In search of membrane-catalyst materials for oxidative coupling of methane: Performance and phase stability studies of gadolinium-doped barium cerate and the impact of Zr doping

Abstract

Oxidative coupling of methane (OCM) is a promising technology for the direct conversion of methane to ethylene and ethane (C2). This process is yet to be commercialized due its poor yield reflected in the formation of undesired products such as CO and CO2 (COx) as methane conversion increases, particularly in conventional packed bed reactors (PBRs). It has been argued that by applying O2− conducting membrane reactors that distribute the oxygen feed, the selectivity to the C2 products can be increased. A practical design for these membrane reactors would include combining a selective catalyst, preferably O2− conducting, with an O2− conducting membrane. In this work, we studied an O2− conducting material, gadolinium-doped barium cerate (BaCe0.8Gd0.2O3-δ or BCG), to evaluate its potential applicability as a catalyst and membrane in OCM membrane reactors. From PBR tests, we found that this material was active for OCM, and achieved a maximum C2+ yield of ∼14% at 1023 K. Furthermore, at low oxygen partial pressures, a C2+ selectivity of ∼90% was obtained at methane conversions of ∼3%. Although the C2+ yield from this material was stable over 48 h on stream at high methane conversions, X-ray diffraction data showed that the BCG perovskite phase, which is required for its conductive (membrane) properties, decomposes into BaCO3, CeO2 and Gd2O3 like phases, due to reactions with CO2. We showed that doping BCG with Zr was effective at suppressing the phase instability in OCM without significantly affecting the C2+ yields.