Applied Catalysis A: General 233 (2002) 21–33
Oxidative dehydrogenation of propane over Cr2 O3/Al2 O3
and Cr2 O3 catalysts: effects of loading, precursor and
surface area
Maymol Cherian a , Musti Someswara Rao a , Wei-Tin Yang b , Jih-Mirn Jehng b ,
Andrew M. Hirt c , Goutam Deo a,∗
a
b
Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208 016, India
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
c Material Research Laboratory Inc., 290 North Bridge Street, Struthers, OH 44471, USA
Received 12 October 2001; received in revised form 26 January 2002; accepted 5 February 2002
Abstract
Several alumina supported chromium oxide catalysts were prepared by varying the chromium oxide loading, precursors and
surface areas of the support. The prepared catalysts were characterized using BET, XRD, XPS and UV–VIS spectroscopic
techniques. The monolayer limit was observed to be ∼9 ␮mol Cr/m2 . Below monolayer limits, surface chromium oxide
species were present irrespective of precursors and surface area of the support. The activity of the prepared samples was tested
for ODH of propane. It was observed that the supported chromium oxide samples were active for the ODH reaction and that
propene was the major product. The activity and selectivity increased with loading up to monolayer limits and decreased for
higher loadings. Bulk Cr2 O3 was also studied for the ODH reaction and found to behave differently than the Cr2 O3 /Al2 O3
catalysts. The efficiency of each surface chromium oxide species on alumina to carry out the ODH of propane, the turn over
frequency (TOF), was relatively independent of chromium oxide loading for the monolayer catalysts. However, it appears
that a constant fraction of the surface chromium oxide species is active for the ODH of propane. This constant fraction of
the surface chromium oxide phase is given by the polymeric chromium oxide species. The inactive fraction is given by the
monomeric chromium oxide species that is not reducible. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Oxidative dehydrogenation; Chromia; Alumina; Propane; Propene; Surface active sites; Catalyst; Precursor; Surface area; Loading
1. Introduction
Oxidative dehydrogenation (ODH) or non-ODH of
hydrocarbons into their corresponding alkenes is an
industrially important process due to the increased
demand of alkenes [1]. Alkenes, especially ethene and
propene, are used as the feedstock for the petrochemical industry. Catalytic dehydrogenation is accepted
∗ Corresponding author. Fax: +91-512-590104.
E-mail address: [email protected] (G. Deo).
as a method for the production of alkenes more selectively even at higher conversion. Many transition and
noble metals and metal oxides have been successfully
tried for several dehydrogenation reactions. Widely
used dehydrogenation catalysts include supported or
unsupported Pt, Pd, Sn, V–Mg or Mo based oxides,
Co, Bi or Sb based oxides [2,3]. However, the deactivation of these catalysts with time-on-stream poses
a problem. In addition, obtaining high conversion
with high selectivity over many of these catalysts was
not always possible. Furthermore, thermodynamic
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 1 3 2 - 1
22
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
constraints limited the universal application of the dehydrogenation reaction. ODH provides an attractive
alternative [4]. Therefore, development of a catalyst that kinetically controls the ODH reaction is a
challenge in recent times.
Chromia-based catalysts have been studied for various petrochemical and environmental applications
[5,6]. Cr2 O3 supported on Al2 O3 is employed as the
commercial catalyst for dehydrogenation of alkanes.
Various studies were performed to gain an insight
about the activity of Cr2 O3 /Al2 O3 for dehydrogenation reactions [7–11]. Bulk Cr2 O3 was studied for the
oxidation of hydrocarbons and it was observed that
Cr2 O3 is efficient for complete combustion. However,
when chromia is supported on another metal oxide
(e.g. Al2 O3 , TiO2 , SiO2 , ZrO2 , etc.), the structure
and reactivity properties are altered and an improvement in the activity/selectivity of these chromia-based
catalysts is observed [12–15]. Various characterization techniques have been applied to study the
supported chromium oxide catalysts: these include
XRD, Raman, UV–VIS, TPR, XPS and ESR studies
[16–20]. Results of these characterization techniques
show that the supported chromia catalysts possess
a two-dimensional surface species below monolayer
loadings that are the active sites in several reactions.
The support-surface interaction stabilizes the Cr
species into different oxidation states, such as Cr3+ ,
Cr5+ and Cr6+ . The relative stability of different
chromium oxidation states depends upon the extent of
support-surface interaction, which is decided by the
acid–base properties of the support and the interaction
energy between the surface-support phases [21].
