Porous ternary Fe-based catalysts for the oxidative dehydrogenation of ethylbenzene in the presence (absence) of carbon dioxide

Article abstract of DOI:10.1039/c4ra14572k

Porous ternary Fe-based catalysts were characterized and their catalytic properties through the oxidative dehydrogenation of ethylbenzene in the presence (ODH) or absence (DH) of carbon dioxide were investigated. The catalysts were characterized by X-ray

Full text of DOI:10.1039/c4ra14572k

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Porous ternary Fe-based catalysts for the oxidative
dehydrogenation of ethylbenzene in the presence
Cite this: RSC Adv., 2015, 5, 20900
(absence) of carbon dioxide
a
b
a
Nuryana A. Ferreira, Josue M. Filho and Alcineia C. Oliveira*
´
Porous ternary Fe-based catalysts were characterized and their catalytic properties through the oxidative
dehydrogenation of ethylbenzene in the presence (ODH) or absence (DH) of carbon dioxide were
investigated. The catalysts were characterized by X-ray diffraction (XRD), chemical analyses,
thermoprogrammed reduction (TPR), physisorption measurements, Raman spectroscopy and scanning
electron microscopy coupled to energy-dispersive X-ray spectrometry (SEM-EDX). The kinetic modeling
of reverse water gas shift reaction (RWGS) and the effects of reaction parameters, such as reaction
temperature and CO
or Ni promoters to a porous Fe-based solid greatly enhanced the ODH reaction, whereas that of the
RWGS is favoured by Ni promotion. The CO /H
¼ 1 ratio and temperature of 850 K were the best
2 2
/H ratio on the catalytic activity, were also investigated. The addition of Zn, La, Mg
2
2
conditions for RWGS occurrence. The implications of these conditions on catalyst application for ODH
and DH reactions were discussed. Ethylbenzene conversions were too low due to the decreased textural
properties of some catalysts as well as the selectivity to styrene is inhibited. A porous FeAlZn catalyst
exhibited higher catalytic performance than the other ternary solids in terms of the ethylbenzene
dehydrogenation and resistance against deactivation, whereas low RGWS conversions were observed
under the abovementioned conditions.
Received 15th November 2014
Accepted 3rd February 2015
DOI: 10.1039/c4ra14572k
www.rsc.org/advances
Another limitation of the process is the low selectivity to
styrene due to the formation of benzene and toluene as by-
1
. Introduction
The chemical recycling of carbon dioxide from combustion products, in addition to thermodynamic equilibrium limita-
6–10
sources is a sustainable energy process because it allows for the tions.
Thus, an ODH reaction in the presence of carbon
strategy of capture and storage of CO in reducing its emissions. dioxide offers advantages over a DH reaction, owing to CO
2
2
In this sense, the use of CO
2
as a mild oxidant in catalytic reducing the energy consumption, accelerating the reaction
reactions has been strongly encouraged. Moreover, alternative rate, prolonging catalyst lifetime, enhancing selectivity, and
technologies for using carbon dioxide, such as wet partial alleviating thermodynamic constraints; moreover, it could
1
–10
oxidation or autothermal reforming, propane dehydrogenation, indeed drive the process towards green chemistry.
ethylbenzene dehydrogenation, and dry reforming, among
The ndings state that the DH reaction is coupled with
others, represent the further remarkable economic advan- reverse water gas shi (RWGS). Because the RWGS reaction is
1–3
ꢀ
1
tages of CO consumption.
mildly endothermic with an enthalpy of 41.1 kJ mol , the
2
In additional, the growing concern about styrene production reaction is carried out at low temperatures. This makes the
costs has directed the interests of researchers toward the devel- coupled process of an ODH reaction slightly endothermic,
9
opment of the oxidative dehydrogenation of ethylbenzene in the compared with that of DH. Although the isolated RWGS reac-
presence of carbon dioxide (ODH). Styrene is a highly valuable tion has no such difficulties, the catalytic stability is oen poor.
monomer used for polymeric polystyrene resins and styrene– Thus, the solid is still not yet sufficiently developed for indus-
1–11
butadiene rubber production.
However, the commercial trial application, although it has been widely investigated on a
process of the direct-steam dehydrogenation of ethylbenzene (DH) laboratory scale. In additional, RWGS reaction occurrence is
to styrene leads to the use of a large amount of superheated steam. undesirable for ethylbenzene dehydrogenation, dry reforming,
and propane oxidation, among other reactions, due to the low
5,12–14
yield of the products from these reactions.
Indeed, the
kinetics of the RWGS reaction is studied at a low conversion and
high hydrogen partial pressure and the reaction should be
a
Universidade Federal do Cear a´ , Campus do Pici-Bloco 940, Fortaleza, Cear a´ , Brazil.
E-mail: alcineia@ufc.br; Fax: +55 85 33 66 9982; Tel: +55 85 33 66 90 51
b
limited by dissociative CO adsorption.
2
Universidade Federal do Cear a´ , Departamento de F ´ı sica, Fortaleza, Cear a´ , Brazil.
Fax: +55 85 33 66 94 83; Tel: +55 85 3366 94 83
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The systematic exploitation of dehydrogenation of ethyl- 2.2. Characterization
RSC Advances
benzene coupled with RGWS would accomplish signicant
efficiency in styrene production and reduction in carbon
dioxide emissions. A great deal of insight into the coupling
reaction has been acquired by studying the catalytic activities
of various solids based on iron oxides. Nanostructured Fe-
containing promoters, such as alumina, zirconia or ceria,
have shown good performance for catalyzing the oxidative
X-ray diffraction patterns (XRD) were measured on a PANalytical
X'PERT HighScore X-ray diffraction equipment, under the
following conditions: Cu target Ka radiation, scanning step of
ꢁ
0
2
.02 , scanning rate of 0.1, and scanning current and voltage of
0 mA and 30 kV, respectively. The diffractograms were
compared with those of JCPDS (Joint Committee on Powder
Diffraction Standards).
dehydrogenation of ethylbenzene (EB) with CO
2
. It is note-
Inductively Coupled Plasma Optic Emission Spectroscopy
worthy to mention that the activity of in situ FeAl O spinel-
2
4
(ICP-OES) was performed with a Varian apparatus. Previously,
phase formation motivated us to investigate the catalytic
properties of a FeAl solid by adding a third element in the
the solids were digested with an equimolar mixture of nitric and
ꢁ
hydrochloric acids at 90 C in a sand bath. The actual metallic
1
0
2 4
catalyst composition. As the stability of the FeAl O phase
content of the solids was then determined by ICP-OES.
in an ODH reaction is limited, owing to carbonaceous
deposition, there is a need to develop new catalyst systems
that allow for the uniform dispersion of the active phase, as
well as high stability and driving the redox mechanism of the
titled reaction to avoid an isolated occurrence of RGWS
reaction. The addition of La, Mg, Zn or Ni in the Fe-based
solids is expected to minimize the deactivation by phase
transformation effects.
