Growing concerns on global warming and the depletion of traditional resources have driven us to look for green and sustainable energy sources. As a biomass- derived platform chemical, the production of 5-HMF has been one of the hottest topics in biomass utilization in these few years, and a large number of publications have appeared. Many works focus on improving 5-HMF yield by applying different substrates, different catalysts, different solvents, different reaction conditions and extraction methods. Application of ionic liquids has been shown to be efficient for the preparation of 5-HMF, and the rapid growing interest in 5-HMF production with ionic liquids from biomass holds great promise for the future. However, this field is still in its infancy and some issues concerning the chemistry and process engineer­ing should be addressed as follows for practical large-scale processes:

1. The high viscosity of ionic liquids leads to some disadvantages in the process, and most of the present used ionic liquids are prepared with petroleum-derived chemicals as starting material. The synthesis and use of ionic liquids in which cation and/or anion are originated from natural resources should be developed in the 5-HMF production. Choline based ionic liquids are good examples that have been reported [38, 63]. The other possible alternatives include glycerol, diethylethanolamine, or amino acid based ionic liquids. The use of these biorenewable ionic liquids permits the processes to be even more green and sustainable. Chapters 2 and 4 provide some exciting new avenues for ionic liquids that dissolve cellulose and that have low viscosity.

2. In the production of 5-HMF from carbohydrates in various solvents, an impor­tant side reaction is the formation of soluble or insoluble polymers known as humins that greatly affect 5-HMF yield. These humins are formed from different intermediates in the reaction and their formation increases with increasing substrate concentration. The formation of humins remains a troublesome prob­lem for 5-HMF synthesis even in ionic liquids. More investigations are required to make clear the detailed formation mechanism of humins and restrain its formation. Interesting results have been reported for the addition of acetonitrile as co-solvent to the glucose — [BMIM][Cl] or [EMIM][Cl] reaction system. Addition of acetonitrile to the reaction system inhibited the formation of humins and enhanced the glucose conversion up to 99 %, with a 98 % 5-HMF selectivity. Moderate amounts of humins of up to 20 % were formed in the absence of acetonitrile as a co-solvent [103]. These results could help to reveal the forma­tion mechanism of humins and find a way to inhibit their formation and to provide a practical method to produce 5-HMF from carbohydrates.

3. To date, efficient production of 5-HMF is mainly from fructose, and the only proposed efficient catalytic system for production of 5-HMF from glucose and cellulose is chromium or tin chlorides that are hazardous to environment. Therefore, more efforts are needed for more environmentally-friendly catalysts that are both efficient and renewable. Functionalized carbonaceous materials possibly could provide new avenues for research in this area [104—106].

4. The starting materials used in most of studies on 5-HMF production are model chemicals such as sugars (glucose, fructose and sucrose) and microcrystalline cellulose, but the use of crude lignocellulosic biomass is relatively lack. How­ever, the composition complexity of the crude biomass will require that the process of 5-HMF production in ionic liquids be greatly different from that when pure chemicals are used. Consequently, more work is required for practical applications in the treatment of actual biomass for 5-HMF production.

5. More works are needed for the separation of 5-HMF product from ionic liquid systems, especially for continuous operation. The extraction of 5-HMF from ionic liquids by supercritical carbon dioxide with co-solvent addition may be a possibility for 5-HMF separation and the creative use of CO2 in forming biphasic systems for separations could provide both an environmental and efficient separation method.

6. Since 5-HMF is unstable, it might be further reacted to produce other chemicals such as furan and esters, and these derivatives would probably be more soluble in CO2 and thus it might be possible to extract them with supercritical CO2.