Furfural synthesis from D-xylose in the presence of sodium chloride: Microwave versus conventional heating

Article abstract of DOI:10.1002/cssc.201600446

We investigate the existence of specific/nonthermal microwave effects for the dehydration reaction of xylose to furfural in the presence of NaCl. Such effects are reported for sugars dehydration reactions in several literature reports. To this end, we adopted three approaches that compare microwave-assisted experiments with a) conventional heating experiments from the literature; b) simulated conventional heating experiments using microwave-irradiated silicon carbide (SiC) vials; and at c) different power levels but the same temperature by using forced cooling. No significant differences in the reaction kinetics are observed using any of these methods. However, microwave heating still proves advantageous as it requires 30% less forward power compared to conventional heating (SiC vial) to achieve the same furfural yield at a laboratory scale.

Full text of DOI:10.1002/cssc.201600446

DOI: 10.1002/cssc.201600446
Full Papers
Furfural Synthesis from d-Xylose in the Presence of
Sodium Chloride: Microwave versus Conventional Heating
+
[a, b]
+ [b, c]
[b]
[a]
Christos Xiouras ,
Norbert Radacsi ,
Guido Sturm, and Georgios D. Stefanidis*
We investigate the existence of specific/nonthermal microwave
effects for the dehydration reaction of xylose to furfural in the
presence of NaCl. Such effects are reported for sugars dehydra-
tion reactions in several literature reports. To this end, we
adopted three approaches that compare microwave-assisted
experiments with a) conventional heating experiments from
the literature; b) simulated conventional heating experiments
using microwave-irradiated silicon carbide (SiC) vials; and at
c) different power levels but the same temperature by using
forced cooling. No significant differences in the reaction kinet-
ics are observed using any of these methods. However, micro-
wave heating still proves advantageous as it requires 30% less
forward power compared to conventional heating (SiC vial) to
achieve the same furfural yield at a laboratory scale.
Introduction
Furfural is a valuable platform chemical derived from renewa-
ble lignocellulosic biomass and agricultural surpluses. It has
several uses, such as an extraction solvent for aromatic com-
pounds or as a precursor for synthesizing specialty chemicals
give rise to degradation products. New processes for furfural
production can potentially circumvent the yield limitations,
lower the energy requirements and minimize the large waste
streams associated with the conventional process. In this way,
the full potential of furfural as a biomass-derived intermediate
could be exploited as a replacement for oil derivatives. Such
intensified processes may be based on chemical activation
through alternative energy forms (e.g., microwave heating). In
many organic syntheses, microwave heating leads to reduced
reaction times and higher reaction efficiency in aqueous sys-
tems compared to conventional heating. As a result, toxic or-
ganic solvents and catalysts may be replaced with more
[
1]
and liquid fuels. As one of a few non-petroleum-derived
chemicals, it can play a vital role in the transition from fossil
[2]
fuel resources to a more sustainable bio-based industry.
Furfural synthesis usually involves the acid hydrolysis of the
pentosan fraction of biomass into pentoses (C-5 sugars), such
as xylose or arabinose, and the subsequent dehydration of the
[
3]
pentoses to furfural. The two reactions can take place in the
same vessel under similar conditions, with the xylose dehydra-
[
4]
[5,6]
tion to furfural as the rate limiting step. Currently, furfural is
produced in industry by an energy intensive process using su-
perheated steam to heat the reaction and mineral acids such
benign aqueous systems.
[7]
Marcotullio and De Jong have shown that the presence of
ꢀ
Cl ions in aqueous acidic solutions could significantly enhance
the dehydration reaction rate of d-xylose under conventional
heating and improve the selectivity and furfural yield. Another
study investigated the effect of microwave heating on furfural
[
2,4]
as HCl and H SO as reaction catalysts. Organic acids such as
2
4
acetic acid can also be used and would be more desirable
from an environmental standpoint, but they usually lead to
lower furfural yield. Furfural product yields are generally limit-
ed to 45–55% owing to the occurrence of side reactions that
[8]
yield using d-xylose in aqueous HCl solutions. Microwave
heating showed no effect on the reaction kinetics of xylose de-
hydration. However, several studies of the dehydration of C-5
and C-6 sugars to furfural and 5-hydroxymethylfurfural
+
[
a] C. Xiouras, Prof. Dr. G. D. Stefanidis
Process Engineering for Sustainable Systems (ProcESS)
Department of Chemical Engineering
KU Leuven
(
5-HMF), respectively, claim significant enhancement of the re-
action rate under the presence of ions and microwave heating
[9–17]
compared to conventional (e.g., oil-bath) heating.
