Elsevier

Catalysis Today

Volume 311, 1 August 2018, Pages 40-47
Catalysis Today

Chemical looping as a reactor concept for the oxidative coupling of methane over the MnxOy-Na2WO4/SiO2 catalyst, benefits and limitation

https://doi.org/10.1016/j.cattod.2017.08.019Get rights and content

Highlights

  • The secondary oxidation of C2 products is successfully suppressed in CLR.

  • The high temperature needed for OCM on Mn-Na2WO4/SiO2 leads to H2 by TDH.

  • H2 formed through the TDH is responsible for the yield limitation.

Abstract

The chemical looping concept (CLC) has been implemented to suppress the formation of COX in an oxidative coupling of methane (OCM) reactor under a wide range of conditions. In comparison to the normal co-feeding strategy, this technique resulted in an enhanced C2 selectivity at the same methane conversions. Nevertheless, the obtained yield never exceeded 25% which is still lower than the minimum value needed for industrializing OCM. The CLC was applied in mechanistic studies to investigate the consecutive reaction of the main products of OCM, i.e. ethane and ethene. The performance of the reaction of C2 components at 750 °C in the chemical looping reactor was compared with that obtained in the co-feeding experiments. The effect of the surface adsorbed oxygen species on the reaction of both ethane and ethene was investigated. The results of these experiments reveal that some of the mechanistic assumptions about the OCM reaction are not compatible with the nature of MnxOy-Na2WO4/SiO2, one of the most stable and best performing catalysts known for this reaction. The yield limitation is shown to be inherent to the catalyst. However, this limitation should be solvable through the modified process concept aiming at the production of ethane in a separated first step.

Introduction

Ethene (C2H4) is the building block for producing a vast range of chemicals from plastics to antifreeze and solvents. It is among the most produced organic compounds of the petrochemical industry. The market demand of ethene was more than 150 million tons in 2016 and its global growth rate is presumed to be around 3.5% over the next 5 years [1]. This chemical is usually produced in steam-cracking units from a range of petroleum-based feedstocks, such as naphtha, and therefore its production capacity and cost are strongly dependent on the availability and price of crude oil. The increasing demand for ethene [1] from the one side and the limitation in the oil reserves [2], [3] from the other side make an alternative process for producing this essential compound highly desirable. Oxidative coupling of methane (OCM), which converts methane directly into C2 products and higher hydrocarbons (Eq. (1) and (2) [4], [5]), may be the way to realize this aim. This reaction has received lots of attention during the last three decades, since the pioneering work of Bhasin and Keller [6]. In the OCM reaction, methane, which is a cheaper and more abundant resource than oil, is used as the feedstock for producing ethane and ethene (C2 hydrocarbons) [7].4 CH4 + O2 → 2 C2H6 + 2 H2O2 CH4 + O2 → C2H4 + 2 H2O

The reaction is performed in the presence of an oxidizing agent (normally oxygen) to change the OCM process from endothermic to exothermic, which is thermodynamically preferable [4], [7]. However, the presence of oxygen in the reactor also brings total and partial hydrocarbon oxidation reactions, which are thermodynamically even more favorable, in competition with the coupling reaction. In these conditions, not only methane, but also the main products of OCM, i.e. C2+ products, tend to react further and produce more of the COX components, see Fig. 1 [4], [8], [9], [10], [11], [12], [13]. Therefore, despite all the research done on OCM, the minimum yield of 30% towards C2+ products, which is required to make the reaction economically feasible, has still not been achieved.

To enhance the performance of this reaction toward C2 products, it is essential to have better control over the formation of carbon oxide components. To reach that point, the origin of the formation of COX should be known more precisely. There is already a consensus that carbon oxides are produced not only through the homogeneous gas phase oxidation but also the heterogeneous catalytic oxidation of the OCM main hydrocarbons [14], [15], [16], [17], [18], [19], [20], [21]. The occurrence of the surface reactions makes the mechanism of OCM a function of the particular catalyst in use. A vast range of metal oxide catalysts are known to activate this reaction [22], [23]. The MnxOy-Na2WO4/SiO2 catalyst, which was first reported by Li [4], is known from a rich literature to be one of the most effective and stable catalysts for OCM [4], [11], [17], [24], [25], [26], [27], [28], [29], [30]. So far, a C2+ yield of 25% has been achieved for OCM performed over this catalyst [4], [31]. This promising performance has encouraged us to choose it as the model catalyst in this study.

