Butanol can be generated biologically by acetone-butanol-ethanol (ABE) fermenta­tion. This metabolic route is present in bacterial species in the genus Clostridium such as C. acetobutylicum and C. beijerinckii. ABE fermentation has been indus­trially applied using starchy biomass as an alternative to chemical synthesis from fossil-fuels. Similarly, the sugars present in lignocellulose hydrolysates can be fer­mented into 1.2-biobutanol, also known as 2,3-butylene glycol (2,3-BD), by mixed acid-butanediol fermentation (Menon and Rao 2012). Several microorganisms have the 2,3-BD pathway but the most commonly used is Klebsiella oxytoca, given its wide sugar spectrum and adaptive potential. Branched alcohol mixtures with high iso-butanol content can also be formed from glucose through synthetic pathways present in bacteria such as Escherichia coli (Rodriguez and Atsumi 2012).

Although these technologies are not as mature as those involved in the production of bioethanol, there is a growing interest in these kinds of biofuels, reflected by the investment in research for their commercial application by companies such as

Dupont, BP, Gevo and Green Biologics. The economic feasibility of production of these biofuels depends on the use of cheaper feedstocks to increase yields and productivities as well as on the development of more efficient recovery processes (Jin et al. 2011). In this context, the use of lignocellulose residues, selection of bacterial strains as well as process development is expected to reduce butanol production costs (Kumar et al. 2012).

Typical process yields for biochemical conversion of woody biomass available in the Southern hemisphere to bio-alcohols are presented in Table 7.4. The impact of biomass properties on biochemical conversion processes is addressed in Chap. 8.