Synthesis of glycerol carbonate from glycerol and urea with gold-based catalysts

Article abstract of DOI:10.1039/c0dt01389g

The reaction of glycerol with urea to form glycerol carbonate is mostly reported in the patent literature and to date there have been very few fundamental studies of the reaction mechanism. Furthermore, most previous studies have involved homogeneous catalysts whereas the identification of heterogeneous catalysts for this reaction would be highly beneficial. This is a very attractive reaction that utilises two inexpensive and readily available raw materials in a chemical cycle that overall, results in the chemical fixation of CO₂. This reaction also provides a route to up-grade waste glycerol produced in large quantities during the production of biodiesel. Previous reports are largely based on the utilisation of high concentrations of metal sulfates or oxides, which suffer from low intrinsic activity and selectivity. We have identified heterogeneous catalysts based on gallium, zinc, and gold supported on a range of oxides and the zeolite ZSM-5, which facilitate this reaction. The addition of each component to ZSM-5 leads to an increase in the reaction yield towards glycerol carbonate, but supported gold catalysts display the highest activity. For gold-based catalysts, MgO is the support of choice. Catalysts have been characterised by XRD, TEM, STEM and XPS, and the reaction has been studied with time-on-line analysis of products via a combination of FT-IR spectroscopy, HPLC, ¹³C NMR and GC-MS analysis to evaluate the reaction pathway. Our proposed mechanism suggests that glycerol carbonate forms via the cyclization of a 2,3-dihydroxypropyl carbamate and that a subsequent reaction of glycerol carbonate with urea yields the carbamate of glycerol carbonate. Stability and reactivity studies indicate that consecutive reactions of glycerol carbonate can limit the selectivity achieved and reaction conditions can be selected to avoid this. The effect of the catalyst in the proposed mechanism is discussed.

Full text of DOI:10.1039/c0dt01389g

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Contributions of Inorganic Chemistry
to Energy Research
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Published in issue 15, 2011 of Dalton Transactions
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Dalton Trans., 2011, DOI: 10.1039/C0DT01688H, Paper
Rapid catalytic water oxidation by a single site, Ru carbene catalyst
Zuofeng Chen, Javier J. Concepcion and Thomas J. Meyer
Dalton Trans., 2011, DOI: 10.1039/C0DT01178A, Communication
Synthesis of glycerol carbonate from glycerol and urea with gold-based catalysts
Graham J. Hutchings et al.
Dalton Trans., 2011, DOI: 10.1039/C0DT01389G, Paper
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An international journal of inorganic chemistry
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Volume 40 | Number 15 | 21 April 2011 | Pages 3761–3996
Themed issue: Contributions of inorganic chemistry to energy research
ISSN 1477-9226
COVER ARTICLE
Hutchings et al.
Synthesis of glycerol carbonate from
glycerol and urea with gold-based
catalysts
1
477-9226(2011)40:15;1-W

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PAPER
Synthesis of glycerol carbonate from glycerol and urea with gold-based
catalysts†
a
a
a
a
a
Ceri Hammond, Jose A. Lopez-Sanchez,‡ Mohd Hasbi Ab Rahim, Nikolaos Dimitratos, Robert L. Jenkins,
a
b
b
a
a
Albert F. Carley, Qian He, Christopher J. Kiely, David W. Knight and Graham J. Hutchings*
Received 14th October 2010, Accepted 14th December 2010
DOI: 10.1039/c0dt01389g
The reaction of glycerol with urea to form glycerol carbonate is mostly reported in the patent literature
and to date there have been very few fundamental studies of the reaction mechanism. Furthermore,
most previous studies have involved homogeneous catalysts whereas the identification of heterogeneous
catalysts for this reaction would be highly beneficial. This is a very attractive reaction that utilises two
inexpensive and readily available raw materials in a chemical cycle that overall, results in the chemical
fixation of CO . This reaction also provides a route to up-grade waste glycerol produced in large
2
quantities during the production of biodiesel. Previous reports are largely based on the utilisation of
high concentrations of metal sulfates or oxides, which suffer from low intrinsic activity and selectivity.
