Page 58 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 37

including a partial reduction of esterified acetic acid, ferulic acid, and p-
coumaric acids as well as a significant cleavage of etherified linkages
between lignin and hemicelluloses [114]. In fact, almost no significant
changes in the chemical structure of hemicelluloses might be observed,
because the oxidation reaction only occurs in the aliphatic part of the
macromolecule [115].
Alkaline peroxide pretreatment has been applied to various biomass
feedstock including wheat straw [116], softwood [117], sugarcane bagasse
[118], bamboo [119], corn stover [120], Miscanthus [121], and other her-
baceous and woody biomass [122]. After alkaline peroxide pretreatment,
the pretreated wheat straw presents 96.7% enzymatic hydrolysis yield.
During the pretreatment, no measurable quantities of furfural and HMF
are produced. The ethanol yield could be 0.46 g/g of available sugars
(0.29 g/g straw) [123]. Recently, the alkaline peroxide pretreatment at
low peroxide loadings has been used to enhance the enzymatic hydrolysis
of softwood [117]. It was found that the presence of H 2 O 2 is essential for
this pretreatment. In order to achieve a better substrate hydrolyzability, it
is more effective to maintain a higher alkaline loading than increasing the
H 2 O 2 loadings. When Douglas-fir was pretreated at 180°C for 30 min
with a low peroxide loading of 0.10 (mass fraction of H 2 O 2 to biomass), a
cellulose to glucose yield of 95% could be achieved [117]. The key factor
is the balance between alkaline and peroxide loadings during the
pretreatment.

2.3.4.3 Peracid oxidation
As powerful oxidants, peracids, such as peroxymonosulfuric acid, peroxy-
formic acid, and peracetic acid (PAA), also have been used for biomass
delignification and pretreatment [68]. Among the peracids used, PAA is
the most promising because it is more stable than performic acid [124]
and easier to recover and prepare than peroxymonosulfuric acid [125].
PAA has been found to be very selective to oxidize lignin. It oxidizes the
hydroxyl groups in lignin side chains to carbonyl groups [126] and cleaves
β-aryl ether bonds, resulting in the decrease of lignin molecular weight
and introduction of hydrophilic groups [127]. In addition, PAA oxidation
increases the water solubility of lignin via several reactions: formation of
hydroquinones by phenolic rings hydroxylation [128], oxidation of
hydroquinones to quinones which undergo ring opening to produce
water-soluble muconic, maleic, and fumaric acid derivatives [129]. The
main reaction types of PAA with lignin-related structures using lignin