Microwave-assisted acylation of aromatic compounds using carboxylic acids and zeolite catalysts

Article abstract of DOI:10.1016/j.molcata.2010.05.016

Acylation of aromatic compounds with carboxylic acids smoothly proceeded at 190-230 °C in the presence of zeolite catalysts under microwave irradiation to give aromatic ketones efficiently. H-Y (SiO₂/Al₂O ₃ = 30-80) and H-beta (25) zeolites were active for the acylation reaction, giving the aromatic ketones in good yields. Carboxylic acids such as hexanoic and butyric acid smoothly underwent the acylation, while propionic acid showed somewhat lower reactivity. Anisole gave the para-acylation products nearly selectively. Anisole, 2,3-dihydrobenzofuran, and thiophene were reactive aromatic compounds. 2,3-Dihydrobenzofuran also reacted at the para position to the oxygen atom predominantly to give the corresponding ketones as the major products. The microwave reactions were generally faster than the conventional oil bath reactions and gave higher yields of the acylation products. Activation energies for the reaction of anisole with butyric acid by microwave and by oil bath heating were also estimated on the basis of the Arrhenius plots.

Full text of DOI:10.1016/j.molcata.2010.05.016

Journal of Molecular Catalysis A: Chemical 327 (2010) 80–86
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Microwave-assisted acylation of aromatic compounds using carboxylic acids and
zeolite catalysts
Hiroshi Yamashita∗, Yumi Mitsukura, Hiroko Kobashi
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e i n f o
a b s t r a c t
Acylation of aromatic compounds with carboxylic acids smoothly proceeded at 190–230 ◦C in the
presence of zeolite catalysts under microwave irradiation to give aromatic ketones efficiently. H–Y
(SiO2/Al2O3 = 30–80) and H–beta (25) zeolites were active for the acylation reaction, giving the aromatic
ketones in good yields. Carboxylic acids such as hexanoic and butyric acid smoothly underwent the acy-
lation, while propionic acid showed somewhat lower reactivity. Anisole gave the para-acylation products
nearly selectively. Anisole, 2,3-dihydrobenzofuran, and thiophene were reactive aromatic compounds.
2,3-Dihydrobenzofuran also reacted at the para position to the oxygen atom predominantly to give the
corresponding ketones as the major products. The microwave reactions were generally faster than the
conventional oil bath reactions and gave higher yields of the acylation products. Activation energies for
the reaction of anisole with butyric acid by microwave and by oil bath heating were also estimated on
the basis of the Arrhenius plots.
Article history:
Received 14 January 2010
Received in revised form 11 May 2010
Accepted 26 May 2010
Available online 1 June 2010
Keywords:
Microwave
Acylation
Carboxylic acid
Aromatic ketone
Zeolite
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
On the other hand, microwave-assisted chemical processes have
attracted increasing interests with hoping to improve the effi-
Aromatic ketones are principal compounds in organic chem-
ical industry. Friedel-Crafts type acylation reactions of aromatic
compounds with acyl halides or carboxylic anhydrides have been
generally utilized for the preparation of aromatic ketones. Con-
cerning the catalysts, utilization of solid acids such as zeolites,
heteropoly acids, etc. has also been investigated because of the
merit of easy separation and/or recyclability of the catalysts [1–3].
Although the method using acyl halides or carboxylic anhydrides
has wide applicability, it has a drawback that equimolar amounts
of hydrogen halides or carboxylic acids are concomitantly formed.
To overcome the drawback, acylation reactions by carboxylic acids
have been investigated using some solid acid catalysts such as zeo-
lites [4–13], heteropoly acids or related compounds [14–20], acidic
clays including metal cation-exchanged montmorillonites [21–24],
and a p-toluenesulfonic acid/graphite system [25]. The method
using carboxylic acids has an advantage that the co-product is only
water, thus providing an environmentally benign method for the
preparation of aromatic ketones. However, the rates and/or the
yields of the acylation reactions using carboxylic acids has not been
necessarily high in comparison with those using acyl halides or
carboxylic anhydrides.
ciency of organic reactions [26–28]. In the course of our study
on the microwave-assisted chemical reactions, we have found
that an intramolecular Friedel-Crafts type dehydrative acylation
reaction using solid acid catalysts has been effectively acceler-
ated by microwave irradiation [29]. Meanwhile, an interesting
microwave-assisted acylation of anisole with carboxylic acids has
been recently reported by Bond et al. using solid acid catalysts of
a silica-supported heteropoly acid, a sulfated zirconia, and a La-
exchanged Y zeolite [30]. To know the efficiency of proton-type
zeolite catalysts in the microwave-assisted acylation reaction, we
have investigated the acylation with carboxylic acids using some
commercially available zeolite catalysts. Herein are reported the
results of the acylation reaction including a consideration on the
kinetics of the reaction.
