L-lysine entrapping in ZSM-5 zeolites with different aluminum ratios: Synchrotron X-ray powder diffraction structural characterization

Amino acids belong to an important class of organic coumpounds exploitable in several application fields, such as the production of pharmaceutical products and biomedical sensors. Among all the materials tested (e.g., activated carbon and polymers), zeolites and their adsorption properties, represent an effective alternative in placing amino acids in form of well ordered-layers on the solid surface [1]. Based on this, it has been decided to test the efficiency of ZSM-5 zeolite (MFI topology) in l-lysine adsorption, locating and quantifying the amount of the biological compound adsorbed within zeolite channels system. With this purpose, two different samples of l-lysine-loaded ZSM-5, with different Si/Al ratio (SAR 51 and 21), were characterized through Synchrotron X-ray powder diffraction analysis. Loaded zeolite samples were obtained from aqueous solution by using batch method and, kinetics and adsorption isotherms data were collected through Capillary Electrophoresis. Powders patterns after contact were collected at the MCX beamline of the Elettra Synchrotron at Room Temperature for both samples. Obtained data were processed through GSAS software [2] and structural refinements revealed that the higher the SAR, the lower the sorption capacity. Difference Fourier maps and occupancies refinements confirmed the presence of higher amount of l-lysine in the SAR 21 ZSM-5: 1.674 l-lysine molecules were detected instead of 1.09 found in the SAR 51 ZSM-5. Also the unit cell analysis confirmed the correct quantification of l-lysine molecules within zeolite channels: a, b and c lattice parameters are lower when higher l-lysine molecules are present. Besides the measurement of ZSM-5 sorption capacity, the localization of the amino acid within the zeolite channel systems, the multidisciplinary approach used has also allowed us to: 1) characterize the sorbent structure after l-lysine adsorption and 3) probe the interaction between the adsorbate and zeolite framework. All the information gained suggest that both analysed samples are suitabale in l-lysine adsorption processes but, among them, the ZSM-5 with the lower Si/Al ratio is particularly performant. [1] Qiang G., Wujung X.,Yao X., Dong W., Yuhan S., Feng D. and Wanling S. (2008) Amino acid adsorption on mesoporous materials: influence of types of amino acids, modification of mesoporous materials, and solution conditions. J. Phys.Chem. B. 112, 2261-2267. [2] Larson, A.C. and von Dreele, R.B. (1994) Report LAUR, 86.