Alkali pretreatment uses bases (sodium, potassium, calcium or ammonia hydroxide) as catalysts. Lime [Ca(OH)2] treatment is most beneficial from an economic and health and safety point of view and lime can be easily recovered for re-use (Yang and Wyman 2008). Alkali treatment induces swelling of celluloses and selective removal of hemicelluloses coupled with lignin solubilisation, which renders the lignocellulose more accessible to enzymes and bacteria. Although the amount of inhibitors generated is lower than in acid pretreatments, hemicelluloses can undergo peeling reactions and be degraded into furans that, together with the solubilised lignin, could negatively impact the functioning of microorganisms. This technology can increase the methane yield of residual materials such as newsprint (Fox et al. 2003) but is less attractive for pretreatment of woody materials given the negative effects of high lignin content on the process.

Acid hydrolysis pretreatment is mostly performed using sulphuric acid catalysts, but other mineral acids such as hydrochloric, nitric and trifluoroacetic acids have also been applied. Processes utilising acid catalysts can either be carried out by concentrated-acid/low temperature or dilute acid/high temperature hydrolysis. Concentrated acid allows the hydrolysis of both cellulose and hemicellulose under moderate temperatures, but requires high acid concentrations (72 % H2SO4, 41 % HCl or 100 % TFA), which makes recovery costly and can lead to equipment corrosion (Gfrio et al. 2010). Lower concentrations (30 %) of acid have been reported to retrieve 41 % of the total theoretical glucose from pine sawdust (Miller and Hester 2007). Dilute acid selectively hydrolyses the hemicellulose fraction, while the cellulose remains in a solid fraction that can be hydrolysed by enzymes or by a second dilute acid step. Dilute acid has been applied to species of Eucalyptus (Mclnstoch et al. 2012; Silva et al. 2011; Wei et al. 2012), Acacia (Ferreira et al. 2011) and Pinus (Huang and Ragauskas 2012). Although dilute acid treatment implies lower acid consumption by the substrate when compared to concentrated acid treatments, the higher temperatures of operation can lead to greater equipment corrosion and higher levels of hemicellulose degradation. Both acid schemes entail a neutralization step prior to biochemical transformation.

Ozonolysis is achieved by the treatment of lignocellulose with oxidizing agents such as ozone, which mainly reduces lignin content, slightly affects hemicellulose and increases the sugar yield of enzymatic hydrolysis. It can be conducted at normal pressure and room temperature, so that it does not generate compounds that are toxic to further hydrolysis and fermentation. Ozonolysis could be effective for the pretreatment of lignocellulose-rich residues such as sawdust (Ncibi 2010), but the amounts of ozone required makes this process costly.

The organosolv process employs organic or aqueous organic solvent mixtures with ethanol, methanol, acetone, ethylene glycol and tetrahydrofurfuryl alcohol as potential components, which can be supplemented with an acid catalyst (HCl, H2SO4, oxalic or salicylic acid) to disrupt the link between hemicellulose and lignin. This improves the susceptibility of cellulose to enzymatic hydrolysis by increasing enzyme accessibility to cellulose in both hardwoods (Romani et al. 2011) and softwoods (Park et al. 2010). An additional advantage of this process is the recovery of relatively pure lignin.

The novel SPORL (Sulphite Pretreatment to Overcome Recalcitrance of Lig — nocellulose) approach to pretreatment is based on the pulping of biomass in the presence of sulphites and was developed to enhance the biochemical conversion of softwoods with large particle sizes (Zhu et al. 2009). This technology consists of sulphite/bisulphite treatment of wood chips under acidic conditions and, contrary to conventional pretreatment technologies, is followed by a reduction of particle size by means of disk milling. The removal of hemicelluloses (pulping spent liquor) and sulfonation of lignin is considered to be critical for enhanced cellulose conversion. Moreover, this technology reduces the energy consumption required for size-reduction to values equivalent to agricultural biomass (Zhu et al. 2010).

Ionic liquids (ILs), also named “green solvents”, are organic salts composed mainly of organic cations with small amounts of either organic or inorganic anions, with the ability to dissolve a wide range of organic and inorganic compounds. IL solvents have several valuable properties including a low melting point, chemical and thermal stability, negligible vapour pressure and relatively low toxicity (Liu et al. 2012). Additionally, its solvent properties can be adapted for a particle substrate by adjusting the ratio of cations to anions. The synergy of ILs with other compounds such as acids (Diedericks at al. 2012) or solid super acids (Br0nsted superacids or Lewis acids) have also been investigated (Gfrio et al. 2010). The most common ILs can be classified according to their cations in four groups: quaternary ammonium ILs, N-alkylpyridinium ILs, N-alkyl-isoquinolinium ILs, and 1-alkyl-3-methylimidazolium ILs. Most of these solvents remove lignin and alter cellulose structure, which increases the accessibility of cellulolytic enzymes. Imidazolium-based ionic liquids have been used to dissolve hardwoods and soft­woods (Mora-Pale et al. 2011). The ionic liquid 1-ethyl-3-methyl imidazolium acetate ([C2mim][OAc]) has been shown to increase cellulose digestibility of species of Eucalyptus (Qetinkol et al. 2010) and Pinus (Torr et al. 2012).

Although the technology based on ILs would require less equipment and energy input compared to conventional pretreatments, efficient methods for both recovery of the different fractions and the recycling of ILs should be developed for large scale application. The development of such processes would circumvent the negative impact that some ILs have on enzyme activity and effective microorganism functioning (Wang et al. 2011).