Sulfonic acid functionalized hydrophobic mesoporous biochar: Design, preparation and acid-catalytic properties

Sulfonic acid functional strong acidic catalysts are largely used in various chemical reactions. However, in many reactions with water as product, the catalytic activity and selectivity are unsatisfactory, because hydrophilic acid sites of SO₃H would suffer from acidity decreasing and some hydrolysis side reactions would occur via water adsorption. Herein, a novel hydrophobic arenesulfonic acid functionalized biochar was successfully prepared for the first time by one-pot diazo reduction method of biochar with amino-arenesulfonic acid (such as, 4-aminbenzenesulfonic acid, 4-amino-1-naphthalenesulfonic acid, 8-amino-1-naphthalenesulfonic acid, 4-amino-3-hydroxy-1-naphthalenesulphonic acid). It has a large specific surface area of 200–400 m²/g, a hydrophobic network with water contact angle higher than 120° and a higher concentration of sulfonic acid over 1.0 mmol/g. Moreover, the hydrophobicity-oleophilicity and acidity are increased with the arene length and grafting amount of arenesulfonic acid. In the esterification reaction of fatty acid with methanol as well as the transesterification reaction of glycerol trioleate with methanol for the production of biodiesel, the sulfonated biochar shows a higher conversion of 96.7% for esterification and 86.3% for transesterification compared with amberlyst-15 (86.7%, 39.9%) and traditional sulfonation biochar-SO₃H (27.4%, 32.6%). In the alkylation reaction of 2-methylfuran with cyclopentanone for the production of high-density biofuel, its catalytic efficiency with target product yield of 76.1% is higher than that of amberlyst-15 (50.2%) and traditional sulfonation biochar-SO₃H (13.2%), because of its hydrophobicity and strong acidity. Furthermore, the catalyst is stable and shows an excellent cycle performance after 6 runs. The successful preparation of hydrophobic biochar-based acidic catalysts not only provides a new way for high-value utilization of biochar, but also eliminates the negative effect of water on many catalytic reactions.