Synthesis of methylal from methanol and formaldehyde catalyzed by Br?nsted acid ionic liquids with different alkyl groups

Article abstract of DOI:10.1039/c5ra18598j

The catalytic reaction of methanol with formaldehyde for the preparation of methylal was investigated in various Br?nsted acid ionic liquids with different carbon chain length of alkyl groups. The structures, acidities, and properties of ionic liquids were experimentally characterized and theoretically analyzed. The Br?nsted acidity-viscosity-activity correlation for the ionic liquids was studied. Among all these ionic liquids, [C₆ImBS][HSO₄] exhibited the best catalytic performance, which was ascribed to its strong Br?nsted acidity and low viscosity. The catalytic activity of the ionic liquid was near that of concentrated sulfuric acid. The influences of ionic liquid dosage, reaction temperature and molar ratio of methanol to formaldehyde were explored using [C₆ImBS]HSO₄ as the catalyst. Under the optimal conditions of n(methanol):n(formaldehyde):n(ILs) = 2.5:1:0.0258, 60 °C, and 4 h, the conversion of formaldehyde can reach 63.37%. The ionic liquid [C₆ImBS]HSO₄ could be reused.

Full text of DOI:10.1039/c5ra18598j

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Synthesis of methylal from methanol and formaldehyde
catalyzed by Brønsted acid ionic liquids with different alkyl
groups
Jiahan Suna, Hansheng Lia, Haoran Songa, Qin Wua,∗, Yun Zhaoa, Qingze Jiaoa,b
aBeijing Key Laboratory for Chemical Power Source and Green Catalysis, School of
Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing
100081, China
bSchool of Chemical Engineering and Material Science, Beijing Institute of
Technology, Zhuhai, Zhuhai 519085, China
Catalytic reaction of methanol with formaldehyde for preparation of methylal was
investigated in various Brønsted acid ionic liquids with different carbon chain length
of alkyl groups. The structures, acidities, and properties of ionic liquids were
experimentally characterized and theoretically analyzed. The Brønsted
acidityꢀviscosityꢀactivity correlation for the ionic liquids was studied. Among all
these ionic liquids, [C6ImBS][HSO4] exhibited the best catalytic performance, which
was ascribed to its strong Brønsted acidity and low viscosity. The catalytic activity of
ionic liquid was near to that of concentrated sulfuric acid. The influences of ionic
liquid dosage, reaction temperature and molar ratio of methanol to formaldehyde were
explored using [C6ImBS]HSO4 as the catalyst. Under the optimal conditions of
n(methanol) : n(formaldehyde) : n(ILs)= 2.5: 1: 0.0258, 60 °C, and 4h, the conversion
of formaldehyde can reach 63.37 %. The ionic liquid [C6ImBS]HSO4 could be reused.
∗ Corresponding author. Tel.: +86 10 68918979; fax: +86 10 68918979.
E-mail addresses: wuqin_bit@126.com.

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Introduction
Methylal (dimethoxymethane, DMM) is an excellent green solvent and chemical
intermediate because of its extremely low toxicity, good solubility and intersolubility
with water. With a boiling point of 315.3 K under atmospheric pressure, DMM can
also be considered as a convenient antifreeze additive in fuel cells1. It has been widely
used in pharmaceutical, perfume industries and organic synthesis2,3. Methylal has low
vapor pressure, high oxygen content, high cetane number and reducing pollutants,
which can be used as promising green diesel additive. Thus methylal has the potential
scientific values and application prospects for petrochemical industry4,5.
Methylal is usually produced through the reaction of methanol with formaldehyde
in the presence of acid catalyst, such as homogeneous or heterogeneous acid
catalyst6,7. Conventionally, this reaction is carried out using homogeneous acid
catalysts, such as concentrated sulfuric acid or pꢀtoluenesulfonic acid. However,
homogeneous liquid catalysts are corrosive and are not easily recovered leading to the
release of environmentally unfriendly effluents, which inevitably leads to a series of
environmental problems8. On account of the detrimental effects of these catalysts,
great efforts directed toward the development of environmentally friendly catalysts
have been made. Solid acid catalyst has been of great interest due to its environment
friendliness. Solid acid catalyst has easy separation, no corrosion of apparatus and no
pollution, but it exists some shortages, such as low activity, rapid loss of activity and
short catalyst life. Therefore, the development of environmental and efficient catalysts
for synthesis of methylal is extremely urgent.
In recent years, ionic liquid (ILs), including functionalized ones, have attracted
considerable research attention9,10. Ionic liquids are organic salts that may be liquid at
room temperature or at relatively low temperatures (<100 °C), generally known as
room temperature ILs11,12. They are finding increasing use and interest owing to their
unique properties such as negligible vapor pressure, thermal stability over a wide
temperature range, large electrochemical window, wide liquidꢀphase temperature

