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

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 W-0 and from Me4+/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 (C-2 hydrocarbons, CO, CO2, water, H-2) 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 degrees C and replenished at much lower temperatures. Its amount (similar to 16 mu mol O-2/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 O-2/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 C-2 hydrocarbons and its lifetime at typical OCM temperatures (around 800 degrees C) exceeds by far the characteristic time of the catalytic reaction. (C) 2016 Elsevier B.V. All rights reserved.