Br?nsted-acidic ionic liquids as efficient catalysts for the synthesis of polyoxymethylene dialkyl ethers

Article abstract of DOI:10.1016/S1872-2067(17)62816-X

Acetalation of formaldehyde (HCHO) with dialkyl formal or aliphatic alcohol to prepare polyoxymethylene dialkyl ethers (RO(CH₂O)_nR, n ≥ 1) catalyzed by Br?nsted-acidic ionic liquids has been developed. The correlation between the structure and acidity activity of various ionic liquids was studied. Among the ionic liquids investigated, 1-(4-sulfonic acid)butyl-3-methylimidazolium hydrogen sulfate ([MIMBs]HSO₄) exhibited the best catalytic performance in the reaction of diethoxymethane (DEM₁) with trioxane. The influences of ionic liquid loading, molar ratio of DEM₁ to HCHO, reaction temperature, pressure, time, and reactant source on the catalytic reaction were explored using [MIMBs]HSO₄ as the catalyst. Under the optimal conditions of n([MIMBs]HSO₄):n(DEM₁):n(HCHO) = 1:80:80, 140 °C, and 4 h, the conversion of HCHO and selectivity for DEM_2–8 were 92.6% and 95.1%, respectively. The [MIMBs]HSO₄ catalyst could be easily separated and reused. A feasible mechanism for the catalytic performance of [MIMBs]HSO₄ was proposed.

Full text of DOI:10.1016/S1872-2067(17)62816-X

Chinese Journal of Catalysis 38 (2017) 853–861
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Brønsted‐acidic ionic liquids as efficient catalysts for the synthesis
of polyoxymethylene dialkyl ethers
Heyuan Song a,b, Meirong Kang a, Fuxiang Jin a, Guoqin Wang a,b, Zhen Li a, Jing Chen a,*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
Gansu, China
b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Acetalation of formaldehyde (HCHO) with dialkyl formal or aliphatic alcohol to prepare polyox‐
ymethylene dialkyl ethers (RO(CH2O)nR, n ≥ 1) catalyzed by Brønsted‐acidic ionic liquids has been
developed. The correlation between the structure and acidity activity of various ionic liquids was
studied. Among the ionic liquids investigated, 1‐(4‐sulfonic acid)butyl‐3‐methylimidazolium hy‐
drogen sulfate ([MIMBs]HSO4) exhibited the best catalytic performance in the reaction of diethox‐
ymethane (DEM1) with trioxane. The influences of ionic liquid loading, molar ratio of DEM1 to
HCHO, reaction temperature, pressure, time, and reactant source on the catalytic reaction were
explored using [MIMBs]HSO4 as the catalyst. Under the optimal conditions of
n([MIMBs]HSO4):n(DEM1):n(HCHO) = 1:80:80, 140 °C, and 4 h, the conversion of HCHO and selec‐
tivity for DEM2−8 were 92.6% and 95.1%, respectively. The [MIMBs]HSO4 catalyst could be easily
separated and reused. A feasible mechanism for the catalytic performance of [MIMBs]HSO4 was
proposed.
Received 10 February 2017
Accepted 17 March 2017
Published 5 May 2017
Keywords:
Acetalation reaction
Polyoxymethylene dialkyl ether
Diethoxymethane
Formaldehyde
Brønsted‐acidic ionic liquid
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
[3]. RO(CH2O)nR also can display excellent solubilizing power,
permeating ability, and miscibility with most organic com‐
Polyoxymethylene dialkyl ethers (RO(CH2O)nR, n ≥ 1) are
polyether compounds with high oxygen contents and cetane
numbers (CN), and are regarded as a promising diesel additive
to improve the combustibility of diesel oil, enhance combustion
efficiency, and lower the release of pollutants [1,2]. It has been
reported that the emission of particulate matter and NOx re‐
leased upon combustion were lowered by 80%–90% and 50%,
respectively, when polyoxymethylene dimethyl ethers
(CH3O(CH2O)nCH3, n = 3–8, PODE3−8, DMM3−8) were added to
diesel oil with a ratio of 20% [2]. The CN, calorific value, and
flashing point gradually increase and density and condensation
point decrease with increasing R chain length of RO(CH2O)nR
pounds. Furthermore, they are very promising for use as green
industrial solvents and pigment dispersants.
RO(CH2O)nR can be synthesized from the end–group (–R)
provider (e.g., aliphatic alcohol or dialkyl formal) and a com‐
pound that offers an oxymethylene group (–CH2O–) as a chain
segment, e.g., 1,3,5‐trioxane (TOX), paraformaldehyde (PF), or
formaldehyde (FA) [4]. In 1948, Gresham et al. [5] reported the
first acetalation reaction of dialkyl formal with PF using a pro‐
ton acid such as sulfuric acid as the catalyst, which mainly pro‐
duced RO(CH2O)nR with n = 2 or 3. Subsequently, there have
been many reports on the synthesis of PODEn. For example,
Renata et al. [6–9] patented the acetalation reaction of PF or
* Corresponding author. Tel: +86‐931‐4968068; Fax: +86‐931‐4968129; E‐mail: chenj@licp.cas.cn
This work was supported by the National Natural Science Foundation of China (21473225).
DOI: 10.1016/S1872‐2067(17)62816‐X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 5, May 2017

854
Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
TOX with methanol or methylal catalyzed by sulfuric or triflic
acid. Heavy corrosion, difficult separation and recovery, and
disposal of the spent catalyst are the disadvantages associated
with the reported methods involving proton acid catalyst sys‐
tems. To solve these problems, more environmentally friendly
and easily separable solid acids, such as ion exchange resin
Amberlyst 36 [10], cation resin NKC‐9 [11], and molecular
sieves [12–14] have been used as catalysts in PODEn formation.
Unfortunately, the reported catalytic systems still suffer from
one or more disadvantages, including poor reactivity (low cat‐
alytic activity and selectivity) and easy deactivation.
