Selective Adsorption of Sterically Hindered Phenols through a Single-Point Binding Mechanism

Abstract

There are growing demands for improved selectivity in industrial separations. Although preorganized three-point binding systems can achieve these selectivities, supramolecular systems are seldom economical for the bulk separations of low-valued chemicals. In this study, we examined the potential of a one-point hydrogen-bonding mechanism to confer selectivity to the adsorption of phenolic species encountered in the industrial synthesis of the antioxidant, 2,6-di-tert-butyl-p-cresol (butylated hydroxytoluene, BHT). The adsorption of 2,6-di-tert-butyl-p-cresol from hexanes onto an acrylic ester sorbent was observed to have a 270-fold lower affinity and a 2.4 kcal/mol less favorable adsorption enthalpy compared to adsorption of the unbutylated species p-cresol. To provide mechanistic explanations for the observed adsorption behavior, we performed infrared spectroscopic and molecular-modeling studies with a small molecule analogue of the acrylic ester sorbent. These small molecule binding studies indicate that the phenolic hydroxyl of p-cresol forms a strong hydrogen bond with an appropriate accepting site, while the bulky tert-butyl groups sterically hinder hydrogen bonding for 2,6-di-tert-butyl-p-cresol.