Xylans are a valuable alternative resource: Production of d-xylose, d-lyxose and furfural under microwave irradiation

Article abstract of DOI:10.1016/j.carbpol.2013.07.066

The influence of microwave irradiation on hydrolysis of xylan and simultaneous epimerization of the d-xylose to d-lyxose has been studied. An acidic solution of xylan was treated with catalytic amount of sodium molybdate and the composition of the reaction mixture was analyzed. Short reaction times of hydrolysis and subsequent epimerization reaction provided an equilibrium reaction mixture of d-xylose and d-lyxose (1.6:1) without significant formation of undesirable side products. Obtained pentoses can be reduced to the corresponding alditols (d-xylitol and d-lyxitol) in very good yields (88% and 85%) or can be further dehydrated to furfural (53%). Combined use of Mo(VI) catalyst and microwave irradiation allows better conversions and substantial reduction of reaction times (400-fold) compared to that obtained by conventional heating. Studied stereospecific transformation of xylan proceeds with high selectivity, short reaction times and very good yields that makes this approach attractive also for preparative purposes.

Full text of DOI:10.1016/j.carbpol.2013.07.066

Carbohydrate Polymers 98 (2013) 1416–1421
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Xylans are a valuable alternative resource: Production of d-xylose,
d-lyxose and furfural under microwave irradiation
Zuzana Hricovíniová∗
Institute of Chemistry, Slovak Academy of Sciences, Dubravská cesta 9, SK-845 38 Bratislava, Slovak Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2013
Received in revised form 18 July 2013
Accepted 26 July 2013
Available online 6 August 2013
The influence of microwave irradiation on hydrolysis of xylan and simultaneous epimerization of the
d-xylose to d-lyxose has been studied. An acidic solution of xylan was treated with catalytic amount of
sodium molybdate and the composition of the reaction mixture was analyzed. Short reaction times of
hydrolysis and subsequent epimerization reaction provided an equilibrium reaction mixture of d-xylose
and d-lyxose (1.6:1) without significant formation of undesirable side products. Obtained pentoses can be
reduced to the corresponding alditols (d-xylitol and d-lyxitol) in very good yields (88% and 85%) or can be
further dehydrated to furfural (53%). Combined use of Mo(VI) catalyst and microwave irradiation allows
better conversions and substantial reduction of reaction times (400-fold) compared to that obtained by
conventional heating. Studied stereospecific transformation of xylan proceeds with high selectivity, short
reaction times and very good yields that makes this approach attractive also for preparative purposes.
© 2013 Elsevier Ltd. All rights reserved.
Keywords:
Xylans
Microwave irradiation
MoVI-catalysis
d-Xylose
d-Lyxose
Furfural
1. Introduction
polysaccharide is mainly found in woods, grasses, algae and cereals.
The major constituents of hemicelluloses are xylans, polysaccha-
During the last decades there has been increased interest
in the development of environmentally friendly processes for
preparation of valuable chemicals from agricultural waste. As the
availability of the fossil fuel reserves will become limited, renew-
able naturally abundant resources are becoming a viable and
important alternative resource. Carbohydrates are the most abun-
dant and inexpensive naturally occurring compounds. They are
mostly present as biopolymers (cellulose, hemicellulose, starch)
and belong to the most intensively studied precursors with a wide
range of properties and applications (Belgacem & Gandini, 2008).
Lignocellulosic biomass, as an abundant renewable resource, has
the potential to become a sustainable supply for the production
of chemical intermediates and biofuels. One important and valu-
able chemicals, furfural, obtained by dehydration of pentoses can
be used to replace petrochemicals or can be further converted to
a range of products including plastics, pharmaceuticals and agro-
chemicals (Xia, Lin, Tonga, & Beltramini, 2011). Utilization of this
potential requires effective conversion of lignocellulose into useful
intermediates, such as monosaccharides, that are ideal alternatives
for the construction of enantiopure target molecules.
rides consisting of d-xylose units linked with (1 → 4)-␤-glycosidic
linkages. They are essentially linear polymers with short branched-
chains. Xylose is always the sugar monomer present in the
largest amount, but some monosaccharides (arabinose, mannose,
rhamnose, glucose) or uronic acids (glucuronic, mannuronic, galac-
turonic) are also present (Ebringerová & Heinze, 2000; Belgacem
& Gandini, 2008). These polymers can be easily hydrolyzed to
monomeric sugars, by the use of mineral acids as well as hemi-
cellulase enzymes (Mansilla et al., 1998; Chen & Gong, 1985). The
challenge for the effective utilization of lignocellulosic waste mate-
rial is to use this bio-based resource for production of high-value
products such as xylose, xylitol, and furfural (Prakasham, Sreenivas,
& Hobbs, 2009; Zhang, Yu, Wang, Dong, & Peng, 2013). Exten-
sive efforts have been expended to achieve efficient polysaccharide
depolymerisation. Conversion should be economical, environmen-
tally friendly and technologically practical at the same time. In
recent years microwave irradiation has received increasing interest
in many chemical reaction studies, including organic and inor-
ganic syntheses, due to remarkable enhancement of the reaction
rates, higher yields and improved selectivity (Loupy, 2006). The
basic mechanisms observed in microwave-assisted processes are
dipolar polarization and conduction. It is known that the pres-
ence of salts can frequently enhance effects of dielectric loss and
microwave coupling could produce superheating (Gabriel, Gabriel,
Grant, Halstead, & Mingos, 1998; Kunlan et al., 2001). Many thermo-
chemical conversion technologies are known and several studies
Hemicellulose is, after cellulose and starch, the third most
abundant biopolymer in Nature (20–35 wt.%). This plant cell wall
∗
Tel.: +421 2 59410282; fax: +421 2 59410222.
