Research On 5-Hydroxymethylfurfural Preparation From Glucose Over Ionic Liquids And Lewis Acids

In recent years, increasing demand of fossil fuel has prompted efficient conversion of carbohydrates to fossil fuels. Especially the conversion of cellulose which widely exists in plants to 5- hydroxymethylfurfural(HMF) has drawn much attention. While many problems still exist in economic costs or environmental protection. As the monomer of cellulose, research of glucose conversion becomes the breach of developing novel catalytic systems. At present, ionic liquids(ILs)/metal chloride is the best system for HMF preparation from glucose. While ionic liquids are expensive and the use of Cr Cl3 will cause heavy metal pollution. So it’s so significant to reveal the existing catalytic mechanism and to provide guidance for the development of new, green catalytic system.In our work, polar aprotic solvent was used as reaction medium, glucose or fructose was used as substrate, metal chloride was used as catalyst, Br?nsted acidic IL was used as cocatalyst and HMF was used as target product to develop a serious of experiments. A complementary computational study was also carried out to reveal the catalytic characteristics of Cr Cl3·6H2O, Al Cl3, Sn Cl4·5H2O and the role of ILs. The specific method is as follows.(1) In the aspect of experiment, we discuss the effect of solvent, time, temperature and catalyst amount. Next, a series of reactions were carried out with glucose or fructose as substrate based on the conclusion that fructose is the main intermediate product from glucose to HMF. Metal chloride and ILs are treated as catalyst and cocatalyst. The conversion of substrates and the yield of product were detected to explore the role of each catalyst.(2) In the aspect of quantitative research, our final purpose is to determine whether ILs promote the isomerization activity of metal in glucose isomerization as cocatalysts or additives. Firstly we discussed the combination mode between metal and solvent o r ILs to speculate effect of solvent on the activity of metal. At last, simulation of glucose isomerization over different complexes was carried out to speculate the synergy mechanism of ILs.(3) The result shows that activity of metal chlorides is affected by organic solvents significantly. For example, only 45% of glucose can be converted in dimethyl sulfoxide DMSO over Cr Cl3 after 90 minutes. The conversion rate is much lower than that in N, N-dimethyl acetamide(DMA). Complementary computational and experimental study show that DMSO can prevent Cr Cl3 from attacking glucose by forming Cr Cl3-3DMSO complex, thus conversion rate of glucose decreases obviously. Al Cl3 and Sn Cl4 showed good activity in DMA and DMSO, respectively. Effect of different solvents on them is more complex.In the reaction systems involved in this work, metal salts work in both glucose isomerization and fructose dehydration. Cr Cl3 can promote the two steps efficiently, resulting in a high HMF yield of 66.5%. Sn Cl4 can promote glucose isomerization to fructose but more byproducts forms from fructose. So HMF yield can only reach 43.9%. Activity of Al Cl3 for glucose isomerization is a little worse than Sn Cl4 and HMF yield can reach 39%.ILs especially acidic ILs are mainly responsible for fructose dehydration. They can also catalyze conversion of glucose but this impact is so weak when metal chloride exists. For Cr Cl3, addition of some ILs can accelerate HMF formation but not increase the yield. It’s because that these ILs can promote fructose dehydration in a short time compare with Cr Cl3. For Al Cl3, addition of N- methyl-2-pyrrolidone bromide([NMP]Br) not only shortens the reaction time but also increase the yield from 39% to 57%. Our study shows that [NMP]Br can promote selective conversion of fructose and Br- can strengthen activity of Al3+ by combining with the metal. For Sn Cl4, addition of some ILs lowers the selectivity of HMF by suppressing glucose conversion and promoting byproduct formation.In a word, metal chloride is indispensable for glucose isomerization. The solvent effect on the activity of the metal shouldn’t be ignored. Appropriate ionic liquid can not only accelerate conversion of fructose to HMF but also promote glucose isomerization by combining with metal.