Functional Ionic Liquids as Efficient and Recyclable Catalysts for the Methylation of Formaldehyde with Aromatics

Article abstract of DOI:10.1007/s10562-016-1750-5

Abstract: Methylation of formaldehyde with various aromatics under functional ionic liquids catalysis has been developed. Among the ionic liquids investigated, triphenyl-(4-sulfobutyl)-phosphonium triflate ([TTPBs][CF₃SO₃]) showed high activity and afforded excellent yields of diarylmethane derivatives. A mechanism for the catalytic performance of [TTPBs][CF₃SO₃] is proposed. Besides, the catalyst can simply be separated from the reaction mixture by centrifugation and be recycled ten times without noticeable loss of activity. Graphical Abstract: Diarylmethane derivatives were successfully synthesized from the methylation of formaldehyde with aromatics using efficient and recyclable functional ionic liquids as catalysts, excellent yields and selectivities were obtained under solvent free conditions. The catalyst was reused at least ten consecutive recycles without noticeable loss in its catalytic activity. Meanwhile, the usability of catalyst was explored.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1750-5

Catal Lett
DOI 10.1007/s10562-016-1750-5
Functional Ionic Liquids as Efficient and Recyclable Catalysts
for the Methylation of Formaldehyde with Aromatics
Heyuan Song1,2 Fuxiang Jin1 Ronghua Jin1 Meirong Kang1 Zhen Li1
Jing Chen1
•
•
•
•
•
Received: 3 April 2016 / Accepted: 13 April 2016
Ó Springer Science+Business Media New York 2016
Abstract Methylation of formaldehyde with various
aromatics under functional ionic liquids catalysis has been
developed. Among the ionic liquids investigated, triphenyl-
(4-sulfobutyl)-phosphonium triflate ([TTPBs][CF3SO3])
showed high activity and afforded excellent yields of
diarylmethane derivatives. A mechanism for the catalytic
Keywords Formaldehyde Á Methylation Á
Diarylmethane Á Ionic liquids
performance of [TTPBs][CF3SO3] is proposed. Besides,
the catalyst can simply be separated from the reaction
mixture by centrifugation and be recycled ten times with-
out noticeable loss of activity.
1 Introduction
Graphical Abstract Diarylmethane derivatives were
successfully synthesized from the methylation of
formaldehyde with aromatics using efficient and recyclable
functional ionic liquids as catalysts, excellent yields and
selectivities were obtained under solvent free conditions.
The catalyst was reused at least ten consecutive recycles
without noticeable loss in its catalytic activity. Meanwhile,
the usability of catalyst was explored.
The methylation reaction of formaldehyde with aromatics
is one of the powerful carbon–carbon bond-forming reac-
tions in organic synthesis for the production of diaryl-
methane derivatives, which are useful in organic synthesis
and polymer synthesis [1–4]. Diarylmethane moiety exists
in many organic compounds and also shows interesting
physiological activity [5–8]. It has already been reported
that diarylmethane derivatives can be synthesized by the
methylation of formaldehyde with aromatics in the pres-
ence of a liquid protonic acid (e.g., sulphuric acid, phos-
phoric acid, hydrochloric acid, etc.) [9–13] or a Lewis acid
(e.g., InCl3Á4H2O, [14] FeCl3, [15] etc.). However, heavy
corrosion, difficulty in separation and recovery, disposal of
the spent catalyst are the disadvantages associated with the
reported methods involving methylation of formaldehyde
with aromatics. In order to solve the above problems, the
more environmentally friendly and easily separable solid
acids, such as montmorillonite KSF [16], heteropoly acids
[17], molecular sieves [18, 19] have been employed in the
methylation reaction of formaldehyde with phenol.
