(683a) Enhancing Selectivity, Rate, and Stability in Lscf Membrane/Na2WO4 Catalyst Reactors for Oxidative Coupling of Methane: Multi-Functional Layered Hierarchical Systems

One of the largest challenges in the oxidative coupling of methane (OCM) is the homogeneous oxidation of the C₂₊ products with O₂, forming CO_x and lowering selectivity. Therefore, membrane/catalyst systems can potentially increase OCM selectivity compared to traditional reactors by replacing O₂ cofeed with controllable O^2- feed. However, the most promising OCM catalysts (based on Na₂WO₄) have been difficult to integrate into O^2--permeable membrane systems: to our knowledge, there has never been a rigorous demonstration of a membrane/Na₂WO₄ catalyst system that outperforms a standard packed bed reactor of the state-of-the-art Mn/Na₂WO₄/SiO₂ catalyst. One key challenge is parasitic reactions of oxygen to form CO_x, whether in the gas phase due to O₂(g), or on the membrane: state-of-the-art O₂ membranes, like Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-x, BaCo_xFe_yZr_zO_3-x or La_0.6Sr_0.4Co_0.2Fe_0.8O_3-x(LSCF), catalyze deeper methane oxidation. Furthermore, Na₂WO₄ is incompatible with these membranes due to formation of additional transport-blocking phases, like BaWO₄ or SrWO₄. Given these challenges (and requirements for high reaction rates), a hybrid multi-layered membrane based on separate oxides with different functionalities is a promising strategy. In this work, we tackle these challenges by applying interlayer films of several oxides to LSCF hollow fiber membranes. These films enhance reaction rates, C₂₊ selectivity, and stability compared to the LSCF-only case. We demonstrate that by using Bi_1.5Y_0.3Sm_0.2O_3-x as a relatively inert buffer interlayer, and Mn/Na₂WO₄/SiO₂ as a catalyst, we can achieve 57% C₂₊ selectivity at 21% CH₄ conversion at 850 °C, with improved stability compared to the LSCF-only case. This is among the best performances demonstrated for an Na₂WO₄-based membrane/catalyst system. Our findings stress the importance of minimizing (i) the detrimental effects of excessive O₂(g), (ii) non-selective membrane OCM contributions, and (iii) transport-inhibiting solid-state reactions between components. Based on these, we present a kinetic-modelling-based framework for optimizing OCM performance of multifunctional membrane/catalyst systems closer towards technoeconomic relevance.