A novel green process for tannic acid hydrolysis using an internally sulfonated hollow polystyrene sphere as catalyst

Article abstract of DOI:10.1039/c8ra02472c

A polystyrene-hollow sphere catalyst was prepared by treating polystyrene-encapsulated calcium carbonate particles with concentrated hydrochloric acid. This catalyst was characterized using TGA, FT-IR, optical microscope, SEM-EDX and XPS. Evidences from SEM-EDX and XPS analyses indicated that the sulfonate groups were on the inner surface of the polystyrene hollow sphere. The polystyrene hollow spheres were used as catalyst in the hydrolysis of tannic acid. Reaction conditions including the reaction temperature and time, loading of catalyst, ratio of tannic acid to H₂O and number of recycles were optimized. A high yield of gallic acid was obtained as the reaction performed under the following conditions: a temperature of 80 °C, a molar ratio of tannic acid to H₂O of 1:3, and a catalyst loading of 7% w/w (based on the mass of tannic acid). This catalyst showed excellent catalytic performance, easy separation, high stability and good reusability. This work provides a new strategy for the controllable synthesis of polystyrene hollow structures with sulfonic groups on the inner surface and an excellent and prospective catalyst for the production of gallic acid through hydrolysis of tannic acid.

Full text of DOI:10.1039/c8ra02472c

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A novel green process for tannic acid hydrolysis
using an internally sulfonated hollow polystyrene
sphere as catalyst
Cite this: RSC Adv., 2018, 8, 17151
ab
b
ab
a
c
Qionglin Luo, Shunqin Zeng, You Shu, Zaihui Fu, Hongran Zhao
a
and Shengpei Su
*
A polystyrene-hollow sphere catalyst was prepared by treating polystyrene-encapsulated calcium
carbonate particles with concentrated hydrochloric acid. This catalyst was characterized using TGA, FT-
IR, optical microscope, SEM-EDX and XPS. Evidences from SEM-EDX and XPS analyses indicated that the
sulfonate groups were on the inner surface of the polystyrene hollow sphere. The polystyrene hollow
spheres were used as catalyst in the hydrolysis of tannic acid. Reaction conditions including the reaction
2
temperature and time, loading of catalyst, ratio of tannic acid to H O and number of recycles were
optimized. A high yield of gallic acid was obtained as the reaction performed under the following
ꢀ
conditions: a temperature of 80 C, a molar ratio of tannic acid to H
2
O of 1 : 3, and a catalyst loading of
Received 21st March 2018
Accepted 27th April 2018
7
% w/w (based on the mass of tannic acid). This catalyst showed excellent catalytic performance, easy
separation, high stability and good reusability. This work provides a new strategy for the controllable
synthesis of polystyrene hollow structures with sulfonic groups on the inner surface and an excellent and
prospective catalyst for the production of gallic acid through hydrolysis of tannic acid.
DOI: 10.1039/c8ra02472c
rsc.li/rsc-advances
cannot improve the efficiency of tannic acid hydrolysis due to
following reasons: the absence of specic affinity between the
Introduction
Gallic acid is an important chemical that has various properties, reactant molecules and the ion exchange resins; the lower
such as anti-fungal, anti-viral, antioxidant, and anti-cancer diffusion and concentration of reactant inside the resin cavities
1–3
activities (Fiuza et al., 2004; Zaheer et al., 2010). It is widely because of the large size of tannic molecules and relative higher
4–6
used in organic synthesis, pharmaceutics, food etc. Gallic acid cross-link density of ionic resins. Thus, environmentally
is mainly produced by the hydrolysis of tannic acid, a specic friendly catalysts with high efficiency are need for the industrial
form of tannin obtained from Chinese gall. There are a few production of gallic acid from tannic acid. In fact, there are
typical catalysts used for hydrolysis of tannic acid including a number of previous studies where sulfonated solid acids were
7
8
13–16
inorganic bases, inorganic acids, enzymes, and sulfonic ionic prepared and used in biomass-related reactions.
