One-pot synthesis of propylene glycol and dipropylene glycol over strong basic catalyst

Article abstract of DOI:10.1016/j.catcom.2010.01.004

The synthesis of propylene glycol (PG) and dipropylene glycol (DPG) was carried out by the hydrolysis of propylene oxide over solid base catalysts. Among them, sol-gel derived Na₂O-ZrO₂ showed the excellent performance. It was found that Na₂O-ZrO₂ had a mesoporous framework in which Na₂O nanoparticles were homogeneously dispersed. Such a structure led to the strong basicity and then the excellent performance in the hydrolysis of propylene oxide. As a result, one-pot synthesis of propylene glycol (PG) and dipropylene glycol (DPG) could take place at a low H₂O/PO ratio of 3 without any condensation reactions. Crown Copyright

Full text of DOI:10.1016/j.catcom.2010.01.004

Catalysis Communications 11 (2010) 675–678
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
One-pot synthesis of propylene glycol and dipropylene glycol over strong
basic catalyst
Zhuo Liu , Wenbo Zhao , Fukui Xiao , Wei Wei a,*, Yuhan Sun
a,b
a
a
a,*
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 April 2009
Received in revised form 4 January 2010
Accepted 7 January 2010
Available online 28 January 2010
The synthesis of propylene glycol (PG) and dipropylene glycol (DPG) was carried out by the hydrolysis of
propylene oxide over solid base catalysts. Among them, sol–gel derived Na
2
O–ZrO
2
showed the excellent
O nanoparticles
performance. It was found that Na O–ZrO had a mesoporous framework in which Na
2
2
2
were homogeneously dispersed. Such a structure led to the strong basicity and then the excellent perfor-
mance in the hydrolysis of propylene oxide. As a result, one-pot synthesis of propylene glycol (PG) and
2
dipropylene glycol (DPG) could take place at a low H O/PO ratio of 3 without any condensation reactions.
Keywords:
Propylene glycol
Dipropylene glycol
Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
2 2
Strong basic catalyst Na O–ZrO
1
. Introduction
Solid base catalysts play a decisive role in a large number of
tion of PG can also be conducted by taking base as the catalyst, but
it was hardly reported. Herein the synthesis of PG and DPG were
carried out with solid base catalysts. This provides an effective
reactions essentially for fine chemical synthesis. Compared to
homogeneous catalysts, which lead to the problems of products
separation and catalysts recycle, solid base catalysts are noncorro-
sive and present fewer disposal problems, and they allow both eas-
ier separation and recovery of the products, catalysts, and solvent.
Therefore, solid base catalysts are expected to offer environmen-
tally benign and more economical pathways for the synthesis of
fine chemicals, which have attracted much attention in recent
years [1–8]. In the present work, several solid base catalysts, espe-
and facile route for the synthesis of PG and DPG at a low H
ratio of 3 and mild condition.
2
O/PO
2. Experimental
2.1. Catalyst preparation
Typically, 1 g of amphiphilicpoly (alkyleneoxide) blockcopoly-
mers PEO20PPO70PEO20, Pluronic P123) was dissolved in a designed
amount of absolute ethanol to form solution A. At the same time,
4.45 g of zirconium(IV) n-propoxide (23–28% free alcohol, Strem
Chemicals) were mixed with 0.5 g of acetylactone (acac) under
stirring to form solution B, in which acetylactone acted as a stabi-
lizer to prevent the zirconium(IV) n–propoxide from uncontrolla-
ble hydrolysis in the following step. Afterwards, solution B was
slowly added to solution A under vigorous stirring. Upon stirring
at room temperature for 1 h, 1.8 g of deionized water was added
dropwise. The mixture, with molar ratio of 1 Zr:0.02 P-123:0.5
2 2
cially Na O–ZrO , which had been used in our laboratory in the
synthesis of both dimethyl carbonate (DMC) and propylene glycol
methyl ether (PGME) [9–11], were investigated for the propylene
oxide hydrolysis.
As known, propylene glycol (PG), which can be used as unsatu-
rated polyester resins, surface coating, non-ionic detergent and
antifreeze, is the main product in the hydrolysis of propylene
oxide. The technical-grade PG is prepared by catalytic hydration
of propylene oxide with sulfuric acid as catalyst, which corrodes
the equipments and pollutes the environment. The production is
also carried out by non-catalytic hydration using a great deal of
water as one of the reactants, which greatly increase the energy
expenditure of product separation [12]. This is the same for the
production of dipropylene glycol (DPG) [13]. Actually, the produc-
2
acac:80 EtOH:10 H O, was gelled in a closed vessel at 40–60 °C
for 24 h. The obtained transparent resin hybrid was smashed and
partly refluxed in a Teflon vessel that contained aqueous solution
À1
of 0.25–1.0 mol L NaOH for another 24 h. Then, the suspension
+
was filtering without wash to remove the free Na in the solution.
For the removal of the surfactant species, finally, the samples were
À1
heated in flowing N
were denoted as xNa
2
at a rate of 1 °C min to 700 °C. The products
*
Corresponding authors. Tel.: +86 351 4049612 (Y. Sun).
E-mail addresses: weiwei@sxicc.ac.cn (W. Wei), yhsun@sxicc.ac.cn (Y. Sun).
2
O–ZrO
2
, herein x stands for the mass fraction
1
566-7367/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.01.004

