Quaternary ammonium functionalized Fe3O4@P(GMA-EGDMA) composite particles as highly efficient and dispersible catalysts for phase transfer reactions

Article abstract of DOI:10.1039/c5ra09088a

This work describes the synthesis and catalytic behavior of magnetic nanoparticle supported quaternary ammonium phase transfer catalysts Fe₃O₄@P(GMA-EGDMA)N⁺(CH₃)₃OH^- (MQPTCs). The MQPTCs prepared by compositing P(GMA-EGDMA) on Fe₃O₄ magnetic nanoparticles via seeded emulsion polymerization and quaternized by trimethylamine were characterized by SEM, TEM, FTIR, TGA and VSM, and core-shell structured MQPTCs with an exchange capacity of 1.14 mmol g^-1 were obtained. The catalytic activity of the MQPTCs was evaluated via the investigation of the efficiency of the conversion of benzyl alcohol and benzyl bromide into dibenzyl ether. In addition, the influences of the concentration of the catalysts and species of the catalysts (different exchange capacities of the MQPTCs) on conversion were studied in detail. The results demonstrated that the MQPTCs showed high efficiency with a conversion of 95% in only 6 h that was comparable with small molecule PTCs, and excellent reusability with a conversion of 94.1% after being reused 8 times.

Full text of DOI:10.1039/c5ra09088a

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Responses to Editor:
1
*
*
. A table of contents entry. This should include:
Colour graphic: maximum size 8 cm x 4 cm
Text: one sentence, of maximum 20 words, highlighting the novelty of the work.
Answer:
OH
Br
+
O
OH-
OH- Br-
Br-
Br-
Br-
Br-
HBr
OH-
OH-
H O
2
+
-
-
O
H
Br-
O
H
Organic phase
Aqueous phase
OH-
-
B
r
-
O
H
Br-
Br-
Br-
Br-
OH-
-
OH-
OH
OH-
CH3
OH-
OH-
OH
H
C
2
N+
OH-
OH-
Br-
Fe
3
O
4
C
H
OH-
OH-
H3
C
CH
3
OH-
OH-
OH-
OH-
P(GMA-EGDMA)
We have revealed a high efficiency, high dispersibility, structural stability, easy
recovery and reusable magnetic phase transfer catalyst.

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DOI: 10.1039/C5RA09088A
RSC Advances
ARTICLE
Quaternary ammonium functionalized Fe O @P(GMA‐EGDMA)
3
4
composite particles as high efficiency and dispersibility catalysts
for phase transfer reaction
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
a
a
Xiangkun Jia , Xinlong Fan , Yin Liu , Wei Li , Lei Tian , Lili Fan , Baoliang Zhang , Hepeng Zhang
DOI: 10.1039/x0xx00000x
a
and Qiuyu Zhang*
www.rsc.org/
This work describes the synthesis and catalysis behavior of magnetic nanoparticles supported quaternary ammonium
+
‐
phase transfer catalysts Fe
EGDMA) on Fe
3
O
4
@P(GMA‐EGDMA)N (CH
3
)3OH (MQPTCs). The MQPTCs prepared by compositing P(GMA‐
3
O
4
magnetic nanoparticle via seeded emulsion polymerization and quaternized by trimethylamine were
characterized by SEM, TEM, FTIR, TGA and VSM, and core‐shell structured MQPTCs with an exchange capacity of 1.14
mmol/g were obtained. Catalytic activity of the MQPTCs was evaluated via the investigation of efficiency of the conversion
of benzyl alcohol and benzyl bromide into dibenzyl ether. In addition, the influences of the concentrations of catalysts and
species of catalysts (different exchange capacities of MQPTCs) on conversion were studied in detail. The results
demonstrated that the MQPTCs showed high efficiency with a conversion of 95% in only 6 h that was comparable with
small molecule PTCs, and excellent reusability with a conversion of 94.1% after reused for 8 times.
1. Introduction
embodied in the high reusability, accordingly, the cost of the
organic synthesis is controllable.
