The effect of ultrasound on the N-alkylation of imidazole over alkaline carbons: Kinetic aspects

Article abstract of DOI:10.1016/j.apcata.2010.01.041

N-Alkylimidazoles have been synthesized by sonochemical irradiation of imidazole and 1-bromobutane using alkaline-promoted carbons. The catalysts were characterized by X-ray photoelectron spectroscopy, thermal analysis and nitrogen adsorption isotherms. Under the experimental conditions, N-alkylimidazoles can be prepared with a high activity and selectivity. It is observed that imidazole conversion increases in parallel with increasing the basicity of the catalyst. For comparison, the alkylation of imidazole has also been performed in a batch reactor system under thermal activation.

Full text of DOI:10.1016/j.apcata.2010.01.041

Applied Catalysis A: General 378 (2010) 26–32
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
The effect of ultrasound on the N-alkylation of imidazole over alkaline
carbons: Kinetic aspects
a
a
b
c
a
´
´
S. Ferrera-Escudero , E. Perozo-Rondon , V. Calvino-Casilda , B. Casal , R.M. Martın-Aranda ,
a
d,
*
´
´
A.J. Lopez-Peinado , C.J. Duran-Valle
a
´
´
´
´
Departamento de Quı´mica Inorganica y Quımica Tecnica, Universidad Nacional de Educacion a Distancia (UNED), C/Senda del Rey, 9, E-28040 Madrid, Spain
b
c
´
´
Instituto de Catalisis y Petroleoquımica (C.S.I.C.), Campus UAM, Cantoblanco, E-28049 Madrid, Spain
´
Centro Nacional de Investigaciones Metalurgicas (CENIM; CSIC), Avda. Gregorio del Amo, 8, 28040 Madrid, Spain
d
´
´
Departamento de Quı´mica Organica e Inorganica, Facultad de Ciencias, Universidad de Extremadura, Avda. de Elvas s/n, E-06071 Badajoz, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
N-Alkylimidazoles have been synthesized by sonochemical irradiation of imidazole and 1-bromobutane
using alkaline-promoted carbons. The catalysts were characterized by X-ray photoelectron spectrosco-
py, thermal analysis and nitrogen adsorption isotherms. Under the experimental conditions, N-
alkylimidazoles can be prepared with a high activity and selectivity. It is observed that imidazole
conversion increases in parallel with increasing the basicity of the catalyst. For comparison, the
alkylation of imidazole has also been performed in a batch reactor system under thermal activation.
ß 2010 Elsevier B.V. All rights reserved.
Received 4 December 2009
Received in revised form 27 January 2010
Accepted 28 January 2010
Available online 4 February 2010
Keywords:
Reaction kinetic
Sonocatalysis
Basicity
Activated carbons
Imidazole
Alkylation
1. Introduction
ultrasonic irradiation has several additional enhancement effects
and this is especially useful when the solid acts as catalyst [7,8]. So,
Ultrasonic irradiation leads to the acceleration of numerous
reactions in homogeneous or heterogeneous systems [1]. Further-
more, significant improvements can be realized with regards to the
yields [2–4].
The sonochemical phenomena are originated from the interac-
tion between a suitable field of acoustic waves and a potentially
reacting chemical system; the interaction takes place through the
intermediate phenomenon of the acoustic cavitation. Three
important factors have to be considered when an ultrasonic
induced reaction is performed: the acoustic field, the bubbles field
and the chemical system [5,6].
The chemical effects of ultrasounds have been attributed to the
implosive collapse of the cavitation period of the sound waves. The
bubbles are generated at localized sites in the liquid mixture that
contain small amounts of dissolved gases. Trapped within a
microbubble, the reactants are exposed to a high pressure and
temperature upon implosion, the molecules are fractured forming
highly reactive species with a great tendency to react with the
surrounding molecules. When one of the phases is a solid, the
the cavitation effects form microjects of solvent which bombard
the solid, increasing the interphase surface able to react. In general,
the sonication presents beneficial effects on the chemical
reactivity, such as to accelerate the reaction, to reduce the
induction period, and to enhance the catalyst efficiency [9].