We have identified heterogeneous catalysts based on gallium, zinc, and gold supported on a range of
oxides and the zeolite ZSM-5, which facilitate this reaction. The addition of each component to ZSM-5
leads to an increase in the reaction yield towards glycerol carbonate, but supported gold catalysts
display the highest activity. For gold-based catalysts, MgO is the support of choice. Catalysts have been
characterised by XRD, TEM, STEM and XPS, and the reaction has been studied with time-on-line
1
3
analysis of products via a combination of FT-IR spectroscopy, HPLC, C NMR and GC-MS analysis
to evaluate the reaction pathway. Our proposed mechanism suggests that glycerol carbonate forms via
the cyclization of a 2,3-dihydroxypropyl carbamate and that a subsequent reaction of glycerol carbonate
with urea yields the carbamate of glycerol carbonate. Stability and reactivity studies indicate that
consecutive reactions of glycerol carbonate can limit the selectivity achieved and reaction conditions
can be selected to avoid this. The effect of the catalyst in the proposed mechanism is discussed.
Introduction
biomass and biodiesels in particular is now well established as a
green alternative to reduce carbon emissions. One major drawback
in the biodiesel industry is the by-production of large amounts of
The impact of CO emissions and global warming is a problem
2
of interest for the whole of society, and the utilisation of CO
as chemical feedstock arises as a valuable alternative to storage
methods aimed at mitigating CO emissions. From a chemical
perspective, CO utilisation is an elegant and sustainable way to
face the challenge of reducing the effect of CO emissions and as a
consequence, it is a matter of major interest for the whole scientific
community and the inorganic chemistry community in particular.
Advances in CO capture and utilisation have been reviewed in the
recent literature intensively. On the other hand, the utilisation of
2
3
undesired glycerol during the transesterification process which
makes for a large surplus in the current glycerol market. Since
the production of biofuels from biomass is increasing, the surplus
production of glycerol is expected to continue for years to come.
The utilisation of glycerol as a platform chemical represents
an opportunity to obtain value added products from a highly
functionalised and cheap raw material, and much research has
recently been dedicated at finding new chemical pathways for
2
2
2
1
2
1,2
3–5
the transformation of glycerol to value added products. In
4,6–8
9
particular, selective oxidation,
reduction to 1,3-propanediol
and etherification for its use as an additive for fuels have been the
subject of much research.
a
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main
Building, Park Place, Cardiff, UK CF10 3AT. E-mail: hutch@cardiff.ac.uk;
Fax: +44 29 2087 4059; Tel: +44 29 2087 4059
Research in the synthesis of glycerol carbonate has only emerged
in the last few years, and this is surprising since glycerol carbonate
has excellent properties such as low toxicity, biodegradability and
a high boiling point which make it a very attractive chemical
for a variety of applications. Glycerol carbonate has found
b
Center for Advanced Materials and Nanotechnology, Lehigh University, 5
East Packer Avenue, Bethlehem, PA 18015-3195, USA
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c0dt01389g
Co-corresponding author: sacjal@cardiff.ac.uk; Fax: +44 29 2087 4059;
10
Tel: +44 29 2087 4061.
application as a high boiling polar solvent, an intermediate
This journal is © The Royal Society of Chemistry 2011
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1
1
12,13
in organic synthesis and in the synthesis of polycarbonates,
the reactivity and stability of the products and, this should be
a target for catalysis research. Secondly, there has only been
limited research addressing the complete identification of reaction
by-products, time on line activity studies and the role of the
catalyst. Problems arising in the separation of the catalyst from the
reaction mixture via the dissolution of the catalyst or formation
of micropowders are important drawbacks. There is therefore a
need for developing new heterogeneous catalysts which are highly
active, fully heterogeneous, recoverable, stable and selective as well
for a better understanding of the reaction mechanism.
