Studies on mechanism for homogeneous catalytic hydration of ethylene oxide: Effects of pH window and esterification

Article abstract of DOI:10.1016/j.catcom.2009.11.020

Selective hydration of ethylene oxide (EO) was investigated with several inorganic salt systems as homogeneous catalysts. By optimizing reaction conditions, the highest monoethylene glycol (MEG) selectivity of 98% was obtained with >99% EO conversion at a water/EO molar ratio = 10. The effects of pH value and anion addition-esterification on MEG selectivity were systematically studied, and a comprehensive mechanism was proposed based on the results. The conclusion should be useful in developing high performance catalysts for the manufacture of MEG by EO hydration at a low water/EO ratio. Crown Copyright

Full text of DOI:10.1016/j.catcom.2009.11.020

Catalysis Communications 11 (2010) 447–450
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Studies on mechanism for homogeneous catalytic hydration of ethylene oxide:
Effects of pH window and esterification
*
*
Zhi-Jian Yang, Nan Ren , Ya-Hong Zhang, Yi Tang
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis, Innovative Materials and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2009
Received in revised form 15 November 2009
Accepted 21 November 2009
Available online 26 November 2009
Selective hydration of ethylene oxide (EO) was investigated with several inorganic salt systems as homo-
geneous catalysts. By optimizing reaction conditions, the highest monoethylene glycol (MEG) selectivity
of 98% was obtained with >99% EO conversion at a water/EO molar ratio = 10. The effects of pH value and
anion addition–esterification on MEG selectivity were systematically studied, and a comprehensive
mechanism was proposed based on the results. The conclusion should be useful in developing high per-
formance catalysts for the manufacture of MEG by EO hydration at a low water/EO ratio.
Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
Keywords:
Ethylene oxide
Catalytic hydration
Monoethylene glycol
pH window
Esterification
1. Introduction
such as carboxylates, carbonates, phosphates and sulfites were
evaluated under both acidic and basic conditions. The effects of
Monoethylene glycol (MEG) has been widely used as an inter-
mediate to make various products, such as polyester fiber, anti-
freeze, lubricant and so on. MEG is commercially produced by
thermal hydration of ethylene oxide (EO) with significant
amounts of by-products diethylene glycol (DEG) and triethylene
glycol (TEG). In pursuit of high selectivity to MEG, a large excess
amount of water (about 30 mol water/mol EO) has to be used,
which ultimately increases production cost due to high enormous
energy consumption occurred during the distillation process to
remove water. Therefore, considerable efforts have been made
to explore efficient catalysts working at a low water/EO ratio.
The catalysts reported in the literature include anion exchange
resins [1–4], supported metal oxides and zeolites [5–8], quater-
nary phosphoniumhalides [9], polymeric organosilane ammonium
salts [10] and macrocyclic chelating compounds [11]. However,
none of the catalysts has been developed to commercial applica-
tion. The resins swell and deactivate under reaction conditions.
The preparation of other catalysts is too complicated or expen-
sive. On the other hand, some low cost soluble salts were re-
ported as good homogeneous catalysts for EO hydration [12–
15]. We chose to study reaction mechanism of EO hydration in
the presence of these low cost soluble salts as homogeneous cat-
alysts. The catalysts, inorganic salts and their acid counterparts
pH value and addition–esterification of anion were also studied.
Based on our observations, a comprehensive mechanism for EO
hydration was proposed, which should be useful in optimizing
catalyst composition and reaction conditions, and exploring more
efficient catalysts.
2. Experimental
2.1. Materials
All the chemicals used in the present work were purchased and
employed without further purification or treatment.
2.2. Catalytic testing
The hydration reaction of EO was carried out in an isothermal
batch autoclave reactor with a H2O/EO molar ratio of 10 under con-
tinuous stirring. The reaction temperature was maintained at
120 °C and the pressure was kept as 1.5 MPa by N2. The amount
of catalyst used was normally 0.20 mol/mol EO. After 1 h reaction,
the reactor was cooled down and the products were analyzed on a
GC920 gas chromatograph equipped with a HP-INNOWAX column
and a FID detector. HP6890/5973 GC–MS was used to verify some
other by-products. As EO conversion with any catalyst was nearly
100%, MEG selectivity was taken into account as the criterion for
comparison of catalyst performance.
* Corresponding authors. Tel.: +86 21 55664125; fax: +86 21 65641740.
E-mail addresses: nanren@fudan.edu.cn (N. Ren), yitang@fudan.edu.cn (Y. Tang).
1566-7367/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.11.020

