Oxidative dehydrogenation of ethylbenzene over γ-Al₂O₃ supported ceria-lanthanum oxide catalysts: Influence of Ce/La composition

3.1.1 BET surface area

BET surface area values of the catalysts were listed in Table 1. During the catalyst preparation via incipient wetness impregnation procedure, we used commercially available γ-Al₂O₃ as support (BET surface area: 200 m²/g), which provided an excellent surface area for dispersing active CeO₂ and/or active solid solution (Ce_xLa_1-xO_2-δ) moieties in LCA samples. This can be manifested by the fact that even after deposition of CeO₂ in variable compositions on γ-Al₂O₃ there is not much decrease in the surface area of the catalysts (165, 151, 142 and 135 m²/g for 5CA, 10CA, 15CA and 20CA catalysts respectively). In addition, after doping of 15CA with various compositions of La₂O₃, no momentous decrease in the surface area (138, 131 and 129 m²_/ g for 1LCA, 3LCA and 5LCA catalysts respectively) was observed. BET surface area per gram of support for all the catalysts is nearly constant (160–174 m²/g of support). This is possible if the active component either CeO₂ or CeO₂/La₂O₃ combination possess more or less same crystallite size even against variable composition. These results evidently endorse that both CeO₂ and La₂O₃ are present in highly dispersed state as isolated CeO₂ and existence of fraction of solid solution (Ce_xLa_1-xO_2-δ) species.

3.1.2 X-ray diffraction studies on CA and LCA catalysts

XRD patterns of CA and LCA catalysts including CeO₂, La₂O₃/CeO₂ were presented in Fig. 1, revealing the presence of cubic CeO₂ (1 1 1) plane. Herein, diffraction signals pertaining to La₂O₃ in the LCA samples were not observed. For the clarity of understanding XRD pattern of La₂O₃ also displayed in Fig. 2. We assume that there is a chance of lanthanum ions entering into CeO₂ (1 1 1) lattice to get (Ce_xLa_1-xO_2-δ) species in LCA catalysts. Additionally, it can stabilize the γ-Al₂O₃ phase by interacting with it (Bettman et al.,1989; Alphonse and Faure, 2014). It can be noted that the diffraction lines of LCA samples are slightly shifted to lower 2θ region compared to that of bulk cubic CeO₂. Doping of lanthanum (La³⁺  = 0.116 nm) ions into CeO₂ (Ce ions = 0.097 nm) lattice causes to increase in the lattice parameter due to the difference in the sizes of La³⁺ and Ce⁴⁺ ions (Lakshmi et al., 2011). Mild intense diffractions lines due to the presence of γ-phase of Al₂O₃ can be seen in the XRD patterns of catalysts with lower ceria contents (Mahipal Reddy et al., 2007; Dolly et al., 2011; Gangar et al., 2012; Martınez-Arias et al., 2000; Damyanova et al., 2002; Kamal, 2007; Parag et al., 2011). In the XRD patterns of catalysts with higher CeO₂ content and in LCA samples, no intense peaks due to γ-Al₂O₃ phase are observed which indicates the presence of this phase is in poorly crystalline form even though it is a known fact that La₂O₃ evidently stabilizes the γ-phase of Al₂O₃ (Lakshmi et al., 2011). Crystallite size of CeO₂ or Ce_xLa_1-xO_2-δ species calculated from the XRD raw data by using Debye-Scherrer equation is shown in Table 1. Herein, (1 1 1) plane at a 2θ value of 28° due to either La₂O₃ inserted into CA to get a solid solution i.e. Ce_xLa_1-xO_2-δ or CeO₂ with similar 2θ value has been taken for the calculation of crystallite size. In addition, a signal at a 2θ value of 46° due to the presence of (4 0 0) plane ascribed to existence of γ-Al₂O₃ phase while, mild intense (4 0 0) plane at a 2θ value of 69° indicates the presence of CeO₂. The CeO₂ and Ce_xLa_1-xO_2-δ crystallite size in the γ-Al₂O₃ supported catalysts (∼20 nm) is significantly lower by a factor 4 compared to that of bulk CeO₂ or La₂O₃/CeO₂ (∼80 nm). This result clearly indicates the fine dispersion of CeO₂ particles on γ-Al₂O₃ surface without compromising the surface area. XRD pattern of 3LCA spent catalyst (collected after 50 h TOS in the EB oxidative dehydrogenation reaction) witnesses more or less similar crystallite size of CeO₂ or Ce_xLa_1-xO_2-δ as in fresh catalyst. In the present study we did not find XRD peaks due to LaAlO₃ which is in line with the literature (Lakshmi et al., 2011) study.

