DFT and GCMC modeling study of the adsorption of glucose and xylose on metal-exchanged Faujasite zeolites

In the frame of the separation processes of biomass-based sugars by means of adsorption, the mechanisms of the adsorption of two key sugars, xylose and glucose, on alkali and alkali-earth exchanged faujasite zeolites is examined by 13 C NMR spectroscopy, Density Functional Theory (DFT) and Gran-Canonical Monte-Carlo (GCMC) molecular modeling simulations. Experimental 13 C NMR results demonstrate that adsorption of glucose and xylose in BaX and BaY zeolites occurs under their pyranose tautomeric forms in proportions reflecting that of the fluid phase. DFT modeling highlighted different driving parameters for the adsorption modes depending on the Si/Al ratio (hence the number of cations present in the zeolites): at high Si/Al, adsorption strength may be rationalized by the Pearson HSAB theory, while for lower Si/Al ratios adsorption to several cations across the zeolite supercage is the main driving parameter. These modeling results explain very well the experimental adsorption isotherms in the Henry regime but fail to predict the xylose/glucose selectivity of adsorption from ternary mixtures (glucose-xylose-water). GCMC modeling were thus carried out and we demonstrate the importance of the entropic contribution in glucose/xylose separation, which is reflected in the observed inversion of selectivity as a function of cage loading.