The role of xylulose as an intermediate in xylose conversion to furfural: insights via experiments and kinetic modelling

Article abstract of DOI:10.1039/c5ra10855a

An experimental work has been performed to study the relevance of xylulose as an intermediate in xylose conversion to furfural in aqueous solution. The furfural formation was investigated at the temperature range from 180 to 220 °C during non-catalyzed and acid-catalyzed conversion of xylose in a stirred microwave-assisted batch reactor. The separate experiments on xylulose and furfural conversions were carried out under similar conditions. The maximum furfural yields obtained from xylose were 48 mol% and 65 mol% for the non-catalyzed and the acid-catalyzed processes, respectively. It was shown that the furfural yield is significantly lower from xylulose than from xylose. Furthermore, the effects of initial xylose concentration and the formation of xylulose were investigated in a mechanistic modeling study. A new reaction mechanism was developed taking into account the xylulose formation from xylose. Based on the experimental results and the proposed reaction model, it was concluded that xylose isomerization to xylulose with subsequent furfural formation is not a primary reaction pathway. The obtained kinetic parameters were further used for plug flow reactor simulations to evaluate furfural yields achievable by an optimized continuous operation.

Full text of DOI:10.1039/c5ra10855a

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The role of xylulose as an intermediate in xylose
conversion to furfural: insights via experiments and
kinetic modelling
Cite this: RSC Adv., 2015, 5, 66727
a
b
a
a
*
O. Ershova, J. Kanervo, S. Hellsten and H. Sixta
An experimental work has been performed to study the relevance of xylulose as an intermediate in xylose
conversion to furfural in aqueous solution. The furfural formation was investigated at the temperature range
from 180 to 220 ꢀC during non-catalyzed and acid-catalyzed conversion of xylose in a stirred microwave-
assisted batch reactor. The separate experiments on xylulose and furfural conversions were carried out
under similar conditions. The maximum furfural yields obtained from xylose were 48 mol% and 65 mol%
for the non-catalyzed and the acid-catalyzed processes, respectively. It was shown that the furfural yield
is significantly lower from xylulose than from xylose. Furthermore, the effects of initial xylose
concentration and the formation of xylulose were investigated in a mechanistic modeling study. A new
reaction mechanism was developed taking into account the xylulose formation from xylose. Based on
the experimental results and the proposed reaction model, it was concluded that xylose isomerization to
xylulose with subsequent furfural formation is not a primary reaction pathway. The obtained kinetic
parameters were further used for plug flow reactor simulations to evaluate furfural yields achievable by
an optimized continuous operation.
Received 8th June 2015
Accepted 30th July 2015
DOI: 10.1039/c5ra10855a
www.rsc.org/advances
efficient catalytic system and the obtaining of the target product
in high yields.
Introduction
Currently, the kinetics of furfural formation under different
conditions are widely studied in order to create a scientic basis
for improving the production efficiency. However, it is chal-
lenging to reconcile all the published kinetic data presented in
the literature due to the different reaction conditions and con-
icting modelling approaches used. Different reaction mecha-
nisms of the furfural formation from pentoses have been
proposed in scientic literature based on different analysis and
reaction modellings. In the majority of research papers, the
kinetics of xylose dehydration to furfural is based on a simpli-
ed reaction scheme with a direct pathway from xylose to
furfural.12–14 The current knowledge on the kinetics of side and
loss reactions has been identied as insufficient as well.15
One of the open questions in the furfural formation reaction
is whether the mechanism proceeds via a cyclic or an acyclic
form of xylose. Antal and co-workers claim that the furfural
formation proceeds from the pyranose form of xylose without a
ring opening stage.16,17 However, most researchers have adopted
a reaction mechanism hypothesis that involves the formation of
an acyclic xylose form, even though this mechanism is not able
to fully explain the formation of some side products.4,15 Antal
et al.17 have also proposed the formation of botꢀh acyclic and
cyclic forms of xylose at temperatures above 250 C.
Furfural is a platform chemical utilized in the chemical industry
for synthesizing solvents, adhesives, medicines, and plastics.1 It
is mainly obtained in acidic aqueous solution by direct dehy-
dration of pentosans derived from different biomass such as
corncob, bagasse, wheat and rice straw. Xylose is the most
utilized monosaccharide in furfural production. It is a pentose
which can be formed either by direct degradation of polymeric
xylan in lignocellulosic material or by depolymerization of the
solubilized oligomeric xylan.2 In addition, several studies
describe the furfural formation from other pentoses3–6 or
hexoses.5 Furfural can be also obtained as a by-product of
different biomass treatment processes.7–9
Currently, furfural is produced industrially with about
50 mol% yield10,11 utilizing sulfuric or hydrochloric acid as a
catalyst. The replacement of these wasteful and energy-inefficient
mineral acid catalysts by solid acid catalysts bearing different
acidic functionalities, which are able to achieve a sufficient
improving in the furfural yield, has a growing interest. However,
an understanding of the reaction pathways as well as the kinetics
of furfural formation in aqueous media in the presence or
absence of acids is crucial for the successful development of the
Irrespective whether the reaction proceeds via cyclic or acyclic
forms of xylose, the existence of some intermediate compounds
is not questionable. The formation of intermediates has been
aDepartment of Forest Products Technology, Aalto University, Finland. E-mail: herbert.