2. Experimental
2.1. Instruments and reagents
Microwave irradiation experiments were carried out with a
CEM Discover instrument (single mode type, microwave max
power 300 W) using closed Pyrex glass tubes (ca. 10 mL) with
Teflon-coated septa. The reaction temperature was monitored by
a radiation thermometer attached to the microwave instrument.
The temperature measured by the radiation thermometer was
calibrated by a fiber optic thermometer and standard mercury ther-
∗
Corresponding author. Tel.: +81 29 861 9412; fax: +81 29 861 6242.
E-mail address: hiro-yamashita@aist.go.jp (H. Yamashita).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.016

H. Yamashita et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 80–86
81
mometers. Centrifugation was performed by a Porta Centrifuge and
a Hsiangtai CN-2060. The yields of the products were analyzed
using gas liquid chromatography (GC) with a Shimadzu GC-17A
system equipped with a capillary column (Zebron ZB-50, 0.25 mm
i.d. × 30 m, an internal standard of dodecane), and a flame ioniza-
tion detector (FID). Identification of the products was made by gas
chromatography/mass spectrometry (GC/MS) analyses with a Shi-
madzu GCMS-QP2000plus system. Complex dielectric constants of
liquid compounds and zeolite catalysts were measured by a pertur-
bation method [31] with a system of Kanto Electronic Application
and Development Inc. comprising of a vector network analyzer of
Agilent 8720ES and a 2.45 GHz cylindrical cavity of TM020 mode
using Teflon-type sample tubes. Confirmation of the structures of
the authentic samples for the acylated products was made on the
basis of 1H and 13C nuclear magnetic resonance (NMR) spectra
measured by a Jeol JNM-LA600 instrument using chloroform-d sol-
vent and Infrared (IR) spectra recorded on a Jasco FT/IR-660plus
spectrometer.
Liquid compounds such as anisole (1a), 2,3-dihydrobenzofuran
(1b), thiophene (1c), hexanoic acid (2a), butyric acid (2b), propi-
onic acid (2c), and 1,2-dichlorobenzene were used after drying with
molecular sieve 4A. Zeolite catalysts were the commercial products
of Tosoh Co., Ltd., UOP, Ltd., and Zeolist International. Their prop-
erties were summarized in Table 1. These zeolite catalysts were
used after heating at 500 ◦C for 5 h under air. An activated carbon
(AC) was obtained from Wako Pure Chem. Ind., Ltd, and used after
heating at 200 ◦C for 2 h.
Fig. 1. A monitoring profile for the reaction of 1a with 2b (190 ◦C, 30 min).
(151 MHz, CDCl3): ı 13.9, 22.5, 24.4, 29.0, 31.6, 38.3, 72.1, 108.9,
125.3, 127.6, 130.0, 130.5, 164.1, 199.2; IR (KBr): ꢀ (cm−1) 1673,
1606, 1587, 1494, 1371, 1324, 1258, 1241, 1205, 1146, 1093, 978,
940, 835, GC–MS: m/z (relative intensity) 218 (M+, 6), 162 (53), 147
(100), 119 (12), 91 (22), 65 (14).
3eꢀ: GC–MS: m/z (relative intensity) 218 (M+, 3), 162 (46), 147
(100), 91 (35), 65 (20).
2.3. Acylation of aromatic compounds with carboxylic acids
A typical procedure for a microwave reaction was as follows.
A mixture of an aromatic compound 1 (12.0 mmol), a carboxylic
acid 2 (2.0 mmol), a zeolite catalyst (100 mg), an activated carbon
(AC, 50 mg), dodecane (internal standard, 50 mg), and a magnetic
bar was sealed in a Pyrex test tube (ca. 10 mL). AC was added as a
susceptor material to absorb microwave energy efficiently; with-
out AC the rate of the temperature increase of the whole reaction
system was lower than that with AC. The mixture was heated by
microwave irradiation (microwave max power 300 W) with mag-
netic stirring under specified conditions shown in Table 2. The
reaction mixture was centrifuged (3000 rpm, 2 min), and the super-
natant solution was separated. The residual solid was washed with
solvents (acetone and toluene, each 2 mL) to extract adsorbed reac-
tants and products. The supernatant solution and the washing were
combined, and analyzed by GC and/or GC–MS. Identification of the
products 3 was made by comparison with their GC retention times
and GC–MS spectral patterns with those of the authentic samples.
