Selectivity Reversal in Oxidative Dehydrogenation of Ethane with CO2 on CaO–NiO/Al2O3 Catalysts

Article abstract of DOI:10.1007/s10562-016-1899-y

Abstract: CaO–NiO/Al₂O₃catalysts were prepared by wet impregnation method, keeping the NiO composition constant at 25?wt.% and varying the CaO composition in the range 6–12?wt.%. The catalysts were evaluated for oxidative dehydrogenation (ODH) of ethane with CO₂at atmospheric pressure and in the temperature range of 600–700 °C. The physicochemical characterization of the catalysts was carried out using X-ray diffraction (XRD), BET surface area, Temperature programmed reduction (TPR), Temperature programmed desorption of CO₂(CO₂-TPD), H₂chemisorption, Carbon analysis and Fourier transform infrared (FT-IR) spectroscopy. NiO/Al₂O₃showed higher ethane and carbon dioxide conversions, but with lower ethene selectivity. The addition of CaO in NiO/Al₂O₃reversed the course of reaction leading to high selectivity to ethene, although at reduced conversions. 9?wt.% CaO containing NiO/Al₂O₃emerged as the most promising catalyst giving the highest yield of ethene. CaO addition increased the basicity and the coke resistance. The shift in selectivity with CaO addition is explained in terms of more probability for reverse water gas shift reaction on a basic site rather than the hydrocracking reaction on the acidic sites of Ni/Al₂O₃. The 9?wt.% CaO-25% NiO/Al₂O₃catalysts were also prepared by changing the sequence of impregnation and by adopting simultaneous deposition of the two oxides on Al₂O₃. The activity of these catalysts was compared to identify the best method of catalyst preparation. Graphical Abstract: Ca promotion of NiAl modifies the surface basic property increasing ethene/methane selectivity ratio from 1 to 4, and enhances coke resistance. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1899-y

Catal Lett
DOI 10.1007/s10562-016-1899-y
Selectivity Reversal in Oxidative Dehydrogenation of Ethane
with CO on CaO–NiO/Al O Catalysts
2
2
3
1
T. V. Sagar · M. Surendar · D. Padmakar · G. Parameswaram · N. Lingaiah ·
1
1
1
1
1
K. S. Rama Rao · I. A. K. Reddy · C. Sumana · P. S. Sai Prasad
2
3
1
Received: 17 March 2016 / Accepted: 27 October 2016
©
Springer Science+Business Media New York 2016
Abstract CaO–NiO/Al O catalysts were prepared by
Ni/Al O . The 9 wt.% CaO-25% NiO/Al O catalysts were
2
3
2
3
2
3
wet impregnation method, keeping the NiO composition
constant at 25 wt.% and varying the CaO composition
in the range 6–12 wt.%. The catalysts were evaluated for
also prepared by changing the sequence of impregnation
and by adopting simultaneous deposition of the two oxides
on Al O . The activity of these catalysts was compared to
2
3
oxidative dehydrogenation (ODH) of ethane with CO at
identify the best method of catalyst preparation.
2
atmospheric pressure and in the temperature range of 600–
Graphical Abstract Ca promotion of NiAl modiꢀes the
surface basic property increasing ethene/methane selectiv-
ity ratio from 1 to 4, and enhances coke resistance.
7
00°C. The physicochemical characterization of the cata-
lysts was carried out using X-ray diꢁraction (XRD), BET
surface area, Temperature programmed reduction (TPR),
Temperature programmed desorption of CO (CO -TPD),
2
2
H chemisorption, Carbon analysis and Fourier transform
2
infrared (FT-IR) spectroscopy. NiO/Al O showed higher
2
3
ethane and carbon dioxide conversions, but with lower
ethene selectivity. The addition of CaO in NiO/Al O
2
3
reversed the course of reaction leading to high selectivity to
ethene, although at reduced conversions. 9 wt.% CaO con-
taining NiO/Al O emerged as the most promising catalyst
2
3
giving the highest yield of ethene. CaO addition increased
the basicity and the coke resistance. The shift in selectiv-
ity with CaO addition is explained in terms of more prob-
ability for reverse water gas shift reaction on a basic site
rather than the hydrocracking reaction on the acidic sites of
Keywords Ethane to ethene · Carbon dioxide · ODH of
ethane · Ca doping
Electronic Supplementary Material The online version
of this article (doi:10.1007/s10562-016-1899-y) contains
supplementary material, which is available to authorized users.
