Tunable synthesis of propylene glycol ether from methanol and propylene oxide under ambient pressure

Article abstract of DOI:10.1134/S0023158411030025

A series of basic and acidic ionic liquids, 1-butyl-3-methylimidazolium hydroxide (BMIMOH), 1-acetyl-3-methylimidazolium chloride (AcMIMCl) and AcMIMCl-FeCl₃, or analogues of AcMIMCl, namely 1-potassium acetate-3-methylimidazolium chloride (KAcMIMCl), 1-potassium (sodium, ammonium) acetate-3-methylimidazolium hydroxides (KAcMIMOH, NaAcMIMOH and NH ₄AcMIMOH), were prepared and used as catalysts for catalytic synthesis of propylene glycol ether via reaction of propylene oxide (PO) with methanol under mild reaction conditions. KAcMIMOH exhibited outstanding catalytic performance with 94.2% of conversion of PO and 99.1% of selectivity to 1-methoxy-2-propanol (MP-2) at 60°C and ambient pressure for 4 h. However, AcMIMCl-FeCl₃ showed a good catalysis performance with high selectivity to 2-methoxy-1-propanol (MP-1). The tunable synthesis of MP-2 or MP-1 catalyzed by basic compound KAcMIMOH or acidic ionic liquid AcMIMCl-FeCl₃ was realized.

Full text of DOI:10.1134/S0023158411030025

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 3, pp. 386–390. © Pleiades Publishing, Ltd., 2011.
Tunable Synthesis of Propylene Glycol Ether from Methanol
and Propylene Oxide under Ambient Pressure1
Yu Bai, Qinghai Cai, Xiaoguang Wang, and Bin Lu
Key Lab for Design and Synthesis of Functionalized materials and Green Catalysis,
School of Chemistry and Chemical Engineering, Harbin Normal University, Harbin, P.R. China
eꢀmail: qinghaic@yahoo.com.cn
Received June 2, 2010
Abstract—A series of basic and acidic ionic liquids, 1ꢀbutylꢀ3ꢀmethylimidazolium hydroxide (BMIMOH),
1ꢀacetylꢀ3ꢀmethylimidazolium chloride (AcMIMCl) and AcMIMClꢀFeCl3, or analogues of AcMIMCl,
namely 1ꢀpotassium acetateꢀ3ꢀmethylimidazolium chloride (KAcMIMCl), 1ꢀpotassium (sodium, ammoꢀ
nium) acetateꢀ3ꢀmethylimidazolium hydroxides (KAcMIMOH, NaAcMIMOH and NH4AcMIMOH),
were prepared and used as catalysts for catalytic synthesis of propylene glycol ether via reaction of propylene
oxide (PO) with methanol under mild reaction conditions. KAcMIMOH exhibited outstanding catalytic perꢀ
formance with 94.2% of conversion of PO and 99.1% of selectivity to 1ꢀmethoxyꢀ2ꢀpropanol (MPꢀ2) at 60°C
and ambient pressure for 4 h. However, AcMIMClꢀFeCl3 showed a good catalysis performance with high
selectivity to 2ꢀmethoxyꢀ1ꢀpropanol (MPꢀ1). The tunable synthesis of MPꢀ2 or MPꢀ1 catalyzed by basic
compound KAcMIMOH or acidic ionic liquid AcMIMClꢀFeCl3 was realized.