Synergis-
Celestijnenlaan 200F, 3001 Leuven (Belgium)
E-mail: georgios.stefanidis@cit.kuleuven.be
tic effects of ions and microwave heating, leading to higher
[16]
furfural yield, have also been reported. A discussion of sever-
al studies comparing microwave and conventional heating for
the synthesis of furfural and 5-HMF from C-5 and C-6 sugars,
respectively, is presented in the Appendix. The presence of
ions in aqueous solutions is known to enhance electromagnet-
ic energy dissipation and consequently, increase the dielectric
heating rate. The rapid temperature increase could explain
some of the observed yield enhancements. However, ions par-
ticipate in the dehydration chemistry as well; therefore, non-
thermal microwave effects (i.e., increase in the number of ef-
+
+
[
b] C. Xiouras, Dr. N. Radacsi, Dr. G. Sturm
Process & Energy Department
Delft University of Technology
Leeghwaterstraat 39, 2628 CB Delft (the Netherlands)
+
[
c] Dr. N. Radacsi
Chemical Engineering
California Institute of Technology
200 E. California Blvd, Pasadena, CA, 91125 (USA)
1
+
[
] These authors contributed equally to this work.
Supporting Information for this article can be found under:
http://dx.doi.org/10.1002/cssc.201600446.
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fective collisions or lowering of activation energy owing to the
direct interaction of the electromagnetic field with the polar
species) have also been put forward to explain the kinetic rate
[
9]
enhancement of these reactions. Nevertheless, it is known
that the existence of such effects is largely speculative.
In this study, the application of microwave-assisted heating
combined with the use of NaCl salt for the dehydration of
xylose in dilute aqueous acidic solutions was investigated and
the results were compared to conventionally heated experi-
ments reported in the literature. In addition, the existence of
nonthermal microwave effects was investigated in an experi-
mental microwave setup that allows the simulation of conven-
tional heating while maintaining all the other process parame-
ters the same (bulk and wall temperature, heating rate, reactor
geometry, and stirring rate). Finally, the rate constants of the
pseudo-first order reactions of xylose conversion to furfural
and xylose and furfural conversion to byproducts were deter-
mined under the two heating modes.
Figure 1. Evolution of xylose and furfural concentration over the course of
the reaction at different NaCl concentrations (2, 3.5, and 5 wt%). The dashed
and solid lines are the kinetic model estimations of the xylose and furfural
concentrations, respectively (black: 2 wt%, red: 3.5 wt%, blue: 5 wt% NaCl
concentrations), based on the experimental results of the present study (tri-
angle, circle, and square marks). Conditions: 35 mm xylose, 50 mm HCl, reac-
tion temperature of 2008C and stirring speed of 600 rpm.
Results and Discussion
Indirect comparison of microwave and conventional heating
using varying amounts of NaCl
Microwave heating in combination with NaCl was investigated
for the dehydration reaction of xylose to furfural. Three differ-
ent NaCl concentrations (2, 3.5 and 5 wt%, or 342, 599 and
tions (towards furfural and byproducts) are enhanced. The rate
constant k3 seems to remain roughly unchanged when the
NaCl concentration is increased from 2 to 3.5 wt%. However,
856 mm, respectively), close to those found in seawater, were
studied in dilute aqueous HCl solutions (50 mm HCl concentra-
tion, initial xylose concentration: 35 mm) at 2008C. The condi-
tions of these experiments are similar to a previous study,
when 5 wt% NaCl was used, k almost doubled, similar to the
3
comparison study, conducted under conventional heating. This
suggests that the addition of NaCl might eventually enhance
furfural degradation reactions, but only after a certain thresh-
old NaCl concentration is exceeded. However, the rates of the
furfural degradation reactions do not seem to have a straight-
[7]
which used a conventionally heated autoclave reactor. This
way, an indirect comparison between the two modes of heat-
ing can be made. The results of these experiments in terms of
the time evolution of the xylose and furfural concentrations
are presented in Figure 1.
ꢀ
forward relation with the Cl concentration, according to the
Based on the experimental results, it appears that NaCl has
a significant catalytic effect on the reaction under microwave
heating leading to complete xylose conversion within 1000 s
in all cases. The maximum experimental furfural yields
were 73, 76, and 72%, for NaCl concentrations of 2, 3.5
and 5 wt%, respectively. Even though increasing the
NaCl concentration within the evaluated range does
not seem to significantly affect the furfural yield and se-
lectivity, the reaction rate is increased considerably.
Conversely, prolonged residence times, particularly at
higher NaCl concentrations, seem to slightly decrease
the furfural concentration. This can possibly be ex-
plained by the higher concentration of furfural or
xylose-to-furfural intermediates that may undergo (en-
hanced) side reactions under these conditions.
proposed kinetics.
The estimated rate constants of this work (Entries 1–3) are
[
7]
compared to the ones derived by Marcotullio and De Jong
Table 1. Derived rate constants (Entries 1–3), by least square regression with
[
7]
their standard errors. Entries 4–6 are retrieved from the literature for compari-
son. The concentrations and reaction temperature are the same as in the com-
[
7]
parison study: 35 mm xylose, 50 mm HCl, reaction temperature of 2008C.
