Efficient and reusable zeolite-immobilized acidic ionic liquids for the synthesis of polyoxymethylene dimethyl ethers

Article abstract of DOI:10.1016/j.mcat.2018.06.012

A series of molecular sieve-immobilized Br?nsted acidic ionic liquids (BAILs@MS) was prepared, characterized and utilized as efficient catalysts for the synthesis of polyoxymethylene dimethyl ethers (PODE_n or DMM_n) from methylal (DMM₁) and trioxane (TOX). Combined characterization results from fourier transform infrared (FT-IR) spectroscopy, elemental analysis, thermogravimetry (TG), N₂ adsorption-desorption (Brunauer-Emmett-Teller, BET) isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed desorption of ammonia (NH₃-TPD), suggested that the synthesized BAILs were successfully immobilized on the surface of molecular sieves through covalent bonds. Moreover, the catalytic performance tests demonstrated that NaZSM-5 immobilized SO₃H-functionalized ionic liquids (ILs) i.e., BAILs@NaZSM-5, exhibited excellent activity for the acetalation of DMM₁ with TOX, compared to the homogeneous catalysis of the precursors ([MIMBs]HSO₄) as well as other molecular sieve-supported ILs. The influence of catalyst concentration, molar ratio of DMM₁ to HCHO, temperature and reaction duration on the catalytic activity were investigated by employing [NaZSM-5IMBs]HSO₄ as the catalyst. It was demonstrated that a superior conversion of formaldehyde (FA) (90.3%) and excellent selectivity for DMM_3-8 (53.5%) has been achieved in mild conditions (110 °C, 2.5 MPa for 2 h). Additionally, the catalyst can be effortlessly separated by filtration and reused more than ten times without significant loss of activity.

Full text of DOI:10.1016/j.mcat.2018.06.012

MolecularCatalysis455(2018)179–187
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Efficient and reusable zeolite-immobilized acidic ionic liquids for the
synthesis of polyoxymethylene dimethyl ethers
Heyuan Songa,b, Ruiyun Lia,b, Fuxiang Jina, Zhen Lia, Jing Chena,⁎
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
University of Chinese Academy of Sciences, Beijing 100049, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Trioxane
Polyoxymethylene dimethyl ethers
Acetalation reaction
Molecular sieve
A series of molecular sieve-immobilized Brønsted acidic ionic liquids (BAILs@MS) was prepared, characterized
and utilized as efficient catalysts for the synthesis of polyoxymethylene dimethyl ethers (PODEn or DMMn) from
methylal (DMM1) and trioxane (TOX). Combined characterization results from fourier transform infrared (FT-IR)
spectroscopy, elemental analysis, thermogravimetry (TG), N2 adsorption-desorption (Brunauer-Emmett-Teller,
BET) isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature-programmed
desorption of ammonia (NH3-TPD), suggested that the synthesized BAILs were successfully immobilized on the
surface of molecular sieves through covalent bonds. Moreover, the catalytic performance tests demonstrated that
NaZSM-5 immobilized SO3H-functionalized ionic liquids (ILs) i.e., BAILs@NaZSM-5, exhibited excellent activity
for the acetalation of DMM1 with TOX, compared to the homogeneous catalysis of the precursors ([MIMBs]
HSO4) as well as other molecular sieve-supported ILs. The influence of catalyst concentration, molar ratio of
DMM1 to HCHO, temperature and reaction duration on the catalytic activity were investigated by employing
[NaZSM-5IMBs]HSO4 as the catalyst. It was demonstrated that a superior conversion of formaldehyde (FA)
(90.3%) and excellent selectivity for DMM3-8 (53.5%) has been achieved in mild conditions (110 °C, 2.5 MPa for
2 h). Additionally, the catalyst can be effortlessly separated by filtration and reused more than ten times without
significant loss of activity.
Supported ionic liquids
1. Introduction
section (–CH2O–) provider (1,3,5-trioxane (TOX), paraformaldehyde
(PF) or formaldehyde (FA)) under acidic conditions [6]. All these raw
Polyoxymethylene dimethyl ethers (CH3O(CH2O)nCH3, n≥2; re-
ferred to as PODEn or DMMn) have been considered as promising diesel
additives because of their high oxygen contents and cetane numbers
(CN), making them suitable for improving the combustibility and en-
hancing the efficiency of combustion, as well as reducing the release of
pollutants [1–3]. Especially, the physicochemical properties of DMM3–8
are comparable to that of diesel oil, thus allowing their use in modern
diesel engines without any modification of the engine infrastructure.
