Conversion of biochar into sulfonate-bearing solid acids used for the hydrolysis of tylosin: the effect of aromaticity and degree of condensation

Abstract In the last few decades, sulfonated carbon materials have garnered significant attention as Brønsted solid acid catalysts. The sulfonation process and catalytic activity of sulfonated biochar can be influenced by the aromaticity and degree of condensation exhibited by biochar. However, the relationships between the aromaticity, sulfonating ability, and resultant catalytic activity are not fully understood. In this study, biochar samples pyrolyzed at 300–650 °C exhibiting different aromaticity and degrees of condensation were sulfonated and employed as sulfonate-bearing solid catalysts for hydrolytically removing tylosin. They exhibited excellent hydrolytic performance and their kinetic constants were positively correlated with the total acidity and negatively correlated with their aromaticity. This study has uncovered the relationship between the structure, properties, sulfonating ability, and subsequent hydrolytic performance of biochar samples. It was observed that the aromaticity of biochar decreased as the pyrolysis temperature increased. Lower pyrolysis temperatures resulted in a reduced degree of condensation, smaller ring size, and an increased number of ring edge sites available for sulfonation, ultimately leading to enhanced catalytic performance. These findings provide valuable insights into the fundamental chemistry behind sulfonation upgrading of biochar, with the aim of developing functional catalysts for mitigating antibiotics in contaminated water. Graphical Abstract