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8/18/2019 Ethylbenzene to Styrene Over Alkali Doped TiO2-ZrO2 With CO2 as Soft Oxidant
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Accepted Manuscript
Title: Ethylbenzene to Styrene over Alkali Doped TiO2-ZrO2with CO2 as Soft Oxidant
Author: Abhishek Burri Nanzhe Jiang Khalid Yahyaoui
Sang-Eon Park
PII: S0926-860X(15)00075-7
DOI: http://dx.doi.org/doi:10.1016/j.apcata.2015.02.003
Reference: APCATA 15240
To appear in: Applied Catalysis A: General
Received date: 25-9-2014
Revised date: 21-1-2015
Accepted date: 5-2-2015
Please cite this article as: A. Burri, N. Jiang, K. Yahyaoui, S.-E. Park, Ethylbenzene to
Styrene over Alkali Doped TiO2-ZrO2 with CO2 as Soft Oxidant, Applied Catalysis A,
General (2015), http://dx.doi.org/10.1016/j.apcata.2015.02.003
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http://dx.doi.org/doi:10.1016/j.apcata.2015.02.003http://dx.doi.org/10.1016/j.apcata.2015.02.003http://dx.doi.org/10.1016/j.apcata.2015.02.003http://dx.doi.org/doi:10.1016/j.apcata.2015.02.003
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Ethylbenzene to Styrene over Alkali Doped TiO2-ZrO2
with CO2 as Soft Oxidant
Abhishek Burri
a
, Nanzhe Jiang
a,b
, Khalid Yahyaoui
c
and Sang-Eon Park
a
*a Laboratory of Nano-Green Catalysis, Department of Chemistry and Chemical Engineering, Inha
University, Incheon 402-751, Republic of Korea.
bDepartment of Chemical Engineering, Yanbian University, China.
cSaudi Basic Industries Corporation, PO Box 5101, Riyadh 11422, Kingdom Of Saudi Arabia.
*Corresponding Author: Tel.: 82-32-860-7675 ; Fax.: 82-32-872-8670
E-mail : [email protected]
Highlights1) Acid-Base bifunctional TiO2-ZrO22) Doping of alkali metal into TiO2-ZrO23) Increasing CO2 conversion by increasing surface basicity
4) Improved surface oxygen species
5) Highly stable alkali doped TiO2-ZrO2 for Oxidehydrogenation of ethylbenzene to styrene
with CO2
Abstract
Oxidative dehydrogenation of ethylbenzene (EB) to styrene monomer (SM) over alkali metal
(Na, K) doped TiO2-ZrO2 (TZ) has been studied. The EB and CO2 conversions observed over
alkali doped TZ are higher than that of non-doped TZ. The enhanced CO 2 conversion
compared with non-doped counterparts is attributed to improved basicity, formation of
TiZrO4 phase with increased CO2 affinity and insertion of K or Na into the lattice which
affects the binding energy of “O” in turn providing more labile oxygen species. Alkali doping
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also effected in fine tuning the surface acid base properties. Moreover, these K and Na doped
binary metal oxides system showed high surface area of 256 m2/g and 199 m
2/g respectively.
There was a 10 fold increase in the CO2 conversions in case of the doped TZ compared to
non-doped TZ increasing the stability of the catalyst by decreasing coking on the surface of
the catalyst in spite of the high conversions.
Keywords: Oxidative Dehydrogenation • CO2 conversion • Soft oxidant • Mixed metal oxides
• Styrene
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Introduction
Establishing a chemical industry based on utilization of CO2 is a long-term goal as well as
a fascinating dream for synthetic and chemical engineers and scientists all over [1]. Several
attempts have been made in the process of development of new systems for the
dehydrogenation of EB and other alkanes like C3-C4 to overcome the energy issues in the
conventional process. EB dehydrogenation to styrene monomer is one of the most prominent
industrial processes producing up to 2.5x107
MT/yea in 2002 [2]. At present, bulk styrene
monomer is produced through dehydrogenation of EB on Fe–K–Cr oxide-based catalysts
with superheated steam at 700 ºC. The disadvantages of the present dehydrogenation process
is endothermic in nature ( ΔH =124.85 kJ mol-1
), highly energy intensive process and catalyst
deactivation [3]. Several investigations were carried on the above problems [3, 4] and on
coke deposition, its negative and positive effects on the EB dehydrogenation both in
oxidative and non-oxidative reactions [5-8]. An estimation of energies required for non-
oxidative (steam) and oxidative (CO2) styrene production by the dehydrogenation of EB was
done by Miura et.al which suggests that the energy required in case of CO2 utilization was
very less (190 kcal/kg of SM) compared to that of H2O (1500 kcal/kg of SM) elucidating that
the process if operated in CO2 must be energy saving process [9]. When O2 was used in
commercial EB dehydrogenation process as an oxidant to overcome the thermodynamic
limitations it eventually lead to burning of the reactants leading to COx and oxygenates [1].