Several recent studies focus on the ODH of alkanes over alumina supported chromia catalysts. Flick
and Huff [22] studied the ODH of ethane over
Pt-Cr2 O3 /Al2 O3 catalysts and observed an increase
in C2 H4 selectivity and C2 H6 conversion compared
to the results over Pt/Al2 O3 catalysts. Grzybowska
et al. [23] and Al-Zahrani et al. [24] studied the influence of loading and preparation methods on the
structure-reactivity properties of Cr2 O3 /Al2 O3 catalysts for ODH of isobutane. These studies reported
that the catalytic properties of supported chromium
oxide are due to the surface chromium oxide species
formed as a result of chromium-support interactions. To the best of our knowledge only four studies
have reported the ODH of propane over chromium
supported on alumina catalysts [25–28]. These studies
are limited to analysis of a single Cr2 O3 /Al2 O3 catalyst. No information regarding the effect of chromium
oxide loading was given for the ODH of propane.
In the present study, the ODH of propane reaction is considered over a series of Cr2 O3 /Al2 O3 and
bulk Cr2 O3 samples to understand the effect of loading. The effects of precursor and surface area on the
structure-reactivity properties of Cr2 O3 /Al2 O3 catalysts are also considered. Finally, the structure property relationships of alumina supported chromium
oxide catalysts are discussed.
2. Experimental
2.1. Catalyst preparation
Supported chromia-alumina catalysts were prepared by incipient wetness impregnation technique.
The precursor used was chromium nitrate nonahydrate (Cr(NO3 )3 ·9H2 O, Aldrich, 99.98% purity) and
alumina (Condea) as support. The support was pretreated with incipient volumes of distilled water and
then calcined at 600 ◦ C for 6 h. The pretreated support and incipient volumes of solutions containing
predetermined amounts of precursor were intimately
mixed in order to prepare the catalysts with different
loadings of chromium oxide. The mixture was kept in
a dessicator overnight, followed by drying at 110 ◦ C
for 8 h, and at 250 ◦ C for another 8 h. Finally, the
samples were calcined at 600 ◦ C for 6 h. The prepared
catalysts were denoted as x% CrAl-600, where x% is
the wt.% loading corresponding to Cr2 O3 .
The effect of precursor was studied by preparing
Cr2 O3 /Al2 O3 catalysts using ammonium dichromate ((NH4 )2 Cr2 O7 ) and adopting the procedure
given above. The prepared samples were denoted as
x% CrAl-AD. Influence of the surface area on the
support-surface interaction was studied by preparing
alumina supports possessing different surface areas.
The original alumina support was subjected to 900 ◦ C
heat treatment and thereafter was repeatedly washed
with NH4 OH in order to remove any sodium impurities. The washed alumina supports were pretreated
and loaded with chromia by following the above procedure. The prepared samples were denoted as x%
CrAl-900.
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
Bulk Cr2 O3 was prepared by decomposing ammonium dichromate at 250 ◦ C. The decomposed sample
was calcined at 600 ◦ C for 6 h. XRD analysis reveals
the presence of Cr2 O3 crystals and no other phases
are present.
2.2. Physical characterization
The BET surface area analysis was done by a single point method using N2 adsorption at 77 K in a
Micromeritics Pulse Chemisorb 2700 apparatus. X-ray
diffraction (XRD) patterns were obtained on a 180 Debye flex-2002 X-ray diffractometer using Ni filtered
K␣ radiation from a Cu target (λ = 1.54056 Å); the
instrument was equipped with a monochromator.
X-ray photoelectron spectroscopy (XPS) studies
were done using Mg K␣ or Al K␣ in fixed analyzer
transmission (FAT) mode. The catalyst powder was
pressed between a stainless steel holder and a polished single crystal silicon wafer; each such sample
was then installed in a vacuum chamber of a Model
DS 800 XPS surface analysis system.
UV–VIS spectroscopy studies were done using a
Hitachi 20/120 spectrophotometer using a deuterium
lamp for studies between 190 and 340 nm and a tungsten lamp for wavelengths exceeding 340 nm. Barium
sulfate was used as the standard. Catalyst pellets were
supported on an absorbing background and then placed
in the sample holder. The spectra were obtained under
ambient conditions.
2.3. Chemical characterization
2.3.1. Temperature programmed reduction (TPR)
The TPR studies were carried out in a microreactor containing ca. 0.05 g of catalyst and attached to a
Micromeritis Pulse Chemisorb 2705 analyzer. Helium
was used as the carrier gas and used also to degas the
samples for 0.5 h prior to the reduction experiments.