Over the past several decades, the investigation of phase
transfer catalysts (PTCs) has attracted much attention owing to
their effective utility in organic synthesis[1‐8]. The major
function of PTCs is to extract ionic reactant out of one phase
and into the other phase where the reaction can take place, as
a result of which the concentration of reactant in reaction
phase is increased, thus accelerating the rate of multiphase
reaction and improving the reaction efficiency[9]. However,
the use of soluble small molecule PTCs in plenty of organic
synthesis reactions is generally inapplicable for the prime
reason that they are difficult to be removed from the reaction
system at the end of the reaction, which leads to the reduction
of the purity and property of the products[3]. Using non‐
recyclable small molecule PTCs is unacceptable in terms of
economy[10] under some circumstances, and causes
eutrophication of water if they were discharged into the
environment because of nitrogen and phosphorus in PTCs[11‐
Although the polymer‐supported PTCs are recyclable after the
reaction, the separation process remains unsatisfactory. The
existing methods to remove polymer‐supported PTCs from
reaction mixture including filtration, natural sedimentation
and centrifugation are time‐consuming and energy‐intensive
processes. While in the past few decades, magnetic
nanoparticles have been introduced to composite with
functional materials to enhance their seperation process on
account of the readily separability and recoverability by
external magnetic field[15‐21]. Kawamura et al.[22] prepared
the first example of magnetic nanoparticles‐supported PTCs
through the immobilization of small molecule quaternary
ammonium and phosphonium salts on magnetic nanoparticles
which were obtained by conventional coprecipitation
method[23], they reported that the catalysts could be
removed easily using an external magnet and were reusable
for four times without significant loss of their catalytic
reactivity by using 3 mol% catalyst based on reactant.
However, the magnetic nanoparticles loaded an unacceptable
low capacity of quaternary ammonium and phosphonium
groups ranging from 0.10 to 0.25 mmol/g. Moreover, the
magnetic nanoparticles‐supported PTCs are difficult to avoid
agglomerating due to their tiny size. The capacity and the
dispersibility were crucial for supported catalysts. The dosage
of catalysts could be decreased when the catalysts had more
effective constituents. High dispersibility guaranteed the
uniform of reaction processes, and further decreased the
amount of catalysts, accelerated the rate of reaction.
1
3]. Fortunately, these problems can be improved when
polymer‐supported PTCs emerged in recent years which were
prepared via loading active‐sites chemically on polymer
supporters[7, 10, 14, 15]. As the properties and structure of
active components on polymer‐supported PTCs are identical
with non‐supported soluble small molecule PTCs, the polymer‐
supported PTCs not only inherit the high‐efficiency of common
PTCs, but also can be removed from the system after reaction.
The preponderant feature of the polymer‐supported PTCs is
a.
Department of Applied Chemistry, School of Science, Northwestern Polytechnical
University, No. 127, West Youyi Road, Xi’an 710072, Shaanxi, China.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 1
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ARTICLE
Eventually, the cost of production could be reduced. stirring at 200 rpm for 12 h, the Fe
Journal Name
3
O
4
@P(GMA‐EGDMA) composite
View Article Online
Therefore, it is necessary to pay much attention to increase particles were obtained by separating in theDpOrIe: s10e.n1t03o9f/mC5aRgAn0e9t0a8n8Ad
the capacity of active‐sites on the magnetic nanoparticles‐ washing with water and ethanol for 5 times. Finally, the separated
supported PTCs and improve their dispersibility so as to make product was freeze‐dried. The percentage of P(GMA‐EGDMA) on
use of them to economically prepare productions on an the composite particles could be obtained by TG.
industrial scale.