The conversion of 2 and the yield of the acylation product 3 were
estimated by GC.
In the microwave reaction, the irradiation power was automat-
ically adjusted so as to keep the reaction temperature. A typical
profile of the changes in the temperature and the microwave irra-
diation power was shown in Fig. 1 for the reaction of 1a with 2b
at 190 ◦C for 30 min. The temperature reached 190 ◦C for about
2.5 min. The irradiation power was 300 W (max power) for the ini-
tial 2.5 min, then decreased to about 50 W by 10 min, and gradually
decreased to 30 W by 30 min with maintaining the temperature of
190 ◦C.
2.2. Authentic samples of 3
Authentic samples of 1-(4-methoxyphenyl)-1-butanone
(3b),
1-(4-methoxyphenyl)-1-propanone
(3c),
4,4ꢀ-
dimethoxybenzophenone (3g), and 4-methoxybenzophenone
(3h) were purchased from Tokyo Chemical Industry Co., Ltd. On
the other hand, authentic samples of 1-(4-methoxyphenyl)-1-
hexanone (3a) [32], 1-(2,3-dihydrobenzofuran-5-yl)-1-butanone
(3d) [33], 1-(2,3-dihydrobenzofuran-5-yl)-1-hexanone (3e), and
1-(2-thienyl)-1-hexanone (3f) [34] were obtained by the reactions
of the corresponding aromatic compounds with suitable carboxylic
anhydrides at 80–100 ◦C using zeolite catalysts; the isolated yields
(reaction conditions) of 3a, 3d, 3e, and 3f were respectively 79% (1a
60 mmol, hexanoic anhydride 10 mmol, CBV760 200 mg, 80 ◦C, 1 h),
57% (1b 10 mmol, butyric anhydride 10 mmol, CBV760 300 mg,
100 ◦C, 2.5 h), 68% (1b 10 mmol, hexanoic anhydride 10 mmol,
CBV760 200 mg, 100 ◦C, 1 h), and 55% (1c 12 mmol, hexanoic
anhydride 10 mmol, CBV720 200 mg, 100 ◦C, 3 h). The NMR data of
the prepared authentic samples were in good agreement with the
reported data or with the proposed structures. The NMR, IR, and
GC–MS spectral data for 3d and 3e and the GC–MS data for their
isomeric products 3dꢀ and 3eꢀ are as follows.
3d: 1H NMR (600 MHz, CDCl3): ı 0.99 (t, J = 7.3 Hz, 3H, CH3), 1.75
(sextet, J = 7.3 Hz, 2H, CH3CH2), 2.88 (t, J = 7.3 Hz, 2H, CH2CO), 3.24
(t, J = 8.8 Hz, 2H, OCH2CH2), 4.65 (t, J = 8.8 Hz, 2H, OCH2), 6.80 (d,
J = 8.4 Hz, 1H, aromatic H), 7.80 (d, J = 8.4 Hz, 1H, aromatic H), 7.85
(s, 1H, aromatic H; 13C NMR (151 MHz, CDCl3): ı 13.9, 18.1, 29.0,
40.3, 72.1, 108.9, 125.3, 127.6, 129.9, 130.6, 164.2, 199.0; IR (KBr): ꢀ
(cm−1) 1672, 1604, 1492, 1439, 1364, 1305, 1285, 1239, 1141, 1120,
1095, 980, 942, 909, 808, 751; GC–MS: m/z (relative intensity) 190
(M+, 13), 147 (100), 119 (12), 91 (21), 65 (15).
In an oil bath reaction also, a Pyrex glass tube with the same
shape as that of the microwave reaction was used. The glass tube
containing starting materials, a catalyst, AC, an internal standard,
and a magnetic bar was immersed into an oil bath preheated at the
specified temperature in Table 2, and heating was continued under
stirring for the specified time. In a separate experiment, the rate of
the temperature increase of the inside solution was checked by a
thermometer. The time for increasing the temperature up to 190 ◦C
was almost the same as that for the microwave reaction.
3dꢀ: GC–MS: m/z (relative intensity) 190 (M+, 9), 162 (11), 147
(100), 91 (30), 65 (20).
