Acid Hydrolysis of Cellulose and Hemicellulose
Direct acid hydrolysis of lignocellulosic biomass is an alternative method to the two-step pretreatment enzymatic hydrolysis. In fact, acid hydrolysis of lignocellulosic biomass with inorganic acid is the oldest method of saccharification of biomass. It dates back to the early 19 th century with its commercialization in the early 20 th century. This method is much simpler and commonly known as direct hydrolysis since no pretreatment is required as in the case of enzymatic hydrolysis described in Chapter 6. A number of more advanced and recent variants of acid hydrolysis are now known and can be divided into four different categories:
1. Concentrated acid hydrolysis
2. Dilute acid hydrolysis
3. Ionic liquid-mediated acid hydrolysis
4. Solid acid hydrolysis
Some of these approaches have been reviewed in a few review articles [1, 2] and will be discussed in this chapter. In the last two
decades, a relatively smaller number of research papers have been published in this field in comparison to enzymatic hydrolysis. This could be attributed to the rapid development of cellulase-based enzymatic saccharification during this period. Even though this direct dilute aqueous acid saccharification gives low sugar yields, in many cases several research groups around the globe have taken an interest in recent times [12-16]. This has led to some promising new directions in the direct hydrolysis technologies. The new trend can be seen as taking a second look at this classical method due to its lower cost and simplicity compared to enzymatic saccharification, which, however, requires an energy intense pretreatment.