Relationship Between Enzymatic Hydrolysis Performance And Structural Characteristics Of Different Plant Derived Celluloses And Influence Of Pretreatment

Lignocellulose material has become a hotspot in the current bio-ethanol energy research area as the most abundant organic resources reserves in the world, due to its advantages including multiple sources, large amount and low cost. However, great differences in enzymatic hydrolysis and fermentation performance may be produced when different kinds of lignocelluloses are used as the raw materials for the production of reducing sugar and ethanol because of their diversity in chemical composition and structural characteristic. Moreover, the structural status and enzymatic hydrolysis process of cellulose has been greatly influenced by other components in the raw material. In the present study, five cellulose samples were separated from two aquatic plants and three terrestrial plants, respectively, hydrolysis experiments of these cellulose samples were conducted at various cellulase loadings (7-200FPU/g cellulose). The enzymatic hydrolysis performance of those cellulose samples and their structural characteristics such as crystallinity index, degree of polymerization and specific surface area were analyzed in order to obtain the relationship between enzymatic hydrolysis performance and structural characteristics independently. Different physical and chemical pretreatment methods were conducted to investigate their effect on the structural characteristics and hydrolysis performance of water hyacinth cellulose, and the influence on the component content and hydrolysis properties of water hyacinth and sugarcane bagasse materials, and the relatively proper pretreatment method for each biomass was obtained, respectively. The main results obtained are as follows.(1) The cellulose contents of aquatic plants (water hyacinth19.55%and water peanut21.78%) are obviously lower than that of terrestrial plants (sugarcane bagasse32.06%, miscanthus35.45%and metasequoia chips47.61%), and cellulose contents of herbaceous plants (sugarcane bagasse and miscanthus) are also lower than that of woody plant (metasequoia chips). The crystallilnity index of sugarcane bagasse cellulose and metasequoia chips cellulose are the lowest (56.42%) and the highest (70.59%), respectively. As a whole, the crystallinity index of those plant celluloses have no huge difference. There is no obvious correlation between the degree of polymerization or specific surface area and cellulose content.(2) The reducing sugar yield of each plant cellulose by enzymatic hydrolysis increased with enzyme loading and hydrolysis time. The hydrolysis efficiency of five kinds of plant celluloses was in the order of sugarcane bagasse> water hyacinth> miscanthus> water peanut> metasequoia chips. In any of the cellulase loadings (7-200FPU/g cellulose), cellulose from metasequoia chips with the highest crystallinity index had the minimum reducing sugar yield (20-60g/100g), meanwhile, the sugarcane bagasse cellulose with the lowest crystallinity index had the maximum reducing sugar yield (>85.72g/100g). The higher the crystallinity index was, the higher degree of crystallization would be, and the more difficult the combination of enzyme and cellulose was, leading to a lower cellulose conversion and a lower reducing sugar yield. An obvious negative correlation between CrI and hydrolysis yield was obtained with the linear correlation coefficient R2as high as0.9828, at sufficiently high enzyme loadings of140and200FPU/g cellulose where the enzyme availability was not a limiting factor of the hydrolysis reaction.(3) It was found that the crystallinity index (CrI) of plant cellulose played a greater role than degree of polymerization (DP) and specific surface area (SSA) did, in enzyme hydrolysis by the multiple linear regression correlation equation fitting between the reducing sugar yield (Y) and structural characteristics (CrI, DP, SSA), meanwhile, the DP and SSA only presented a minor degree of positive correlation with the reducing sugar yield, thus had a minor influence on the hydrolysis of plant celluloses. The influence of structural characteristics on enzymatic hydrolysis was in the order of CrI> DP> SSA. This points to the potential advantages in practical application of utilizing water hyacinth as feedstock for bio-ethanol production as it has relatively low CrI and relatively high SSA. When water hyacinth is applied to enzymatic hydrolysis, it can not only reduce the cost of enzyme reagent as well as the cost of bio-ethanol production, but also control the aquatic environmental problems.(4) Different intensity of phosphoric acid and microwave pretreatments were employed to water hyacinth cellulose separated by ethanol-nitrate method in order to change its structural feature. The SSA of the pretreated materials decreased significantly, and the reducing sugar yields of these materials were also lower than those of untreated cellulose. Therefore, water hyacinth cellulose separated by ethanol-nitrate method itself was already very suitable for the subsequent enzymatic hydrolysis. Its structural features would change to be unfavorable for enzymatic hydrolysis reaction after pretreatment with the different methods furthermore.(5) Three chemical pretreatments (2%H2SO4,2%NaOH,1%H2O2+2%NaOH) were employed to pretreat water hyacinth and sugarcane bagasse biomasses, and the pretreated biomasses were subjected to enzymatic hydrolysis. The reducing sugar yields of water hyacinth pretreated by three chemical pretreatments had no obvious differences (58.48, 60.39and62.88g/100g at12h, respectively). However, the yields of sugarcane bagasse pretreated by2%NaOH and1%H2O2+2%NaOH (80.08and82.54g/100g at12h) were significantly higher than that pretreated by2%H2SO4(44.64g/100g at12h), and were higher than those of water hyacinth under three treatments, indicating its superiority over water hyacinth. Cellulose content, lignin and ash removal had a positive effect on reducing sugar yield of pretreated samples. High reducing sugar yield of90.6g/100g could be achieved by enzymatic hydrolysis for12hours of sugarcane bagasse pretreated by1%H2O2+2%NaOH.1%H2O2+2%NaOH is an effective pretreatment method for lignocellulose materials as it is relatively mild and obtains higher utilization efficiency of raw material.