Scholarly article on topic 'The advantage of ceria loading over V2O5/Al2O3 catalyst for vapor phase oxidative dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant'

The advantage of ceria loading over V2O5/Al2O3 catalyst for vapor phase oxidative dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant Academic research paper on "Nano-technology"

Share paper
Academic journal
Appl Petrochem Res
OECD Field of science

Academic research paper on topic "The advantage of ceria loading over V2O5/Al2O3 catalyst for vapor phase oxidative dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant"

Appl Petrochem Res

DOI 10.1007/sl3203-016-0163-0



The advantage of ceria loading over V2O5/Al2O3 catalyst for vapor phase oxidative dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant

Venkata Ramesh Babu Gurram1,2 • Siva Sankar Enumula2 • Suresh Mutyala2 • Ramudu Pochamoni2 • P. S. Sai Prasad2 • David Raju Burri2 • Seetha Rama Rao Kamaraju1,2

Received: 24 February 2016/Accepted: 9 June 2016

© The Author(s) 2016. This article is published with open access at

Abstract The present work highlights the influence of ceria over vanadia/alumina for the oxidative dehydrogenation of ethylbenzene to styrene with CO2 as a soft oxidant. Various weight loadings (0, 3, 5, and 7 %) of ceria were incorporated into 10wt % V2O5/Al2O3 catalyst by wet impregnation process. Structural and textural characterizations of the catalysts were performed by means of powder X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption of CO2, N2 adsorption-desorption analysis. Over 10 % V2O5/Al2O3, ethylbenzene conversion decreases from 62 to 49 % in a spam of 12 h. Ceria-incorporated catalyst (3 % CeO2/10 % V2O5/Al2O3) showed steady ethylbenzene conversion of nearly 65 % with a styrene selectivity of 96 % for a period of 12 h time. Improved catalytic activity observed on this catalyst can be attributed to the increase in the number of redox/active VOx species after incorporation of ceria. The redox properties of 3 % CeO2/10 % V2O5/Al2O3 and 10wt % V2O5/Al2O3 were examined by reaction of CO2 in pulses on to the partially reduced catalysts. The spent catalysts were characterized using XRD & TPR analysis to assess the reason for the deactivation of catalysts.

Electronic supplementary material The online version of this article (doi:10.1007/s13203-016-0163-0) contains supplementary material, which is available to authorized users.

& Seetha Rama Rao Kamaraju;

1 CSIR-Academy of Scientific and Innovative Research (CSIR-AcSIR), New Delhi, India

2 Catalysis Laboratory, Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India

Published online: 24 June 2016

Keywords Ceria • V2O5/Al2O3 • Oxidative dehydrogenation • Ethylbenzene • CO2 as soft oxidant


Styrene (ST) is an important monomer of many petrochemical industries for the production of polystyrene, styrene-acrylonitrile, styrene-butadiene rubber and acry-lonitrile-butadiene-styrene [1]. ST is being produced via a commercial process using catalytic dehydrogenation of ethylbenzene (EB) with large quantity of steam as heat supplier, diluent and coke gasifier. The use of steam is highly energy consuming, and several alternatives to steam have been studied [2]. The high reaction temperature (873-973 K) and latent heat of steam condensation lead to excessive energy consumption for the steam-assisted EB dehydrogenation process. This process also shows the characters of the low atom economy, thermodynamic equilibrium limitations, short lifetime of catalysts, etc. [2, 3]. In this context, oxidative dehydrogenation with various types of oxidants like carbon dioxide, nitrous oxide, oxygen, sulfur dioxide and dry air gains momentum as alternatives for steam. Among these, carbon dioxide has enormous advantage because it reduces one of the major global warming gases via reverse water gas shift reaction (RWGSR). CO2 employed in this reaction reacts with H2 produced in the EB dehydrogenation reaction to yield CO and H2O via RWGSR. Because of coupling of EB dehydrogenation reaction with RWGSR, enhancement in the catalytic activity is possible. Most importantly, in this process the CO2 is converted to a useful co-product, carbon monoxide (2), which is one of the useful chemicals for Fischer-Tropsch synthesis and other organic transformations. Hydrogen can be recovered by the reaction of the

formed CO with steam via water gas shift reaction (WGSR). Thus, CO2 is considered as a promising soft oxidant to replace steam [4-7]. CO2 has an advantage over steam in the form of equilibrium alleviation due to coupling of dehydrogenation reaction with RWGSR which increases the ST yield and also requires lower CO2 consumption when compared to steam [8]. Reports say usage of oxygen overcomes thermodynamic limitations; the reaction can be operating at lower temperatures than the commercial process due to exothemicity of the process with oxygen. There are two main reactions occurred with O2: (1) ODH reaction, (2) the coke gasification reaction [9-11]. The consumption of oxygen in EB dehydrogenation is very less (0.5 mol for a mole of EB). But the process with oxygen as an oxidant has not been realized yet because of significant loss of ST selectivity due to the production of carbon oxides and oxygenates [12].

EBD using CO2 (3) follows via a two-step mechanism: the first one being a simple dehydrogenation of EB to ST (1) followed by RWGSR (2). Dehydrogenation of EB in the presence of CO2 has attracted much attention after the pioneering works of Sato et al. [13] and Matsui et al. [14]. CO2 can also be regarded as coke removal agent (4). Even though RWGSR is endothermic in nature, the energy required for oxidative dehydrogenation with CO2 (1.5-1.9 x 105 kcal) is lower by an order of one magnitude when compared to steam process (1.5 x 106 kcal) for the production of one ton of ST [15].

the surface/pores of the sample that leads to severe fall in the ST selectivity [17, 18].