The helium flow was set at 30 ml/min. A 10% H2 /Ar
mixture flowing at 40 ml/min was used for reduction
and the temperature was ramped from ∼100 ◦ C at a
rate of 10 ◦ C/min to 700 ◦ C. The amount of hydrogen
consumed was detected using a TCD.
2.3.2. Reactivity of the Cr2 O3 /Al2 O3
The samples were tested for the ODH of propane
in a down-flow quartz reactor at atmospheric pressure.
23
The reactor was a single piece of quartz with an inlet of 10 mm i.d. and 15 cm length and an outlet of
5 mm i.d. and 15 cm length. The two sections were tapered and the catalyst bed was placed just above the
tapered region on quartz wool. The reactor tube was
mounted vertically in a tubular furnace. The temperature of the reactor and the catalyst bed was measured by a thermocouple located inside the reactor
tube just above the catalyst bed and was controlled by
a PID temperature controller (FUJI Micro-controller
X Model PXZ 4). The product gases were sent for online analysis to a gas chromatograph (AIMIL-NUCON
5765) equipped with a methanizer. The carbon oxides
and hydrocarbons are analyzed in an FID mode using an activated alumina column. The propane flow
rate was adjusted through an electronic flow controller
(Bronkhost Hi-Tec, El-Flow Mass flow controller) and
the air flow rate was adjusted through a rotameter
(Eureka, Model SRS/MG-5) to maintain a 3:1 propane
to oxygen ratio. A physical mixture of 0.1 g of the
catalyst and amount of quartz glass powder required
to form a bed height of 1 cm was loaded into the reactor. Runs were performed at different temperatures
starting from 300 to 450 ◦ C with a constant total flow
rate of 42.7 sccm. The experiments were performed in
a differential mode and the conversions are limited to
below 5%.
Based on the inlet and outlet concentrations and
assuming differential reactor conditions one can calculate the conversion, selectivity, yield and TOF as
follows:
nc
conversion (%) = 100
nf
nhc
Nhc
selectivity (%) = 100
nc
Np
Nhc
nhc
yield (%) = 100
nf
Np
TOF (s−1 ) =
FA0 XA
nCr
where nc and nf are the number of moles of propane
consumed and fed, respectively, nhc the number of
moles of products (eg. propene, ethene, carbon oxides,
etc.) formed, Nhc and Np the number of carbon atoms
present in the products and propane, respectively, FA0
the molar flow rate, moles of propane fed per second,
24
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
XA the conversion of propane per gram of the catalyst, and nCr the moles of Cr per gram of the catalyst.
Since propene is the desired product, the propene selectivity is reported. The by-products are CO and CO2
and the by-product selectivity can be obtained by difference. Blank reactor runs were conducted and no
significant conversions were observed under the experimental conditions. For each catalyst, several runs
were taken at a particular temperature and the average
value is reported.
constant. Others have observed similar variations in
surface area [23,29,30]. The initial decrease in surface
area might be attributed to the plugging of pores. In
contrast, the surface area for pure Cr2 O3 sample is
27 m2 /g. The surface area of the Al2 O3 -900 sample is
134 m2 /g. The third column in Table 1 gives the surface concentration of chromium oxide, ␮mol Cr/m2 ,
based on the wt.% Cr2 O3 and the surface area of the
sample.
3.2. XRD
3. Results
Several Cr2 O3 /Al2 O3 catalysts were prepared as a
function of loading, precursor used and surface area
and were analyzed by XRD, XPS and UV–VIS spectroscopic techniques. The characterized samples were
then studied by TPR and ODH of propane. Initially,
the results of the CrAl-600 samples are presented, followed by the results related to the effect of precursor
used and surface area.
3.1. BET
The BET surface areas of pure alumina (Al2 O3 -600
and -900) and the Cr2 O3 /Al2 O3 samples were obtained
and are tabulated as the second column in Table 1.
The surface area values for CrAl-600 samples are between 140 and 185 m2 /g. The surface area initially
decreased with loading and then remained relatively
Table 1
BET surface area and TPR data of CrAl-600 and bulk Cr2 O3
samples
Samples
Surface
area (m2 /g
catalyst)
Concentration
(␮mol Cr/m2 )
Tmax
(◦ C)
H/Cr
Al2 O3 -600
Al2 O3 -900
5% CrAl-600
7.5% CrAl-600
10% CrAl-600
15% CrAl-600
20% CrAl-600
Bulk Cr2 O3
201
134
185
170
156
141
155
27
–
–
3.6
5.8
8.4
14.0
17.0
18.0a
n.d.
n.d.
