A chemical looping technology for the synthesis of 2,2′-dibenzothiazole disulfide

Article abstract of DOI:10.1039/d0gc00260g

A novel chemical looping technology for the synthesis of 2,2′-dibenzothiazole disulfide (MBTS) with zero emissions has been developed. Through chemical cycling between carbonate and bicarbonate in the reaction system, the insoluble 2-mercaptobenzothiazole (MBT) can become soluble in the aqueous phase of sodium carbonate and then be oxidized efficiently by hydrogen peroxide and the absorption of CO₂ with high selectivity. The mother liquor can be totally recycled by the desorption of CO₂ without any generation of salt-containing wastewater. With the new synthesis technology, the product has high purity and reaches the standard of a superior product that can be applied as a vulcanization accelerator. When 5.0 wt% hydrogen peroxide was added dropwise over 40 min at 50 °C, the conversion ratio of MBT was over 98%. The mother liquor was recycled 5 times and no side-products were found. The whole process is clean and pollution-free, which greatly reduces the burden of hazardous waste treatment and brings great environmental benefits.

Full text of DOI:10.1039/d0gc00260g

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DOI: 10.1039/D0GC00260G
ARTICLE
A Chemical Looping Technology for the Synthesis of 2,2'-
Dibenzothiazole Disulfide
Jiaxin Tian, Jiayu Hu, Kai Wang, Jian Deng, Guangsheng Luo*a
Received 00th January 20xx,
Accepted 00th January 20xx
A novel chemical looping technology for the synthesis of 2,2'-dibenzothiazole disulfide (MBTS) with zero emission has been
developed. Through the chemical cycling between carbonate and bicarbonate in the reaction system, the insoluble 2-
mercaptobenzothiazole (MBT) can be soluble in the aqueous phase of sodium carbonate, and oxidized efficiently by
hydrogen peroxide by absorption of CO2 with high selectivity. The mother liquor can be totally recycled by desorption of CO2
without any generation of salt-containing wastewater. With the new synthesis technology, the product has high purity and
reaches the standard of superior product as vulcanization accelerators. When 5.0 wt.% hydrogen peroxide was added
dropwise in 40 min at 50 oC, the conversion ratio of MBT was over 98 %. The mother liquor was recycled for 5 times and no
side-products were found. The whole process is clean and pollution-free, which greatly reduces the burden of hazardous
waste treatment and brings great environmental benefits.
DOI: 10.1039/x0xx00000x
seriously pollutes the air. What’s worse, the sodium nitrite is
oncogenic. The second process is suggested by US 4482720-A.
Accordingly, MBT is dissolved in NaOH solution to produce the
Introduction
2,2'-dibenzothiazole disulfide (MBTS) as a kind of accelerator is
widely used in sulfur vulcanization, which can greatly improve
the vulcanization efficiency of natural rubber1-3. It is mainly used
to process some non-food products such as tires and belts due
to its bitter taste4. In addition, MBTS with high purity is also a
pharmaceutical intermediate used in the manufacture of
cephalosporin antibiotics5. Disulfide has a broad range of
applications in pharmaceuticals, chemical biology and food
chemistry6. Many studies have focused on the methodologies
and designs for disulfide syntheses6-8. As a symmetrical
solution of sodium 2-mercaptobenzothiazole (M-Na), and then
the product is prepared by introducing chlorine gas diluted with
air14. However, this process requires strict control in order to
prevent the further oxidation of MBTS to benzothiazyl-2-
sulfinate and -sulfonate. Of course, the process generates a
large amount of salt-containing wastewater too. Other
developed methods in some patents include dissolving MBT
directly in alcohol solutions with hydrogen peroxide as an
oxidant17,18
. The use of organic solvents, however, is
disadvantageous for an industrial process owing to their high
inflammability and high solvent consumption. Thus, the
technology has not been applied to any industrial processes
until now. Instead, using aqueous media for the organic
synthesis processes is regarded as a crucial way for developing
disulfide,
MBTS
is
generally
prepared
with
2-
mercaptobenzothiazole (MBT) as the raw material. The
sulfhydryl group in MBT has weak acidity and can be easily
oxidized to disulfides under specific conditions to form the
target product9-12. In order to ensure the quality of accelerators,
it is stipulated that the initial melting point of MBTS for superior
products should be no less than 170 oC. However, high purity is
relatively difficult to reach in the industrial production
process13.