There are many reports on EB dehydrogenation; still a lot of research is needed to improve the process because of its huge industrial importance. Recently, Abhishek and et al. have reported improved CO2 conversions in EB dehydrogenation over potassium- and sodium-doped Tita-nia-Zirconia mixed oxide supports. The increase in the conversion of EB and CO2 in case of doped TZ is because the dopants increase the stability of the catalysts by decreasing coke formation on the surface of the catalyst [19]. Sara and et al. have reported ethylene as the main coke precursor in EB dehydrogenation over CrOx/Al2O3 catalysts and the formation of coke is due to the formation of para-substituted aromatic species. They reported that precooking treatment is one of the options to inhibit undesired reactions (cracking, hydrogenolysis, and dealkylation) and thereby enhancement in the selectivity to ST [20]. Kovacevic et al. reported that ceria cubes exhibited a remarkable higher activity per m2 for dehydrogena-tion of EB as compared to ceria rods and particles [21].

The physical and chemical properties of the support play a keen role in the ethylbenzene dehydrogenation reaction as reported by many researchers. Among several reported supports, alumina has major contribution because of their surface area, better redox properties, and high thermal stability. The molecular structure and reactivity of Vanadia supported alumina have been keenly investigated for few

C6HsCH2-CH3 M-► C6HsCH=CH2 + H2 AH298K=117.6 kJmol"1 1

CO2 + H2 <-► CO + H2O AH298K=41.6 kJmol-1 2

C6H5CH2-CH3 + CO2 M-► C6HsCH=CH2 + CO + H2O AH298K=159.2 kJmol-1 3

C + CO2 *-► 2CO 4

EB dehydrogenation reaction without any oxidants (in the presence of inert gases) leads to nonselective products and facilitates faster deactivation of the catalyst. If CO2 or N2O is introduced, the deactivation of catalysts drops down drastically and selectivity towards ST not only increases but also sustains for prolonged times [16]. However, with increase in reaction temperature (above 873 K), the selectivity of ST considerably decreases due to thermal cracking of EB and other products. At higher temperatures, there is a possibility of the formation of higher amounts of toluene, benzene and other polyaromatics which deposit on

¿1 Snrinopr

KAcsToAoUiq r^QjiU ^ springer

decades over dehydrogenation of EB [22-25]. Also, the CeO2-based mixed oxides are effective catalysts for the oxidation of different hydrocarbons and for the removal of total organic carbon from polluted waters from different sources [26, 27]. The presence of CeO2 promotes various catalytic reactions such as CO2 activation [28], CO oxidation [29, 30], CO/NO removal [31] and low-temperature water-gas shift [32]. The success of ceria in various applications is mainly due to its unique properties like oxygen transport capacity, neutralizes the acidic sites of alumina, ability to shift easily between reduced and

oxidized states (Ce3+ $ Ce4+) can create a better platform of acid-base bifunctionality with alumina which will play a pivotal role in EB dehydrogenation [33].

In our recent publications, utilization of CO2 for EB dehydrogenation has been explained in detail over molybdenum- and cobalt-based catalysts. MoO3 and Co3O4 supported on COK-12 (mesoporous silica) showed good activity towards EBD in the presence of CO2 due to the formation of MoO3 monolayer on the surface of the support [34] and the support COK-12 facilitates a better platform for Co3O4 [35]. Cobalt molybdenum bimetallic nitride catalyst supported on y-Al2O3 showed high and stable activity in the presence of CO2 and N2. The high activity is due to the concurrent reactions of RWGSR and ammonia synthesis. The insitu generated ammonia (produced from N2 and H2) neutralizes the strong acidic sites of the catalyst which helps in maintaing the stable activity [36]. In continuation of our previous work, oxidative dehydrogenation of EB using CO2 as a soft oxidant [34-36], the present study examines the promotive role of ceria to V2O5/Al2O3 catalyst.

Experimental Materials

Ammonium metavanadate AR grade, NH4VO3 (M/s. LOBA Chemie, India), y-Al2O3 (M/s. Harshaw, BET Surface area: 260 m2 g-1), ammonium ceric nitrate AR grade, H8N8CeOi8 (M/s. SD fine chemicals, India), ethyl-benzene (M/s. Sigma-Aldrich, anhydrous, 99.8 % purity) were used without further purification.

Catalyst preparation

10wt % V2O5/Al2O3 (VA) was prepared by dissolving requisite amount of ammonium metavanadate in Millipore water. After complete dissolution of the salt, finely powdered y-Al2O3 was added. The excess water was evaporated on a hot plate under vigorous stirring. The resulting powder was dried overnight at 373 K and calcined at 773 K for 5 h with a ramping rate of 2 K min- in static air. 3, 5, and 7 weight % CeO2 loaded V2O5/Al2O3 catalysts were prepared according to the procedure adopted for V2O5/Al2O3 catalysts. Briefly, requisite quantities of ammonium ceric nitrate and ammonium metavanadate were dissolved in Millipore water. After dissolution of salts, y-Al2O3 powder was added to this solution and stirred the contents at 333 K for complete evaporation of water and dried at 373 K for overnight followed by calcination at 773 K for 5 h at a ramping rate of

2 K min 1 in static air. Final catalysts with corresponding loadings of ceria (0, 3, 5, and 7) were denoted as VA, 3VCA, 5VCA, and 7VCA, respectively.

