Esterification of sodium 4-hydroxybenzoate by ultrasound-assisted solid-liquid phase-transfer catalysis using dual-site phase-transfer catalyst

Article abstract of DOI:10.1016/j.ultsonch.2013.08.004

The catalytic esterification of sodium 4-hydroxybenzoate with benzyl bromide by ultrasound-assisted solid-liquid phase-transfer catalysis (U-SLPTC) was investigated using the novel dual-site phase-transfer catalyst 4,4′-bis(tributylammoniomethyl)-1,1′-bip

Full text of DOI:10.1016/j.ultsonch.2013.08.004

Ultrasonics Sonochemistry 21 (2014) 395–400
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Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Esterification of sodium 4-hydroxybenzoate by ultrasound-assisted
solid–liquid phase-transfer catalysis using dual-site phase-transfer
catalyst
⇑
Hung-Ming Yang , Wei-Ming Chu
Department of Chemical Engineering, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 April 2013
Received in revised form 29 July 2013
Accepted 5 August 2013
Available online 13 August 2013
The catalytic esterification of sodium 4-hydroxybenzoate with benzyl bromide by ultrasound-assisted
solid–liquid phase-transfer catalysis (U-SLPTC) was investigated using the novel dual-site phase-transfer
catalyst 4,40-bis(tributylammoniomethyl)-1,10-biphenyl dichloride (BTBAMBC), which was synthesized
from the reaction of 4,40-bis(chloromethyl)-1,10-biphenyl and tributylamine. Without catalyst and in
the absence of water, the product yield at 60 °C was only 0.36% in 30 min of reaction even under
ultrasound irradiation (28 kHz/300 W) and 250 rpm of stirring speed. When 1 cm3 of water and 0.5 mmol
of BTBAMBC were added, the yield increased to 84.3%. The catalytic intermediate 4,40-bis(tributylam-
moniomethyl)-1,10-biphenyl di-4-hydroxybenzoate was also synthesized to verify the intrinsic reaction
which was mainly conducted in the quasi-aqueous phase locating between solid and organic phases.
Pseudo-first-order kinetic equation was used to correlate the overall reaction, and the apparent rate
Keywords:
Solid–liquid phase-transfer catalysis
Dual-site phase-transfer catalyst
Ultrasound irradiation
Esterification
coefficient with ultrasound (28 kHz/300 W) was 0.1057 minꢀ1, with 88% higher than that (0.0563 minꢀ1
)
Benzyl 4-hydroxybenzoate
without ultrasound. The esterification under ultrasonic irradiation using BTBAMBC by solid–liquid phase-
transfer catalysis was developed.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
vent is important to achieve a high catalytic activity. The decay of
reaction rate by SLPTC was commonly observed due to the side-
Phase-transfer catalysts can conduct the reactions between
mutually immiscible reactants under mild conditions. It has been
widely applied in manufacturing pharmaceuticals, agricultural
chemicals, flavorants, dyes, perfumes and environmental processes,
etc [1]. Phase-transfer catalysis (PTC) can be divided into several
categories, including liquid–liquid, solid–liquid, gas–liquid, liquid–
liquid–solid, and liquid–liquid–liquid types. For solid–liquid
phase-transfer catalysis (SLPTC), water is seldom used or only intro-
duced in a very small quantity, thus the hydrolysis side-reaction can
be effectively inhibited and the reaction rate can be promoted. SLPTC
exhibits the advantages of easy separation of products, easy selec-
tion of organic solvents, easy recovery of catalysts and prevention
of unfavorable side reactions and behaves as a green technology in
organic synthesis [2,3].
In SLPTC, the nucleophile reactant is in solid form and the other
reactant exists in the organic solvent [4]. The catalytic intermediate
is formed from the reaction of solid reactant and the catalyst dissolv-
ing in organic solvent, and transferred into the bulk organic phase to
conduct the intrinsic reaction with the organic substrate [5,6].
Hence, the content of the catalytic intermediate in the organic sol-
product salt deposited on the surface of solid reactant particles,
reducing the formation of the catalytic intermediate [7,8]. The way
to remove the deposited side-product from the surface of particles
during the progress of reaction is also important to gain a favorable
product synthesis by SLPTC. Recently, in PTC, multi-site phase-
transfer catalyst shows significant enhancement in reaction rate
[9]. Multi-site phase-transfer catalyst contains more than one active
center, and has the potential to be used in solid–liquid system.
Ultrasonic irradiation in liquid–liquid phases can increase inter-
facial area coupled with local hot-spot generation, and has been
demonstrated to promote high reaction rate in organic synthesis
[10–13]. Under ultrasonic irradiation, the liquid jet could be favor-
able to employ for removing the surface-deposited side-product in
SLPTC and increasing the formation of the catalytic intermediate.
