Microwave induced thermal gradients in solventless reaction systems

Article abstract of DOI:10.1016/j.tet.2006.07.038

Development of thermal heterogeneity under microwave irradiation for solventless solid-liquid phase-transfer catalytic (PTC) reactions has been studied by means of a thermovision camera and fiber-optics thermometer.

Full text of DOI:10.1016/j.tet.2006.07.038

Tetrahedron 62 (2006) 9440–9445
Microwave induced thermal gradients in solventless reaction
systems
b
c
Dariusz Bogdal,a, Szczepan Bednarz and Marcin Lukasiewicz
*
aFaculty of Chemical Engineering and Technology, Politechnika Krakowska, ul. Warszawska 24, 31-155 Krakow, Poland
bDepartment of Engineering and Machinery for Food Industry, University of Agricultural, ul. Balicka 122, 30-149 Krakow, Poland
cDepartment of Carbohydrate, University of Agricultural, ul. Balicka 122, 30-149 Krakow, Poland
Received 13 April 2006; revised 8 July 2006; accepted 11 July 2006
Available online 10 August 2006
Abstract—Development of thermal heterogeneity under microwave irradiation for solventless solid–liquid phase-transfer catalytic (PTC)
reactions has been studied by means of a thermovision camera and fiber-optics thermometer.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
processing of materials. However, there is a general agree-
ment that the application of fiber-optics thermometers is
the only reliable way to determine temperature under micro-
wave conditions; using a thermovision camera, we reported
that the application of a pyrometer and fiber-optics ther-
mometer did not give correct values while high temperature
gradients were developed within reaction mixtures.11–13 For
example, it was shown that heterogeneous support like Mag-
trieve, which is a strong microwave absorber, could reach
higher temperature than the boiling point of a solvent ap-
plied in the synthesis, but a pyrometer and fiber-optics ther-
mometer did not show this effect.11,12 In another example, in
the case of viscous homogenous reaction media like epoxy
resins, a thermovision camera showed high temperature gra-
dients on the surface of the reaction mixtures. Since in some
parts of the sample local temperatures under microwave irra-
diation can be different, pyrometers as well as fiber-optics
thermometers gave only information about the local temper-
atures but bulk temperature of the reaction mixture was hard
to estimate.13
Recently, microwave irradiation has been found to be a very
efficient tool to improve yields of a great number of chemi-
cal transformations.1–3 Many authors suppose that it might
be a result of specific microwave interaction with reagents
on molecular scale when the reaction system’s polarity is in-
creased from a ground state to a transition state4 or on macro-
molecular scale, which in turn influences yield5 as well as
selectivity.6 Most recent critical reviews concerned with
these effects were published by Perreux and Loupy,4
Nuchter et al.,5 and de la Hoz et al.6
The most successful examples of microwave applications
were found to be related to the use of heterogeneous sol-
vent-free systems, in which microwaves interact directly
with reagents and, therefore, can more efficiently drive
chemical reactions. The possible accelerations of such reac-
tions are expected to be optimal since they are not moderate
or impeded by solvents.7 On the other hand, in diluted
homogenous chemical systems in which it is possible to
obtain a good thermal homogeneity by use of an effective
mechanical stirring or/and boiling chips, it was proved that
acceleration of microwave-assisted reactions was rather
negligible;8,9 however, it could be also dependent on the
mechanism of the reaction.10
2. Results and discussion
In the present study, we decided to apply the microwave pro-
tocol to investigate the reaction of salicylaldehyde with
ethyl ester of chloroacetic acid under solid–liquid PTC con-
ditions in the presence of K2CO3 and tetrabutylammonium
bromide (TBAB) as a catalyst. The reaction resulted in the
formation of (2-formylphenoxy)acetic acid ethyl ester (1),
which after further intermolecular condensation gave benzo-
furan-2-carboxylic acid ethyl ester (2)—the final product
(Scheme 1).
Temperature measurement during microwave irradiation of
materials is the major problem in microwave-enhanced
Keywords: Microwave irradiation; Benzofuran; Solventless solid–liquid
reaction.
