Acid-Catalyzed Hydrolysis: Hydrolysis of cellulosic materials can be catalyzed by either a dilute or concentrated acid. Sulfuric acid is the most investigated acid, although other acids such as hydrochloric acid have also been applied. The acid reacts with cellulose to produce glucose and short chain molecules which is further transformed to the desired fuel grade bioethanol.

Dilute acid hydrolysis is a method that can be used either as a pretreatment preced­ing enzymatic hydrolysis or as the actual method of hydrolyzing lignocelluloses to sugars (Qureshi and Manderson 1995). The process is conducted under high tempera­ture and pressure, and the reaction time is in the range of seconds or minutes, which facilitates continuous processing (Lee et al. 1999). Although the process shows low acid consumption, the combination of acid and high temperature and pressure may cause the degradation of glucose to undesirable by-products such as levulinic acid, acetic acid, formic acid, and furfural, decreasing the yield of glucose.

On the other hand, concentrated acid hydrolysis occurs in relatively mild tem­peratures and atmospheric pressure. The process is more efficient and produces high sugar recovery efficiency, which can be on the order of over 90 % of both hemicel- lulose and cellulose sugars. Typically, 1 % of sulfuric acid is used at elevated pres­sure (42.0 MPa) and temperature (493 K). An advantage of this process over enzymatic hydrolysis is the high hydrolysis rate, with residence times of the order of several seconds. The mild temperatures and pressures employed allow the use of relatively low-cost materials. Unfortunately, the process is time-consuming, taking up to 120 h to complete. Also, the acid catalyst after the reaction is difficult to recover generating acid wastes (Fig. 20.3).

Solid Acid-Catalyzed Hydrolysis: Homogeneous catalysts such as sulfuric acid and enzymes produce glucose from cellulose in high yields; however, these processes suffer from the complicated product recovery and high production costs. The low selectivity of product due to the further degradation of glucose in harsh conditions should also be improved. Solid acid catalysts are expected to overcome these prob­lems as various types of the catalysts can be designed and applied in a wide range of reaction conditions. Furthermore, solid catalysts are easily recovered and reused.

Solid acids, e. g., zeolites, sulfonated zirconia and sulfonated carbons, can carry a high concentration of strong Bronsted acid sites, creating a very acidic environment in the catalyst pores or close to the catalyst surface. H-form zeolites and sulfonated mesoporous silicas were used as solid acid catalysts for the hydrolysis of soluble oligosaccharides and starch. Sulfonated activated-carbon catalysts showed a remarkably high yield of glucose, which was due to the high hydrothermal stability

Fig. 20.3 Reaction route of cellulose to glucose

and the excellent catalytic property attributed to the strong acid sites of SO3H func­tional groups and the hydrophobic planes, selectivity 90 % (Onda et al. 2008). The hydrolysis of cellulose with a highly active solid acid catalyst bearing SO3H, COOH, and OH groups was also investigated at 323-393 K using an artificial neural net­work (ANN) and a response surface methodology (RSM). The study shows that hydrolysis reaction depends largely on the amount of water as the solvent. The glucose yield by the solid acid catalyst reaches a maximum with an amount of water comparable to the catalyst weight. The reaction efficiency increases with increasing reaction temperature up to 363 K and does not increase in proportion to the reaction temperature above 363 K, suggesting that degradation of the cellulose surface by the acid catalyst prevents efficient hydrolysis of cellulose (Yamaguchi et al. 2009).

Recently, many new catalysts have emerged as powerful tools for the hydrolysis of cellulose to sugars. Ferrate CaFe2O4 was used as a solid catalyst for cellulose hydrolysis giving a glucose yield of 37 and 74 % selectivity. Ionic liquid was used for pretreatment to reduce the crystallinity of cellulose. The reaction was carried out at 423 K over 24 h. Hydrolysis of cellulose using hydrotalcite [Mg4Al2(OH)12CO3-4 H2O] activated by saturated Ca(OH)2 showed the conversion and glucose selectivity of 46.6 % and 85.3 %, respectively. The reaction was carried out at 423 K over 24 h with ball-milled cellulose. The solid acid catalyst could be reused four times and the catalytic activity remained. Compared with carbon-based solid acids, the activated hydrotalcite catalyst is more stable and can be separated more easily from the reaction mixture (Fang et al. 2011).

20.3.1 Fermentation