*2.1. Influence of CO2 on Oxidation of Cyclohexene*

The impact of CO2, at various concentrations, was investigated on the oxidation of cyclohexene which is a small and symmetric molecule, similar to many starting compounds in chemical synthesis (Scheme 2) [30]. The results revealed that O2/CO2 conversion (%) was higher than O2/N2 conversion (%) rate. However, at a gas ratio of 0.066 O2:CO2/N2 (Table 1, entry 1), cyclohexene was not converted. Park et al. revealed the positive impact of carbon dioxide on mesoporous metal-free oxidation carbon nitride (MCN) catalysts [31]. These mesoporous MCN elements exhibit oxygen-carrying capabilities which are effective sites for oxidation. Additionally, the large nitrogen quantity in the CN matrix acts as a CO2-philic exterior for the incitation of CO2. Molecular oxygen promotes this synergy, allowing for the oxidation of cyclic olefins and improving the conversion of cyclic olefins with better selectivity. In-between the conversion of the O2/CO2 and the O2/N2, Park et al. observed the enhancive performance as a premier time, which can be expressed as ΔC (%) and can be calculated using the Equation (1):

$$
\Delta\text{C}(\%) = \frac{\left(\text{C}\_{\text{O}\_2/\text{CO}\_2}\right) - \left(\text{C}\_{\text{O}\_2/\text{N}\_2}\right)}{\left(\text{C}\_{\text{O}\_2/\text{CO}\_2}\right) + \left(\text{C}\_{\text{O}\_2/\text{N}\_2}\right)} \times 100\tag{1}
$$

where,

$$\text{H}\_{\text{(}C\_{\text{2}/C\text{O}\_{2})}} = \text{Conversion in O}\_{2}/\text{CO}\_{2} \text{ and} \left(\text{C}\_{\text{O}\_{2}/\text{N}\_{2}}\right) = \text{Conversion in O}\_{2}/\text{N}\_{2}$$

**Scheme 2.** Cyclohexene oxidation reaction over catalyst. (Redrawn from [30]; copyright (2018), WILEY-VCH). (**A**) = 2-cyclohexene-1-one, (**B**) = cyclohexene oxide, (**C**) = 2-cyclohexene-1-ol, (**D**) = 2-cyclohexene-1-hydroperoxide). Reaction conditions: 10 bar O2; 2.5 mL cyclohexene; 0.5 mL cyclohexane(IS); 10 mg catalyst; 15 mL MeCN; stirred in an autoclave (1000 rpm); 70 ◦C; 16 h.


**Table 1.** Effect of the CO2 on oxidation of cyclohexene over MCN Ref [31] (Reproduced from [31]; copyright (2011), Royal Society of Chemistry).

Reaction conditions: 20 mg Melamine mesoporous carbon nitride (M-MCN), 10 mL Dimethylformamide (DMF), temperature 373 K, Pressure 80 PSI, time 10 h; Estimated by Gas Chromatography (GC) analysis. <sup>a</sup> PSI = Pounds per Square Inch, <sup>b</sup> Conversion (%) of cyclic olefin.

The efficiency of CO2 in the oxidation of cyclohexene at varying CO2 concentrations is shown in Table 1. Higher conversions were achieved by the O2/CO2 system. The results showed that the conversion of cyclohexene was nothing at a content of 0.066 O2 (entry 1). This is Possibly due to the low frictional pressure of O2, which is deficient to drive the reaction. Further, the ΔC% value was higher for higher concentrations of CO2. No meaningful change of ΔC% was demonstrated for gas ratios beyond 0.333 in the catalytic process, demonstrating the impregnation of activity.

CO2 has been used with metal-supported systems that were observed to produce a per-oxycarbonate species which are highly active in oxidation reactions. Aresta et al. were reported the composition of a metal per-oxycarbonate species, as determined by spectroscopic analysis [32]. A process for the production of per ox-carbonate has acceded in Scheme 3. Park et al. investigated the oxidation of alkyl aromatics via an EPR analysis using a metal carbonate catalyst. They demonstrated the production of metal per-oxycarbonate groups in the presence of carbon dioxide by the hyperfine cracking of manganese. Yoo et al. [20] observed the production of per-oxycarbonate on Fe/Mo/DBH (deboronated borosilicate molecular sieve); the production of per-oxycarbonate is illustrated in Figure 1. All of the catalytic schemes discussed above involve transitional metal catalysts and CO2 coupled with oxygen. The resulting enhancement over traditional metal oxide systems in O2/CO2 mixtures may occur because of an oxygen exchange between O2 and CO2, which would increase the rate of the reaction. During isotope-labeling studies, these types of exchanges have been detected by Iwata et al. [33] using different metal oxide structures.

**Scheme 3.** Per-oxycarbonate production reaction mechanisms (Redrawn from [32]; copyright (1996), American Chemical Society).

**Figure 1.** Per ox-carbonate over Fe/Mo/DBH in the O2/CO2 system (Reproduced from [20]; copyright (1993), Elsevier (Amsterdam, The Netherlands)).
