Structure evolution of layered double hydroxides activated by ultrasound induced reconstruction

Abstract

Reconstruction of mixed oxides resulting from the thermal decomposition at 723K of ZnAl and ZnMgAl layered double hydroxides (LDHs) has been studied performing mechanical stirring or ultrasonic treatments for different periods of time in aqueous media. These materials were fully characterized by several physico-chemical methods including powder X-Ray diffraction and Rietveld refinements, XANES and XASF spectroscopies. The results show that the crystallinity increases in the LDH samples after the reconstruction. The crystallite sizes of the reconstructed LDHs increase in comparison to the as-prepared LDHs and this effect is improved upon ultrasound treatments. Moreover, reconstructed samples did not completely recover the original structure and ZnO phase was formed at the expense of an amorphous phase present in the as-prepared LDH. The application of ultrasound favors the formation of ZnO. Incorporation of Mg as ternary cation to obtain Mg/Zn/Al LDH decreases the crystallinity and leads to more disordered materials. The Mg/Zn/Al LDHs present lower amount of amorphous phase than ZnAl LDHs which also decreases in the reconstructed samples. But, in contrast to the behavior observed for Zn/Al, ultrasonication alters the crystallinity and increase of the amount of amorphous phase when sonication time increases. The basicity (number, strength and nature of sites) increases after reconstruction of the mixed oxides into the LDH structures as well as with the sonication time; while the opposite trend was observed for the acid sites. Moreover the basicity is greatly enhanced upon introduction of Mg in the LDH. The changes in the population of acid–base sites affect the behavior of the catalysts in the carbonylation of glycerol.