Synthesis of propylene glycol methyl ether catalyzed by MCM-41

Article abstract of DOI:10.1080/00397911003707089

In this work, we found that MCM-41 prepared using cetyltrimethyl ammonium bromide (CTAB) as the template could be used as a heterogeneous catalyst for the reaction of methanol with propylene oxide to produce propylene glycol methyl ether. 1-Methoxy-2-propanol was the predominant product. The influence of ratio of the reactants, reaction temperature, and time on the yield and selectivity was studied. The as-prepared MCM-41 proved to be an efficient and reusable catalyst, and the separation of the catalyst form the product was very easy.

Full text of DOI:10.1080/00397911003707089

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Synthesis of Propylene Glycol Methyl
Ether Catalyzed by MCM-41
a
a
a
a
Shuguang Liang , Yinxi Zhou , Huizhen Liu , Tao Jiang & Buxing
a
Han
a
Beijing National Laboratory for Molecular Sciences (BNLMS) ,
Center for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences , Beijing , China
Published online: 25 Feb 2011.
To cite this article: Shuguang Liang , Yinxi Zhou , Huizhen Liu , Tao Jiang & Buxing Han (2011)
Synthesis of Propylene Glycol Methyl Ether Catalyzed by MCM-41, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:6, 891-897, DOI:
10.1080/00397911003707089
To link to this article: http://dx.doi.org/10.1080/00397911003707089
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1
Synthetic Communications , 41: 891–897, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003707089
SYNTHESIS OF PROPYLENE GLYCOL METHYL ETHER
CATALYZED BY MCM-41
Shuguang Liang, Yinxi Zhou, Huizhen Liu, Tao Jiang, and
Buxing Han
Beijing National Laboratory for Molecular Sciences (BNLMS), Center for
Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, China
GRAPHICAL ABSTRACT
Abstract In this work, we found that MCM-41 prepared using cetyltrimethyl ammonium
bromide (CTAB) as the template could be used as a heterogeneous catalyst for the reaction
of methanol with propylene oxide to produce propylene glycol methyl ether. 1-Methoxy-2-
propanol was the predominant product. The influence of ratio of the reactants, reaction tem-
perature, and time on the yield and selectivity was studied. The as-prepared MCM-41
proved to be an efficient and reusable catalyst, and the separation of the catalyst form
the product was very easy.
Keywords CTAB; MCM-41; methanol; propylene glycol methyl ether (PGME); propy-
lene oxide (PO)
INTRODUCTION
Propylene glycol methyl ether (PGME) is widely used as a solvent because of
its ether bond and hydroxyl group, which are hydrophobic and hydrophilic, respect-
ively. It is considered to be a safe substitute for toxic ethylene glycol ether because of
[
1]
the negligible toxicity of propylene glycol ether. PGME is generally derived from
the addition of methanol (MeOH) to propylene oxide (PO) catalyzed by an acid
or base, and the product obtained by a basic catalyst is different from that produced
[
2,3]
by an acidic catalyst because of the different mechanisms,
as shown in Scheme 1.
The epoxide ring of PO might open at either of the C-O bonds to form primary
or secondary alcohols. Compared to secondary alcohol, the primary alcohol (such as
[
-methoxy-1-propanol) has a much greater toxicity. Therefore, high selectivity to
4]
2
Received August 9, 2009.
Address correspondence to Tao Jiang or Buxing Han, Beijing National Laboratory for Molecular
Sciences (BNLMS), Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: jiangt@iccas.ac.cn; Hanbx@iccas.ac.cn
8
91

