Vol.2 No.4 2010

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Research paper : Bioethanol production from woods with the aid of nanotechnology (T. Endo)−276−Synthesiology - English edition Vol.2 No.4 (2010) shear force to the water-dispersible slurry of pulp. The water enters the minute gaps of pulp fiber by shear force, acts like wedges to unravel the fibers, and the surrounding water prevents the aggregation of the fluffed fibers. We thought the wood could be broken down to microfibrils by using some treatment similar to the beating process. Therefore, as basic experiment, wet mechanochemical treatment was done using the ball mill with wood powder dispersed in water at 20 times the quantity by weight, and we obtained some highly viscous, creamy product. The product was dried and observed under the scanning electron microscope. It was found that the microfibril formation progressed, and fine fibers of 100 nm or smaller, some of 20 nm at the narrow parts, were produced (Fig. 7). The degree of cellulose crystallinity was studied by x-ray diffraction, and it was found that the same degree of crystallinity was maintained as the raw material. Since the microfibril itself is the crystal body of cellulose, the retention of crystal structure indicated that the microfibril could be unraveled without much damage. Although this wet mechanochemical product (microfibrilized substance) had high crystallinity, degree of saccharification was 70 % or above. This result showed that decreasing the crystallinity of cellulose was not important to increase the enzymatic saccharification, but it was more important to increase the surface area with which the enzymes could react by separating the microfibrils from each other. While the concentration of solids was about 5 % in the wet mechanochemical product, the cellulose microfibril with specific gravity of 1.5 was dispersed without settling out as the water molecules were retained around it. Therefore, space where the enzymes could act freely was formed around the microfibrils. Since the main residue of the saccharification experiment were large fiber structures in the samples with insufficient wet treatment and low saccharification, it was found that the enzymatic saccharification could be increased greatly if the wood were thoroughly broken down into microfibrils.As presented above, the working hypothesis that the separation of microfibril was effective for enzymatic saccharification based on the solid-state NMR, was proven by actually creating microfibrils from wood by the wet mechanochemical treatment. The crystallinity of cellulose was not important in this pretreatment. In section 4.1, the refined wood pulp had high degree of enzymatic saccharification while it was highly crystallized, but in this case, it was thought that the microfibrils separated from each other in the refining process of breaking down and removing the hemicellulose and lignin, and the surface to which the enzymes could approach and react increased. The wood structure is an assembly of microfibrils, and the assembly force is a weak bond such as a hydrogen bond. Therefore, by using the water molecule like a wedge in the wet mechanochemical treatment to break the hydrogen bonds between the microfibrils that keep them together, the microfibril building blocks can be disassembled easily. This method is not troublesome from the wood chemistry perspective.6 Microfibrilization technology for practical application6.1 Investigation of continuous and mass treatment methodThe ball mill used in the fundamental experiment for the microfibrilization treatment is useful for experiments with small amounts of samples, but is done as a batch process, so it is not practical in terms of upsizing and cost reduction. Therefore, we considered a treatment method where shear force and pressure could be applied to the raw material as a wet method. We decided continuous and mass production would be possible using a disk mill that had a similar milling mechanism as a stone mortar.We used a disk mill (Supermasscolloider, Masuko Sangyo Co., Ltd.) to repeatedly mill the wood powder slurry (wood powder concentration 5 wt%), and found that the microfibrilization progressed as in the ball mill, and the enzymatic saccharification of the product greatly increased. In the disk mill treatment, the upper and lower disks were set at about 10 m distance, and the processing efficiency was 10~20 times or higher than the ball mill. However, in the disk mill, there was a dependency on the wood species that was not significant in the ball mill. In some cases, the hardwood did not microfibrilize sufficiently and the saccharification did not improve compared to softwood. This was because the milling energy was smaller in the disk mill compared to the ball mill. Therefore, treatment to weaken the wood structure was necessary before the disk mill treatment.6.2 Combination treatment for weakening the wood structureThe origin of wood strength is the robust layer structure as mentioned before. Although the microfibrils are held together by weak bonds such as hydrogen bonds, separation to microfibril units is not easy due to its board-and-hoop layer structure. Therefore, we thought the wood structure would weaken and the disk mill treatment could be conducted effectively by first destroying the structure that corresponded to the hoop of the tub to increase the water permeability, and then removing the hemicellulose that glued the microfibrils together.The method for destroying the structure that corresponded to the hoop was preliminarily investigated using the ball mill. After conducting dry mechanochemical treatment of the raw wood powder for a certain time, water was added for wet treatment, and the microfibrilization and saccharification of the product were studied. As a result, when the time of dry mechanochemical treatment was 15

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