E-mail addresses: Zuzana.Hricoviniova@savba.sk, chemhric@savba.sk
0144-8617/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbpol.2013.07.066

Z. Hricovíniová / Carbohydrate Polymers 98 (2013) 1416–1421
1417
have appeared that describe the transformation of hemicellulose in
a microwave field (Binder & Raines, 2009; De, Dutta & Saha, 2011).
We have recently reported that the formation of rare aldoses can
be facilitated by combined action of transition-metal catalyst and
microwave irradiation. Molybdate ions were investigated as a cata-
lyst for the isomerization of various reducing sugars. It was shown
that transformation proceeds very efficiently in microwave field
(Hricovíniová, 2006, 2008, 2009). The rapid and convenient produc-
tion of d-mannose from starch (Hricovíniová, 2011) prompted us to
investigate this approach on other types of polysaccharides and to
study this catalyzed transformation in detail. Herein we described a
novel approach applied to hydrolysis of xylans to produce rare pen-
toses employing the catalytic effect of Mo(VI) ions and microwave
irradiation as the thermal energy source. This method is directed
to rapid release of d-xylose from xylans and its immediate trans-
formation to rare aldose, d-lyxose. The combination of two steps
makes the method possible to achieve reaction mixture of two
valuable pentoses, d-xylose and d-lyxose, which could be easily
transformed to the corresponding alditols (d-xylitol and d-lyxitol)
or furfural.
measurements and the ratio of xylan/xylose/lyxose was deter-
mined by integration of selected resonances in 1H NMR spectra.
2.3. Hydrolysis of xylan in the presence of sodium molybdate
with conventional heating
Xylan (500 mg) was dissolved in 0.25 M HCl (10 ml) and
Na2MoO4·2H2O (50 mg) was added. The tube was sealed and heated
in an oil-bath at 90 ◦C for 0–25 h. Samples (0.5 ml) were taken at
−
selected intervals, treated with Amberlite IRA-400 in the HCO3
and H+ form to remove the catalyst. The composition of the reaction
mixture was analyzed by 1H NMR spectroscopy.
2.4. Xylan hydrolysis and epimerization reaction in
semi-preparative scale
For a typical procedure beechwood xylan (1 g) was dissolved
in 0.25 M HCl (20 ml) and Na2MoO4·2H2O (100 mg) was added.
The weight ratio of reactants is the same as applied in previous
model experiments. The sealed tube was exposed to microwave
irradiation (200 W) for 3 min. The reaction mixture was treated
batch-wise with an excess of the cation/anion ion-exchange resin,
filtered off, washed with water and combined filtrates were evapo-
2. Experimental
rated. The composition of the reaction mixture was analyzed by 1
H
2.1. Materials and methods
NMR spectroscopy. The syrupy residue was fractionated by column
chromatography on Dowex 50W X8 (200–400mesh) in Ba2+ form
with water as eluent. Fractionation of the syrupy residue afforded
d-xylose (0. 604 g, 60%) and d-lyxose (0.314 g, 31%). Analytical data
were in accordance with literature cited (Collins, 1998).
Beechwood xylan, birchwood xylan and 4-O-methyl-d-
glucurono-xylan isolated from beechwood. Standards: d-xylose,
d-lyxose, furfural (2-furalaldehyde, 99%). All chemicals and mate-
rials used were purchased from commercial suppliers. Microwave
reactions were performed in a multimode microwave reactor CEM
Discover consisting of a continuous focused microwave power
delivery system with operator-selectable power; microwave fre-
quency source of 2.45 GHz. The reactions were performed in sealed
glass tubes and were stirred magnetically to prevent hot spots.