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Z.-J. Yang et al. / Catalysis Communications 11 (2010) 447–450
Table 1
3. Results and discussion
3.1. Reaction results
Effects of alkyl length and b-chlorine substitution for carboxyl acid systems on MEG
selectivity.
Catalyst type
Catalyst amount
(mol/mol EO)
pH
MEG selectivity
(%)
3.1.1. The effect of pH value
HCOONa–HCOOH
0.2
0.2
0.2
0.1
0.1
0.1
3.7
3.8
3.8
3.8
3.5
3.5
97
93
91
89
91
94
Five soluble salts and their conjugated acids were used as cata-
lysts for the hydration of EO. Fig. 1 shows MEG selectivities of these
five catalysts versus pH value. The pH values of the reaction solu-
tions were controlled by adjusting the ratio of the salts and their
conjugated acids. To avoid the possible influence of catalyst con-
CH3COONa–CH3COOH
C2H5COONa–C2H5COOH
CH3COONa–CH3COOH
CHCl2COONa–CHCl2COOH
CCl3COONa–CCl3COOH
centration on the reaction, the total amount of the anions (AÀ
=
HCOOÀ, CH3COOÀ, CO32À, PO43À or SO32À) in each reaction solution
was kept constant at 0.20 mol/mol EO. Although MEG selectivities
with these catalysts are quite different, their variations with pH va-
lue are similar. The best selectivity with each catalyst was obtained
when the pH value is in the ranges of 3.0–6.0 and 7.5–10.0, respec-
tively, which are defined as two pH windows. Similar results were
reported previously [16,17]. Although strong acidic or basic cata-
lysts can accelerate reaction rate of the hydration, a low MEG
selectivity was obtained. The same group also reported the EO
hydration can be catalyzed by soluble amine, bifunctional (EDTA
family) and salen compounds, and a catalyst with high MEG selec-
tivity requires a pH value ranging from weakly acidic to weakly ba-
sic [17].
and consequently increase their esterification capability. The ob-
served high MEG selectivity in the present might be attributed to
the esterification effect of carboxyl groups.
The esterification effect of acid anions on MEG selectivity is
shown in Fig. 2. The pH value was controlled by fixing CH3COO-
Na/CH3COOH (pH = 3.7) and Na2CO3/NaHCO3 (pH = 9.8) ratios.
The two anions have the same effect. The selectivity rapidly in-
creases with the amount of catalyst used till its concentration
reaches 0.15 mol/mol EO after which no significant increase is ob-
served. The highest MEG selectivity in both systems is over 97%.
Again, almost complete EO conversion was achieved.
The acetates with Na+, K+ and Ca2+ were examined for the
hydration under identical conditions to investigate the effect of
cations. It was found that the MEG selectivity varied very little in
the presence of the acetate catalysts solutions with different cat-
ions, indicating that the influence of the cations on this reaction
can be ignored.
3.1.2. Effects of anion and concentration
Besides the effect of pH value, it is also observed in Fig. 1 that
the catalysts with different acid anions give different selectivities.
The formate and carbonate are the best in the acidic and basic pH
windows, respectively, whereas the sulfite results in the lowest
selectivity in both windows. In order to study the effect of the an-
ions on the hydration, carboxyl acid with different alkyl chain
length and b-chlorine substitution were investigated (Table 1).
The pH value of the reaction solution was kept at 3.5–3.8 by adding
conjugated salts, and the total anion concentrations were 0.20 and
0.10 mol/mol EO in these two cases, respectively. The results show
MEG selectivity decreases with increasing alkyl length and in-
creases with increasing chlorine substitution (Table 1). Liu et al.
[18] investigated the effect of the alkyl chain length of carboxylic
acid on the kinetics of acid-catalyzed esterification in liquid-phase,
and found that esterification rate decreased as alkyl chain length
increased. It was suggested that the chlorine substitution on b-car-
bon can decrease the electron density on the carbonyl in chloroace-
tic acids due to the electron-withdrawing substituent group [19],
3.1.3. Effects of reaction temperature and water/EO ratio
Fig. 3 shows the effects of reaction temperature and water/EO
ratio on EO conversion and MEG selectivity using sodium formate
as catalyst. MEG selectivity slightly increases with reaction tem-
perature up to 120 °C, but decreases rapidly at higher tempera-
tures. The highest MEG selectivity is about 97% at 100–120 °C.
Water/EO ratio strongly influences MEG selectivity. At a water/
EO ratio of 10, the MEG selectivity is ca. 97%, further increasing
of water/EO ratio has negligible influence on the MEG selectivity.
In any case, EO conversion was high.
For comparison, the additive results of thermal hydration on the
condition of different reaction temperature and water/EO ratio are
shown in Fig. 3. It is found that the EO conversion in thermal
hydration is much lower than the formate catalyzed system up
to a reaction temperature of 120 °C for its lower reaction rate at
low temperature, and almost no difference is found between blank
Fig. 2. Effect of catalyst concentration on MEG selectivity and EO conversion for
Fig. 1. MEG selectivities of different catalysts versus pH value.
carbonate and acetate systems.