892
S. LIANG ET AL.
Scheme 1. Mechanism of addition of methanol to propylene oxide.
the secondary alcohol is preferred for this process. The ring of PO might preferentially
open at the least-sterically-hindered position over a basic catalyst, leading to the pre-
dominant production of secondary alcohol 1-methoxy-2-propanol. However, acidic
catalysts provide a mixture of secondary and primary alcohols; the proportion of
the two products depends on the acid strength. So, utilization of basic catalysts has
attracted much attention. To overcome the separation and corrosion problems of
[
5]
[6]
liquid catalysts, solid bases, such as anionic double hydroxide clays, basic zeolites,
and basic metal oxide,
[7–9]
have been used to synthesize propylene glycol ether.
Until now, various micelle-templated silicas (MTS), such as MCM-41,
[
10]
[
MCM-48, SBA-3,
11]
[12]
[13]
SBA-15, and MSU, have been reported, and many of
[14]
them have been used as heterogeneous basic catalysts or as supports for immobiliz-
ing acidic and=or basic catalysts. Both organic and inorganic acids and bases have
[
15–18]
been used as the active components in the supported catalysts.
Quaternary cationic surfactant C H
N(CH ) Br (n ¼ 8–22) solutions are
n
2nþ1
3 3
commonly used as templates for the synthesis of ordered mesoporous silicate materi-
als. Most of the surfactant stays in the pores of the as-synthesized materials after sep-
aration from the mother liquor. The as-synthesized MCM-41 has been used as a
reusable, heterogeneous catalyst for the synthesis of cyclic carbonates via cycloaddi-
tion of CO with epoxides and for N-alkyl or N-aryl carbamates via the reactions of
2
[
19]
amines, CO , and alkyl halides.
In this paper, we found that the as-synthesized
MCM-41 was an effective catalyst for the reaction of PO and methanol to produce
-methoxy-2-propanol predominantly. This catalyst can be easily separated from the
2
1
product and reused. The method to prepare the catalyst is simple.
EXPERIMENTAL
MCM-41 was prepared according to the procedures described in the litera-
10,11,20]
and the product was extracted using ethanol by Soxhlet extraction to
[
ture,
ꢀ
remove the extractable surfactant. MCM-41 was dried under 50 C in a vacuum.
Then MCM-41 (3 g) was added into 120 mL (0.5 M) of tetrahydrofuran (THF) sol-
ution of NH ꢁ H O (28% aqueous solution), and the resulting mixture was vigor-
3
2
ously stirred at room temperature for 12 h. The solid was collected by filtration,
ꢀ
washed with water, and dried at 80 C for 5 h under vacuum to afford the catalyst.
Infrared (IR) spectra of the catalyst were recorded on a Bruker Tensor Fourier

SYNTHESIS OF PGME
893
transform 27 (FT)–IR spectrometer as KBr pellets. Transmission electron micro-
graph (TEM) images were obtained on a Philips TECNAI F-30 FEG instrument
at an accelerating voltage of 300 kV. The sample was dispersed with ethanol.
Thermogravimetric (TG) analysis of the catalysts were carried out on a TA Q50
instrument using a platinum pan under an air (60 ml=min) atmosphere from ambient
ꢀ
ꢀ
tempera here to 800 C with a heating rate of 20 C=min.
The catalytic reaction was carried out in a stainless-steel batch reactor of
6
.5 mL at the desired MeOH-to-PO molar ratios under magnetic stirring. After reac-
ꢀ
tion for 3 h at 90–140 C, the reactor was cooled to room temperature. Product
analysis was performed on an Agilent 6820 GC instrument with a FID detector
and a Supelcowax 10 capillary column (30 m ꢂ 0.25 mm ꢂ 0.25 mm). 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
-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.
1
RESULTS AND DISCUSSION
A TEM image of the as-prepared MCM-41 is shown in Fig. 1. Regular and
uniform porous structure can be observed. Solvent is extracted by a mild and
efficient method to remove surfactants and get porosities without distinct effect on
[21]
the frameworks.
Most of the surfactant could be removed, and a small amount
of surfactant molecules, which were more strongly bonded to the inorganic hosts,
remained in the pores. In this work, the as-prepared MCM-41 was extracted for dif-
ꢀ
ferent times, and the activity of the catalysts was tested at 110 C with a reaction time
of 3 h. 1-Methoxy-2-propanol was formed as the main product. As shown in Fig. 2,
the extraction time did not affect the yield of PGME and the selectivity.
The FT-IR analysis was performed to characterize the catalyst further. The
ꢃ1
ꢃ1
ꢃ1
bands at 2927 cm and 2851 cm represented the stretching mode of C-H, and
ꢃ1
the band at 1481 cm was also assigned to C-H. The band at 1229 cm was attrib-
uted to the stretching mode of C-N. This confirmed the existence of cetyltrimethyl
ammonium bromide (CTAB) in the pores of MCM-41. TG analysis indicated that
Figure 1. TEM image of the MCM-41 synthesized in this work.

