Enhanced Hydrogen Bonding for the Adsorptive Recovery and Separations of Oxygenated Aromatic Compounds from Renewable Resources

Abstract

Chemicals that are available from renewable resources typically contain heteroatoms that make hydrogen bonding a prominent intermolecular interaction. We are examining how hydrogen bonding can be exploited as an adsorption mechanism to facilitate the recovery and separations of oxygenated aromatic compounds from renewable resources. Specifically, we examined the adsorption of compounds that have weak or attenuated hydrogen-bonding abilities. The solvent in our study was hexane, which is commonly used in the food industry to extract chemicals from plant material (e.g., for vegetable oils and flavor extracts). The low dielectric constant of hexane also facilitates the hydrogen-bonding adsorption mechanism. Three polymeric adsorbents of differing basicities were studied, an acrylic ester sorbent (XAD-7, Rohm and Haas), a pyridine sorbent (Reillex 425, Reilly Industries), and a tertiary amine sorbent (IRA-93, Rohm and Haas). Adsorption affinities (related to the adsorption equilibrium constant, Keq) and adsorption enthalpies (−ΔH°) were observed to increase with increasing basicity of the sorbent's binding site. Infrared spectroscopy was used to study the interaction mechanism between the solutes and small-molecule analogues of the binding sites for the three sorbents. These studies support the conclusion that the observed increase in adsorption results from increased hydrogen-bond strengths. These results indicate that, by adjusting the hydrogen-bond accepting abilities of the adsorbent, it is possible to alter adsorption affinities to balance the dual objectives of recovery and selectivity.