Degradation of Sulfated Polysaccharides into Platform Chemicals: Catalysis Mechanism by a Bro̷nsted Acid and the Effect of Protonated Polar Additives

Sulfated polysaccharides (SPSs) are the main components of algal biomass, which represents a promising feed stock for biorefinary. Molecular-level understanding of the Bro̷nsted acid-catalyzed conversion of SPS into platform chemicals is essential for efficient algal biomass utilization. The calculated results reveal that SPS tends to break the glycosidic bond through the Koshland mechanism in the liquid phase, leading to the formation of monomers. The participation of Cl- ions significantly increases the reaction rate. Thereafter, the Bro̷nsted acid displays favorable catalytic activity in a series of cascading steps for the conversion of the monosaccharides to furfurans, including isomerization, dehydration, decarbonylation, and desulfuration. The rate-determining steps are related to the [1,2]-H shift between C1 and C2 for the stepwise isomerism mechanism. We first proposed the formation pathways of 4-hydroxyvaleric acid (HVA) and succinic acid (SA) initiated from acyclic intermediates, which include site-selective C-C bond fission, dehydration, isomerization, hydration, and SN2 reactions. The concerted dehydration mechanism is the rate-controlling step for the formation of HVA or SA. It is found that the synergistic effect between the products leads to the formation of acid platform compounds. The effect of polar solvent additives is evaluated, and the result suggests that DMSO and THF can promote the formation of furan products and inhibit the formation of acid products.