Catalytic Solid Oxide Membrane Reactors: Development and Application in the Oxidative Coupling of Methane

Abstract

Developing direct routes for upgrading methane (the principal component of natural gas) into value-added chemicals is crucial for integrating natural gas into the chemical industry in a more environmentally sustainable way. Unlike commercial multi-step methane conversion routes that rely on economies of scale, the direct routes can be potentially applied in upgrading natural gas from small-scale sources, thereby mitigating their underutilization and flaring to deleterious greenhouse gases. The oxidative coupling of methane (OCM) is a promising direct route for converting methane to valuable C2 hydrocarbons (ethylene and ethane). This reaction occurs at high temperatures in the presence of oxygen and an active catalyst. The main challenge with OCM is the formation of undesired byproducts (CO and CO2) from over-oxidation reactions, which limits the C2 selectivity. This selectivity problem is particularly significant in conventional co-fed reactors (where oxygen is fed together with methane) because the high gas-phase oxygen concentrations at the reactor inlet accelerates the undesired over-oxidation reactions. In this dissertation, the thermodynamic and kinetic constraints on the C2 selectivity and yield in OCM are elucidated. With the aid of reactor models, we demonstrated that an oxygen-ion (O2-) conducting catalytic solid oxide membrane reactor, with distributed oxygen feed along the reactor length, can achieve significantly higher C2 selectivity and yield compared to a conventional co-fed reactor. From systematic experimental studies, BaCe0.8Gd0.2O3-δ (BCG) was identified as an O2- conducting material that is catalytically active for OCM and shows high resistance to deactivation via solid carbon deposition. These properties make BCG a promising membrane-catalyst material for OCM membrane reactors. Further work included fabricating tubular BCG membranes via a combined slip-casting and solid-state reactive sintering technique using Cu as a sintering additive. OCM results from the Cu-modified BCG tubular membrane reactors (with distributed oxygen feed) showed greater C2+ selectivity compared to co-fed reactor operation, at similar methane conversions. This conclusive experimental result supports the main hypothesis of this study; that catalytic solid oxide membrane reactors can improve OCM performance compared to conventional co-fed reactors.