Conversions and the purities of the products were determined by
NMR spectroscopy. High-resolution NMR spectra were recorded in
a 5 mm cryoprobe on Varian 600 VNMRS spectrometer. The exper-
iments were carried out at 25 ◦C or 30 ◦C in D2O. One-dimensional
1H and 13C NMR spectra as well as two-dimensional COSY and
HSQC were used to determine 1H and 13C chemical shifts. FT-IR
spectra were measured on a Nicolet 6700 spectrometer with DTGS
detector and OMNIC 8.0 software using 128 scans at the resolution
of 4 cm−1 with diamond ATR technique. UV absorption spectra
were recorded on Beckman DU-7 UV–Visible spectrophotometer.
Optical rotations were determined at 20 ◦C with an automatic
polarimeter Perkin–Elmer Model 141 using 1-cm cell. Melting
points were measured on a Kofler hotstage microscope. Separations
of the free sugars were accomplished by column chromatography
on Dowex 50W X8 resin in the Ba2+ form (200–400 mesh). Paper
chromatography was performed by the descending method on
Whatman No. 1 paper using ethyl acetate–pyridine–water (8:2:1)
as the mobile phase. The chromatograms were made visible by
means of alkaline silver nitrate. All chemicals were reagent grade
and used without further purification.
2.5. Reduction of d-xylose to d-xylitol
To a solution of d-xylose (1 g) in water (10 ml) was added
an aqueous solution of sodium borohydride (0.2 g/5 ml H2O). The
reaction mixture was kept at room temperature for 3 h. When
the reduction was complete the reaction mixture was acidified
with a few drops of acetic acid, deionized with cation/anion ion-
exchange resin and evaporated to dryness. Crystallization from
ethanol afforded optically inactive d-xylitol (880 mg; 88%). Ana-
lytical data and NMR spectra were in accordance with literature
(Collins, 1998; Angyal & Le Fur, 1980).
2.6. Reduction of d-lyxose to d-lyxitol
To a solution of d-lyxose (1 g) in water (10 ml) was added an
aqueous solution of sodium borohydride (0.2 g/5 ml H2O). Reaction
was worked up in exactly the same way as mentioned in previ-
ous procedure. Crystallization from methanol afforded d-lyxitol.
Yield 85%; analytical data and NMR spectra were in accordance with
literature (Collins, 1998; Angyal & Le Fur, 1980).
2.7. Microwave-assisted dehydration of xylan to furfural
Xylan (2 g) was dissolved in 0.1 M HCl (10 ml) and
Na2MoO4·2H2O (50 mg) was added to obtain a stock solution.
The sealed tube (1 ml) was inserted into the microwave reactor
and exposed to microwave irradiation (300 W) for different lengths
of time (1–10 min). Upon completion of the reaction the sample
was cooled down to room temperature and reaction mixture was
extracted with diethylether and evaporated. Furfural, isolated
as yellow oil was analyzed by UV, IR and NMR techniques. Yield
53%; UV 278 nm (Martinez, Rodriguez, York, Preston, & Ingram,
2.2. Hydrolysis of xylan in the presence of sodium molybdate
under microwave irradiation
Xylan (500 mg) was dissolved in 0.25 M HCl (10 ml) and
Na2MoO4·2H2O (50 mg) was added to obtain a stock solution.
Sealed tubes (1 ml) were exposed to microwave irradiation 200 W
for different lengths of time (10 s–5 min). The vessels were removed
from the microwave reactor, the reaction mixtures were treated
2000); ꢀmax (ATR, diamond), 1462 cm−1
, C of
1473 cm−1 (C
with Amberlite IRA-400 HCO3 and H+ form to remove the cata-
aromatic ring), 1666 cm−1 (Ar C O), (Sashikala & Ong, 2007); ıH
−
lyst. The reaction mixtures were analyzed by NMR spectroscopy
(D2O, 599.84 MHz): ring protons were shown in aromatic region

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Z. Hricovíniová / Carbohydrate Polymers 98 (2013) 1416–1421
Xylan
6.6–9.6 ppm (Shin & Cho, 2008). Analytical data and NMR spectra
were in accordance with literature cited.
4
O
O
O
O
O
HO
O
HO
O
HO
1
2.8. Dehydration of xylan with conventional heating
OH
OH
OH
Xylan (2 g) was dissolved in 0.1 M HCl (10 ml) and
Na2MoO4·2H2O (50 mg) was added. The round bottom flask
was placed in an oil-bath and refluxed at 150 ◦C (10–120 min). The
reaction was worked up in exactly the same way as mentioned in
previous experiment. Furfural yield 36%.
MoVI H3O+
MW 3 min
n
OH
O
O
HO
HO
HO
HO
OH
+
OH
D-Xylose
OH
D-Lyxose
3. Results and discussion
Scheme 1. Reaction scheme of xylan hydrolysis and simultaneous epimerization
reaction of obtained d-xylose to d-lyxose in microwave field.
