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

摘要

Binary oxides, mainly Ca-Mn, Sr-Mn and Ba-Mn, have been studied as catalysts for the coupling of CH4 to C-2 hydrocarbons (C2H6 and C2H4) using CO2 as oxidant. At temperatures of greater than or equal to 840 degreesC, 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 degreesC increase remarkably with increasing partial pressure of CO2, 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 degreesC, CH4 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 degreesC changes only slightly with partial pressure of CO2, and the activation energies are constant over the whole temperature range examined and notably lower. Characterizations reveal that solid solution of Ca0.48Mn0.52O is the main phase for the Ca-Mn catalyst after reaction at 850 degreesC, while, at 800 degreesC, some Ca2+ ions separate from the solid solution to form CaCO3, 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, SrCO3 and BaCO3 are formed along with MnO after reaction, and the carbonates are suggested to react with MnO to form SrMnO2.5 and BaMnO2.5 in the conversion process of CH4 with CO2. The mechanism for C-2 formation involving SrMnO2.5 and BaMnO2.5 as intermediates is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.