Design of Brønsted acidic ionic liquid functionalized mesoporous organosilica nanospheres for efficient synthesis of ethyl levulinate and levulinic acid from 5-hydroxymethylfurfural†
Brønsted acidic ionic liquids (BAILs) have brought new vitality in catalytic transformation of biomass to fuels and chemicals, but practical applications of BAILs suffer from drawbacks of slow diffusion and difficulty in separation. Chemical immobilization of BAILs is an effective way to circumvent these problems. Here, we demonstrate a series of monodispersed mesoporous organosilica nanosphere-immobilized BAIL catalysts, [C3PrIm][OTf]-MONSs (C3 = PrSO3H, OTf = SO3CF3), by a quaternary ammonium surfactant micelle-directed liquid-interface assembly strategy followed by successive chemical modifications, and the particle size (180–360 nm) and pore morphology (periodic centrally radialized and 3D interconnected mesopores) of the catalysts are well-adjusted by changing the cations and/or anions of the surfactants as well as the preparation conditions. As-prepared [C3PrIm][OTf]-MONSs serve as nanoreactors to transform an important biomass-derived platform molecule, 5-hydroxymethylfurfural, in ethanol and water media to valuable chemicals, ethyl levulinate (EL) and levulinic acid (LA). By the combination of their superstrong Brønsted acidic nature, outstanding open mesoporous spherical nanostructures and excellent textural properties, the [C3PrIm][OTf]-MONSs exhibit high ethanolysis and hydrolysis activity and selectivity. The particle size and pore morphology of the catalysts significantly influence the selectivity and thereby the yield of the products. The cetyltrimethylammonium tosylate-directed [C3PrIm][OTf]-MONSs catalyst with the smallest particle size (210 nm) and a wormhole-like interconnected mesostructure shows the highest yield of EL (93.6%) and LA (72.8%) under the optimum reaction conditions. The catalyst also displays good reusability in ethanolysis reaction, originating from chemical bonding [C3PrIm][OTf] within the hydrophobic silica framework.