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range and being also recyclable13ꢀ15. They are also described as “designer solvents”16
since their properties may be appropriately tuned and developed for specific needs by
the proper selection of the anion–cation couples. Many organic reactions involving
alkylation17 ꢀ 20 , esterification21 , 22 , nitration23 , hydrogenation24 , epoxidation25 or
DielsꢀAlder26 have been performed in room temperature ionic liquids with excellent
yields and selectivity.
In this work, Brønsted acid ionic liquids with different carbon chain length of alkyl
groups were synthesized. They were characterized by infrared spectrum (IR), nuclear
magnetic resonance (NMR), electrospray ionization mass spectrum (ESIꢀMS),
differential thermal analysis (TGꢀDTA). Their Brønsted acidity was determined
through UVꢀvis spectroscopy. Their catalytic performances for the synthesis of
methylal from methanol and formaldehyde were investigated in various Brønsted acid
ionic liquids with different carbon chain length of alkyl groups by gas
chromatography. Correlations between the carbon chain length of alkyl groups of
ionic liquids, Brønsted acidic strength, the viscosity and catalytic activity were
discussed. The effects of reaction conditions on the production in the selected ionic
liquids were extensively studied.
Experimental
Materials
Sodium,
formaldehyde,
bromomethane,
bromoethane,
1ꢀbromobutane,
1ꢀbromohexane and 1ꢀbromononane were purchased from Sionpharm Chemical
Reagent Co., Ltd. 1ꢀbromodecane, 1ꢀbromododecane, methylal were obtained from
Alfa Aesa. 1,4ꢀbutane sultone(99 %) were purchased from Wuhan Fengfan Chemical
Co, Ltd. Imidazole was obtained from Linhai Kaile Chemical Factory. Ethanol,
acetone, dichloromethane, ethyl acetate, pꢀtoluenesulfonic acid, toluene, methanol and
concentrated sulfuric acid were received from Beijing Chemical Co., Ltd. All reagents
and raw materials were commercially available and used without any further
purification.

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Methods
IR spectra were run on a PerkinꢀElmer Spectrum, which used KBr windows suitable
for Fourier transform infrared (FTIR) transmittance technology to form a liquid film.
1H NMR spectra were obtained on a varian mercuryꢀplus 400MHz nuclear magnetic
resonance spectrometer. Chemical shifts were reported in parts per million(ppm, δ).
ESIꢀMS spectra were obtained on Vavian 500MS instrument. The thermal
decomposition temperature was determined by PerkinꢀElmer TGA7 TGꢀDTA analysis,
with a heating rate of 10 °C/min from room temperature to 700 °C.
Preparation of Brønsted acid ionic liquids with different alkyl groups
The synthetic procedure of Brønsted acid ionic liquids with different alkyl groups is
illustrated in Scheme 1.
N Na
N
RꢀBr
S
O
R
N
N
O
O
ꢀ
C4H8SO3
R
N
N
N
N
H2SO4
C4H8SO3H
R
HSO4
R:CnH2n+1
n=1、 2、 4、 6、 8、 10、 12、 14、 16
Scheme 1 Synthetic procedure of Brønsted acid ionic liquids with different alkyl groups
（1）Preparation of alkylimidazole with different carbon chain(RIm)
The sodium was slowly added to ethanol. The produced sodium ethoxide solution,
which was colorless transparent liquid, was purified through filtration.

