processes Review Mixed Ionic-Electronic Conducting Membranes (MIEC) for Their Application in Membrane Reactors: A Review Alba Arratibel Plazaola 1,2, Aitor Cruellas Labella 1, Yuliang Liu 1, Nerea Badiola Porras 1,2,3, David Alfredo Pacheco Tanaka 2 , Martin Van Sint Annaland 1 and Fausto Gallucci 1,* 1 Inorganic Membranes and Membrane Reactors, Sustainable Process Engineering, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology (TU/e), Den Dolech 2, 5612AD, Eindhoven, The Netherlands; [email protected] (A.A.P.); [email protected] (A.C.L.); [email protected] (Y.L.); [email protected] (N.B.P.); [email protected] (M.V.S.A.) 2 TECNALIA, Energy and Environment Division, Mikeletegi Pasealekua 2, 20009, San Sebastian-Donostia, Spain; [email protected] 3 Chemical Engineering and Environmental Department, University of the Basque Country UPV/EHU, C/Alameda Urquijo s/n, Bilbao, 48013, Spain * Correspondence: [email protected] Received: 15 January 2019; Accepted: 12 February 2019; Published: 1 March 2019 Abstract: Mixed ionic-electronic conducting membranes have seen significant progress over the last 25 years as efficient ways to obtain oxygen separation from air and for their integration in chemical production systems where pure oxygen in small amounts is needed. Perovskite materials are the most employed materials for membrane preparation. However, they have poor phase stability and are prone to poisoning when subjected to CO2 and SO2, which limits their industrial application. To solve this, the so-called dual-phase membranes are attracting greater attention. In this review, recent advances on self-supported and supported oxygen membranes and factors that affect the oxygen permeation and membrane stability are presented. Possible ways for further improvements that can be pursued to increase the oxygen permeation rate are also indicated. Lastly, an overview of the most relevant examples of membrane reactors in which oxygen membranes have been integrated are provided. Keywords: oxygen separation; membrane; fluorite; perovskite; MIEC; membrane reactor 1. Introduction Oxygen, which is a key reactant in many industrial processes, is among the top five major chemicals produced worldwide [1]. Industrial oxygen production is performed mainly by cryogenic distillation and pressure swing adsorption (PSA), which both require high investments and operating costs [2–5]. Cryogenic distillation technology provides high purity oxygen (>99%), while, generally, the purity is lower for PSA at around 95% [6]. Recent international policies about energy security and greenhouse gas emissions are challenging the conventional oxygen production systems. Oxygen separation using membrane technology is less energy intensive [7] and can, thus, help reduce greenhouse gas emissions related to the chemical industries. When oxygen selective membranes are coupled with high temperature reactions (e.g., syngas production from natural gas) and even higher benefits can be achieved due to process integration [8]. Actually, companies like “Air Products” or “Praxair” are currently doing big efforts on this field, developing novel oxygen membrane units for both air separation and for their integration into chemical processes [9,10]. In literature, two principal types of ceramic membranes have been pointed as possible oxygen separation devices including pure oxygen conducting membranes and Processes 2019, 7, 128; doi:10.3390/pr7030128 www.mdpi.com/journal/processes Processes 2019, 7, 128 2 of 50 mixedProcesses ionic–electronic 2019, 7, x FOR conducting PEER REVIEW membranes (MIEC). In the first case, the oxygen ions are transported2 of 51 by the membrane material while the electrons are transported by an external electron material (electrodes). oxygen conducting membranes and mixed ionic–electronic conducting membranes (MIEC). In the The oxygen generated is controlled by an electric current applied to the system. In contrast, in MIEC first case, the oxygen ions are transported by the membrane material while the electrons are membranes,transported the membrane by an external material electron transports material both (ele ionsctrodes). and electrons,The oxygen which generated can be is eithercontrolled a single by an phase or a dual-phaseelectric current (where applied one materialto the system. phase In transports contrast, thein MIEC electrons membranes, and the the other membrane transports material the ions). In the twotransports cases, both the drivingions and force electrons, for the which oxygen can permeationbe either a single is the phase oxygen or a chemical dual-phase potential/partial (where one pressurematerial difference phase on transports both sides the of electrons the membrane and the other [11,12 transports]. Compared