(666e) Selective Adsorption of Biofuels From Dilute Aqueous Solution with Grafted Calixarenes

Fermentation-derived bioalcohols such as n-butanol and ethanol have potential as sustainable fuels, but their production is hindered by inefficiencies in separating from the dilute fermentation broth.  The standard method of distillation is a major energy sink because of the low concentrations and unfavorable vapor-liquid equilibrium properties typical of A.B.E. fermentation broths.  Thus, more efficient methods are needed in order to realize the potential of bioalcohols as renewable fuels.  Adsorption is one such method, but conventional sorbents such as activated carbons have only limited ability to be fine-tuned at the molecular level, and as such, selectivities are typically low.  Herein we investigate the use of calixarenes, intrinsically porous organic macrocycles, covalently immobilized to porous hydrophilic oxide supports, as tunable and uniform adsorption sites for separation of small organics from aqueous solutions.  We previously showed that with nonpolar substituents at the upper rim of the calixarene, adsorption on these materials is entropy-driven, characterized by low net energy differences, and practically independent of temperature, indicating van der Waals interactions as the governing force.  Presently we explore the effects of adding polar substituents to the calixarene structure to closer match the dipole moment of the intended guests, as well as the addition of a Lewis acid site at the bottom of the cavity, which is predicted to act as a coordination center for the guest’s alcohol moiety.  In addition to their potential uses in separations, these materials enable the study of calixarenes in water, where they are otherwise generally insoluble.  Moreover, calixarenes are believed to be constrained to a cone-like conformation due to multiple lower-rim surface attachments as well as steric hindrance from the surface itself.  These materials provide uniform and uniquely tunable adsorption sites on a bulk hydrophilic surface, and thus have great potential as selective adsorbents for biofuel separations as well as a broad range of other aqueous systems.