ZnCl2-modified ion exchange resin as an efficient catalyst for the bisphenol-A production

Article abstract of DOI:10.1016/j.cclet.2014.06.004

A ZnCl₂-modified ion exchange resin as the catalyst for bisphenol-A synthesis was prepared by the ion exchange method. Scanning electron microscope (SEM), Fourier transform infrared spectrophotometer (FT-IR), thermo gravimetric analyzer (TGA) and pyridine adsorbed IR were employed to characterize the catalyst. As a result, the modified catalyst showed high acidity and good thermal stability. Zn²⁺ coordinated with a sulfonic acid group to form a stable active site, which effectively decreased the deactivation caused by the degradation of sulfonic acid. Thus the prepared catalyst exhibited excellent catalytic activity, selectivity and stability compared to the unmodified counterpart.

Full text of DOI:10.1016/j.cclet.2014.06.004

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Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
ZnCl2-modified ion exchange resin as an efficient catalyst for the
bisphenol-A production
*
Bao-He Wang, Jin-Shi Dong, Shuang Chen, Li-Li Wang, Jing Zhu
Key Laboratory for Green Chemical Technology of Ministry of Education, Research and Development Center of Petrochemical Technology, Tianjin University,
Tianjin 300072, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A ZnCl2-modified ion exchange resin as the catalyst for bisphenol-A synthesis was prepared by the ion
exchange method. Scanning electron microscope (SEM), Fourier transform infrared spectrophotometer
(FT-IR), thermo gravimetric analyzer (TGA) and pyridine adsorbed IR were employed to characterize the
catalyst. As a result, the modified catalyst showed high acidity and good thermal stability. Zn2+
coordinated with a sulfonic acid group to form a stable active site, which effectively decreased the
deactivation caused by the degradation of sulfonic acid. Thus the prepared catalyst exhibited excellent
catalytic activity, selectivity and stability compared to the unmodified counterpart.
Received 1 April 2014
Received in revised form 26 May 2014
Accepted 29 May 2014
Available online xxx
Keywords:
Zinc chloride
Ion exchange resin
Modification
Bisphenol-A
ß 2014 Jing Zhu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
1. Introduction
and Toshitaka [14] successfully synthesized a modified catalyst
with mercapto alkyl amine, which showed a great improvement in
Bisphenol-A [2,2-bis(4-hydroxyphenyl)propane; BPA] is
mainly used for the production of polycarbonate and epoxy
resins. It is commonly prepared by the condensation of phenol and
acetone and the catalyst plays a very important role in this
reaction. Till now, some novel catalysts have increasingly drawn
attention, such as layered silicate [1], mesoporous MCM silica [2–
4], heteropoly acids [5–7], and zeolites [8]. These catalysts,
although promising, still require considerable R&D efforts prior to
the commercial scale applications. Since 1960 when U.C.
Corporation [9] first brought the cation exchange resins into
industrialized application for the BPA production, ion exchange
resins have been widely used in industry due to their high
selectivity, easy separation and minimal equipment corrosion.
However, poor thermal stability and low acidity are their intrinsic
drawbacks. At the same time, partial exchange of H+ between
cationic impurities from reactants and the resin accelerates the
deactivation of the catalyst.
the condensation of phenol and acetone. Carvill et al. [15]
discovered that 4-(2-mercaptoethyl)-pyridine was a good modi-
fied reagent. Terajima et al. [16] invented a novel thiol-modified
resin catalyst with a phosphate structure, and it proved to possess
very high activity. These methods have obviously achieved great
improvements, but they are also greatly limited by the complexity
in synthesis and easy deactivation.
Ion exchange resins have been used in other reactions, such as
esterification [17], transesterification [18], oligomerization [19].
Low acid strength is also one of their main drawbacks affecting the
reaction efficiency. Some researchers attempted to solve this issue
by introducing Lewis acids into the resins [20–23]. Magnotta and
Gates [20] reported that the acidic property of the complex formed
by AlCl3–sulfonic acid can be similar to that of the superacid
solution of SbF5 + FSO3H. Shi et al. [23] showed that the efficiency
of acid-catalyzed transesterification and esterification reactions
depend on the subtle balance between Lewis and Brønsted
acidities. It has been proved that the coordination of a Lewis acid
with a Brønsted acid can increase its original acidity [24].
Therefore, the design of dual acid catalysts based on resins can
be advantageous in the BPA production. In this study, ZnCl2 acting
as a Lewis acid was added into cationic ion exchange resins to
fabricate a more efficient catalyst. To the best of our knowledge,
this is the first resin catalyst for producing BPA that contains both
a Brønsted acid and a Lewis acid.
To overcome the shortcomings of ion exchange resins,
numerous efforts have been made, including loading thiol groups
and/or amine groups by means of reduction [10], esterification
[11], neutralization [12] or ion exchange method [13]. Takahim
*
Corresponding author.
E-mail address: cj_zhu1975@tju.edu.cn (J. Zhu).
http://dx.doi.org/10.1016/j.cclet.2014.06.004
1001-8417/ß 2014 Jing Zhu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: B.-H. Wang, et al., ZnCl2-modified ion exchange resin as an efficient catalyst for the bisphenol-A
production, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.004