Thus, the aim of this study is to investigate the performance
of nanostructured FeAlZn, FeAlLa and FeAlNi catalysts for
styrene production. FeMgZn is used for comparison purposes.
In addition, a deep comprehension of the DH and RWGS
reaction conditions for improving the reaction yields or
avoiding their occurrence is highly desirable, depending on
the investigative focus. This study presents kinetic modeling
and catalytic results for evaluating the effect of temperature
The adsorption–desorption isotherm experiments were
carried out with an ASAP 2000 Micromeritics instrument to
determine the specic surface areas and pore structure
parameters of the solids. The probe molecule was nitrogen at
77 K in a surface area analyzer. Samples were degassed at 423 K
for 12 h prior to measurement. The BET equation was used to
calculate the total specic surface areas of fresh and spent
catalysts. The pore distributions and surface areas of mesopores
were calculated by the Barrett–Joyner–Halenda (BJH) method
from the desorption branch of the isotherms.
The morphological aspects of the sample were determined
by Scanning Electron Microscopy (SEM) measurements using
an FEG Quanta 450 electron microscope equipped with an EDS
Bruker QUANTAX system coupled to the SEM microscope, using
an acceleration voltage of 2 kV.
2
Temperature programmed reduction (H -TPR) experiments
2 2
and CO /H ratio on the catalytic properties of Fe-based cata-
were performed with homemade equipment using a quartz tube
reactor possessing an inner diameter of 6 mm coupled to a
thermal conductivity detector (TCD) for monitoring hydrogen
consumption. The mass of the catalyst was 50 mg, and the
experiment was carried out in the range of 323–1273 K. A mixture
lysts through RWGS and DH reactions. The investigations of
the adsorption and deactivation constants, as well as the rate
of the RWGS reaction in different temperatures, are also
studied by the model.
2 2
of 8% H /N was used as a reducing gas with a rate of 100 mL
ꢀ
1
2. Experimental
min , aer passing through a 13ꢂ molecular sieve trap to
remove water. Before the analysis, samples of ca. 0.1 g were placed
in a tube reactor and heated under nitrogen at 373 K for 2 h.
The spent catalysts were characterized by Raman spectros-
copy. The Raman measurements were obtained on a LabRam
spectrometer (JobinYvon) under ambient conditions. A 532 nm
argon ion laser was used as the exciting source on the sample
surface with a power of 2 mW. Ten accumulated spectra were
obtained in each spectral range, and the spectral resolution was
2
.1. Catalyst preparation
The mixed oxides were prepared by the precipitation method
using aluminum tri-sec-butoxide (Al(OC sec) ) and ferric
O as precursors. In brief, aluminum tri-
4
H
9
3
10
3 3 2
nitrate Fe(NO ) 9H
sec-butoxide was rst dissolved into an excess of isopropanol
and vigorously stirred at 333 K until the solution became
homogeneous. Then, a mixture of 2.9 mol of water, ferric
nitrate, and 6.5 mol of absolute ethanol together with
lanthanum nitrate solution was added to the stirred mixture of
aluminium through a peristaltic pump. The reactants were
maintained under constant stirring and reuxing for 24 h. The
gel was subsequently washed with ethanol, dried at room
ꢀ1
ꢀ1
3
cm in the 5–2000 cm range. The Olympus lens focus was
at 100 times magnication.
2.3. Catalytic testing
2
temperature and calcined at 873 K under air ow at a heating The dehydrogenation of ethylbenzene by CO was carried out
ꢁ
ꢀ1
rate of 5 C min for 2 h. The abovementioned methodology under steady-state conditions in a quartz xed-bed reactor. The
was used to obtain the FeAlLa. Other solids, such as FeAlZn, reactant mixture was composed of carbon dioxide and ethyl-
2
FeAlNi and FeMgZn were also prepared, in which zinc, nickel benzene at a 30 : 1 ratio. The ow rates used were N ,
ꢀ1
ꢀ1
ꢀ1
and magnesium nitrate, were the active component precursors. 11.7 mmol h ; CO , 58 mmol h ; and EB, 1.98 mmol h
2
The metal contents measured by chemical analyses were (CO /EB molar ratio of 30). The catalyst (50 mg) was crushed
2
60 : 20 : 20 mol%, respectively, for iron, aluminium and the and activated in situ under a ow of nitrogen and was heated
third metal added to the solid.
from room temperature to 823 K over the course of 1 h to
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remove any gaseous impurities from the surface of the catalyst. phase, derived from the Eley–Rideal mechanism expression, as
15
The reaction was performed under atmospheric pressure at a given below in reaction (V):
temperature of 823 K, as previously dened by theoretical and
experimental studies. The product was analyzed by gas chro-
5
H (g) + Oc # H O(g) + c
(V)
2
2
matography (GC) using a simple chrome chromatograph
equipped with ame ionization detector (FID) and thermal
conductivity detector (TCD). GC analyses were carried out at
isothermal conditions from 295 to 523 K at a rate of 283 K min
and an injector temperature of 523 K.
0
Thus, the rate of water produced and consumed, r , is
H2O
expressed in eqn (2):
ꢀ
ꢁ
ꢀ1
PH2OCV
KH2O
0
r
¼ kH O P 2
H
C
V
ꢀ
(2)
H2O
2
The conversion and selectivity were dened in the following
manner:
In a second approach, the kinetics for the system under study
adopts the steady-state condition for RWGS reaction, in which the
rate of distinct and discreet site generation is zero and the
following expression is presented for the rate of carbon dioxide:
EBin ꢀ EBout
%
EB conversion ¼
ꢂ 100
(I)
EBin
mol of desired product
ꢀ
ꢁ
%
EB selectivity ¼
ꢂ 100
(II)
P
COPH2O
mol of reacted ethylbenzene
k
CO2
C
T
P
CO2 ꢀ P
ꢀ 0
r
¼
H2
KCO2 KH2O
(3)
CO2
PH2O
5
1
þ
P 2
Prior to the catalytic tests, modeling of DH and RWGS
reactions without the catalyst was performed to identify the best
reaction conditions to perform the experimental analyses.
