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F. Chemat, M. Poux / Tetrahedron Letters 42 (2001) 3693–3695
Table 1. Microwave assisted pyrolysis of urea under homogeneous and heterogeneous conditions
Phase (reactants)
T (°C)
Time (min)
Yield (%)
Selectivity (%)
Reaction rate (103 s−1
)
Homogeneous phase (urea 20 g)
Microwave heating
Classical heating
Microwave heating
Classical heating
200
200
200
200
30
30
1
68.4
67.9
5.2
73.6
72.2
20.2
30.3
8.8
8.7
8.8
8.7
1
4.5
Heterogeneous phase (urea 20 g+graphite 5 g)
Microwave heating
Classical heating
Microwave heating
Classical heating
300
300
200
200
3
3
1
1
61.2
15.2
9.9
93.5
45.6
56.3
33.6
12028
7156
21.9
8.7
4.6
The reaction rate is connected to the temperature by
Arrhenius Eq. (1):
The traditional process of producing cyanuric acid 4
using urea 1 as the starting raw material consists basi-
cally in heating urea at 180–300°C to convert it to 4
with evolution of ammonia gas. An excessive heating
necessary for converting the intermediates into 4 causes
decomposition of the product formed in the mass.8
Identification of the components of the reaction mix-
ture, initially containing urea, allowed the reaction
pathway illustrated in Scheme 1 to be established.
k=A exp(−Ea/RT)
(1)
For two temperatures, the ratio of the respective reac-
tion rates is related to Eq. (2):
ln(k2/k1)=(Ea/R)×(1/T1−1/T2)
(2)
From Eq. (2), it follows that the apparent elevated
temperature of the graphite support for the batch reac-
tions was calculated to be 11 1 K higher than the
measured bulk temperature.
Kinetic studies have been carried out under carefully
monitored reaction conditions. Within experimental
errors, our results fit with the first order reaction equa-
tion. Using Arrhenius equation, the activation energy
and the pre-exponential constant were determined for
both microwave and conventional heating: Ea
(microwave)=159 3 kJ/mol, Ea (conv. heat)=160 3
kJ/mol; ln A (microwave)=35 1, ln A (conv. heat)=
34 1.
In the steady state, the microwave heat transfer to the
support is equal to the heat loss of the support to its
surroundings. The resulting DT will be linearly depen-
dent on the difference in loss factor and the radius of
the support.
In summary, it has been found that heterogeneous
reactions, taking place on the surface of a dissipative
and/or catalytic solid, are more likely to show
microwave activation. Higher yields and reaction rates
were in most cases caused by localised superheating, i.e.
by creating hot zones frequently called ‘hot spots’.9,10
Table 1 summarises the effect of the heating mode
when the experiments are carried out under homoge-
neous and heterogeneous conditions. When the reaction
is conducted in the homogeneous phase, identical reac-
tion rates and equilibrium yields are obtained with
classical heating and with microwave irradiation. The
question of
a specific non-thermal activation by
Acknowledgements
microwaves has been addressed during the past decade.
In our case of the homogeneous pyrolysis, the answer is
clearly that there is no non-thermal effect. When
graphite is added to the reaction mixture, the yield and
the rate of the pyrolysis reaction increases with
microwave heating as compared to conventional heat-
ing under the same conditions (temperature, concentra-
tion and pressure).
The authors thank Professor Jacqueline Smadja for her
valuable comments and helpful discussions.
References
1. Loupy, A.; Petit, A.; Hamelin, J.; Texier-boullet, F.;
Jacquault, P.; Mathe´, D. Synthesis 1998, 1213–1234.
2. Varma, R. S. Green Chem. 1999, 43–55.
3. Caddick, S. Tetrahedron 1995, 51, 10403–10432.
4. Walkiewicz, J. M.; Kazonich, G.; McGill, S. L. Miner.
Metall. Process. 1988, 5, 39–42.
The results show an increase yield of 200% when the
reaction is conducted in the microwave batch reactor.
The increase in reaction rate corresponds to a virtual
difference in reaction temperature. Since the bulk tem-
perature is equal for both the conventional and
microwave heated systems, there must be an elevated
temperature at the local heterogeneous site: the support
surface. The apparent temperature of the catalytic site
under microwave irradiation can be estimated from the
initial reaction rates.
5. Garrigues, B.; Laporte, C.; Laurent, R.; Laporterie, A.;
Dubac, J. Leibigs Ann. 1996, 739–741.
6. The microwave experiments were accomplished in Syn-
thewave 402 microwave reactor supplied from Merck
Eurolab and nowadays from CEM. This single mode
Chemat, Farid
