KAORU FUJIMOTO
SACHIO ASAOKA
TAISEKI KUNUGI
Department of Synthetic Chemistry
Faculty of Engineering — University of Tokyo
Hongo — Bun kyo-ku
Tokyo — JAPAN
REVERSE SPILLOVER OF
HYDROGEN AS A KEY
PROCESS OF THE
DEHYDROGENATION OF
PARAFFIN OVER METAL
SUPPORTED CARBON
CATALYSTS
1. INTRODUCTION
Active carbon has been known as an effective catalyst for the
dehydrogenation of cycloparaffins. We have already reported that
active carbon is a fairly good catalyst for the dehydrogenation and
dehydroaromatization of aliphatic paraffins (1). However, active
carbon exhibits a low catalytic activity at around 450 ° C because of
the slow desorption of hydrogen from the carbon surface. Thus, if a
suitable hydrogen acceptor such as oxygen, nitric oxide or ethylene
is incorporated in the reaction system the rate of dehydrogenation
increases several times (2). On the other hand the active carbon
impregnated with metals which can activate molecular hydrogen
adsorb a large amount of hydrogen fairly rapidly, while the
adsorption of hydrogen on active carbon itself is extremely slow.
This phenomenon is known as «spillover» of hydrogen (3).
We report in this study the promotion effect of several metals on the
dehydrogenation of paraffins over active carbon catalysts and the
analysis of the effect based on the reversal of hydrogen spillover.
2. EXPERIMENTAL
Catalyst : The active carbon used as the catalyst was commer-
SPILLOVER INVERSO DO
HIDROGÊNIO COMO
FASE CONTROLADORA
DA DESIDROGENAÇÃO
CATALÍTICA DE
PARAFINAS SOBRE
CARVÃO COM METAIS
cially available one (Shirasagi C) which was manufactured by Takeda
Pharmaceutical Co., made of wood, activated by steam and
has the specific surface area of 1200 m 2 /g. A calculated amount
of a metal nitrate was impregnated on the active carbon, which had
been boiled in the deionized water, from aqueous solution. The
active charcoal which was impregnated with a metal nitrate was
dried at 150 ° C for 3 hours in vacuo. When the reaction was
performed the catalyst was packed in a reactor and reduced in-situ
with hydrocarbon reactant under the same condition as that of the
reaction. Within one hour the metal nitrate was reduced to metal
and neither metal nitrate nor metal oxide was detected by X ray
analysis.
Reaction apparatus: The reaction apparatus used was a
flow type fixed bed set up operated under the atmospheric pressure.
The reactor was made of pyrex glass tube (12 mm i.d.) and
externally heated with electric furnace. Carrier gas (nitrogen or
hydrogen) was fed from a compressed gas cylinder after being
regulated and metered the flow rate and being dried by silicagel.
Isopentan was fed with a carry over method by feeding the carrier
gas into the isopentan container which is held at a predetermined
temperature. Gaseous product was analyzed by gaschromatgraphy. A
30 m capillary column cated with di-n-butyl maleate was used for
hydrocarbon analysis. AO 1.5 m MS. 5A column was used for
nitrogen, hydrogen and methane analysis.
The catalytic activity of active carbon for the dehydrogenation of aliphatic
paraffins were determined in the temperature range from 400 ° C to
507 ° C. It was found that the active carbon which were impregnated with
small amounts of metals such as Co, Ni, Fe and Cu exhibited very high
activities compared to the unsupported one although the metals
themselves showed little activities. The catalyst promoted by the metal
had a saturated activity which value was independent of the quantity and
the kind of metal. Carbon which were impregnated with the metals
adsorbed hydrogen reversibly. The phenomenon has been known as
'spillover,. While the amount of adsorbed hydrogen was independent of
the quantity and the kind of metal, the rate of adsorption depended on
them. The rate of hydrogen adsorption on the metal supported carbon
related well to the catalytic activity. The high activity of the metal
supported carbon catalyst is concluded to be based on the rapid release of
hydrogen from the carbon to the gas phase through the metal.
286
3. RESULTS AND DISCUSSION
3.1. DEHYDROGENATION OF ISOPENTANE IN THE
PRESENCE OF HYDROGEN ACCEPTOR
Fig. 1 shows the results of dehydrogenation of isopentane on an
unsupported active carbon in the presence and sbsence of hydrogen
acceptor. The selectivity of dehydrogenation was higher than 95
mole percent in every case. In the presence of 0 2, NO and C 2H4
little hydrogen was formed but water (in the case of O 2), water,
nitrogen and ammonia (in the case of NO) and ethane (in the case of
a)
-
30
ro
u
a -cnP
Q+ r - i
Cu
00
N ^
20
Cu
o
o
o
N
—95
None
Fe
•^
—90
10
v
G
o
U
500
450
400
Reaction temperature(°C)
450 °C
PI = 1 /6atm
None
50
100
W /F(g—cat•hr•mol)
Fig. 1
Dehydrogenation of isopentane by hydrogen acceptor.