Ultrasound irradiation combined with liquid–liquid PTC has
revealed significant improvement in the reaction rate. In third-li-
quid PTC system, the overall reaction rate can also be effectively
raised by ultrasound in two-phase flow reactor [14] or with dual-
site phase-transfer catalyst [15,16]. But the application of ultra-
sound in SLPTC especially catalyzed by dual-site phase-transfer
catalyst was rarely reported and understood.
In the present study, the efficient synthesis of benzyl
4-hydroxybenzoate was developed. Benzyl 4-hydroxybenzoate is
⇑
Corresponding author. Tel.: +886 4 22840510x609; fax: +886 4 22854734.
E-mail address: hmyang@dragon.nchu.edu.tw (H.-M. Yang).
1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2013.08.004

396
H.-M. Yang, W.-M. Chu / Ultrasonics Sonochemistry 21 (2014) 395–400
a chemically stable, insoluble in water, low melting point and low
price compound and is used as colorless dye, color developing
agent, intermediate of liquid crystal and is also widely applied in
cosmetics industry [17]. In the past, the substituted hydroxybenzo-
ates are commonly produced from esterification of hydroxybenzoic
acid and alcohols in the presence of sulfuric acid, but having disad-
vantages of severe reaction conditions, low yields, hydrolysis side
reactions and special consideration in reactor materials due to
sulfuric acid [17,18]. Those drawbacks can be overcome by phase-
transfer catalysis and the reaction rate can be enhanced by PTC
accompanying with ultrasonic irradiation. The purpose of this work
is to investigate esterification process of sodium 4-hydroxybenzo-
ate and benzyl bromide by novel dual-site phase-transfer catalyst
BTBAMBC in SLPTC under ultrasonic irradiation (termed as
U-SLPTC). The reaction mechanism was also verified by analyzing
the variation of the catalytic intermediate in this U-SLPTC system.
under vacuum, the final white solid product was obtained for
identification.
2.3. Synthesis of catalytic intermediate and determination of Q2+
To synthesize the catalytic intermediate Q(ArCOO)2, 0.001 mol
of BTBAMBC and 0.02 mol of ArCOONa were co-dissolved in
40 cm3 of de-ionized water, and reacted at 60 °C and 250 rpm for
at least 1 h. The intermediate Q(ArCOO)2 was precipitated, sepa-
rated by filtration and washed with water and acetone to eliminate
un-reacted residues for at least three times. The purified
Q(ArCOO)2 was identified with NMR. To determine the content of
Q(ArCOO)2 in the third phase, the third phase was first separated
and added into methanol containing pre-dissolved diphenyl meth-
ane (used as internal standard). A volume 0.2 cm3 of the methanol
solution was sampled and further diluted in 4 cm3 of methanol for
HPLC analysis. To determine Q2+ in the organic phase or the third
phase before and after reaction, titration method was applied by
using sodium tetraphenyl borate (0.05 N of Na+TBPꢀ) and bromo-
phenol blue (0.2% in ethanol) as titrant and indicator in water/
dichloromethane system. When the end point was reached, the
color of dichloromethane phase would change from blue to
colorless.
2. Experimental
The reagents 4,40-bis(chloromethyl)-1,10-biphenyl (95%, from
Aldrich, USA), tributylamine (99%, from ACROS, Belgium), benzyl
bromide (99%, from Alfa Aesar, USA, denoted as RBr), and
4-hydroxybenzoic acid sodium salt (98%, from Aldrich, USA,
denoted as ArCOONa) and phase-transfer catalysts (PTCs) tetra-
n-butylphosphonium bromide (99%, from Alfa, USA, denoted as
TBPB), tetra-n-butylammonium bromide (99%, from Alfa, USA,
denoted as TBAB) and benzyltributylammonium bromide (98%,
from ACROS, USA, denoted as BTBAB) were used without further
treatments. Other reagents are all reagent-grade chemicals from
Lancaster, SHOWA, ACROS, TEDIA, Mallinckvodt, J.T. Baker, Fluka,
ECHO and TCI. The ultrasonic generator was a thermostatic bath
equipped with dual frequencies (28/50 kHz or 40/80 kHz) and
variable electric powers (max. 300 W with 0.0126 W/cm3 of power
density).
2.4. Ultrasound-assisted solid–liquid phase-transfer catalyzed
esterification
The kinetic measurement of the esterification in solid–liquid
system was carried out in
a thermo-stated batch reactor
equipped with ultrasonic generator for each reaction condition.
Definite quantities of BTBAMBC (or other PTCs), RBr and diphe-
nyl methane (internal standard) were dissolved in organic sol-
vent and stirred for 15 min in a batch reactor installed in the
ultrasonic bath, and reaction temperature was controlled at
the desired value. To start the reaction, a definite amount of
ArCOONa was introduced into the reactor and the ultrasonic
irradiation was activated. At each chosen time, 0.2 cm3 of the
organic phase was sampled and diluted in 4 cm3 of methanol
for HPLC analysis using C18 column and UV detector at
254 nm with eluent of acetonitrile/methanol/water at 1.0 cm3/
min and 37/37/26 (v/v/v).