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Fig. 1 XRD patterns of different catalysts: (a) Al₂O₃, (b) CeO₂, (c) CeO₂/La₂O₃, (d) 5CA, (e) 10CA, (f) 15CA, (g) 20CA, (h) 1LCA, (i) 3LCA, (j) 5LCA and (k) 3LCA spent catalyst.

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Fig. 2 XRD pattern of bulk La₂O₃ catalyst.

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3.1.3 Transmission electron microscopic investigation

TEM images of the representative sample; 3LCA was shown in Fig. 3. Fine particles of CeO₂, La₂O₃ as well as Ce_xLa_1-xO_2-δ can be seen in the fresh sample (Fig. 3a). The SAED image shows various planes of CeO₂ or Ce_xLa_1-xO_2-δ (Fig. 3b). From the TEM picture (Fig. 3c), of the spent catalyst, (collected after time on stream study of EB oxidative dehydrogenation), did not show much variation in size compared to that of fresh catalyst (no phase changes were observed from the XRD results of spent 3LCA catalyst). This is an apparent sign that there was no drastic agglomeration of particles even after the prolonged TOS measurement.

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Fig. 3 TEM images of 3LCA catalyst, (a) fresh catalyst, (b) SAED image of fresh catalyst and (c) spent catalyst.

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3.1.4 Temperature programmed reduction studies on CA and LCA catalysts

The H₂-TPR profiles of CA and LCA along with CeO₂ and La₂O₃/CeO₂ were shown in Fig. 4. Generally, bulk CeO₂ displays two stage reduction with peaks pertaining to surface capped oxygen and bulk ceria reduction. (Shung et al., 2011) have observed that these two reduction maxima are centered at ∼693 and >973 K respectively. The difference in these two successive reduction temperatures could be due to dissimilar binding energy of oxygen to cerium cations in crystal lattice. In the TPR plot of 15CA catalyst, the two T_max values are shifted to lower temperature region compared to those in either 10CA or 20CA catalysts. Comparison of TPR patterns of CeO₂ (Fig. 4a) and CeO₂-La₂O₃ (Fig. 4b) reveals that the low temperature signal (T_max of ∼800 K in CeO₂ has been shifted to ∼1000 K when La₂O₃ has been introduced. This is a clear indication of existence of interacted species between CeO₂ and La₂O₃. The fine dispersion of CeO₂ particles and proper interaction with the support could be the reason for facile reduction of Ce⁴⁺ species. The intensity of reduction maxima (T_max) in the TPR patterns of LCA samples is higher than those in CA samples. Thus one can presume that the role of La₂O₃ is to increase the number reducible species of CeO₂ during reduction event. Among the two reduction maxima, the low temperature signal could be due to reduction of CeO₂ species interacted with La₂O₃ and the high temperature signal ascribed to reduction of Ce_xLa_1-xO_2-δ species. Higher loadings of La₂O₃ in LCA catalysts cause shifting of T_max to lower temperature region and these results are in line with those reported by Shuang et al. (2011) and Lakshmi et al. (2011).

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Fig. 4 Temperature programmed reduction (TPR) profiles: (a) CeO₂, (b) CeO₂/La₂O, (c) 10CA, (d) 15CA, (e) 20CA, (f) 1LCA, (g) 3LCA, (h) 5LCA.

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3.1.5 Ultra violet–visible diffuse reflectance spectroscopic (UV–vis DRS) study on CA and LCA