[
a]
ꢀ4
ꢀ4
ꢀ4
ꢀ4
[b]
[c]
Entry NaCl
k
x
ꢁ10
]
k
[s
1
ꢀ1
ꢁ10
]
k
[s
2
ꢀ1
ꢁ10
]
k
[s ]
3
ꢀ1
ꢁ10
Select.
[%]
Yield
[%]
ꢀ
1
[
wt%] [s
1
2
3
4
5
6
2.0
3.5
5.0
2.0
3.5
5.0
52.3ꢁ4.1
93.3ꢁ8.4
43.2ꢁ1.8 9.1ꢁ2.3 1.53ꢁ0.9 82.6ꢁ7.3 74.2ꢁ8.2
76.9ꢁ4.1 16.4ꢁ4.3 1.50ꢁ0.9 82.4ꢁ8.6 77.2ꢁ8.6
112.8ꢁ10.8 93.0ꢁ5.4 19.8ꢁ5.4 2.85ꢁ0.9 82.4ꢁ9.2 74.9ꢁ8.7
64.5
74.4
119.1
52.8
63.1
107.4
11.7
11.3
11.6
2.2
2.2
3.4
81.9
84.8
90.2
72.8
76.1
81.3
The first order rate constants k , k , and k were de-
1
2
3
rived based on the experimental data and are present-
ed in Table 1. The estimated rate constants (Entries 1–3,
Table 1) reveal that increasing salt concentration, at
a constant acidity, leads to an almost proportional in-
crease in both k and k , implying that both xylose reac-
[
a] k
x
=k
1
+k
2
. [b] Selectivity calculated as k
1
/k
x
. [c] Maximum theoretical yield
ꢂ
ꢃ
ꢀ
ꢁ
k3
k1
k1 þk2
k1þk2 k3 ꢀk1 ꢀk2
calculated as:
mation.
, 95% confidence interval in parameter esti-
k3
1
2
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under conventional heating (Entries 4–6). The derived values of
periments were 50 mm initial xylose concentration, 50 mm HCl
concentration, 500 mm NaCl concentration and 600 rpm stir-
ring speed. A temperature of 1708C was selected to study the
reaction at longer residence times (5–60 min). Therefore, the
preheating and cooling times for both vials are negligible com-
pared to the residence time at the reaction temperature.
k , k , k are similar in the two studies. The small differences
1
2
3
may be attributed to differences between the conventional
and microwave setups, causing small variations in the resi-
dence time in the experiments, as well as the different temper-
ature measurement systems. However, the estimated values
are close enough to suggest that microwave heating does not
show any significant effect on the dehydration reaction of
xylose to furfural under dilute acidic conditions compared with
conventional heating. This finding contradicts previous studies
on the dehydration of C-5 and C-6 sugars, which claimed sig-
nificant enhancement of the yield of furfural or 5-HMF in aque-
ous systems by using acid catalysis and/or salts in combination
As expected, the temperature and pressure profiles recorded
throughout the experiments using the glass and SiC vials are
very similar, as shown in Figure 2. This enables a reliable com-
parison of microwave and conventional heating by keeping
the rest of the process parameters similar. However, in the SiC
vial, the temperature fluctuated in a cyclic manner. The vial
wall of the SIC vial is heated directly, which leads to faster dy-
namics. Specifically, the temperature rises and drops faster as
the microwave power increases and decreases, respectively.
This causes the temperature controller to continuously over-
and undershoot its reference, leading to an oscillatory temper-
ature trend. Re-tuning the controller may prevent this from
happening, although the device does not allow for this. De-
pending on the activation energies of the reactions involved,
even short times at higher temperatures might have an influ-
ence on the observed reaction kinetics. To make sure that this
is not the case for the SiC vial, we calculated the mean kinetic
temperature (MKT), based on the residence time at the high
and low temperatures. For the calculation, we used the typical
[
10,11,13,16]
with microwave heating.
Direct comparison of microwave and conventional heating
The results presented in the previous section do not seem to
indicate that a strong microwave effect is present for the dehy-
dration reaction of xylose to furfural. However, the comparison
was based on experiments performed in different setups. It
should be remarked that accurate comparison between micro-
wave and conventional heating often proves challenging, not
only because of inherent difficulties in determining the actual
temperature in microwave heated transformations, but also
due to difficulties in reproducing heating rates attained with
ꢀ1
range of pseudo-activation energies (70–140 kJmol ), which is
reported for the dehydration reaction of xylose to furfural in
[
18]
microwaves in a conventional heating setup.
