Oxidative carbonylation of glycerol to glycerol carbonate catalyzed by PdCl2(phen)/KI

Article abstract of DOI:10.1016/j.apcata.2010.07.059

We present a direct and highly efficient approach for synthesizing glycerol carbonate, via the catalytic oxidative carbonylation of glycerol, using PdCl₂(phen) (phen = 1,10-phenanthroline) as catalyst with the aid of KI. The palladium catalyst loading as low as 0.25 mol% is sufficient for high conversion (92%) and selectivity (99%) at the reaction condition: 2.0 MPa CO, 1.0 MPa O₂, 140 °C, 2 h. The turnover frequency (TOF) reaches 184 h^-1. Furthermore, using crude glycerol, we achieved 85% conversion of glycerol. We discuss in detail a plausible mechanism based on PdI ₂(phen) as an intermediate. We propose that there is synergistic effect of I^- and 1,10-phenanthroline on the performance of the Pd complex catalyst. Based on the cyclic voltammograms, the reduction of PdI ₂(phen) occurred at a more positive potential than that of PdCl ₂(phen). This may explain why the catalytic activity of PdI ₂(phen) was more effective than that of PdCl₂(phen).

Full text of DOI:10.1016/j.apcata.2010.07.059

Applied Catalysis A: General 386 (2010) 188–193
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Oxidative carbonylation of glycerol to glycerol carbonate catalyzed by
PdCl (phen)/KI
2
a
a
a,b
a
a
Jianglin Hu , Jinjin Li , Yanlong Gu , Zhenhong Guan , Wanling Mo ,
a
a,b
a,b,∗
Youming Ni , Tao Li , Guangxing Li
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, Hubei 430074, PR China
Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2010
Received in revised form 27 July 2010
Accepted 28 July 2010
Available online 6 August 2010
We present a direct and highly efficient approach for synthesizing glycerol carbonate, via the catalytic
oxidative carbonylation of glycerol, using PdCl2(phen) (phen = 1,10-phenanthroline) as catalyst with the
aid of KI. The palladium catalyst loading as low as 0.25 mol% is sufficient for high conversion (92%) and
◦
selectivity (99%) at the reaction condition: 2.0 MPa CO, 1.0 MPa O2, 140 C, 2 h. The turnover frequency
−1
(
TOF) reaches 184 h . Furthermore, using crude glycerol, we achieved 85% conversion of glycerol. We
discuss in detail a plausible mechanism based on PdI2(phen) as an intermediate. We propose that there is
synergistic effect of I and 1,10-phenanthroline on the performance of the Pd complex catalyst. Based on
the cyclic voltammograms, the reduction of PdI2(phen) occurred at a more positive potential than that
of PdCl2(phen). This may explain why the catalytic activity of PdI2(phen) was more effective than that of
PdCl2(phen).
Keywords:
−
Oxidative carbonylation
Glycerol carbonate
PdCl2(phen)/KI
Valorize glycerol
Cyclic voltammetry
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
The large surplus of bio-glycerol generated by the biodiesel
tries [10]. Glycerol carbonate may also be used in place of ethylene
and propylene carbonates as a main ingredient of an electrolyte of
secondary batteries, like lithium ion batteries [11].
industry has gained considerable attention. Using bio-glycerol as
a raw material for the production of value-added chemicals may
extend and broaden the practical applications of biomass energy
sources. Therefore, it is both important and desirable [1–3]. It is
possible to transform glycerol into various glycerol derivatives,
including acrolein [4], glycerol ether [5], and 1,2-propanediol [6].
Glycerol can also be used directly as a solvent [7]. However, fur-
ther research is needed to in order to develop new and innovative
processes using glycerol.