The objective of this contribution is to investigate the kinetics
and the mechanism of the imidazole alkylation reaction
(Scheme 1) catalyzed by basic carbons under both ultrasound
and conventional thermal activation. The authors have previously
proved that this reaction can be carried out over acid zeolites and
under ultrasound activation following a different reaction mecha-
nism [10]. In this report, the authors study the influence of
different external parameters in this reaction such as basicity
(alkaline promoter on the carbon), porosity, and amount of catalyst
and reaction temperature. The combination of these parameters of
the catalysts enabled to be used for the highly selective synthesis of
N-alkylimidazoles.
The N-alkylation of heterocycles, particularly imidazole, has
been reported as a key route to prepare important bactericidal,
fungicidal [11] and anticonvulsant [12] compounds.
Activated carbons have been employed as selective catalysts in
related reactions, because of their microporous structure, high
degree of surface reactivity and extended surface area [13].
* Corresponding author. Tel.: +34 924289300.
E-mail address: carlosdv@unex.es (C.J. Dura´n-Valle).
0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.01.041

S. Ferrera-Escudero et al. / Applied Catalysis A: General 378 (2010) 26–32
27
Scheme 1. Alkylation of imidazole with 1-bromobutane.
Moreover, the inclusion of alkaline promoters on the carbons
results in the generation of basic sites on their surface [14,15].
in the absence of any solvent. Then, the catalyst was added to the
reaction mixture and the reaction started.
The reaction was followed by gas chromatography–mass
spectrometry (GC–MS), using a 60-m-long phenyl-silicone capil-
lary column and a flame ionization detector. The conversion is
expressed in terms of amount of A product (Scheme 1).
In previous experiments, it was found that for stirring speeds
above 1000 rpm, and particle sizes of diameter minor than
0.250 mm, nor diffusion processes control exists, either external
or internal [15].
2. Experimental
2.1. Catalysts preparation and characterization
RX-1-EXTRA Norit carbon from Norit Company was treated with
the corresponding aqueous alkaline chloride solution (2 M) at 353 K
for 1 h. The liquid/solid ratio was 10. The samples were filtered and
washed with distilled water until chloride free (AgNO3 test). After
drying at 373 K, overnight, the resulting carbons were pelletized,
crushed and sieved to particle size of 0.074–0.140 mm diameter.
The two series of alkali-loaded catalysts are named: M-N (Na-
Norit, K-Norit and Cs-Norit), and MM0-N (NaK-Norit, NaCs-Norit
and KCs-Norit). The ash contents of the catalysts were obtained by
thermogravimetric analysis (TG/DTA Seiko System 320). The pH of
the samples was measured following the method described by
3. Results and discussion
3.1. Catalyst characterization
The pH of samples is given in Table 1. The pristine carbon, RX-1-
EXTRA Norit, exhibits a basic pH (8.0) that increases only slightly in
the sample exchanged with sodium, but considerably in the Cs-
Norit sample (pH = 10.3). In spite that the amount of cesium in the
solid is around one-third of the sodium cation, the higher basicity
of the Cs ions could be the cause of the increase observed in the pH
of the cesium samples. The pH of the series MM0-N is within the
values of M-N series, being all of them around 10 pH units.
As it was above mentioned the amount of sodium ion into the
Norit is around 3 times the amount of cesium ions. This is expected
because the surface of the active carbon has a determined density
of active groups [17] which interact with the M+ ions by
electrostatic forces during the adsorption–impregnation process.
For steric hindrance, mainly at the micropores of the substrate, it is
obvious that the Na+ ions (ionic radius = 0.095 nm) can access the
centers located at the micropores, whereas the Cs+ ions (ionic
radius = 0.169 nm) have to overcome a diffusional barrier.
´
Rivera-Utrilla and Ferro-Garcıa [16] using an Omega pH-meter,
model PHB-62.
Specific surface areas of the carbon samples were determined
by nitrogen adsorption isotherms at 77 K, applying the BET method
[17] in a Micromeritics ASAP 2010 Volumetric System. Volume
adsorbed in the different types of pores was calculated by the DFT
(density functional theory) method [18] by means of DFT plus
software.