To address
[2]
these potential limitations, we performed two series of experi-
ments using a different microwave reactor setup (Anton Paar
Monowave 300) that allows for variation of the heating mode
while keeping all the other process parameters similar. This is
achieved by using a silicon carbide (SiC) vial to mimic conven-
tional heating and a glass vial with the same geometry to
study microwave heating. The SiC vial “shields” the reaction
mixture from the microwave field but still enables high heating
rates to be achieved because of the electromagnetic dissipa-
tion in the SiC vial wall itself (further discussion about the SiC
vials is provided in the Supporting Information). In this case,
instead of volumetric heat generation owing to microwave dis-
sipation in the reactant mixture, the heating of the reaction
mixture occurs conventionally, that is, due to heat transfer
dilute aqueous acidic solutions. The calculated MKTs (169.83–
169.858C) are in close agreement to the average reaction tem-
perature of 1708C, thus the small cyclic temperature behavior
in the SiC vial can be safely neglected.
Figure 3 presents the xylose conversion and furfural yield
measured for the glass vial (microwave heating) and SiC vial
(conventional heating) experiments and Table 2 shows the esti-
mated rate constants for the same experiments. By comparing
the values, it appears that microwave heating has no influence
on the reaction kinetics of the dehydration of xylose to furfu-
ral. This finding contradicts previous publications that have
compared microwave and conventional heating for xylose de-
hydration under acidic conditions with or without the presence
[16,17]
[17]
of salts.
For example, Yang et al.
reported a 16% in-
[
18]
from the wall to the mixture. The conditions for these ex-
crease in furfural selectivity when applying microwave heating
Figure 2. Left: Temperature and pressure profile of a typical glass vial experiment. Right: Temperature and pressure profile of a typical SiC vial experiment.
Conditions: 50 mm xylose, 50 mm HCl, 500 mm NaCl, stirring speed of 600 rpm. The power control system was the same for both vials and was set to reach
the target temperature of 1708C as fast as possible, preventing overshooting of the temperature.
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modes, the results are almost identical, indicating
that xylose conversion and furfural yield are only de-
pendent on the temperature profile attained either
by conventional or microwave heating.
Increasing microwave power under constant tem-
perature
To probe further into the existence of potential spe-
cific/nonthermal microwave effects on the dehydra-
tion reaction of xylose to furfural, the microwave
power was varied, this time using only the glass vial
Figure 3. Analytical results (mean values with standard error bars) of the microwave
glass vial) and conventional (SiC vial) heating experiments at 1708C (xylose: 50 mm,
HCl: 50 mm, NaCl: 500 mm, stirring at 600 rpm) at different residence times. Left: Xylose
conversion versus residence time for glass and SiC vials. Right: Furfural yield versus resi-
dence time for glass and SiC vials. The almost identical results indicate that the kinetics
of xylose dehydration are unaffected by the heating mode.
(
(
microwave heating). If the temperature profile re-
mains unchanged as the applied microwave power is
varied, any measured yield/selectivity differences can
be attributed to specific/nonthermal microwave ef-
fects. To increase the microwave power at constant
reaction temperature, the so-called enhanced micro-
Table 2. Derived rate constants for conventional and microwave heating.
[20]
wave synthesis (EMS) technique was used. It is based on the
simultaneous application of microwaves to the reaction vessel
and forced external cooling using compressed air for the entire
duration of the experiment. This type of experiment allows for
the examination of the effect of microwave power alone at
constant temperature, and consequently, for implicit verifica-
tion of nonthermal microwave effects. According to the instru-
ment readings of the forward-emitted microwave power, the
high power (forced cooling) mode required approximately 5
times more power to maintain the temperature at 1708C com-
pared to the low power mode (no cooling). The preheating
times are the same in both experiments. Even though the ap-
plied power was significantly higher when forced cooling was
applied (10 W versus 50 W), xylose conversion and furfural
yield were not affected, as shown in Figure 4. Slightly different
Conditions: 50 mm xylose, 50 mm HCl, 500 mm NaCl, stirring speed of
00 rpm and a raction temperature of 1708C. The derived values are very
similar for the conventional and microwave heating.
6
ꢀ4
ꢀ4
ꢀ4
ꢀ4
[a]
[b]
Vial
k
x
ꢀ
ꢁ10
]
k
[s
1
ꢀ1
ꢁ10
]
k
[s
2
ꢀ1
ꢁ10
]
k
[s ]
3
ꢀ1
ꢁ10
Select.
[%]
Yield
[%]
1
[
s
Glass 8.01ꢁ0.53 6.37ꢁ0.23 1.64ꢁ0.30 0.60ꢁ0.23 79.5ꢁ5.9 64.5ꢁ7.5
SiC
8.05ꢁ0.38 6.22ꢁ0.17 1.83ꢁ0.21 0.61ꢁ0.19 77.2ꢁ4.2 62.5ꢁ5.6
[
a] Selectivity calculated as k /k . [b] Maximum theoretical yield calculated
1
x
ꢂ
ꢃ
ꢀ
ꢁ
k3
k1
k1 þk2
k1 þk2 k3 ꢀk1 ꢀk2
as:
, 95% confidence interval in parameter estimation.
k3
instead of conventional heating under the same process condi-
tions (1408C, 45 min, AlCl and NaCl in a water/tetrahydrofuran
[21]
3
results have been reported by Serrano-Ruiz et al., where the
(
THF) system). However, the exact temperature profile of the
authors observed a small increase in xylose conversion and fur-
fural yield (10% and 7% respectively) by increasing the applied
microwave power from 100 W to 300 W at 1508C in 30 min.
