Oxidative dehydrogenation of ethane to ethylene over NiO loaded on high surface area MgO

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Abstract

The oxidative dehydrogenation of ethane over NiO-loaded MgO with high surface area was carried out using a fixed-bed flow reactor at 600 °C under atmospheric pressure.

At 600 °C, the oxidative dehydrogenation of ethane (C2H6/O2 = 1) without dilution with an inert gas resulted in C2H6 conversion of 68.8% and a high C2H4 selectivity of 52.8%, which corresponds to a C2H4 yield of 36.3%. In addition, the catalytic activity did not decrease for at least 10 h. X-ray photoelectron spectra of the catalysts after the reaction exhibited that the initial valence state of Ni2+ (NiO) was maintained during the oxidative dehydrogenation of ethane. However, when NiO-loaded MgO was reduced with H2 prior to the reaction, C2H4 selectivity decreased to nearly zero and high CO and H2 selectivities were observed with the C2H6 conversion of 50 %, indicating that partial oxidation of C2H6 proceeded. Therefore, it seems important to keep Ni species as an oxide phase on the support, and for this purpose, use of the high surface area of MgO is essential.

Graphical abstract

NiO loaded high surface area magnesium oxide afforded 36.8% of ethylene in the oxidative dehydrogenation of thane without dilution.

Introduction

Increasing demand on light alkenes is of current importance. Ethylene and propylene are the most important petrochemical feed stocks, and they are produced via steam cracking of naptha and the heavier fraction of natural gas. This process, however, is operated at a high temperature and consumes large amount of energy with low selectivities to alkenes and unavoidable coke formation [1].

Recently, an efficient utilization of light gases from oil fields has attracted a great deal of attention from petrochemical industries, and as a result, intensive development of more efficient and selective processes has been carried out. One of such processes is an oxidative dehydrogenation of ethane to ethylene (ODHE; reaction (1)):C2H6+12O2C2H4+H2O,ΔH298°=149kJ/molOxidative dehydrogenation of ethane to ethylene could be achieved at much lower temperatures than those with steam cracking due to an exothermic reaction.

Catalytic oxidative dehydrogenation of ethane was first reported by Lunsford et al. over MoO3 on SiO2 using N2O as an oxidant [2]. A large number of researchers have proposed various catalysts effective for ODHE; they are classified as non-reducible catalysts, reducible metal-oxide catalysts and complex metal-oxide catalysts.

Aika et al. [3] have shown that Co2+-doped MgO is effective for the ODHE with N2O as an oxidant. However, it is difficult to industrialize this process because of the high cost of N2O as an oxidant. From an industrial viewpoint, catalytic oxidative dehydrogenation of ethane using oxygen as an oxidant should be exploited.

Thorsteinson et al. [4] have reported that mixed oxide of Mo and V is active for the ODHE, with very high selectivity to ethylene with a low conversion.

Lunsford et al. reported that Li+/MgO catalyst was active in the oxygen and ethane mixed gas at a low O2 partial pressure. They proposed that ethoxide formed on MgO surface via ethyl radicals afforded a high yield of ethylene up to 34% at a reaction temperature above 600 °C [5], [6]. However, the catalyst exhibited only a short lifetime.

The ODHE reaction over V2O5- or MoO3-loaded catalysts has achieved high ethylene selectivity via the redox cycle (Mars and Krevelen mechanism) and/or acid–base properties [7], [8], [9], [10]. However, a high activity always proceeded with deep oxidation of produced ethylene.

Layered complex metal chloride oxide SrBi3O4Cl3 with SrCl2 and KCl afforded very high selectivity to ethylene in the reaction of ethane and oxygen at 660 °C [11], [12].

Non-reducible mixed metal oxides like Mo–V–Te oxides for ODHE have been reported to exhibit high activity at a temperature lower than 400 °C [13], [14], [15], [16], [17]. In these studies, most of the researchers employed conditions in which ethane and oxygen were highly diluted with an inert gas. From a practical stand point, dilution with inert gases must be avoided.

We have reported that carbon dioxide, as a mild oxidant, markedly promotes the dehydrogenation of ethane over Ga2O3-loaded TiO2 and Cr2O3-loaded oxidized diamond catalysts [18], [19]. However, with these catalyst systems, ethylene yields decreased with an increase in the reaction time, due to carbon deposition onto the catalyst. To overcome this, development of an active and selective new catalyst using oxygen as an oxidant is essential.

Recently, NiO-loaded on Al2O3 catalysts for ODHE have been reported to exhibit high activity [20], [21], [22]. NiO-loaded Al2O3 catalyst with highly dispersed NiO produces a high yield of ethylene. However, details regarding active sites for ODHE have not yet been clarified. Heracleous et al. have reported that Nb for NiO–NbnOm catalyst improves the dispersion of the nickel phase and facilitates C–H bond activation by acting as an electron transfer promoter [23], [24].

Heracleous et al. have reported that in the comparison between V- or Mo-oxide loaded on Al2O3 and those loaded on TiO2, the presence of basic sites enhances the fast desorption of the produced olefins on the catalyst surface, resulting in higher selectivities [10]. However, NiO loaded on basic support for ODHE has not been reported. Ruckenstein et al. have described that Ni species loaded on MgO catalyst exist in the form of a solid solution, which stabilizes nickel against sintering during the CO2 reforming of methane [25], [26]. This finding seems to indicate that the highly dispersed NiO loaded on high-surface area MgO would be resistant to sintering and would promote the ODHE.

The present study focused on the ODHE on NiO-loaded catalysts without dilution with an inert gas, and here we report that MgO with high surface area is a promising support for this reaction.

Section snippets

Catalyst preparation

The catalyst supports used in this study were MgO (100, 1000 A; Ube Industries Ltd.), Al2O3 (Sumitomo Chemical Co.), SiO2 (Wako Pure Chemical Industries Ltd.), TiO2 (P25; Japan Aerosil Co.), La2O3, and Y2O3 (Nacalai Tesque, Inc.).

The supported nickel oxide catalysts containing 5 wt.% of Ni as metal were prepared by impregnating an ethanol solution of Ni(acac)2 (Nacalai Tesque, Inc.) to a suspended support, followed by evaporation-to-dryness under vacuum. Supported catalyst precursors were

Activities of NiO-loaded catalysts in the oxidative dehydrogenation of ethane

Table 1 summarizes the effects of various supports on ODHE at 600 °C. As seen in Run 1, even without catalyst ODHE slightly proceeded to give an ethylene yield of 5%. NiO loaded on Al2O3, SiO2, Y2O3, La2O3, and TiO2 and low-surface area MgO (LS-MgO) (Runs 2–8) afforded high C2H6 conversions with high CO and H2 selectivities. This finding indicates that partial oxidation of ethane to CO and H2 proceeded (POE; reaction (2)). There was also carbon formation on these supports as a side reaction:C2H6+

Conclusion

For the oxidative dehydrogenation of ethane to ethylene (ODHE), NiO catalyst loaded on high-surface area MgO (HS-MgO) afforded high C2H6 conversion of 68.8%, C2H4 selectivity of 52.8%, and C2H4 yield of 36.8% at 600 °C, without dilution of the feed.

XRD, XPS, and H2-TPR revealed that NiO phase surrounded by NiO–MgO solid solution was the active phase for the ODHE. To keep Ni species as NiO, a high surface area of MgO was essential. When the NiO was reduced to metallic Ni, the partial oxidation of

Acknowledgements

This work was supported in part by the “High-Tech Research Center” Project for Private Universities: matching fund subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2001–2006 and grant-in-aid for Scientific Research (B) from Japan Society for the Promotion of Science.

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