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The imidazole was added to sodium ethoxide solution, and the mixture was stirred
in the oil bath pan for eight hours, resulting in the formationof the sodium imidazole.
The sodium imidazole was purified through filtration.
A stoichiometric amount of RBr was added to the ethanol solution of sodium
imidazole, and the mixture was stirred in the oil bath pan for two days, resulting in the
formation of the alkylimidazole (RIm). The RIm was washed repeatedly with ethanol,
acetone and dichloromethane, leached and distilled accordingly. Finally, the RIm was
purified by column chromatography on silica gel.
（2）Preparation of acidic ionic liquids ([RImBs][HSO4])
A stoichiometric amount of 1,4ꢀbutane sultone was added dropwise to the RIm at 0 °C
and the mixture was stirred at 50 °C for 2ꢀ3 days. The zwitterions BsImR produced,
all of which were white solids, were washed three times with ethyl acetate and ether
to remove unreacted material and dried for 12 hours in a vacuum.
A stoichiometric amount of concentrated sulfuric acid was added dropwise to the
zwitterions at 0 °C, and the mixture was stirred at 50 °C for 1ꢀ2 days, resulting in the
formation of the [RImBs][HSO4] ionic liquids. The ionic liquids were washed
repeatedly with ethyl acetate and ether to remove unreacted material and dried under
vacuum27.
The ionic liquids were fully characterized by FTꢀIR, NMR, ESIꢀMS and TGꢀDTA,
and the spectral data agreed with their structures.
Spectral data for [C1ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3108, 1515, 1234, 1167, 1054,
1
571. H NMR (400 MHz, DMSOꢀd6): δ 1.540(m, 2H), 1.877(m, 2H), 2.496(t, 2H),
3.852(s,3H), 4.183(t, 2H), 7. 707( s, 1H) , 7.772( s, 1H) , 9.137( s, 1H). Positiveꢀion
ꢀ
ESIꢀMS m/z: 317.1 (C1ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9 (HSO4 ). TGꢀDTA
(N2, 10 °C/min): Td 311 °C.
Spectral data for [C2ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3105, 1513, 1216, 1167, 1054,
580. 1H NMR (400 MHz, DMSOꢀd6): δ 1.420(m, 3H), 1.547(m, 2H ,), 1.871 (m, 2H,),
2.510 (t, 2H ), 4.201(m, 4H) , 7.807 (s, 1H), 7.833 (s, 1H), 9.250 (s, 1H). Positiveꢀion
ꢀ
ESIꢀMS m/z: 233.1 (C2ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9 (HSO4 ). TGꢀDTA
(N2, 10 °C/min): Td 301 °C.

