Development of Mixed Metal Oxide Catalysts for Oxidative Dehydrogenation of Ethane to Ethylene

Abstract

The increasing demand for light olefins and the changing nature of upstream feedstock boosted up the substantial research activity into the development of alternative process routes. Ethylene is one of the most widely required primary building blocks for the preparation of value-added chemical products (e.g., polyethylene, ethylene oxide, ethylene glycol, styrene, vinyl acetate monomers, and polyvinyl chloride etc.) and many other intermediate products in current society. A steam cracking method, under high temperature pyrolysis in the presence of diluting steam, is the most settled industrial process for the manufacture of ethylene. Feedstocks for the steam cracking method have mostly been naphtha and natural gases in this process. Currently the basic feedstock for steam cracking has only shifted to ethane in the last decade, thus leading to attractive production costs. It should be noted that the emerging availability of shale gas will remarkably shift the overall petrochemical industry towards processes that use light alkanes to production of ethylene. The increase in the price of crude oil, in particular, and the availability of ethane from shale gas, has led to the interest in alternative processes for ethylene production, such as oxidative dehydrogenation of ethane (ODHE). This process offers diverse advantages, thus it has been attracting considerable attentions as a subject of substantial research activities. However, the lack of suitable catalysts that combine high activity and selectivity has prevented their industrial realization so far. In an attempt to develop efficient catalyst for the reaction, both the exterior and interior design of active catalysts was performed as below:  Ni-Nb-O mixed oxide embedded on CexZr1-xO2 (denoted herein as Ni-Nb-O/CexZr1-xO2), which is a concept of exterior design, were designed based on the insights into reaction pathways. Compared with using Ni-Nb-O alone, the introduction of CexZr1-xO2 to the Ni-Nb-O active catalyst contributes on suppressing the formation of byproducts (CO, CO2, and CH4) at relatively high reaction temperature (450 °C). The increased reaction temperature also leads to an enhancement of ethane conversion (~55%) and subsequent increase in the production of ethylene (6.3 μmolgactive cat-1 s-1), compared to that of conventional Ni-Nb-O catalyst (1.5 μmolgactive cat-1 s-1). Relevant control tests and the electrochemical tests suggest that the promotion effect of CexZr1-xO2 additive is attributed to the compensation of lattice oxygen from CexZr1-xO2 into the lattice oxygen vacancy in Ni-Nb-O active catalysts, which originates from consumed lattice oxygen during the ethane conversion.  Ce-incorporated MoVTeNbO, which is a concept of interior design, was designed to improve catalytic activity with exhibiting almost 100% ethylene selectivity from ODHE process. In this study, the effect of Ce in MoVTeNbO was intensively characterized. Activation temperature (600 °C) and amount of Ce atom (0.1 atom%) are optimized as attempts to maximize the ratio of reactive phase, M1 phase (unique structure for Mo-V based mixed metal oxide), for the selective production of ethylene. As a result, the Ce-incorporated MoVTeNbO catalyst exhibited comparable ethylene yield (~60 %) with reducing reaction temperature (~50 °C) during the ODHE process, compared to pure MoVTeNbO catalyst. Results of physicochemical characterizations suggest that the improved catalytic performance of Ce-incorporated MoVTeNbO should corresponds with the location of Ce atoms in the lattice structure of M1 phase MoVTeNbO, and subsequent improvements in redox property of active sites.