Immobilized 1,1,3,3-tetramethylguanidine ionic liquids as the catalyst for synthesizing propylene glycol methyl ether

Article abstract of DOI:10.1007/s10562-010-0426-9

In this work, we immobilized ionic liquid 1,1,3,3-tetramethylguanidium lactate on two solid supports, bentonite and SBA-15, by different methods. The prepared materials were used to catalyze the reaction of propylene oxide and methanol to produce propylene glycol methyl ether. 1-Methoxy-2-propanol was the predominant product. The influence of the amount of the catalyst, molar ratio of the reactants, reaction temperature and time on the yield and selectivity was studied. The two catalysts were proved to be efficient and reusable catalysts for the reaction. Graphical Abstract: 1,1,3,3-Tetramethylguanidium lactate was supported on bentonite and SBA-15, which were effective and recyclable catalysts for the synthesis of propylene glycol methyl ether from methanol and propylene oxide.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0426-9

Catal Lett (2010) 140:49–54
DOI 10.1007/s10562-010-0426-9
Immobilized 1,1,3,3-Tetramethylguanidine Ionic Liquids
as the Catalyst for Synthesizing Propylene Glycol Methyl Ether
•
Shuguang Liang Yinxi Zhou Huizhen Liu
•
•
•
Tao Jiang Buxing Han
Received: 21 June 2010 / Accepted: 30 July 2010 / Published online: 17 August 2010
Ó Springer Science+Business Media, LLC 2010
Abstract In this work, we immobilized ionic liquid
1,1,3,3-tetramethylguanidium lactate on two solid supports,
bentonite and SBA-15, by different methods. The prepared
materials were used to catalyze the reaction of propylene
oxide and methanol to produce propylene glycol methyl
ether. 1-Methoxy-2-propanol was the predominant product.
The influence of the amount of the catalyst, molar ratio of
the reactants, reaction temperature and time on the yield
and selectivity was studied. The two catalysts were proved
to be efficient and reusable catalysts for the reaction.
recent years, a great number of task-specific ILs was syn-
thesized and applied [8, 9]. We previously reported the
preparation and characterization of 1, 1, 3, 3-tetramethyl-
guanidium (TMG)-based ILs [9]. They have been suc-
cessfully applied to catalyze Henry [10] and direct aldol
reaction [11, 12], immobilize metals to prepare catalysts
[13, 14] and absorb SO2 from simulated flue gases [15].
These uses are based on the electron lone pair of the
guanidine cation. But there are very few work about the
immobilized TMG-based ILs used as catalysts. Zhang et al.
[16] reported that silica-supported hexaakylguanidinium
chloride was the effective and recyclable catalysts for CO2
fixation to produce cyclic carbonates.
Keywords Propylene glycol methyl ether (PGME) Á
Ionic liquids Á Propylene oxide (PO) Á SBA-15 Á
Tetramethylguanidine Á Methanol Á Bentonite
Propylene glycol methyl ether (PGME) is one kind of
fine chemicals widely used as solvent because of the ether
bond and hydroxyl group, which is hydrophobic and
hydrophilic, respectively. It is expected as a safe sub-
stitute for toxic ethylene glycol ether because of the
negligible toxicity of propylene glycol ether [17]. PGME
is conventionally derived from addition of methanol to
propylene oxide (PO) catalyzed by an acid or a base.
Generally, the mechanism was considered as follows
[18, 19].
1 Introduction
Ionic liquids (ILs) are organic salts that possess many
different properties compared with traditional organic sol-
vents, such as nonflammability, negligible vapor pressure,
reusability, and high thermal stability [1–3]. They have
been investigated extensively in organic synthesis as sol-
vents or catalysts [1, 3–7]. As both the anionic and the
cationic part of ILs can be easily varied, the properties of
these compounds can be adjusted for specific purposes. In
Basic catalysis
S. Liang Á Y. Zhou Á H. Liu Á T. Jiang (&) Á B. Han (&)
Beijing National Laboratory for Molecular Sciences (BNLMS),
Centre for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
Acidic catalysis
e-mail: jiangt@iccas.ac.cn
B. Han
e-mail: Hanbx@iccas.ac.cn
123