H
KH2O
Eqn (3) can be simplied by considering that kCO2 is the ratio
between adsorption and desorption constants because the
former constant is much higher than the second constant, the
expression is summarized as:
2
.4. Model
The modeling of the experimental data is well suited to quantify
the kinetic and thermodynamic effects of the RWGS reaction
without using the catalysts. To include possible temperature,
ꢀ 0
r
¼ kCO2
P
CO
C
T
(4)
CO2
2
H /CO ratio and pressure effects on the Fe-catalyzed RWGS
reaction, it is necessary to model the data. Thus, the thermo-
2.4.2. Adsorption constants. Most simulation studies
employ previously published adsorption constant expressions
for similar catalysts in the RWGS reaction. The dependence of
dynamic–kinetic assessment was implemented and solved in
+
+
the C program in the 400–1100 K temperature range. The
reaction rates have been taken into consideration for dening
the optimal conditions to operate the reaction.
temperature on the adsorption of water and CO constants can
2
be calculated based on the following equations
"
#
2
.4.1. Kinetic modeling. The kinetic model considers that
the reverse water gas shi reaction (RWGS) is formally regarded
as a single-step surface reaction (reaction (III)). CO is assumed
ꢀ
ꢁ
DHH2O
1
1
*
KH2O ¼ K
exp
exp
ꢀ
ꢀ
(5)
(6)
H2O
R
T
298:15
2
to be directly transformed into carbon monoxide and an oxygen
radical by dissociative adsorption (reaction (IV)):
"
#
ꢀ
ꢁ
DHCO2
1
1
*
CO2
KCO2 ¼ K
R
T
298:15
CO (g) + H (g) # CO(g) + H O(g),
2
2
2
DH298.15 K ¼ 41.17 kJ molꢀ1 (III)
ꢁ
where KH2O and KCO2 are the adsorption of water and CO
2
*
*
constants, respectively; and K
and K
values are 3.52 ꢂ
H2O
CO2
4
1
ꢀ1
69
ꢀ1
CO
2
(g) # CO(g) + Oc
(IV) 10 bar and 1.24 ꢂ 10 bar , respectively. The enthalpy
values for water and CO
Hence, the rate of carbon dioxide consumed, ꢀr , is 393.5 kJ mol and 393.8 kJ mol
2
at 298.5 K (DHH O and DHCO2) are
2
0
ꢀ1
ꢀ1 16
CO2
, respectively.
0
proportional to carbon monoxide produced, r , with reaction
The term used to quantify the attenuation of the velocity rate
CO
(
IV) being the determining step of the reaction:
by of CO and water adsorption on the catalyst surface, q, is
given in eqn (7)
2
ꢀ
ꢁ
P
CO
C
OS
0
¼ r0 ¼ kCO2
ꢀ
r
P
CO2
C
V
ꢀ
(1)
CO2
CO
K
CO
1
q ¼ 1
(7)
þ KH O
f
H O þ KCO2
f
CO2
2
2
where kCO2 is the kinetic constant of velocity for CO
2
, and PCO is
the partial pressure of CO. The ratio between the adsorption
and desorption constants of CO is given as KCO. In addition, it is
Accordingly, the term used to quantify the attenuation of the
assumed that the total amount of active sites occupied by velocity rate by of CO2 and water desorption on the catalyst
oxygen atom and the number of active surface sites available per surface, q , is described in the following manner in eqn (8):
d
catalyst mass is equal to COS and C , respectively.
The rst approach used for RGWS reaction assumes the fact
that the adsorbed oxygen reacts with hydrogen from the gas
v
1
q
d
¼ 1
(8)
þ ðKH OÞ fH O þ ðKCO2 Þ fCO2
2
d
2
d
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where fH O and fCO2 represent the fugacity of water and CO
2
ꢀ3
,
The reverse water gas shi reaction was carried out in a
microcatalytic system in a quartz tubular xed-bed reactor.
2
*
*
respectively. The values of K
and K
are 2.02 ꢂ 10
H2Od
CO2d
ꢀ1
ꢀ3
ꢀ1
bar and 9.89 ꢂ 10 bar , respectively. The enthalpy values About 150 mg of catalysts were used with the reactor operating
for water and CO at 298.5 K (DHH
and DHCO2d) are at various temperatures and a mixture of 5% CO /N2 in a
2
2
Od
2
ꢀ1
ꢀ1
16–18
4
1.3 kJ mol and 204.0 kJ mol , respectively.
These values hydrogen atmosphere was introduced into the catalyst bed
ꢀ
1
were also used to determine the terms that quantify both the using a ow of 40 mL min . The H
water and CO adsorption capacities during the deactivation of and the products of the reaction were analysed in a Varian
the catalyst, particularly KH Od and KCO2d chromatograph. Previously, the catalysts were in situ activated at
2 2
/CO molar ratio was 1 : 1,
2
.
2
2 2
By summing up the reaction rates, the values of the kinetic 873 K under 5% H /N for 1 h.
constants for adsorption, q, and desorption, q , are determined
d
in eqn (9) and (10).
3
. Results and discussion
k0
¼ k
¼ k
q
(9)
a
a
a
3.1. Characterizations of the catalysts
0
d
k
d
q
d
(10)
3.1.1. XRD, textural properties and SEM-EDX. Fig. 1 shows
the XRD patterns of the catalysts. All the XRD patterns possess
are previously deter- relatively high intensity lines describing the crystalline char-
It is important to note that k
mined by eqn (4) at 850 K, being 0.32 s
a
and k
d
ꢀ1
ꢀ1
and 0.33 s
,
acter of the solids, except for FeAlLa. The latter exhibits diffuse
ꢁ
respectively.
diffraction peaks at 2q ¼ ca. 35 , which is attributed to the
20
The velocity rate of RGWS is also predicted by varying the typical amorphous character of La O –Al O . In additional,
2
3
2 3
partial pressure of carbon dioxide and hydrogen to obtain the characteristic diffractogram of FeAlLa could be attributed to
18
optimal conditions for CO /H trough reaction (III), as shown La O (JCPDS, 05-0602), LaAlO (JCPDS 85-1071) or LaAl O
2
2
2
3
3
12 19
in eqn (11).
2 3
(JCPDS, 77-0335) phases or yet g-Al O (JCPDS, 10-04625).
ꢀ73600
RT
P
T3
However, due to the broadness of the peaks, their presence is
not determined. In addition, the reections belonging to iron
phases are not observed, which is probably due to their nano-
RGWS ¼ ꢀ 322 ꢂ 10ꢀ
6
e
p
H2
P
(11)
r
CO2
metric sizes or good dispersion in the La
2 3 2 3
O –Al O matrix. This
2
.4.3. Experimental results in RWGS. Experimental results
feature is commonly found in materials prepared by the sol–gel
in RWGS were carried out by using the most active solids. The
Fe–Co-based catalyst, possessing a metal-to-iron ratio of 3 was
21,22
and nanocasting methods.
It is evident that FeAlZn has diffraction peaks assigned to
g-Fe (JCPDS 39-1346), ZnO (JCPDS, 89-0510) and g-Al
JCPDS, 10-04625). Mixed-oxide phases such as ZnAl O4 or
15
synthesised according to a method previously reported and
calcined at 873 K under air ow. This catalyst was chosen due to
its excellent structural and textural properties, which promoted
the dry reforming of methane and it showed to be a potential
catalyst for RWGS.