Previous mechanistic studies have revealed two important facts regarding the formation of carbon oxides in the presence of MnxOy-Na2WO4/SiO2 catalyst [11], [17]. Firstly, the results of the pump probe experiments have indicated that the oxygen is activated on the surface of the catalyst and forms either selective or unselective surface species [8]. The former species were reported to be strongly adsorbed on the surface while the latter were weakly bounded there. Therefore, purging the reactor with an inert gas was observed to remove a part of the unselective oxygen species from the surface of the catalyst while the concentration of the selective ones remained almost unchanged. Under these circumstances, an increase in the selectivity of the reaction toward the C2 products was noticed [8], [17]. Secondly, the total and partial oxidation of C2 products rather than that of methane are revealed to be the main routes to the formation of COX in the OCM reactor [17], [32]. The secondary oxidation of C2 components has been shown to mostly occur in the presence of the gas phase oxygen [32].

The discussed mechanistic features of the reaction clearly indicate the necessity for removing both these oxygen species, those in the gas phase and the unselective weakly bounded surface species, for enhancing the performance of OCM. This can be realized by applying the chemical looping concept (CLC) to the reaction. This concept has existed for about a hundred years and is well-known for combustion processes in order to not only separate the formed carbon dioxide from nitrogen but also to avoid the formation of NOX [33], [34], [35], [36], [37], [38], [39]. To take advantage of such a reactor concept for OCM, the catalyst material needs to have the ability to provide a sufficient storage capacity for one of the reactants, in this case, oxygen. The MnxOy-Na2WO4/SiO2 catalyst is already proven to have a good oxygen storage capacity (almost 20 μmol/gcat) [40], [41], [42]. To realize this reactor concept on the laboratory scale two pneumatic pulse valves were installed at the inlet of a fixed bed reactor. All the details regarding the operation and construction of the reactor set up can be found in our recent publication [41].

The performance of oxidative coupling of methane over a wide range of reaction conditions was the first issue tested in this reactor set up [41]. Since a yield limitation of 25% was observed during these experiments, the reactor was applied for mechanistic studies to investigate the reasons causing this behaviour.

Section snippets

Catalyst preparation

All catalytic reactions were performed using MnxOy-Na2WO4/SiO2 catalyst. The preparation procedure and analytical information of the catalyst are described in detail elsewhere [32], [43]. The final catalyst consists of 5 wt.% Na2WO4, 2 wt.% Mn(II) on SiO2 and has a specific surface area of 3.2 m2/g and a particle size of 200–300 micrometer.

Feed stream

Methane (99.95%), Ethane (99.95%), Ethene (99.90%), Oxygen (99.998%) and Helium (99.999%) were purchased from Air Liquide and used in experiments as received,

Effect of CLC on consecutive reactions of ethane and ethene (chemical looping tests)

The performance of OCM at various reaction conditions over the MnxOy-Na2WO4 has already been tested in the chemical looping reactor (CLR) [41]. These results show that at the same level of methane conversion, the selectivity toward C2+ products (ethane, ethene, propane, and propene) in the chemical looping reactor is higher than that measured in the co-feeding reactors. This effect is even more noticeable at lower methane conversion. However, this reactor concept didn’t result in the desired

Conclusion

The chemical looping concept can successfully suppress the rate of unselective oxidation of OCM main hydrocarbons, i.e., methane, ethane, ethene. However, the maximum yield obtained by applying this concept to the reaction is still below the minimum value required for industrializing the process. It was observed that both the inherent nature of the MnxOy-Na2WO4 catalyst and the characteristics of the reaction mechanism cause this limitation. It is shown that the conversion of methane in the CLR

Acknowledgment

Financial support by the DFG (grant no. EXC 314) (UniCat Cluster of Excellence) is gratefully acknowledged.

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