(395s) Recovery of Biofuels From Dilute Aqueous Solution With Oxide-Supported Intrinsically Porous Oligomers
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 ABE fermentation broths. More efficient methods are therefore 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 intrinsically porous oligomers covalently immobilized to hydrophilic oxide supports as tunable adsorbents for separation of small organics from aqueous solutions. We previously synthesized these hybrid materials by grafting cup-shaped macrocyclic oligomers known as calixarenes, intended to act as the adsorption sites, directly to silica. We studied the effects of polarizing the calixarene cavities and found that the strongest adsorption occurs when both the adsorption site and the adsorbate are least polar, and that the adsorption energy decreases as the polarity of either the site or the adsorbate increases. These findings suggest that van der Waals forces drive the adsorption process and that permanent dipole-dipole interactions may hinder adsorption on calixarene-silica materials. Presently we investigate the effect of placing a Lewis acidic metal center such as Ti or Ta at the bottom of the calixarene cavity, which is intended to act as a weak coordination site for n-alkanols and allow for possible stronger, more directional interactions. In addition to their potential uses in separations, these materials enable the study of hydrophobic calixarenes in water, where they are otherwise insoluble. The synthesis schemes used here are generalizable and applicable for direct grafting of calixarenes, as well as other types of intrinsically porous oligomers, to different metal oxide supports. This leads to a large number of possible hybrid materials, and thus, the ability to fine-tune adsorbents for a given application.