Sugar derived hydrochar catalysts for enhanced biodiesel production via esterification

Sugars and sugar alcohols are renewable raw materials which can be used for char production. Their acid-catalyzed hydro-carbonization results in the formation of carbon materials with acidic functional groups on the surface, which makes them suitable to be used as heterogeneous catalysts in esterification reactions. Hydrochars materials were prepared by low-temperature slow carbonization of glycerol, xylitol, sucrose, and glucose in the presence of different amounts of H₂SO₄ (mass ratios from 1 to 4). The hydrochars were extensively characterized and tested in the methanolysis reaction of oleic acid. All the carbon materials were amorphous with features of polyaromatic sheets with hydroxyl (–OH), carboxylic acid (–COOH), and sulfonic (−SO₃H) functional groups. The effect of the oleic acid water content on catalytic performance was studied, showing a strong inhibition effect due to the hygroscopic character of catalysts induced by sulfonic groups and other surface groups with oxygen. Under the conditions evaluated (5 % wt. of catalyst, methanol/oleic acid = 9 M ratio, methanol reflux temperature) the sucrose-derived catalyst prepared with the highest H₂SO₄ content showed the best performance, leading to methyl oleate yield > 90 % after 2 h of reaction, which is only achieved by the majority of analogous catalysts reported in the literature after 4 to 8 h of reaction. The data showed that there is a correlation between the methyl ester yield and the graphitization of the catalyst, as this makes the catalyst more hydrophobic, which is an advantage in esterification catalysts.