3e: 1H NMR (600 MHz, CDCl3): ı 0.86–0.98 (m, 3H, CH3),
1.30–1.46 (m, 4H, CH3CH2CH2), 1.66–1.76 (m, 2H, CH2CH2CO), 2.89
(t, J = 7.3 Hz, 2H, CH2CO), 3.25 (t, J = 8.6 Hz, 2H, OCH2CH2), 4.65
(t, J = 8.6 Hz, 2H, OCH2), 6.80 (d, J = 8.4 Hz, 1H, aromatic H), 7.80
(d, J = 8.4 Hz, 1H, aromatic H), 7.85 (s, 1H, aromatic H); 13C NMR
Experiments of the Arrhenius plots for the microwave and the
oil bath reactions of 1a with 2b were carried out in the temperature

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H. Yamashita et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 80–86
Table 1
Properties of commercial zeolites used in the acylation reaction.
Zeolite
Product codea
SiO2/Al2O3
BET surface area (m2/g)
Na2O (%)
LOIb (%)
H–Y (30)
H–Y (60)
H–Y (80)
H–beta (25)
H–beta (40)
H–M (90)
H–ZSM-5 (29)
H–ZSM-5 (50)
CBV720
CBV760
CBV780
UOP-Beta
CP811C
CBV90A
HSZ-830HOA
CBV5525
28
55
81
25
40
89
29
58
850
910
860
650
700
570
360
460
0.03
0.02
0.01
9.6
9.2
9.6
–
2.1
–
0.5
<0.01
<0.04
<0.01
<0.01
2.7
5.5
a
UOP-Beta: UOP, Ltd., HSZ-830HOA: Tosoh Co., Ltd., others: Zeolyst International.
Loss on ignition.
b
range of 160–200 ◦C. The kinetic constants k were derived by taking
pseudo first-order kinetics for the carboxylic acid 2b.
Table 2). Survey of the catalyst activity revealed that some H–Y and
H–beta zeolites gave relatively high yields for 3a among the H–Y,
H–beta, ZSM-5, and H–mordenite (H–M) zeolites (Fig. 2). H–Y zeo-
lites with the SiO2/Al2O3 ratios being 30:80 gave the best results,
forming 3a in good yields; 61.4% for H–Y (30), 61.7% for H–Y (60),
and 57.8% for H–Y (80) (Runs 1–3). In addition to the H–Y zeo-
lites, H–beta zeolites also provided 3a in reasonably good yields,
although the efficiency was lower than the H–Y catalysts; the yields
of 3a using H–beta zeolites were 49.6% (SiO2/Al2O3 = 25) and 40.6%
(40) (Runs 4 and 5). In these reactions, formation of small amounts
(≤1.2%) of an isomeric product of 3a, presumably the ortho-form
3. Results and discussion
3.1. Catalysts
The reaction of anisole 1a with hexanoic acid 2a proceeded at
190 ◦C in the presence of acidic zeolite catalyst under 2.45 GHz
microwave irradiation to give a para-acylation product, 1-(4-
methoxyphenyl)-1-hexanone (3a), along with water (Scheme 1,
Table 2
Reactions of 1 with 2.a
Run
1
2
Catalyst
Methodb
Temp. (◦C)
3
Yieldc (%)
3
Isomer
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1c
1c
1a
1a
1a
1a
1a
1b
1b
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2c
2c
2c
2c
2c
2b
2b
2a
2a
2a
2d
2d
2e
2e
2b
2b
2a
H–Y (30)
H–Y (60)
H–Y (80)
H–beta (25)
H–beta (40)
H–M (90)
H–ZSM-5 (29)
H–ZSM-5 (50)
H–Y (30)
MW
MW
MW
MW
MW
MW
MW
MW
OB
OB
OB
OB
OB
190
190
190
190
190
190
190
190
190
190
190
190
190
190
190
190
190
190
200
190
190
210
230
210
190
230
230
190
190
190
230
190
230
190
230
230
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
3c
3c
3c
3c
3c
3d
3d
3e
3f
61.4
61.7
57.8
49.6
40.6
17.2
2.8
17.6
17.2
32.5
19.5
17.5
28.1
14.5
2.4
10.1
59.6
70.9
74.0
33.6
33.2
44.2
52.0
52.1
25.8
69.6
57.7
56.8
77.8
29.9
55.4
26.3
49.1
26.6
31.4
25.9
0.8
1.2
1.2
≤0.1
0.2
≤0.1
≤0.1
≤0.1
0.4
0.5
0.6
≤0.1
≤0.1
≤0.1
≤0.1
≤0.1
1.0
1.4
1.4
0.9
1.1
≤0.1
≤0.1
0.9
1.0
3.1
9
10
11
12
13
14
15
16
17
18d
19
20
21
22
23
24e
25
26
27
28e,f
29e,f,g
30
31
32
33
34
35
36
H–Y (60)
H–Y (80)
H–beta (25)
H–beta (40)
H–M (90)
H–ZSM-5 (29)
H–ZSM-5 (50)
H–Y (60)
H–Y (60)
H–Y (60)
H–Y (60)
H–beta (40)
H–beta (40)
H–beta (40)
H–beta (40)
H–Y (30)
H–Y (60)
H–Y (60)
H–Y (30)
H–Y (30)
OB
OB
OB
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
OB
3.7
0.7
1.2
1.1
2.5
1.0
1.8
0.4
3f
H–Y (30)
H–Y (30)
H–Y (30)
H–Y (30)
H–Y (60)
H–Y (60)
H–Y (60)
3g
3g
3h
3h
3b
3d
3e
OB
OB
1.5
1.7
a
Reaction conditions: 1 12.0 mmol, 2 2.0 mmol, catalyst 100 mg, activated carbon (AC) 50 mg, 190–230 ◦C, 30 min.