sixta@aalto.
bDepartment of Biotechnology and Chemical Technology, Aalto University, Finland
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considered as the rate-limiting stage in the reaction of furfural
The samples were prepared by heating 3 ml of the reaction
formation from xylose.15,18 Firstly, it was stated that an essential mixture in a Monowave 300 single-mode microwave reactor
intermediate is a 2,3-(a,b-)unsaturated aldehyde as formed by a (Anton Paar GmbH, Graz, Austria) using a borosilicate glass vial
b-elimination reaction from pentose sugars.10,19,20 Several other of 10 ml capacity. A magnetic stirrer at 600 rpm utilized to mix
authors explained the reaction to take place via a 1,2-enediol the solution during the reaction. Time-to-maximum tempera-
intermediate with subsequent dehydration.4,18 More recently, the ture was set to 1.5 min with 850 W maximum output power
isomerization of aldopentoses to ketose sugar (luxose17,21 or employed. The temperature inside the reaction vessel was
xylulose22–24) as an intermediate step in furfural formation has controlled by a ber optic sensor. The prepared solutions were
been studied intensively. The current developments of tested for furfural yield and monosugars conversion at the
bi-functional catalysts24–28 and other catalytic systems29 consider treatment temperatures of 220, 200 and 180 C with different
ꢀ
an implementation of such two stage process which includes the reaction times in the range of 1–240 min. The reaction vial was
xylose isomerization as the rst step followed by the second step rapidly cooled aer the treatment by compressed air inside the
– the ketose sugar dehydration to furfural. However, the role of reactor. The highest temperature and the longest reaction time
these intermediate ketose sugars (luxose or xylulose) in the furfural studied at the present work were limited to 220 ꢀC and 240 min,
formation reactions remains unclear. Only few studies have respectively, due to the technical limitations of the Monowave
focused on the determination of the intermediate xylulose 300 reactor.
concentration during the process4,27,29 and considered its formation
The liquid samples were analyzed by High Performance
and conversion for kinetic parameters evaluation.22 The formation Liquid Chromatography (HPLC) using a Dionex UltiMate 3000
of xylulose during xylose conversion in high temperature water has HPLC (Dionex, Sunnyvale, CA, USA) device equipped with
been conrmed in a study made by Aida et al.22 with imple- refractive index (RI) and ultraviolet (UV) diode array detectors
mentation of GC-MS analysis. They observed the formation of and HyperREZ XP Carbohydrate Ca+ column (Thermo Scientic,
about 20% of D-xylulose at 350–400 ꢀC aer 0.5–1 s reaction time. Waltham, MA, USA). 0.0025 mol lꢁ1 sulfuric acid solution was
In addition to conventional reactor systems, microwave used as eluent at a ow rate of 0.8 ml minꢁ1. The column
ꢀ
irradiation can be utilized as an alternative rapid and efficient temperature and the RI detector temperature were set to 70 C
heating method during furfural production. It has been shown and 55 ꢀC, respectively. The furfural concentration in the liquid
to accelerate reaction rates and to give a slightly higher furfural samples was determined by UV-detector at wavelength 280 nm,
yield, which is, nonetheless, comparable to the conventional and the residual xylose concentration was analyzed by RI
heating methods.12 The aim of this study is to devise kinetic detector. The xylulose concentration was measured by
models for the non-catalyzed and acid-catalyzed dehydration of RI-detector with a crosscheck using UV detector at 210 nm.30
xylose to furfural under microwave irradiation at the tempera- The HPLC system was calibrated within the compounds
ture range 180–220 C. A special focus is placed to address the concentrations from 10 to 150 mg lꢁ1
.
ꢀ
role of xylulose in the total reaction scheme.
The following equations have been used for the mathemat-
ical evaluation of the obtained results:
cp
Experimental
Yp ¼
ꢂ 100 ½%ꢃ;
(1)
cr
Materials
cirn ꢁ cfr
D-Xylose powder ($99%, Sigma Aldrich), D-xylulose ($98%,
syrup, HPLC grade, Sigma Aldrich), furfural (99%, Sigma
Aldrich), sulfuric acid (49–51%, HPLC grade, Sigma Aldrich)
were used in the experiments and as calibration standards as
purchased, without further purication. Formic acid (98%,
Sigma Aldrich) and acetic acid (99%, HPLC grade, Sigma
Aldrich) were used for the preparation of calibration standards
for HPLC analysis. Millipore grade water was used for the
solutions preparation.
Xr ¼
Srp ¼
ꢂ 100 ½%ꢃ;
ꢂ 100 ½%ꢃ;
(2)
(3)
cirn
cp
cirn ꢁ cfr
where Y, X, S – yield, conversion, and selectivity to product,
respectively; c – concentration in mmol lꢁ1 (the abbreviations
to be read as follows: r, p, in, f – reactant, product, initial,
nal).
