Hydrothermal process for increasing acetic acid yield from lignocellulosic wastes

Article abstract of DOI:10.1246/cl.2002.504

To increase the acetic acid yield in a wet oxidation of lignocellulosic wastes, a new two-step reaction process is proposed. The first step produces 5-hydroxymethyl-2-furaldehyde (HMF) and 2-furaldehyde (2-FA) by dehydration of monosaccharides which are f

Full text of DOI:10.1246/cl.2002.504

504
Chemistry Letters 2002
Hydrothermal Process for Increasing Acetic Acid Yield from Lignocellulosic Wastes
Fangming Jin,ꢀ Junchao Zheng, Heiji Enomoto, Takehiko Moriya,y Naohiro Sato,yy and Hisao Higashijimayyy
Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University, Sendai 980-8579
yResearch and Development Center, Tohoku Electric Power Co., Inc., Sendai 981-0952
yyIndustrial Technology Institute, Miyagi Prefectural Government, Sendai 981-3206
yyyNiigata Engineering Co., Ltd., Tokyo 144-8640
(Received January 25, 2002; CL-020097)
To increase the acetic acid yield in a wet oxidation of
lignocellulosic wastes, a new two-step reaction process is proposed.
The first step produces 5-hydroxymethyl-2-furaldehyde (HMF) and
2-furaldehyde (2-FA) by dehydration of monosaccharides which
are formed by hydrolysis of polysaccharides under the condition of
non-supply of oxygen. In the second step, HMF and 2-FA are
oxidized to acetic acid with oxygen newly added. The yield
increased about twofold.
Acetic acid production by wet oxidation (WO) of various food
wastes and lignocellulosic wastes had been investigated to produce
calcium/magnesium acetate (CMA)1 known as an environmentally
friendly deicer.2;3 Results indicate that the yield of acetic acid is
limited to only 12–13% on a TOC/TOC basis at most, by a usual wet
oxidation procedure.1 In order to improve the conversion efficiency
to acetic acid, other reaction pathways of lignocellulosic wastes in
hydrothermal reaction including WO were investigated.
Figure 1. HPLC chromatograms of intermediate products for
oxidation of rice hulls, cellulose and glucose at 300 ꢁC, 1 min of
reaction time and 100% oxygen supply (UV detection at 210 nm).
Rice hulls, as a representative of lignocellulosic wastes, and
cellulose, as a representative of the main component of lignocellu-
losic wastes, were used as test materials, as well as glucose as an
intermediate product of cellulose by hydrolysis. A 0.07 g (dry base)
of test material was used in each run. As an oxidant, liquid H2O2was
used. A 100% oxygen supply was defined on the basis of the
stoichiometric demand of oxygen for complete oxidation of carbon
in the test material to carbon dioxide. Previous studies revealed that
an oxygen supply of 100% was sufficient for complete oxidation of
lignocellulosic wastes.4
All experiments were carried out with a batch reactor system
described earlier.5 For WO, the experimental procedure was as
follows: a desired amount of test material and H2O2-water were
added into a microreactor, and then the reactor was put into a salt
bath that had been preheated to a desired temperature. After a
certain reaction time, the reactor was removed from the salt bath,
and then put into a cold-water bath to quench the reaction.
In a two-step reaction process, which is a new process proposed
in this study, the procedure described above was repeated twice.
That is, in the first step, only the test material and water were put into
the reactor for reaction. After this reaction, H2O2 was added to the
cooled reactor, and the second step took place.
Figure 2. Reaction pathways for oxidation of cellulose/lignocellulosic
wastes.
each other, i.e., many low molecular weight carboxylic acids
including acetic acid were identified. These results may suggest the
main WO pathways of cellulose/lignocellulosic wastes as shown in
Figure 2(A). That is to say, cellulose and lignocellulosic wastes are
first hydrolyzed to monosaccharides (mainly glucose) and then the
oxidation of monosaccharides takes place. Oxidation of mono-
saccharides leads to the formation of both monocarboxylic acids
and dicarboxylic acids, because the treatment of monosaccharides
with a more vigorous oxidizing agent brings about the oxidation of
not only the –CHO group but also the –CH2OH group.
Experimental conditions are as follows: temperature 280–
350 ꢁ, reaction time 0.5–6 min, oxygen supply 0–150%, and water
fill 30% (70% was also used in the two step reaction of rice hulls).
After the reaction was quenched, a solution sample was collected
and analyzed by HPLC, GC/MS and 1H-NMR.
Intermediate products in the WO of rice hulls, cellulose and
glucose were identified to see the main WO pathway of
lignocellulosic wastes. Results of HPLC analysis are shown in
Figure 1. Rice hulls, cellulose and glucose show similar results to
