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.
Scopus Subject Areas
- Physical and Theoretical Chemistry