Recovery of n-Butanol From Dilute Aqueous Solution with Grafted Calixarenes

Hybrid adsorbent materials have been synthesized with the goal of selective aqueous separations. These materials consist of a high surface area oxide support (e.g. SiO₂, Al₂O₃) with rigidly- and covalently-attached calix(n)arenes, which are intrinsically cavity-containing small molecules. (Fig. 1) This method of surface attachment allows for a high density of these calixarene sites, which in turn each act as a strong adsorption site. In this sense, these materials are proposed to act as ideal Langmuiran adsorbents, where the number of adsorption sites corresponds to the synthesized number of calixarenes.

In Fig. 1, the identity of the R groups, the number of phenol units in the macrocycle (n), the X bridging species, and the surface density of calixarene (s) are all systematically tuned to optimize uptake from the aqueous phase and to understand the mechanism of uptake. A large library of calixarenes are commercially available and have known host-guest chemistry in organic solvents, but have generally not been studied for these types of applications, as these hydrophobic calixarenes cannot typically be dispersed in water or the absence of solvent.

Here we study separation of n-butanol from water at binary concentrations relevant to typical fermentation processes (< 0.1 M).  Butanol is a renewable energy source that has advantages over other bioalcohols, but separation by distillation is not practical, opening opportunities for adsorption. If able to be made selective, as this methodology for highly-tailorable adsorbent surfaces promises, adsorbents could also be added directly to fermentation broths for enhanced yield.

Adsorption uptake and interaction energies are shown to increase monotonically with the number of hydrophobic groups at the upper rim, indicating that adsorption on this first class of materials is dominated by van der Waals interactions. The possibility is also floated of more specific OH-p or CH-p, or in some cases, OH-S interactions between butanol and the calixarene cavity. Fractional uptake is weakly dependent on the surface density of the calixarene adsorption sites, implying that background effects are small. The net adsorption process (exchange of butanol for water within the calixarene cavity) is net exothermic process with a low heat of adsorption. Equilibrated adsorption is demonstrated by reversible uptake upon desorption into water or temperature programmed desorption into the vapor. These organic layers on these surfaces are robust and stable up to 250 °C, which suggests they may be capable of multiple regeneration cycles. In general, these materials are not suitable only for butanol separations. The synthetic protocols presented here can be generalized to a wide variety of functionalized calixarenes and supports, potentially enabling the design of new calixarene-based adsorbents, sensors, and other functional materials.