1 Introduction
Ethene is one of the largest-volume petrochemical mono-
mer used in the production of plastics, ꢀbres and other
organic chemicals [1, 2]. The global production of ethene
touched 160 million tonnes in 2015, increasing at a rate
of 4% per annum [3]. Commercially, ethene is produced
by steam cracking of higher alkanes or naphtha, a highly
endothermic and energy intensive reaction [4–6]. Catalytic
oxidative dehydrogenation (ODH) of ethane to ethene has
emerged as an alternative to steam cracking due to its many
*
P. S. Sai Prasad
saiprasad@iict.res.in
1
2
3
I&PC Division, CSIR-Indian Institute of Chemical
Technology (CSIR-IICT), Hyderabad 500 007, India
Chemistry Department, National Institute of Technology
(NITW), Warangal 506004, India
Chem. Engg. Division, CSIR-Indian Institute of Chemical
Technology (CSIR-IICT), Hyderabad 500 007, India
1
3

T. V. Sagar et al.
advantages like overcoming the equilibrium limitation and
a considerable reduction in energy demand [7]. The pres-
120°C overnight and ꢀnally calcined in air at 750°C for
4 h. It is denoted as NiAl in the present text. The NiAl thus
prepared was then impregnated with required quantities of
aqueous Ca(NO ) ·4H O (M/s. SD Fine, India, A.R. grade),
ence of O as a co-reactant decreases the endothermicity of
2
the cracking reaction, but leads to the production of over
3
2
2
oxidation products like CO . In order to obviate this disad-
such that the ꢀnal catalysts contained 6, 9 and 12 wt.% of
CaO (represented as 6CaNiAl, 9CaNiAl and 12CaNiAl,
respectively). A control catalyst with 9 wt.% CaO on Al O
x
vantage, the mild oxidant CO is preferred to increase the
2
selectivity and hence the yield of ethene [8–10]. The uti-
2
3
lization of CO for ODH also helps to mitigate the global
warming problem [11]. However, catalyst deactivation due
to coking during the reaction is realized in the ODH of
was prepared by wet impregnation and is denoted as 9CaAl.
Two more catalysts were prepared, one by depositing CaO
ꢀrst followed by NiO and the other by a simultaneous depo-
sition of CaO and NiO on Al O support. Evaporation, dry-
2
ethane even with CO as the oxidant [12, 13]. Thus, any
2
2
3
catalyst development activity on ODH of ethane should
address coke formation during the reaction, apart from
achieving high activity and selectivity.
ing and calcination steps for all the Ca containing catalysts
were similar to that of NiAl catalyst.
Ni based catalysts have been extensively used for the ODH
of ethane [14, 15]. Literature reveals that in the case of Ni,
the free availability of electrophilic O species on its surface
2.2 Catalyst Characterization
−
BET surface area of the catalysts was determined by N2
adsorption at liquid nitrogen temperature on a SMART SORB
92/93 instrument (M/s. SMART Instruments, India). Prior to
the measurement, degasiꢀcation of samples was carried out at
150°C for 2 h. XRD patterns of the catalysts were obtained
on an Ultima-IV diꢁractometer (M/s. Rigaku Corporation,
Japan) using nickel-ꢀltered Cu Kα radiation (λ=1.54 Å). The
measurements were recorded in steps of 0.045° with a count
time of 0.5 s and in the 2θ range of 10–80°. Identiꢀcation of
the crystalline phases was carried out with the help of JCPDS
ꢀles. TPR studies were performed using a home-made appa-
ratus. Catalyst sample (50 mg) taken in a quartz reactor, was
ꢀrst treated in argon ꢃow at 300°C for 2 h. Then the reduc-
decreases the ethene selectivity [16–18]. In order to obviate
this disadvantage the catalysts are modiꢀed with Al ions.
