*3.4. Role of CO2 on Dehydrogenation of Propane*

Propylene is the most prominent raw material in the chemical industries. It is primarily manufactured by steam cracking and propane oxidative dehydrogenation [104–106]. Oxidative dehydrogenation (ODH) is preferred due to its low energy requirements and lack of thermodynamic limitations [107,108]. However, the ODH reaction with O2 occurs under potentially flammable conditions and forms of carbon oxides due to over-oxidation with low selectivity [109,110]. This complication can be resolved using CO2 as a mild, safer oxidant. Thus, this reaction is a favorable example of CO2 utilization. Interestingly, CO2 was used as a mild oxidant to shift the equilibrium more toward the products, as well as enhance the dehydrogenation over the coupling between propane oxidative dehydrogenation to propylene and the reverse water gas shift (RWGS) reaction [111–113]. Dehydrogenation of propane occurred on the acid site of the catalyst. The SiO2/Al2O3 proportion is critical in determining both the catalyst physicochemical properties and its reactivity characteristics [114–117]. The HZSM-5, SBA-15, MCM-41, SBA-1 catalyst which is a two-dimensional microchannel system, has been used in the oxidative dehydrogenation of alkanes especially for the conversion of methane to propane in the existence of CO2. Various research groups have reported

on the influence of catalyst acidity in the oxidative dehydrogenation reaction with CO2. The activity of the zeolites decreased with increasing Si/Al proportion in HZSM-5 based Ga2O3, although the selectivity increased, as shown is in Figure 7 [118]. Lewis acidity is present in the metal oxide (Ga2O3) catalyst, while Bronsted acidity is present in HZSM-5. Thus, extracting the aluminum from HZSM-5 declines the Bronsted acidity more than it decreases the Lewis acidity. Several transition metals, such as vanadium, molybdenum, and chromium, have been used to support catalysts for ODH of light alkanes including propane [105,112,119,120]. Among these, chromium oxide provided high catalytic performance with CO2, despite fractional deactivation by coke production. Chromium oxide enhanced propane conversion and the propylene selectivity by expelling H2 produced in the ODH reaction [112]. The catalytic performance of Cr-supported catalysts was observed by the character of chromium categories on the support surface of the catalysts [121–125] Park et al. found that different Cr doping of Cr-TUD-1 catalysts (3, 5, 7 and 9 wt.%) with soft oxidant (CO2) were formed by MW irradiation and investigated the propane oxidative dehydrogenation [126]. The effect of reaction temperature on the oxidative dehydrogenation of propane in the existence of CO2 as a mild oxidant over the Cr-TUD-1 catalyst (7 wt.%) was investigated thoroughly to improve the catalytic activity. The conversion of CO2 was 3.5% at 550 ◦C and improved to 5.5% at 650 ◦C. To demonstrate the importance of CO2 in the propane oxidative dehydrogenation on Cr-TUD-1 catalysts, the process was carried out at 550 ◦C on 7 wt.% catalyst under the same conditions in the presence of CO2 as well as He. The decline in the catalytic activity of the catalyst with helium may be due to coke production and the reduction of the Cr groups on the surface of the zeolite. The proposed mechanism of propane oxidative dehydrogenation over metal oxide surfaces with the CO2 stream is shown below [112]:

A weak exclusive propane adsorption on the lattice oxygen

$$\rm C\_3H\_8 + O^\* \to C\_3H\_8O^\* \tag{2}$$

C-H schism via H-abstraction from propane utilizing an abutting lattice oxygen

$$\rm C\_3H\_8O^\* + O^\* \to \rm C\_3H\_7O^\* \tag{3}$$

Propylene desorption by hybrid expulsion from adsorbed alkoxide groups

$$\rm C\_3H\_7O^\* \to C\_3H\_6 + OH^\* \tag{4}$$

Reconsolidation of OH groups to produce H2O, reduced metal center (\*)

$$\rm OH^\* + OH^\* \rightarrow H\_2O + O^\* + \* \tag{5}$$

Re-oxidation of abridged M-centers by separating chemisorptions of CO2

$$2\text{CO}\_2 + \* + \* \to 2\text{CO} + 2\text{O}^\*\tag{6}$$

To evaluate the deactivation of the catalyst by coke creation and the enhancement of CO2, (Equation (7)) can be used as the deactivation parameter:

$$\begin{aligned} \text{Deactivation parameter (\%)} &= \text{Conversion of propane (initial amount} - \text{final amount)} \prime \\ &\quad (\text{initial amount}) \, \* \, 100 \end{aligned} \tag{7}$$

The rate of Cr degradation by H2 liberated from dehydrogenation is faster than the rate at which CO2 re-oxidizes the degraded Cr species, resulting in catalytic deactivation. Selective adsorption properties can be improved by surface functional groups on activated carbons. Thus, surface treatment of activated carbon may result in more selective and efficient adsorption of the gas, liquid and the alleviation of pollution [127].