Compared with the usage of diesel fuel alone, the particulate matter
and NOx emissions are reduced by 80–90% and 50%, respectively with
20% blend [2–4]. DMM2, DMM3, and DMM4 also exhibit low toxicity
[5], excellent volatility, low condensation point, great permeating
ability, good solubilizing power and miscibility with nearly all organic
compounds and water. Additionally, they have the potential to be used
as environment-friendly industrial solvents. The conventional process
for the synthesis of DMMn is based on the acetalation of the end-group
(–CH3, –OCH3) provider (methanol or methylal (DMM1)) with a chain
materials can be obtained economically on large scale from coal and
natural gas. In recent years, many routes have been reported to syn-
thesize DMMn from methanol and FA, PF or TOX [7–14]. However,
when methanol and water are present in the process, the separation of
DMMn is difficult because many side-products are produced. To avoid
introducing or generating water, one particularly favorable route is,
therefore, the water-free process using DMM1 and TOX as starting
materials. So far, many catalysts have been developed for the acetala-
tion reaction of DMM1 with TOX, such as liquid acids [15,16], ion-
exchange resins [17–19], molecular sieves [20], metal oxides [21], etc.
In this context, our group has recently proposed a process catalyzed by
Brønsted-acidic ionic liquids (BAILs) [22–25], which combines the ad-
vantages of both homogeneous and heterogeneous catalysts. Even
though BAILs possess inherent advantages such as high acid density,
broad liquid range, and uniform active catalytic sites, there are still
some disadvantages, including high viscosity and losses in polar sol-
vents.
⁎
Corresponding author.
E-mail address: chenj@licp.cas.cn (J. Chen).
https://doi.org/10.1016/j.mcat.2018.06.012
Received 2 March 2018; Received in revised form 10 May 2018; Accepted 9 June 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

H. Song et al.
MolecularCatalysis455(2018)179–187
To overcome these drawbacks of ILs, there has been a great deal of
attention recently to immobilization of ILs on a variety of polymeric
[26–32] and inorganic supports [33–40]. The immobilization process
aims to combine the desired catalytic properties of ILs with the easy
recovery features of heterogeneous supports. Moreover, it is desirable
to utilize properties of support materials such as surface and pore
structure, achieving synergistic catalysis in the reaction system. Among
the heterogeneous solid supports employed for ILs, molecular sieves are
the most widely used, convenient, and acid stable, possessing several
attractive advantages in terms of structure stability and recycling. W.G.
Cheng et al. [34] have successfully synthesized an SBA-15 supported
1,2,4-triazolium-based IL for the preparation of cyclic carbonates from
CO2 and epoxides, 80–99% yield and 97–99% selectivity of the pro-
ducts were obtained under mild conditions. J.B. Yang et al. [35] have
developed –SO3H functionalized BAILs supported onto SBA-15 and
applied them as efficient catalysts in the transesterification of sec-butyl
acetate and methanol for the synthesis of 2-butanol. Z.M. Li et al. [36]
reported MOR zeolite immobilized –SO3H functionalized BAILs as ef-
ficient catalysts for the ketalization of cyclohexanone with diols.
Therefore, we choose the hydrothermally stable molecular sieves as the
solid support which can effectively promote the catalytic reactivity of
acetalation reactions when compared to the corresponding homo-
geneous IL catalysts.
Herein, we report a novel heterogeneous catalyst i.e., molecular
sieve-supported -SO3H functionalized BAILs (BAILs@MS) and its ap-
plication in the acetalation reaction of DMM1 with TOX (Scheme 1).
Fourier transform infrared (FT-IR) spectroscopy, elemental analysis,
thermogravimetry analysis (TG), N2 adsorption-desorption (Brunauer-
Emmett-Teller, BET) isotherm, X-ray diffraction (XRD), scanning elec-
tron microscopy (SEM) and temperature-programmed desorption of
ammonia (NH3-TPD) were then utilized to characterize the catalyst in
detail. Further, the effect of the reaction parameters including catalyst
loading, material ratios, temperature, and reaction time were studied to
obtain the optimum conditions and the recyclability of the catalyst was
also investigated.
Scheme 2. Synthetic route of BAILs@MS.
recorded on the INOVA-400 MHz spectrometer (Bruker, Switzerland)
employing tetramethylsilane (TMS) as the internal standard. NMR
spectra data in C6D6 is shown as follows. 1H NMR (400 MHz, C6D6): δ
0.11–0.15 (t, J = 8.0 Hz, 2 H), 0.86–0.90 (t, J = 8.0 Hz, 9 H), 1.32–1.40
(h, J = 8.0 Hz, 2 H), 2.98–3.02(t, J = 8.0 Hz, 2 H), 3.43–3.49 (q,
J = 8.0 Hz, 6 H), 6.26 (s, 1 H), 6.92 (s, 2 H). 13C NMR (100 MHz, C6D6):
δ 7.32, 18.16, 24.90, 48.31, 58.20, 118.08, 129.79, 137.06.
