Efficiently Catalytic Conversion of Cellulose to Levulinic Acid in Biphasic Molten Salt Hydrate and Methyl Isobutyl Ketone
An efficient approach for synthesis of levulinic acid (LA) was reported by converting cellulose in biphasic system consisting of molten salt hydrate (MSH), (LiCl·3H2O) and methyl isobutyl ketone (MIBK), and using NbOPO4/HZSM-5 as a catalyst. A series of molten salt hydrates (LiBr·3H2O, LiCl·3H2O, ZnCl2·3H2O, CaCl2·3H2O and Ca(NO3)2·3H2O) were evaluated in terms of their dissolvability to cellulose as well as the catalytic activity without any additional catalyst in conversion of cellulose to LA. Among them, LiBr·3H2O, LiCl·3H2O and ZnCl2·3H2O were capable of dissolving and swelling cellulose and thus demonstrated excellent performance in cellulose conversion to LA even without additional catalyst. High temperature favors the formation of LA while low temperature is beneficial to saccharides and 5-hydroxymethylfurfural (HMF). With NbOPO4/HZSM-5, efficient conversion of cellulose to LA was achieved in biphasic system, affording a LA yield as high as 94.0 %. The partition coefficient of HMF between MIBK and LiCl·3H2O phases increased notably with elevating the temperature, proving biphasic solvent restrained the secondary reaction of HMF to humins. Reaction mechanism study based on the investigation of converting glucose, fructose and HMF to LA found that the side reactions of saccharides to humins are the key factors affecting the selectivity of LA in the investigated reaction system. Therefore, it is crucial to match the cellulose hydrolysis reaction with the subsequent reactions to maintain saccharides including glucose and fructose at low concentrations for highly efficient conversion of cellulose to LA.