Catalytic performance of Na-Mn2O3-based catalysts towards oxidative coupling of methane

Various active materials (NaCl and Na2WO4 doped on Mn2O3) and supports (SiC, SiO2, WO4, CoO, La2O3, Ce2O3, ZnO, Al2O3, Cr2O3, La0.3Sr0.7Cr0.7Fe0.3O3-delta (LSCF), and clinker) were prepared and tested for use in the OCM reaction in a packed-bed reactor. NaCl-Mn2O3/SiO2 showed the best performance among the catalysts, with 37.5 % methane conversion, 60.5 % C-2 selectivity, and 22.7 C-2 yield at an optimum temperature of 750 degrees C under atmospheric pressure. The ethylene-to-ethane ratio of NaCl-Mn2O3/SiO2 (7.5) was found to be the highest, and higher than that of Na2WO4-Mn2O3/SiO2 (1.4), possibly due to its high basicity. Na2WO4-Mn2O3/SiO2 showed good catalytic stability during 12 h time-on-stream experiment whereas slight deactivation was observed in NaCl-Mn2O3/SiO2 due to the decomposition of NaCl, evidenced from traces of gaseous chloride compounds such as CH3CH2Cl, CH3Cl, and Cl-2 in the outlet stream. However, the degree of deactivation was found to be more severe when (1) level of NaCl was lower (from 60 % to 10 %) and (2) time-on-stream experiment was tested for the longer period of time (12 h-62 h). The loss of chloride was estimated at 57 and 69 % (by weight) after 12 and 62 h TOS, respectively. The results were in good agreement with information received from XRD where Na2SiO3 was observed due to the loss of chloride, when the deactivation was extreme. From O-2-TPSR result, Na2WO4Mn2O3/SiO2 showed no coke deposition, which could be the result of the high surface oxygen (Os) contained in Na2WO4. Carbon formation at 56.36 mu mol/g.hr was detected when using NaCl-Mn2O3/SiO2 as catalyst. The solid-gas mechanism of the catalytic OCM reaction was predicted using the CH4-TPSR technique.