Highly efficient production of l-rhamnose from catalytic hydrolysis of marine polysaccharide ulvan over carbon embedded sulfonated resins

Herein, we report hydrolysis of crude ulvan sourced from Ulva fasciata using active carbon embedded sulfated resins as efficient catalysts affording high yield commercially important rare sugar l-rhamnose (upto 85 %) under mild conditions (90–120 °C, 8–24 h). These low-cost (hybrid) solid acid catalysts could be obtained from commercial grade macroporous sulfated resins and waste derived activated carbons by simple mechanochemical approach. Most importantly these catalysts outperformed the benchmark sulfated resins, HCl, solution mixtures of acids and carbon, and enzymes reported in prior art in terms of activity, monomeric sugar selectivity, and reusability (retained 90 % activity upto 5 cycles with the possibility of regeneration). Most importantly the use of such catalyst also afforded a high-quality hydrolysate free from furanic and oligomer impurities which enabled downstream l-rhamnose recovery by crystallization as l-rhamnose hydrate. A preliminary techno-economic analysis also indicated feasibility of a ulvan based l-rhamnose process with the developed catalysts.