Microwave activation of homogeneous and heterogeneous catalytic reactions

Article abstract of DOI:10.1135/cccc19970347

Microwave healing was applied in homogeneous and in heterogeneous reactions and the results were compared from the point of view of activation of chemical reactions. Reactions including the addition of halo compounds to alkenes catalyzed by copper and ruthenium complexes in different solvents and NaY zeolite catalyzed alkylation of secondary amine in the absence of solvent were studied as model reactions to compare possibilities of microwave activation of reactants and catalysts. Rate enhancement of over one order of magnitude in homogeneous reactions was caused mainly by thermal dielectric heating effect which resulted from the effective coupling of microwaves to polar solvents. Activation of reactants and catalysts was very low if any. In heterogeneously catalyzed alkylation reactions highly efficient activation of zeolite catalyst was recorded. The results indicated that the best reaction conditions were in experiments when both activation of catalyst and performance of reaction were carried out under microwave conditions. Rate enhancement was most probably caused by "hot spots" or by "selective heating" of active sites. In both homogeneous and heterogeneous reactions non-thermal activation (specific effect) was excluded.

Full text of DOI:10.1135/cccc19970347

Microwave Activation of Catalytic Reactions
347
MICROWAVE ACTIVATION OF HOMOGENEOUS AND HETEROGENEOUS
CATALYTIC REACTIONS
Milan HAJEK
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic,
165 02 Prague 6-Suchdol, Czech Republic; e-mail: hajek@icpf.cas.cz
Received October 11, 1996
Accepted October 25, 1996
Dedicated to Dr Karel Mach on the occasion of his 60th birthday.
Microwave heating was applied in homogeneous and in heterogeneous reactions and the results were
compared from the point of view of activation of chemical reactions. Reactions including the addition
of halo compounds to alkenes catalyzed by copper and ruthenium complexes in different solvents and
NaY zeolite catalyzed alkylation of secondary amine in the absence of solvent were studied as model
reactions to compare possibilities of microwave activation of reactants and catalysts. Rate enhance-
ment of over one order of magnitude in homogeneous reactions was caused mainly by thermal di-
electric heating effect which resulted from the effective coupling of microwaves to polar solvents.
Activation of reactants and catalysts was very low if any. In heterogeneously catalyzed alkylation
reactions highly efficient activation of zeolite catalyst was recorded. The results indicated that the
best reaction conditions were in experiments when both activation of catalyst and performance of
reaction were carried out under microwave conditions. Rate enhancement was most probably caused by
“hot spots” or by “selective heating” of active sites. In both homogeneous and heterogeneous reactions
non-thermal activation (specific effect) was excluded.
Key words: Microwave activation; Catalysis; Addition of halo compounds; Alkylation of secondary
amines; NaY zeolite as catalyst.
In recent years increasing number of papers concerning acceleration of chemical reac-
tions by microwave irradiation have been reported. However, satisfactory explanation
of drastic reduction of reaction time by microwave irradiation in comparison with con-
ventional heating methods has yet not been reported. According to our opinion and in
agreement with some other authors, two models of the mechanism of microwave acti-
vation of chemical reactions can be proposed. The first can be called thermal activation
resulting from mass heating, i.e. dielectric heating, and the second non-thermal activa-
tion, i.e. specific activation resulting from non-thermal interactions by microwave
irradiation.
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

348
Hajek:
The first assumes that whilst the reaction time is drastically reduced, the kinetics or
mechanism of the reaction is not altered. This proposal assumes that the rate enhance-
ment is simply due to thermal dielectric heating effect.
The second alternative assumes that there is a specific activation effect due to micro-
wave irradiation that operates in addition to the dielectric heating.
Thermal interactions of microwave irradiation can include the following categories:
1. Reaction rate enhancement due to “hot spots” (localized heating effects).
2. Reaction rate enhancement due to superheating (superheating effect).
3. Reaction rate enhancement due to improved transport properties of organic mole-
cules (diffusion enhancement).
4. Reaction rate enhancement due to selective heating (active sites heating, heteroge-
neous catalysis).
5. Changes of selectivity of chemical reactions (fast heating).
Specific microwave activation has been a debated concept which actually refers to a
unique interaction, reaction or activation, specific to the microwave irradiation. Some
of the speculation about non-thermal microwave effect appears to have stemmed from
a widely held misconception that microwave at 2.45 GHz can excite rotational transi-
tions. It is necessary to point out that when a compound absorbs microwaves at 2.45 GHz,
dielectric heating accounts for the temperature rise and the irradiation does not directly
excite the molecule to higher vibrational or rotational energy levels. Moreover, the
energy carried by microwaves at 2.45 GHz is too small (0.3 cal/mol) to split off chemi-
cal bonds. It means that direct molecular activation such as in photochemistry is evi-
dently improbable. On the other hand if we assume that there can be some
non-conventional effect which results in kinetic, yield or selectivity improvement, true
microwave activation may occur.
The published results indicated that strong differences exist when microwave heating
is applied in reactions carried out under homogeneous or under heterogeneous condi-
tions1. The aim of this study was to examine microwave heating effects in homo-
geneous and heterogeneous catalytic reactions and to compare possibilities of
microwave activation of reactants and catalysts.
EXPERIMENTAL
Methods
The reactions were carried out in Philips Whirlpool ovens, M 306 (600 W, mode stirrer) for homo-
geneous reactions and AVM 902 (1 000 W, turntable, double emission) for heterogeneous reactions.
1
GC analysis was performed on an HP 5890 A instrument with HP Ultra-1 capillary column, H NMR
spectra of the products were measured on a Varian XL 200 spectrometer with CDCl3 as solvent.
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

