Hydrolysis of Cellulose and Hemicellulose

Cellulosic biomass includes agricultural (e.g., corn stover and sugarcane bagasse) and forestry (e.g., sawdust, thinnings, andmill wastes) residues, portions ofmunicipal solid waste (e.g., waste paper), and herbaceous (e.g., switchgrass) and woody (e.g., poplar trees) crops. Such materials are abundant and competitive in price with petroleum, and cellulosic biomass can provide a sustainable resource that is truly unique for making organic products [1]. Furthermore, cellulosic biomass can be produced in many regions of the world that do not have much petroleum, opening up a new route to manufacturing organic fuels and chemicals. The structural portion of cellulosic biomass is a composite of cellulose chains joined together by hydrogen bonding. These long cellulose fibers are in turn held together with hemicellulose and lignin, allowing growth to large aerial plants that canwithstand weather and attack by microorganisms and insects. Traditionally, humans have employed cellulosic biomass for such uses as construction, soil stabilization, animal feed, and paper manufacture. However, the cellulose and hemicellulose portions of biomass, representing about 40–50% and 20–30% by dry weight of plants, respectively, are polysaccharides that can be broken down into sugars and fermented or chemically altered to valuable fuels and chemicals. Sugars from such sources have been used to make ethanol and other products during periods of war by approaches such as the Scholler process during the 1940s and in controlled economies such as the former Soviet Union. Research over the last two decades has advanced biological conversion of cellulosic biomass to the point of becoming economically competitive for production of fuels and chemicals that offer important strategic, environmental, and economic advantages [2]. The challenge is to overcome the risk of commercializing first-of-a-kind technology and to continue to advance hydrolysis processes so that fuels and chemicals from cellulosic resources are competitive without subsidies. This chapter presents a comprehensive overview of the technology and economic status for cellulose and hemicellulose hydrolysis, including a description of important structural features of cellulosic materials, applications, process steps, and stoichiometry for hydrolysis reactions. Included is an overview of the basic chemistry and processes followed by consideration of both chemical and enzymatic approaches to these reactions. The structure and functions of enzymes are described along with pretreatment of biomass to achieve high yields for these systems. The chapter continues with an overview of acid