In connection with adding value to lignocellulose biomass,
the application of appropriate catalytic system in combina-
tion with microwave field represents an innovative approach
with processing advantages. Here we show the application of
microwaves in combination with hexavalent molybdenum cata-
lyst as an efficient method for the production of commercially
interesting chemicals. d-Xylose, d-xylitol, d-lyxose, d-lyxitol or
furfural can be prepared from xylan-based polysaccharides from
various sources. Molybdate ions act as a powerful catalyst for
the epimerization of reducing sugars (Bílik, 1972, 1983) and have
a number of unique features (Porai-Koshits & Atovmyan, 1974;
Petrusˇ, Petrusˇová, & Hricovíniová, 2001). Reaction conditions are
the prime factors that affect the yield and composition of the reac-
tion mixture. The influence of different variables, such as acid
concentration, amount of catalyst, microwave power and reaction
time, were evaluated for identification of xylan conversion and
product distribution.
Several experiments were designed on exploitation of the
catalytic system that would improve xylan hydrolysis with subse-
quent epimerization of d-xylose monomer. Beechwood xylan was
selected as the model compound for determination of the influence
of microwave-induced Mo(VI)-catalyzed conversion of hemicellu-
loses to epimeric pentoses, d-xylose and d-lyxose. Experiments on
the combined hydrolysis and isomerization reaction in one step
have been carried out using a catalytic amount of sodium molyb-
date and dilute hydrochloric acid. It was found that a 0.5% solution
of sodium molybdate was a sufficiently high concentration of this
catalyst and led to very good conversion to desired products.
The efficiency of thermochemical conversion, selectivity and
product distribution was examined and the best results were
obtained by performing the reaction at 200 W of microwave power.
In the optimized reaction conditions, a 5% solution of xylan in
0.25 M hydrochloric acid with a catalytic amount of sodium molyb-
date was exposed to microwave irradiation for different time
periods. Selected reaction conditions led to complete depolymer-
ization of xylan and high conversion by means of d-xylose and
d-lyxose content. The analysis of 1H NMR spectra indicated that
xylan was completely hydrolyzed and converted to the equilib-
rium mixture of d-xylose and d-lyxose (1.6:1) within 3 min. The
reaction scheme of the combined xylan hydrolysis and simulta-
neous epimerization reaction of obtained d-xylose to d-lyxose is
illustrated in Scheme 1.
3.1. Microwave-assisted conversion of xylan
Many polysaccharides require harsh conditions to degrade.
While cellulose is crystalline and strongly resistant to hydroly-
sis, hemicellulose containing predominantly d-xylose units (linked
as in cellulose by (1 → 4)-␤-glycosidic linkages), has an amor-
phous structure with little strength (Nishiyama, Sugiyama, Chanzy,
& Langan, 2003). Microwave irradiation results in polarization of
polar bonds (such as C
O
C in glycosidic linkages) and increases
In the present work, the yields of d-xylose/d-lyxose were com-
pared using three different xylan substrates. Xylans from beech
wood, birch wood and 4-O-methyl glucuronoxylan from beech-
wood were tested. The solution of each xylan sample was exposed
to microwave irradiation under previously optimized conditions.
The ratio of sugars present in the equilibrium reaction mixture with
respect to the production of d-xylose and d-lyxose was determined
by 1H NMR spectroscopy. The transformation occurs in 3 min reach-
ing comparable final concentrations. Table 1 summarizes all the
data obtained for various xylan samples heated in microwave field
or by conventional heating. Mo(VI) catalyst in combination with
microwave power, caused only minor differences of higher over-
all conversion to d-xylose/d-lyxose among three different xylans
tested. It should be also noted that the increasing amount of catalyst
did not influence the product composition.
their reactivity. Therefore, a microwave field is able to enhance the
hydrolytic cleavage of macromolecular chains. Experimental trials
of the acid hydrolysis of xylan to sugar monomers in a microwave
field were carried out by means of a multimode microwave reac-
tor consisting of a continuous focused microwave power delivery
system with operator-selectable power. Various concentrations of
hydrochloric acid (0.1 M, 0.25 M, 0.5 M, 0.75 M and 1.0 M) were
tested for the hydrolysis of xylan samples. The depolymerization of
(1 → 4)-␤-xylopyranosyl linkages to d-xylose units worked already
in 0.1 M hydrochloric acid. We observed that hydrolysis of xylan is
predominantly determined by acid concentration independently
of xylan source used. As expected, faster conversion of xylan to
d-xylose can be reached in the reaction performed using higher
concentrations of acid.
Table 1
Comparison of microwave and conduction-heated samples of the combined xylan hydrolysis and simultaneous epimerization reaction of obtained d-xylose to d-lyxose.
Both partial and full conversions are shown for conventional oil-bath heating to demonstrate considerable differences between microwave and conventional approaches in
reaction kinetics and amounts of xylose/lyxose formations.
