*3.1. Influence of CO2 on Dehydrogenation of Ethyl Benzene*

Styrene is typically formed by the dehydrogenation of ethyl benzene under the steam on a metal oxide catalyst in an adiabatic reactor [39]. There are several limitations to this process, including thermodynamic drawbacks, low conversion rates, high endothermic energy (Δ*H*<sup>o</sup> <sup>298</sup> = 123.6 kJ mol<sup>−</sup>1), huge energy destruction, and catalyst deactivation by coke production [40]. An alternative method of styrene production is the oxidative dehydrogenation reaction with O2; however, this results in the burning of large quantities of valuable hydrocarbons. In this context, the use of CO2 in the oxidative dehydrogenation of ethyl benzene may prove useful [39–61]. *Zhang* et al. [43] confirmed coke deposition using spectroscopy and reported the deactivation of a ceria catalyst without CO2 present. In a two-step, reaction mechanism for the dehydrogenation of ethyl benzene to produce styrene with H2 in the initial step and in the presence of CO2, ejection of H2 through a reverse water-gas shift (RWGS) reaction was also demonstrated [41]. Kovacevic et al. revealed the results of CeO2 catalyst morphology (i.e., rods vs. cubes vs. particles) in the presence and absence of CO2 [42]. They reported that in the presence of CO2 cubic catalysts showed higher initial benzene selectivity, and about two times more activity per m<sup>2</sup> compared to the reaction without CO2. Interestingly, the number of oxygen species was increased by the presence of CO2. They also observed that these additional oxygen molecules were expended in the ethyl benzene conversion, demonstrating their performance as active sites for styrene formation. Periyasamy et al. reported that in the ODH reaction the conversion of ethyl benzene (EB) was 50% and the selectivity for styrene was 93% at gas hourly space velocity (GHSV) 2400 h<sup>−</sup>1. They also observed that the conversion and selectivity increased with enhancing oxidant flow ratio, up to GHSV 2400 h<sup>−</sup>1.

Park et al. [49] reported on the use of SBA-15 as a beneficial backing for a ceria-zirconium (25:75) combined oxide catalyst for oxidative dehydrogenation of ethyl benzene utilizing carbon dioxide. Ce-Mn oxide nanoparticles enclosed inside carbon nanotubes (CNTs) were used for the oxidative dehydrogenation of ethyl benzene with CO2 acting as a soft oxidant. The high diffusion and the encapsulation effect of CNTs resulted in excellent performance of the entrapped catalysts. Correlated to CeO2 support CNTs, the restriction result of CNT pathways enhanced the communication between carbon nanotube (CNT) inner walls and CeO2 particles, which is orderly, convinced the misrepresentation of CeO2 crystal lattice which is advertised CeO2 reduction and invigoration of CeO2 surface lattice oxygen. The unique process of promoting oxidative catalytic activity the addition of CO2 was reported by Zhang et al. [44]. They observed that multi-walled carbon nanotubes (MWCNTs) have a significant quantity of surface hydroxyl groups which are produced by an alkali-supported hydrothermal method after ball milling. The MWCNTs can mostly arrange the active sites for the oxidative dehydrogenation of ethyl benzene (EB) in the existence of CO2. Figure 3a shows the conversion of ethyl benzene over various types of MWCNTs at 3 hr. The HMWCNTs-OH exhibits significant catalytic activity, indicating that the surface hydroxyl groups are the active sites for the oxidative dehydrogenation of ethyl benzene. The O1s spectra of HMWCNTs-B-OH identified by XPS is shown in Figure 3b. Figure 3c demonstrates the production of carbonyl groups in the reaction. The results indicate that CO2 acts effectively as a soft oxidant, directly oxidizing -OH groups into carbonyl groups. As shown in Figure 3d, CO and H2 were also identified as byproducts for the reaction, indicating that CO2 is reduced in the RWGS reaction. CO2 activation occurs via electron donation from the surface of the catalyst to the anti-bonding orbital of CO2 [62]. However, ethyl benzene (EB) can be activated for oxidative dehydrogenation (ODH) by donating an electron to the acidic portion of the catalyst surface.

**Figure 3.** (**a**) Conversion of EB using CO2 as a soft oxidant; (**b**) O1s spectra of HMWCNTs-B-OH after oxidative dehydrogenation by XPS for 3 hr; (**c**) Oxygen substance (mol%) of HMWCNTs-B-OH earlier and later oxidative dehydrogenation reaction at 3 h; (**d**) Gas derivatives of HMWCNTs-B-OH after oxidative dehydrogenation at 3 h. (Reprinted from [44]; copyright (2013), Royal Society of Chemistry).