Recently, ionic liquids (ILs) have attracted great interest as a
novel catalyst because of their favorable properties. ILs com‐
bine the advantageous characteristics of both homogenous and
heterogeneous catalysts, such as high acidic or alkali density,
wide liquid range, uniform catalytic active sites, easy separa‐
tion, and reusability [15]. As a result, ILs have been widely used
in catalytic processes such as material synthesis, organic reac‐
tions, and biomass conversion [16–22]. Our group developed a
method to synthesize PODE3−8 using acidic ILs as catalysts with
TOX, PF, or FA and methanol or methylal as raw materials
[23–28]. Recently, acid ILs [PyBs]HSO4 and [MIMBs]HSO4 have
been reported as catalysts for acetalation of TOX and methylal
[29]. The conversion of TOX and selectivity of PODE3−8 were
91.2% and 70.9%, respectively, under the conditions of
n(ILs):n(methylal):n(TOX) = 1:180:60, 170 °C, and 10 h. Here,
we investigate the potential applications of Brønsted‐acidic IL
in the acetalation reaction of FA and diethoxymethane
(C2H5OCH2OC2H5, DEM1) or aliphatic alcohol (CnH2n+1OH, n ≥ 2)
(Scheme 1). Ideal yields of RO(CH2O)nR (n = 2–8) are obtained
in the presence of –SO3H functionalized ILs and a possible reac‐
tion mechanism for this reaction system is proposed based on
the experimental results. The recyclability of the catalyst sys‐
tem is also examined.
O
BAILs
O
O
O
O
+ (n-1)
R
R
R
R
n
H
H
O
BAILs
O
R
O
O
+ n
H
+ H O
R
2
2
R
n
H
H
R = C2H5, C3H7, C4H9
BAILs:
SO3H
N
N
m
X-
-
m = 4, X = HSO4
[MIMBs]HSO4
[MIMBs]p-TSA
[MIMBs]CH3SO3
[MIMPs]HSO4
-
m = 4, X = p-CH3C6H4SO3
-
m = 4, X = CH3SO3
m = 3, X = HSO4
-
[PyBs]HSO4
N
SO3H
-
-
HSO4
P
SO3H
[TTPBs]HSO4
[TENBs]HSO4
HSO4
3
SO3H
N
-
HSO4
[MIMAc]HSO4
[MIMB]HSO4
COOH
N
N
N
N
-
HSO4
-
HSO4
Scheme 1. Acetalation reaction of FA with dialkyl formal or aliphatic
alcohol catalyzed by Brønsted‐acidic ionic liquids.
then the mixture was heated at 60 °C for 12 h. The formed solid
zwitterion was centrifuged and washed three times with tolu‐
ene to remove unreacted non‐ionic residues. After drying un‐
der vacuum (70 °C, 5.3 kPa, 12 h), white solid zwitterionic
samples were obtained. A stoichiometric amount of concen‐
trated sulfuric acid or methanesulfonic acid was then added
dropwise to each zwitterionic sample in anhydrous toluene,
and stirred at 80 °C for 12 h to form the IL. The IL phase was
then washed repeatedly with toluene and dried under vacuum
(70 °C, 5.3 kPa, 12 h) to give each viscous clear IL. When using
p‐toluenesulfonic acid (p‐TSA) as the anion source, the prepa‐
ration of ILs was carried out in water and the system was
heated under reflux for 12 h. The mixture was dried under
vacuum to form corresponding ILs.
2. Experimental
2.1. General
All chemicals were analytical grade and used without fur‐
ther purification. Both 1H and 13C NMR spectra were recorded
on a Bruker AVIII HD 400 MHz NMR spectrometer (Switzer‐
land) using tetramethylsilane as an internal standard. Fourier
transform infrared (FT‐IR) measurements were performed
using KBr tablets on a Nicolet NEXUS 870 FT‐IR infrared spec‐
trometer (Madison, America). Absorbance spectra of
4‐nitroaniline were obtained with a PerkinElmer Lambda 35
UV/VIS spectrometer (America).
NMR spectral data in CD3OD or D2O and FT‐IR spectral data
for ILs are presented as follows. Chemical shifts are reported in
parts per million (ppm, δ) and referenced to D2O (δ = 4.73) or
CD3OD (δ = 3.31).
2.2. Ionic liquid preparation
[MIMBs]HSO4. 1H NMR (400 MHz, D2O): δ 1.66 (h, J = 8.0 Hz,
2H), 1.94 (h, J = 7.0 Hz, 2H), 2.86 (t, J = 8.0 Hz, 2H), 3.81 (s, 3H),
4.16 (t, J = 8.0 Hz, 2H), 7.35 (d, J = 4.0 Hz, 1H), 7.40 (d, J = 4.0 Hz,
1H), 8.64 (s, 1H). 13C NMR (100 MHz, D2O): δ 21.0, 28.14, 35.75,
49.0, 50.14, 122.25, 123.75, 136.09. IR (KBr, ν/cm−1): 3156
(C–H stretching vibration, MIM), 2963 (C–H stretching vibra‐
ILs were prepared according to a previous report [30] and
their structures are shown in Scheme 1. A stoichiometric
amount of 1,4‐butane sultone, 1,3‐propane sultone, bromobu‐
tane, or a solution of chloroacetic acid in chloroform was added
dropwise to a stirred solution of N‐methylimidazole, pyridine,
or triphenylphosphine in toluene at room temperature, and

Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
855
tion, CH2), 1572 (C=N stretching vibration, MIM), 1461 (C–H
bending vibration, CH3–N), 1229 (SO2 stretching vibration,
–SO3H), 1168, 1054 (S=O stretching vibration, –SO3H), 746
(C–C rocking vibration, (CH2)n, n ≥ 4), 584 (–SO3H, absorption
peak).
(s, 2H), 7.38 (d, J = 8.0 Hz, 2H), 8.69 (s, 1H). 13C NMR (100 MHz,
D2O): δ 35.94, 49.85, 123.52, 137.40, 169.93. IR (KBr, ν/cm−1):
3119 (C–H, MIM), 2965 (C–H, CH2), 2543–2620 (O–H stretching
vibration, –COOH), 1742 (C=O, stretching vibration, –COOH),
1579 (C=N, MIM), 1413 (C–H, CH3–N), 1173 (CO2, stretching
vibration, –COOH).
