Determination of mechanisms via computational chemistry for xylene and hydroxynaphthalene separations on beta-cyclodextrin

Abstract

Equilibrium and dynamic separations for the three xylene and two hydroxynaphthalenes (naphthol) isomers were experimentally obtained with beta-cyclodextrin (β-CD) on mesoporous glass bead and silica gel supports. β-CD–support attachment was either by direct covalent bonding or through tethering with 3-glycidoxypropyltrimethoxysilane. The obtained guest/host separation selectivity ratios were as high as 4.1/1.0 for 1-naphthol from 2-naphthol. Maximum separation ratios were 2.4/1.0 for m-xylene from p-xylene and 0.75/1.0 for o-xylene from p-xylene. β-CD tethering decreased the separation efficiencies of the xylene and naphthol isomers over those obtained by β-CD covalent bonding. Molecular mechanics and semi-empirical methods were used to determine the mechanisms for the above guest–host separation efficiencies. The three mechanisms evaluated were guest–host inclusion energy, charge transfer and steric hindrance. Computational results show that the steric hindrance of the guest entering the host was the controlling mechanism. Levels of steric hindrance were determined by guest–host overlap of electrostatic potential surfaces. A key component concept was used to develop a model for predicting separation selectivities for other guests with β-CD and other guest–host combinations. This model is based on a linear correlation between component/key component selectivity ratios and component/key component electrostatic potential overlap ratios.