Page 344 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Pretreatment of agroindustry waste by ozonolysis for synthesis of biorefinery products 313

14.2.3 Chemical reactions between ozone and lignocellulosic
biomass
Ozone is one of the oxidants, which is highly reactive toward lignin and carbohy-
drates. In the ozonolysis process the lignin, carbohydrate, and other compounds are
degraded, generating by-products that may act as inhibitors in downstream pro-
cesses. The reaction of the ozone with aromatic and phenolic compounds changes
according to the substituents since the ozone attacks the initial electrophilic and
later the aromatic ring for hydroxylation which increase the electrophilic substitu-
tion reactivity of the ring (Travaini et al., 2016c). The addition of 1,3-cyclo is the
probable subsequent steps followed with the Criegee mechanism that involves ionic
1,3-dipolar cycloaddition. This opens the olefinic double bonds, and 1,1-cycloaddi-
tion occur via p and r complexes (Souza-Corrˆ ea et al., 2013). Moreover, other reac-
tions could be produced by ozone reacting with carbon hydrogen bonds in
aldehyde-, alcohol-, and ether type structure. The reaction also occurs in the cleav-
age of the ether bond for aryl and alkyl ethers (Olkkonen et al., 2000).
According to Travaini et al (2013), the ozone partially attacks the acid insoluble
lignin to produce acid soluble lignin. Chemical analysis of samples from experi-
ments with bagasse sugarcane showed a reduction of 66.8% of acid insoluble lignin,
increase of acid soluble lignin from 3.13% to 7.21%, and total degradation of lignin
of 39.6% (Travaini et al., 2013).
On the other hand, the reaction between carbohydrates is slower than reaction of
ozone with lignin (Travaini et al., 2016c). It produces random cleavage of glyco-
sidic bonds by the reaction of hydroxyl radicals through the formation of superox-
ide (Bule et al., 2013; Gierer, 1982). The reaction of carbohydrate and ozone forms
carbonyl and carboxyl group. The ester-type compound undergoes fragmentation
reaction after generating hydrotrioxide hemiortho ester by the attack of β-glucoside
involves the 1,3-dipolar addition on the anomeric carbon in the C 2 H bond
(Olkkonen et al., 2000).
During the ozonolysis process the ozonated products react with excess ozone,
generating inhibitory compounds depending on its functional group reactivity. The
sugars degrade during ozonolysis, generating oxalic acid, acetic acid, formic acid,
and levulinic acid (LA), but no furfural and hydroxymethylfurfural (HMF) was
detected (Travaini et al., 2016b; Travaini et al., 2015). Other than that, the aromatic
aldehydes, carboxylic acids, and acids are also found as oxidation products
(Schultz-Jensen et al., 2011).
After finishing the ozonation process, the ozonated samples would be put
through washing steps to enhance the sugar release yield after hydrolysis and
remove inhibitory compounds. Around 30 types of lignin products including car-
boxylic acids and phenolic compounds were found in the washing water after the
ozonolysis of straw (Schultz-Jensen et al., 2011). Water washing also removes for-
mic acid and lactic acids generated during ozonolysis and partially remove xylan
while retaining xylitol and acetic acid. The water washing steps also improve the
glucose release from 35.22% to 45.39% and reduce xylose release from 52.44% to
26.40% (Travaini et al., 2013). The glucose release only increases 10% for