Elucidation of the layered structures of AMO solvent-washed Mg–Al layered double hydroxides using Mg- and Al K-edge XANES and EXAFS: Impacts of washing solvents on structures and catalytic esterification of benzoic acid with 2-ethylhexanol to 2-ethylhexyl benzoate

Abstract

Recently, aqueous miscible organic-layered double hydroxides (AMO-LDHs) have been found to possess impressively-enhanced surface area and pore volume. Although computer simulations were adopted to understand the interaction between an AMO solvent and an LDH, their layered structures have not yet been clearly depicted. Therefore, in this work, Mg- and Al K-edge X-ray absorption spectroscopy (XAS), in association with characterization techniques such as XRD, BET, TEM, and STA-FTIR, were employed to visualize the layered structures in the atomic level of AMO-LDHs synthesized using two AMO solvents; that are, ethanol and acetone. XANES revealed that Mg (II) and Al (III) had octahedral coordination with oxygen atoms, whereas EXAFS revealed that the coordination numbers of Mg–O and Al–O bonds in the ethanol-washed LDH were distinguished from those of acetone- and water-washed LDHs. In addition, the radial distribution around cations of Mg–O–Al bonds in the layers was different from that in the conventional LDH, due to the differences in charge balance between the LDH sheets and AMO solvents. The illustrations of AMO-LDH layered structures were then constructed, in association with the information from the Molecular Dynamic simulation. As a result, the linear combination fitting based on the Mg K-edge XANES results showed that the change in phase composition of AMO-LDHs was attributed to the significantly-enhanced surface area, acid density, and base density, and consequently to the catalytic activity of AMO LDH-derived mixed oxides on the esterification of benzoic acid, a major component in chemical wastes, with 2-ethylhexanol to valuable 2-ethylhexyl benzoate.