Herein, we carried out studies in an effort to enhance the 2.3 Preparation of magnetic nanoparticles‐supported quaternary
dispersibility and increase the capacity of active‐sites on ammonium phase transfer catalysts
magnetic nanoparticles‐supported PTCs which were prepared
by compositing and quaternizing of poly(glycidyl methacrylate‐
Through the TG data, we could gain the contents of P(GMA‐
EGDMA) on the composite particles, and then the molar weight of
ethyleneglycol dimethacrylate (P(GMA‐EGDMA)) on Fe O
3
4
epoxy group would be estimated. Fe O @P(GMA‐EGDMA)
3
4
nanoparticle surfaces. Since GMA was general reaction
scaffold due to the high activity and easy post‐modification,
the introduction of GMA could not only improve the
dispersibility of composite nanoparticles but also bring the
functional groups to increase the exchange capacity and
further enhance the catalytic efficiency of magnetic
nanoparticles‐supported PTCs composite microspheres. The
ability of the separation of PTCs composite microspheres could
be greatly raised by external magnet at the end of reaction,
which was attributed to the high magnetic responsibility of
Fe O nanoparticles prepared by solvothermal method. The
composite particles with different thicknesses of P(GMA‐EGDMA)
0.1 g in 25 mL water) and TMA (approximately excessive 60 molar
times of epoxy group, dispersed in 125 mL 1,4‐dioxane) were
blended into a three‐neck round‐bottomed flask at 200 rpm and
heated to 60 °C. After stirring for 12 h, magnetic nanoparticles‐
supported quaternary ammonium phase transfer catalysts
(
(MQPTCs) were obtained by separating in the present of magnet
and washing with water and ethanol for 5 times, then freeze‐dried.
The MQPTCs were sealed and stored at room temperature.
3
4
200.4 Evaluation of the catalytic activity of MQPTCs
catalytic activity of this magnetic nanoparticles‐supported
PTCs was investigated through catalyzing the nucleophilic
reaction between benzyl alcohol and benzyl bromide.
The oil phase contained benzyl alcohol (0.81 g, 7.5 mmol), benzyl
bromide (1.28 g, 7.5 mmol) and toluene (13.05 g) was added into a
three‐neck round‐bottomed flask equipped with a paddle agitator
and reflux condenser, then aqueous phase consisted of MQPTCs
2. Experimental
2.1 Materials
ultrasonically dispersed into a mixture of H O (15 g) and NaOH (15
2
Glycidyl methacrylate (GMA, 95%, Sartomer Company) and g) was added into the oil phase. The phase transfer reaction was
o
ethyleneglycol dimethacrylate (EGDMA, 98%, J&K Scientific, Ltd.) carried out at a stirring rate of 400 rpm and a temperature of 40 C.
were purified to remove inhibitor by passing through alkaline Al O
In order to evaluate the catalytic activity of MQPTCs, oil samples
2
3
column and stored in a refrigerator prior to use. 2,2'‐Azobis(2‐ taken at time intervals were used for determining the conversion
methylpropionamide) dihydrochloride (V‐50), hexadecyl trimethyl rate of the phase transfer reaction. 0.1 g oil phase (accurate to 0.1
ammonium bromide (CTAB), γ‐(methactyloxyl)propyltrimethoxyl mg) was dissolved in 20 mL methanol, then diluted to 100 mL for
silane (MPS), trimethylamine (TMA), 1,4‐dioxane, benzyl alcohol, measurement by high performance liquid chromatography (HPLC).
benzyl bromide, toluene, ethanol, sodium hydroxide (NaOH) and The ultimate yield was confirmed by HPLC with the flowing phase
tetrabutyl ammonium bromide (TBAB) were procured from composed of methanol and water (v:v=80:20). The yields (y) of this
commercial sources and used as received. Methanol (HPLC grade) phase transfer catalytic reaction were calculated by the following
was purchased from J & K Scientific Ltd. Deionized water was used formulae:
throughout the whole experiment. Fe O nanoparticles were
3
4
prepared according to the procedure reported in the early ꢀ ꢁ ꢂꢃ ꢄ ꢅꢆ⁄ꢇꢈꢃ ꢄ ꢉꢊꢋꢌ ꢍ ꢉꢎꢏꢐꢑꢒꢓꢔꢕꢖꢑꢗꢘꢑꢌꢙ ꢄ ꢚꢛꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢂꢜꢆ
literature of our research group[17].