However, the authors do not mention whether the same heat-
ing rate was maintained during these experiments. Because
microwave-heated transformation could not be accurately re-
produced using an oil-bath; in addition, 18 more minutes were
required to bring the temperature to 1408C using convention-
al heating. Therefore, any observed selectivity improvement
could be a result of the increased heating rate in the micro-
[17]
wave heated reaction, as the authors correctly suggest. In
[
10]
another study, the authors report that the application of mi-
crowaves to fructose dehydration in an aqueous HCl system re-
sults in a 16% increase in the 5-HMF yield in comparison to
conventional heating. However, the authors also report signifi-
cant dissimilarities between the heating rates in the microwave
and conventional heating setup. Such discrepancies in the
heating rate may be even more pronounced if large concentra-
tions of salts or ionic liquids are used (see further discussion in
the Appendix). Ionic liquids couple very efficiently with micro-
ꢀ
1
waves, leading to heating rates that easily exceed 108Cs ,
which are difficult to reproduce using conventional heating
methods. If the heating rates between various experimental
setups cannot be kept similar, the use of a non-isothermal ki-
netic analysis might be more suitable, for example, by estimat-
Figure 4. Xylose conversion and furfural yield without cooling (10 W) and
with cooling (50 W). The results show that the applied microwave power
has a negligible effect on the xylose dehydration and furfural yield. Condi-
tions: 50 mm xylose, 50 mm HCl, 500 mm NaCl, stirring speed of 600 rpm, re-
action temperature of 1708C, and a reaction time of 15 min.
[
19]
ing an equivalent isothermal temperature. In this study, we
show that at similar heating rates between the two heating
&
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ionic liquids were used in their work, the heating rate would
probably be more readily affected by changes in the applied
microwave power. If the reaction temperature in their high
power experiment was attained faster, this would explain the
higher yield/conversion. Another study of HCl-catalyzed fruc-
tose dehydration to 5-HMF under microwave irradiation
showed that the 5-HMF yield and fructose conversion were in-
[10]
dependent of the applied microwave power (100–300 W). In
the present study, it is shown that furfural yield and xylose
conversion are independent of the applied microwave power,
provided that enough microwave power is supplied to sustain
the reaction temperature.
Figure 5. Cumulative forward microwave power supplied in glass and SiC re-
actor vials. The use of the SiC vial (simulated conventional heating) results in
significantly higher power input compared to the glass vial (microwave heat-
ing) for the xylose dehydration reaction.
Comparison of energy consumption
We have shown that the two heating modes result in the
same xylose conversion and furfural yield if the rest of the pro-
cess conditions are kept the same. However, a critical evalua-
tion of the process also requires an assessment of the energy
consumption of the two heating modes. However, such precise
assessment is difficult as it has been shown that the total heat
generation in microwave cavities employing resonant micro-
wave fields is very sensitive to even slight variations in the
geometric aspects of the load and the process conditions and
can also deviate significantly from the power readings provid-
the total power supplied during the entire course of the reac-
tion was 30% higher. The reflected microwave power is un-
known and can be different in the two modes of heating.
However, the difference in the supplied power can be ex-
plained by the differences in the properties of the materials.
The microwave-transparent borosilicate glass vial, which is
used in the case of volumetric microwave heating, has a signifi-
cantly lower density than the highly conductive SiC vial
[
22]
ꢀ1
ꢀ1
ed by these instruments. In addition, the microwave power
generated in such cavities is not exclusively dissipated in the
load, but also in other locations of the microwave circuit (e.g.,
the magnetron) leading to significant energy losses. Detailed
energy balances in small scale microwave reactors, also ac-
counting for energy losses, have shown that the power ab-
(2.23 gmL versus 3.1 gmL , respectively) that is used to sim-
ulate conductive heating at similar heating rates as in micro-
wave heating. Consequently, more energy is required to reach
and maintain the same reaction temperature during conduc-
tive heating owing to the higher mass of the SiC vial. The ther-
mophysical properties of SiC and borosilicate glass are present-
ed in Table S1.
[
23]
sorbed by the load can be as low as 44%. An accurate esti-
mation of the total heat generation in the load requires tailor-
made microwave applicators, such as the ones used in
Refs. [24,25], in which the electromagnetic field is well defined
and the reflected power can be accurately estimated (and
minimized), for instance, by including power transfer sensors
in different parts of the system. Commercial microwave heat-
ing devices for laboratory applications do not provide such ca-
pability.
