Condensation of 9-fluorenone and phenol using an ionic liquid and a mercapto compound synergistic catalyst

Article abstract of DOI:10.1039/c9nj02581b

A series of ionic liquids (ILs) were synthesized and their Hammett acidities (H₀) were determined using 4-nitroaniline as the indicator. The relationship among IL's structure, the acid strength, and the catalytic performance in the condensation reaction of 9-fluorenone with phenol was discussed. The effective H₀ range of ionic liquids that can catalyse the condensation reaction was obtained. Moreover, the catalysis of the mercapto compound co-catalyst was also systematically studied. According to the analysis of how the structure of the sulfydryl co-catalyst affects the percent conversion of 9-fluorenone and the selectivity of BHPF, a mechanism for the reaction in the IL-thiol cooperative catalytic system was proposed. The present work gave a clear clue to design novel IL catalysts for the synthesis of BHPF.

Full text of DOI:10.1039/c9nj02581b

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DOI: 10.1039/C9NJ02581B
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Condensation of 9-fluorenone and phenol using the ionic liquid
and mercapto compound synergistic catalyst
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Received 00th January 20xx,
Accepted 00th January 20xx
Lei Yan a b, Limei Yu* a b, Maochang Shen, Shikang Luo and Zhanxian Gao b
DOI: 10.1039/x0xx00000x
A series of ionic liquids (ILs) were synthesized and their Hammett acidities (H0) were determined using 4-nitroaniline as the
indicator. The relationship among IL’s structure, the acid strength, and the catalytic performance in the condensation
reaction of 9-fluorenone with phenol was discussed. The effective H0 range of ionic liquids that can catalyse the
condensation reaction was obtained. Moreover, the catalysis of the mercapto compound co-catalyst was also
systematically studied. According to the analysis of how the structure of the sulfydryl co-catalyst affects the percent
conversion of 9-fluorenone and the selectivity of BHPF, a mechanism for the reaction in the IL-thiol cooperative catalytic
system was proposed. The present work gave a clear clue to design novel IL catalysts for the synthesis of BHPF.
are used widely in alkylation17, esterification18, 19, 20, nitration21,
hydrogenation22, 23, hydration24, oxidation25, 26, 27, cyclization28
and Diels-Alder reaction29, 30, and so on. However, it has
seldom reported on ILs catalysing the synthesis of bisphenol
fluorenone.
In this work, we synthesized nine SO3H-functionalized IL
catalysts, and characterized the structures of the ILs by Fourier
transform infrared spectrum (FT-IR), nuclear magnetic
resonance spectrum (NMR), and electrospray ionization mass
spectrometry (EI-MS). The thermal stability of ILs was detected
by thermogravimetry - differential thermogravimetry (TG-DTG).
The acidity of the ILs was determined through UV-vis
spectroscopy. In the condensation of 9-fluorenone with phenol,
the relationship between the structures of the acidic ILs and
thiol co-catalysts and their catalytic effect has been scrutinized.
The minimum acidity H0 initiating reaction was investigated.
Meanwhile the mechanism of the condensation reaction was
given, and the recyclability of the IL catalyst was also examined.
Introduction
The bisphenol compounds, a series of fine chemical raw
materials, can be applied in many fields and prepared by
condensation of aldehyde, ketone or keto acid with phenol.
For example, 9,9-Bis (4-hydroxyphenyl) fluorene (BHPF) has
been studied due to its unique cardo-ring structure, which acts
as the monomer or modifying agent to manufacture polymers,
such as epoxy resin1, 2, polycarbonate3 and polyethersulfone4, 5
etc.. The polymers exhibit good rigidity and thermal stability,
good optical properties and excellent formability6, so they
have been widely used in aerospace, electronics and
automotive industries7-10
.
Synthesis of BHPF from condensation of 9-fluorenone with
phenol is a complex process, which uses acid-thiol cooperative
catalytic system, the acid as catalyst and mercapto compound
as co-catalyst. The classical acid catalyst includes strong
mineral acid11, 12, heteropoly acid13, and acidic cation exchange
resin14 etc. However, these catalysts have several
disadvantages, such as severe corrosion of the equipment,
serious environmental pollution, long reaction time and low
yield, massive usage of catalyst while difficulty of recycle. At
present, the commonly used co-catalyst is thioglycolic acid or
mercaptopropionic acid. The co-catalyst can accelerate the
reaction rate, but the mechanism is not clear yet. Thus, the
development of an efficient catalyst system and the
exploration of the mechanism in the synthetic reaction of
BHPF are still deemed worthy of pursuit.