the ions). with In traditional the two cases, oxygen the driving separation technologies,force for membrane the oxygen permeation technology is features the oxygen improved chemical energy potential/partial efficiency, pressu superiorre difference oxygen on selectivity both (thus highsides oxygen of the purity),membrane and compatibility[11,12]. Compared with manywith industrialtraditional reactionoxygen systems,separation including technologies, selective oxidationmembrane of ethane technology (SOE) [13 features], oxidative improved coupling energy of efficiency, methane superior (OCM) [oxygen14,15], andselectivity partial (thus oxidation high of oxygen purity), and compatibility with many industrial reaction systems, including selective methane (POM) [16,17] among other reactions. oxidation of ethane (SOE) [13], oxidative coupling of methane (OCM) [14,15], and partial oxidation The mixed ionic and electronic conduction of solid oxide materials was initially described by of methane (POM) [16,17] among other reactions. Takahashi etThe al. mixed [18] in ionic 1976. and Afterward, electronic conduction Cales and Baumardof solid oxide [19, 20materials] introduced was initially the mixed described conducting by oxide conceptTakahashi to et oxygenal. [18] in transport 1976. Afterward, membranes. Cales and However, Baumard favorable[19,20] introduced oxygen the permeability mixed conducting was not achievedoxide until concept Teraoka to oxygen et al. transport [21] made membranes. a breakthrough However, on favorable oxygen separationoxygen permeability membranes was innot 1985 by introducingachieved until the perovskiteTeraoka et al. membranes. [21] made a breakthrou One of thegh drawbacks on oxygen ofseparation oxygen membranes membranes in has1985 always by been theintroducing thermal stabilitythe perovskite at the membranes. high temperatures One of the requireddrawbacks for of achievingoxygen membranes appreciable has always oxygen been fluxes. Attemptsthe ofthermal increasing stability the at thermal the high stability temperatures include required doping for the achieving perovskite appreciable (and MIEC) oxygen with fluxes. different elements,Attempts which of clearlyincreasing improved the thermal the stability thermal inclu andde chemical doping the stability perovskite while (and resulting MIEC) with in a different decrease in elements, which clearly improved the thermal and chemical stability while resulting in a decrease in the oxygen permeation rate [22,23]. the oxygen permeation rate [22,23]. It is generally accepted that materials with superior oxygen ionic conductivity usually have It is generally accepted that materials with superior oxygen ionic conductivity usually have correspondinglycorrespondingly lower lower chemical chemical stability stability when when exposed exposed toto reducingreducing atmospheres. atmospheres. This This is easily is easily explainedexplained by the by fact the thatfact that weak weak chemical chemical bonding bonding of of metal metal andand oxygen could could be be favorable favorable for forthe thefast fast oxygenoxygen mobility mobility in membranes, in membranes, but but it alsoit also increases increases the the chanceschances for for metals metals to to be be reduced reduced at a at high a high temperaturetemperature in reducing in reducing atmosphere. atmosphere. Therefore, Therefore, this this decreases decreases thethe chemical stability stability [24]. [24 ]. ThereThere are, however, are, however, other other ways ways to increase to increase the the oxygen oxygen flux. flux. In In fact, fact, the the oxygenoxygen permeation rate rate in MIEC membranesin MIEC membranes is controlled is controlled by two different by two different mechanisms, mechanisms, such as ionicsuch as transport ionic transport within thewithin membrane the and surfacemembrane membrane and surface exchange. membrane When theexchange. permeation When process the permeation is controlled process by bulk is controlled diffusion, by the bulk oxygen diffusion, the oxygen permeate flux will increase when decreasing the thickness of the membrane permeate flux will increase when decreasing the thickness of the membrane [25,26]. Membranes with a [25,26]. Membranes with a thickness in the order of 400 to 500 μm is, however, sufficiently small, thickness in the order of 400 to 500 µm is, however, sufficiently small, because the surface exchange rate because the surface exchange rate becomes limiting for smaller membrane thicknesses, as shown in becomesFigure limiting 1 [27]. for smaller membrane thicknesses, as shown in Figure1[27]. Figure 1. Thickness dependence of oxygen permeation of self-supported (Bi2O3)-(Er2O3)-Ag Figure 1. Thickness dependence of oxygen