Heteropoly acid and ZrO2 bifunctionalized organosilica hollow nanospheres for esterification and transesterification

Abstract

Single-micelle-templated preparation of heteropoly acid and ZrO₂ bifunctionalized organosilica hollow nanospheres (H₃PW₁₂O₄₀/ZrO₂-Et-HNS) is developed by co-hydrolysis and -condensation of bissilylated organic precursor, 1,2-bis(trimethoxysilyl)ethane (BTMSE), with a zirconium source (Zr(OC₄H₉)₄) in the presence of H₃PW₁₂O₄₀, triblock copolymer surfactant F127 and 1,3,5-trimethylbenzene (TMB) followed by boiling ethanol washing. Through tuning the molar ratios of Si/Zr in the initial gel mixture, the morphology transformation from the 3D interconnected mesostructure to the hollow spherical nanostructure is realized. The inner diameter of the H₃PW₁₂O₄₀/ZrO₂-Et-HNS materials is in the range of 6–12 nm, and their shell thickness is ca. 2 nm. As novel organic–inorganic hybrid catalysts, the catalytic activity of H₃PW₁₂O₄₀/ZrO₂-Et-HNS is evaluated by the model reactions of esterification of levulinic acid (LA) with methanol to methyl levulinate and transesterification of yellow horn oil with methanol to biodiesel at refluxing temperature (65 °C) and atmospheric pressure. The obtained excellent heterogeneous acid catalytic activity of H₃PW₁₂O₄₀/ZrO₂-Et-HNS is explained in terms of their strong Brönsted and Lewis acidity, unique hollow nanospherical morphology and hydrophobic surface. Finally, the recyclability of the hybrid catalysts is tested through three consecutive catalytic runs.