Adsorption of n-butanol from dilute aqueous solution with grafted calixarenes

Materials were synthesized for the recovery of n-butanol from dilute aqueous solutions, as may be useful for applications in biofuel-water separations. These materials are composed of hydrophobic, cavity-containing calixarenes covalently bound directly to porous, hydrophilic silica supports through a Si linker atom rather than a flexible organic linker, as is common, at surface coverages of up to ∼0.25 calixarenes/nm(2) (∼250 μmol calix/g matl). The calixarene ring size, upper rim groups, bridging group (calixarene vs thiacalixarene), and surface density were varied. The materials were characterized by NMR, UV-vis, and TGA. The absolute butanol uptake reached ∼0.16 mmol butanol per gram of material at equilibrium concentrations below 0.12 M and increased monotonically with the calixarene surface density. The background adsorption onto the silica surface was small at high calixarene loading. At 298 K, the free energy of adsorption in the calixarene cavities became more favorable by 3 kJ/mol as the surface area of the hydrophobic calixarene upper rim groups increased from H to methyl to tert-butyl, consistent with adsorption driven by van der Waals interactions. A thiacalix[4]arene-SiO(2) material, containing polarizable sulfur bridges and a larger, more conformationally mobile calixarene structure, had slightly stronger adsorption still. All materials except this thiacalixarene exhibited fully reversible adsorption into solution. As a representative material, the adsorption of n-butanol from aqueous solution at a tert-butylcalix[4]arene site was accompanied by a negligible enthalpy change but a small, favorable entropy change of +50 ± 20 J/mol/K, indicating that adsorption is driven by desolvation. Butanol desorbed from tert-butylcalix[4]arene materials at ∼150 °C into the gas phase, well within the range of stability of calixarenes (<300 °C), indicating that these materials have promise as regenerable adsorbents.