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Oxidative coupling of methane (OCM), which transforms CH 4 into C 2 products (C 2 H 6 and C 2 H 4) with molecular O 2 as the oxidant, is one of the most studied direct methane conversions (DMCs). However, a major technical hurdle to the OCM process is to achieve high CH 4 conversion at high C 2 (C 2 H 6 and C 2 H 4) selectivity. One rudimentary cause for this "tradeoff" behavior is the high chemical reactivity of the products (C 2 H 6 or C 2 H 4), which can be re-oxidized by O 2. To overcome this thermodynamic challenge, minimizing the oxidizing power of the oxidant and lowering the local oxygen partial pressure are keys. In this work, we demonstrate a new membrane reactor that capture CO 2 /O 2 from a flue gas and uses it for OCM conversion. The results show that the co-captured CO 2 /O 2 mixture converts CH 4 into C 2 H 6 in the presence of a 2%Mn–5%Na 2 WO 4 /SiO 2 catalyst, followed by thermal cracking of C 2 H 6 into C 2 H 4 and H 2. The presence of CO 2 decreases the local partial pressure of O 2 , thus reducing the propensity of C 2 -products re-oxidation and leading to a higher C 2 selectivity. We show that a small button-type membrane reactor can achieve 12% C 2 yield with ∼57% C 2 -selectivity using a diluted CH 4 -Ar mixture as the feedstock. We expect higher C 2 yield with tubular plug-flow membrane reactors in the future. We also highlight the unique advantage of the membrane reactor in intensifying CO 2 capture from both flue gas and OCM purification process into one single step. A dual-phase CO 2 transport membrane for oxidative coupling of methane conversion. [Display omitted] • First demonstration of CO 2 transport membranes for oxidative coupling of methane (OCM) conversion. • Identification of CO 2 transport pathways in the membrane. • Thermodynamic analysis of main and side reactions in the OCM conversion. [ABSTRACT FROM AUTHOR]

High-temperature membranes for CO 2 transport and separation has attracted significant interest from academia and industries due to their potential to mitigate the emissions of CO 2 and ultimately global warming/climate change. In this study, we report a dual-phase CO 2 membrane fabricated from a porous proton conducting BaZr 0.8 Y 0.2 O 3-δ (BZY) matrix and eutectic mixture of Li 2 CO 3 –Na 2 CO 3 (denoted as MC). The membrane exhibits a high CO 2 permeation flux density in the range of 550–750 °C in both dry and wet conditions. Through microstructural optimization, a CO 2 flux density as high as 0.34 mL ⋅ cm−2 ⋅ min−1 at 650 °C and 0.53 mL ⋅ cm−2 ⋅ min−1 at 750 °C have been achieved with an 0.8 mm thick BZY-MC membrane containing 52% porosity and 50%CO 2 –N 2 feed gas. The high flux is attributed to synergistic effects of microstructure, MC loading and high bulk conductivity of BZY. In addition, we also demonstrate the positive effect of H 2 O in the permeate side on CO 2 flux density and proposed a reasonable mechanism to explain the H 2 O-enhanced CO 2 flux density. With 3% H 2 O-added into the sweeping gas, the membrane exhibits 30% CO 2 flux density enhancement and good stability over 250 h at 650 °C. A new dual-phase membrane with a porous proton conducting BaZr 0.8 Y 0.2 O 3-δ (BZY) matrix and molten carbonate (denoted as MC) exhibits high CO 2 flux density at 650 °C with H 2 O as a booster. [Display omitted] • A new intermediate-temperature CO2 separation membrane. • Enhanced CO 2 flux by H 2 O in sweep gas. • A new dual-ion transport mechanism for H 2 O-enhanced CO2 flux. • Direct experimental evidence supporting the proposed dual-ion transport mechanism. [ABSTRACT FROM AUTHOR]

Natural gas (NG) fired power plants emit low concentration (4–5%) of CO 2 , which presents additional technical and economic challenges to the current benchmark amine absorption technology. The newly emerged high-temperature multiphase membranes operated on molten carbonate (MC) chemistry for CO 2 capture/separation/conversion have been demonstrated with great potential to meet this challenge. In this study, we report on the CO 2 capture performance of such a membrane in tubular geometry from a mockup NG flue gas. The membrane is comprised of a mixture of Gd 0.20 Ce 0.80 O 1.95 (GDC) and MC, in which GDC forms a porous skeleton to contain MC. We show that the membrane with a dimension of 6.1 mm in outer diameter, 5.1 mm in inner diameter and 5 cm in effective length (resulting in 4 cm2 effective surface area) can achieve a CO 2 flux density of 0.46–0.55 mL/min·cm2 at 650 °C, capturing 97% pure CO 2 at a rate of 37–42% from 5%CO 2 –N 2 using moistened Ar as the sweep gas. The level of performance demonstrated by this study suites the membrane well for stationary CO 2 capture from NG power plants. [Display omitted] • Demonstration of high-performance of tubular membrane for low CO 2 capture. • Demonstration of beneficial effect of steam on CO 2 flux. • Confirmation of CO 2 –H 2 O co-transport mechanisms. [ABSTRACT FROM AUTHOR]