Sulfonic acid functionalized silica nanoparticles as catalysts for the esterification of linoleic acid

Abstract

In this work we describe a comprehensive study on sulfonic acid functionalized silica nanoparticles (SiO₂NPs) as novel catalysts for esterification of linoleic acid in the presence of methanol. SiO₂NPs were covalently functionalized with propyl- or aryl-sulfonic acid groups (SO₃H–) by several post-grafting methodologies through a sylilation reaction involving methoxy/ethoxy derivative organosilanes. Alkyl-sulfonic SiO₂NPs were prepared by a two-step protocol using 3-mercaptopropyltrimethoxysilane followed by oxidation using H₂O₂ (1) or HNO₃ (2) and chlorosulfonation (3). Aryl sulfonic acid SiO₂NPs were prepared by a two-step protocol using phenyltriethoxysilane (a) and phenyltrimethoxysilane (b), followed by chlorosulfonation (4a and 4b) and sulfonation (5a and 5b); direct silylation with (4-chlorosulfonylphenyl)ethyltrimethoxysilane allowed the preparation of 6. The physicochemical characterization of all SO₃H–SiO₂NPs showed that the nanocatalysts 3, 4b and 6 exhibited the highest acidity capacity (estimated by EA sulphur content and potentiometric titration), with pH_pzc lower than 2.0, suggesting that the use of either methoxy derivative organosilanes and chlorosulfonation steps played key roles in the surface acidity fine-tuning of SiO₂NPs. These catalysts exhibited excellent performance in the heterogeneous esterification of linoleic acid with 100% conversion in a 2 h reaction time, with rate constants in the range of 1.80–5.55 h⁻¹ and TOF values in the interval 53–498 h⁻¹. The nanocatalyst 6 showed the highest stability and reusability for 5 catalytic cycles.