DOI: 10.1002/cssc.201403385
Full Papers
Governing Chemistry of Cellulose Hydrolysis in
Supercritical Water
[
a]
Danilo A. Cantero, M. Dolores Bermejo, and M. Josꢀ Cocero*
At extremely low reaction times (0.02 s), cellulose was hydro-
lyzed in supercritical water (T=4008C and P=25 MPa) to
obtain a sugar yield higher than 95 wt%, whereas the 5-hy-
droxymethylfurfural (5-HMF) yield was lower than 0.01 wt%. If
the reaction time was increased to 1 s, the main product was
glycolaldehyde (60 wt%). Independently of the reaction time,
the yield of 5-HMF was always lower than 0.01 wt%. To evalu-
ate the reaction mechanism of biomass hydrolysis in pressur-
ized water, several parameters (temperature, pressure, reaction
time, and reaction medium) were studied for different bio-
masses (cellulose, glucose, fructose, and wheat bran). It was
+
ꢀ
found that the H and OH ion concentration in the reaction
medium as a result of water dissociation is the determining
factor in the selectivity. The reaction of glucose isomerization
to fructose and the further dehydration to 5-HMF are highly
dependent on the ion concentration. By an increase in the
pOH/pH value, these reactions were minimized to allow con-
trol of 5-HMF production. Under these conditions, the retroal-
dol condensation pathway was enhanced, instead of the iso-
merization/dehydration pathway.
Introduction
Biomass exploitation as a raw material is growing as an alter-
native sustainable method of production of fuels and chemi-
of process represents an advantageous intensification that will
reduce the energetic and equipment requirements in the scale
up.
[
1]
cals. Cellulose is one of the main compounds of biomass and
[
2]
represents the most abundant biopolymer. An important
challenge in the processing of cellulosic biomass is to hydro-
lyze the b(1ꢀ4) glucose–glucose bond to produce a stream of
sugars with a low concentration of byproducts, by using an ef-
Cellulose depolymerization in hot pressurized water has
been done in different kinds of reactors (batch, semibatch, and
continuous) at different temperatures and pressures, with or
[10]
without catalysts. The yield of sugars after biomass hydroly-
sis is enhanced by using supercritical water reactors operated
[
3]
ficient process. These sugar streams could be further trans-
formed into valuable chemicals such as pyruvaldehyde, glyco-
in continuous mode at high temperature for short reaction
[
4]
[5]
[9b,11]
laldehyde, 5-hydroxymethylfurfural (5-HMF), organic acids,
times.
The combination of these two parameters is crucial
[
6]
or polyalcohols. Acid and enzymatic hydrolysis of cellulose
are two conventional methods that need long treatment times
for obtaining high yields of sugars. With long reaction times,
the sugars are derived; at low reaction temperatures, several
side reactions take place and produce many compounds. In
fact, it was observed that some reactions are avoided at super-
critical conditions. In particular, attention should be paid to the
formation of 5-HMF. The production of 5-HMF from cellulose in
pressurized water is highly dependent on the reaction temper-
ature. In Figure 1, several experimental results are shown for
cellulose hydrolysis in pressurized water from T=300–4008C at
different pressures and reaction times.
[7]
(
tꢁ3 h) to obtain a product with poor selectivity (<60 wt%).
The use of ionic liquids as both solvent and reaction medium
has been intensively studied owing to the possibility of dis-
solving cellulose and making it more “accessible” for the hy-
[
8]
drolysis reaction. However, this kind of process takes at least
h of hydrolysis to obtain a selectivity near to 30 wt% of re-
3
[
8a]
ducing sugars. These processing methods require long reac-
tion times (hours), which will demand big reactors during the
scale up. The use of pressurized water as the reaction medium
for the processing of cellulosic biomass in a one-step fast pro-
cess is an alternative. Total hydrolysis of cellulose can be ach-
ieved in a reaction time of 0.02 s in a supercritical water
medium and produces a stream of water-soluble sugars with
It can be observed that 5-HMF production was faster, but
the yield was lower, when the reaction temperature was in-
creased from 300 to 3508C. The reaction time was reduced
from 40 to 10 s by increasing the reaction temperature. This
behavior was expected and it follows the Arrhenius law. How-
ever, an expected behavior was detected if the reaction tem-
perature was increased over the critical point of water: the
production of 5-HMF was highly avoided. Although this behav-
[
9]
a low concentration of derived products (<2 wt%). This kind
[
a] Dr. D. A. Cantero, Dr. M. D. Bermejo, Prof. Dr. M. J. Cocero
Department of Chemical Engineering and Environmental Technology
University of Valladolid
[4a,b,9a,12]
ior was previously described in the literature,
a clear
and quantitative explanation has not been developed yet. The
different discovered behaviors can be classified into three
main groups. 1) The maximum amount of 5-HMF from cellu-
lose in pressurized water without catalyst is produced at tem-
Dr. Mergelina S/N, 47005, Valladolid (Spain)
E-mail: mjcocero@iq.uva.es
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201403385.
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