14
polyurethanes, and surfactants. The traditional routes for syn-
thesis of glycerol carbonate are typical of other low molecu-
lar weight organic carbonates. The reaction of glycerol with
phosgene suffers from the drawbacks of using a dangerous and
environmentally unfriendly reactant. The transesterification of
15
glycerol can also be readily performed with dimethyl carbonate,
16,17
18
ethylene carbonate,
or propylene carbonate. However, the
carbonates utilised during the transesterification are typically
generated via phosgene utilisation or energy intensive routes
employing epoxides. While there are some reports addressing
the direct carbonylation of glycerol with carbon dioxide with
tin complexes, there are serious limitations for the applicability
of such processes, particularly the unfavorable thermodynamic
equilibrium and low yields.
supercritical CO was also attempted using zeolites as catalysts,
however there is no evidence that a direct CO insertion occurred
as glycerol carbonate was only produced when another organic
carbonate was added as a reactant. The above observations lead
us to utilising urea as an alternative source for carbonylation.
It can be argued that urea represents an activated form of CO
The reaction between CO
most important process involving the chemical utilisation of CO
Gold has attracted a huge amount of interest due to its
33
activity as both a homogeneous and heterogeneous catalysis.
In particular, gold has been shown to selectively oxidise glycerol
34
to glyceric acid in aqueous solution, but also that it is a good
19,20
35
The direct reaction of glycerol with
catalyst for solvent-free selective oxidation of alcohols. When
21
2
gold is suitably supported on an appropriate oxide, it can also
behave as a Lewis acid catalyst and these observations lead us
to consider gold for the glycerolysis of urea. In this paper our
initial results for this catalysed reaction are described. We have
discovered that a number of heterogeneous catalysts are both very
active and reusable, and in particular that gold can promote the
formation of glycerol carbonate in the reaction of glycerol with
2
2
.
2
and ammonia producing urea is the
32
2
urea.
8
and about 10 tons of urea are produced per year worldwide.
The industrial process utilises the Bosch–Meiser urea process,
Experimental
22
developed in 1922. Overall, the reaction utilises two readily
available and cheap reactants in a simple process that operates
without a solvent, with high selectivity and yields under very
mild conditions (Scheme 1). Until recently, most of the research
Materials
The metal sulfate salts utilised in this work were tested as received
23–27
from Sigma-Aldrich. Their purities are as follows: ZnSO
9.9+% trace metal basis, MgSO 99.99+% trace metal basis
and VOSO O 99.99+% trace metal basis. HAuCl ·3H O,
·3H
PdCl , (99.99% purity) and activated carbon (G60) were supplied
by Johnson Matthey. Titania (P25) and SiO were supplied
by Degussa, zinc oxide (ZnO), magnesium oxide (MgO) and
niobium oxide (Nb ) were supplied by Aldrich. ZSM-5 with a
SiO /Al molar ratio of 30 was supplied by Zeolyst and used
in its ammonium form in the synthesis of catalysts. The following
metal salts were used: gallium(III) nitrate hydrate (Ga(NO O)
0) were obtained from
4
·H
2
O
was reported in the patent literature,
can proceed in the absence of a catalyst, several catalysts have
been found to improve its rate. Homogeneous catalysis has been
and while the reaction
9
4
4
2
4
2
26,28
24,29
2
described with inorganic salts such as ZnSO
4
,
and MgSO
4
,
2
and more recently some heterogeneous systems based on oxides
1
7,30,31
have been described.
To the best of our knowledge, no reports
2
O
5
have been published using supported nanoparticles.
TM
2
2
O
3
3
)
3
·H
2
and zinc nitrate hexahydrate (Zn(NO
Sigma-Aldrich.