MBTS has a variety of synthetic processes depending on the
different kinds of oxidant used14-16. The most traditional
method in industry is to use sodium nitrite as the oxidant
agent15. Sulfuric acid is dripped into the slurry of MBT and
sodium nitrite, air is introduced in the presence of nitric oxide,
and the oxidation occurred at 60 oC. The serious problem of this
method is that the reaction produces a large amount of salt-
containing wastewater, while the nitrogen oxide tail gas
more green, effective and sustainable methodologies19-21
.
Hydrogen peroxide is widely used as an environmentally
friendly green oxidant because its reduction product is water
without any pollution22-26. In industrial, oxidation process in the
aqueous phase is carried out by mixing hydrogen peroxide and
sulfuric acid. The mixture is dripped into the M-Na solution for
the homogeneous reaction16. In this method, some of the
insoluble MBT would be precipitated in the presence of acid
before being oxidized, which could reduce the purity of final
product greatly. Besides, a large amount of water is required
and a corresponding large amount of salt-containing waste
water of sodium sulphate would be produced as well.
It is well known that the disposal of solid waste and hazardous
waste has become a major problem restricting the green
development of chemical enterprises27. In the above process,
about 0.43 tons of sodium sulfate waste could be generated for
each ton of the product. According to incomplete statistics of
a. The State Key Laboratory of Chemical Engineering, Department of Chemical
Engineering, Tsinghua University, Beijing 100084, China
E-mail: gsluo@tsinghua.edu.cn; Fax: +86 010 6278 3870;
Tel: +86 010 6278 3870
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major rubber chemical manufacturers in China, the annual water was obtained from a laboratory waterViepwuArritficilceaOtniolinne
output of MBTS in 2018 was up to 49000 tons. The heavy system, Center 120 FV-5, purchased froDmOIT: H10E.10L3A9B/DI0nGstCr0u0m26e0nGt
burden of waste salt treatment therefrom can be imagined. If Technology (Shanghai, China). All chemicals were used without
the salt-containing waste water is only treated by evaporation any further purification.
Hydrolysis Constant Kh Measurement
crystallization, the toxic substances contained will exist in the
waste salt. Furthermore, if the waste salt is not treated
properly, it will bring greater risk to the surrounding
environment and also bring great environmental protection
burden to chemical enterprises28. To solve this problem, the
most effective way is to avoid the discharge of wastewater from
the production process. Another issue in the existing production
processes is the generation of superoxidized side-products15.
The reaction selectivity and the effective utilization ratio of
oxidant are reduced as a result. At the same time, the COD value
of wastewater will increase. Therefore, it is necessary and
meaningful to develop a clean and green synthesis process with
zero emission for MBTS.
With regard to the problems above, a novel chemical looping
process of recycling mother liquor using carbon dioxide as the
acid agent is developed. Recently, the effective utilization of
carbon dioxide has become a research hotspot29-32. In this
process, carbon dioxide is selected to replace the inorganic and
organic acids used in the previous methods. On the one hand, it
is cheap, widely available and avoids the requirements for the
corrosion resistance to strong acids of reactors. On the other
hand, carbonic acid is relatively mild, which can greatly reduce
the precipitation of insoluble MBT in the acidification process.
Firstly, MBT is dissolved in the sodium carbonate solution and
converted into M-Na solution. Secondly, hydrogen peroxide is
used as the oxidant and carbon dioxide is introduced to regulate
the pH of the reaction system. In this way, the mercaptan group
can be homogeneously oxidized to disulfide to produce MBTS in
the aqueous solution without any catalyst. At the end of this
process, the mother liquor mainly containing sodium carbonate
and sodium bicarbonate is heated to decompose sodium
bicarbonate by desorption of CO233,34, and then be recycled to
the first step of the reaction. We aim to realize the mother
liquor recycling on the basis of ensuring the product yield and
purity by this environment-friendly novel process. The
generation of salt-containing wastewater can be totally
avoided.