Catalyst characterization

Powder X-ray diffraction patterns of the catalysts were recorded on a Ultima-IV X-ray diffractometer (M/s. Rigaku Corporation, Japan) using Ni filtered Cu Ka radiation (l = 1.5406 A) with a 26 scan range of 10-80° and 26 scan speed of 2° min-1 at 40 kV and 30 mA. Crystalline phases identified were compared with the reference data from International Center for Diffraction Data (ICDD) files. TPR analysis of catalysts was carried out using a quartz micro-reactor interfaced to a thermal conductivity detector (TCD) equipped gas chromatograph (M/s. CIC Instruments, India). Approximately, 50 mg of the catalyst sample is loaded in an isothermal zone of a quartz reactor (i.d. = 6 mm, length = 300 mm) and heated by an electric furnace to 573 K with ramping rate of 10 K min- and kept at that temperature for 1 h in helium gas flow (30 cm min- ) to facilitate the desorption of physisorbed water. After degassing, the sample was cooled to 373 K and the helium gas was replaced with a reducing gas (5 % H2 in argon) at a flow rate of 30 cm3 min-1 and the temperature was linearly increased to 1073 K at a ramping rate of 10 K min-1 and maintained at the same temperature for 30 min. The hydrogen consumption was monitored by standard GC software. The basicity of the catalyst was measured by temperature-programmed desorption of CO2 using an automated gas sorption analyzer, AUTOSORB-iQ (M/s. Quantachrome Instruments, USA). Prior to desorption, the catalyst has been pre-treated in He flow for 1 h at 573 K and cooled down to 373 K in the same helium flow. After that, the catalyst has been saturated with 10 % CO2/ He at 373 K for 0.5 h, followed by stripping with helium for 0.5 h to remove the physisorbed CO2 at the same temperature. Then, the temperature was raised from 373 to 1073 K at a ramping rate of 10 K min- while maintaining the He flow (30 ml min- ) and the desorbed gas was monitored with the TCD equipped GC. Re-oxidizability of the catalysts was examined by CO2 pulse chemisorption experiment conducted at 873 K in a stainless steel reactor with 100 mg of the catalyst. Prior to this, the catalyst was reduced at 873 K in H2 flow for 30 min. 100 il volume of carbon dioxide was injected onto the catalyst sample in pulses through an assembled six-port valve and the outlet gas was analyzed by a TCD equipped GC-6890 (M/s. Agilent Corporation, USA) with Helium as a carrier gas and porapak T 80/100 column, 3 m length) [12]. The formation of coke was studied by CHNS analysis.

Catalyst activity studies

Catalytic activity study was performed in a fixed bed down flow quartz reactor at atmospheric pressure. 1 g of the catalyst was loaded at the center of the reactor with the help of two quartz wool plugs and subjected to pre-treat-ment with N2 for 30 min at 873 K. Dehydrogenation of EB was performed in the temperature range of 723-923 K using CO2 as a soft oxidant. The ratio of CO2 to EB (ml/ ml) was maintained as 30. EB was introduced by a liquid feed pump (M/s. B. Braun, Germany) at a feed rate of 8.2 mmol h-1 (1 ml/h). To assess the beneficial role of CO2, a blank experiment with best catalyst under N2 flow was also conducted. The products were collected at regular intervals in an ice cold trap and analyzed on a flame ion-ization detector (FID) equipped gas chromatograph, GC-17A (M/s. Shimadzu Instruments, Japan) using a capillary column, OV1-G43 (30 m length, 0.53 mm Inner diameter and 3 mm film thickness). The gaseous products were analyzed by injecting gas mixture into a thermal conductivity detector (TCD) equipped gas chromatograph, Agi-lent-6890 (M/S. Agilent Technologies, USA) having porapak-Q column (80-100 mesh size, 1 m length). EB conversion and ST selectivity are calculated as per the following Eqs. (5) and (6), respectively.

ethylbenzene out


ethylbenzene in


ethylbenzene in x 100

Styrene out

ethylbenzene out

Results and discussion

Textual properties

The BET surface area and pore volume of the fresh catalysts are presented in Table 1. Surface area of the

corresponding catalysts was measured by BET equation, which is inscribed below:

C - 1 VmC

where P is the partial vapor pressure of adsorbate gas in equilibrium with the surface at 77.4 K (b.p. of liquid nitrogen), in pascals; Po is the saturated pressure of adsorbate gas, in pascals; Va is the volume of gas adsorbed at standard temperature and pressure (STP) [273.15 K and atmospheric pressure (1.013 x 105 Pa)], in milliliters; Vm is the volume of gas adsorbed at STP to produce an apparent monolayer on the sample surface, in milliliters; and C is the dimensionless constant that is related to the enthalpy of adsorption of the adsorbate gas on the powder sample.

Incorporation of 10 weight % V2O5 decreases the BET surface area of alumina (260 m g- ) to 230 m g- due to the blocking of support pores by V2O5 particles. After ceria addition, the surface areas further decreased to 183, 146, and 144 m2 g-1 for 3VCA, 5VCA and 7VCA catalysts, respectively. For 7VCA, there was not much noticeable loss in surface area when compared to 5VCA, indicating that the alumina surface is saturated with vanadia and ceria particles in 5VCA and 7VCA. A similar trend in pore volume is also observed. It clearly says that the ceria particles after 5VCA were simply settling down on the surface of alumina. The normalized surface area values (surface area of catalyst/surface area of support component) of all the catalysts are close to unity, which is an identification of homogeneous distribution of VOx species [37].

XRD analysis

XRD patterns of the catalysts VA, 3VCA, 5VCA, and 7VCA catalysts are shown in Fig. 1. XRD patterns of VA show that four diffraction peaks at 29 values of 37.5 (311),

Table 1 Ceria loading, corresponding textual properties and ethylbenzene conversions of VA, 3VCA, 5VCA, and 7VCA catalysts

Catalyst Ceria loading (wt %) Surface area (m2/g)a Surface area (m2/g-support) NSBet Pore volume (cc/g)c X (EB) (%)d

VA 0 230 256 0.90 0.29 65.3

3VCA 3 183 210 0.87 0.21 69.1

5VCA 5 146 172 0.85 0.19 68.4

7VCA 7 144 174 0.83 0.18 65.6

a BET Surface area b Normalized BET surface area

c Total pore volume at relative pressure P/Po by single point method

d X ethylbenzene conversion. Reaction conditions 1 g catalyst, T = 600 °C, 1 atm, 1 ml/h EB flow, 30 ml/min CO2 flow