Microwave Activation of Catalytic Reactions
Homogeneous Reactions
349
In a typical experiment a mixture of a halo compound, alkene, copper or ruthenium catalyst and a
solvent in 2.5 : 1 : 0.01 : 5 molar ratio was irradiated for 15–40 min (conversion up to 90%). Screw-
tap Teflon closed vessel was used for experiments under microwave conditions (oven) and sealed
glass ampoule was used for experiments under conventional conditions (thermostated oil bath). Both
reactors were equipped with a septum to take samples for GC analysis and to measure the tempera-
ture of reaction mixture. The temperature was measured by quick-response digital thermometer (thin
thermocouple was placed inside the needle) immediately after irradiation intervals. Each reaction was
carried out both under microwave irradiation (600 W, full power) and conventional heating at the
same temperature. The quantity of the reactants and catalysts were kept the same under both sets of
reaction conditions. The samples of the reaction mixture were analyzed by GC in regular intervals,
and rate constants km (microwave heating) and kc (conventional heating) were determined by known
method2. The results are presented in Table I.
TABLE I
Rate constants of addition of chloro compounds to alkenes under microwave (km) and conventional
(kc) conditions in the presence of [Cu(2-PhNH2)2Cl] catalyst
Chloro
compound
Temperature
km . 104
kc . 104
Run
Alkene
Solventa
f b
°C
s–1
s–1
1
2
3
4
5
6
7
C6H5CH=CH2
CCl4
none
30
73
72
85
83
82
85
0.0
1.2
0.00
0.37
0.44
1.30
1.20
1.10
6.8
–
3.2
CCl4
DCHE
ACN
CCl4
3.3
7.5
CCl3COOEt
CCl3COOEt
CCl3COOEt
CCl3COOEt
none
9.3
7.1
DCHE
ACN
8.7
7.2
23.0
8.2
21.0
1.2c
DCHE
8
9
CH2=CHCOOMe CCl4
none
DCHE
ACN
none
DCHE
ACN
B
51
74
0.2
1.2
1.9
5.0
4.8
5.5
0.7
0.18
0.25
0.35
0.93
0.85
0.83
0.66
1.1
4.8
5.4
5.4
5.6
6.6
1.1
CCl4
CCl4
10
11
12
13
14
85
CCl3COOEt
88
CCl3COOEt
CCl3COOEt
CCl3COOEt
90
102
80
a
c
b
DCHE, 1,2-dichloroethane; ACN, acetonitrile; B, benzene. f, rate enhancement factor (km/kc).
[RuCl(PPh3)3] catalyst.
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