Sodium 2-mercaptobenzothiazole (M-Na) has hydrolytic
equilibrium35. The pH values of M-Na aqueous solutions with
different concentrations at different temperatures were
measured by a pH meter (SevenCompact pH meter S210,
METTLER TOLEDO). The hydrolysis equilibrium constant Kh is
calculated by the following formula. In the calculation, c0 is the
total concentration of 2-mercaptobenzothiazole and the
thiolate anion in the aqueous solution. The ionization of water
is ignored, and it is considered that the concentration of
conjugated acid is equal to that of hydroxyl ion.
RS + H2O
RSH + OH
(1)
[RSH][OH- ]
[OH- ]2
(Kw /[H+ ])2
(2)
Kh =


[RS- ]
c0 [OH- ] c0 (Kw /[H+ ])
Solubility Measurement
2-mercaptobenzothiazole was added in the 15% sodium
carbonate solution at different temperatures until there was no
more solid dissolved. The solubility of 2-mercaptobenzothiazole
in the sodium carbonate solution is calculated according to the
concentration in the solution.
MBTS Preparation
The aqueous solution of sodium carbonate (15 wt.%, 50 g) was
added into a 100 mL three-neck flask with magnetic stirring (500
r/min) in
a
50 oC thermostatic water bath. Then, 2-
mercaptobenzothiazole (3 g) was added and stirred for about 5
min until completely dissolved. Then, the hydrogen peroxide
solution (5.0 wt.%, 6.10 g) was added dropwise in 40 min.
Meanwhile, pure carbon dioxide was introduced into the
bottom of the flask and regulated the pH to 8.5~9.5. The molar
ratio of M-Na to hydrogen peroxide was fed according to the
stoichiometric ratio. When the dripping of hydrogen peroxide
solution was finished, the reaction continued for a short period
of time. Then, the product of MBTS was obtained after filtration,
twice washed with IO water (30 mL), and dried at 80 °C for 8 h.
Mother Liquor Recycle
The filtrate separated from oxidation reaction system was
collected and heated to 90 oC for 40 min to release CO2 from the
filtrate. Then a certain amount of sodium carbonate solution (15
wt.%) was replenished. MBT was fed and stirred at the reaction
temperature until all the solid was dissolved and the M-Na
solution with approximate initial mass fraction was obtained
correspondingly. The hydrogen peroxide (5.0 wt.%) solution
was added dropwise according to stoichiometric ratio. And the
subsequent operation was the same as the previous part.
Analysis and Characterization
Experimental section
Materials and Chemicals
Hydrogen peroxide (30 wt.%) was purchased from Sinopharm
Chemical Reagent (Shanghai, China). Carbon dioxide (99.99 %
vol) was supplied by Beijing Hua Yuan Gas Chemical Industry
(Beijing, China). 2-Mercaptobenzothiazole (98 wt.%) was
provided by Willing New Materials Technology Co.,Ltd. (Henan,
China). 2,2'-Dibenzothiazole disulfide (≥99.0 wt.%) was
purchased from Shanghai Dibai Biotechnology Co., Ltd
(Shanghai, China). N, N-Dimethylformamide (A.R ≥99.5 wt.%)
from Shanghai Titan Scientific Co.,Ltd. (Shanghai, China) was
used as the solvent and acetonitrile (HPLC grade) from J. T.
Baker (USA) was used as the mobile phase for HPLC. Deionized
The concentration of M-Na in the solution was determined by
UV-Vis spectroscopy (Shimadzu UV-2450) at 280 nm. When no
side reactions occurred, only residual M-Na and recyclable
inorganic salt were found in the mother liquor. Therefore, the
conversion of M-Na was calculated by measuring the
2 | J. Name., 2012, 00, 1-3
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concentration in the filtrate. The solid product separated was dioxide that can escape from the reaction system.VTiehwuAsr,ticclae rObnolinne
DOI: 10.1039/D0GC00260G
characterized by IR (TENSOR 27) and Raman (LabRAM HR conservation is maintained in the whole chemical looping
Evolution). The purity was analyzed by HPLC (Agilent 1260). The process essentially. Consequently, the recycle of the mother
particle size distribution of MBTS particles was determined by liquor is reasonable in theory. Also, the co-product produced by
laser particle size analyzer (Mastersizer2000). And the the introduction of carbon dioxide in the oxidation step is
morphology was recorded by SEM (Hitachi TM3000). The sodium bicarbonate instead of sodium carbonate. The reaction
melting points of all samples were obtained with a capillary principle is shown as follows.
melting point apparatus.