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2. Experimental
where A is the indicator; H+ is the acidic site; Ka is the equilibrium
constant; aA, aH and aAH
are the activity of A, H+ and AH+,
þ
þ
2.1. Catalyst preparation
respectively; H0 is the function of acid strength.
Phenol (99.8%), acetone (ꢀ99.9%) and ethanol (ꢀ99.5%) were
purchased from Tianjin Reagent Company (China). AlCl3 (ꢀ97.0%),
FeCl3 (ꢀ99.0%), SnCl2 (ꢀ98.0%), ZnCl2 (ꢀ98.0%) were purchased
from Tianjin Guangfu Company (China).
2.3. Catalytic reaction
The synthesis of BPA was carried out in the liquid phase under
atmospheric pressure in a 250 mL three-neck round-bottom flask
equipped with a condenser, a magnetic stirrer and a thermometer.
Phenol (13.6 g) and catalyst (2.0 g) were added into the reactor,
and the reactants were heated at 70 8C for 5 h with a stirring rate
of 800 revolutions per minute (rpm), then acetone (1.7 g) was
added. After 150 min, the reaction mixture was cooled to 30 8C,
then the catalyst was separated from the reaction solution.
Reaction products were analyzed by an HPLC (Agilient 810) and
methanol/water (60:40, v/v) was used as the mobile phase with
the flow rate of 1.0 mL/min.
A strong acidic ion-exchange resin (Amberlyst-15, Rohm and
Haas Company) was washed by ethanol and deionized water with
the speed of 15 cm/min until the effluent was colorless, and was
subsequently dried in an oven for 24 h at 70 8C. Certain amount of
ZnCl2 was dissolved in ethanol to produce different concentrations
(w/w); then 5 g of treated ion exchange resin was added into the
ZnCl2 solution (100 mL) and kept for 6 h at room temperature.
Finally the modified catalyst was washed with deionized water
until no Clꢁ exists in the solution, and then was dried in a vacuum
oven for 48 h at 70 8C.
Catalytic results were recorded as conversion (CAcetone, wt%,
The preparation of AlCl3, FeCl3 and SnCl2 modified catalysts
employed the same procedure as that of ZnCl2 modified one
mentioned above.
based on acetone) and selectivity (SBPA
calculated by HPLC analysis. These parameters are defined as
, wt%), which were
CBPAðMPhenolR þ MAcetone
Þ
CAcetone
¼
ꢃ 100%
2.2. Catalyst characterization
MBPA
where CBPA is the content of BPA in the product, R is the molar ratio
of the starting phenol to acetone, MPhenol is the molecular weight of
phenol, MAcetone is the molecular weight of acetone and MBPA is the
molecular weight of BPA
The crystal morphology was observed on a Hitachi S-4800 SEM
at 5.0 kV. FT-IR spectra were recorded in a Nicolet NEXUS
spectrophotometer using KBr pellets in the 4000–400 cmꢁ1 region.
IR spectrum of pyridine adsorption was also used to obtain the type
of acid sites ranging from 1700 cmꢁ1 to 1300 cmꢁ1; the samples
were pre-treated under vacuum at 70 8C for 2 h prior to the
adsorption of pyridine at room temperature for 48 h and
subsequently desorbed at 100 8C for 1 h to physically remove
adsorbed pyridine. Thermo gravimetric analysis curves were
collected using a Perkin Elmer Diamond instrument with a heating
rate of 10 8C/min from 30 to 800 8C, and the flow rate of N2 is
40 mL/min.
CBPA
SBPA
¼
ꢃ 100%
CBPA þ C2 þ C3 þ C4 þ CR
where CBPA, C2, C3, C4 and CR are the contents of BPA, 2,4-bisphenol-
A, triphenol, chroman, and other impurities in the reaction
product, respectively.
The acid strength of the catalysts was determined by the
Hammett indicator method.
3. Results and discussion
Fig.
1 illustrates the catalytic activity affected by the
concentration of ZnCl2 (Fig. 1a) and exchange time (Fig. 1b),
respectively. Catalytic activity was enhanced greatly with in-
creased ZnCl2 concentration before 2.0% (Fig. 1a). The coordination
of Zn2+ with a sulfonic acid group formed a stable Brønsted–Lewis
For AHþ Ð A þ Hþ
þ
aA ꢂ aH
Ka ¼
H0 ¼ pKa
þ
aAH
100
90
80
70
60
50
40
30
20
100
(a)
(b)
90
80
70
60
50
40
Conversion
Conversion
Selectivity
Selectivity
30
20
0
2
4
6
8
0.0
0.5
1.0
1.5
2.0
2.5
Exchange time (h)
ZnCl2 concertration (%)
Fig. 1. Effects of preparation conditions on catalytic activity of ZnCl2-modified ion exchange resin catalyst for phenol and acetone condensation. (a) ZnCl2 concentration; (b)
exchange time. Reaction conditions: phenol 13.6 g, acetone 1.7 g, catalyst 2.0 g, 70 8C, 150 min, stirring rate 800 rpm.
Please cite this article in press as: B.-H. Wang, et al., ZnCl2-modified ion exchange resin as an efficient catalyst for the bisphenol-A
production, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.004

