AN AGGREGATED UNDERSTANDING OF ALKALINE HYDROGEN PEROXIDE (AHP) PRETREATMENT OF BIOMASS AND HEMICELLULOSE CONVERSION
Alvarez-Vasco, Carlos Andres
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Despite significant efforts to achieve an efficient deconstruction of plant cell-wall and decrease its recalcitrance, biomass pretreatment continues to be challenging. The main biomass pretreatment strategies have not only been inefficient on decreasing cellulose recalcitrance to enzymatic attack, but also, in providing alternatives to convert hemicellulose, the second most abundant non-food carbohydrate accounting for up to 30 % wt of biomass. In this dissertation, we present an aggregated understanding of alkaline hydrogen peroxide (AHP), a promising biomass pretreatment method with potential for the valorization of highly recalcitrant softwood cellulose and hemicellulose. To understand the mechanism of hemicellulose degradation and have a “reference” pretreatment for cellulose recalcitrance, we study the dilute acid (DA) pretreatment of softwood (Chapter Three). Unfortunately, DA led to excessive hemicellulose degradation to furanic compounds and humins, and partial cellulose decomposition. The ensuing degradation compounds were recognized as inhibitory for enzymatic hydrolysis and fermentation. This study prompted the need to use a different pretreatment chemistry in order to guide the eminent degradation of hemicellulose to valuable compounds while selectively avoiding cellulose degradation. AHP appears as a promising pretreatment alternative to overcome DA limitations. The recalcitrance factors of softwood to enzymatic attack during pretreatment were first identified (Chapter Four). We described glucomannan gelation and high affinity to cellulases as an important underlying recalcitrance factor. The role of pretreatment reagents and conditions on hydrolysability were also study. Results show that it is possible to obtain a 95 % cellulose-to-glucose conversion yield using low peroxide loading AHP. During AHP pretreatment hemicellulose was selectively converted to valuable products (Chapter Five). The conversion pathways for the formation of these products (mainly from glucomannan) where elucidated. Under tested conditions cellulose does not depolymerize but 22% lignin was removed. Finally, we further studied the conversion of carbohydrates during AHP using biomass model compounds and demonstrate that it is possible to increase the conversion to desirable products using metal ions to catalyze AHP (Chapter Six). The results strongly support that alkaline hydrogen peroxide is a pretreatment method that can guide hemicellulose degradation to the formation of valuable compounds while selectively decreasing biomass recalcitrance to enzymatic attack.