2. Experimental
2.1. General
The molecular sieves ZSM-5 (Si/Al = 500), Y (Si/Al = 5), and Si-
MCM-41 were purchased from Nanjing XFNAON Materials Tech Co.,
Ltd. DMM2 (> 99.5%), DMM3 (> 99.5%), DMM4 (> 99.5%) and
DMM5 (> 99.5%) were separated by rectification. Other chemicals
were analytical grade and utilized without additional purification.
2.2.2. Preparation of NaZSM-5
In a typical procedure, 1 M NaNO3 solution (200 mL) was mixed
with ZSM-5 (20 g) over a water bath at 80 °C with stirring for 4 h. The
product was filtered, washed thoroughly with water and dried at 60 °C
for 12 h. Next, the product was calcined in air at 550 °C for 5 h. The test
was repeated three times.
2.2. Catalyst preparation
2.2.3. Preparation of molecular sieve supported ionic liquids (ILs@MS)
The molecular sieve supported ionic liquids (ILs@MS) were syn-
thesized in three steps, as shown in Scheme 2. Firstly, a mixture of
molecular sieves (10.0 g) with 1-(3-(triethoxysilyl)propyl)-1H-imida-
zole or 4-(2-(triethoxysilyl)ethyl)pyridine (0.04 mol) in anhydrous to-
luene (100 mL) was stirred and refluxed for 48 h. Secondly, the reaction
mixture was cooled to 80 °C and 1,4-butylene sulfone (0.04 mol) was
added dropwise to the reaction system, the mixture was stirred for 48 h
at 80 °C. After the reaction has completed, the reaction mixture was
collected by filtration and washed thrice with toluene to remove any
unconverted reactants. After drying in vacuum (70 °C, 5.3 K Pa for
12 h), the white powder (intermediate) was obtained. Finally, the in-
termediate was added in a round-bottom flask containing ethanol
2.2.1. Preparation of 1-(3-(triethoxysilyl)propyl)-1H-imidazole
A stoichiometric amount of imidazole (1 mol) in anhydrous fur-
anidine (200 mL) was added dropwise to a suspension of NaH (1 mol) in
anhydrous furanidine (100 mL) at room temperature under stirring, and
the mixture was further heated at 40 °C for 12 h. Then, a stoichiometric
amount of (3-chloropropyl)triethoxysilane (1 mol) was added dropwise
to the mixture and stirred at 80 °C for 48 h to yield 1-(3-(triethoxysilyl)
propyl)-1H-imidazole. After the reaction has completed, the reaction
mixture was filtered and the filter cake was washed three times with
furanidine. The filtrate was collected and dried for 12 h with anhydrous
magnesium sulfate and 1-(3-(triethoxysilyl)propyl)-1H-imidazole was
separated by distillation. The 1H NMR and 13C NMR spectra were
Scheme 1. Acetalation reaction of methylal with trioxane
catalyzed by BAILs@MS.
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H. Song et al.
MolecularCatalysis455(2018)179–187
(100 mL) and sulphuric acid (0.04 mol) in ethanol (50 mL) was gradu-
ally added to the flask at room temperature while stirring. Next, the
mixture was steadily heated to reflux and allowed to react for 36 h. The
mixture was further filtered and washed with toluene (50 mL × 2) and
ethanol (50 mL × 12) until the pH reached 7.0. Finally, the prepared
ILs@MS was dried under vacuum to entirely eliminate the residual
volatiles and moisture prior to use.
2.3. Catalyst characterization
FT-IR spectra were recorded on a NEXUS 870 FT-IR infrared spec-
trometer (Nicolet Instruments Co., USA) in dry air at room temperature
with anhydrous KBr as standard. Sulfur (S, wt.%) was determined by
elemental microanalysis technique using a Vario EL cube elemental
analyzer (Elementar Analysensysteme Co., Ltd., Hanau, Germany), and
the loading amount of ILs on the molecular sieve was calculated by the
content of S element. TG analysis was carried out using a Netzsch
STA449F3 simultaneous thermal analyzer (Netzsch Co., Ltd., Selb,
Germany) in a N2 atmosphere between 50 and 1000 °C. The BET
sorption of N2 was determined using Micromeritics ASAP2010 equip-
ment (Micromeritics Instrument Co., Ltd., Norcross, USA). The XRD was
measured on
a Panalytical X’PERT PRO X-ray diffractometer
(Panalytical B V, Almelo, Netherlands). The particle size and mor-
phology was assessed by SEM using Hitachi SU8020 equipment
(Hitachi, Tokyo, Japan). The acidity of BAIL@NaZSM-5 was measured
by NH3-TPD using an AutoChem 2910 apparatus (Micromeritics
Instrument Co., Ltd., Norcross, USA). A 100 mg sample was treated in
situ at 300 °C for 1 h under a Helium (He) stream (30 mL/min), purged
by He (30 mL/min) for 0.5 h to clean its surface, and then cooled to
50 °C. The sample was further saturated with NH3 for 1 h, followed by
flushing with He until a stable baseline was obtained. TPD measure-
ments were then conducted from 50 to 800 °C with a rate of 10 °C/min.
