Activated carbon as a catalyst for the synthesis of N-alkylimidazoles and imidazolium ionic liquids

Article abstract of DOI:10.1016/j.cattod.2011.12.021

An activated carbon Norit RX3 has been treated with acids (sulphuric, nitric and sulphonitric mixture). The modified activated carbons have been characterized by elemental analysis, point of zero charge, N₂ adsorption, XPS and FTIR. The solids obtained have been used as acidic heterogeneous catalyst in the reaction between imidazole and haloalkanes. The reaction with 1-bromobutane yields an ionic liquid, N,N-di-n-butyl-imidazolium bromide (1). Reaction with 2-bromobutane and 1-chlorohexane yields the corresponding N-alkyl-imidazoles (2) and (3). Sulphuric acid treated carbon is the most active catalyst.

Full text of DOI:10.1016/j.cattod.2011.12.021

Catalysis Today 187 (2012) 108–114
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Activated carbon as a catalyst for the synthesis of N-alkylimidazoles and
imidazolium ionic liquids
a,∗
a
a
b
b
C.J. Durán-Valle , M. Madrigal-Martínez , M. Martínez-Gallego , I.M. Fonseca , I. Matos ,
c
A.M. Botelho do Rego
Departamento de Química Orgánica e Inorgánica, Universidad de Extremadura, Avda. de Elvas, s/n, E-06006 Badajoz, Spain
REQUIMTE, CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal
Centro de Química-Física Molecular and Institute of Nanoscience and Nanotechnology (IN), IST, Technical University of Lisbon, 1049-001 Lisboa, Portugal
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2011
Received in revised form
An activated carbon Norit RX3 has been treated with acids (sulphuric, nitric and sulphonitric mixture).
The modified activated carbons have been characterized by elemental analysis, point of zero charge, N2
adsorption, XPS and FTIR. The solids obtained have been used as acidic heterogeneous catalyst in the
reaction between imidazole and haloalkanes. The reaction with 1-bromobutane yields an ionic liquid,
N,N-di-n-butyl-imidazolium bromide (1). Reaction with 2-bromobutane and 1-chlorohexane yields the
corresponding N-alkyl-imidazoles (2) and (3). Sulphuric acid treated carbon is the most active catalyst.
1
0 December 2011
Accepted 12 December 2011
Available online 20 January 2012
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Activated carbon
N-heterocycles alkylation
Ionic liquid
Catalysis
1
. Introduction
The alkylation of N-heterocycles is the primary route to obtain
pharmaceutically important intermediate products. In an attempt
to meet effective conditions for the N-alkylation of heterocycles,
attention has turned to the use of heterogeneous catalysts, which
can be easily removed from the reaction mixture by filtration. In this
context, we have shown earlier that the alkaline-doped carbons
[6–8] are highly effective in the imidazole alkylation. The use of
acidic catalyst in this reaction is not usual. Ono [9] uses zeolytes
as catalyst and alcohol as alkylating agent but in gas phase and at
higher temperatures (near 500 K) than in this work. Calvino Casilda
et al. [10] use anacidicsolid, hydratedniobia Nb O5·nH O.
The design of chemical products and processes that reduce or
eliminate the use and generation of hazardous substances is the
main goal of green chemistry. The 9th principle of green chem-
istry [1] is the use of catalytic reagents instead of stoichiometric
ones. The design and implementation of new catalysts and catalytic
systems allow simultaneously achieving two objectives: environ-
mental protection and economic benefit [2]. The waste production
increases [3] dramatically on going from bulk to fine chemicals and
pharmaceuticals. That is because of, the latter production involves,
both multistep and complex syntheses as the use of stoichiometric
reagents, rather than catalytic methodologies. Catalysis is of crucial
importance for chemical industry and particularly nowadays in the
field of fine chemicals. In heterogeneous catalysis, carbon materi-
als have been employed [4,5] because they can be used as catalysts
directly, and moreover, they can satisfy most of the properties
desired for a suitable support. It has been shown that although
the surface area and the shape of carbon porosity may be very
important in the preparation and properties of the corresponding
catalysts, the role of carbon surface chemistry is also extremely
important.