1 Springer

jjjjiJIiir dial cùjiû MCSTcuïoIJIg fTjqjkU

Fig. 1 Powder XRD patterns of a y-Al2O3, b VA, c 3VCA, d 5VCA, and e 7VCA catalysts

45.6 (400), 60.5 (511) and 66.6 (440) correspond to y-Al2O3 (ICDD No 50-0741), and no diffraction peaks corresponding to crystalline vanadia were observed, suggesting that vanadia is highly dispersed or in amorphous phase on the surface of alumina. Assuming a cross-sectional area of V-atom as 6.803 x 10-20 m2, y-Al2O3 with a surface area of 260 m2 g-1 can accommodate above 50 wt % of V2O5 in a monolayer provided that the pore size of alumina is sufficient enough to accommodate V-atoms. With incorporation of ceria on VA, diffraction peaks corresponding to ceria at 26 values of 28.5 (111), 33.0 (200), 47.5 (220), 56.3 (311), 59.1 (222), 69.4 (400), 76.7 (331), and 79.1 (420) were observed at a ceria weight loading of 3 %. The diffraction planes are in good agreement with ICDD No. 81-0792. With increase in the composition of ceria from 3 to 7 %, the intensity of diffraction peaks of ceria increases due to the increase in crystallinity of the catalysts. Incorporation of large size of Ce3+ ions in the vacant positions of small size alumina might be the reason for the presence of both alumina and ceria peaks in XRD spectrum [38]. The CeO2 crystallite size calculated from Debye-Scherrer equation in all the ceria promoted catalysts comes to be * 50 nm. From XRD results, it is clear that alumina surface is covered by VOx species and further addition of CeO2 was settling down on the surface.

H2-TPR results

To investigate the reducibility of the vanadium species dispersed on the support in the presence of ceria, VCA samples are subjected to temperature-programmed reduction (TPR) in the temperature range of 373-1173 K and the

Fig. 2 TPR patterns for a VA, b 3VCA, c 5VCA, and d 7VCA catalysts

results are compiled in Fig. 2. TPR patterns of supported vanadium species result in yielding a single Tmax at around 723-873 K depending on the nature of support. VOx species is more easily reduced when deposited on ZrO2 (Tmax = 817 K) rather than on MgO surface (Tmax = 933 K) [39, 40]. The significantly higher temperature needed for the reduction of VOx species on MgO is due to the higher degree of interactions between VOx and oxidic support which are confirmed by the identification of MgV2O6 and VMgO phases [41].

Shiju and et al. reported that lower vanadia loadings (2-10 %) reveal a broad reduction peak below 773 K. A decrease in the reduction temperature with increase in the vanadia loading was observed by them. The shift in Tmax value with increasing vanadia loading results from the reduction of monomeric, dimeric or low oligomeric surface vanadia species. The shift in reduction peaks with increase in vanadium content may be ascribed to the progressive increase in polymeric vanadium species [22]. In our present study, the catalyst VA showed a high intense signal at around 803 K and another small intense shoulder peak at around 943 K. The low-temperature peak might be attributed to the reduction of dispersed surface vanadium species mostly due to the presence of monomeric phase (tetragonal phase) of the vanadia species, while the high temperature peak should be ascribed to the reduction of bulk V2O5-like V5? species with lower intensity [41-43]. With incorporation of 3 weight % of ceria (3VCA), Tmax shifted to higher temperature suggesting increase in the interaction between CeO2 and VOx. With increase in ceria loading (5VCA), Tmax increases to 906 K due to further increase in interactions between CeO2 and VOx species. 7VCA catalyst also showed same kind of reduction behavior

Fig. 3 CO2 TPD profiles for a VA, b 3VCA, c 5VCA, and d 7VCA catalysts

indicating the saturation of interaction between ceria and vanadia in 5VCA catalyst.

CO2-TPD results

To explain surface basicity of samples, CO2-TPD profiles are recorded (Fig. 3). CO2-TPD profile of VA shows an intense peak at *473 K and two shoulder peaks in the range of 673-873 K. The two peaks at *443 and 673-873 K ascribed to the presence of weak and moderate basic sites. CO2-TPD profile of 3VCA sample shows the presence of large population of week basic sites (Tmax *440 K) and also moderate basic sites (Tmax *735 K) compared to VA. It appears that incorporation of ceria increases basicity of VA. Similar kind of desorption behavior was observed from the CO2-TPD profiles of 5VCA and 7VCA, which shows the presence of weak and moderate basic sites (Tmax *473 and 833 K), respectively. Shifting of Tmax towards right hand side (higher Tmax) might be attributed to increased interactions between CeO2 and VOx in 5VCA and 7VCA compared to 3VCA. The increase in the number of surface basic sites can be attributed to the doping effect as it is evinced from XRD results that surface was covered with ceria and the vanadia, dispersed on the alumina surface. The surface basicity is one of the major factors for the adsorption of CO2 [19].

CO2 pulse chemisorption study

CO2 pulse injection experiments are planned to know the oxidizing power of a catalyst. Since EB dehydrogenation has been conducted with CO2 as soft oxidant, it is a good

Fig. 4 CO2 pulse treatment for partially reduced a (dark circle) VA, b (open circle) 3VCA catalysts

idea to choose CO2 as a probe molecule to assess the oxidizing power of a catalyst [12]. The selection of temperature for CO2 pulse injection experiments is based on the EB dehydrogenation reaction temperature. Thus, CO2 pulse chemisorption experiments were conducted at 873 K. Since the catalyst prior to the CO2 chemisorption has been reduced in H2 flow, during the pulse chemisorption, CO2 gets reduced to CO and the lower oxidation of metal oxide (V/CeOx) gets oxidized. The amount of CO produced is same as that of CO2 consumed during the successive injections of CO2 pulses at 873 K on a partially reduced catalyst with and without ceria. The amount of CO2 consumed against pulse number is shown in Fig. 4. The reactivity of a catalyst towards CO2 pulse reaction reflects its re-oxidizability of reduced surfaces. CO2 uptake over 3VCA catalyst is conspicuously higher than that on VA implying that the oxidizing power of 3VCA catalyst is 55 % greater than that of VA. This clearly indicates that the amount of CO2 conversion over 3VCA is larger compared to VA. The unique combination of excellent oxygen storage/release capacity features of CeO2 and the oxidizing ability of dispersed vanadium oxide is an expected result which reflects in yielding good EB conversions with CO2.