The release of NH3 was monitored by a thermal conductivity detector.
2.4. Typical acetalation procedure
Catalytic activity experiments were conducted in a 100 mL Teflon-
lined stainless-steel autoclave provided with a thermometer and me-
chanical stirrer. It is thermostated using an electric heater. Initially,
DMM1 (0.36 mol), TOX (0.1 mol), and catalyst (1.8 g) were added into
the autoclave, which was then closed and flushed thrice with N2
(1.5 MPa) at room temperature. The reaction mixture was preheated to
the desired temperature before starting the reaction, after which, the
mechanical stirrer was immediately turned on, and the timing began.
After completion of the reaction, the products have been characterized
and quantitatively determined using gas chromatography/mass spec-
trometry (GC/MS) (Agilent 7890 A/5975C) and GC (Agilent 6890),
respectively. An SE-54 capillary column with a FID detector was used in
an Agilent 6890 gas chromatograph. The GC quantitative analysis of
TOX, DMMn, methanol, and methyl formate was determined with the
internal standard method: a specific quantity of furanidine was used as
the internal standard for the samples prior to GC analyses. FA was
analyzed using titration by the sodium sulfite method and formic acid
content was analyzed using titration by potassium methylate.
Fig. 1. Fourier transform infrared spectroscopy spectra of molecular sieves and
BAILs@MS.
3. Results and discussion
Conversion of FA (Con. FA) and the selectivity for products (Sele.
DMM3–8) were determined using the following equations:
3.1. Catalyst characterization
w
−w
(TOX+FA),0
(TOX+FA)
Con. FA(%) =
Sele. DMM3−8
× 100%
w
(TOX+FA),0
3.1.1. Fourier transform infrared (FT-IR) spectroscopy
The FT-IR spectra of NaZSM-5, Si-MCM-41, Y, [NaZSM-5IMBs]
HSO4, [NaZSM-5PyBs]HSO4, [Si-MCM-41IMBs]HSO4 and [YIMBs]
HSO4 are illustrated in Fig. 1. As shown in Fig. 1(a), the absorption
bands at about 447.2, 801.7 and 1089.6.0 cm–1 were characteristic of
SiO2 [36,37], whereas the bands at 1640.0 and 3443.8 cm−1 were
wDMM
3−8
=
× 100%
wDMM
+ wMeOH + wMA + wMF
2−10
FA, formaldehyde; MA, formic acid; MF, methyl formate.
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H. Song et al.
MolecularCatalysis455(2018)179–187
attributed to the stretching frequency of Si–OH and physisorbed water
[36,38]. For [NaZSM-5IMBs]HSO4, the characteristic peaks were found
at 3154.4, 1566.6 and 1458.0 cm–1, which can be assigned to the CeH,
C]N and C]C stretching vibrations of the imidazole ring [36,39]. The
bands around 1232.1 and 1092.9 cm–1 were associated with the signals
of CeS and S]O bonds, which overlap with the characteristic peaks of
SiO2, where the peaks become wider and stronger. These results in-
dicate the existence of the –SO3H group [40]. In comparison with
NaZSM-5, [NaZSM-5IMBs]HSO4 exhibited the same characteristic
bands such as the Si–O–Si vibrations (789.3 and 1092.9 cm–1) and the
stretching vibration of –OH (1646.9, 3445.0 cm–1). Moreover, the
bands around 3445.0 cm–1 become wider and stronger after loading ILs,
which indicates the existence hydrogen bonds with ILs on the surface of
the zeolite. Similar results were obtained in [NaZSM-5PyBs]HSO4; it
displays peaks at 3060.5, 1515.8 and 1471.4 cm–1 related to the
stretching frequencies of CeH, C]N and C]C on the pyridine ring
while the characteristic peaks of the –SO3H group are detected at
1232.1 and 1095.5 cm–1. Therefore, these results confirm that the ILs
have been successfully immobilized on the NaZSM-5 via covalent
bonds.
Fig. 2. Thermal stability of molecular sieves and BAILs@MS.