* Corresponding author. Tel.: +48 12 6282572; fax: +48 12 6282038;
e-mail: pcbogdal@cyf-kr.edu.pl
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.07.038

D. Bogdal et al. / Tetrahedron 62 (2006) 9440–9445
9441
TBAB/K2CO3
∆
OCH2COOEt
CHO
OH
Cl CH2 COOEt
+
-KCl, -0.5 CO2, -0.5 H2O
CHO
(1)
O
OCH2COOEt
CHO
∆
COOEt
-H2O
(2)
Scheme 1. Preparation of benzofuran-2-carboxylic acid ethyl ester.
This reaction was chosen as the test reaction because in our
previous study we had found that it was an effective way for
the synthesis of a number of benzofuran derivatives.14 When
the reaction was run with the same reaction temperature pro-
files under both conventional (oil bath) and microwave con-
ditions, different distributions of the intermediate (1) and
final product (2) were determined (Table 1). It is worth
stressing that the product distribution was much strongly af-
fected by the microwaves, and the yield of 2 was higher in
comparison with conventional experiments when the reac-
tion was run at lower temperatures; for example, 85 ꢀC
(Table 1, entries 4 and 5) versus 110 ꢀC (Table 1, entries 2
and 3). Thus, in order to observe the influence of microwave
irradiation on chemical reactions if the yields were high
under both microwave and conventional conditions it was
required not to increase but decrease the reaction tempera-
tures, which have been shown in previous reports.15 In our
case, it allowed finding a temperature range where yields
or/and product distributions were different for both protocols
(Table 1). Furthermore, the addition of a small amount of
polar or non-polar solvents also influences the product distri-
bution. In this case, two solvents were applied: one capable
of strongly coupling and the second not coupling with micro-
waves, i.e., ethanol and cyclohexane, respectively. The addi-
tion of ethanol strongly shifted the product distribution
toward the final product (2), whereas the addition of cyclo-
hexane resulted in much lower yield of 2.16 The extensive
discussion on the influence of reaction medium, reaction
mechanism, temperature, temperature measurement method,
and stirring on the reaction yield and selectivity under micro-
wave irradiation in comparison with conventional thermal
heating can be found in the literature.17,18
of such gradients has been confirmed by both theoretical cal-
culations as well as experimental results,19–21 we decided to
repeat our previous experiments14 under slightly modified
conditions so that it was possible to use a thermovision cam-
era. The development of a temperature gradient (superheat-
ing) within a cross-section of alumina within a Petri dish has
been already presented by means of a thermovision camera.
Then kinetic calculations of a theoretical reaction indicated
that the yield of this reaction might be higher because of the
development of temperature gradients under microwave
irradiation.20 In this paper, we were able to prove these
conclusions experimentally.
First of all, we have observed that crude compounds 1 and 2
adsorbed on the surface of K2CO3 gave different colors (i.e.,
yellow (1) and brown (2)), which were utmost of importance
and permitted to visually follow the progress of the reaction
(Figs. 1 and 2). At the same time, temperature of the reaction
mixtures and temperature of the surface were determined by
fiber-optics thermometer (ReFlex, Nortech) and thermo-
vision camera (V-20, VigoSystem), respectively. The results
of the experiments under conventional conditions that were
run in order to find optimal conditions for microwave exper-
iments are shown in Table 2. Obviously, the reactions were
carried out without a mechanical stirring of the reaction
mixtures in order to monitor them with the thermovision
camera.
For the purpose of the present investigation on thermal gra-
dients in heterogeneous systems in which the development
Table 1. The distribution of the intermediate (1) and the final product (2) in
the synthesis of benzo[b]furans under both conventional (D) and microwave
(MW) conditions
No. Temperature Time
(ꢀC)
Solvent (ml)
Yield (%)
(min)
1
2
1
2
3
4
5
6
7
8
9
110
110
110
85
85
85
85
85
85
10/MW
20/MW
20/D
20/MW
20/D
30/MW Cyclohexane (1.5)
30/MW EtOH (100%) (1.5)
30/MW EtOH (96%) (1.5)
—
—
—
—
—
12
—
4
15
90
62
0
66
100
96
85
10
38
100
100
53
0
Figure 1. Photograph of the surface of the reaction mixture (Table 3,
entry 1).
