Novel Separations Media Based On Grafted Calixarenes

Calixarenes are cavity-containing small molecules often suggested to be useful for separations. In solution, these molecules exhibit ‘host-guest’ chemistry wherein a small molecule is included within the calixarene cavity. This property is used, for example, to purify fullerenes.[1] Here, calixarenes are covalently attached to solid oxide supports and examined for the selective uptake of aromatics and butanol from aqueous solution.[2] The former may have applications in sensing or remediation, while the latter may lead to selective adsorbents for this potential biofuel.[3] Calixarenes of different ring sizes, substituents, and bridging groups can be readily synthesized or purchased. The attachment of the calixarene is described and the resulting hybrid organic-inorganic materials are characterized primarily by TGA, UV-visible and photoluminescence spectroscopy, and SS ¹³C CP/MAS NMR. Working adsorbent materials contain up to ~2 adsorption sites per 10 nm² of oxide support, or 0.2 meq adsorption sites per g of a typical SiO₂ sample. Uptake onto solid adsorbents is measured by depletion of bulk solution followed by changes in refractive index. Calixarene substituents control the strength of adsorption of butanol, and the increased uptake with increasing temperature suggests that adsorption is driven by a net favorable entropy change with adsorption, instead of enthalpy, as is typical.

[1] J. L. Atwood, G. A. Koutsantonis, C. L. Raston, Nature (1994) 368, 229-231.

[2] J. M. Notestein, A. Katz, E. Iglesia, Langmuir (2006) 22, 4004-4014.

[3] L. M. Vane, Biofuels Bioproducts & Biorefining (2008) 2, 553-588.