The oxidation of monocarboxylic acids and dicarboxylic acids
may proceed as shown in Figure 2(B). When the oxidation of
monocarboxylic acids takes place, acetic acid and formic acid are
formed as final organics, while for the oxidation of dicarboxylic
acids, either lower molecular weight monocarboxylic acids or
dicarboxylic acids are formed. Our quantitative analysis showed
that the concentration of oxalic acid was overwhelmingly high, and
additional experiments also showed that oxalic acid was easily
decomposed to CO2 and H2O by WO, so that the total amount of
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
505
oxalic acid produced could be much more than that we expected
from the analytical data. This indicates that the formation of
dicarboxylic acid takes precedence over the formation of mono-
carboxylic acids.
half of HMF, was produced.
Other products such as 1-hydroxy-2-propanone, which is a
thermal decomposition product of glucose, levulinic acid, which is a
rehydration product of HMF, 2-butanone and 2,5-hexanedione were
also detected as shown in Figure 3. They also form monocarboxylic
acids by oxidation, although their amounts are small.
On thebasis of these results, a newtwo-step reaction process for
promoting acetic acid production is proposed, as shown in Figure 5.
In the first step, cellulose/lignocellulosic wastes are hydrolyzed to
monosaccharides, which subsequently undergo dehydration to form
HMF and 2-FA under the condition of nonsupply of oxygen. In the
second step, the HMF and 2-FA produced are further converted to
acetic acid by oxidation with newly added oxygen.
From the discussion above, it is understandable that the
conversion efficiency of acetic acid from cellulose/lignocellulosic
wastes is hardly improved by changing the reaction condition in
conventional WO, because WO of cellulose and lignocellulosic
wastes easily forms dicarboxylic acids, only a little of which is
decomposed to form monocarboxylic acids by further oxidation.
Therefore, in order to increase the acetic acid yield, oxidative
decomposition pathways via dicarboxylic acids should be avoided.
In other words, direct oxidation of monosaccharides should be
inhibited.
In order to find some intermediate products, which can form a
large amount of monocarboxylic acids rather than dicarboxylic
acids by their oxidation, decomposition experiments were
performed with a lower oxygen supply. Large quantities of 5-
hydroxymethyl-2-furaldehyde (HMF) and 2-furaldehyde (2-FA)
were detected in all cases (Figure 3). It was also found that the
oxidation of HMF and 2-FA forms acetic acid as well as formic acid,
and oxalic acid was not detected in their oxidation reaction (Figure
4).
Figure 5. Two-step reaction process proposed for production of more
acetic acid.
To obtained better conditions for producing HMF and 2-FA
from cellulose in the first step and then converting the HMF and 2-
FA into acetic acid in the second step, the firststep experiments were
performed using cellulose at temperatures between 280 and 370 ꢁC
and reaction time from 30 s to 6 min, and the second step
experiments with HMF and 2-FA as test material at temperatures
between 280 and 370 ꢁC, reaction time from 30 s to 6 min and
oxygen supply from 50 to 130%. Conditions so far obtained as
optimum are as follows: temperature 300 ꢁC and reaction time 2 min
for the first step, and temperature 300 ꢁC, reaction time 1 min and
oxygen supply 70% for the second step.
Under the conditions mentioned above, rice hulls were treated
in the two-step reaction process. Results showed that the acetic acid
yield obtained in the two-step reaction was approximately twice that
obtained by the usual WO procedure (Table 1).
Figure 3. GC/MS chromatograms for glucose (A) and cellulose (B)
after reaction at 300 ꢁC and 1 min for glucose, 2 min for cellulose.
Table 1. Comparisonof acetic acid yieldsa by a conventional wet oxidation
(WO) and a two-step reaction proposed in this study
Figure 4. HPLC chromatograms of liquid samples after
oxidation of HMF (A) and 2-FA (B) at 300 ꢁC for 1 min and
with 100% oxygen supply (UV detection at 210 nm).
References and Notes
1
2
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4
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(1999).
It is generally known that, on heating with or without acid,
hexoses undergo dehydration and cyclization to form HMF while
pentoses form 2-FA. But 2-FA, as well as HMF, has been reported as
a product of dehydration of glucose, although the amount of 2-FA is
less.6 In the present study, a relatively large amount of 2-FA, about a
5F. Jin, A. Kishita, T. Moriya, and H. Enomoto,
(2001).
J. Supercrit. Fluids, 19, 25 1
6
7
G. Bonn and O. Bobleter, J. Radioanal. Chem., 79, 171 (1983).
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