Catalytic activity of the nanoporous MCM-41 surface for the Paal-Knorr pyrrole cyclocondensation

Article abstract of DOI:10.1515/znb-2014-0259

The investigation of different oxide surfaces revealed that nanoporous silica (MCM-41) had the best catalytic activity for Paal-Knorr pyrrole synthesis. Despite the same composition, MCM-41 proved to be more effective than SiO₂ itself, probably due to a significantly higher surface area of the SiO₂ nanopores. The important features of this "clean" solvent-free protocol are the ease of recovery and the reuse of the catalyst for several cycles, operational simplicity, and easy product isolation and purification.

Full text of DOI:10.1515/znb-2014-0259

Z. Naturforsch. 2015; aop
Kioumars Aghapoor*, Mostafa M. Amini, Khosrow Jadidi, Farshid Mohsenzadeh
and Hossein Reza Darabi
Catalytic activity of the nanoporous MCM-41 surface
for the Paal–Knorr pyrrole cyclocondensation
Abstract: The investigation of different oxide surfaces the reaction occurs between the surface of these solid
revealed that nanoporous silica (MCM-41) had the best catalysts and reactants, scientific research aims to have a
catalytic activity for Paal–Knorr pyrrole synthesis. Despite better understanding of these kinds of catalyzed chemical
the same composition, MCM-41 proved to be more effective processes [3].
than SiO itself, probably due to a significantly higher sur-
Pyrroles and their derivatives constitute an important
2
face area of the SiO nanopores. The important features of class of heterocyclic compounds. The biological and medic-
2
this “clean” solvent-free protocol are the ease of recovery inal properties of these compounds have been well estab-
and the reuse of the catalyst for several cycles, operational lished in the literature [4–6]. Several methodologies have
simplicity, and easy product isolation and purification.
already been reported to construct the pyrrole skeleton. The
Paal–Knorr reaction is by far the simplest method employed
Keywords: MCM-41; Paal–Knorr reaction; pyrrole; recycla- for the condensation of 1,4-dicarbonyl compounds with
ble catalyst; thermally stable surface.
primary amines in order to generate pyrroles [7–9].
Numerous catalysts and protocols have already been
implemented to synthesize 2,5-disubstituted pyrroles, e.g.,
Lewis acids [10–14], heterogeneous solids having active
acidic sites [15–21], ionic liquids [22, 23], deep eutectic
solvents [24], an enzyme [25], and organocatalysts [26, 27].
Nevertheless, from the standpoint of synthesis, some of
these procedures have restricted activity and suffer from
operational and practical problems; they are costly to
prepare and time consuming (causing high labor time).
The sensitivity of some substituted substrates to the use
of Lewis acid catalysts is also considered as a serious limi-
tation. Therefore, the development of a mild and greener
alternative remains essential in terms of sustainability for
the Paal–Knorr synthesis of pyrroles.
Since the discovery of MCM-41 in 1992 [28, 29], a pleth-
ora of studies have been carried out on these mesoporous
materials. These materials are well characterized and
their physical properties are usually provided by X-ray
diffraction (XRD) and nitrogen physisorption studies. The
prominent features of MCM-41 reside in their large specific
surface area and pore volume, uniform pore diameter, and
high thermal stability. Due to these interesting character-
istics, the scientific research in the preparation and use
of MCM-41 (absorbent or catalyst) as a host for incorpo-
ration of different metals or organic moieties in chemical
processes has increased exponentially [30–34]. However,
the use of MCM-41 as a catalyst (by itself) has not received
much attention.
DOI 10.1515/znb-2014-0259
Received November 23, 2014; accepted February 5, 2015
1
Introduction
The importance of heterogeneous catalysis has been well
established for many processes in chemical industry [1, 2].
