Metal chlorides or sulfuric acid in ionic liquid solvents convert catechol to p-tert-Butylcatechol

Article abstract of DOI:10.1016/j.catcom.2015.03.001

A series of novel acidic ionic liquids (ILs) mediated in metal chloride or sulfuric acid (IL-RCl_x or IL-H₂SO₄, R = Na, K, Al, Ba, Ca) were investigated as catalysts in the alkylation of catechol and methyl tert-butyl ether to p-tert-Butylcatechol. Based on the ILs screening, it was found that N-(4-sulfonic acid butyl) pyridine hydrogen bisulfate (IL₂) was the most efficient catalyst (in the reaction). Moreover IL₂ modified by sulfuric acid showed the best performance with the yield of p-tert-Butylcatechol approximately 81.7% and the selectivity of 92.5%. For a clear perspective to the influence of H₂SO₄, IL₂ ([C₄SO₃HPy]HSO₄) was systematically studied by the Density Functional Theory at 6-31 + G? level and the results showed that hydrogen bonds inside the ILs were probably the key for acidity enhancement of the modifications.

Full text of DOI:10.1016/j.catcom.2015.03.001

Metal chlorides or sulfuric acid in ionic liquid solvents convert catechol to
p-tert-Butylcatechol
Wenlin Zhang, Ying Yue, Weiyi Su, Wei Du, Xiaowen Wang, Guanyu
Zhu, Chunli Li
PII:
DOI:
Reference:
S1566-7367(15)00096-5
doi: 10.1016/j.catcom.2015.03.001
CATCOM 4248
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 December 2014
2 March 2015
3 March 2015
Please cite this article as: Wenlin Zhang, Ying Yue, Weiyi Su, Wei Du, Xiaowen
Wang, Guanyu Zhu, Chunli Li, Metal chlorides or sulfuric acid in ionic liquid sol-
vents convert catechol to p-tert-Butylcatechol, Catalysis Communications (2015), doi:
10.1016/j.catcom.2015.03.001
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ACCEPTED MANUSCRIPT
Metal chlorides or sulfuric acid in ionic liquid solvents convert
catechol to p-tert-Butylcatechol
Wenlin Zhang a, Ying Yue a, Weiyi Su a, Wei Du a,b, Xiaowen Wang a,
Guanyu Zhu a , Chunli Li a,*
a Institute of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China;
b State Key Laboratory of Chemical Engineering, School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China
A B S T R A C T
A series of novel acidic ionic liquids(ILs) mediated in metal chloride or sulfuric
acid(IL-RClx or IL-H2SO4, R= Na, K, Al, Ba, Ca) were investigated as catalysts in
the alkylation of catechol and methyl tert-butyl ether to p-tert-Butylcatechol. Based
on the ILs screening, it was found that N-(4-sulfonic acid butyl) pyridine hydrogen
bisulfate (IL2) was the most efficient catalyst(in the reaction). Moreover IL2 modified
by sulfuric acid showed the best performance with the yield of p-tert-Butylcatechol
approximately 81.7% and the selectivity of 92.5%. For a clear perspective to the
influence of H2SO4, IL2([C4SO3HPy]HSO4) was systematically studied by the
Density Functional Theory at 6-31+G* level and the results showed that hydrogen
bonds inside the ILs were probably the key for acidity enhancement of the
modifications.
Keywords: Ionic Liquids, N-(4-Sulfonic Acid Butyl) Pyridine Bisulfate, Alkylation, Metal
Chlorides
1. Introduction
p-tert-Butylcatechol (4-TBC) as an important fine chemical in petroleum
chemicals and materials industry [1] is commonly used as a polymer antioxidant, an
insecticide stabilizer, and also a polymerization inhibitor for distillation processes and
for the storage of monomers of olefine. 4-TBC has been successfully synthesized by
using tert-butanol method[2], isobutylene method, and methyl tert-butyl ether (MTBE)
method, among which the last has been widely applied in industry due to the low cost
of production and the short time of the reaction. Generally catalyst is very important
to the MTBE method, and mineral acids, Lewis acids, and solid acids [3] have been
reported to be used as catalysts, but with which the reaction typically requires high
temperature, strong Brønsted acid environment, high-performance equipments, and
* Corresponding author. Address: Institute of Chemical Engineering, Hebei University of
Technology, Tianjin 300130, China. Tel.: +86 22 60202246; fax: +86 22 60202246.
E-mail addresses: ctstzwl@hebut.edu.cn, ctstzwl@163.com.