2
2
Imidazolium-based ionic liquids are becoming increasingly pop-
ular as catalysts and solvents [11–15]. Ionic liquids offer several
beneficial attributes as fixed charge, potential as green solvents,
relatively high thermal stability and large spectral transparency
[16]. Water-miscible ionic liquids can [17] enhance solubility of
substrates or products in biocatalysis with the advantage of not
inactivating enzymes. Also, ionic liquids are now applied [18] in
other scientific fields as electrochemistry, nanotechnology and
industrial processes.
In the present work, our goal is to study the use of acidic car-
bon materials as catalysts in the N-alkylation of imidazole. These
catalysts have been employed in other reactions as epoxide ring-
opening [19,20], ester synthesis [21,22] and acetals synthesis [23]
under mild conditions. Acidic activated carbons have been used
in catalysis as catalyst supports [24,25] and oxidation/reduction
∗ Corresponding author. Tel.: +34 924289300; fax: +34 924289404.
E-mail address: carlosdv@unex.es (C.J. Durán-Valle).
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.12.021

C.J. Durán-Valle et al. / Catalysis Today 187 (2012) 108–114
109
Table 1
Table 2
Catalysts preparation method and yield.
Elemental composition of catalysts.
Catalyst
Raw
Preparation (mixture)
Yield (wt%)
Elemental composition (wt%)
material
Catalyst
RX3
N
SN
S
C
H
N
S
Odiff
RX-3
N
SN
S
–
Pristine carbon, not treated
HNO3, 69%
Sulphonitric mixture 1:1 vol
H2SO4, 98%
–
89.05
1.36
0.59
1.10
0.98
0.50
1.02
0.79
0.51
0.47
0.31
0.27
1.09
8.62
RX-3
RX-3
RX-3
98.00
99.42
102.52
90.08
82.33
89.19
8.01
15.51
8.23
catalysts [26,27] but rarely [19–23] in the synthesis of fine
chemicals.
[10], that in the conditions of this work, the reaction is neither under
external nor internal diffusion control.
2.4. Products characterization
2
. Materials and methods
The products obtained in the catalysed reactions were analysed
2
.1. Catalyst preparation
in an Agilent 6890 GC/MS. The products of reaction of imidazole
1
with 1-bromobutane and 2-bromobutane were also studied by
H
An activated carbon Norit RX-3 has been employed as a pris-
NMR and 13C NMR on a Bruker 400 AC/PC spectrometer.
tine carbon (named as RX3). The acidic catalysts (N, SN and S [28])
were prepared by treating the carbon with mineral acids (a mixture
of 20 mL of acids with 1 g of activated carbon) at room temper-
ature and stirring for 90 min (Table 1). Later, the catalysts were
filtered, washed with water until a constant pH was reached and
then, dried for 24 h at 383 K. Additionally, the catalyst S was washed
out with distilled water, until no sulphate ions were detected (test
with BaCl2 solution).
3. Results and discussion
3.1. Catalyst characterization
The catalysts were obtained with a yield near 100% (Table 1).
The elemental composition changes depend on mineral acid
used (Table 2). In all cases, hydrogen content decreases. This can be
due to the loss of aliphatic chains (with higher H/C rate than aro-
matic structures) more reactive with oxidant acids than aromatics
or to the substitution of H atoms by heteroatoms. The nitrogen per-
cent increases when nitric acid is used (catalyst N and SN), and the
sulphur percent increases only in the preparation of catalyst S.
Reaction with sulphuric acid is a traditional sulphonation
method of aromatic compounds. Titration with BaCl2 of the water
showed that when a constant pH was reached, the sulphur lix-
2.2. Catalyst characterization methods
Elemental analysis (C, H, N, S, O) was carried out using a LECO
CHMS-932 elemental analyser. C, H, N and S were analysed, and
the difference was assigned to ash (proximate analysis) and oxygen
content. Proximate analysis (fixed carbon, volatile matter, ash and
humidity) was determined as described previously [29]. The points
of zero charge (PZC) values were determined using the method pro-
posed by Valente Nabais and Carrott [30]. Specific surface areas of
the carbon samples were determined by N2 adsorption isotherms
at 77 Kin a Quantachrome Autosorb-1, applying the BET method.
