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Research paper : Bioethanol production from woods with the aid of nanotechnology (T. Endo)−271−Synthesiology - English edition Vol.2 No.4 (2010) carry water in the middle and then layering the microfibrils around them (Fig. 1 left top). The toughness of wood results from this layer structure. This is often likened to a barrel or tub. A tub is strong because the boards are arranged circularly in a vertical direction, and the hoop is used to bundle them together in a direction 90 degrees of the boards. In the wood structure, the nanosize cellulose microfibrils act like the boards and hoops of a tub, and form a highly layered nanostructure. This tough wood structure makes the pretreatment for enzymatic saccharification very difficult.3 Issues in conventional technology and scenario for the development of new technology3.1 Acid saccharification and enzymatic saccharificationThe method for saccharification of cellulose in the wood can be divided roughly into acid and enzyme methods. Figure 2 shows the advantages and issues of the two methods. The oldest method used is the acid saccharification using sulfuric acid. Currently, large-scale bioethanol manufacturing experiments incorporating new technologies is in progress for this method. The greatest advantage of the sulfuric acid saccharification is that the reaction occurs in a short time using inexpensive sulfuric acid as the catalyst.However, the facilities must be resistant to sulfuric acid, and the recovery and removal of sulfuric acid from the saccharified solution and waste liquid are remaining issues. While these can be solved with the advancement of engineering technology, the greatest problem is that the produced sugar is susceptible, in principle, to over-decomposition into furfural by the action of coexisting sulfuric acid[2]. When over-decomposition occurs, the yield of sugar that enables ethanol fermentation is reduced, and the product of over-decomposition even at relatively small amounts (few %) may inhibit yeast fermentation.On the other hand, since, in principle, no side reaction occurs in enzymatic saccharification, the yield of ethanol end product is expected to increase. The enzymatic reaction progresses at mild conditions of about 50 ºC, and the environmental load is low since it does not require large amounts of chemicals. The enzymes for the saccharification of cellulose are generally called cellulase. There have been over 500 kinds identified, and reactivity of some of them is affected greatly by the crystallinity and structure of cellulose. In actual enzymatic saccharification, a mixture of several enzymes including the ones for hemicellulose is used rather than a single species of enzymes.However, there are also issues in enzymatic saccharification. Enzymatic saccharification requires much longer time compared to acid saccharification. Also, while cellulase is used in the food and textile industries, it is actually fairly expensive. As mentioned earlier, in some cases, large quantity of enzyme is required because the enzymatic reaction may be affected by the structure of cellulose.3.2 Pretreatment technology for enzymatic saccharificationSince untreated wood does not react with enzymes, it is necessary to conduct the pretreatment to increase the reactivity of wood and cellulose and to promote enzymatic saccharification. According to the empirical rules of pretreatment gathered from past R&Ds, the points considered were: to increase of surface area by breaking up the wood into small pieces; to increase reactivity by amorphization of highly crystalline cellulose; and to breakdown and Fig. 1 Diagram of cellulose microfibril and wood structure.Primary wallIntercellular layerSecondary wallS3 layerDirections of microfibril layers are differentAssembly using hemicellulose and lignin as adhesives0.5 nm2~60 µmCellulose molecule20~500 µmWood structureCellulose microfibrilHemicellulose lignin~100 nm~10 µm~3~5 nm6 fibers6 fibersS2 layerS1 layerWood fiber

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