Reaction results

EB dehydrogenation reaction was carried out in vapor phase at atmospheric pressure over different weight loadings of ceria by maintaining the vanadium loading constant (VA, 3VCA, 5VCA, and 7VCA) at temperatures ranging from 723 to 923 K using CO2 as a soft oxidant. The catalytic activity is shown in Figs. 5 and 6. Figure 5 depicts EB conversion whereas Fig. 6 depicts ST selectivity. Since

jjjilliir dlloJI ¿in» c •

KAcsToAoUiq rnqiiU ^ springer

Fig. 5 Ethylbenzene converison over different ceria loadings VA, 3VCA, 5VCA, and 7VCA catalysts

Fig. 6 Styrene selectivity over different ceria loadings VA, 3VCA, 5VCA, and 7VCA catalysts

EB dehydrogenation is an endothermic reaction, with increasing reaction temperature an increase in the EB conversion was observed. The catalyst VA exhibited low EB conversions at 723, 773, and 823 K. Further increase in temperatures (873 and 923 K) elevates the EB conversion to higher values, but the selectivity towards ST drops down due to the formation of more byproducts at higher temperatures. With the addition of 3 % ceria to VA (3VCA), a slight increase in EB conversion was observed that of VA. The slight improvement in EB conversion and ST selectivity is might be due to the enhancement of redox properties of the active species of VA catalyst with the addition of ceria. In this connection, it is worth to mention that the

Time(in Hrs.)

Fig. 7 Time on stream study over VA and 3VCA catalysts at 873 K

CeOx addition increases the oxidizing power of the catalyst (CO2 pulse chemisorption results). Further additions of CeOx to VA (5VCA and 7VCA) showed no further increase in EB conversions. From these results, 3VCA was selected as optimum loading chosen for on stream studies. To understand the influence of CO2 as a soft oxidant, the reaction was also carried out under N2 flow on 3VCA under similar reaction conditions in the range of 723-923 K. The activity results with N2 flow are far below than those with CO2 flow. At 873 K, the EB conversion and ST selectivity are 57.3 and 95.6, respectively which are lower than those with CO2 (EB conversion 69.1 % and ST selectivity 92.2 %). The lower EB conversion in N2 feed is due to simple dehydrogenation reaction, while in the case of CO2 feed gas better conversion and selectivity are due to the participation of both RWGSR and dehydrogenation reactions.

Since the catalysts were calcined at 773 K and tested in the temperature zone of 723-923 K, a set of catalysts were calcined at 973 K (above the reaction temperature) and these catalysts were characterized by XRD and TPR. The effect of reaction temperature on these catalysts was also performed. All the results have been shown in Electronic supplementary material (ES1for XRD figure, ES2 for TPR and ES3 for activity studies-temperature effect). Compared to the catalysts calcined at 773 K, the catalysts calcined at 973 K show slightly decreased activity; little bit more intense crystalline d-lines for ceria and slight shifting of Tmax towards higher temperature were observed. These are due to the interaction between active phase/promoter with support. However, V2O5 remains in the highly dispersed form.

To understand the catalyst stability, time on stream experiment was performed with CO2 feed gas for VA and

3VCA catalysts. On both VA and VCA catalysts, the major gaseous components observed are CO along with unconverted CO2. H2, CH4, C2H6 are observed in minute quantities. Generally, a faster catalytic deactivation often was encountered in most of the catalytic systems in ODH reactions due to coke formation. Thus, time-on-stream study is very useful to understand the behavior of the catalysts. Figure 7 shows the effect of time-on-stream study in the presence of CO2 feed gas at 873 K. From the figure, it is clear that during initial hours over VA, the conversion of EB increases from 58 to 62.2 %. The increase in conversion during initial hours is due to the formation of carbonaceous species over VA surface [44, 45]. Beyond second hour, the EB conversion decreases gradually and reaches 49.2 % at the end of 12th hour with an ST selectivity of 96 %. The presence of acidic sites prompts the formation of cracking products and thereby coke on the surface of the catalyst which results in decrease in the EB conversion. The continuous 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 in addition enhance the cracking by adsorption of the reactant on the coke present on the catalyst surface [44]. The activity of 3VCA catalyst shows almost stable EB conversion for 12 h (67-64 %) with 96 % ST selectivity. The activities of VA and 3VCA do not differ substantially during the initial hours. It is generally accepted that active sites for these systems are to be the surface vanadium cations operating according to a Mars-Van-Krevelen redox mechanism [33]. However, the catalyst stability is much improved by the addition of ceria to VOX. It is suggested that easy redox cycle is an important factor to design more effective catalysts. Due to the addition of ceria to vanadia, the amount of reactive oxygen is increased and the reduction-reoxidization cycles of vanadium proceed more efficiently. The addition of ceria improved the catalytic activity in terms of conversion and improved the selectivity by minimizing the formation of coke deposition over the surface of catalyst. The stable activity of ceria-based catalysts is mainly due to facile oxygen transport from the bulk of CeO2 [45-47]. Vanadium reduction from V5? to V3? also seems to be one of the main reasons of the deactivation of VA because the reoxidization of these strongly reduced active sites can become the rate-limiting step. This was confirmed by pulse and DSC techniques for the V2O5/Al2O3 catalyst for oxydehydrogenation of ethane [48].

