Synergistic catalytic effect of various species in SnCl4(aq) for conversion of Enteromorpha prolifera derived rhamnose to 5-methylfurfural

The growing demand for 5-methylfurfural (5-MF) across various applications has driven the pursuit of environmentally friendly production methods. Herein, we proposed a facile and highly efficient method for converting Enteromorpha prolifera -derived rhamnose into 5-methylfurfural (5-MF) in H 2 O/MIBK media using metal chloride salts as catalysts. SnCl 4 was capable of achieving a high 5-MF yield of 68.8 % under optimized conditions. The ionization level of metal cations in an aqueous solution might be impacted by the presence of counter anions or charge density, which in turn could affect the yield of 5-MF. The experimental and DFT study results revealed that Sn(IV)-OH species, specifically [Sn(OH) 2 (H 2 O) 4 ] 2 + species as a Lewis acid, predominantly facilitated the tautomerization of chain rhamnose to ketose (rhamnulose), a key step in 5-MF formation, rather than Sn(IV)-Cl species. The Brønsted acid (H 3 O + ), formed from the SnCl 4 hydrolysis, facilitated the conversion of cyclic rhamnose to chain rhamnose and the final dehydration to yield 5-MF. The chloride ion (Cl - ) might interact with ether groups in cyclic rhamnose or aldehyde group in chain rhamnose to promote the formation of 5-MF. This work might provide some valuable insights for converting Enteromorpha prolifera -derived rhamnose to 5-MF and improving the utilization of algal biomass. • SnCl 4 ·5 H 2 O exhibited outstanding efficiency for 5-MF production from rhamnose. • As high as 68.8 % yield of 5-MF was achieved. • The key cooperative roles of various species in SnCl 4 (aq) on the catalytic activity were disclosed. • Sn(IV)-OH species ([Sn(OH) 2 (H 2 O) 4 ] 2+ ) mainly promote tautomerization reaction.