Hydrolysis of alginate into its monomers over sulfonated carbon catalyst derived from glucose

Abstract

To mitigate the global pollution and reliance on petroleum­based fuels and chemicals, various types of biomass, such as agricultural crops, wood, and algae, have been investigated. Among these biomass feedstocks, algae is considered as a promising renewable resource due to its rapid growth, inedibility, and ligin­free composition. Alginate, the major constituent of algae, has found its wide applications in numerous fields. Similar to the structure of cellulose composed of glucoses via 1,4-­glycosidic linkage, alginate consists of two hexuronic acids, β–D–mannuronic acid and α–L–guluronic acid by the ether bond. With the structural analogy, alginate can also be subjected to the current biorefinery technology, namely, hydrothermal conversion, to produce value-added organic compounds. To hydrolyze biomass effectively, employing of acid catalysts is essential. The use of homogeneous catalysts, however, causes critical issues, such as separation, purification, and neutralization. In this sense, developing solid acid catalysts easily separable and having a high catalytic activity has drawn interest worldwide. In this work, hydrothermal depolymerization of alginates into its two monomers over sugar­derived solid acid catalysts was performed to investigate effects of catalyst properties, such as acid density, specific surface area, and thermal stability, on the yield of monomers. The catalysts bearing three functional groups, phenolic OH, ­COOH and ­SO3H, were synthesized by partial carbonization of D­glucose and sulfonation. It was found that the presence of strong brønsted acid sites(-SO3H) catalyzed hydrolysis of alginate, whereas weak acid sites(­OH and ­COOH) had negligible effect on the reaction. The heterogeneous carbon catalyst shows a comparable activity with that of homogeneous catalysts such as sulfuric acid. The high activity is explained by high acid density and hydrophilic surface of the catalyst, despite low surface area. The sulfonated carbon shows the possibility of replacing homogeneous catalysts and could propose a green route to decompose alginate.