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.

Acknowledgments

This research was sponsored by the Colorado Advanced Materials Institute, Chevron Research and Technology Company, and the Department of Energy Office of Science through Grant #DE-FG03-93ER14363. J. D. Way would like to thank Dr John Newsam and the staff at Accelerys, Inc. for their hospitality and computing resources during his sabbatical at MSI during the fall of 1999. The authors would like to thank Johns-Manville for supplying the porous glass beads from their Schüller AG subsidiary and IBM for supplying the computing equipment used for the molecular simulations.