The making and characterization of BaO- and BaCl₂-promoted Y₂O₃ catalysts for the OCM reaction

The performances of the 30 mol% MO/Y₂O₃ (M = Mg, Ca, Sr, Ba) catalysts in OCM reactions have been investigated. The BaO-promoted one has out-performed the others. We found that the 30 mol% BaX₂/Y₂O₃ (X=F, Cl, Br) catalysts could do even better. Both BaF₂ and BaCl₂ are good materials to promote Y₂O₃. At 800°C and with CH₄:O₂:N₂ = 2.47:1:11.4, total flow rate = 50 ml min^-1, contact time = 0.6 g s ml^-1, and weight of catalyst = 0.5 g, the C₂ yields over the catalysts were respectively 19.7 and 22.3%. In this paper, we concentrate on the BaCl₂/Y₂O₃ catalysts and compare them with the BaO/Y₂O₃ ones. It is obvious that the 30 mol% BaCl₂/Y₂O₃ catalyst is superior to the 30 mol% BaO/Y₂O₃ one both in C₂ selectivity and C₂H₄/C₂H₆ ratio. XRD examinations revealed that the addition of 10-50 mol% of BaO or BaCl₂ has greatly altered the surface and bulk natures of Y₂O₃. Crystal phase of orthorhombic BaCO₃ was found to exist in the 30 mol% BaO/Y₂O₃ catalyst. Although no significant amount of any barium compound was detected over the fresh 30 mol% BaCl₂/Y₂O₃ catalyst, crystal phases of orthorhombic Ba₃Cl₄CO₃ and BaCO₃ were detected after OCM reactions. Also, the cubic Y₂O₃ lattices of the BaO- and BaCl₂-promoted catalysts were found to have undergone expansion which is a result of ionic substitution(s). The reactivity of the 30 mol% BaCl₂/Y₂O₃ catalyst has been monitored for 40 h at 750°C and found to be stable. Although XPS results suggested no significant change in surface chlorine concentration, analysis of Cl content of the catalyst revealed that up to ca 15.5% of the original content of chlorine was lost after 20 h of reaction. Y₂O₃ by itself lacks active oxygen for the oxidation reaction. O₂ TPD and reduction studies of the fresh 30 mol% BaO- and BaCl₂-promoted catalysts showed that the amount of active oxygen in the former was at least 4 times that in the latter. However, during OCM reaction, the 30 mol% BaCl₂/Y₂O₃ catalyst was changing somewhat like a 30 mol% BaO/Y₂O₃ catalyst. It is apparent that the presence of Cl^- ions in the 30 mol% BaCl₂/Y₂O₃ catalyst is the reason for the superiority in C₂ selectivity over the BaCl₂-promoted Y₂O₃ catalysts. We conclude that the presence of Ba²⁺ ions can enhance the amount of oxidative oxygen for CH₄ activation while the presence of Cl^- suppresses deep oxidation. A careful balance of the two can optimize the performance of the Y-Ba-Cl catalysts in the OCM reaction.