Aresta et.al reported the benefits of CO2 utilization such as inexpensive, nontoxic,
renewable feedstock which can lead to new routes to existing chemical processes. This would
lead to efficient and economical chemicals. Moreover, it may provide significant positive
impact on the global carbon balance [10]. Alternatively, it has been realized that the carbon
dioxide utilization as a diluent as well as soft oxidant will be technologically a great deal of
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development. The CO2 utilization offers several advantages, like acceleration of the reaction
rate, enhancement in the product selectivity, diminishing of thermodynamic limitations,
suppression of total oxidation, prolonging of catalyst life, prevention of hotspots and so many
more. Based on the above investigations CO2 was proposed as one of the most advantageous
feedstock. Hence, several research groups including us have been exploring the possibility of
CO2 utilization in the dehydrogenation of EB over different catalyst such as, Fe 2O3, V2O5,
Sb2O5, Cr 2O3, CuO, CeO2, ZrO2, La2O3, alkali metals and then promoting these active metal
oxides onto supports like mesoporous silica, carbon and clays [11-16].
CO2 activation is widely investigated on materials like alkali/alkaline earth metal, Zr, Ti,
Ga, Ni, MgO and many other catalysts [17-21]. Since the first report on CO2 as oxidant in the
dehydrogenation of EB it is studied in different aspects from the past two decades [22-28]. A
great deal of work has been contributed by Park et.al on the oxidative dehydrogenation of
alkyl aromatics and alkanes in the presence of CO2, a recent review summarized all the work
that has been contributed on the CO2 utilization as oxidant and promoter [18]. The activation
of carbon dioxide on catalyst surfaces is an important elementary step in CO2-mediated
oxidative reactions and has been investigated in a number of surface science studies. Even
though the catalysts are able to activate carbon dioxide in soft oxidant systems, the utilization
of some activators, such as alkali metals, promotes the activation and conversion of CO2
which, in turn, has a prominent effect on the catalytic performance.
The robustness, variability and high tuning ability of the physical and chemical properties
has given a wide scope for intense studies on TZ [29]. The properties of TZ can be controlled
by the preparation methods. The controlled acid base properties of TZ are well discussed by
Manríquez [30]. Doping of alkali or alkaline earth metals is viable in case of ZrO2 or TiO2.
Other reports on ODH of lower alkanes using promoters suggest that alkali promoters either
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reduce the coke deposits or facilitate in quicker desorption of the product on the surface of
the catalyst [31]. Stabilizing the tetragonal phase of ZrO2 by doping technique is very much
in practice [32, 33]. Amorphization by doping alkali metal into TZ was discussed in detail in
our previous publication [34].
Sugino et al. have reported the oxidative dehydrogenation of EB with CO2 over iron
supported on alkali or alkaline earth metals (Li, Na, Ca, Ba and K) impregnated on activated
carbon. The highest yield of styrene (40.4%) with more than 90% selectivity with a specific
activity of 2% was observed over a lithium-impregnated carbon support, which was
comparatively higher than other alkali-impregnated supports [35].
In continuation of our earlier studies on oxidative dehydrogenation of EB using CO2 as
soft oxidant, herein we report the effect of K and Na doping enhancing the surface basicity
and also the labile oxygen species. This in turn enhances the CO2 conversions relative to the
EB dehydrogenation leading to higher conversion and selectivity.
2. Experimental Section
2.1 Synthesis of catalysts
TiO2 –ZrO2 (1:1) mixed oxide catalysts were prepared by co-precipitation-digestion method.
The requisite amounts of TiCl4 (0.09M TiCl4 in 20% HCL solution, Aldrich, USA) and
ZrOCl2. 8H2O (Aldrich, USA) are dissolved in 100 ml distilled water and pH was adjusted to
14 by 5M KOH/NaOH solution added drop wise under vigorous stirring at room temperature.