Fig. 1(b) and (c) depict the FT-IR spectra of [Si-MCM-41IMBs]HSO4
and [YIMBs]HSO4, as well as Si-MCM-41 and Y for comparison, re-
spectively. Similar results were obtained, providing further evidence of
ILs grafted onto the surface of molecular sieves.
3.1.2. Elemental analysis
The fraction of the ILs grafted onto molecular sieves was measured
by an elemental analyzer through the amount of S element and the
results were presented in Table 1. The results of [NaZSM-5IMBs]HSO4
show that 17.7% of the IL ([PIM]HSO4) were immobilized on NaZSM-5
according to the content of S element (3.3 wt.%) in the catalyst. Fur-
thermore, the grafted percentages of [NaZSM-5PyBs]HSO4, [Si-MCM-
41IMBs]HSO4 and [YIMBs]HSO4 were 9.6%, 28.5% and 8.1%, respec-
tively.
3.1.3. Thermal stability
The thermal stability of molecular sieve-grafted acidic ILs was de-
termined by TG, and the weight losses of the samples were depicted in
Fig. 2. For NaZSM-5, a slight weight loss of 2.8% at the temperature
range of 50–1000 °C occurred due to the residual moisture in pores.
When –SO3H functionalized imidazolium ILs was immobilized on zeo-
lite, an initial weight loss of ∼5.3% occurred at the temperature range
of 50–310 °C, which was attributed to the release of physisorbed water
as well as the dehydration of unreacted silanol groups. Further, a
considerable weight loss of ∼18.5% was seen at 310–510 °C as the
organic structure of the ILs decomposed from the surface of NaZSM-5.
Therefore, the catalyst can be operated stably at temperatures under
310 °C. Similar results were obtained when –SO3H functionalized pyr-
idine ILs was immobilized on NaZSM-5 ([NaZSM-5PyBs]HSO4); a
weight loss of ∼5.6% at 50–280 °C was attributed to the loss of water
and unreacted silanol groups. Further, a weight loss of ∼9.7% was
found at 280–500 °C brought about by the decomposition of ILs. When
–SO3H functionalized imidazolium ILs was immobilized on Si-MCM–41
and Y, the weight loss temperatures were 320 °C and 350 °C, respec-
tively. Consequently, we arrived at a conclusion that all of these
BAILs@MS demonstrate high thermal stability and could be potentially
Fig. 3. N2 Adsorption–desorption isotherms of NaZSM-5 and BAILs@NaZSM-5.
employed as catalysts.
3.1.4. N2 adsorption-desorption (BET)
The BET isotherms of the support before and after grafting are
presented in Fig. 3. It can be observed that all the curves exhibit typical
type IV isotherms and the H1 hysteresis loop of NaZSM-5 was at the
pressures of P/P0 = 0.45 − 0.8, which was characteristic of a meso-
pores structure [41]. Moreover, the hysteresis loops of BAILs@NaZSM-5
became smaller, which was attributed to the immobilization of ILs onto
the zeolite and filling into some mesopores of the zeolite. The surface
area and pore size of the samples were calculated by the BET equation
and shown in Table 2. It can be seen that the surface area and total pore
volume were reduced after ILs were grafted on the molecular sieves,
Table 2
BET surface properties of molecular sieves before and after grafting.
Sample
BET Surface
area
Total pore
volume
Average pore
width
(Å)
Table 1
(m2/g)
(cm3/g)
Elemental analysis of NaZSN–5 and BAILs@MS.
NaZSM–5
416.43
40.19
218.06
1122.95
10.69
0.22
0.06
0.11
1.02
0.02
0.37
0.02
20.89
56.39
20.67
36.32
60.84
20.36
82.40
Sample
S content (wt.%)
IL content (wt.%)
[NaZSM–5IMBs]HSO4
[NaZSM–5PyBs]HSO4
Si-MCM–41
[Si-MCM–41IMBs]HSO4
Y
NaZSM–5
0
0
[NaZSM–5IMBs]HSO4
[NaZSM–5PyBs]HSO4
[Si-MCM–41IMBs]HSO4
[YIMBs]HSO4
3.3
1.8
5.3
1.5
17.7
9.6
28.5
8.1
723.47
9.20
[YIMBs]HSO4
182

H. Song et al.
MolecularCatalysis455(2018)179–187
Fig. 4. XRD patterns of NaZSM–5 and BAILs@NaZSM-5.
Fig. 6. NH3–TPD profiles of NaZSM–5 and BAILs@NaZSM–5.
while the average pore width increased as a small fraction of the pores
were blocked off after grafting [PIMBs]HSO4.
possibly due to the binding of BAILs to the zeolite crystals.