Recently, based on biomass conversion, Zhang and coau-
thors [20, 21] reported a series of solid-acid as catalysts for
the alkylation between furan and carbonyl compounds and
& Zhen Li
zhenli@licp.cas.cn
& Jing Chen
chenj@licp.cas.cn
1
State Key Laboratory for Oxo Synthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, People’s Republic of
China
2
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
123

H. Song et al.
found that Nafion-212 resin is the best catalyst given 67 %
yield at 50 °C for 2 h. Nevertheless, the rapid deactivation
of solid acids might be the major drawbacks from the
viewpoint of industrial application.
standard. Chemical shifts are reported in parts per million
(ppm, d) and referenced to D2O (d = 4.79) or CD3OD
(d = 3.31). Elementary analyses were obtained with an
Elementar Vario EL cube instrument.
Ionic liquids (ILs) have attracted interest as a novel
catalyst because of their favorable properties. ILs possess
the advantageous characteristics of both homogenous and
heterogeneous catalysts, such as high acid or alkali density,
uniform catalytic active centers, easy separation and
recyclability [22]. Therefore, they had been widely used in
the catalytic processes such as material synthesis, organic
reaction and the recent biomass conversion [23–26].
However, it has not yet seen in the literature reports about
the methylation of formaldehyde with arenes under the
catalysis of functionalized ILs (FILs). In our previous work
concerning the application of acid-functionalized ILs in
esterification, [27] cyclotrimerization, [28] carbonylation
[29, 30] and other acid catalyzed reactions, we found that
they especially those with an alkane sulfuric acid group are
advanced alternatives to traditional liquid acid catalysts.
This promoted us to explore the potential applications of
such SO3H-functionalized ILs (Scheme 1) in the methy-
lation of formaldehyde with aromatics. Moderate to good
yields were obtained for the methylation of arenes con-
taining various groups with formaldehyde in the presence
of [TTPBs][CF3SO3] catalyst. Moreover, the recyclability
of the catalyst system was also examined. Finally, a pos-
sible reaction mechanism for this reaction system was
discussed based on the experimental results.
2.1.1 [TTPBs][CF3SO3]
1H NMR (400 MHz, CD3OD): d 1.86 (h, J = 7.8 Hz, 2H),
2.00 (h, J = 8.3 Hz, 2H), 2.86 (t, J = 3.5 Hz, 2H), 3.44 (t,
J = 7.5 Hz, 2H), 7.72–7.77 (m, J = 5.5 Hz, 9H), 7.80 (t,
J = 2.2 Hz, 3H), 7.87 (t, J = 4.7 Hz, 3H), 13C NMR
(100 MHz, CD3OD): d 20.84, 25.33, 47.18, 47.28, 47.82,
48.24, 49.70, 118.99 (JC*F = 315.1 Hz), 130.13, 133.50,
134.74. Anal. Calcd for C23H24F3O6PS2: C, 50.36; H, 4.41;
O, 17.50. Found: C, 49.91; H, 4.39; O, 17.91.
2.1.2 [TTPBs][HSO4]
1H NMR (400 MHz, CD3OD): d 1.87 (h, J = 7.8 Hz, 2H), 1.98
(h, J = 7.2 Hz, 2H), 2.86 (t, J = 3.6 Hz, 2H), 3.44 (t,
J = 7.6 Hz, 2H), 7.72–7.77 (m, J = 5.5 Hz, 9H), 7.80 (t,
J = 2.1 Hz, 3H), 7.86 (t, J = 4.1 Hz, 3H), 13C NMR
(100 MHz, CD3OD): d 19.89, 25.43, 47.18, 47.39, 47.60, 47.82,
48.03, 130.0, 133.50, 134.87. Anal. Calcd for C22H25O7PS2: C,
53.21; H, 5.08; O, 22.56. Found: C, 53.12; H, 5.12; O, 22.63.