Additionally, basic sites abstract hydrogen from ethyl benzene. Thus, the aggregated effect of the basic and acidic sites of the catalyst face is the oxidative dehydrogenation reaction, resulting in high catalytic efficiency in the existence of CO2 [63]. Sato et al. reported on the use of CO2 as a mild oxidant in the oxidative dehydrogenation reaction as well as the typical dehydrogenation process in the absence of CO2. Two mechanisms utilizing acidic and basic sites were proposed, as depicted in Figure 4. Vanadium-embed catalysts also used in CO2 based oxidative dehydrogenation of oxidative dehydrogenation of ethyl benzene (ODHEB) reactions [21,45]. CO2, being a mild oxidant, cannot reproduce the active sites on the V2O5 (001) surface of the catalyst quickly enough due to the large activation energy (3.16 eV) [46]. A ceria-supported vanadium catalyst floated on a titania-zirconia combined oxide (TiO2-ZrO2) has moderate constancy which was reported by Reddy et al. [47]. XPS analysis of Ce 3d indicated the presence of Ce4<sup>+</sup> and Ce3<sup>+</sup> on the Ti-Zr catalyst. They also reported that CeO2-V2O5/TiO2-ZrO2 (TZ) catalysts resulted in 56% conversion of ethylbenzene and 98% selectivity of styrene. Liu et al. [48] illustrated the red-ox mechanism for the CO2-oxidative dehydrogenation of ethyl benzene (CO2-ODEB) using a ceria promoted vanadium catalyst, as shown in Figure 5. In the CO2-ODEB case, CO2 directly oxidizes Ce3<sup>+</sup> to Ce4<sup>+</sup>, and ethylbenzene reduces of V5<sup>+</sup> to V4<sup>+</sup>. Then, the reduction of Ce4<sup>+</sup> to Ce3<sup>+</sup> and the oxidation of V4<sup>+</sup> to V5<sup>+</sup> completes the full cycle. In the existence of CO2, modified vanadium catalysts are effective, selective, and stable for the ODEB, as reported by Park et al. [49] Rapid regeneration of active sites on a silica-assisted vanadium catalyst along in the presence of CO2 has also been reported [64]. 10% La2O3-15%V2O5/SBA-15 (wt.%)

catalyst resulted in a 74% styrene yield, with La3<sup>+</sup> resisting coke ejection [50]. The use of supporting materials, such as Aluminum mesoporous cylindrical molecular sieve (Al MCM-41) also resulted in substantial EB conversion in the ODEB using a VOx/Al MCM-41 catalyst in the presence of CO2 [51]. ZrO2-containing combined oxide catalysts for oxidative dehydrogenation of ethylbenzene with CO2 in the presence of MnO2, CeO2 and TiO2 have exhibited high activity. The styrene yield was also increased over the MnO2-ZrO2 dual oxide catalyst at a high temperature. Significant enhancement of catalytic activity was checked with increasing CO2/EB ratios [52]. A TiO2-ZrO2 catalyst was used, and the proportion of TiO2/ZrO2 determined the catalytic activity [53–55,65]. A 60% titania content resulted the best performance for the ODEB [65,66]. Commercial Fe-supported catalysts are unsuitable for the oxidative dehydrogenation of ethyl benzene in the existence of carbon dioxide due to the atomization of the active catalytic site [56]. However, the use of appropriate dopants' support materials might enhance the activity by promoting re-oxidation of Fe2<sup>+</sup> and preventing coke deposition [57,58]. High product yield stability was observed in a mesoporous silica COK12-assisted CoO3 catalyst [59]. The performance of several effective catalysts in the oxidative dehydrogenation of ethyl benzene to styrene in the existence of CO2 is shown in Table 7.

**Figure 4.** The procedure of oxidative dehydrogenation of ethylbenzene to styrene (**a**) without CO2 and (**b**) with CO2. (Reproduced with permission from [62]; copyright (2016), Elsevier).

**Figure 5.** Red-ox cycle for CO2-ODEB over the ceria-promoted vanadium catalyst. (Reproduced from [48]; copyright (2011), WILEY-VCH).


**Table 7.** Performance of CO2 on oxidative dehydrogenation of ethylbenzene to styrene.
