comparative study of catalytic performance in ODH of butane, we evaluated
pristine SWCNT, oxidized CNT, graphene, glassy carbon, active carbon, graphite,
and 2 wt% P2O5-loaded graphite. The carbon samples were analyzed by SEM and
TEM; the photographs are shown in Fig. 1. The graphene was very transparent
because of its thin layers, as can be seen in Fig. 1a. It was composed of
approximately 10 carbon layers (Fig. 1b). The graphite has a flake shape with an
average size of 45 lm (Fig. 1c). In addition, single-wall CNT was chosen as a
typical carbon catalyst. The characteristic of single-wall CNT can be seen in the
TEM image (Fig. 1d). XPS analysis showed that all the carbon samples contained
metal compounds in trace amounts (less than 0.5%). As a result, co-operative
Fig. 1 SEM and TEM images of carbon catalysts; a and b graphene, c graphite, d single-wall CNT
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C4 alkenes Selectivity
Propylene Selectivity
Ethylene Selectivity
CO+ CO2 Selectivity
Butane Conversion
50
40
30
20
10
0
0
50
100
150
200
250
300
Reaction Time(min)
Fig. 2 Stability with reaction time of graphite catalyst in ODH of butane (0.5 g catalyst, O2-to-butane
ratio = 0.8, 500 °C)
catalysis of the reactions by metal species could be ruled out. Reactions in the gas
phase could also be excluded by blank experiments. All the samples had a stable
catalytic performance after a few hours on-stream, and their activity and selectivity
were investigated in detail by variation of reaction temperature and oxygen-to-
butane ratio. We conducted the reaction at 400–600 °C with O2-to-butane ratios of
0.4–0.8. The product mixture contained methane, ethene, propene, butenes,
butadiene, CO, CO2, and residual reactants; the resulting carbon balance was
almost 100%. At the O2-to-butane ratio of 0.8, SWCNT, graphite, and graphene
catalysts had outstanding stability in the ODH of butane. During reaction for 24 h
on the graphite catalyst, butane and oxygen conversion remained almost unchanged,
and selectivity for butenes stayed above 35%, as shown in Fig. 2. Dissociative
conversion of butane into ethene and propene was observed under these conditions.
Considering the high stability of graphite in O2, we conclude that the CO2 during the
reaction mainly originated from oxidation of the hydrocarbon feedstock and not
from burning of the carbon catalysts.
In Fig. 3, we show the ODH activity of different carbon catalysts as a function of
reaction temperature and O2-to-butane ratio. Conversion increased as the reaction
temperature increased whereas selectivity for total olefins decreased. Conversion
values improved on increasing the O2-to-butane ratio from 0.4 to 0.8 at the same
reaction temperature. All the catalysts had almost the same selectivity for alkenes as
the concentration of oxygen was doubled. As shown in the figure, amounts of the
olefins obtained on the graphite and graphene catalysts were higher than on the other
types of carbon. For glassy carbon, although conversion of the reactant butane was
high, total olefin selectivity was lowest among the carbon catalysts. Conversion
values were highest for active carbon, with promising selectivity, but this catalyst
was consumed by combustion during the reaction, because of the presence of
oxygen.
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Oxidative dehydrogenation of n-butane on nano-carbon catalysts
1149
O /butane = 0.4
2
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
graphite
conversion
CP
SW
AC
graphene
graphite
selectivity
CP
SW
AC
graphene
300
400
500
600
Temperature(°C)
O2/butane = 0.8
100
80
90
80
70
60
50
70
60
50
40
30
40
30
20
10
0
20
10
0
300
400
500
600
Temperature(°C)
Fig. 3 Effect of reaction temperature and O2-to-butane ration on olefin selectivity and butane conversion
(0.5 g catalyst, O2-to-butane = 0.4 and 0.8, 300–600 °C)
We then functionalized surfaces of pristine CNT by heating under reflux and
oxidation in concentrated HNO3. With the resulting oxidized CNT as a catalyst, we
observed no improvement in the yield of alkenes. The oxidized CNT was unstable
during the reaction, and was combusted in the ODH of butane. Only 10% of the
CNT catalyst remained after ODH reaction at 600 °C. Over the oxidized CNT, more
than 80% of the converted butane was combusted, yielding less than 2% alkenes.
After heating CNT under reflux in HNO3, defect sites were functionalized by use
of O2- anions into various functional groups. Then, at the reaction temperature,
desorption of less stable groups may result in many defects that subsequently
generate new electrophilic oxygen sites, thus partially limiting the selectivity for
alkenes of the oxidized CNT.
To compare catalytic activity, we evaluated different types of carbon in ODH of
n-butane at different reaction temperatures. The effects of reaction temperature and
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D. Y. Jang et al.