Modulation of the chemical properties of the highly reactive
−
O species around Ni results in the formation of high selec-
tive sites useful for the ODH of ethane [19, 20]. Smolakova
et al. have also substantiated this claim showing higher selec-
tivity on Ni-Al hydrotalcites [21]. Of late, there have been
reports highlighting the advantages of Ca in the ODH cata-
lysts. For example, Ca ions, in solid solution with CeO , have
2
been shown to improve the eꢂciency of CO used as a soft
2
oxidant [22]. Hence, it is prudent to study the inꢃuence of Ca
addition to Ni-Al catalysts on ethene selectivity.
In this study, an attempt has been made to delineate the
inꢃuence of CaO addition on ethane conversion and ethene
yield of NiO/Al O catalysts during the ODH of ethane
tion preceded under 10% H /Ar gas mixture at a ꢃow rate of
2
30 ml/min and a heating rate of 5°C/min from 25 to 800°C,
keeping the temperature constant for 1 h after reaching the
ꢀnal temperature. The hydrogen consumption was monitored
using a TCD equipped gas chromatograph (Varian, 8301). H2
chemisorption experiments were conducted in a quartz reactor
system with 50 mg of catalyst. The catalyst was ꢀrst reduced
2
3
with CO . Identifying the optimum composition of CaO in
2
the catalyst to get maximum yield of ethene, and the fac-
tors involved in achieving this, are the main objectives of
this work. Besides, studying the eꢁect of the addition of
this basic species on the extent of coke formation during
the reaction is yet another objective.
under H /Ar gas mixture at a ꢃow rate of 30 ml/min at 600°C
2
for 1 h, and then the reactor was cooled to room temperature.
A series of H pulses were given until the saturation point was
2
reached. The hydrogen consumption in the pulses was esti-
2
Experimental
mated gas chromatographically. CO -TPD was carried out on
2
a BELCAT II (M/s. BEL Japan, Inc.) apparatus using a TCD
detector. In a typical experiment about 50 mg of the oven
dried sample was taken in a quartz tube. The sample was ꢀrst
pre-treated at 300°C for 1 h in pure helium (99.9%, 50 ml/
min) and then saturated with CO (10% CO –90% He mixture
2
.1 Material Synthesis
Heracleous et al. in a study on Ni based catalysts for ODH
of ethane, reported an optimum composition of 25 wt.% for
NiO [15]. Based on this report, NiO/Al O with a constant
2
2
gas) at 80°C, passed at a ꢃow rate of 50 ml/min for 1 h. After
ꢃushing with He at the same temperature to remove phy-
2
3,
NiO composition of 25 wt.% in the ꢀnished catalyst was
ꢀ
rst prepared by the impregnation of commercial γ-Al O
sisorbed CO , the TPD analysis was carried out from ambi-
ent temperature to 800°C at a heating rate of 10°C/min. The
2
3
2
2
(
M/s. Sud Chemie, S.A.: 201 m /g) with the required quan-
tity of an aqueous solution of Ni(NO ) ·6H O (M/s. SD
amount of CO evolved was calculated from the peak area of
3
2
2
2
Fine, India, A.R. grade). After evaporating the excess solu-
tion on a water bath, the solid was dried in an air oven at
the already calibrated TCD signal. FT-IR spectra of the used
catalysts were recorded on a Biorad-Excalibur series (USA)
1
3

Selectivity Reversal in Oxidative Dehydrogenation of Ethane with CO on CaO–NiO/Al O …
2
2
3
spectrometer using the KBr disc method. Enough caution was
exercised to see that the samples were not exposed to external
atmosphere. The coke content in the spent catalysts was deter-
mined on a CHNS analyzer (M/s. ElementaV, Germany).
trend is due to possible pore ꢀlling, as also observed by Dias
et al. [23]. Catalysts with low surface area are found to be
advantageous in terms of controlling the side reactions [24].
3
.2 X-ray Diꢁraction Studies
2
.3 Activity Test
Figure 1 shows the XRD patterns of the fresh catalysts.
