Page 97 - Biofuels Refining and Performance

80 Chapter Three

Acetic acid, formic acid, and levulinic acid are the most common car-
boxylic 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, for-
mation 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.
Alarge 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 com-
pounds, 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,
is added to hydrolyzates to increase the pH (up to 9–12) and
Ca(OH) 2
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 param-
eters 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 fer-
mentable hydrolyzate without any loss of the sugar [21, 26].

3.6.2 Pretreatment prior to enzymatic
hydrolysis of lignocellulosic materials
Native (indigenous) cellulose fractions of cellulosic materials are recal-
citrant to enzymatic breakdown, so a pretreatment step is required to
render them amenable to enzymatic hydrolysis to glucose. A number of
pretreatment processes have been developed in laboratories, including:
Physical pretreatment—mechanical comminution, irradiation, and
pyrolysis