Direct Growth on Si(100) of Isolated Octahedral Mil-101(Fe) Crystals for the Separation of Aromatic Vapors

Octahedral crystals of Fe(III)-based Mil-101 and amino-functionalized Mil-101 are directly grown on Si(100) using easy solution routes under mild conditions. Images from microscopy analyses show that samples consist of a layer of isolated octahedral crystals in which the [111]-direction develops vertically with respect to the Si(100) substrate. Structural and chemical characterizations indicate that crystals consist of Mil-101(Fe) with the following stoichiometry: Fe-3(O)Cl-(DMF)(2)(Terepht)(3) (DMF = dimethylformamide and Terepht = terephthalate or aminoterephthalate). Both Si-grown Mil-101(Fe) and NH2-Mil-101(Fe) layers are capable of adsorbing two aromatic gases with different polarities, toluene and nitrobenzene. Their characteristic thermal desorption profiles depend on the amino-functionalization of the terephthalate ligand as well as on the mutual interactions between the two guests when adsorbed from a mixture. It is also shown that, by adapting suitable experimental conditions, Mil-101(Fe) layers are capable of separating a toluene/nitrobenzene gas mixture through selective thermal desorption. A model based on the evaluation of the involved host-guest interactions through the density functional theory (DFT) approach is proposed to explain the observed selective desorption and to account for the key parameters involved in the desorption process of aromatic vapors.