Vol.2 No.4 2010

43/68

Research paper : Bioethanol production from woods with the aid of nanotechnology (T. Endo)−273−Synthesiology - English edition Vol.2 No.4 (2010) materials thoroughly in the dry state using equipments such as the ball mill to produce fine wood powder, and to destroy the crystal structure of cellulose (amorphization).Consideration of the mechanism for improving the saccharification by such mechanochemical treatment from the perspective of size, the process is as follows. The saccharification reaction of cellulose by cellulase is the breaking of the bond between glucose with length of 1 nm or less at the active site of cellulase, a protein. As a first step, cellulase must adhere to cellulose. This means cellulase first adheres to the microfibril, since cellulose is basically a microfibril. Cellulase is about 5 nm as a sphere, but the wood powder obtained by dry milling is much bigger. The wood powder obtained by the general dry mechanochemical treatment is about 10 m even if the processing condition is optimized, and hardly any submicron or nanosize wood powder is produced even by prolonged processing[9]. The reason is because the aggregation of particles occurs when the wood is finely broken down in the mechanochemical treatment, and the particle size may increase depending on the material.Another point from the size perspective is the crystallinity of cellulose. The evaluation is done in most cases by powder x-ray diffraction. As mentioned in section 2.1, the cellulose crystal is a microfibril, and the amorphization of cellulose is simply looking at the disarrangement of the hydrogen bond with length of 1 nm or less in the region of 3~5 nm or less. The change of cellulose from crystal to amorphous is a change in a smaller region, considering the size of cellulase. While there are many reports on the enzymatic saccharification of cellulose, the reaction mechanism from the size perspective of cellulose and cellulase had not been clarified.Figure 3 is a summary of the points of the conventional pretreatment technologies from the size perspective. According to the knowledge so far, the destruction and fiber production of wood structure at the level of tens of m, the compositional change by decomposition and removal of hemicellulose and lignin, and the change of crystallinity at the nano level were considered important. There were hardly any discussion for the several nm region at the initial phase of the enzymatic saccharification reaction (adsorption of cellulase to cellulose microfibril), and there was not much viewpoint from the microfibril side.From the above, concerning the factors that improve the enzymatic saccharification of wood through mechanochemical treatment, we thought some guide can be obtained to construct the optimal pretreatment technology by carefully analyzing from various perspectives including the analysis at the microfibril level.4.1 Re-investigation of classic technologyFor the mechanochemical treatment of wood, we conducted basic experiments using a planetary ball mill, and investigated the relationship between the physical and chemical properties and the saccharification of the product[10]. When the mechanochemical treatment was conducted using roughly milled hardwood powder (Eucalyptus < 0.2 mm), the breakdown of raw material wood powder progressed but Fig. 3 Size image of (conventional) pretreatment technology for enzymatic saccharification.Classic mechanochemical treatmentSize1 m1 mm100 µm100 nm1 nmGlucoseWoodWood structureParticle size of wood powderCellulose moleculeRaw materialViewpoint of microfibril is smallCrystallinity of celluloseAcid saccharificationDecomposition and removal of wood component (compositional change)Acid hydrolysisHydrothermal saccharificationHydrothermal treatmentDecomposition and removal of wood component (compositional change)Enzymatic saccharificationCooking treatment, Steam explosion treatmentBreakdown to fiber, decomposition and removal of wood components (compositional change)Enzymatic saccharificationEnzymatic saccharificationGoalWood fiberCellulose microfibrilPrimary wallIntercellular layerSecondary wallS3 layerS2 layerS1 layer20~500 µm0.5 nm2~60 µmHemicellulose lignin ~100 nm~10 µm~3~5 nm6 fibers6 fibers

※このページを正しく表示するにはFlashPlayer9以上が必要です