1
[MIMBs]p‐TSA. H NMR (400 MHz, D2O): δ 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): δ 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. IR (KBr, ν/cm−1): 3154 (C–H, MIM),
2958 (C–H, CH2), 1570 (C=N, MIM), 1459 (C–H, CH3–N), 1229
(SO2, –SO3H), 1168, 1033 (S=O, –SO3H), 760 (C–C, (CH2)n, n ≥ 4),
616 (–SO3H).
[MIMB]HSO4. 1H NMR (400 MHz, D2O): δ 0.79 (t, J = 8.0 Hz,
3H), 1.18 (m, J = 4.8 Hz, 2H), 1.72 (h, J = 8.0 Hz, 2H), 3.76 (s,
3H), 4.07 (t, J = 8.0 Hz, 2H), 7.30 (s, 1H), 7.55 (s, 1H), 8.57 (s,
1H). 13C NMR (100 MHz, D2O): δ 12.64, 18.75, 31.26, 35.67,
49.30, 122.25, 123.52, 135.83. IR (KBr, ν/cm−1): 3152 (C–H,
MIM), 2963 (C–H, CH2), 1571 (C=N, MIM), 1464 (C–H, CH3–N),
753 (C–C, (CH2)n, n ≥ 4).
2.3. Typical acetalation procedure
[MIMBs]CH3SO3. 1H NMR (400 MHz, D2O): δ 1.60 (h, J = 8.0
Hz, 2H), 1.88 (h, J = 7.0 Hz, 2H), 2.66 (s, 3H), 2.80 (t, J = 8.0 Hz,
2H), 3.75 (s, 3H), 4.11 (t, J = 6.0 Hz, 2H), 7.30 (s, 1H), 7.36 (s,
1H), 8.60 (s, 1H). 13C NMR (100 MHz, D2O): δ 20.96, 28.11,
35.70, 38.50, 48.95, 50.10, 122.21, 123.70, 135.97. IR (KBr,
ν/cm−1): 3154 (C–H, MIM), 2958 (C–H, CH2), 1573 (C=N, MIM),
1461 (C–H, CH3–N), 1194 (SO2, –SO3H), 1172, 1052 (S=O,
–SO3H), 651 (C–C, (CH2)n, n ≥ 4), 557 (–SO3H).
In a typical experiment, DEM1 (0.6 mol), TOX (0.2 mol), and
IL (7.5 mmol) were mixed together in a 120‐mL Teflon‐lined
stainless‐steel autoclave equipped with a thermometer and
mechanical stirrer, then the autoclave was sealed up and
flushed three times with N2. N2 was introduced with initial
pressure of 1.0–1.2 MPa at room temperature and the reaction
was performed at 140 °C/2.0 MPa for the specified time. After
the reaction, the autoclave was cooled to room temperature
and the IL was separated from the reaction mixture through
layering. 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 fu‐
ranidine was added as an internal standard to the product
mixture before GC analysis. To recycle the catalyst, two meth‐
ods were used to separate the IL from the reaction system. The
first was decantation, which recovered the IL so that it could be
reused directly in subsequent reactions. The second involved
extraction of the products with benzene and then recovery of
the IL by rotary evaporation to remove the extractant before
reuse in the next run.
[MIMPs]HSO4. 1H NMR (400 MHz, D2O): δ 2.17 (h, J = 8.0 Hz,
2H), 2.78 (t, J = 8.0 Hz, 2H), 3.75 (s, 3H), 4.22 (t, J = 6.0 Hz, 2H),
7.31 (d, J = 3.6 Hz, 1H), 7.38 (d, J = 3.6 Hz, 1H), 8.60 (s, 1H). 13
C
NMR (100 MHz, D2O): δ 25.08, 35.74, 47.26, 47.77, 122.20,
123.81, 136.15. IR (KBr, ν/cm−1): 3158 (C–H, MIM), 2965 (C–H,
CH2), 1572 (C=N, MIM), 1461 (C–H, CH3–N), 1131 (SO2, –SO3H),
1170, 1028 (S=O, –SO3H), 749 (C–C rocking vibration, (CH2)n, n
= 3), 579 (–SO3H).
1
[PyBs]HSO4. H NMR (400 MHz, D2O): δ 1.68 (h, J = 8.0 Hz,
2H), 2.06 (h, J = 7.0 Hz, 2H), 2.84 (t, J = 8.0 Hz, 2H), 4.54 (t, J =
8.0 Hz, 2H), 7.96 (t, J = 6.0 Hz, 2H), 8.43 (t, J = 8.0 Hz, 1H), 8.73
(d, J = 8.0 Hz, 2H). 13C NMR (100 MHz, D2O): δ 20.90, 29.31,
50.0, 61.23, 128.36, 144.25, 145.74. IR (KBr, ν/cm−1): 3070
(C–H stretching vibration, Py), 2948 (C–H, CH2), 1636 (C=N
stretching vibration, Py), 1490 (C=C stretching vibration, Py),
1232 (SO2, –SO3H), 1171, 1033 (S=O, –SO3H), 729 (C–C, (CH2)n,
n ≥ 4), 577 (–SO3H).
3. Results and discussion
[TENBs]HSO4. 1H NMR (400 MHz, D2O): δ 1.12 (t, J = 8.0 Hz,
9H), 1.68 (h, J = 8.0 Hz, 4H), 2.82 (t, J = 6.0 Hz, 2H), 3.06 (t, J =
6.0 Hz, 2H), 3.13 (q, J = 4.0 Hz, 6H). 13C NMR (100 MHz, D2O): δ
6.66, 19.97, 21.25, 50.06, 52.66, 56.03. IR (KBr, ν/cm−1): 2994
(C–H stretching vibration, R), 1488, 1397 (C–H bending vibra‐
tion, R), 1232 (SO2, –SO3H), 1163, 1034 (S=O, –SO3H), 730 (C–C,
(CH2)n, n ≥ 4), 578 (–SO3H).
