Organosolv Pretreatment
In the organosolv pretreatment (OP) process an organic solvent and water with or without added acid/base catalyst is used as the pretreatment media. A number of solvents including methanol, ethanol, acetone, ethylene glycol, triethylene glycol, glycerol, and tetrahydro- furfuryl alcohol have been used as the organic solvent in this technique. Lignocellulosic biomass is exposed to organic solvent water mixture at high temperature for a short time, typically 100-200°C, but the exact temperature depends on the type of organic solvents used. This solvent mixture hydrolyzes the internal lignin bonds, as well as the ether and 4-O-methylglucuronic acids ester bonds between lignin and hemicellulose. Additionally, glycosidic bonds in hemicellulose are also partially hydrolyzed during the organosolv pretreatment depending on process conditions [118-120].
The organosolv process is known to be able to produce a large amount of a high-quality lignin that is primarily unaltered and less condensed than Kraft lignins. This type of lignin is partially soluble in many organic solvents and could be utilized in many industrial applications such as adhesives, films and biodegradable polymers. Low boiling point alcohols, like methanol and ethanol, appear to be the most suitable organic liquids for use in organosolv processes, due to their low cost and facile recovery, and ethanol is safer and preferred over methanol due to toxicity. Many forms of woody and softer non-wood biomass materials have been tested in the organo — solv process including poplar [119], pine [120], Miscanthus gigan — teus [121, 122], bamboo [123], wheat straw [124], olive tree wood biomass [125, 126], and sugarcane bagasse [127]. The use of bases as catalysts in the organosolv process is common. For instance, sodium hydroxide as a catalyst agent during organosolv ethanol pretreatment is known to improve the selectivity with respect to lignin. This may be due to the fact that ethanol can reduce the surface tension of the pulping liquor, favoring the alkali penetration into the material, consequently removing the lignin. Some selected examples of organosolv pretreatment are summarized in Table 5.6.
The main advantage of the organosolv pretreatment process is the multi-product extraction phase capability, which is very attractive in terms of an integrated biorefinery approach of producing cellulosic ethanol. The output of organosolv pretreatment consists of three separate fractions (1) dry lignin, (2) an aqueous hemicel — lulose stream, and (3) a relatively pure cellulose fraction, all of which can be utilized or converted to feedstock for the chemical industry [132]. In particular, lignin separated in the process is sulfur free with high purity and low molecular weight. It can be used as a fuel to power cellulosic ethanol plants, or further purified to obtain high quality lignin, which can be used for preparation or as a substitute for polymeric materials such as phenolic resins, poly — isocyante foams, polyurethane, and epoxy resins [133]. The separation and uses of lignin are presented in detail in Chapter 10 of this book. The organosolv pretreatment method is very effective for the pretreatment of high-lignin lignocellulosic materials such as soft woods. Furthermore, organosolv pretreatment can be combined with other pretreatment techniques to obtain a clean and effective biomass fractionation process or multi-stage pretreatment processes for more recalcitrant biomass as means for improving pretreatment yield.
A main drawback of the organosolv pretreatment (OP) process is the cost of chemicals, and occasionally catalyst makes organo — solv pretreatment more costly than other pretreatment processes. Another concern is the side reactions such as acid-catalyzed degradation of monosaccharides into furfural and 5-hydroxymethylfuru — ral that are inhibitory to fermentation microorganisms. As volatile organic liquids are used at high temperature and pressure it is essential to use high-quality containment vessels, since pretreatment-digester leaks can cause fire, explosion hazards, environmental and health and safety concerns. In the organosolv pretreatment process it is essential to recover and reuse solvents to reduce costs. Furthermore, this is compulsory since the solvent may be inhibitory to growth of microorganisms, subsequent enzymatic hydrolysis, and fermentation process.