Autocatalytic Depolymerization of Protonated Ulvan into Monosaccharides and Furfurals

Ulvan, a major component polysaccharide of the cell wall of the fast-growing Ulva species, is a promising biorefinery feedstock. We developed an autocatalytic strategy for selective depolymerization of ulvan. Protonated ulvan was prepared by exchanging 99 % of the intrinsic metal cations with protons using an H-form ion-exchange resin with unaltered structure. Hydrothermal treatment of the protonated ulvan at 140 °C in an aqueous medium caused efficient depolymerization into the constituent monosaccharides, including rhamnose (32 C-%), xylose (12 C-%), glucose (6 C-%), and glucuronic acid (10 C-%). This autocatalytic approach afforded comparable, near-stoichiometric yields of neutral sugars relative to conventional solid acid catalysts, significantly enhanced glucuronic acid yield, and suppressed furfural-based by-product formation. Interestingly, hydrothermal treatment in γ-butyrolactone solvent at 140 °C promoted depolymerization into valuable furfurals, yielding 5-methylfurfural (16 C-%) and furfural (5 C-%), corresponding to approximately 40 % of the theoretical yields. These results demonstrate that the autocatalytic depolymerization of protonated ulvan can be optimized to effectively yield either monosaccharides or furfurals, with the product selectivity being controlled by the choice of solvent.