The solution was then divided into two parts. The first part is filtered immediately, washed
several times with warm water and dried at 120oC for 12 h and calcined at 600
oC for 6 h.
The second was kept under reflux for 24 h at 100oC. It was filtered to separate the precipitate
followed by washing, drying in an air oven for 12 h at 120 ºC and calcined in a muffle
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furnace at 600 ºC for 6 h. The samples were named as Na-x and K–x, where x is the digestion
time. TiO2-ZrO2 was prepared in a method reported earlier [36] and is denoted as TZ.
2.2 Characterization
Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku diffractometer using Cu
Ka radiation (λ = 0.154 nm) radiation source and a scintillation counter detector. The
intensity data were collected over a 2θ range of 2–80° with a 0.02°/min step size and using a
counting time of 1 s per point. Crystalline phases were identified by comparison with the
reference data from International Center for Diffraction Data (ICDD) files. Raman spectra
were recorded on a LabRam HR800UV Raman spectrometer (Horiba Jobin-Yvon) equipped
with a confocal microscope and liquid-nitrogen cooled CCD detector at ambient temperature
and pressure. The emission line at 325 nm from He-Cd laser (Melles Griot Laser) was
focused on the sample under microscope. The time of acquisition was adjusted according to
the intensity of Raman scattering. N2 adsorption–desorption isotherms and pore
characterizations were done by using a Micromeritics ASAP 2020 apparatus at liquid N2
temperature. Before each measurement, the samples were degassed at 150 oC for 5 h. The
BET specific surface areas were calculated from adsorption data in the relative pressure
(P/Po) range = 0.04–0.25. The total pore volumes were estimated from the amount adsorbed
at the relative pressure of 0.99. The pore size distributions were calculated by using Barret–
Joyner–Halenda (BJH) method from adsorption branches, and the pore sizes were obtained
from the peak positions of the pore distribution peaks. The TEM images were obtained on a
JEM-2010 (JEOL) instrument equipped with a slow-scan CCD camera and at an accelerating
voltage of 400 kV. Samples were sonically dispersed in ethanol and deposited on a carbon
coated copper grid before examination. The Acidity measurements of the materials were
performed by temperature programmed desorption of NH3, using Chemisorp 2705 unit
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(Micromeritics Instrument. Co., USA) equipped with thermal conductivity detector (TCD).
Typically, c.a. 50 mg of catalyst was pretreated in flowing He at 500oC for 1 h, cooled to
100oC and allowed to expose 5% NH3 in helium gas mixture with a flow rate 20 mL/min for
30 min and subsequently the adsorbed NH3 was purged with helium at the same temperature
for 1 h to remove the physisorbed NH3. The chemisorbed NH3 was measured in flowing
helium with the flow rate of 20 mL/min from 100oC to 600
oC with the heating rate of 10
oC
per min. The XPS measurements were made on a KRATOS (ESCA AXIS 165) spectrometer
by using Mg Kα (1253.6 eV) radiation as the excitation source. The SEM images were
collected with a JEOL 630-F microscope. Before measurements, samples were dispersed on a
steel plate surface and coated with Pt metal. Thermo-Gravimetric analysis was made using a
Bruker 2010 SA instrument under Air.