Reaction kinetics were modelled by nonlinear regression
analysis by tting the simulated ideal batch reactor composi-
tion to the experimental one covering xylose, xylulose and
furfural molar concentrations. All the computations were
implemented in MATLAB 2013b (Mathworks Inc.). The ordinary
differential equations were numerically solved by ‘ode15s’
(variable order method for stiff odes) and optimization to
minimize the weighed sum of squared residuals was done by
‘fminsearch’ (Nelder-Mead algorithm). Weighing factors were
applied to compensate the weight of the experiments with very
dilute concentration scale.
Methods
Single component solutions of D-xylose (51, 196, 498 mmol lꢁ1),
D-xylulose (7 mmol lꢁ1) and furfural (85 mmol lꢁ1) were freshly
prepared before experiments. The rst set of experiments was
performed without addition of sulfuric acid as catalysts. These
experiments can be considered as auto-catalyzed reaction
system where some side products (namely carboxylic acids) or
intermediates, formed during the treatment, may have a cata-
lytic effect. The second set of experiments was performed using
0.1 mol lꢁ1 H2SO4 as a catalyst.
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The kinetic parameters for each tested reaction temperature of the treatment the furfural yield was increased up to 4.5–5 times
were expressed by the following equation:
by increasing the temperature from 180 to 200 ꢀC. A further 2–2.5-
fold increase was observed with the temperature change from
200 to 220 ꢀC.
ꢀ
ꢀ
ꢁꢁ
Â
Ã
Ea;i
R
1
1
ki ¼ Aref;i exp
ꢁ
ꢁ
minꢁ1
;
(4)
T
Tref
It was observed that at the highest temperature studied
(220 ꢀC) the furfural yield from xylose dehydration goes through
a maximum and thereaer decreases with increasing reaction
time. The maximum furfural yield (45–48%) was reached aer
the rst 35 min at 220 ꢀC, corresponding to a xylose conversion
where Tref and T [K] – reference temperature (473 K) and
experimental temperature respectively, Ea,i [kJ molꢁ1] – activa-
tion energy, Aref,i [minꢁ1] – temperature-mean-centered Arrhe-
nius pre-exponential constant.
The plug ow reactor simulations were implemented in
MATLAB 2013b. The set of boundary value equations were
numerically solved by ‘ode23’. Optimization of the temperature
and the residence time to maximize the simulated furfural yield
was carried out using ‘fminsearch’ routine.
ꢀ
of 96% (Fig. 1c). However, at the temperature 200 C the same
conversion was achieved only aer 115–125 min resulting in
similar maximum furfural yield.
These results are in close agreement with those reported by
34
¨
¨
Moller and Schroder. The decrease of furfural yield with a
further increase in the reaction time can occur due to decom-
position35–37 and polymerization with char formation.10,38
At the lower reaction temperatures (180 and 200 ꢀC), no rapid
decrease in the furfural yield aer certain reaction time was
observed during the reaction time range studied. It is, however,
possible that a yield decrease similar to that observed at 220 ꢀC
could be observed also at these temperatures during prolonged
treatment times. Nevertheless, the maximum yield clearly shis
to a longer reaction time with a decline of the treatment
temperature.
Results and discussions
Experimental results
Un-catalyzed decomposition of xylose. The furfural yield,
pH, xylose conversion and xylulose yield at various reaction
times for the un-catalyzed D-xylose conversion experiments
ꢀ
conducted at 180, 200 and 220 C are shown in Fig. 1.
In agreement with previous studies,31–33 furfural yield and
xylose conversion were observed to be strongly inuenced by the
treatment temperature. As seen in Fig. 1a, aer the rst 25–35 min
Fig. 1 The furfural yield (a), pH (b), xylose conversion (c) and xylulose yield (d) at various initial xylose concentrations, temperatures and reaction
times during un-catalyzed xylose conversion (red – 51 mmol lꢁ1 xylose, green – 196 mmol lꢁ1 xylose, blue – 498 mmol lꢁ1; square with straight
connection line – 180 ꢀC, circle with dashed connection line – 200 ꢀC, triangle with doted connection line – 220 ꢀC).
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ꢀ
The maximum selectivity to furfural formation during of the process. At the temperature of 180 C, some amount of
un-catalyzed xylose conversion was around 48–52% in the xylulose was present even aer a 4 h treatment. The extent of
studied range of reaction temperatures. These results are in xylulose formation was highest for the solution with the lowest
agreement with the previous reports,31,34 indicating either a initial xylose concentration. As can be seen from Fig. 1d, the
small or even a negligible effect of microwave irradiation on highest amount of xylulose formed was experimentally found at
furfural formation.
200 C with the initial xylose concentration 51 mmol lꢁ1. The
ꢀ
The xylose conversion and furfural yield are also dependent obtained results show that the xylulose formation is accelerated
on the initial xylose concentration as seen in Fig. 1a and c. The by increasing the reaction temperature, particularly from 180 ꢀC
dependence of xylose conversion on the initial concentration is, to 200 ꢀC. However, at 220 ꢀC the presence of furfural in a
however, less signicant at higher temperatures. During the sufficient amount increases the probability of condensation
rst 15–25 min of the experiment, both furfural yield and xylose reactions with xylulose.