**Figure 7.** Influence of Si/Al proportion on the efficiency of (**a**) Ga2O3/ZSM-48 zeolites (Reproduced from [118]; copyright (2012), Elsevier), (**b**) ZnO-HZSM-5 zeolites in the oxidative dehydrogenation of propane along with CO2 (Reproduced from [105]; copyright (2009), Elsevier), (**c**) Ga2O3/M-HZSM-5 zeolites in the absence of CO2 (Reproduced from [118]; copyright (2012), Elsevier), (**d**) Influence of Cr substance on the effectiveness of Cr/SBA-15 in the carriage of CO2 (Reproduced from [128]; copyright (2012), Elsevier).

#### **4. Conclusions**

This review article has comprised a number of CO2 conversions, which are still in the research scale. These promising technologies are mitigating the continuously increasing atmospheric CO2 concentration. Among the methods employing CO2, the ethyl benzene ODH process has seen significant progress. Currently, most of the ethylbenzene dehydrogenation plants apply the oxidative dehydrogenation method, which leads to large heat losses upon compression at the gas–liquid separator. Further, this reaction is thermodynamically restrictive and energy intensive. Several industrial companies such as SABIC (Saudi Basic Industry Corporation, Saudi Arabia), Samsung General Co. in south Korea have tested the catalytic consummation for this method. The commercial implementation of such a process may support the economics of styrene monomer production. According to European Rubber Journal (ERJ), Asahi Kasei Chemical Company 's (Japan) 6th generation SBR (Styrene-butadiene rubber) is currently being tested by many customers in the world with positive feedback and company is planning to commercialize some grades in 2021. Moreover, Trinseo's highly functionalized SPRINTANTM 918S Solution-Styrene Butadiene Rubber (S-SBR) has awarded second position in the elastomers for sustainability initiative of the European Rubber Journal. Based on lab indicator data confirmed by tire customers, grade 918S (compared to non-functionalized high-grip SSBR) improves fuel efficiency of the whole car approximately 1.5%. Considering in Europe alone, the benefit of this increased fuel efficiency would translate in approximately 540 tons less fuel consumed or a reduction of CO2 emissions by 1.3 million tons.

Several methods using CO2 as a mild oxidant have appeared in the technology sector. It is a long-term goal and alluring dream for chemical engineers to establish commercial industries based on the utilization of CO2. Challenges for the commercial utilization of this technology include the process rate required to ensure CO2 conversion with low coke deposition, the need to decrease energy expenditure, and the need for improved catalysts offering higher conversion. Despite the challenges, there is great room for catalyst improvement in these sectors. Recently, the carbon XPRIZE is a \$20 million competition to capture and CO2 conversion which is jointly funded by COSIA (Canada's Oil Sends Innovation Alliance) [129]. Most of the countries' governments are concern about climate changes with a high priority. China, the world's largest energy consumer and carbon emitter, announced USD 360 billion in renewable energy investments by 2020 to reduce carbon emissions [130]. Canada has implemented federally a carbon pricing policy with a current tax of USD 10/ton CO2 and a steady rise to USD 50/ton CO2 nationwide by 2022. However, the positive effects of CO2 in benzene hydroxylation over commercial and hierarchical zeolites in the liquid phase as well as the gas phase are under investigation by our group, wherein the byproducts are various aromatic compounds. The recycling of CO2 from the atmosphere to fuels, chemicals will lead to a real sustainable future for humanity. We expect that the use of CO2 as a promoter and as a mild oxidant at the laboratory level can be translated to the industrial scale in the future, thus contributing also to the world economy.

**Author Contributions:** S.T.R.: Writing original draft; J.-R.C.: Editing; J.-H.L.: Editing; S.-J.P.: Writing review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program-Development of technology on materials and components) (20010881, Development of ACF for rigid (COG)/ flexible (COP) and secured mass production by developing core material technology for localizing latent hardener for low temperature fast curing) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea) and supported by Korea Evaluation institute of Industrial Technology (KEIT) through the Carbon Cluster Construction project [10083586, Development of petroleum based graphite fibers with ultra-high thermal conductivity] funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

**Conflicts of Interest:** The authors declare that they have no conflicts of interest.

## **References**


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