NiAl displays broad diꢁraction peaks at 2θ values of 37.2,
45.6 and 66.6° corresponding to NiAl O (JCPDF:78-1601)
The evaluation of the catalysts was performed in a ꢀxed-bed
stainless steel reactor passing a mixture of He/C H /CO
2
6
2
2
4
at a ratio of 30/20/10, at atmospheric pressure. The activ-
ity tests were carried out using 0.5 g of catalyst diluted with
the same amount of ceramic beads and placed between two
quartz wool plugs in the reactor. Prior to the activity meas-
urements, the sample was pretreated in He ꢃowing at 30 ml/
min at 500°C for 4 h. The activity tests were conducted in the
temperature range of 600–700°C. The outlet gases were ana-
lyzed by an online Nucon 5765 gas chromatograph equipped
with a Porapak-Q column, using He gas as carrier and a TCD
detector. After attaining the required temperature and reach-
ing the steady-state over a period of 1 h, the product analysis
was duplicated and the average value considered. The accu-
racy was within the error margin of ±3%.
and weak peaks due to NiO. However, the presence of
γ-Al O (JCPDF:29-0063) cannot be excluded as its peak
2
3
positions are very close to that of the spinel. The agglom-
erated CaO(Al O ) phase is seen with peaks at 37.6, 46.0
2
3 6
and 67.1° (JCPDF-76-0665) in the patterns of CaO con-
taining catalysts; 9CaAl showing its predominant forma-
tion. In the presence of CaO the samples show noticeable
levels of NiO. The basically broad and low intense peaks in
the patterns indicate high interactions of active metal oxide
species with the support giving the possibility for the for-
mation of spinel structures.
3.3 Temperature Programmed Reduction
The following formulae were used:
The TPR patterns shown in Fig. 2 distinguish two major
peaks appearing at low (500°C) and high (800°C) tem-
Moles of i(in) − Moles of i (out)
Conversion of i =
× 100%,
Moles of i (in)
2
+
peratures, respectively. Both the peaks represent the Ni to
0
where i=reactant C H , CO
Ni reduction; the low temperature peak due to the reduc-
2
6
2
tion of NiO with medium oxide-support interaction and the
high temperature one due to the reduction of the Ni phase
in NiAl O . With the addition of CaO the intensity of the
j
Selectivity of j = ∑ × 100%,
j
2
4
where j=product species C H , CH
4
high temperature peak decreases, revealing that the reduc-
tion of NiO in the spinel is considerably decreased. In the
case of high CaO containing catalyst (12CaNiAl) a new
peak appears at 280°C, which may be due to the reduction
of free NiO on the surface of CaO agglomerated on Al O ,
2
4
ꢀ
ꢁ
%.
1
Yield = (Conversion × Selectivity) ×
1
00
2
3
as evidenced by the XRD studies and also reported in the
literature [23–27].
3
Results and Discussions
3
.1 Speciꢀc Surface Area
3
.4 CO -Temperature Programmed Desorption
2
Table 1 displays the speciꢀc surface area values of the
catalysts which trace a decreasing trend after the addition
of CaO. 9CaAl shows a surface area of 101.1 m g . This
CO -TPD study reveals the acid/base nature of the cata-
lysts. The patterns obtained on the catalysts are shown in
Fig. 3. The desorption bands with peaks appearing at about
2
2
−1
Table 1 Speciꢀc surface area,
Catalysts
Speciꢀc surface
area (m /g)
Dispersion (%)
Particle size
(nm)
Basicity (m
mol/g)
Extent of
coking
(
Ni dispersion and particle size
of the fresh and carbon content
of the used catalysts
2
wt.%)a
NiAl
114.5
101.9
53.3
4.1
3.4
4.8
3.7
24
29
20
26
0.184
0.618
0.866
0.711
2.89
2.07
0.53
0.78
6CaNiAl
9CaNiAl
12CaNiAl
73.6
aCoke formation expressed in terms of the C content of the used catalyst
1
3

T. V. Sagar et al.
Fig. 1 XRD patterns of cal-
cined catalysts
Fig. 2 TPR patterns of calcined catalysts
Fig. 3 CO -TPD patters of calcined catalysts
2
2
00, 400–500 and 675–700°C on the CaO containing cata-
of the catalysts. Therefore, the metal dispersion and par-
ticle size of Ni in the catalysts were determined to reꢃect
the metal oxide content on the surface and the values are
reported in Table 1. Ni dispersion on NiAl is obtained as
4.1% with the corresponding particle size of 24 nm. CaO
promotion has not varied the Ni dispersion (3.4–4.8%) and
the Ni particle size (20–29 nm) much. It appears that CaO
has covered the surface sites. However, in numerical terms
the dispersion is the highest and the corresponding particle
size is the lowest in 9CaNiAl.
