Li/MgO

Li/MgO(100) Mo(lOO) thin film (selective oxidation of methane to ethane) (7)... 

For economic reasons the oxidative dimerization of CH at about 750°C was extensively investigated. Li+/MgO was the most studied catalyst, but a wide variety of oxides are effective, none lamentably with enough selectivity to be interesting commercially. The reaction which probably involves oxidation at the surface with liberation of CH3 to the gas phase where it dimerizes is, like the Rideal-Eley step, an exception to the earlier statement about mechanism. 

Oxidative methane coupling. MngCa r OjK catalyst placed in the pores of the tube. Ethylene and ethane synthesis from methane. Li/MgO (3 wt. % Li) catalyst placed in the pores of the tube. T 850 C. Conversion 30%. Selectivity to Cj products 60% T 700-750°C. Conversion 40%. Selectivity to Cj products 55%. ... 

Samples of Table 2 were tested for the citral/acetone reaction. The reaction formed essentially pseudoionones and the selectivity to PS was higher than 96 % over all the catalysts during the 6 h catalytic tests. Figure 3 shows the evolution of the relative concentration of citral and the PS yield (tips) as a function of time on Li/MgO-2 and typically illustrates the catalytic behavior of the samples during the reaction. Citral concentration continuously decreases reaching almost complete conversion at the end of the evaluation test while pps value concomitantly increases up to more than 90%. 

Figure 3. Evolution of relative citral concentration and PS yield on Li/MgO-2 sample...

Using a similar procedure, we determined the initial PS formation rate ( Tpg, mol/hm ) from T ps vs tW/n , curves (not shown here). The obtained and r g values for all the catalysts are shown in Table 3. It is observed that r , and r g increase when small amounts of Li are added, reaching a maximum at ca. 0.5 wt. % Li (sample Li/MgO-3) then, the initial catalyst activity decreases for higher Li concentrations. A similar trend with the amount of Li on the catalyst was verified for the pseudoionone yield. In fact, in Fig. 5 we have represented the evolution of T ps values obtained at the end of the catalytic runs as a function of % Li and it is clearly observed that T ps depend on the Li content, reaching a maximum of 93 % for about 0.5 wt. % Li. 

Figure 5. Effect of Li amount in Li/MgO samples on pseudoionone yield...

These data show that small amounts of Mn (0.1 mol %) incorporated into Li/MgO considerably enhance the selectivity to C2 hydrocarbons. Additives of Zn also enhance the C2 selectivity, however, not as significantly as Mn dopants. 

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