b
c
d
e
f
MW: microwave, OB: oil bath.
Estimated by GC.
1a 24.0 mmol, 2b 2.0 mmol.
1,2-Dichlorobenzene (0.5 mL) was added.
1c 4.0 mmol, 2a 2.0 mmol.
120 min.
g

H. Yamashita et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 80–86
83
acylation reactions [4–7,13]. On the other hand, the effect of the
SiO2/Al2O3 on the catalyst activity is not so clear at the moment.
However, lower SiO2/Al2O3 ratios tended to show higher activity
in the H–Y and H–beta series with the SiO2/Al2O3 ratios of 30–80.
H–Y and H–beta zeolites with reasonably low SiO2/Al2O3 ratios
have appropriately hydrophilic and protic natures, which seem
to be favorable for inclusion and activation of intermediary polar
oxygen-containing substrates.
When external oil bath heating was used in place of microwave
irradiation, significant decrease in the yield of 3a was observed.
In all the above-mentioned zeolite catalysts, the yields obtained
by oil bath heating were lower than those by microwave irradi-
ation (Runs 9–16, Fig. 2). The degree of the acceleration effect of
microwave irradiation was dependent on the nature of the cat-
alyst. Thus, the difference in the yield between microwave and
oil bath heating was relatively large for H–Y (SiO2/Al2O3 = 30–80)
and H–beta (25), while the difference was rather small for H–beta
(40). Consequently, H–beta (40) showed a relatively high efficiency
similar to that of H–Y (60) in the oil bath reaction (Runs 10 and
13). It is worth noting that the tendency for the catalyst activity
in the microwave reaction is different from that in the oil bath
reaction. The degree of microwave absorption is proportionally cor-
related with the dielectric loss factor of the material (vide infra).
Measurements of the dielectric loss factors of the zeolite catalysts
showed that the values of H–Y (30), H–Y (60), H–Y (80), H–beta
(25), and H–beta (40) were respectively 0.21, 0.25, 0.23, 0.29, and
0.17, revealing that H–beta (40) zeolite had the lowest dielectric
loss factor. This means that H–beta (40) will not absorb microwave
energy as efficiently as other catalysts. The relatively small differ-
ence in the activity might be partly related to the low efficiency
of the microwave absorption for H–beta (40) zeolite, although fur-
ther study is required to clarify the reason of the difference in the
activity more certainly.
Scheme 1.
of 3a, was also observed, while other isomeric products were not
found in GC and GC–MS.