DOI: 10.1134/S0023158411030025
As an important solvent, glycol ether is widely used in under atmospheric pressure. High selectivity to 1ꢀmetoxyꢀ
various industries such as coating, printing ink, leather dyeꢀ 2ꢀpropanol (MPꢀ2) or 2ꢀmetoxyꢀ1ꢀpropanol (MPꢀ1) was
achieved by the catalysis of KAcMIMOH or KAcMIMClꢀ
FeCl3, respectively. This phenomenon is very rare in glycol
ether synthesis to our knowledge. Moreover, highly selecꢀ
tive processes of manufacture are among the most imporꢀ
tant targets pursued in green chemistry.
ing and so on. Propylene glycol ether is expected to be a safe
substitute for toxic ethylene glycol ether due to its negligible
toxicity [1]. There are several methods for synthesis of proꢀ
pylene glycol ether. Among them, the propylene oxide
route is the most convenient and industrially feasible. In this
synthetic process, propylene oxide (PO) reacts with fatty
alcohols to form propylene glycol ether, and the reaction
catalyzed by acid or base catalysts, including at first homoꢀ
geneous acids or bases, such as NaOH, alcoholic sodium,
H2SO4 and BF3 etc [2–4], and later solid acids and bases,
namely acidic zeolites, CaO, MgO and amine modified
porous silica [1, 5–7] etc. Although the catalysis of the solid
acids or bases can help to avoid separation of the catalysts,
liquid waste treatment and corrosion problems, they also
suffer from some drawbacks such as lower activity and
selectivity to target ether, high reaction temperature and
pressure. Hence, there is still a need to develop a new cataꢀ
lytic system and/or an easyꢀclean technology more suitable
for the synthesis of the ether and its derivatives. In light of
this idea, a series of basic and acidic ionic liquids, 1ꢀbutylꢀ
EXPERIMENTAL
Preparation of the Basic Compound
Ionic liquid AcMIMCl was prepared by previously
reported method [8]: in a threeꢀneck flask equipped
with a condenser and a magnetic stirrer, 47.25 g
(0.5 mol) of chloroacetic acid was dissolved in 100 ml
of chloroform under stirring. 40 ml (0.5 mol) of
methylimidazole was dropped in the solution at 40°C
Nꢀ
,
followed by reaction for 50 h. After cooling to room
temperature, the reaction mixture was washed by ethꢀ
anol, filtered and dried under vacuum to afford a white
solid AcMIMCl with yield of 36.8%.
8.8 g (0.05 mol) of AcMIMCl was slowly added to
3ꢀmethylimidazolium hydroxide (BMIMOH), 1ꢀacetylꢀ3ꢀ a solution of KOH (0.1 mol dissolved in 40 ml H2O) in
methylimidazolium chloride (AcMIMCl) and AcMIMClꢀ
FeCl3, or analogues, namely KAcMIMCl, KAcMIMOH,
NaAcMIMOH and NH4AcMIMOH (see Scheme below
as an example), were prepared and used as catalysts for the
synthesis of propylene glycol ether from PO and methanol
a threeꢀneck flask with iceꢀwater bath and retained for
5 h under stirring. After the reaction, the mixture soluꢀ
tion was evaporated under reduced pressure and the
remainder was dissolved in dehydrated methanol, and
then filtered to remove precipitated KCl. The filtrate was
again evaporated under reduced pressure to eliminate
methanol. After drying a white solid KAcMIMOH was
1
The article is published in the original.
386

TUNABLE SYNTHESIS OF PROPYLENE GLYCOL ETHER FROM METHANOL
Reaction of KAcMIMOH preparation
387
Cl–
H2
OH–
H2
+
+
+ 2KOH
+ KCl
H3C N N C COOH
H3C N N C COOK
H2
H2
AcMIMCl
KAcMIMOH
Scheme.
obtained with the yield of 65.3%. The FTꢀIR spectra of 98.1% of selectivity and AcMIMClꢀFeCl3 exhibited
AcMIMCl and KAcMIMOH are listed in Fig. 1 and
the data of their elemental analysis are the next (the
data in parenthesis represent theoretical calculation
results):
AcMIMCl (%): C 38.95 (40.79), H 5.37 (5.10),
N 15.03 (15.86);
KAcMIMOH (%): C 35.07 (36.73), H 4.81 (4.59),
N 13.58 (14.29).
the best reactivity with conversion of 65.7 and selectivꢀ
ity to MPꢀ1 of 99.4% due to the order of their acidity:
AcMIMClꢀFeCl3 > AcMIMCl > KAcMIMCl. It was
worthy of special mention that the acidity or basicity of
the catalyst plays an important role in the homogeꢀ
neous catalytic synthesis of propylene glycol ether
from propylene oxide and methanol under ambient
pressure. When using basic catalyst KAcMIMOH, the
main product of the reaction is MPꢀ2 with selectivity
of 99.1% and when the acidic catalyst was used, high
selectivity of MPꢀ1 was achieved in this process.