Spent catalyst studies

The remarkable ability of CeO2 in V2O5/Al2O3 catalysts to obtain a high and stable activity is clearly apparent from


KAcsToAoUiq r^QjiU ^ springer

Fig. 8 Powder XRD patterns for a VA, b VA spent, c 3VCA, and d 3VCA spent catalysts

Temperature (K)

Fig. 9 TPR patterns for a VA and b 3VCA spent catalysts

the present study. To understand the deactivation phenomenon of these catalysts, the spent catalysts were subjected to XRD and TPR analysis. XRD patterns for the spent catalysts (collected after time on stream study over VA and 3VCA) are shown in Fig. 8. Peaks correspond to ceria and alumina retained and there is no such noticeable change in the peak positions or their intensities after TOS indicating that the structure is retained even after 12 h reaction. In fresh catalysts, no peaks corresponding to vanadia were observed both in VA and 3VCA. The diffraction peaks corresponding to vanadia are absent in the XRD patterns of spent catalysts which indicates that vanadia retains its amorphous nature in both VA and 3VCA catalysts after 12 h reaction.

TPR patterns of the fresh VA and 3VCA catalysts (Fig. 9) show two Tmax values at 803,943 K for VA and 843, 983 K for 3VCA catalysts, respectively. While the spent catalysts show the same two-stage reducibility with

Table 2 Comparison of ethylbenzene dehydrogenation activity of various catalysts with the present catalyst

Catalyst Feed flow Reaction conditions Ethylbenzene conversion (%) Styrene selectivity (%) Refs.

VSbOx/Al2O3 CO2 868 K, 1 atm 79.9 95.1 [37]

V2O5/MgO CO2 823 K, 1 atm 73.8 90.1 [7]

CeO2-V2O5/TiO2-ZrO2 CO2 823 K, 1 atm 56 98 [47]

V2O5/CeO2/Al2O3 CO2 723 K, 1 atm 19 94 [39]

CeO2 cubes CO2 833 K, 1 atm 58 80 [21]

V2O5/activated carbon CO2 823 K, 1 atm 52 97.3 [8]

V2O5/CeO2-Al2O3 CO2 873 K, 1 atm 68.4 92.2 Present

higher Tmax values. The increase in Tmax values is ascribed to the formation of coke deposition on the surface of the catalyst which will hinder the reducibility of a catalyst by forming a carbon layer on the surface of the support. Along with these two peaks, one more negative peak was observed at around 1073 K in both the catalysts. This negative peak might be due to the reduction of H2-rich CHX species formed during the reaction [36, 49].

The CHNS results indicate that the rate of formation of carbon in the spent VA catalyst is 1.48 mmol-Carbon/h/gm catalyst which is much higher than that on 3 VCA catalyst (0.92 mmol-Carbon/h/gm catalyst). Coking of catalysts generally occurs due to cracking products. The addition of ceria to VA results in low coke formation, which suggests that the ceria is preventing the coke formation by decreasing the formation of side products.

Activity comparison with reported catalysts

Table 2 presents the comparison of activity of different catalysts cited in the literature with the present catalyst system for EB dehydrogenation with CO2. It was reported that SbOX species increases the redox capability of V2O5 on Al2O3 which in turn increases the EB conversion with high ST selectivity [12]. It appears that the role of CeOX in the present case is similar to that of SbOX. It was reported that V2O5 supported on MgO catalyst exhibited high initial EB conversion with high ST selectivity, but suffers reduction in the ST yield with in 6 h [7]. The advantage of CeOX on V2O5/TiO2-ZrO2 catalyst in maintaining steady EB conversion with high ST selectivity for a period of 10 h time on stream study was reported [46]. Another study highlighted the advantage of CeOX to V2O5/Al2O3 catalyst in promoting the EB conversion in CO2 at low temperatures [38]. Another study indicated that among different phases of CeOX, CeO2 cubes are active for EB dehydro-genation [21]. Comparison of [21] with [8] indicates that vanadia's role is in maintaining higher selectivity to ST than with CeOX. Thus, the EB dehydrogenation activity of the present catalyst system, 3VCA, is either on par or more active compared to the reported catalysts.


XRD results indicate that VOX species are present either in amorphous phase or highly dispersed on the surface of the support. TPR results suggest that the increased Tmax values in VCA catalysts compared to VA catalyst are due to the interactions between ceria and VOx species. CO2 TPD results suggest that the addition of ceria to VOX-Al2O3 increased their basicity. CO2 pulse chemisorption studies indicate that the addition of ceria increased the redox properties of VOX component. The addition of 3 % ceria to VOx/Al2O3 is helpful in not only improving the catalytic activity but also yielding stable results up to 12 h during time on stream studies. The stable and high catalytic activity of ceria-incorporated VOx/Al2O3 catalysts is mainly due to the facile oxygen transport from bulk of CeO2.

Acknowledgments GVRB and ESS acknowledge the Department of Science and Technology (DST), Government of India, New Delhi for the award of fellowship and all authors are grateful to DST for sanctioning a project (Project No. DST/IS-STAC/CO2-SR-136/ 12(G)).

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.