9
resins. In the cases using inorganic bases and acids as cata-
In this paper, a polystyrene hollow sphere with sulfonic
lysts, a lot of wastewater is produced, which is difficult to be groups on the inner surface was prepared to improve the
treated. In addition, corrosion of the equipment and tedious diffusion of reactants, and the structure of the polystyrene
purication are also faced in typical production process. In the hollow sphere was characterized. In addition, its catalytic
9–12
case using enzyme
as catalyst, shortages including high cost performance was tested for the hydrolysis of tannic acid.
of tannase, deactivation of enzyme and non-recyclable, low
efficiency limit the application of the enzymatic method for
gallic acid production in industry. Strongly acidic ion exchange
resins could be useful in the sulfonic catalyzed hydrolysis of
Experimental section
Materials
tannic acid, but commercialized ionic exchange resins still Light calcium carbonate (2 mm < d < 5 mm) was purchased from
Zhangjiajie Hengliang New Material Technology Co., Ltd.
(
Hunan Province, China). Analytical grade styrene and divinyl-
a
Key Lab for Fine Processing of Resources and Advanced Materials of Hunan Province,
benzene were purchased from Shanghai Yongda Chemical
Reagent Co., Ltd. (Shanghai, China). Analytical grade vinyl
benzenesulfonic acid was purchased from Shanghai Aladdin
Reagent Co., Ltd. (Shanghai, China). Benzoyl peroxide (BPO),
Hunan Normal University, Changsha 410081, P. R. China
b
Hunan Engineering Laboratory for Preparation Technology of Polyvinyl Alcohol Fiber
Material, Huaihua University, Huaihua 418000, P. R. China
c
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials
3
methyl alcohol, phosphoric acid, AgNO , NaOH and NaCl were
purchased from Tianjin Chemical Reagent Co., Ltd. (Tianjin,
Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R.
China
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China). Tannic acid (>95%) was purchased from Zhangjiajie
JiuRui Biological Technology Co., Ltd. (Hunan Province, China).
Distilled water was used throughout this experiment. All
chemicals were used without further purication.
Synthesis of internally sulfonated polystyrene hollow sphere
50 g of light calcium carbonate was added into 100 mL of ethyl
alcohol in a three-necked ask equipped with a mechanical stir.
ꢀ
This mixture was homogenized for 30 min at 80 C at a speed of
2000 rpm. Then 3.5 g of vinyl benzenesulfonic acid was added.
The mixture continued to be stirred at a speed of 400 rpm at
ꢀ
8
0 C for 2 h. Subsequently, 0.105 g of styrene, 0.105 g of divinyl
benzene, and 0.07 g of BPO were added independently. Aer
h, this suspended mixture was cooled to room temperature,
and the resulted solid particles were separated and collected by
2
centrifugation. Then, they were washed with ethyl alcohol, and Fig. 2 Typical TGA curve of polystyrene hollow sphere.
ꢀ
subsequently dried at 80 C under vacuum for 12 h to obtain the
polystyrene-coated calcium carbonate. The internally
sulfonated polystyrene hollow spheres were obtained by with KBr and pressed in the form of pellets for FT-IR analysis.
removing calcium carbonate from polystyrene-coated calcium Optical microscopic images of the samples were obtained using
carbonate using hydrochloric acid (36% w/w). This polystyrene a N-180M microscope (Shanghai, China). The scanning electron
hollow spheres were further treated by repeatedly washing microscopy (SEM) was performed on a FEI Nova NanoSEM 450
using distilled water until no hydrogen chloride was detected by instrument (Hillsboro, America), and analyses were carried out
ꢀ
AgNO solution test. Aer being dried at 40 C under vacuum on a Nova NanoSEM 450 equipped with an energy-dispersive X-
3
for 12 h, 4.8 g of polystyrene hollow sphere with catalytic ray analyzer. XPS measurements were carried out on a Shi-
property was obtained. The scheme for the preparation of madzu ESCA-3200 (Tokyo, Japan) using Al Ka radiation. The
sulfonated polystyrene hollow sphere is shown in Fig. 1.
