Deactivation of Alkali Promoted Magnesia in Oxidative Coupling of Methane

doi:10.1016/S0167-2991(09)60359-1

Studies in Surface Science and Catalysis

Volume 34, 1987, Pages 183-195

https://doi.org/10.1016/S0167-2991(09)60359-1 Get rights and content

Abstract

The deactivation of MgO and Li/MgO catalysts during oxidative coupling of methane is studied together with the morphological and chemical changes of the solids. Evidence of different processes through magnesia sintering and loss of alkali are presented and the nature of the active surface is discussed in terms of Li/Mg interface.

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Cited by (35)

Oxidative coupling of methane on Li/CeO<inf>2</inf> based catalysts: Investigation of the effect of Mg- and La-doping of the CeO<inf>2</inf> support
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CO32−+O2−→CO42−+O−2O−→O22− On the other hand, Mirodatos et al. [106] and Xu et al. [107] argued that carbonate species, formed by the presence of CO2 during the reaction of OCM, have a negative effect in the catalytic activity and selectivity. In particular, according to the study of Xu et al. [107] catalyst deactivation could be avoided by having the appropriate concentration of CO2 and high reaction temperature.
The work presented herein reports on the oxidative coupling of methane (OCM) performance of a series of Li-free and Li-doped CeO₂ and CeO₂ modified with Mg²⁺ and La³⁺ catalysts. The supporting materials (Ce, Mg-Ce and La-Mg-Ce metal oxides) were synthesized using the microwave assisted sol-gel method, while lithium ions were added using the wet impregnation technique, to further affect the physicochemical properties, activity and selectivity of the materials, in terms of the desirable hydrocarbon products (C₂H₄ and C₂H₆). The materials were characterized towards their textural, structural and redox properties, surface basicity, and surface morphology using N₂ adsorption/desorption, X-Ray Diffraction (XRD), Raman spectroscopy, CO₂-Temperature Programmed Desorption (CO₂-TPD), H₂-Temperature Programmed Reduction (H₂-TPR), Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS). Catalytic activity was assessed between 600 and 870 °C, at atmospheric pressure and different CH₄:O₂ molar ratios and Weigh-basis Gas Hourly Space Velocity (WGHSV). It is concluded that low specific surface area values, the existence of surface moderate basic sites, increased concentration of oxygen vacancies and the presence of electrophilic oxygen species (O₂^– and O₂^2–) on the catalyst surface had a crucial role on the improvement of the catalytic performance in terms of the desirable products, mainly ethylene. It is noted that the addition of Li changed thoroughly the reaction pathway and the products’ distribution, with the C₂ selectivity values exceeding 85%. The Li/Mg-Ce catalyst, presenting the higher population of intermediate basic sites and surface superoxide species, showed an improved catalytic activity in terms of X_CH4 and production of ethylene, while the incorporation of La³⁺ into the crystal structure of CeO₂ suppressed the production of ethylene. Finally, smaller CH₄:O₂ molar ratios suppressed the production of hydrocarbons, while enhanced residence times, favoured the dehydrogenation of C₂H₆ to C₂H₄.
Oxidative coupling of methane (OCM): An overview of the challenges and opportunities for developing new technologies
2021, Journal of Natural Gas Science and Engineering
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However, although catalyst stability is crucial to ensure the economic viability of OCM, early studies rarely probed this property. Mirodatos et al. (1987) alerted the Li-doped MgO catalyst deactivation, which occurred along with morphological and chemical changes during OCM reactions. The authors observed that the catalyst surface was stable until 907 K under oxidizing conditions, but the admission of the reagent mixture promoted sintering by forming Li2CO3.
This review discusses the thermodynamic and kinetic limitations that currently make the oxidative coupling of methane (OCM) industrially inviable. We analyze some promising strategies to overcome such limitations. We present a brief screening of the OCM catalysts, focusing on crucial aspects to improve their catalytic performance: methane and oxygen activation, acid-base properties, tuning electronic properties by doping, mixed oxides, and nanostructured materials. We especially discuss the current understanding of the Mn–Na₂WO₄/SiO₂ catalyst by contrasting the well-consolidated literature against recent fast-developing in situ/operando studies. Finally, we analyze membrane reactors and monolith-supported catalysts as engineering approaches to manage thermal and kinetic OCM issues.
Effects of low molar concentrations of low-valence dopants on samarium oxide xerogels in the oxidative coupling of methane
2021, Catalysis Today
Citation Excerpt :
Low valence dopants (LVDs) have been shown to be effective dopants for the OCM reaction with a variety of host oxides, and the alkali metals typically result in the largest improvements [39–44]. One issue with alkali metal dopants, particularly Li and Na, is that they are volatile under reaction conditions and are therefore prone to rapid deactivation [34,37,45,46]. In comparison, low-valence transition metals such as silver or nickel, are typically ineffective at promoting coupling over complete oxidation [47–50].
The effects of low-valence dopants on the catalytic properties of samarium oxide xerogel catalysts were investigated in the oxidative coupling of methane (OCM). More specifically, very low concentrations (0.1 and 1.0 % by mol) of transition metal (Ag, Ni, and Cu) and traditional alkali metal (Li and K) dopants were investigated. At these low loadings, it was shown that transition metal dopants have potential to improve the activity and selectivity over an undoped Sm₂O₃ xerogel, but these dopants can only outperform alkali metal dopants under certain conditions. Even at a concentration of 0.1 mol %, the dopants significantly increased the number of basic sites compared with the pure Sm₂O₃ xerogel. However, no trend is evident between the number or strength of the basic sites and the activity or selectivity in the methane coupling reaction. The XRD data reveal a lattice expansion upon addition of the low valence dopants, which is consistent with substitutional doping and the formation of oxygen vacancies due to charge compensation. In most cases the