Abstract

The development of efficient pretreatments for lignocellulosic biomass is a key step to decrease the cost for the production of biofuels and biomaterials. The enzymatic hydrolysis stage for bioethanol production is highly sensitive to the physicochemical characteristics of the pretreated fibers, which determines the adsorption and efficiency of the cellulase enzyme complex over the cellulose microfibrils. These features can increase the accessibility to the substrate or cause an increase in the recalcitrance. Soft woods such as Pinus radiata are particularly complex to pretreat mainly because the structure of lignin presented in the matrix, with approximately 90% to 95% of guayacyl groups. During the pretreatment processes lignin is susceptible to condensation reactions, generating a more hydrophobic, recalcitrant and difficult to solubilize lignin. In addition, the percentage of acetyl groups in their hemicellulose is lower than in hard woods, so the extraction of the hemicelluloses is also inefficient by methods such as diluted acid hydrolysis or autohydrolysis. This work present the ASA and SEW pretreatments as alternatives for the production of bioethanol from P. radiata fibers, evaluating the chemical composition of the pretreated material, its accessibility by enzymatic hydrolysis and analysis of microscopy to reveal the differences in morphology of the fibers and the distribution of the lignin. The conversion of cellulose to glucose from the pretreated materials obtained by ASA and SEW were 78.3% and 84.3% in dry wood basis, respectively. Also, both pretreated materials contains high percentages of lignin (ASA = 20.4% and SEW= 26.5%) indicating that its removal is not a relevant aspect, rather its re-location and chemical modification. The formation of microparticles of lignin during the SEW pretreatment is the most important phenomenon that influence the accessibility of the material by an important redistribution of lignin. On the other hand, with the ASA pretreatment also a high accessibility is achieved but unlike the SEW pretreatment, with a homogeneous distribution of the lignin. This can be attributed to the possible sulfonation of lignin making it more hydrophilic, decreasing non-specific adsorption of the cellulases.