50
S. Liang et al.
The epoxide ring of propylene oxide might open at
used in this work is a smectite rich white bentonite pro-
vided by Zhejiang Sanding Scientific and Technology Co.,
Ltd., China. The composition of the clay was 58.98% SiO2,
19.82% Al2O3, 3.73% MgO, 5.18% Na2O, 0.42% K2O,
0.87% CaO, 1.31% Fe2O3, 0.10% TiO2, 0.74% P2O5, and
0.08% FeO. It was microporous and cation-rich (Na?). The
cation exchange capacity (CEC) of the bentonite was
99 mmol/100 g. (3-Mercaptopropyl) trimethoxysilane (3-
MPTS) (98%) was obtained from Alfa. Pluronic triblock
copolymer (EO20-PO70-EO20, P123) was obtained from
Badische Anilin- and Soda-Fabrik (BASF). Other reagents
and solvents were analytical grade and produced by Beijing
Chemical Reagents Company. All chemicals were used as
received.
either of the C–O bonds, to form primary or secondary
alcohols. Compared to secondary alcohols, the primary
alcohols (such as 2-methoxy-1-propanol), revealed the
reproduction and developmental toxicity [20]. Therefore,
the high selectivity to secondary alcohols is required for
this process. The ring of propylene oxide might preferen-
tially open at the least sterically hindered position over a
basic catalyst leading to most secondary alcohol 1-meth-
oxy-2-propanol. But an acid catalyst provides a mixture of
secondary and primary alcohols, and the proportion of the
isomers depends on the acid strength. So, the base catalysts
attract much attention in the research and application.
In the past decades, a lot of homogenous base catalysts
were exploited and showed high selectivity to secondary
alcohol, but they had the drawbacks of separation, liquid
waste treating and corrosion problems. Thus solid bases
were developed to catalyze the synthesis of propylene
glycol ether. Those heterogeneous catalysts included
anionic double hydroxide clays [21], basic zeolites [22] and
basic metal oxide [23–25].
2.2 Preparation of the Catalysts
2.2.1 TMG/Bentonite
[TMG][Lac] ionic liquid was prepared directly by neu-
tralization of 1,1,3,3-tetramethylguanidine with lactic acid
at room temperature [9]. Bentonite was treated with
[TMG][Lac] to exchange the Na cations with the IL cat-
ions. 20.0 g of bentonite was dispersed in 150 mL of
aqueous solution containing 4.87 g of [TMG][Lac], and
stirred for about 6 h. Then the bentonite was separated by
filtration, and treated with fresh IL aqueous solution again.
The bentonite treated with IL aqueous solution was filtered,
washed with a large amount of deionized water, dried at
378 K for 24 h and named as TMG/Bentonite.
Natural clay minerals are a type of environmentally
benign material. Bentonite is a clay consisting predomi-
nantly of montmorillonite, which has layered structure,
large surface areas, and cation exchange capacity. The
special properties of bentonite make it a valuable material
for a wide range of applications, such as environment and
catalysis. Bentonite is potentially a good catalyst support.
For example, ion-exchanged montmorillonite has been used
to prepare excellent catalysts for the hydrogenolysis of
glycerol [26]. A montmorillonite-enwrapped scandium has
been used as a heterogeneous catalyst for Michael reaction
[27]. Mesoporous SiO2 is attractive supports due to their
advantageous properties, such as excellent chemical and
thermal stability, high porosity, large surface area, and high
surface concentration of silanols. In this work, ionic liquid
1,1,3,3-tetramethylguanidium lactate ([TMG][Lac]) was
immobilized on two kinds of solid supports, i.e. bentonite
and SBA-15. The as-prepared materials were used as the
basic and heterogeneous catalysts for preparing PGME
from methanol and PO. Good yield and isomer selectivity
were obtained over the two types of catalysts. The two
catalysts can be separated with the products easily by cen-
trifugation and reused without decrease in activity and
selectivity.