Glycerol carbonate, an important glycerol derivative, can be
polymerized or reacted with isocyanates or acrylates for use in coat-
ings, adhesives and lubricants [8]. Glycerol carbonate’s low toxicity,
low evaporation rate, low flammability, and moisturizing proper-
ties make it ideal for use as a solvent in cosmetics, personal care
items, and medicine [9]. Furthermore, glycerol carbonate is a valu-
able intermediate for the production of glycidol, used in resins,
plastics, and throughout the pharmaceutical and cosmetics indus-
Several methods for the synthesis of glycerol carbonate have
been described. These methods are commonly based on the reac-
tion of glycerol with carbonyl sources like phosgene [12], dialkyl
carbonate [13–15], alkylene carbonate [16], urea [17], and CO2
[18,19]. However, transesterification is not entirely appropriate
because alkylene carbonate is typically prepared through a petro-
chemical process. The direct carboxylation of glycerol with CO2 is
an interesting process that can transform two waste products into
a valuable product. However, it produces a low glycerol carbonate
yield. Therefore, finding a new, direct and highly efficient approach
for synthesizing glycerol carbonate is important.
Oxidative carbonylation is one of the most important reactions
in the field of organic transformations. The reaction can produce
a variety of oxygen-containing compounds, particularly carbon-
ates [20–24]. The wide availability of CO and the attractiveness of
the process in light of its high atom economy and environmental
friendliness merit further study.
Herein, we report a direct and highly efficient approach for
synthesizing glycerol carbonate via the oxidative carbonylation of
glycerol using PdCl (phen) (phen = 1,10-phenanthroline) as cata-
lyst with the aid of KI (Scheme 1). In addition, we evaluate the
2
∗ Corresponding author at: School of Chemistry and Chemical Engineering,
Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, Hubei
mechanistic pathway using the possible intermediate PdI (phen)
4
30074, PR China. Tel.: +86 27 87543732; fax: +86 27 87544532.
2
E-mail address: ligxabc@163.com (G. Li).
as a direct catalyst in the catalytic cycle. To the best of our knowl-
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.07.059

J. Hu et al. / Applied Catalysis A: General 386 (2010) 188–193
189
Scheme 1. Catalytic oxidative carbonylation of glycerol to glycerol carbonate.
◦
edge, this promising result has not previously been reported for the
synthesis of glycerol carbonate in the literature.
pinkish yellow suspension was stirred at 40 C for 1 h, then filtered,
washed with cold water, and dried in a vacuum. A light yellow
complex was obtained.
2
2
2
. Experimental
2
.1.2. PdI (phen) and [Pd(phen) ][PF ]
Pd(OAc)2 (0.25 mmol) dissolved in ethanol/H2O (10 mL, 1:1, v/v)
2 2 6 2
.1. Catalyst preparation
was mixed with a 10 mL ethanol solution containing KI (0.60 mmol;
I/Pd = 2.4) and phen·H2O (0.25 mmol). After 30 min the resulting
precipitated pinkish brown product was filtered, washed with
ethanol, and dried in a vacuum [25]. The [Pd(phen)2][PF6]2 was
synthesized according to the literature [26].
.1.1. PdCl (phen)
2
PdCl (phen) was synthesized according to the literature [25]
2
with some modifications. The mixture of a suspension of PdCl2
1.46 mmol) and KCl (2.92 mmol, 2 eq.) in methanol (10 mL)
was stirred at room temperature overnight, and a brown yel-
low solid (K PdCl ) was generated. Then, 10 mL of water was
(
2.2. Catalytic oxidative carbonylation of glycerol
2
4
added and the solid was dissolved. Finally 1,10-phenanthroline
monohydrate(phen·H O) (1.46 mmol) was added, and the pH
The catalytic reactions were carried out in a 250 mL stainless
steel autoclave lined with Teflon and equipped with a mechani-
2
adjusted to 4.47 with hydrochloric acid (37 wt%). The resulting
Fig. 1. GC–MS chromatogram of silylated sample obtained after oxidative carbonylation of glycerol: (a) GC chromatogram; (b) MS, 1,2,3-Tris-(trimethylsilyloxy)- propane
silylated glycerol); (c) MS, 4-(((trimethylsilyl)oxy)methyl)-1,3-dioxolan-2-one (silylated glycerol carbonate); (d) MS, bis(2,2,8,8-tetramethyl-3,7-dioxa-2,8-disilanonan-5-yl)
carbonate (silylated bis(1,3-dihydroxypropan-2-yl) carbonate).
