*3.2. Performance of CO2 on Dehydrogenation of Ethane*

Ethylene is one of the most prominent raw materials in the chemical industry. Presently, it is used to produce industrial products such as PVC, ethylene glycol, ethylbenzene, ethylene oxide, and vinyl acetate. Commercially, ethylene is formed by steam cracking dehydrogenation of hydrocarbons and fluid catalytic cracking (FCC). These conventional methods have several major limitations including the reaction endothermicity, thermodynamic drawbacks, rapid coke formation, and high energy consumption. The oxidative dehydrogenation of ethane (ODHE) to ethylene in the presence of CO2 as a mild oxidant is an environmentally friendly alternative method for the production of ethylene. A Cr-oxide catalyst with zeolite support was successfully used for the oxidative dehydrogenation of ethane in the presence of CO2 as a soft oxidant. A novel Clinoptilolite-based Cr-oxide (Cr/CLT-IA) catalyst for the ODHE in the existence of CO2 was investigated by Rahamani et al. [70]. This Cr-supported catalyst exhibits high selectivity and catalytic activity which was expected due to its acidity. Homogeneous, tunable smaller Clinoptilolite-based Cr catalyst particles with higher surface area can be generated. Thus, using a Cr/CLT-IA nano-catalyst may be feasible and favorable for the oxidative dehydrogenation of ethane to ethylene in the existence of CO2. Cr/H-ZSM-5 (SiO2/Al2O3 ≥ 190) outperformed the SiO2-based catalyst in the oxidative dehydrogenation of ethane to ethylene with CO2 [71]. CO2 is a promising soft oxidant for the ODHE reaction acting as a channel for transporting heat to the endothermic dehydrogenation. Further, CO2 improves the conversion by modifying alkanes and maintains the catalytic activity by eliminating coke from the catalyst surface. the texture of the Cr active sites and the catalyst activity are determined by the SiO2/Al2O3 ratio. The presence of more alumina amount in the zeolite negatively affected the activity of the catalyst, due to the incorporation of alumina with the Cr into the catalyst structure, affecting the red-ox properties of Cr. Mimura et al. [71] reported on the dehydrogenation of ethane on a Cr-doped HZSM-5 catalyst which is established on the redox phase of the eminent oxidation type Cr species. In their work, C2H6 was absorbed on the acidic site of CrOx and H-ZSM-5. Then, the activated C2H6 reacted with CrOx (active O species) to produce ethylene. The CrOx−<sup>1</sup> species is then re-oxidized by the soft oxidant CO2 regenerating the active O species and eliminating coke from the surface of the catalyst. The catalytic performance of the Cr-supported mesoporous catalyst, as well as a Cr-doped silicate MSU-1 catalyst, in the ethane oxidative dehydrogenation to ethylene in the presence of CO2 was reported on by Liu et al. [72]. They initially observed high catalytic activity due to the Cr(VI) active species. However, even in the existence of CO2, the reduction of Cr(VI) to Cr(III) occurred, resulting in the deactivation of the catalyst during the dehydrogenation reaction. Shi et al. [73] reported that Cr-supported Ce/SBA-15 catalysts were comprised of hexagonally ordered mesoporous frameworks and exhibited high catalytic activity in the oxidative dehydrogenation of ethane in the existence of CO2. They confirmed the addition of Ce species using high-angle XRD, which increased the Cr species distribution in the Cr-Ce based SBA-15 zeolite. TPR results determined that Cr species in SBA-15-type zeolites are Cr6<sup>+</sup> and Cr3<sup>+</sup> groups. Among those two ions, Cr6<sup>+</sup> exhibited significant activity for the oxidative dehydrogenation reaction in the existence of CO2. Including a Ce-supported in 5Cr/SBA-15 catalysts modified the red-ox properties and enhanced the activity of the catalyst. Ethane conversion was 55% and ethylene