Indeed these heterogeneous catalysts play a key role in
the development of environmentally benign processes
and have several advantages such as easy handling and
separation from reactants, improved recovery in order
to be recycled, and cost-effectiveness in large scale. As
*
Corresponding author: Kioumars Aghapoor, Faculty of Science,
Department of Chemistry, Shahid Beheshti University, G. C. Tehran,
1
9839-63113, Islamic Republic of Iran; and Applied Chemicals
Synthesis Laboratory, Chemistry and Chemical Engineering
Research Center of Iran, Pajoohesh Blvd., km 17, Karaj Hwy, Tehran,
14968-13151, Islamic Republic of Iran, Tel.: +98-21-44787720,
Fax: +98-21-44787762, E-mail: k.aghapoor@yahoo.com or
aghapoor@ccerci.ac.ir
Mostafa M. Amini and Khosrow Jadidi: Faculty of Science,
Department of Chemistry, Shahid Beheshti University, G.C. Tehran,
1
9839-63113, Islamic Republic of Iran
Farshid Mohsenzadeh and Hossein Reza Darabi: Applied Chemicals
Synthesis Laboratory, Chemistry and Chemical Engineering
Research Center of Iran, Pajoohesh Blvd., km 17, Karaj Hwy, Tehran,
Recently, we have observed the remarkable inherent
catalytic activity of MCM-41 mesoporous materials for the
14968-13151, Islamic Republic of Iran
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2ꢀ ꢀK. Aghapoor et al.: Catalytic activity of the nanoporous MCM-41 surface
preparation of aza heterocyclic compounds, e.g., polyhyd- classified as type IV according to the IUPAC convention,
roquinolines [35], benzoxazoles [36], and tetra-substituted and is typical of mesoporous materials [42].
imidazoles [37].
Taking into account the importance of MCM-41 mes-
oporous molecular sieves as potential solid catalysts for 2.2 Catalyst screening of various mineral
the synthesis of N-containing heterocycles and our interest
in the development of greener protocols for N-heterocyclic
oxides
condensation [38–40], we decided to explore the viability Initially, we examined the condensation of hexane-2,5-di-
of using MCM-41 to promote the Paal–Knorr pyrrole syn- one (1.2 mmol) with 4-bromoaniline (1 mmol) to evalu-
thesis (Scheme 1) that, to the best of our knowledge, has ate the efficiency of various mineral oxides as catalysts
not been reported yet.
(Table 1). The reaction was screened under solvent-free
conditions at room temperature within 1 h.
Among these solid surfaces, Fe O , ZnO, and basic,
3
4
neutral, and acidic Al O showed poor catalytic activi-
2
3
2
Results and discussion
ties (Table 1, entries 7–11). TiO and SiO gave moderate
2
2
yields, whereas MCM-41 catalyzed the reaction to afford
the desired product in good yield.
The mesoporous MCM-41 material was prepared through
hydrothermal treatment following a methodology estab-
lished by Gonçalves and co-workers [41]. The prepared
material was characterized by XRD and nitrogen phys-
isorption measurements.
2
.3 Catalyst screening of various MCM-41
materials
Various MCM-41 surfaces prepared using the conventional
hydrothermal method and microwave heating were tested
for the same reaction (Table 2). The reaction in the pres-
ence of various MCM-41 materials resulted in 57–65 % yield
of product. However, a 10–18 % decrease was observed
for the sample prepared using microwaves at 480 W for
2
.1 Characterization of MCM-41
A small-angle powder XRD pattern of MCM-41 is presented
in Fig. 1. The XRD pattern of MCM-41 shows four reflec-
tions: a very intense peak (1 0 0) and three additional
high-order peaks (1 1 0), (2 0 0), and (2 1 0) with lower
intensities. This pattern is characteristic of a hexagonal
pore structure.
2
0 min (Table 2, entry 8). Porosimetry data showed that
the catalyst had a high surface area in the range of 1054–
2
−1
1
275 m g . Nevertheless, it seems that the yield of product
Nitrogen sorption isotherms at 77 K for the prepared
sample are presented in Fig. 2. The isotherm can be
is a function of the surface area of the catalyst, and below
2
−1
1
100 m g , a notable decrease in the catalytic activity is
observed (Table 2, entries 1–8).
O
Ar
N
MCM-41
+
Ar NH2
neat/70 °C
2.4 Optimization of the model reaction
O
To investigate the role of mesoporous MCM-41, the same
reaction was carried out in the absence or presence of
a catalyst at different temperatures. In the absence of
MCM-41 under ambient (i.e., room temperature) and
thermal conditions (i.e., 50 °C and 70 °C), the uncata-
lyzed reaction afforded the product 1j in 38 %, 70 %, and
Scheme 1:ꢁCondensation of hexane-2,5-dione with amines catalyzed
by MCM-41.