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complicated operations. Additionally these catalysts usually bring serious corrosion
problems to the device. Therefore, it is crucial to find an effective environmental
friendly catalyst to lower the reaction requirements with high convert ratio and high
selectivity to the desired product.
Ionic liquids(ILs) as alternatives to organic agents have attracted extensive
attentions due to the favorable properties, such as non-volatility(101.325 KPa), high
stability, strong dissolving ability etc [4, 5, 6]. Specifically Brønsted acidic ionic
liquids, which possess the advantages of both Lewis acids and solid acids, have been
designed to replace the traditional acid catalysts [7]. However, compared with the
mineral acids, the acid strength of ILs is not strong enough for the alkylation reaction.
Thus this paper focuses on searching for a relatively eco-friendly, simple, and
effective method to enhance the acidity of ILs. 4-Sulfobutyl pyridine bisulfate (IL2)
was chosen through the screening tests for the study, which was then mediated by
metal chlorides [8] or sulfuric acid [9] in order to improve the ionicity [10] on the
basis of literatures [11]. Moreover, the results showed that the acidity of IL2-RClx or
IL2-H2SO4 (R= Na, K, Al, Ba, Ca) increased compared with IL2. As a result, the
catalytic performance of IL2-RClx or IL2-H2SO4 on the synthesis of 4-TBC was found
improved dramatically under the optimum reactive conditions which have been
particularly investigated in our previous work [12]. For example, the mediated
catalysts brought high yields and excellent chemoselectivity to the product, as well as
low dosage and reusability to the catalyst itself. With these benefits, the mediated IL2
were efficiently used as catalysts for the alkylation. The results were validated by
Density Functional Theory (DFT) [13,14].
2. Experiments
All organic reagents were commercially available with the purity of more than
98% and used in the paper without further purification. The ionic liquids used in the
paper were provided by Shanghai Cheng Jie Chemical Co. LTD.. The other chemicals
were provided by Tianjin Guangfu Fine Chemical Research Institute and Tianjin
Fuchen Chemical Reagents Factory.
2.1. Screening of ionic liquids
The synthesis of 4-TBC obeys the catalytic mechanism with protonic acids, in
which the proton source in the reaction plays an important role in improving the
reaction rate. Thus in this study, different types of ionic liquids of proton acid were
studied. The acid strength of 1-sulfobutyl-3-methylimidazolium hydrogen sulphate
(IL1), N-(4-sulfobutyl) pyridine bisulfate (IL2), N-(4-sulfobutyl) triethylammonium
amine bisulfate (IL3), and 1-ethyl-3-methylimidazolium hydrogen sulphate (IL4) were
determined by Hammett indicator associated with UV spectroscopy. Firstly, the
solutions of each IL in water were prepared at different concentrations (50 6mmol/L,
100 mmol/L, 150 mmol/L, 200 mmol/L, and 250 mmol/L). Secondly, the indicator
(4-nitroaniline) was mixed with the solution leaving the content of 4-nitroaniline
0.015mg/mL in the mixture. At last the Hammett functions (acidity) were measured
for all samples. The acid strength of the system was specifically calculated by

