Highly Efficient Alkylation Using Hydrophobic Sulfonic Acid-Functionalized Biochar as a Catalyst for Synthesis of High-Density Biofuels

Because H₂O decreases the acidity of sulfonic acid-based catalysts and initiates catalytic side reactions, the catalytic activity and selectivity of these catalysts are often unsatisfactory in many reactions that generate H₂O as a byproduct. Herein, for the first time, hydrophobic acid-functionalized biochar is prepared by a new route based on a two-step diazo grafting method using 4-aminobenzenesulfonic acid as the sulfonated agent and 4-tert-butylaniline as the hydrophobic agent. The acid density and hydrophobicity can be easily regulated by the amounts of benzenesulfonic acid and tert-butylbenzene groups, respectively. The resulting catalysts have a large specific surface area of 400–700 m²/g, high acid density of greater than 2.0 mmol/g, and strong hydrophobicity with an H₂O contact angle of up to 136°. Compared with Amberlyst-15 and H₂SO₄-sulfonated biochar, the hydrophobic acidic biochar exhibits higher activity and selectivity in the alkylation of 4-ethylphenol with phenylmethanol and of 2-methylfuran with cyclic ketones (cyclopentanone and cyclohexanone) for the production of high-density biofuels. The successful preparation of the hydrophobic acidic biochar not only results in an efficient catalyst for reaction systems in which H₂O has a deleterious effect but also paves a way for the functional modification of carbonaceous materials.