Therefore, synthesis of MPꢀ2 or MPꢀ1 can be tuned
by using basic catalyst KAcMIMOH or acidic one
AcMIMClꢀFeCl3. This controllable nature for organic
synthesis is final goal pursued in chemistry.
Catalytic Performance
20 ml of methanol (0.5 mol), 5.8 ml of PO (0.025 mol),
and 3.2 g of KAcMIMOH (0.016 mol) were charged in
a roundꢀbottomed flask (100 ml) equipped with a conꢀ
denser and magnetic stirrer. The reactor was then
heated to 60°C in water bath under stirring. The reacꢀ
tion was carried out at ambient pressure for 4 h. After
completion of the reaction, the liquid mixture was
cooled, distilled to separate the catalyst from the reaction
mixture, sampled and analyzed by GC and GCꢀMS.
Influence of Various Reaction Conditions
Reaction temperature. Figure 2 shows the catalytic
performance of KAcMIMOH at different temperaꢀ
RESULTS AND DISCUSSION
Table 1. The catalytic performances of different catalysts
Catalytic Performances of Various Catalysts
We examined the activity of KAcMIMOH and
other basic or acidic compounds as catalysts in the
reaction of PO with methanol at 60°C and ambient
pressure for 4 h. Our results are listed in Table 1.
Among the catalysts screened, KAcMIMOH exhibꢀ
ited a surprising high activity for the reaction, giving
94.2% conversion of PO and selectivity of 99.1% to
MPꢀ2 (entry 1). The main byꢀproduct of the reaction
is identified to be MPꢀ1. On the contrary, its analogues
NaAcMIMOH, NH4AcMIMOH and a basic ionic liqꢀ
uid BMIMOH were found to have relatively low activꢀ
ity with conversion of 59–73% and low selectivity
(entries 2–4). This difference in catalytic activity
between them is likely caused by the composition and
structure of cations composed of these salts, the
former possesses stronger basicity than the latter three
salts does. Surprisingly, KAcMIMCl exhibited a high
selectivity to MPꢀ1 even though it possesses lower
activity for this reaction (entry 5). Moreover, the
increase in the catalyst acidity resulted in higher activꢀ
ity for the reaction producing MPꢀ1 (entries 6, 7). For
example, AcMIMCl showed higher activity with
Entry
Catalyst
Conversion, % Selectivity, %
1
2
3
4
5
6
7
KAcMIMOH
94.2
72.7
61.7
59.8
20.7
51.6
65.7
99.1
85.8
NaAcMIMOH
NH AcMIMOH
67.7
4
BMIMOH
KAcMIMCl
AcMIMCl
86.8
93.8 (MPꢀ1)
99.4 (MPꢀ1)
98.1 (MPꢀ1)
AcMIMClꢀFeCl (1 : 3)
3
KINETICS AND CATALYSIS Vol. 52
No. 3
2011

388
Transmittance, %
BAI et al.
Characteristics, %
100
100
80
60
40
20
0
90
80
70
60
50
Yield
Selectivity
Conversion
AcMIMCl
KAcMIMOH
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber, cm–1
40
50 60
Temperature,
70
80
°
C
Fig. 1. FTꢀIR spectra of AcMIMCl and KAcMIMOH.
Fig. 2. Effect of reaction temperature on the synthesis of MPꢀ2.