2.1.3 [TTPBs][Ts-OH]
1H NMR (400 MHz, CD3OD): d 1.87 (h, J = 7.9 Hz, 2H),
2.01 (h, J = 7.2 Hz, 2H), 2.36 (s, 3H), 2.86 (t, J = 3.6 Hz,
2H), 3.44 (t, J = 7.5 Hz, 2H), 7.24(d, J = 2.0 Hz, 2H),
7.71(d, J = 1.1 Hz, 2H), 7.72–7.77 (m, J = 3.5 Hz, 9H),
7.80 (t, J = 2.8 Hz, 3H), 7.87 (t, J = 4.0 Hz, 3H), 13C
NMR (100 MHz, CD3OD): d 19.89, 21.0, 25.43, 46.97,
47.18, 47.39, 47.60, 47.82, 48.03, 48.24, 125.60, 128.45,
130.18, 133.40. Anal. Calcd for C29H31O6PS2: C, 61.04; H,
5.48; O, 16.82. Found: C, 60.93; H, 5.60; O, 16.90.
2 Experimental
2.1 ILs Preparation
ILs were prepared according to the previous literature [31].
A stoichiometric amount of 1,4-butane sultone, 1,3-pro-
pane sultone, or bromobutane was added dropwise to a
solution of triphenylphosphine, N-methyl imidazole or
pyridine in toluene, and then the mixture was stirred at
60 °C for 12 h. The white solid zwitterion was washed
repeatedly with toluene to remove non-ionic residues and
dried in vacuo. Then the zwitterion and CF3SO3H or
H2SO4 were mixed in a molar ratio of 1:1 in anhydrous
toluene, and stirred magnetically at 60 °C for 8 h, followed
by washing with toluene and drying in vacuo to obtain the
final ILs. When using p-toluenesulfonic acid as source of
anion, the preparation of FIL was conducted by water as
solvent and the system was at reflux for 8 h. The mixture
2.1.4 [TTPTs][CF3SO3]
1H NMR (400 MHz, CD3OD): d 2.13 (h, J = 6.7 Hz, 2H),
2.99 (t, J = 3.5 Hz, 2H), 3.63 (t, J = 7.6 Hz, 2H),
7.72–7.80 (m, J = 7.5 Hz, 9H), 7.82 (t, J = 3.2 Hz, 3H),
7.88 (t, J = 4.0 Hz, 3H), 13C NMR (100 MHz, CD3OD): d
18.61, 47.18, 47.38, 47.60, 47.82, 48.03, 118.84 (JC*F
=
315.1 Hz), 130.25, 133.61, 134.92. Anal. Calcd for C22H22
F3O6PS2: C, 49.44; H, 4.15; O, 18.0. Found: C, 49.32; H,
4.21; O, 18.87.
1
was dried in vacuo to form corresponding ILs. The H
2.1.5 [TTPB][CF3SO3]
NMR spectra and 13C NMR spectra in CD3OD or D2O
were recorded on an Avance TM III-400 MHz NMR
spectrometer using tetramethylsilane (TMS) as internal
1H NMR (400 MHz, CD3OD): d 0.96 (t, J = 3.6 Hz, 3H),
1.58 (m, J = 12.2 Hz, 4H), 3.38 (t, J = 7.3 Hz, 2H),
123

Functional Ionic Liquids as Efficient and Recyclable Catalysts for the Methylation of…
Scheme 1 Methylation of
formaldehyde with aromatics
catalyzed by acidic ionic liquids
7.72–7.76 (m, J = 5.2 Hz, 9H), 7.81 (t, J = 3.1 Hz,
3H),7.88 (t, J = 4.0 Hz, 3H), 13C NMR (100 MHz, CD3-
OD): d18.48, 20.18, 20.70, 47.18, 47.39, 47.60, 47.82,
48.03, 118.75 (JC*F = 315.1 Hz), 130.25, 133.61. Anal.
Calcd for C23H24F3O3PS: C, 58.97; H, 5.16; O, 10.25.
Found: C, 58.99; H, 5.16; O, 10.30.
118.05 (JC*F = 315.1 Hz), 122.11, 123.61, 135.83. Anal.
Calcd for C9H15N2F3O6S2: C, 29.35; H, 4.10; N, 7.61.