catalyst was titrated with aqueous NaOH solution to determine
the amount of catalytic –SO H group in it. In a typical proce-
3
ꢁ1
Characterization
dure, 20 mg of the sample was suspended in 10 mL of 2 mol L
aqueous NaCl solution for 24 h at room temperature, then this
Thermogravimetric analysis (TGA) was performed on a Netzsch
suspension was ltrated, and the resulting solution was titrated
STA409PC instrument (Bavaria, Germany) under a owing
ꢁ
1
12
ꢀ
ꢁ1
with 0.05 mol L aqueous NaOH solution. The amount of
catalytic –SO H group was calculated from the amount of NaOH
nitrogen atmosphere at a scan rate of 20 C min from room
ꢀ
3
temperature to 1000 C using a-Al
2
O
3
as the standard material.
used in the above titration reaction. The samples were quanti-
tatively analyzed by HPLC with UV detector. The HPLC column
was Phenomenex C18 (5 mm, 250 mm ꢃ 4.6 mm ID). The
mobile phase was a mixture of methyl alcohol, phosphoric acid
TGA results reported were the average of a minimum of three
ꢀ
determinations. Temperatures were reproducible to ꢂ3 C, and
the error bars on the fraction of nonvolatile material were ꢂ1%.
Fourier-transform infrared (FT-IR) spectra of the samples were
obtained using a WQF-200 spectrometer (Rayleigh, China) over
and H
2
O in a volume ratio of 15 : 0.5 : 100 at a ow rate of 0.5
ꢁ1
a frequency range of 400–4000 cm . The samples were mixed
Fig. 1 The scheme for the preparation of sulfonated polystyrene hollow sphere.
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tannic acid, and the recycling of catalyst were optimized and
evaluated.
Results and discussion
Characterization of catalysts
TGA analysis. In this experiment, the thermal stability of
polystyrene hollow sphere was evaluated using TGA, and the
experimental results were shown in Fig. 2.
TGA analysis information includes the 5% weight loss
temperature (T5%), the initial degradation temperature, the 50%
weight loss temperature (T50%), the degradation temperature,
ꢀ
13
and the char content (residual weight percent at 950 C). Fig. 2
ꢀ
indicates that the samples began to lose weight at 300 C, which
is due to the thermal degradation of polystyrene hollow sphere.
It is evident that the prepared polystyrene hollow sphere is
ꢀ
thermally stable below 300 C, which would be the upper limit
Fig. 3 FT-IR spectra of polystyrene hollow sphere.
temperature of the hydrolysis of tannic acid.
FT-IR analysis. FT-IR analysis was conducted to obtain the
chemical structural information of the polystyrene hollow
sphere. FT-IR spectra of polystyrene hollow sphere are shown in
Fig. 3.
ꢁ
1
ꢀ
mL min . The temperature of column was 30 C and the
wavelength was 280 nm.
The yield of gallic acid was calculated by the following
equation:
ꢁ1
The characteristic absorption bands at 1190–1200 cm and
ꢁ
1
1020–1130 cm
can be attributed to the asymmetric and
m
GA
Y ¼ m
ꢃ 100%
symmetric stretching modes of O]S]O, respectively, and the
TA
w
TA
ꢁ
1
absorption band at 573–629 cm can be attributed to the
where Y is yield of gallic acid; mGA is mass of gallic acid
determined by HPLC); mTA is mass of tannic acid; wTA is mass
17
stretching mode of S–O. These above absorption bands are the
(
3
evidence for the presence of –SO H in the prepared polystyrene
percentage of gallic acid to tannic acid aer complete hydro-
lysis. wTA was experimentally obtained as 80.3%.
