Oxidative coupling of methane: the role of solid state chemistry

Abstract

Oxidative coupling of methane by a mixed manganesemagnesium oxide is governed in part by the solid state structure and chemistry.

Conversion of Methane by Oxidative Coupling

“…The Mn/MgO system, which more properly falls within a discussion of the transition metals, is highly effective. In the solid-state domains of Mg 6 MnO K other phases are formed, and appear to be the active components [12]. These results suggest that other such mixed metal oxide phases would be worth the significant effort required for their study as catalysts.…”

“…In the cyclic mode of operation, Mn/MgO was found to be effective, with C0 2 and C 2 H ft predominating at low conversions, but C 2 H 4 and C 3+ at high conversions [11]. Na 2 MgSi0 4 is an effective promoter [12], and Mg 6 MnO s has been identified in the catalyst and may be the effective component [12], Molten salts are formed under oxidative coupling conditions (1072-1203 K, oxygen/methane = 50-90%) using MnS0 4 /Li 2 S0 4 , MnS0 4 / Na 2 S0 4 , NaMnOVLbB.oOK,, NaMn0 4 /Na 2 Si 2 0 5 , NaMn0 4 /Li 3 P0 4 , or NaMn0 4 /Na 2 B 2 0 7 (each 1:1). Selectivity to C 2 varied with both catalyst composition and methane conversion, from 13% to 57%.…”

“…As a result, the coupling reaction is not steady -the conversion and selectivity change with time. Because of these features, only a few studies have employed the cyclic mode [6][7][8][9][10][11][12], mainly for the mechanistic study of the reaction. Some oxides, however, have been claimed in patents for the cyclic process of oxidative coupling [13].…”

“…Other alkaline earth oxides. Group 3 metal, transition metal, or posttransition metal oxide admixes, as supports or promoters, do not seem to be as effective as Li in promoting MgO, with the notable exceptions of Mn/MgO promoted with Na [ 12] and a mixed PbO-MgO catalyst [36]. For the latter system selectivity to C 2+ greater than 80% is attainable, but only at low oxygen/methane ratios.…”

A study of the kinetics of the oxidative coupling of methane over a Li/Sn/MgO catalyst

“…Labinger et al [17] have also found an increase in the reaction rate at a certain methane partial pressure for a Mn/MgO catalyst used in the cyclic reaction mode, analogous to the effect shown in Figure 1, but gave no explanation.…”