ꢅ ꢁ ꢝ ꢍ ꢝ ꢍ ꢝꢟꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢂꢞꢆ
ꢜ
ꢞ
2
.2 Synthesis of Fe O @P(GMA‐EGDMA) composite particles
3 4
ꢃ ꢁ ꢂꢠ ꢄ ꢜꢡꢢꢣ ꢄ ꢤ. ꢡꢡꢡꢥ ꢍ ꢡ. ꢡꢡꢡꢞꢆ⁄ꢦꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢖꢂꢟꢆꢖ
Fe
3 4
O nanoparticles prepared in advance were modified with
double bond by grafting MPS in ethanol[24]. GMA as functional w , w and w were the weight of benzyl alcohol, benzyl bromide
1
2
3
monomer and EGDMA as crosslinker were employed to produce
and toluene added into the reaction system, respectively. M was
the less mole between benzyl alcohol and benzyl bromide added
into the reaction system. MHBr=80.904 g/mol, Mdibenzyl ether=198.26
g/mol. ω was the mass fraction of dibenzyl ether produced in oil
Fe O @P(GMA‐EGDMA) composite particles via a modified seeded
3
4
emulsion polymerization technique[25]. The detailed experimental
conditions in this paper are listed in Table S1. Typically, MPS
modified Fe O particles (0.05g) dispersed in ethanol (8mL) by
3
4
ultrasonic were added into a three‐neck round‐bottomed flask
equipped with a paddle agitator and a reflux condenser, then GMA phase. The formula was matched by a series of dada points of mass
and EGDMA emulsified in aqueous solution of CTAB (0.02g in 50 mL
water) by ultrasonic were added into the mixture. 0.02 g V‐50
fraction versus the integral area of dibenzyl ether which were
measured by HPLC (Fig. S1). M was the accurate weight of sample
dissolved in 10 mL water was added into the emulsion to start the
o
polymerization when the temperature was increased to 75 C. After
2
| J. Name., 2012, 00, 1‐3
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Journal Name
ARTICLE
of oil phase. A was the integral area of dibenzyl ether measured by
View Article Online
DOI: 10.1039/C5RA09088A
HPLC.
After phase transfer catalytic reaction, the MQPTCs were separated
from the system by an external magnetic field, washed with water
and ethanol for 5 times, and then freeze‐dried for next use.
2.5 Characterization
Scheme 1. Schematic illustration of the preparation of MQPTCs.
The P(GMA‐EGDMA) contents on the magnetic composite particle
surfaces were measured by thermogravimetric analysis (TGA) on a
Q50 (TA Instruments) with a temperature from room temperature
o
o
to 800 C and a heating rate of 10 C/min. Fourier transform
infrared spectra (FTIR) were performed on a TENSOR27 FTIR
spectrometer (Bruker). The morphologies of composite particles
were observed by scanning electron microscopy (SEM, JSM‐6700F,
JEOL, Japan), samples dispersed in ethanol were dropped onto
silicon wafers and then sputtered with platinum by a JFC‐1600 auto
fine coater at a current of 20 mA for 180 s before examination.
Transmission electron microscopy (TEM, JEM‐3010, JEOL, Japan)
was operated at an accelerating voltage of 200 kV. The samples for
TEM observations were prepared by depositing particles on copper
grids, and evaporating the water at room temperature before
measurement. The magnetic properties of magnetic nanoparticles
were evaluated by a vibrating sample magnetometer (VSM,
VersaLab, Quantum Design, USA). HPLC from Hitachi was used to
measure the yield of the phase transfer catalytic reaction with the
flowing phase composed of methanol and water (v:v=80:20).
