Can two allylic hydrogen react with a divalent cation ?
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Answer:
n and Al layered double hydroxide with interlayer carbonate
When the element of divalent metal cation is changed, the different results are observed in these measurements. Figure 6 shows a general view of in situ HTXRD of ZnAl/CO3-LDH on increasing the temperature from 30°C and 1000°C. The hydrotalcite-like layered structure is observed up to the temperature of 160°C with the intense diffraction line at 2θ= 11.3 Å (d = 7.8 Å) associated with some discrete lines, which is very similar to that of MgAl/CO3-LDH in the temperature between 30°C and 180°C of Figure 1. A metastable phase appears at 180°C with diffraction lines slightly shifted to the higher 2θ angle thus giving a smaller basal spacing d = 7.4 Å (2θ= 11.9 Å).
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Figure 6. In situ HTXRRD patterns of ZnA/CO3-LDH on increasing the temperature; at room temperature, 100°C, 120°C, 140°C, 160°C, 180°C, 200°C, 220°C, 240°C, 260°C, 280°C, 300°C, 320°C, 340°C, 360°C, 380°C, 400°C, 500°C, 600°C, 700°C, 800°C, 900°C and 1000°C.
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The reduced basal spacing has been also reported at 120°C in the HTXRD measurement of ZnAl/Cl-LDH whereas the reduced value recovers on further increasing the sample temperature followed by the collapse of the layer structure [18]. The reason for appearing a metastable phase having smaller basal spacing is assumed to be the same as that of MgAl/CO3-LDH described before. It is common to the three LDHs above that the basal spacing is reduced at lower temperature than that of the layer collapse. This metastable phase disappears promptly at 200°C which is lower than the temperature that Phase II disappears in Figure 1, suggesting the thermal instability of the metastable phase of ZnAl/CO3-LDH. The layered structure collapses and the dehydroxylation occurs at this temperature producing metal oxides, finally a spinel phase appears above 800°C. The temperature at which the layered structure collapses differs depending on the divalent metal species in the layers; 200°C for Zn and 379°C for Mg.
DTA/TG thermal analysis in Figure 7 also illustrates the thermal change of the hydrotalcite-like layered structure of ZnAl/CO3-LDH. A sharp endotherm at 180°C appears with a large mass loss. In the DTA curve of ZnAl/Cl-LDH, however, it has been reported that the intense endotherm splits in two; one is assigned to the elimination of interlayer water and the other to that of lattice one due to the layer collapse [19]. The endotherm in Figure 7, however, includes the elimination of the interlayer water/carbonate and dehydroxylation of the layers of the double hydroxide which induces the collapse of the layered structure. The endotherm at 180°C in Figure 7 corresponds to the metastable phase also observed at 180°C in Figure 6 and this phase promptly disappears on increasing the temperature.
Explanation: