Page 64 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 43

increased from 80% to 97% [163]. However, the cost still should be
reduced for a commercial application.

2.3.5.3 Ionic liquid
ILs are molten salts, which consist of paired ions and have tunable proper-
ties. ILs are considered “green solvents” due to their negligible vapor pres-
sure, nonflammability, and good thermal and chemical stability [164]. ILs
containing imidazolium or pyridinium cations paired with chloride, car-
boxylates, phosphates, and phosphonates are able to dissolve cellulose
through high hydrogen bond basicity [165]. However, low-basicity
anions, such as [DCA] and [Tos], are not effective for cellulose dissolution
[166]. The dissolution of lignocellulose in ILs disrupts the three-
dimensional network, therefore making biomass more digestible by
enzymes [167]. A number of researches have been reported for the pre-
treatment of kinds of lignocellulose with various ILs [168].
Liu and Chen [169] obtained a significant improvement of enzymatic
hydrolysis yield using [BMIM]Cl to treat raw and steam-exploded wheat
straw. The degree of hydrolysis of pretreated wheat straw reached 70.4%
while the pretreated steam-exploded wheat straw could be completely
hydrolyzed. It has been found that the hydrolyzability improvement by IL
pretreatment is attributed to the decrease in the DP of cellulose and hemi-
cellulose and decrystallization of cellulose resulting in the increased acces-
sibility. Compared to dilute acid pretreatment, switchgrass by [EMIM]Ac
pretreatment showed greatly increased surface area, reduced cellulose crys-
tallinity, and lignin content [170]. In addition, after 12 h enzymatic
hydrolysis, the glucose yield of [EMIM]Ac pretreated samples reached
90%, while the glucose yield of dilute acid pretreated samples was only
80% after 72 h saccharification. Cox et al. [171] used 1-H-3-methylimida-
zolium chloride for pretreatment of yellow pine wood chips at 110°
C 150°C for up to 5 h. Dissolution of hemicelluloses and lignin from
cell walls of pine wood was observed. Although the dissolution was faster
at higher temperatures, significant cellulose degradation was also found at
the highest temperatures tested. The cellulose-rich fraction was easily sac-
charified by enzymatic hydrolysis with cellulases from Trichoderma viride,
and higher glucose yields were obtained at longer pretreatment duration
at 130°C. Another approach was developed in which simultaneous pre-
treatment and saccharification of biomass in ILs were conducted [172].
However, in this integrated process, enzymatic hydrolysis is performed
with the presence of ILs, and therefore, the cellulase may be easily