350
Hajek:
Heterogeneous Reactions
NaY zeolite (Aldrich, cat. No. 33, 444-8, powder) was activated3 both classically (350 °C, 9 h, elec-
tric oven) and by microwaves (350 °C maximum, 10 min, 1 000 W, microwave oven). The results are
presented in Figs 1 and 2. Activated zeolite was impregnated by dibenzylamine and 2-phenylethyl
chloride (1 : 1.5 molar ratio) and exposed to microwave (oven) or conventional (oil bath) heating.
The samples of reaction mixture were treated by solution of NaHCO3, extracted 3 times by petroleum
ether and analyzed by GC and NMR. NMR spectra of the product were in agreement with published
data4. Alkylation reaction was carried out at two temperatures 80 and 150 °C, both under microwave
and conventional conditions. The temperature of the solid reaction mixture under microwave heating
varied in differences ±10 °C. Lower temperatures (70–90 °C) were established by lowering micro-
wave power with a beaker of water (200 ml) to absorb excess microwave energy. Because of varying
temperatures which prevented precise kinetic measurements, microwave activation of alkylation reac-
tion was evaluated by the yield of product. The results are presented in Table II.
TABLE II
Alkylation of dibenzylamine under microwave (MW) and conventional (CL) conditions
Activation of zeolite, (350 °C)
Alkylation of dibenzylamine
Run
Conditions
Time, h
Conditions Temperature,°C
Time, h
Yield, %
1
2
CL
9.0
CL
80
80
0.5
7.0
0.5
7.0
0.5
0.5
0.5
7.0
0.5
7.0
0.5
0.5
7.0
13.3
23.9
45.3
67.9
14.8
70.1
15.7
40.0
44.3
66.8
47.4
69.7
2.7a
CL
9.0
CL
3
CL
9.0
CL
150
4
CL
9.0
CL
150
5
CL
9.0
MW
MW
CL
70–80
140–160
80
6
CL
9.0
7
MW
MW
MW
MW
MW
MW
–
0.17
0.17
0.17
0.17
0.17
0.17
–
8
CL
80
9
CL
150
10
11
12
13
CL
150
MW
MW
CL
70–90
140–160
80
a
In the absence of zeolite (Na2CO3 solution).
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

Microwave Activation of Catalytic Reactions
351
RESULTS AND DISCUSSION
Homogeneous Reactions
MW
Cat
R–CH=CH2 + CXCl3
R–CHCl–CH 2–CXCl2
(A)
R = Ph, COOMe; X = Cl, COOEt; Cat = [Cu(2-PrNH2)2Cl], [RuCl(PPh3)3]
The experiments included combination of polar (ethyl trichloroacetate, methyl acrylate)
and non-polar (styrene, tetrachloromethane) reactants, and polar (1,2-dichloroethane,
acetonitrile) and non-polar (benzene) solvents with different abilities to absorb micro-
waves. Transition metal complexes of copper [Cu(2-PrNH2)2Cl] and ruthenium
[RuCl(PPh3)3] were used as catalysts. Preliminary results indicated that a certain acti-
vation of homogeneously catalyzed addition reactions is possible2. Because almost no
data on application of microwave heating in reactions catalyzed by transition metal
complexes was available, we have examined this subject in more detail. The results are
summarized in Table I.
Rate enhancement up to 21-fold (run 6) was observed with [Cu(2-PrNH2)2Cl] cata-
lyst when acetonitrile was used as a polar solvent. In non-polar benzene no rate en-
hancement was observed (run 14). No reaction proceeded with non-polar reactants
(styrene, tetrachloromethane) in non-polar media (excess of tetrachloromethane, run 1).
This reaction mixture containing homogeneous catalyst did not absorb microwaves and
therefore no reaction occurred.
From the values of rate constant under conventional condition can be seen that sol-
vent effect on reaction rate was not too significant (kc = 0.37–0.44, 0.18–0.35 for tetra-
chloromethane, runs 2, 3; 8–10 and kc = 1.10–1.30, 0.66–0.93 for ethyl trichloroacetate,
runs 4–6; 11–14). On the other hand reaction rate under microwave condition was
much more dependent on the polarity of solvent ( km = 1.2–3.3, 0.2–1.9 for tetrachloro-
methane, runs 2, 3; 8–10 and km = 8.7–23.0 for ethyl trichloroacetate, runs 4–6) except
in the case when both reactants were polar (4.8–5.5, runs 12, 13). In non-polar benzene
no rate enhancement was observed (run 14). Ruthenium catalyst was more active than
copper catalyst, however, only slight rate enhancement was observed (f = 1.2, run 7) in
comparison with copper catalyst (f = 7.2, run 5). Reactivity of reactants followed the
same order under conventional and under microwave conditions (styrene > methyl acry-
late; ethyl trichloroacetate > tetrachloromethane) as expected.
The highest differences in the rate enhancement by microwave heating were recorded
in the presence of polar solvents. Non or low rate enhancement in the absence of polar
solvents indicate that activation of reactants and catalysts in homogeneous reactions by
microwaves is very low if any. Specific, i.e. non-thermal activation of reactants and
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