Scheme 1 Structures of ionic liquids
.
Experimental
Reagents and solvents
Ionic liquids (ILs) have excellent physicochemical natures
and are considered to be a new type of green solvent and
catalyst15, 16. Recently, the applications of functionalized acidic
ILs instead of traditional liquid or solid acid catalysts in diverse
organic reactions have been reported. The acidic IL catalysts
Pyridine, triethylamine, imidazole, benzyl mercaptan, 1, 3-
propane sultone, p-mercaptobenzoic acid, 1-butanethiol,
thioglycolic acid and 3-mercaptopropionic acid were obtained
from Aladdin. p-Toluenesulfonic acid was obtained from
Shanghai Macklin Biochemical Co.. Sulphuric acid, phosphoric
acid, tetrafluoroborate acid, diethyl ether and phenol were
obtained from Sino pharm Chemical Keagent Co.. Methanol
was obtained from Tianjin KeMiou Chemical Co.. The 9-
a. State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian, Liaoning
116024, China. *Email: ochem@dlut.edu.cn; Tel: +86 13942698335.
b. School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning
116024, China.

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fluorenone was self-made, and its purity was greater than 99%.
All reagents were used directly without further purification.
Synthesis of ionic liquids
The experimental procedure (Scheme 2) was cVaierwrieArdticloeuOtnlianes
following: 9-fluorenone(0.02 mol), phenol(0.12 mol), IL(0.0025
mol), and co-catalyst(1.0 mmol) were added to a 50 mL three-
necked flask equipped with a thermometer, a spherical
condenser and a magnetic stirrer. Under stirring, the reactants
DOI: 10.1039/C9NJ02581B
The SO3H-functionalized ILs (IL1-IL8 as shown in Scheme 1)
were synthesized using two-step process32-34. Firstly, (0.2 mol)
pyridine or triethylamine was added to a 100 mL round
bottom flask containing absolute ethanol (20 mL), and then
equimolar 1, 3-propane sultone was slowly added to the round
bottom flask under stirring at 298 K. After reacting 4-12 h, a
large amount of solid precipitate emerged. Then the
precipitate was washed with ether for 3 times and dried at 353
K under vacuum to get the white solid precursor zwitterions.
The yield of precursor is about 85-95%.
were heated at 110
℃ for 6 hours. The qualitative analysis of
crude product was conducted on a LC/MS (Agilent 1100 / 6130)
with a Hypersil ODS-2 column. To further distinguish the target
molecule and by-products, the crude products were purified
by column chromatography on silica gel (eluent: V (petroleum
ether) / V (dichloromethane) = 5:1) to obtain the relative pure
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compounds, and their structures were characterized by H and
13C NMR spectra (see the Supporting Information). The
quantitative analysis of the reaction was carried out on an
HPLC (Agilent HP1100) with a XB-C18 column (5μm × 250 mm ×
4.6 mm), and the chromatographic conditions: 30 °C constant
temperatures; mobile phase: methanol / water (volume ratio:
60:40 to 100:0 within 30 min); flow rate: 1.0 mL / min;
injection volume: 5μL; detector: UV (257 nm and 275 nm).
Secondly,
a stoichiometric amount of sulfuric acid,
phosphoric acid, tetrafluoroboric acid or p-toluenesulfonic acid
(0.05 mol) was added to the aqueous solution of the precursor
(0.05 mol). The mixture was stirred at 353 K for 8 h to form the
ionic liquid. The product mixed liquor was distilled under
reduced pressure to remove water and then afforded a viscous
liquid. The liquid was washed thoroughly with diethyl ether to
remove residual acid, and finally dried at 373 K under vacuum
to obtain the corresponding acidic ionic liquid. The yield of
ionic liquid is nearly 90-98%.
The IL9 (Scheme 1) was synthesized according to the
literature35. Sodium ethoxide (0.21 mol) was added to a
solution of imidazole (0.2 mol) in ethanol (50 mL), and the
mixture was stirred at room temperature for 4 h. Then 1, 3-
propanesultone (0.42 mol) was added and the mixture was
stirred at the same temperature for 6 h. After the solvent was
removed by vacuum distillation, the obtained mixture was
repeatedly washed with ethanol, and subjected to evaporation
under vacuum to give the white solid precursor.