3
)
2
·6H
2
Catalyst synthesis
Synthesis of catalysts by the impregnation method (I). Sup-
ported gold catalysts (2.5 wt% Au) were prepared by wet impreg-
nation as follows. An aqueous solution of HAuCl
4
·3H
2
O (5.1 mL,
5
g dissolved in 250 mL) was added dropwise with stirring to
◦
the support (1.95 g). The resulting material was dried (110 C,
◦
1
6 h) and calcined (400 C, 3 h, static air). Monometallic 1 wt.%
Au/ZSM-5 catalysts were prepared following the same procedure
using ZSM-5 (30) from Zeolyst in its ammonium form. Powder
X-ray diffraction displayed the reflections due to gold metal on
the various oxides utilised (Fig. S1 in ESI†).
Scheme 1 Reaction setwork for glycerol carbonate synthesis using urea
as CO donor.
2
There are still many issues arising from these reports which
highlight the need for developing new catalytic materials. Firstly,
the reaction yield has only increased two fold from that of
the uncatalysed reaction. Also the catalysed and uncatalysed
reactions operate at the same temperature; clearly, reducing the
reaction temperature could have important benefits considering
Synthesis of catalysts by the deposition precipitation method
(DP) (ref. 36). For the preparation of gold catalysts, an aqueous
solution of HAuCl
water) was added to distilled water (400 mL) containing a zeolite
support (ZSM-5, 2.97 g) and urea (10 g). Upon pH stabilisation,
4
·3H
2
O (3 mL, 2 g dissolved in 100 mL
3
928 | Dalton Trans., 2011, 40, 3927–3937
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◦
the suspension was heated to 85 C for 3 h in order to raise
The stability of glycerol carbonate was determined as follows;
glycerol carbonate (13.8 g, 0.117 moles) was placed in a round
bottom flask (100 mL) and heated at 150 C for 24 h under a flow
of dry nitrogen. Samples were subsequently analysed by HPLC
(previously described). The reaction between glycerol carbonate
and urea was performed as described above, except that glycerol
was substituted by glycerol carbonate (13.8 g, 0.117 moles).
the resultant pH to 6.8–7.2. The suspension was subsequently
filtered hot and washed with distilled water. The paste was dried
overnight at 110 C 16 h. The resulting material was dried (110 C,
◦
◦
◦
◦
1
6 h) and calcined (400 C, 3 h, static air). Ga-doped ZSM-5 was
prepared in a similar manner, with the desired amount of gallium
nitrate replacing aqueous HAuCl O. Zn-doped ZSM-5 was
4
·3H
2
also prepared in accordance to the procedure described above,
with the exception that urea was substituted for a 0.1 M aqueous
ammonia solution, and the desired amount of zinc nitrate replaced
Analysis of products
1
13
H and C NMR spectra were recorded at room temperature
aqueous HAuCl
Preparation of vanadium phosphate catalyst (ref. 37).
11.8 g, Strem) was refluxed with H PO (16.49 g, 85 wt.%,
Aldrich) in isobutanol (250 mL) for 16 h. The light blue solid
was recovered by filtration, washed with isobutanol (200 mL) and
4
·3H
2
O.
1
3
on a Bruker DPX 500 ( C 125.77 MHz) for dilute solutions.
Proton chemical shifts are reported in parts per million relative
to Me
V
2
O
5
(
3
4
13
13
4
Si. C Spectra were referenced to the C resonance of the
OD d = 48.97). Mass spectrometric analyses
NMR solvent (CD
3
were carried out on a Waters LCT Premier XE operating in the
-
1
ethanol (150 mL, 100%), refluxed in water (9 mL H
2
O (g solid) )
APCI (+ve) mode. The reaction mixture was infused into the
◦
for 2 h, filtered and dried in air (110 C, 16 h). Finally, the sample
-1
MS source via a syringe pump at a rate of 10 ml min . FT-IR
◦
was calcined at 750 C under a flow of nitrogen for 24 h in order
spectroscopy was performed by placing the neat reaction mixture
to obtain the active (VO)
Reference gold catalysts. The two standard gold catalysts were
obtained from the World Gold Council (4.5 wt.% Au/Fe (Lot
2
P
2
O
7
phase.
between two NaCl plates, with the spectra recorded on a Jasco FT-
-
1
-1
IR660 Plus over a range of 4000–400 cm at a resolution of 2 cm .