The UV–vis DRS of the CA and LCA including bulk CeO₂ and La₂O₃/CeO₂ samples were depicted in Fig. 5. A split in the absorption maxima is visible for pure CeO₂ and La₂O₃/CeO₂, centering at ∼ 253 and 287 nm corresponding to charge-transfer (CT) of O²⁻ → Ce³⁺, O²⁻ → Ce⁴⁺ and 339 nm absorption ascribed to inter band transitions (IBT) (Lakshmi et al., 2011). Bensalem et al. (1995) have pointed out that the UV–vis DRS spectrum for CeO₂ is affected by the peculiar reflectance effects produced by the strong absorption of CeO₂ in the UV range. In the present investigation, UV–vis DR spectra of all the CA catalysts displayed unresolved absorption trend with mild intense bands at 325, 335, 345 and 355 nm respectively. Shifting of absorption band to lower wavelength with decrease in CeO₂ content in UV region (≤355 nm) indicates the presence of defects produced in the lattice during CeO₂ incorporation. Besides, a partially resolved peak related to IBT can also be observed in UV region at ≤355 nm in all CA and LCA catalysts due to significant influence of CeO₂ accompanied with La₂O₃ on γ-Al₂O₃. The formation of solid solution in the La₂O₃ doped CA samples not only alter the crystalline nature but also affects the charge transfer band positions leads to optical properties of the catalyst. Interestingly, shifting of absorption signals (blue shift) in CA and LCA (average crystallite size 20 nm) may be due to the smaller crystallites than in the bulk CeO₂ (80 nm), La₂O₃/CeO₂ (78 nm) catalysts (Martınez-Arias et al., 2000).

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Fig. 5 UV–vis DRS of catalysts: (a) CeO₂, (b) CeO₂/La₂O, (c) 5CA, (d) 10CA, (e) 15CA, (f) 20CA, (g) 1LCA, (h) 3LCA, (i) 5LCA.

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3.1.6 Fourier Transform infrared spectroscopic investigation on CA and LCA catalysts

The FT-IR patterns of CA and LCA including CeO₂ and La₂O₃/CeO₂ obtained in the region of 400 and 4000 cm⁻¹ are presented Fig. 6. The mild intense signal at 3432 cm⁻¹ can be attributed to absorption of OH stretching mode. The absorption signal at 1631 cm⁻¹ is due to the deformation band of interlayer H-O-H molecules. Besides metal-oxygen, Al-O or Ce-O vibrations can be expected in the 500–900 cm^–1 region. CeO₂ bands in the range of 400 to 750 cm⁻¹ due to stretching vibration (Dolly et al., 2011). FT-IR patterns of La₂O₃ incorporated samples demonstrated similar absorptions due merging of La-O into CeO₂/γ-Al₂O₃ absorption net work to get a solid solution. Therefore, we assumed the absorption reflection at 580 cm⁻¹ due to stretching mode of La₂O₃ assisted AlO₆ or bending mode of CeO₂ (Ce-O-C). The principal vibration mode of La-O usually occurs in the range of 920–940 cm⁻¹. However in the present investigation, La-O absorptions in LCA are not visible, indicates the formation of solid solution. A sharp band observed at around 846 cm^–1 in CA and LCA samples is due to the stretching mode of AlO₄. Whereas, strong band marked at around 570 cm^–1 can be attributed to stretching mode of AlO₆ or bending mode of CeO₂ (Kamal, 2007).

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Fig. 6 FT-IR spectra of catalysts: (a) CeO₂, (b) CeO₂/La₂O, (c) 5CA, (d) 10CA, (e) 15CA, (f) 20CA, (g) 3LCA, (h) 5LCA.

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3.1.7 Temperature programmed desorption of NH₃

To find out the acid site distribution of CA and LCA catalysts, temperature programmed desorption of NH₃ experiments were conducted and the TPD profiles of 15CA, 1LCA, 3LCA and 5LCA catalysts were shown in Fig. 7. The acidic sites can be classified based on the desorption temperature. If NH₃ desorbs below 523 K these acidic sites are weak acidic sites, 523–673 K range corresponds to moderate acidic sites and beyond 673 K represents strong acidic sites (Chada et al., 2017). In the NH₃-TPD pattern of 15CA sample, a very broad desorption peak with its peak maximum (T_max) at around 723 K is observed. This peak is distributed in a wide temperature range of 573–873 K with strong signal intensity. This strong and wide temperature range peak is primarily due to the generation of acidic sites owing to interaction between CeO₂ and γ-Al₂O₃ leading to the formation Al- and Ce- linkages through bridging of oxygen bonds. In addition an intense board peak at a T_max of ∼473 K is observed. Generation of these weak acidic sites in 15CA and LCA catalysts can be attributed to the presence of well dispersed solid solution species with γ-Al₂O_3. The amount of NH₃ desorbed in different temperature regions implying the strength of acidic sites (Table 2).

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Fig. 7 Temperature programmed desorption of NH₃: (a) 15CA, (b) 1LCA, (c) 3LCA, (d) 5LCA.