2
O
3
2 3
O
(
2
ZnFe O , Fe O –Al O could be formed, but calcination
2
4
2
3
2 3
19
temperatures are too low to obtain these mixed oxides. The XRD
pattern of FeMgZn displays MgO (JCPDS 18-1022), in addition to
Fig. 1 XRD patterns of the fresh catalysts that were studied. The solids were calcined at 873 K under air.
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compounds are highly dispersed over the bulk. Particle sizes
estimated by the Scherrer equation show the nanosized features
of the FeAlLa and FeAlZn, whereas the aggregation of FeMgZn
and FeAlNi particles implies that the solids have large sizes.
The chemical analyses by ICP-OES results show that the
obtained oxide composition is close to that predicted theoreti-
cally and corresponds to 68 wt% of Me
that of each Me or Me specie. If one considers all oxides
having or Me Me Me general formulae, the percentage of the
1
, whereas 16 wt% for
2
3
1
2
3
elements matches well with the calculated values.
Nitrogen sorption isotherms show that FeAlLa, FeAlZn and
FeAlNi catalysts have type IV isotherms with the hysteresis loop
2 3
between H and H (Fig. 2), which is typical of mesoporous
solids.
FeMgZn is an exception because its isotherm has a type II
feature sorption curve. The specic surface area (SgBET) and pore
volume (V ) of FeAlLa are the largest among the oxides studied
p
2
ꢀ1
3
ꢀ1
˚
(
SgBET ¼ 70 m g ; V ¼ 0.10 cm g ; D ¼ 39 A), as illustrated
p
p
in Table 1. This might be due to their oxides being uniformly
dispersed as nanocrystallites that are not observed by XRD.
FeMgZn catalyst has a rather low surface area (ca. 46 m g ),
2
ꢀ1
3
ꢀ1
p
and the other textural parameters, such as V of ca. 0.10 cm g
˚
and pore diameter of 11 A, as well, indicate that this solid is
microporous and possesses larger particle sizes. Furthermore,
the pore diameters of FeAlZn and FeAlNi are reasonably large,
and both SgBET and pore volumes slightly decreased.
The differences among the SgBET of the solids are not
signicant, rendering it a highly suitable surface area for solids
21,24
obtained by the sol–gel method.
The particles sizes
measured by XRD follows the same trends of the textural
parameters values; moreover, the sintering effects during the
calcination process could reduce the textural properties of
FeAlNi and FeMgZn. This is in line with their particles sizes
values of 21 and 44 nm, respectively, which are measured by
XRD.
Fig. 2 (a) Nitrogen adsorption isotherms of the fresh catalysts. (b) The
corresponding BJH pore-size distribution of the solids.
SEM-EDX images of the solids are illustrated in Fig. 3. The
morphology of FeAlLa (Fig. 3a and a
. The diffraction lines do not show any LaAl-containing phases (e.g., La –Al
2 3
diffraction peak relative to MgFe O due to the low heating yet g-Al ), in which the g-Fe O nanoparticles are mostly
1
2
) exhibits a platelet of
those of ZnO and g-Fe
2
O
3
2
O
3
2
O
3
, La or LaAlO or
2
O
3
3
2
4
2
O
3
23
temperature of the solid to generate this phase. For FeAlNi, dispersed. This result indicates that FeAlLa appears to be
peaks ascribed to NiO (JCPDS 75-0197) and g-Fe O are visible, composed of small particles of iron with size of ca. 2–20 nm,
2
3
whereas those of a-Fe
observed. The reections of NiAl
2
O
3
(JCPDS 79-1741) are obscurely and the mean particle size of 30 nanoparticles is about 15 nm.
and FeAl
cannot be This is further conrmed by EDX analysis that displays the
2
O
4
2 4
O
ruled out, and this implies that these generated spinel oxides uniform dispersion of iron nanoparticles on the La–Al surface
Table 1 Textural parameters obtained from the nitrogen adsorption–desorption isotherms. SgBET is the specific surface area calculated from the
BET method in a relative pressure range of 0.05–0.2; V
maximum of the pore-size distribution calculated by the BJH method from the adsorption branch. Ethylbenzene conversion and styrene
selectivity were obtained using 50 mg of fresh catalyst at 823 K, CO /EB molar ratio of 30 for 5 h
p p
is the total volume calculated at a relative pressure of 0.99; D is the pore diameter at
2
a
2
ꢀ1
3
ꢀ1
b
p
c
c
˚
(A)
Catalyst
L (nm)
SgBET (m g
)
V
p
(cm g
)
D
EB conversion (%)
Styrene selectivity (%)
FeAlNi
FeMgZn
FeAlZn
FeAlLa
21
44
16
—
52
46
61
70
0.07
0.06
0.08
0.10
17
11
20
39
10
6
25
17
2
100
99
95
a
b
c
From the (311) reection of g-Fe
2
O
3
observed by XRD. From the desorption branch of the isotherms. Steady-state condition.
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(Fig. 3a), which is consistent with the XRD and textural prop- 1, which are obtained from XRD. They are also in agreement
erties results. with the previous N physisorption results that show the lowest
2
On the other hand, the morphology of FeAlNi is markedly textural properties for FeMgZn. Additional EDX analysis
different from other solids. Plate-like crystallites are clearly conrms the non-uniform distribution of MgO, ZnO and g-
visible (Fig. 3b
composed of a-Fe
1
and b
2
). The crystallites are believed to be Fe
and g-Al , as suggested by EDX
2
O
3
in some regions.
2
O
3
, g-Fe
2
O
3
2 3
O
3.1.2. TPR analyses. TPR curves were obtained to deter-
analyses. The increased magnication to examine the plates mine the reducibility of the ternary oxide catalysts, as shown in
shows that the homogeneously dispersed particles of NiO are on Fig. 4.
their surfaces. These results are in line with XRD
measurements.
The curves show two major peaks with maxima at low
temperatures, centered at around 811 K, and high-
a
The SEM-EDX micrographs of FeAlZn reveals well-formed, temperature reduction peak at 920 K, as for FeMgZn. It
thin plate-like crystals with sharp edges (Fig. 3c1 and c2), should be assumed that the TPR proles of a-Fe O and g-Fe O
2 3
2
3
3
+
2+
which are indicative of a-Fe
2 3 2 3 2 3
O or g-Fe O and g-Al O pres- relate a rst-reduction process of Fe to Fe at around 673 K,
ence. The existence of some of these phases is suggested by whereas a second peak at about 723 K is ascribed to the
2
+
9,21
XRD. In additional, it is clearly observed that nely dispersed reduction of Fe to metallic Fe. Furthermore, pure ZnO and
25,26
ZnO crystallites are superposed in the platelet at a size of ca. 100 MgO do not reduce at temperatures as low as 1073 K.
