Design of Brønsted acidic ionic liquid functionalized mesoporous organosilica nanospheres for efficient synthesis of ethyl levulinate and levulinic acid from 5-hydroxymethylfurfural

Abstract

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.