(591c) Aiding Catalysis with Additive Manufacturing: Fabrication and Optimization of Hierarchical Membrane/Catalyst Systems for Oxidative Coupling of Methane

One of the largest detriments to C₂₊ hydrocarbon selectivity in the oxidative coupling of methane (OCM) is the undesired homogeneous oxidation of the C₂₊ products with O₂ gas. Therefore, membrane/catalyst systems (which feed O^2- species) can potentially allow higher C₂₊ selectivity than equivalent packed bed reactors (which feed O₂(g)). However, parasitic undesired O₂(g) evolution on the OCM side of the membrane can nullify much of the selectivity enhancement. Furthermore, issues of insufficient C₂₊ yields, low reaction rates, and high membrane capital costs have also hindered the development of these membrane/catalyst systems. Here, we present a streamlined, tunable synthesis approach for the design of catalytic hollow fiber membrane OCM systems, inspired by additive manufacturing. The presented approach allows us to carefully co-design membrane and catalyst functionalities with tunable layer porosity and tunable thickness, helping to address the above challenges. As an OCM case study, we use BaCe_0.8Gd_0.2O_3-δ (BCG) as the basis of both the catalyst and membrane layers. Due to the tuned BCG catalyst surface area for methane activation on the OCM side, the C₂₊ selectivity, yield, and reaction rates were greatly improved compared to a thick, symmetric BCG hollow fiber with non-tunable catalytic properties. A maximum C₂₊ yield of 22.7% was achieved at 845 °C using the optimized system. The enhanced performance of the asymmetric system is attributed to an increased heterogeneous CH₄ activation rate that stoichiometrically matches the incoming O₂ flux, thereby minimizing O₂(g) effects. We use OCM kinetic studies and dimensional analysis to argue that this “rate matching” of O₂ flux and CH₄ activation rate is critical to maximizing the C₂₊ yield, and that this requires careful co-designing of membrane and catalyst functionalities.