2.1. Synthesis of dual-site phase-transfer catalyst BTBAMBC
The dual-site phase-transfer catalyst BTBAMBC was synthesized
as follows. A definite amount 0.005 mol of 4,40-bis(chloromethyl)-
1,10-biphenyl was completely dissolved in 10 cm3 of acetonitrile to
react with 0.1 mol of tributylamine at 70 °C and 150 rpm for 24 h.
After completing the reaction, acetonitrile solvent was removed to
get light yellow raw catalyst for purification. The raw catalyst was
first dissolved in 10 cm3 of n-propanol at 70 °C. A quantity 50 cm3
of methyl tert-butyl ether was then introduced very slowly under
ultrasonic irradiation for at least 1 h to precipitate white solid cat-
alyst. The solvent was removed by centrifugation. The re-precipita-
tion step was repeated for at least three times and the final solid
catalyst was rinsed with n-hexane, followed by drying at 80 °C
for 1 h to obtain BTBAMBC.
2.5. Reaction mechanism and kinetic equation
In this solid–liquid reaction system, a thin-layer phase termed
as ‘‘quasi-aqueous phase’’ can be formed by adding small amount
of water, and is locating between the solid phase and organic
phase. In this quasi-aqueous phase, the dissolved reactant salt
ArCOONa and catalyst BTBAMBC would react to produce Q(Ar-
COO)Cl and Q(ArCOO)2. The organic reactant RBr would transfer
from the bulk organic phase to the organic/quasi-aqueous interface
to conduct the intrinsic reaction with Q(ArCOO)2. The reaction
mechanism is shown as follows.
2.2. Synthesis of the product benzyl 4-hydroxybenzoate
The product benzyl 4-hydroxybenzoate (ArCOOR) was synthe-
sized as follows. An amount 0.013 mol of ArCOONa and
0.0065 mol of TBPB were first dissolved in 40 cm3 of water. Then
0.01 mol of benzyl bromide in 40 cm3 of dichloromethane was
added into the above aqueous solution to react at 35 °C and
350 rpm for 24 h. After completing the reaction, the organic phase
was separated and rinsed with de-ionized water several times to
remove the catalyst, and then concentrated under vacuum to get
crude product, which was further purified by column chromatogra-
phy in the condition of eluent at the ratio of ethyl acetate to
n-hexane equal to 1:5. After separating the solvent and drying

H.-M. Yang, W.-M. Chu / Ultrasonics Sonochemistry 21 (2014) 395–400
397
intermediate Q(ArCOO)2 and catalyst cation Q2+ in the organic/
quasi-aqueous phases before and after reactions were analyzed
to identify the reaction mechanism. The results are shown in
Table 1. In silence condition, before reaction 97% of the added cat-
alyst Q2+ existing in the quasi-aqueous phase, within which 77.4%
of Q2+ was in the form of Q(ArCOO)2, and after 30 min of reaction,
only 52% of added Q2+ retained in this phase with 1.1% of Q2+ as
Q(ArCOO)2. For the case of ultrasonic irradiation, before reaction
67% of the added Q2+ was found in the quasi-aqueous phase with
43.7% of Q2+ to be Q(ArCOO)2, and after 30 min of reaction, there
was only 50.5% of added Q2+ retained in this phase with 1.1% of
Q2+ as Q(ArCOO)2. From these results, ultrasonic irradiation was
demonstrated to be able to improve the transfer of both Q2+ and
Q(ArCOO)2 from the quasi-aqueous phase into organic phase.
When the organic reactant benzyl bromide was introduced to start
the reaction, the content of Q(ArCOO)2 in the quasi-aqueous phase
quickly decreased to a fraction of 1.1% of the total added Q2+. This
phenomenon described that the intrinsic reaction of Q(ArCOO)2
and RBr mainly conducted in this quasi-aqueous phase, locating
in the interfacial region of solid/organic phases.
After the formation of the catalytic intermediate Q(ArCOO)Br
and Q(ArCOO)2 in the quasi-aqueous phase, the intrinsic reactions
are mainly conducted at the region between the quasi-aqueous
and organic phases:
k1
RBrðorgÞ þ QðArCOOÞ2ðqꢀaqÞ ! ArCOORðorgÞ þ QðArCOOÞBrðqꢀaqÞ
;
ð1Þ
k2
RBrðorgÞ þ QðArCOOÞBrðqꢀaqÞ ! ArCOORðorgÞ þ QBr2ðqꢀaqÞ
:
ð2Þ
Thus, the rate of formation of ArCOOR in the organic phase is
expressed as
org
ArCOOR
dC
org
RBr
¼ ðk1
ꢀ
1Cqꢀaq
þ k2
ꢀ
2Cqꢀaq
ÞC
QðArCOORÞBr
;
ð3Þ
QðArCOORÞ2
dt
where k1 and k2 are reaction rate constants of reactions (1) and (2),
and 1 and 2 are enhancement factors for reactions with ultrasound
e
e
irradiation. From the experimental results, the content of
Q(ArCOO)Br in the quasi-aqueous phase is much less than
Q(ArCOO)2 and is not observable, Eq. (1) can be reduced to
org
dC
ArCOOR
¼ ðk1ꢀ1 þ k2
ꢀ
2kÞCqꢀaq
Corg
;
ð4Þ
QðArCOORÞ2 RBr
dt
3.2. Effect of water in U-SLPTC catalyzed by BTBAMBC
where k is the molar ratio of Q(ArCOO)Br to Q(ArCOO)2.Taking
molar balance for ArCOONa and neglecting the trace amounts of
Q(ACOO)Br and Q(ArCOO)2 in the organic phase, we have
In solid–liquid system, the addition of small amount of water
can be useful in forming the catalytic intermediate for conducting
intrinsic reactions. The effect of water on this SLPTC was explored
using BTBAMBC under 250 rpm and ultrasonic irradiation (28 kHz/
300 W). Fig. 1 is the plot of ꢀlnð1 ꢀ YÞ versus time for different
amounts of water added, showing that pseudo-first-order kinetic
equation can be successfully used to describe the U-SLPTC system.