82 % yield, respectively (Table 3, entries 1–3). When the
same reaction was catalyzed by MCM-41, the reaction
yield sharply increased under various conditions (Table 3,
entries 4–6); e.g., in the presence of MCM-41 as a catalyst,
the product 1j was obtained in 65 % yield (instead of 38 %)
at room temperature. Accordingly, the MCM-41 catalyzed
reaction at 70 °C after 1 h exhibited the highest conversion
1
2
3
4
5
2
6
-θ (°)
7
8
9
10
Fig. 1:ꢁSmall-angle powder XRD pattern of MCM-41.
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K. Aghapoor et al.: Catalytic activity of the nanoporous MCM-41 surfaceꢀꢂꢁꢁꢁꢀ3
900
800
700
600
500
400
300
200
100
0
ADS
DES
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P0
Fig. 2:ꢁN adsorption–desorption isotherm of MCM-41.
2
Table 1:ꢁCatalyst screening of various mineral oxides for the synthesis of 1j under neat conditions at room temperature within 1 h.
O
NH2
Catalyst
+
Br
N
1
h/RT
Br
neat
O
0
.14 g (1.2 mmol)
0.17 g (1 mmol)
1j
Entryꢀ
Catalyst, g
ꢀ
Time, minꢀ
Conversion, %a
ꢃ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
–
ꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢇꢈ
ꢅꢆ
ꢅꢋ
ꢅꢃ
ꢊꢉ
ꢊꢃ
ꢃꢋ
ꢃꢈ
ꢃꢆ
ꢃꢅ
ꢉꢉ
b
ꢉ
MCM-ꢊꢃ-HT (ꢆ.ꢆꢉ) ꢄ
MCM-ꢊꢃ-HT (ꢆ.ꢆꢊ) ꢄ
MCM-ꢊꢃ-HT (ꢆ.ꢆꢅ) ꢄ
ꢇ
ꢊ
ꢋ
SiO (ꢆ.ꢆꢊ)
ꢄ
ꢉ
ꢅ
TiO anatase (ꢆ.ꢆꢊ)ꢄ
ꢉ
ꢌ
Al O neutral (ꢆ.ꢆꢊ)ꢄ
ꢉ
ꢇ
ꢈ
Al O acidic (ꢆ.ꢆꢊ) ꢄ
ꢉ ꢇ
ꢍ
Al O basic (ꢆ.ꢆꢊ)
ꢄ
ꢄ
ꢄ
ꢉ
ꢇ
ꢃꢆ
ꢃꢃ
ZnO (ꢆ.ꢆꢊ)
Fe O (ꢆ.ꢆꢊ)
ꢇ
ꢊ
a
Gas chromatography assay (%).
b
MCM-41 prepared using a hydrothermal method.
rate and afforded the product 1j quantitatively (Table 3, optimized conditions and the results are summarized in
entry 6).
Table 4, indicating that both electron-donating (such as
methoxy, methyl, and dimethyl) and electron-withdraw-
ing (such as bromo, chloro, dichloro, and nitro) substitu-
ents on the aromatic ring of aniline underwent smooth
reaction with hexane-2,5-dione giving excellent conver-
2
.5 Evaluation of the reaction scope
To expand the generality of this novel catalytic method, sions in most cases. However, an extended reaction time is
various aromatic primary amines were tested under the observed for nitro-substituted aniline derivatives (Table 4,
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4ꢀ ꢀK. Aghapoor et al.: Catalytic activity of the nanoporous MCM-41 surface
Table 2:ꢁCatalytic effect of various MCM-41 materials on the synthesis of 1j under neat conditions.