[TTPBs]HSO4. 1H NMR (400 MHz, CD3OD): δ 1.87 (h, J = 8.0
Hz, 2H), 1.98 (h, J = 7.0 Hz, 2H), 2.86 (t, J = 6.0 Hz, 2H), 3.44 (t, J
= 8.0 Hz, 2H), 7.72–7.77 (m, J = 4.0 Hz, 9H), 7.80 (t, J = 6.0 Hz,
3H), 7.86 (t, J = 6.0 Hz, 3H), 13C NMR (100 MHz, CD3OD): δ
19.89, 25.43, 47.18, 47.39, 47.60, 47.82, 48.03, 130.0, 133.50,
134.87. IR (KBr, ν/cm−1): 3061 (C–H stretching vibration, Ph),
2927 (C–H, R), 1568, 1484, 1438 (benzene skeleton vibration),
1241 (SO2, –SO3H), 1114, 1019 (S=O, –SO3H), 725 (C–C, (CH2)n,
n ≥ 4), 605 (–SO3H).
3.1. Acidity analysis of –SO3H functionalized ILs
To investigate the relationship between the acidity of ILs
and anions, the acid strengths of –SO3H functionalized ILs were
determined by UV/VIS spectroscopy using 4‐nitroaniline as a
basic indicator according to a previous report [31]. The Ham‐
mett function (H0), which could be regarded as the relative
acidity of the IL, was calculated using the equation H0 = pK(A)aq
+ lg([A]s/[AH+]s).
Using the same concentrations of 4‐nitroaniline (2.5 mg/L,
pKa = 0.99) and IL (10 mmol/L) in ethanol, the H0 values of the
ILs were determined. The absorption maximum of the unpro‐
tonated form of 4‐nitroaniline was observed at 372.8 nm in
ethanol. When an IL was added, the absorbance of the unpro‐
tonated form of 4‐nitroaniline decreased. As shown in Fig. 1,
the absorbance of the unprotonated form of 4‐nitroaniline in
the presence of the three ILs decreased as follows:
[MIMAc]HSO4. 1H NMR (400 MHz, D2O): δ 3.83 (s, 3H), 5.03

856
Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Table 1
A
Calculation and comparison of H0 values of different –SO3H functional‐
ized ILs in ethanol.
A+[MIMBs]HSO
A+[MIMBs]p-TSA
A+[MIMBs]CH3SO3
4
Ionic liquid
A a
[MIMBs]HSO4
[MIMBs]p‐TSA
[MIMBs]CH3SO3
a 4‐Nitroaniline.
[A] (%)
100
89.7
92.1
98.1
[AH+] (%)
H0
—
1.93
2.06
2.70
0
10.3
7.9
1.9
efficiency (Table 2, entry 1 vs. entry 2) and [MIMBs]HSO4 func‐
tionalized with –SO3H appeared to be the best catalyst with
87.4% conversion of FA and 95.0% selectivity for DEM2–8 (Ta‐
ble 2, entry 1). The effect of ILs with different anions on the
catalytic reaction was also studied. The catalytic performance
of ILs functionalized with –SO3H and with HSO4 as the anion
appeared to be superior to that of the ILs with p‐(CH3)C6H4SO3
or CH3SO3 as the anion in this reaction when they have the
same cation (Table 2, entry 1 vs. entries 8 and 9). The catalysts
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. Absorbance spectra of 4‐nitroaniline (A) for various –SO3H func‐
tionalized ILs in ethanol.
−
−
−
[MIMBs]HSO4 > [MIMBs]p‐TSA > [MIMBs]CH3SO3. After the
calculation, we obtained the acidity order of the three ILs with
the following H0 values (Table 1): [MIMBs]HSO4 (1.93) >
[MIMBs]p‐TSA (2.06) > [MIMBs]CH3SO3 (2.70), which is con‐
sistent with the acidity sequence of the anion precursors.
displayed activities with the order of [MIMBs]HSO4
>
[MIMBs]p‐TSA > [MIMBs]CH3SO3, which is consistent with the
acidity sequence determined using H0 (Table 1). These results
suggest that the acidity of the ILs influenced its catalytic activity
for acetalation reactions.
3.2. Effect of ionic liquid type on acetalation of DEM1 and TOX
According to previous reports [11,12,32,33] and our results,
a plausible mechanism for the formation of DEMn is presented
in Scheme 2. TOX is first converted to FA via hydrogen transfer
along hydrogen bonds between the IL cation and TOX. The FA
monomer reacts with DEM1 via a carbocation mechanism:
First, a probe reaction of DEM1 and TOX was carried out at
140 °C and 2.0 MPa for 2 h catalyzed by these ILs (IL:HCHO =
1:80 molar ratio, a TOX molecule is considered as three HCHO
ones) and the results are shown in Table 2. The main products
were polyoxymethylene diethyl ethers (C2H5O(CH2O)nC2H5,
DEMn, n = 2–8); byproducts of ethanol, ethyl formate, and ether
were also detected in the reaction mixture. The catalytic activi‐
ties of the ILs were affected by their structures. ILs bearing an
alkyl sulfonic acid group on the cations exhibited higher activi‐
ty than the one without this group even though it had the same
+
DEM1 is first protonated to generate the carbocation EtOCH2 .
Adding FA molecules one by one to EtOCH2+ will cause the car‐
bocation chain length to increase, and end‐capping of the
formed carbocations with ethanol will result in the generation
of DEMn with different chain lengths. Based on the analysis of
the products, ethanol and FA were detected in the reaction
mixture and the contents of DEMn had the sequence of n = 2 > n
= 3 > n = 4 > n = 5 > n = 6 > n = 7 > n = 8, which verifies that the
reaction mechanism is reliable. The reaction rates for the ac‐
id‐catalyzed decomposition of TOX correlated exactly with the
Hammett acidity of the catalysts [34]. In addition, the insertion
of FA into DEMn−1 to form DEMn via a carbocation mechanism is
−
anion (HSO4 ) (Table 2, entries 1–5 vs. entries 6 and 7). Good
conversion of FA (containing TOX and FA, > 83%) and selectiv‐
ity for DEM2–8 (> 94%) were obtained at 140 °C and 2.0 MPa for
2 h, and the selectivity for DEMn decreased with increasing
polymerization degree. The carbon chain length between the
–SO3H group and cation core had a slight effect on catalytic
Table 2
Effect of ILs with different structures on the reaction.