3. Results and Discussion
3.1 Synthesis and Structure of Fe
3 4
O @P(GMA‐EGDMA) and
MQPTCs composite particles
The preparation procedure of MQPTCs was exhibited in Scheme 1,
Fe particles were modified by MPS, then coated with P(GMA‐
3 4
O
EGDMA) via a seeded emulsion polymerization, finally, the MQPTCs
were produced after quaternized by TMA. Typical structures of
Fe O , MPS modified Fe O and Fe O @P(GMA‐EGDMA) with
3
4
3
4
3 4
different amounts of P(GMA‐EGDMA) particles are characterized by
TEM and SEM, and shown in Fig. 1. Image in Fig. 1a shows that
many primary crystals aggregated and assembled to form Fe O
3
4
nanoparticles with rough surfaces. The surface morphologies and
particle sizes of the magnetic nanoparticles had no obvious change
when MPS was introduced (Fig. 1b). While obvious smooth surface
was obtained once P(GMA‐EGDMA) was coated on the Fe O
3
4
surface. From the TEM images, it could be found that the thickness
of P(GMA‐EGDMA) around the core particles was increased along
with the increasing of the amounts of GMA and EGDMA in the
emulsion system (Fig. 1c‐f). When the percentage of P(GMA‐
EGDMA) was up to 50.8%, some small protuberances formed by
polymers were attached to the magnetic composite particles,
leading to unsmooth surfaces (Fig. 1g), which was possibly because
that P(GMA‐EGDMA) nanoparticles formed by the emulsion
polymerization of radical decomposed in water were captured by
the magnetic composite particles. Fig. 1h presents the morphology
of MQPTCs with 44.6% P(GMA‐EGDMA), it could be found that the
surface of MQPTCs had no change comparing with corresponding
Fig. 1. TEM and SEM images of Fe O nanoparticles (a); Fe O
3 4
3
4
modified by MPS (b); Fe
with different percentages of P(GMA‐EGDMA): 19.9%(c); 30.4%(d);
5.9%(e); 44.6%(f); 50.8%(g) and MQPTCs with 44.6% P(GMA‐
3 4
O @P(GMA‐EGDMA) composite particles
3
EGDMA)(h).
3 4
Fig. 2 shows TGA curves of Fe O nanoparticles, MPS modified
3 4
Fe O @P(GMA‐EGDMA) composite particles.
Fe O and Fe O @P(GMA‐EGDMA) with different amounts of
3
4
3 4
P(GMA‐EGDMA). The DTA curves of Fe O and Fe O @P(GMA‐
3
4
3 4
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ARTICLE
Journal Name
EGDMA) were displayed at Fig. S2. It was obvious that there was a surface of Fe
O
magnetic particles, and the epoxy group was
3
4
View Article Online
DOI: 10.1039/C5RA09088A
o
small weight loss from initial temperature to about 280 C, which successfully quaternized by TMA.
could be attributed to the evaporation of water and residual
o
o
organic solvent. From 280 C to 550 C, the tiny weight loss of Fe O
3
4
nanoparticles was due to the decomposition of small molecules
such as sodium acetate absorbed during the preparation process
(
Fig. 2a). MPS modified Fe O had more weight loss than Fe O
3
4
3
4
nanoparticles because of the silane coupling agent attached (Fig.
b), and the magnetic contents of curve b is about 93.4%. When the
2
amounts of GMA were increased during the seeded emulsion
polymerization, the magnetic contents of Fe O @P(GMA‐EGDMA)
3
4
composite particles were decreased to 74.8%, 65.0%, 59.9%, 51.8%
and 46.0% (Fig. 2c‐g), respectively. And the percentage of P(GMA‐
EGDMA) on the composite particles could be calculated by the
following formula:
Fig. 3. FTIR spectra of Fe
3 4 3 4
O nanoparticles(a), Fe O modified by
MPS(b); Fe O @P(GMA‐EGDMA) (c) and quaternized composite
3
4
particles(d).
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ꢜ
ꢞ
ꢜ
The magnetic responsivity of the magnetic particles which is usually
evaluated by specific saturation magnetization is an important
property for practical application. Fig. 4 presents the hysteresis
loops of Fe O nanoparticles (Fig. 4a), MPS modified Fe O particles
η and η were the percentage of inorganic component of Fe O
3 4
1
2
modified by MPS and Fe O @P(GMA‐EGDMA) with different
3
4
thicknesses of P(GMA‐EGDMA), respectively. The percentages of
P(GMA‐EGDMA) on the composite particles were 19.9%, 30.4%,
3
4
3 4
(
Fig. 4b) and Fe O @P(GMA‐EGDMA) with different amounts of
3 4
35.9%, 44.6% and 50.8%, respectively.
