Processing, stability and oxygen permeability of Sr(Fe, Al)O3-based ceramic membranes

Minor additions of alumina into perovskite-type SrFe0.7Al0.3O3−δ, a composition close to the solid solution formation limits in SrFe1−xAlxO3−δ system, result in decreasing thermal expansion and increasing oxygen permeability. The improved sinterability of SrFe0.7Al0.3O3-based composite with 3 wt.% Al2O3 addition enables to fabricate high-quality tubular membranes for the methane conversion reactors. No essential degradation in the performance of SrFe0.7Al0.3O3−δ membranes under air/CH4 or air/H2H2O gradients at 973-1223 K was observed during 200-700 h. The stable operation under high oxygen chemical potential gradients is possible due to surface-limited oxygen transport, indicated by the dependencies of oxygen permeability on the membrane thickness. Applying porous layers of the same composition, synthesized via cellulose-precursor technique, onto the permeate-side surface leads to substantially higher oxygen fluxes. For a model reactor with the surface-modified SrFe0.7Al0.3O3−δ membrane and commercial Ni/Al2O3 catalyst, the CH4 conversion rate achieved 90-97% at 1073-1123 K, when the CO selectivity was almost 100%.