conversion seem to be independent of the initial xylose
Un-catalyzed decomposition of xylulose. Several authors
concentration. However, with extended reaction times, a clear suggested xylulose to be the key intermediate in the reaction of
concentration dependence can be observed. The furfural yield furfural formation from xylose22,23 and its formation as the limiting
increased by 12% aer 100 min at 200 ꢀC and 14% aer 175 min step in the furfural formation process. In order to shed some light
at 180 ꢀC when the initial concentration of xylose was increased to the role of xylulose in the subsequent dehydration process to
from 51 to 196 mmol lꢁ1. A further increase of the initial xylose furfural, a pure solution of 7 mmol lꢁ1 xylulose in Millipore water
concentration to 498 mmol lꢁ1 did not lead to higher furfural has been tested in the Monowave300 reactor at temperatures
ꢀ
production; but, at the temperatures of 200 and 220 ꢀC, it led to 180–220 C. The results were compared to those obtained from
a reduction of 5–7% in the furfural yield in comparison to the 7 mmol lꢁ1 pure xylose solution and summarized in the Fig. 2.
experiments with a xylose concentration of 196 mmol lꢁ1
The maximum furfural yield from xylulose obtained during
(Fig. 1a). Fig. 1a also shows that at 220 ꢀC, the furfural yield the un-catalyzed process was 25%, which is almost 10 mol%
starts to decrease earlier in the solution with increasing initial lower than the furfural yield obtained from solution with
xylose concentration. This phenomenon could be well similar concentration of xylose (Fig. 2a). Xylulose was
explained by a side reaction between furfural and xylose completely transformed into products aer 10–15 min at
(or intermediate compounds),10,38 which is, probably, promoted 220 and 200 ꢀC, respectively (Fig. 2b), while no formation of
by increasing temperature. These results suggest that differ- xylose was detected. However, it is possible that its amount was
ences in reaction temperature and other conditions might below the HPLC detection limit. At the same time, the full
ꢀ
explain the contradiction between previous reports either conversion of xylose was achieved only aer 55 min at 220 C.
concluding the furfural yield to be independent on the xylose The results show that the xylulose is more reactive and converts
concentration12 or observing an almost linear decline in furfural to furfural and other products faster than xylose giving lower pH
yield when initial xylose concentration was increased.14,39 In the during rst 10 min of the reaction. However, the nal pH of the
work published by Weingarten et al.,12 the experiments were obtained solutions was very similar in both cases with equal
performed at relatively low temperatures and very short exper- initial concentrations of the xylose and xylulose. This implies
imental time, while other authors investigated the furfural that almost equal amount of acids were generated from
formation at more severe conditions including higher temper- pentoses during degradation experiments.
ature, pressure and prolonged reaction time.14,39
The formation of solid products was observed at the latest
As can be seen in Fig. 1b, the pH of the solution decreases stages of the reaction experiments. These so-called humin-like
rapidly during the rst 5 min of the experiment and levels off at substances are formed either by resinication and polymeriza-
about 3–2.5 in all of the temperatures studied. A similar tion reactions of furfural10 or by condensation reactions
decrease in pH has been earlier reported for the xylose degra- between furfural and pentose.38 The obtained results show that
dation in subcritical6,40 or high temperature41 water, and it can xylulose is not a key intermediate in the un-catalyzed furfural
be explained by the formation of acidic compounds such as formation. Xylulose rather leads to a loss reaction than to
formic, glycolic, lactic and acetic acids41 during the degradation furfural formation.
of the pentose sugars and furfural. The nal pH is observed to
Acid-catalyzed decomposition of xylose and xylulose. The
be dependent on the reagents concentrations in the solution. A furfural yield and the xylose conversion at various reaction
lower nal pH was obtained in the solution with higher initial times and temperatures during acid-catalyzed conversion of
xylose concentration, supporting the results of Oefner et al.,41 xylose and xylulose are shown in Fig. 3a–c. As in the
who observed the formation of these organic acids with a yield un-catalyzed process, the furfural yield and the xylose conver-
of about 15% of all xylose conversion products. The generated sion are strongly inuenced by the treatment temperature. The
acidic conditions may promote further furfural formation as yield of furfural shows a maximum at each experimental
well as the formation of side products.41,42
temperature, and this maximum is shied to shorter reaction
The xylulose formation was monitored by means of HPLC times with an increase in the temperature. The complete xylose
ꢀ
utilizing a pure xylulose solution as a reference. No detectable conversion aer 2 min reaction at 220 C leads to a maximum
amounts of xylulose were observed in the initial xylose solu- furfural yield of about 65% (Fig. 3b). Thꢀis result is comparable
tions. Fig. 1d shows that the highest amount of xylulose to one reported previously (62% at 250 C (ref. 17)). Similar to
(3–3.5%) was formed during the initial stage (the rst 1–3 min) the un-catalyzed process, at a lower reaction temperature, a
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During acid-catalyzed xylose conversion, the maximum
selectivity to the furfural formation was around 64% in
contrast to 50–51% obtained in un-catalyzed reaction system.
