Understanding the Impact of Hydrogen Activation by SrCe0.8Zr0.2O3-δ Perovskite Membrane Material on Direct Non-Oxidative Methane Conversion.

Sichao Cheng,Yuxia Diao,Limei Qiu,Mann Sakbodin,Su Cheun Oh,Dongxia Liu

doi:10.3389/fchem.2021.806464

Sichao Cheng, Yuxia Diao + Show 4 more

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https://doi.org/10.3389/fchem.2021.806464

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Abstract

Direct non-oxidative methane conversion (DNMC) converts methane (CH4) in one step to olefin and aromatic hydrocarbons and hydrogen (H2) co-product. Membrane reactors comprising methane activation catalysts and H2-permeable membranes can enhance methane conversion by in situ H2 removal via Le Chatelier's principle. Rigorous description of H2 kinetic effects on both membrane and catalyst materials in the membrane reactor, however, has been rarely studied. In this work, we report the impact of hydrogen activation by hydrogen-permeable SrCe0.8Zr0.2O3−δ (SCZO) perovskite oxide material on DNMC over an iron/silica catalyst. The SCZO oxide has mixed ionic and electronic conductivity and is capable of H2 activation into protons and electrons for H2 permeation. In the fixed-bed reactor packed with a mixture of SCZO oxide and iron/silica catalyst, stable and high methane conversion and low coke selectivity in DNMC was achieved by co-feeding of H2 in methane stream. The characterizations show that SCZO activates H2 to favor “soft coke” formation on the catalyst. The SCZO could absorb H2 in situ to lower its local concentration to mitigate the reverse reaction of DNMC in the tested conditions. The co-existence of H2 co-feed, SCZO oxide, and DNMC catalyst in the present study mimics the conditions of DNMC in the H2-permeable SCZO membrane reactor. The findings in this work offer the mechanistic understanding of and guidance for the design of H2-permeable membrane reactors for DNMC and other alkane dehydrogenation reactions.

Highlights

Direct non-oxidative methane conversion (DNMC) has received intense attention in the past decades since it directly converts methane into value-added hydrocarbons such as ethylene (C2H4) and benzene (C6H6), and hydrogen (H2) co-product (Borry et al, 1999; Lunsford, 2000; Xu et al, 2003; Alvarez-Galvan et al, 2011)
The peak intensity did not reduce obviously with H2 co-feed until the 20% H2 concentration was used in the DNMC reaction. These results showed that SCZO oxide was more active in methane activation, and it was not influenced by the additional H2 presence due to its mixed ionicelectronic conductivity (MIEC) property
Characterizations on the spent Fe/SiO2 catalyst, SCZO oxide, and their mixture samples show the coexistence of Fe/SiO2, SCZO, and H2 co-feed favors low-temperature “soft coke” formation

Summary

Introduction

Direct non-oxidative methane conversion (DNMC) has received intense attention in the past decades since it directly converts methane into value-added hydrocarbons such as ethylene (C2H4) and benzene (C6H6), and hydrogen (H2) co-product (Borry et al, 1999; Lunsford, 2000; Xu et al, 2003; Alvarez-Galvan et al, 2011). A strategy to increase methane conversion in DNMC is to conduct the reaction in a hydrogen-permeable. Hydrogen (H2) is the smallest molecule in DNMC, and its yield reaches up to ∼50% in the product effluent, which directly influences the kinetics and thermodynamics of the reaction. According to the Le Chatelier’s principle, the removal of hydrogen produced in DNMC can shift the thermodynamic equilibrium to higher methane conversion. Membrane reactors comprising methane activation catalysts and H2-permeable membranes, have been intensively studied since the 1990s (Li et al, 2001; Li et al, 2002; Morejudo et al, 2016)

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