In contrast to H–Y and H–beta zeolite catalysts, H–ZSM-5 and
H–mordenite zeolites afforded 3a with much lower efficiency;
the yield of 3a using H–mordenite (SiO2/Al2O3 = 90) was 17.2%
(Run 6) and the yields of 3a using H–ZSM-5 catalysts were 2.8%
(SiO2/Al2O3 = 29) and 17.6% (50) (Runs 7 and 8). In the present
acylation reaction of a carboxylic acid with an aromatic com-
pound, both molecules will be included into the zeolite channels
with acidic catalytic sites. H–Y and H–beta zeolites have large
three-dimensional channels with 12-membered rings and would
be favorable for such inclusion process, while the use of ZSM-5
possessing smaller channels with 10-membered rings is not so
advantageous. The framework of H–M zeolite also has channels
with 12-membered rings, but the structure of the channel is nearly
one-dimensional, and H–M zeolite will not include the substrates as
easily as H–Y and H–beta zeolites. Effective uses of Y- or beta-type
zeolite catalysts have been previously reported in the Friedel-Crafts
3.2. Carboxylic acids and aromatic substrates
Butyric acid 2b also smoothly reacted with 1a at 190 ◦C in
the presence of the H–Y (60) catalyst to give the corresponding
aromatic ketone, 1-(4-methoxyphenyl)-1-butanone (3b), in 59.6%
yield (Run 17). The yield was improved to 70.9% by raising the
ratio of 1a/2b from 6/1 to 12/1 (Run 18). Heightening the reaction
temperature to 200 ◦C also increased the yield up to 74.0% (Run
19).
In contrast, propionic acid (3c) with a shorter hydrocarbon
chain gave the corresponding ketone, 1-(4-methoxyphenyl)-1-
propanone (3c), in a lower yield of 33.6% (Run 20). The relatively
low reactivity of carboxylic acids with short hydrocarbon chains
were previously reported in the reactions using solid acid catalysts
[4,19]. Although the reaction using H–beta (40) also gave 3c in a
similar yield, 33.2% (Run 21), the yields were improved to 44.2%
and to 52.0% by increasing the temperature to 210 ◦C and 230 ◦C,
respectively (Runs 22 and 23). Addition of 1,2-dichlorobenzene in
the reaction at 190 ◦C also showed a positive effect to improve the
yield from 44.2% to 52.1% (Run 24).
As another aromatic substrate, 2,3-dihydrobenzofuran (1b),
could react with carboxylic acid 2b to form the corresponding acy-
lated product 3d as the major product, although the reactivity was
lower than 1a and the yield of 3d in the reaction at 190 ◦C was
25.8% (Run 25). Sterically larger size of 1b would be unfavorable
for the acylation reaction. The yield was improved by conducting
the reaction at a higher temperature of 230 ◦C, giving 69.6% yield
for 3d (Run 26). Similarly, 1b reacted with 2a at 230 ◦C to pro-
vide the corresponding ketone 3e in 57.7% yield (Run 27). Although
small amounts of the regioisomeric products of 3d and 3e were
also formed (ca. 3.1% and 3.7% yields, respectively), the acylation
of 1b took place at the para position to the oxygen atom of 1b pre-
Fig. 2. Comparison of catalyst efficiency in the reactions of 1a with 2a (190 ◦C,
30 min).

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H. Yamashita et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 80–86
dominantly, as observed in the reaction of 1b with acetic anhydride
using solid acid catalysts [35].
In the acylation of benzene derivatives, the presence of alkoxy
groups at the aromatic rings significantly promoted the reaction.
Exchange of the methoxy group by methyl group resulted in much
lower yield; toluene reacted with 2a to form the corresponding
ketone only in a low yield (3.8%) even at a high temperature of
230 ◦C. This suggested that the electron-donating nature of methyl
group was not enough to undergo the present acylation reaction
smoothly.
A heteroaromatic compound also underwent the acylation reac-
tion. Thus, thiophene (1c) reacted with 2a (2 equiv.) at 190 ◦C for
30 min in the presence of the H–Y (30) catalyst under microwave
irradiation to form the acylated product, 1-(2-thienyl)-1-hexanone
(3f), in 56.8% yield (Run 28). The yield was increased up to 77.8%
by prolonging the reaction time to 120 min (Run 29). The regiose-
lective acylation of 1c by zeolite catalysts was previously reported
in the reactions of butyryl chloride [36] and acetic anhydride [37].
Besides aliphatic carboxylic acids, aromatic acids also reacted
with 1a to give the corresponding diaryl ketones, although the
reaction conditions have not been optimized. Thus, treatment of
4-methoxybenzoic acid (2d) with 1a using the H–Y (30) catalyst at
190 ◦C provided a diaryl ketone 3g in 29.9% yield (Run 30). Benzoic
acid (2e) also reacted at 190 ◦C with 1a to form the corresponding
ketone 3h in 26.3% yield (Run 32). The yields of 3g and 3h were
improved to 55.4% and 49.1%, respectively, by raising the reaction
temperature from 190 ◦C to 230 ◦C (Runs 31 and 33).
