Selective Adsorption of Methoxyphenol Positional Isomers

Disubstituted aromatics are commonly synthesized by electrophilic substitution of the ring. However, these reactions typically yield a mixture of ortho and para isomers. In this study we examined whether adsorption from a nonpolar solvent onto a polar sorbent could offer regioselectivity for separating such positional isomers. Specifically, we examined adsorption of substituted phenols onto an acrylic ester sorbent that has previously been observed to adsorb phenolic compounds through a hydrogen-bonding mechanism. The isotherms for the individual isomers indicated small regioselectivities for the adsorption of the alkylphenols cresol and propylphenol. Considerably larger regioselectivities were observed for methoxyphenol adsorption, with the para isomer being preferentially adsorbed compared to o-methoxyphenol. In mixture studies, the separation factor for p-/o-methoxyphenol separations was observed to exceed 15. Infrared measurements suggest that these high separation factors were attained because o-methoxyphenol can form an intramolecular hydrogen bond, and this intramolecular hydrogen-bonding mechanism competes with the hydrogen-bonding adsorption mechanism. These results demonstrate that when hydrogen bonding is the primary adsorption mechanism, solution-phase intramolecular hydrogen bonding suppresses adsorption and this suppression can be exploited to enhance adsorptive separation factors.