Oxidative coupling of methane (OCM) conversion into C2 products through a CO2/O2 co-transport membrane reactor

Abstract

Oxidative coupling of methane (OCM), which transforms CH4 into C2 products (C2H6 and C2H4) with molecular O2 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 CH4 conversion at high C2 (C2H6 and C2H4) selectivity. One rudimentary cause for this “tradeoff” behavior is the high chemical reactivity of the products (C2H6 or C2H4), which can be re-oxidized by O2. 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 CO2/O2 from a flue gas and uses it for OCM conversion. The results show that the co-captured CO2/O2 mixture converts CH4 into C2H6 in the presence of a 2%Mn–5%Na2WO4/SiO2 catalyst, followed by thermal cracking of C2H6 into C2H4 and H2. The presence of CO2 decreases the local partial pressure of O2, thus reducing the propensity of C2-products re-oxidation and leading to a higher C2 selectivity. We show that a small button-type membrane reactor can achieve 12% C2 yield with ∼57% C2-selectivity using a diluted CH4-Ar mixture as the feedstock. We expect higher C2 yield with tubular plug-flow membrane reactors in the future. We also highlight the unique advantage of the membrane reactor in intensifying CO2 capture from both flue gas and OCM purification process into one single step.