Cyclohexane oxidation and carbon deposition over metal oxide catalysts

Article abstract of DOI:10.1016/S0045-6535(00)00195-8

Catalytic activity of V, Mn, Ni, Cu, Zn, Mo, Zr and Ce oxides over an α-alumina support was evaluated for cyclohexane oxidation under oxygen deficient conditions in order to understand the relation between carbon deposition and catalytic activity/selectiv

Full text of DOI:10.1016/S0045-6535(00)00195-8

Chemosphere 43 /2001) 1079±1083
Cyclohexane oxidation and carbon deposition over metal oxide
catalysts
*
C. Hettige, K.R.R. Mahanama, D.P. Dissanayake
Department of Chemistry, University of Colombo, Colombo 3, Sri Lanka
Received 7 March 2000; accepted 19 May 2000
Abstract
Catalytic activity of V, Mn, Ni, Cu, Zn, Mo, Zr and Ce oxides over an a-alumina support was evaluated for cy-
clohexane oxidation under oxygen de®cient conditions in order to understand the relation between carbon deposition
and catalytic activity/selectivity. Carbon formation over the catalysts during the oxidation reaction was measured by
means of Fourier transformed infrared spectroscopy /FTIR). Catalysts Mn/Al2O3 and Ce/Al2O3, which are selective for
deep oxidation of cyclohexane, possessed relatively carbon free surfaces. The catalysts with relatively high carbon
deposition /V, Ni, Cu, Zn, Mo and Zr) produced CO in addition to CO2. Traces of formaldehyde were produced over
the catalysts Mo and V. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Carbon formation; VOC; Oxidation catalysts; Cyclohexane oxidation; Combustion
1. Introduction
A catalyst to be employed for VOC oxidation must
be highly active, resistant to carbon deposition, selective
for deep oxidation and structurally strong. In addition
to those properties, the catalyst itself must be non-toxic,
non-volatile and non-corrosive. Many of the studies on
oxide catalysts have been conducted under conditions
where ample oxygen is available for oxidation. However,
such conditions might not reveal the propensity of a
catalyst for carbon deposition because oxygen in the gas
phase can also participate in the oxidation of carbon
deposits. Secondly, many metal oxides produce partially
oxygenated compounds when hydrocarbons are oxi-
dized under low oxygen partial pressures. These partially
oxygenated compounds include carbonmonoxide, alco-
hols, aldehydes, ketones, acids and aromatic compounds
and a catalyst selective to such products may not be
desirable as a VOC oxidation catalyst. Therefore, one
should pay attention to the above aspects in designing
catalysts for VOC oxidation.
Exhaust emissions of internal combustion engines
and many industrial processes contain volatile organic
compounds /VOCs). Ceramic supported solid catalysts
containing precious metals has been the method of
choice to control pollutants emitted from such processes
/Koltsakis and Stamatelos, 1997). The use of precious
metals has made their application expensive and prac-
tically unwelcome in many cases as a method of pollu-
tion control. Therefore, in the recent times attention has
been drawn on research to develop catalyst systems that
are ecient, low cost and easily replaceable /Syczewska
and Musialik-Piotrowska, 1995; Cordi et al., 1997; Tahir
and Koh, 1997; Baldi et al., 1998; Ferri and Forni, 1998;
Hamoudi et al., 1998; Lahousse et al., 1998; Tra-
wczynski, 1998; Tsyrulnikov et al., 1998).
* Corresponding author. Tel.: +94-1-503367; fax: +94-1-
503148.
E-mail address: dpd@chem.cmb.ac.lk /D.P. Dissanayake).
In the present study, we investigated the catalysts
containing oxides of V, Mn, Ni, Cu Zn, Mo, Zr and Ce
over an a-alumina support for oxidation of cyclohexane
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 / 0 0 ) 0 0 1 9 5 - 8

