Page 130 - Lignocellulosic Biomass to Liquid Biofuels

104 Lignocellulosic Biomass to Liquid Biofuels

herbicides, antifreeze, and pharmaceutical agents, by different chemical
reaction (such as esterification, halogenation, hydrogenation, and conden-
sation) [272]. The production of LA is mainly achieved by acid-catalyzed
dehydration of hexose sugars to HMF, followed by a rehydration step of
HMF to LA [271].
An often overlooked pathway in the LA production from lignocellu-
losic material is derived from the furfural. The furfural can be reduced to
furfuryl alcohol that can in turn be converted to LA by acid hydrolysis
[273,274].
In addition, the hydroxymethylation process of furfural with formalde-
hyde determines the HMF production. The HMF can be subsequently
converted to LA [275,276].

3.5.3 Phenols
Phenolic compounds, or phenols, are the most abundant secondary meta-
bolites in plants, biosynthesized via the PP, shikimate (SK), and phenyl-
propanoid (PhP) pathways in plants [277].
In the synthesis of phenolic compounds in plants the glucose is initially
committed in the PP pathway, and the glucose-6-phosphate is irreversibly
dehydrogenated to ribulose-5-phosphate by glucose-6-phosphate dehy-
drogenase (G6PD). In the last reactions of the PP pathway, ribose-5-
phosphate is converted to erythrose-4-phosphate that enters in the SK
pathway along with phosphoenol pyruvate obtained from glycolysis [278].
The phenylalanine produced in the SK pathway is used through the
PhP pathway to generate phenolic compounds [278]. The phenols range
from simple phenolic molecules to highly polymerized compounds and
include various classes of compounds, such as phenolic acids, colored
anthocyanins, simple, and complex flavonoids [279]; their structure com-
prises an aromatic ring and various functional groups such as aldehyde,
ketone, acid and the side groups such as methoxy and hydroxyl [277].
The phenols make up the lignin building blocks and play a key role in
plant defense systems against microbial infections, in addition to being
involved in several plant physiological processes [277]. Different phenols
are indeed formed as residues chiefly by lignin degradation during biomass
pretreatment, depending on both the plant source and the pretreatment
method [278]. In general, the phenolic compounds formed during
pretreatment processes are a mix of phenolic acids, phenolic aldehydes,
phenolic alcohols, and phenolic ketones [277].