Friedel-Crafts acylation of pyrrole over ultrasonic-assisted phosphoric acid-modified Hβ zeolite

Article abstract of DOI:10.1007/s11164-011-0418-4

Friedel-Crafts acylation of pyrrole with acetic anhydride was studied over different zeolites and an ultrasonic-assisted phosphoric acid-modified Hβ (P-Hβ-US) zeolite was found to have the best catalytic performance among all the zeolites examined. The conversion of acetic anhydride reached 98.8% and the selectivity to acetylpyrroles was 97.0% under the optimized conditions. The high activity of the P-Hβ-US zeolite is attributed to the increase of weak acid sites, caused by the synergistic effect between the phosphoric acid modification and ultrasound. Moreover, the carbonaceous deposits, mainly due to the adsorption of reactants and products, on the surface acid sites and the blockage of the pores is believed to be the reason for the deactivation of the reused P-Hβ-US zeolite, confirmed from the Brunauer-Emmett-Teller (BET) method, scanning electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the catalyst activity can be recovered effectively by a subsequent calcination. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0418-4

Res Chem Intermed (2012) 38:795–806
DOI 10.1007/s11164-011-0418-4
Friedel–Crafts acylation of pyrrole over ultrasonic-
assisted phosphoric acid-modified Hb zeolite
•
Guo Yi Bai Yong Hui Yang Zheng Ma
•
•
•
Jian Zhong Xu Man De Qiu Fei He
•
•
•
Xi Long Yan Hai Yang Dou
Received: 17 September 2011 / Accepted: 30 September 2011 / Published online: 15 October 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Friedel–Crafts acylation of pyrrole with acetic anhydride was studied over
different zeolites and an ultrasonic-assisted phosphoric acid-modified Hb (P-Hb-US)
zeolite was found to have the best catalytic performance among all the zeolites examined.
The conversion of acetic anhydride reached 98.8% and the selectivity to acetylpyrroles
was 97.0% under the optimized conditions. The high activity of the P-Hb-US zeolite is
attributed to the increase of weak acid sites, caused by the synergistic effect between the
phosphoric acid modification and ultrasound. Moreover, the carbonaceous deposits,
mainly due to the adsorption of reactants and products, on the surface acid sites and the
blockage of the pores is believed to be the reason for the deactivation of the reused
P-Hb-US zeolite, confirmed from the Brunauer–Emmett–Teller (BET) method, scanning
electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the catalyst
activity can be recovered effectively by a subsequent calcination.
Keywords Acylation Á Pyrrole Á Zeolite Á Ultrasonic-assisted Á
Phosphoric acid-modified
Introduction
Friedel–Crafts acylation of pyrrole is by far the most popular method for the
synthesis of acetylpyrroles, which are key pharmaceutical intermediates, especially
G. Y. Bai (&) Á Y. H. Yang Á Z. Ma Á J. Z. Xu Á M. D. Qiu Á H. Y. Dou
Key Laboratory of Medicinal Chemistry and Molecular Diagnosis of Ministry of Education,
College of Chemistry and Environmental Science, Hebei University, No.180 Wusi East Road,
Baoding 071002, Hebei, People’s Republic of China
e-mail: baiguoyi@hotmail.com
F. He Á X. L. Yan
School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, People’s Republic of China
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G. Y. Bai et al.
for 2-acetylpyrrole [1, 2]. Zeolites have been proven to be viable alternatives to
conventional liquid acids in Friedel–Crafts type reactions due to their high thermal
stability and environmentally friendly nature [3–5]. Although there have been many
reports on the Friedel–Crafts acylation of arenes [6–10], only a few results are
available on the acylation of pyrrole [11–13]. For example, Reddy and colleagues
[11] investigated the influence of zeolites and reaction parameters on the vapor
phase acylation of pyrrole and 2-acetylpyrrole was obtained in a 75.5% yield on
HZSM-5(280) at 250 °C, but the catalyst deactivated sharply within 6 h. Kantam
et al. [12] recently reported the acylation of pyrrole over different beta zeolites with
low 2-acetylpyrrole yields, but no results were available about the stability of the
zeolites. Therefore, it is desirable to develop an effective method on the mild
acylation of pyrrole with good stability and investigate the relationship of activity
with the structure of the catalysts. On the other hand, there has been an increasing
interest in the use of ultrasound in chemical reactions and catalysts preparation
because of its unique chemical effects arising from acoustic cavitation [14–17].