1080
C. Hettige et al. / Chemosphere 43 $2001) 1079±1083
under oxygen de®cient conditions. Carbon deposition
over oxide catalysts was measured using Fourier trans-
formed infrared spectroscopy /FTIR) and correlated
with their selectivity to deep oxidation.
was calculated using GC analysis of the product stream.
Temperature of the catalyst bed was controlled to ‹2°C
using a tubular furnace and a temperature controller
/Cole±Palmer). Control experiments were carried out by
passing the same reactant mixture over a bed of quartz
chips at each temperature.
2. Experimental
Carbon deposition over catalysts was measured by
FTIR using a Jasco, model-5300 FTIR spectrometer.
Catalyst samples for FTIR measurements were pressed
into thin pellets transparent to the IR beam. These
pellets were then heated to 600°C in a ¯ow of oxygen to
oxidize any hydrocarbon residues. IR spectra of the
catalysts after the oxygen treatment showed no carbon
deposits on them. The same pellet was then treated in
the reactant gas stream at the desired temperature for 15
min. After the treatment, the catalyst sample was
transferred to the IR instrument and the spectrum was
recorded. The amount of carbon formed was estimated
from a previously prepared plot of percentage weight
gain, calculated by weighing the catalyst pellet Vs the
area under the absorption band in the region 1400±1700
cmÀ1. The values calculated from the plot were ran-
domly checked by weighing the catalyst before and after
the treatment in cyclohexane.
Catalysts were prepared using metal carbonates or
nitrates /general purpose grade) from BDH. All the
gases were obtained from Ceylon Oxygen company and
the purity was 99.99% or better. Cyclohexane used was
from Fluka Chemie AG. and was of general purpose
grade.
Catalysts were prepared by impregnating the support
/a-Alumina) with an aqueous solution of the active
metal to give a metal loading of 25% by weight. For this,
the required weight of the metal /in the form of nitrate
or the carbonate) was dissolved in a minimum amount
of distilled water. This was then added to the slurry of
the support made by mixing the support with distilled
water. The resultant metal salt and alumina slurry was
stirred well and evaporated to dryness. The solid was
dried in an oven at 120°C and then calcined at 700°C for
two hours, which was sucient to yield a stable metal
oxide catalyst.
Catalytic activity for oxidation of cyclohexane was
measured by passing a mixture of helium /100 ml
minÀ1), oxygen /10 ml minÀ1) and vapor of cyclohexane
/10 ml minÀ1 at standard temperature and pressure) over
300 mg of the catalyst in a quartz tubular reactor. All
the catalytic experiments were performed using this
composition of the reactants.
3. Results and discussion
The activities of metal/a-alumina catalysts are dis-
played in Figs. 1 and 2. All the experiments were carried
out under oxygen de®cient conditions as described in the
experimental section.
The gas ¯ows were controlled using needle valves and
the ¯ow rates were measured using bubble ¯ow meters
calibrated for respective gases. Cyclohexane was added
to the gas stream containing He and O2 by bubbling the
gas stream through cyclohexane in a container. The
temperature of the container was controlled to maintain
the desired composition of cyclohexane in the reactant
gas stream. The amount of cyclohexane in the reactant
gas stream was analyzed by gas chromatography and
this was equivalent to a gas phase volume of 10 ml minÀ1
at standard temperature and pressure. All the tubing
prior to and after the reactor was heated and maintained
above 120°C in order to prevent condensation of water
and cyclohexane. The reactor was maintained at the
desired temperature and 1 ml gas samples of the product
stream were injected to the gas chromatograph /GC) for
analysis using an online gas sampling valve. Under the
testing conditions, the amount of oxygen in the reactant
gas stream was sucient to convert only ꢀ10% of cy-
clohexane completely into carbondioxide and water. The
catalyst evaluation system was coupled to a GL science,
model GC-380 equipped with a thermal conductivity
detector. Conversion of cyclohexane into carbondioxide
Fig. 1. Percent conversion of cyclohexane over metal oxide
catalysts.

