Chemical hydrolysis involves exposure of lignocellulosic materials to a chemical for a period of time, at a specific temperature, and results in sugar monomers from cellulose and hemicellulose polymers. Acids are predominantly applied in chemical hydrolyses. Sulfuric acid is the most investigated acid, although other acids such as hydrochloric acid (HCl) have also been used. Acid hydrolyses can be divided into two groups: concentrated-acid hydrolysis and dilute-acid hydrolysis [18].

Concentrated-acid hydrolysis. Hydrolysis of lignocellulose by concen­trated sulfuric or hydrochloric acids is a relatively old process. Concentrated-acid processes are generally reported to give higher sugar and ethanol yield, compared to dilute-acid processes. Furthermore, they do not need a very high pressure and temperature. Although this is a successful method for cellulose hydrolysis, concentrated acids are toxic, corrosive, and hazardous, and these acids require reactors that are highly resistant to corrosion. High investment and maintenance costs have greatly reduced the commercial potential for this process. In addi­tion, the concentrated acid must be recovered after hydrolysis to make the process economically feasible. Furthermore, the environmental impact strongly limits the application of hydrochloric acid [12, 15].

Dilute-acid hydrolysis. Dilute-sulfuric acid hydrolysis is a favorable method for either the pretreatment before enzymatic hydrolysis or the conversion of lignocellulose to sugars. This pretreatment method gives high reaction rates and significantly improves enzymatic hydrolysis.

Depending on the substrate used and the conditions applied, up to 95% of the hemicellulosic sugars can be recovered by dilute-acid hydrolysis from the lignocellulosic feedstock [2, 13]. Of all dilute-acid processes, the processes using sulfuric acid have been the most extensively studied. Sulfuric acid is typically used in 0.5-1.0% concentration. However, the time and temperature of the process can be varied. It is common to use one of the following conditions in dilute-acid hydrolysis:

■ Mild conditions, i. e., low pressure and long retention time

■ Severe conditions, i. e., high pressure and short retention time

In dilute-acid hydrolysis, the hemicellulose fraction is depolymerized at temperatures lower than the cellulose fraction. If higher temperature or longer retention times are applied, the monosaccharides formed will be fur­ther hydrolyzed to other compounds. It is therefore suggested that the hydrolysis process be carried out in at least two stages. The first stage is carried out at relatively milder conditions during which the hemicellulose fraction is hydrolyzed, and a second stage can be carried out by enzymatic hydrolysis or dilute-acid hydrolysis, at higher temperatures, during which the cellulose is hydrolyzed [13]. These first and second stages are some­times called “pretreatment” and “hydrolysis,” respectively.

Hydrolyzates of first-stage dilute-acid hydrolysis usually consist of hemicellulosic carbohydrates. The dominant sugar in the first-stage hydrolyzate of hardwoods (such as alder, aspen, and birch) and most agri­cultural residues such as straw is xylose, whereas first-stage hydrolyzates of softwoods (e. g., pine and spruce) predominantly contain mannose. However, the dominant sugar in the second-stage hydrolyzate of all lig­nocellulosic materials, either by enzymatic or dilute-acid hydrolysis, is glu­cose, which originates from cellulose.

Detoxification of acid hydrolyzates. In addition to sugars, several by-products are formed or released in the acid hydrolysis process. The most impor­tant by-products are carboxylic acids, furans, and phenolic compounds (see Fig. 3.6).

‘ Mannan—► Mannose —► HMF ► Acids

Xylan—- ► Xylose— ►Furfural—► Acids

I Glucan — ► Glucose— ► HMF — ► Acids

——————— ► Phenolic Compounds

Acetyl groups————————— ► Acetic acid

Figure 3.6 Formation of inhibitory compounds from ligno­cellulosic materials during acid hydrolysis.

Acetic acid, formic acid, and levulinic acid are the most common car­boxylic acids found in hydrolyzates. Acetic acid is mainly formed from acetylated sugars in the hemicellulose, which are cleaved off already at mild hydrolysis conditions. Since the acid is not further hydrolyzed, for­mation of acetic acid is dependent on the temperature and pressure of dilute-acid hydrolysis, until the acetyl groups are fully hydrolyzed. Therefore, the acetic acid yield in the hydrolysis does not significantly depend on the severity of the hydrolysis process [13, 19].

Furfural and HMF are the only furans usually found in hydrolyzates in significant amounts. They are hydrolysis products of pentoses and hexoses, respectively [13]. Formation of these by-products is affected by the type and size of lignocellulose, as well as hydrolysis variables such as acid type and concentration, pressure and temperature, and the retention time.

A large number of phenolic compounds have been found in hydrolyzates. However, reported concentrations are normally a few milligrams per liter. This could be due to the low water solubility of many of the phenolic com­pounds, or a limited degradation of lignin during the hydrolysis process. Vanillin, syringaldehyde, hydroxybenzaldehyde, phenol, vanillic acid, and 4-hydroxybenzoic acid are among the phenolic compounds found in dilute- acid hydrolyzates [18].

Biological (e. g., using enzymes peroxidase and laccase), physical (e. g., evaporation of volatile fraction and extraction of nonvolatile fraction by diethyl ether), and chemical (e. g., alkali treatment) methods have been employed for detoxification of lignocellulosic hydrolyzates [20, 21].

Detoxification of lignocellulosic hydrolyzates by overliming is a common method used to improve fermentability [22-25]. In this method, Ca(OH)2 is added to hydrolyzates to increase the pH (up to 9-12) and keep this condition for a period of time (from 15 min up to several days), followed by decreasing the pH to 5 or 5.5. Recently, it has been found that time, pH, and temperature of overliming are the effective param­eters in detoxification [26]. However, the drawback of this treatment is that part of the sugar is also degraded during the overliming process. Therefore, it is necessary to optimize the process to achieve a fer­mentable hydrolyzate without any loss of the sugar [21, 26].