(

1
90
J. Hu et al. / Applied Catalysis A: General 386 (2010) 188–193
Table 1
a
Synthesis of glycerol carbonate by catalytic oxidative carbonylation of glycerol .
Entry
1
Catalyst
Glycerol:catalyst molar ratio
Conversion (%)
Selectivityb (%)
TOFc (h−1
)
CuCl
CuCl/phen
CuI/phen
PdCl2
PdCl2(phen)
PdCl2(phen)/PPh3
PdCl2(phen)/CuCl2
PdCl2(phen)/KI
PdCl2(phen)/KI
PdCl2/KI
10
10
10
4.4
6.8
8.4
47
36
42
56
92
92
58
93
95
95
96
96
98
97
>99
99
96
98
0.22
0.34
0.42
47
36
42
56
92
184
58
170
d
2
3
d
4
5
200
200
200
200
200
400
200
400
e
6
f
7
8
9
0
g
g
g
1
g,h
1
1
PdCl2(phen)/KI
85
a
◦
Reaction conditions: solvent DMF 30 mL, glycerol 4.60 g (50 mmol), CO 2.0 MPa, O2 1.0 MPa, 140 C, 2 h, stirring speed 800 rpm.
b
c
d
e
f
The possible byproduct was bis(1,3-dihydroxypropan-2-yl) carbonate (see Fig. 1d).
TOF (mol converted glycerol/mol cat. h).
Cu:phen = 1:1 (molar ratio).
Pd:PPh3 = 1:1 (molar ratio).
Pd:CuCl2 = 1:5 (molar ratio).
Pd:KI = 1:10 (molar ratio).
g
h
Using industrial grade glycerol 5.75 g (80 wt% glycerol, generally contains about 15 wt% of water and 5 wt% of soap).
cal stirrer. In the experiment, 44.7 mg (0.125 mmol) PdCl (phen),
dard. The mixture was centrifuged at 2500 rpm for 5 min and the
clear supernatant was analyzed with gas chromatography.
2
2
07.5 mg (1.25 mmol, 10 eq.) KI, 30 mL N,N-dimethyl formamide
(
DMF) solvent, and 4.60 g (50 mmol) glycerol were loaded into the
GC–MS analyses were performed in an Agilent 6890/5973
GC–MS apparatus equipped with a split/splitless injection system
and a flame ionization detector (FID). The capillary column was
an Agilent HP-5MS, 30 m × 0.25 mm × 0.25 ␮m. Oven temperature
reactor. The autoclave was purged three times with CO and then
pressurized to 3.0 MPa with CO and O2 (P(CO):P(O ) = 2:1) at room
temperature. The reaction was carried out at 140 C for 2 h. During
2
◦
◦
◦
◦
the reaction, the CO and O2 mixture was added to maintain a total
pressure of 3.0 MPa. After the reaction, the reactor was cooled to
room temperature and depressurized.
Safety advice: Although oxidative carbonylation of methanol
has been industrialized by ENIchem for 30 years [20] and no
security incident has been reported up to date, high-pressure
carbonylation experiments with compressed gases represent a sig-
nificant safety risk and should only be conducted in conjunction
with the use of suitable equipment and special care.
started at 50 C and was increased to 260 C at a rate of 20 C/min.