O
NH2
MCM-41 (0.04 g)
+
Br
N
1
h/RT
Br
neat
O
0
.14 g (1.2 mmol)
0.17 g (1 mmol)
1j
ꢁ
–ꢂ
ꢀ Conversion, %a
Entryꢀ
Catalyst
ꢀ
BET surface area, m g
ꢃ
ꢉ
ꢇ
ꢊ
ꢋ
ꢅ
ꢌ
ꢈ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
MCM-ꢊꢃ-HTb
ꢄ
ꢃꢉꢌꢋ
ꢃꢃꢋꢃ
ꢃꢃꢇꢌ
ꢃꢃꢉꢍ
ꢃꢃꢃꢍ
ꢃꢃꢃꢍ
ꢃꢃꢌꢌ
ꢃꢆꢋꢊ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢅꢋ
ꢅꢆ
ꢋꢈ
ꢋꢌ
ꢋꢌ
ꢋꢌ
ꢅꢆ
ꢊꢌ
c
MCM-ꢊꢃ-ꢇꢅꢆ W-ꢉꢆ min ꢄ
c
MCM-ꢊꢃ-ꢇꢅꢆ W-ꢊꢆ min ꢄ
c
MCM-ꢊꢃ-ꢇꢅꢆ W-ꢅꢆ min ꢄ
c
MCM-ꢊꢃ-ꢇꢅꢆ W-ꢍꢆ min ꢄ
c
MCM-ꢊꢃ-ꢇꢆꢆ W-ꢉꢆ min ꢄ
c
MCM-ꢊꢃ-ꢊꢉꢆ W-ꢉꢆ min ꢄ
c
MCM-ꢊꢃ-ꢊꢈꢆ W-ꢉꢆ min ꢄ
a
Gas chromatography assay (%).
b
MCM-41 prepared using a hydrothermal method.
MCM-41 prepared using microwave heating.
c
Table 3:ꢁTemperature effect on the synthesis of 1j under neat
conditions.
Table 4:ꢁSolvent-free synthesis of N-substituted pyrroles catalyzed
by MCM-41 at 70 °C.
O
Entryꢀ Catalyst
ꢀ
Temperature,ꢀ Time,ꢀ Conversion,
MCM-41 (0.04 g)
a
°C
min
%
+
Ar NH2
Ar
N
Neat/70°C
ꢃ
ꢉ
ꢇ
ꢊ
ꢋ
ꢅ
ꢌ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
–
–
–
ꢄ
ꢄ
ꢄ
ꢉꢋꢄ
ꢋꢆꢄ
ꢌꢆꢄ
ꢉꢋꢄ
ꢋꢆꢄ
ꢌꢆꢄ
ꢌꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢅꢆꢄ
ꢇꢆꢄ
ꢇꢈ
ꢌꢆ
ꢈꢉ
O
1
mmol
1
.2 mmol
b
MCM-ꢊꢃ-HT (ꢆ.ꢆꢊ)ꢄ
MCM-ꢊꢃ-HT (ꢆ.ꢆꢊ) ꢄ
MCM-ꢊꢃ-HT (ꢆ.ꢆꢊ) ꢄ
MCM-ꢊꢃ-HT (ꢆ.ꢆꢊ) ꢄ
ꢅꢋ Entryꢀ Ar
ꢀ
Time,ꢀ Conversion,ꢀ Yield,ꢀ M.p., °C
ꢍꢉ
ꢃꢆꢆ
ꢈꢈ
h
%
a
%
b
ꢂa ꢄ ꢊ-Methoxyphenyl
ꢂb ꢄ Phenyl
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢃꢄ
ꢃꢄ
ꢃꢆꢆꢄ
ꢃꢆꢆꢄ
ꢃꢆꢆꢄ
ꢃꢆꢆꢄ
ꢍꢌꢄ
ꢍꢋꢄ
ꢍꢇꢄ
ꢍꢊꢄ
ꢍꢇꢄ
ꢍꢃꢄ
ꢍꢉꢄ
ꢋꢅ–ꢋꢌ
ꢊꢈ–ꢋꢆ
ꢊꢊ–ꢊꢋ
Oil
a
Gas chromatography assay (%).
ꢂc ꢄ ꢊ-Methylphenyl
ꢂd ꢄ ꢇ-Methylphenyl
ꢂe ꢄ ꢉ-Methylphenyl
ꢃꢄ
b
MCM-41 prepared using a hydrothermal method.