(1)
Results and discussion
Reaction principle and possibility of the chemical looping process
(2)
The values of pKh calculated from the measured data at
different temperatures are shown in Fig. 1(a). The results at
The solubility of MBT in the sodium carbonate solution at
lower concentrations were chosen to reduce the difference in
different temperatures is shown in Fig. 2. MBT can be dissolved
concentration and activity due to non-ideality. And the
in the sodium carbonate solution and reach a high mass fraction
hydrolysis equilibrium constants of sodium carbonate solution
of M-Na, and the solubility is significantly increased with the rise
of temperature. This is because the sodium bicarbonate
produced can releases carbon dioxide at high temperatures to
were determined in the same way for comparison.
According to Van 't Hoff equation, the equilibrium data of this
hydrolysis reaction was fitted, as shown in Fig. 1(b). The results
form sodium carbonate33, which is more alkaline so that more
obtained by fitting straight line and the relevant literature
MBT can be well dissolved.
The above two parts have proved the feasibility of the recycling
process from the reaction route. Based on this, a chemical
reports are listed as Table 1.
Based on the above results, it could be concluded that the order
of alkalinity from strong to weak is sodium carbonate, M-Na and
looping for MBTS synthesis is proposed as Fig. 3, a green process
sodium bicarbonate36. Correspondingly, the order of acidity
with zero emission could be reached if the selectively of the
from strong to weak is carbonic acid, MBT and bicarbonate.
reaction process is high enough.
MBTS preparation and the pH effect
First of all, it provides a theoretical basis for the process of
replacing sodium hydroxide with sodium carbonate to dissolve
Under the conditions described in the above experimental part,
MBT so that homogeneous aqueous solution can be obtained
the white product continuously precipitated with the dripping
using sodium carbonate as the base agent. Secondly, since MBT
of hydrogen peroxide. At the end of the reaction, the potassium
is less acidic than carbonic acid, it can be judged that the co-
iodide/ starch test paper showed no hydrogen peroxide
product generated is sodium bicarbonate rather than carbon
residue. From the HPLC chromatograms shown in Fig. 4(a), the
Table 1 pKh at different temperatures
T (K)
288.15
7.86
298.15
7.65
308.15
7.56
318.15
7.46
a
pKa
pKh in this
work
6.97
6.70
6.45
6.22
a. in an aqueous medium containing 0.10 mole fraction of ethanol at
I=0.02 mol·dm-3 (KNO3)35
Fig.
2 Saturated mass fraction of M-Na in 15% Na2CO3 solution at different
temperatures
Fig. 1 The values of pKh at different temperatures. (a) The calculated pKh of sodium
carbonate and M-Na. (b) Fitting results of hydrolysis equilibrium constants.
Fig. 3 The chemical looping technology for MBTS synthesis
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retention time of MBT is 2.35 min. MBTS (≥99.0 %) purchased It can be seen that without carbon dioxide duringVtiehweArpticrloecOenslinse,
DOI: 10.1039/D0GC00260G
for commercial and the product both appear at 2.80 min. It is the pH value increased slowly to about 10.1. Considerable solid
proved that the white solid is the product of MBTS with high product could still be obtained under this condition. Therefore,
purity. From the IR spectra shown in Fig. 4(b), it can be seen that the carbonate / bicarbonate ions in the mother liquor play a
the stretching vibration absorption peaks of S-H in MBT certain role in buffering, so that the alkalinity could not be too
structure appear at 2700~2850 cm-1 which are not found in the high to hinder the oxidation in the later stage. Coincidentally,
product or the MBTS sample. Fig. 4(c) shows that the product the co-product bicarbonate during the reaction also has a
with symmetrical molecular structure has the characteristic certain activation effect on hydrogen peroxide according to
Raman bands of -S-S- at around 500 cm-1 and 530 cm-1 which are relevant literatures38,39. When carbon dioxide was introduced,
not found in MBT as the raw material37. The conversion ratio of the pH decreased gradually, and the conversion ratio increased
MBT is up to 98.6% and the purity of the prepared MBTS is 99.05 greatly. It is believed that the regulation of neutralizer promotes
o
wt.%. The initial melting point of the product is 172.5 C, much the formation of weak acid MBT by hydrolysis in the aqueous
higher than the standard of superior product (170 oC).
phase, which is conducive to the formation of MBTS.