X-ray photoelectron spectroscopy (XPS) analyses were performed
with a XSAM800 X-ray spectrometer, operated in the fixed analyser
transmission (FAT) mode, with a pass energy of 20 eV, a power of
−
8
iviated in 24 h was only the 10
of all sulphur contained in
the catalyst. Such strong bond can be explained by formation
of sulphonic groups in catalyst S. Otherwise, the reaction with
sulphonitric mixture is a typical method to nitration of organic
compounds, but do not yield sulphonic groups. Treatment with
nitric acid generally produces only an oxidation. Such oxidation
is produced by the other acidic reagents too. So, some authors
1
30 W and using a non-monochromatic radiation from Mg anode
[
26,31–33] detect oxidation but no nitration in the reaction of
carbons with nitric acid; but other authors have described nitro
NO ) groups detected by FTIR [34] or XPS [35]. Also, Papirer and
(
main hꢀ = 1486.6 eV). Spectra were collected and stored in 300
channels with a step of 0.1 eV, and 60 s of acquisition by sweep,
using a Sun SPARC Station 4 with Vision software (Kratos). The
curve fitting (through the freeware XPSPeak 4.1) for component
peaks was carried out with a non-linear least-squares algorithm
using a mixture of Gaussian and Lorentzian peak. FTIR spectra
were registered from a disc, prepared by mixing 0.8 mg of acti-
vated carbon with 400 mg of KBr, and then pressing the resulting
mixture successively at 5 and 10 tonnes cm for 5 min under vac-
uum. Water was removed from the disc by oven-drying at 383 K for
a time longer than 24 h. Using a Perkin-Elmer 1720 spectrometer,
(
2
Guyon [36] in a nitration with nitronium tetrafluoroborate con-
clude that only some carbons are nitrated. Thus, it is difficult to say
if nitro groups are formed in catalyst N or not.
Proximate analysis (Table 3) shows that the content in volatile
matter increases. This change is because the oxidation, sulphona-
tion and perhaps nitration give more labile functional groups than
in Norit RX3. The greater effect is caused by the sulphonitric mix-
ture that also produces the greater oxygen content, Table 2. Also, the
content of mineral matter (ash) in the carbons decreases. Acid treat-
ment is a known leaching method for minerals in carbonaceous
material.
−2
−
1
the FTIR spectrum was recorded between 4000 and 400 cm .From
the spectrum of each sample, the reference spectrum of a similar
thickness pure KBr disc was subtracted.
Table 3
2.3. Reaction procedure
Proximate analysis of catalyst.
Proximate analysis (d.b.) (wt%)
Imidazole (5 mmol, 0.3404 g) and alkylating agent (15 mmol; 1-
bromobutane, 1.61 mL; 2-bromobutane, 1.61 mL; 1-chlorohexane,
.92 mL) were mixed in a 10 mL Pyrex flask, generally without any
solvent. The flask was suspended in an oil bath at the reaction tem-
perature, and stirred at 1200 rpm. Then, the corresponding catalyst
was added and the reaction time started. It has been demonstrated
Catalyst
Fixed carbon
Volatile matter
Ash
1
RX3
N
SN
S
92.35
87.26
84.70
88.18
2.46
9.96
12.38
7.92
5.19
2.77
2.90
3.89

1
10
C.J. Durán-Valle et al. / Catalysis Today 187 (2012) 108–114
Table 4
XPS peak positions, assignment and atomic percentage.
Peak
C 1s
RX3
N
SN
S
assignment
BE (eV)
%at.
BE (eV)
%at.
BE (eV)
%at.
BE (eV)
%at.