lysts correspond to the weak, medium and strong basic
sites [28]. The intensity of the high temperature desorption
peak is seen increasing upto 9CaNiAl due to increase in
the strong basic sites on the surface of the catalysts. Fur-
ther addition of CaO to 12 wt.% (12CaNiAl) has shown a
decreasing trend. Though NiAl has shown a similar pattern,
the intensities of the peaks are much lower than the CaO
containing catalysts.
3
.5 H Chemisorption
2
3
.6 Catalytic Activity
Studying Ni-Nb-O mixed oxide catalysts, Heracleous and
Lemonidou [29] observed segregation phenomena between
Nb O and NiO phases for Nb/Ni atomic ratio >0.176. The
same authors also identiꢀed a correlation between activity
of the catalysts and surface exposed Ni content. Thus, metal
oxide dispersion is a crucial factor in the ODH performance
The activity results of the catalysts are reported in Table 2.
At a reaction temperature of 700°C, NiAl shows higher con-
version values for both ethane and carbon dioxide (26.2 and
45.1%, respectively) compared to the CaO containing cata-
lysts. However, its ethene selectivity is much lower (52%).
2
5
1
3

Selectivity Reversal in Oxidative Dehydrogenation of Ethane with CO on CaO–NiO/Al O …
2
2
3
CaO addition has lead to drop in conversion of the reac-
tants. What is interesting is the attainment of high selectiv-
ity towards ethene. 9CaNiAl has appeared to be promising in
terms of ethane conversion and ethene selectivity among the
CaO containing catalysts. This catalyst has given the maxi-
mum ethene yield of 14.9%.
The eꢁect of reaction temperature on ethene selectivity
and yield obtained on 9CaNiAl catalyst is shown in Fig. 4.
Increase in reaction temperature increases the conversion.
Higher CO conversions are realised at high reaction tem-
2
peratures, indicating that activation of this reactant requires
higher temperatures. ODH of ethane is a complicated reac-
tion involving simultaneous occurrence of several reactions,
as shown below. While Reactions 1 and 2 increase ethylene
selectivity, Reactions 3 and 4 are deleterious as they lead
to the production of unwanted product, methane. Under the
reaction conditions the Reverse Water Gas Shift (Reaction 5)
can also occur. Reaction 6 is a deactivation reaction resulting
in coking on the surface of the catalysts. Ultimately, the prod-
uct stream contains ethene, water vapour, carbon monoxide
and unreacted ethane and carbon dioxide.
Fig. 4 Temperature study of 9CaNiAl catalyst
observed on this catalyst compared to the others indicates
that CO methanation (Reaction 4) is also feasible. Even
2
though the conversion values are high on this catalyst,
its methane selectivity is almost equal to that of ethene
(Table 2), which is not very advantageous for the selec-
tive synthesis of the alkene. Compared with this catalyst,
the CaO containing catalysts show very high ethene/meth-
ane ratio (4:1) in the product which is signiꢀcant. Particu-
larly, the overall yield of ethene is the highest in the case
of 9CaNiAl catalyst. Very recently Myint et al. [30] have
reported that the reaction pathway can also depend on the
nature of catalyst. Whereas CoMo/CeO and NiMo/CeO
Oxidative dehydrogenation.
C H + CO → C H + CO + H O
6
2
2
2
4
2
(1)
(2)
(3)
(4)
(5)
(6)
Dehydrogenation of ethane.
C H → C H + H
2
6
2
4
2
Hydrocracking of ethane.
C H + H → 2CH
2
2
followed the reforming pathway, FeNi/CeO followed the
oxidative dehydrogenation pathway to produce ethylene.
