Emulsion-templated hierarchical silica monoliths functionalized with sulfonic acid for esterification and acetalization reactions.

Abstract

Efficient heterogeneous catalysts have been developed owing to their advantages in reusability and sustainability. However, conventional catalysts composed of fine or nanosized particles suffered from a high pressure drop and catalyst loss in fixed-bed or batch reactors, limiting their practical applications. In this work, macro-mesoporous hierarchical silica monoliths were fabricated via an emulsion-templating method with easily available silica colloids as building blocks. Post-grafting of -SH groups and subsequent oxidation with H₂O₂ generated hierarchically porous silica monoliths with sulfonic acid sites. The resulting H-(0.9) SO₃-75%-silica exhibited an interconnected macroporous morphology with a pore volume of 2.67 cm³ g^-1 and a surface area of 232.7 m² g^-1. NH₃ temperature programmed desorption analysis confirmed the presence of acidic sites, while the sulfonic acid site density was estimated from the sulphur content determined by elemental analysis. The H-(0.9) SO₃-75%-silica catalyst demonstrated high performance in both oleic acid-rich fatty acid feedstock esterification and glycerol acetalization. The free fatty acid conversion reached 95% at 90 °C in 6 h with a turnover frequency (TOF) of 32.1 h^-1, while glycerol acetalization achieved 98.6% conversion and 98% solketal selectivity at 20 °C within 30 min (TOF = 1155 h^-1). Moreover, the catalyst maintained high activity over five consecutive cycles, and its monolithic form allowed easy separation from the reaction system, indicating its potential suitability for practical continuous or fixed-bed applications.