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Spectral data for [C4ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3107, 1514, 1221, 1166, 1037,
1
595. H NMR (400 MHz, DMSOꢀd6): δ 0.9160(m, 3H), 1.276 (m, 2H,), 1.536 (m,
2H,), 1.776(m, 2H), 1.886(m, 2H), 2.506 (t, 2H ), 4.187(m, 4H), 7.815 ( s, 2H), 9.237
( s, 1H). Positiveꢀion ESIꢀMS m/z: 261.1 (C4ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9
ꢀ
(HSO4 ). TGꢀDTA (N2, 10 °C/min): Td 288 °C.
Spectral data for [C6ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3107, 1510, 1225, 1165, 1046,
1
591. H NMR (400 MHz, DMSOꢀd6): δ 0.8020(m, 3H), 1.210(m, 6H), 1.510(m,
2H),1.738(m, 2H), 1.848 (m, 2H), 2.529 (t, 2H ), 4.139(m, 4H), 7.775 (s, 2H), 9.228
(s, 1H). Positiveꢀion ESIꢀMS m/z: 289.1 (C6ImBs+), Negativeꢀion ESIꢀMS m/z: 97.0
ꢀ
(HSO4 ). TGꢀDTA (N2, 10 °C/min): Td 274 °C.
Spectral data for [C8ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3105, 1508, 1228, 1167, 1051,
596. 1H NMR (400 MHz, DMSOꢀd6): δ 0.9120(m, 3H), 1.250(m, 10H), 1.521(m, 2H),
1.752(m, 2H), 1. 872 (m, 2H,) , 2.509 (t, 2H), 4.187(m, 4H), 7.812 (s, 2H) , 9.227 (s,
1H). Positiveꢀion ESIꢀMS m/z: 317.1 (C8ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9
ꢀ
(HSO4 ). TGꢀDTA (N2, 10 °C/min): Td 273 °C.
Spectral data for [C10ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3108, 1507, 1223, 1164, 1046,
580. 1H NMR (400 MHz, DMSOꢀd6): δ 0.8610(m, 3H), 1.282(m, 14H), 1.532(m,2H),
1.781(m, 2H), 1.852 (m, 2H), 2.521(t, 2H), 4.334(m, 4H), 7.781(s, 2H) , 9.208(s, 1H).
ꢀ
Positiveꢀion ESIꢀMS m/z: 345.2 (C10ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9 (HSO4 ).
TGꢀDTA (N2, 10 °C/min): Td 280 °C.
Spectral data for [C12ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3100, 1509, 1243, 1164, 1050,
592. 1H NMR (400 MHz, DMSOꢀd6): δ 0.8112(m, 3H), 1.253(m, 10H),
1.521ꢀ1.853(m,10H), 2.532 (t, 4H) ,4.139(m, 6H) , 7.775 ( s, 2H) , 9.205 (s, 1H).
ꢀ
Positiveꢀion ESIꢀMS m/z: 373.2 (C12ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9 (HSO4 ).
TGꢀDTA (N2, 10 °C/min): Td 296 °C.
Spectral data for [C14ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3107, 1509, 1232, 1167, 1057,
596. 1H NMR (400 MHz, DMSOꢀd6): δ 0.8921(m, 3H), 1.117(m, 20H),
1.511ꢀ1.832(m, 6H), 2.532(t, 4H), 4.176(m, 4H), 7.871 (s, 2H), 9.204 (s, 1H).
ꢀ
Positiveꢀion ESIꢀMS m/z: 401.3 (C14ImBs+), Negativeꢀion ESIꢀMS m/z: 97.0 (HSO4 ).
TGꢀDTA (N2, 10 °C/min): Td 265 °C.

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Spectral data for [C16ImBs]HSO4. IR(KBr, ν/cmꢀ1): ν 3113, 1503, 1239, 1168, 1055,
593. 1H NMR (400 MHz, DMSOꢀd6): δ 0.9612(m, 3H), 1.031(m, 22H),
1.511ꢀ1.801(m, 6H), 2.521 (t, 6H), 4.210(m, 4H), 7.775 (s, 2H)，9.119 (s, 1H).
ꢀ
Positiveꢀion ESIꢀMS m/z: 429.3 (C16ImBs+), Negativeꢀion ESIꢀMS m/z: 96.9 (HSO4 ).
TGꢀDTA (N2, 10 °C/min): Td 253 °C.
Brønsted acidity analysis of the ionic liquids
The Brønsted acidic strengths of ionic liquids were determined using UVꢀvis
spectroscopy28. A series of different concentration of ionic liquids solutions were
prepared, using pꢀnitroaniline as an indicator, and then the Hammett function (H0)
was calculated as follows:
H0 = pKa + log([I]/[HI])
Where pKa is the electrolytic constant of pꢀnitroaniline ( pKa =0.99), [I] and
[HI] are percentage of unprotonated and protonated pꢀnitroaniline respectively.
Synthesis of methylal catalyzed by the [RImBs][HSO4] ionic liquids
The reaction of formaldehyde with methanol was carried out in a 50 mL teflonꢀlined
stainlessꢀsteel autoclave, equipped with thermostat and mechanical stirring.
Formaldehyde, methanol, and [RImBs][HSO4] ionic liquid catalyst with different
molar rations were quantitatively introduced into the reactor successively. The
reaction was allowed to proceed for 4h and kept at the desired temperature. After the
reaction completed, the mixture was cooled and kept at 0 °C for 1 h and two phases
were formed. The upper phase consisted of the produced methylal and the unreacted
materials, and the lower phase contained the ionic liquid catalyst. The lower phase
was simply separated from the upper phase by decantation.
The upper solution was analyzed by a Tianmei GCꢀ7980T equipped with a Thermal
Conductivity Detector (TCD). A packed columnꢀ Porapak T (60/80mesh 5m
(L)*3mm (OD)*2mm (ID)) was used. Analytical conditons were as follows: injection

