Influences of polyhydric alcohol co-solvents on the hydration and thermal stability of MgAl-LDH obtained via hydrothermal synthesis

Magnesium–aluminum layered double hydroxides (MgAl-LDH) was prepared by hydrothermal synthesis. Five kinds of solvents, including pure water, water/ethanol, water/ethylene glycol, water/glycerin, and water/pentaerythritol, were respectively employed to synthesize the LDH grains. The microstructures and thermal stabilities of the obtained LDH were then contrastively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), differential scanning calorimetry (DSC), and thermogravimetry (TG). It was shown that the incorption of polyhydric alcohol co-solvents benefit in the combination of crystal water through in-situ reaction. In comparison with the water molecules entered via diffusion, the in-situ combined crystal waters are characterized with high hydration level. But the excessive electrostatic interactions results in the decrease of crystallinity. With the increase of the hydroxyl group numbers in the co-solvents, the crystallinity of the obtained LDH became worse. At the same time, the dehydration temperatures of combined crystal water were elevated, while the deprivation of CO2 from intermediate layers and the dehydration of basic layers should occurred at relative lower temperatures. In the case of the LDH prepared by using pentaerythritol as co-solvents, the crystallinity and thermal stability were the worst because of the poor stacking of the layer structure.

The electrostatic interactions in the hydrothermal systems vary with the employment of various polyhydric alcohol co-solvents including ethanol, ethylene glycol, glycerin, and pentaerythritol. The enhanced hydrogen bonding results in the elevation of hydration degree, and the excessive hydration leads to the decrease of crystallinity and thermal stability.Figure optionsDownload as PowerPoint slideHighlights
► The electrostatic interactions vary with the change of different alcohol co-solvents.
► The hydration level of the LDH can be changed by the use of alcohol co-solvents.
► The co-solvents have important influence on the crystallinity and thermal properties.