Furthermore, acid modifications have been proven to be important for improving the
activity of zeolites [18, 19].
In this article, we report our recent studies on the Friedel–Crafts acylation of
pyrrole with acetic anhydride over zeolites and an ultrasonic-assisted phosphoric
acid-modified Hb (P-Hb-US) zeolite was found to have the best catalytic
performance in this reaction. The activity and stability of the modified and parent
Hb zeolites were investigated and clarified based on necessary characterizations.
Experimental
Catalyst preparation
P-Hb-US zeolite was prepared by impregnating 1 g of the parent Hb zeolite
(Nankai University Catalyst) into 25 mL of an aqueous solution of phosphoric acid
(1.0 mol/L), and then induced by 40 kHz ultrasound at 40 °C for 1 h. After that,
the zeolite was dried at 110 °C and calcined at 550 °C for 4 h. Moreover, citric
acid-, tartaric acid-, and oxalic acid-modified Hb zeolites were prepared with the
same method and labeled CA-Hb-US, TA-Hb-US, and OA-Hb-US, respectively.
Catalyst characterization
X-ray diffraction (XRD) patterns were recorded on a Y-2000 X-ray diffractometer
with Cu Ka radiation. The Brunauer–Emmett–Teller (BET) surface area and total
pore volume were calculated using a Micromeritics Tristar II 3020 surface area and
pore analyzer. NH3 temperature-programmed desorption (NH3-TPD) experiments
were carried out in a TP-5000 instrument with a thermal conductivity detector. The
surface acidity was monitored by FT-IR spectroscopy after adsorption of pyridine
(Py-IR) using a Nicolet Nexus 470 spectrometer coupled to a conventional high
vacuum system. The morphology of the samples was investigated using a JEOL
JSM-7500 Scanning electron microscopy (SEM) microscope. Thermogravimetric
123

Friedel–Crafts acylation of pyrrole
797
O
O
O
O
Zeolite
+
CH3COOH
+
+
N
N
N
O
H
H
H
Scheme 1 Friedel–Crafts acylation of pyrrole with acetic anhydride
(TG) analysis was conducted on a NETZSCH QMS 403C TG/DSC/MS instrument
in an argon gas flow. X-ray photoelectron spectroscopy (XPS) measurements were
performed on a PHI 1600 spectrometer using an Mg Ka X-ray source for excitation.
Catalyst activity test
The Friedel–Crafts acylation of pyrrole with acetic anhydride, as shown in Scheme 1,
was carried out in a 25-mL round-bottom flask. In a typical run, pyrrole (0.67 g,
10 mmol), acetic anhydride (0.34 g, 3.3 mmol), and catalyst (0.2 g) were charged to
the flask and stirred at 60 °C for 6 h under nitrogen atmosphere. The reaction
mixtures were analyzed by gas chromatography using a 30-m SE-30 capillary column
and the products were confirmed by GC–MS (Agilent 5975C). The reused P-Hb-US
zeolite was obtained by filtrating, multiple washing and drying at 110 °C, and the
regenerated zeolite was obtained by a further calcination at 550 °C for 4 h.
Results and discussion
Catalysts selection and characterization
In order to find the suitable catalyst for the acylation of pyrrole, some commercial
zeolites were tested first. It was clear that Hb zeolite was the most effective one
among them (Table 1, entries 1–4), although still lower than our expectations.
Considering the benefit of acid modification and the use of ultrasound, a series of
ultrasonic-assisted acid-modified Hb zeolites were then prepared and investigated in
the reaction (Table 1, entries 5–8). Just as we expected, the modified zeolites had
better catalytic performance than Hb zeolite, especially for the P-Hb-US zeolite.
The conversion of acetic anhydride was 81.8% with P-Hb-US zeolite as the catalyst,
better than 64.2% catalyzed by Hb zeolite. Thus, P-Hb-US zeolite was proven to
have the highest activity and was chosen as the catalyst for further investigation.
XRD patterns of Hb and P-Hb-US zeolites are shown in Fig. 1. The characteristic
diffraction peaks of the samples were both at about 7.5 and 22.5°, indicating that the
ultrasonic-assisted phosphoric acid modification had not changed the basic structure
of Hb zeolite. In addition, the reflections of P-Hb-US zeolite were weaker than those
of Hb zeolite. This suggests that the crystallinity of P-Hb-US zeolite decreases as a
result of modification, and this may favor the desired transformation.