30/D
EtOH (100%) (1.5) 47

9442
D. Bogdal et al. / Tetrahedron 62 (2006) 9440–9445
Table 3. Averagea reaction selectivity depending on experimental mode
No.
Selectivitya (%)
Description
1
2
1
2
3
4
5
6
33
70
84
100
67
77
67
30
16
0
33
23
No stirring
Vessel rotation
Vessel rotation and stirring
No stirring, addition of 6 ml of decane
No stirring, vessel rotation
No stirring, vessel rotation
a
Average selectivity determined by GC/MS from entire reaction mixture.
All microwave experiments were carried out in the same condition: max-
imal microwave power 240 W, maximal temperature 110 ꢀC.
The first three experiments (Table 3, entries 1–3) were car-
ried out in order to check the influence of the rotation of
the reaction vessels and stirring of the reaction mixture on
process selectivity. In the case of the first experiment (entry
1), the reaction mixture was placed in the microwave reactor
without any rotation of the reactionvessel. In the second case
(entry 2), there was only applied a vessel rotation, while in
the third case (entry 3) there was used the vessel rotation
and mechanical stirring by immobilizing the quartz spatula
inside the reaction vessel.
Figure 2. Photograph of the surface of the reaction mixture (Table 3,
entry 2).
Table 2. The influence of temperature on the product selectivity of 1 and 2 in
the synthesis of benzo[b]furans under conventional conditions (5 min)
No. Temperature (ꢀC) Catalyst (0.5 mmol)
Selectivitya (%)
At the beginning, the development of thermal gradients dur-
ing microwave experiments on the surface of reaction mix-
tures was confirmed visually, i.e., it was possible to
observe the formation of brown spots in all the places where
the compound 1 was converted to 2 (Figs. 1 and 2). These
observations were later proved by the analysis of the samples
taken from different places from the surface of the reaction
mixtures. Also, thermal gradients were revealed by means of
the thermovision camera, which detected strong temperature
increase in the central part (on the side of the microwave
waveguide in the reactor) of the reaction mixture in compar-
ison to outer regions (Fig. 4).
1
2
1
2
3
4
5
6
7
8
70
95
115
135
145
95
110
150
80b
80b
90c
90c
110d
85d
TBAB
TBAB
TBAB
TBAB
TBAB
—
—
—
TBAB
—
100
48
12
0
0
52
88
100
100
0
26
89
0
0
100
74
11
0
9
10
11
12
13
14
0
0
0
0
0
0
TBAB
—
TBAB
TBAB
3
71
97
29
Eventually, the existence of thermal gradient was proved by
the quantitative analysis by taking samples from different
places of reaction mixtures that exhibited different tempera-
tures during the reactions (Figs. 1 and 4, entries P1, P2, and
P3). The results are summarized in Table 4. It can be seen
that the highest conversion of 1 to 2 was found for the sample
P3, which exhibited the highest temperature (ca. 200 ꢀC)
during the microwave experiment (Table 4, entry 3). Then
the conversions of 1 to 2 45% and 0% were observed for
the samples P2 and P1, which gained lower temperatures
than P3, i.e., 125 ꢀC (Table 4, entry 2) and 70 ꢀC (Table 4,
entry 1), respectively. These results are in good agreement
with the experiments under conventional conditions (Table
1), in which it was shown that only the samples that reached
higher temperature (more than 150 ꢀC) can give high
a
The reaction’s selectivity is determined by GC/MS.
Al2O3 was used instead of K2CO3.
Without any solid support.
b
c
d
The reaction time was 20 min.
However, the addition of the catalyst (i.e., TBAB) strongly
increased the yield of the final product (2) (Table 2, entries
1–5), it was also possible to carry out the reaction without
the catalyst at elevated temperatures (Table 2, entries 6–8).