284.2
285
31.9
2.0
12.6
33.5
4.5
4.9
1.5
2.3
6.6
Aliphatic and graphitic carbon
2
2
2
2
2
85
86.6
88
89.5
91.3
58.2
13.1
10.6
5.6
5.0
2.3
2.8
2
284.8
286.2
287.7
289.4
291.4
532.1
534.2
536
41.5
12.9
21.6
5.4
4.8
4.5
4.8
3.8
0.3
0.2
284.9
286.4
287.8
289.2
291.1
531.8
534.2
536.4
47.0
14.4
15.7
9.0
4.6
2.8
285.6
287.5
289.4
291.4
531.2
533
C
C
O
O
O, C
C O
N
␲–␲ + more oxidized C
O 1s
N 1s
S 2p
531.7
C
C
O
O-
5
5
33.8
36.1
3.0
3.0
535.3
Ocluded H2O
400.4
402.3
406.0
N
N
C
+
401.9
0.4
405.7
0.2
0.2
NOx
168.3
1
0.24
0.12
90.8
8.8
0.0
0.4
Oxidized sulphur
69.5
Total C
Total O
Total N
Total S
92.5
7.1
0.4
89.4
10.4
0.2
86.2
13.1
0.7
0.0
0.0
0.0
1700 cm 1 (C O) increase, especially in SN catalyst. In catalyst N, a
−
Results of analysis by X-ray photoelectron spectroscopy and
assignment of fitted peaks [37] as well as the overall are given
in Table 4. Despite giving an information much more specific of
the extreme surface (10 nm in depth), the overall composition
follows the same trend both the elemental and the proximate anal-
yses (Tables 2 and 3). Some exceptions in the trend have been
detected related with the different analysed zone. The character-
istics binding energies (BE) of activated carbons in C 1s spectra
are observed in all samples. The peaks centred at 284.2–285.0 eV
are dominant in all the materials and correspond to a mixture
of graphitic (BE = 284.3 ± 0.3 eV) and aliphatic (BE = 285.0 ± 0.2 eV)
−
1
narrow band at 1380 cm can be assigned to phenol groups [41].
Again, the analysed zones by infrared spectroscopy and XPS are
not the same (bulk and surface, respectively). This fact can be used
for explaining in catalyst N, the presence of phenols not present in
pristine carbon, but a decrease of peaks C 1s and O 1s related to
C
O. The C O bond can be due to ether, ester or phenols groups.
In catalyst N preparation, on the surface the process of destruction
of ether and ester groups must be larger than formation of phenols
groups. So, the number of former decreases with respect to pris-
tine carbon, as XPS showed. Likewise, XPS data related with ester
(or lactone) groups (O 1s 531.7 eV and C 1s 289.5 eV peaks) sup-
port the decrease of ester groups. The increase of the broad band
[
38]. However, their intensity decreases with the acidic treatment,
especially when the nitric acid is used. The decrease in this band
can be explained by the oxidation of the pristine carbon Norit RX3,
and it agrees with the increase in the oxygen amount also ver-
ified though the elemental composition and proximate analysis.
Therefore, in C 1s region, the peaks corresponding to oxidized car-
bon functional groups (285.8–289.5 eV) are more intense for all
catalysts in particular for N and SN catalysts. The main oxidized
functional group formed is the carbonyl (C O) (287.7 ± 0.3 eV),
meanwhile the ether/alcohol peak (286.0–286.7 eV) and carboxyl
band (289.2–289.5 eV) increase slightly. The O 1s XPS spectra reveal
a higher content of oxygen, but not remarkable differences between
samples. These differences exist in N 1s and S 2p spectra. Reduced
form of protonated nitrogen (401.1–401.9 eV) exists in the raw
material and it is also detected in SN catalyst. Likewise, the oxidized
form of nitrogen (405.3–406.1 eV) as in nitro groups [35] is detected
in N and SN catalyst, but not in S sample. And the S 2p region only
displays sulphur in the catalyst S. The bond energy (168.3–169.5 eV)
is characteristic of sulphonic or sulphate groups [35].
