Placing of amino acids in microporous materials: a ZSM-5 and L zeolites Synchrotron x-ray characterization after l-lysine adsorption

Amino acids, which represent the proteins primary structural unit, belong to an important class of organic compounds exploited in several application fields, such as pharmaceutical, biomedical and food processing industries. The global market based on the amino acids production is constantly growing and cutting-edge sorbent materials for separation processes are continuously required to decrease amino acids losses during adsorption/desorption cycles. Zeolites and their adsorption properties represent an effective material in placing amino acids in form of well ordered-layers on the solid surface (Qiang et al, 2008). Based on this, two samples of l-lysine loaded ZSM-5 (SiO2/Al2O3 ratio (SAR) = 51 and 21, respectively) and one sample of L (SAR 6.1) were characterized through a multidisciplinary approach. Batch method (pH=5.5) was used to enable l-lysine sorption process, capillary electrophoresis separation technique was applied to obtain adsorption isotherms, while thermal analysis were used to quantify the total weight loss. Furthermore, Synchrotron X-ray powder diffraction (carried out at the MCX Beamline of Elettra Synchrotron, at Room Temperature and using a fix wavelength of 0.82700 Å) was performed to study ZSM-5 and L sorption capacity and structural modifications driven l-lysine adsorption. Rietveld structural refinements (processed through the GSAS-EXPGUI software package; Larson and von Dreele, 1994; Toby, 2001) were carried out in monoclinic (s.g. P21/n) and hexagonal symmetry (s.g. P6/mmm) for ZSM-5 and L zeolite, respectively. Results obtained through difference Fourier map analysis highlighted that: in both ZSM-5 samples, l-lysine is located at the intersection of sinusoidal and straight channel and within the sinusoidal channel, while in L zeolite the molecule was detected within the monodimensional channel (i.e., 6 possible orientations). Refined occupancies revealed an amount of extraframework species of about 4.85, 6.51 and 17.63 % for ZSM-5 SAR 51, ZSM-5 SAR 21 and L zeolite, respectively. The content quantified is in good agreement with adsorption isotherms data and thermal analysis results. After l-lysine adsorption, no noteworthy structural deformations were detected: channels retain their circular geometry. All the information gained revealed that ZSM-5 as well as L zeolites are efficient in l-lysine adsorption and this suggests a possible exploitation of both as sorbent materials in biocatalytic processes.