Xylan source
MW field
Conventional
Time (h)
Heating
Time (h)
Time (min)
xylan/Xyl/Lyx (%)
xylan/Xyl/Lyx (%)
Xylan/Xyl/Lyx (%)
Beechwood xylan
Birchwood xylan
4-O-Methyl-glucuronoxylan
3
3
3
0/62/38
0/63/37
0/61/39
10
10
10
6/68/25
7/68/24
7/67/25
20
20
20
0/70/30
0/69/31
0/68/32

Z. Hricovíniová / Carbohydrate Polymers 98 (2013) 1416–1421
1419
D-xylose
+ MoVI
O
O
O
+H3O+
O
Mo
O
Mo
O
O
OH
O
1
C
O
H
H
H
C 4
2
C
C5
C3
HO
H
H
H
A
=
O
O
O
O
Mo
O
Mo
OH
O
O
O
1
C
O
H
H
2
C
C 4
C5
C
OH
HO
3
H
H
H
H
B
O
O
O
O
Mo
O
Mo
O
O
O
O
H
H
H
C
1
C
2
C 4
OH
C5
C3
HO
H
H
H
- MoVI
C
- H3O+
D-lyxose
Scheme 2. Mechanism of the Mo(VI)-catalyzed epimerization reaction of d-xylose to d-lyxose. The equilibrium reaction mixture of two epimeric aldoses is reached in
microwave field after 3 min.
In order to compare these data with semi-preparative experi-
ment we performed the reaction in microwave field using 1 g of
beechwood xylan. Fractionation of the reaction mixture by col-
umn chromatography afforded pure epimers d-xylose (60%) and
d-lyxose (31%). Xylan hydrolysis is predominantly determined by
acid concentration but the simultaneous mutual interconversion of
d-xylose and d-lyxose is governed by highly stereospecific isomer-
ization reaction that is catalyzed by molybdate ions (Bílik, 1983).
The mechanism of this stereospecific transformation is based
on the fact that d-xylose units create catalytically active com-
plexes in the presence of molybdate ions. Catalytically active
complexes lead to isomerization of d-xylose to d-lyxose through
the rearrangement of carbohydrate carbon skeleton. The accepted
mechanism is depicted in Scheme 2. The first step is the com-
plex formation of binuclear molybdate species with four adjacent
hydroxyl groups (on C-1, C-2, C-3 and C-4) of the acyclic hydrated
form of d-xylose (Scheme 2A). This rigid framework holds the
acyclic carbohydrate skeleton in required conformation for stere-
ospecific carbon backbone rearrangement. Rearrangement occurs
through a transition state (Scheme 2B) in which C-1 and C-2
are enantiomeric. Bond formation between C-2 and C-3 regener-
ates the starting d-xylose, while bond formation between C-1 and
C-3 produces the 2-epimer, d-lyxose (Scheme 2C). These results
matched well with our previous observations and are also in agree-
ment with NMR experiments analyzing the mechanism of this
transformation with 13C-enriched saccharides (Petrusˇ et al., 2001).
Microwave irradiation, as a non-conventional energy source,
has a strong influence upon the kinetics and thermodynamics
of this reaction. The obtained data suggest that the coordination
ability of the Mo(VI)-catalyst under the given reaction condi-
tions enables simultaneous efficient depolymerisation of xylan
and formation of epimeric d-lyxose in one step. It should be also
noted that the presence of molybdate salts improves efficiency
of energy absorption from microwave source and thus, the reac-
tion proceeds faster compared to the reaction without the catalyst
(Hricovíniová, 2006). Under identical reaction conditions (0.5%
Na2MoO4 in 0.25 M HCl) the microwave irradiated samples of
xylan were compared with samples prepared by conduction heat-
ing (90 ◦C, oil-bath). Under conventional conditions the equilibrium
reaction mixtures were obtained after long heating. Xylan was com-
pletely hydrolyzed to d-xylose after 3–4 h. Formation of d-lyxose
started after 2 h and the maximum was reached in about 20 h. The
final equilibrium mixture is comparable to that found in previous
studies of d-xylose/d-lyxose epimerization (Hricovíniová, 2006).

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Z. Hricovíniová / Carbohydrate Polymers 98 (2013) 1416–1421
MoVI H3O+
CHO
O
H+
90
75
60
45
30
15
0
Xylan
Pentoses
Xylan
-3 H2O
MW
Xylose
Lyxose
Furfural
Scheme 3. The microwave-assisted transformation of xylan to furfural.
time were observed. The reaction equilibrium was shifted in both
cases to the aldose with the lower value of conformational insta-
bility, thus a larger amount of d-xylose was present. A high input
of energy allows the reaction to overcome the energy barrier for
products formation very fast (2–3 min) compared to conventional
conditions (20–25 h).