6
76
Z. Liu et al. / Catalysis Communications 11 (2010) 675–678
of Na in Na
2
O–ZrO
2
. ZrO
2
, MgO–ZrO
2
and CaO–ZrO
2
were synthe-
920, Shanghai Haixin Chromatograph Instrument Co. Ltd.) with a
sized following the literatures [14,15].
flame ionization detector and a HP-5 column after filtration from
P
the catalyst. The selectivity was defined as m
i
/
m
i
 100, where
P
2.2. Characterization
m
i
was the molar of product of i, and
m
i
was the total molars
of the products.
The materials were characterized by a Rigaku D/max-A X-ray
diffractometer (XRD) with k = 0.1541 nm, Cu K radiation in the
h range of 10–90° with the step of 0.02° at room temperature.
a
2
3
. Results and discussion
Their specific surface areas were measured with a Tristar 3000 ana-
lyzer using the multipoint Brunauer, Emmett and Teller (BET)
adsorption. Powder X-ray diffraction (XRD) experiments were car-
ried out on a Rigaku Miniflex diffractometer using a Cu target with
a Ni filter in a 2h range of 10–80°. And the X-ray gun was operated
at 50 kV and 30 mA, using a scan speed rate of 0.2°/min. The total
3
.1. Textural structure and phase
The porosity of Na
contents were determined by N
As shown in Fig. 1, their N adsorption–desorption isotherms dis-
played type IV isotherms with clear hysteresis loops associated
with capillary condensation, indicating the existence of mesopor-
ous framework. Furthermore, BET surface area of those samples
gradually decreased with the Na content. It was also found that
with the Na content increased, the type IV adsorption isotherms
became unconspicuous (see Table 1). This suggested the increase
of Na content had a negative effect on the mesoporous framework
2
O–ZrO
2
solid bases with different sodium
adsorption–desorption technique.
2
2
2
basicity and base strength of the samples were measured by CO -
TPD. About 0.1 g of sample were heated in flowing Ar (99.99%) at a
À1
rate of 5 °C min to 700 °C and kept at 700 °C for 1 h. When the
2
temperature elevated, the CO desorbed was detected by a Balza
Q-Mass spectrometer.
2.3. Catalytic test
of Na
2 2
O–ZrO solid bases.
The catalytic performance of so-produced solid base was evalu-
ated in the synthesis of propylene glycol and dipropylene glycol
from propylene oxide and H O. The reaction was carried out in a
stainless steel autoclave reactor with an inner volume of 150 ml.
The standard procedure is as follows: 5.80 g of propylene oxide
Fig. 2 illustrates the wide-angle XRD patterns of Na
2
O–ZrO
2
with different sodium content. It could be seen that only the dif-
fraction peak of tetragonal zirconia was observed for the catalyst