Table 2 shows the apparent rate constants for the water effect.
Without adding water, only 3.7% of product yield was obtained
in 30 min of reaction; but merely with 1 cm3 of water in the sys-
tem, the product yield largely increased to 84.3%. Continuing to in-
crease the quantity of water, the reaction rate was gradually
reduced. When the amount of water was added to 15 cm3, the so-
lid–liquid system became a liquid–liquid type, and the product
yield greatly decreased to 19.5%, much lower than that in SLPTC.
The increase of reaction rate with small amount of water was
mainly due to water solubilizing a small part of solid reactant to
increase the production of Q(ArCOO)2 in the quasi-aqueous phase,
thus enhancing the intrinsic reaction rate. When more water was
added, the effective concentration of Q(ArCOO)2 in the quasi-
aqueous phase would be decreased and the reaction rate was
reduced. This phenomenon can be verified by the analysis of
Q(ArCOO)2 in the organic phase and in the quasi-aqueous phase,
as shown in Table 1 and Fig. 2a and b.
The variations of Q(ArCOO)2 in the organic phase and in the qua-
si-aqueous phase are shown in Fig. 2a and b. It is noted that most of
the formed Q(ArCOO)2 mainly stayed in the quasi-aqueous phase.
In the absence of water, the reaction rate of solid reactant and
BTBAMBC in the organic phase was very slow, resulting in small
product yield 3.7% after 30 min of reaction. With only 1 cm3 of
water, the formation of Q(ArCOO)2 greatly increased and concen-
trated in the quasi-aqueous phase. As shown in Fig. 2b, the molar
fraction of Q(ArCOO)2 in the quasi-aqueous phase relative to the to-
tal added catalyst Q2+ was quickly reduced to about 3.7% in 10 min
of reaction, meaning that the intrinsic reaction was fast in the qua-
si-aqueous phase to get above 65% of product yield.
NArCOONa;0 ¼ NASrCOONa þ Nqꢀaq
ArCOONa þ Vqꢀaqð2 þ kÞCqꢀaq
QðArCOOÞ2
org org
þ V
C
;
ð5Þ
ðArCOORÞ
where Vq-aq is the volume of quasi-aqueous phase. NASrCOONa and
qꢀaq
N
are the reactant in solid form and the dissolved in the qua-
ArCOONa
si-aqueous phase with the distribution NASrCOONa
¼
aN
, in
qꢀaq
ArCOONa
which Nqꢀaq
ArCOONa would be further converted to Q(ArCOO)2 in the qua-
si-aqueous phase. Hence, for simplicity, the efficiency factor
g
N
is used
qꢀaq
qꢀaq
qꢀaq
qꢀaq
to relate N
and N
by the equation N
¼
g
.
ArCOONa
ðArCOOÞ2
ðArCOOÞ2
ArCOONa
Substituting this equation into Eq. (5) and rearranging it, the expres-
sion for Q(ArCOO)2 is,
ꢀ
ꢁ
qꢀaq
org org
C
1
V
C
ðArCOOÞ2
ArCOOR
¼
1 ꢀ
ð6Þ
þ 2 þ kÞVqꢀaq
NArCOONa;0
NArCOONa;0
ðð1 þ
aÞ=g
The product yield (Y) based on ArCOONa is defined as the molar
ratio of produced ArCOOR to initial addition of limiting reactant
ArCOONa, Y = NArCOOR/NArCOONa,0. For largely excessive molar ratio
of RBr to ArCOONa (>14), the change of RBr in the organic phase
can be neglected. Thus, substitution of Eq. (6) into (4) yields the
pseudo-first-order kinetic equation,
ðk1ꢀ1 þ k2
ꢀ
2kÞCorg
dY
dt
RBr;0
¼
ð1 ꢀ YÞ ¼ kappð1 ꢀ YÞ
ðð1 þ
aÞ=
g
þ 2 þ kÞVqꢀaq
with the apparent rate coefficient kapp to be
ðk1ꢀ1 þ k2
ꢀ
2kÞCorg
RBr;0
kapp
¼
ð7Þ
ðð1 þ
a
Þ=
g
þ 2 þ kÞVqꢀaq
Eq. (7) is solved to get ꢀlnð1 ꢀ YÞ ¼ kapp
t
for correlating the
experimental data.