3. Results and Discussion
3.1 XRD Pattern
The XRD patterns of K/Na doped TiO2 –ZrO2 (calcined at 600 ºC) and pure TZ catalysts are
shown in Fig. 1. Highly crystalline phases of ZrO2 are observed for the undigested K-0 and
Na-0 catalyst. The XRD reflections at 2 = 30.24º (111), 2 = 34.58º (002), 2 = 35.28º (200),
2 = 50.24º (202), 2 = 50.74º (220), 2 = 59.3º (113) and 2 = 60.22º (311) revealed the
presence of tetragonal ZrO2, which is the predominant phase in undigested K-0 and Na-0
catalysts. In the K-0 catalyst an intense XRD reflection appeared at 2 = 28.22º and 40o
which belongs to monoclinic phase of ZrO2. The XRD patterns show that the digestion play a
critical role in stabilizing the ZrO2 phases in TiO2 –ZrO2 catalysts [33]. As it can be seen in
the comparative TZ and K/Na doped TZ the predominant ZrO2 phase is observed after
digestion for 24 h. According to the Ostwald ripening method reported earlier [34], the
doping allows the rearrangement of the bonds. In the process of digestion the Ti enters the
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lattice of Zr, the sizes of Ti+4
and Zr +4
are 63 and 71 pico meters respectively. Due to the size
factors, there is a possibility of Ti entering the Zr lattice. As there is no evidence of Ti phase
on the surface from the XRD patterns, it can be explained that the Ti enters the lattice of Zr
(evidenced in XPS and TPD). The FWHM of XRD in case of TZ, K-24 and Na-24 cannot be
exactly correlated with the particle size (Observed from TEM, Fig. 4) because of the
agglomerates of nanoparticles in case of K-24 and Na-24 which show clusters which are
subjected to 24h of aging (discussed in our previous work [15]
). The digestion induces the
formation of the TiZrO4 structure as confirmed in our previous report [34]. The tetragonal
phase of the ZrO2 is stabilized by doping [33]. The presence of trace amount of potassium
impurity or certain portion of TiO2 might have contributed to stabilize the tetragonal phase of
ZrO2 similar to La, Ca and Mg dopants [37, 38]. The amount of K in K-24 is 1.24% and
amount of Na in Na-24 is 1.86% determined by ICP-EOS analysis.
3.2 BET surface area
From Fig. 2, the nitrogen sorption analysis yields typical IV curves with type H1 hysteresis
loops for products of K and Na doped TZ. But the undigested K-0 showed no porosity in the
adsorption curve. The mixed oxides obtained by this methodology of preparation using
KOH/NaOH as precipitating agent showed vital changes in the surface area and porosity of
the materials. The high surface of the digested samples is as discussed, the high pH
conditions during the digestion process facilitate the recrystallization of the precipitate
formed during the addition of the KOH and NaOH [34]. In the course of recrystallization
process there is emergence of mesoporosity to the materials. K-0 and K-24 exhibited a
surface area of 118 and 166 m2/g, while the Na-0 and Na-24 showed a surface area of 112
and 188 m2/g respectively. The pure TZ prepared in the conventional method using NH4OH
as the hydrolyzing agent of 144 m2/g. The NaOH precipitated samples showed
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comparatively lower surface area than the KOH precipitated samples as the amount of the
dopant and the nature of the dopant is different. The Na (116 pico) is smaller in size than the
K (152 pico) which may lead to the lower doping amount of the alkali into the crystal
structure of TiO2-ZrO2. This effect may cause the higher amorphization as it can be observed
in the XRD patterns as the Na doped samples are more amorphous compared to those of the
K doped. The size of the ion affects the properties of the catalyst in the unit cell level.
3.3 X-ray Photoelectron Spectroscopy
Depending on the preparation procedure there is a possibility of embedding the alkali
metal into the lattice of the ZrO2. As the samples were subjected to digestion for 24 h, the
following phenomenon may occur a) The monoclinic and unorganized TiO2 and K + are well
dispersed and the monoclinic zirconia transforms into tetragonal zirconia evidenced in XRD
patterns. b) The migration of Ti+4
into the crystal structure of Zr which indeed causing the
formation of TiZrO4 solid solution and decrease in the surface Ti+4
content evidenced in Fig.3.
c) The doping of K or Na into the crystal structure of the Zr causing an increasing in the unit
cell size. Tsunekawa et al. [39] investigated the lattice expansion of the CeO2 nanoparticles
by doping and they found that “The reduction of the nominal ionic valence induces an
increase in the lattice constant due to the decrease in electrostatic forces”. The surface
properties of the catalyst are changed after the digestion process. The surface peak intensities
of Ti reduced in the K and Na doped samples indicating the decrease in the surface Ti4+
content. The Ti 2p spectra show the core binding energy (BE) levels at 463.64 eV and 458.2
eV for 2p1/2 and p3/2, respectively. The obtained BE core level of the Zr, Ti and O agrees
well with the earlier reported results of the ZrTiO4[40]. The binding energy of O 1s spectra in
TZ sample is 531 eV whereas in the K doped TZ the, K-24 sample the binding energy of O 1s
spectra 530.8 eV and in case of Na doped TZ, Na-24 sample it is 529.5 eV. The decrease in
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the binding energy of the O 1s can be attributed to the expansion of the unit cell causing a
decrease in the electrostatic forces between the atoms.