Found: C, 29.32; H, 4.21; N, 7.56.
2.1.7 [MIMBs][HSO4]
1H NMR (400 MHz, D2O): d 1.58 (h, J = 4.8 Hz, 2H),
1.85 (h, J = 7.8 Hz, 2H), 2.78 (t, J = 7.6 Hz, 2H), 3.72 (s,
3H), 4.08 (t, J = 7.0 Hz, 2H), 7.26 (d, J = 3.6 Hz, 1H),
7.32 (d, J = 3.6 Hz, 1H), 8.57 (s, 1H). 13C NMR
(100 MHz, D2O): d 20.86, 28.04, 35.62, 48.85, 55.36,
122.08, 123.67, 135.94. Anal. Calcd for C8H16N2O7S2: C,
30.37; H, 5.10; N, 8.86. Found: C, 29.53; H, 5.20; N, 8.78.
2.1.6 [MIMBs][CF3SO3]
1H NMR (400 MHz, D2O): d 1.29 (h, J = 7.8 Hz, 2H),
1.56 (h, J = 7.5 Hz, 2H), 2.48 (t, J = 7.6 Hz, 2H), 3.43 (s,
3H), 3.78 (t, J = 7.2 Hz, 2H), 6.98 (d, J = 3.6 Hz, 1H),
7.03 (d, J = 4.0 Hz, 1H), 8.26 (s, 1H). 13C NMR
(100 MHz, D2O): d 20.91, 28.07, 35.61, 48.84, 50.06,
123

H. Song et al.
2.1.8 [MIMBs][Ts-OH]
3 Results and Discussion
1H NMR (400 MHz, D2O): d 1.56 (h, J = 7.7 Hz, 2H),
1.84 (h, J = 7.5 Hz, 2H), 2.23 (s, 3H), 2.77 (t, J = 7.6 Hz,
2H), 3.70 (s, 3H), 4.05 (t, J = 7.0 Hz, 2H), 7.19 (d,
J = 6.4 Hz, 2H), 7.24 (d, J = 3.6 Hz, 1H), 7.30 (d,
J = 3.6 Hz, 1H), 7.51 (d, J = 8.4 Hz, 2H), 8.54 (s, 1H).
13C NMR (100 MHz, D2O): d 20.39, 20.82, 27.99, 35.53,
48.79, 49.95, 122.03, 123.53, 125.23, 129.34, 135.79,
139.34, 142.35. Anal. Calcd for C15H22N2O6S2: C, 46.02;
H, 5.66; N, 7.16. Found: C, 45.96; H, 5.72; N, 7.15.
Initially, a probe reaction of trioxane (10 mmol) and o-
xylene (60 mmol) was carried out at 160 °C under solvent-
free conditions catalyzed by these ILs (2 mol %) and the
results are shown in Table 1. Besides the main product
bis(dimethylphenyl)methane, small amount of 1,1,2-tris(-
dimethylphenyl)ethane was also detected in the reaction
mixture. It is obviously to see that the catalytic activities of
ILs with the same cations were mainly affected by the
anions. ILs with CF3SO3- as anion appeared to be superior
-
to those of the corresponding ILs with HSO4 or p-
(CH3)C6H5SO3- as anion (Table 1, entries 1–3 and entries
6–8). This may be due to the strong acidity of ILs with
CF3SO3- compared with other anions when possessing the
same cation [29]. Those of [CF3SO3]-type ILs bearing one
alkyl sulfonic acid group on cations exhibited better
activity than the one without –SO3H group on cation
(Table 1, entries 1, 4, 6, 9, 10 vs. entry 5), and the carbon
chain length between –SO3H group and cation core has
only a slight effect on catalytic efficiency (Table 1, entries
1, 4 vs. entries 6, 9). Among the ten ILs investigated,
2.1.9 [MIMTs][CF3SO3]
1H NMR (400 MHz, D2O): d 2.23 (h, J = 7.3 Hz, 2H),
2.84 (t, J = 7.6 Hz, 2H), 3.81 (s, 3H), 4.28 (t, J = 7.0 Hz,
2H), 7.36 (d, J = 3.6 Hz, 1H), 7.43 (d, J = 3.6 Hz, 1H),
8.67 (s, 1H). 13C NMR (100 MHz, D2O): d 24.99, 35.60,
47.10, 47.63, 117.97 (JC*F = 315.1 Hz), 122.10, 123.67,
136.09. Anal. Calcd for C8H13N2F3O6S2: C, 27.12; H, 3.70;
N, 7.91. Found: C, 27.46; H, 3.08; N, 7.92.