To investigate the possible furfural formation from xylulose in
the presence of the acid catalyst, a solution containing
6.6 mmol lꢁ1 of xylulose and 0.1 mol lꢁ1 H2SO4 was tested
using the same procedure as for the xylose solution. The
results are shown in Fig. 3a. For all the tested temperatures,
the maximum furfural yield produced was 58–59% and it was
obtained in less than 2 min. This maximum yield is almost
2.4 times higher than in the case of the un-catalyzed system. In
the range of the studied reaction temperatures, the complete
xylulose conversion during the acid-catalyzed reaction was
achieved already aer 1 min treatment. At the time when all
the xylulose was converted, a part of formed furfural was
already degraded. The furfural yields from xylulose in acid-
catalyzed reactions obtained in previous studies were
40–50% (ref. 4) at 96 ꢀC, 50–65% (ref. 27) at temperatures
105–135 ꢀC using 0.1 mol lꢁ1 HCl, and 70% (ref. 29) at 110–130 ꢀC
with HCl at pH 1.
Aer acid-catalyzed conversion, neither formation of xylu-
lose from xylose nor formation of xylose from xylulose was
detected by HPLC. However, it is possible that the amounts were
below the detection limit.
The results of the acid-catalyzed process suggest that two
different reaction pathways of furfural formation from
pentoses possibly coexist. In the un-catalyzed xylose conver-
sion, the intermediate xylulose seemingly leads to the side
products formation, while with the addition of sulfuric acid a
prevailing mechanism includes fast ketose dehydration to
furfural.
Furfural degradation. In order to extend the understanding
about the behavior of furfural under the conditions used in the
microwave-assisted reaction, knowledge on its degradation rate
is needed. The furfural degradation experiments were per-
formed for the un-catalyzed as well as for the acid-catalyzed
reactions using 85 mmol lꢁ1 furfural solution at the tempera-
tures of 180, 200 and 220 ꢀC. The experimental data showing the
remaining fractions of furfural at various reaction times are
presented in Fig. 4.
Fig. 4 illustrates the effect of the treatment temperature and
the usage of 0.1 mol lꢁ1 H2SO4 as catalyst on the degradation
rate of furfural. The results show a clear dependency of furfural
degradation on the temperature, similarly to the data presented
in earlier reports.31,36,37,42,43 It can be seen that the imple-
mentation of a higher temperature increases the furfural
degradation for both un-catalyzed and acid-catalyzed reactions.
In addition, the results show that furfural is decomposed more
rapidly in the presence of 0.1 mol lꢁ1 H2SO4. The highest degree
of degradation, 68%, was observed at 220 ꢀC aer 35 min in
0.1 mol lꢁ1 sulfuric acid solution. Without the acid catalyst, only
15% of furfural was converted at the same temperature and
reaction time. The formation of furfural resinication products
– humins – was observed at the late stages of both un-catalyzed
and acid-catalyzed degradation. The acetic and formic acids
were also found in the solution during non-catalyzed and acid-
catalyzed furfural degradation reactions.
Fig. 2 The furfural yield (a), conversion (b), and pH (c) at various
reaction times during un-catalyzed conversion of xylose and xylulose
(red circle – 180 ꢀC, green square – 200 ꢀC, blue triangle – 220 ꢀC;
doted connection line – experiments with xylose, straight connection
line – experiments with xylulose).
longer reaction time is required to convert the same amount of
xylose (Fig. 3c). At the same time it was observed that in the
acid-catalyzed process the maximum furfural yield depends on
the treatment temperature: it decreases from 65% to 55% when
temperature is decreased from 220 ꢀC to 180 ꢀC (Fig. 3b). A
further increase in the reaction time leads to a decrease in the
product yield due to furfural decomposition and solid products
formation. These furfural yield losses were observed in the
whole range of the studied reaction temperatures.
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Fig. 3 The furfural yield from xylulose (a), xylose (b) and xylose conversion (c) at various reaction times during acid-catalyzed conversion of
xylose and xylulose (red circle – 180 ꢀC, green square – 200 ꢀC, blue triangle – 220 ꢀC; doted connection line – experiments with xylose, straight
connection line – experiments with xylulose).
of the model to experimental data was obtained taking into
account pathways via xylulose and via another intermediate
compound leading to the formation of furfural in the
un-catalyzed and the acid-catalyzed reactions. The developed
reaction mechanism taking into account the xylulose formation
from xylose is as follows:
(5)
In the reaction model presented in eqn (5), xylose can be
converted to furfural either stepwise with xylulose as an inter-
mediate product (k1 + k2), or via a direct or pseudo-direct reac-
Fig. 4 The remaining furfural concentration at various reaction times
tion pathway (k3), which may involve the formation of some
other intermediate compound but not xylulose. The produced
during un-catalyzed and acid-catalyzed degradation (red circle – 180 ꢀC,
green square – 200 ꢀC, blue triangle – 220 ꢀC; doted connection line
furfural further forms degradation products (DP3). At the same
– un-catalyzed process, straight connection line – 0.1 mol lꢁ1 H2SO4
added as catalyst).
time, some parts of the xylose and xylulose form degradation
products, DP1 and DP2, respectively. The possible side reactions
between xylose and furfural10,44 and the isomerization of xylu-
lose back to xylose were not identiable from the present data.