Photoelectron spectra (XPS) were acquired with a VG ESCALAB
200R spectrometer equipped with
analyzer and MgK
ray source. The powder samples were pressed into aluminum
holders and mounted on a sample rod placed in the pretreatment
chamber of the spectrometer. After outgassing 1 h at room
temperature, they were placed into the analysis chamber. The
residual pressure in the ion-pumped analysis chamber was
maintained below 5 ꢀ 10ꢁ9 Torr during data acquisition. The
intensities of C 1s, O 1s, and Na 1s or Cs 3d5/2 peaks were estimated
by calculating the integral of each peak after smoothing and
subtraction of the ‘‘S’’-shaped background and fitting the
experimental curve to a combination of Gaussian and Lorentzian
lines of variable proportion. The binding energies (BE) were
reference to the major C 1s component at 284.9 eV, this reference
giving BE values with an accuracy of ꢂ0.1 eV.
a hemispherical electron
a
(hn
= 1253.6 eV, 1 eV = 1.6302 ꢀ 10ꢁ19 J) X-
Table 1
Some characterization data of the catalysts.
Catalyst
pH
8.0
Ash (%)
M2O (%)
Metal (at-g/100 g cat.)
Norit
Na-N
Cs-N
3.1
4.8
5.5
5.1
5.2
5.3
4.4
–
–
8.4
10.4
10.1
9.8
1.7
2.4
2.0
2.1
2.2
1.3
0.055
0.017
0.035
–
K-N
NaK-N
NaCs-N
KCs-N
10.0
10.4
–
–
2.2. Reaction procedure
2.2.1. Ultrasonic induced reactions
Table 2
Imidazole (5 mmol) and 1-bromobutane (15 mmol) were
mixed in a flask suspended into the ultrasonic bath (Selecta
Ultrasound-H, with a heating system, 40 kHz of frequency, and
550 W of power). Then, the corresponding catalyst was added and
the reaction started. The reactions were carried out at the reaction
temperatures: 293, 313 and 333 K, respectively in the absence of
any solvent.
Specific area (SBET) and adsorbed volume data obtained by DFT.
Sample
SBET (m2/g)
Vmicropore (cm3/g)
Vmesopore (cm3/g)
Vtot (cm3/g)
Norit
1450
1375
1447
1415
1338
1416
0.467 (81.8%)
0.440 (77.2%)
0.460 (84.7%)
0.455 (74.8%)
0.434 (78.2%)
0.463 (80.9%)
0.094 (16.5%)
0.111 (19.5%)
0.074 (13.6%)
0.135 (22.2%)
0.104 (18.7%)
0.095 (16.6%)
0.571
0.570
0.543
0.608
0.555
0.572
Na–N
Cs-N
NaK-N
NaCs-N
KCs-N
2.2.2. Thermal induced reactions
Values in parenthesis indicate the percentage of each type of pore in carbon sample.
Similar procedure was adopted for conventional thermal
heating. The samples were magnetically stirred, in a batch reactor
˚
˚
˚
Micropore: Ø < 20 A; mesopore: 20 A < Ø < 500 A. Vtot: volume adsorbed by pores
˚
Ø < 2000 A.

28
S. Ferrera-Escudero et al. / Applied Catalysis A: General 378 (2010) 26–32
Table 3
Binding energies (eV) of core electrons and atomic ratios calculated by XPS.
Catalyst
Norit
C 1s
O 1s
M (BE)
M/C atomic ratio
Na
–
Cs
K
284.9 (57)
286.4 (23)
288.9 (7)
290.6 (6)
292.5 (7)
531.2 (29)
532.5 (41)
533.9 (19)
535.2 (11)
–
Na-N
K-N
284.9 (57)
286.2 (15)
287.5 (15)
530.9 (44)
532.7 (56)
1072.5a
293.0b
0.0050
289.4 (13)
284.9 (70)
286.8 (30)
531.0 (24)
532.8 (51)
534.5 (25)
0.0030
0.0005
0.0006
Cs-N
284.9 (59)
286.5 (20)
0.0027
530.9 (47)
532.6 (53)
Fig. 1. N2 adsorption isotherm of pristine Norit carbon.