Quantification of the reaction products was carried out on a Varian
920LC HPLC using ultraviolet and refractive index detectors. The
2
O
3
8
,38
No. 02-04) and 1 wt.% Au/TiO ).
2
eluent was an aqueous solution of H
3
PO (0.01 M) and the flow
4
-
1
was 0.45 ml min . Samples of the reaction mixture were diluted
using the eluent to a concentration of 0.0375 M concentration.
Reactants and products were separated using a Metacarb 67H
column and the products identified by comparison with authentic
samples. For quantitative analysis, an external standard method
was used.
Catalyst characterisation
Powder X-ray diffraction (XRPD) was performed using a PANa-
lytical X’PertPRO X-ray diffractometer, with a Cu-Ka radiation
source (40 kV and 40 mA). The diffraction patterns obtained
were baseline corrected. Samples for examination by transmission
electron microscopy (TEM) and scanning transmission electron
microscopy (STEM) were prepared by dispersing the catalyst
powder in high purity ethanol, then allowing a drop of the
suspension to evaporate on a holey carbon film supported by a
Results and discussion
Microscopy of the zeolite-supported catalysts
3
00 mesh copper TEM grid. Samples were then subjected to bright
field imaging experiments using a JEOL 2000FX TEM operating
at 200 kV. Phase contrast lattice imaging and high-angle annular
dark field (HAADF) imaging experiments carried out using a
Fig. 1(a) is a typical bright field TEM micrograph of the 3.5 wt.%
Ga/ZSM-5 catalyst, prepared by deposition precipitation in
urea. This image shows Ga-oxide particles about 3–5 nm in
size uniformly distributed on the zeolite support. Fig. 1(b)
corresponds to the 3.5 wt.% Zn/ZSM-5 material, prepared by
deposition precipitation in ammonia. Large agglomerates of bulk
ZnO particles as well as a more dilute distribution of smaller
(~5 nm) ZnO nanoparticles scattered over the ZSM-5 support are
apparent.
The Au/ZSM-5 samples were also examined by TEM in
order to compare the morphologies of the samples made by wet
impregnation and deposition precipitation with urea. It was found
that these different preparation methods produced very different
results in terms of Au dispersion on the zeolite support (see
Fig. 2). In the deposition precipitation (urea) sample, the Au
formed characteristic sintered ‘chain-like’ structures about 5–8 nm
in width (see Fig. 2(a)). In contrast, discrete Au particles around
2
00 kV JEOL 2200FS transmission electron microscope equipped
with a CEOS aberration corrector. X-Ray photoelectron spectra
were recorded on a Kratos Axis Ultra DLD spectrometer employ-
ing a monochromatic AlK X-ray source (75–150 W) and analyser
a
pass energies of 160 eV (for survey scans) or 40 eV (for detailed
scans). Samples were mounted using double-sided adhesive tape
and binding energies referenced to the C(1 s) binding energy
of adventitious carbon contamination which was taken to be
284.7 eV.
Catalyst testing
The synthesis of glycerol carbonate was performed as follows.
Glycerol (typically 13.8 g, 0.15 moles) was added to a round
bottom flask (100 mL) and dried at the desired reaction tem-
2–5 nm in diameter were the predominant Au morphology present
◦
perature (150 C) under a flow of dry nitrogen for 20 min. The
in the sample prepared by impregnation (see Fig. 2(b)).
desired amount of urea (typically 13.5 g, 0.225 moles (molar ratio
glycerol : urea 1 : 1.5)) was then added, and once solubilised, the
desired catalyst (250 mg) was added. Dry nitrogen was passed
Microscopy of the magnesia-supported Au catalysts
through the reaction mixture in order to remove any NH
The reaction was sampled periodically, and the samples obtained
were diluted to 0.0375 M for analysis by HPLC.