As reported by Sandra et. al [41], due to the doping of alkali earth metals into the Ce
there is an expansion in the lattice. The decreasing in the binding energy enhances its
reactivity in other words the labile oxygen species quantity increased. The peak broadening in
the O 1s spectra in K and Na doped samples indicate the doping of K and Na into the
interstitial positions of the framework. The FWHM is >1.5 in all the cases indicating the
contribution of the O species from the different species. The peak shift of 0.2eV and 1.07 of
Na doped and K doped TZ show a clear evidence of different oxides available on the surface
revealing the presence of K and Na oxides on the surface of the catalyst [42].
3.4 TEM Images
The TEM images shown in the Fig. 4 show the decrease in the particle size of the catalysts
from the TZ-NH3 to Na-24 and K-24. The TZ-NH3 catalyst showing a particle size of 25 nm
whereas the digested samples K-24 and Na-24 show a particle size of 3-5 nm. The
nanoparticles seem to agglomerate and form clusters of the size 10-15nm as it can be
observed in the case of K-24[15]
. The particle sizes are in line with the BET surface area.
3.5 Acidity and basicity measurements
Figure 5 depicts the TPD spectra of the catalysts. The CO2 TPD (Fig. a) CO2) of TZ has
weak to moderate basic site distribution from 200oC to 450
oC. The shoulder between 100-
200oC is ascribed to surface –OH groups[43]. The two peaks concentrated at 250
oC and
400oC show the weak and moderate basic site distribution. The Na and K doped samples
showed a weak basic site distribution ranging from 100 oC to 350 oC. The Na-24 sample
showed the highest adsorption of CO2 at lower basic sites with peak centered at 250oC and a
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shoulder peak at 160oC. The K-24 sample showed a similar kind of adsorption of CO2 in the
range of 100oC to 350
oC but the intensity is lower compared to Na-24 sample. On the whole
the basicity of the doped samples is higher in intensity of lower basic sites in comparison
with pure TiO2-ZrO2. As observed in the Fig. 6, the quantity of the basic sites decreased in
accordance with the doping. The Na-24 sample showed 0.46 mmol.g-1
, K-24 showed 0.45
mmol.g-1
which is 0.05 mmol.g-1
in case of TZ sample. The increase in the surface basic sites
can be attributed to the doping effect, as it is evidenced from the XRD and the XPS that the
surface is covered with Zr and the K and Na are inside the structure. This K and Na present
inside the structure not only stabilizes the structure but also decreases the surface electron
density over all enhancing the CO2 adsorption ability. The surface basicity is one of the major
factors for the adsorption of CO2 [44].
The temperature programmed adsorption and desorption of 5% NH3 studies have provided
the acidity of the surface of the catalyst. The Na-24 sample showing broad peak ranging from
60 to 500oC indicating large quantity of lower and medium acidic sites. But in case of the K-
24 due to the agglomeration of the smaller nanoparticles no peaks are observed in the range
of 100-600oC. Probably, the presence of higher amounts of K on the surface of the catalyst
than in the structure has reduced the acidic strength in case of K-24. TiO2-ZrO2 doped with
K 2O by wet impregnation and its acidic and basic properties were studied by I. Wang et.al
[45], which explains that the increase in the doping amount decreases the NH3 desorption
amount as the K 2O firstly adsorbs on the strong acidic sites and later on as the loading
increases it was also absorbed on the moderate and low strength acidic sites. The amount of
the dopant may vary according to the digestion time [46].
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3.6 Raman spectra
From Fig. 7, shows the Raman spectra of the samples. The fluctuations in the peaks or shifts
in peaks are related to different synthesis methodologies, in particular due to the difference in
the heat treatments. The undigested K-0 and Na-0 show peaks of ZrO2 and TiO2
independently. Digested samples K-24 and Na-24 show the formation of TiZrO4. The
spectrum exhibited bands (263, 337, 478, 635, and 784 cm-1
) at low frequency range which
can be assigned to TiZrO4 phase. The Raman spectroscopy reveal the formation of the
TiZrO4 induced by the doping of the alkali metals[47]. The quenching effect induced the
formation of a clear solid solution of TiZrO4. In case of K-24 and Na-24, the formation of
TiZrO4 is confirmed by the peaks 299,446, 624 and 707 cm-1
which are the characteristic
peaks of TiZrO4. The peaks seen at 250, 450 and 620 cm-1
in case of K-0 and Na-0 are due to
Ti-O-Ti [48]. The peak at 778 cm-1
which is a characteristic peak observed in the undigested
sample showed a shift as compared with the digested samples. This peak shift can be
attributed to the smaller particulate size formation evidencing the phase transformation from
crystalline to amorphous. The digestion provides the media for the transformation of the
larger particles to nanosized particles.