723.9c
288.1 (10)
289.7 (11)
Norit carbon used as pristine solid has a high specific surface
area (1450 m2/g) and the total adsorbed volume of nitrogen
(0.571 cm3/g) (Table 2). The pore size distribution of this carbon is
principally micropores (81.8%) with an important contribution of
mesopores (16.5%). This data is reflected in N2 isotherms profile
(Fig. 1), which is type I in BDDT classification [18]. The hysteresis
loop, classified as type H4 [18], is typically of activated carbons as it
is explained by the slit-shaped pores, present in the carbon [19,20].
The impregnation with different alkaline metals does not
change significantly the specific surface area characteristics of the
substrate (Table 2). The isotherm curves show the same profile,
and slight differences in specific surface area and micropore
volume (Table 2). The Cs-N sample presents the higher value of
surface area and nitrogen adsorption into the micropores.
NaK-N
NaCs-N
KCs-N
284.9 (74)
531.4 (25)
1071.2a;
292.9b
0.0002
0.0001
286.6 (20)
288.3 (6)
533.0 (44)
534.8 (28)
284.9 (73)
531.4 (27)
1071.2a;
723.9c
0.0001
0.0001
286.6 (18)
288.3 (9)
533.0 (45)
534.8 (28)
284.9 (74)
531.4 (25)
292.8b;
723.9c
286.6 (18)
288.3 (8)
533.0 (47)
534.8 (28)
Values in parenthesis indicate the percentage of each peak.
a
Na 1s.
b
In order to get a more precise idea about the chemical state
and the relative dispersion of the alkaline metals at the surface of
the carbon, a study of the different samples by X-ray photoelec-
tron spectroscopy was carried out. The binding energies of C 1s
and O 1s core levels and the characteristic inner levels of the
alkaline elements are given in Table 3, together with the M/C
atomic ratios, determined from the peak intensities and the
tabulated sensibility atomic factors [18]. The C 1s core level is
complex, in which several components can be revealed. A major
peak at 284.9 eV and another three (four in the case of the Norit
pristine) centered at higher binding energies can be discerned.
The peak located at 284.9 eV can be assigned to the C–C bonds of
graphitic-like structure of the carbon, and even to H-containing
species (–CH–), because the small chemical shift of these last
species with respect to the first ones makes it difficult to
distinguish. A second peak with a 15–23% ratio of the total area is
observed around 286.4 eV, which can be associated with C–O
bonds in alcohols or ethers. The third component close to 288 eV,
in general less intense than the former, is attributed to ketonic
species (C55O) and the last one, above 289 eV, to more oxidized (–
COO–) or carbonates species [21]. In the Norit sample, besides
this peak, which is observe at 290.6 eV, another component
around 292.5 eV is detected. This last peak, due to a shake-up
K 2p3/2
.
c
Cs 3d5/2
.
to molecular water [21], probably with different interaction
degrees with the surface.
The binding energies of the inner electrons of the alkaline
elements fit well with hydroxide species, although carbonates
species cannot be discarded, due to the proximity of the binding
energies of these last species. Nevertheless, it does not mean that
Na+ and Cs+ are not ion exchanged on surface negative groups. As it
is known, the surface of carbon materials presents negative groups
that easily interact with alkaline cations when the corresponding
ion exchange treatment is carried out. This ion exchange generates
the active sites. By contrast, the exposure to ambient condition can
generate a partial carbonation of the surface by reaction with CO2,
as detected by XPS. This is a frequent process in basic solid
materials. Hence, the alkaline carbon is considered as the true
catalyst [21].
With respect to the M/C atomic ratio, this ratio diminishes
when the size of the alkaline cation increases as a consequence of
the less exchange in the case of cesium than in the case of sodium,
as determined by thermogravimetric analysis. However, the XPS
results show a slight cesium enrichment at the surface, because the
M/C ratio is around 2.4 times less for the cesium than for the
sodium, meanwhile the content in metal (at-g M/100 g carbon)
determined by thermogravimetric analysis is 3 times higher for the
Na-Norit than for the Cs-Norit.
satellite (
p
!
p*) produced in the photoionization process of
graphitic structure, is not observed in the alkaline-containing
samples.
