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Ceria-based dual-phase membranes showing high oxygen permeation fluxes and stability under a CO 2 environment are promising materials for CO 2 capture via an oxyfuel route. The high oxygen permeation fluxes compared with other dual-phase membranes are derived from the mixed conducting properties of the perovskite oxides used in the dual-phase membranes.Climate change induced by the emission of CO 2 is attracting more and more attention worldwide. CO 2 , as the major greenhouse gas originating from the combustion of fossil fuels, is mainly produced during power generation.1 There are three routes for the capture of CO 2 in power plants: post-combustion, pre-combustion and oxyfuel processes.2 Mixed conducting membranes, which are usually perovskite oxides, can transport oxygen with 100% selectivity and can be integrated with oxyfuel technology for CO 2 capture with great improvement in efficiency.3 This concept is illustrated in Fig. 1. Here, fossil fuels combust with the permeated oxygen carried by CO 2 , and part of the CO 2 is recycled as a sweeping gas for the oxygen separation process, whereas the remaining CO 2 is captured and sequestered after steam condensation. Mixed conducting membranes have been investigated for oxygen separation 4 and as membrane reactors for catalytic conversion of light hydrocarbons to high-added-value chemicals.5 Such membranes can be integrated into an oxyfuel process if they are stable enough under a CO 2 atmosphere at elevated temperatures. However, few membranes can sustain their crystal structure under such harsh working conditions because membranes with the perovskitetype structure usually are doped by large amounts (20-100%) of Ba 2+ or Sr 2+ in the A-site of the perovskite, which are easily converted into carbonates in a CO 2 atmosphere. 6 A decrease in permeation flux with time, even to zero, is usually reported for these perovskite membranes when the feed gases or sweeping gases contain CO 2 .7 Perovskite or perovskite-related membranes without doped alkaline-earth metal ions in the A-site are stable towards CO 2 , but their permeability is far lower than that required for practical applications. Recently, mixed conducting dual-phase membranes, made of ionic conducting oxides and electronic conducting oxides, have attracted considerable attention since the two phases can be separately tailored according to the special requirements of a given application. Usually, the fluorite-type oxide ionic conductors are used for the ionic transport and noble metals (such as Pt, Au, Ag etc.) 9 or pure electronic conducting oxides (such as La 1Àx Sr x MnO 3 , MnFe 2 O 4 etc.)10 are used for electronic transport. We have proposed that dual-phase membranes that are made of ionic conducting oxides and mixed conducting oxides show oxygen fluxes one order of magnitude higher than those of the traditional dual-phase membranes, 11 together with high stability for partial oxidation of methane in membrane reactors.12 Here, we adopt the design concept of dual-phase membranes to develop new materi...

in Wiley InterScience (www.interscience.wiley.com).A composite membrane, which comprises of one fluorite oxide phase (Ce 0.8 Gd 0.2 O 1.9 ) for oxygen ionic transport and one perovskite oxide phase (Gd 0.2 Sr 0.8 FeO 32d ) for both oxygen ionic and electronic transport, was investigated. XRD results revealed that the two oxides are compatible and showed good oxygen permeation stability at 9508C (more than 1100 h). Oxygen flux of the composite membrane was two times higher than that of the Gd 0.2 Sr 0.8 FeO 32d mixed conducting membrane at the same conditions. At steady state, oxygen flux of a 0.5 mm membrane was 0.80 ml/(cm 2 min) under oxygen partial pressure gradient of air/He and ;5.0 ml/(cm 2 min) for syngas production at 9508C, respectively. After ;440 h operation under syngas conditions, SEM analysis revealed that the syngas side of the membrane still kept dense and the EDX showed the transfer of elements occurred in the depth of a few microns. 2008 American Institute of Chemical Engineers AIChE J, 54: [665][666][667][668][669][670][671][672] 2008

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In this paper, oxygen permeable membrane used in membrane reactor for selective oxidation of alkanes will be discussed in detail. The recent developments for the membrane materials will be presented, and the strategy for the selection of the membrane materials will be outlined. The main applications of oxygen permeable membrane in selective oxidation of light alkanes will be summarized, which includes partial oxidation of methane (POM) to syngas and partial oxidation of heptane (POH) to produce H 2 , oxidative coupling of methane (OCM) to C 2 , oxidative dehydrogenation of ethane (ODE) to ethylene and oxidative dehydrogenation of propane (ODP) to propylene. Achievements for the membrane material developments and selective oxidation of light alkanes in membrane reactor in our group are highlighted.