388
374
373
377
–
–
1.9
1.6
1.5
1.2
n.d.: not detected.
a Based on a value 11 Cr/nm2 [9].
b Not shown.
b
b
292
0.7
The XRD patterns of pure Al2 O3 , 5, 10, 15, 20%
CrAl-600 samples and pure Cr2 O3 samples are shown
as Fig. 1. For all the CrAl-600 samples the major peaks
are observed at 46.1 and 67.8◦ with an additional broad
peak between 36 and 38◦ . These peaks are due to the
␥-Al2 O3 support. The broadness of the peak suggests
that this alumina support is amorphous. For the 20%
CrAl-600 sample additional peaks appear at 24.8, 32.8
and 54.6◦ , corresponding to peaks due to Cr2 O3 crystals. Thus, from the XRD it appears that the monolayer coverage is exceeded for the 20% Cr2 O3 /Al2 O3
sample and that an XRD amorphous chromium
oxide species is present for lower loadings. Similar
changes in XRD patterns are reported in the literature
[31,32].
3.3. XPS
The CrAl-600 and -900 samples were analyzed
using XPS. The XPS results reveal the presence of
only Cr, Al, O and C on the samples. Trace amounts
of F, Cl, and Sn were also detected. From the XPS
data the surface atomic concentration of Cr and Al
were calculated and the Cr/(Cr+Al)surface ratio was
determined. From the knowledge of chromia in the
sample the Cr/(Cr + Al)bulk ratio was also calculated. The Cr/(Cr+Al)surface ratio is plotted versus the
Cr/(Cr + Al)bulk ratio in Fig. 2. Fig. 2 shows that the
Cr/(Cr+Al)surface ratio increases with Cr/(Cr+Al)bulk
ratio up to a Cr/(Cr + Al)bulk value of 0.09 after
which the Cr/(Cr + Al)surface ratio remains constant. A Cr/(Cr + Al)bulk value of 0.09 corresponds
to ∼12% Cr2 O3 loading. Plateauing of such type
of plots has been attributed to multilayer formation
[33]. Similar results have been observed by others
[34,35].
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
25
Fig. 1. X-ray diffractograms of Al2 O3 , x% CrAl-600 samples and bulk Cr2 O3 .
3.4. UV–VIS
The spectra obtained in the diffuse reflectance
modes of the supported chromium oxide samples
and of pure Cr2 O3 physically mixed with Al2 O3 are
shown in Fig. 3. For the 5 and 10% loading, two
peaks centered at 276 and 370 nm are observed that
correspond to Cr6+ . These peaks of lesser intensity are also present in the 15% CrAl-600 samples.
As the loading increases above 15% Cr2 O3 loading, new broad peaks centered at 460 and 600 nm,
similar to those of bulk Cr2 O3 (top spectrum) are
observed. Thus, from UV–VIS also it appears that
the monolayer coverage is exceeded for the 20%
CrAl-600 sample. Furthermore, UV–VIS spectra reveal the presence of Cr6+ for low loadings (up to
15%) and gradual development of Cr3+ octahedral
symmetry transitions due to Cr2 O3 with increase
in loading. The studies are similar to the results
reported in the literature for Cr2 O3 /Al2 O3 catalysts
[36,37].
3.5. Temperature programmed reduction
TPR studies were performed for the prepared
Cr2 O3 /Al2 O3 samples and for pure Cr2 O3 . The TPR
profile of pure Al2 O3 sample does not reveal any features in the temperature range considered. The TPR
profiles for 5, 7.5, 10 and 15% CrAl-600 are shown
in Fig. 4, where the TCD signal is plotted versus the
temperature. A single Tmax is observed for all samples; the value is between 373 and 388 ◦ C. From the
peak area and the calibration amounts of hydrogen,
the hydrogen consumption can be calculated; the
value is then used to calculate the H/Cr ratio. Based
on the data in Fig. 4, the Tmax temperature and H/Cr
ratios are tabulated in the fourth and fifth columns
of Table 1 for the CrAl-600 catalysts. From the data
26
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
Fig. 2. (Cr/Cr + Al)XPS vs. (Cr/Cr + Al)bulk for x% CrAl-600 samples.
presented in Table 1, one may observe that the Tmax
temperature appears to be constant within experimental error. Furthermore, the H/Cr ratio appears to
decrease somewhat with loading; however, this variation is within experimental error. For bulk Cr2 O3 , the
Tmax and H/Cr ratio are 292 ◦ C and 0.7, respectively;
these have been included in Table 1 for reference.
