*2.3. Influence of CO2 on Oxidation of p-Xylene*

It was proposed that in the O2-CO2 system, metal peroxy-carbonate groups assist as oxygen transfer promoters to the oxyphilic substrate. Aresta et al. [32] also reported that the presence of O-O bonds in Rh (η2-O2) complexes imply the accumulation of metal per-oxycarbonate during the other oxidation reaction. They demonstrated that CO2 promotes the oxidative ability of O2 over the RhCl(Pet2-Ph)3 catalyst. In the presence of CO2 over the metal-based structure was found to be formation of peroxycarbonate species which are more active than hydrogen peroxide in oxidation reaction [37]. Park et al. [22] reported the performance of carbon dioxide in the liquid-phase oxidation reaction of toluene, *p*-tolu-aldehyde, and *p*-xylene with O2 over an MC-based catalyst (Co/Mn/Br). The reaction rate, selectivity, and the conversion were all enhanced by the co-presence of CO2. This enhancement was attributed to the creation of per-oxycarbonate species, as determined by electron paramagnetic resonance (EPR) analysis of the reaction with and without carbon dioxide. A hyperfine manganese arrangement was noticed in the existence of CO2, confirming the formation of a per-oxycarbonate species.

Additionally, Park et al. observed the oxidation of different alkyl aromatics applying MC-supported catalysts [22]. Oxidations were carried out using O2 as the oxidant (with N2) and compared to reactions in the presence of both O2 and CO2. The conversion of *p*-xylene without CO2 (Table 3) was 57.2%, whereas the conversion of *p*-xylene was increased to 66.8% in the presence of CO2. Moreover, in O2/CO2, the yield of terephthalic acid was improved. The Amoco Chemical Research Laboratory studied the activation of CO2 in the gas-state of *p*-xylene oxidation to *p*-tolualdehyde and terephthaldehyde over the chemical vapor deposition (CVD) of Fe/Mo/DBH [20]. The oxidation reaction was performed in two feed streams varying compositions, including *p*-xylene with O2/N2/He and *p*-xylene with O2/N2/CO2. The catalytic activity is shown in both the feeds at various temperatures in Figure 2, as shown in the figure, *p*-xylene conversion in the existence of CO2 in O2 was greater than the absence of CO2 in O2. This improved conversion was connected to the production of per-oxycarbonate groups over the catalyst surface. Furthermore, in the existence of CO2, the secondary reactions also emerged more remarkable, possibly due to the acidity of the CO2 molecules adsorbed onto the DBH matrix. In comparison with O2 alone, the conversion of *p*-xylene was higher in the co-presence of CO2 at all temperatures (Figure 2). The O2/N2/CO2 feed system, resulted in a higher conversion of *p*-xylene and greater selectivity towards benzaldehyde at temperatures from 300 ◦C to 375 ◦C (Table 4). It was observed that no carbon dioxide was formed by the burning of *p*-xylene over the catalyst at 375 ◦C; however, in the O2/N2/He feed system, the formation of CO2 started (10.7%) at 300 ◦C and significantly increased (20.2%) at 375 ◦C. Thus, CO2 performed as a co-oxidant for the gas-phase *p*-xylene oxidation reaction with oxygen. Yoo et al. [20] also reported a significant enhancement in the conversion of *p*-xylene, *p*-ethyl toluene, and *o*-xylene in the presence of CO2 at varying temperatures.

Reaction conditions: Temperature 170 ◦C, time 3 h, Mesoporus carbon (MC) type catalyst with transition metal additive (Co/Mn/Br), Co 100 ppm, Mn 200 ppm, Br 300 ppm [38].

**Figure 2.** Promotional role of CO2 on Fe/Mo/DBH for the oxidation of *p*-xylene.

**Table 4.** Enhancive effect of CO2 on oxidation of *p*-xylene (Reproduced from [20]; copyright (1993), Elsevier). Reaction conditions: WHSV: 0.22 h<sup>−</sup>1, contact time: 0.21 s, gas flowrate: 400 sccm, Feed gas 1: 0.1% *p*-xylene, 1% O2, 1% N2 in He. Feed gas 2: 0.1% *p*-xylene, 1% O2, 1% N2 in commercial grade CO2.


<sup>a</sup> Feed gas 1: O2/N2/He, Feed gas 2: O2/N2/CO2.

## *2.4. Oxidation of p-Toluic Acid and p-Methyl-Anisole*

CO2 acts as a promoter in catalytic systems and as a co-oxidant with O2 resulting in improved reaction kinetics, more desirable product distributions, better selectivity, and higher conversion. Initially, Aresta et al. [32] reported that carbon dioxide enhanced the oxidative characteristics of dioxide in transition metal systems. Park et al. [38] studied the use of Co/Mn/Br catalysts in the fluid- phase oxidation of olefins. Interestingly, they observed the expansion effect of carbon dioxide on mesoporous carbon nitride (MCN) catalytic systems, whereas the CO2-promoted system was fabricated by them on the oxidation of alkyl-aromatics. In the presence of CO2, the conversion of *p*-toluic acid over the metal carbonate (MC) catalyst was increased by 12% (Table 5) compared to oxidation in O2 alone. Furthermore, the yield of terephthalic acid increased from 58.2% to 64.9%. These data demonstrate that the catalytic activity is significantly enhanced by CO2. Interestingly, over an MC-supported catalytic system, the main product of the oxidation of *p*-methyl-anisole is *p*-methoxy phenol (Table 6) along with a limited number of other products, such as *p*-anisaldehyde and *p*-anisic acid. However, the yield of *p*-anisaldehyde has increased the presence of CO2, again demonstrating the capacity of CO2 to sustain mono-oxygen transfer.

**Table 5.** *p*-toluic acid oxidation with CO2 on an MC-type catalyst (Reproduced from [22]; copyright (2012), Royal Society of Chemistry).

Reaction conditions: 6 mL *p*-toluic acid, 0.1183 g CoBr2, 0.1587 g Mn(OAc)2·4H2O in 24 mL HOAc, temperature 190 ◦C, time 3 h; PCO2 = 0-6 atm, PO2 = 2 atm [38].

**Table 6.** Performance of CO2 on oxidation of *p*-methylanisole (Reproduced from [22]; copyright (2012), Royal Society of Chemistry).

Reaction conditions: 43.5 mmol *p*-methylanisole, 0.6 mmol CoBr2, 0.6 mmol Co(OAc)2, 0.6 mmol Mn(OAc)2·4H2O in 30 g HOAc, total pressure 12 atm (PCO2 = 0-2 atm, PO2 = 2,3,6 atm, PN2 balance). temperature 120 ◦C, time 3 h [38].