Characteristics, %
100
Characteristics, %
100
95
90
85
80
95
90
85
80
75
Yield
Selectivity
Conversion
75
Yield
Selectivity
Conversion
70
65
1
2
3
Time, h
4
5
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Amount of catalyst, g
Fig. 3. Effect of reaction time on the synthesis of MPꢀ2.
Fig. 4. Effect of catalyst amount on the synthesis of MPꢀ2.
tures and ambient pressure. As the reaction temperaꢀ which implied that a long contact time between the
ture was raised, the conversion of PO was increased to substrate and the catalyst may cause further converꢀ
94.2% at 60°C. Thereafter, conversion was decreased sion of the product into byꢀproducts.
in the temperature range from 60 to 80°C, which is
Amount of the catalyst. Amount of the catalyst used
likely attributed to a part of PO volatilizing out of the
condenser at high temperature. The decrease in selecꢀ
tivity to MPꢀ2 beyond 60°C was likely attributed to
side reaction taking place.
in the reaction system has a great influence on the
reaction. When the etherification of PO and methanol
was carried out with various amount of KAcMIMOH
from 0.8 to 4.0 g under the same reaction conditions,
Reaction time. The dependence of PO conversion the conversion of PO was linearly increased up to
and selectivity to MPꢀ2 on the reaction time is invesꢀ 94.2% with catalyst amount of 3.2 g, and then it retained
tigated in the range of 1–6 h using KAcMIMOH as unchangeable with the amount raised (Fig. 4). However,
catalyst (Fig. 3). The conversion was found to be the decline of the selectivity to MPꢀ2 was observed
greatly increased with prolonging reaction time from 1 above 3.2 g of the catalyst used in the system. As we
to 4 h. When the reaction proceeded, the conversion know, an excess amount of the catalyst could result in
was almost unchangeable and the decrease of the the increase of overall amount of basicity in the sysꢀ
selectivity was observed in the time range from 4 to 6 h, tem, promoting undesirable sideꢀreaction.
KINETICS AND CATALYSIS Vol. 52
No. 3
2011

TUNABLE SYNTHESIS OF PROPYLENE GLYCOL ETHER FROM METHANOL
389
Molar ratio of CH3OH : PO. Figure 5 exhibits the
effect of the methanol : PO molar ratio on the catalytic
performance of KAcMIMOH. As can be seen from
the figure, the conversion of PO was increased up to
97.6% with the molar ratio, and then it was decreased
to 94.2% at 20 : 1. At this ratio, the highest selectivity
to 99.1% was observed. An evident decline of the conꢀ
version and the selectivity was likely due to dilute
effect of excess methanol to PO.
In short, the effect of various reaction conditions
on the synthesis of MPꢀ2 was investigated and the
optimal operational conditions for the system were
obtained: reaction temperature 60°C, time 4 h, methꢀ
anol : PO molar ratio 20 : 1 and amount of the catalyst
3.2 g. The PO conversion of 94.2% and the selectivity
of 99.1%, giving yield of 93.4%, were achieved under
the optimal conditions.
Characteristics, %
100
95
90
85
Yield
Selectivity
Conversion
8 : 1 12 : 1 15 : 1 20 : 1 30 : 1
Molar ratio of methanol : PO
Fig. 5. Effect of methanol : PO molar ratio on the reaction.
Reactions of PO with Other Alcohols
When other alcohols such as ethanol, propanol,
isopropanol, butanol, isobutanol and amyl alcohol
were used as alternatives of methanol, the etherificaꢀ
tion reactions were also investigated in the presence of
KAcMIMOH as catalyst (Table 2). As compared with
the system using methanol as reactant, relative low
conversion of 53–78% and high selectivity of 88–
100% to the corresponding products were observed
with primary alcohols, such as ethanol, propanol,
next run under the same conditions. The results shown
in Table 3 indicated that the conversion of PO and the
selectivity to MPꢀ2 were almost not affected even at
the third run with the reused KAcMIMOH. This findꢀ
ing implied that the catalyst can be recovered and
recycled efficiently. By this token, the catalyst posꢀ
sesses potential application in industry.