Similar TPR results showing the difference between
Cr2 O3 and Cr2 O3 /Al2 O3 have also been observed
before [23].
3.6. ODH of propane
The ODH of propane reaction over the 5, 7.5, 10, 15
and 20% CrAl-600 samples, and that of pure Cr2 O3
and Al2 O3 were performed at different temperatures
ranging from 380 to 500 ◦ C. The conversion and selectivity data at a reaction temperature of 400 ◦ C are
plotted in Fig. 5 as a function of loading. The pure
Al2 O3 support did not show any activity towards
ODH under the present operating conditions. As the
chromia loading is increased, the propane conversion
increases, reaches a maximum at 15% loading, and de-
creases at higher loadings. The maximum conversion
for 15% CrAl-600 at 400 ◦ C is 3.1%. The selectivity is
observed to follow the same trend as that of conversion
with loading. The selectivity increases with loading
up to 15% and decreases slightly at higher loadings.
To observe the difference in reactivity over alumina
supported chromia oxide catalysts and bulk Cr2 O3 ,
the conversion and selectivity during ODH of propane
of 15% CrAl-600 and bulk chromia between 380 and
500 ◦ C is shown in Fig. 6. The conversion of both catalysts increases with temperature. However, the conversions of the supported chromium oxide catalysts are
greater than the conversions of bulk Cr2 O3 at all temperatures. For bulk Cr2 O3 catalysts, the selectivity decreases with temperature, however, for 15% CrAl-600
catalyst it reaches a maximum and remains constant at
higher temperatures. Conversion trends similar to the
15% CrAl-600 catalysts shown in Fig. 6 are observed
for the other CrAl-600 catalysts.
The yield of propene is calculated based on the data
given in Fig. 5 and values are tabulated in Table 2. The
turn over frequency (TOF, s−1 ) calculated based on the
total moles of Cr present in sample is also tabulated.
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
27
Fig. 3. UV–VIS spectra of x% CrAl-600 samples, and Cr 2 O3 + Al2 O3 physical mixture.
Table 2 shows that the propene yield increases with
loading up to 15% loading and then decreases. The
TOF, however, is observed to remain constant below
15% loading and then to decrease. The activation
energies of the Cr2 O3 /Al2 O3 samples based on a
first-order reaction and differential conditions were
also calculated and varied between 90 and 110 kJ/mol,
Table 2
Reactivity data for CrAl-600 and Cr2 O3 catalysts at 400 ◦ C
Samples
Activity
(× 106 mol
C3 H8 /(g s))
Yield
(%)
TOF
(× 103 , s−1 )a
5% CrAl-600
7.5% CrAl-600
10% CrAl-600
15% CrAl-600
20% CrAl-600
Bulk Cr2 O3
1.2
1.7
2.3
3.2
1.7
0.7
0.8
1.2
1.7
2.4
1.2
0.5
1.9
1.8
1.7
1.6
0.7
0.2
a
Related to chromium concentration given in Table 1.
which are similar to values reported earlier [38,39].
However, the activation energy for pure Cr2 O3 was
∼40 kJ/mol.
3.7. Effect of the precursor
The influence of the precursor on the structure and
activity of chromia species was studied by performing BET, TPR, XPS, Raman and activity studies on
5 and 10 wt.% of Cr2 O3 on Al2 O3 catalysts. These
Cr2 O3 /Al2 O3 samples are prepared using chromium
nitrate and ammonium dichromate as the different
precursors. Comparisons of the results of the above
studies are summarized in Table 3. It is observed that
the highest chromium–oxygen peak positions in the
ambient Raman spectra and the XPS surface to bulk
atomic ratio are similar. The H/Cr ratio from TPR data
shows a small variation, however, the Tmax positions
for the samples prepared from the two precursors
were similar. The surface area of the Cr2 O3 /Al2 O3
28
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
in Table 3. The difference in their values is within
5%.
3.8. Effect of surface area
Fig. 4. TPR profile for x% CrAl-600 catalysts.
catalysts prepared from ammonium dichromate had
a lower surface area, 165 and 145 m2 /g, compared
to the samples prepared from chromium nitrate. The
reactivity properties of Cr2 O3 /Al2 O3 samples prepared using both chromium nitrate and ammonium
dichromate were also studied for the ODH of propane
and the activity, selectivity and TOF are tabulated
The effect of surface area on the structure and activity properties of Cr2 O3 /Al2 O3 catalysts were studied
by using alumina supports possessing different surface
areas. The BET surface areas for CrAl-900 samples
also show a behavior similar to that of the CrAl-600
samples. The surface area for the CrAl-900 samples
varied between 92 and 134 m2 /g. The XRD patterns
for the CrAl-900 samples reveal strong peaks corresponding to crystals of θ -Al2 O3 , for which the major peaks are present at 2θ values at 46 and 67.7◦ .
