Ethylbenzene to Styrene Over Alkali Doped TiO2-ZrO2 With CO2 as Soft Oxidant

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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References

[1] F. Cavani, F. Trifirò, App. Catal., A., 133 (1995) 219-239.

[2] Y. Tamsilian, A.N. Ebrahimi, A. Ramazani S.A, H. Abdollahzadeh, Comp. & Chem.

Engg. 40 (2012) 1-11.

[3] I. Rossetti, E. Bencini, L. Trentini, L. Forni, App. Catal., A., 292 (2005) 118-123.

[4] G.R. Meima, P.G. Menon, App. Catal., A., 212 (2001) 239-245.

[5] R. Fiedorow, W. Przystajko, M. Sopa, I.G. Dalla Lana, J. Catal., 68 (1981) 33-41.

[6] K.R. Devoldere, G.F. Froment, Ind. Eng. Chem. Res., 38 (1999) 2626-2633.

[7] L.E. Cadus, O.F. Gorriz, J.B. Rivarola, Ind. Eng. Chem. Res., 29 (2002) 1143-1146.

[8] R.M. Freire, F.F. de Sousa, A.L. Pinheiro, E. Longhinotti, J.M. Filho, A.C. Oliveira,

P.d.T.C. Freire, A.P. Ayala, A.C. Oliveira, App. Catal., A., 359 (2009) 165-179.

[9] H. Miura, H. Kawai, T. Taguchi, in: M.M. Hideshi Hattori, O. Yoshio (Eds.) Studies in

Surface Science and Catalysis, Elsevier (1994) 461-466.

[10] H. Arakawa, M. Aresta, J.N. Armor, M.A. Barteau, E.J. Beckman, A.T. Bell, J.E.

Bercaw, C. Creutz, E. Dinjus, D.A. Dixon, K. Domen, D.L. DuBois, J. Eckert, E. Fujita, D.H.

Gibson, W.A. Goddard, D.W. Goodman, J. Keller, G.J. Kubas, H.H. Kung, J.E. Lyons, L.E.

Manzer, T.J. Marks, K. Morokuma, K.M. Nicholas, R. Periana, L. Que, J. Rostrup-Nielson,

W.M.H. Sachtler, L.D. Schmidt, A. Sen, G.A. Somorjai, P.C. Stair, B.R. Stults, W. Tumas, in

Progress, Challenges, and Opportunities, Chem. Rev. (Washington, DC, U. S.), 101 (2001)

953-996.

[11] N. Dulamita, A. Maicaneanu, D.C. Sayle, M. Stanca, R. Craciun, M. Olea, C. Afloroaei,

A. Fodor, App. Catal., A., 287 (2005) 9-18.

[12] B. Xiang, H. Xu, W. Li, Chinese J. Catal, 28 (2007) 841-843.

[13] Z.-W. Liu, C. Wang, W.-B. Fan, Z.-T. Liu, Q.-Q. Hao, X. Long, J. Lu, J.-G. Wang, Z.-F.

Qin, D.S. Su, ChemSusChem, 4 (2011) 341-345.


18/30

Page 17 o

A c c e p t e

d M a n u

s c r i p t

17

[14] J. Hanuza, B. Jeżowska-Trzebiatowska, W. Oganowski, J. Mol. Catal., 29 (1985) 109-

143.

[15] B.M. Reddy, K.N. Rao, G.K. Reddy, A. Khan, S.-E. Park, J. Phys. Chem. C, 111 (2007)

18751-18758.

[16] B.S. Liu, R.Z. Chang, L. Jiang, W. Liu, C.T. Au, J. Phys. Chem. C, 112 (2008) 15490-

15501.

[17] T. Badstube, H. Papp, R. Dziembaj, P. Kustrowski, App. Catal., A., 204 (2000) 153-165.

[18] M.B. Ansari, S.-E. Park, Energy & Environ. Sci., 5 (2012) 9419-9437.

[19] Y. Sakurai, T. Suzaki, K. Nakagawa, N.-o. Ikenaga, H. Aota, T. Suzuki, J. Catal., 209

(2002) 16-24.

[20] M. Chiesa, E. Giamello, Chem. -Eur. J., 13 (2007) 1261-1267.

[21] G. Preda, G. Pacchioni, M. Chiesa, E. Giamello, Phys. Chem. Chem. Phys., 11 (2009)

8156-8164.

[22] N. Mimura, M. Saito, Catal. Today, 55 (2000) 173-178.

[23] I. Wang, W.-F. Chang, R.-J. Shiau, J.-C. Wu, C.-S. Chung, J. Catal., 83 (1983) 428-436.

[24] J.-C. Wu, C.-S. Chung, C.-L. Ay, I. Wang, J. Catal., 87 (1984) 98-107.

