Tetragonal LaOBr is a better catalyst than La2O3 for the oxidative coupling of methane (OCM), whereas BaCO3 is a good promoter for LaOBr. With contact time equal to 0.2 g s ml-1 and a CH4:O2:N2 ratio of 1:1:12, a C2 yield of 26.3% with a C2H4/C2H6 ratio of 4.3 could be achieved over the 50 mol% BaCO3/LaOBr catalyst at 800°C. When the CH4:O2:N2 ratio was 0.5:1:12 and the contact time was 0.6 g s ml-1, the C2 yield became 18.9% and C2H4 was the sole C2 product. Fixing the CH4:O2:N2 ratio to 2.6:1:12 and contact time to 0.6 g s ml-1, the C2H4/C2H6 ratios over the LaOF, LaOBr, and 30 mol% BaCO3/LaOBr catalysts were, respectively, 1.6, 3.3, and 11.3 at 800°C. Generally speaking, the performance of BaCO3/LaOBr catalysts may be considered quite competitive, especially from the point of view of high ethylene production from methane. Detail XRD investigations reveal that new phases such as La2O3, La(OH)3, and Ba3Br4CO3 were generated in the BaCO3/LaOBr catalysts. In situ Raman studies showed that the LaOBr and 10 mol% BaCO3/LaOBr catalysts were capable of forming O22- and O2n (1 < n < 2) species during OCM reaction, while LaOF could form O2n- and O2- species. We suggest that for a mixed catalyst like BaCO3/LaOBr, nonstochiometric behavior is expected and defects such as O centers and trapped electrons which would bring about the generation of surface dioxygen species could be present within the constituent structures or at the interfaces. We suggest that the direct interaction of CH4 with O22- is an efficient way of producing carbene, and the coupling of carbene, rather than methyl radical, could be the major step for C2H4 generation over these highly C2H4 selective BaCO3/LaOBr catalysts.
Scopus Subject Areas
- Physical and Theoretical Chemistry