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3
acid site. But we should note that an excessive amount of loaded
ZnCl2 could occupy active sites and decrease effective catalytic
surface area. Consequently, the optimal concentration of ZnCl2 is
2.0%. The same tendency was observed for the exchange time
(Fig. 1b). When the exchange time exceeded 6 h the conversion and
selectivity changed little, for Amberlyst-15 ion exchange resin has
a fixed exchange volume of 4.7 mmol/g [25].
and d). Fig. 2a shows that there are many cracks on the surface of
the unmodified catalyst due to the introduction of the sulfonic acid
groups during the sulfonation process [26]. After being modified, it
had fewer or no cracks (Fig. 2b). The pore diameter became much
smaller than the unmodified ones (Fig. 2c and d). This can be
attributed to the fact that ZnCl2 and sulfonic acid groups can form a
new network structure.
The SEM images were obtained to show surface morphologies
of the catalysts before (Fig. 2a and c) and after modification (Fig. 2b
The IR spectra of pyridine adsorbed on the samples are shown
in Fig. 3a. The intense peak at 1550 cmꢁ1 was attributed to the
Fig. 2. SEM images of (a, c) unmodified and (b, d) ZnCl2-modified catalysts.
Fig. 3. (a) IR spectra of pyridine adsorbed on unmodified and ZnCl2-modified catalysts. B: Brønsted acid site, L: Lewis acid site. (b) FT-IR spectra of unmodified and ZnCl2-
modified catalysts.
Fig. 4. TGA patterns of (a) unmodified and (b) ZnCl2-modified catalysts.
Please cite this article in press as: B.-H. Wang, et al., ZnCl2-modified ion exchange resin as an efficient catalyst for the bisphenol-A
production, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.004

