8.3.1 Dilute Acid Pretreatment (DAP)

Among the numerous pretreatment techniques, dilute acid pretreatment (DAP) has been shown as a leading pretreatment process that is currently under commercial de­velopment. DAP can significantly reduce lignocellulosic recalcitrance by disrupting the composite material linkage, such as the covalent bonds, hydrogen bonds, and van der Waals forces [44]. The most widely used and tested approaches in DAP are based on dilute sulfuric acid (H2SO4) since it is inexpensive and effective [45, 46].

However, nitric acid [47], hydrochloric acid (HCl) [48], and phosphoric acid [49] have also been examined. In addition, it was shown that sulfur dioxide (SO2) was also an efficient acid catalyst in the DAP, especially for softwood [50-52]. How­ever, there are certain drawbacks with such an approach. It is difficult to handle SO2 (gas) at large scales, as safety issues may constitute a concern, and it is also a more expensive option as compared to similar alternatives such as using H2SO4.