Catalyst : active charcoal, W/F = 24 g-cat.hr/mol
Fig. 2
C2 H4) was formed in addition to isopentenes. It is obvious from
the figure that the dehydrogenation is markedly accelerated by
adding acceptors. Elementary reactions of the dehydrogenation and
the hydrogen transfer on the carbon catalyst are described as
follows :
C5 H12 + X
C5 H11 + X-H
C5 H11 + X —4—
C5H10 + X-H
(2)
2X H
•
2X + H2
(3)
2X-H + A
- 2X + AH 2
(1)
Effect of contact time
the high activity is considered to be caused of the interaction
between the metal and the carbon. The primary product is
substantially composed of thermodynamical equilibrium mixture of
isopentenes. The formation of isoprene is very small and skeletal
isomerization does not occur. Isopentenes decompose successively,
to form smaller molecules. The dehydrogenation of isopentane
follows the first order rate equation up to about 25% of conversion..
Above it the hydrogenation of isopentenes is not negligible.
(4)
2X-H + C5 H10
Where X is the active site of carbon and A is hydrogen acceptor.
The dehydrogenation is composed of elementary reactions (1), (2)
and (3) while the hydrogen transfer reaction is composed of
reactions (1), (2) and (4). From the thermodynamical view point
reaction (4) is far more easy to proceed than reaction (3). For
example, when the hydrogen acceptor is ethylene reaction (4) is
more exothermic by 33 kcal/mol than reaction (3). This suggest a
smaller activation energy and higher rate of reaction (4) than
reaction (3). Since hydrogen acceptors, especially nitric oxide and
ethylene, accelerate only the desorption of hydrogen from carbon
surface through reaction (4) the slowest step in the dehydrogenation
of paraffin (reactions (1) — (3)) is considered to be reaction (3).
Thus, if reaction (3) is accelerated by some method the overall
reaction should be accelerated.
2X + C5 H12(5)
Fig. 3 shows the catalytic activities of metal supported carbon
25
Fe
Co
Ni
Au
3.2, DEHYDROGENATION OF ISOPENTANE ON
METAL SUPPORTED CARBON CATALYSTS
5
Fig. 2 shows the results of dehydrogenation of isopentane on
carbon, Cu-carbon and Fe-carbon catalysts in the absence of
hydrogen acceptor. Under the same reaction condition Cu — C or
Fe —Ccatalystgivesmuch higher conversion than the unsupported
carbon catalyst. Since these metals exhibit little catalytic activities
when they are used on other support such as silicagel or porous glass
O
n^—
5
4—SS-+--'
450 °C
P1 = 1 /6atm
10 15
20
Metal content (wt%)
Fig. 3
Effects of metals — W/F = 24 g.hr/mol
287
catalysts as a function of metal content. Every metal tested exhibited
an promotional effect on the catalytic activity. The activity increases
with the increase in the metal content. However, when the activity
reaches a certain level, which value is independent of the kind of
metal, it never increases above the value even if more metal is added.
The saturated activity is never controlled by the thermodynamical
equilibrium as evidently from the fact that whereas the conversion
of isopentane obtained on a fully promoted catalyst is about 20 %,
the equilibrium conversion is about 60% under the same condition.
On the fully promoted catalyst the desorption of hydrogen from the
surface is considered to be not rate determining any longer. This is
discussed in more detail, later.
The smallest value of metal content in the catalyst which exhibites
the saturated catalytic activity differs from metal to metal. The
smaller the critical metal content is, the higher is the metal
efficiency. The efficiency of metal is in the following order: Co(0.3
wt %)>Ni(0.5 wt %)>Fe1(2 wt 96)>Cu(10 wt %)>Au(>10
wt %). Values in the brackets show the critical points of metal
content where the activities reach the saturated value. While the
order of the efficiency has no relation to the heat of hydrogen
adsorption on the metal, it consists well with the order of the
particle size of supported metal.
equilibrium amount of adsorbed hydrogen is independent of it. The
equilibrium amount of adsorbed hydrogen (0.12 mg-atm/g-cat) is far
excess to the amount of surface metal atom (0.016 mg-atom/g-cat),
when the metal content is 1 wt %). It shows that hydrogen spill over
from metal to carbon and the most part of adsorbed hydrogen exhist
on the carbon. The idea described above is also supported by the
facts shown in fig. 5 which are obtained on Co — C, Ni — C, Cu — C
20
Co < (40 A) < Ni(45 A) < Fe(200 A) < Cu(205 A) < Au(1000 A)
It suggests that the metal which disperses more effectively exhibits
higher efficiency.
