Page 124 - Lignocellulosic Biomass to Liquid Biofuels

98 Lignocellulosic Biomass to Liquid Biofuels

as mannose, galactose, rhamnose, and arabinose, are obtained by hemicel-
luloses decomposition [233].
HMF and furfural are the most process-derived inhibitors, formed by
the dehydration of different hexoses and pentoses, respectively [234].
Consequently, simple sugars, such as glucose, mannose, and galactose,
with the exception of rhamnose [235], can be converted to HMF, while
xylose and arabinose can follow different reaction mechanisms resulting in
the formation of furfural [236,237].
A study of Rasmussen et al. [238] reviewed different routes of furfural
and HMF formation from xylose and glucose, respectively. The mecha-
nism and degradation route depend on the protonation site initiating the
decomposition of monosaccharides. In particular, for the degradation of
xylose to furfural it was proposed one acyclic mechanism and two differ-
ent types of cyclic reaction mechanisms, resulting from protonation arising
from xylose protonation of either hydroxyl (1-OH or 2-OH) or the
O-pyranose sites. For the formation of HMF from glucose they were
suggested acyclic and cyclic mechanisms with or without fructose isomeri-
zation. These mechanisms are caused by 2-OH and O-pyranose proton-
ation of glucose.
Furfural and HMF are commonly generated during acidic pretreat-
ment conditions, thus at low pH, in combination with a high temperature
[230,239]. Limayem [33] reports that a pretreatment of lignocellulosic
material by using dilute sulfuric acid under 4 wt.% and temperatures more
than 160°C determines the two furans formation in high concentrations.
A furan formation of 8 25 g/kg initial lignocellulose dry weight (DW)
can be reached by acid pretreatment of monocot lignocellulose, while an
acid pretreatment of softwood lignocellulose can result in a furans forma-
tion of 35 g/kg initial lignocellulose DW [230]. Furthermore, as discussed
by Monlau [236], thermophysical and thermochemical pretreatments
release furfural and HMF at low pH (i.e., thermo-acid pretreatment);
their release is instead negligible at high pH (i.e., thermo-alkaline
pretreatment).
The methods of pretreatment carried out at high pH (i.e., wet oxida-
tion, alkali hydrolysis, and AFEX) lead to a relatively low furans formation
[230]. The presence of furfural and HMF is also extensively reported after
the thermo-mechanicochemical pretreatment known as steam explosion
[240,241], although the furan quantities produced are relatively low,
about 4 g/kg initial lignocellulose DW [230]. The furans formation
depends on the pretreatment method used to decompose the