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G. Y. Bai et al.
Table 1 Catalytic results for the acylation of pyrrole with acetic anhydride over different zeolites
Entry
Zeolite
Conversion of acetic
anhydride (%)
Selectivity to
2-acetylpyrrole (%)
Selectivity to
acetylpyrrolesa (%)
1
2
3
4
5
6
7
8
Hb
64.2
49.4
34.6
50.9
81.8
69.8
72.1
72.4
83.6
93.1
93.1
77.5
77.8
77.2
77.8
75.4
97.8
97.0
94.0
82.4
92.6
92.3
94.0
90.4
HY
ZSM-5
MCM-41
P-Hb-US
OA-Hb-US
CA-Hb-US
TA-Hb-US
Reaction conditions: pyrrole-to-acetic anhydride molar ratio: 2:1, catalyst amount: 0.2 g, reaction tem-
perature: 60 °C, reaction time: 3 h
a
Acetylpyrroles refer to 2-acetylpyrrole and 3-acetylpyrrole
900
800
700
600
500
400
Hβ
300
200
100
P-Hβ-US
0
10
20
30
40
50
60
2 Theta (degree)
Fig. 1 X-ray diffraction (XRD) patterns of Hb and P-Hb-US zeolites
N2 adsorption and desorption results of Hb and P-Hb-US zeolites are listed in
Table 2. There are certain decreases both in the surface area and pore volume for
¨
P-Hb-US zeolite compared with Hb zeolite. The presence of Bronsted and Lewis
acid sites in Hb and P-Hb-US zeolites are exhibited by the FT-IR spectra of
adsorbed pyridine, and the results are also shown in Table 2. It is found that P-Hb-
¨
US zeolite has more Bronsted acid sites but fewer Lewis acid sites compared to
¨
Hb zeolite. Therefore, Bronsted acid sites are suggested to be advantageous to this
reaction, similar to the previous report [20].
Figure 2 illustrates the NH3-TPD curves of Hb and P-Hb-US samples (curves a
and b). The strong peaks centered at about 250 °C are assigned to the weak acid
sites in the catalysts and the peaks centered approximately at 400 °C are assigned to
the moderately-strong acid sites according to the literature [21, 22]. The amount of
123

Friedel–Crafts acylation of pyrrole
799
Table 2 Textural properties and acidic properties of zeolites
Zeolite
BET (m2/g)
Pore volume (cm3/g)
B acid sitea
L acid sitea
Hb
395
347
0.32
0.28
12.2
13.1
14.5
12.4
P-Hb-US
a
Relative concentration
weak acid sites of P-Hb-US zeolite increases compared with Hb zeolite, indicating
that ultrasonic-assisted phosphoric acid modification favors the formation of weak
acid sites, which can be ascribed to the synergistic effect between the phosphoric
acid modification and ultrasound. Thus, the catalyst activity appears to increase
proportionally with the increase of the weak acid sites.
Optimization of reaction conditions
The effect of the reaction time on Friedel–Crafts acylation of pyrrole catalyzed by
P-Hb-US zeolite is shown in Fig. 3. It was found that the conversion of acetic
anhydride increased considerably from 55.7 to 90.2%, with the reaction time
increasing from 1 to 6 h, and then kept nearly stable with a further increase of the
reaction time up to 7 h; furthermore, the selectivity to acetylpyrroles decreased after
6 h. Thus, 6 h is chosen as the proper reaction time.
The effect of the pyrrole-to-acetic anhydride molar ratio on Friedel–Crafts
acylation of pyrrole was studied and the results are shown in Fig. 4. The conversion
of acetic anhydride was found to increase from 82.4 to 98.8% as the pyrrole-to-
acetic anhydride molar ratio increased from 1:1 to 3:1. The increasing conversion of
acetic anhydride is attributed to the easy desorption of acetylpyrroles and acetic acid
formed through the zeolite pores assisted by the excess pyrrole as self-solvent.
60
50
b
a
d
40
c
30
20
10
0
-10
100
200
300
400
500
600
700
800
Temperature of desorption (°C)
Fig. 2 NH3 temperature-programmed desorption (NH3-TPD) profiles of Hb (a), P-Hb-US (b), reused
P-Hb-US (c) and regenerated P-Hb-US (d) zeolites
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G. Y. Bai et al.