3. Results and discussion
3.1. Reaction mechanism of U-SLPTC
The interaction of water and catalyst was explored. Four condi-
tions, including case (1) with both water and catalyst, case (2) with
water, no catalyst, case (3) with catalyst, no water, and case (4)
without water and catalyst, were performed. The results are shown
in Fig. 3. For the case (4), only 0.36% of product yield was obtained
in 30 min of reaction, and by adding 0.5 mmol of BTBAMBC for case
(3), the product yield merely increased to 3.7%. When 1.0 cm3 of
In this U-SLPTC system for reacting sodium 4-hydroxybenzoate
and benzyl bromide, the quasi-aqueous phase was found to be
located between the organic phase and solid phase by adding small
amount of water. The intrinsic reaction was mainly conducted in
this organic/quasi-aqueous phases. The distributions of catalytic

398
H.-M. Yang, W.-M. Chu / Ultrasonics Sonochemistry 21 (2014) 395–400
Table 1
Distribution of catalytic intermediate and catalyst between phases.
2þ
2þ
2þ
Q2qꢀþaq=Q2þ
total
Item
Q(ArCOO)2,qꢀaq
(mmol)
Q(ArCOO)2,org
(mmol)
Q
Q(ArCOO)2,q-aq/Q
Q(ArCOO)2,q-aq/Q
qꢀaq
qꢀaq
total
(mmol)
BR#
AR#
BR*
AR*
0.3755
0.0055
0.2183
0.0054
0.0018
0.0059
0.0746
0.0115
0.485
0.26
0.335
0.253
0.774
0.021
0.652
0.021
0.751
0.011
0.437
0.0107
0.97
0.52
0.67
0.51
Conditions: ArCOONa 1 mmol, BTBAMBC 0.5 mmol, RBr 15 mmol, MIBK 15 cm3, water 1 cm3, 250 rpm, 60 °C, reaction time 30 min.
AR: after reaction; BR: before reaction; subscript q-aq: quasi-aqueous phase.
#
Without ultrasound.
With ultrasound (28 kHz/300 W).
*
10
9
8
7
6
5
4
3
2
1
0
1.8
1.6
1.4
1.2
1
(a)
0.8
0.6
0.4
0.2
0
0
10
20
30
0
10
20
30
Time (min)
Time (min)
80
Fig. 1. Plot of ꢀlnð1 ꢀ YÞ versus time for different amounts of water added; 1 mmol
of ArCOONa, 15 mmol of RBr, 0.5 mmol of BTBAMBC, 15 cm3 of MIBK, 60 °C,
250 rpm, ultrasound (28 kHz/300 W); water (cm3): (ꢀ) 0, (d) 0.5, (j) 1, (N) 3, (h) 5,
(4) 15.
(b)
70
60
50
40
30
20
10
0
Table 2
Apparent reaction rate for different amounts of water and PTC.
PTC
Amounts of water kapp ꢁ 102 (minꢀ1
)
Yield of ArCOOR (%) at
reaction time
0
10
20
30
(cm3)
15 min
30 min
Time (min)
BTBAMBC
0
0.16
7.31
10.57
4.86
1.18
0.72
8.89
7.89
6.81
3.17
70.6
78.9
46.5
13.6
9.92
70.6
70.3
67.4
3.70
73.1
84.3
78.3
32.9
19.5
78.2
81.1
77.8
0.5
1.0
3.0
5.0
15.0
1.0
1.0
1.0
Fig. 2. Effect of water on the variation of Q(ArCOO)2 in different phases; conditions
the same as that in Fig. 1; water (cm3): (a) in organic phase, (ꢀ) 0, (j) 1, (4) 15; (b)
in quasi-aqueous phase, (ꢀ) 1, (j) 3.
100
90
80
70
60
50
40
30
20
10
0
TBPB
BTBAB
TBAB
Conditions: ArCOONa 1 mmol, PTC 0.5 mmol, RBr 15 mmol, MIBK 15 cm3, 250 rpm,
60 °C, ultrasound 28 kHz/300 W.
water was employed for case (2), the product yield would increase
to 12.0%; by using 1.0 cm3 of water and 0.5 mmol of BTBAMBC, the
product yield greatly raised to 84.3%. The reaction system followed
the pseudo-first-order kinetics, except for the slight variation in
the late reaction period (>20 min) for faster reaction rate that
would generate much side product NaBr deposited on the surface
of solid reactant to retard the formation of Q(ArCOO)2 and quickly
terminate the reaction. The results strongly reveal that the small
amount of water can efficiently promote the solubilization of the
solid reactant and thus enhance the formation of catalytic interme-
diate Q(ArCOO)2.