2.1.10 [PyBs][CF3SO3]
–SO3H
functionalized
phosphonium
based
IL,
[TTPBs][CF3SO3] appeared to be the best catalyst with
89.3 % total yield of bis(dimethylphenyl)methanes
(Table 1, entry 1).
1H NMR D2O (d ppm): 1.64 (h, J = 7.8 Hz, 2H), 2.03 (h,
J = 7.6 Hz, 2H), 2.82 (t, J = 7.6 Hz, 2H), 4.51 (t,
J = 7.6 Hz, 2H), 7.92 (t, J = 7.2 Hz, 2H), 8.42 (t,
J = 4.6 Hz, 1H), 8.73 (d, J = 5.6 Hz, 2H); 13C NMR
D2O (d ppm): 20.78, 29.24, 49.85, 61.11, 117.96
(JC*F = 315.4 Hz CF3), 128.21, 144.16, 145.59; Anal.
Calcd for C10H14NF3O6S2: C, 32.88; H, 3.86; N, 3.83.
Found: C, 32.71; H, 3.73; N, 3.87.
Methylation of formaldehyde with aromatics mostly
proceeds via a two-step conversion, that is hydrox-
ymethylation of formaldehyde with aromatics to produce
benzyl alcohol followed by subsequent alkylation of benzyl
alcohol with aromatics [16, 20, 21, 32, 33]. The hydroxy-
methylation step requires a strong acid to protonate
formaldehyde because of its low proton affinity, in addition
alkylation of benzyl alcohol and aromatic is also a typical
acid-catalyzed reaction. So SO3H-functionalized ILs with
strong acidity can be suitable catalysts for the aimed
reaction.
2.2 General Procedure for the Methylation
of Formaldehyde and Aromatics
The reaction was carried out using a 25 mL round-bot-
tomed flask equipped with a magnetic stirrer. In a typical
experiment, substituted aromatics (80 mmol), formalde-
hyde (10 mmol) and IL (1.8 mmol) were charged into the
flask, then the flask was sealed up and the reaction was
performed at 140–170 °C for 2–12 h. After reaction, the
flask was cooled to room temperature and the IL was
separated from the reaction mixture by centrifugation. The
final products were identified and quantitatively analyzed
by gas chromatography/mass spectrometry (GC/MS)
(Agilent 7890A/5975C) and GC (Agilent 6890 equipped
with a SE-54 capillary column), respectively. A known
amount of n-heptane was added as an internal standard to
the product mixture before the GC analysis. For the recy-
cling of catalyst, the IL was extracted with n-hexane
(5 mL 9 3) and dried in a vacuo for 8 h before reusing.
Using [TTPBs][CF3SO3] as catalyst, the source of
formaldehyde, i.e. solid state (trioxane and paraformalde-
hyde) and liquid state (50 % of aqueous solution of
formaldehyde) was firstly examined and the results are
shown in Table 2. We found that although similar selec-
tivity of 1a was obtained under different form of
formaldehyde, trioxane was the best supplier of
formaldehyde for the reaction that gave the product bis(-
dimethylphenyl)methane in high yield (91.3 %). This
phenomenon may be explained in the light of better solu-
bility of trioxane in the other reactant of o-xylene.