Similarly, the O 1s line profile is quite complex, especially in the
Norit sample and several components can be distinguished. The
first component centered at 531 eV, can be assigned to C–O and/or
COO species of the carbonaceous support [21]. The second one, at
532 eV, is attributed to the hydroxides (and carbonates) of the
corresponding alkaline metals. In the Norit sample, another two
components above 533 eV are observed, which could be assigned
3.2. Sonochemical synthesis of N-alkylimidazoles
Under our experimental conditions, N-substituted imidazole of
type A is selectively obtained. The mass spectrum of the reaction
product confirms that A (MS m/s: 124 (M+), 97, 81, 55(100), 41) is

S. Ferrera-Escudero et al. / Applied Catalysis A: General 378 (2010) 26–32
29
Scheme 2. Reaction mechanism between imidazole and electrophiles.
Fig. 2. Alkylation of imidazole (5 mmol) with 1-bromobutane (15 mmol) using 0.025 g of alkaline-promoted carbons under sonochemical activation at 298 K. (A) M-N series
and (B) MM0-N series.
produced. Under base catalysis, imidazole reacts with electrophiles
at the N-position (Scheme 2). The nature of the products depends
upon the choice of the alkylating conditions and of catalyst type. As
we have shown, alkaline-promoted carbons are appropriate
materials to catalyze basic organic reactions showing, under our
experimental conditions, basic sites up to pKa = 16.5 [22].
obtained from the alkylation of imidazole reactions under both
conventional thermal and sonochemical activation by using
different basic solid catalysts. In the present work N-substituted
imidazole (A) was obtained as the only product of the alkylation of
imidazole. The reaction proceeds with 100% selectivity to N-
butylimidazole under all reaction conditions. The reported
selectivity was determined in the following way [yield of desired
product]/([yield of detected product] + [detected reactant]). The
total balance calculated to carbon atoms was in all cases better
than 98%.
Considering that the NH group of imidazole presents
a
pKa = 14.5, these carbons could be appropriate catalysts to perform
the alkylation [23].
3.2.1. Kinetic study
The results obtained in Table 4 were obtained by fitting a
pseudo-first order kinetic model considering that one of the
reactant is in excess [10], in our case 1-bromobutane concentra-
tion, not showing any significant change during the reaction. From
these results it can be said that Na-Norit catalyst is approximately
half as active as catalysts containing caesium, these are Cs-, NaCs-
and KCs-Norit and slightly lower than that containing potassium,
this is NaK-Norit. It can be observed that the rate constants
decrease in the order: Cs-Norit > KCs-Norit > NaCs-Norit > NaK-
Norit > K-Norit < Na-Norit.
Under our experimental conditions, the reaction was neither
controlled by external nor internal diffusion as it was confirmed by
using different particle sizes of the catalysts (0.074, 0.140 and
0.250 mm) and different stirring rates (600 and 1500 rpm). From
the Arrhenius equation, the activation energies were also
calculated. The results obtained for the five catalysts employed
in this study confirm that the reaction rate is not limited by pore
diffusion and that it is controlled chemically (Ea > 20 kJ/mol).
The alkylation of imidazole with alkyl halides proceeds under
both acid [10] and basic (Scheme 2) media following a different
reaction mechanism. In Table 4 is shown the kinetic parameters
Table 4
Kinetic rate constants for the alkylation of imidazole (5 mmol) with 1-bromobutane
(15 mmol) under sonochemical activation using alkaline carbons (0.05 g).
Catalyst
Na-Norit
T (K)
Batch k
Ultrasounds k
ðꢀ104 Mꢁ1
s
g
Þ
ðꢀ104 Mꢁ1
s
Þ
ꢁ1 ꢁ1
ꢁ1 ꢁ1
g
cat
cat
293
313
333
0.82
2.00
2.78
3.65
9.85
15.3
Cs-Norit
293
313
333
2.28
4.36
5.11
7.48
8.61
18.7
K-Norit
293
313
333
1.68
2.11
3.53
4.37
8.57
8.87
NaCs-Norit
KCs-Norit
NaK-Norit
293
313
333
2.22
3.10
4.79
7.18
7.80
8.77
3.2.2. Influence of external parameters
3.2.2.1. Influence of the basicity of the catalyst and temperature. The
alkylation of imidazole with 1-bromobutane was performed on
both series of catalysts, M-N and MM0-N at different temperatures.
