Enantioselective separation of tryptophan by Mg–Al layered double hydroxides intercalated with tartaric acid derivative

A series of tartaric acid derivatives, such as di-p-toluoyl-L-tartaric acid (L-DTTA), di-p-toluoyl-D-tartaric acid (D-DTTA), dibenzoyl-L-tartaric acid (L-DBTA) and dibenzoyl-D-tartaric acid (D-DBTA), were intercalated into layered double hydroxides (LDHs) by the ion-exchange method in this paper. The supramolecular products, DTTA-LDHs and DBTA-LDHs characterized by XRD and FTIR, were explored to recognize tryptophan enantiomers according to the different opticity active of different tartaric acid derivatives in the interlayer of LDHs. Batch studies were carried out to address various experimental parameters such as contact time, pH, temperature and initial tryptophan concentration. The results show that levo compounds modified LDH trend to adsorb D-tryptophan, while L-tryptophan trend to be adsorbed by LDHs intercalated with dextrorotary compounds. The recognition process was confirmed to be a chemisorption process according to the kinetic studies. Separation of tryptophan from aqueous solution using DTTA-LDHs and DBTA-LDHs has been well described by pseudo-second-order kinetic model. The adsorption isotherms showed that the adsorption data fitted the Freundlich isotherm equations well. The thermodynamic parameters indicated that the adsorption process was a spontaneous and endothermic chemisorption process.