1. Meima GR, Menon PG (2001) Catalyst deactivation phenomena in styrene production. Appl Catal A Gen 212:239-245

2. Cavani F, Trifiro F (1995) Alternative processes for the production of styrene. Appl Catal A Gen 133:219-239

3. Jiang N, Han DS, Park SE (2009) direct synthesis of mesoporous silicalite-1 supported TiO2-ZrO2 for the dehydrogenation of ethylbenzene to styrene with CO2. Catal Today 141:344-348

4. Chang JS, Park SE, Park MS (1997) Beneficial effect of carbon dioxide in dehydrogenation of ethylbenzene to styrene over zeolite-supported iron oxide catalyst. Chem Lett 26:1123-1124

5. Sugino M, Shimada H, Turuda T, Miura H, Igenaga N, Suzuki T (1995) Oxidative dehydrogenation of ethylbenzene with carbon dioxide. Appl Catal A Gen 121:125-137

6. Sakurai Y, Suzaki T, Nakagawa K, Ikenaga NO, Aota H, Suzuki T (2000) Oxidation capability of carbon dioxide in the dehy-drogenation of ethylbenzene over vanadium oxide-loaded MgO catalyst. Chem Lett 29:526-527

7. Sakurai Y, Suzaki T, Nakagawa K, Ikenaga NO, Aota H, Suzuki T (2002) Dehydrogenation of ethylbenzene over vanadium oxide-loaded MgO catalyst: promoting effect of carbon dioxide. J Catal 209:16-24

8. Chen S, Qin Z, Sun A, Wang J (2006) Theoretical and experimental study on reaction coupling: dehydrogenation of ethyl-benzene in the presence of carbon dioxide. J Nat Gas Chem 15:11-20

9. Christian N, Valeriya Z, Ignacio VMC, Erik HJH, Freek K, Michiel M (2014) Application of staged O2 feeding in the oxidative dehydrogenation of ethylbenzene to styrene over Al2O3 and P2O5/SiO2 catalysts. Appl Catal A Gen 476:204-214

10. Christian N, Valeriya Z, Ignacio MC, Hero JH, Freek K, Michiel M (2013) Oxidative dehydrogenation of ethylbenzene to styrene over alumina: effect of calcination. Catal Sci Technol 3:519-526

11. Zarubina V, Nederlof C, Van der Linden B, Kapteijn F, Heeres HJ, Makkee M, Melian-Cabrera I (2014) Making coke a more efficient catalyst in the oxidative dehydrogenation of ethylben-zene using wide-pore transitional aluminas. J Mol Cat A Chem 381:179-187

12. Park ME, Vislovskiy VP, Chang JS, Shul YG, Yoo JS, Park SE (2003) Catalytic dehydrogenation of ethylbenzene with carbon dioxide: promotional effect of antimony in supported vanadium-antimony oxide catalyst. Catal Today 87:205-212

13. Sato S, Ohhara M, Sodesawa T, Nozaki F (1988) Combination of ethylbenzene dehydrogenation and carbon dioxide shift-reaction over a sodium oxide/alumina catalyst. Appl Catal A Gen 37:207-215

14. Matsui J, Sodesawa T, Nozaki F (1990) Influence of carbon dioxide addition upon decay of activity of a potassium-promoted iron oxide catalyst for dehydrogenation of ethylbenzene. Appl Catal A Gen 67:179-188

15. Mimura N, Saito M (1999) Dehydrogenation of ethylbenzene to styrene over Fe2O3/Al2O3 catalysts in the presence of carbon dioxide. Catal Lett 58:59-62

16. de Morais Batista A H, de Sousa FF, Honorato SB, Ayala AP, Filho JM, de Sousa FW, Pinheiro AN, de Araujo JCS, Nasci-mento RF, Valentini A, Oliveira AC (2010) Ethylbenzene to chemicals: catalytic conversion of ethylbenzene into styrene over metal-containing MCM-41. J Mol Catal A: Chem 315:86-98

17. Rao KN, Reddy BM, Park SE (2010) Novel CeO2 promoted TiO2-ZrO2 nano-oxide catalysts for oxidative dehydrogenation of p-diethylbenzene utilizing CO2 as soft oxidant. Appl Catal B Environ 100:472-480

18. Lisovskii AE, Aharoni C (1994) Carbonaceous deposits as catalysts for oxydehydrogenation of Alkylbenzenes. Catal Rev Sci Eng 36:25-74

19. Burri A, Jiang N, Yahyaoui K, Park SE (2015) Ethylbenzene to styrene over alkali doped TiO2-ZrO2 with CO2 as soft oxidant. Appl Catal A Gen 495:192-199

20. Sanz SG, McMillan L, McGregor J, Zeitler JA, Al Yassir N, Al Khattafb S, Gladdena LF (2016) The enhancement of the catalytic performance of CrOx/Al2O3 catalysts for ethylbenzene dehydrogenation through tailored coke deposition. Catal Sci Technol 6:1120-1133

21. Kovacevic M, Agarwal S, Mojet BL, van Ommen Jan G, Lefferts L (2015) The effects of morphology of cerium oxide catalysts for dehydrogenation of ethylbenzene to styrene. Appl Catal A Gen 505:354-364

22. Shiju NR, Anilkumar M, Mirajkar SP, Gopinath CS, Rao BS, Satyanarayana CV (2005) Oxidative dehydrogenation of

ethylbenzene over vanadia-alumina catalysts in the presence of nitrous oxide: structure-activity relationship. J Catal 230:484-492

23. Chen S, Qin Z, Xu X, Wang J (2006) Structure and properties of the alumina-supported vanadia catalysts for ethylbenzene dehy-drogenation in the presence of carbon dioxide. Appl Catal A Gen 302:185-192

24. Itika K, Jayesh TB, Rangappa SK, Nagaraja BM (2015) Activity studies of vanadium, iron, carbon and mixed oxides based catalysts for the oxidative dehydrogenation of ethylbenzene to styrene: a review. Catal Sci Technol 5:5062-5076

25. Reddy BN, Reddy BM, Subrahmanyam M (1991) Dispersion and 3-picoline ammoxidation investigation of V2O5/a-Al2O3 catalysts. J Chem Soc, Faraday Trans 87:1649-1655

26. Imamura S, Fakuda I, Ishida S (1988) Wet oxidation catalyzed by ruthenium supported on cerium (IV) oxides. Ind Eng Chem Res 27:718-721