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As shown in Table 2, in 3LCA the total acidic sites are higher compared to 1LCA and 5LCA, but exhibited higher conversion of EB and selectivity of styrene, revealing that apart from acidity, other parameters like dispersed species might have contributed for the catalytic activity.

3.1.8 CO₂ Pulse Chemisorption studies on CA and LCA catalysts

The cumulative amounts of CO₂ obtained in the pulse injections at 873 K over partially reduced 1LCA, 3LCA, 5LCA and 15CA catalysts were shown in Fig. 8. CO₂ Pulse Chemisorption on partially reduced CeO₂ i.e. (CeO_X) catalysts gives a scope to find out the reoxidizability of the catalyst at 873 K. Similar observation was found over vanadium-antimony oxide catalysts (Jong-San et al., 2003). Fig. 8 reveals the amount CO₂ adsorbed over 3LCA is more than 1LCA, 5LCA and 15CA. The total amount of CO₂ adsorbed over 3LCA is 55 µmoles/g catalyst at 873 K, which is higher than that of 15CA (30 µmoles/g). CO₂ Pulse Chemisorption data concluding that 3LCA possesses large number of reoxidizable Ce clusters. Higher amount of CO₂ uptake is observed in LCA catalysts compared either CeO₂ or La₂O₃ containing catalysts reported in the literature (Liu et al., 2008; Gurram et al., 2016).

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Fig. 8 CO₂ pulse chemisorption profiles: (a) 15CA, (b) 5LCA, (c) 1LCA, (d) 3LCA, Conditions: Catalyst: 100 mg, CO₂ flow: 35 mL/min, Temperature: 873 K.

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3.1.9 Oxygen Pulse Chemisorption studies on CA and LCA catalysts

The total oxygen storage capacity (OSC) of the catalysts 1LCA, 3LC, 5LCA, 20CA and 15CA has been investigated at 873 K. The consumption and replenishment of the lattice oxygen is dynamic between reduction and oxidation environments for ODH reaction. The contribution of the total OSC to the catalytic behavior gives valuable information to assess the activity of the catalyst. As illustrated in Fig. 9, 15CA possesses a limited oxygen storage capacity (200 μmol/g). However, doping with La₂O₃ results in increase of oxygen storage capacity as shown in Fig. 8. Doping small amount of La causes a remarkable modification in the oxygen storage behavior of the CA materials. The 3LCA sample gives the highest value of OSC (>400 μmol/g. The number of surface lattice oxygen species in the ceria-lanthanum solid solutions contributes to their catalytic activity in the ODH of EB reaction (Jie et al., 2011). Both OSC and CO₂ uptakes indicate that 3LCA would be a better catalyst in EB oxidative dehydrogenation with CO₂ as a soft oxidant.

