Page 56 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 35

conversion yield was significantly improved from 2.4% to 45.1%. In addi-
tion, the concentrations of fermentation inhibitors, for instance HMF and
furfural, were negligible in the spent liquor, which were much lower than
those from sulfuric acid catalyzed water or ethanosolv pretreatments. The
organosolv pretreatment of Liriodendron tulipifera with sodium hydroxide
could enhance the enzymatic hydrolysis and ethanol yield at lower tem-
perature [74]. Wang et al. compared the effects of various catalysts (formic
acid, triethylamine, and sodium hydroxide) and solvents (methanol,
ethanol, n-propanol, and n-butanol) on the substrate’s physicochemical
characteristics under mild organosolv pretreatment [104]. They found that
the pretreatment with NaOH was more efficient on hemicelluloses disso-
lution, and therefore higher glucose yield was obtained than formic acid
and triethylamine catalyzed pretreatments.

2.3.4 Oxidative pretreatment
Oxidative pretreatment refers to using oxidant to pretreat the biomass, in
which the biomass components, especially lignin, undergo oxidative
depolymerization. Various oxidants have been used for oxidative delignifi-
cation; however, the oxidants to be used for pretreatment must be cheap
but effective. Promising oxidative pretreatments include wet oxidation,
alkaline H 2 O 2 , and peracid pretreatments.

2.3.4.1 Wet oxidation
Wet oxidation process was proposed to remove organic compounds in
the liquid phase by Zimmerman in the year of 1950, in which oxygen or
air was used as an oxidant to oxidize the organic matters completely to
carbon dioxide and water [105]. The wet oxidation pretreatment involves
treating biomass with water and oxygen or air at elevated temperatures
(120°C 238°C) and pressure (45 480 psi) [106]. The first reaction dur-
ing wet oxidation refers to the formation of acids. Due to the deesterifica-
tion of the acetate groups in hemicellulose and oxidation of some
hemicellulose fragments, the hemicelluloses are solubilized with formation
of organic acids. With the increase in acid concentration, the hydrolytic
reactions are facilitated. More and more hemicelluloses are broken down
into lower molecular weight fragments and dissolved in the water [107].
The wet oxidation not only shows effects on the hemicelluloses but also
on the cellulose and lignin fractions. When wet oxidation is performed
under a relatively mild condition (120°C 172°C), hemicellulose and par-
tial lignin are solubilized, resulting in a solid fraction enriched in cellulose