To optimize the methylation reaction conditions of tri-
oxane with o-xylene, the effects of catalyst dosage, molar
ratio of o-xylene to formaldehyde, reaction temperature,
and reaction time were also investigated with
123

Functional Ionic Liquids as Efficient and Recyclable Catalysts for the Methylation of…
Table 1 Catalytic activities of different ILs on the methylation of formaldehyde with o-xylene
Entrya
Catalysts
Yield (%)
Selectivity (%)
a
1a
2a
3a
1
2
3
4
5
6
7
8
9
10
a
[TTPBs][CF3SO3]
[TTPBs][HSO4]
[TTPBs][p-TSA]
[TTPTs][CF3SO3]
[TTPB][CF3SO3]
[MIMBs][CF3SO3]
[MIMBs][HSO4]
[MIMBs][p-TSA]
[MIMTs][CF3SO3]
[PyBs][CF3SO3]
89.3
71.1
21.4
87.5
69.1
72.2
57.1
41.7
69.9
71.7
86.0
93.7
91.7
85.0
80.1
83.5
92.9
94.0
82.7
86.7
1.0
1.3
1.8
1.1
1.0
2.1
1.3
1.6
1.0
1.1
3.4
3.8
3.3
3.2
3.2
3.4
3.8
4.5
3.7
3.5
Reaction conditions: amount of catalyst is 2 mol%, using trioxane as monomeric formaldehyde sources, o-xylene/formaldehyde =
6 (mol ratio), 160 °C, 4 h
respectively (Table 3, entries 6, 10–12). Further elevate of
Table 2 Influence of different monomeric formaldehyde sources on
the reaction
the temperature, the yield of products decreased due to the
formation of more byproducts at higher temperature. The
dependence of the total yield and selectivity of 1a on the
reaction time was investigated in the range of 2–6 h
(Table 3, entries 6, 13 and 14). The yield and selectivity
increased with prolonging reaction time from 2 to 4 h, and
then decreased slightly when the reaction proceeded, which
implied occurrence of the side-reactions with time exten-
sion. When increased the dosage of [TTPBs][CF3SO3] and
reactants to ten times, the yield of a and the selectivity of
1a were still up to 90.9 and 87.3 %, respectively, which
showed that methylation of formaldehyde with aromatics
has good synthetic application (Table 3, entry 6 vs. entry
7).
Entrya
Source of
formaldehyde
Yield (%)
Selectivity (%)
a
1a
2a
3a
1
2
3
a
Trioxane
91.3
81.5
76.0
87.1
86.7
87.1
1.2
1.2
2.2
3.6
3.5
3.5
Paraformaldehyde
Formalin (50 %)
Reaction conditions: amount of [TTPBs][CF3SO3] is 2 mol%, o-
xylene/formaldehyde = 8 (mol ratio), 160 °C, 4 h
[TTPBs][CF3SO3] as catalyst and the results are summa-
rized in Table 3. The catalyst dosage had a great effect on
the reaction. When molar percent content of
[TTPBs][CF3SO3] increased from 1 to 2 mol%, (Table 3,
entries 1 and 2) the total yield of products and the selec-
tivity of 1a increased from 72.6 and 82.9 % to 89.3 and
86.0 % respectively, and a further increase to 4 mol%
resulted in decrease in yield and selectivity (Table 3, entry
3). Hence, 2 mol% of [TTPBs][CF3SO3] was the optimal
amount to efficient production of methylation products.
