(495b) Surface Anchoring of Nano-Sized Organic Cavities to Create Selective Adsorbents

As economically-relevant chemicals become more structurally diverse, new adsorbent materials are required to selectively interact with the target species. Molecules targeted by our group include small molecule pharmaceuticals for purification, delivery, and recovery applications; heteroaromatic impurities for extractive purification of heavy hydrocarbon fuel sources; and long-chain alcohols for removal from fermentation broths. In contrast to bulk adsorbents that tend either to be disordered at all length scales (e.g. carbons) or be macroscopically crystalline (e.g. zeolites), we design nanometer-size, cavity-containing molecules for the selective host-guest interaction with a target molecule. The molecular cavities are then covalently attached to high-surface area materials such as SiO₂, which synthesizes working adsorbent materials containing up to 3 adsorption sites per 10 nm² of oxide support, or 0.3 meq adsorption sites per g of a typical SiO₂ sample. This synthetic route creates pre-defined numbers of uniform and atom-precise adsorption sites that enable bulk uptake measurements to be connected to individual molecular interactions. We will present results using grafted calixarenes, which possess an internal cavity of ~1 nm, for the selective uptake of alcohols from aqueous solution. We probe the effect of 1) the groups lining the calixarene cavity: bulky, hydrophobic tert-butyl groups or only the aromatic core of the parent calixarene, 2) the role of polarizable groups in the backbone of the calixarene: methylene-bridged vs S-bridged calixarenes, and 3) the use of oxophilic metal sites covalently attached at the base of the calixarene cavity. Preliminary results show saturation of metal-anchored sites when exposed to solutions of 50 mM aqueous butanol solutions, whereas purely hydrophobic sites show weak uptake with adsorption constants near 1 M^-1. Synthesized materials are characterized by TGA, solid-state NMR, UV-visible, gas physisorption, and other techniques where relevant. Uptake onto solid adsorbents is measured by depletion of bulk solution followed by gas chromatography or spectrophotometric titration. Semi-empirical calculations shed light on the important molecular interactions within the host-guest complex.