Sele. for Con. of
Product distribution (%)
Entry
Catalyst
DEM2–8
(%)
FA
(%)
DEM1 DEM2 DEM3 DEM4 DEM5 DEM6 DEM7 DEM8 EtOH DE a
EF b TOX FA
44.1
44.5
46.3
44.0
45.4
45.7
63.2
51.9
24.9
25.3
24.8
24.6
24.8
23.9
15.1
22.7
17.0
14.2
14.0
13.0
13.6
13.2
12.6
3.5
6.8
6.7
6.0
6.8
6.4
6.2
0.7
3.8
1.3
3.0
2.8
2.4
3.2
2.8
2.8
0.1
1.3
0.4
1.2
1.0
0.9
1.4
1.1
1.2
0
0.4
0.4
0.3
0.5
0.4
0.4
0
0.1
0.08
0.1
0.2
0.1
2.0
1.5
1.9
2.0
1.8
1.6
1.7
1.5
0.4
0.10
0.04
0.04
0.04
0.03
0.06
0.03
0.06
0.06
0.2
0.2
0.2
0.1
0.2
0.06
0.3
0.05
0.2
2.5 0.3
2.1 0.9
2.9 0.7
2.9 0.5
3.4 0.4
4.1 1.2
14.0 1.3
7.2 0.7
14.6 0.4
1
2
3
4
5
6
7
8
9
[MIMBs]HSO4
[MIMPs]HSO4
[PyBs]HSO4
[TPPBs]HSO4
[TENBs]HSO4
[MIMAc]HSO4
[MIMB]HSO4
[MIMBs]p‐TSA
95.0
95.4
94.2
95.0
95.3
95.9
89.0
94.3
95.4
87.4
86.3
83.7
84.9
83.0
76.5
31.8
64.9
33.1
0.2
0
0.04
0.03
9.8
4.7
0.4
0.2
0.1
0.05
[MIMBs]CH3SO3 60.0
Reaction conditions: ILs:HCHO(TOX):DEM1 = 1:80:80 molar ratio, 140 °C, 2.0 MPa, 2 h.
a Ether. b Ethyl formate.

Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
857
H+
O
45
40
H C
O
3
2
[MIMBs]HSO4:HCHO = 1:60
[MIMBs]HSO4:HCHO = 1:80
[MIMBs]HSO4:HCHO = 1:100
[MIMBs]HSO4:HCHO = 1:120
O
O
H+
- EtOH
HCHO
HCHO
35
... ...
EtOCH2
EtOCH2OCH2
EtO(CH2O)n-1CH2
EtOCH2OEt
30
25
20
15
10
5
- H+
H+
- H+
EtOH
H+
Et = -CH2CH3
EtOH
- EtOH
- EtOH
EtOCH2OCH2OEt
EtO(CH2O)nEt
Scheme 2. The formation process of DEMn from DEM1 and TOX.
a typical acid‐catalyzed reaction. Therefore, –SO3H functional‐
ized ILs with strong acidity may be suitable catalysts for the
target reaction.
0
1
2
3
4
5
6
7
8
n value of DEMn (n = 18)
3.3. Effect of reaction conditions on acetalation of DEM1 and
TOX
Fig. 2. Contents of DEMn dependence of [MIMBs]HSO4:HCHO molar
ratio. Reaction conditions: HCHO(TOX):DEM1 = 1:1 molar ratio, 140 °C,
2.0 MPa, 2 h.
To optimize the reaction conditions of the acetalation of
DEM1 and TOX, the effects of catalyst loading, molar ratio of
DEM1 to HCHO (a TOX molecule is considered as three HCHO
ones), reaction temperature, reaction pressure, and reaction
time were also investigated using [MIMBs]HSO4 as the catalyst;
the results are summarized in Table 3.
The conversion of FA and the selectivity of DEM2−8 in‐
creased with catalyst mass (Table 3, entries 1–3), and the high‐
est conversion of FA (87.4%) and selectivity for DEM2–8
(95.0%) were obtained when the molar ratio of [MIMBs]HSO4
to HCHO was 1:80 (Table 3, entry 3). Upon further increasing
the catalyst mass, the conversion of FA and selectivity for
DEM2−8 decreased. This behavior indicates that it was im‐
portant to use a suitable amount of catalyst in this acetalation
reaction. Excess acid may promote depolymerization reactions,
which produce a large number of byproducts such as ethanol
and FA. In addition, as shown in Fig. 2, the contents of DEM1–8
in the reaction solution remained almost unchanged as the
molar ratio of [MIMBs]HSO4 to HCHO changed from 1:120 to
1:80, and the contents of DEM3–6 increased as the molar ratio of
[MIMBs]HSO4 to HCHO increased from 1:80 to 1:60. Therefore,
the optimum molar ratio of [MIMBs]HSO4 to HCHO is 1:80.
The molar ratio of DEM1 to HCHO also played an important
role in the efficiency of the acetalation reaction. The results
showed that the conversion of FA increased with the molar
ratio of DEM1 to HCHO, and reached its maximum of 87.4%
when the DEM1:HCHO molar ratio was 1:1 (Table 3, entry 3).
Table 3
Effect of reaction conditions using [MIMBs]HSO4 as catalyst.