majority of the dopant stayed in the lattice during reaction. The dopants were also shown to influence the Sm₂O₃ structure, and the dopants that were more effective in suppressing the transformation from cubic to monoclinic Sm₂O₃ in general resulted in the more active and selective catalysts. While the Ag- and Ni-dopants could outperform the alkali metal doped catalysts in narrow temperature ranges, the best performing catalysts were still the K-doped Sm₂O₃ catalysts, as the 1.0 % K catalyst exhibited the highest activity at the lowest temperature (500 °C) and the 0.1 % K-doped catalyst was the most stable during extended operation. These results indicate that transition metal dopants, at low concentrations, can positively affect the activity and selectivity of a methane coupling catalyst, such as Sm₂O₃, and suggests that there may be benefits to other OCM catalyst systems from traditionally non-selective dopants, as long as the concentrations are kept very low and stability issues are addressed.
Doped samarium oxide xerogels for oxidative coupling of methane—Effects of high-valence dopants at very low concentrations
2021, Catalysis Today
Citation Excerpt :
Most of the previous literature has focused on doping with alkali metals, typically with high concentrations of alkali metal dopants, or mixtures of rare earth oxides [52,55,59,60]. One issue with many of the alkali metal dopants is that they readily deactivate resulting in poor catalytic activity with time on stream [57,59,61,62], and this may be why higher concentrations are typically used. Few OCM studies of Sm2O3 have focused on transition metal dopants [51,63–65] as their ability to change oxidation state (reducibility) can lead to over-oxidation and thus poor C2+ selectivity [64,66].
The effects of high-valance dopants on the catalytic properties of samarium oxide xerogels were investigated at concentrations of 0.1 and 1.0 % (by mol) in the oxidative coupling of methane (OCM). Gd, Y, Zr, and V dopants were selected to examine the influence of oxidation states between +3 and +5 on the OCM performance. Even at these low loadings, the high-valance dopants were observed to have a significant impact on the OCM reaction. At the lowest loading, 0.1 mol %, and below 700 °C, all dopants improved the activity over that of the pure Sm₂O₃ xerogel, and most also improved the selectivity. In particular, the ethylene yield was significantly improved over these catalysts between 500 and 700 °C. However, the stability of the doped catalysts compared to the undoped Sm₂O₃ xerogel were inferior above 700 °C, and the higher concentration (1.0 mol %) resulted in catalysts with a lower stability. Time-on-stream experiments revealed that the 0.1 mol % high valence dopants improved the stability of the Sm₂O₃ xerogel at 700 °C. As a result of the higher stability, the doped catalysts retain more of the original specific surface area and appear to stabilize the more active cubic Sm₂O₃ phase compared with the undoped catalyst. Therefore, the doped catalysts have a higher number of available basic sites during reaction. This study reveals that high-valence dopants have potential to improve the low temperature (500–700 °C) activity of OCM catalysts. However, the concentrations must be kept very low, as dopants that increase the activity and selectivity at concentrations of 0.1 mol % can result in inferior catalysts at 1.0 mol %, and temperatures above 700 °C must be avoided or rapid deactivation can occur.
Porous silicon carbide as a support for Mn/Na/W/SiC catalyst in the oxidative coupling of methane
2017, Applied Catalysis A: General
Citation Excerpt :
Systems based on Li/MgO and on Mn/Na/W/SiO2 represent the two most frequently studied catalysts. Li-doped MgO exhibits high initial activity and selectivity [7], but suffers from an intrinsic instability, which prevents its industrial application [8–10]. Silica-supported oxidic Mn/Na/W-phases reported e.g. by Li et al. [11] show better long-term stability and are therefore frequently studied with respect to active sites, reaction mechanism [12–15] and process engineering [16].
Efficient conversion of methane to higher hydrocarbons via oxidative coupling of methane “OCM” is one of the dream reactions in heterogeneous catalysis. Promising yields of C₂ hydrocarbons were reported for nanostructured catalysts based on Mn/Na/W-SiO₂. However, the exact role of the nanostructure could not be studied so far due to collapse of the pore structure of the SiO₂ support at typical OCM temperatures. We investigate porous SiC as an alternative catalyst support for OCM catalysis. Mn/Na/W/SiC catalysts with different pore size were synthesized via impregnation, calcined under different atmospheres and then studied in OCM. The calcination critically impacts the pore structure and surface area of the obtained catalysts. Calcination in nitrogen preserves the support’s structure significantly better than calcination in air. However, the nitrogen-calcined catalysts show a strong loss of porosity during OCM testing. The loss of porosity is caused by the presence of Na, which induces a melting of the surface layer that typically protects SiC against bulk oxidation. OCM tests suggest that the activity and C₂ yield of Mn/Na/W/SiC catalysts depend critically on the supports stability, i.e. the surface area retained under reaction conditions.
Oxidative methylation of toluene with methane over alkali modified X zeolite catalysts
1999, Studies in Surface Science and Catalysis
The oxidative methylation of toluene with methane was carried out at 750°C on zeolites, containing lithium and cesium introduced both by ion exchange and/or impregnation. The catalytic behaviour was shown to be favoured by: the additional impregnation of the already ion exchanged zeolite; the modification with cesium (as compared to lithium); the simultaneous impregnation of both lithium and cesium in the parent zeolite. The study of the basicity shows that it guides the catalytic activity in the direction that the more basic zeolite displays better catalytic properties. Non-framework species which are detected in the completely preserved structure of all modified zeolites, are supposed to be responsible for the best basicity and activity of the zeolite prepared by simultaneous impregnation with lithium plus cesium.

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Deactivation of Alkali Promoted Magnesia in Oxidative Coupling of Methane

J. Catal.

J. Catal.

J. Am. Chem. Soc.

J. Phys. Chem.