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port temperature, 140 °C；oven temperature, 150 °C; TCD temperature, 140 °C;
carrier gas, hydrogen.
Results and discussion
Brønsted acidity analyses of the ionic liquids
The Brønsted acidic strengths of various ionic liquids are shown in Table 1. As could
be seen from Table 1, with increasing the carbon chain length of ionic liquids, the
Hammett function (H0) are slightly increased at first and then slightly decreased.
[C6ImBs][HSO4] shows the weakest Brønsted acidity. When the length of carbon
chain increases from 4 to 12, the variation of acidic strength was in agreement with
the results of references29ꢀ31. But the Hammett function of these ionic liquids have no
great difference. The Brønsted acidic strength of these ionic liquids with different
carbon chain of alkyl groups changes small. Therefore, for ionic liquids with the same
anion, carbon chain length of alkyl groups has little effect on the Brønsted acidic
strengths.
Table 1 The Brønsted Acidic Strengths of Various Ionic Liquids
Ionic liquids
0
[I](%)
100
[IH+](%)
0
H0
0
A
max（AU）
2.483
C1ꢀIL
C2ꢀIL
C4ꢀIL
C6ꢀIL
C8ꢀIL
C10ꢀIL
C12ꢀIL
2.145
2.218
2.275
2.298
2.194
2.228
2.185
86.40
89.35
91.64
92.58
88.36
89.73
87.98
13.59
10.64
8.351
7.413
11.63
10.27
12.01
1.793
1.913
2.030
2.086
1.870
1.931
1.854

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Concentration: 20mmol/L; indicator: pꢀnitroaniline; Amax: Maximum absorbance of the
indicator in various ionic liquids solutions; [I]: The percentage of unprotonated pꢀnitroaniline;
[IH+]: The percentage of protonated pꢀnitroaniline; H0: The Hammett function.
Reaction of methanol with formaldehyde for preparation of methylal
Effect of ionic liquids with different carbon chain of alkyl groups on the catalytic
reaction
The effect of ionic liquids with different carbon chain of alkyl groups on the catalytic
reaction was studied (see Fig. 1). It is found that the conversion of formaldehyde is 0
and no methylal is obtained when there is no catalyst. The conversions of
formaldehyde are distinctly different for the different carbon chain of ionic liquids.
When the carbon chain length of ionic liquids is less than 6, the conversion of
formaldehyde has a distinct increase with the increasing of carbon chain length.
However, when the carbon chain length of ionic liquids increases from 6 to 16, the
conversion of formaldehyde sharply decreases. The carbon chain length of ionic
liquid has a significant effect on its catalytic activity. Among all the ionic liquids,
[C6ImBS][HSO4] shows the best catalytic performance. The catalytic activity of the
ionic liquids is related to the viscosity of the ionic liquids32. When the viscosity of the
ionic liquids is high, the collision rate between the ionic liquid catalyst and reactants
becomes small. So, the reaction rate is slow. On the contrary ,when the viscosity of
the ionic liquids is low, the reaction rate is fast. On the basis of the study of the
viscosity of the ionic liquids, the carbon chain length of ionic liquids has important
effect on the viscosity of the ionic liquids. When the carbon chain length of ionic
liquids is less than 6, the viscosity of ionic liquid has a slight decrease. However,
when the carbon chain length of ionic liquids increases from 6 to 16, the viscosity of
ionic liquid sharply increases. The viscosity of [C6ImBS][HSO4] ionic liquids is the
lowest among all the ionic liquids. Therefore, the carbon chain length of ionic liquids
have effect on the catalytic activity of the ionic liquids. Among all the ionic liquids,
[C6ImBS][HSO4] shows the best catalytic performance. With increasing the carbon