Xylose and lyxose were obtained from various xylan samples
in a one-pot transformation. d-Lyxose occurs rarely in Nature and
it can be obtained only by chemical synthesis (Bílik & Caplovic,
1973; Giudici & Griffin, 1974). d-Xylose is an important compound
mainly for its conversion to d-xylitol with its wide variety of appli-
cations. It metabolizes easily and independently of insulin in the
human body and produces the same amount of energy which
highlights its application in all diabetic foods. Xylitol, with sweet-
ening power matching that of sucrose, is applicable as a sugar
substitute in the food processing industry. It can be used as an
additive in foods, beverages, pharmaceuticals and nutraceuticals
(Granström, Izumori, & Leisola, 2007). Industrially, xylitol is pro-
duced by catalytic hydrogenation or by enzymatic reduction of
d-xylose. However, industrial bioconversion of detoxified hemi-
cellulosic hydrolysate by yeast strains to xylitol is complicated
by inhibitory effects of the often detected phenolic by-products
(Wisniak, Hershkowitz, Leibowitz, & Stein, 1974; Zhang, Geng,
Yao, Lu, & Li, 2012). Compared to enzymatic production, chemical
catalysis employing Mo(VI) salts could offer attractive advantages.
Utilizing an efficient catalytic system and water as the solvent
has many advantages. Moreover, this approach is environmentally
friendly and inexpensive. Xylitol and lyxitol can be prepared by the
reduction of d-xylose or d-lyxose with sodium borohydride which
leads to these alditols in very good yields (88% and 85%).
0
50
100
150
200
250
300
reaction time [s]
Fig. 1. Conversion of beechwood xylan (ꢀ) to d-xylose (᭹) and d-lyxose (ꢁ) as a
function of time in microwave field.
Different reaction kinetics and thermodynamics under conven-
tional conditions, compared to microwave conditions, are seen
in large differences in both time course and the composition
of reaction mixture during reaction. After 10 h of heating the
ratio of xylose/lyxose was 2.7:1 and the equilibrium mixture of
xylose/lyxose 2.2:1 was reached after 20 h of heating. The com-
position of the reaction mixture in terms of xylan, d-xylose and
d-lyxose content as a function of time is depicted in Figs. 1 and 2. As
shown in Fig. 1, after 25 s more than 80% of xylan is hydrolyzed to d-
xylose monomers in microwave field. Epimerization of d-xylose to
d-lyxose starts in 30 s, simultaneously with the hydrolysis of xylan.
Results indicate that sufficient reaction time (3 min) facilitated the
epimerization of d-xylose to d-lyxose, whereas prolonged duration
might lead to the occurrence of side reactions and traces of fur-
fural were also observed. Fig. 2 shows the kinetics for the reaction
under conventional, oil-bath heating. After 2 h, about 45% of xylan is
hydrolyzed to d-xylose monomers and formation of d-lyxose starts.
The maximum value of d-lyxose is reached after 20 h.
3.2. Microwave-assisted dehydration of monosaccharides from
xylan to furfural
The results obtained by application of two heating techniques
clearly show that microwave irradiation caused the differences in
the equilibration of the reaction mixtures and thus led to faster
conversions and higher yields. Owing to differences in the mode
of energy transfer different ratios of stereoisomers as a function of
The production of furfural from xylan in microwave field was
also analyzed. Pentoses, as precursors for furfural, are formed
during the hydrolysis of hemicellulose. The hydrolysis reaction
proceeds about 50 times faster than the dehydratation reaction,
thus the latter becomes the limiting step. The use of microwaves
in the dehydration of pentoses revealed a notable effect on the
reaction rate. It is important to conclude that the yield of furfural
also depends on the substrate concentration therefore maximum
generation of pentoses during hydrolysis is necessary. The opti-
mal reaction conditions were found a 20% solution of xylan in
0.1 M hydrochloric acid. Microwave-assisted acid-catalyzed ther-
mochemical conversion was carried out by irradiation of xylan
samples for a different time periods at 300 W. The obtained reac-
tion mixture was extracted with diethyl ether and furfural was
determined by UV spectrophotometry (Martinez et al., 2000). Com-
position of the reaction mixture was analyzed also by 1H NMR
spectroscopy. The direct production of furfural from xylan via
hydrolysis followed by dehydration reaction was about 42% after
5 min. The reaction scheme of the combined xylan hydrolysis,
epimerization reaction and dehydration to furfural is illustrated in
Scheme 3. The synthesis of furfural from xylan was also carried out
using oil-bath heating (150 ◦C, 30 min). Under these conditions we
obtained maximum 28% yield of furfural. Longer reaction times and
nonselective heat transfer at the surface of reaction vessels resulted
100
Xylan
90
80
Xylose
Lyxose
70
60
50
40
30
20
10
0
0
3
6
9
12
15
18
21
24
reaction time [h]
Fig. 2. Conversion of beechwood xylan (ꢀ) to d-xylose (᭹) and d-lyxose (ꢁ) as a
function of time under conventional, oil-bath heating.

Z. Hricovíniová / Carbohydrate Polymers 98 (2013) 1416–1421
1421
in the loss of furfural due to the formation of secondary reactions
and undesired by-products that decreased its final yield.