The increase of the molar ratio of o-xylene to formalde-
hyde is propitious for the reaction (Table 3, entries 2, 4–9)
because the excess of one of the reactants makes the
equilibrium shifting towards the products side. From the
data of Table 3, the reaction was also influenced by reac-
tion temperature. When the temperature increased from
140 to 160 °C, the yield of products and selectivity of 1a
increased from 87.8 to 91.3 % and 84.5 to 87.1 %,
A series of recycle experiments were conducted to
investigate the recoverability and recyclability of the
SO3H-functionalized ILs. The ILs with –SO3H functional
group are hardly soluble in common organic solvents
except for methanol and ethanol and have good solubility
in water. [TTPBs][CF3SO3] dissolved in water as a sepa-
rate phase could be centrifugated out from the reaction
mixture at the end of the reaction. The amount of water in
ILs phase is about 15 %, which is by-product of the reac-
tion. The ILs phase was extracted by n-hexane (5 mL 9 3)
and then vacuumized for 12 h at 80 °C. After that the ILs
phase was used for another cycle under the same condi-
tions. The reusability of the recycled catalyst is shown in
Fig. 1. As expected, [TTPBs][CF3SO3] could be reused for
ten times at least and there was no obvious decrease in the
123

H. Song et al.
Table 3 Effects of different reaction conditions using [TTPBs][CF3SO3] as catalyst
Entrya
Amount of
catalyst (mol%)
O-xylene:
formaldehyde (mol ratio)
Temp. (°C)
Time (h)
Yield (%)
Selectivity (%)
a
1a
2a
3a
1
1
2
4
2
2
2
2
2
2
2
2
2
2
2
6
160
160
160
160
160
160
160
160
160
140
150
170
160
160
4
4
4
4
4
4
4
4
4
4
4
4
2
6
72.6
89.3
82.6
43.3
70.1
91.3
90.9
93.1
98.6
87.8
90.6
89.4
91.0
83.6
82.9
86.0
82.3
67.4
78.7
87.1
87.3
91.3
93.9
84.5
87.3
86.0
86.2
83.6
1.1
1.0
1.1
0.9
1.6
1.2
1.3
1.1
1.9
2.4
1.2
1.1
1.1
1.1
4.9
3.4
3.7
2.4
3.2
3.6
3.1
3.4
3.7
3.3
3.5
3.6
3.4
3.4
2
6
3
6
4
2
5
4
6
7b
8
8
8
10
12
8
9
10
11
12
13
14
a
8
8
8
8
Reaction conditions: using trioxane as monomeric formaldehyde sources
b
The dosage of o-xylene is 0.8 mol, formaldehyde is 0.1 mol, [TTPBs][CF3SO3] is 0.018 mol
yield of products and selectivity of 1a, which indicated that
[TTPBs][CF3SO3] was stable in the reaction system.
The applicability of this catalytic system for the reaction
of various aromatics with formaldehyde was also studied
and the results were summarized in Table 4. The results
indicated that aromatics containing electron donating sub-
stituents such as –OCH3 and –CH3 gave corresponding
methylation products in excellent yields within 4–8 h at
160 °C, and no significant effect was observed by changing
the number and the position of the –CH3 group in the
benzene ring (Table 4, entries 1–3, 6, 8). Astonishingly,
benzene which does not have any substituent also affords
methylation product in 31.1 yield and 76.0 % selectivity
(Table 4, entry 7). The methylation of aromatics substi-
tuted with an electron-withdrawing group such as –Cl and
–Br was also successful using 4 mol % [TTPBs][CF3SO3]
as catalyst. The yield of bis(2-methoxyl-5-chlor-
ophenyl)methane was found to be 87.3 % under 160 °C for
12 h, as well as 100 % of selectivity to the corresponding
products (Table 4, entry 4). As for the p-bromoanisole, the
yield and selectivity were 76.7 and 98.7 %, respectively
(Table 4, entry 5). Thus, the feasibility of the FILs cat-
alyzed methylation reaction of formaldehyde with aro-
matics, is dependent on the nucleophilicity of the
respective aromatics. To obtain more insight into the cat-
alytic possibilities of FILs, we also investigated the reac-
tion of N,N-dimethylaniline with formaldehyde for
synthesis of 4,40-methylenebis( N,N-dimethylaniline), 68.2
yield and 96.2 % selectivity were obtained under 160 °C
for 12 h (Table 4, entry 10).