Z.-J. Yang et al. / Catalysis Communications 11 (2010) 447–450
449
Fig. 3. Effects of (a) reaction temperature and (b) water/EO ratio on EO conversion and MEG selectivity in formate catalyzed and thermal hydration (blank test).
and catalyzed reactions with the changing of water/EO ratios.
However, MEG selectivity in thermal hydration is much lower than
the catalyzed system. Even at a water/EO of 20, the MEG selectivity
of blank system is still lower than formate catalyzed system at a
water/EO of 5.
3.2. Mechanism of EO hydration in acidic and basic solutions
As shown in Section 3.1, the pH value plays a key role on MEG
selectivity, and esterification effect of acid anions further enhances
the selectivity. Similarly, Shvets et al. [12,20–22] studied the kinet-
ics of EO hydration using homogeneous salts as catalysts and sug-
gested a possible process of ester formation. van Hal et al. [16]
proposed a reaction mechanism to explain a good correlation be-
tween catalyst performance and its acidity/basicity. However, it
seems difficult to explain why MEG selectivity increases with the
increasing of the catalyst concentration since pH value of the reac-
tion solution was kept constant. On the basis of the facts found in
this paper and those reported previously, a more comprehensive
mechanism was proposed, as shown in Fig. 4.
For acid-catalyzed reaction (Fig. 4a), a protonated EO (EOH+) is
formed as an initial active intermediate. In thermal hydration
without any catalyst, H2O can attack a carbon atom in the EOH+
through SN2 mechanism, producing one MEG with releasing of a
proton (Route 1). At the same time, the EOH+ can also be attacked
by an unprotonated EO to form undesirable by-products of oligo-
mers through Route 3. Therefore a large amount of excess water
Fig. 4. Schematic diagrams of reaction mechanisms in (a) acidic and (b) basic
is necessary to ensure the high MEG selectivity by promoting Route
1 [23]. In the presence of an acid catalyst, the anion AÀ can also at-
tack the carbon atom in the EOH+ to form an ester intermediate
leading to MEG with releasing of a proton (Route 2). These three
routes proceed in parallel and are competitive, and the MEG selec-
tivity is ultimately determined by their relative rates which de-
pend on pH value and esterification capability of the anions.
The results reported in this paper may well be explained in this
mechanism. For acidic systems, when its pH value is low, the
strong acidity may accelerate the rate of the EOH+ formation. How-
ever, Route 2 would be suppressed because the anion species pre-
dominantly exist in the form of unionized form in the solution at a
low pH value, thus resulting in the relatively low MEG selectivity.
On the other hand, if the acidity is too weak, the rate of EOH+
formation would be low because of the low concentration of
H3O+, consequently a large amount of unprotonated EO will favor
the oligomerization through Route 3. When the AÀ anions with
high esterification capability exist in the reaction system, such as
formate and acetate, the route via the ester intermediate (Route
2) will occur in addition to the traditional hydration by water
systems.
(Route 1), enhancing overall MEG selectivity. Such a promotion ef-
fect could be optimized by careful selection of the type (Table 1) or
concentration (Fig. 2) of acid anions.
The mechanism of EO hydration in basic systems (Fig. 4b) is
rather similar to that in acidic solutions. The EO molecule is first
activated by hydroxyl anion in the solution to form an intermedi-
ate that will further react via three competitive routes in parallel.
When a catalyst is added, Route 2 via ester intermediate will
greatly enhance the formation of MEG. A catalyst concentration
up to 0.15 mol/mol EO will benefit for the esterification and there-
by enhance the MEG selectivity.
It is also noted that MEG selectivity in some solutions, such as
sulfite, is even lower than that of thermal process without any cat-
alyst (Fig. 1). To understand the reason for the low MEG selectivity,
pH values of all the catalyst solutions were measured before and
after the reaction. It is found that the pH values of the solutions