For CrAl-900 samples, crystals of Cr2 O3 were not observed by XRD. XPS analysis of these samples reveals
the absence of impurities.
UV–VIS, Raman, TPR and the ODH of propane
studies were used to analyze the structure and reactivity properties of the CrAl-900 samples. The results of
these studies are summarized in Table 4. The Raman
band of the highest chromium–oxygen vibration is at
882–890 cm−1 , corresponding to monochromate and
polychromate species. The 10% CrAl-900 samples
had a Raman band of lesser intensity at 550 cm−1 corresponding to a small amount of Cr2 O3 crystals. The
spectroscopy studies on the CrAl-900 samples reveal
the presence of Cr6+ peak at ∼278 and ∼372 nm. The
Raman and UV–VIS data for CrAl-900 samples are
similar to those for CrAl-600 samples. The H/Cr and
Tmax values from the TPR experiments reveal similar
values for the CrAl-900 samples and the CrAl-600
samples, except for the H/Cr value for the 10%
CrAl-900 sample. For this sample, small amounts of
Cr2 O3 crystals were detected in the Raman, which
Table 3
Effect of precursor used on the properties of Cr2 O3 /Al2 O3 catalyst
Precursors
(CrNO3 )3 ·9H2 O
(NH4 )2 Cr2 O7
Wt.%
Cr2 O3
Ambient
Raman
(cm−1 )
XPS
Cr/Al
TPR
(H/Cr)
Tmax
(◦ C)
TOF
(× 103 ,
s−1 )a
Selectivity
(%)a
Ambient
Raman
(cm−1 )
XPS
Cr/Al
TPR
(H/Cr)
Tmax TOE
(× 103 ,
s−1 )a
Selectivity
(%)a
5
10
880
882
0.07
0.13
1.9
1.5
351
337
1.9
1.7
63
72
882
887
0.07
0.12
2.11
1.34
354
340
60
71
a
Based on propane ODH at 400 ◦ C.
1.7
1.8
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
29
Fig. 5. Variation of conversion (solid symbol) and selectivity (open symbol) with loading (wt.% Cr2 O3 ) for the CrAl-600 catalysts for
ODH of propane at 400 ◦ C.
gives a smaller H/Cr value (see Table 1). The selectivity and TOF for CrAl-900 samples for ODH of
propane at 400 ◦ C were calculated and are also shown
in Table 4. It is observed that the CrAl-900 samples
have a slightly higher conversion and selectivity than
the CrAl-600 samples.
4. Discussion
The
Cr2 O3
Cr2 O3
reveal
XRD and UV–VIS studies reveal that bulk
is formed on the alumina support at 20%
loading. Raman spectra on the same samples
that crystals of Cr2 O3 are present for the
Table 4
Effect of surface area on the properties of Cr2 O3 /Al2 O3 catalysts
CrAl-600 Al2 O3 -201 m2 /g
CrAl-900 Al2 O3 -134 m2 /g
Ambient
Raman
UV–VIS TPR
Tmax
(nm)
(H/Cr) (◦ C)
TOF
(× 103 ,
s−1 )a
Selectivity Ambient
(%)a
Raman
(cm−1 )
UV–VIS
(nm)
TPR
(H/Cr)
Tmax
(◦ C)
TOF
(× 103 ,
s−1 )a
Selectivity
(%)a
5
880
280
376
1.92
351
1.9
63
890
276
370
1.7
341
2.8
67
10
882
282
374
1.52
337
1.7
72
862
278
372
1.1
328
2.0
76
Wt.%
Cr2 O3
a
Based on propane ODH at 400 ◦ C.
30
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
Fig. 6. Variation of conversion (solid symbols) and selectivity (open symbols) with temperature for 15% CrAl-600 and bulk Cr2 O3 catalysts
for ODH of propane.
synthesized 15% CrAl-600 sample [40]. Consequently, Raman spectroscopy is more sensitive than
XRD and UV–VIS for the detection of Cr2 O3 crystals.
Data from XPS suggest that monolayer coverages,
defined as the coverages where crystals of Cr2 O3 are
first observed, is achieved at 12% Cr2 O3 loading.
