Catalytic properties of oxygen semipermeable perovskite-type ceramic membrane materials for oxidative coupling of methane

The catalytic properties for the oxidative coupling of methane (OCM) of La_0.8Sr_0.2CoO₃ (LSC) and SrCo_0.8Fe_0.2O₃ (SCF) in solid solution were studied and compared with those of 5 wt% Li/MgO, using a steady/unsteady state packed-bed reactor and a transient microbalance. The results of the steady-state cofeed experiments show that LSC possesses OCM catalytic properties similar to those of Li/MgO in terms of C₂ yield and selectivity at temperatures of around 800°C. The former gives a larger C₂ space time yield than the latter. SCF exhibits poor OCM catalytic properties at 700-850°C. To further examine the suitability of LSC as a membrane material for use in a dense membrane reactor for OCM, the instant OCM selectivity and activity and oxygen consumption rate for LSC and 5% Li/MgO on exposure to pure methane in cyclic feed mode were measured respectively at 850°C and 800°C. For both materials, the unsteady-state cyclic feed operation gives smaller initial OCM activity and larger initial C₂ selectivity than the cofeed steady state operation. Li/MgO quickly loses its OCM activity and selectivity in the unsteady state operation due to rapid consumption of the active sites. Up to 5 min of methane run time, LSC maintains appreciable OCM activity with poorer C₂ selectivity as compared to the steady state cofeed operation. The surface of LSC membrane allow oxygen partial pressure may become nonselective for OCM in membrane reactor applications.