hollow sphere. Furthermore, absorption bands at 1610–
ꢁ
1
ꢁ1
1623 cm
and 2928–2932 cm
are the evidences for the
presence of phenyl groups. The absorption band at 3000–
ꢁ
1
18
3
460 cm indicates the presence of –OH group. All above
Catalytic performance
evidences indicate that benzenesulfonic acid groups are present
The polystyrene hollow spheres were used as catalyst in the in the polystyrene hollow sphere.
hydrolysis of tannic acid. In a typical reaction, 200 g of tannic Optical microscopic analysis. Optical microscope was used
acid and 600 mL of H O were added in a three-necked ask to obtain the particle sizes of light calcium carbonate,
2
equipped with magnetic stirring bars and water-separation polystyrene-coated calcium carbonate, and polystyrene hollow
apparatus. Aer being stirred for 30 min, 7% w/w of catalyst sphere. The images are shown in Fig. 4.
(
based on the mass of tannic acid) was added. The reaction
Light calcium carbonates have irregular shapes and the
ꢀ
temperature and time were set at 80 C and 6 h, respectively. particle sizes are in the range of 2–5 mm (Fig. 4A). The light
Then, the solution was cooled down. Reaction process was calcium carbonate was coated with polystyrene to obtain
monitored using HPLC. The reaction temperature and time, the polystyrene-coated calcium carbonate. The polystyrene-coated
amount of polystyrene hollow sphere, the concentration of calcium carbonate has smooth surface and the sizes increase
Fig. 4 The optical microscopic images of (A) light calcium carbonate, (B) polystyrene-coated calcium carbonate, and (C) polystyrene hollow
sphere.
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Fig. 5 SEM image (A) and EDX analysis results (B) of polystyrene hollow sphere. Red box in (A) denotes the region analyzed by EDX (B).
SEM-EDX and XPS analyses. SEM-EDX and XPS analyses
were performed on the polystyrene hollow sphere to determine
the position of –SO H groups. SEM-EDX is an X-ray technique
3
used to reveal the elemental composition of materials. SEM-
EDX analysis results consist of spectra showing peaks corre-
sponding to the elements making up the true composition of
19
the analyzed sample. SEM-EDX analysis of the polystyrene
hollow sphere are shown in Fig. 5. There is no evidence for the
presence of S element on the outer surface of polystyrene hollow
sphere.
This polystyrene hollow sphere was also characterized by
XPS to determine the elementary composition and valence
state.
Fig. 6 XPS spectra of polystyrene hollow sphere.
As shown in Fig. 6, the peak at binding energy of 1196.7 eV
was assigned to C1s, conrming that the catalyst was composed
of C. There is no peak at the binding energy of S, indicating no
sulfonic group on the interior of the polystyrene hollow
20
sphere. We magnied the XPS spectra near the positions
where peaks of S occur, but the peaks of S were still not found
(see the following Fig. 7). Both XPS and SEM-EDX analysis
results demonstrate that there is no element S present on the
outer surface of the polystyrene hollow sphere. Considering the
IR analysis results, we inferred that SO H groups should be on
3
the inner surface of the polystyrene hollow sphere.
Application in the hydrolysis of tannic acid. Weighed
amounts of tannic acid and H O (in a molar ratio of 1 : 3), as
2
well as polystyrene hollow sphere (7% w/w, based on the mass of
tannic acid) were added to a 500 mL three-necked round
bottomed glass ask equipped with a water-cooled condenser
and a magnetic stirrer. The temperature of the reaction mixture
Fig. 7 XPS spectra of polystyrene hollow sphere (magnified the XPS
spectra near the positions where peaks of S occur).
was controlled using a temperature controller with an accuracy
ꢀ
to be in the range of 5–10 mm (Fig. 4B). Polystyrene-coated of ꢂ0.5 C. The hydrolysis reaction of tannic acid (Fig. 8) was
calcium carbonate was treated with hydrochloric acid solution allowed to proceed for 0.5–6 h with vigorous stirring at the
to obtain polystyrene hollow sphere. Polystyrene-coated calcium designed temperature. Then, the solution was cooled down.
carbonate and polystyrene hollow sphere have no signicant Subsequently, the amount of tannic acid in the reaction mixture
difference in particle size and shape. The prepared polystyrene was determined by HPLC.
hollow sphere has a smooth surface with a size of 5–10 mm
Fig. 4C). From the perspective of catalyst, the size of poly- (C) molar ratio of reactants, (D) loading of polystyrene hollow
styrene hollow sphere meets the size requirement of a high- sphere and (E) number of recycles on the tannic acid hydrolysis
performance catalyst.
were evaluated (Fig. 9).
