CO2 Assisted Oxidative Dehydrogenation of Ethane to Ethylene over FeOx-Based Catalysts

Introduction

In recent years, significant efforts have been made for utilizing CO₂ in high temperature reactions, such as dry methane reforming, oxidative coupling of methane, in addition to the conversion of CO₂ into fuels such as methanol and DME. From the plethora of processes that are either in industrial scale or under development, promising are those in which CO₂ is used as a mild oxidant instead of O₂ [1]. Characteristic examples are the dehydrogenation of ethylbenzene to styrene and the dehydrogenation of light paraffins to the corresponding olefins [2]. Ethylene is the cornerstone of the petrochemical industry with over 180 million tons production and annual growth of 3-4% [3]. Moreover, the increasing availability of ethane from shale gas has revived the interest in the development of new sustainable, low environmental impact ethylene production processes [4]. Oxidative dehydrogenation (ODH) of ethane is an alternative process for ethylene production with significant advantages over the energy-intensive steam cracking process [5]. The use of CO₂ as mild oxidant in the dehydrogenation of ethane is a promising concept, with the overall reaction being the following: C₂H₆ + CO₂ ↔ C₂H₄ + CO + H₂O

The main advantage of CO₂ over O₂ is the absence of undesirable secondary oxidation reactions to CO_x and the safer reactor operation due to the avoided use of flammable oxygen/hydrocarbon mixtures. In addition, CO₂ is simultaneously converted to CO, a valuable raw material for the production of chemicals (e.g. oxo-alcohols) and liquid fuels through the F-T reaction. The CO₂-assisted ethane dehydrogenation has been mainly studied over redox catalysts that generally have good performance in alkane ODH [6]. Despite the extensive research, the relatively low turnover frequencies, the low ethylene selectivity and the extensive catalyst deactivation still remain major challenges in the pursuit of an active and stable catalyst.

In this study, we present the promising performance of FeO_x-based catalysts supported on (NiO-)ZrO₂-MgO in the CO₂-assisted ethane dehydrogenation to ethylene. Temperature programmed tests provide insight of the optimum temperature window for attaining maximum selectivity. Parametric analysis was performed to explore the effect of the basic operating parameters on the efficiency of the reaction.

Experimental

The FeO_x-based catalysts with 5 and 10 wt% Fe loading were synthesized by combining the methods of sol-gel auto-combustion and wet impregnation on mixed Ni(5%), Mg(70%) and Zr(25%) oxide. The supports were calcined at 800°C for 3h. After impregnation of the Fe precursor salts and drying, the catalysts were further calcined at 600^oC for 3h. Characterization included BET, XRD, TPR, TPO and TEM. TP reaction tests were also conducted in a transient unit equipped with a mass analyzer. Testing of the FeO_x catalysts on ZrO₂-MgO with and without 5 wt% NiO was conducted in a lab-scale continuous flow unit with fixed bed reactor. The catalysts were tested at 600°C with CO₂/C₂H₆ feed at 1.3/1 molar ratio in mixture with N₂, and W/F values ranging from 0.013 to 0.043 g.min/cm³. The products of the reaction were C₂H₄, CO, CH₄, H₂ and H₂O.

Results

The surface areas of all four catalysts are >100 m²/g. The XRD patterns of the bare supports reveal the presence of cubic crystalline structures of MgO and ZrO₂. No peak corresponding to NiO is detected, probably due to its low concentration and high dispersion. Over the catalysts, a low-intensity peak at 35.5-35.7°, characteristic of ã-Fe₂O₃ and Fe₃O₄ phases, is observed. TEM images of the 5% Fe/NiO-ZrO₂-MgO sample show the presence of ã-Fe₂O₃ nanocrystals (10-15 nm). The TPR of the catalysts show a reduction peak centered at ~400^oC, ascribed to the reduction of Fe₂O₃ to FeO. Complete reduction to metallic Fe occurs at temperatures over 700^oC, outside the temperature window of the dehydrogenation reaction. No reduction peaks appear in the TPR profiles of the upromoted and NiO-promoted ZrO₂-MgO supports, implying the formation of a solid solution between NiO and MgO that prevents NiO reduction.

TP reaction tests in C₂H₆/CO₂ flow over the bare NiO-ZrO₂-MgO support demonstrate that temperatures >620^ïC are necessary for the activation of the reactants. Similar tests with the catalyst show that in the presence of FeO_x the onset temperature drops by 100^ïC. In the absence of CO₂, FeO_x is quickly reduced to Fe by C₂H₆ and catalyzes the reforming ethane to CO and H₂ at temperatures as low as 600°C. Therefore, the presence of CO₂ is crucial in maintaining the active phase in oxidized form and drive the dehydrogenation reaction to ethylene. The beneficial effect of CO₂ in reducing coke, possibly via the Boudouard reaction, is also evident.   