Scheme 2 The condensation of 9-fluorenone with phenol
Results and discussion
Determination of ionic liquid acidity
The acidities of ILs were determined on the basis of the
Hammett acidity function method36, 37. The Hammett function
(H0) was calculated by the following equation:
Sulfuric acid (0.1 mol) was added to the solution of the
precursor (0.05 mol) in water (15 mL) at 298 K. The mixture
was stirred at 363 K for 10 h and concentrated under vacuum.
The obtained precipitate was washed with a mixture solution
of methanol and Et2O, then filtrated and dried under vacuum
at 373 K to gain IL9.
H0 = pK (B) aq + log ([B] / [BH+])
[1]
Where pK (B)aq is the pKa value (0.99) of the aqueous
solution of 4-nitroaniline, [BH+] and [B] are respectively the
molar concentrations of the protonated and unprotonated
indicator, which can be determined by UV-visible spectroscopy.
Ionic liquids, H2SO4 and HSCH2COOH were prepared under
the same concentration of aqueous solution (c = 0.02 mol/L),
and the absorbance was measured with 4-nitroaniline as the
indicator. The Hammett function values (H0) were calculated
according to the equation [1] and the results were shown in
Table 1.
In Table 1, IL1 - IL4 have different anions but the same cation
(PyN+ (CH2)3SO3H), in which the H0 values are 1.46, 1.70, 1.94
and 1.60, respectively. The IL3 with H2PO4ˉ anion has the
largest H0 value indicating that the IL3 is a weaker acid. The H0
values of IL5 - IL8 show the same pattern. So the anionic
structure of the IL has crucial influence on its acidity. In
addition, the H0 values of IL1 and IL5 with different cations and
same anion (HSO4ˉ) are similar to that of sulfuric acid. Thus the
acidity of IL is determined by the anion of IL and H0 value is
almost equivalent to that of the corresponding inorganic acid.
Characterization of ionic liquids
The structures of precursors and the SO3H-functionalized ILs
were fully characterized by Fourier transform infrared (FT-IR),
1H, 13C NMR spectroscopy analysis and electrospray ionization
mass spectrometry (ESI-MS; Q-TOF Micro Mecromous, VK).
Among them, FT-IR spectra in the frequency range of 4000–
400 cm-1 were recorded on a Nicolet 460 spectrometer
(Nicolet, USA) using KBr pellets and liquid membrane method.
1H and 13C NMR spectra were recorded on a Bruker Avance II
400 MHz with TMS as an internal standard. The parameters
about the thermal stability of ILs were obtained by TGA
(Shimadzu TGA-50) at a heating rate of 10 K/min under
nitrogen. The structural information of ILs is in good
agreement with those reported in literatures33-35. The spectral
data were given in the Supporting Information. UV-vis spectra
were recorded on an UV (HP-8453) spectrophotometer at
room temperature.
Catalytic experiment of functional ILs
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The other series of ILs sharing same anion but different
cations exhibit similar trend. So the cation of IL affects less on
the acid strength of IL.
Con(9-fluorenone) = -101.41 H0 + 203.88
RV2ie=w0A.r9tic9le Online
Where, the point of intersection for thDeOliIn: 1e0a.1n03d9t/Ch9eNxJ-0a2x5i8s1iBs
(2.01, 0). It illustrates when the H0 value of acid catalyst is
greater than 2.01, the condensation of 9-fluorenone with
phenol cannot be initiated. Due to the fact that the H0 of
thioglycolic acid is 2.34, larger than 2.01; when no IL and only
thioglycolic acid were added, 9-fluorenone could not react
with phenol (see Table 3). In brief, a limited H0 value of acid
catalyst is given from Fig 1, which could be used as the rough
criterion of selecting acid catalyst for the synthesis of BHPF.
Using thioglycolic acid as a co-catalyst, the catalytic activities
of ILs in the condensation reaction were further investigated,
and the results are shown in Table 2.