3.2.1 Oxidative dehydrogenation of ethylbenzene with CO₂

Ethylbenzene dehydrogenation is an endothermic reaction and hence high temperatures are required to get enhanced yields of ST. Co-feeding of CO₂ along with EB helps in improving the yields of ST through RWGS reaction. The EB oxidative dehydrogenation was carried over CeO₂, La₂O_3, γ-Al₂O_3, La₂O₃/CeO₂ and CeO₂/γ-Al₂O₃ catalysts. The catalytic efficiency of various catalysts for EB conversion in CO₂ atmosphere was depicted in Table 3. Bulk CeO₂, La₂ O₃ and La₂O₃/CeO₂ catalysts exhibit poor EB conversion (25, 30 and 40% respectively). Lower EB conversion on CeO₂, La₂O₃ and La₂O₃/CeO₂ is due to mild oxygen releasing ability and considerably bigger crystallites (Jie et al., 2009, 2011). Among the CA catalysts, 15CA shows 73% EB conversion and higher ST selectivity (94%) compared to that on γ-Al₂O₃ (62% EB conversion and 85% ST selectivity). Both 5CA and 10CA catalysts are inferior compared to 15CA catalyst due to lower number of active ceria sites. Higher EB conversion over 15CA catalyst is due to the presence of more number of active CeO₂ species. Though the amount of Ce species is more in 20CA (23 nm), its activity is lower compared 15CA (21), which may be due slight increase in crystallite size. Based on the activity point of view 15CA is the best catalyst, hence different amounts (1, 3 and 5 wt%) of La₂O₃ doped on 15CA catalyst. Among the LCA catalyst, 3LCA exhibited higher EB conversion (82%) and ST selectivity (99%) mostly due to formation of suitable solid solution clusters (Ce_xLa_1-xO_2-δ). 5LCA exhibited lower EB conversion (67.1%) compared to that of 3LCA due to lowering of acidic sites (Table 2) lower O₂ uptake (Fig. 9) and mild CO₂ adsorption efficiency (Fig. 8). Similarly, 1LCA catalyst is also exhibited lower activity (73% EB conversion) compared to 3LCA catalyst, which might be due insufficient amount of La₂O₃ for the formation solid solution cluster. It is reported that the solid solution (Ce_xLa_1-xO_2-δ) is an essential requirement for excellent CO oxidation and higher resistance ability towards lower coke deposits along with the formation of CeAlO₃ phase in La₂O₃ doped CA samples (Mahipal Reddy et al., 2006, 2009; Lakshmi et al., 2011). It was reported by Liu et al. (2008) in the EB oxidative dehydrogenation with CO₂, introduction of La₂O₃ to V₂O₅/SBA-15 catalyst improves the activity and stability of the catalyst via the formation of La₂O₂CO₃ species, which promote the RWGSR. Lakshmi et al. (2011) have reported the role of La₂O₃ in Ce/γ-Al₂O₃ catalysts to increase the number of oxygen vacancies with increased oxygen mobility and increase the reducibility by forming a defect structure Ce_xLa1_-xO_2-δ. Hence, Ce_xLa_1-xO_2-δ solid solution clusters facilitates the mobility of oxygen species from bulk to surface and thereby creating more number of active sites on the surface which assists the favorable adsorption of EB. Both moderate and strong acidic sites are more in 15CA catalyst compare to LCA catalysts, leads to formation of benzene and toluene as byproducts, resultantly deceased selectivity is noticed.

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Fig. 9 Oxygen pulse chemisorption profiles: (a) 15CA, (b) 5LCA, (c) 1LCA, (d) 3LCA; (b) Conditions: Catalyst: 100 mg, Gas flow (1%O₂ + 99% He): 60 mL/min, Temperature: 873 K.

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3.2.2 Effect of reaction temperature on 3LCA catalyst with CO₂

The conversion of EB over 3LCA sample at different reaction temperatures was depicted in Fig. 10, indicates the increase in EB conversion with a rise in reaction temperature, both in N₂ and CO₂ flow. The higher EB conversion in CO₂ flow is due to the removal of H₂ produced in EB dehydrogenation via reverse water gas shift reaction (RWGSR). Hence, CO₂ is regarded as a soft oxidant for the EB oxidative dehydrogenation. The following reactions clearly indicate the advantage of CO₂ as a soft oxidant.

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Fig. 10 Effect of reaction temperature in the oxidative dehydrogenation of ethylbenzene over 3LCA in N₂ and CO₂ flow. Conditions: Catalyst: 1 g, CO₂ and N₂ flow: 30 mL/min, EB feed flow: 1 mL/h, Temperature: 723,773, 823, 873 and 923 K.

