Adsorption of L-lysine on zeolites: effect of different framework topology and different Si/Al ratio

The study of the ability of zeolites to adsorb amino acids is significant to improve the knowledge of the interactions between these molecules and solid surfaces. This is of fundamental importance to increase the level of understanding of much more complex systems and to select adsorbent materials able to selectively capture amino acids from aqueous solutions.[1] Zeolites are aluminosilicate materials characterized by regular crystalline structure with microporous cavities which confer a high superficial area. The physico-chemical proprieties of zeolites depend on the composition of their framework that consist of tetrahedral units of SiO4 and AlO4 and, in particular, their ratio (SAR) characterizes the hydrophilic/hydrophobic behavior of the material. Moreover, the different conformation and dimension of the internal channels and cages influence the adsorption selectivity towards host molecules.[2] The ability of zeolites to adsorb biologically active biomolecules such as amino acids is of particular interest in industrial biotechnology[3] due to the fact that these adsorbent materials could be used as solid solvents to stabilize the different charged forms of the amino acids. In this study we tested the different adsorption capacity of zeolites L and ZSM-5 towards the amino acid L-lysine. These zeolites were chosen since they present different framework, with 2D and 3D channels structures, and different pore dimensions. Moreover, for the zeolite ZSM-5, two materials having different SAR were selected to study the effect of zeolite chemical composition on the adsorption capacity. Kinetic and isothermal experiments were carried out at different L-lysine initial concentrations to study the adsorption processes. The results show high capacity towards the adsorption of L-lysine for every zeolite investigated. We found a higher capacity for zeolite L with respect to ZSM-5. Moreover, zeolite ZSM-5 with greater SAR has shown less adsorption, probably a cause of different surface charge. [1][4] Moreover, the results obtained by thermal analyses (TGDTG- DTA) showed different interactions within the amino acid-zeolite adduct and different stability, depending on the peculiarities of the zeolites studied. Further investigation of the refinements of X ray powder diffraction patterns showed the adsorption sites, the orientation and the interactions of the amino acid molecules within the zeolite channels. This work allowed to improve the understanding of the formation and interactions of the adducts that originate from the adsorption of amino acids on zeolites. The information obtained are useful for separation of amino acids from complex mixtures. [1] G. Beltrami, A. Martucci, L. Pasti, T. Chenet, M. Ardit, L. Gigli, M. Cescon, E. Suard, ChemistryOpen 2020, 9(10), 978–982. [2] A. Martucci, L. Pasti, N. Marchetti, A. Cavazzini, F. Dondi, A. Alberti, Micropor. Mesopor. Mat. 2012, 148(1), 174-183. [3] K. Stückenschneider, J. Merz, and G. Schembecker, J. Phys. Chem. 2014, 118, 5810-5819. [4] T. Chenet, A. Martucci, M. Cescon, G. Vergine, G. Beltrami, L. Gigli, M. Ardit, M. Migliori, E. Catizzone, G. Giordano, L. Pasti, Micropor. Mesopor. Mat., 2021, 323, 111183.