3
formed.
Fig. 3(a) shows a representative bright field TEM image of the fresh
2.5 wt% Au/MgO sample prepared by impregnation which was
calcined at 400 C. Discrete Au particles ranging between about
◦
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Fig. 2 Bright field micrographs of 1 wt.% Au/ZSM-5 catalysts prepared
by (a) deposition-precipitation with urea and (b) and via wet impregnation.
during reaction, nitrogen was bubbled through the reaction
mixture instead of applying vacuum. Homogeneous metal sulfates
Fig. 1 Bright field micrographs of (a) 3.5 wt.% Ga/ZSM-5 and (b)
.5 wt.% Zn/ZSM-5, both prepared by deposition precipitation.
3
(
ZnSO in particular) were shown to be particularly active for the
4
5
and 20 nm in size are clearly visible against the flake-like MgO
support material. HAADF imaging shows that discrete Au atoms
Fig. 3(b)) and occasional 1–2 nm raft-like Au clusters (Fig. 3(c))
reaction, and this was corroborated in the experiments plotted
in Fig. 4. In addition to reproducing the activity of ZnSO
4
23,28
(
and MgSO reported in the literature,
it was also found that
4
also co-exist on the MgO support. We were unable to acquire the
corresponding electron HAADF images from the used 2.5 wt%
Au/MgO sample material as the support had become extremely
beam sensitive and drilled rapidly under the electron probe.
In order to understand this effect, XRD of the 2.5%Au/MgO
catalyst was performed before and after use (Fig. S2 in ESI†). The
XRD revealed the appearance of the reflections typical of gold
nanoparticles in the fresh sample, which are attributable to gold
in the metallic state (2q = 38.1 , 44.2 and 64.6 ) and that no
significant changes in the crystalline structure of the magnesium
oxide resulted from the synthesis. However, after 9 uses, the sample
displays only the gold reflections, indicating that the support has
changed during reaction and lost crystallinity, however, that the
gold nanoparticles are still supported.
VOSO4 was an active catalyst for this reaction. It is notable
that all the metal sulfate catalysts studied resulted in an increase
in glycerol carbonate yield, this suggests that the study of the
role of cation and anion in these systems are worth of study
in future work. It can be seen in Fig. 4 that while the yield of
glycerol carbonate increases linearly for all the catalysts for the
first 6 h of reaction, there is however, a significant contribution
of the uncatalysed homogeneous reaction. In these experiments
8.6 wt% of catalyst was used with respect to glycerol, and this
high percentage emphasises the poor activity of these catalysts. In
order to develop precious metal supported catalysts and reduce the
amount of wasted catalyst, later reactions were carried out with a
decreased mass of catalyst (1.8 wt% with respect to glycerol).
◦
◦
◦
Activity of a range of heterogeneous catalysts. A number
of heterogeneous catalysts were screened (Fig. 5). The catalyst
Catalytic activity
loading was lower than that previously utilised for ZnSO and
4
17
Activity of homogeneous catalysts for glycerol carbonate forma-
tion. Catalytic testing was performed under previously reported
reaction conditions but, in order to remove the ammonia formed
other recent publications, and because of this, the relative
ratio between activity for the catalysed and uncatalysed reaction
decreased significantly when compared to the results shown
3
930 | Dalton Trans., 2011, 40, 3927–3937
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Fig. 3 Representative BF-TEM (a) and STEM-HAADF images (b,c) of sample MH-01: 2.5% Au/MgO (imp, cal 400 fresh).
Fig. 4 Glycerol carbonate yield for a series of homogeneous metal sulfate
catalysts. Conditions: catalyst: (4.74 g), reactants: glycerol/urea: 1 : 1 molar
◦
ratio, temperature: 150 C, time: 6 h. Key: ᭜ = ZnSO
4
; ꢀ = MgSO ; ᭡ =
4
Fig. 5 Glycerol carbonate yield obtained with various zeolite supported
VOSO ; ᭹ = No catalyst.
