Mn-based binary oxides as catalysts for the conversion of methane to C2 hydrocarbons with carbon dioxide as oxidant

Binary oxides, mainly Ca-Mn, Sr-Mn and Ba-Mn, have been studied as catalysts for the coupling of CH 4 to C 2 hydrocarbons (C 2 H 6 and C 2 H 4 ) using CO 2 as oxidant. At temperatures of ≧840°C, the Ca-Mn catalyst exhibits quite similar performances to those of other Ca-containing binary oxide catalysts (Ca-Ce, Ca-Cr and Ca-Zn) reported previously; C 2 selectivity and yield at 850°C increase remarkably with increasing partial pressure of CO 2 , and apparent activation energies observed over these catalysts are roughly the same (190–220 kJ mol −1 ). When the temperature is decreased from 840 to 825°C, CH 4 conversion and C 2 selectivity over the Ca-Mn catalyst abruptly drop; here a discontinuous change also is seen in the Arrhenius plots. On the other hand, the Sr-Mn and Ba-Mn catalysts show different kinetic features from the Ca-Mn system; C 2 selectivity at 850°C changes only slightly with partial pressure of CO 2 , and the activation energies are constant over the whole temperature range examined and notably lower. Characterizations reveal that solid solution of Ca 0.48 Mn 0.52 O is the main phase for the Ca-Mn catalyst after reaction at 850°C, while, at 800°C, some Ca 2+ ions separate from the solid solution to form CaCO 3 , which covers the catalyst surface. Such a difference probably accounts for the discontinuous change in the catalytic behavior with temperature. With the Sr-Mn and Ba-Mn catalysts, SrCO 3 and BaCO 3 are formed along with MnO after reaction, and the carbonates are suggested to react with MnO to form SrMnO 2.5 and BaMnO 2.5 in the conversion process of CH 4 with CO 2 . The mechanism for C 2 formation involving SrMnO 2.5 and BaMnO 2.5 as intermediates is discussed.