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The increase in the EB conversion with increasing temperature is mainly due to endothermic nature of the reaction (Lee, 1973). The EB conversion and ST selectivity respectively in CO₂ flow are: at 723 K (19%, 97%), 773 K (29%, 97%) 823 K (40%, 99%), 873 K (82.1%, 99.1%) and 923 K (92%, 98%). The EB conversion at 923 K is higher than the reported vanadium/ceria-alumina catalyst (Mahipal Reddy et al., 2007) because of the synergistic effect between CeO₂ and La₂O₃ in LCA catalyst. Even at 873 K, the present catalytic system exhibited 82% EB conversion with 99% ST selectivity and the comparison of the activity of these result with those cited in the literature were shown in Table 4. Activity comparison of these cited catalysts such as, meso porous CeO₂ (723 K, 50%), vanadium/ceria-alumina (823 K, 20%), MnO₂-ZrO₂ (873 K, 54%), K₂O/TiO₂-MnO₂ (873 K, 60%), Cr/MCM-41, (823 K, 55%), Co-Mo nitride (873 K, 67%) and Co/COK-12 (873 K, 55%), as reported by Jie et al. (2009), Van et al. (2014), Do-Young et al. (2008, 2005), David Raju et al. (2006), Madhavi et al. (2014) and Ramudu et al. (2015) respectively indicate that the activity of 3LCA catalyst of the present investigation is much higher in the oxidative dehydrogenation of EB with CO₂. Simplistic redox properties in LCA samples promotes higher EB conversion due to RWGSR with CO₂ as co-feed (Yoshihiko et al., 2005; Liu et al., 2008; David Raju et al., 2006; Lakshmi et al., 2011; Li et al., 2015). Thus CO₂ as a soft oxidant in the EB oxidative dehydrogenation is advantageous in achieving good EB conversions and ST selectivity (Madhavi et al., 2014; David Raju et al., 2006). Enriched oxygen vacancies created by active (Ce_xLa_1-xO_2-δ) solid solution clusters over catalytic surface efficiently utilizes oxygen density (Li et al., 2015. The role of CeO₂ as a support in number of catalysts for EB oxidative dehydrogenation in presence of several oxidants such as O₂, N₂O and CO₂ were reported (Ashok Kumar et al., 2013; Jie et al., 2011; Van et al., 2014; Li et al., 2015). During EB oxidative dehydrogenation in presence of CO₂, the online gas phase analysis confirms the generation of CO via RWGSR and the results are displayed in Fig. 11. Even though the initial CO formation is very high, it gets stabilized at 9thh. The yield of CO formation is higher on 3LCA than on 15CA catalyst at 600 °C. This clearly indicates, the RWGSR ability of 3LCA is higher than that of 15CA. This is one of the reasons for 3LCA to exhibit higher activity in oxidative dehydrogenation of EB.

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Fig. 11 CO formation on 3LCA & 15CA catalysts: Conditions: Catalyst: 1.5 g, CO₂ flow: 35 mL/min, 1.5 ml/h EB feed flow: 1.5 mL, Temperature: 873 K.

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3.2.3 Time - on- stream (TOS) study on 3LCA catalyst

The time on stream study conducted over 3LCA and 15CA in CO₂ was shown in Fig. 12. In presence of N₂, 3LCA maintains EB conversion of ∼65% right from the first hour to few hours (not shown in the figure) then smoothly decreases to 45% at 50 h) with high (94%) ST selectivity. On the other hand, in CO₂ flow the initial EB conversion (65%) steadily goes up and reaches to >80% at 10th hour and maintains up to 30 h then gradually decreases to 67% at 50th hour. The high ST selectivity (>99%) remains unaltered from the first hour to 50th hour which is higher than on 15CA. RWGSR plays an imperative role for getting superior styrene yields in CO₂ flow rather than in presence of N₂ flow. Hence, dramatic CO formation achieved on 3LCA (10% at 1 h) rather than 15CA catalyst (7% at 1 h) owing to strong interface between CO₂ molecules and adequate solid solution clusters during TOS measurements. The 10% CO formation over 3LCA via RWGSR is due to adequate redox properties (in terms of excellent OSC after incorporation of considerable lanthanum into CA) compared to 15CA catalyst. Recently a research group found that the gasification of solid carbonaceous deposits over larger surface area CeO₂ in CO₂ environment is due to the extensive utilization of carbon deposits into oxidized gaseous components (Yoshihiko et al., 2005; David Raju et al., 2006; Li et al., 2015). In the current study, ST selectivity (99.1%) remain constant until 50 h TOS because of the presence of adequate solid solution clusters (Ce_xLa_1-xO_2-δ) in 3LCA catalyst. TOS after 50 h, the amount of coke deposited is 23 wt% and 37 wt% in 3LCA and 15CA respectively and the EB conversions on 15CA and 3LCA are 47 and 67%. According to the aforementioned activity results, active ceria or solid solution (Ce_xLa_1-xO_2-δ) clusters on surface of support might have helpful in getting good activity. Mobile oxygen density and redox properties (better CO₂ utilization through RWGSR) helped in maintain higher EB conversion in 3LCA. In the present study amount of CO formation is similar to the reported literature (Liu et al., 2008). CO₂ uptake during TOS study over lanthanum doped vanadium-SBA-15 and vanadium-SBA-15catalysts was observed, wherein lanthanum incorporated catalysts consumed maximum CO₂, revealing that the lanthanum content plays a crucial role for better CO₂ conversion.

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Fig. 12 Time-on-stream (TOS) for the oxidative dehydrogenation of ethylbenzene over 3LCA and 15CA catalysts: Conditions: Catalyst: 1 g, CO₂ flow: 30 mL/min: EB feed flow: 1 mL/h, Temperature: 873 K.

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