3.1.5. X-Ray diffraction (XRD) studies
3.1.7. Acidity analysis
Fig. 4 displays the high-angle powder XRD patterns of NaZSM-5 and
NaZSM-5 grafted with various ILs. It was clear that the diffraction peak
decreased apparently after the ILs was grafted on NaZSM-5; moreover,
the type of XRD pattern has not altered. The higher the amount of ILs
loaded, the weaker the peak intensity. This result indicates that the
framework of the zeolite was not damaged and only the crystallinity
was reduced when ILs were grafted on NaZSM-5.
NH3-TPD was employed to determine the acidity of BAILs@NaZSM-
5 and NaZSM-5, and the images were depicted in Fig. 6. As shown in the
NH3-TPD profiles, the ammonia desorption peaks at about 120–250 °C
and 300–450 °C correspond to the weak acid sites and the strong acid
sites, respectively [20]. The peak intensity has obviously increased
when ILs were grafted onto the surface of the zeolite. Furthermore, the
peak intensity representing the density of acid sites was enhanced with
an increase in the loading of ILs, especially for the strong acid sites.
Based on the aforementioned results, we have obtained the order of
acidity for BAILs@NaZSM-5 and NaZSM-5: [NaZSM-5IMBs]
HSO4 > [NaZSM-5PyBs]HSO4 > NaZSM-5.
3.1.6. Scanning electron microscopy (SEM) studies
SEM was performed to characterize the morphology of BAILs@
NaZSM-5 and NaZSM-5, and the images were shown in Fig. 5. The SEM
image showed that there was no obvious change in the size of NaZSM-5
particles after the introduction of the ILs. These results indicated that
the morphological homogeneity of the NaZSM-5 particles was main-
tained after BAILs grafting. However, the SEM images also revealed that
some NaZSM-5 particles have coalesced in BAILs@NaZSM-5, which is
3.2. Performance of the synthesized BAILs@MS catalysts
The catalytic activity of BAILs@MS for the preparation of
Fig. 5. The whole and magnified SEM images of NaZSM–5 and BAILs@NaZSM–5. (a) NaZSM–5, (b)[NaZSM–5IMBs]HSO4, (c) [NaZSM–5PyBs]HSO4.
183

H. Song et al.
MolecularCatalysis455(2018)179–187
Table 3
Effect of catalysts on the reaction.
Catalysts
Product distribution (%)
CH3O(CH2O)nCH3, DMMn, n=
Con.
of FA
(%)
Sele. for
DMM3-8
(%)
MeOH
MF
TOX
FA
1
2
3
4
5
6
7
8
9
10
NaZSM–5
*[MIMBs]HSO4
[NaZSM–5IMBs]HSO4
[NaZSM–5PyBs]HSO4
[YIMBs]HSO4
74.0
65.4
43.1
48.0
72.6
75.1
1.1
0.1
3.0
14.8
12.4
0.3
0.09
0.7
8.3
6.5
0.03
0.03
0
0
0
1.6
1.4
0
0
0
0.6
0.6
0
0
0
0.2
0.2
0
0
0
0.07
0.1
0
0
0
0.01
0.02
0
0.06
0.6
0.8
1.6
0.7
0.04
0.3
0.1
0.08
0.4
0.02
24.5
19.8
2.2
2.0
22.4
24.0
0.1
0.6
0.2
0.5
0.9
0.1
0.8
16.8
21.5
53.5
49.8
10.0
17.9
13.6
24.8
23.8
2.7
0.1
3.4
2.7
0
17.8
90.3
89.9
6.1
[Si-MCM–41IMBs]HSO4
0.5
0.09
0
0
0
0
0
0
0.04
2.8
Reaction conditions: Catalyst: 5.0 wt.%, HCHO (TOX):DMM1 = 1:1.2 M ratio, 110℃, 2.5 M Pa, 2 h. *[MIMBs]HSO4: 0.9 wt.%.