selectivity was 96% on the 5Cr-10Ce/SBA-15 catalyst in the existence of CO2 (Table 8). Cr6<sup>+</sup> is reduced to Cr3<sup>+</sup> during the oxidative dehydrogenation method reaction, however, in the presence of CO2, Cr3<sup>+</sup> is re-oxidized to Cr6<sup>+</sup>. Cr2O3/ZrO2 supported catalysts with Fe, Co, Mn was also investigated in an effort to fully understand the excellent catalytic activity for the ethane dehydrogenation reaction to ethylene under CO2 treatment [64,65,74]. The Cr6+/Cr3<sup>+</sup> red-ox cycle is crucial in the oxidative dehydrogenation reaction, as is a Fe<sup>3</sup>+/Fe2<sup>+</sup> red-ox cycle which was removes H2 from the lattice oxygen. An SBA-15-based, Cr-modified catalyst using a Fe-Cr-Al alloy [75] also exhibited remarkable selectivity of ethylene and high ethane conversion in the oxidative dehydrogenation reaction with CO2. Wang et al. [64] observed the red-ox properties and the acidity/basicity of the Cr-supported catalyst in the oxidative dehydrogenation of ethane to ethylene with CO2. They found that Cr-supported catalysts exhibited different activities in the ODHE with CO2. Cr2O3/SiO2 showed higher ethane conversion and ethylene selectivity. The catalytic activities were ranked as follows Cr/SiO2 > Cr/ZrO2 > Cr/Al2O3 > Cr/TiO2 [76,77]. Notably, Cr2O3 interacted more with Al2O3 than with SiO2, resulting in tetrahedral Cr6<sup>+</sup> sites and declining activity [78]. Cr is one of the vital elements of various types of nano-catalysts (Table 9). The active site of these catalysts contains both Cr3<sup>+</sup> and Cr6<sup>+</sup>. The Cr<sup>6</sup>+/Cr3<sup>+</sup> ratio strongly influences the reducibility of Cr/H-ZSM-5 catalysts. The red-ox performance of Cr-supported catalysts is crucial for the oxidative dehydrogenation of ethane to ethylene in the presence of CO2 as a soft oxidant. Cr6<sup>+</sup> (or Cr5<sup>+</sup>) sites are reduced to Cr3<sup>+</sup> as ethane is dehydrogenated. Then, the reduced Cr3<sup>+</sup> sites are re-oxidized by carbon dioxide treatment. Mimura et al. reported that the highly active Cr-based catalysts had Cr6<sup>+</sup> or Cr5<sup>+</sup> species on the surface of the catalyst [79]. Apart from Cr-supported catalysts, several other effective catalysts have been used in research on ethane oxidative dehydrogenation. Among these, the Ni-Nb-mixed oxide catalyst performed very well at relatively low temperatures [80–82]. Additionally, a TiO2-based Ga catalyst proved applicable for oxidative dehydrogenation with CO2 [83].


**Table 8.** Catalytic activity for the dehydrogenation of ethane (Reproduced from [73]; copyright (2008), Springer (Berlin, Germany)).

Reaction conditions: GHSV = 3600 mL/g h, T = 700 ◦C.

**Table 9.** Influence of CO2 on oxidative dehydrogenation of ethane.


Step-1

$$\text{CH}\_3\text{C}\text{\---\text{CH}}\_3 + \text{CrO}\_3 \rightleftharpoons \text{H}\_2\text{C=CH}\_2 + \text{H}\_2\text{O} + \text{CrO}\_{\text{x-1}} \text{ (oxidation development)}$$

Step-2

$$\text{H}\_3\text{C}-\text{CH}\_3 \rightleftharpoons \text{H}\_2\text{C=CH}\_2 + \text{H}\_2 \text{ (Simple depletion)}$$

$$\text{CrO}\_3 + \text{H}\_3\text{C}-\text{CH}\_3 \rightleftharpoons \text{CH}\_4 + \text{C} + \text{H}\_2\text{O} + \text{CrO}\_{x-1} \text{ (methane and coke formation)}$$

$$\text{H}\_3\text{C}-\text{CH}\_3 + \text{H}\_2 \rightleftharpoons 2\text{CH}\_4 \text{ (hydrogenaching)}$$

Step-3

CrOx−<sup>1</sup> <sup>+</sup>CO2 ֎ CrOx <sup>+</sup> CO (reoxidizing)

C + CO2 ֎ 2CO