IL:HCHO
(molar ratio)
1:120
DEM1:HCHO
(molar ratio)
1:1
Temperature
(°C)
Pressure
(MPa)
2.0
Time
(min)
120
Con. of FA
(%)
76.7
Sele. for DEM2–8
Entry
(%)
94.8
95.8
1
2
140
140
1:100
1:1
2.0
120
82.0
3
4
5
1:80
1:60
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:80
1:1
1:1
140
140
140
140
140
140
100
120
130
150
140
140
140
140
140
140
140
140
140
140
2.0
2.0
2.0
120
120
120
120
120
120
120
120
120
120
120
120
120
20
87.4
85.6
85.9
84.6
81.3
76.8
35.4
68.7
82.5
84.0
81.4
85.1
87.3
40.4
56.4
60.7
73.8
79.6
90.1
92.6
95.0
93.7
94.6
94.8
95.8
94.9
91.2
94.2
95.1
90.6
94.1
93.7
95.1
92.8
92.3
93.5
93.0
92.8
95.0
95.1
0.8:1
1.2:1
1.4:1
1.6:1
1:1
6
7
8
9
2.0
2.0
2.0
2.0
2.0
2.0
10
11
12
13
14
15
16
17
18
19
20
21
22
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
2.0
Self‐pressure
1.0
3.0
2.0
2.0
40
1:1
1:1
1:1
1:1
2.0
2.0
2.0
2.0
60
80
100
180
240
1:1
2.0
Reaction conditions: the source of formaldehyde is trioxane.

858
Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
60
50
40
30
20
10
0
self-pressure
1.0 MPa
2.0 MPa
DEM1:H2CO = 0.8:1
DEM1:H2CO = 1:1
DEM1:H2CO = 1.2:1
DEM1:H2CO = 1.4:1
40
3.0 MPa
30
DEM1:H2CO = 1.6:1
20
10
0
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
n value of DEMn (n = 18)
n value of DEMn (n = 18)
Fig. 3. Contents of DEMn dependence of DEM1:HCHO molar ratio.
Reaction conditions: ILs:HCHO(TOX) = 1:80 molar ratio, 140 °C, 2.0
MPa, 2 h.
Fig. 5. Contents of DEMn under different pressures. Reaction conditions:
ILs:HCHO(TOX):DEM1 = 1:80:80 molar ratio, 140 °C, 2 h.
The influence of pressure on the reaction was not obvious.
At self‐pressure, the conversion of FA and selectivity for
DEM2−8 were 81.4% and 94.1%, respectively (Table 3, entry
13). With increasing reaction pressure, the reaction was slight‐
ly promoted, reaching good conversion of FA and selectivity for
DEM2–8 of 87.4% and 95.0% at 2.0 MPa, respectively (Table 3,
entry 3). Further increasing the pressure to 3.0 MPa caused the
conversion and selectivity to change very little (Table 3, entry
15). Fig. 5 reveals that pressure had little influence on the con‐
Then, the conversion gradually decreased as the amount of
DEM1 continued to increase, which was probably caused by the
dilution of the IL catalyst with the excess reactant. The selectiv‐
ity for DEM2–8 was unchanged regardless of the molar ratio of
DEM1 to HCHO. The optimum molar ratio of DEM1:HCHO is 1:1,
at which the conversion of FA is 87.4%, and the selectivity for
DEM2–8 is 95.0%. As illustrated in Fig. 3, with increasing molar
ratio of DEM1 to HCHO, the contents of DEM3–6 in the reaction
solution gradually decreased.
tents of DEM2–8
.
Reaction temperature had a dramatic effect on the reaction
outcome. When the temperature was increased from 100 to
140 °C, the conversion of FA and selectivity for DEM2–8 in‐
creased from 35.4% to 87.4% and 91.2% to 95.0%, respective‐
ly (Table 3, entries 3 and 9–11). As the temperature rose fur‐
ther, the conversion of FA and selectivity for DEM2–8 decreased
because of the depolymerization reactions of the products to
form more byproducts at higher temperature. Fig. 4 reveals
that the contents of DEM2–8 first increased and then decreased,
approaching the maximum value when the reaction tempera‐
ture was 140 °C. The depolymerization of TOX is an endother‐
mic reaction, so higher temperature is favorable for it. Howev‐
er, higher temperature will limit the chain length increase of
DEMn, which is an exothermic reaction. For this reason, the
optimal reaction temperature is 140 °C.
The effect of reaction time on the synthesis of DEMn was al‐
so investigated in the range from 20 min to 4 h. Prolonging
reaction time was propitious for the reaction (Table 3, entries 3
and 16–22). Increased conversion of FA and selectivity for
DEM2−8 were observed when the reaction time was extended,
and reached 87.4% and 95.0%, respectively, at 2 h (Table 3,
entry 3). Further prolonging the reaction time resulted in only
slight increases of the conversion of FA and selectivity for
DEM2−8. As shown in Fig. 6, with lengthening reaction time, the
content of DEM2 in the reaction solution first increased and
then decreased, reaching the maximum value when the reac‐
tion time was 100 min. When the reaction time was less than 2
h, with extending reaction time, the contents of DEM3−6 gradu‐
25
60
100 oC
DEM2
DEM3
DEM4
DEM5
DEM6
DEM7
DEM8
120 oC
50
20
15
10
5
130 oC
40
140 oC
150 oC
30
20
10
0
0
0
1
2
3
4
5
6
7
8
9
40
80
120
160
200
240
n value of DEMn (n = 18)
Time (min)
Fig. 6. Contents of DEMn at different time. Reaction conditions:
Fig. 4. Contents of DEMn at different temperature. Reaction conditions:
ILs:HCHO(TOX):DEM1 = 1:80:80 molar ratio, 140 °C, 2.0 MPa.
ILs:HCHO(TOX):DEM1 = 1:80:80 molar ratio, 2.0 MPa, 2 h.

Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
859
ally increased. However, when the reaction time was more than
Table 5
2 h, they increased only slightly, which indicates that the reac‐
tion rate lowered and the reaction probably approached equi‐
librium. DEM7 and DEM8 were also detected in the reaction
mixture after 1 and 2 h, respectively, and their contents in‐
creased over time. These experimental phenomena are at‐
tributed to the consecutive reaction mechanism of acetalation
of DEM1 with TOX. As the reaction time lengthened, the conver‐
sion and content of FA increased slightly, which indicates that
the decomposition of TOX to FA becomes stable. The FA mon‐
omer inserted one by one into DEMn−1 to form DEMn as the
reaction proceeded.
Recycling of [MIMBs]HSO4 in the acetalation reaction of DEM1 with TOX
using different separation methods (layering and extraction).
