STEIN RESEARCH GROUP

POROUS SOLIDS — NANOCOMPOSITES — SELF-ASSEMBLED FRAMEWORKS

Mesoporous Solids with Organic Functionality

Mesoporous silicates, such as MCM-41 (a silicate with hexagonal arrays of 2-10 nm channels), can be synthesized by exploiting cooperative self-assembly interactions between surfactants and a silica source. These materials possess high surface areas, ordered channel structures, and narrow pore size distributions — characteristics which can be beneficial in applications such as sorbents, host structures and catalysts for reactions involving large molecules. We have developed and investigated several synthetic approaches that permit modification of the mesoporous sieve surface in order to effect such applications. One method is a direct, one-pot synthesis of organically functionalized mesoporous silicates. We demonstrated that vinyl, thiol, and other organic functions could be incorporated in these well-ordered materials in-situ, and proved that they are located within the channels. Through further functionalization it was possible to attach other surface moieties. Surface grafting with vinyltrichlorosilane resulted in hydrothermal stabilization of the mesoporous sieve and converted the sieve into a good sorbent for non-polar solvents in aqueous mixtures or emulsions.

 
   Fig. 1. Transmission electron micrograph of
a vinyl-functionalized MCM-41 sample

By employing quaternary ammonium surfactants with bis(alkoxysilyl) precursors such as bis(triethoxysilyl)ethylene, mesoporous materials with hybrid frameworks were synthesized. In these solids, called Unified Organically Functionalized Mesoporous Networks (UOFMNs), the inorganic and organic wall components have a fixed stoichiometry and are covalently bonded. The surfactant is removed by extraction with acid, resulting in a high surface area porous organosilicate framework in which Si atoms are bridged by ethane or ethylene groups. The pore structures of UOFMN materials consist of worm-like channels that are uniform in diameter. Ethylene groups could be partially brominated with little or no damage to the pore structure, the brominated product itself being reactive as a bromide source. UOFMN materials were shown to absorb organic compounds from aqueous mixtures.

     Large organic molecules, such as water-soluble, cationic porphyrins, could be incorporated in mesoporous silicate sieves directly during a hydrothermal surfactant-templated synthesis of the sieve or by ion-exchange with the cationic surfactants after the synthesis. Metallation of the porphyrin with Cu²⁺, Ni²⁺, or Fe²⁺ was possible without leaching the porphyrin from the mesoporous sieve. The copper-loaded porphyrin was stabilized by the support towards auto-oxidation in an oxidative bleaching reaction of an azo dye.