As an example the conversions at 333 K are given in Fig. 2.
From these results it can be said that the order of activity is
293
313
333
2.36
4.32
4.89
7.27
7.95
9.10
293
313
333
1.83
2.26
3.06
4.80
7.52
8.99
Na-N < K-N < NaK-N < NaCs-N < KCs-N ꢃ Cs-N:

30
S. Ferrera-Escudero et al. / Applied Catalysis A: General 378 (2010) 26–32
Table 5
Alkylation of imidazole (5 mmol) with 1-bromobutane (15 mmol) under sono-
chemical activation. Influence of the catalyst amount. M-N and MM0-N series.
Reaction temperature: 293 K.
Catalyst
Na–N
Time (min)
Conversion (%)
0.025 g
0.050 g
15
30
22.2
27.5
33.7
60.7
32.4
39.6
41.8
70.6
60
120
Cs-N
15
30
40.1
53.6
70.3
80.4
44.6
57.3
78.3
87.3
60
120
K-N
15
30
24.2
28.9
40.2
49.1
32.0
50.1
63.4
72.2
60
Fig. 3. Alkylation of imidazole with 1-bromobutane under sonochemical activation
with 0.025 g of catalyst after 60 min at different reaction temperatures. (A) M-N
series and (B) MM0-N series.
120
NaK-N
NaCs-N
KCs-N
15
30
25.3
30.1
42.6
50.2
35.6
53.2
65.6
74.7
Cs-N is around 1.3 times more active than Na–N. Thus, conversions
of 99% are reached in only 120 min of reaction when Cs-N is the
catalyst at 333 K. Closer conversions (around 93%) are obtained
when the catalyst is doped with K and Cs (KCs-N). The activity of
the samples NaK-N and NaCs-N is intermediate between Cs-N and
Na-N catalysts. This trend indicates that the type of promoter is
decisive to define the basicity of these catalysts. Moreover, the
reaction is extremely sensitive to slight modification of the catalyst
basicity. Thus, the Lewis basic character is determined by the
alkaline dopant. The same trend is observed at 293 and 313 K. Fig. 3
displays the conversion values at 120 min of reaction as an
example of the influence of the temperature on the ultrasonic
induced reactions. As this figure shows, from the results obtained
at 293 and 313 K, the conversion increases with the reaction
temperature, keeping the selectivity 100% in all cases due to the
cavitation phenomena during the sonochemical reactions. How-
ever no significant changes in the conversion are observed when
temperature of sonication is increased from 313 to 333 K. This fact
confirms some previous works in the literature [10,25] which
describes that a liquid sonicated at a temperature near the boiling
point of the medium, enhanced effect is diminished or even no
important sonochemical effect may be expected.
60
120
15
30
37.0
52.6
60.6
80.0
45.0
52.6
72.5
86.7
60
120
15
30
40.0
52.3
63.6
81.3
45.3
53.2
73.3
87.0
60
120
In general, under ultrasound activation the conversion
increases by a factor of 1.2 when duplicates the catalyst amount.
3.2.2.3. Comparison of ultrasonic activation with thermal activation:
influence of the reaction temperature. The effect of the reaction
temperature has been studied during the alkylation of imidazole
with 1-bromobutane under ultrasonic waves and in a batch
reaction system using different amounts of catalysts. As an
example, Figs. 4 and 5 show the activity of the catalyst at
60 min of reaction using 0.05 g of the two series of catalysts.
The comparison evidences that the ultrasonic activation
enhances N-alkylation of imidazole much better than thermal
activation at any temperature since higher conversion levels are
reached at the same reaction times. The improvement of the
reactivity is more drastic at low temperatures. This can be
explained due to the fact that an increase of the temperature will
raise the vapor pressure of the liquid medium of the ultrasonic
bath, leading to an easier cavitation but to a less violent collapse,
which makes the cavitation less effective [24]. On the other hand, a
As expected, an increase of the reaction temperature implies an
increase of the yield of the N-alkylimidazole, keeping the
selectivity 100% in all cases. High yields (between 75% and 99%)
are obtained when Cs-N is employed as catalyst.