Moreover, the use of TBAB resulted in the release of volatile
products of its decomposition upon the surface of the reac-
tion mixtures that in turn strongly influenced the temperature
measurements by means of the thermovision camera (i.e., re-
lease of any vapor within the reaction vessel immediately
leads to disability of the thermovision camera).
It was also found that the reaction did not occur when K2CO3
was substituted with another commonly used mineral sup-
port, i.e., Al2O3 or when the reagents were applied without
any support (Table 2, entries 9 and 10, respectively). Finally,
for the purpose of microwave investigation with the thermo-
vision camera it was decided to carry out the reaction on
K2CO3 support at 110 ꢀC without the catalyst. The results
of the microwave experiments are presented in Table 3.
Table 4. Local composition of the reaction mixture (Figs. 1 and 4)
Sample
Color
Temperature (ꢀC)
Yield (%)
1
2
P1
P2
P3
Light
Brown
Dark
70
125
200
100
55
0
0
45
100

D. Bogdal et al. / Tetrahedron 62 (2006) 9440–9445
9443
conversion of 1 to 2 (Table 1, entry 8) without a catalyst. For
the samples that reached temperatures lower than 100 ꢀC
(Table 1, entry 6), no conversion of 1 to 2 was observed at all.
It has been shown that the investigated reaction system is
characterized by the strong interaction with microwaves,
which results in very high heating rates. Therefore, a lot of
heat can be generated within a small amount of the material
in a relatively short time (3 min). This behavior leads to
overheating and difficulties in proper temperature measure-
ment. The investigation has proved the existence of thermal
heterogeneity in microwave irradiated reaction mixtures
without rotation, occurring as a result of significant differ-
ences in intensity of microwave field in a reactor cavity.
Figure 4. Thermovision photograph of the surface of the reaction mixture
during the experiment (Table 3, entry 1).
The rotation of the reaction vessel within the microwave cav-
ity decreases substantially the temperature gradient; how-
ever, still the regions are characterized by different colors,
i.e., different concentrations of 1 and 2 in the reaction mix-
ture (Fig. 2). In fact, the rotation of reaction vessels reduced
overheating, the reaction mixture was exposed to more ho-
mogenous microwave field, and, in turn, the yield of 2 was
lower (Table 3, entry 2) than for the experiments without
rotation (Table 3, entry 1) but much closer (i.e., comparable)
to the yield of the reaction under conventional conditions
(Table 2, entry 7).
microwave protocol and they gave satisfactory results. It
was also shown that the addition of an inert liquid, n-decane
(Table 3, entry 4), which was added in such an amount that it
formed 1–2 mm layer over the top of the reaction mixture,
improved temperature homogeneity too; however, it was
a bit difficult to observe with the thermovision camera and
visually, but the fiber-optics thermometer showed the same
temperature (i.e., ca. 110 ꢀC) at every region of the reaction
mixture.
The mechanical stirring with a quartz spatula placed within
the reaction mixture improved the thermal homogeneity of
the reaction mixture to a greater extent than the rotation
of the reaction vessels (Table 3, entry 3); the results are com-
parable to those obtained under conventional conditions
(Table 2, entry 7). Although, it is hard to jump to further con-
clusions since we did not run conventional protocol with
the stirring of reaction mixtures, it can be seen (Fig. 3) that
the surface of the reaction mixture was more homogenous
in comparison with the experiments with vessel rotation
(Fig. 2) and, in particular, without rotation (Fig. 1). Two
additional experiments (Table 3, entries 5 and 6) were
performed in order to observe the repeatability of the
The results presented above are important because one can
expect that overall reaction yields given in Table 3 should
be similar for all the experiments and do not show significant
differences during additional rotation of the vessels and/or
stirring of the reaction mixtures. It can be assumed that the
overall energy input given by microwaves is similar in all
the cases, thus the reaction rate can be different in different
regions of the reaction mixture because temperature is dif-
ferent but overall reaction rates should be comparable. In
fact, we observed an opposite situation, the increase in tem-
perature homogeneity caused a decrease of the overall reac-
tion yields and made them comparable to those obtained
under conventional conditions.