Fig. 3. Comparison of the yields between MW and OB heating in the reactions of
1a,b with 2a,b affording 3a, 3b and 3d, 3e (1a: 190 ◦C, 30 min; 1b: 230 ◦C, 30 min).
Since the zeolite catalyst was easily recovered by centrifugation,
reuse of the catalyst was attempted in the reaction of 1a with 2a
at 200 ◦C for 30 min using H–beta (25) catalyst. After the reaction
was finished, the yield of 3a was estimated at 51.8%. The catalyst
was recovered by centrifugation, washed with acetone and toluene,
and reactivated by heating at 500 ◦C for 5 h. When the reaction of
1a with 2a was carried out using the recovered catalyst, however,
the yield of 3a was 27.9%, which was lower than that obtained by
the pristine catalyst, 51.8%, showing a significant decrease in the
activity of the catalyst. In the present reaction using a batch-type
closed vessel, the generated water did not go out of the reaction
system, and might cause dealumination of the zeolite to reduce
the catalyst acidity, as previously demonstrated by Timken et al. in
the steam treatment of a H–beta zeolite (SiO2/Al2O3 = 36) [38]. To
reduce the effect of the generated water, a batch-type open system
or a flow reaction system, which is able to remove the generated
water continuously, may be better than the present closed system.
Fig. 4. Time dependency of the conversion of the carboxylic acid in the microwave
(MW) and oil bath (OB) reactions of 1a with 2b using H–Y (60) catalyst in the range
of 160–200 ◦C; the conversion was estimated on the basis of the yield of the acylated
product.
sity of electric field [39]. In this sense, the dielectric properties of
the starting compounds were analyzed by a perturbation method
using a 2.45 GHz cylindrical cavity. As the results, aromatic com-
pounds 1a and 1b exhibited relatively large values of the dielectric
loss factors (0.289 and 1.007, respectively), while the values of the
carboxylicacids 2a and 2b were moderateor small(0.135 and 0.071,
respectively). The values of 1a and 1b were much larger than those
of normal hydrocarbons such as toluene and hexane (0.012 and
0.0008, respectively). The large values of 1a and 1b indicate that
starting aromatic compounds are suitable for absorbing microwave
energy. In addition, the water, which is produced as a co-product
in the acylation reaction, has a very large value of the dielectric loss
factor, 10.1. Activation of water by microwave irradiation may lead
to effective desorption of the water from the surface of the catalyst,
resulting in acceleration of the acylation reaction. Bond et al. also
provided a similar suggestion on the effect of microwave for des-
orption of water in the solid acid-catalyzed dehydrative acylation
reaction [30].
In addition, to make a consideration from a kinetic view point for
the microwave-assisted acylation reaction, the Arrhenius plots for
the oil bath and the microwave reactions of 1a with 2b using H–Y
(60) catalyst were derived on the basis of the kinetic data taken in
the temperature range of 160–200 ◦C (Fig. 4, Table 3). Consequently,
nearly straight lines were observed in both reactions (Fig. 5). Note-
worthy is that the line of the microwave reaction is placed in a upper
position with an almost parallel manner to the line of the oil bath
reaction. The parallel location means that the activation energy of
the microwave reaction is similar to that of the oil bath reaction;
the estimated energy is 103.9 kJ/mol for the microwave reaction,
which is close to the energy of the oil bath reaction, 102.4 kJ/mol.
On the other hand, when the line of the microwave reaction was
replotted by modifying the temperature with an increase of 17◦, the
simulated line was revealed to be close to the line of the oil bath
3.3. Comparison with external heating
As mentioned above for 3a in Section 3.1, the reactions using
external heating by oil bath in place of microwave irradiation sig-
nificantly decreased the yields of other aromatic ketones 3 as well.
Thus, the yield of 3b in the reaction using oil bath was 26.6%, which
was much lower than that obtained by microwave irradiation,
59.6% (Runs 17 and 34, Fig. 3). Similarly, in the reactions of 1b also,
the yields of 3d and 3e by oil bath heating were lower than those by
microwave irradiation; the yields of 3d and 3e were respectively
31.4% and 25.9% (oil bath) and 69.6% and 57.7% (microwave) (Runs
26, 27, 35, and 36). This shows that microwave irradiation more
effectively promotes the acylation reaction than the conventional
oil bath heating.
