Determination Of Mechanisms
Via Computational Chemistry For Xylene And Hydroxynaphthalene Separations
Static and dynamic separations for the three xylene and two hydroxylnaphthalene
(naphthol) isomers were experimentally obtained with beta-cyclodextrin
(beta-CD) on mesoporous glass bead and silica gel supports. Beta-CD/support
attachment was either by direct covalent bonding or through tethering
with 3-glycidoxypropyltrimethoxysilane. 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. Beta-CD tethering decreased the separation
efficiencies of the xylene and naphthol isomers over those obtained by
beta-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 steric hindrance of guest entering 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 beta-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.