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Scheme 1. Coordination reaction between sulfonic acid group and ZnCl2.
Brønsted acid site. After modification two new peaks appeared at
1486 cmꢁ1 and 1435 cmꢁ1, which corresponded to the Brønsted–
Lewis site and the Lewis acid site, respectively [27]. The
measurement result indicates that the Lewis acid was successfully
introduced to the resin catalyst by partially exchanging H+ with
AlCl3, FeCl3 and SnCl2 modified catalysts were also prepared
as references, and the preparation conditions were the same as
that of ZnCl2 modified one. The electronegativities of Al3+, Fe3+
,
Zn2+ and Sn2+ are 26.72, 15.38, 10.38 and 8.12, respectively [30],
and the electron acceptability of these cations is in parallel to the
electronegativity. So the sequence of these Lewis acids’ strength is
AlCl3 > FeCl3 > ZnCl2 > SnCl2. Table 1 demonstrates the catalytic
activity of different Lewis acid modified catalysts. Moderate
strength Lewis acid modified catalyst was found to produce the
best catalytic activity. A certain strength of Lewis acid is obviously
needed to form stable Brønsted–Lewis active sites to increase the
proton-donating ability, thus making acetone more easily proton-
ated to form a stable carbenium ion to initiate the reaction
(Scheme 2, step I). However, it is important to note here that a
stronger Lewis acid do not necessarily results in a better catalytic
activity. If a metal cation is an overly strong Lewis acid, it is
speculated to be prone to coordinate with acetone, which prohibits
Zn2+
.
The FT-IR spectrum indicates that the basic skeleton of the resin
did not changed except the modified one contained a new
absorption peak at 2358 cmꢁ1 shown in Fig. 4b, which could be
due to the asymmetric stretching vibration of S55O in –SO3H [28].
After the resin modified with ZnCl2, Zn2+ interacted with the lone
pair electron of the O to form a p–d coordinate bond, which could
make the two S55O asymmetric. This bond can exist either in one
molecule or between two molecules, thus forming a peripheral
shielding that stabilizes the sulfonic acid groups. Moreover, the
p
.
electron on the S can be transferred to the empty orbitals of Zn2+
The positive charge can be delocalized in the sulfonic acid group
[27] thereby greatly improved the stability of the ion exchange
resin.
As shown in Scheme 1, ZnCl2 can coordinate with the sulfonic
acid group to form structure (I), which is unstable and can
reversibly form structure (II) and structure (III) [28]. It can be seen
that structure (III) is a strong proton-donor that possesses a high
acid strength. The Hammett indicator method was used to
measure the acid strength of the catalysts. The ZnCl2 modified
catalyst (ꢁ8.2 < H0 < ꢁ5.6) exhibited higher acid strength than the
unmodified one (ꢁ5.6 < H0 < ꢁ3.0). Therefore, we believe that the
coordination of the Lewis acid with Brønsted acid led to high
acidity and good catalytic activity.
Fig. 4 shows the TGA patterns of unmodified catalyst and 2.0%
ZnCl2 modified one. Three kinds of weight loss exist in this process
for the unmodified catalyst (Fig. 4a, curve DTG), and it is generally
considered that the first peak (80 8C) is produced by the removal of
solvent, the second (300 8C) corresponds to the removal of the
sulfonic acid group and the last (520 8C) is responsible for the
resolving of the styrene–divinyl phenyl body [29].
Table 1
Catalytic activity of different Lewis acid modified catalysts.a
Modified reagentb
Electronegativity
–
Conversion (%)
Selectivity (%)
None
AlCl3
FeCl3
ZnCl2
SnCl2
25.7
37.2
44.1
69.5
58.4
80.6
86.3
89.9
95.2
93.2
26.72 (Al3+
15.38 (Fe3+
10.38 (Zn2+
8.12 (Sn2+
)
)
)
)
a
Reaction conditions: phenol 13.6 g, acetone 1.7 g, catalyst 2.0 g, 70 8C, 150 min,
stirring rate 800 rpm.
b
Preparation methods are the same as ZnCl2 modified one.
After being modified the second peak split into two peaks
(Fig. 4b, curve DTG). According to the analysis of the FT-IR spectra
above, the coordination of Zn2+ with the sulfonic acid group
changed the structure of the catalyst, leading to a two-step process
for the sulfonation removal (the removal of single sulfonic acid
groups and the removal of sulfonic acid groups coordinating
with metal ions). Thus the sulfonic acid group was protected by
coordination and still functioned well at
a relatively high
temperature. Accordingly, the thermal stability of the catalyst
has been significantly improved.
Scheme 2. Reaction mechanism of bisphenol-A synthesis on acidic catalyst.
Please cite this article in press as: B.-H. Wang, et al., ZnCl2-modified ion exchange resin as an efficient catalyst for the bisphenol-A
production, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.004

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4. Conclusion
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We have successfully fabricated a ZnCl2-modified ion exchange
resin that possesses excellent catalytic activity for the synthesis of
bisphenol-A. The final conversion of acetone and selectivity of BPA
are 69.5% and 95.7%, respectively. The increased activity and
stability after adding Lewis acid have been preliminarily explained
by analyzing the IR spectra and the rein structure coordinating
with ZnCl2. The TGA patterns also indicated that the thermal
stability of the resin catalyst was dramatically increased after
modification. Compared with the widely used thiol-modified ion
exchange resin, the ZnCl2-modified catalyst is easier to prepare and
less susceptible to oxidation. The synergistic effect of Brønsted and
Lewis acids not only extends the use of resin catalysts in industry,
but also provides a guideline for the design of new catalysts.
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Please cite this article in press as: B.-H. Wang, et al., ZnCl2-modified ion exchange resin as an efficient catalyst for the bisphenol-A
production, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.06.004