Figure 1. Ethane conversion (T=600°C, W/F=0.026 g.min/cm³)

Figure 2. Ethylene selectivity vs ethane conversion (T=600°C, W/F=0.013-0.043 g.min/cm³)

Testing of the materials at 600°C with CO₂/C₂H₆ molar ratio 1.3 in the lab-scale unit confirm the high activity of the catalysts. Ethane conversions vary in the range of 20-30% (Fig. 1), with similar CO₂ conversion (22-32%). The increase of the FeO_x loading has a positive effect only in the catalysts supported on ZrO₂-MgO. When the support is promoted with NiO, activity is boosted significantly (50% increase), probably masking the loading effect. Ethylene selectivity is very high, in the range of 60-80%. Ethylene selectivity as a function of ethane conversion is presented in Figure 2. Selectivity decreases with increasing conversion due to the increased extent of secondary reactions. The different loading catalysts show similar selectivity-conversion curves, indicating that only the number, and not the nature of the active sites, is altered with increasing loading. On the contrary, incorporation of NiO in the ZrO₂-MgO support enhances selectivity, with the 5 wt% Fe/NiO-ZrO₂-MgO catalyst exhibiting the highest selectivity equal to ~80% at 20-30% conversion. The product distribution allows a first estimation of the reaction network over the FeO_x-based catalysts, which comprises the dehydrogenation of ethane to ethylene, the consecutive oxidation of H₂ to H₂O via the reduction of CO₂ to CO (Reverse Water Gas Shift Reaction), the dry reforming of ethane from CO₂ to CO and H₂ and, to a small extent, the hydrogenolysis of ethane to methane. Figure 1 also shows the contribution of the three main reactions to the total measured ethane conversion. It is apparent that the desired route – dehydrogenation to ethylene – constitutes the main route of ethane consumption, followed by reforming and finally cracking to methane. The dehydrogenation contribution is highest in the 5 wt% Fe/NiO-ZrO₂-MgO catalyst, consistent with its highest selectivity.

The optimum catalyst was further testing at different operating conditions. The effect of temperature on conversion and selectivity is of particular interest. Conversion, as expected, increases significantly with temperature. Ethylene selectivity however, reduces by 10% by only a small temperature increase from 600°C to 625°C at constant conversion (27%). This decrease can be attributed to the side reaction of dry reforming. The apparent activation energy for the consumption of ethane and CO₂, calculated from the Arrhenius plots_, is equal to 115 and 123 kJ/mol respectively. These values, although not directly comparable, are consistent with corresponding values over Cr-containing catalysts [7]. A stability test was conducted for 17.5h of reaction to monitor catalyst stability. A 25% activity loss occurs at the early stages of the reaction, with conversion stabilizing at longer reaction times. Characterization of the used catalyst did not show appreciable coke deposition. Loss of activity is probably related to sintering of the active phase and/or partial reduction of Fe₂O₃ to Fe₃O₄. The latter was confirmed by TEM and by visual observations, as the catalyst color changed from orange to green-gray, indicative of iron oxide reduction.

Conclusions

This work demonstrates that FeO_x catalysts on mixed (NiO)-ZrO₂-MgO support constitute a promising catalytic system for the CO₂-mediated ethane conversion to ethylene, exhibiting relatively high ethane and CO₂ conversion with ethylene selectivity up to 80% at mild reaction temperature. Promotion of the ZrO₂-MgO support with NiO enhances both activity and selectivity of the reaction, indicating a participation and/or modification of the FeO_x active sites by NiO. Increase in iron loading increases activity in the FeO_x-based catalysts on unmodified ZrO₂-MgO, without substantial influence on selectivity. This suggests that the active centers remain the same, with the additional loading simply increasing their concentration on the surface. The optimum material is the 5 wt% Fe/NiO-ZrO₂-MgO catalyst that exhibits the highest activity and ethylene selectivity. The stability test reveals moderate catalyst deactivation in the first hours of reaction, attributed to sintering and partial reduction of Fe, with the performance stabilizing at longer operating times. It is worth mentioning that although the research is in early stages, the conversion and selectivity values ​​recorded are among the highest reported in the open literature on Cr-free catalysts.

References

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