(453d) New Insights into the Effect of CO2 on Oxidative Coupling of Methane (OCM)

CO₂ being a soft oxidant and one of the major side products in oxidative coupling of methane (OCM), its impact on OCM is of great interest. Up to now, significant efforts and progresses have been made in this respect, however, no consensus has yet been reached. On the contrary, the effect of CO₂ on OCM appears to be diverse and catalyst-dependent^1,2. In this work, new insights into the effect of CO₂ on OCM have been acquired based on experimental investigations.

The reaction performances exhibited by two benchmark OCM catalysts, i.e. La-Sr/CaO and NaMnW/SiO₂, were investigated in both O₂-rich and O₂-lean conditions. Specifically, in O₂-rich conditions CO₂ was added by substituting inert balance gas while in O₂-lean conditions O₂ was substituted by equal amount of CO₂. In addition, a gas mixture with equimolar amount of C₂H₆ and CO₂ was also fed in the attempt to study the occurrence of CO₂ assisted oxidative dehydrogenation of ethane (CO₂-ODH) on the OCM catalysts.

The catalyst-testing results in O₂-rich conditions are shown in figure 1. Overall, CO₂ co-feeding has a detrimental impact on both catalysts in terms of C₂ selectivity. This effect is more pronounced on La-Sr/CaO, which is deactivated already by 8% CO₂. In contrast, NaMnW/SiO₂ has a higher tolerance to CO₂ up to 20%. Contrary to the observations in O₂-rich conditions, a positive effect of CO₂ has been observed in O₂-lean conditions. Compared with the reference test without co-feeding CO₂, the selectivity towards C₂H₄ increases, and this effect becomes more significant with the CO₂ fraction. The increase in C₂H₄ selectivity is always achieved at the expense of C₂H₆ selectivity, and is accompanied by a decrease in CO₂ selectivity. Seemingly, the addition of CO₂ in O₂-lean conditions promotes the dehydrogenation of C₂H₆ to C₂H₄. The results obtained by feeding a CO₂-C₂H₆ mixture are summarized in table 1. In comparison with the blank tests, the conversion of C₂H₆ and CO₂ are evidently increased in the presence of a catalyst, verifying the occurrence of CO₂-ODH on the OCM catalysts.

Based on the experimental investigations, a mechanism of the CO₂ effect on OCM is proposed. Firstly, CO₂ as a soft oxidant competes for the active sites with O₂ and produces a new type of oxygen surface species [O*]’. Different from the oxygen surface species [O*] generated by O₂ chemisorption, [O*]’ is less active but is capable of triggering CO₂-ODH. In O₂-rich conditions, the competition between O₂ and CO₂ makes O₂ less consumed in catalytic reactions, which lowers the catalyst activity and indirectly intensifies the gas-phase oxidation of C₂ species eventually leading to the decrease of C₂ yield. Therefore in this condition, CO₂ only exhibits negative effect. On the other hand, in O₂-lean conditions, the low O₂ fraction makes it possible to have OCM and CO₂-ODH taking place in parallel. The [O*]’ promotes the conversion of C₂H₆ to C₂H₄ and prevents C₂H₆ from being further oxidized into CO₂. In the meantime, with lower O₂ fraction, the C₂ species also have higher chance to survive from the gas-phase oxidation. Consequently, positive effect of CO₂ can be observed in the O₂-lean condition.

With the investigations conducted in this work, it is concluded that the effect of CO₂ is not only catalyst-dependent but also conditions-dependent.

Reference

1. Yide, X., Lin, Y. & Xiexian, G. Effect of basicity and adding CO₂ in the feed on the oxidative coupling of methane over K₂O and SrO promoted La₂O₃/ZnO catalysts. Appl. Catal. A Gen. 164, 47–57 (1997).
2. Shi, J., Yao, L. & Hu, C. Effect of CO₂ on the structural variation of Na₂WO₄/Mn/SiO₂ catalyst for oxidative coupling of methane to ethylene. J. Energy Chem. 24, 394–400 (2015).