(488b) 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.