ON THE NATURE OF THE ACTIVE CARBON CATALYST FOR OXIDATIVE
DEHYDROGENATION OF ETHYLBENZENE TO STYRENE
MAKSIMOVA NADEZHDA, RODDATIS VLADIMIR, MESTL GERHARD,
SHLÖGL ROBERT
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, Germany
Tel: +49 (30) 8413 4499; Fax: +49 (30) 8413 4405; E-mail: schloegl@fhi-berlin.mpg.de
Recently, it was found that activated carbon is a catalyst for oxidative dehydrogenation
(ODH) of ethylbenzene (EB) to styrene (ST) [1-2]. Different compounds, i.e. transition metal
oxides [3], phosphates [4], and polymers [5], were found to be active and selective catalysts
for the ODH of EB to ST. The fact that different substances, which are mainly oxides and
which tend to coke deposition, are catalysts for ODH of EB indicates that carbon is probably
an active phase common for all ODH catalysts. In this context, these catalytic compounds are
rather supports for the active carbon phase. The characteristic induction period of all catalysts
in the ODH, during which coke builds up and activity increases, also points into this direction.
In order to test the hypothesis of the active carbon, different kinds of carbon, i. e.
carbon black, graphite and carbon nanotubes, were investigated for ODH to understand the
nature of the carbon species active as catalyst for this reaction. The catalytic behavior of those
carbons with time on stream was tried to correlate with the physico-chemical characterization
before and after the reaction by TEM, XPS and BET. It was found that the catalytic behavior
of different carbons for this reaction depended on the structure of the carbon surface and the
possibility to form the active Cn-Om species on the surface.
Carbon black – Lamp Black 101 (Degussa), a natural graphite – “AF special”
(Kropfmühl AG), and multiwalled carbon nanotubes obtained by decomposition of acetylene
over Fe catalysts after purification were used in this study. The reaction was performed at
temperatures between 450 and 550C with addition of oxygen in the stoichiometric ratio to EB
of 1:1. Ethylbenzene was evaporated at 35C and fed with a He flow (total flow 10 ml/min) to
the quartz tube reactor (4 mm i.d. x 200 mm). Catalyst particles (0.02 g) were held between
two quartz wool plugs in the isothermal zone of the reactor. Products were analyzed by an oneline gas chromatograph equipped with two columns for simultaneous analysis of aromatics and
permanent gases. The characterization of the carbons used before and after the catalytic tests
was done by TEM (Phillips CM200 FEG operated at 200kV), XPS (MgK line (h=1253.6
eV), and BET area analysis (nitrogen isotherms, Quantachrome JR II instrument).
Time on stream experiments with carbon black, graphite and carbon nanotubes as
catalysts (Fig. 1) have shown for carbon black (a) a high activity and then fast deactivation; for
graphite (b) an almost constant activity; and for carbon nanotubes (c) an induction period
during which the activity increases.
80
70
60
c
ST yield, %
50
40
b
30
20
a
10
0
0
20
40
60
80
100
120
140
160
180
200
220
240
260
Time on stream, min
Fig. 1. Catalytic activity in time on stream experiments at 550 of (a) carbon black;
(b) graphite; (c) carbon nanotubes.
BET analysis has shown (Tab. 1) a decrease of the specific surface area for carbon
black, a constant specific surface area for graphite, and increase of the specific surface area for
carbon nanotubes during the reaction. These changes of the BET surface area could be
correlated with catalytic behavior of these carbons during the reaction (Fig. 1).
Table 2.
Specific surface area of the samples before and after the reaction.
Sample
Carbon black
Initial
ST yield,
%
27
BET before
reaction,
m2/g
26
Graphite
37
11
37
13
Carbon nanotubes
18
26
45
47
Steady state
ST yield,
%
17
BET after
reaction,
m2/g
19
The comparative TEM characterization of the fresh and tested samples of carbon black
and graphite does not indicate important modifications in their structure. TEM indicates only
some modifications of the carbon nanotubes surface during the reaction (Fig. 2, a and b). Fig.
2, a shows that the walls of carbon nanotubes before the reaction consist of two layers: an
inner layer with conical graphite layers and an outer layer with amorphous carbon layers
parallel to the axis. After the reaction, the outer amorphous layer has disappeared (Fig. 2, b).
The ends of the inner conical layer were partially oxidized, but the wall of the nanotube was
still covered by amorphous carbon.
Fig. 2. TEM image of the wall of carbon nanotube (a) before and (b) after the reaction.
The XPS characterization of the fresh and used carbon nanotubes is shown in the Fig.
3. XPS proved a presence of two different carbon species on the surface of the fresh carbon
samples (spectra a). The main species has a binding energy of 2848 eV. This carbon species
can be assigned to graphitic sp2 carbon.
The second species has a binding energy of ~286 eV and is attributed to carbon in
carboxylic groups. After reaction (spectra b) the signal intensity of the carboxylic groups has
strongly decreased indicating their particular burn-off during the reaction. This implies that
these carboxylic species are not taking part in the catalytic ODH, which increased with time on
stream. It is here suggested that the catalytic ODH reaction takes place on some active surface
sites of graphene-like layers within the catalyst material, i.e. on the nanotubes.
To determine the nature of the active carbon species is important to design an efficient
and stable catalyst for oxidative dehydrogenation of ethylbenzene to styrene. Future
experiments will hopefully clarify the nature of such active species on a carbon surface.
Fig. 3. Carbon 1s core level spectra (Mg K) of the fresh (a) and used carbon nanotubes (b).
References
1.
2.
3.
4.
5.
Alkhazov T.G. Kinet. Katal. 13 (1972) 509.
Pereira M.F.R., Orfao J.J.M., Figueiredo J.L. Appl. Catal. A: General 184 (1999) 153.
Cortes A., Seoane J.L. J. Catal. 34 (1974) 7.
Vrieland G.E., Menon P.G. Appl. Catal. 77 (1991) 1.
Grunewald G.C., Drago R.S. J. Mol. Catal. 58 (1990) 227.