0
10
20
30
Time (min)
Fig. 3. Effects of water on the reaction rate; 1 mmol of ArCOONa, 15 mmol of RBr,
15 cm3 of MIBK, 60 °C, 250 rpm, ultrasound (28 kHz/300 W); condition: (N) 1 cm3 of
water, 0.5 mmol of BTBAMBC, (ꢀ) no water, 0.5 mmol of BTBAMBC, (d) 1 cm3 of
water, no PTC, (j) no PTC, no water.
3.3. Activities of BTBAMBC and single-site phase-transfer catalysts
tion, BTBAMBC exhibited the best catalytic activity than TBPB, TBAB
and BTBAB. The order of reaction rate was BTBAMBC > TBPB > BT-
BAB > TBPB, and the apparent rate coefficient of BTBAMBC was al-
most 1.5 times of that of TBAB.
To compare the catalytic activity of the prepared BTBAMBC with
single-site catalysts in U-SLPTC system, the experiments using
1 cm3 of water and ultrasound (28 kHz/300 W) were performed, as
shown in Table 2. From the product yield in 15 and 30 min of reac-
Fig. 4 shows the effect of amounts of BTBAMBC for water at
1.0 cm3 under ultrasonic irradiation (28 kHz/300 W). Without

H.-M. Yang, W.-M. Chu / Ultrasonics Sonochemistry 21 (2014) 395–400
399
100
90
80
70
60
50
40
30
20
10
0
yield was 79.5% in 30 min of reaction. By applying ultrasound
(28 kHz/300 W) and at 250 rpm of stirring, 84.3% of product yield
was achieved. This implies that the synergetic effect of ultrasound
and stirring enhanced the intrinsic reaction occurring in the quasi-
aqueous phase (Fig. 5).
For temperature effect, at 55 °C, the yield was only 58.4% in
30 min, while at 60 °C, the yield raised to 78.9% only in 15 min of
reaction, showing significant increase in reaction rate by tempera-
ture rise. The apparent rate coefficients using MIBK solvent are
shown in Table 3. The Arrhenius’ equation kapp ¼ AexpðꢀEapp=RTÞ
was used to correlate the experimental data. The apparent
activation energy was obtained to be 24.05 kcal/mol, and
kapp ¼ 4:85 ꢁ 1014expðꢀ24050=RTÞ. This high activation energy
demonstrates that this U-SLPTC with dual-site phase-transfer cata-
lyst BTBAMBC was reaction-controlled and the mass-transfer resis-
tance between phases was unimportant under ultrasonic
irradiation. Fig. 6 shows the profiles of Q(ArCOO)2 in the organic
phase for different temperatures. Although the main reaction was
conducted in the quasi-aqueous phase, but the content of
Q(ArCOO)2 in the organic phase would be increased during the pro-
gress of reaction, and more catalytic intermediate was transferred
into the organic phase.
0
10
20
30
Time (min)
Fig. 4. Product yield for different amounts of catalyst; 1 mmol of ArCOONa,
15 mmol of RBr, l cm3 of water, 15 cm3 of MIBK, 60 °C, 250 rpm, ultrasound
(28 kHz/300 W); BTBAMBC (mmol): (ꢀ) 0, (j) 0.1, (d) 0.5, (h) 0.7, (4) 1.
using BTBAMBC, it still gave 12.0% of the product yield. The appar-
ent rate coefficients for different amounts of BTBAMBC were
0.045 minꢀ1 (0 mol), 0.143 minꢀ1 (0.1 mol), 1.057 minꢀ1 (0.5 mol),
0.808 minꢀ1 (0.7 mol) and 0.693 minꢀ1 (1.0 mol), respectively. As
seen in Eq. (7), the factors that affected apparent rate coefficient
Ultrasonic frequency affects the surface morphology of the
particulate phase from the impact of liquid-jet and hot-spot gener-
ated by ultrasonic waves, that in turn influences the reaction rate,
asshownin Eq. (7). Theresultsare depictedin Fig. 7a and b. Theorder
of apparent rate coefficient for tested frequency (kapp, kHz) was
by BTBAMBC was mainly reflected in the factors a, g, ke, and the
concentration of Q(ArCOO)2 in the quasi-aqueous phase. The reac-
tion rate increased with increasing the usage of BTBAMBC, up to
0.5 mol, then decreased even increasing the catalyst amount. It re-
vealed that more catalyst resulted in much faster initial reaction
rate, leading to more inorganic salt produced and deposited on
the particle surface, hence reducing the contact of the catalyst
and the solid reactant, and the reaction rate was quickly diminished
and terminated in the late reaction period (>20 min) with slight
deviation to pseudo-first-order kinetics.