Helium was used as the carrier gas (1.0 mL/min). The GC–MS chro-
matogram of silylated samples is shown in Fig. 1. MS (m/z) (1)
+
+
+
silylated glycerol: 293 (M –CH ), 263 (M –3CH ), 218 (M –6CH ),
3 3 3
+
+
205 (M –CH OSi(CH ) ), 191 (M –CH OSi(CH ) –2CH ), 175
2
3
3
2
3
3
3
+
+
(M –CH OSi(CH ) –3CH ), 147 (M –CH OSi(CH ) –Si(CH ) ), 117
2
3
3
3
2
3
3
3 3
+
+
+
+
(M –Si(CH ) ), 103 (CH OSi(CH ) ), 101 (M –OSi(CH ) ), 89
3
+
3
2
3
3
3 3
+
(OSi(CH ) ), 73 (Si(CH ) ); (2) silylated glycerol carbonate:
3
+
3
3 3
+
+
+
189 (M –1), 175 (M –CH ), 160 (M –2CH ), 145 (M –3CH ),
131 (M –Si(CH ) ), 116 (M –Si(CH ) ), 103 (CH OSi(CH ) ), 101
(M –OSi(CH ) ), 89 (OSi(CH3)3 ), 73 (Si(CH3)3 ). The bis(1,3-
3
+
3
3
+
+
Gas chromatography analyses were performed in an Agilent GC
790 apparatus equipped with a HP-5 (30 m × 0.32 mm × 0.25 ␮m)
3
+
2
3
3
2
3 3
+
+
+
1
3
3
capillary column and a flame ionization detector (FID). The sam-
ple taken after reaction of 0.50 g was weighed into 5 mL tubes
with caps. 2.00 g of silyl reagent (a 3:1:9 hexamethyldisilazane,
trimethylchlorosilane, and pyridine mixture) was added and the
tubes were closed, gently agitated for 5 min and allowed to incu-
dihydroxypropan-2-yl) carbonatebyproductwas also detected (see
Fig. 1d).
2.3. Electrochemical properties
bate at room temperature for 30 min. A white, NH Cl precipitate
was formed, and 40 mg of cumene was added as an internal stan-
Cyclic voltammetry (CV) was utilized to elucidate the electro-
chemical behavior of the palladium complexes. CV measurement
was performed in DMF using tetrabutylammonium tetrafluorobo-
4
−
1
rate (n-Bu NBF , 0.1 M = 0.1 mol L ) as the supporting electrolyte
4
4
◦
with an Electrochemical Workstation CS300 at 20 C. A con-
ventional three-electrode system was employed. A stationary
platinum-disk served as the working electrode, and a large plat-
inum foil was used as the counter electrode. A commercial
saturated calomel electrode (SCE) was used as the reference elec-
trode. Current-voltage curves were measured at 0.1 V s . All
potentials were reported in volts vs SCE. The palladium complex’s
concentration was 1 mM. All measurements were performed in
well-deaerated solutions under nitrogen atmosphere.
−
1
3. Results and discussion
3.1. The catalytic performance of different catalysts
Glycerol carbonate was prepared by oxidative carbonylation of
glycerol with CO (2.0 MPa) and O2 (1.0 MPa) using different cata-
lyst systems, in presence of a DMF solvent at 140 C. The results are
Fig. 2. Effect of temperature on oxidative carbonylation catalyzed by
PdCl2(phen)/KI, glycerol/Pd = 400 molar ratio, CO 2.0 MPa, O2 1.0 MPa, 2 h,
stirring speed 800 rpm. (a) Conversion of glycerol and (b) selectivity of glycerol
carbonate.
◦
presented in Table 1. Initially, CuCl was employed as the catalyst.
−
1
The catalytic efficiency was very low (TOF was only 0.22 h ), even

J. Hu et al. / Applied Catalysis A: General 386 (2010) 188–193
191
Table 2
when using a nitrogen-containing ligand (1,10-phenanthroline,
phen) that has proven effective in promoting both the activity and
selectivity of the CuCl-based system for the synthesis of dimethyl
carbonate [24]. This result was consistent with the work of Teles et
al. reported in a US patent in 1994 [27].
Recently, a number of oxidative carbonylation reactions have
been achieved with various palladium catalysts [21,23], perhaps
the most active and versatile in organic synthesis [28]. Therefore, a
Effect of pressure on oxidative carbonylation of glycerol catalyzed by
PdCl2(phen)/KI .
a
Entry Ptotal (MPa) P(CO)/P(O2) Conversion (%) Selectivity (%) TOFb (h−1
)
1
2
3
4
2.0
2.5
3.0
3.5
2:1
2:1
2:1
2:1
23
67
92
94
97
98
99
99
46
134
184
188
a
Reaction conditions: solvent DMF 30 mL, glycerol 4.60 g (50 mmol), PdCl2(phen)
simple palladium salt, PdCl , and its 1,10-phenanthroline complex
2
◦
0.125 mmol (glycerol/Pd = 400 molar ratio), KI 1.25 mmol, 10 eq., 140 C, 2 h, stirring
PdCl (phen), were adopted. Remarkably, we observed a consider-
2
speed 800 rpm.