Bílik, V. (1972). Reactions of saccharides catalyzed by molybdate ions. II., III. Chemicke
Zvesti, 26, 187–189.
Bílik, V. (1983). Epimerization of aldoses catalyzed by molybdate ions. Chemicke
Listy, 77, 496–505.
However, the furfural formation in the molybdate-catalyzed
transformation of xylan sample using microwave field was higher.
The process involves combined hydrolysis, epimerization and
dehydration reactions in a single step and thus provided higher
amounts of furfural (53%) compared to reaction without Mo(VI)
ions (42%). Xylan dehydration with conventional heating in the
presence of molybdate yielded 36% of furfural that is also higher
compared to 28% without Mo(VI) ions. The presence of molybdate
ions in acidic solution may lead to the complexation with d-xylose
released during hydrolysis. It is very likely that this interaction pro-
motes the stabilization of the acyclic form of d-xylose that is quickly
equilibrated with its furanose and pyranose forms. The less stable
pyranose ring leads via 2,5-anhydride in dehydratation pathway to
2-furaldehyde (Antal, Leesomboon, & Mok, 1991). Microwave irra-
diation as a non-conventional energy source in combination with
catalyst has a strong influence on the course of reaction. Conver-
sion to furfural is in accordance with literature data where furfural
yields rarely exceeded 50–60% when produced from hemicellulose
biomass via one step procedure. Furfural yields are limited by side
reactions such as homopolymerizarion, degradation or condensa-
tion (Yemis & Mazza, 2011). The molybdate salts thus play a double
role in the presented reaction: Mo(VI) acts as a catalyst that enables
stereospecific conversion of d-xylose to d-lyxose and at the same
time molybdate salts cause improvement of the dielectric proper-
ties of the solution, that lead to higher energy transfer to solution
from a microwave source. The present approach should provide
access to useful alternatives for the preparation of many interesting
chemical intermediates and biofuels based on sugar derivatives.
Bílik, V.,
& Caplovic, J. (1973). Reactions of saccharides catalyzed by molyb-
date ions. VII: Preparation of l-ribose, d- and l-lyxose. Chemicke Zvesti, 27,
547–550.
Binder, J. B., & Raines, R. T. (2009). Simple chemical transformation of lignocellulosic
biomass into furans for fuels and chemicals. Journal of the American Chemical
Society, 131, 1979–1985.
Chen, L. F., & Gong, C. S. (1985). Fermentation of sugarcane bagasse hemicellulose
hydrolysate to xylitol by a hydrolsate acclimatized yeast. Journal of Food Science,
50, 226–228.
Collins, P. M. (1998). Dictionary of Carbohydrates. London: Chapman and Hall.
De, S., Dutta, S., & Saha, B. (2011). Microwave assisted conversion of carbohydrates
and biopolymers to 5-hydroxymethylfurfural with aluminium chloride catalyst
in water. Green Chemistry, 13, 2859–2868.
Ebringerová, A., & Heinze, T. (2000). Xylan and xylan derivatives—biopolymers
with valuable properties, 1. Naturally occurring xylans: sructures, isola-
tion, procedures and properties. Macromolecular Rapid Communications, 21,
542–556.
Gabriel, C., Gabriel, S., Grant, E. H., Halstead, B. S. J., & Mingos, D. M. P. (1998).
Dielectric parameters relevant to microwave dielectric heating. Chemical Society
Reviews, 27, 213–223.
Giudici, T. A., & Griffin, J. J. (1974). The interconversion of monosaccharide configu-
rations: arabinose to lyxose. Carbohydrate Research, 33, 287–295.
Granström, T. B., Izumori, K., & Leisola, M. (2007). A rare sugar xylitol. Part II: biotech-
nological production and future applications of xylitol. Applied Microbiology and
Biotechnology, 74, 273–276.
Hricovíniová, Z. (2006). The effect of microwave irradiation on Mo(VI) catalyzed
transformations of reducing saccharide. Carbohydrate Research, 341, 2131–2134.
Hricovíniová, Z. (2008). Isomerization as a route to rare ketoses: the beneficial effect
of microwave irradiation on Mo(VI)-catalyzed stereospecific rearrangement.
Tetrahedron: Asymmetry, 19, 204–208.
Hricovíniová, Z. (2009). The influence of microwave irradiation on stereospecific
Mo(VI) catalyzed transformation of deoxysugars. Tetrahedron: Asymmetry, 20,
1239–1242.
Hricovíniová, Z. (2011). Rapid, one pot preparation of d-mannose and d-mannitol
from starch: the effect of microwave irradiation and MoVI catalyst. Tetrahedron:
Asymmetry, 22, 1184–1188.
Kunlan, L., Lixin, X., Jun, L., Jun, P., Guoying, C., & Zuwei, X. (2001). Salt-assisted
acid hydrolysis of starch to d-glucose under microwave irradiation. Carbohydrate
Research, 331, 9–12.