894
S. LIANG ET AL.
Figure 2. Effect of the extraction time of the catalyst using ethanol on the reaction. Reaction conditions:
ꢀ
PO, 20 mmol; MeOH=PO molar ratio, 3:1; catalyst, 0.1 g; temperature, 110 C; reaction time, 3 h.
the contents of CTAB in MCM-41 extracted for different times were 1.19 mmol=g
for 2 days, 1.03 mmol=g for 4 days, and 0.94 mmol=g for 7 days, respectively. As
mentioned, in the experiments, MCM-41 was treated with the THF solution of
NH ꢁ H O. The ion exchange between Br- and OH- (NH OH) took place, and
3
2
4
CTAB should exist in the form of a base, cetyltrimethyl ammonium hydroxide
(
CTAOH), which was responsible for the catalytic activity. The MCM-41 calcined
ꢀ
at 550 C for 8 h was not active for the reaction, while the noncalcined MCM-41
exhibited excellent activity (Fig. 2). This further confirmed that the CTAOH was
the active component in the catalyst.
Figure 3 shows the influence of MeOH=PO molar ratio on the yield of PGME
ꢀ
and the selectivity to 1-methoxy-2-propanol at 110 C for 3 h. With the increase of
Figure 3. Effect of MeOH=PO molar ratio on the reaction. Reaction conditions: PO, 20 mmol; catalyst,
ꢀ
0.1 g; temperature, 110 C; reaction time, 3 h.

SYNTHESIS OF PGME
895
Figure 4. Effect of the reaction time on the reaction. Reaction conditions: PO, 20 mmol; MeOH=PO molar
ꢀ
ratio, 3:1; catalyst, 0.1 g; temperature, 110 C.
the molar ratio, PO could completely react with MeOH, and the yield of PGME
rapidly increased from 62% to 92%. When the molar ratio was more than 3:1, the
yield did not increase further. The selectivity decreased by only 1.5% when the molar
ratio increased from 1:1 to 5:1.
The dependence of the yield and selectivity on the reaction time is presented in
Fig. 4. The yield of PGME increased with increasing reaction time up to 3 h. It can
be seen that a yield of 92% could be achieved at 3 h. No further increase in the yields
was observed with longer reaction times. The selectivity remained at about 93.5%.
Therefore, a reaction time of 3 h was suitable for this reaction system.
Figure 5 shows the dependence of the yield of PGME and the selectivity on the
reaction temperature. The yield increased from 69% to 92% when the temperature
Figure 5. Effect of reaction temperature on the reaction. Reaction conditions: PO, 20 mmol; MeOH=PO
molar ratio, 3:1; catalyst, 0.1 g; reaction time, 3 h.

896
S. LIANG ET AL.
Figure 6. Recyclability of the catalyst. Reaction conditions: PO, 20 mmol; MeOH=PO molar ratio, 3:1;
ꢀ
catalyst, 0.1 g; reaction time, 3 h; temperature, 110 C.
ꢀ
ꢀ
increased from 90 C to 110 C and did not change when the temperature was
increased further. The selectivity was almost independent of temperature in the
temperature range studied.
The reusability of the catalyst was also examined at the optimized reaction con-
ditions, and the results are shown in Fig. 6. The yield of PGME decreased slightly
after the catalyst was reused four times, and the selectivity stayed constant during
the each run.
CONCLUSION
The CTAOH encapsulated in MCM-41 is an excellent catalyst for the reaction
of methanol with PO to produce PGME. 1-Methoxy-2-propanol is the predominant
product. At the optimized reaction conditions, the yield of PGME and the selectivity
to 1-methoxy-2-propanol are as high as 92% and 93.5%, respectively. The method to
prepare the catalyst is very simple. The catalyst can be recovered easily and reused
for at least four cycles.
ACKNOWLEDGMENTS
The authors are grateful to the Ministry of Science and Technology of China
2006CB202504, 2009CB930802) and the National Natural Science Foundation of
(
China (20932002).
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