O
0
2
1
3
4
5
6 7
Time (hr)
3.3. ADSORPTION OF HYDROGEN ON THE METAL
SUPPORTED AND THE UNSUPPORTED ACTIVE
CARBON CATALYSTS
Fig. 4 shows hydrogen uptake on Co-carbon catalysts at 400
°
76--
p
A^`r
Hydrogen uptake on metal 5 wt % / active charcoal
C
5(wt%)
c3P°
Fig. 5
o-
0.25
and Au — C catalysts which support 5 wt % of metal and have been
pretreated as Co — C catalysts shown in fig. 4. From fig. 5 the initial
rate of hydrogen adsorption on the metal supported carbon lies in
the following order: Co> Ni> Fe> Cu> Au. This order is well
consistent with the order of the extent metal dispersion. These are
summarized in table 1.
Table 1
Characteristics of metal- supported carbon catalysts
400 °C
PH O =4 cmHg
Catalyst
Metal
content
(wt %)
10
5
Time
15
(hr)
Fig. 4
Hydrogen uptake on Co / active charcoal
which have been used for the dehydrogenation of isopentane and
degassed at 450 ° C for 5 hours in vacuo. While the initial rate of
hydrogen uptake is nearly proportional to the metal content, the
288
Initial rate
Catalytic
of H2
activity
adsorption
(450°C)
(mmol/g.hr)
Metal
particle
size (4)
1
1.6
4.4
7.3
6.7
5.5
—
1000
205
200
45
0.25
0.01
0.03
0.04
—
0.40
0.05
1
0.10
40
5
0.42
6.9
7.0
C
Au — C
Cu — C
Fe — C
Ni — C
0
5
5
5
5
Co — C
Co — C
Co — C
Critical
point
(wt%)
—
10
10
2
1.0
—
40
0.3
4
3.4. DESORPTION OF HYDROGEN FROM METAL
SUPPORTED CARBON CATALYSTS
Fig. 6 shows the rate of hydrogen desorption from Co — C catalysts
as a function of the time after the isopentane feed cut. After the
feed of isopentane was stopped, the concentration of hydrocarbon
in the purge gas became negligible in a few minutes, however,
hydrogen could be detected in it for a long time. It suggests that
hydrogen comes out from the catalyst, not from hydrocarbon. The
rates of hydrogen desorption from the metal supported carbon
catalysts are much higher than that of the unsupported carbon. The
rate of desorption is higher when the metal content in the catalyst is
higher. These facts suggest that hydrogen atoms on the carbon
supplied from isopentane transfer to metal surface and recombine to
form hydrogen molecules and release to gas phase. It is just a
reverse process of hydrogen spillover.
0
1
4
2
3
R D (a.u.)
5
450 °C
• •
Cu /A.C.
•
Fig. 7
Relationship between the rate of hydrogen desorption, RD, and
catalytic activity
•
n
•
•
•
5 (wt%)
•
1 (wt% )
•
0.5(wt %)
•
•
^ _
Isopentane o
stopped 0♦
0
1
2
Time (hr)
Fig. 6
Effect of metal content on hydrogen desorption
3.5. THE RELATIONSHIPS BETWEEN THE CATALYTIC ACTIVITIES AND THE RATES OF SPILLOVER AND REVERSE SPILLOVER OF HYDROGEN
Fig. 7 and fig. 8 show the relationships between the catalytic
activities and the rates of spillover and reverse spillover of hydrogen.
The rates of reverse spillover which are measured as the rate of
hydrogen desorption at 10 minutes after the isopentane feed cut,
relate very well to the rates of isopentane dehydrogenation over the
same catalysts. As the rate of reverse spillover thus measured can be
considered to represents well the rate of hydrogen desorption during
the dehydrogenation the concept that the rate of dehydrogenation is
controlled by the process is strongly supported by the result shown
in fig. 7. On the other hand, the rates of hydrogen spillover
measured as the initial rates. of hydrogen adsorption corelate well to
,
0
ss
,
0.05
0.1 0.40 0.45
RA(mmol•g—cat -1 •hr -1 )
Fig. 8
Relationship between the initial rate of hydrogen spillover, RA, and
catalytic activity
the rates of dehydrogenation up to a certain point which value is
shown by the content and the kind of supported metal. The facts in
fig. 7 and fig. 8 are well under stood by the interpretation that the
initial rate of hydrogen spillover includes only the transfer step of
hydrogen atom from metal to carbon as the possible rate
determining process which should increase almost proportionally to
the number of contact point or to the area of contact face, while the
rate of dehydrogenation include more steps such as the transfer of
289
hydrogen from isopentans to carbon, the diffusion of hydrogen
atom on the carbon surface and the transfer of hydrogen atom on
the carbon to metal.
