Environmental Engineering Reference
In-Depth Information
'cocktails' of enzymes can be used to overcome this issue [56]. Ionic liquids [57],
supercritical water [58], plasmas [59] and microwaves [60, 61] have also been
utilised in the pretreatment or selective hydrolysis of cellulose, though these
protocols require further development to be applied on an industrially significant
scale. Achieving selective, economically viable and reproducible methods for the
conversion of cellulose to fermentable sugars is a vital requirement for the
development of biorefineries that produce platform molecules by fermentation,
especially if competition with food continues to remain high on the bio-economy's
agenda. Despite the difficulties of cellulose utilisation, it still represents an ideal
feedstock for the formation of platform molecules from biomass, being cheap,
widely abundant and often the major waste product of food production.
An alternative to the saccharification of polysaccharides is the chemical and/or
thermal conversion from their polymeric form direct to platform chemicals [62].
Examples of platform molecules obtained directly from the chemical and/or
thermal treatment of polysaccharides include HMF, levulinic acid, furfural and
sorbitol (Figure 4.3). The production of furans (furfural, HMF and (chloromethyl)
furfural or CMF) and their derivatives (e.g. levulinic acid) requires conditions to
promote both hydrolysis and dehydration, and this is most commonly achieved using
soluble or solid acid catalysts in water or the application of ionic liquids [63, 64].
Formation of the sugar alcohols requires hydrolytic hydrogenation conditions (i.e.
hydrolysis in the presence of H
2
or a source of H
2
) [52], cellulose being converted
to sorbitol and mannitol [65] and hemicellulose predominately forming xylitol
and arabitol (Figure 4.4) [66]. These hydrolytic hydrogenation conditions can fur-
ther lead to dehydration products such as sorbitan, 1,4-dianhydroxylitol and
isosorbide [67]. Even more extreme conditions for hydrolytic hydrogenation
OH
OH
H
OH
OH
O
O
HO
OH
Δ
Δ
-H
2
O, H
+
HO
O
-H
2
O, H
+
Pt or Ru catalyst
~ 200°C
Cellulose
OH
OH
HO
OH
H
HO
Sorbitol
1,4-sorbitan
Isosorbide
Pt catalyst
~250°C, H
2
10-100 bar H
2
,
H
2
O, H
+
C1-C6: alkanes, alkenes, alcohols,
diols, aldehydes, ketones,
tetrahydrofuran derivatives
OH
OH
OH
O
HO
OH
Δ
H
2
, H
2
O, H
+
Δ
-H
2
O, H
+
Hemicellulose
OH
OH
Xylitol
HO
1,4-dianhydroxylitol
Figure 4.4
Isosorbide and 1,4-anhydroxylitol from cellulose and hemicellulose, respectively.