The pH change and the necessity of introducing neutralizer in Utilization ratio of hydrogen peroxide and generation of side
reactions
the reaction process were studied. 5.0 wt.% H2O2 was added to
the M-Na aqueous solution (6.40 wt.%) prepared by sodium
hydroxide and MBT, and white solid precipitated as soon as the
oxidant was dripped into. At the beginning, the pH value of the
reaction system raised rapidly to about 10.9. Subsequently, the
pH value decreased slightly due to the increase of the H2O2
amount, as shown in Fig. 5(a), during which there was no more
solid precipitation. After 40 min for reaction, the conversion
ratio was only 44.6%.
It is proved that the oxidation reaction cannot continue under
the strong alkaline environment. Next, the pH change was
monitored during the reaction in the sodium carbonate
solution, the results are shown in Fig. 5(b). The conversion ratios
under corresponding conditions are listed in Table 2.
Different multiples of stoichiometric equivalent hydrogen
peroxide were added into the 6.40 wt.% M-Na solution
(obtained as described in the experimental section) by one
time, and carbon dioxide was introduced meanwhile. The
conversion of M-Na and the effective utilization of hydrogen
peroxide are shown in Fig. 6(a).
When hydrogen peroxide was added by one-stage addition, the
effective utilization ratio was less than 100 % and decreased
with the increase of the amount of hydrogen peroxide added
(i.e. the initial concentration in the solution). It is proved that
hydrogen peroxide will decompose in alkaline condition40, and
the higher the concentration is, the faster the decomposition is.
The initial pH value of the M-Na solution is about 9.9. We
believe that even if the whole reaction system is in a weak
alkaline state, the oxygen-containing radicals released by
hydrogen peroxide can oxidize sulfhydryl group, which
competes with the reaction of decomposing to oxygen. When
hydrogen peroxide was added dropwise, the concentration was
greatly reduced, which can effectively reduce the invalid
decomposition.
Table 2 The convention ratios with or without carbon dioxide
Temperature
CO2 flow rate
(mL/min)
Conversion %
(oC)
50
50
0
15
88.8
98.6
6.40% M-Na, 5.0 wt.% H2O2, 40 min for dripping
Fig. 5 The pH value with time during the reactions. (a) M-Na aqueous solution as the
substrate. (b) The mixed solution of M and sodium carbonate (6.40 wt.% M-Na) as the
substrate.
Fig.
4 Characterization of MBT, the MBTS sample and the product. (a) HPLC
chromatograms. (b) IR spectra. (c) Raman spectra.
Fig. 6 The effect of the oxidant on the reaction. (a) Conversion of M-Na and utilization
of H2O2 under different oxidant dosage (T=50 oC, 5.0 wt.% H2O2, 20 min for reaction, 15
mL/min for CO2 flow rate, pH=8.5~9.5). (b) UV spectra of mother liquor with different
4 | J. Name., 2012, 00, 1-3
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Then, different amounts of the oxidant were added dropwise. dropwise. The amounts of nucleation are closeViaewccAortircdleinOgnllinye.
DOI: 10.1039/D0GC00260G
When the hydrogen peroxide drops were added to excess, the Continuous crystallization occurs during the dripping process.
absorbance at 295 nm (the maximum absorption wavelength of At high initial concentration, the subsequently generated MBTS
M-Na) decreased gradually, and a new maximum absorption molecules grow rapidly along the two-dimensional space, and
wavelength appeared at about 260nm, as shown in Fig. 6(b). the particles are more likely to agglomerate. Thus, larger
Many studies have reported the oxidation of mercaptan into particles become easier to form.
sulfonate by hydrogen peroxide41,42
, and the maximum The effect of reaction temperature on the reaction is shown in
absorption wavelength of sulfonate is just about 260 nm43. It is Table 4. It is considered that the oxidation process belongs to
believed that a small amount of sulfonate is produced when free radical reaction. Because RSH is easy to form RS·, the
hydrogen peroxide is excessive.
reaction occurs even at room temperature. As the temperature
Therefore, in order to reduce the decomposition of oxidant and increases, the effective collision of free radicals produced by
avoid side reactions, the hydrogen peroxide solution should be activated molecules increases, and the conversion ratio
added dropwise and the total amount should be strictly increases accordingly. However, when the temperature is too
controlled.