Layering
Extraction
Run
Con. of FA Sele. for DEM2–8
Con. of FA Sele. for DEM2–8
(%)
87.7
82.6
75.8
67.3
63.6
57.4
54.0
47.8
(%)
95.0
94.5
96.1
95.1
95.3
95.5
95.7
95.2
(%)
87.7
83.5
77.3
68.5
66.2
64.4
62.7
61.8
(%)
95.0
95.7
95.4
94.8
93.2
94.5
93.9
94.5
1
2
3
4
5
6
7
8
3.4. Sources of FA and end‐group providers
Reaction conditions: [MIMBs]HSO4:HCHO(TOX):DEM1 = 1:80:80 molar
ratio, 140 °C, 2.0 MPa, 2 h.
Using [MIMBs]HSO4 as a catalyst, the sources of FA (i.e., TOX
and PF) and the end‐group (–R) provider (such as DEM1, etha‐
nol, propanol, and butanol) were examined; the results are
presented in Table 4. We found that although similar selectivity
for DEM2−8 was obtained using different forms of FA, TOX was
the best supplier of FA for the reaction with the highest conver‐
sion (87.7%). When ethanol was used as the –R provider, the
main products showed a lower polymerization degree of DEMn
and the selectivity for DEM2−8 decreased markedly, and the
conversion of FA decreased to 80.4%. Water is a byproduct of
the acetalation of FA with aliphatic alcohols, which easily caus‐
es hydrolysis of products and inhibits the occurrence of con‐
secutive reactions. We studied the scope of the IL‐catalyzed
acetalation towards various aliphatic alcohols with TOX. The
conversion of FA was 65.6% and the selectivities for
C3H7O(CH2O)nC3H7 with n = 1, 2, 3, 4, 5, 6, and 7 were 60.0%,
23.8%, 8.9%, 2.9%, 0.9%, 0.2%, and 0.05%, respectively, when
propanol was used as the –R provider at 140 °C and 2.0 MPa for
2 h. As for butanol, the conversion of FA was 73.2% and the
selectivities for C4H9O(CH2O)nC4H9 with n = 1, 2, 3, 4, 5, 6, and 7
were 57.7%, 24.3%, 9.6%, 3.5%, 1.2%, 0.4%, and 0.1%, respec‐
tively. Reactivity increased with decreasing chain length of the
aliphatic alcohols, and aliphatic alcohols with an even number
of carbons displayed higher reactivity compared with those
with an odd number of carbons.
with FA. In this paper, two methods, i.e., layering and extrac‐
tion, were used to separate ILs from the reaction system. First,
[MIMBs]HSO4 was separated from the reaction mixture
through layering and reused in another cycle under the same
conditions without any treatment. The reusability of the recy‐
cled catalyst is shown in Table 5. No obvious change was ob‐
served in the selectivity for DEM2−8, and the conversion of FA
decreased from 87.7% to 47.8% after eight runs. Recovering
the used catalyst by extraction of the products with n‐hexane (5
mL×3) and vacuum drying for 12 h at 70 °C recovered 85.6% of
[MIMBs]HSO4. Second, [MIMBs]HSO4 was separated by extrac‐
tion of the products with benzene and then dried by rotary
evaporation before reuse. After eight runs, the conversion of FA
decreased from 87.7% to 61.8% and the recovery of
[MIMBs]HSO4 was 91.8%. In addition, the FT‐IR spectrum of
fresh [MIMBs]HSO4 is compared with that reused eight times in
Fig. 7. The lack of obvious change of the three FT‐IR spectra
indicates that the structure of the [MIMBs]HSO4 catalyst hardly
changed after reuse eight times. Therefore, the main reason for
the observed decrease of catalytic activity is the loss of catalyst.
4. Conclusions
We demonstrated that Brønsted‐acidic ILs can catalyze the
acetalation reactions for the formation of polyoxymethylene
dialkyl ethers under solvent–free conditions. ILs functionalized
with –SO3H showed the highest catalytic performance for the
synthesis of polyoxymethylene dialkyl ethers. [MIMBs]HSO4
showed the highest catalytic activity for acetalation of DEM1
with TOX, giving the high conversion of FA (92.6%) and selec‐
3.5. Reusability of [MIMBs]HSO4 as a catalyst
A series of recycling experiments were conducted to inves‐
tigate the recoverability and recyclability of the ILs functional‐
ized with –SO3H in the acetalation reaction of dialkyl formal
Table 4
Influence of different FA sources and the end‐group (–R) providers on the reaction.
Sele. for RO(CH2O)nR (%)
Con. of FA
–R provider
–CH2O– provider
(%)
87.7
86.6
80.4
65.6
73.2
n = 1
—
—
56.9
60.0
57.7
n = 2
47.7
48.4
23.0
23.8
24.3
n = 3
26.5
26.1
8.3
n = 4
13.3
12.9
2.6
n = 5
5.8
6.0
0.7
0.9
1.2
n = 6
2.3
2.5
0.2
0.2
0.4
n = 7
1.2
1.0
0
0.05
0.1
n = 8
0.2
0.4
0
DEM1
DEM1
CH3CH2OH
CH3(CH2)2OH
CH3(CH2)3OH
TOX
PF
TOX
TOX
TOX
8.9
2.9
0
9.6
3.5
0
Reaction conditions: [MIMBs]HSO4:HCHO:R provider = 1:80:80 molar ratio, 140 °C, 2.0 MPa, 2 h.

860
Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
Hasse, S. Blagov, US Patent 20 070 260 094 A1, 2007.
100
300
[9] E. Stroefer, H. Hasse, S. Blagov, US Patent 20 080 207 954 A1,
2008.
(1)
[10] J. Burger, M. Siegert, E. Ströfer, H. Hasse, Fuel, 2010, 89,
3315–3319.
[11] Y. Y. Zheng, Q. Tang, T. F. Wang, Y. H. Liao, J. F Wang, Chem. Eng.
Technol., 2013, 36, 1951–1956.
[12] Q. Zhao, H. Wang, Z. F. Qin, Z. W. Wu, J. B. Wu, W. B. Fan, J. G. Wang,
J. Fuel Chem. Technol., 2011, 39, 918–923.
[13] J. Cao, H. Q. Zhu, H. Wang, L. Z. Huang, Z. F. Qin, W. B. Fan, J. G.