3.2.2.2. Influence of the catalyst amount. In order to study the effect
of the catalyst amount, imidazole was alkylated with 1-bromo-
butane using two different amounts of catalyst (0.025 and 0.050 g).
The results are shown in Table 5.
Fig. 4. Alkylation of imidazole with 1-bromobutane under sonochemical and thermal activation with 0.05 g of catalyst after 60 min at different reaction temperatures. (A) Na-
N and (B) Cs-N.

S. Ferrera-Escudero et al. / Applied Catalysis A: General 378 (2010) 26–32
31
Fig. 5. Alkylation of imidazole with 1-bromobutane under thermal (A) and sonochemical (B) activation with 0.05 g of catalyst after 60 min at different reaction temperatures
for the catalysts of the MM-N series.
Fig. 6. Alkylation of imidazole with 1-bromobutane under thermal and sonochemical activation with 0.025 g of catalyst at 293 K (A) and 333 K (B) on Cs-N catalyst.
further factor to be considered is that at temperatures near boiling
point of the ultrasonic liquid medium, large number of
enhancing effect in the yield when ultrasound was used to activate
the reaction. Enhancement effect on the reaction rate by
combining the basicity of the carbon catalysts and ultrasound is
presented as an alternative method for the production of N-
substituted imidazoles and, in general, for the production of other
fine chemicals due to the mild conditions that this system offers. In
spite of the large body of work dealing with sonocatalysis, only a
relatively small number of papers report the influence of the
reaction conditions on the rate and yields of chemical reactions. It
is well documented that, external parameters have a great
influence on the cavitation and, since the cavitation is necessary
to induce sonochemical reactions. It is important that those
parameters are fine tuned and controlled, to maximize the
sonochemical effects. We have found here that among all the
experimental procedures induced by heterogeneous media
employed in this synthesis, the combination of ultrasound
irradiation with alkaline carbons offers interesting prospects
because excellent yields are achieved under very mild conditions.
a
microbubbles are generated concurrently in it, acting as a barrier
to the sound transmission. These bubbles dampen the effective
ultrasonic energy from the source that enters the reaction liquid
medium. Thus, our experimental results confirm some previous
works published in the literature [25], which describe that when a
liquid is sonicated at a temperature near its boiling point,
enhanced effect are diminished or even any important sonochem-
ical effects may be expected.
As a final example, the evolution of the reaction under
sonochemical and thermal activation at 293 and 333 K using the
Cs-N carbon, which is the most basic catalysts are displayed in
Fig. 6. It is evidenced that at lower temperatures, the differences of
activity between ultrasound and thermal activation are higher
than at higher temperatures again because there is no sonochem-
ical effect during the ultrasound activation when the temperature
is increased up to 333 K. In any case our basic carbons turned out as
suitable catalysts to carry out the proposed reaction under both
conventional thermal and ultrasound activation, in short times and
under mild conditions being also able to be reused in four cycles of
reaction without important changes in the final yield of the
reaction.
Acknowledgements
V.C.C. acknowledges CSIC for the award of a JAE-Doctor
contract. E.P.R. also acknowledges the grant supported by the
Carabobo University (Valencia, Rep. Bolivariana Venezuela). This
work has been supported by Spanish Minister of Science and
Innovation (project CTM2007-60577/TECNO, MAT-2006-04486 y
NAN2006-27758-E) and Junta de Extremadura (GRU09027).
Authors thank Prof. JLG. Fierro for the fruitful discussion of XPS
data. Pristine Norit Carbon was kindly supplied by Norit Company.
4. Conclusions
The effect of ultrasound waves in the activation of alkaline-
promoted carbons has been explored in the alkylation of imidazole
with 1-bromobutane. It was found that there is a substantial

32
S. Ferrera-Escudero et al. / Applied Catalysis A: General 378 (2010) 26–32
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