352
Hajek:
catalysts which was considered in our previous communication2 can be excluded. Rate
enhancement of over one order of magnitude seems to be caused mainly by thermal
dielectric heating effect which resulted from the effective coupling of microwaves to
polar solvents.
Heterogeneous Reactions
NaY, MW
(PhCH2)2NH + PhCH2CH2Cl
(PhCH 2)2NCH2CH2Ph
(B)
–HCl
Alkylation of a sterically hindered amine (dibenzylamine) with a low reactive alkyl
chloride (2-phenylethyl chloride) in the presence of a solid catalyst (NaY zeolite) was
chosen as a convenient model reaction to study possibility of microwave activation.
This reaction has not been described in the literature. In an independent experiment we
have proved that this reaction almost does not proceed in the absence of catalyst (Table II,
run 13).
It was found that activation of NaY zeolite reduced drastically the time from 9 h to
10 min when microwave dehydration method was used in comparison with known con-
ventional heating3. The same effect was observed by Roussy5 and it is explained by
direct desorption of water molecules by the electromagnetic field, this process being
independent of the temperature of the solid (diffusion enhancement). The results are
presented in Figs 1 and 2.
In order to evaluate the effect of microwaves both on activation of catalyst and on
heterogeneous reaction, cross-experiments have been carried out by combination of
microwave and conventional method of heating. The results are summarized in Table II.
500
100
H O
2
t, °C
%
400
80
300
200
60
40
20
100
0
FIG. 1
Activation (dehydration) of NaY zeolite by
microwave heating (❍) (temperature against
time curve designated by ■)
0
0
2
4
6
8
10
τ, min
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

Microwave Activation of Catalytic Reactions
353
It was found that the activity of zeolite was almost the same irrespective if the catalyst
was activated by conventional or by microwave method.
More significant effect of microwaves was found in alkylation reaction especially at
higher temperatures. The yield of product was much higher when the reaction was carried
out under microwave condition in comparison with conventional method (45.3 → 70.1%,
runs 3, 6 and 44.3 → 69.7%, runs 9, 12). To achieve high yield under conventional
conditions longer reaction time from 0.5 to 7 h was necessary (runs 4, 10).
The results indicated that alkylation reaction was significantly affected by micro-
waves. The best evidences were obtained in experiments when the activation of catalyst
and the activation of reaction were both carried out under microwave conditions (runs
11, 12).
The explanation for activation of heterogeneous reactions by microwaves seems to
be more complex than activation of reaction in solution. It is known that zeolites con-
tain polar active sites (dipoles) on the surface which are mainly responsible for a strong
absorption of microwaves and therefore for a fast efficient heating of this material.
These sites interact with microwaves and their temperature may be somewhat higher
than in the bulk. This is the case of activation by “selective heating”. The second possi-
bility is the presence of “hot spots”. We have observed temperature gradients up to 20 °C
in the samples, the elimination of which is not a simple operation and they can not be
totally eliminated even by stirring. These “hot spots” may be therefore always present
in the solid sample in the absence of solvent and they may be responsible for accelera-
tion of alkylation reaction.
Both these cases belong to the thermal activation of chemical reactions. The possi-
bility of non-thermal activation may also occur, e.g. in the case when reactants ad-
500
t, °C
100
H O, %
2
400
300
200
80
60
40
20
0
100
0
0
1
2
3
4
5
τ, h
FIG. 2
Activation (dehydration) of NaY zeolite by conventional heating (❍) (temperature against time curve
designated by ■)
Collect. Czech. Chem. Commun. (Vol. 62) (1997)

354
Hajek:
sorbed on the active sites are activated by microwave energy transfer from the sites to
adsorbed molecules. In our described alkylation reaction the rate enhancement was not
so high as would be expected (less than one order) and therefore non-thermal activation
may be excluded.
It can be concluded that for the study of microwave activation of catalytic reactions
heterogeneous reactions taking place on the surface of solid catalyst are more interes-
ting than reactions in solution where possibility for microwave effects is very limited.
The results of this study documented that much broader possibilities exist for both
thermal or non-thermal activation in heterogeneous reactions in the absence of solvent
than for homogeneous reactions in solution where microwave effects are practically
reduced to thermal “superheating” effect.
Financial support for this work by the Grant Agency of the Czech Republic (Grant No. 203/94/1017)
is gratefully acknowledged.
REFERENCES
1. Bram G., Loupy A., Villemin D.: Solid Support and Catalysis of Organic Synthesis (K. Smith,
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2. Adamek F., Hajek M.: Tetrahedron Lett. 33, 2039 (1992).
3. Ipaktschi J., Brück M.: Chem. Ber. 123, 1591 (1990).
4. Katritzky A. R., Yannakopoulou K., Lue P., Rasala D., Urogdi L.: J. Chem. Soc., Perkin. Trans.
1 1989, 225.
5. Roussy G., Zoulalian A., Charreyre M., Thiebaut J. M.: J. Phys. Chem. 88, 5702 (1984).
Collect. Czech. Chem. Commun. (Vol. 62) (1997)