NiAl exhibits very low basicity (Table 1), as revealed by
2
2
6
2
4
CO methanation.
2
its CO -TPD proꢀle. Its ethene selectivity is also low. On
2
CO + 4H → CH + 2H O
4
2
2
2
the other hand, the highest selectivity to ethene is obtained
on the most basic 9CaNiAl catalyst. Thus, basicity appears
to be a reason for obtaining high ethene yield. The conver-
sion-yield and conversion-basicity relationships are shown
in Fig. 5. The parallelism between the two curves indicates
that yield is proportional to basicity. Valenzuela et al. [31]
have observed increase of oxygen-ion mobility and the sur-
face basicity of the catalyst in the presence of Ca in CeO2.
Xin et al. [32] have characterized the structure and surface
acidity/basicity of CeO /γ-Al O catalysts and reported that
Reverse Water Gas Shift (RWGS) Reaction.
H + CO → CO + H O
2
2
2
Carbon formation.
2
CO → C + CO2
In the case of NiAl catalyst, apart from the ODH reac-
tion (Reaction 1) giving ethene, the facile ethane conver-
sion may be due to the predominance of dehydrogenation
2
2
3
(
Reaction 2) and hydrocracking (Reaction 3) of ethane
surface acidity decreases while surface basicity increases
after the addition of CeO to γ-Al O . The selectivity to
occurring on the Ni sites. The higher conversion of CO2
2
2
3
Table 2 The performance of
the ODH catalysts at 700°C
Catalysts
Conversion (%)
C H
Selectivity (%)
C H
Yield (%)
C H
CO2
CH4
CH4
2
6
2
4
2
4
NiAl
26.2
17.1
18.4
12.5
45.1
31.9
22.3
18.9
52.5
63.9
80.7
80.2
47.5
36.1
19.3
19.8
13.7
10.9
14.9
10.0
12.5
6.2
3.6
2.5
6CaNiAl
9CaNiAl
12CaNiAl
1
3

T. V. Sagar et al.
Table 3 compares the activity of 9CaNiAl catalyst with
that of the catalysts prepared by changing the method of
preparation. It can be seen that in the case of catalyst
prepared by depositing CaO ꢀrst and NiO later on Al O
2
3
(
NiCaAl) and the catalyst prepared by simultaneous
deposition of CaO and NiO on Al O [(Ni + Ca)Al], the
2
3
ethane conversion is higher than that of the 9CaNiAl cat-
alyst. However, there is a distinct decrease in the selectiv-
ity towards ethene. Whereas 9CaNiAl shows high ethene
selectivity, the other catalysts are selective towards meth-
ane formation. Therefore, the sequence of depositing NiO
ꢀ
rst followed by CaO on Al O appears to be the best
2 3
method for the catalyst preparation.
Fig. 5 Variation of ethene yield and catalyst basicity with ethane
conversion
3
.7 Eꢁect of CO Partial Pressure on the Activity
2
of the Catalysts
ethene is shown to increase for all the CeO , γ-Al O and
2
2
3
CeO /γ-Al O catalysts. In order to further substantiate the
Experiments were conducted by varying CO2 par-
tial pressure in the feed and the results are included in
2
2
3
claim that basicity is responsible for ethene selectivity and
in turn its yield, the control catalyst, 9CaAl (without Ni)
was also tested for the ODH reaction. At 700°C, the cata-
lyst has shown 13% ethane conversion with 84% selectiv-
ity to ethene (ethene yield: 11%), reiterating the fact that
increase in basicity increases the ethene selectivity.
Table 4. It can be observed that the CO conversion
2
increases exponentially with its partial pressure, par-
ticularly at high reaction temperature. The ethane con-
version initially increases and then decreases. The eꢁect
of CO partial pressure is particularly felt on the ethene
2
Increased yield of ethene with increased basicity needs
to be understood. One probability is the occurrence of
basic site activated RWGS reaction (Reaction 5) that con-
sumes the hydrogen produced during the decomposition of
ethane pushing the reaction forward. It is known that CO2
enhances the equilibrium conversion of C H formation
selectivity which shows a tremendous decrease from 80
to 20%. Methane formation on the other hand increases
with increase in CO concentration in the feed. This is
2
an interesting observation. The competitive adsorption of
CO in preference to ethane has decreased ethane conver-
2
sion (Table 4) with simultaneous increase in methanation
of carbon dioxide.
