Catalytic Transformation Of Carbohydrate Into5-hydroxymethylfurfural In Ionic Liquids

Concerns about energy crisis and global warming have led to the exploration ofvalue-added chemical production from carbon-neutral renewable biomass. As one ofthe top platform chemicals derived from biomass,5-hydroxymethylfurfural (HMF)can be used to synthesize a variety of useful acids, aldehydes, alcohols, aminescurrently obtained from petroleum. Herein, conversion of glucose and cellulose wasstudied in ionic liquids，fousing on kinetics of glucose dehydration, influence ofcatalysis coordination and role of halide anions.The effects of reaction conditions on conversion of glucose into HMF werestudied in1-allyl-3-methylimidazolium chloride ([Amim]Cl) with CrCl3·6H2O as thecatalyst. The highest HMF yield was62.5%, obtained at an initial glucoseconcentration of10wt.%, reaction temperature of373K, catalyst loading of6mol%and reaction time of4h. The HMF was fully separated from ionic liquids byextraction with water as the dilutent. The recycle of the CrCl3/[Amim]Cl system wasconducted, finding that the catalytic activity decreased with the increased reuse times.Kinetic studies were performed to understand the glucose conversion mechanism.The reaction is second order in glucose, with an activation energy of134.9kJ mol-1.The order in chromium is first. Thus, the rate-determining reaction of glucoseconversion was supposed to involve two molecules of glucose which was catalyzedby a mononuclear chromium species. A simplified kinetic model was developed todescribe the behaviors of glucose conversion and HMF formation.To investigate the influence of chromium coordination on the catalytic activity,basic compounds were added into the system. It was found that the HMF yieldincreased with a small amount of basic compounds added. However,superstoichiometric amounts of such compounds could render the coordinatelysaturated Cr(Ⅲ) centre, thereby negatively impacting or even completely shuttingdown the catalyst activity.It was found that halide anions were also important for the glucose conversion.The yield of HMF from fructose declined in the order of Br-> Cl-> I-, while HMFyield from glucose declined in the order of Cl-> Br-> I-. Addition ofN,N-dimethylacetamide (DMAc) into bromide type ILs increased the HMF yieldfrom glucose into78.0%, which could be attributed to the increased basicity of Br- according to UV/Vis spectra. Hence, halide anions therefore not only act asnucleophile to facilitate fructose dehydration, but also serve as base to promoteglucose isomerization.The transformation of cellulose into HMF in a single step was also conductedwith [Amim]Cl as the solvent and CrCl3·6H2O as the catalyst. The maximum HMFyield was52.1%, obtained at an initial cellulose concentration of10wt.%, reactiontemperature of413K, catalyst loading of15mol%, reaction time of20min. Theinfluence of halide anions on the transformation of cellobiose and cellulose wasinvestigated. The highest yield of HMF from cellobiose was obtained in[Amim]Br-DMAc mixed solvent. However, dissolution and conversion of celluloseonly occurred in [Amim]Cl and [Amim]Cl-DMAc systems. Therefore, the basicityand nucleofugality of halide anions are also important for cellulose conversion.