Assuming a sequence of pseudo-rst order reactions,22,41 the
Reaction mechanism and its mathematical modelling
proposed reaction model can be expressed by differential
equations as follows:
Based on the obtained experimental results, several reaction
mechanisms were screened in order to nd the optimal kinetic
model explaining the xylose dehydration and furfural forma-
d½xyloseꢃ
¼ ꢁðk1 þ k3 þ k6Þ½xyloseꢃ;
(6)
(7)
(8)
tion. The model screening was performed by formulating
mechanism candidates, implementing corresponding mathe-
matical models and by carrying out nonlinear regression anal-
ysis to test the compatibility of a model to the experimental data
and to receive values for kinetic parameters. These reaction
mechanisms considered a pathway of furfural formation path-
ways from xylose via intermediate xylulose or tentatively via
some other intermediates to furfural. Using the xylose conver-
sion data only in the modeling resulted in multiple equally well-
tting models with differing mechanistic assumptions. There-
fore, the data set was appended with the independent xylulose
conversion and furfural degradation experiments that enabled
the model screening and mathematical identication of the
values for all the related rate constants. Finally, the best tting
dt
d½xyluloseꢃ
dt
¼ k1½xyloseꢃ ꢁ ðk2 þ k5Þ½xyluloseꢃ;
d½furfuralꢃ
dt
¼ k2½xyluloseꢃ þ k3½xyloseꢃ ꢁ k4½furfuralꢃ;
where k1 is the rate constant of xylose isomerization to xylulose;
k2 and k3 are the rate constants of furfural formation from
xylulose and xylose, respectively; k4, k5 and k6 are the degrada-
tion rate constants for furfural, xylulose and xylose,
respectively.
The ts of the proposed kinetic model for acid-catalyzed and
un-catalyzed reactions are shown in Fig. 5 and 6, respectively.
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The tting is relatively good for all experimental series with
This is in line with the activation energies given in Table 2.
different initial xylose concentration ranging from 50 to The ratio of the reaction rate constants k2/k3 shows that in both
500 mmol lꢁ1. The model reconciles all the xylose conversion reaction types furfural formation is more than six times faster
data as well as the independent experiments for xylulose from xylulose than from xylose. At the same time, the high ratio
conversion and furfural degradation. This indicates that the k2/k1 indicates that the xylulose formation from xylose can be
reaction behavior of furfural and monosugars at high temper- considered as the rate-limiting step in furfural production via
atures can be adequately predicted by rst order kinetic model. this reaction pathway. The values of k3/k1 show a substantial
It should be noted that the validity of rst-order models may be difference between the un-catalyzed and acid-catalyzed
attributed to the relatively dilute concentrations and non- processes. The rate of the xylose isomerization to xylulose
simultaneous presence of abundance of furfural and sugars. during un-catalyzed process is comparable to the rate of furfural
To the author's knowledge in some solutions, relevant in formation from xylose (k3/k1 is near to 1), whereas in the acid-
industry, the high xylose concentration, e.g. 1.5–2 mol lꢁ1, have catalyzed process isomerization is substantially slower in
a strong impact on the furfural yield. Future experiments tar- these two competing reactions (k3/k1 values from 44.5 to 5.3).
geting at the investigation of the inuence of the initial xylose
From these results, it can be concluded that the addition of
concentration on furfural formation encompassing a much 0.1 mol lꢁ1 H2SO4 shis the xylose conversion reaction towards
broader concentration range need to be carried out in order to furfural formation via some other intermediate than xylulose.
elucidate the effect of initial xylose concentration on furfural For the acid-catalyzed reactions, the role of xylulose formation
formation.
in the furfural production is insignicant in the range of
The obtained kinetic parameters and the activation energies studied reaction conditions.
for the each reaction step are shown in Tables 1–3. Some liter-
The ratios k5/k6 and k5/k4 for the un-catalyzed and acid-
ature data for the kinetic parameters related to similar reaction catalyzed reactions reveals that at temperatures 180–220 ꢀC
steps are shown for comparison in Tables 2 and 3.
xylulose is intend to degradation reaction more than xylose and
As seen from Table 1, the implementation of sulfuric acid as furfural. Moreover, the ratio k5/k2 indicates that in un-catalyzed
a catalyst leads to an increase of more than one order of process the degradation of xylulose is three times more prefer-
magnitude for all the reaction rates in comparison to the able than its conversion to furfural. However, when sulfuric acid
respective step in the un-catalyzed process. As an exception, the is added as a catalyst, about 70% of the reacting xylulose is
rate constants for the xylose isomerization to xylulose (k1) are of dehydrated to furfural. However, as the isomerization rate from
the same order of magnitude in both catalytic and non-catalytic xylose to xylulose is rather low in the acid catalyzed case the
systems. This observation is supported by the fact that sugar overall role of xylulose as an intermediate remains modest.
isomerization reactions are catalyzed by Lewis acids24,26 or
bases18 but not by sulfuric acid.
The calculated frequency factors and activation energies for
proposed kinetic model for un-catalyzed and acid-catalyzed
The rate constants for un-catalyzed and catalyzed reactions conversions are presented in Tables 2 and 3, respectively.
are evaluated at experimental temperatures to be able to eluci- To the authors' knowledge, the activation energies of xylose
date relative contributions of individual steps (Table 1). The isomerization to xylulose, furfural and degradation products
temperature dependence of ki is more pronounced in the acid- formation from xylulose have not been previously reported in
catalyzed processes, indicating that the reaction rates in the literature for neither un-catalyzed nor acid-catalyzed reactions.
absence of sulfuric acid are less sensitive to the reaction Aida et al.22 have presented respective rate constants, but only
ꢀ
temperature.
for the temperatures of 350 and 400 C. The high temperature
difference and implementation of high pressure by Aida et al.