2.2.2 TMG/SBA-15
The siliceous SBA-15 mesoporous material was synthe-
sized according to the procedures described by Zhao et al.
[28] using pluronic triblock copolymer (EO20-PO70-
EO20, P123) as the structure-directing agent and tetraethyl
orthosilicate (TEOS, 98%) as a source of silica. The tri-
block copolymer was dissolved in HCl aqueous solution
(150 mL, 1.6 M) under stirring, after the required amount
of TEOS was added to the solution at 308 K the solution
was stirred for 20 h. The mixture was heated at 363 K for
24 h. After synthesis, the solid obtained was filtered,
washed thoroughly with deionized water, dried at 373 K
overnight and finally calcined at 773 K for 6 h to remove
the organic template.
4 g of SBA-15 suspended in 25 g of dry toluene was
refluxed and magnetically stirred for 1 h, under nitrogen
atmosphere. To this suspension, excess 3-MPTS was added
drop wise using a syringe. To ensure a complete covalent
anchoring of the precursor over silica, the mixture was kept
at reflux conditions for 48 h. Finally, the material was fil-
tered, washed with toluene and ethanol and dried at 353 K
2 Experimental
2.1 Materials
1,1,3,3-Tetramethylguanidine was purchased from Baigui
Chemical Company (Shijiazhuang, China). The clay mineral
123

Immobilized 1,1,3,3-Tetramethylguanidine Ionic Liquids as the Catalyst
51
for 2 h. The oxidation of the mercapto propyl groups
anchored on the silica surface to the corresponding sul-
phonic acids was carried out by treating 2 g of the sample
with 20 mL of H2O2 (30 wt% in water) for 24 h. In order to
confirm that all the sulphonic groups are protonated, the
solid material was further acidified with 0.05 M H2SO4 for
2 h. The solid material obtained was then filtered, washed
with water for several times and dried in air at 353 K for
24 h.
capillary column (SUPELCOWAX 10, 30 m in length,
0.25 mm in diameter) after centrifugal separation from the
catalyst. 1-Propanol was used as the internal standard to
calculate the amount of products. Yield of PGME (primary
and second alcohols) is defined as the ratio of number of
moles of PGME produced in the reaction to the total moles
of PO initially added. Selectivity to 1-methoxy-2-propanol
is defined as the ratio of number of moles of 1-methoxy-2-
propanol to the number of moles of the two isomers.
2 g of the sample and 100 g of deionized water was
loaded into a 250 mL beaker, and the 5% aqueous solution
of [TMG][Lac] was dropped into the beaker under stirring,
until the pH of the solution in the beaker was basic. The
solid was filtered off, washed with a large amount of
deionized water, until the pH of the filtrate was about 7,
and dried under vacuum at 353 K for 24 h to afford TMG/
SBA-15.
3 Results and Discussion
Before testing the catalytic activity of the catalysts, TMG/
Bentonite and TMG/SBA-15 were characterized with
FT-IR, and TG techniques to confirm the existence of TMG
cations ([TMG]?) on the surface of the catalyst. The
presence of [TMG]? in the catalyst was supported by the
fact that the catalysts contained organic compound (8.2
wt% for TMG/Bentonite, and 12.9 wt% for TMG/SBA-15)
as determined by TGA (Fig. 1). The existence of the
[TMG]? was also supported by the FT-IR analysis (Fig. 2).
The specific peaks of TMG (TMG/Bentonite: 3242, 3418
m(N–H), 2937 m(C–H), 1622 m(C=N), 1563 d(N–H), 1463,
1422 d(C–H) cm-1; TMG/SBA-15: 3258, 3362 m(N–H),
2905, 2944, 2980 m(C–H), 1614 m(C=N), 1658, 1570
d(N–H), 1453, 1417 cm-1 d(C–H)) can be observed in the
spectra of the two catalysts.