C. Hettige et al. / Chemosphere 43 $2001) 1079±1083
1081
Table 1
Product distribution at 500°C
Catalyst
% Selectivity to products
CO2
CO
H2CO
V/Al2O3
77
100
95
21
)
2
Mn/Al2O3
Ni/Al2O3
Cu/Al2O3
Zn/Al2O3
Mo/Al2O3
Zr/Al2O3
Ce/Al2O3
)
)
)
)
2
5
86
83
14
17
18
7
80
93
)
)
100
)
monoxide /Table 1). Had cyclohexane been converted
completely and only into carbondioxide and water over
Cu/Al2O3 catalyst, the conversion would have been low.
The catalysts Mo/Al2O3 Zn/Al2O3, Ni/Al2O3, V/Al2O3
and Zr/Al2O3 displayed moderate activities /Figs. 1 and
2). These catalysts recorded less than 100% selectivity
for deep oxidation of cyclohexane and produced car-
bonmonoxide in small quantities /Table 1). Although,
Ce/Al2O3 displayed a low activity, this catalyst recorded
100% selectivity to carbondioxide /Figs. 1±3).
Fig. 2. Percent conversion of oxygen over metal oxide catalysts.
As can be seen from the Figs. 1 and 2, the catalysts
Cu/Al2O3 and Mn/Al2O3 displayed the highest activity.
The activity over these catalysts reached the maximum
limit /10%) at 500°C and at this temperature almost 99%
oxygen conversion was observed /Fig. 2). The catalyst
Mn/Al2O3 displayed 100% selectivity to carbondioxide
and water /Fig. 3). However, the selectivity of Cu/Al2O3
catalyst for deep oxidation of cyclohexane was less than
90% at all the temperatures tested /Fig. 3). This shows
that the apparent high conversion of cyclohexane over
Cu/Al2O3 is mainly due to the formation of carbon-
FTIR measurements of catalysts were carried out in
the wave number region of 800±2400 cmÀ1. Catalyst
pellets were not transparent to the IR beam at the wave
numbers below 1000 cmÀ1. When the used catalysts were
analyzed by FTIR, growth of a broad band in the region
1400±1700 cmÀ1 was observed. This region covers aro-
matics and unsaturated hydrocarbons. A growth of this
band was observed as carbon was deposited over the
catalyst when exposed to the reacting gases at high
temperatures. Further, the catalysts V/Al2O3, Ni/Al2O3,
Zn/Al2O3, Mo/Al2O3 and Zr/Al2O3 became opaque to
the IR beam when heated in the reactant stream to
500°C or to higher temperatures due to carbon forma-
tion /indicated by `À' in Table 2). The amount of carbon
formed was estimated by measuring the area under the
absorption band. Carbon deposition results of the cat-
alysts are shown in the Table 2. An interesting obser-
vation made here was that the catalysts, which are
selective for deep oxidation, possess relatively carbon
free surfaces.
As can be seen from Table 2, carbon formation over
Mn/Al2O3 is very low and this is in accordance with its
high activity /Figs. 1 and 2) and selectivity /Fig. 3) for
deep oxidation. Mn has been reported as one of the most
e€ective catalysts for deep oxidation of hydrocarbons
/Syczewska and Musialik-Piotrowska, 1995; Tahir and
Koh, 1997; Baldi et al., 1998; Lahousse et al., 1998).
This may be due to oxygen storage ability of manganese
oxide. It has been reported that MnOx supported on
LaAlO3 has the ability to store oxygen /Chang and
McCarty, 1996). Stored oxygen may be eciently uti-
lized in the deep oxidation of surface carbon species
Fig. 3. Percent selectivity to carbondioxide over metal oxide
catalysts.

1082
C. Hettige et al. / Chemosphere 43 $2001) 1079±1083
Table 2
Carbon deposition over metal oxide catalysts
oxidize VOCs selectively to carbondioxide and water.
Among the catalysts tested, over the Mn/Al2O3 catalyst,
the oxidation of cyclohexane produced only carbondi-
oxide and water. Further, this catalyst recorded a very
low carbon deposition. The catalysts Ni/Al2O3, Mo/
Al2O3 and Zr/Al2O3 recorded heavy carbon deposition,
which resulted in the fragmentation of the catalyst into a
®ne powder. In comparison, carbon deposition was
moderate over the catalysts V/Al2O3, Cu/Al2O3, and Zn/
Al2O3, which produced partially oxidized products. It
can be suggested that deep oxidation is facilitated by
oxygen storage capacity of the catalyst. Strong adsorp-
tion of hydrocarbon leads to heavy carbon deposition.
Catalysts selective for deep oxidation of VOCs retain
relatively carbon free surfaces during reaction.
Catalyst/
temperature
% Weight gain due to carbon deposition
300°C
400°C
500°C
600°C
Al2O3
3
3
3
8
4
4
3
4
4
121215
19
13
V/Al2O3
Mn/Al2O3
Ni/Al2O3
Cu/Al2O3
Zn/Al2O3
Mo/Al2O3
Zr/Al2O3
Ce/Al2O3
2
)5a
17
)
14
30
2
18
18
5
30
18
16
2
)0
)
)0
17
)
2
16
17
15
a ± Indicates heavy carbon deposition, the pellet was not
transparent to the IR beam.
resulting a surface free of carbon deposits. Therefore, it
may be possible that all the hydrocarbon molecules that
are adsorbed over the surface of the catalyst be oxidized
completely into carbondioxide, without forming carbon
deposits. When carbon deposits are formed, inecient
oxidation yields carbonmonoxide /Table 1).
Acknowledgements
This work was funded by the National Science
Foundation, Sri Lanka under the grant RG/97/C/02.
The catalysts Ni/Al2O3, Mo/Al2O3 and Zr/Al2O3
have shown heavy carbon deposition /Table 2). The
metals Ni, Mo and Zr are known to adsorb hydrocar-
bons very strongly /Bond, 1997). Strong adsorption of
hydrocarbons over these catalysts could have made the
catalyst depleted of oxygen facilitating continuous
growth of carbon. Formation of carbon over Ni/Al2O3,
Mo/Al2O3 and Zr/Al2O3 catalysts caused the fragmen-
tation of the catalysts into a ®ne powder. Compared to
Ni, Mo and Zr containing catalysts, the carbon forma-
tion over V/Al2O3 and Zn/Al2O3 was low. These cata-
lysts did not fragment due to carbon deposition.
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