Eventually, Figure 5 illustrates the relation between temper-
ature measured by the fiber-optics thermometer and thermo-
vision camera versus reaction time for the two experiments
without and with an inert solvent (Table 3, entries 1 and 4,
respectively). In all the cases, temperatures measured by
the thermovision camera were lower than those measured
by the fiber-optics thermometer. It can be also seen that
the temperature differences between the fiber-optics ther-
mometer and thermovision camera were higher for the
experiment with the inert solvent (Table 3, entry 4) in
comparison with the experiments without the solvent (Table
3, entry 1). It can be explained by the difficulties in temper-
ature measurements by the thermovision camera in the pres-
ence of solvent, which was mentioned earlier, as well as heat
transfer process of non-polar liquids that proceeds only via
conventional heat transport (i.e., thermal conduction) that
is slower than microwave heating. Thus, in such a case tem-
perature measurements under microwave irradiation were
inaccurate for two possible reasons. First, the rate of heating
is so high and measurement devices have some specific ther-
mal inertia during temperature measurements. Second, there
Figure 3. Photograph of the surface of the reaction mixture (Table 3,
entry 3).

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D. Bogdal et al. / Tetrahedron 62 (2006) 9440–9445
240
180
120
60
4. Experimental
All chemicals were purchased from Aldrich and used as
received. The reactions were carried out in a single-mode
microwave reactor with a continuous power regulation
(SynthWave 402, Prolabo). The inlet at the top of the reactor
allowed the application of the thermovision camera (Vigo V-
20E2) and the introduction of the fiber-optics thermometer
(ReFlex, Nortech), which was used to control the tem-
perature during microwave experiments. GC/MS spectra
were determined on GC/MS 5890 SERIES II HEWLETT–
PACKARD gas chromatograph equipped with Ultra 2
(25 mÂ0.25 mmÂ0.25 mm) column with HEWLETT–
PACKARD 5971 Series Mass Detector.
In a typical experiment, the reactions were carried out by
simply mixing K2CO3 (2.70 g, 20 mmol), salicylaldehyde
(0.61 g, 5 mmol), chloroacetic acid ethyl ester (1.22 g,
10 mmol), and a catalyst (TBAB 0.16 g, 0.50 mmol). Then
the mixtures were irradiated for 3 min in an open quartz ves-
sel (4 cm of diameter) in the microwave reactor or heated for
5 min in a thermostated oil bath (2 min were added in
comparison to microwave experiments because of a thermal
inertia of the vessel). Finally, the reaction mixtures were
extracted with 20 ml of acetone to estimate overall yields
by means of GC/MS.
0
0
1
2
3
Time / min
Figure 5. Relation between time and temperature, during reaction progress,
measured by fiber-optics thermometer in the hot zone—P3 (>) and shown
by thermovision camera as maximal (A) (Table 3, entry 1). For the sake of
comparison two curves were added: temperature measured by the fiber-
optics thermometer placed in the position identified as the hot zone (B) and
the camera (d) for the experiments with an inert solvent (Table 3, entry 4).
Acknowledgements
are only appointed local temperatures (fiber-optics ther-
mometer) or average temperatures (thermovision camera)
that in the latter case correspond only to the temperature
of surface, which is lower than the bulk temperature.
This work was undertaken as part of the EU sponsored D32
COST Program (Chemistry in High-Energy Microenviron-
ments).
References and notes
3. Conclusion
In conclusion, a proper temperature measurement in case of
heterogeneous reaction mixture is very difficult. In order to
maintain a good temperature homogeneity and make some
comparison with the experiments under conventional condi-
tions, an effective stirring has to be provided, perhaps,
together with a small amount of an inert solvent. There is
a general agreement that the application of fiber-optics ther-
mometers is the reliable way to determine temperature under
microwave conditions. Applying the thermovision camera,
we found that for the reactions in heterogeneous systems un-
der microwave irradiation, the temperature measurement
with a fiber-optics thermometer can lead to serious errors
like pyrometry; in particular for those experiments that are
planned without any attention being paid to temperature ho-
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