27. Mishra VS, Mahajani VV, Joshi JB (1995) Wet air oxidation. Ind Eng Chem Res 34:2-48

28. Trovarelli A, Dolcetti G, de Leitenburg C, Kaspar J, Finetti P, Santoni A (1992) Rh-CeO2 interaction induced by high-temperature reduction. Characterization and catalytic behavior in transient and continuous conditions. J Chem Soc, Faraday Trans 88:1311-1319

29. Reddy BM, Lakshmanan P, Bharali P, Saikia P, Thrimurthulu G, Muhler MGrunert W (2007) Influence of alumina, silica, and titania supports on the structure and CO oxidation activity of CexZr1-xO2 nanocomposite oxides. J Phys Chem C111:10478-10483

30. Monteiro RS, Dieguez LC, Schmal M (2001) The role of Pd precursors in the oxidation of carbon monoxide over Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts. Catal Today 65:77-89

31. Zhang H, Zhu A, Wang X, Wang Y, Shi C (2007) Catalytic performance of Ag-Co/CeO2 catalyst in NO-CO and NO-CO-O2 system. Catal Commun 8:612-618

32. Andreeva D, Ivanov I, Ilieva L, Abrashov MV (2006) Gold catalysts supported on ceria and ceria-alumina for water-gas shift reaction. Appl Catal A Gen 302:127-132

33. Trovarelli A, Leitenburg C, Dolcetti G (1997) Design better cerium based oxidation catalysts. Chem tech 27:32-37

34. Ramudu P, Anand N, Venkata RBG, Murali DG, Sai Prasad PS, David Raju B, Seetha Rama Rao K (2014) Molybdenum oxide supported on COK-12: a novel catalyst for oxidative dehydro-genation of ethylbenzene to styrene using CO2. Indian J Chem 53A:493-498

35. Ramudu P, Anand N, Mohan V, Murali DG, Sai Prasad PS, David Raju B, Seetha Rama Rao K (2015) Studies on ethylbenzene dehydrogenation with CO2 as soft oxidant over Co3O4/COK-12 catalysts. J ChemSci 127:701-709

36. Madhavi J, Suresh M, Ramesh Babu GV, Sai Prasad PS, David Raju B, Rama Rao KS (2014) N2 as a co-soft oxidant along with CO2 in ethylbenzene dehydrogenation to styrene over y-Al2O3 supported Co-Mo nitride catalysts. J CO2 Util 8:21-26

37. Beitha MA, Rabie AM, Elfadl AM, Yehia FZ (2016) microwave assisted synthesis of a VOx modified disordered mesoporous silica for ethylbenzene dehydrogenation in presence of CO2. Microporous Mesoporous Mater 222:44-54

38. Reddy BM, Rao KN, Reddy GK, Khan A, Park SE (2007) Structural characterization and oxidehydrogenation activity of CeO2/Al2O3 and V2O5/CeO2/Al2O3 catalysts. J Phy Chem C111:18751-18758

39. Roozeboom F, Mittelmeijer-Hazeleger MC, Moulijn JA, Medema J, de Beer VHJ, Gellings PJ (1980) Vanadium oxide monolayer catalysts. 3. A Raman spectroscopic and temperature-programmed reduction study of monolayer and crystal-type vanadia on various supports. J Phys Chem 84:2783-2791

jjjilliir dllall ¿in» c •

KAcsTäLLäJiq rnqiiU ^ springer

40. Erdohelyi A, Solymosi F (1991) Oxidation of ethane over silica-supported alkali metal vanadate catalysts. J Catal 129:497-510

41. Lemonidou AA, Nalbandian L, Vasalos IA (2000) Oxidative dehydrogenation of propane over vanadium oxide based catalysts: effect of support and alkali promoter. Catal Today 61:333-341

42. Liu YM, Cao Y, Yi N, Feng WL, Dai WL, Yan SR, He HY, Fan KN (2004) Vanadium oxide supported on mesoporous SBA-15 as highly selective catalysts in the oxidative dehydrogenation of propane. J Catal 224:417-428

43. Reddy EP, Varma RS (2004) Preparation, characterization, and activity of Al2O3-supported V2O5 catalysts. J Catal 221:93-101

44. Fiedorow R, Przystajko W, Sopa M, Dalla Lanal G (1981) The nature and catalytic influence of coke formed on alumina: oxidative dehydrogenation of ethylbenzene. J Catal 68:33-41

45. Murugan B, Ramaswamy AV (2007) Defect-site promoted surface reorganization in nanocrystalline ceria for the low-temperature activation of ethylbenzene. J Am Chem Soc 129:3062-3063

46. Reddy BM, Lee SC, Han DS, Park SE (2009) Utilization of carbon dioxide as soft oxidant for oxidehydrogenation of ethyl-benzene to styrene over V2O5-CeO2/TiO2-ZrO2 catalyst. Appl Catal B Environ 87:230-238

47. Rao KN, Reddy BM, Abhishek B, Seo YH, Jiang N, Park SE (2009) Effect of ceria on the structure and catalytic activity of V2O5/TiO2-ZrO2 for oxidehydrogenation of ethylbenzene to styrene utilizing CO2 as soft oxidant. Appl Catal B Environ 91:649-656

48. Le Bars J, Auroux A, Forissier M, Vedrine JC (1996) Active sites of V2O5/y-Al2O3 catalysts in the oxidative dehydrogenation of ethane. J Catal 162:250-259

49. Chandra Shekar S, Krishna Murthy J, Kanta Rao P, Rama Rao KS (2003) Selective hydrogenolysis of dichlorodifluoromethane on carbon covered alumina supported palladium catalyst. J Mol Catal A: Chem 191:45-59

jjj_sJlxLc clLLoJ cujoo KACSTojISLIIg foqlaJJ