dos Santos-Durndell, VC; Durndell, LJ; Isaacs, MA; Lee, AF; Wilson, K; (2023) WOx/ZrOx functionalised periodic mesoporous organosilicas as water-tolerant catalysts for carboxylic acid esterification. Sustainable Energy and Fuels , 7 (7) pp. 1677-1686. 10.1039/d2se01724e.

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Abstract

Catalyst hydrophobicity is an oft-neglected property despite its significance in aqueous phase reactions and those wherein water is a by-product, such as condensation and esterification. Here we synthesise WOx/ZrOx impregnated periodic mesoporous organosilicas (PMOs) of varying organic framework content, through the stepwise substitution of bis(triethoxysilyl)benzene (BTSB) for tetraethyl orthosilicate (TEOS), followed by tungsten and zirconium co-grafting. Incorporation of phenyl groups into the framework of mesoporous SBA-15 silica imparts surface hydrophobicity and tunes the solid acidity, while preserving the textural properties of the parent silica. The resulting WOx/ZrOx/PMO catalysts exhibit excellent turnover frequencies (TOFs) for the esterification of C3–C16 carboxylic acids in methanol at 60 °C, with TOFs inversely proportional to fatty acid chain length. The superior activity and stability (water tolerance up to 50 mol%) of WOx/ZrOx/PMO versus WOx/ZrOx/SBA-15 is attributed to the displacement of water from in-pore active sites, mitigating the reverse ester hydrolysis reaction. Such hydrophobic, solid acid catalysts are anticipated to find widespread application in aqueous phase synthesis, particularly of biorefinery output streams.

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