2.3 Characterization of the Catalysts
FT-IR spectra of the samples were taken in the range of
4,000–400 cm-1 on Bruker Tensor 27 FT-IR spectrometer
as KBr pellets. TG-DTA and DTG analysis of the catalysts
were carried out on a TA Q50 using a platinum pan under
(60 mL/min) atmosphere from ambient to 1,073 K with a
heating rate of 20 K/min. The specific surface area, total
pore volume and average pore diameter were measured by
N2 adsorption–desorption method using NOVA 1200
instrument (Quanta chrome). The samples were treated at
453 K for 3 h under vacuum and then the adsorption–
desorption was conducted by passing nitrogen into the
sample, which was kept under liquid.
Figure 3 is the N2 adsorption–desorption isotherm of
TMG/Bentonite and TMG/SBA-15. Because of the layered
structure of bentonite, TMG/Bentonite shows a small sur-
face area of 9.7 m2/g, while TMG/SBA-15 shows a high
surface area of 550 m2/g due to its mesoporous structure.
In addition, from TG analysis, the loading content of TMG
cation was 0.7 mmol/g on TMG/Bentonite and 0.58 mmol/
g on TMG/SBA-15, respectively.
2.4 Catalytic Performance Test
The catalytic performance of the catalysts was evaluated in
a 6.5 mL batch reactor with molar ratio of methanol to PO
of 1:1 to 5:1. After running at 373–413 K for a required
time under magnetic stirring, the reactor was cooled down
to room temperature. The products were analyzed by GC
(Agilent 6820) equipped with a FID detector and a
The effect of the amount of the catalysts on the yield of
PGME and the isomer selectivity to 1-methoxy-2-propanol
is shown in Fig. 4. The PGME yield increases with the
amount of TMG/Bentonite; while the yield arrives at the
maximum when the amount of TMG/SBA-15 is 0.04 g.
Fig. 1 TG analysis of the
supports, fresh and reused
catalysts
100
100
SBA-15
Bentonite
95
95
TMG/SBA-15(after five runs)
TMG/Bentonite (fresh)
90
90
85
80
TMG/SBA-15(fresh)
TMG/Bentonite (after five runs)
85
80
400
600
800
1000
400
600
800
1000
Temperature/K
Temperature/K
123

52
S. Liang et al.
Fig. 2 FT-IR spectra of catalyst
TMG/Bentonite
and TMG/SBA-15
1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
SBA-15
TMG/SBA-15
Bentonite
TMG/Bentonite
1500 2000 2500 3000 3500 4000
1500
2000 2500
3000 3500 4000
-1
-1
Wavenumber/cm
Wavenumber/cm
Fig. 3 N2 adsorption–
desorption isotherm of TMG/
Bentonite and TMG/SBA-15
600
25
20
15
10
5
TMG/SBA-15
adsorption
desorption
TMG/Bentonite
adsorption
500
400
300
200
100
0
desorption
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure, P/P
Relative Pressure, P/P
0
0
Fig. 4 The effect of the amount
of the catalyst on the yield of
PGME and isomer selectivity.
Reaction conditions: PO,
20 mmol; MeOH/PO molar
ratio, 3; reaction temperature,
383 K; reaction time, 3 h
100
80
60
40
20
0
100
80
60
40
20
0
TMG/SBA-15
TMG/Bentonite
yield
selectivity
yield
selectivity
0.02
0.04
0.06
0.08
0.10
0.02
0.04
0.06
0.08
0.10
catalyst / g
catalyst / g
The isomer selectivity does not change with the amount of
catalysts over both catalysts, indicating that the supported
TMG catalysts are effective for producing 1-methoxy-2-
propanol isomer.
As shown in Fig. 6, the dependence of the catalytic
activity on the reaction temperature is different over the
two catalysts. The effect of the reaction temperature on the
activity of TMG/SBA-15 is not obvious. When increased
the reaction temperature from 363 to 373 K, the yield is
increased by 16% (72–88%, TMG/SBA-15). And then, the
yield does not change with the increasing temperature. But
the situation was different for the TMG/Bentonite. The
yield is only 3% at 363 K and reaches the highest value at
383 K. The activity difference at low temperature might be
due to the different of the surface area of two catalysts,
which is important to the activity of the catalysts. The
surface area of TMG/SBA-15 is almost sixty times of that
Figure 5 shows the influence of molar ratio of methanol
to PO (MeOH/PO) on the yield of PGME and the isomer
selectivity to 1-methoxy-2-propanol at 383 K for 3 h. With
the increase of the molar ratio of MeOH/PO, PO could
completely react with MeOH and the yield of PGME
rapidly reaches a maximum value. When the molar ratio
exceeds 3:1, the yield decreases slightly. The selectivity to
1-methoxy-2-propanol almost keeps constant over the two
catalysts when changing the molar ratio of MeOH/PO.
