WOx/ZrOx functionalised periodic mesoporous organosilicas as water-tolerant catalysts for carboxylic acid esterification

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 C-3-C-16 carboxylic acids in methanol at 60 degrees 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.

Automated summary

Despite its significance in aqueous phase reactions and those wherein water is a by-product, the hydrophobicity of catalysts is often neglected. In this study, we synthesized WOx/ZrOx impregnated periodic mesoporous organosilicas (PMOs) with varying organic framework content. The incorporation of phenyl groups into the framework of mesoporous SBA-15 silica imparted surface hydrophobicity and tuned the solid acidity, leading to excellent turnover frequencies (TOFs) for the esterification reactions. The hydrophobic, solid acid catalysts showed superior activity and stability compared to the hydrophilic catalysts, making them suitable for aqueous phase synthesis.