Fig. 5 Experimental (circle) and modeled (solid lines) concentrations of xylose, furfural and xylulose at different temperatures in acid-catalyzed
conditions (green – xylose, red – furfural, blue – xylulose).
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Fig. 6 Experimental (circle) and modeled (solid lines) concentrations of xylose, furfural and xylulose at different temperatures and initial xylose
concentrations in un-catalyzed conditions (green – xylose, red – furfural, blue – xylulose).
makes their results incomparable with the values obtained in good agreement with the results previously published by Jing
31
¨
this work.
and Lu and Chen et al.40 The activation energy of furfural
As seen from Tables 2 and 3 the higher activation energies of degradation (53.5 kJ molꢁ1) is also in a good agreement with
steps 2 and 3 in case of the catalysed reactions in comparison to data published by Jing and Lu,31 which can be expected because
¨
the uncatalysed ones reveals that furfural formation is favored of the similarity in the experimental and procedure design.
at higher temperatures in the case of catalyzed reactions. This
fact is supported by the results in Fig. 1 and 3a and b.
As it can be seen from Table 3, the activation energies
calculated for the acid-catalyzed process in the present study are
As seen in Table 3, the activation energy of furfural forma- much higher than those published earlier.3,12,41,45 These
tion from xylose (113.2 kJ molꢁ1) obtained in our work is in a discrepancies between data could be possibly explained by
Table 1 Kinetic rate constants ki (minꢁ1) at each experimental temperature for un-catalyzed and acid-catalyzed systems and ratio of selected
rate constants
T, (ꢀC)
Acid
k1
k2
0.268
0.081
0.022
16.151
4.100
0.922
k3
k4
k5
1.016
0.270
0.064
11.214
2.800
0.619
k6
k3/k1
k2/k3
k2/k1
k5/k6
k5/k2
k5/k4
220
200
180
220
200
180
ꢁ
ꢁ
ꢁ
+
+
+
0.039
0.009
0.002
0.056
0.028
0.013
0.042
0.013
0.004
2.499
0.450
0.070
0.0059
0.0034
0.0019
0.0315
0.0150
0.0067
0.013
0.004
0.001
1.235
0.230
0.037
1.1
1.5
2.0
44.5
16.1
5.3
6.4
6.2
6.0
6.5
9.1
6.9
9.1
12.3
287.8
146.4
70.2
79.8
73.0
66.2
9.1
12.2
16.7
3.8
3.3
2.9
0.69
0.68
0.67
172.1
79.4
34.2
355.7
186.7
92.5
13.2
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Table 2 Frequency factors (Aref,i minꢁ1) and activation energies (Ea,i, kJ molꢁ1) for the kinetic model proposed in eqn (5); un-catalyzed system
i
Aref,i
Ea,i
Ea,i, literature data
1
2
3
4
5
6
0.0089
0.0810
0.0130
0.0034
0.2700
0.0037
143.2
116.1
113.2
53.5
128.5
119.8
—
—
76.6 (ref. 14), 111.5 (ref. 31), 110–127 (ref. 40), 119.4 (ref. 41), 129.9 (ref. 13)
24.2 (ref. 14), 36.5 (ref. 6), 58.8 (ref. 31), 75.5 (ref. 47)
—
58.8 (ref. 14), 101–119 (ref. 22), 143.1 (ref. 31), 168.5 (ref. 13)
Table 3 Frequency factors (Aref,i, minꢁ1) and activation energies (Ea,i, kJ molꢁ1) for the kinetic model proposed in eqn (5); acid-catalyzed system
with 0.1 mol lꢁ1 H2SO4
i
Aref,i
Ea,i
67.4
132.9
166.2
72
Ea,i, literature data
1
2
3
4
5
6
0.03
4.10
0.45
0.02
2.80
0.23
—
—
123.9 (ref. 12), 126.8 (ref. 3), 120.6–130.8 (ref. 41)
48.1 (ref. 46), 67.6 (ref. 12), 83.6 (ref. 36), 92.3 (ref. 44), 102–115 (ref. 42 and 43), 125.1 (ref. 37)
—
134.5
162.9
148 (ref. 45)
either utilization of simplied reaction mechanism3,12–14,31,41,45 from 200 ꢀC up to 280 ꢀC and residence times between one
or implementation of subcritical and supercritical reaction second up to 150 min. In the simulations, a constant feed
conditions22 in the earlier studies.
concentration of 200 mmol lꢁ1 of xylose was assumed.
Simulation results revealed that for the uncatalyzed system
the optimal process parameters are T ¼ 218 ꢀC and t ¼ 34.8 min
which resulted in 44.34% furfural yield. Fig. 7 illustrates the
furfural yield as a function of the residence time and the reac-
tion temperature. As can be seen, in addition to the reported
optimum, there is a relatively wide region of temperature and
time combinations that provide a furfural yield above 40%.
