Functionalized Biochar with Superacidity and Hydrophobicity as a Highly Efficient Catalyst in the Synthesis of Renewable High-Density Fuels

Developing sulfonic-acid-supported catalysts with superacidity and hydrophobicity is of great importance for many acid-catalyzed reactions where H2O is generated. Herein, a novel hydrophobic superacidic biochar is first reported via a diazo grafting method using halogen-substituted amino-benzenesulfonic acids and 4-tert-butylaniline as sulfonating and hydrophobic reagents, respectively. The resultant biochar has a high Brunauer–Emmett–Teller (BET) surface area (400–700 m2·g–1), acid concentration (up to 1.5 mmol·g–1), and H2O contact angle (above 134°). The synergistic effect of superacidity and hydrophobicity greatly improves the catalytic alkylation reactions of 4-ethylphenol-phenylmethanol and 2-methylfuran-cyclic ketones for the synthesis of renewable high-density fuels. The hydrophobic superacidic biochar shows higher catalytic conversion and selectivity than those of its counterparts without hydrophobicity or superacidity. Meanwhile, it is also far superior to commercial Amberlyst-15 and the traditional sulfonated biochar. Finally, the catalyst is stable without obvious activity reduction after cycling.