4 Reaction results
4.1 Comparison of catalytic activity in case of TZ with K/Na doped TZ
As it can be seen in Fig. 8 the activity of the doped material especially in case of K-24
and Na-24 the styrene yield is much higher compared to that of the TZ. The activity results
directly coincide with the above discussion on XPS, TPD and BET. All the reactions were
carried out in a fixed bed continuous flow reactor system at 600oC with CO2 to EB ratio of
5.1. The catalysts K-0 and Na-0 showed a catalytic conversion of EB 35 % and 32%
respectively with a selectivity of 95% towards styrene. The presence of lesser acidic sites
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reduces the EB adsorption on the catalyst decreasing the conversion as evidenced by NH 3-
TPD.
The basic sites present on the K-0 and Na-0 catalyst are not effecting the conversion of EB or
CO2 (SI, Fig 1). Though the CO2 can be activated on alkali metals the dissociation is difficult
unless until there is another probe molecule to abstract the oxygen from CO2 and convert it to
CO. Direct dissociation of CO2 on the catalysts at the reaction conditions without the EB as
reactant was not observed. This clearly indicates that the CO2 enhances the
Oxidehydrogenation process rather than itself undergoing the direct dissociation without
much effect on the catalytic reaction EB to SM.
The Na-24 and K-24 show higher conversion initially similar to that of TZ. The EB
conversions of K-24, Na-24 and TZ are 75, 72 and 55% respectively in the second hour of the
reaction. The selectivity towards styrene is higher in case of TZ up to 98% whereas in case of
K-24 and Na-24 the selectivities towards styrene remained between 95-97% showing not
much significant difference. The higher conversion of the K-24, Na-24 can be attributed to
the TiZrO4 solid solution formation which has higher amount of active sites for both CO2 and
EB activation. The TPD results indicate the presence of higher acidic sites in case of TZ and
lesser basic sites shows the lesser EB conversion whereas the balanced acidic and basic sites
in Na-24 showed stable activity compared to that of K-24. In the K-24 sample, the acidic sites
are much lower than that of the TZ and Na-24 but the presence of the higher basic sites
enhanced the Oxidehydrogenation reaction enhancing the conversions of EB as well. With
the time profile the drastic drop in the activity in case of K-24 may be due to the coke
deposition on the active sites. The Na-24 sample balanced with both acidic and basic sites
indicated the lesser deactivation rate compared to that of the other catalysts. The balanced
acidic and basic site reduces the coke formation on the surface of the catalyst stabilizing the
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catalytic stability. The CO2 conversions of the catalysts are shown in fig. 8b. As you can
clearly see the CO2 conversions in case of K and Na doped TZ are highly enhanced compared
to that of the TZ. The improved basic site quantity and concentration in the moderate basic
site region in case of K-24 and Na-24 catalysts enhanced the CO2 conversion which indeed
enhanced the activity of the catalyst. The CO2 conversions are mainly based on the CO2
adsorptions as reported earlier[49], the enhancement in the basicity is one major reason that
enhances the CO2 adsorption. The CO2 conversion is likely related to the EB conversion, as
reported by Saito et.al, [50] the lattice oxygen species is being replaced by the CO2. In alkali
doped materials this behavior is very facile due to highly reactive surface oxygen species
with high basicity. This helps in the Oxidehydrogenation process enhancing the CO2
conversion. The major product in the reaction was Styrene (Minor products such as Benzene,
Toluene, CO, CH4, H2O and traces of CH4 were also observed). The CO (SI, Fig 3) clearly
indicates the CO2 conversion into CO and nascent oxygen species which can be replaced in
the depleted oxygen spaces on the surface of the catalyst. The stability of the K and Na doped
TZ catalysts can be attributed to the decrease in the coke on the surface of the catalyst and
due to the balance of the acidic and basic sites reducing the cracking process. Amount of
coke on TZ was up to 25 wt. % but in case of K-24 and Na-24 it is 15 wt. % and 10 wt. %
respectively (Calculated from TGA). In case of the TZ because of the highly acidic nature of
the catalyst the deactivation rate is higher due to the coke formation. The coke deposition on
the surface of the catalyst is the major cause for the deactivation of the catalyst. The surface
coke deposits hinder the adsorption of reactants on the active sites and furthermore enhance
the cracking by adsorption of the reactant on the coke present on the catalyst surface [5, 51].