In the microwave reaction, it seems important to know the
degree of microwave absorption for the chemical materials in the
reaction system. For transfer of microwave energy, the dielectric
loss factor of the material is a key parameter; the calorific power
(P) by microwave absorption is postulated to be proportional to the
dielectric loss factor (εꢀꢀ); P = 2ꢁfε0εꢀꢀE2, where f is the microwave
frequency, ε0 is the dielectric constant of vacuum, and E is the inten-

H. Yamashita et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 80–86
85
Table 3
Kinetic data in the reaction of 1a with 2b.a
Methodb
Temp. (K) [(◦C)]
1/temp. (K−1
)
Kinetic constant (k)c
ln(k)
MW
MW
MW
MW
MW
OB
OB
OB
OB
473 [200]
463 [190]
453 [180]
443 [170]
433 [160]
473 [200]
458 [185]
443 [170]
433 [160]
490 [217]
480 [207]
470 [197]
460 [187]
450 [177]
0.00211
0.00216
0.00221
0.00226
0.00231
0.00211
0.00218
0.00226
0.00231
0.00204
0.00208
0.00213
0.00217
0.00222
0.0456
0.0307
0.0181
0.0091
0.0040
0.0177
0.0084
0.0033
0.0016
–
−3.09
−3.48
−4.01
−4.70
−5.52
−4.03
−4.78
−5.71
−6.44
−3.09
−3.42
−4.01
−4.70
−5.52
MWd
MWd
MWd
MWd
MWd
–
–
–
–
a
Reaction conditions: 1a 12.0 mmol, 2b 2.0 mmol, H–Y (60) zeolite catalyst 100 mg, activated carbon (AC) 50 mg, 160–200 ◦C.
MW: microwave, OB: oil bath.
Kinetic constants (k) were estimated by taking a pseudo first-order approximation on the basis of the conversion of the carboxylic acid at the reaction times of 10, 20,
b
c
and 30 min (Fig. 4); the value of k for each temperature was calculated as the gradient in the straight line passing through the origin of the coordinates in the graph of
ln[100/(100 − conversion)] versus time.
d
The values of Temp. and 1/temp. were calculated by adding 17◦ to the temperatures in the microwave reaction, while the values of ln(k) were the same as those of the
original data.
how accelerate the catalytic process by specific activation of the
catalyst, the reactant, and/or the product molecules in the reaction
system.
4. Conclusion
Dehydrative acylation reactions of aromatic compounds (alkoxy
unit-containing benzene derivatives, thiophene) with aliphatic and
aromatic carboxylic acids smoothly proceeded in the presence of
H–Y and H–beta zeolite catalysts under microwave irradiation. The
microwave-assisted reactions generally gave higher yields than
the conventional oil bath reactions. Further study on the develop-
ment of efficient chemical reactions using solid acid catalysts and
microwave irradiation is under way.
Acknowledgement
Fig. 5. Arrhenius plots for the microwave (MW, ꢀ) and oil bath (OB, ꢁ) reactions
of 1a with 2b using H–Y (60) catalyst in the range of 160–200 ◦C; the line of MW
This research was partially supported by the New Energy and
Industrial Technology Development Organization (NEDO), Project
of Development of Microspace and Nanospace Reaction Environ-
ment Technology for Functional Materials.
(simulated, ꢂ) was drawn by adding 17◦ to the temperatures of the MW reaction.
reaction. These results seem to suggest that (1) the reaction path-
way by microwave heating is not so different from that by oil bath
heating, but (2) the temperature of the catalyst active center in the
microwave reaction may be higher than that in the oil bath reac-
tion. Recently, Winé et al. also have provided an assumption that
the temperature of the catalyst is higher than that of the solution in
the microwave-assisted reaction of anisole with acetic anhydride
or benzoyl chloride, catalyzed by a beta/SiC composite system [40].
Concerning the ability of microwave absorption, even if the zeolite
catalyst is not a good absorber as a bulk form [40], there would be
a possibility that the protic catalytic sites on the zeolite surface can
absorb microwaves well because of their polar structures. This may
provide an explanation on the increased reaction rate observed by
microwave irradiation, although future investigation is necessary
to evaluate the microwave absorption of the surface of the zeolite
catalyst. On the other hand, heat-transfer is also an important key
factor, and we have to think whether non-uniform temperature
distribution really exists or not and, if it exits, whether the temper-
ature of the catalytic sites is reasonably high enough to promote the
acylation reaction. Although it is difficult to give confirmed ideas
at the moment and further study is required to elucidate the origin
of the microwave effect, we suppose that selective heating itself is
a characteristic nature of microwave irradiation and would some-
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