(0.1057 minꢀ1, 28 kHz) > (0.0742 minꢀ1, 40 kHz) > (0.0635 minꢀ1
,
80 kHz) > (0.0563 minꢀ1, 0 kHz). The higher ultrasonic frequency
induced less powerful liquid-jet due to the shorter time for bubble
growth. The defect occurred on the solid surface was relatively
The polarity of organic solvent affects the dissolution of solid
reactant anion in organic solvent. With adding 1 cm3 of water,
the more polar solvent induced the higher activity of phase-trans-
fer catalyst, because a higher content of Q(ArCOO)2 was acquired.
As shown in Table 3, the order of product yield in 30 min of reac-
tion for the tested solvents is MIBK (84.3%) > toluene (16.7%), and
n-heptane (3.2%).
100
90
80
70
60
50
40
30
20
10
0
3.4. Effects of stirring and ultrasound irradiation
0
10
20
30
The comparison for the effect of agitation and ultrasound was
performed. For applying agitation alone, the apparent rate coeffi-
cients for different stirring speeds were 0.0669 minꢀ1 (0 rpm),
0.0843 minꢀ1 (150 rpm), 0.1 minꢀ1 (200 rpm), 0.1057 minꢀ1
(250 rpm) and 0.0937 minꢀ1 (300 rpm). The factors influencing
Time (min)
Fig. 5. Effects of ultrasound and stirring on the reaction; 1 mmol of ArCOONa,
15 mmol of RBr, 0.5 mmol of BTBAMBC, 15 cm3 of MIBK, 1 cm3 of water, 60 °C;
condition: (ꢀ) ultrasound (28 kHz/300 W), (j) stirring (250 rpm), (N) ultrasound
(28 kHz/300 W) and stirring (250 rpm).
the kapp were mainly the distribution factor
a
and the ultrasonic
enhancement factor
e. At 250 rpm without ultrasound, the product
4
3
2
1
0
Table 3
Apparent reaction rate for different organic solvents and temperatures.
Solvent (#DC at
25 °C)
Temperature
(°C)
kapp ꢁ 102
Yield of ArCOOR
(%)
(minꢀ1
)
MIBK (13.11)
40
50
60
0.88
2.75
10.57
0.57
1.49
5.63
0.57
0.09
21.1
51.1
84.3
16.6
31.5
79.5
16.7
3.2
40*
50*
60*
60
0
10
20
30
Toluene (2.4)
Heptane (1.92)
Time (min)
60
Conditions: ArCOONa 1 mmol, BTBAMBC 0.5 mmol, RBr 15 mmol, solvent 15 cm3,
Fig. 6. Temperature effect for Q(ArCOO)2 in the organic phase; 1 mmol of ArCOONa,
15 mmol of RBr, 0.5 mmol of BTBAMBC, l cm3 of water, 15 cm3 of MIBK, 60 °C,
250 rpm, ultrasound (28 kHz/300 W); temperature (°C): (e) 45, (h) 50, (s) 60, (4)
65.
water 1 cm3, 250 rpm, 60 °C, ultrasound 28 kHz/300 W, reaction time 30 min.
#
DC: dielectric constant.
*
Without ultrasound.

400
H.-M. Yang, W.-M. Chu / Ultrasonics Sonochemistry 21 (2014) 395–400
2
1.8
1.6
1.4
1.2
1
sodium 4-hydroxybenzoate and benzyl bromide under ultrasonic
(a)
(b)
irradiation in solid–liquid system. The quasi-aqueous phase was
generated by adding water and the prepared catalyst. Ultrasound
can effectively enhance the overall reaction and the surface depos-
ited side-product can be removed by liquid-jet generated from
ultrasonic waves. The reaction mechanism of U-SLPTC was identi-
fied by analyzing the variation and distribution of the catalytic
intermediate between phases. The esterification catalyzed by
BTBAMBC using U-SLPTC was developed.
0.8
0.6
0.4
0.2
0
0
10
20
30
Time (min)
Acknowledgments
4
3
2
1
0
The authors acknowledge the financial support of the National
Science Council, Taiwan, ROC (Grant No. NSC 99-2221-E-005-
092). This work is also supported in part by the Ministry of Educa-
tion, Taiwan, ROC, under the ATU plan.
References
[1] C.M. Starks, C.L. Liotta, M. Halpern, Phase-Transfer Catalysis: Fundamental,
Applications and Industrial Perspectives, Chapman & Hall, New York, 1994.
[2] H.M. Yang, H.S. Wu, Interfacial mechanism and kinetics of phase-transfer
catalysis, Catal. Rev. -Sci. Eng. 45 (2003) 463–540.
[3] N. Ge, Y. Zhang, D. Shi, Q. Gu, X. Zhu, Z. Ding, Novelties of solid–liquid phase
transfer catalyzed synthesis of benzyl diethyl phosphate from the sodium salt
of diethyl phosphate, Eur. J. Chem. 2 (2011) 269–271.
[4] S. Baj, A. Chrobak, I. Gottwald, Application of solid–liquid phase transfer
catalysis system for peroxyester synthesis: a kinetic study of hydroperoxides
acylation in the presence of solid sodium carbonate, Appl. Catal., A 224 (2002)
89–95.