Table 1 Calculation of the Hammett Function (H0) for different ILs in H2O
ILs
Bank
H2SO4
HSCH2COOH
IL1
Amax
2.09
1.54
2.00
1.59
1.75
1.88
1.68
1.59
1.74
1.87
1.68
1.50
[B] / %
100
[BH+] / %
0
26.32
4.31
25.36
16.27
9.57
19.62
25.36
16.75
10.53
19.62
29.40
H0
-
73.68
95.69
74.64
83.73
90.43
80.38
74.64
83.25
89.47
80.38
70.60
1.44
2.34
1.46
1.70
1.94
1.60
1.46
1.69
1.92
1.60
1.37
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IL2
IL3
IL4
IL5
IL6
IL7
IL8
IL9
Table 2 Catalytic activities of ILs in the condensation of 9-fluorenone and phenol
Con(9-
fluorenone)
/ % [a]
59.4
Con(9-
fluorenone)
/ % [b]
100
Selectivity / % [b]
Catalyst
A(BHPF)
B
C
Amax: absorbance at maximum absorption wavelength, 382nm.
H2SO4
IL1
87.9
87.4
86.5
84.0
86.6
87.7
86.8
84.5
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86.9
8.5
8.8
9.4
16.0
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8.5
9.1
15.5
9.1
9.1
3.6
3.8
4.1
0
4.0
3.8
4.1
0
53.7
100
The IL9 bearing two alkyl sulfonic acid groups has much
stronger acidity than the IL1 and IL5 with only one alkyl
sulfonic acid group, which suggests the number of alkyl
sulfonic acid group of the IL will influence the acidity. However
the H0 value of thioglycolic acid is 2.34 resulting in a very weak
acidity.
IL2
29.9
100
IL3
6.9
10.4
IL4
46.1
100
IL5
54.8
100
IL6
31.4
100
IL7
9.1
37.1
IL8
42.5
100
4.0
4.0
Catalytic effect of ILs
IL9
64.2
100
To investigate how the H0 of acid catalyst influences the
condensation reaction of 9-fluorenone with phenol, a series of
experiments with absence of co-catalyst were carried. The
reaction conditions are as follows: 9-fluorenone 0.02 mol,
molar ratio of phenol / 9-fluorenone / catalysts = 6: 1: 0.125, 6
Reaction conditions: 9-fluorenone 0.02 mol, molar ratio of phenol / 9-fluorenone
/ SO3H-functionalized IL = 6: 1: 0.125, 6 h, 383 K. a: without co-catalyst [38]; b: with
co-catalyst (thioglycolic acid 1 mmol, IL / thioglycolic acid = 5:2).
Table 2 shows that SO3H-functionalized ILs and H2SO4 do
work well in the condensation of 9-fluorenone, the conversion
of 9-fluorenone is nearly 100% and the selectivity of BHPF is
h, 110
℃. From Table 2 it can be seen that, for different acid
catalysts (H2SO4 and IL1 - IL9), the percent conversion of 9-
fluorenone (Con(9-fluorenone)/%[a]) is 59.4%, 53.7%, 29.9%,
6.9%, 46.1%, 54.8%, 31.4%, 9.1%, 42.5% and 64.2%,
respectively.
We do simply linear association between the H0 of ILs and
the conversion of 9-fluorenone in Fig.1.
-
about 87%, except IL3 and IL7 with same anion H2PO4 . The
conversion of 9-fluorenone for IL3 and IL7 is separately 10.4%
and 37%. It reinforces further that the lower acidity of acid
catalyst may strongly affect the condensation reaction.
The H0 values of IL1, IL2, IL4, IL5, IL6, IL8, IL9 and H2SO4 are
in the range of 1.45
～ 1.70, which shows that all of them can
fully convert 9-fluorenone. Thus the acid catalyst that H0 value
is less than 1.70 may have practical significance in the
condensation reaction.
Moreover, the co-catalyst plays an important role in the
condensation of 9-fluorenone. Because IL1 catalyzes the
condensation reaction, without or with thioglycolic acid, the
conversion of 9-fluorenone changes from 53.7% to 100%.
Catalytic effect of the mercapto compounds
Using IL1, IL5 as acid catalyst, and p-HSC6H4COOH,
HSCH2COOH,
HSCH2CH2COOH,
CH3CH2CH2CH2SH
and
C6H5CH2SH as co-catalyst, several experiments were carried to
find the catalytic effect of co-catalyst. The results are shown in
Table 3.
Fig 1 The correlation for the H0 of ILs and the conversion of 9-fluorenone
As shown in Table 3, the co-catalysts will obviously affect
The following equation could be obtained through fitting the the conversion of 9-fluorenone and the selectivity of BHPF
experimental data, as:
when the reaction time changes from 1 h to 6 h.
