Aqueous phase catalytic conversion of agarose to 5-hydroxymethylfurfural by metal chlorides
The production of 5-hydroxymethylfurfural (5-HMF) from agarose catalyzed by metal chlorides was studied in aqueous phase. A series of metal chlorides, including NaCl, CaCl2, MgCl2, ZnCl2, CrCl3, CuCl2 and FeCl3, were comparatively investigated to catalyze agarose degradation for the production of 5-HMF at temperatures of 180 °C, 200 °C and 220 °C, catalyst concentration of 0.5% (w/w), 1% (w/w) and 5% (w/w), time of 0–50 min, and substrate concentration of 2% (w/w). Results revealed that alkali and alkaline earth metal chlorides, including NaCl, CaCl2 and MgCl2, resulted in higher 5-HMF yields from agarose with negligible amount of byproducts, such as levulinic acid and lactic acid, derived from further degradation reactions. 1% (w/w) MgCl2 was the most efficient catalyst among the tested metal chlorides for 5-HMF production from agarose and resulted in both the highest yield of 40.7% and highest selectivity of 49.1% at 200 °C for 35 min. The cleavage of C–O–C bond in agarose with subsequent isomerization of galactose to its ketose was considered as a possible mechanism for formation of 5-HMF under MgCl2 catalyzed conditions.