According to the previous papers reported before, [16,
20, 21, 32, 33] a plausible mechanism for methylation of
formaldehyde with aromatics catalyzed by SO3H-func-
tionalized ILs is shown in Scheme 2. We consider that
SO3H-functionalized ILs catalyst plays a crucial role in the
decomposition process of trioxane and monomeric
formaldehyde activation, which has been studied with the
density functional theory (DFT) calculations [34]. That is
to say, formaldehyde is firstly activated via hydrogen-
bonding interaction between H atom of –SO3H group in
catalyst and O atom of CH2O. Subsequently, a proton is
transferred from catalyst to CH2O to form the intermediate
Fig. 1 Recycling of [TTPBs][CF3SO3] in the methylation reaction of
formaldehyde with o-xylene. Reaction conditions: amount of
[TTPBs][CF3SO3] is 2 mol%, o-xylene/formaldehyde = 8 (mol
ratio), 160 °C, 4 h
123

Functional Ionic Liquids as Efficient and Recyclable Catalysts for the Methylation of…
Table 4 Methylation of formaldehyde with various aromatics catalyzed by FILs
Aroma Time Yield
Entrya
Products (Selectivity )
tics
(h)
(%)
1
2
8
91.7
8
4
80.6
95.8
87.3
76.7
96.0
3
4 b
5 b
6
12
12
4
7
8
8
4
31.1
93.6
9
8
66.4
68.2
10 b
12
a
Reaction conditions: amount of [TTPBs][CF3SO3] is 2 mol%, using trioxane as monomeric formaldehyde sources, aromatic/formalde-
hyde = 8 (mol ratio), 160 °C
b
Reaction conditions: amount of [TTPBs][CF3SO3] is 4 mol %
methylol cation (CH2?OH). The electrophilic attack of
methylol cation (CH2?OH) on aromatic ring generates a
benzyl alcohol. And then hydroxymethyl group of the
benzyl alcohol is further activated by the SO3H-function-
alized ILs catalyst to form hydroxyl benzyl carbocation,
which reacts with the second molecule of aromatic to give
diarylmethanes and eliminates a molecule of water
simultaneously.
Based on the analysis of the products of the reaction of
o-xylene with formaldehyde by using GC/MS, bis(di-
methylphenyl)methane was identified as the major product
and 1,1,2-tris(dimethylphenyl)ethane as byproduct, 3,4-
123

H. Song et al.
Scheme 2 Plausible
mechanistic pathway for
methylation of formaldehyde
with aromatics catalyzed by
[TTPBs][CF3SO3]
dimethylbenzyl alcohol was hardly detected in the reaction
mixture. We then investigated the reaction of 3,4-
dimethylbenzyl alcohol with o-xylene under 160 °C for
30 min, the conversion of carbinol and the yield of bis(3,4-
dimethylphenyl)methane were up to 100 and 99.1 %
respectively, which verifies the rate of the alkylation
reaction is much higher than that of the hydroxymethyla-
tion reaction step.
4 Conclusions
In conclusion, 2 mol% [TTPBs][CF3SO3] was found to be an
effective catalyst for methylation reaction of formaldehyde
with aromatics, giving the highest yield (98.6 %) to methy-
lation products and selectivity (93.9 %) to bis(3,4-dime-
thylphenyl)methane. The catalyst could simply be separated
by centrifugated out from the reaction mixture. After
123

Functional Ionic Liquids as Efficient and Recyclable Catalysts for the Methylation of…
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5,395,915
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extracted with n-hexane and removal of water under vacuum
the catalyst could be recycled and reused for ten times
without decreasing the catalytic activity. The present study
shows that the SO3H-functionalized ILs has a potential
application in the procedures for C–H transformation and C–
C formation of aromatics in organic synthesis.
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Acknowledgments This work was supported by the National Nat-
ural Science Foundation of China (Project No. 21473225 and
21133011).
21. Li GY, Li N, Li SS, Wang A, Cong Y, Wang XD, Zhang T (2013)
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