P(GMA‐EGDMA) (Fig. 4c‐f). Apparently, the coercivity of all samples
is almost negligible, indicating that all of the magnetic particles
prepared in this paper possess superparamagnetic property and
could be easily separated by external field. The specific saturation
magnetization of Fe O particles was 62.6 emu/g, which proved that
3
4
Fe O particles prepared by solvothermal method had an excellent
3
4
magnetic responsivity and were easy to be manipulated by external
magnetic field. When Fe O particles were modified by MPS, the
3
4
specific saturation magnetization had a few decline (58.7 emu/g)
probably because of the coating of silane coupling agent and mild
oxidation under high temperature. Once coated by P(GMA‐
EGDMA), the specific saturation magnetization decreased along
with the increasing of amounts of P(GMA‐EGDMA). When the
percentages of polymers on Fe O @P(GMA‐EGDMA) composite
Fig. 2. TGA curves of Fe O nanoparticles(a), Fe O modified by
3
4
3 4
MPS(b) and Fe O @P(GMA‐EGDMA) with different amounts of
3
4
3
4
P(GMA‐EGDMA): 19.9%(c); 30.4%(d); 35.9%(e); 44.6%(f); 50.8%(g).
particles were 19.9%, 30.4%, 35.9%, 44.6% and 50.8%, the specific
saturation magnetizations of them were 49.7, 44.4, 41.6, 35.7 and
In order to further prove the successful coating of P(GMA‐EGDMA)
and quaternization of epoxy groups, FTIR was used to research the
alteration of functional groups of the composite particles. Fig. 3
shows the FTIR spectra of Fe O nanoparticles (Fig. 3a), Fe O
3
2.0 emu/g, respectively. The insert picture in Fig. 4 indicated that
the MQPTCs dispersed in aqueous phase gathered together
completely within 15s under an external magnetic field and
redispersed quickly with a slight shake once the magnetic field was
removed. There is a strong linear relationship between the
percentages of inorganic component pure Fe O and the specific
3
4
3 4
modified by MPS (Fig. 3b), Fe O @P(GMA‐EGDMA) composite
3
4
particles (Fig. 3c) and quaternized magnetic composite particles(Fig.
3
4
‐
1
3
d). It is quite clear that the strong peak at 579 cm corresponded
saturation magnetizations (Fig. S3), which proved that the specific
saturation magnetizations of the composite particles decreased
linearly as the increasing of polymer coating.
to the characteristic of Fe‐O vibration. The adsorption peaks at
−
1
1
186 and 960 cm could be attributed to the stretching vibration of
C‐C and Si‐O of MPS attached to Fe O particles, respectively(Fig.
3
4
3
1
b). In curve c, the distinct C=O and C‐H stretching vibrations at
−
1
724 and 2941 cm proved the successful coating of P(GMA‐
−
1
EGDMA) on the magnetic particles. The peak at 1167 cm is the
stretching vibrations of C‐O‐C of ester group. The existence of
−
1
epoxy groups is confirmed by the peaks at 856 cm of ring
stretching vibration. When the magnetic composite particles were
−
1
quaternized, the strength of peaks at 856 cm was greatly
−
1
weakened, and C‐N adsorption peak appeared at 1562 cm . All the
results proved that P(GMA‐EGDMA) was successfully coated on the
4
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‐
and the MQPTCs adsorbed OH in the aqueous phase transfer to oil
View Article Online
DOI: 10.1039/C5RA09088A
‐
phase. In the alkaline environment, abundant OH in aqueous phase
replaced Br on the MQPTCs to regenerate the MQPTCs, then the
‐
MQPTCs turned into oil phase to catalyze the reaction again.
OH
Br
+
OO
OH-
-
Br
OH-
OH OH-
-
OH
OH-
Br-
Br-
Br-
Br-
HBr
H
2
O
+
--
Br-
Organic phase
Aqueousphase
3 4
Fig. 4. The hysteresis loops of Fe O nanoparticles (a),MPS modified
OH-
OH--
-
Br
Br-
Br
OH-
Br-
Br-
OH-
Fe
3
O
4
(b) Fe
3
O
4
@P(GMA‐EGDMA) with different percentages of
OH-
OH-
-
O HH -
-
OH
OH-
Br
-
B
P(GMA‐EGDMA): 19.9%(c); 30.4%(d); 35.9%(e); 44.6%(f); 50.8%(g).