500°C, O2/C4=0.4
400°C, O2/C4=0.4
100
90
80
100
2
1
1
90
80
70
60
50
40
30
20
10
0
2
1
2
1
3
1
1
70
60
29
28
33
42
1,3-butadiene
butene
34
64
76
1,3-butadiene
butene
68
62
61
50
40
30
20
10
0
32
19
propylene
ethylene
29
18
propylene
ethylene
25
16
32
12
23
15
13
13
15
12
13
13
10
13
10
10
600°C ,O2/C4=0.4
500°C, O2/C4=0.8
100
90
80
70
60
50
40
30
100
90
80
1
2
1
2
1
1
2
11
1
11
9
70
60
1
24
32
24
1
8
20
10
22
35
36
1,3-butadiene
butene
50
40
30
20
35
1,3-butadiene
butene
30
25
32
23
33
22
31
23
30
25
31
21
propylene
ethylene
24
19
21
17
20
10
0
propylene
ethylene
26
10
0
Fig. 4 The product distribution obtained in the ODH of butane over different carbon catalysts at reaction
temperatures of 400–600 °C (0.5 g catalyst, O2-to-butane ratio = 0.4 and 0.8)
O2-to-butane ratio on olefin selectivity were investigated in detail; the product
distribution obtained over the different carbon catalysts is summarized in Fig. 4. As
mentioned above, higher selectivity for total alkenes was obtained with at decreased
temperature and lower O2-to-butane ratio. In the product olefins, the higher
proportion was butenes at a reaction temperature of 400 °C. The amount of butenes
decreased as the reaction temperature increased. Instead, selectivity for ethene and
propene increased with increasing temperature. This means that dissociation of CH
bonds was predominant at the higher reaction temperature in the presence of carbon
catalysts. The trend in the change of product distribution was very similar for every
carbon catalyst.
Selectivity for CO and CO2 was determined over the carbon catalysts, and the
result is shown in Fig. 5. In general, CO and CO2 selectivity increased as the O2-to-
butane ratio and the reaction temperature were increased. At relatively low O2-to-
butane ratios, for example 0.4, graphite and graphene had low CO, CO2 selectivity
compared with the other catalysts. The low yield of CO2 means improvement in the
selectivity for alkenes in the ODH reaction of butane.
Figure 6 shows the relationship between conversion and selectivity for total
olefins in ODH of butane over different catalysts. At the higher level of conversion,
alkene selectivity decreased. However the carbon catalysts had enhanced selectivity
for olefins at the high oxygen concentration, as indicated in the results shown in
Fig. 6. This phenomenon was investigated in detail by comparing conversion vs.
selectivity for each olefin, for example dissociated product ethene and propene, and
123

Oxidative dehydrogenation of n-butane on nano-carbon catalysts
1151
O
2
/butane=0.4
O2/butane=0.8
16
14
12
10
8
16
14
12
10
8
400
500
600
6
6
4
4
2
2
0
0
Fig. 5 Selectivity for CO and CO2 in the ODH of butane over different carbon catalysts at reaction
temperatures of 400–600 °C (0.5 g catalyst, O2-to-butane ratio = 0.4 and 0.8)
Fig. 6 Plot of conversion vs. total olefin selectivity in the ODH of butane over carbon catalysts (0.5 g
catalyst, O2-to-butane ratio = 0.4 and 0.8, 400–600 °C)
butene. Selectivity for butenes decreased with progressively increasing conversion
values. The same trend and the similar values in the plot of conversion vs. C4 olefin
selectivity have been reported in the paper by the Su group, as shown additionally in
Fig. 7 [14]. However concentrations of ethene and propene increased at the high
level of butane conversion.
The effect of addition of phosphorus as a promoter was investigated in the
ODH reaction. Selectivity for CO, CO2, and alkenes in the ODH of butane
were determined over graphite and 2 wt% P-doped graphite catalyst at reaction
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D. Y. Jang et al.
Fig. 7 Plot of conversion vs. butenes (b) and ethane ? propene selectivity (a) in the ODH of butane
over different carbon catalysts (0.5 g catalyst, O2-to-butane ratio = 0.4 and 0.8, 400–600 °C, Ref [14])
Graphite
P doped-Gaphite
16
14
12
10
8
16
14
12
10
8
400
500
600
400
500
600
6
6
4
4
2
2
0
0
0.4
0.8
0.4
0.8
O2/ butane ratio
Fig. 8 Selectivity for CO and CO2 in the ODH of butane over graphite and 2 wt% P-doped graphite
catalysts at reaction temperatures of 400–600 °C (0.5 g catalyst, O2-to-butane ratio = 0.4 and 0.8)
temperatures of 400–600 °C. Selectivity for CO and CO2 is shown in Fig. 8.
The P-loading on CNT reduced selectivity for CO and CO2 by suppressing the
combustion rate, rather than enhancing the rate of formation of alkenes.
Modification with phosphate is indeed reported to block the combustion sites
serving as a point of attack for gaseous oxygen, and thus to suppress the combustion
of the carbon framework at elevated temperatures [19].
Figure 9 shows the relationship between conversion and selectivity for each
olefin, e.g. ethene, propene, and butene. At the higher level of conversion, alkene
selectivity decreased for both carbon catalysts. At the same conversion of butane,
fresh graphite had the same selectivity for olefins as the 2 wt% P-doped graphite
catalyst.
