(532bl) Aiding Methane Activation Catalysis with Reactor Engineering: Synthesis and Optimization of Catalytic Hollow Fiber Membrane Systems for Oxidative Coupling of Methane

Design of traditional catalysts in packed bed reactors has been insufficient to meet technoeconomic targets for the oxidative coupling of methane (OCM), in large part due to gas phase reactions of O₂(g) with the C₂₊ hydrocarbons and their intermediates. Therefore, membrane/catalyst systems that feed O^2- species can enable higher C₂₊ selectivity than equivalent packed bed reactors that feed O₂(g). However, issues of insufficient C₂₊ yields, low reaction rates, and high membrane capital costs are major hurdles to practical membrane reactor implementation. Here, we present a synthesis approach to aid the optimized design of catalytic membrane OCM systems and address the above challenges. The approach allows us to design hybrid membrane/catalyst hollow fiber reactors with tunable parameters of layer porosity and thickness. As an OCM case study, we use BaCe_0.8Gd_0.2O_3-δ (BCG) as the basis of both the catalyst and separation layers. Due to the tuned BCG catalyst surface area for methane activation on the membrane inner surface, the OCM performance was greatly improved compared to a thick, symmetric BCG hollow fiber with a non-porous inner surface. 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 more closely matches the inherent membrane O₂ diffusion rate, thereby minimizing O₂(g) effects. We use reactor performance comparisons, kinetic studies and dimensional analysis to argue that this “rate matching” of O₂ permeation and CH₄ activation rates is critical to maximizing the OCM performance in membrane reactor systems.