The previous named exceptions in the trend (for oxygen and
nitrogen contents) can be explained if we take into account the
different analysed zones by elemental analysis and XPS, bulk and
surface, respectively. The larger nitrogen content obtained by ele-
mental analysis than by XPS must be due to residual nitric acid
or reduced forms of nitrogen strongly adsorbed, mainly placed
inside the sample and not on the surface. The lower oxygen con-
tent obtained by elemental analysis than by XPS can be due to a
removal of oxygenated functional groups existing in the pristine
carbon. This process is parallel to the oxidation produced by nitric
acid.
Assignation of FTIR bands is based in both own and other authors
papers [35,39,40]. FTIR spectra (Fig. 1) show the oxidation produced
−
1
by the acid treatment. So, the bands about 3400 cm (O H) and
Fig. 1. FTIR spectra of activated carbons. All spectra are in the same scale.

C.J. Durán-Valle et al. / Catalysis Today 187 (2012) 108–114
111
Table 5
100
PZC and BET specific surface area of catalysts.
90
Catalyst
PZC
BET (m2
g
−1
)
80
70
60
50
40
30
20
RX3
N
SN
S
7.09
3.85
3.60
2.85
1306
1178
1078
1114
−
1
10
0
near 1550 cm could be due to the presence of nitro groups. Nev-
ertheless, an alternative explanation is the loss of symmetry due
to oxidation (aromatic C C bonds). It corresponds with the lack-
No cat.
RX-3
N
SN
S
Catalyst
−
1
ing N catalyst spectrum of a band near 1350 cm [34] assigned to
nitro groups. In catalyst SN, the spectrum is similar but with more
intense bands at 3400 cm (O H) and 1700 cm (C O), as can be
expected of a more oxidized sample (Tables 2 and 4). A double band
at 1383 and 1365 cm can be assigned to nitro groups and phe-
nols. A band near to 1575 cm would confirm the presence of NO2
group in catalyst SN. The band with a strong increase and two max-
imum, 1563 and 1575 cm , is very similar to the expected. So we
can conclude that the probabilities of nitration with sulphonitric
mixture are high. Finally, the catalyst S showed in its FTIR spec-
trum oxidation, but the low resolution, typical of these materials,
does not allow observe band clearly assignable to sulphonic groups.
Only the displacement of the band near 1150 cm and the change
in form can indicate the presence of sulphonic groups. In an earlier
paper [39] bands related to the introduction of sulphonic groups
can be observed in the FTIR spectra of carbon fibres treated with
sulphuric acid.
Fig. 2. Influence of catalyst. Yields (%). Imidazole + 2-bromobutane, 333 K, 180 min,
0.040 g catalyst.
−1
−1
−1
in elemental composition (carbon increases, hydrogen decreases
and oxygen content is similar) for catalyst N and S indicate that
no significant destruction of pore walls occurs. It is more probable
the other option: fixation of oxygen groups at the entrance of the
pores reducing the accesibility. So, the more oxidized catalyst (SN)
has the lower specific surface area (Table 5).
−1
−1
3
.2. Products structure
−
1
The product obtained from 1-bromobutane and imidazole
Scheme 1) is an imidazolium ionic liquid, N,N-di-n-butyl-
imidazole bromide (1). The structure of this compound was
(
determined by mass spectra [EI-MS: m/z 124(59%), 97(100%),
The acidity of catalyst was evaluated by PZC measurements
Table 5). The use of sulphuric acid (98%) yields the more acidic cat-
2(97%), 81(94%), 68(32%), 55 (53%)] and 1H NMR and C NMR
13
8
(
experiences (Tables 7 and 8). The value of shift and multiplic-
ity in 1H NMR spectrum agree with the proposed structure. It is
important to note that the intensity of signal (integer) of aliphatic
alyst, and the use of nitric acid (69%) and sulphonitric mixture yield
catalyst with similar PZC. These results can be explained consider-
ing the functional groups formed in the preparation of catalyst. So,
the hydroxyls are acidic but weaker than carboxyls. Both are formed
in the oxidation of Norit RX-3 with mineral acids. In Table 6, the
pKa of some simple organic acids [42] is shown. If nitro groups are
formed (and this is more probable in the SN than in the other cata-
lysts), the acidity increases. But the more acidic functional group is
sulphonic acid, which only can be formed in catalyst S. This is the
reason because the catalyst S with lower amount of volatile matter
1
hydrogens ( H NMR) corresponds with two lateral chains, and that
the shift (10.4 ppm) of the hydrogen near to both nitrogen atoms is
typical of ionic liquids [45–47]. The shift and type of signals in 13
NMR spectrum also agree with the proposed structure.