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Table 3 Catalytic effect of co-catalysts at different reaction time in the condensation of 9-fluorenone with phenol
Catalyst
Blank
Co-catalyst
RSH*
[Con(9-flurenone)/%]a
[S(BHPF)/%]a
0
[Con(9flureone)/%]b
[S(BHPF)/%]b
0
0
0
Blank
20.5
28.6
81.5
98.4
100
100
25.0
32.9
89.7
97.9
100
100
89.3
84.2
82.4
89.2
92.6
91.5
89.9
84.0
81.9
89.5
92.4
91.4
46.5
80.2
100
100
100
100
53.7
84.7
100
100
100
100
87.6
88.9
87.4
90.8
91.6
91.5
87.7
88.7
87.7
90.8
91.6
91.6
p-HOOCC6H4SH
HOOCCH2SH
HOOCCH2CH2SH
CH3CH2CH2CH2SH
C6H5CH2SH
IL1
IL5
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p-HOOCC6H4SH
HOOCCH2SH
HOOCCH2CH2SH
CH3CH2CH2CH2SH
C6H5CH2SH
* HSR，R = -CH2COOH，-CH2CH2COOH，-CH2CH2CH2CH3，-CH2C6H5，-C6H4COOH; HSR 2.5 mmol; IL1: [PyN+(CH2)3SO3H] [HSO4ˉ]; IL5：[(CH3CH2)3N+(CH2)3SO3H]
[HSO4ˉ]; a: 1 h; b: 6 h. Reaction conditions: 9-fluorenone 0.02 mol, molar ratio of phenol / 9-fluorenone / SO3H-functionalized ionic liquid = 6:1: 0.125, Co-catalysts 1.0
mmol, 110℃.
As previously mentioned when only mercapto compound co-catalyst, the conversion of 9-fluorenone closes to 100% and
was present without any ILs, the reaction would not occur. Yet the selectivity of BHPF have no change over different reaction
if IL was only added without any co-catalysts, the reaction time. However the selectivity of BHPF slight drops with the
would be carried out in a low efficiency. In the IL catalyzing prolongation of reaction time in IL-only system. In summary,
reaction system, even a small amount of thiol co-catalyst the electron density of the -SH group and the size of mercapto
(except 4-mercaptobenzoic acid) was added, the conversion of compound affect the selectivity of BHPF. The fact reconfirms
9-fluorenone could be remarkably increased. This illustrates that -SH group in co-catalyst plays an important role in the
that the IL catalyst and the thiol co-catalyst have a synergistic reaction process.
effect in the reaction process.
Comparing the experimental data of catalytic reactions at 1
The mechanism discussion
According to the above results and analysis as well as the
h, it is shown that the type and structure of co-catalyst have
existing references39, 40, a possible mechanism is proposed for
vital influence on the conversion of 9-fluorenone. In the
the condensation reaction of 9-fluorenone under the IL-thiol
reactions using IL1 as catalyst and 4-mercaptobenzoic acid,
composite catalyst system (Scheme 3).
thioglycolic acid, 3-mercaptopropionic acid, 1-butanethiol,
benzyl mercaptan as co-catalyst, the conversion of 9-
fluorenone is 28.6%, 81.5%, 98.4%, 100% and 100%,
respectively. The three mercaptan acid co-catalysts have lower
conversion of 9-fluorenone, compared to that of the two
mercaptan co-catalysts. As far as the three mercaptan acids
are concerned, there is a consistent augment for the
conversion of 9-fluorenone while the electron density
(nucleophilicity) of -SH group in co-catalyst increases in turn.
That means that -SH in co-catalyst directly participates in the
reactive process.
When the reaction time extends from 1h to 6 h, 9-
fluorenone is fully converted except reactions with 4-
mercaptobenzoic acid as co-catalyst. This is due to the -SH
group of 4-mercaptobenzoic acid having the lower
nucleophilicity. Even so, in the reactions with IL1 as catalyst
and 4-mercaptobenzoic acid as co-catalyst, along with the
reaction time changing from 1h to 6h, the conversion of 9-
fluorenone also increases from 28.6% to 84.2%.
Using IL1 or IL5 as the catalyst and 4-mercaptobenzoic acid
or thioglycolic acid as co-catalyst, the conversion of 9-
fluorenone is lower than 98% after reacting 1h. And the
selectivity of BHPF changes nearly from 84% to 89% when
reaction time expends to 6h. As to the reactions with 3-
mercaptopropionic acid, 1-butanethiol or benzyl mercaptan as
Scheme 3 The mechanism for the condensation of 9-fluorenone with phenol
catalysing by the SO3H-functionalized IL and thiol synergistic catalyst system.