Based on the surface area of the 12% Cr2 O3 /Al2 O3
sample, this corresponds to ∼9 ␮mol Cr/m2 . Similar values of monolayer coverage have also been
observed before [41].
Prior to bulk Cr2 O3 formation, the chromium oxide
species is molecularly dispersed. The presence of dispersed chromium oxide is observed as Cr6+ species
in the UV–VIS spectra and as mono- and polychromate species in the Raman spectra obtained on the
same samples under ambient condition [40]. Surface
chromium oxide in the +6 oxidation state is the predominant species present below monolayer coverages
[23,42]. Minor amounts of Cr5+ have also been proposed [43]. When the surface moisture is removed the
surface chromium oxide species on alumina is present
as tetrahedral monomeric and polymeric species different from those present under ambient conditions
[44]. Furthermore, the ratio of the two species does
not change as a function of surface coverage below monolayer loadings. Since XPS reveals that no
impurities are present in the Cr2 O3 /Al2 O3 samples,
similar results can be expected. Indeed Raman spectra of 15% CrAl-600 under dehydrated conditions
also reveals bands at 866 and 1002 cm−1 corresponding to monomeric and polymeric tetrahedral species
[45]. Thus, below monolayer coverage, which in the
present study is achieved at ∼12% Cr2 O3 loading,
the surface chromium oxide species is predominantly
present as Cr6+ species monomeric and polymeric
species.
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
The reactivity studies reveal that the activity and
selectivity of the Cr2 O3 /Al2 O3 catalysts depend upon
the type of the chromium oxide species present.
The maximum conversion is observed for the 15%
Cr/Al-600 catalyst. At this loading, the physical characterization studies suggest that the maximum amount
of surface chromia species is present and a small
amount of bulk Cr2 O3 is formed. Below 15% loading,
the alumina surface is also exposed to the reactant and
product molecules. It is this exposed alumina surface
below monolayer coverages that is partially responsible for the decrease in selectivity. As the loading
increases above 15%, the presence of bulk Cr2 O3
crystals of the sample increases, which results in a decrease in propane conversion and propene selectivity
and consequently in the yield. These results suggest
that the bulk Cr2 O3 species that is observed by XRD,
Raman and UV–VIS spectroscopies is less active than
the two-dimensional surface phase that is present below monolayer loadings. The difference in propane reactivity between the surface chromium oxide species
and bulk Cr2 O3 is clearly observed in Fig. 6, where
the variation of conversion and selectivity with temperature were significantly different. Furthermore, the
smaller activation energy for bulk Cr2 O3 suggests
that a different rate-determining step is involved. The
difference between the surface chromium oxide phase
and bulk Cr2 O3 is also observed in the TPR experiment. For bulk Cr2 O3 the Tmax is at 292 ◦ C, whereas
the surface chromium oxide species possesses a Tmax
temperature between 373 and 388 ◦ C. The H/Cr ratio
is also different, since the value for bulk Cr2 O3 is
0.7 and for the surface chromium oxide species it is
1.3–1.9. All of these factors clearly suggest that the
surface chromium oxide species that are present below
monolayer coverages are more active and behave differently than bulk Cr2 O3 towards the ODH of propane.
The TPR experiments reveal that the Tmax temperature and the H/Cr ratio are relatively insensitive to
the loading for the monolayer and sub-monolayer catalysts. Furthermore, the H/Cr ratio is less than 3. An
H/Cr value of 3 corresponds to the reduction of the
surface chromium oxide species from +6 to +3 oxidation state. In situ Raman spectroscopy studies reveal
that the polymeric surface chromium oxide species is
reducible and the monomeric species is not [46]. Correlating the H/Cr ratio and the in situ Raman studies
suggests that only a fraction of the surface chromium
31
oxide species is reducible and, consequently, the H/Cr
ratio is less than 3. This reducible fraction is given by
the polymeric chromium oxide species. Furthermore,
since the ratio of monomeric and polymeric species is
independent of coverage, a constant H/Cr ratio is observed for the monolayer and sub-monolayer catalysts.
To understand the effect of coverage of the
two-dimensional surface chromium oxide species, we
calculated the TOF based on the total chromium oxide
loading (see Table 2). The TOF is relatively constant
for the monolayer Cr2 O3 /Al2 O3 catalysts. A constant
TOF value is also observed for supported vanadium
oxide catalysts for the ODH of propane [47,48] and
ethane [49]. If one assumes that a redox mechanism
occurs, it appears that reducible polymeric species
are active for the ODH of propane. Consequently, the
TOF is independent of coverages, since the polymeric
chromium oxide species form a constant fraction of
the total surface chromium oxide sites. Thus, a constant fraction of the total surface chromium oxide
phase represented by the polymeric chromium oxide
species is active for the ODH of propane, which gives
rise to a TOF value that is independent of coverage.