with the Na content of 0.05–0.20. Soler-Illia and Ozin [16,17] found
that the presence of hetero-atom such as Si and Y in the ZrO skel-
2
eton could reduce the contraction of the mesostructure, which in-
creased the structural stability of the mesoporous zirconia. In the
present case, the introduction of Na element into ZrO
lize the tetragonal zirconia and then Na O nanoparticles might be
homogeneously dispersed in the mesoporous zirconia framework
at the Na content of 0.05–0.20. As a result, the structure of
2
(
PO), 5.40 g of distilled water and a certain amount of catalyst were
introduced into the autoclave. The reaction was carried out at 90–
30 °C for 1–4 h under autogeneous pressure, and the autoclave
1
2
might stabi-
was heated and magnetically stirred constantly during the reac-
tion. Moreover, other 3 heterogeneous bases were used as refer-
ences. The products were analyzed by a gas chromatograph (GC-
2
Na
2
O–ZrO
2
solid bases was greatly influenced by their
composition.
2
1
1
00
50
00
0
0
0
0
.05Na-ZrO2
.10Na-ZrO2
.15Na-ZrO2
.20Na-ZrO2
3.2. Basicity
The basic strength and basicity of different solid basic catalysts
were estimated by CO -TPD (see Fig. 3). Except ZrO , two distinct
desorption peaks were observed for other samples, indicating
two kinds of basic sites with different basic intensity present on
their surface. Among them, the peak at 120 °C could be contributed
2
2
to the weak basic site of zirconia [18]. CaO–ZrO
basic sites due to a sharp desorption peak at 600 °C, while MgO–
ZrO had the moderate strength basic sites with a sharp desorption
peak at 300 °C. Mesoporous Na O–ZrO showed the strongest basi-
city with the peak at 700 °C. During the sol–gel process, zirconium
alkoxide and the template formed the gel after ageing, and Na
was incorporated into the ZrO and then partially inserted into
the oxygen vacancy on the ZrO surface. However, they were too
tiny to be detected by XRD (see Fig. 2). Thus, those highly-dis-
2
showed the strong
5
0
2
2
2
0
2
O
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
2
2
Fig. 1. Nitrogen adsorption–desorption isotherm of Na
sodium content.
2
O–ZrO
2
with different
persed Na
2
O gave rise to the high basicity.
Table 1
The surface area, porous channel structure and CO
2
uptake of Na
2
O–ZrO
2
solid bases.
BET (m2
g
À1
)
D
BJH (nm)
Vp (cm3
g
À1
)
CO
uptake
Samples
S
2
(l
mol/g)
(l
mol/m2)
0
0
0
0
.05 Na
.10 Na
.15 Na
2
2
2
O–ZrO
O–ZrO
O–ZrO
2
2
2
168.2
151.3
105.0
86.6
3.2
6.2
7.4
5.6
0.2
0.3
0.3
0.2
101.3
171.3
244.0
268.7
0.6
1.1
2.3
3.1
2 2
.2 Na O–ZrO
xNa
2
O–ZrO
2
, herein x stands for the mass fraction of Na in Na
2
O–ZrO2.

