Lewis Acid-catalyzed Conversion Of Carbohydrate Into 5-hydroxymethylfurfural

With the rapid growth in fossil energy consumption and the imited storage of fossil fuel resources, renewable and clean energy development and application are widely recognized and valued. In a variety of renewable resources, biomass-derived carbohydrates are a promising carbon-based alternative energy source and constitute a sustainable chemical feedstock. Recently, increasing interests and efforts are focused on the dehydration of carbohydrates into 5-hydroxymethylfurfural which is an important"bridge"molecule connecting biomass and petrochemical industry. This study is aimed to develop a novel and efficient method to make the reaction more environmentally friendly, more green,more suitable for commercial production and being more widely used. The main raw material for dehydration of carbohydrates into 5-hydroxymethylfurfural are glucose and fructose. Compared with glucose, it is easier to convert fructose to 5-hydroxymethylfurfural by direct dehydration. The catalysts commonly used in reaction are including solid acid catalyst, Lewis acids, organic acids and inorganic acids. Unlike fructose, glucose can not be converted to 5-hydroxymethylfurfural by simple dehydration. In the presence of catalyst, the glucopyranose was converted to fructofuranose by isomerization at first and then dehydrated to 5-hydroxymethylfurfural, and the first step was the rate-determining. Someone found that CrCl2 is the most effective catalyst for the conversion of glucose into 5-hydroxymethylfurfural in ILs. Based on previous research, a systematic study on the conversion of glucose and fructose to 5-hydroxymethylfurfural in integrated ILs and Lewis acid system has been presented herein. A variety of Lewis acids have been examined for the transformation of glucose and fructose into 5-hydroxymethylfurfural in the ionic liquids and found that SnCln and CrCln are effective catalysts for the glucose conversion. In the fructose conversion, acidity is the key factor which influence the 5-hydroxymethylfurfural yield. Lewis acid with stronger acidity tends to provide a higher 5-hydroxymethylfurfural yield. Furthermore, the late lanthanide chlorides show high activity and good selectivity in the conversion. The effect of ILs structures to the conversion has been discussed. When we used glucose as substrate, we observed that a distinct odd-even carbon-atom-number effect for short side-chains in imidazole-based ionic liquids. The higher yield of 5-hydroxymethylfurfural was achieved in the ILs when alkyl side-chain has a even number of carbons. In the fructose conversion, the imidazole-based ionic liquids with short alkyl side-chains can provide a relatively higher yield of 5-hydroxymethylfurfural. In the presence of 1-ethyl-3- methylimidazolium bromide ([C2MIM]Br), the yields of 5-hydroxymethylfurfural are 65% catalyzed by SnCl2 from glucose and 90% catalyzed by ErCl3 from fructose, respectively.Compared to homogeneous reactions, heterogeneous catalytic reaction is more suitable for continuous production, catalyst recycling and reduce the cost of reaction. Therefore, another research is conducted on the dehydration of carbohydrates into 5-hydroxymethylfurfural catalyzed by heterogeneous catalyst to overcome the shortcomings of homogeneous reactions. In the fructose conversion, solid acid catalysts are widely used, such as acidic ion exchange resins, heteropoly acid and so on. However, there are few studies on glucose conversion. In this part, the framework of MCM-41 mesoporous molecular sieves which incorporated with heteroatoms are used to catalyze glucose and fructose into 5-hydroxymethylfurfural. It is found that only Sn-MCM-41 could catalyze the conversion of glucose to 5-HMF with amazing yield but the others do not work. The maximal yields of 5-hydroxymethylfurfural are 51% from glucose and 63% from fructose catalyzed by Sn-MCM-41 at 110°C with 6 h, respectively. After the reaction, the catalyst can be separated by centrifugation, washed with water and ethanol, calcined at 673 K to remove adsorbed by-products prior to the reuse and the activity of catalyst is stable in the next run.