100
95
90
85
80
75
70
65
60
55
50
Conversion
Selectivity
1
2
3
4
5
6
7
Reaction time (h)
Fig. 3 Effect of reaction time on acylation of pyrrole (catalyst amount 0.2 g; reaction temperature
60 °C; pyrrole-to-acetic anhydride molar ratio 2:1)
100
95
90
Conversion
Selectivity
85
80
1
2
3
4
5
Molar ratio
Fig. 4 Effect of pyrrole-to-acetic anhydride molar ratio on acylation of pyrrole (catalyst amount 0.2 g;
reaction temperature 60 °C; reaction time 6 h)
The conversion remains almost constant while the molar ratio further increases from
3 to 5, and the selectivity to acetylpyrroles even decreases slightly. So we choose
the molar ratio of 3:1 as the optimized molar ratio.
The influence of reaction temperature on the acylation of pyrrole is shown in
Fig. 5. The conversion of acetic anhydride is 90.1% at 50 °C and it increases to
98.8% at 60 °C. Further increase in reaction temperature to 70 °C has no
appreciable effect on the conversion of acetic anhydride, together with a slight
123

Friedel–Crafts acylation of pyrrole
801
100
98
96
94
92
90
88
86
84
82
Conversion
Selectivity
80
50
55
60
65
70
Reaction temperature (°C)
Fig. 5 Effect of reaction temperature on acylation of pyrrole (catalyst amount 0.2 g; pyrrole-to-acetic
anhydride molar ratio 3:1; reaction time 6 h)
100
98
96
94
92
90
Conversion
Selectivity
88
86
84
82
0.10
0.15
0.20
0.25
0.30
Catalyst amount (g)
Fig. 6 Effect of catalyst amount on acylation of pyrrole (pyrrole-to-acetic anhydride molar ratio 3:1;
reaction temperature 60 °C; reaction time 6 h)
lowering in the selectivity to acetylpyrroles, as frequently observed in similar
experiments. Therefore, 60 °C is chosen as the appropriate reaction temperature.
The effect of catalyst amount was also investigated (Fig. 6). The conversion first
increases with an increase in catalyst amount from 0.1 to 0.2 g and then decreases
slightly with a further increase in the catalyst amount from 0.2 to 0.3 g. Moreover,
the selectivity to acetylpyrroles is also the highest when the catalyst amount is 0.2 g.
So 0.2 g is the proper dosage in view of the conversion and selectivity at the same
time.
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G. Y. Bai et al.
Stability test of P-Hb-US zeolite
The stability of P-Hb-US zeolite was then checked under the optimum reaction
conditions (Table 3). An obvious deactivation occurred with the reused P-Hb-US
zeolite with a conversion of acetic anhydride dropping from 98.8% in the third run
to 48.3%. In contrast, the activity of the regenerated P-Hb-US zeolite remained
nearly constant with the fresh one. The conversion of acetic anhydride and the
selectivity to acetylpyrroles both remained at roughly 93.0% in six subsequent
cycles. Thus, P-Hb-US zeolite is proven to have good stability in this reaction.
In order to find the reason for the above results, the reused and regenerated P-Hb-
US zeolites were characterized and compared with the fresh one. The regenerated
P-Hb-US zeolite has a similar BET surface area (334 m2/g) to the fresh one
(347 m2/g), whereas the reused P-Hb-US zeolite has a much lower BET surface
area (153 m2/g) than the fresh one. The pore volume of the reused P-Hb-US zeolite
also decreases remarkably from 0.28 to 0.19 cm3/g, caused by carbonaceous
deposits on the surface and within the pores of the zeolite, and responsible for its
deactivation. SEM images of the fresh and reused P-Hb-US zeolites show the
differences directly (Fig. 7). There are obvious carbonaceous deposits on the
surface of the reused P-Hb-US zeolite compared with the fresh one, which has
uniform crystals and clear channels. In addition, the almost 10% weight loss of the
reused P-Hb-US zeolite and the detection of carbon dioxide and pyrrole in the gas
phase, based on TG-MS, support this conclusion.