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chain length of ionic liquids, the variation trend of viscosity is contrary to that of
catalytic activity. In addition, the hydrophobicity of ionic liquids becomes stronger
with the increase of carbon chain length32. The catalytic activity of the ionic liquid
may be related to the hydrophobicity. As the carbon chain length of ionic liquid
increases, the solubility of ionic liquid catalyst in reactants decreases. The collision
rate between the ionic liquid catalyst and reactants becomes small. Therefore, when
the carbon chain length of ionic liquids increases from 6 to 16, the conversion of
formaldehyde sharply decreases. Moreover, when the carbon chain length of ionic
liquids is greater than 6, the effect of steric hindrance on reaction reduces catalytic
activity. Therefore, the carbon chain length of ionic liquids have effect on the
catalytic activity of the ionic liquids. Among all the ionic liquids, [C6ImBS][HSO4]
shows the best catalytic performance and the conversion of formaldehyde is 55.03 %.
To investigate the influence of various factors synthesizing of methylal from
methanol and formaldehyde, [C6ImBS][HSO4] was chosen as the catalyst.
In addition, as could be seen from Fig. 1, the catalytic activity of the ionic liquid
[C6ImBS][HSO4] was near to that of concentrated sulfuric acid and pꢀtoluenesulfonic
acid. Concentrated sulfuric acid is corrosive and as a catalyst, is not easily recovered
and its recovery gives the release of environmentally unfriendly effluents. It
inevitably leads to a series of serious environment problems. On the contrary, ionic
liquids are environmentally friendly catalysts that possess important attributes, such
as negligible vapor pressure, high catalytic activity, excellent chemical and thermal
stability, potential recoverability, design possibilities and ease of separation of the
products from reactants. Therefore, considering the catalytic activity and
environmental benefit, the ionic liquid catalyst is superior to concentrated sulfuric
acid.

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60
50
40
30
20
10
0
no
H2SO4
pꢀTSA C1
C2
C4
C6
C8 C10 C12 C14 C16
catalyst
Fig. 1. Effect of ionic liquids with different carbon chain of alkyl groups on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) : n(ILs) =2.4: 1: 0.0186, 60 °C, 4h.
Effect of catalyst dosage on the catalytic reaction
A series of experiments were conducted using the best catalyst [C6ImBS][HSO4] with
different dosages (Fig. 2). The amount of ionic liquid is denoted by ILs/ formaldehyde
molar ratio. As shown in Fig. 2，when the amount of ionic liquid is less than 0.0258,
with an increase in the relative amount of ionic liquids, the rate of reaction is
enhanced, resulting in a higher conversion of formaldehyde. The amount of ionic
liquids had a significant effect on the catalytic performance. This is attributed to the
increase in acidity in the reaction system. However, when the amount of ionic liquid
is more than 0.0258, the conversion of formaldehyde is basically unchanged, which
indicates that the reaction has probably approached equilibrium. From the point of
view of economy in the industrial applications, the increase in catalyst dosage would
undoubtedly increase operation cost. Thus, considering the reaction rate and the cost
of ionic liquids, the optimum catalyst dosage is 0.0258.