[5] H.M. Yang, C.M. Wu, Phase-transfer catalyzed allylation of sodium phenoxide
in solid–liquid system, J. Mol. Catal. A: Chem. 153 (2010) 83–91.
[6] H.M. Yang, P.I. Wu, Kinetics of phase-transfer catalyzed esterification of
0
10
20
30
Time (min)
Fig. 7. Effect of ultrasonic frequency (a) on the product yield, (b) on the variation of
Q(ArCOO)2 in the organic phase; 1 mmol of ArCOONa, 15 mmol of RBr, 0.5 mmol of
BTBAMBC, l cm3 of water, 15 cm3 of MIBK, 60 °C, 250 rpm, 300 W of ultrasonic
power; ultrasonic frequency (kHz): (s) 0, (e) 28, (h) 40, (4) 80.
1.8
1.6
1.4
1.2
1
aliphatic dicarboxylate in solid–liquid system, Appl. Catal.,
17–26.
A 209 (2001)
[7] H.M. Yang, C.M. Wu, H.E. Wu, Kinetics for phenolysis of ethyl
2-bromoisobutyrate via phase-transfer catalysis in
J. Chem. Technol. Biotechnol. 75 (2000) 387–393.
a solid–liquid system,
[8] H.M. Yang, P.I. Wu, C.M. Li, Etherification of halo-eater by phase-transfer
catalysis in solid–liquid system, Appl. Catal., A 193 (2000) 129–137.
[9] H.E.-S. Ali, Cycloalkylation reactions of fatty amines with a, x-dihaloalkanes:
role of bis-quaternary ammonium salts as phase-transfer catalysts, Catal.
Commun. 8 (2007) 855–860.
0.8
0.6
0.4
0.2
0
[10] J.L. Luche, Synthetic Organic Sonochemistry, Plenum Press, NewYork, 1998.
[11] M.L. Wang, V. Rajendran, Ultrasound assisted phase-transfer catalytic
0
10
20
30
epoxidation of 1,7-octadiene
(2007) 46–54.
– a kinetic study, Ultrason. Sonochem. 14
Time (min)
[12] J.T. Li, X.L. Li, An efficient and practical synthesis of methylene dioximes by
combination of ultrasound and phase transfer catalyst, Ultrason. Sonochem. 14
(2007) 677–679.
[13] H. Xu, Y. Chen, An efficient and practical synthesis of mandelic acid by
combination of complex phase transfer catalyst and ultrasonic irradiation,
Ultrason. Sonochem. 15 (2008) 930–932.
Fig. 8. Plot of ꢀlnð1 ꢀ YÞ versus time for effect of ultrasonic power; 1 mmol of
ArCOONa, 15 mmol of RBr, 0.5 mmol of BTBAMBC, l cm3 of water, 15 cm3 of MIBK,
60 °C, 250 rpm, 28 kHz of ultrasonic frequency; ultrasonic power (W): (s) 0, (e)
100, (h) 200, (4) 300.
[14] H.M. Yang, G.Y. Peng, Ultrasound-assisted third-liquid phase-transfer
catalyzed esterification of sodium salicylate in a continuous two-phase flow
reactor, Ultrason. Sonochem. 17 (2010) 239–245.
small and the deposited inorganic salts would be less sheared,
leading to slower formation of Q(ArCOO)2 (Fig. 7b).
[15] H.M. Yang, C.C. Chiu, Ultrasound-assisted phase-transfer catalysis:
benzoylation of sodium 4-acetylphenoxide by dual-site phase-transfer
catalyst in a tri-liquid system, Ultrason. Sonochem. 18 (2011) 363–369.
[16] H.M. Yang, Y.C. Chen, Ultrasound-assisted third-liquid phase-transfer
catalyzed esterification of potassium 4-methoxyphenylacetate by dual-site
phase-transfer catalyst, J. Taiwan Inst. Chem. Eng. 43 (2012) 897–903.
[17] Y. Hu, X. Liu, L. Wang, Study on synthesis of benzyl p-hydroxybenzoate by
phase transfer catalysis, Huaxue Shiji 26 (2004) 363–365.
The effect of electric power of ultrasound was evaluated for 0,
100, 200 and 300 W, as shown in Fig. 8. From the pseudo-first-order
plots, the apparent rate coefficients were found to be 0.0563 minꢀ1
for 0 W, 0.0759 minꢀ1 for 100 W, 0.092 minꢀ1 for 200 W and
0.1057 minꢀ1 for 300 W, respectively. With no doubt, the reaction
rate increased with increasing ultrasonic power, showing the 88%
increase of reaction rate by 28 kHz/300 W of ultrasonic irradiation.
[18] D. Peng, G. Xiong, W. Xu, New method for synthesis of benzyl
p-hydroxybenzoate, Shipin Gongye Keji 25 (2004) 113–115.
4. Conclusion
In the present study, the novel dual-site phase-transfer catalyst
BTBAMBC was synthesized and used to catalyze esterification of