4
Au, Ga and Zn catalysts prepared by deposition precipitation (DP) and
◦
impregnation (I). Typical catalyst pre-treatment conditions: 110 C for
1
6 h, with the exception of 1 wt.% Au/ZSM-5 (I) (c) which was calcined
in Fig. 4. In order to emulate the activity of ZnSO
4
with a
◦
at 400 C in static air for 3 h. Reaction conditions: glycerol/urea molar
ratio: 1 : 1, temperature: 150 C, catalyst: 0.25 g, reaction time: 4 h.
heterogeneous catalyst, our first approach was to deposit Zn on
zeolite ZSM-5 by deposition-precipitation, taking advantage of
the high surface area of ZSM-5 as a support to disperse Zn.
Firstly it became apparent that while ZSM-5 had some level of
activity, it resulted in slightly lower glycerol carbonate yields with
respect to the uncatalysed reaction. However, the deposition of
Zn, Ga and Au onto ZSM-5 by deposition precipitation resulted
in an enhancement in glycerol carbonate yield in all cases, with the
order of promotion following the order: Au > Ga > Zn.
◦
It is also clear that the preparation procedure used during the
synthesis of the gold catalyst has an effect on the catalytic activity,
as Au/ZSM-5 was shown to be more active when prepared by
impregnation as opposed to deposition precipitation (Fig. 5). It
has been described that the catalytic activity of homogeneous
23,28
metal sulfates may be due to their Lewis acid character,
our initial XPS results with the above series of heterogeneous
and
This journal is © The Royal Society of Chemistry 2011
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a
Au/ZSM-5 catalysts appears in agreement with this. In Fig. 6
the Au(4f) spectra from the Au/ZSM-5 catalysts prepared by
impregnation (IMP) and deposition-precipitation (DP) before and
Table 1 Catalytic activity of gold catalysts supported on several supports
Selectivity (%)
◦
after calcination at 400 C are shown. The dried materials contain
Conv. Glycerol
(%) carbonate 5 7 9 yield (%)
Glycerol carbonate
+
Entry Catalyst
cationic gold species: the IMP sample comprises mainly Au
3
+
+
ions and the DP sample a mixture of Au and Au species. In
contrast, both catalysts contain only Au species after calcination
at 400 C. It is clear from Fig. 5 that the calcination of Au/ZSM-5
1
2
3
4
5
6
7
8
9
1
1
Blank
ZnSO
Au/TiO
Au/Fe O
3
2.5wt%Au/C
2.5wt%Au/SiO
2.5wt%Au/Nb
2.5wt%Au/TiO
2.5wt%Au/ZnO 88
2.5wt%Au/MgO 81
2.5wt%Au/MgO 80
After 10th use
59
83
69
80
66
73
66
77
36
58
37
48
34
39
32
37
56
68
69
44 7 13 21
20 5 17 48
36 9 18 26
26 9 17 39
38 9 19 22
33 6 22 23
37 10 21 21
32 11 20 29
17 7 20 49
16 4 12 56
17 4 10 55
0
4
◦
b
2
b
has a negative impact on the catalytic activity, and this may be
due to the decrease in cationic gold in the catalyst. As well as the
possible influence of cation distribution on activity, we must also
consider the morphology of the gold deposited on to ZSM-5. It
can be observed in Fig. 6, that while the impregnation of gold on to
ZSM-5 results in the formation of discrete Au particles around 2–
2
2
2
O
5
2
0
1
5
nm in diameter, deposition precipitation results in the formation
1
2
MgO
69
37
37 10 16 26
of sintered ‘chain-like’ structure around 5–8 nm in width, but
over 100 nm in length. While the deposition precipitation sample
exhibits a 50% increase in glycerol carbonate yield over the un-
catalysed reaction, its catalytic activity is significantly lower than
that of the impregnated sample. The large chains of gold present on
the catalyst surface would presumably result in increased metallic
character and a potential decrease in catalytic activity versus the
discrete nanoparticles present in the impregnated sample. As a
consequence, later materials were prepared by the impregnation
method.
a
Reaction conditions: glycerol/urea molar ratio: 1 : 1.5, temperature:
◦
150 C, catalyst: 0.25 g, time: 4 h. Product (5) = 2,3-dihydroxylpropyl
carbamate, product (7) = 4-(hydroxymethyl) oxazolidin-2-one, product
b
(
9) = (2-oxo-1,3-dioxolan-4-yl) methyl carbamate. World Gold Council
reference catalysts.
