Pretreatment of poplar
Most of the popular pretreatment techniques have been tested on poplar, in many examples these are single stage pretreatments such as dilute mineral acid [197, 196, 198-200], steam explosion [201, 202], and wet oxidation [203]. In recent years a number of researchers have identified the advantages of using two stage pretreatments on woody biomass, for example, combinations like steam + organosolv [204], fungus + alkaline fractionation [205], and ionic liquid + alkaline [206] have been used successfully on poplar varieties.
A two-stage pretreatment approach employing steam followed by organosolv treatment was employed by Panagiotopoulos and coworkers to fractionate and recover most of the hemicellulose, lignin, and cellulose components of poplar wood chips [204]. In this technique, a mild steaming stage was initially used to maximize hemicellulose sugar recovery, with 63% of the original xylan solubilized and recovered after this stage and close to 90% recovered in total. They reported that, prior steam treatment enhanced lignin solubilization with more than 66% of the original lignin removed after two-stage pretreatment, rather than hindering subsequent organosolv delignification. The extracted lignin contained at least equal or greater amounts of functional groups as compared to the lignin solubilized after a singlestage organosolv pretreatment. Furthermore, more than 98% of the original cellulose was recovered after the two-stage pretreatment and 88% of cellulose could be hydrolyzed to glucose at cellulase enzyme loading of 5 FPU/g, and after 72 hr treatment [204].