ꢃꢄ
ꢃꢄ
Oil
ꢂf ꢄ ꢉ,ꢋ-Dimethylphenylꢄ
ꢂg ꢄ ꢊ-Nitrophenyl
ꢃꢄ
ꢃꢆꢆꢄ
Oil
ꢄ
ꢄ
ꢄ
ꢄ
ꢄ
ꢃ/ꢊꢄ
ꢃ/ꢊꢄ
ꢊꢄ
ꢃꢃ/ꢇꢋꢄ ꢌ/ꢉꢍꢄ ꢃꢊꢉ–ꢃꢊꢊ
ꢋꢊ/ꢈꢈꢄ ꢊꢈ/ꢈꢆꢄ
ꢊꢄ
ꢃꢆꢆꢄ
ꢃꢆꢆꢄ
ꢍꢅꢄ
entries 1g–1i) having a negative influence on the reaction ꢂh ꢄ ꢇ-Nitrophenyl
ꢈꢋ
–
ꢌꢋ
ꢂi
ꢂj
ꢄ ꢉ-Nitrophenyl
–ꢄ
ꢍꢊꢄ
ꢍꢇꢄ
ꢈꢈꢄ
ꢌꢇꢄ
–ꢄ
rate.
For comparison, the reactions with sterically more
demanding primary aliphatic amines were also studied
Table 4, entries 1m–1o). In particular, with Ar ꢀ=ꢀ R ꢀ=ꢀ tert-
butyl, no reaction was observed under these conditions.
ꢄ ꢊ-Bromophenyl
ꢃꢄ
ꢂk ꢄ ꢊ-Chlorophenyl
ꢃꢄ
ꢋꢋ–ꢋꢌ
Oil
ꢂl
ꢄ ꢇ,ꢊ-Dichlorophenyl ꢄ
ꢊꢄ
(
ꢂm ꢄ Cyclohexyl
ꢂn ꢄ tert-Butyl
ꢂo ꢄ iso-Propyl
ꢄ
ꢄ
ꢄ
ꢃꢄ
ꢃꢆꢆꢄ
–ꢄ
ꢃꢆꢆꢄ
Oil
c
ꢊ ꢄ
–
Oil
c
ꢊ ꢄ
ꢌꢉꢄ
a
Gas chromatography assay (%).
2.6 Reusability of the catalyst
b
Yields refer to those of pure isolated products.
c
The reaction was carried out at 30 °C.
The feasibility of repeated use of MCM-41 was also exam-
ined. The recovery of the catalyst was very easy. After
completion of the reaction, ethanol was added to the the reaction conditions, and its activity in terms of yields
reaction mixture. The catalyst was simply filtered from slightly decreased with increasing number of cycles of
the resulting mixture, and dried at 120 °C under reduced the reaction (conversion for six subsequent runs: 100 %–
pressure for 2 h. The reused catalyst was stable under 100 %–100 %–99 %–95 %–92 %).
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inert gas flow prior to analysis. Specific surface area, total
pore volume, and pore diameter of samples were obtained
3
Conclusion
The direct condensation of 4-bromoaniline and hexane- by the Brunauer–Emmett–Teller (BET) method using
,5-dione as a model reaction in the presence of a few BELSORP analysis software.
mineral oxides was investigated. Among these surfaces, A laboratory microwave oven MW 3100 (Landgraf
2
nanoporous silica MCM-41 prepared through hydrother- Laborsysteme HLL GmbH, Langenhagen, Germany)
mal treatment (MCM-41-HT) exhibited the highest cata- equipped with a magnetic stirrer operating at 2450 MHz
lytic activity with regard to the transformation at 70 °C was used for syntheses of MCM-41 samples. In order to
within 1 h.
prepare MCM-41 under microwave conditions, the top of
Among various nanoporous MCM-41 materials (pre- the reaction vessel (placed inside the microwave chamber)
pared under different conditions), it was observed that was attached to a water-cooled reflux condenser, located
MCM-41-HT had the best performance in terms of con- outside of the microwave chamber, for reflux purpose. The
version and yield. It is noteworthy to mention that MCM- temperature inside the vessel was monitored using a Pre-
4
1-HT had the highest surface area in comparison to other cision Temperature Logger EBI-2 T Type 311 (possessing an
®
MCM-41 materials and much higher surface area than silica external probe) from ebro Electronic GmbH and Co KG,
2
–1
2
–1
gel itself [43] (MCM-41 ≈ 1500 m g ; SiO ≈ 400 m g ); it Ingolstadt, Germany.
2
may be deducted that the increased catalytic activity may
be due to a significantly higher surface area.