4. Conclusions
Loupy, A. (2006). Microwaves in Organic Synthesis. Verlag GmbH & Co. KGaA: Wiley-
VCH.
Mansilla, H. D., Baeza, J., Urzua, S., Maturana, G., Villasenor, J., & Duran, N. (1998). Acid
catalyzed hydrolysis of rice hull: evaluation of furfural production. Bioresource
Technology, 66, 189–193.
Martinez, A., Rodriguez, M. E., York, S. W., Preston, J. F., & Ingram, L. O. (2000).
Use of UV absorbance to monitor furans in dilute acid hydrolysates of biomass.
Biotechnology Progress, 16, 637–641.
Nishiyama, Y., Sugiyama, J., Chanzy, H., & Langan, P. (2003). Crystal structure
and hydrogen bonding system in cellulose 1(alpha) from synchrotron X-ray
and neutron fiber diffraction. Journal of the American Chemical Society, 125,
14300–14306.
In summary, microwave irradiation in combination with Mo(VI)
catalyst has been shown to be a very effective route of transfor-
mation of cheap natural lignocellulosic biopolymers. The influence
of microwave-induced Mo(VI)-catalyzed conversion of xylans to
epimeric pentoses was studied. d-Xylose and d-lyxose were
obtained in one step procedure by the combined hydrolysis and
epimerization reaction in very good yields. Moreover, this catalyst
was also efficient in the transformation of mixed source of pentoses
to furfural, where three simultaneous steps (hydrolysis, epimeriza-
tion and dehydration reaction) are combined in a one-pot reaction.
Dramatically shorter reaction times (about two orders) and higher
yields of products compared to conventional methods make the
method attractive and applicable also on a semi-preparative scale.
Experimental results reveal that Mo(VI)-catalyzed isomerization
reactions may provide an interesting alternative toward achieving
platform chemicals from abundant renewable resources.
Petrusˇ, L., Petrusˇová, M.,
& Hricovíniová, Z. (2001). Glycoscience: Epimerisa-
tion Isomerisation and Rearrangement Reactions of Carbohydrates (vol. 215)
Berlin/Heidelberg: Springer-Verlag.
Porai-Koshits, M. A., & Atovmyan, L. O. (1974). Kristallokhimiya I stereokhimiya koor-
dinatzionnykh soedinenii molibdena. Moscow: Nauka.
Prakasham, R. S., Sreenivas, R. R., & Hobbs, P. J. (2009). Current trends in biotechno-
logical production of xylitol and future prospects. Current Trends in Biotechnology
and Pharmacy, 3, 8–36.
Sashikala, M., & Ong, H. K. (2007). Synthesis and identification of furfural from rice
straw. Journal of Tropical Agriculture and Food Science, 35, 165–172.
Shin, S. J., & Cho, N. S. (2008). Conversion factors for carbohydrate analysis by hydrol-
ysis and 1H NMR spectroscopy. Cellulose, 15, 255–260.
Acknowledgements
Wisniak, J., Hershkowitz, M., Leibowitz, R., & Stein, S. (1974). Hydrogenation of xylose
to xylitol. Industrial and engineering chemistry. Product research development, 13,
75–79.
Yemis, O., & Mazza, G. (2011). Acid-catalyzed conversion of xylose xylan and
straw into furfural by microwave-assisted reaction. Bioresource Technology, 102,
7371–7378.
This research was supported by VEGA Grant 2/0087/11, SP Grant
2003SP200280203 and in part by CEGreen I, supported by the
Research & Development Operational Program funded bythe ERDF.
Zhang, J., Geng, A., Yao, C., Lu, Y., & Li, Q. (2012). Effects of lignin-derived phenolic
compounds on xylitol production and key enzyme activities by a xylose utilizing
yeast Candida athensensis SB18. Bioresource Technology, 121, 369–378.
Zhang, L., Yu, H., Wang, P., Dong, H., & Peng, X. (2013). Conversion of xylan, xylose
and lignocellulosic biomass into furfural using AlCl3 as catalyst in ionic liquid.
Bioresource Technology, 130, 110–116.
Zhou, Ch. H., Xia, X., Lin Ch, X., Tonga, D.-S., & Beltramini, J. (2011). Catalytic con-
version of lignocellulosic biomass to fine chemicals and fuels. Chemical Society
Reviews, 40, 5588–5617.
References
Antal, M. J., Leesomboon, T., & Mok, W. S. (1991). Mechanism of formation of 2-
furaldehyde from d-xylose. Carbohydrate Research, 217, 71–85.
Angyal, S. J., & Le Fur, R. (1980). The 13C-NMR spectra of alditols. Carbohydrate
Research, 84, 201–209.
Belgacem, M. N., & Gandini, A. (2008). Monomers, Polymers and Composites from
Renewable Resources. Elsevier.