450
Z.-J. Yang et al. / Catalysis Communications 11 (2010) 447–450
Table 2
Difference of pH value before and after reaction corresponding to the MEG selectivity for different systems.
Carboxylate systems
Selectivity (%)
Phosphate system
Selectivity (%)
Carbonate system
Selectivity (%)
Sulfite system
Selectivity (%)
pH rising
pH rising
pH rising
pH rising
84
92
94
96
96
97
97
98
96
96
97
0.8
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.5
0.3
83
85
85
86
88
69
57
64
62
59
54
0.1
0.3
0.1
À0.1
1.0
2.2
4.3
4.6
3.6
2.0
1.6
96
97
97
95
90
–
–
–
–
–
0.2
0.3
0.3
0.3
0.5
–
–
–
–
–
63
67
61
67
–
–
–
–
–
3.2
5.9
6.7
4.8
–
–
–
–
–
–
–
–
–
–
–
with low MEG selectivity dramatically increased after the reaction,
while those with high MEG selectivity only had minor change
throughout the whole reaction process (Table 2). As can be seen,
the pH value increase after the reaction is 2.0 or more for the sulfite
system with low MEG selectivity (<70%), while that for the systems
with high selectivities (>90%), such as carboxylate and carbonate, is
typically 50.5. The same correlation between pH value increase
and MEG selectivity was also observed with the phosphate system
covering a wider range of pH value. It is interesting to note the
sudden transition of pH value as the selectivity decreases from
>85% to <70%. GC–MS results confirmed the formation of glycol
oligomers in solutions encountered pH rising, such as DEG, TEG
and tetraethylene glycol. These by-products containing more ester
linkages can strongly adsorb water and capture H3O+, leading to
the pH of reaction solution alkalization. Once the alkalization hap-
pens, the solution might jump out the pH window to produce more
by-products. And the by-products will further alkalize the solution
through capture H3O+, causing the reaction system to enter a vi-
cious circle and further lower MEG selectivity.
Acknowledgements
This work was supported by NSFC (20721063), STCSM
(075211013 and 08DZ2270500) and Major State Basic Research
Development
Program
of
China
(2009CB623502
and
2009CB623506).
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Several inorganic salt systems were investigated as homoge-
neous catalysts for EO hydration. The comprehensive mechanisms
with both acidic and basic catalysts were proposed based on the
experimental observations. Two pH windows having pH values of
3.0–6.0 and 7.5–10.0 for optimizing the MEG selectivity were dis-
covered. Moreover, the esterification effect of the acid anion is also
proved to enhance the MEG selectivity. Formate, acetate and
carbonate are found to be the best catalysts because of their
adjustable pH range and suitable esterification capability. By opti-
mizing the reaction conditions, the highest MEG selectivity of 98%
was obtained with >99% EO conversion under the water/EO molar
ratio of 10. The results revealed in this paper should be useful in
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