Direct Intercalation of Amino Acids into Layered Double Hydroxides by Coprecipitation

First, the direct intercalation of amino acids into various layered double hydroxides (Mg–Al, Mn–Al, Ni–Al, Zn–Al, and Zn–Cr LDHs) by coprecipitation has been investigated using phenylalanine (Phe) as a guest amino acid. The degree of Phe coprecipitation was strongly influenced by the solution pH and kind of LDH system and reached a maximum in the pH 8–10 region. The solid products were found to have an expanded LDH structure, which supported the belief that Phe was intercalated into LDHs as an amphoteric ion or anion. Two kinds of configuration for Phe in the gallery, either vertical (Mn–Al, Zn–Al, and Zn–Cr LDHs) or horizontal (Mg–Al LDH) in orientation were observed. Only in the Ni–Al system was the solid product a mixture of Phe/Ni–Al LDH and Phe/Ni(OH)2 composite. Next, the thermal decomposition of the intercalated Phe was revealed to be accelerated by the catalytic action of metal ions in the host basal layer. The formation of Phe/Mg–Al, Mn–Al, and Zn–Al LDHs was also elucidated as being appreciably different by the kind of LDH system. Moreover, for various amino acids, the degree of coprecipitation was found to be strongly influenced by amino acid sidechains, i.e., length, structure, and physicochemical properties.