4
.2 Preparation of MCM-41 using a
hydrothermal method (MCM-41-HT)
Following these results, various aromatic primary
amines were tested with hexane-2,5-dione under the opti-
mized conditions. In most cases, MCM-41 showed high to
excellent catalytic activity for the reaction. The ease of
MCM-41 isolation from substrates and products and its
reuse as a catalyst for several cycles are very beneficial
from the economic and industrial point of view and can
be further expanded into other catalysis areas.
MCM-41 was prepared through hydrothermal treatment by
following a similar methodology according to Gonçalves
and co-workers [41]. The following solutions were used:
(
A) a solution of 9.9 g sodium silicate in 30 mL deionized
water; (B) a suspension of 8.12 g of cetyltrimethylammo-
nium bromide in 80 mL deionized water.
In a typical experiment, solution A was added drop-
wise to a rapidly stirred suspension B. After 1-h stirring at
ambient temperature, the obtained pH was 12.0. The pH
was adjusted to 10.0 using dilute sulfuric acid (2 ꢁ) within
4
Experimental section
1
h.
4
.1 Materials and methods
The mixture was then autoclaved at 373 K for 2 days
in Teflon-lined stainless steel reaction vessels. The solid
product was recovered by filtration and washed with
deionized water, until the filtrate had the final pH 7.0. The
white precipitate was dried at 373 K under vacuum and
calcined at 813 K for 6 h to remove the surfactant leading
to MCM-41-HT.
Sodium silicate solution (7.5–8.5 % Na O, 25.5–28.5 % SiO ,
2
2
Merck-105621) and cetyltrimethylammonium bromide
96 %, Fluka-52370) were used for the preparation of
(
MCM-41 without additional purification. All mineral
oxides were available commercially. All other chemicals
(
H SO , HCl, and organic solvents) were of analytical
2 4
quality, and water was purified in an SG Water purifica-
tion system (Barsbüttel, Germany).
All of the products were characterized by a compari-
son of their spectroscopic data with those of authentic
samples [38–40, 44].
4.3 Preparation of MCM-41 using microwave
heating
In this procedure, after adjusting the pH of the suspension
®
The powder XRD pattern was recorded at room tem- to 10.0 (as mentioned above), antifoam SILFAR S184
perature from 1° to 10° in 2θ by using a D8 Advance instru- (0.2 g) was added to the obtained suspension and stirred
ment (Bruker AXS GmbH, Karlsruhe, Germany) with CuKα for a further 1 h. The resulting mixture was loaded into the
radiation (λ ꢀ=ꢀ 0.15406 nm).
microwave vessel. The vessel was then inserted into the
The N2 adsorption/desorption analyses were per- microwave chamber (equipped with a water-cooled reflux
formed on a BELSORP-miniII instrument at 77 K (BEL condenser), and the reaction was carried out maintaining
Japan). MCM-41 was degassed at 120 °C for 1.5 h under the power at 300, 360, 420, and 480 W (50 %, 60 %, 70 %,
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6ꢀ ꢀK. Aghapoor et al.: Catalytic activity of the nanoporous MCM-41 surface
and 80 % of maximum power, respectively) for an appro-
priate time (recorded temperature during irradiation was
[7] V. F. Ferreira, M. C. B. V. De Souza, A. C. Cunha, L. O. R. Pereira,
M. L. G. Ferreira, Org. Prep. Proced. Int. 2001, 33, 411.
[
[
8] G. Minetto, L. F. Raveglia, M. Taddei, Org. Lett. 2004, 6, 389.
9] U. Joshi, M. Pipelier, S. Naud, D. Dubreuil, Curr. Org. Chem.
9
6.5 °C). The solid product was recovered by filtration
and washed with deionized water, until the filtrate had
the final pH 7.0. The white precipitate was dried at 373 K
under vacuum and calcined at 813 K for 6 h to remove
the surfactant leading to MCM-41 prepared by microwave
irradiation.
2
005, 9, 261.
[
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Z. Naturforsch.ꢀ
ꢀ2015 | Volume x | Issue x(b)
Graphical synopsis
Kioumars Aghapoor, Mostafa M.
Amini, Khosrow Jadidi, Farshid
Mohsenzadeh and Hossein Reza
Darabi
Catalytic activity of the nanoporous
MCM-41 surface for the Paal–Knorr
pyrrole cyclocondensation
DOI 10.1515/znb-2014-0259
Z. Naturforsch. 2015; x(x)b: xxx–xxx
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