VANADIUM MIXED-OXIDE CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE

The improvement of methane conversion and C2+ selectivity for the oxidative coupling of methane has been pursued by evaluating a series of new polyfunctional oxide catalysts. The catalysts included: (i) a transition metal oxide, (ii) an alkaline earth metal oxide (CaO or MgO), and (iii) an inert oxide support (Al2O3, SiO2, TiO2, or ZrO2) to stabilize the active phases. Vanadium oxide was chosen from the set of transition metal oxides having a standard free energy change lower than zero for the reaction methane+metal oxide→ 1 2 ethene+water+reduced metal oxide. The molar composition of the catalysts was the following: 68Mg(Ca)/2V/30Al(Ti,Si,Zr). Several catalysts were tested before and after alkali impregnation (lithium with MgO and sodium with CaO). Impregnation of alkali caused an increase in the methane conversion in the CaO-based catalysts. The catalysts were examined by electron spin resonance and UV-visible spectroscopy to find relationships between vanadium species and catalytic activity. The spectroscopic analyses did not show a correlation between VIV species and catalytic activity, whereas the (V = O)3+ species seems to play an important role in the reaction.