C
The products obtained from imidazole with 2-bromobutane
and 1-chlorohexane are similar N-alkylimidazoles, N-sec-
butyl-imidazole (2) and N-n-hexyl-imidazole (3), respectively
Scheme 2). The structures of these compounds were determined
by mass spectra [(2) EI-MS: m/z 124(86%), 95(97%), 81(10%),
(
(
Table 3) and oxygen (Table 2), both indicators of oxidation, is the
most acidic catalyst. Some authors describe [26] that the nitric acid
treatment yield a more acidic carbon that the sulphuric acid treat-
ment, but with lower activity as catalyst. These different properties
can be explained by the use of a different acid concentration that
in this work.
The specific surface area (Table 5) decreases with the treatment,
but does not influence the activity of catalysts. Li et al. have shown
25] that the specific surface area increases in the treatment with
low concentration and decreases with high concentration of nitric
acid, and Teng et al. [27] observe a increase in specific surface area
with diluted (2 N) nitric and sulphuric acids. The decrease of the sur-
face has been assigned to a preferential fixation of oxygen groups
at the entrance of the pores [43] or to the destruction of the pore
walls by the oxidizing treatment [44]. But the high yield in the
preparation of the catalyst (Table 1) and the variations (Table 2)
6
8
(
8(100%), 57(14%)] [(3) EI-MS: m/z 152(16%), 125(33%), 96(26%),
2(53%), 68(100%), 55(19%)]. Also, 1H NMR and C NMR spectra
13
Tables 7 and 8) were registered for compound (2) and the results
obtained agree with the structure proposed. It must be noted that,
in contrast to the compound (1): (a) integral in 1H NMR spectrum
of aliphatic hydrogen corresponds to a single lateral chain; (b)
shift of the hydrogen near to both nitrogen atoms (7.72 ppm)
[
differs from the same value in compound (1), and it is typical
1
of N-alkylheterocycles; (c) the existence of two signals in
H
NMR and 13C NMR spectra for hydrogen and carbon, respectively,
both in positions 4 and 5 in the ring, indicates an asymetrical
compound.
Calvino Casilda et al. [10] describe the production of anionic
liquid using an acidic catalyst and simple alkylation of imidazole
or imidazole derivatives) using a basic catalyst [6–8,10].
(
Table 6
3.3. Catalysed reactions
pKa of some organic compounds.
Compound
pKa
Under our experimental conditions, N-substituted and N,N-
substituted imidazoles are selectively obtained.
In order to study the effect of different acid solid catalysts, imi-
dazole was alkylated with 2-bromobutane using 0.040 g of them at
Phenol
Benzoic acid
p-Nitrophenol
p-Nitrobenzoic acid
Benzosulphonic acid
9.89
4.19
7.15
3.41
0.70
3
33 K during 180 min. From the results, Fig. 2, it can be concluded
that the use as catalyst of activated carbons, with or without acid

1
12
C.J. Durán-Valle et al. / Catalysis Today 187 (2012) 108–114
Br-
cat.
+
2
Br
N
N
N
N
H
(
1)
Scheme 1. Synthesis of N,N-di-n-butyl-imidazolium bromide.
Table 7
1
H NMR data of compounds (1) and (2).