2
4
via dehydrogenation of C H and reacts with the carbon
2
6
deposited on the surface. Both the functions are positive in
nature. The former stabilizes the activity and the latter pro-
longs the catalyst life, as reported by Li et al. [33].
3.8 Estimation of the Thermodynamic Equilibrium
Composition
CO is one of the products of RWGS reaction. However,
it was not detected in suꢂcient quantities in the product in
the present work. The absence of CO can be explained by
considering the methanation reaction (Reaction 4). Fechete
and Vedrine, in a recent review, have proposed diꢁerent
The thermodynamic equilibrium composition of the prod-
uct was calculated for the input gas conditions (molar ratio:
C H /CO /He=2:1:3) using HSC Chemistry Version 9.0
(Outotec Technologies, USA). The variation in product
composition against temperature is depicted in Figs. S1
and S2. Preliminary results have indicated the presence of
2
6
2
mechanisms for CO methanation. In one of the mecha-
2
nisms, CO is ꢀrst converted to CO and then hydrogenated
2
to methane [34]. Taking this into consideration we may
propose that in the presence of hydrogen not only the CO
produced during methanation but also that produced from
RWGS reaction would have been converted to methane.
An examination of the performance stability of 9CaN-
iAl catalyst by continuing the evaluation for 300 min at
Table 3 Activity comparison of diꢁerent catalysts
Catalysts
Conversion Selectivity Selectivity
Extent of
C H (%)
C H (%)
CH (%)
coking
2
6
2
4
4
a
(
%)
9
CaNiAl
NiCaAl
Ni+Ca)Al
18.4
19.1
55.1
80.4
61.7
–
19.3
38.3
99.0
0.53
7.43
7
00 °C, reveals that the catalyst has reasonable stability
till 210 min. The ethane conversion is seen to increase
from 21.9% after 210 min to 23.2% after 300 min,
whereas the ethene yield decreased from 12.8 to 12.0%
during the same period.
(
54.54
aCoke formation expressed in terms of the C content of the used cata-
lyst
1
3

Selectivity Reversal in Oxidative Dehydrogenation of Ethane with CO on CaO–NiO/Al O …
2
2
3
Table 4 Eꢁect of CO2
concentration in the feed on the
performance of the catalyst
Catalyst
Tempera-
ture (°C)
Conversion (%)
Selectivity (%)
Yield (%)
C H
C H
CO2
C H
CH4
CH4
2
6
2
4
2
4
9
9
9
CaNiAl (He:C H :CO 30:20:10)
600
1.6
5.1
7.8
1.0
3.2
63.2
69.7
79.6
80.7
0
36.8
30.2
20.4
19.3
100
1.0
3.3
6.2
14.8
0
0.6
1.5
1.6
3.6
2.1
8.1
8.0
12.2
2.5
3.8
8.5
16.9
2
6
2
650
675
700
9.5
18.4
2.3
22.3
2.1
CaNiAl (He:C H :CO 25:20:15)
600
2
6
2
650
675
700
11.9
19.3
27.8
2.1
9.1
32.1
58.4
56.2
0
67.9
41.6
43.8
100
3.8
11.4
15.6
0
17.6
40.9
7.3
CaNiAl (He:C H :CO 20:20:20)
600
2
6
2
650
675
700
4.9
24.5
47.9
76.5
21.1
18.3
20.1
78.6
81.6
79.9
1.0
1.9
4.3
10.5
21.2
two temperature regions. In the low temperature (<400°C)
region ethane cracking to methane is found to be predomi-
nant, whereas in the high temperature region (>400°C)
ODH of ethane predominates. The inference is consistent
with the experimental results. Between 600 and 700°C, the
composition of ethene is found to be considerable. How-
ever, the ethene/methane ratio varies in the presence of
catalyst. Another interesting observation from the thermo-
dynamic analysis is that when C(s) formation is taken into
consideration coke formation appears to be predominating
ethene and methane formation during the entire tempera-
ture region (Fig. S2). The very fact that the ethene selectiv-
ity is high on 9CaNiAl implies that the catalyst resists coke
formation.