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60
50
40
30
20
10
0
0.00
0.01
0.02
0.03
0.04
0.05
n[C6ImBS][HSO4] /nformaldehyde(mol/mol)
Fig. 2. Effect of catalyst dosage on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) =2:1, 60 °C, 4h.
Effect of Reaction Temperature on the Catalytic Reaction
In general, the reaction temperature has an important effect on the kinetics and
thermodynamics of the reaction. Therefore, the effect of reaction temperature on the
synthesis of methylal was investigated in detail (see Fig. 3). With the increasing of
reaction temperature, the conversion of formaldehyde increases at first and then
slightly decreases, which is up to maximum value when reaction temperature is 60 °C.
When reaction temperature is high, the collision rate between the ionic liquid catalyst
and reactants becomes fast. So, the reaction rate is fast and the conversion of
formaldehyde increases. In addition, when reaction temperature is high, the viscosity
of the ionic liquid becomes low. So, the collision rate between the ionic liquid catalyst
and reactants becomes fast. The reaction rate is fast and the conversion of
formaldehyde increases. However, the reaction is a slight exothermic reaction and the
increase of temperature will reduce the reaction performance. Therefore, when
reaction temperature is more than 60 °C, the conversion of formaldehyde slightly
decreases. For this reason, the preferred reaction temperature is 60 °C.

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80
60
40
20
0
20
40
60
80
100
Reaction temperature (℃)
Fig. 3. Effect of the reaction temperature on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) : n(ILs)=2:1: 0.0258, 4h.
Effect of the molar ratio of methanol to formaldehyde on the catalytic reaction
A series of experiments were conducted using the best catalyst [C6ImBS][HSO4]. Fig.
4 shows the effect of the molar ratio of methanol to formaldehyde on the catalytic
reaction. As the molar ratio of methanol to formaldehyde increases, the conversion of
formaldehyde was increased. When the mole ratio of methanol to formaldehyde is
less than 2.5, the conversion of formaldehyde has a obvious increase with the
increasing of methanol. However, when the mole ratio of methanol to formaldehyde is
more than 2.5, the conversion of formaldehyde has a slight increase. Based on the
chemical equilibrium theory, increasing the amount of methanol is beneficial to the
forward reaction, resulting in a higher rate of reaction. However, if this ratio was
further increased, the concentration of catalyst will be greatly diluted, resulting in a
slight increase in the rate of reaction. Therefore, when the molar ratio of methanol to
formaldehyde was more than 2.5, the amount of methanol had a slight effect on the
catalytic performance. Moreover, a higher molar ratio of methanol to formaldehyde
will cause a larger amount of methanol to be separated from the ionic liquid during

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recycling. For this reason, the optimal molar ratio of methanol to formaldehyde 2.5 is
preferable. The conversion of formaldehyde was obtained as high as 63.37 %.
80
60
40
20
0
1.75
2.00
2.25
2.50
2.75
3.00
3.25
nmethanol /n/formaldehyde( mol/mol)
Fig. 4. Effect of methanol/formaldehyde molar ration on the catalytic reaction
Reaction conditions: n(ILs): n(formaldehyde) = 0.0258, 60 °C, 4h.
The reuse of ionic liquid
In order to investigate the potential reusability of the ionic liquids in the synthesis of
methylal, a series of recycle experiments were conducted with a methanol to
formaldehyde molar ratio of 2.5 and ionic liquid to formaldehyde molar ratio of
0.0258 at 60°C for 4h. After each recycle reaction, the ionic liquid was simply
separated from the reactants by distillation and was dried in vacuum. As shown in Fig.
5, the [C6ImBS][HSO4] was used five times and its catalytic activity changed very
little. Therefore, the [C6ImBS][HSO4] as the catalyst for the synthesis of methylal is
recyclable and stable.
In addition, the catalytic activity of the ionic liquid [C6ImBS][HSO4] was near to
that of concentrated sulfuric acid and pꢀtoluenesulfonic acid from Fig. 1.
Concentrated sulfuric acid is corrosive and as a catalyst, is not easily recovered and its
recovery gives the release of environmentally unfriendly effluents. It inevitably leads
to a series of serious environment problems. Although the cost of [C6ImBS][HSO4] is

