Messi Biology states that magnesium oxide (MgO) is an important catalyst support or co-catalyst in methane reforming reactions. Due to its high thermal stability, strong surface basicity, and good coke resistance, it is widely used in various methane conversion reaction systems, such as:
- Steam Reforming (SRM)
- Dry Reforming (DRM)
- Partial Oxidation (POM)
- Tri-reforming
The goal of these reactions is to react CH₄ with H₂O, CO₂, or O₂ to produce syngas (H₂ + CO), which is used for synthesizing hydrogen, methanol, Fischer-Tropsch fuels, and more.
- Mechanism of Magnesium Oxide’s Action
In methane reforming, magnesium oxide typically acts as a support or forms composite catalysts with other metal oxides (such as Ni, Co, La, Ce, etc.), playing the following roles:(1) Surface Basicity Regulation- The basicity of the magnesium oxide surface helps to adsorb and activate CO₂, promoting the DRM reaction.
- Reduces the tendency of carbon deposition (coke) and extends catalyst life.
- Magnesium oxide’s high melting point (approximately 2852℃) ensures that the catalyst does not undergo structural collapse during high-temperature reforming reactions (700–900℃).
- Forms strong metal-support interactions (SMSI) with metals, enhancing metal dispersion.
- Prevents the aggregation of active metals like Ni or Co at high temperatures, maintaining high catalyst activity.
- Applications of Magnesium Oxide in Different Methane Reforming Reactions(1) Dry Reforming of Methane (DRM)
- Reaction Equation: CH₄ + CO₂ → 2H₂ + 2CO (ΔH = +247 kJ/mol)
- Characteristics: Endothermic reaction, usually carried out at 700–900℃, can utilize two greenhouse gases simultaneously (CH₄ + CO₂).
- Advantages of Magnesium Oxide:
- Inhibits coke formation: The basic surface can adsorb CO₂, participating in carbon removal (C + CO₂ → 2CO).
- Combines with Ni to form Ni-Mg-O solid solutions, enhancing anti-coking and thermal stability.
- When synergized with oxides like CeO₂ and ZrO₂, it can improve oxygen migration capacity and anti-sintering properties.
- Examples:
- Ni/MgO catalysts exhibit high stability and anti-coking ability.
- Ni–MgO–La₂O₃ ternary composite systems improve the H₂/CO ratio and catalyst life.
- Reaction Equation: CH₄ + H₂O → CO + 3H₂ (ΔH = +206 kJ/mol)
- Role of Magnesium Oxide:
- Acts as a basic support to regulate the balance between metal activity and the water-gas shift reaction (CO + H₂O → CO₂ + H₂).
- Avoids metal sintering or catalyst poisoning caused by excessive acidity.
- Examples:
- Magnesium oxide can be used with Al₂O₃ as a composite support for Ni (Ni/MgO-Al₂O₃), offering strong stability.
- Ni-MgO catalysts show good selectivity for high H₂ yields and tolerance to partial sulfur poisoning.
- Reaction Equation: CH₄ + ½O₂ → CO + 2H₂ (ΔH = -38 kJ/mol)
- Features:
- Exothermic reaction, often used for rapid reaction conditions.
- Syngas ratio (H₂/CO) is close to 2, suitable for Fischer-Tropsch synthesis.
- Contribution of Magnesium Oxide:
- Improves the redox ability of metal oxides like Ni.
- Helps inhibit complete oxidation (CH₄ → CO₂ + H₂O), enhancing selectivity for CO and H₂.
- When combined with ZrO₂, it improves oxygen transfer capacity and catalytic efficiency.
- Overall Reaction Equation: CH₄ + CO₂ + H₂O + O₂ → syngas (H₂ + CO)
- Features:
- Combines DRM, SRM, and POM to improve thermodynamics and reaction rates.
- Better control of syngas composition (H₂/CO) to meet downstream demands.
- Role of Magnesium Oxide:
- Participates in the basic activation of multiple reaction pathways.
- Maintains a stable structure, especially in intense reactions involving oxygen.
- Used with CeO₂, La₂O₃, ZrO₂, etc., to improve oxygen vacancy content and redox performance.