Glycerol valorization into STBE as an oxygenated fuel additive: influence of propylsulfonic-functionalized silica synthesis on catalytic activity and stability

Glycerol valorization, an important challenge associated with biodiesel production, offers a sustainable route to oxygenated fuel additives. Solketal, obtained by glycerol acetalization with acetone, improves fuel properties but its high polarity limits blending with diesel. This limitation can be addressed through O-alkylated solketal derivatives such as solketal tert-butyl ether (STBE). In this work, STBE was synthesized from 1-mono-tert-butyl glycerol ether (1-MTBG) and acetone using two amorphous silica catalysts functionalized with propylsulfonic acid groups: Am-PrSO₃H_G, prepared by grafting, and Am-PrSO₃H_OP, obtained by a one-pot method. Fresh and used catalysts were characterized by N₂ adsorption–desorption, SEM-EDX and XPS, and catalytic performance was evaluated under optimized batch conditions: 40 °C, 30 min, 1100 rpm, acetone:1-MTBG molar ratio of 10:1 and 2.5 wt.% catalyst.

The one-pot catalyst showed higher surface area and acidity (517 m²/g and 0.56 mmol SO₃H/g) than the grafted material (421 m²/g and 0.11 mmol SO₃H/g), while XPS confirmed that sulfur was fully present as sulfonic acid groups in both solids. Despite these differences, both catalysts achieved high STBE yields, 84% for Am-PrSO₃H_OP and 83% for Am-PrSO₃H_G, indicating sufficient acidity for efficient acetalization. Reusability tests showed stable performance over nine batch cycles, although used catalysts exhibited decreased acidity, suggesting partial leaching of sulfonic groups. In continuous fixed-bed operation, Am-PrSO₃H_OP maintained 100% selectivity but its STBE yield decreased from 68% to 40% after 8 h. These results demonstrate the potential of Am-PrSO₃H_OP as an efficient, reusable and highly selective catalyst for batch STBE production, while further strategies are needed to improve long-term stability under continuous-flow conditions.