Hydrothermal Conversion of Alginate into Uronic Acids over a Sulfonated Glucose-Derived Carbon Catalyst

Alginate, the major constituent of algal biomass, was hydrolyzed into uronic acids over sulfonated glucose- derived carbon catalysts using water as a green solvent under mild temperature conditions (100, 120, and 140 degrees C). The catalytic activity was compared with that of sulfuric acid and other commercial solid acid catalysts such as carbon black, activated carbon, Amberlyst-15, and zeolites. The carbon catalysts were further treated with nitric acid or sulfuric acid to impart acid properties. The sulfonic group is an active site in the catalysis of the hydrolysis of alginate, and phenolic hydroxyl and carboxylic groups on the carbon surface showed negligible effects on the hydrolysis reactions in the temperature region. A high local acid density of the sulfonated glucose- derived carbon catalyst (Glu-SO3H) facilitates the cleavage of the 1,4-glycosidic bond as alginate adsorbs onto the carbon catalyst surface by a carbohydrate-p interaction. Mannuronic acid and guluronic acid were further degraded into humins or epimerized into other uronic acids, which led to the decrease in the yield of target monomers. The highest monomer yield of 19.6% was obtained if alginate was treated hydrothermally over the sulfonated carbon at 140 degrees C for 1 h. The catalytic activity of Glu-SO3H for the alginate conversion is higher than that of commercial acid catalysts used in this study.