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about 30 times the cost of H2SO4, ionic liquids are environmentally friendly catalysts
that possess important attributes, such as negligible vapor pressure, high catalytic
activity, excellent chemical and thermal stability, potential recoverability and ease of
separation of the products from reactants. Therefore, considering the catalytic activity
and environmental benefit, the ionic liquid catalyst is superior to concentrated sulfuric
acid.
100
80
60
40
20
0
0
1
2
3
4
5
Run number
Fig. 5. The reuse of ionic liquid
Conclusions
Several ionic liquids with different carbon chain length of alkyl groups were
synthesized successfully and were characterized by IR, NMR, ESIꢀMS, TGꢀDTA,
UVꢀvis spectroscopy and capillary viscometer. Catalytic reaction of methanol with
formaldehyde for preparation of methylal was investigated in various ionic liquids
with different carbon chain length of alkyl groups. It was found that their structures
were consistent with the designed structures. These acidic ionic liquids showed
excellent catalytic performance for synthesis of methylal. The catalytic activity of
each ILs is dependent on the carbon chain length of alkyl groups. The conversion of
formaldehyde increased at first and then decreased with lengthening carbon chain of
ionic liquids. The ionic liquid [C6ImBs][HSO4] showed the best catalytic performance.

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From various parametric studies, such as catalyst dosage, reaction temperature and the
methanol/ formaldehyde mole ratio, an optimum reaction condition was established.
The Brønsted acidic ionic liquids had a potential application in the production of
methylal.
Acknowledgments
This work is supported by the National Nature Science Foundation of China
(20706006), the Fundamental Research Foundation of Beijing Institute of Technology
(20121042007) and the Foundation of Beijing Key Laboratory for Chemical Power
Source and Green Catalysis (2013CX02031, 2014CX02026).
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Captions
Scheme 1 Synthetic procedure of Brønsted acid ionic liquids with different alkyl
groups
Fig. 1. Effect of ionic liquids with different carbon chain of alkyl groups on the
catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) : n(ILs) =2.4: 1: 0.0186,
60 °C, 4h.
Fig. 2. Effect of catalyst dosage on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) =2:1, 60 °C, 4h.
Fig. 3. Effect of the reaction temperature on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) : n(ILs)=2:1: 0.0258, 4h.
Fig. 4. Effect of methanol/formaldehyde molar ration on the catalytic reaction
Reaction conditions: n(ILs): n(formaldehyde) = 0.0258, 60 °C, 4h.
Fig. 5. The reuse of ionic liquid

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N Na
N
R-Br
S
R
N
N
O
O
O
-
C4H8SO3
R
N
N
N
N
H2SO4
C4H8SO3H
HSO4
R
R:CnH2n+1
n=1、 2、 4、 6、 8、 10、 12、 14、 16
Scheme 1 Synthetic procedure of Brønsted acid ionic liquids with different alkyl groups

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60
50
40
30
20
10
0
no
H2SO4
p-TSA C1
C2
C4
C6
C8 C10 C12 C14 C16
catalyst
Fig. 1. Effect of ionic liquids with different carbon chain of alkyl groups on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) : n(ILs) =2.4: 1: 0.0186, 60 °C, 4h.

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60
50
40
30
20
10
0
0.00
0.01
0.02
0.03
0.04
0.05
n[C6ImBS][HSO4] /nformaldehyde(mol/mol)
Fig. 2. Effect of catalyst dosage on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) =2:1, 60 °C, 4h.

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80
60
40
20
0
20
40
60
80
100
Reaction temperature (℃)
Fig. 3. Effect of the reaction temperature on the catalytic reaction
Reaction conditions: n(methanol) : n(formaldehyde) : n(ILs)=2:1: 0.0258, 4h.

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80
60
40
20
0
1.75
2.00
2.25
2.50
2.75
3.00
3.25
nmethanol /n/formaldehyde( mol/mol)
Fig. 4. Effect of methanol/formaldehyde molar ration on the catalytic reaction
Reaction conditions: n(ILs): n(formaldehyde) = 0.0258, 60 °C, 4h.

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100
80
60
40
20
0
0
1
2
3
4
5
Run number
Fig. 5. The reuse of ionic liquid