As the
nm, in line with XRD analysis. These features are associated temperatures of reduction take place at higher temperatures
with the elevated textural properties of the solid. In addition, a than those of the literature, the reduction of the iron species
small amount of carbon from the aluminum precursor was could be delayed due to a synergic interaction between iron,
found on the Al-containing samples.
From Fig. 3d and d , it can be observed that FeMgZn is material. Another possibility is the interaction between surface
formed by a small agglomeration of particles, which are iron and MgO and ZnO species, forming other compounds in a
magnesium and zinc, favoring the redox properties of the
1
2
27
superposed in a platelet. It consists of rather heterogeneous, reduction environment. In addition, a third reduction peak up
large spherical-like particles with a mean size of 50 nm. These to 1073 K is suggested by TPR curve of FeMgZn. This curve could
results are consistent with the crystallite size included in Table be attributed to the direct reduction of nely dispersed
Fig. 3 SEM images of the fresh catalysts: FeAlLa (a
1 2 1 2 1 2 1 2
and a ), FeAlNi (b and b ), FeAlZn (c and c ) and FeMgZn (d and d ). The EDX images are
represented by a , b , c and d for FeAlLa, FeAlNi, FeAlZn and FeMgZn catalysts, respectively.
3
3
3
3
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Fig. 4 TPR profiles of the fresh catalysts.
2 4 2 4
MgFe O or ZnFe O phases; these species show elevated limitation. Combining with the XRD results, the latter peak is
temperatures of reduction on comparing with their bulk not visible because it can be mainly dispersed in the solid
counterparts because they are strongly interacting with the matrix as nanoparticles.
28,29
support.
Because g-Al O , La O , LaAlO or LaAl O or even
12 19
2
3
2
3
3
3
.2. Catalytic results in the dehydrogenation of
ethylbenzene in the presence (absence) of CO
FeAlLa are associated to the nely dispersed iron-oxide reduc- The catalytic performance is evaluated by the dehydrogenation
tion on the Al and/or La matrix. The latter enables the of ethylbenzene to styrene over various catalysts. Blank runs
difficulty of iron-oxide reduction in the catalyst and thus provided almost negligible conversion aer 5 h of reaction time
Al O –La O phases do not exhibit reduction peaks at temper-
2
3
2 3
2
atures as low as 1273 K, the aforementioned reduction peaks of
2
O
3
2 3
O
decreases its degree of reduction, compared with FeMgZn.
in the absence of CO
2
, whereas its presence gave 2% conversion.
The TPR prole of FeAlZn exhibits an asymmetric peak at ca.
Fig. 5a shows the overall conversion and selectivity obtained
6
80 K, which can attribute for the reduction of g-Fe O to Fe O . in 5 h of reaction time, when CO is co-fed in the reaction.
2
3
3
4
2
A broad peak in a much wider range from 440 to 943 K consists
The conversions are high (e.g., up to 10%) in 1 h of reaction
of two components with maxima at ca. 766 K and ca. 877 K and time over all solids. A possible reason for this performance is
2
+
0
corresponds to the reduction of Fe to Fe , respectively. With the cracking of ethylbenzene molecules due to the thermal
9,21
regard to this point, it should be emphasized that the reduction effects at the beginning of the reaction.
The behavior of
of magnetite to metallic iron is affected by the ZnO presence, solids follows distinct trends as the reaction proceeds. FeAlNi
9
probably due to FeO (wustite) formation, whereas FeAl
reduction does not occur in concomitance with that of other whereas FeMgZn does not display signicant catalytic activity in
iron or zinc species. the same testing period, and its conversion gradually decreases
The TPR prole of FeAlNi obviously displays that the with stabilization at 6% in the steady state (Table 1).
reduction peaks of iron species shis to the higher temperature Judging from the fact that FeAlNi and FeMgZn possess the
and exhibits a remarkable broadening of the hydrogen uptake same active Fe /Fe sites from g-Fe
2
O
4
retains 10% of the conversion along with the reaction time,
3
+
2+
2 3
O and that their textural
peak due to the formation of reduced metallic iron reduction properties (Table 1) are closer, the catalytic behavior can be
over nickel aluminate or iron aluminate supports. According to rationally explained by the presence of their promoters. The
31,32
the ndings, both iron and nickel aluminate are formed by a ndings
proposed a mixed acid–basic and reduction–
solid-state reaction between g-Al O and iron or nickel coun- oxidation mechanism for the reaction. There is a formation of
2
3
3
+
terparts under moderate temperatures and oxidative environ- p-adsorbed intermediate on Fe , which is a Lewis acid center
30–33
ments.
peak at 573 and 673 K.
water formation is not visible over all solids due to the detector centers; the subsequent electron transfer to Fe results in
Typically, pure NiO is characterized by a single TPR obtained from a-Fe O . This step is followed by the elimination
2 3
34–36
In addition, the peak position of of two hydrogen ions from two C–H ethylic groups on basic
3
+
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other light products are also found over FeAlNi, indicating the
predominance of ethylbenzene cracking and condensation
0
34
reactions, as well as Boudouard reaction over Ni sites. For the
purposes of comparison, a binary FeCo catalyst prepared by the
35
co-precipitation method is also used as a catalyst, obtaining
ca. 5% of conversion and ca. 9% of styrene selectivity. According
36
0
0
to our previous study, both Co and Ni sites are indispensible
elements for coking.
The catalytic reaction rate depends strongly on the disper-
sion degree of active components. FeAlZn and FeAlLa possess
3
+
Fe well dispersed in their large surfaces (XRD and textural
properties), and the results depicted in Fig. 5 show that high
conversions are achieved at relatively low reaction times (typi-
cally under 30 min). The conversions over the these solids
decrease monotonically and a maximum of ca. 30% is observed
for FeAlZn, whereas FeAlLa conversion is about 20% in 4 h of
reaction time, with styrene being the main product. Neverthe-
less, the styrene selectivity is not entirely obtained over FeAlLa
in 5 h and styrene conversion falls simultaneously to 17%. A
rational reason for explaining the aforementioned results is that
the dispersed nanoparticles expose terrace, corner and edge
atoms, in addition to step atoms, as observed for Fe-based
37
solids obtained by the same preparation method. These
types of defects may contribute to an increase of ethylbenzene
3
+
3+
adsorption on Fe , whereas this action is not observed on La
3
+
3+
or Al sites. As the active species containing Fe are consumed,
conversion is decreased, and the parallel reaction of ethyl-
benzene conversion to benzene, ethylene and methane, among
others, accounts for slightly low selectivity over FeAlLa,
compared with FeAlZn.