characteristic features of the carbonate ion in calcium car-
−
1
bonate [36]. The bands at 1082 and 858cm represent
symmetric stretching and out-of-plane bending modes of
the carbonate ion, respectively. The peak at 1082cm− usu-
ally observed in the spectrum is of aragonite-phase of cal-
1
−
1
cium carbonate, whereas the one at 713cm can be attrib-
uted to the in-plane bending mode of ions, which indicates
a structural change in the calcium ions from the symmetry
−
1
of the calcite phase. The band at 1455cm represents the
asymmetric stretching mode of carbonate ion. Thus, the Ca
containing catalysts exhibit peaks due to calcium carbon-
ate. Therefore, it could be expected that as the CaO con-
tent increases, the carbonate formation increases, in turn
decreasing the coke formation. The data on the extent of
coke formation on diꢁerent catalysts reported in Table 1
supports this observation. When the reaction temperature is
3
.9 Coke Formation in used Catalysts
The coke accumulation was examined by determining the
carbon composition of the used catalysts. The values are
presented in Table 1. NiAl reveals a C content of 2.89%
whereas the CaO containing catalysts show lower values
indicating good coke resistance. According to Heracleous
et al. [35] coking during the ODH reaction is due to C–C
3
+
cracking of ethane on acidic centers like –OH and Al –
2
−
O . These sites are normally neutralized by the addition
of an alkaline earth species such as CaO. Particularly, the
method of preparation adopting the sequence of depositing
NiO ꢀrst followed by CaO on Al O is also advantageous
2
3
compared to the other two methods in containing the coke
formation, as seen from Table 3.
In order to conꢀrm the formation of calcium carbonate
in the present study, the used catalysts were examined by
FTIR analysis. The spectra are reported in Fig. 6. The data
obtained for the CaO containing samples reveal a broad
−1
absorption peak at 1455cm and clear peaks at 1082, 858
−1
and 713cm , which have been reported to be the common
Fig. 6 FTIR spectra of used catalysts
1
3

T. V. Sagar et al.
9. Ansari MB, Park SE (2012) Energy Environ Sci 5:9419
low, the CaCO formed covers the Ni sites. As a result the
hydrocracking is limited, eventually decreasing the overall
conversion. At high reaction temperatures the decompo-
3
1
1
1
0. Shi X, Ji S, Wang K (2008) Catal Lett 125:331
1. Wang S, Zhu ZH (2004) Energy Fuels 18:1126
2. Blasco T, Lopez-Nieto JM (1997) Appl Catal A 157:117
sition of CaCO opens up the Ni sites on which the oxi-
13. Wang S, Murata K, Hayakawa T, Hamakawa S, Suzuki K (1999)
Chem Commun 1:103
3
dative dehydrogenation takes place, again increasing the
conversion.
1
4. Schuurman Y, Ducarme V, Chen T, Li W, Mirodatos C, Martin
GA (1997) Appl Catal A Gen 163:227
1
5. Heracleous E, Lee AF, Wilson K, Lemonidou AA (2005) J Catal
2
31:159
4
Conclusions
16. Skoufa Z, Xantri G, Heracleous E, Lemonidou AA (2014) Appl
Catal A Gen 471:107
1
7. Heracleous E, Delimitis A, Nalbandian L, Lemonidou AA
2007) Appl Catal A Gen 325:220
NiAl oꢁers high ethane and carbon dioxide conversions
predominantly due to hydrocracking of ethane and metha-
nation of carbon dioxide. Thus, it gives a gas product with
its ethene/methane selectivity ratio almost equal to 1. The
addition of CaO increases the basicity thereby increasing
the selectivity towards ethene and bringing the ethene/
methane ratio to 4. High selectivity and yield of ethene
could be achieved on CaO-containing catalysts. Ca addition
also decreases the coke formation. The sequence of addi-
tion of NiO and CaO to Al O also inꢃuences the catalyst
(
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1
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