Mn-based binary oxides as catalysts for the conversion of methane to C 2 hydrocarbons with carbon dioxide as oxidant

Abstract

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 2H 6 and C 2H 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 0.48Mn 0.52O 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.