So, a series of basic and acidic ionic liquids or the
butanol and amyl alcohol as alternatives at the same analogues, namely, KAcMIMOH, NaAcMIMOH,
conditions. And for secondary alcohols, isopropanol NH4AcMIMOH KAcMIMCl, AcMIMCl,
,
and isobutanol, the catalyst KAcMIMOH showed difꢀ
ferences in the activity. For example, lowest activity
was observed for isopropanol and higher activity for
isobutanol, which is ascribed to the difference in their
molecular structure, namely, isopropanol possesses
greater steric effect than isobutanol.
AcMIMClꢀFeCl3 and BMIMOH were prepared and
used as catalysts for the synthesis of propylene glycol
ether via reaction of PO with methanol under mild
reaction conditions. KAcMIMOH exhibited outꢀ
standing catalytic performance for the synthesis of
MPꢀ2 with 94.2% of yield and 99.1% of selectivity at
60°C and ambient pressure for 4 h. Its high activity is
attributed to its stronger basicity with Brönsted and
Lewis basic sites. However, the ionic liquid
AcMIMClꢀFeCl3 shows a good catalysis performance
for the etherification with high selectivity to MPꢀ1.
Therefore, synthesis of MPꢀ2 or MPꢀ1 can be tuned
by using basic catalyst KAcMIMOH or acidic catalyst
AcMIMClꢀFeCl3. In addition, the synthesis of MPꢀ2
Reusability of the Catalyst
In order to further evaluate performance of the catꢀ
alyst, reuse experiments of the catalyst were carried
out. The catalyst was separated out by distillation
under reduced pressure in the end of the reaction due
to the homogeneous nature and then reused for the
Table 2. The reactions of PO with different alcohols
Table 3. The reusability of catalyst
Alcohol
Methanol
Conversion, %
Selectivity, %
Times
Conversion, %
Selectivity, %
94.2
77.7
76.5
53.3
69.0
35.1
82.6
99.1
97.5
99.9
94.9
88.3
86.8
70.6
1
2
3
4
94.2
96.7
92.9
55.2
99.1
91.8
91.2
32.1
Ethanol
Propanol
Butanol
Amyl alcohol
Isopropanol
Isobutanol
KINETICS AND CATALYSIS Vol. 52
No. 3
2011

390
BAI et al.
2. Pecorini, H.A. and Banchero, J.T., Ind. Eng. Chem.
,
,
catalyzed by KAcMIMOH was investigated and the
optimal operational conditions for the system were
obtained. The catalyst KAcMIMOH is easily regenerꢀ
ated and reused.
1956, vol. 48, p. 1287.
3. Chitwood, H.C. and Freurec, B.T., J. Am. Chem. Soc.
1948, vol. 68, p. 680.
4. Reeve, W. and Sadle, A., J. Am. Chem. Soc., 1950,
ACKNOWLEDGMENTS
vol. 72, p. 1251.
We make a great acknowledgement for the financial
support of this work by the Natural Science Foundation
of Heilongjiang Province, P.R. China (no. ZD200820ꢀ01
and B200814).
5. US Patent 4 440 958, 1984.
6. Zhang, W., Wang, H., Wei, W., and Sun, Y., J. Mol.
Catal. A. Chem., 2005, vol. 231, p. 83.
7. Zhang, X., Zhang, W., Li, J., Zhao, N., Wei, W., and
REFERENCES
Sun, Y., Catal. Commun., 2007, vol. 8, p. 437.
1. Zhang, W.Y., Wang, H., Li, Q.B., Dong, Q.N.,
Zhao, N., Wei, W., and Sun, Y.H., Appl. Catal., A, 2005,
vol. 294, p. 188.
8. Dubreuil, J.F. and Bazureau, J.P., Tetrahedron Lett.
,
2000, vol. 41, p. 7351.
KINETICS AND CATALYSIS Vol. 52
No. 3
2011