Best results are achieved over FeAlZn because of the FeAl
2 4
O
active-phase formation, which could be stabilized by ZnO as a
ence of CO . The open symbols represent the conversion, whereas the textural and structural promoter of the iron species. This
Fig. 5 (a) Catalytic results of the dehydrogenation of EB in the pres-
33
2
closed symbols are the selectivities to styrene. (b) Overall selectivity of
the products formed during the reaction in 5 h. The reaction was
catalyst exhibits better results in ODH reaction, compared with
FeCo and FeNi, whose active phases are CoFe O and NiFe O
2 4
spinel oxides, respectively. The latter phases are not selective
to styrene, and thus, stability is restricted to 2 h of reaction time.
Another factor may be the occurrence of a parallel RGWS reac-
2
4
performed under atmospheric pressure at 823 K and a CO
ratio of 30.
2
/EB molar
34
tion increasing the CO
2
conversion at the expense of ethyl-
33
styrene and H
2
production. Bonm et al. reported that bulk benzene dehydrogenation, in some cases. This was
Fe-containing ZnO oxides are seen as active phases for ethyl- subsequently conrmed by the modeling and experimental
benzene conversion in the presence of steam because of the assay of RWGS reaction studies with FeAlNi and FeCo catalysts.
interaction between the acidic ZnO and a-Fe
ZnFe
. Nevertheless, the activity decreases signicantly with on FeAlNi is sufficiently reactive to be converted on metallic Ni
Zn sites in the iron-based catalysts in the dehydrogenation of sites, whereas MgO adsorption ability for CO is elevated rather
ethylbenzene in a He atmosphere due to the easy reduction of than by other elements. The reason for this behavior is
the iron species. The same fact can probably be attributed to the believed to be due to the presence of either MgO or MgAl on
FeMgZn, as previously stated. For FaAlZn, the ability of Zn sites
2
O
3
, forming
It is important to note that CO formed during the reaction
2
o
2 4
O
2
+
2
32
38
2 4
O
deactivation behavior of FeAlZn.
However, using NiO as promoter, FeAlNi activity does not in converting CO can be quite low, and thus, poor CO
2 2
changed signicantly, suggesting that the active Fe is not conversion is expected. In the case of FeAlLa, lanthanum
39
3
+
3
+
2+
40
sufficiently reducible, and the Fe /Fe reduction–oxidation carbonates can be formed by the adsorption of CO
, and it can
2
couple is stabilized on the catalysts (shown by TPR), whereas justify the catalytic performance of the solid.
2
+
0
Ni is reduced to Ni , acting as active sites by increasing the
As can be observed in Table 2, the catalytic performance is
signicantly affected in the absence of CO , i.e., a DH reaction.
Moreover, the selectivity of styrene over FeMgZn reaches 99%, The catalytic runs are performed far from equilibrium. When
whereas the FeAlNi production of styrene is only 2% (Fig. 5b). CO is co-feeding the reaction, an enhancement of the EB
By-products, such as toluene, benzene, aromatics, methane and conversion to 20% is obtained over FeAlNi, whereas the
34,36
occurrence of side reactions such as ethylbenzene cracking.
2
2
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conversion over FeMgZn is about 9% in 5 h of time on stream.
The deactivation of FeAlNi observed in the ODH reaction
(Fig. 5a) is probably due to the reduction of Ni particles by the
products H and CO in the RGWS reaction.
2
Catalytic performance in the absence of CO
2
(Table 2)
displays twice as little as that of the parent solids used in the
presence of the gas (Table 1). However, no general trend or
correlation among iron dispersion, promoter nature and
activity can be drawn from EB conversion data. From these
results, it can be concluded that simple ethylbenzene dehy-
drogenation (DH reaction) occurrence is limited in the absence
of the mild oxidant due to the reduction of the active iron phase.
In addition, it appears that the styrene product is unstable in
Fig. 6 Raman measurements for the spent catalysts. The solids were
the absence of CO
will be further investigated over the most active solids.
.2.1. Characterization of spent solids. The Raman
2
and reacts with the loss of its selectivity. This
used in the reaction under atmospheric pressure at 823 K and a
CO /EB molar ratio of 30 for 5 h.
2
3
measurements of the spent solids are performed to describe the
structural features of the solids and conrm the existence of
carbonaceous deposits aer being tested in the reaction (Fig. 6).
FeAlLa exhibits two broad bands at around 1345 and
Fe-based catalysts induced by metallic nickel species, which
provides the cracking of the ethylbenzene molecule to form
carbon on the solid surface. These results could not be
explained without taking into account the damage of the solid
surface. As shown in Table 2, the values of the textural proper-
ties of the solids decrease signicantly, compared with the fresh
solid, because of coking on the solid surface.
Investigating the Raman spectrum of FeMgZn more closely, it
can be found in only one band, namely that of D. It implies that the
amorphous carbon deposition from ethylbenzene or CO decom-
position reactions could be responsible for the poor activity of the
solid. A quite satisfactory relationship between Raman results and
surface properties is evidenced in Table 2. The low textural prop-
erties of the FeMgZn can be ascribed to the much more amor-
phous carbon deposition on pores and/or solid surfaces.
ꢀ1
1620 cm , which are attributed to the D and G bands,
respectively; these bands are associated with the deposition of
carbonaceous species on the solid surface. The D band is
ascribed to the defects in the structure or disordered carbon
species, whereas that of G is originated from the in-plane C–C
1
bond stretching of more ordered graphitized carbon. Most
probably, some well-dispersed iron nanoparticles of FeAlLa are
more prone to be reduced during the reaction, and thus, form
metallic iron due to hydrogen presence. Assuming that coking
formation is inevitable under the aforementioned conditions,
the reason for a lower activity decay of the FeAlLa could be a
higher resistance against a full reduction of the nanoparticles
and leaching of the coking by oxi-lanthanum carbonate species
No carbon bands are observed for FeAlZn, which is assumed
to be due to resistance to the coking of the solid. This inter-
pretation is reasonable if the textual properties are still main-
tained for the spent solid (Table 2). It can be assumed that
FeAlZn is a stable catalyst for the reaction.
41
formed by CO2 from the solid surface. More interestingly,
Table 2 shows that the textural properties of spent FeAlLa are
only slightly affected aer the reaction, as compared with the
fresh solid in Table 1.
3
.2.2. Kinetic constant and RWGS reaction rate determi-
In the case of FeAlNi, the shiing of D and G bands to higher
wavenumbers is also observed, as compared with FeAlLa,
++
nations. Using the C
temperature on the RGWS reaction rate is performed with the
CO /H of 1 at 10 bar (Tables 3 and 4). Not surprisingly, at
programming tool, the effect of
ꢀ
1
particularly as the D band displays a 35 cm red-shi. Raman
results are in reasonable harmony with those aforementioned
catalytic results that suggested the deactivation of this solid,
owing to ethylbenzene cracking. This is in a good agreement
2
2
different operating temperatures (from 400 to 1050 K), the
results show that the non-catalyzed reaction is favored by
increasing the temperatures. However, temperatures higher
than 850 K signicantly slow kRWGS due to the thermodynamic
limitations of the process at elevated temperatures (Table 3).