3.6. THE MODEL AND THE ENERGETIC ANALYSIS
OF THE DEHYDROGENATION OF PARAFFIN ON
METAL SUPPORTED CARBON CATALYSTS
The dehydrogenation on metal supported carbon catalysts is
represented by the following sequence of elementary reactions :
RH + X
X H + R'(a)
X + M
+ RH2
(6)
(7)
Table 2
X-H + M
X + MH
(8)
2M H
NI+ H2
Apparent activation energy, E, and activities
of dehydrogenation of isopentane
(9)
R•la) + X —•- R'+ X-H
Where R (a) is an adsorbed alkyl radial and M is the metal on
carbon. The schematic model of the hydrogen transfer is shown
in fig. 9. Reactions (7) and (9) are considered to be very rapid
Catalyst
Metal
content
(wt %)
.
E
(kcal.mo1 -1 )
C
O
26
Fe — C
Co — C
5
5
18
16
Ni — C
Cu — C
Cu — C
Cu — E
Au — C
5
16
24
2
5
10
5
22
19
19
Relative
activity
( 450 ° C
7.3
7.0
6.7
2.8
4.4
6.5
1.6
Fig. 9
Model of hydrogen reverse-spillover
because of reaction (7) being the decomposition of a radical and
reaction (9) being the desorption of hydrogen from metal surface.
Energy diagram of reaction (6) to (9) is described as follows : The
activation energy of reaction (8) (AE 2 ) should be smaller than
A E3, which is the activation energy of the desorption of hydrogen
from the unsupported carbon because the process being divided into
two steps on the metal supported carbon. It suggests elementary
reaction (8) being more rapid than reaction (3).
In the region where the catalytic activity depends on the metal
content, reaction (8) is considered to be rate determining because of
the low concentration of metal sites. However, if there exists a
sufficient amount of metal sites through which hydrogen atoms
transfer from carbon to metal, reaction (8) is no longer rate
determining but reaction (6) should determine the overall rate. The
concept described above is also supported by the fact that as shown
in table 2 the apparent activation energy where the catalytic activity
depends on the metal content decreases with the increase in the
metal content while it is independent of the content and the kind of
metals where the saturated catalytic activity is obtained and the rate
is considered to be determined by reaction (6). In the case the
apparent activation energy represents the activation entropy of
elementary reaction (6) (AE 1 ).
290
REFERENCES
1. K. FUJIMOTO, H. HAMADA, T. KUNUGI, Intern. Chem.
Engng., 13, 377 (19731.
2. K. FUJIMOTO, M. MASAMIZU, S. ASAOKA, T. KUNUGI,
Nippon Kagakukaisi, Japan, in press.
3. A. J. ROBELL, E. V. BALLOU, M. BOUDART, J. Phys. Chem.,
68, 2748 (1964).
RESUMO
A actividade catalítica do carvão activado para a desidrogenação de
parafinas alifáticas foi ensaiada na gama de temperaturas de 400 °C a
500 ° C. Verificou-se que o carvão activado impregnado com pequenas
quantidades de metais tais como o Co, Ni, Fe e Cu apresentava actividades
bastante elevadas comparadas com a do carvão sem aditivos, embora os
próprios metais mostrassem actividades baixas. O catalisador com
promotor metálico tem uma actividade que se satura, com valor que é
independente da quantidade e da qualidade do metal. O carvão
impregnado com os metais acima indicados adsorve o hidrogénio
reversivelmente. O fenómeno é conhecido como «spillover».
A quantidade de hidrogénio adsorvido é independente da quantidade e da
espécie dos . metais, mas a velocidade de adsorção depende deles. A
velocidade de adsorção do hidrogénio no carvão com suporte metálico está
bem relacionada com a actividade catalítica. Concluiu-se que a elevada
actividade do carvão com suporte metálico é devida à rápida passagem do
hidrogénio do carvão para a fase gasosa através do metal.
DISCUSSION
E. H. TAYLOR : Have you done experiments on the
combined action of an added metal and an acceptor of hydrogen ?
K. FUJIMOTO : Yes, we have done experiment using ethylene
as a hydrogen acceptor and Cu-C catalysts. Rate of isopentane
dehydrogenation increases slightly and the rate of hydrogen
formation increases almost linearly with the increase in the copper
content. On the other hand the rate of ethane formation decreases
with the copper content. So it is concluded that the supported metal
act as the port hole of hydrogen release even in the presence of the
hydrogen acceptor. However, the over-all reaction rate is not
accelerated so much by the combination of metal and hydrogen
acceptor, may be due to the rate determination at other step.
291