Synthesis of NiCuMgAl-layered double hydroxides using advanced microwave and ultrasound methods

The effect of using ultrasound vs magnetic stirring, microwaves vs conventional heating, and autoclave vs refluxing on the synthesis of NiCuMgAl-layered double hydroxides (LDH) was widely studied. The use of ultrasounds during coprecipitation resulted in lower LDH crystallinity, difficulted the incorporation of the divalent cations in the layer and led to low specific surface area and low basicity. This could be attributed to smaller crystallization nuclei formed under ultrasound; that is, more efficient agitation due to cavitation phenomena, led to less crystal growth. The use of microwaves refluxing during the aging step instead of conventional heating resulted in higher crystallinity, allowing a better incorporation of the cations in the layer, obtaining higher specific surface area and higher amount of stronger basic sites. Interestingly, the use of autoclave at higher temperature (180 °C) but at shorter time (1 h) improved the crystallinity of the LDH samples, especially in the stacking direction and applying microwaves, favoring the incorporation of the cations in the layer. Autoclave under microwaves led to higher amount of basic sites but lower surface area than autoclave by conventional heating at the same conditions. This suggests that microwaves favored a better incorporation of the hydroxyl groups into the layers and/or the appearance of surface-defective sites.