5
with the report of Menezes that illustrated the deactivation of
Table 2 Catalytic performance in the absence of CO
2
for the steady- Therefore, side reactions such as CO dissociation into CO and
2
state condition. Reaction conditions: 50 mg of fresh catalyst and a
temperature of 823 K over the course of 5 h. The textural properties of
the spent catalysts, after being used in the reaction in the aforemen-
tioned reaction conditions
its further conversion to coking (reaction (VI)) prevails over the
RGWS reaction at around 1100 K. Up to this temperature, the
overall process becomes close to the maximum allowed by
equilibrium, probably due to the equal velocity of WGS and
EB conversion in the
RWGS reactions or CO
2
dissociation (VI).
2
ꢀ1
3
ꢀ1
Catalyst
absence of CO
2
(%)
SgBET (m g
)
p
V (cm g )
+ coke, DH298.15 K ¼ ꢀ172 kJ molꢀ1
ꢁ
(VI)
2
CO # CO
2
FeAlNi
FeMgZn
FeAlZn
FeAlLa
20
9
2
19
14
54
63
0.03
0.02
0.07
0.09
From Arrhenius plots used to establish a good set of
temperature-dependent rate parameters, a resultant activation
—
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3
ꢀ1
energy parameter of 4.01 ꢂ 10 J mol is obtained in accor- rates. These differences are appreciable at temperatures less
43
4
2–45
dance with those found elsewhere.
than 950 K, in which RWGS is thermodynamically favored.
Many mathematical models have been developed by investiga-
and 42.3 kJ mol , respectively, at 823 K. These values decrease tors to predict the evolution of the effective RGWS reaction on a
with an increase in temperature, suggesting that the CO hydro- solid surface and mostly agreed that CO adsorption is favored
genation is favored at high temperatures, otherwise the RGWS at temperatures lower than 850 K, due to mono and bidentade
ꢁ
ꢁ
r
ꢀ1
Thermodynamic parameters DG and DH are ꢀ3.2 kJ mol
r
ꢀ1
2
2
44
side reaction occurs inspite of the main reaction under these compound formation with CO
2
, on the catalyst surface.
conditions. According to the DFT-rened microkinetic model It has been assumed that when temperatures are higher than
studies and mechanistic predictions, the WGS reaction proceeds 950 K, the parameter tends to reect values close to zero
via a carboxyl (COOH) mechanism, whereas the RGWS reaction because of the difficulty in adsorbing CO on the solid surface.
2
proceeds according to that of a redox (reaction (IV)) at moderate Thus, the CO more rapidly decomposes to carbon monoxide
2
42,45
temperatures,
.2.3. Adsorption and desorption constants by varying the resulting in reduced desorption constants.
temperature. The CO and water adsorption constants (e.g., The inuence of the reaction temperature on the KH2O
CO2 and KH O) are obtained by the means of eqn (5) and (6). The behavior is examined in temperatures ranging from 850 to
which is in good agreement with our results.
and the traces of the effluents are detected in low amounts,
3
2
K
2
plots of predicted adsorption constants as a function of 1000 K (Fig. 7b). The model considering water adsorption gives
temperature are shown in Fig. 7. reasonable results, and the main reason is the good capture of
Because of the slight endothermicity of the RWGS reaction, water at temperatures as low as 950 K. Furthermore, the curves
KCO2 gradually increases with increasing temperature (Fig. 7a). provide reasonable accuracy and good agreement with the fact
In addition, elevated temperatures lead to higher KCO2 values that RWGS reaction is favored upon using these conditions, as
than those of KH O, and this is also reected in their velocity
2
Table
constant (kRGWS) values for RWGS reactions. The results were obtained
from 400 to 1050 K by using a CO /H molar ratio of 1
3 Number of assays (NPt), temperature (T) and velocity
2
2
2
ꢀ1
NPt
T (K)
k
RGWS ꢂ 10 (s
)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
0.07
0.3
0.8
2.0
4.0
7.0
11.0
18.0
26.0
32.0
33.0
35.0
36.0
37.0
0
1
2
3
4
Table 4 Number of assays (NPt), carbon dioxide to hydrogen partial
pressure (pCO2/pH2) and RWGS reaction rates
19
ꢀ1
NPt
pCO2/pH2
r
RGWS ꢂ 10 (kmol kg
)
1
2
3
4
5
6
7
8
9
0.5
1
2
3
4
5
6
7
8
0.39
1.57
3.14
4.71
6.29
7.86
9.43
11.01
12.58
14.15
15.72
2
Fig. 7 (a) CO adsorption constant at various temperatures obtained
1
1
0
1
9
10
for a hypothetic catalyst surface. (b) Water adsorption constant as a
function of the temperature for the aforementioned hypothetical
catalyst surface.
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47
observed for catalytic runs over Ru and carried out at 850 K. It plots of adsorption constants vs. temperature result in an
is interesting to consider the aforementioned observations in exponential curve, which is regarded as sufficiently good to
light of the recently reported Cu–Fe catalysts tested under the indicate that temperatures below 950 K are adequate to show
same conditions, which demonstrate the abilities of CO and that the CO2 adsorption rate is the determining step of the
water to desorb from Cu–Fe surfaces during the steam refor- reaction, which is sufficiently elevated to allow for RWGS
19
mation of methane.
2
parallel reactions. However, the CO deactivation rate is also
Fig. 8 displays the plots of deactivation constants (e.g., KH Od enhanced at these temperature conditions. In addition, the
2
and KCO2d) versus temperature.
model shows a strong tendency to underestimate the conver-
Of all the models considering a hypothetic surface where CO sions at elevated temperatures.
39,42,48
and H O species react,
the curve provides the best t to
3.2.4. Effect of CO /H on the occurrence of RWGS. The
2 2
2
corroborate that the deactivation of these entities on a solid reverse water gas shi reaction is studied under distinct CO /H2
2
surface is faster over CO than H O at temperatures as low as ratios by varying the temperature. RWGS reaction rates is
2
2
950 K. This result indicates that the reaction is more favorable gradually enhanced by increasing the partial pressure of the
at temperatures lower than 950 K, as the reactant molecules of reactants (Table 4) due to the shi of equilibrium to form CO
4
9,50
2
CO disproportionate to form CO, and the rate of this reaction is and water, as it has already been stressed in the literature.
42,48
too high, compared with other parallel reactions.