polyoxymethylene dimethyl ethers (CH3O(CH2O)nCH3, DMMn,
n = 2–8) via acetalation of DMM1 with TOX were evaluated and com-
pared with that of NaZSM-5 and [MIMBs]HSO4. The probe reaction of
DMM1 and TOX was carried out at 110 °C and 2.5 MPa for 2 h (DMM1:
HCHO = 1.2 M ratio, one TOX molecule is equivalent to three HCHO
molecules) and the observations are presented in Table 3. The major
products were DMMn (n = 2–8), while the byproducts of methanol,
methyl formate and formic acid were also detected in the reaction
mixture. As shown in Table 3, only traces of DMM2-4 were formed when
NaZSM-5 zeolite was used as the catalyst, which proves that the support
has almost no catalytic activity. In contrast, [NaZSM-5IMBs]HSO4
fabricated through the immobilization of –SO3H functionalized ILs onto
the NaZSM-5 support, produced a drastic increase in the catalytic ac-
tivity while the FA conversion and selectivity for DMM3-8 were at
90.3% and 53.5%, respectively. However, the corresponding homo-
geneous catalysis of [MIMBs]HSO4 (0.9 wt.%, with the same amount of
loading on [NaZSM-5IMBs]HSO4) under the same conditions produced
only 17.8% and 21.5%, respectively. This is mainly due to the co-
operation of Brønsted acid site from BAILs and the pore structure of the
NaZSM-5 zeolite, besides, [MIMBs]HSO4 is viscous and immiscible with
the substrate which gose against mass transfer. Furthermore, when
[NaZSM-5PyBs]HSO4 was used as the catalyst, 89.9% FA conversion
and 49.8% DMM3-8 selectivity were obtained, which were slightly in-
ferior to those of [NaZSM-5IMBs]HSO4. This is mainly attributed to the
stronger surface acidity displayed by [NaZSM-5IMBs]HSO4 compared
to [NaZSM-5PyBs]HSO4, which can effectively promote the dissociation
of TOX to FA species [42]. Moreover, the insertion step of FA species
into DMM1 and lower DMMn to form higher DMMn is an acid-catalyzed
carbocation mechanism [22]. Hence, it was inferred that the catalytic
performance of BAILs@NaZSM-5 was related to its acidic properties.
For comparison, the acetalation of DMM1 with TOX was also studied
using [YIMBs]HSO4 and [Si-MCM-41IMBs]HSO4 as catalysts. It has
been found that the conversion of FA for these catalysts were 6.1% and
2.8%, respectively. This result is attributed to the fact that the support
confinement in the channel has a significant effect on the activity of a
reaction. The increase in the orbital energy of organic molecules is more
significant when the size of the guest molecule is similar to the pore size
of the support, which indicates their improved ability to provide elec-
trons in the reaction along with superior reactivity [43–45]. The
structure of DMMn is linear with ∼3.4 Å width, which was determined
using a density functional theory (DFT) method (Fig. 7) [46]. The
zeolite of NaZSM-5 with the three-dimensional intersecting channel
system (a straight channel of 5.6 × 5.3 Å and a sinusoidal channel of
5.5 × 5.1 Å) [47], MCM-41 possesses a hexagonal array of uniform
mesopores with regularly channels of 20–100 Å [48]. The pore struc-
ture of zeolite Y is characterized by supercages approximately 12 Å in
diameter and open channel width of about 7.4 Å [49,50]. The pore
structure and size of NaZSM-5 are close to those of the reactant and
product molecules which possess linear chain structure. The diffusion
rate of the reactants and products in NaZSM-5 channels is faster, so
[NaZSM-5IMBs]HSO4 exhibited superior catalytic activity.
Using [NaZSM-5IMBs]HSO4 as the catalyst, the effect of reaction
parameters such as catalyst concentration, the M ratio of DMM1 to
HCHO, temperature and duration were examined in detail and the re-
sults were presented in Table 4. Initially, we studied the effect of cat-
alyst concentration on the acetalation of DMM1 with TOX and found
that the conversion of FA increased with an increase in catalyst loading
(Table 4, entries 1–4), and the highest conversion of FA (90.7%) was
obtained when the amount of [NaZSM-5IMBs]HSO4 was 6 wt.%
(Table 4, entry 4). Initially, the selectivity for DMM3-8 increased and
then decreased, which approached a maximum (53.5%) when the
amount of [NaZSM-5IMBs]HSO4 was 5 wt.% (Table 4, entry 3). This
behavior indicates the importance of using an appropriate quantity of
catalyst in this reaction system. On the other hand, the excess acid may
enhance the formation of byproducts such as methanol, FA and methyl
formate. In addition, the M ratio of DMM1 to HCHO had a remarkable
impact on the reaction; the conversion of FA gradually increased from
84.6% to 92.3% and the selectivity for DMM3-8 decreased from 55.6%
to 43.9% when the M ratio of DMM1 to HCHO was increased from 1:1 to
1.6:1 (Table 4, entries 3, 5–7). The most favorable M ratio of DMM1 to
HCHO is determined to be 1.2:1, at which the FA conversion was
90.3%, and the selectivity for DMM3-8 was 53.5%. The reaction tem-
perature also had an immense effect on the results; the conversion of FA
enhanced with an increase in the reaction temperature (from 100 to
120 °C) and the selectivity for DMM3-8 reached the maximum at 110 °C
(Table 4, entries 3 and 8–9). The results suggested that the products
depolymerized into byproducts at higher temperatures and the best
reaction temperature was 110 °C. The impact of reaction time on the
preparation of DMMn was also explored ranging from 1 to 4 h. (Table 4,
entries 3 and 10–12). The FA conversion and selectivity for DMM3-8 has
improved rapidly by extending the reaction time from 1 to 2 h and
reached 90.3% and 53.5%, respectively, at 2 h. Further extending the
reaction time led to only a slight augmentation in the conversion of FA,
hence, an optimum reaction time of 2 h was selected for saving energy.
