Methane activation on superacidic catalysts based on oxoanion modified zirconium oxide

A series of metal promoted catalysts was prepared by impregnation of metal precursor solutions on oxoanion modified zirconias (sulfated, molybdated and tungstated), well recognized, in the literature, for their ability in catalyzing the isomerization of light alkanes at low temperatures. Structural characterization and nature of surface active sites were attempted to explain the methane activation reaction on these catalysts. Although Lewis and Brönsted acid sites were observed on the oxoanion modified zirconias, as detected by infrared of pyridine adsorption, the methane activation occurred only when promoted with specific metals, such as Fe-Mn, Co-Mn, Al, Ni and Cu, for sulfated zirconias, and Ni for molybdated and tugstated zirconias. The results obtained suggest different kinds of active surface sites, depending on the oxoanion present and thus different mechanisms could be proposed for the methane activation reaction. It was suggested that on metal promoted sulfated zirconias, methane was converted into ethane by an acid catalyzed mechanism, more specifically, by SOH groups as proton donors. On NiO promoted molybdated and tungstated zirconias, methane was not simultaneously converted into ethane but only after an induction period. The XRD patterns of tugstated zirconias after the reaction with methane, showed only reflections at 2θ = 42.6°, 53.8° and 77.6°, characteristics of nickel on zero oxidation state. Although these reflections were absent for Ni promoted molybdated zirconias, these catalysts were black in color after reaction. TPR experiments using CH4 as reduction gas gave support to the proposal for a mechanism catalyzed by metal for the methane conversion into ethane on Ni promoted molybdated and tungstated zirconias.