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carbonyl of 9-fluorenone to form intermediate (I). Because the
hydroxyl group of the intermediate (I) is a poor leaving group,
and the ortho (o-) or para (p-) carbon of phenol has a weak
nucleophilicity, so it makes phenol difficult to directly react
with the intermediate (I). Therefore, in the absence of co-
catalyst, the overall reaction rate is slow and the conversion of
9-fluorenone is low. When the mercapto co-catalyst is added,
the nucleophilicity of the -SH group is greater than that of o- or
p- carbon of phenol, the intermediate (I) is converted to the
sulfonium salt intermediate (II). The o- or p- carbon in phenol
with partially negative charge acts as nucleophile to attack the
intermediate (II) to form the tertiary carbocation intermediate
(III, IV), meanwhile the thiol co-catalyst is released back to the
reaction system.
Finally, the tertiary carbocation intermediate (III, IV) is
combined with the other phenol molecule to obtain BHPF (A)
or by-product 1 (B), and a proton is also released back and
make the IL catalyst recruit. The tertiary carbocation (III) can
also be combined with BHPF (A) to form by-product 2 (C).
The important step in the mechanism is that thiol co-
catalyst takes part in the forming of the intermediate (II). Due
to the strong nucleophilicity of -SH group in the thiol
compound, the intermediate (II) is easily formed, and thus the
subsequent reaction can proceed smoothly. This explains the
significantly increasing of the conversion of 9-fluorenone after
the addition of the thiol co-catalyst. According to the different
electronic effect of thiol co-catalysts, if the nucleophilicity of -
SH group in thiol compounds is larger, then the intermediate (I)
is more easily converted to intermediate (II). At the same time
the size of the mercapto compounds may determine the
proportion of the intermediates (III) and the intermediates (IV),
and thereby changes the selectivity of the BHPF.
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Fig. 2 Recycling of [PyN+(CH2)3SO3H][HSO4 ] in the condensation reaction. Reaction
conditions: 9-fluorenone 0.02 mol, molar ratio of phenol / 9-fluorenone / SO3H-
functionalized ionic liquid (the first dose) = 6: 1: 0.125, thioglycolic acid 1 mmol, 6 h,
383 K.
-
Conclusions
In the present work, a reusable SO3H-functionalized IL is
used in the condensation of 9-fluorenone with phenol to
replace normal acid catalysts. The H0 of ILs is mainly controlled
by its anion, and also affects its catalytic performance in the
condensation reaction. If the H0 of acid catalyst is greater than
2.01, the catalyst has no catalysis. When the H0 of ILs is less
than 1.70, 9-fluorenone can be fully converted and the best
selectivity of BHPF is about 92% combinded with thiol co-
catalyst. In addition, a series of mercapto compounds are used
in the reaction system, and it is discussed in details how the
electronic and spatial effect of co-catalysts works on the
activity of the reaction and the selectivity of BHPF. Based on
the experimental results, a mechanism is promoted and the
significance of -SH group in the co-catalyst is clearly illustrated.
Under the mechanism of the synergistic catalysis, the
nucleophilicity of -SH group in the thiol co-catalyst and the H0
of the IL catalyst have important influences on the conversion
of 9-fluorenone and the selectivity of BHPF. The study on the
synthesis of bisphenolfluorene under IL-thiol composite
catalyst system provides valuable clues for the design of new IL
catalysts that contain the bifunction of the composite catalysts
in single IL structure.
Recycling of the acidic IL catalyst
In order to investigate the recyclable possibility and ability
of the SO3H-functionalized IL, the recycling experiment was
conducted. After the reaction, deionized water is added to the
reaction mixture, which is stirred at room temperature about
15 min, and then stands until the mixture separates to two
layers. The IL catalyst is in the upper aqueous solution. The
upper liquid is transferred to a 50 mL one-neck round bottom
flask, and water is removed by rotary evaporator. The residue
is washed three times with diethyl ether. Finally the residue is
dried under vacuum drying oven at 100°C to obtain recycled IL
catalyst. The recycled IL catalyst again is employed in the
condensation of 9-fluorenone. The procedure is repeated five
times at the same reaction conditions, the conversion of 9-
fluorenone firstly begins to reduce, and the result is shown in
Fig. 2.
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DOI: 10.1039/C9NJ02581B
The SO3H- functionalized ionic liquid and thiol co-catalyst synergistically catalyze
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