640 BOLETIN DE LA SOCIEDAD ESPAÑOLA DE A R T I C U L O Cerámica y Vidrio Mixed conducting materials for partial oxidation of hydrocarbons J.R.FRADE(*), V.V.KHARTON(*, + ), A.A.YAREMCHENKO(*), E.V.TSIPIS(*), A.L.SHAULA(*), E.N.NAUMOVICH(*, + ), A.V.KOVALEVSKY(+), F.M.B.MARQUES(*) (*)Ceramics and Glass Eng. Dep. (CICECO), University of Aveiro, 3810-193 Aveiro, Portugal ( + ) Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus Thermodynamic calculations with additional conditions for the conservation of carbon and hydrogen were used to predict the gas composition obtained by partial oxidation of methane as a function of oxygen partial pressure and temperature; this was used to assess the stability and oxygen permeability requirements of mixed conducting membrane materials proposed for this purpose. A re-examination of known mixed conductors shows that most materials with highest permeability still fail to fulil the requirements of stability under reducing conditions. Other materials possess suficient stability but their oxygen permeability is insuficient. Different approaches were thus used to attempt to overcome those limitations, including changes in composition in the A and B site positions of ABO 3 perovskites, and tests of materials with different structure types. Promising results were obtained mainly for some materials with perovskite or related K 2 NiF 4 -type structures. Limited stability of the most promising materials shows that one should rely mainly on kinetic limitations in the permeate side to protect the mixed conductor from severe reducing conditions. Materiales conductores mixtos para la Oxidación Parcial de Hidrocarburos Se han usado cálculos termodinámicos con condiciones adicionales para la conservación del carbono e hidrógeno para predecir la compos- ición del gas obtenido mediante la oxidación parcial del metano en función de la presión parcial de oxígeno y de la temperatura; esto se ha usado para asegurar los requerimientos de estabilidad y permeabilidad al oxígeno de los materiales conductores mixtos empleados como membrana para este propósito. Un nuevo exámen de los conductores mixtos conocidos muestra que la mayoría de los materiales con la mayor permeabilidad todavía fallan en el cumplimiento de los requerimientos de estabilidad bajo condiciones reductoras. Otros materiales poseen suiciente estabilidad, pero su permeabilidad al oxígeno es insuiciente. Por ello se han empleado diferentes aproximaciones para intentar superar esas limitaciones, incluyendo cambios en la composición en las posiciones A y B de de las perovsquitas ABO 3 , y pruebas con mate- riales con estructuras diferentes. Se han obtenido resultados prometedores principalmente en algunos materiales con estructura perovsquita o estructuras relacionadas con K 2 NiF 4 . La estabilidad limitada de los materiales más prometedores muestra que los estudios deben hacerse principalmente sobre las limitaciones cinéticas del lado permeable para proteger el conductor mixto en condiciones reductoras severas. 1. INTRODUCTION The main methods proposed to convert hydrocarbons to a mixture of hydrogen and carbon monoxide (syngas), have been steam reforming and partial oxidation by means of mesoporous membranes, mixed conductors with high electrochemical oxygen permeability, or ionic conductors working under short circuited conditions or assisted by an external applied ield. The mixed conducting membranes possess interesting advantages relative to endothermic steam reforming, or electrically assisted partial oxidation. In addition to high oxygen permeability the mixed conductor must meet other requirements of catalytic activity, stability under wide ranges of chemical potential and in the presence of CO 2 , steam, SO x , etc., and low thermal and chemical expansion under larger gradients of oxygen chemical potential. A relatively wide range of mixed conductors has been studied for these membrane reactors [1-17], including single phase and composite materials consisting of a metallic conductor and a good ionic conductor. This concept is widely discarded because these composite materials are either based on noble metals of prohibitive cost, or fail to meet the required stability (e.g.Ag/Bi(Er)O 1.5 ) or oxidation resistance conditions. The emphasis has thus been on mixed conducting ceramics, mainly single phase [1-16], and occasionally 2-phase ceramic/ceramic composites [17]. Perovskite materials Sr 1-x La x CoO 3-δ [3-5] and Sr 1- x La x FeO 3-δ possess the highest permeability, limited stability, and excessive thermal and chemical expansion. Other mixed conductors are based on perovskite materials with higher stability (e.g. gallates [6,10] and titanates [7-9]) with mixed valent additives. Some La 2 NiO 4 - based materials combine good permeability and relatively low thermal thermal expansion [12-16]. 2. SIMULATION OF LIMITING WORKING CONDITIONS IN MEMBRANE REACTORS The chemical stability requirements of mixed conducting materials can be estimated by predicting the equilibrium gas phase composition expected under typical conditions of methane partial oxidation, in which case: CH 4 + 1/2O 2 ⇔ CO + 2H 2 . (1) CO + 1/2O 2 ⇔ CO 2 ,. (2) H 2 + 1/2O 2 ⇔ H 2 O.. (3) Methane cracking, CH 4 →C+2H 2 , is mainly catalysed by metallic surfaces, and one thus assumes that this is entirely suppressed on all ceramic membranes. Limiting conditions were estimated by combining the equilibrium constants of reactions 1, 2, 3, with additional relations for the conservation of relevant elements (C and H). One thus obtained the following dependence of the fraction of methane converted α, and gas composition on the oxygen partial pressure pO 2 : α(w o +2α) 2 /[(1+z+w o +2α) 2 (1-α)] = K 1 .(pO 2 ) 1/2 [1+K 2 .(pO 2 ) 12 ][1+K 3 .(pO 2 ) 1/2 ] 2 (4) Bol. Soc. Esp. Ceram. V., 43 [3] 640-643 (2004) (c) Sociedad Española de Cerámica y Vidrio, Consejo Superior de Investigaciones Científicas. Licencia Creative Commons 3.0 España (by-nc) http://ceramicayvidrio.revistas.csic.es