2 2 2
Lower CO /H inferior to 1 corresponds to an increase in H
) is found to be equal to content in the feed and this can favor the competition among
at 850 K by using eqn (7). The reaction in this the following reactions: WGS (backward reaction (III)), hydro-
study is more inuenced by an increase in the reaction genation of CO2 to methanol or ethanol (reactions (VII–IX)),
0
The value of adsorption constant (k
a
ꢀ
25 ꢀ1
7
.9 ꢂ 10
s
temperature. In addition, the deactivation on the external methane formation (reaction (X)), alkenes formation (reactions
25,26
surface of the catalyst is assisted by a CO
parallel reaction more than its own CO adsorption. Finally, the Moreover, RWGS is not thermodynamically favored under lower
CO /H , indicating that the kinetic factors prevail over the
thermodynamic ones.
2
decomposition (XI and XII)) and coking by CO reduction (reaction (XIII)).
2
2
2
CO + 3H / CH OH + H O
(VII)
(VIII)
(IX)
2
2
3
2
2 2 3 2
CO + 4H / CH OH + 2H O
2
CO + 6H / C H OH + 3H O
2
2
2
5
2
CO
2
+ 4H
2
/ CH
/ C
4
+ 2H
2
O
(X)
2
CO
2
+ 7H
2
2
H
6
+ 4H
2
O
(XI)
3
CO + 9H / C H + 6H O
(XII)
(XIII)
2
2
3
6
2
CO
2
2 2
+ 2H / C + 2H O
Nevertheless, the rates reach a plateau and a maximum value
of CO /H ¼ 1, which tends to be favored due to the stoichio-
2
2
metric relations of the RGWS reaction even if elevated condi-
tions are obtained at CO
reaction rate levels up 1.0 ꢂ 10
reaching 1.
2 2
/H ratios superior to 1. Therefore,
ꢀ19
ꢀ1
mol h are achieved at
p
CO /p
H
2 2
Apart from the modeling results, RWGS reaction occurrence
is preferred at a CO /H molar ratio of 1 and a temperature of
2
2
850 K.
3.3. Experimental studies in RWGS over the catalysts studied
Catalytic runs in the RGWS reaction were carried out over
FeAlNi and FeCo (Fig. 9), the latter being a reference catalyst.
The reaction conditions are the CO
temperature of 850 K. A detailed description of FeCo reference
obtained for a hypothetic catalyst and its textural and structural features of the solids
2
2 2
/H molar ratio of 1 and a
Fig. 8 (a) Deactivation constants of CO
catalyst. (b) Deactivation constants of water for a hypothetic catalyst.
have been given in ref. 42. The FeMgZn and FeAlLa catalysts are
not active in the RGWS reaction due to the lack of active sites to
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catalyze the reaction, whereas the conversion of FeAlZn shows extremely high temperatures are impractical for the commercial
activity in the rst minutes of the reaction and then falls to zero. application of catalysts in the RWGS reaction, thermodynami-
Experimental and both experimental and predicted conver- cally favorable parallel reactions, such as Boudouard (reaction
sion have shown good t.
(X)) and methanation (reaction (XIII)), could be likely over the
The CO conversion of Fe-based catalysts increases rapidly FeAlNi catalyst. In the case of FeAlZn, the oxidation of iron
2
from 0 to 2.5% at 600 K, and thereaer, it appears that best nanoparticles may not be likely; as a result, there is a loss of the
catalytic activity can be attained with 20.3% conversion at 1400 K active sites needed for the reaction, and thus, CO selectivity is
2
(Fig. 9a). Both the CO conversion and CO selectivity do not meaningless in the range of temperatures studied. The results
follow the same trends due to WGS reaction predominance over indicated that the metallic Co nanoparticles are mainly
certain catalysts, as shown in Fig. 9b. Instead, FeAlZn is inactive responsible for the catalytic performance because the FeCo
even at high temperatures. It is well known that Ni in iron-based shows two times higher activity than the FeAlNi catalyst. Such
catalysts promotes the WGS reaction at temperatures as high as an observation hints towards how the reaction conditions
6
00 K due to reaction kinetic control, and the catalyst is very (temperature and composition) can affect the catalytic conver-
49–52
active, stable and selective to CO at high temperatures.
The sion. The results are indeed in excellent agreement with those
FeAlNi is activated more rapidly; however, aer 1 h of reaction, it obtained theoretically.
quickly got deactivated mainly due to the formation of coke
From these results, it can be concluded that FeAlZn is very
residues on the catalysts during the test. Carbon monoxide advantageous in ethylbenzene conversion in the presence of
selectivity linearly decreases with an increase in temperature, CO for producing styrene because the coupled reaction and the
2
ensuring that the catalytic activity for CO decomposition to stability of the dispersed iron-active phase makes the occur-
coking at elevated temperatures is likely. This is favored when rence of RGWS reaction effectively reduced.
reduced nickel particles are present on the surface of the support.
Moreover, the inverse relationship between CO selectivity
4
. Conclusions
and temperature also suggests that the RWGS endothermic
reaction needs heat to achieve high CO
2
conversion. Because The ethylbenzene dehydrogenation in the presence of CO (or its
2
absence) was investigated over Fe-based catalysts. Among the
various studied ternary systems that contained La and Zn
promoters, FeAlZn showed the best results in ODH due to the
dispersionofa-Fe
in the presence or absence of CO
2
O
3
andg-Fe
2
O
3
nanoparticles ontheirmatrices
. FeAlZn was the most active
2
solid in the ODH reaction, whereas FeAlNi exhibited the best
performance in the DH reaction among the catalysts studied.
Catalytic results in the RWGS reaction were performed by the
meansofkineticmodelingandexperimentalstudies. Theoptimal
conditions for RWGS reaction occurrence were at 850 K, using a
ratio of CO
resultsforFeAlNi. TheCO
increase in temperature; this factor was responsible for the
highest reaction rate at temperatures close to 850 K, due to CO
2 2
/H
¼ 1, which were proved by the experimental
2
adsorptionconstantdecreasedwithan
2
disproportionatereactionoccurrences. Althoughthe reactionrate
was elevated at high hydrogen and carbon dioxide partial pres-
sures, the CO /H ratio values less than 1 led to the formation of
2
2
hydrocarbons, whereas those superior to the unity gave CO2
degradation. A CO /H
¼ 1 ratio and temperature of 850 K were
2
2
the best condition for RWGS reaction occurrence, whereas that of
the ODH was not favoured under these conditions. From the
catalytic results, the FeAlZn showed a poor performance in RGWS
reaction, compared with that of the FeAlNi catalyst due to the
active-phase degradation of the former solid. Thus, FeAlZn is best
suited for ethylbenzene dehydrogenation coupled to RGWS, and
this solid exhibited 30% of ethylbenzene conversion, being
entirely selective to styrene.
Abbreviations
COS
CT
Active sites occupied by oxygen atom
Total amount of active sites occupied
Fig. 9 Experimental assays of RWGS reactions for FeAlNi and FeCo
catalysts: (a) CO conversion and (b) selectivity to CO.
2
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