OH- OH-
OH-
OH-
OH-
OH-
The exchange capacity measured by acid‐base titration method of
MQPTCs with different percentages of polymer are given in Fig. 5. Scheme 2. Schematic illustration of the phase transfer reaction
When the percentages of polymers on Fe
@P(GMA‐EGDMA) catalyzed by MQPTCs.
O
3 4
composite particles were 19.9%, 30.4%, 35.9%, 44.6% and 50.8%,
the exchange capacities of quaternary ammonium composite
particles were 0.15, 0.48, 0.61, 0.99 and 1.14 mmol/g, respectively,
which were much larger than the capacity of previous report[22].
While the data of exchange capacity measured by acid‐base
titration were still smaller than theoretical value of quaternized
groups calculated from percentages of polymers, mostly because
the polymer could not be quaternized completely by TMA.
The effect of variation of exchange capacity of MQPTCs on the
phase transfer catalytic reaction was investigated in the range of
0
.15 to 1.14 mmol/g (Fig. 6). By controlling an equal mole of
quaternary ammonium groups at 0.6 mol% of the reactant added
into the reaction system. It was found that the exchange capacity of
MQPTCs had little impact on the phase transfer catalytic system. No
matter how low the exchange capacity was, the conversion could
reach about 95% in only 6‐8 h, and could be comparable to the
small molecule catalyst TBAB. While the initial slope of TBAB was a
little bigger than MQPTCs maybe because that TBAB could migrate
in two phases quickly owing to their small sizes. The control
experiments were consist of two sections, one was catalyzed by
Fe
quaternized by TMA, and the other was no PTCs added in the
reaction. When the MQPTCs were replaced by Fe @P(GMA‐
3 4
O @P(GMA‐EGDMA) composite particles which was not
3 4
O
EGDMA), the conversion was 61.9% after 24 h because of the
migration of HBr absorbed on organic‐inorganic composite particles
Fe O @P(GMA‐EGDMA) from oil phase to aqueous phase. While no
3 4
PTCs were added, the conversion just was 38.3% after 24 h, thus
the reaction rate was much lower obviously.
Fig. 5. The exchange capacity of MQPTCs with different percentages
of P(GMA‐EGDMA)
3.2 Evaluation of the catalytic activity of MQPTCs
In order to examine the catalytic activity of MQPTCs, the
nucleophilic reaction between benzyl alcohol and benzyl bromide
with removing HBr and producing dibenzyl ether in alkaline
environment was chosen as model reaction (Scheme 2). Most of the
MQPTCs dispersed in the aqueous phase or stayed at the interface
between oil and aqueous phase and minor amount of the MQPTCs
could enter into the oil phase because of the relatively stronger
hydrophilicity and weaker hydrophobicity of the polymer. Three
equilibriums including the distributions of MQPTCs, hydroxyl ions
‐
Fig. 6. The variation of conversion with the extended reaction time
of different exchange capacities of MQPTCs, TBAB and without
PTCs.