In comparison to the un-catalyzed system, the strong acid-
catalyzed system behaved differently in PFR simulations. The
Plug ow reactor simulations for xylose conversion
The determined kinetics for uncatalyzed and acid-catalyzed
system (Table 1) were utilised to simulate a ow reactor
system described by the plug ow reactor model (PFR). The
objective was to assess possibilities to maximize the furfural
yield by adjusting the reactor residence time and the tempera-
ture. A steady state and isothermal operation were assumed.
The PFR model with the determined kinetics incorporated for
each component is expressed as:
1 d½xyloseꢃ
¼ ð ꢁ k1 ꢁ k3 ꢁ k6Þ½xyloseꢃ
(9)
t
dz
1 d½xyluloseꢃ
¼ ðk1½xyloseꢃ ꢁ ðk2 þ k5Þ½xyluloseꢃÞ
(10)
t
dz
1 d½furfuralꢃ
¼ ðk2½xyluloseꢃ þ k3½xyloseꢃ ꢁ k4½furfuralꢃÞ (11)
t
dz
where a component's name in brackets corresponds to its molar
concentration (mmol lꢁ1), z is the dimensionless axial coordi-
nate of the reactor and t is the reactor residence time (min), i.e.
the reactor volume divided by the volumetric ow rate. The
temperature affects the outcome via Arrhenius dependencies of
the rate constants ki. The numerical simulations provide the
molar concentrations as a function of reactor axial coordinate.
The key quantity for simulations was chosen to be the furfural
yield (mol%), calculated from the reactor outlet composition.
The yield quantity covers both the efficient usage of the reactant
xylose and its selectivity to the targeted product. The investi-
Fig. 7 Simulated furfural yield as a function of residence time and
gated ranges in the simulations were chosen to be temperatures temperature for uncatalyzed xylose conversion in plug flow reactor.
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Table 4 Plug flow reactor simulated at selected temperatures for
strong acid catalyzed xylose conversion. The residence times have
been optimized for maximal furfural yields at the chosen temperatures
Xylulose was identied during the xylose conversion in
varying yields depending on the reaction conditions, especially
on the temperature. In the light of the results obtained, the role
of xylulose as a key intermediate for furfural production from
xylose was rejected under the tested reaction conditions. The
experimental data obtained for the un-catalyzed xylose conver-
sion as such ruled out the possibility of xylulose being the key
intermediate. Modeling helped to establish the role of xylulose
along a parallel reaction pathway. The primary reaction route
involves another short-living intermediate that reacts rapidly to
the furfural.
T, ꢀ
C
t, min
Y(furfural) mol%
200
220
230
240
250
260
270
280
5.3
1.3
60.4
64.0
65.1
66.1
66.7
67.4
67.9
68.3
0.62 ¼ 37 s
0.31 ¼ 18.6 s
0.16 ¼ 9.6 s
0.084 ¼ 5.0 s
0.045 ¼ 2.7 s
0.025 ¼ 1.5 s
The results of the batch reactor experiments in conjunction
with the plug-ow reactor simulations suggest that the
maximum obtained furfural yield for the un-catalyzed system
remains below 50% and requires approximately 35 minutes at
highest furfural yield was calculated for the maximum allowed
temperature and with extremely short residence time. Table 4
displays the maximum furfural yields at selected temperatures
and the required residence times for each temperature for
attaining the optimal yield. The simulation results suggest that
operating a PFR for 65% furfural yield requires the residence
time slightly below 40 seconds at the temperature of 230 ꢀC.
Further increase in temperature and decrease in residence time
can promote the furfural yield further up to 68%, provided that
the extrapolation from 220 ꢀC and 280 ꢀC is valid. The dynamics
of the strong acid catalyzed xylose conversion in a ow system
would call for small-diameter-devices such as millireactors or,
alternatively, an efficiently heat-controlled section in a tubular
reactor followed by an efficient effluent cooling.
ꢀ
ꢀ
220 C or 90 minutes at 200 C. The presence of a strong acid
accelerates most of the reaction steps and increases the
maximum furfural yield. In the batch reactor experiments the
maximum furfural yield of 62–65 mol% was obtained corre-
ꢀ
sponding to 2–3 min reaction time at 220 C in the acid cata-
lyzed case. The plug ow reactor simulations for the acid
catalyzed conversion suggested furfural yields up to 68 mol%
with further increase in the reaction temperature along with an
appropriate reduction in the residence time in the continuous
operation.
Acknowledgements
The authors would like to acknowledge nancial support from
The International Doctoral Programme in Bioproducts Tech-
nology (PaPSaT) and from the Academy of Finland. We also
sincerely thank Rita Hatakka for her contribution in the HPLC
method development.
The plug ow reactor simulations underline how the un-
catalyzed and strong acid catalyzed kinetics dene the theo-
retical bounds for harnessing the intrinsic kinetics for furfural
production, and they simply illustrate the interplay between the
temperature and the residence time in ow systems for opti-
mizing the furfural yield. If a yield of 44 mol% is considered
acceptable, furfural production could be implemented without
catalysts. On the other hand, 68 mol% cannot likely be exceeded
without either inventive furfural recovery strategies or the
inhibition of the loss reactions related to furfural and xylose.
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