Several mechanisms have been proposed for the
ODH of propane [1,50,51]. One of the mechanisms
proposed for supported metal oxide catalysts involves
three routes [51]. In this mechanism, propane is directly converted to propene (k1 ) and carbon oxides
(k2 ), and a secondary reaction in which propene is
converted to carbon oxides (k3 ). It was suggested that,
as loading increases, the selectivity increases due to
the formation of large domains of poly vanadates or
poly molybdates, which decrease the k2 /k1 ratio. In the
present study, however, the ratio of monochromates to
polychromates does not change with respect to loading [44]. Therefore, it appears that the increase in selectivity with loading for supported chromium oxide
catalysts is not controlled by the decrease in k2 /k1 ratio associated with the polychromates. Moreover, the
selectivity is improved with an increase in chromium
oxide loading due to the coverage of exposed support
sites, which otherwise would degrade propene to carbon oxides. For a more detailed kinetic model for the
ODH of propane over supported chromium oxide sites,
additional in situ characterization studies are required.
The choice of the precursor used to form the surface chromium oxide species appears to be immaterial, since the catalysts prepared by using ammonium
32
M. Cherian et al. / Applied Catalysis A: General 233 (2002) 21–33
dichromate or chromium nitrate do not differ significantly in their physical and chemical characteristics
(see Table 3). Different surface areas of the alumina
support also do not affect the physical nature of the
surface chromium oxide species, since the Raman and
UV–VIS spectra are not significantly different. Due to
a lower surface area of the support, a small amount
of Cr2 O3 crystals are observed for the 10% CrAl-900
sample, since about 9% Cr2 O3 can be accommodated
on this alumina. However, minor differences in reactivity between the different surface area Cr2 O3 /Al2 O3
catalysts are observed. These differences in TOF may
be related to a slightly higher effectiveness factor of the
CrAl-900 samples [52]. The higher effectiveness factor arises from larger pores associated with lower surface areas of the same material. As mentioned before,
the selectivity to propene for the CrAl-900 samples is
higher, since the amount of exposed support-surface
is lower when compared with the CrAl-600 samples.
These observations regarding the effect of precursors
and surface area are supported by the hypothesis that
the preparation method or surface area do not play a
significant role in preparing the surface metal oxide
phase provided the support matrix remains unchanged
[53]. Consequently, the physical and chemical characteristics are similar for both catalysts, since the same
surface chromium species is present.
In summary, below monolayer coverage, molecularly dispersed surface chromium oxide species are
formed independent of the precursor or surface area
of the support. These surface species are active for
the ODH of propane to propene and show a greater
activity and selectivity compared to bulk Cr2 O3 . As
the concentration of the surface chromium oxide is
increased, the average effectiveness of the chromium
oxide species, given by the TOF, is relatively constant
and the selectivity to propene increases. The selectivity increases with increase in loading, since the exposed Al2 O3 surface decreases. The independence of
the TOF for surface chromium oxide species is related
to the constant fraction of the polymeric chromium
oxide species present.
5. Conclusions
Several Cr2 O3 supported Al2 O3 catalysts were
prepared by incipient wetness impregnation method.
The samples were analyzed by BET, XRD, TPR,
XPS and UV–VIS characterization techniques. The
presence of surface chromates was confirmed and the
monolayer limit was estimated at ∼9 ␮mol Cr/m2 .
The characterization studies suggest that the surface
chromia species are predominantly present as Cr6+
species independent of the precursor used and of the
surface area of the support. The prepared samples
were observed to be chemically active for the ODH
of propane. The propane conversion increases with
loading up to monolayer coverages and decreases for
higher loadings since bulk Cr2 O3 are less active than
the surface chromium oxide species. The propene
selectivity increases with loading due to the decrease
in the amount of exposed support-surfaces. It appears
that a constant fraction of surface chromium oxide
sites are active for the ODH of propane reaction; such
sites give a TOF that is independent of surface coverage. This constant fraction is given by the polymeric
chromium oxide species, which form the active sites
for the ODH of propane.
Acknowledgements
The authors gratefully acknowledge Prof. Jitendra
Kumar for assisting in obtaining the UV–VIS and Prof.
I.E. Wachs for providing the Raman data. This work
has been partially supported by the financial assistance
of The Department of Science and Technology, India.
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