[25] N. Mimura, I. Takahara, M. Saito, T. Hattori, K. Ohkuma, M. Ando, in: T. Inui, M.

Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Eds.) Stud. Surf. Sci. Catal., Elsevier (1998) 415-

418.

[26] J.-S. Chang, S.-E. Park, M.S. Park, Beneficial Chem. Lett., 26 (1997) 1123.

[27] J.Noh, J. -S. Chang, S.-E. Park, W.-Y. Kim, and C.-W. Lee, Bull. Korean Chem. Soc, 19

(1998) 1342-1346.

[28] J.-N. Park, J. Noh, J.-S. Chang, S.-E. Park, Catal. Lett., 65 (2000) 75-78.

[29] B.M. Reddy, A. Khan, Catal. Rev., 47 (2005) 257-296.


19/30

Page 18 o

A c c e p t e

d M a n u

s c r i p t

18

[30] M.E. Manríquez, T. López, R. Gómez, J. Navarrete, J. Mol. Catal. A: Chem., 220 (2004)

229-237.

[31] S. Wang, K. Murata, T. Hayakawa, S. Hamakawa, K. Suzuki, Catal. Lett., 73 (2001)

107-111.

[32] G.K. Chuah, S. Jaenicke, B.K. Pong, J. Catal., 175 (1998) 80-92.

[33] G.K. Chuah, S. Jaenicke, S.A. Catal. Sci. & Technol.,2 (2012) 514-520.

[34]

[35] M.-o. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga, T. Suzuki, App. Catal., A.,

121 (1995) 125-137.

[36] K.-M.Choi, D. R. Burri, S.-C. Han, A. Burri, S.-E. Park, Bull. Korean Chem. Soc, 28

(2007) 53.

[37] D. R. Burri, K.-M. Choi, S.-C. Han, A. Burri and S.-E. Park, J. Mol. Catal. A: Chem.,

269 (2007) 58-63.

[38] B. Karmakar, A. Nayak, B. Chowdhury, J. Banerji, - Part XXIII, Arkivoc, 2009 (2009)

209-216.

[39] J. Zhou, Z. Hua, J. Shi, Q. He, L. Guo, M. Ruan, Chem. -Eur. J., 15 (2009) 12949-12954.

[40] S. Tsunekawa, K. Ishikawa, Z.Q. Li, Y. Kawazoe, A. Kasuya, Phys. Rev. Lett., 85

(2000) 3440-3443.

[41] K.N. Rao, B.M. Reddy, B. Abhishek, Y.-H. Seo, N. Jiang, S.-E. Park, App. Catal., B., 91

(2009) 649-656.

[42] C. Santra, S. Rahman, S. Bojja, O.O. James, D. Sen, S. Maity, A.K. Mohanty, S.

Mazumder, B. Chowdhury, Catal. Sci. & Technol., 3 (2012) 360-370.

[43] D. Zhang, J. C. Yu, G. Li, Phys. Chem. Chem. Phys., 11 (2009) 3775-3782.

[44] L. She, J. Li, Y. Wan, X. Yao, B. Tu, D. Zhao, J. Mater. Chem., 21 (2011) 795-800.


20/30

Page 19 o

A c c e p t e

d M a n u

s c r i p t

19

[45] F. Papa, P. Luminita, P. Osiceanu, R. Birjega, M. Akane, I. Balint, J. Mol. Catal. A:

Chem., 346 (2011) 46-54.

[46] J. Fung, I. Wang, App. Catal., A., 166 (1998) 327-334.

[47] D. Liu, X.Y. Quek, S. Hu, L. Li, H.M. Lim, Y. Yang, Catal. Today, 147 (2009) S51-S57.

[48] W.D. Mross, Catal. Rev. : Sci. Engi., 25 (1983) 591-637.

[49] B.M. Reddy, S.-C. Lee, D.-S. Han, S.-E. Park, App. Catal., B., 87 (2009) 230-238.

[50] F. Rondinelli, N. Russo, M. Toscano, Theor. Chem. Acc., 115 (2006) 434-440.

[51] K. Saito, K. Okuda, N.-o. Ikenaga, T. Miyake, T. Suzuki, J. Phys. Chem. A, 114 (2010)

3845-3854.

[52] Z. Dziewicki, M. Jagiello, A. Makowski, React. Funct. Polym., 33 (1997) 185-191.

[53] R. Rao, M. Yang, Q. Ling, C. Li, Q. Zhang, H. Yang, A. Zhang, Catal. Sci. & Technol.,

4 (2014) 665-671.


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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:

Ethylbenzene to Styrene Over Alkali Doped TiO2-ZrO2 With CO2 as Soft Oxidant

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Transcript of Ethylbenzene to Styrene Over Alkali Doped TiO2-ZrO2 With CO2 as Soft Oxidant