Conclusions
In this work, the dehydration of xylose to furfural using micro-
wave heating in the presence of NaCl at concentrations close
to that of seawater concentrations was studied and compared
with conventional heating. The highest furfural yield of 76%
was obtained using 3.5 wt% NaCl at 2008C, which led to com-
plete xylose conversion in 440 s. The existence of nonthermal
microwave effects was investigated in three ways: a) compari-
son of microwave heating experiments with literature results
from conventional heating experiments; b) direct comparison
of microwave and conventional heating in the same mono-
mode cavity using strongly microwave absorbing SiC vials to
simulate conventional heating; and c) comparison between ex-
periments at different microwave power levels at the same re-
action mixture temperature. The results indicate that micro-
wave heating does not have a prominent effect on the dehy-
dration reaction kinetics under the examined conditions. How-
ever, microwave heating eliminates the need for long preheat-
ing times and can thus reduce the uncertainty of kinetic
measurements in cases of slow conductive heating during the
initial stage of the experiments. Furthermore, at a laboratory
scale, the forward-emitted microwave power was 30% higher
in the case of the denser SiC vials owing to heating of a larger
The cumulative forward-emitted microwave power supplied
to a typical glass vial (microwave heating) and SiC vial (simulat-
ed conventional heating) during the preheating and the reac-
tion time was recorded, as presented in Figure 5. At the initial
stages of the experiment, high power is supplied to quickly
reach the reaction temperature of 1708C. In turn, the power is
reduced to hold the temperature constant, balancing the ther-
mal losses to the surroundings and the reaction heat. The
same power control system was used to regulate the tempera-
ture using both vials and it is based on a proportional-integral-
derivative (PID) controller built-in to the device. This system is
designed to reach the reaction temperature as fast as possible,
but also to prevent overshooting the temperature, and it leads
to the same heating rate in both vials under the conditions ex-
amined here. However, in the case of the SiC vial, the power
required to reach the set reaction temperature (preheating
period) was 43% higher compared to the glass vial, whereas
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thermal mass compared to the glass vials. However, the overall
energy consumption might not always be lower in the case of
microwave heating processes. The overall energy efficiency de-
pends mainly on two factors: a) the magnetron efficiency and
b) the reflected (undissipated) power. Magnetron efficiency
can be maximized by operating at alternative operating fre-
quencies (up to 85% electric-to-microwave power efficiency at
the sugars to undesired byproducts). In the case of combined
hydrolysis of xylan or biomass and subsequent dehydration of
the obtained pentoses, the latter is considered to be the rate-
limiting step. The resulting yield of 5-HMF and furfural under
microwave and conventional heating for these studies, as well
as the temperatures and catalysts used, are summarized in
Table 3 and Table 4. Because not all studies include the calcula-
tion of the kinetic parameters, these are not included here
either. Instead, the process times to achieve the respective fur-
fural yield are presented. The studies mentioned herein are
only limited to cases in which homogeneous catalysis is em-
ployed, although there is a number of studies that compare
microwave and conventional heating in the presence of heter-
915 MHz). The reflected power can be minimized through ap-
propriate design and operation of the overall microwave appli-
cator including the reactor itself. Furthermore, if a different sol-
vent that couples more effectively with microwaves (e.g., ionic
liquids or higher salt concentration aqueous solutions) is se-
lected, the heating rate of the solution can be significantly
higher and difficult to attain by conventional heating, possibly
resulting in enhanced product selectivity. Scale-up of the mi-
crowave process is also possible using either novel, non-cavity
based, reactor designs, such as internal transmission line tech-
[27–31]
ogeneous catalysts as well.
The effect of microwave heat-
ing in such heterogeneous systems is beyond the scope of this
work. For the sake of brevity, studies that compare microwave
and conventional heating for the synthesis of other furan com-
[
23]
[32]
nology (INTLI, up to 500 L of batch processing), or continu-
ous flow microwave reactors at high temperature and pressure
pounds, such as 5-chloromethyl furfural (5-CMF),
not included.
are also
[
26]
conditions, as those commonly applied in laboratory batch
Comparison between the data in Table 3 and Table 4 is not
straightforward owing to the diverse systems and conditions
used by the various research groups. However, it is clear that
contradicting results exist regarding the role of microwave
heating in these studies. For instance, one study (Table 4,
Entry 1) reported no significant difference in furfural yield
using microwave heating and conventional heating with HCl
microwave equipment (up to 3008C and 60 bar).
Appendix
Discussion of studies on microwave-assisted dehydration of
C-5 and C-6 sugars
[8]
as a catalyst. Conversely, two other studies claimed a 16% in-
crease in the yield of 5-HMF and a 14% increase in the yield of
furfural from fructose and xylan, respectively, using HCl and
A number of studies have been published comparing conven-
tional and microwave heating for the synthesis of furfural and
[10,16]
5-HMF by dehydration of C-5 and C-6 sugars, respectively.
microwave heating (Table 3, Entry 3 and Table 4, Entry 3).