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equation 1.1, where the non-protonated indicator in water was named as I.
Ho  pKa  log[I]/[HI]
(1.1)
Ho—function value of acid strength
pKa—electrolysis constant of Hammett indicator
[HI]—the concentration of the protonated indicator
[I]—the concentration of the non-protonated indicator
2.2. Preparation of the acidic ionic liquids modified by chloride or sulfuric acid
A stoichiometric amount of metal chloride（NaCl, KCl, AlCl3, BaCl2, CaCl2） or
sulfuric acid were added to IL2 and the mixtures were stirred for 20 minutes in order
to form new ionic liquids (IL-RClx, IL-H2SO4, R= Na, K, Al, Ba, Ca). The acid
strength of IL-RClx and IL-H2SO4 were measured respectively, while the structure of
those were analyzed by infrared spectroscopy (IR).
2.3. The synthesis of 4-TBC catalyzed by IL2-RClx or IL2-H2SO4
During the alkylation, IL2-BaCl2 or IL2-H2SO4, catechol, and methyl tert-butyl
ether (MTBE) were mixed in a 100ml of glass reactor with the molar ratio of n
(catechol): n (MTBE) : n (IL2- BaCl2 or IL2-H2SO4) = 1: 3: 0.02, and then the mixture
was stirred magnetically for 2h at 393K [15]. After that, the system was cooled to
room temperature and stabilized for a few hours. Finally two phases appeared, and the
products on the top were analyzed by gas chromatography (GC), while the ionic liquid
in the bottom was collected and recycled.
3. Results and discussion
3.1. Selection of ILs
The absorbance of ILs was measured at 380nm in order to calculate the
corresponding acid strength (Ho) by equation 1.1. The changes of Ho with the
concentration of different ILs are show in Fig. 1. It can be seen that the acid strength
of ILs is IL1> IL2> IL3> IL4 in the whole concentration range.
These ILs were then used to catalyze the alkylation of MTBE. The amount of
MTBE left was calibrated by 1,4-dioxane and measured by GC after the reaction.
Thus the conversion of MTBE (XMTBE) could be calculated and the results are shown
in Fig. 2. It is clear in Fig. 2 that the activity of ILs is IL2> IL1> IL3> IL4 under the
same conditions (n(MTBT):n(IL)=1:0.12, T=110℃, t=2h). This phenomenon might
be related to the protonated MTBE and carbenium ions, the amount of which could be
influenced by the structure of different ILs. This is discussed in details later.
In conclusion, considering the industrial costs, acid strength, and activity,
N-(4-acid butyl) pyridine bisulfate (IL2) is chosen as the (promising catalyst) in the
synthesis of 4-TBC in this work.
3.2. The acid strength of Ho of IL2-RClx and IL2-H2SO4

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The Ho of IL2-RClx and IL2-H2SO4 with various amount of RClx and H2SO4 were
measured through Hammett indicator associated with UV spectroscopy, and the
results are shown in Fig. 3. On one hand, it can be seen that the Ho in Fig.3 was much
larger than that in Fig.1, which indicated that RClx and H2SO4 were efficient
modifications to enhance the acid strength of ionic liquids. In some particular points,
the Ho of IL2-RClx or IL2-H2SO4 was even as high as that of H2SO4, for example
when n(AlCl3)：n(IL2) was 0.361, n(BaCl2)：n(IL2) was 0.305, n(CaCl2)：n(IL2) was
0.482, and ω(H2SO4) was 6.04%, the corresponding Ho curves crossed to the
horizontal line of H2SO4 as shown in Fig. 3. On the other hand, it is clear that
different RClx showed different influence to the Ho, and the approximate order of the
Ho was IL2-BaCl2>IL2-CaCl2>IL2-AlCl3>H2SO4>IL2-NaCl>IL2-KCl with the same
molar amount of chloride. Therefore the most effective IL2-BaCl2 was chosen to
catalyze the synthesis of 4-TBC whilst various amount of BaCl2 was used to modify
the ionic liquid.
3.3. The synthesis of 4-TBC catalyzed by IL2- BaCl2 or IL2-H2SO4
The alkylation of catechol was performed with a catalytic amount of IL2- BaCl2 or
IL2-H2SO4 due to the high acid strength, and the reaction mechanism is shown in
Scheme 1. The products were quantitative analyzed by gas chromatography
(GC) ,shown in Table 1.1and qualitative analyzed by 13C nuclear magnetic resonance
(NMR) spectroscopy. It can be seen in Table 1.1 that 98.0%H2SO4 brought 91.7% of
4-TBC yield and 84.8% of selectivity due to high acid strength. Whilst using
IL2-BaCl2 or IL2-H2SO4 as catalyst, the yields ranged from 76.1 to 91.7% as shown in
the Table1.1. Among those, IL2-H2SO4 with 7.8 wt% H2SO4 has the best selectivity
(92.5%) to 4-TBC, and the yield of 81.7% is also reasonable. It is worth noting that
even though the acid strength of some catalytic systems (n(BaCl2):n(IL2)=0.3,
[C4SO3HPy]HSO4+16.9%BaCl2) equals to that of sulfuric acid, the convert ratio and
selectivity both did not achieve the desired results, and the causes of this still need
further exploration.
The catalysts were reused at least 8 times, and the residual of IL2-RClx or
IL2-H2SO4 in the product was consistently less than 10%.
Scheme 1. The reaction mechanism of the synthesis of 4-TBC
To understand the results, the infrared spectroscopy was used to investigate the
IL2-RClx and IL2-H2SO4, and the results are shown in Fig. S4. It can be seen in Fig.

