The structural properties of Mn-Na2WO4/SiO2-type catalysts and the reaction mechanism for controlling the oxidative coupling of methane (OCM) have been studied since 1980s but the roles of Na, Mn, and W, and their synergistic effects in the catalytic system are not understood in great detail. In this work, the role of each component in oxygen and methane activation, as well as selective and nonselective reaction pathways, have been investigated using transient kinetic techniques. Of the prepared samples listed in Figure 1A, Mn/SiO2 showed the lowest oxygen activation energy, Ea(O2), which demonstrates the important role of Mn in promoting oxygen activation. The result of CH4/D2 co-pulsing experiments suggests that both CH3 and CH2 are the CH4 activation intermediate products (Figure 1B). Monoxo species were measured as the main oxygen species. Na and W alone do not provide active sites for oxygen and methane activation. The synergistic effect between Na and W was observed on the 5%Na2WO4/SiO2 sample in both lowering the Ea(O2) and more active towards CH4 activation. Samples with Mn loading onto Na2WO4/SiO2 further lowered the observed Ea(O2). Moreover, the activity of CH4 activation was also clearly enhanced. The result of CH4/D2 co-pulsing experiments suggests that CH3 is the main CH4 activation product (Figure 1B). The dioxo species was predominant over samples of Mn-Na2WO4/SiO2 and 5%Na2WO4/SiO2. By combining the results of Ea(O2), methane activation intermediates, and types of surface oxygen species, it is suggested that the dioxo species were the active oxygen species for OCM reaction leading to C2+3 formation. Steady state studies were found to support the conclusions derived from transient kinetic studies. From both methods we conclude that new active sites were formed by combination of Na and W which were responsible for both of methane activation and oxygen activation. Impregnation of Mn into Na2WO4/SiO2 further lowered the Ea(O2) and provided more active and selective sites for OCM reaction.

Mechanistic understanding the role of catalyst components in the Mn-Na2WO4/SiO2 system for oxidative coupling of methane

ACS Spring Meeting 2021

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