Solventless esterification of fatty acids with trimethylolpropane using sulfonated amorphous carbons derived from wood powder

Polyolesters synthesized by an esterification between polyols and fatty acids are value-added oleochemicals widely used as lubricants, cosmetics, and food additives among other applications. However, homogeneous acid catalysts are still preferred in their industrial production, despite their associated energy cost and the environmental issues that they present. In this paper, we describe lignocellulose-derived amorphous carbons with a high loading level of SO₃H and their application to the synthesis of polyolesters as a biobased heterogeneous acid catalyst. These sulfonated amorphous carbons could be readily prepared via (i) heat treatment at 400 °C for 1 h and (ii) sulfonation with chlorosulfuric acid. XRD and BET analyses demonstrated that these carbonaceous materials were not crystalline but were amorphous structures with a low surface area. The attachment of SO₃H groups was confirmed by FT-IR and XPS, and the loading level of SO₃H was determined by CHNS elemental analysis. Chlorosulfuric acid gave a higher loading level of SO₃H than other sulfating agents, such as conc. sulfuric acid and fuming sulfuric acid. The higher SO₃H-loaded amorphous carbons exhibited a greater catalytic activity for esterification between trimethylolpropane and fatty acids under solventless conditions. Esterification of oleic acid derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant at over 93% yield in 3 h. The sulfonated amorphous carbons could be reused three times without any significant loss of catalytic activity.