X-ray spectra were measured after the reaction which shows no significant changes in case of
TZ, K-24 and Na-24.
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Based on the CO2 conversions and EB conversions, the degree of oxidehydrogenation
(ODH) can be evaluated. The increase in the EB conversions in case of K-24 and Na-24
compared to that of TZ can be attributed to the modification of the surface acid base
properties, nanosize, TiZrO4 formation which indeed caused enhanced the CO2 conversions
along with. Oxidehydrogenation process involving two steps as demonstrated recently by
Zheng et.al., over MWCNT’s [52] can also be elucidated in this case. The increase in the
surface labile oxygen species along with the surface basicity by doping helps in the high
interaction of CO2 on the surface showing high CO2 conversions. The CO2 not only reduces
the coke deposits on the surface of the catalyst but may also help in quick desorption of the
reactants and products from the surface of the catalyst which indeed helps in retaining the
surface active sites increasing stability and conversion.
5. Conclusion
In conclusion, the effect of doping alkali metal K and Na into the structure of TZ is studied.
The presence of this K and Na in the structure enhanced the basic nature of the catalysts
which in turn enhances the interactions of CO2 on the surface of the catalyst resulting in
improved CO2 conversions in the Oxidehydrogenation of EB to styrene reaction. The acid-
base bi-functionality of the catalyst depends on the formation of TiZrO4 solid solution which
is improved by doping alkali metals. The TiZrO4 solid solution formation enhances the
stability of the catalyst. All the effects of doping put together disclosed the importance of
doping in the TZ structure stabilization and improvement of surface basicity enhances the
CO2 conversion and the Oxidehydrogenation process in the EB dehydrogenation reaction.
Acknowledgements
This work is funded by Saudi Arabia Basic Industries Corporation, Kingdom of Saudi Arabia and INHA University, Korea.
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Figure Captions
Table of contents
1) Figure 1 XRD patterns of TZ, K-0, K-24, Na-0 and Na-24.
2) Figure 2 N2 adsorption desorption isotherms of the samples K-0, K-24, Na-0 and Na-
24.
3) Figure 3 XPS spectra of O1s and T2p of TZ, K-24 and Na-24.
4) Figure 4 TEM images of TZ, K-24 and Na-24.
5) Figure 5 a) CO2 and b) NH3 TPD profiles of TZ, K-24 and Na-24.
6) Figure 6 Quantitative analysis of acidic and basic sites of TZ, K-24 and Na-24.
7) Figure 7 Raman spectra of TZ, K-0, K-24, Na-0 and Na-24 catalysts.
8) Figure a) Comparison of Catalytic activity performance of TZ, K-24, and Na-24
catalysts in EBD at 600oC, WHSV =1
-1h, Pressure = 1atm, CO2:EB = 5.1(molar)
Catalyst: 1g b) CO2 conversions based on the catalysts.
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Figure 1
Figure 1 XRD patterns of TZ, K-0, K-24, Na-0 and Na-24.
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Figure 2
Figure 2 N2 adsorption desorption isotherms of the samples K-0, K-24, Na-0 and Na-24.
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Figure 3
Figure 3 XPS spectra of Ti 2p and O 1s of TZ, K-24 and Na-24.
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Figure 4
Figure 4 TEM images of TZ, K-24 and Na-24.
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Figure 5
Figure 5 a) CO2 and b) NH3 TPD profiles of TZ, K-24, and Na-24.
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Figure 6
Figure 6 Quantitative analysis of acidic and basic sites of TZ, K-24 and Na-24
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Figure 7
Figure 7 Raman spectra of TZ, K-0, Na-0, K-24 and Na-24.
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Figure 8 Comparison of catalytic activity, selectivity and CO2 conversions of the catalysts TZ,
K-24 and Na-24 in EBD at 600oC.
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Graphical abstract: