Ca2MnO4-layered perovskite modified by NaNO3 for chemical-looping oxidative dehydrogenation of ethane to ethylene

Abstract

Chemical-looping oxidative dehydrogenation (CL-ODH) is a process designed for the conversion of alkanes into olefins through cyclic redox reactions, eliminating the need for gaseous O2. In this work, we investigated the use of Ca2MnO4-layered perovskites modified with NaNO3 dopants, serving as redox catalysts (also known as oxygen carriers), for the CL-ODH of ethane within a temperature range of 700-780 °C. Our findings revealed that the incorporation of NaNO3 as a modifier significantly enhanced the selectivity for ethylene generation from Ca2MnO4. At 750 °C and a gas hourly space velocity of 1300 h−1, we achieved an ethane conversion up to 68.17%, accompanied by a corresponding ethylene yield of 57.39%. X-ray photoelectron spectroscopy analysis unveiled that the doping NaNO3 onto Ca2MnO4 not only played a role in reducing the oxidation state of Mn ions but also increased the lattice oxygen content of the redox catalyst. Furthermore, formation of NaNO3 shell on the surface of Ca2MnO4 led to a reduction in the concentration of manganese sites and modulated the oxygen-releasing behavior in a step-wise manner. This modulation contributed significantly to the enhanced selectivity for ethylene of the NaNO3-doped Ca2MnO4 catalyst. These findings provide compelling evidence for the potential of Ca2MnO4-layered perovskites as promising redox catalysts in the context of CL-ODH reactions.

1. Introduction

Ethylene is a basic building block for the world's chemical industry, standing as one of the most abundantly produced organic chemicals worldwide. Its primary application lies in serving as a reactive monomer for the synthesis of polyethylene. Beyond that, ethylene functions as a crucial intermediate in the production of various compounds, including ethylbenzene, vinyl acetate, ethylene oxide, polyethylene, polyvinyl chloride, and other important chemical compounds. The production of ethylene holds immense significance as an indicator of a country's level of petrochemical development [1e3]. In industry, ethylene is typically produced through the steam cracking of hydrocarbon feedstocks such as naphtha or ethane, due to the shortage of petroleum [4].

Ethane, owing to its abundant presence in natural gas reservoirs, rightfully stands out as the most appealing feedstock for ethylene production. It is noted that ethane steam cracking is a highly energy-intensive and environmentally impactful procedure. This is primarily attributed to its heavy endothermic nature, requiring a substantial input of energy (143 kJ·mol−1) and operating at elevated temperatures exceeding 1000 °C. Additionally, downstream separation processes add to the overall energy demand [5]. An alternative approach for steam cracking is oxidative dehydrogenation (ODH) of ethane, in which ethane and gaseous oxygen are co-fed in the presence of a heterogeneous catalyst (Eq. (1)) [6]. The reaction of ODH can break the limitation for thermodynamic equilibrium by adding gaseous oxygen. In contrast to the traditional steam cracking method, the ODH method enhances the kinetics of the reaction, resulting in a lower Gibbs free energy. This fundamental difference can significantly impact the overall efficiency and environmental footprint of the ethane-to-ethylene conversion process.