Compound (1)
Compound (2)
Assignment
Shift (ppm)
Ma
Integer
Assignment
Shift (ppm)
Ma
Integer
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
Hring positions 4 and 5
N
N
N
N
0.97
1.36
1.82
4.38
7.43
10.4
t
6
4
4
4
2
1
CH3 CH2 CH(N) CH3
CH3 CH2 CH(N) CH3
CH3 CH2 CH(N) CH3
CH3 CH2 CH(N) CH3
Hring positions 4 and 5
0.83
1.48
1.78
4.17
7.09/7.02
7.72
t
3
3
2
1
1 + 1
1
m
q
t
d
s
d
m
m
s
N
CH
N
N
CH
N
s
a
Multiplicity: s – singlet, d – doublet, t – triplet, q – quintet, m – multiplet.
Table 8
1
3C NMR data of compound (1) and (2).
Compound (1)
Assignement
Compound (2)
Assignement
Shift (ppm)
Typea
Shift (ppm)
Type1
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
CH3 CH2 CH2 CH2
N
N
N
N
13.37
19.41
32.54
49.83
121.98
136.47
CH3
CH2
CH2
CH2
CH
CH3 CH2 CH(N) CH3
CH3 CH2 CH(N) CH3
CH3 CH2 CH(N) CH3
CH3 CH2 CH(N) CH3
9.70
20.80
29.84
55.04
116.70/126.50
135.23
CH3
CH3
CH2
CH
CH
CH
N
N
CH CH N (ring)
CH
N
N
CH CH N (ring)
CH
N
CH
N
a
DEPT experiment.
treatments, enhances the yield of the process. The order of activity
of catalyst is S > SN ≥ N > pristine carbon (RX3). This trend indicates
that the activity increases with the acidity of them, Table 5. High
yields (between 100 and 81%) are obtained when acid catalysts are
used.
In order to study the effect of catalyst amount, two reactions
have been studied. The first one, imidazole was alkylated with 1-
bromobutane. For conditions of reaction of 333 K, 15 min and an
increase of amount of catalyst from 0.005 to 0.020 g, Fig. 3, con-
version factors increase 2, 1.6 and 1.2 for N, SN and S catalyst,
respectively. The increase of four times of the catalyst amount is
significant for all catalysts.
100
90
80
70
60
50
40
30
20
10
0
0.005 g
0.020 g
No cat.
RX3
N
SN
S
Catalyst
The second one, imidazole was alkylated with 2-bromobutane.
The conditions of reaction are 333 K, 180 min and amounts of cata-
lyst of 0.020 and 0.040 g, Fig. 4. The increases of reaction time and
amount of catalyst with respect to the first reaction are because
the 2-bromobutane is less reactive than 1-bromobutane for imi-
dazole by steric effect of the former. Mechanism reaction SN2,
sensible to steric effects, can be proposed in this reaction. The
increase of two times of amount of catalyst produces conversion
Fig. 3. Effect of catalyst quantity (0.005 and 0.020 g). Yields (%). Imidazole + 1-
bromobutane, 333 K, 15 min.
factor increase of 1.5, 1.4 and 1.2 for N, SN and S catalyst, respec-
tively. From these results can be concluded that yields increase
with mass of catalyst. Nevertheless, a saving of catalyst S can
get in both reactions because the conversion factors are of small
Br
cat.
+
N
N
N
N
N
N
N
N
H
H
(
2)
cat.
+
Cl
(3)
Scheme 2. Synthesis of N-sec-butyl-imidazole (2) and N-n-hexyl-imidazole (3).

C.J. Durán-Valle et al. / Catalysis Today 187 (2012) 108–114
113
100
2-bromobutane
1-bromobutane
90
80
70
60
50
40
30
20
10
0
1
00
9
8
7
6
5
4
0
0
0
0
0
0
0.020 g
.040 g
0
30
20
10
0
RX-3
N
SN
S
Catalyst
N
SN
S
Fig. 4. Effect of catalyst quantity (0.020 and 0.040 g). Yields (%). Imidazole + 2-
bromobutane, 333 K, 180 min.