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Oxidative dehydrogenation of n-butane on nano-carbon catalysts
1153
O2/butane=0.4-0.8(graphite)
100
600
400
500
80
60
40
20
0
C2
C3
C4
0
10
20
30
40
50
conversion(%)
O2/butane=0.4-0.8(p-graphite)
100
80
60
40
20
0
600
400
500
C2
C3
C4
0
10
20
30
40
50
60
conversion(%)
Fig. 9 Plot of conversion vs. butane, ethane, and propene selectivity in the ODH of butane over graphite
and 2 wt% P-doped graphite catalysts at reaction temperatures of 400–600 °C (0.5 g catalyst, O2-to-
butane ratio = 0.4 and 0.8)
We have synthesized CNF on Pd particles supported on silica. The nano-sized Pd
particles were embedded at the end of CNF, as shown in Fig. 10. After dissolution
of Pd by nitric acid, we conducted the ODH reaction of butane using the
as-synthesized Pd-containing CNF and HNO3 treated CNF as catalysts. Different
product distributions were observed. The results obtained are compared in Fig. 11,
from which it can be seen that Pd-containing CNF have superior selectivity toward
butadiene formation than Pd-free CNF catalyst. Such a pronounced enhancement
can be related to the high dehydrogenation activity of Pd species. However,
selectivity for butenes was dramatically reduced in the presence of Pd-containing
CNF. At the high O2-to-butane ratio, selectivity for ethene and propene was
enhanced for both catalysts.
However, interaction of butane with oxygen-treated surfaces of the carbon
catalysts is important to yield alkenes such as propene and butenes. Butane
molecules are activated by the surface oxygen groups to yield alkenes with
remarkable selectivity. The surface of carbon materials is terminated by a variety of
functionalized oxygen groups [11]. In these surface species, ketonic groups have
great potential to coordinate a redox process, because they are rich in electrons. It is
well known that Lewis basic sites can abstract hydrogen atoms from CH bonds of
alkanes to produce the corresponding alkenes. The reaction between the gas-phase
oxygen and the abstracted hydrogen will regenerate the active sites with the
formation of water [14, 15].
In XPS spectra for C1s, the ketonic C=O bond has a peak at 287.8 eV. Fresh
graphite and that used in the ODH reaction were analyzed by XPS. The small
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D. Y. Jang et al.
Fig. 10 TEM images of carbon nanofiber (CNF) as-synthesized on the nano-size Pd particles: this
Pd-containing sample and acid-treated Pd-free CNF were used as catalysts in the ODH of butane
2wt%Pd-CNF,500°C
CNF,500°C
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
9
5
40
31
14
10
8
1,3-butadiene
butene
46
40
33
propylene
ethylene
18
18
19
16
0.4
0.8
0.4
0.8
O2/ butane ratio
O2/ butane ratio
Fig. 11 Selectivity for olefins in the ODH of butane over the Pd-containing carbon nanofiber (Pd–CNF)
and HNO3-treated Pd–CNF carbon catalysts at a reaction temperature of 500 °C (0.5 g catalyst, O2-to-
butane ratio = 0.4–0.8)
shoulder near 288 eV for ketonic C=O bonds was observed in the C1s peak as
shown in Fig. 12. It is believed that these groups present on the carbon surfaces may
be critical components of the active sites, as reported by Su et al. [14]. The presence
of oxygen was confirmed in the same XPS analysis, and an increase of oxygen
species on the surface was found after use in the ODH reaction.
Conclusion
In this work, the catalytic activity of well-nanostructured carbon, for example glassy
carbon spheres, SWCNT, graphene, and graphite were demonstrated in the ODH
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Oxidative dehydrogenation of n-butane on nano-carbon catalysts
1155
C1s
A
C1s Scan
Graphite
C-C
O1s
C= O
Survey
F1s ???
1.00E+05
In3d
N1s
S2p
C1s
Br3d ??
S
?
i2p
1200
1000
800
600
400
200
0
Binding Energy (eV)
298 296 294 292 290 288 286 284 282 280
Binding Energy (eV)
O1s
B
S2p ???
Si2p ???
1200
1000
800
600
400
200
0
Binding Energy (eV)
Fig. 12 XPS spectra of fresh graphite (b) and of graphite used (a) in the ODH of butane
reaction of butane to obtain olefins. These carbon samples had stable catalytic
performance with prolonged reaction time. Higher selectivity for total alkenes was
obtained at lower temperatures and lower O2-to-butane ratios. The proportion of
butenes in the product decreased as the reaction temperature was increased, whereas
selectivity for ethene and propene increased with increasing temperature. Further-
more butene selectivity decreased with progressively increasing conversion.
P-loading of the CNT reduced the selectivity for CO and CO2 by suppressing
combustion. Pd-containing CNF has superior selectivity for butadiene formation
than Pd-free CNF catalyst. Carbon with graphitic structures had high selectivity to
propylene and butenes without severe combustion of butane in ODH.
Acknowledgment This research is supported by Korea Ministry of Environment as ‘‘Converging
technology project’’.
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