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-
S4 that the characterized absorption peak of -HSO3 and HSO4 in IL2 shifted in
different mediates (except for NaCl and KCl). The reason of this should be the
-
fracture of hydrogen bonds related to -HSO3 and HSO4 inside the IL. Thus it is
speculated that the addition of H2SO4 or RClx into IL2 caused the break of hydrogen
bonds in the ionic liquid, which might release the hydrogen ions and bring the
enhancement of the acid strength.
The [C4SO3HPy]HSO4 was then studied systematically by the Density Functional
Theory(DFT) at 6-31+G* level to validate the speculation above. The structure
optimization, frequency analysis, and energy calculation were performed to IL2, and
the interaction energy and atomic charge were analyzed. Then the stable structure of
IL2 was obtained and illustrated in Fig.4. The results indicated that there are apparent
intramolecular and intermolecular hydrogen bonds in the ionic liquid as shown in
-
Fig.4. Moreover, the strongest interaction was found between -HSO3 and HSO4 .
Therefore the speculation is reasonable to explain the enhancement of the acidity in
this paper. However, the structure of the modified ionic liquid with H2SO4 or RClx is
still unknown at the moment. And the mechanism of the catalytic reaction with
modified ILs needs more work as well. Currently the investigation on the
microstructures of modified ILs is proceeding in our group, and more results will be
published later.
4. Conclusion
The work in this paper provided a new method for the modification of ionic
liquids to be a relatively eco-friendly catalyst. A highly effective catalytic alkylation
of catechol using [C4SO3HPy]HSO4+7.8% H2SO4 as a catalyst was successfully
processed with the yield of near 81.7% and selectivity of 92.5%. It was found that
ionic liquids modified with RClx or H2SO4 had better catalytic activities than other
acidic ionic liquids [16] while they were also more environment friendly. Among all
the chlorides, BaCl2 was found to be the most effective on acid strength. By the
comparison of the performance of IL2-BaCl2 or IL2-H2SO4 during the catalysis to
catechol alkylation, the [C4SO3HPy]HSO4+7.8%H2SO4 was picked as the best
catalyst. The Density Functional Theory (DFT) was used to explain the influence, and
it indicated that hydrogen bonds inside the ILs might play an important role during the
modification of ionic liquids.

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Fig. 1. Hammett functions (acidity) of ionic liquids in water with 4-nitroaniline as an
indicator
1.8
IL1
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
IL2
IL3
IL4
0
50
100
150
200
250
300
Concentration of IL / mmol/L

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Fig. 2. Conversion of MTBE with different ILs as catalysts
18
16
14
12
10
8
6
4
2
0
IL1
IL2
IL3
IL4
IL

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Fig. 3. Effect of the amount of RClx and H2SO4 on the Ho of modified ionic liquid
IL+KCl
H SO
IL+NaCl
IL+AlCl
0.95
0.90
0.85
0.80
0.75
0.70
0.65
2
4
3
IL+CaCl
IL+BaCl
2
2
0.0
0.5
1.0
1.5
2.0
2.5
3.0
n(metal chloride):n(IL)
1.00
0.95
0.90
0.85
0.80
0.75
0.70
H SO
2
4
IL+H SO
2
4
0
3
6
9
12
15
18
21
H SO (Wt%)
2
4

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Fig.4. The stable configuration of [C4SO3HPy]HSO4

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Table 1
Effect of catalysts on the catalytic performance of 4-TBC synthesis
Entry a Catalyst
Yield b (%) Selectivity(%)
1
2
3
4
5
6
7
[C4SO3HPy]HSO4
76.1
79.9
80.7
77.6
78.3
81.7
91.7
80.3
66.3
68.5
81.7
84.1
92.5
84.8
[C4SO3HPy]HSO4+16.9%BaCl2
[C4SO3HPy]HSO4+20.3%BaCl2
[C4SO3HPy]HSO4+ 2.1%H2SO4
[C4SO3HPy]HSO4+5.5% H2SO4
[C4SO3HPy]HSO4+7.8% H2SO4
98.0%H2SO4
a. The reaction conditions: catechol 100 mmol, methyl tert-butyl ether 300 mmol,
catalyst 2 mmol, reaction temperature 393K, reaction time 2h.
b. Isolated yield.

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Highlights




We report a method to transform from acid to super-acid.
Ionic liquids mediated chloride or sulfuric acid show superior catalytic activity.
Activity relies on hydrogen bonds changes of molecules and proton-transfer.
[C4SO3HPy]HSO4+7.8%H2SO4 was picked as the best catalyst.