able improvement in the conversion of glycerol compared with that
of CuCl-based system (Table 1, entries 4 and 5). It should be noted
b
TOF (mol converted glycerol/mol cat. h).
that decomposition of the catalyst, i.e., reduction of Pd2 to Pd ,
+
0
occurred in the case of PdCl . Palladium black was deposited on the
2
bottom of the Teflon tube after the reaction. However, it was not
3.2. Effect of temperature on the oxidative carbonylation of
glycerol
observed in the case of PdCl (phen), which may be attributed to the
2
0
ligand phen. The ligand may prevent Pd from being deposited as
the bulk metal palladium black [29]. Furthermore, when PPh3 and
We also studied the effect of varying the temperature from
◦ ◦ ◦
110 C to 160 C (Fig. 2). Below 120 C, only a small amount of
CuCl2 were introduced into the PdCl (phen) system as cocatalysts,
2
◦
◦
the conversion and selectivity were moderately enhanced. When
glycerol carbonate was obtained. From 120 C to 140 C, the con-
version and yield increased rapidly, reaching a maximum value
the reaction was performed on 0.5 mol% PdCl (phen) with the aid
2
◦
◦
of KI, we observed a considerable improvement in the reaction rate
along with a high conversion of glycerol (92%) and selectivity of
glycerol carbonate (>99%). Moreover, with a catalyst loading as
low as 0.25 mol%, high conversion and selectivity were achieved,
at 140 C. Above 140 C, the conversion and selectivity decreased.
Stahl reported that at higher temperatures the palladium complex
was prone to rapid decomposition that consequently decreased
the activity [28]. Indeed, we found that the palladium black was
deposited at the bottom of the Teflon tube after the reaction when
−1
and the TOF reached 184 h . To the best of our knowledge, this
attractive result has not yet been reported for the synthesis of
◦
it was carried out at 160 C, resulting in the low catalytic activity.
glycerol carbonate. For comparison, the PdCl /KI catalyst system
2
[
30] was also examined in this reaction. We found that 58% glyc-
erol was converted, considerably lower than occurred when using
3.3. Effect of pressure on the oxidative carbonylation of glycerol
PdCl (phen)/KI.
2
Considering the possible commercial uses of this process, we
also carried out the reaction using industrial grade, crude glyc-
erol, which is more economical and eco-friendly. Fortunately, the
impurities present in crude glycerol do not significantly change its
physical properties or the activity of the palladium catalyst. We
obtained a very good result using crude glycerol: the conversion
and selectivity were 85% and 98%, respectively (Table 1, entry 11).
The results obtained by varying the pressure are shown in
Table 2. We found that there is a significant relationship between
the conversion of glycerol and total pressure. Since the total pres-
sure varied from 2.0 to 3.5 MPa at the same CO/O2 ratio, using
0
.125 mmol PdCl (phen)/1.25 mmol KI, the glycerol conversion
2
increased notably from 23% to 92%. After 3.0 MPa, increasing the
pressure had little impact on the conversion rate.
Fig. 3. Proposed mechanism for oxidative carbonylation of glycerol.

1
92
J. Hu et al. / Applied Catalysis A: General 386 (2010) 188–193
Table 3
a
Comparison of different Pd-catalysts in oxidative carbonylation .
Entry
Catalyst system
Conversion (%)
Selectivity (%)
TOFb (h−1
)
1c
2
3
PdCl2(phen)/KI
PdI2(phen)
PdI2(phen)/KI
[Pd(phen)2][PF6]2
92
65
87
38
99
99
99
97
184
130
174
76
d
4
a
Reaction conditions: solvent DMF 30 mL, glycerol 4.60 g (50 mmol), palladium
◦
catalyst 0.125 mmol (glycerol/Pd = 400 molar ratio), CO 2.0 MPa, O2 1.0 MPa, 140 C,
2
h, stirring speed 800 rpm.
b
TOF (mol converted glycerol/mol cat. h).
