The main pathways for the production of liquid transportation fuels from biomass are shown in Fig. 4. As indicated in previous sections, food crops such as corn grain or sugar cane can be converted into ethanol by fer­mentation processes. Alternatively, second generation ethanol can be pro­duced from lignocellulosic sources by means of pretreatment-hydrolysis and subsequent fermentation of soluble sugars. Butanol, with energy den­sity and polarity similar to gasoline, can be also produced by this route, representing an interesting alternative to overcome many of the technical shortcomings of ethanol as a fuel. [43,44] Interestingly, the microorgan­isms utilized for fermentation can be engineered to convert sugars to liq­uid alkanes instead of alcohols. [45] This new technology could achieve improvements over classical fermentation approaches, because hydrocar­bons separate spontaneously from the aqueous phase, thereby avoiding poisoning of microbes by the accumulated products and facilitating sepa­ration/ collection of alkanes from the reaction medium.

Vegetable oils, obtained from food sources such as soybeans, palm or sunflower, can serve as feedstocks for the production of first-generation biodiesel through transesterification processes. Since vegetable oils are expensive and compete with food sources, the challenge of the biodiesel industry is to find non-edible sources of lipids. Algae crops are receiv­ing interest in this respect, [46] although the high cost associated with feedstock production is an important barrier, and related technologies are presently at an early stage of development. Green diesel can be produced from plant oils and animal fats by means of deoxygenation reactions under hydrogen pressure in hydrotreating processes. [47,48] This recent technol­ogy has potential in that it can be carried out in existing petroleum refinery infrastructure. [49]

Representative examples of non-food lignocellulosic feedstocks such as forest wastes, agricultural residues like corn stover, or municipal paper wastes are shown in Fig. 4. Apart from their intrinsic recalcitrance, these feedstocks are characterized by a high degree of chemical and structural complexity, and, consequently, technologies for the conversion of these resources into liquid hydrocarbon fuels typically involve a combination of different processes. The methodology most commonly used to overcome lignocellulose complexity involves the transformation of non-edible feed­stocks into simpler fractions that are subsequently more easily converted into a variety of useful products. This approach, similar to that used in conventional petroleum refineries, would allow the simultaneous produc­tion of fuels, power, and chemicals from lignocellulose in an integrated facility denoted as a biorefinery. [50,51] Current technologies for convert­ing lignocellulose to liquid hydrocarbon transportation fuels involve three major routes: gasification, pyrolysis and pretreatment-hydrolysis (Fig. 4). By means of these primary routes, lignocellulose is converted into gaseous and liquid fractions that are subsequently upgraded to liquid hydrocarbon fuels. Thus, gasification converts solid biomass to synthesis gas (syngas), a valuable mixture of CO and H2 which serves as a precursor of liquid hydrocarbon fuels by Fischer — Tropsch (F-T) reactions. This pathway is commonly known as biomass to liquids (BTL). Pyrolysis allows trans­formation of lignocellulosic biomass into a liquid fraction known as bio­oil that can be subsequently upgraded to hydrocarbon fuels by a variety of catalytic processes. The third route involves pretreatment-hydrolysis steps to yield aqueous solutions of C5 and C6 sugars derived from lig — nocellulose. While gasification and pyrolysis are pure thermal routes in which lignocellulose is decomposed with temperature under controlled atmosphere, aqueous-phase processing, in contrast, involves a series of catalytic reactions to selectively convert sugars and important platform chemicals derived from them into targeted liquid hydrocarbon fuels with


FIGURE 4: Routes for the conversion of biomass into liquid fuels. Red arrows refer to thermal routes, green arrows refer to biological routes, and blue arrows refer to catalytic routes. Adapted from ref. 25.

molecular weights and structures appropriate for gasoline, diesel and jet fuel applications.