3.3. Reusability of [NaZSM-5IMBs]HSO4
From environmental and economic points of view, the stability and
sustained activity of the catalyst are of great importance. Thus, a series
of repetitive experiments were carried out to explore the reusability of
the zeolite-immobilized acidic ILs in the acetalation reaction of DMM1
with TOX (the loading of [NaZSM-5IMBs]HSO4 is 5.0 wt.%, HCHO
(TOX): DMM1 = 1:1.2 M ratio, 110 °C, 2.5 MPa, 2 h). In each cycle,
[NaZSM-5IMBs]HSO4 was separated from the reaction mixture by fil-
tration and reused in the next run under the same conditions without
any further treatment. The FA conversion and selectivity for DMM3-8
from ten consecutive runs is shown in Fig. 8. The results demonstrated
that [NaZSM-5IMBs]HSO4 can be recycled up to ten times with no
appreciable decrease in the FA conversion and selectivity for DMM3-8
,
indicating that [NaZSM-5IMBs]HSO4 owns excellent reusability and
sustained activity. Additionally, the FT-IR and XRD spectrum of fresh
[NaZSM–5IMBs]HSO4 were compared with that of the catalyst which
184

H. Song et al.
MolecularCatalysis455(2018)179–187
Fig. 7. Structure of DMMn calculated by density function theory (DFT) method.
Table 4
Effect of reaction conditions using [NaZSM–5IMBs]HSO4 as catalyst.
Entry [NaZSM-
5IMBs]
DMM1:HCHO Temperature Time Con. of Sele. for
(M ratio)
(oC)
(h)
FA (%) DMM3-8
(%)
HSO4
(wt.%)
1
2
3
4
5
6
7
8
2
4
5
6
5
5
5
5
5
5
5
5
1.2
1.2
1.2
1.2
1
1.4
1.6
1.2
1.2
1.2
1.2
1.2
110
110
110
110
110
110
110
100
120
110
110
110
2
2
2
2
2
2
2
2
2
1
3
4
78.9
89.0
90.3
90.7
84.6
90.6
92.3
82.1
90.7
85.2
91.0
91.8
46.6
52.1
53.5
50.5
55.6
46.6
43.9
47.5
50.0
49.4
50.0
50.8
9
10
11
12
Reaction conditions: the source of formaldehyde is TOX.
Fig. 9. FT–IR spectra comparison of the fresh and the ten times reused
[NaZSM–5IMBs]HSO4.
Fig. 8. The recycle test of [NaZSM–5IMBs]HSO4 in the acetalation reaction of
DMM1 with TOX. Reaction conditions:[NaZSM–5IMBs]HSO4. 5.0 wt.%, HCHO
(TOX):DMM1 = 1:1.2 M ratio, 110℃, 2.5 M Pa, 2 h.
Fig. 10. XRD spectrum comparison of the fresh and the ten times reused
[NaZSM–5IMBs]HSO4.
Table 5
was reused ten times. As shown in Figs. 9 and 10, the absence of ap-
parent change in the FT-IR and XRD spectra reveals that the structure of
[NaZSM-5IMBs]HSO4 has not altered even after reusing ten times. To
investigate the real loss of acidity, acid-base titration was adopted to
determine the acid capacity of the fresh and the ten times reused
[NaZSM–5IMBs]HSO4 using NaCl solution as an ion-exchange agent,
the loading of ILs was also compared and the results were presented in
Table 5. After ten runs, the acid capacity of [NaZSM–5IMBs]HSO4 has
slightly decreased from 9.8 mmol/g to 9.1 mmol/g and the fraction of
the ILs grafted onto NaZSM-5 decreased from 17.7% to 16.7%. Ac-
cording these comparative experiments, the prepared zeolite–immobi-
lized acidic ILs possesses excellent stability and reusability. In this
Acid capacity and elemental analysis comparison of the fresh and the ten times
reused [NaZSM–5IMBs]HSO4.
Sample
Acid capacity
(mmol/g)
Results of elemental analysis
(wt.%)
S content
3.3
IL content
17.7
Fresh [NaZSM-5IMBs]
HSO4
[NaZSM-5IMBs]HSO4
reused for 10 times
9.8
9.1
3.1
16.7
185

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MolecularCatalysis455(2018)179–187
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