Pd:KI = 1:10 (molar ratio).
c
d
Pd:KI = 1:8 (molar ratio).
4
. Proposed mechanism
Taking into account what we know about the Pd-catalyzed
oxidative carbonylation reaction mechanism [21,30], we propose
a plausible mechanism for the oxidative carbonylation of glycerol
to glycerol carbonate catalyzed by PdCl (phen)/KI (Fig. 3). In this
2
Fig. 4. Cyclic voltammogram performed in DMF (0.1 M n-Bu4NBF4) at a stationary
platinum-disk electrode with a scan rate of 0.1 V s , at 20 C: (a) PdCl2(phen) (1 mM)
and (b) PdI2(phen) (1 mM).
catalytic cycle, formation of an intermediate PdI (phen) species I
2
−
1
◦
takes place through the halide anion exchange of PdCl (phen) with
2
KI, which may be transformed into the ((1,3-dihydroxypropan-
2
-yl)oxy)palladium(II) iodide 1,10-phenanthroline complex II via
+
results could explain why the catalytic activity of PdI2(phen) was
the abstraction of H from the glycerol OH group. The glycerox-
ycarbonyl intermediate III is generated by reaction of II and CO.
The species III then undergoes intramolecular nucleophilic dis-
placement by the second hydroxyl group and is convert into the
palladiouscycle derivative IV by eliminating of HI. Reductive elimi-
nation eventually leads to the final product, glycerol carbonate, and
more effective than that of PdCl (phen) [33].
2
5. Conclusion
In conclusion, we showed that glycerol carbonate can be read-
ily synthesized via the catalytic oxidative carbonylation of glycerol
0
0
[
Pd (phen)] species V. [Pd (phen)] is then reoxidized to PdI (phen),
2
using PdCl (phen) as catalyst with the aid of KI, even when using
involving the initial oxidation of HI by O2 to give I2 followed by the
oxidative addition of the latter to [Pd (phen)].
2
0
crude glycerol. Excellent conversion (92–94%), selectivity (>99%),
−
1
and TOF (184–188 h ) were achieved using 2.0 MPa CO, 1.0 MPa O2
The
additional
experiments
using
PdI (phen)
and
2
◦
at 140 C with a very low dosage of palladium catalyst (0.25 mol%).
[
Pd(phen) ][PF ] as catalysts were carried out in the oxidative
2
6 2
We discussed in detail a plausible mechanism using PdI (phen) as
carbonylation of glycerol. As summarized in Table 3, PdI (phen)
exhibited high catalytic activity and a 65% conversion rate. This
percentage increased to 87% when KI was used as a cocatalyst. This
was comparable to the percentage obtained with PdCl (phen)/KI.
However, [Pd(phen) ][PF ] , an excellent catalyst in the carbonyla-
2
2
−
an intermediate, and the I and 1,10-phenanthroline ligands have
a synergistic effect on the catalytic performance of palladium. In
general, the catalytic oxidative carbonylation of glycerol to glyc-
erol carbonate would not only present a new platform to valorize
glycerol but also offer an attractive way to conduct environmentally
friendly organic synthesis.
2
2
6 2
tion of styrene [26] or nitrobenzene [29], had low catalytic activity
in this reaction. It is worth mentioning that a long induction period
(
about 30 min) was observed when the reaction was carried out
using PdCl (phen)/KI as the catalyst. This can be explained by
Acknowledgements
2
the slow halide anion exchange of PdCl (phen) with KI. With
2
these results in hand, we consider the proposed mechanism was
reasonable. Indeed, iodide is one of the “softest” ligands, binding
more strongly to soft metals (low oxidation state, polarizable, and
electron rich, such as the later transition metals), than do the other
halides. Hence in a catalytic cycle that includes these metals in low
oxidation states, the metal is less likely to precipitate and will be
We are grateful for the spectroscopic analysis conducted at
the Analytical and Testing Center, Huazhong University of Science
and Technology, Wuhan, China. We also thank Dr. Lijuan Chen for
invaluable discussions regarding preparation of catalysts.
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−
removed from the cycle in the presence of I when compared with
[
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[
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ꢀ
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2
2
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