The inuences of (A) reaction time, (B) reaction temperature,
(
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Fig. 8 Hydrolysis of tannic acid using polystyrene hollow sphere as catalyst.
ꢀ
ꢀ
Fig. 9A shows the time course of yield of gallic acid at 80 C, temperature of 80 C, reaction time of 6 h, and molar ratio of
molar ratio of tannic acid to H O of 1 : 3, and polystyrene tannic to H O of 1 : 3. All loadings of polystyrene hollow sphere
2
2
hollow sphere loading of 7% w/w (based on the mass of tannic could ensure a yield no lower than 90.5% (Fig. 9D). All above
acid). At rst, the yield of gallic acid increased with the reaction experimental results indicate a high catalytic performance of
21
time and reached 96.99% at 6 h of reaction. With further polystyrene hollow sphere. Note that the yield of gallic acid
extension of reaction time, the yield remained almost decreases when the polystyryl hollow sphere loading is above
unchanged and the yield at 10 h increased little compared with about 10%. This might be related to the calcium ions in poly-
that at 6 h.
styrene hollow sphere. As the polystyrene hollow sphere loading
Fig. 9B shows the effect of reaction temperature on the yield increases, the amount of calcium ions also increases. Gallic acid
of gallic acid. Reaction temperature was set in the range of 60– produced by hydrolysis of tannic acid might react with calcium
ꢀ
1
20 C, while other reaction conditions were set as follows: ions to form by-products, thus affecting the yield of gallic acid.
molar ratio of tannic acid to H
2
O of 1 : 3, reaction time of 6 h Considering both the reaction rate and the cost of polystyrene
ꢀ
and polystyrene hollow sphere loading of 7% w/w. At 60 C, the hollow sphere, 7% w/w was taken as the optimum polystyrene
yield of gallic acid was 91.5%. As the temperature increased hollow sphere loading and used in most of the hydrolysis
ꢀ
from 60 to 80 C, the hydrolysis rate and the yield of gallic acid experiments.
increased rapidly and signicantly. As the temperature further
Fig. 9E shows the effect of the number of recycle times on the
ꢀ
increased from 110 to 120 C, the yield of gallic acid slightly activity and stability of polystyrene hollow sphere. Aer each
decreased. This indicates that gallic acid might be converted to cycle, the polystyrene hollow spheres were separated from the
pyrogallic acid or other by-products. Fig. 9C shows the effect of reaction mixture by decantation and then washed with water,
the molar ratio of tannic acid to H
2
O on the hydrolysis reaction. followed by drying before the next run. Other reaction condi-
ꢀ
The molar ratio ranged from 1 : 1 to 1 : 6, at a reaction tions included a reaction temperature of 80 C, a reaction time
ꢀ
temperature of 80 C, a reaction time of 6 h, polystyrene hollow of 6 h, a molar ratio of tannic acid to H
2
2
O of 1 : 3, and poly-
2–25
sphere loading of 7% w/w. Fig. 8C indicates that the yield of styrene hollow sphere loading of 7% w/w.