Optimization of process parameters
high, the decomposition rate of hydrogen peroxide is
accelerated. The decomposed molecular oxygen has weak
oxidizing capacity without catalyst. And the product MBTS is
unstable in alkaline environment at high temperature. The
above factors lead to the decrease of conversion ratio.
After dripping, the aging was continued for a period of time by
stirring at the constant temperature. As shown in Fig. 8, the
change of conversion ratio is not significant. It is notable that
the calculated conversion ratio decreases slightly when the
aging time is too long. It may be caused by the decomposition
of MBTS back into M-Na during the long aging process.
Therefore, it is meaningless to prolong the aging time.
Table 5 shows the effect of H2O2 concentration on the reaction.
When 30 wt.% H2O2 was used, the conversion is obviously low.
H2O2 in higher concentration led to more decomposition in
alkaline environment40.
The process conditions including the concentration of raw
materials, reaction temperature, aging time and H2O2
concentration are discussed below.
Table 3 shows the change of conversion ratio, product particle
size and purity under different initial concentrations of M-Na.
The particle size distribution is shown in Fig. 7(a). With the
increase of the initial concentration, the volume and size of
MBTS particles increase, and the crystal morphology changes
from acicular to lamellar, as shown in Fig. 7(b-d). The final
particle size distribution is determined by the relative kinetics
of nucleation and growth44. Although the initial concentrations
of M-Na are different, the supersaturations of locally formed
product molecules are close because the oxidant is added
Realization of mother liquor recycle
Table 3 Experimental results under different initial concentrations of M-Na
In order to realize the recycle of mother liquor, some
experiments have been tried to optimize the condition
parameters. When the initial concentration of M-Na is high, the
generated sodium bicarbonate will crystallize and precipitate
due to its limited solubility. At the end of the reaction, a large
amount of solid product in the mother liquor is not easy to
M-Na mass
fraction %
3.30
6.40
10.29
M-Na
conversion %
94.3
D [4,3]
(μm)
Purity %
13.878
27.389
29.887
99.1
98.2
97.7
98.6
97.9
T=50 oC, 5.0 wt.% H2O2, 40 min for dripping, 15 mL/min for CO2 flow
Table 4 The effect of temperature on the reaction
Temperature
Dripping time
Conversion %
(oC)
(min)
40
40
30
50
70
97.2
98.6
80.7
40
6.40% M-Na, 5.0 wt.% H2O2
Fig. 7 Particle morphology at different initial concentrations of M-Na. (a) Particle size
distribution of solid products. (b-d) SEM photos of the solid product (The initial mass
fraction of M-Na (b) 3.30 %, (c)6.40 %, (d)10.29 %).
Fig. 8 Conversion ratio with time under different initial concentrations.
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Table 5 The effect of H2O2 concentration on the reaction
oxidation reaction was carried out in the aqueous VsioewluAtritoiclne Owniltinhe
the introduction of carbon dioxide. The conversion ratio
H2O2
mass fraction %
5.0
Dripping time
M-Na
conversion %
98.5
DOI: 10.1039/D0GC00260G
Purity %
(min)
40
40
reached 98.8 % under the optimized conditions with high
selectivity, and the purity of the product was up to the standard
of superior product. First of all, the novel process can be
realized by the conversion between carbonate and bicarbonate,
avoiding the large amount of inorganic waste salt produced.