Catalyst
Fig. 6. Yields of N,N-di-n-butyl-imidazolium bromide(1) and N-sec-butyl-
imidazole(2), 333 K, 15 min, with catalyst (0.020 g).
significance. Saving of N and SN catalysts can get just only for reac-
tion of 2-bromobutane.
The effect of alkylating agents on the type of reaction was stud-
ied using three alkylating agents: 1-bromobutane, 2-bromobutane
and 1-chlorohexane, without or with catalyst. From the results
of the reactions of bromoalkanes without catalyst, Fig. 5, it can
be concluded that the reaction go faster for 1-bromobutane than
for 2-bromobutane, this is because the aforementioned less reac-
tivity of the second alkylating agent. The rate of reaction for the
first one is large with a 60% of conversion, at the first 30 min.
Later, till 150 min, the rate decrease. It can be due to a drastic
decrease of reactive concentrations at the beginning of the reaction
because the high reactivity of the 1-bromobutane. Alkylation with
1-chlorohexane
2-bromobutane
1
00
9
8
7
6
0
0
0
0
50
40
30
2
0
0
0
1
-chlorohexane results has not been shown because conversion has
1
not been detected.
Using catalyst, 0.020 g, at 333 K and 15 min, Fig. 6, for 1-
bromobutane reaction, the time of conversion 100% decreases, from
more than 150 min, Fig. 5, to 15 min. For 2-bromobutane reaction, at
No cat.
RX-3
N
SN
S
Catalyst
1
5 min, yield is lower (12% with catalyst, Fig. 6) but better compared
Fig. 7. Influence of the alkylating agent. Yields of N-sec-butyl-imidazole (2) and
N-n-hexyl-imidazole (3), 333 K, 0.040 g cat., 180 min.
to that without catalyst (3%, Fig. 5).
In the same way, imidazole was alkylated with 2-bromobutane
and 1-chlorohexane. Mass spectra of the last reaction product con-
firm that N-n-hexyl-imidazole is the only product formed, so it
is selectively 100% obtained. From the results of both reactions
of the former. The experimental results show that steric effect at
2
-bromobutane is not significant for reaction compared with bond
strength. The order of activity of catalyst found for these two reac-
tions is S > SN ≥ N like 1-bromobutane, Fig. 3. The reactivity order in
haloalkanes, as can be observed in Figs. 5–7 is 1-bromobutane > 2-
bromobutane > 1-chlorohexane.
The effect of reaction temperature has been studied during the
alkylation with 2-bromobutane using 0.040 g of the different types
of catalyst, during 15 min and temperatures of 333 and 353 K, Fig. 8.
As this figure shows, the conversion, lower than 100%, enhances
with the temperature and the acidity of the catalyst.
(
0.040 g of catalyst, 333 K and 180 min), Fig. 7, can be concluded
that the reactivity of 2-bromobutane is larger than 1-chlorohexane
because a smaller bond strength C Br with respect to C Cl [42].
The larger difference of energy between C Br orbitals bond than
Cl ones produces a lower overlapping orbitals and bond strength
1-bromobutane
2-bromobutane
100
90
80
70
60
50
40
30
20
10
0
7
0
60
50
40
30
20
10
0
333 K
353 K
0
50
100
150
N
SN
S
t, min
Catalyst
Fig. 5. Yields of N,N-di-n-butyl-imidazolium bromide (1)and N-sec-butyl-
Fig. 8. Effect of temperature. Yields (%). Imidazole + 2-bromobutane, 333 K and
imidazole(2), 333 K without catalyst.
353 K, 0.040 g cat., 30 min.

1
14
C.J. Durán-Valle et al. / Catalysis Today 187 (2012) 108–114
with dissolvent
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(
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Acidic carbons were found to be very efficient and selective cat-
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enhanced by the acid mineral treatment. The best catalyst is the
one treated with sulphuric acid (S). The acidity of catalyst is more
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liquids and N-alkyl-heterocycles and, in general, for the produc-
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This work has been supported by Spanish Minister of Science
and Innovation (Project CTM2010-14883/TECNO) and Junta de
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