Water is employed as a solvent in most of these studies, al-
though ionic liquids have also been tested in some cases. Bi-
phasic systems have also been used; in these systems, an or-
ganic water-immiscible phase [e.g., tetrahydrofuran (THF) or
methylisobutylketone (MIBK)] is employed to extract the pro-
duced furfural or 5-HMF immediately, preventing its further de-
composition to undesirable products and improving the yield.
The catalysts employed in most studies are chloride salts in
combination with mineral acids and/or ionic liquids. Most ki-
netic studies assume a reaction mechanism that includes two
first order reactions for the disappearance of the sugars (the
reaction of the sugars to furfural or 5-HMF and the reaction of
Even more striking differences between microwave and con-
ventional heating yields are reported if chloride salts or ionic
[13]
liquids are added, with a study reporting a striking 42% in-
crease in the yield of 5-HMF from dehydration of fructose
using AlCl in water (Table 3, Entry 7).
3
To rationalize such yield improvements, several authors have
proposed the existence of nonthermal microwave effects, such
as lowering of the activation energy or an increase of the pre-
exponential factor in the Arrhenius law owing to possible ori-
entation effects of polar species in response to the electromag-
[9,12,33]
netic field.
The existence of such effects has not been
proven yet and is highly debated in the literature.
Table 3. Studies comparing microwave (micr.) and conventional (conv.) heating for the dehydration of hexoses to 5-HMF in terms of 5-HMF yield and
sugar conversion.
Entry
Feed
amt. [mm])
Solvent
Catalyst
(amt. [mm])
T
[ C]
t
Yield [%]
micr.
Convers. [%]
Ref.
o
(
[min]
conv.
micr.
conv.
1
2
3
4
5
6
7
glucose (500)
glucose (500)
fructose (1766)
glucose (278)
fructose (500)
glucose (730)
fructose (278)
[BMIM]Cl
[BMIM]Cl
Water
Water/MIKB 1:1 (v/v)
[BMIM]Cl
[BMIM]Cl
Water
CrCl
CrCl
3
3
·6H
·6H
2
O (50)
O (50)
120
140
160
120
155
80
5
0.5
5, 10
5, 30
67.0
71.0
28.0
42.0
98.0
85.0
55.7
45.0
48.0
12.0
24.0
82.0
57.0
14.1
–
–
–
–
[9]
[9]
2
[
a]
[b]
[b]
HCl (100)
Zr(O)Cl (10 mol%)
No catalyst
CrCl ·6H O (50)
AlCl (50 mol%)
48.0
–
99.0
93.0
–
29.0
–
84.0
84.0
–
[10]
[11]
[12]
[14]
[13]
[
a]
2
[
a] [b]
1,
5
[
a]
[b]
3
2
2.5, 180
20, 60
[
a]
[b]
3
120
[a] Residence time with microwave heating. [b] Residence time with conventional heating.
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Table 4. Studies comparing microwave (micr.) and conventional (conv.) heating for the dehydration of pentoses, pentosans, and biomass to furfural, in
terms of furfural yield and sugar conversion.
Entry
Feed
amt. [mm])
Solvent
Catalyst
(amt. [mm])
T
[ C]
t
Yield [%]
micr.
Convers. [%]
Ref.
o
(
[min]
conv.
micr.
conv.
1
2
3
4
5
6
7
8
9
xylose (740)
xylose (33)
xylose (667)
pine wood (100 mg)
xylan (2000 mg)
xylan (2000 mg)
xylose (667)
xylose (67)
xylose (250)
water
water
water
[BMIM]Cl
water
water
water/THF (1:2 w/w)
water
water/THF (1:3 v/v)
[BMIM]Cl
water
HCl (100)
HCl (100)
HCl (100)
170
180
180
30
30
30
~40
59.8
39
31
42
53
85
67
~35
–
–
18
28
36
–
–
12.8
-
–
–
~80
–
–
–
–
~75
–
–
–
–
–
–
–
98
-
[8]
[34]
[34]
[15]
[16]
[16]
[21]
[35]
[17]
[33]
[36]
[37]
[
a]
[b]
CrCl
HCl (100)
HCl (100)+Na
SO H-ionic liquids
maleic acid (250)
3
·6H
2
O (10 mg)
200
3
, 6
[
a]
[b]
[b]
~150
~150
180
200
140
160
200
170
5
5
, 30
, 30
60
28
45
1.5
60
20
[
a]
2
MoO
4
·6H
2
O (206)
–
3
>95
100
81
>95
89
100
AlCl
AlCl
3
3
·6H
(~62.5)
2
O (100)+NaCl (6000)
15.2
82.2
49
10
xylose (~125)
xylose (57)
xylose (312.5)
1
1
none
FeCl
–
–
12
water-CPME (1:3 v/v)
3
(31)+NaCl (312.5)
74
[a] Residence time with microwave heating. [b] Residence time with conventional heating. The ~ indicates that the values were not clearly mentioned in
the studies and are interpolated based on figures or other data.