Second, the sodium carbonate solution is used to dissolve the
insoluble MBT, so that it can be oxidized into MBTS efficiently
in the aqueous media, which avoids the potential safety hazard
and high cost caused by the use of organic solvents. Moreover,
CO2 is used as a neutralizer to provide a suitable pH
environment for the reaction, which has sufficient sources and
avoids equipment corrosion. Based on the above advantages,
the whole reaction system becomes clean, safe and pollution-
free. Applied to industrial production, this process could greatly
reduce the burden of waste treatment and bring great
economic and environmental benefits. Significantly, the
amount of hydrogen peroxide should be strictly controlled to
avoid the generation of side-products. In our previous work,
relevant studies on thiuram disulfide syntheses have been
carried out26, showing the compatibility to symmetrical
disulfides of this process and the potential of developing
chemical looping methodologies for other similar chemicals.
Based on the above results, the daily output of a single stirring
reactor can reach over 100 g in the laboratory scale. As for the
production scale per batch, the previous process using sulfuric
acid has been successfully scaled up to 10000 t/a with the
reactor volume of more than 10 m3. The stirring reactor applied
in our novel process is basically the same as the previous one.
Therefore, it is believed that the practical application in industry
could be reached without serious scaling effect.
98.2
99.8
99.4
10.0
30.0
98.8
89.7
20
6.40% M-Na, T=50 oC, 15 mL/min for CO2 flow
completely separate and its wet weight is high, resulting in the
loss of the mother liquor. In addition, a small amount of water
will be brought into the reaction system with the addition of
oxidants, which will reduce the concentration of sodium
carbonate. Therefore, appropriate initial concentration of M-Na
(i.e. the feed amount of MBT in the dissolution step) can make
sodium carbonate greatly excessive and alkaline enough, so as
to dissolve MBT much faster in the next time. Besides, the
amount and dripping rate of oxidant were strictly controlled to
avoid or reduce the formation of side-products.
Based on the above considerations, the following reaction
conditions were determined. 5.0 wt.% H2O2 was added
dropwise at 50 C for 40 min, and the flow rate of CO2 was 15
o
mL/min. Five cycles of the mother liquor have been carried out.
The related data of each cycle are shown in Table 6. Each time
of the reaction has good performance and high conversion ratio
over 95%. The purity of each batch of the products is more than
99%. It should be mentioned that a certain amount of sodium
carbonate solution was added to the approximate initial
amount before each reaction due to the solution loss by
filtration. It is proved that the clean process without salt
discharged by mother liquor recycle strategy is feasible.
As far as we know, in the previous process, the costs of sodium
hydroxide and sulfuric acid consumed per ton of the product are
around 900 CNY and 200 CNY respectively, and the cost of
wastewater treatment is around 1100 CNY per ton of the
product. In the novel chemical looping process, all the mother
liquor could be recycled. Thus, it is roughly estimated that the
cost saving of the novel process might be around 2200 CNY per
ton of the product.
Further research will be devoted to the continuous synthesis
technology of MBTS. In this way, the advantages above can be
combined with the continuous technology to further intensify
the reaction process and greatly improve the production
efficiency. Of course, the reaction mechanism and kinetics
should be studied in our future work.
Conclusions
To realize the clean process of MBTS synthesis with high quality
and without waste salt, we have developed a novel chemical
looping technology. Using hydrogen peroxide as oxidant, the
Table 6 Experimental results of mother liquor cycle
Initial amount of M-Na
Initial mass fraction
Dosage of H2O2
(g)
Mass fraction
of M-Na in the filtrate %
per-pass conversion
%
Cycle times
Solution
(g)
of M-Na %
1st.
2nd.
3rd.
4th.
5th.
53.08
53.08
54.50
52.67
54.9
6.39
6.59
6.49
6.39
6.36
6.14
6.33
6.40
6.09
6.32
0.25
0.35
0.31
0.26
0.31
96.3
94.8
95.3
95.7
95.3
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Green Chemistry
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DOI: 10.1039/D0GC00260G
ARTICLE
26 J. Hu, K. Wang, J. Deng and G. Luo, IND ENG CHEM RES, 2018,
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27 P. DANG, S. SAI, N. ZHANG, S. LI, Y. ZHENG, H. LIU, T. HANG, L.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We gratefully acknowledge financial supports from National
Natural Science Foundation of China (Nos. 21991100,
21991104).
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Green Chemistry
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DOI: 10.1039/D0GC00260G
A novel chemical looping technology for the synthesis of 2,2'-dibenzothiazole
disulfide (MBTS) with zero emission.