Can high temperature calcined Mg-Al layered double hydroxides (LDHs) fully rehydrate at room temperature in vapor or liquid condition?

Abstract

The rehydration behavior of metal oxides (LDOs) derived from calcination of Mg-Al layered double hydroxides (LDHs) at high temperatures of 600-900 degrees C was studied in the presence of deionized water under vapor or liquid conditions. XRD reflections showed that the LDO metal oxides obtained from thermally treated LDHs can rehydrate into LDHs at room temperature even after calcination as high as 900 degrees C, both in vapor and liquid conditions, although faster in the latter. Only when the calcination temperature was increased to 900 degrees C an additional spinel phase appeared, which remained in the particles under both vapor and liquid rehydration conditions. This is proven by 27Al MAS NMR showing the presence of a peak representing tetrahedral Al. Dif-ferences between the as-synthesized LDHs and rehydrated LDOs were observed via additional analysis with FTIR, SEM and nitrogen sorption. This illustrated that the process of the rehydration of the LDO into the LDH resulted in a new LDH rather than reconstructing it into the former LDH. Based on the results, it is suggested that even after high temperature calcination (600-900 degrees C), samples need to be stored carefully to avoid rehydration re-actions to altered LDH materials, occurring at room temperature under ambient, moist containing atmosphere.