Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study
Graphical abstract
Introduction
Activation of methane over heterogeneous catalysts remains an attractive subject in view of the abundance of natural gas and renewable methane resources. The scientific interest in oxidative coupling of methane (OCM) to ethane, ethylene, and higher hydrocarbons (C2+ products), however, went down noticeably during the past years. One reason might be that in spite of numerous attempts using chemically quite different catalysts, C2+ yields of about 30% have not been significantly surpassed so far. In addition, the diversity of known active catalyst masses complicates the identification of a general functional model [2]. The not yet fully resolved relationship between surface and gas phase chemistry of CH4 at the high reaction temperatures inspired us to deal with the fundamental questions of methane activation on the surface of heterogeneous catalysts in a systematic approach addressing in particular the function of surface defects using magnesium oxide as a model.
Li-doped MgO was discovered by Lunsford et al. as an active catalyst in the oxidative coupling of methane using molecular oxygen 28 years ago [3]. The originally proposed reaction mechanism involves the activation of gas-phase O2 on either intrinsic cationic vacancies on the surface of Li-free MgO or on substitutional Li+ ions under the formation of O− or [Li+O−] centers, respectively [3], [4], which can abstract a hydrogen atom from methane. The resulting methyl radical is released to the gas phase to undergo selective coupling to ethane. The selectivity is, presumably, to a large extent governed by consecutive reactions, since methyl radicals may not exclusively collide with themselves. By reaction with gas phase oxygen or with O2− ions on the catalyst surface, CH3O2 radicals or surface methoxy species CH3O− can be formed, respectively, which are considered as intermediates in the undesired formation of CO2 that limits the C2+ yield. The mechanism, which has been proposed by Lunsford et al. for MgO and Li–MgO, has been, meanwhile, applied to chemically very different catalysts [5].
The outstanding activity of Li–MgO catalysts compared to pure MgO in methane activation has been explicitly attributed to the presence of the specific [Li–O]− centers [6]. However, lithium as a fluxing agent causes sintering of magnesium oxide at the high reaction temperatures [6]. A clear impact of the varying morphology of MgO, which changes in the course of the sintering process, on undesired secondary surface reactions of re-adsorbed methyl radicals was not observed [6]. In return it was shown that lithium, which completely desorbs by the formation of volatile compounds during the oxidative pretreatment of the catalyst (which involves the loss of all [Li–O]− centers), acts as a structural promoter that favors the formation of terminating higher index planes such as {1 1 1} or {1 1 0} [2], [7]. The highest selectivity to C2+ products was found for pure MgO catalysts, which expose a greater fraction of {1 1 1} planes [8].
Inconsistent findings have also been reported with respect to activity. From studies of magnesium oxide catalysts, which show similar morphology, but different cube size, it was concluded that edge and corner sites are not catalytically significant under steady-state conditions [8]. However, Ito et al. reported that low coordination ions on the surface of MgO play an important role in the dissociation of adsorbed methane [9].
Defects will definitely fulfill a key function with respect to the activation of either methane or oxygen by changing the electronic structure of the wide band gap magnesium oxide particularly with regard to facilitate the transfer of electrons between the solid surface and the adsorbed molecules, which undergo a redox reaction [10], [11], [12], [13], [14], [15]. The present work addresses relations between the nature and abundance of morphological surface defects such as steps and corners on pure magnesium oxide and its reactivity in the oxidative coupling of methane. Various synthetic techniques have been applied to prepare nano-structured MgO catalysts with different morphology. In Part I of this work, we will report about synthesis, microstructural, and kinetic analysis of the morphologically different MgO catalysts. Part II deals with the spectroscopic investigation of coordinatively unsaturated surface sites and the electronic structure. We propose a reaction path for the activation of methane on freshly activated MgO that is confirmed by the detection of reaction intermediates using EPR spectroscopy. The observed fast deactivation of magnesium oxide is interpreted in terms of a change from a concerted reaction that involves co-adsorption of methane and oxygen on mono-atomic step sites to a sequential reaction in which methane and oxygen are activated independently. The latter scenario dominates under steady-state conditions.
Section snippets
Starting materials
Magnesium chips (99.98%, Sigma Aldrich), magnesium oxide (Puratronic®, 99.99%, Alfa Aesar), methanol (ROTIPURAN®, ⩾99.9%, p.a., ROTH), and toluene (ROTIPURAN®, ⩾99.9%, p.a., ROTH) were used as received. Ultrapure water was obtained by using the Milli-Q Synthesis System (MQ). All gases for the catalytic reaction were purchased at Westfalen AG. The purity of nitrogen, argon, and oxygen was 99.999%, and the purity of hydrocarbons was 99.95%.
Catalyst synthesis
Magnesium oxide was prepared applying various dry and wet
Synthesis of nano-structured MgO catalysts
Different preparation methods and post-synthetic treatments have been applied with the objective to obtain magnesium oxide nano-particles with varying primary particle size, shape, and surface area. Sol–gel synthesis (SG-MgO), and oxidation of metallic magnesium (S-MgO) were used starting from metallic magnesium. Furthermore, commercially available magnesium oxide (C-MgO) was modified by hydrothermal treatment at normal pressure (HT-MgO), and at elevated pressure in a microwave autoclave
Discussion
MgO catalysts exhibiting different materials properties, such as mean crystallite size, particle size, surface area, and concentration of surface defects were compared in view of their reactivity in the oxidative coupling of methane. The importance of structural aspects in the oxidative coupling of methane over magnesium oxide has been reported previously [8]. Hargreaves et al. studied the morphology of three differently prepared MgO catalysts by transmission electron microscopy and the results
Conclusions
At this point, we postulate that two different mechanisms occur in the oxidative coupling of methane over MgO catalysts: (i) simultaneous methane and oxygen activation that is surface-mediated and characterized by high methane conversion and high C2H4 selectivity; and (ii) a reaction mechanism under steady-state conditions involving gas phase combination of methyl radicals to C2H6 and C2H6 partial oxidation. Over fresh MgO catalysts, the surface-mediated coupling mechanism is predominant. Based
Acknowledgments
The authors thank Gisela Lorenz and Pia Kjaer Nielsen for their help with the surface area measurements, Dr. Frank Girgsdies and Edith Kitzelmann for performing the XRD analysis, and Iris Pieper at the Technical University Berlin for chemical analysis. This work was conducted in the framework of the COE “UniCat” (www. unicat.tu-berlin.de) of the German Science Foundation.
References (42)
- et al.
J. Catal.
(2015) - et al.
J. Catal.
(1992) - et al.
J. Cryst. Growth
(2011) - et al.
Catal. Today
(1994) - et al.
Chem. Eng. Process.
(2011) - et al.
J. Catal.
(1991) - et al.
J. Catal.
(1989) - et al.
Fuel Process. Technol.
(1995) - et al.
J. Catal.
(1995) - et al.
Catal. Today
(1992)