Oxygen availability and catalytic performance of NaWMn/SiO2 mixed oxide and its components in oxidative coupling of methane

Abstract

Relationships between catalytic performance in oxidative coupling of methane (OCM) and properties of lattice oxygen of the mixed NaWMn/SiO2 oxide and its components are studied. It is demonstrated that in hydrogen and methane flows tungsten and manganese can be practically completely reduced from W6+ to W0 and from Mn4+/Mn3+ to Mn2+ states, respectively. Reduction in hydrogen proceeds at substantially lower temperatures. If the system is reduced in methane, the formation of methane oxidation products (C2 hydrocarbons, CO, CO2, water, H2) is observed, and the product distribution substantially changes at increasing degree of reduction. In addition to strongly-bonded oxygen which can be removed from the NaWMn/SiO2 system by reduction, a more weakly-bonded form can be detected using temperature programmed desorption (TPD). This form of oxygen can be reversibly removed at temperatures above 640°C and replenished at much lower temperatures. Its amount (∼16μmol O2/g) is about 4.5% of that removable by reduction in hydrogen and methane, or about 10% of the formal oxygen surface monolayer. Using the sequential O2/CH4 pulse technique, it is also shown that this weakly-bonded form of oxygen can participate in the steady-state catalytic OCM reaction because its presence substantially increases the rate of selective formation of C2 hydrocarbons and its lifetime at typical OCM temperatures (around 800°C) exceeds by far the characteristic time of the catalytic reaction.