and bromide ions were created between the two phases. The OH
carried by the MQPTCs entered into the oil combined with HBr
‐
produced by the nucleophilic reaction to generate H O and Br . As a
2
‐
result, the amount of OH in the oil phase was decreased, while the
amount of Br in the oil phase was increased. The equilibriums of
the distributions of OH and Br were broken, leading to the
MQPTCs adsorbed Br in the oil phase transfer to aqueous phase
The MQPTCs in the range of 0.1 mol% to 2.0 mol% (based on the
mole of reactant) of the catalyst with an exchange capacity of 0.99
mmol/g were used to study the effect of concentrations of catalysts
on the rate of phase transfer catalytic reaction. From Fig. 7, it could
‐
‐
‐
‐
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be seen that when the amounts of catalyst were increased, the
reaction rate increased distinctly. Nevertheless when the amount of
catalysts was increased to 2.0 mol%, the conversion rate curve was
close to the case of 1.0 mol%, which was probably because of the
saturation of catalysts. The conversion ratio could reach 96.8%
when the reaction was catalyzed by 0.1 mol% MQPTCs for 24 h. By
raising the amounts of catalysts to 0.2 mol%, the conversion ratio
attained to 95.1% after 12 h and 97.4% after 24 h. When 0.6 mol%
MQPTCs was added, the conversion ratio reached 95.4% in 6 h, and
increased to 97.5% after 24 h. When the amounts of catalysts
reached 1.0 mol%, the conversion ratio became 96.3% in only 3.5 h,
and 98.8% in 24 h. By further increasing the MQPTCs to 2.0 mol%,
View Article Online
DOI: 10.1039/C5RA09088A
Fig. 8. Recycling of the MQPTCs for the preparation of dibenzyl
ether.
9
5.0% conversion ratio was achieved after 2.5 h, and after 24 h, the
conversion ratio was increased to 98.4%. The efficiency of
producing dibenzyl ether catalyzed by MQPTCs was much higher
than previous reports[26, 27].
b
I
2941
1724 1562
1167
II
4000
3500
3000
2500
2000
1500
1000
500
-
1
Wavenumber (cm
)
Fig. 9. (a)TEM and SEM image of MQPTCs after reused 8 times;
b)FTIR spectra of quaternized composite particles(I) and MQPTCs
after reused 8 times(II).
(
Fig. 7. The variation of conversion with the extended reaction time
of different dosages of MQPTCs.
Conclusions
We have demonstrated the preparation and the efficiency on
producing of dibenzyl ether by using magnetic particles‐supported
quaternary ammonium phase transfer catalysts in this paper. The
results of the experiments indicated that the conversion of phase
transfer reaction catalyzed by MQPTCs composite particles could
reach to 95% in only 6 h, showing high efficiency comparing with
small molecule PTCs. More importantly, the catalysts have a high
exchange capacity of 1.14 mmol/g and could be recycled easily and
rapidly via an external magnetic field. Moreover, the catalytic
activity can be kept well after catalyzing the phase transfer reaction
Separated easily and rapidly by external magnet is the most
favorable feature of the magnetic particles‐supported quaternary
ammonium phase transfer catalysts. After the reaction, the
MQPTCs were collected by using a magnet, and the catalysts stayed
at the bottom of the reaction vessel was washed and dried to reuse
directly. The recycle and reuse of the MQPTCs for the phase
transfer catalytic reaction were examined at 40 C for 24 h with 0.6
mol% MQPTCs. As presented in Fig. 8, the MQPTCs showed no
significant loss of catalytic activity and reached a conversion ratio of
o
9
4.1% when they were reused for 8 times. The MQPTCs reused 8
times were characterized by TEM, SEM and FTIR (Fig. 9). From Fig.
a, we could find that the morphologies of the used MQPTCs had
no change comparing with the unused MQPTCs. The FTIR data (Fig.
8
times. Furthermore, the morphology and dispersibility of MQPTCs
reused 8 times have no significant change comparing with the
unused MQPTCs. To sum up, we have revealed a high efficiency,
high dispersibility, structural stability, easy recovery and reusable
catalyst which is expected to play an important role in economically
preparation on an industrial scale.
9
−
1
9
b) showed that the C‐N adsorption peak at 1562 cm of used
MQPTCs was still existed, and the important peak of hydroxyl was
also strong enough, the structure was not broken after the phase
transfer catalytic reaction. And the MQPTCs remained excellent
dispersibility without any agglomeration even after 8 cycles (Fig.
S4), which was much better than previous report[22].
Acknowledgements
This work was supported by the National High Technology Research
and Development Program of China (863 Program) (Grant No.
2
012AA02A404), the Key Program of the National Natural Science
Foundation of China (Grant No. 51433008), the General Program of
the National Natural Science Foundation of China (Grant No.
51173146) and the Basic Research of Northwestern Polytechnical
University (Grant No. 3102014JCQ01094, JC20120248, 3102014ZD).
6
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