The experimental
gallic acid increases rst and then decreases as the molar ratio results indicate that the catalyst is stable enough to be recycled
changes from 1 : 1 to 1 : 6. The optimum molar ratio was found for many times. However, it is worth noting that the yield rate
to be 1 : 3 in order to obtain the maximum yield of gallic acid. In decreases slightly aer the catalyst has been reused for 8 times.
the reaction system, water serves as both solvent and reactant. In order to nd the reason for this trend, we performed XPS and
At rst, with the increase in water content, more tannic acids SEM-EDX analyses of this catalyst before and aer use (Fig. 10
are dissolved into water and the viscosity of the reaction system and 11). Fig. 10 suggests that the intensity of the peak assigned
decreases, which is conducive to dispersion of reactants and to S decreases aer the catalyst has been reused for 8 times.
enhancement of mass transfer. As a result, the yield of gallic Fig. 11 shows that the peak corresponding to S shrinks aer the
acid increases. However, with further increase in water content, catalyst has been reused for 8 times. It is thus inferred that the
the concentration of tannic acid decreases, which results in content of sulfonic groups in the catalyst decreases as the
decrease in yield of gallic acid. Fig. 9D shows the effect of number of recycle times increases. This can lead to the degra-
polystyrene hollow sphere loading on the yield of tannic acid. dation of the performance of catalyst and therefore decrease in
The polystyryl hollow sphere loading was set at 3%, 5%, 7%, the yield of gallic acid.
10%, 15% and 20% w/w (based on the mass of tannic acid),
The hydrolysis of tannic acids in the presence of four
respectively. Other reaction conditions included reaction different catalysts including polystyrene hollow sphere,
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Fig. 9 Yield of gallic acid using polystyrene hollow sphere as catalyst (A) yield vs. reaction time; (B) yield vs. reaction temperature; (C) yield vs.
molar ratio of reactants; (D) yield vs. catalyst loadings; (E) yield vs. number of recycles.
amberlite XAD-16, BC-SO
3
H, PRPC-SO
3
H were conducted to
The comparison experimental results indicate that the
assess their catalytic activities under reaction conditions polystyrene hollow sphere has the highest performance as
ꢀ
including a temperature of 80 C, molar ratio of tannic acid to a catalyst for hydrolysis of tannic acid under the same reaction
H
2
O of 1 : 3, and polystyrene hollow sphere loading of 7% w/w conditions with the same amount of sulfonic groups. The
(based on the mass of tannic acid). The experimental data are excellent catalytic performance of polystyrene hollow sphere
listed in Table 1.
might be attributed to its porous hollow structure, which is
conducive to enhancing mass transfer.
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Fig. 10 XPS spectra of polystyrene hollow sphere (A), reused for 8 times; (B), not used.
Fig. 11 SEM-EDX analysis results of polystyrene hollow sphere (A), reused for 8 times; (B), not used.
a
Table 1 Experimental results on the synthesis of hydrolysis of tannic acid is catalyzed by polystyryl hollow sphere and other common catalysts
Conversion rate/%
ꢁ1
Catalysts (SO
3
H/mmol g
)
1 h
2 h
3 h
4 h
5 h
6 h
Yields
ꢁ1
ꢁ1
ꢁ1
ꢁ1
1
2
3
4
(1.24 mmol g
)
)
)
)
90.90
74.70
80.45
84.56
93.84
84.29
83.87
85.73
94.90
91.97
97.12
87.26
96.92
92.00
97.87
86.29
99.92
93.07
97.71
87.98
99.85
93.14
97.20
87.54
98.9
94.0
97.0
87.6
(1.24 mmol g
(1.24 mmol g
(1.24 mmol g
a
(1) polystyrene hollow spheres; (2) amberlite XAD-16; (3) BC-SO
3 3
H; (4) PRPC-SO H.
Conclusions
Acknowledgements
This study provides a new strategy for the controllable synthesis This work was supported by the Xiaoxiang Scholar Sponsorship
of hollow structures with sulfonic groups on the inner surface. of Hunan Normal University and the National Natural Science
And this polystyrene hollow sphere has high performance as Foundation of China (No. 21606082).
a green catalyst in the hydrolysis of tannic acid to obtain gallic
acid. This polystyrene hollow sphere prepared in this study
might be used as an excellent and prospective catalyst for the
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