Z. Liu et al. / Catalysis Communications 11 (2010) 675–678
677
In general, PG could be synthesized by PO and water, and then
PG and PO reacted into DPG. Afterwards, TPG was produced by PO
and DPG. During these reactions, no PG or DPG self-condensation
were observed. As a result, the sodium contents had a great influ-
ence on the PO conversion (see Fig. 4). The conversion of PO in-
creased with the Na content, which was due to the improvement
of basicity by the sodium. The conversion reached almost 90% for
Tetragonal 0.05Na
O-ZrO
2
2
0
.10Na O-ZrO
2
2
0
.15Na O-ZrO
2 2
.20Na O-ZrO
0
2
2
0
2 2 2 2
.20 Na O–ZrO , but 0.05 Na O–ZrO only gained no more than
4
0% at 30 min. In any case, the PO conversion finally reached al-
most 100% at 4 h.
Moreover, the reaction parameters also showed the different ef-
fect on both PO conversion and product selectivity with 0.10 Na
2
O–
ZrO as the catalyst (see Fig. 5(a–d)). With the reaction in the tem-
2
perature region from 90 to 130 °C, the selectivity of DPG decreased
a little with the temperature (see Fig. 5(a)), since the reaction was
an exothermic reaction with the higher thermodynamic equilib-
rium constant than PG. Furthermore, both PO conversion and
selectivity almost had no change after 2 h (see Fig. 5(b)). This could
be attributed to the 100% conversion of PO without any condensa-
1
0
20
30
40
50
60
70
80
2θ
2 2
Fig. 2. XRD patterns of Na O–ZrO with different sodium contents.
tion reactions. Moreover, at the molar ratio of H
DPG selectivity sharply decreased, indicating that the excessive
O was favorable for the production of the incipient product PG
see Fig. 5(c)). In addition, the PO conversion increased a little at
the amount of catalyst up to 2.0 wt% (see Fig. 5(d)), but the selec-
tivity of DPG ascended with the increase of the amount of catalyst.
2
O to PO over 3, the
H
(
2
Na O-ZrO
2
2
MgO-ZrO2
100
ZrO
2
CaO-ZrO
2
80
60
0
100
200
300
400
500
600
700
800
900
o
Temperature/ C
40
Fig. 3. CO
2
-TPD profiles of different solid bases.
0
0
0
0
.05Na O-ZrO
2
2
2
2
2
2
0
.10Na O-ZrO
2
.15Na O-ZrO
Table 2
The performance of the catalysts.
2
.20Na O-ZrO
2
0
Samples
PO conversion (%)
Selectivity (%)
0
50
100
150
200
250
t/min
PG
DPG
TPG
ZrO
MgO–ZrO
CaO–ZrO
Na O–ZrO
2
5.1
10.2
23.2
99.9
100
100
94.8
46.2
0
0
5.2
41.3
0
0
0
12.5
Fig. 4. The influence of different sodium contents of Na O–ZrO2 on the PO
conversion.
2
2
2
2
2
Reaction condition: propylene oxide 5.8 g, deionized water 5.4 g, cat 0.3 g, 2 h,
00 °C.
1
100
100
90
90
a
b
c
d
3.3. Catalytic performance
80
80
7
6
5
4
0
0
0
0
70
Table 2 gives the catalytic performance of solid bases in the
hydrolysis of propylene oxide. The main products were propylene
glycol (PG), dipropylene glycol (DPG) and tripropylene glycol
6
0
0
5
(
TPG). Among different solid bases, as-prepared mesoporous
Na O–ZrO had a remarkably activity. Compared with MgO–ZrO
and CaO–ZrO , Na O–ZrO showed the high PO conversion of
9.9%. This could be attributed to the higher charge density of
40
30
20
2
2
2
3
0
0
2
2
2
2
9
O
10
2
À
+
2À
2+
2+
10
with Na than that of O with Mg or Ca , which gave rise
0
to the increase in the basic strength (see Fig. 3). Taking the advan-
tages of both mesoporous structure and super strong base, Na O–
ZrO showed the best performance in the reaction among the
catalysts.
90 100110120130
Temperature(°C)
0
2
4
6
8
1.0 0.8 0.6 0.4 0.2
1
2
3
2
n(PO)/n(H2O) Amount of catalyst(wt%)
t/h
2
Fig. 5. (a–d) Effects of the reaction parameters.

6
78
Z. Liu et al. / Catalysis Communications 11 (2010) 675–678
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2
O/PO ratio without any condensation
[
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