Separation of amino acids by selective sorption through reconstructive intercalation in calcined MgAl-layered double hydroxide

Abstract

Mg–Al layered double hydroxide (MgAl-LDH) exhibits selectivity in the intercalation of amino acids (AAs). When the MgAl-LDH derived mixed metal oxide was treated with different mixtures of AAs, preferential sorption of one AA over the other(s) was observed as indicated by XRD analysis of the products and HPLC analysis of the interlayer AA contents in the products. The order of preference was aspartic acid, glutamic acid (acidic AAs) > glycine, alanine (neutral AAs) > hystidine, and arginine (basic AAs). Among the acidic AAs, aspartic acid was preferred over glutamic acid and among the basic AAs, histidine was preferred over arginine. LDH shows equal preference for glycine and alanine. The selectivity can be explained on the basis of the isoelectric pH (pI) of the AA. A similar selectivity order was obtained when the mixtures of AAs were treated with nitrate-intercalated LDH (direct anion exchange) although the net AA intercalated is much lower due to competition with carbonate derived from atmospheric CO₂. The high selectivity observed in some cases (such as aspartic acid and glycine) would result in the quantitative separation of the individual AAs from their mixture.