Wang, J. Fuel Chem. Technol., 2014, 42, 986–993.
[14] J. Q. Zhang, D. Y. Fang, D. H. Liu, Ind. Eng. Chem. Res., 2014, 53,
13589–13597.
[15] X. Zhang, M. B. Tu, M. G. Paice, Bioenergy Res., 2011, 4, 246–257.
[16] A. S. Amarasekara, Chem. Rev., 2016, 116, 6133–6183.
[17] A. Pinkert, K. N. Marsh, S. S. Pang, M. P. Staiger, Chem. Rev., 2009,
109, 6712–6728.
[18] M. E. Zakrzewska, E. Bogel‐Lukasik, R. Bogel‐Lukasik, Chem. Rev.,
2011, 111, 397–417.
[19] V. I. Parvulescu, C. Hardacre, Chem. Rev., 2007, 107, 2615–2665.
[20] J. P. Hallett, T. Welton, Chem. Rev., 2011, 111, 3508–3576.
[21] D. Yang, D. L. Wang, H. Liu, X. L. Zhao, Y. Lu, S. J. Lai, Y. Liu, Chin. J.
Catal., 2016, 37, 405–411.
[22] A. H. Tamboli, A. A. Chaugule, F. A. Sheikh, W. J. Chung, H. Kim,
Chin. J. Catal., 2015, 36, 1365–1371.
50
200
100
(2)
(3)
50
100
50
0
500 1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm1)
Fig. 7. FT‐IR spectra comparison of the fresh and the eight times reused
[MIMBs]HSO4. (1) Fresh [MIMBs]HSO4; (2) [MIMBs]HSO4 reused eight
times (separation by layering); (3) [MIMBs]HSO4 reused eight times
(separation by extraction).
tivity for DEM2−8 (95.1%). The source of FA and –R provider
were examined and a plausible reaction mechanism was pro‐
posed. The present study provides an environmentally friendly
and high‐yielding synthetic methodology to obtain polyox‐
ymethylene dialkyl ethers.
[23] J. Chen, H. Y. Song, C. G. Xia, Z. H. Tang, US Patent 8 344 183 B2,
2013.
[24] J. Chen, H. Y. Song, C. G. Xia, Z. Li, US Patent 8 816 131 B2, 2014.
[25] C. G. Xia, H. Y. Song, J. Chen, Z. Li, US Patent 8 987 521 B2, 2015.
[26] J. Chen, H. Y. Song, C. G. Xia, M. R. Kang, R. H. Jin, US Patent
9 067 188 B2, 2015.
[27] C. G. Xia, H. Y. Song, J. Chen, F. X. Jin, M. R. Kang, US Patent
9 169 186 B2, 2015.
References
[1] Y. H. Lei, Q. Sun, Z. X. Chen, J. Y. Shen, Acta Chim. Sin., 2009, 67,
767–772.
[2] T. H. Fleisch, R. A. Sills, Stud. Surf. Sci. Catal., 2004, 147, 31–36.
[3] W. Mashiro, U. Hiroyuki, B. Steve, D. Jean‐Lue, EP Patent
1 993 159, 2008.
[28] J. Chen, H. Y. Song, C. G. Xia, M. R. Kang, US Patent 9 168 503 B2,
2015.
[4] H. Staudinger, M. Lüthy, Helv. Chim. Acta, 1925, 8, 41–64.
[5] W. F. Gresham, R. E. Brooks, US Patent 2 449 469, 1948.
[6] P. Renata, M. Marchionna, EP Patent 1 505 049 A1, 2005.
[7] R. Patrial, M. Marchionna, US Patent 6 534 685 B1, 2003.
[8] H. Schelling, E. Stroefer, R. Pinkos, A. Haunert, G. D. Tebben, H.
[29] Q. Wu, M. Wang, Y. Hao, H. S. Li, Y. Zhao, Q. Z. Jiao, Ind. Eng. Chem.
Res., 2014, 53, 16254–16260.
[30] A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver, D. C. Forbes,
J. H. Davis Jr, J. Am. Chem. Soc., 2002, 124, 5962–5963.
[31] D. Fang, Q. R. Shi, J. Cheng, K. Gong, Z. L. Liu, Appl. Catal. A, 2008,
Graphical Abstract
Chin. J. Catal., 2017, 38: 853–861 doi: 10.1016/S1872‐2067(17)62816‐X
Brønsted‐acidic ionic liquids as efficient catalysts for the synthesis of
polyoxymethylene dialkyl ethers
BAILs
SO3H
N
N
X-
m
SO3H
Heyuan Song, Meirong Kang, Fuxiang Jin, Guoqin Wang, Zhen Li, Jing Chen *
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences;
University of Chinese Academy of Sciences
N
-
-
m = 4, X = HSO4-
m = 3, X = HSO4
HSO4
N
SO3H
P
3
SO3H
-
-
HSO4
HSO4
O
BAILs
O
O
O
O
n
(n-1)
+
R
2
R
R
R
H
H
Polyoxymethylene dialkyl ethers (RO(CH2O)nR, n ≥ 1) were synthesized by
acetalation of formaldehyde with dialkyl formal or aliphatic alcohol using effi‐
cient, recyclable Brønsted‐acidic ionic liquids as catalysts; excellent yields and
selectivities were obtained under solvent‐free conditions.
O
BAILs
O
O
R
O
n
n
+
H
+ H2O
R
R
H
H
R = C2H5, C3H7, C4H9
Polyoxymethylene
dialkyl ethers

Heyuan Song et al. / Chinese Journal of Catalysis 38 (2017) 853–861
861
345, 158–163.
[33] F. Wang, G. L. Zhu, Z. Li, F. Zhao, C. G. Xia, J. Chen, J. Mol. Catal. A,
2015, 408, 228–236.
[32] Y. P. Zhao, Z. Xu, H. Chen, Y. C. Fu, J. Y. Shen, J. Energy Chem., 2013,
22, 833–836.
[34] M. A. Paul, J. Am. Chem. Soc., 1950, 72, 3813–3814.