Analysis
Experimental Section
The reaction products were analyzed using an HPLC apparatus
equipped with a Rezex RHM-Monosaccharide column. The xylose
concentration in the samples was measured using a Varian Pro Star
In all experiments, 4 mL of a solution containing d-xylose, 50 mm
HCl (constant) and an appropriate amount of NaCl was transferred
into a 10 mL borosilicate (glass) or SiC vial and subsequently to the
TM
3
50 refractive index (RI) detector at 358C whereas furfural was de-
microwave applicator cavity (CEM Discover or Anton Paar Mono-
termined using both the RI and UV detector at 254 nm. The eluent
wave 300) for the required reaction time. All reactions were per-
formed under pressure owing to the solution vapor pressure at the
reaction temperature (1708C or 2008C). Stirring was applied at
was 0.005n sulfuric acid solution in ultrapure (MilliQ) water at
ꢀ
1
a flow rate of 0.6 mLmin and the column temperature was at
08C. The injection volume was 10 mL and the total analysis time
8
6
00 rpm using a magnetic stirring bar. At the completion of the ex-
was 40 min for each sample. Under these conditions, xylose and
furfural had approximate retention times of 11.5 and 35.5 min, re-
spectively. The concentrations of the samples were quantified by
integrating the detector response peak area on the basis of calibra-
tion curves developed by analyzing known external standards.
periment, microwaves were stopped and compressed air cooling
was applied. For the experiments at a higher power, air-cooling
was applied from the beginning and for the entire duration of the
experiment. After the temperature of the mixture dropped below
7
08C, air-cooling was terminated and the vial containing the reac-
tion mixture was immediately quenched in ice for further cooling.
Once the temperature dropped below room temperature, samples
were taken for HPLC analysis. All experiments were performed at
least twice and the results did not deviate by more than 10%.
Modeling
Variation of the NaCl concentration
The degradation rate of xylose was modeled using the following
first-order rate expression [Eq. (1)]:
[7]
TM
This set of experiments was performed in the CEM Discover mi-
crowave reactor at 2008C. The NaCl concentrations were varied at
XðtÞ ¼ X eꢀ
ðk1þk2Þt
ð1Þ
2
, 3.5, and 5 wt%, or 342, 599, and 856 mm, respectively The reac-
0
tion temperature was measured using the built-in infrared (IR)
sensor of the instrument. Prior to the experiments, the IR sensor
was calibrated to the reaction temperature and vessel using an in-
ternal optical fiber temperature sensor (FISO Technologies, Inc.).
Furfural formation and degradation to byproducts is expressed by
the following first-order rate expression [Eq. (2)]:
[7]
ꢄ
ꢅ
k1
À
ꢀ eꢀk3tÁ
ꢀðk1þk2Þt
FðtÞ ¼ X0
e
ð2Þ
k ꢀ k ꢀ k
3
1
2
Comparison of microwave and conventional heating
These experiments were performed in the Anton Paar Mono-
wave 300 microwave reactor at 1708C, as this instrument allows
for a direct comparison of conventional and microwave heating
using the strongly absorbing SiC vials that are available with this
device. The temperature was measured and controlled using the
internal optical fiber (Ruby) sensor of the instrument and the built-
in IR sensor measurements were also recorded to allow for com-
parison. For these experiments, the concentration of NaCl was kept
at 500 mm.
where X is the xylose concentration [mm], X0 is the initial xylose
concentration [mm], F is the furfural concentration, t is the time (s)
ꢀ1
and k k and k are the reaction rate constants (s ). k is the reac-
1,
2
3
1
tion rate constant towards furfural production and k is the reac-
2
tion rate constant accounting for side reactions of xylose. k is the
3
decomposition rate constant of furfural to byproducts. The values
of the rate constants were estimated based on the experimental
results by least square regression (95% confidence interval in pa-
rameter estimation).
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&
These are not the final page numbers! ÞÞ

Full Papers
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Published online on && &&, 0000
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FULL PAPERS
Microwave adventure! The dehydration
of xylose to furfural in the presence of
NaCl is investigated under microwave
and conventional heating. The highest
furfural yield of 76% is obtained using
C. Xiouras, N. Radacsi, G. Sturm,
G. D. Stefanidis*
&& – &&
Furfural Synthesis from d-Xylose in
the Presence of Sodium Chloride:
Microwave versus Conventional
Heating
3.5 wt% NaCl at 2008C. Microwave
heating does not show a prominent
effect on the reaction kinetics, but
lowers the energy consumption com-
pared to simulated conventional heat-
ing (using SiC vials) at similar heating
rates.
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www.chemsuschem.org
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