Alkali-promoted metal oxide , methane

Alkali-Metal-Promoted Metal Oxides. Researchers at Texas A M University studied methane oxidative coupling using MgO promoted with Oxygen cofeeding was used in all... 

The Institute of Gas Technology recently patented a boron/alkali metal- promoted, metal oxide catalyst for oxidative coupling of methane. The catalytic studies were performed between 700 and 820 °C. The best activity (21%) and selectivity (86%) were achieved using 1% Li/0.2% B promoted on MgO. 

Besides oxidative coupling of methane and double bond isomerization reactions (242), a limited number of organic transformations have been carried out with alkali-doped alkaline earth metal oxides, including the gas-phase condensation of acetone on MgO promoted with alkali (Li, Na, K, or Cs) or alkaline earth (Ca, Sr, or Ba) (14,120). The basic properties of the samples were characterized by chemisorption of CO2 (Table VI). 

From the results of both pulse reaction and adsorption experiments, it could be confirmed that Ni has a strong affinity with methane, while alkali promoters with carbon dioxide. The retardation of coke deposition on KNiCa/ZSI catalyst must be ascribed to the abundantly adsorbed CO2 species. This explanation is similar to the suggestion of Horiuchi et al. [5], showing that the surface of the Ni cat2ilyst with basic metal oxides was labile to CO2 adsorption, while the surface without them was labile to CH4 adsorption. Since coke deposition was mainly caused by methane decomposition, the catalyst surface covered with adsorbed CO2 or reactive oxygen species from the dissociation of CO2 would suppress coke deposition. The addition of alkaline promoters also seemed to greatly suppress the activity of supported Ni catalyst for the direct decomposition of methane. 

Catalysts considered in the present discussion cover a wide spectrum of solids reducible multivalent metal oxides as well as non reducible basic compounds Reducible metal oxides possess some inherent problems whereas these problems are less for the alkali ions promoted alkaline earth oxides. Alkaline earth oxides seem to be more suitable for working at low partial pressure of oxygen. By doping alkaline earth oxides with alkali metal compounds it is conceivable that O species can be stabilized for dissociative absorption of methane. Reducible metal oxides will tend to transform into lower valent oxides or even upto metallic state partly under applied reaction conditions specially at low partial pressure of O2. Both activity and selectivity will be deteriorated. But for the non reducible basic oxides structural changes will be quite different. They will tend to reach an equilibrium state in the surface level amongst the oxide, hydroxide and carbonate phases on reacting with evolved H2O and CO. Both the lattice distortion and the formation of O species can occur in the alkali earth oxides in doping with alkali ions as they can not build a mixed oxide lattice. 

The structural promoter functions to provide a stable, high-area catalyst, while the chemical promoter alters the selectivity of the process. The effectiveness of the alkali metal oxide promoter increases with increasing basicity. Increasing the basicity of the catalyst shifts the selectivity of the reaction toward the heavier or longer-chain hydrocarbon products (Dry and Ferreira, 1967). By the proper choice of catalyst basicity and ratio, the product selectivity in the Fischer-Tropsch process can be adjnsted to yield from 5% to 75% methane. Likewise, the proportion of hydrocarbons in the gasoline range ronghly can be adjnsted to produce 0%-40% of the total hydrocarbon yield. 

Promoters also have an important influence on activity. Alkali metal oxides and copper are common promoters, but the formulation depends on the primary metal, iron versus cobalt (Spath and Dayton, 2003). Alkali oxides on cobalt catalysts generally cause activity to drop severely even with very low alkali loadings. C5+ and carbon dioxide selectivity increase while methane and C2-C4 selectivity decrease. In addition, the olefin to paraffin ratio increases. 

Depending on the mode of feeding of methane and oxidant, different catalytic materials are applied. When the OCM reaction is performed with cofed CH4 and O2, nonreducible metal oxides show higher C2 selectivity than reducible metal oxides. Promoting with alkaline earth and alkali metal oxides significantly enhances the selectivity to C2 hydrocarbons of reducible and nonreducible metal oxides [8,10-12]. Very recently, Ivanov et al. [22] demonstrated that MgO and SrO formed under reaction conditions and stabilized on the surface of Mg-, A1-,... 

In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of alkali and alkaline earth metal doped ceria... 

Halogen species can also be important bonding modifiers, because they are powerful electron acceptors. Indeed, they are used as promoters in several catalytic processes (for example, ethylene oxidation to ethylene oxide over silver, or during partial oxidation of methane). Nevertheless, their molecular and atomic chemisorption behavior has been studied less and therefore is not as well understood as the role of coadsorbed alkali-metal ions. 

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