123

Immobilized 1,1,3,3-Tetramethylguanidine Ionic Liquids as the Catalyst
53
Fig. 5 The effect of molar ratio
of MeOH/PO on the yield of
PGME and the isomer
selectivity. Reaction conditions:
PO, 20 mmol; catalyst, 0.1 g;
reaction temperature, 383 K;
reaction time, 3 h
100
80
60
40
20
0
100
80
60
TMG/Bentonite
TMG/SBA-15
yield
40
20
0
yield
selectivity
selectivity
1
2
3
4
5
1
2
3
4
5
molar ratio of MeOH/PO
molar ratio of MeOH/PO
Fig. 6 The effect of the
reaction temperature on the
yield of PGME and the isomer
selectivity. Reaction conditions:
PO, 20 mmol; MeOH/PO molar
ratio, 3; catalyst, 0.1 g; reaction
time, 3 h
100
80
60
40
20
0
100
80
60
40
20
TMG/SBA-15
TMG/Bentonite
yield
selectivity
yield
selectivity
0
360
370
380
390
400
410
360
370
380
390
400
410
Temperature / K
Temperature / K
Fig. 7 The effect of reaction
time on the yield of PGME and
the isomer selectivity. Reaction
conditions: PO, 20 mmol;
MeOH/PO molar ratio, 3;
catalyst, 0.1 g; reaction
100
80
60
40
20
0
100
80
60
40
20
0
temperature, 383 K
TMG/SBA-15
yield
TMG/Bentonite
yield
selectivity
selectivity
1
2
3
4
5
1
2
3
4
5
time / h
time / h
yield
selectivity
Fig. 8 Recycle of the catalyst
TMG/Bentonite and TMG/
SBA-15. Reaction conditions:
PO, 20 mmol; MeOH/PO molar
ratio, 3; catalyst, 0.1 g; reaction
time, 3 h; temperature, 383 K
TMG/Bentonite
yield
selectivity
TMG/SBA-15
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
4
5
1
2
3
4
5
n
n
of TMG/Bentonite. So TMG/SBA-15 is more active than
TMG/Bentonite at low temperature. At high temperature,
the effect of temperature on the activity exceeds the effect
of surface area, and the activity of two catalysts tends to the
same. In addition, the effect of temperature on the selec-
tivity is not obvious.
The dependence of the yield of PGME and the selec-
tivity to 1-methoxy-2-propanol on the reaction time is
123

54
S. Liang et al.
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conversion of PO is incomplete and the PGME yield is low.
It can be seen that the maximum yield could be achieved at
3 h and no further increase in the yields of PGME is
observed with prolonged reaction time over the two cata-
lysts. But the yield over TMG/SBA-15 is higher than that
over TMG/Bentonite when the reaction time was 1 h. This
might be attributed to the enormous difference of the sur-
face area of the two supports. The high surface area favors
the contact of reactants on the catalyst surface. This effect
is especially obvious at short reaction time.
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The ionic liquid 1,1,3,3-tetramethylguanidium lactate was
immobilized on two types of solid supports, bentonite and
SBA-15, with different method. The obtained materials
were proved to be excellent heterogeneous catalysts for the
reaction of methanol with PO to produce PGME. 1-Meth-
oxy-2-propanol was the predominant product. At the opti-
mized reaction conditions, the yield of PGME and the
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Acknowledgments The authors are grateful to the financial sup-
ports from the Ministry of Science and Technology of China
(2006CB202504), Chinese Academy of Sciences (KJCX2.YW.H16)
and National Natural Science Foundation of China (20932002,
20733010, 20633080).
123