Effect of particle size and local disorder on specific surface area of layered double hydroxides upon calcination-reconstruction

Abstract

The specific surface area changes in MgAl-layered double hydroxide (LDH), which is also known as hydrotalcite, were monitored along with their changes in particle size, crystallinity and local disorder around Al. Two kinds of LDHs were prepared by hydrothermal reaction during 1 day (H1) and 3 days (H3). The lateral size and thickness of H3 was twice than those of H1; however, the crystallinity and ordering around Al was comparable for both LDHs. Upon calcination at 500 °C, the local disorder around octahedral Al in both LDHs significantly increased with migrating Al to tetrahedral coordination. The degree of disorders and the migration to tetrahedral sites was more severe in H3 than H1, and this was related to the much higher increase of specific surface area in H3 (16.5 times) than in H1 (11.1 times). During the calcination, dehydroxylation of LDH gave rise to the crystal deformation and interstratified structure of metal oxides, resulting in the development of mesopores and enhancement of specific surface area. The calcined samples could recover original LDH phase upon reconstruction reaction, while the crystallinity and ordering around Al was not completely recovered. Although the small mesopores in calcined LDHs seemed to be partially blocked upon reconstruction, the specific surface area of reconstructed LDHs was still higher than pristine LDHs, possibly due to the incomplete recovery of Al ordering in LDHs.