As shown in XPS spectra, the amount of surface carbon of the reused P-Hb-US
zeolite is much higher than that of the fresh one (Fig. 8). This provides evidence for
carbonaceous deposits on the surface of the reused P-Hb-US zeolite, in total
agreement with the BET and SEM results. Furthermore, three carbon species exist
on the surfaces of catalysts by applying the curve fitting procedure, which gives
Table 3 Stability of P-Hb-US zeolite
P-Hb-US zeolite
Conversion of acetic
anhydride (%)
Selectivity to
2-acetylpyrrole (%)
Selectivity to
acetylpyrrolesa (%)
Fresh
98.8
77.3
54.9
48.3
95.8
95.9
95.9
93.0
92.9
93.4
78.7
72.6
72.1
73.3
74.4
74.4
71.5
73.6
73.2
71.3
97.0
94.5
94.5
93.6
95.3
95.0
92.7
95.0
94.5
95.1
Reused first cycle
Reused second cycle
Reused third cycle
Regenerated first cycle
Regenerated second cycle
Regenerated third cycle
Regenerated fourth cycle
Regenerated fifth cycle
Regenerated sixth cycle
Reaction conditions: pyrrole-to-acetic anhydride molar ratio: 3:1, catalyst amount: 0.2 g, reaction tem-
perature: 60 °C, reaction time: 6 h
a
Acetylpyrroles refer to 2-acetylpyrrole and 3-acetylpyrrole
123

Friedel–Crafts acylation of pyrrole
803
Fig. 7 Scanning electron microscopy (SEM) images of fresh P-Hb-US (a) and reused P-Hb-US
(b) zeolites
binding energies of about 284, 286, and 288 eV, respectively. The amount of carbon
species corresponding to 286 eV of the reused P-Hb-US zeolite is higher than that
of the fresh one. Thus, the surface carbon adjoining 286 eV is believed to be the
main carbonaceous species, which can mainly be ascribed to the adsorption of
reactants and products on the surface acid sites of the reused zeolites and, in turn,
result in the deactivation of them.
The NH3-TPD profile of the reused P-Hb-US zeolite (Fig. 2, curve c) shows that
the amount of weak acid sites decreases sharply compared with the fresh catalyst.
However, the amount of weak acid sites on the regenerated P-Hb-US zeolite only
partly decreases (Fig. 2, curve d). Thus, the decrease of weak acid sites due to
carbonaceous deposition is believed to be the major reason of deactivation.
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G. Y. Bai et al.
a 2.5x105
2.0x105
1.5x105
1.0x105
Atomic %
C1s 46.1
O1s 35.7
Al2p 9.2
Si2p 8.0
P2p 1.0
5.0x104
0.0
1000
800
600
400
200
0
Binding Energy (eV)
5
b 1.4x10
Atomic %
C1s 60.6
O1s 26.9
Al2p 5.9
Si2p 5.9
P2p 0.6
1.2x105
1.0x105
8.0x104
6.0x104
4.0x104
2.0x104
0.0
1000
800
600
400
200
0
Binding Energy (eV)
Fig. 8 X-ray photoelectron spectroscopy (XPS) spectroscopy of fresh P-Hb-US (a, c) and reused P-Hb-
US (b, d) zeolites
Conclusions
In summary, P-Hb-US zeolite exhibits better catalytic performance than Hb zeolite
in the Friedel–Crafts acylation of pyrrole. Under the optimized reaction conditions,
the conversion of acetic anhydride and the selectivity to acetylpyrroles were 98.8
and 97.0%, respectively. The weak acid sites, which can be increased by the
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Friedel–Crafts acylation of pyrrole
805
c
22000
20000
Pos. Sep. %Area
18000
16000
14000
12000
10000
8000
284.65 0.00 84.51
286.57 1.92 8.54
288.56 3.90 6.94
6000
4000
2000
292
290
288
286
284
282
Binding Energy (eV)
d
18000
16000
14000
12000
10000
8000
Pos. Sep. %Area
284.78 0.00 79.83
286.41 1.63 12.72
288.71 3.93 7.45
6000
4000
2000
292
290
288
286
284
282
Binding Energy (eV)
Fig. 8 continued
synergistic effect between the phosphoric acid modification and ultrasound, play a
significant role in this reaction. The carbonaceous deposits, mainly due to the
adsorption of reactants and products, on the surface acid sites and their blockage of
the zeolite pores account for the deactivation of P-Hb-US zeolite, whereas the
regenerated P-Hb-US zeolite shows similar activity to the fresh one.
Acknowledgments The authors thank Professor Baoting Liu for his assistance. Financial support by the
National Natural Science Foundation of China (20806018), the Natural Science Foundation of Hebei
Province (B2011201017), and the Science Project of the Hebei Education Department (ZD200910) are
gratefully acknowledged.
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