The Research On The Law Of Carbohydrates Catalyzed By The Extremely Low Acid Concentration In Ethanol Medium

Biomass is renewable resources on the earth that can replace the petroleum-based chemicals. Carbohydrates, are the most plentiful element in biomass resources, can be converted into a variety of high valuable chemicals by biological or chemical pathways. Recently the research on ethyl levulinate and 5-ethoxymethyfurfural from biomass becomes hot.In this research, the distribution of products from glucose and fructose ethanolysis catalyzed by extremely low sulfuric acid in ethanol, ethanol/n-hexane, ethanol/n-hexane/water medium was investigated in low boiling point organic solvent. A series studies aiming at three types of solvent systems for biomass conversion were performed, including the distribution of products from biomass in different processing conditions, the reaction pathway of biomass alcoholysis. Specifically, conversion of glucose and fructose in different reaction systems was selected as the representative route for detailed study.Extremely low sulfuric acid(≤0.1wt%) was used as catalyst. The distribution of products from glucose and fructose ethanolysis was studied. The experiment results indicated the fructose can be converted easily and the yield of ethyl levulinate was much higher compared with the result obtained from glucose with lower reaction temperature and shorter reaction time. The main byproducts of glucose and fructose were ethyl glucoside and 5-ethoxymethylfurfural, whose content gradually decreased when increasing the reaction temperature and prolonging the reaction time. The ethyl levulinate yield of 40.46% was obtained under the reaction condition of glucose concentration 20 g·L-1, reaction temperature 200 ℃, sulfuric acid concentration 0.1 wt% and reaction time 120 min. The ethyl levulinate yield of 70.74% was obtained at fructose concentration 20 g·L-1, sulfuric acid concentration 0.1 wt%, 180 ℃ within 60 min.The effect of reaction temperature, reaction time, sulfuric acid concentration and the different ratio of ethanol/n-hexane were investigated when n-hexane with lower boiling point as a co-solvent. The similar results of the distribution of products were obtained compared with the results in pure ethanol system. The experimental results showed that the addition of n-hexane can not increase the yield of ethyl levulinate effectively, but it is helpful to the accumulation of intermediate of 5-ethoxymethylfurfural, and also can weaken the further hydration of 5-ethoxymethylfurfural to ethyl levulinate. According to the rules of distribution of products, a reaction pathway for the extremely low sulfuric acid catalyzed conversion of glucose in ethanol/n-hexane medium was further proposed.The effect of reaction temperature, reaction time, sulfuric acid concentration and the different ratio of ethanol/n-hexane for fructose ethanolysis was researched. The main product is 5-ethoxymethylfurfural with lower reaction temperature or shorter reaction time. The yield of 5-ethoxymethylfurfural of 66.29% was obtained when the reaction condition of fructose concentration 20 g·L-1, reaction temperature 120 ℃, sulfuric acid concentration 0.1 wt%, reaction time 180 min and the ratio of ethanol/n-hexane 6:4. The influence on products distribution from fructose is more remarkable at higher temperature than lower temperature in ethanol/n-hexane medium.The distribution of products from glucose and fructose in ethanol/n-hexane/water was further studied. In two phase reaction system, products are mainly concentrated in lower reaction liquid and less in the upper solution. When the amount of ethanol was fixed in the reaction system, the yield of levulinic acid from glucose increased significantly, while the yield of ethyl levulinate and 5-ethoxymethylfurfural decreased with the increase of the amount of water in the reaction system. When the amount of water was fixed in the reaction system, the yield of levulinic acid remained almost unchanged while the yield of ethyl levulinate and 5-ethoxymethylfurfural decreased with the decrease of the content of ethanol. The distribution of products from fructose had the similar result from glucose at 120 ℃. The yield of 5-ethoxymethylfurfural increased firstly and then decreased When the water volume fraction of 10% in the reaction system and the increase of the amount of n-hexane, or the ethanol volume fraction of 50% and the increase of the content of water. Glucose and fructose are prone to convert into levulinic acid with the more content of water in the solvent system. Ethanol content has obvious influence on the yield of ethyl levulinate.