Activity of gold catalysts. In the following catalytic studies, full
quantitative analysis of all reaction products was obtained, and
therefore selectivities are now calculated in addition to glycerol
carbonate yields. The possible reaction products have been drawn
and numbered in Scheme 2. The reaction products detected were
2
,3-dihydroxypropyl carbamate (5), glycerol carbonate (6), 4-
(hydroxymethyl) oxazolidin-2-one (7) and (2-oxo-1,3-dioxolan-4-
yl)methyl carbamate (9). The results obtained with a range of gold
catalysts supported on oxides or carbon are displayed in Table 1.
To study the effect of the support on catalyst performance,
six gold catalysts were prepared by impregnation of gold onto
carbon, silica, niobium oxide, titania, magnesium oxide and zinc
oxide with a 2.5 wt% loading. The catalytic activity of each
catalyst after 4 h reaction is displayed in Table 1 in entries 5–
1
(
0. For comparison, the catalytic activity of homogeneous ZnSO
entry 2) tested under our standard reaction conditions, and the
activity of two reference catalysts supplied by the World Gold
Council (Au/TiO and Au/Fe ; entries 3 and 4 respectively) are
4
2
2
O
3
displayed. The authors have previously shown that the two gold
reference materials are very active for oxidation and hydrogenation
Fig. 6 Au(4f) spectra for a series of Au/ZSM-5 catalysts prepared by
deposition precipitation (DP) and impregnation (IMP).
8,38
reactions and it is evident that all the gold catalysts produced
higher glycerol carbonate yields compared to the uncatalysed
reaction (entry 1). This appears to be due to an increase in
conversion but also to an increase in reaction selectivity toward
glycerol carbonate, especially for the catalysts supported on
magnesium oxide. The results indicate large differences in catalytic
activity and the support seems to play a fundamental role. In
particular, 2.5wt%Au/MgO gave the highest glycerol carbonate
yield as it displayed over 80% conversion and selectivity to glycerol
carbonate of almost 70% after 4 h. When compared to the activity
of the support alone (entry 12), the presence of gold improves
both glycerol conversion and glycerol carbonate selectivity. As
a result, the catalytic performance and morphology of 2.5 wt%
Au/MgO was studied further in detail. The re-usability of the
Due to the activity displayed by VOSO
reaction (Fig. 4), the activity of a vanadium phosphorous oxide
VPO) catalyst was evaluated. VPO materials are known for
4
in the homogeneous
(
providing Lewis acid and basic functionalities, and some recent
reports have highlighted their activity for glycerol conversion
3
9,40
reactions.
The catalyst was prepared by the organic route
◦
and treated at 750 C under nitrogen to produce the (VO)
2
P
2
O
7
catalytic phase. The calcination process ensures that the vanadyl
hemihydrate phase present in the precursor fully transforms to
the vanadyl pyrophosphate, which is insoluble under reaction
conditions. The results were surprisingly good as displayed in
Fig. 5, improving the glycerol carbonate yield by some 50%,
relative to the uncatalysed reaction. However, a gold catalyst
supported on ZSM-5 displayed better glycerol carbonate yield
and the performance of gold supported catalyst will be the main
focus for the rest of this research paper.
◦
catalyst (2.5 wt% Au/MgO calcined at 400 C) was evaluated for
a total of ten catalytic cycles. Entry 11 in Table 1 indicates that
the catalyst retains both its high activity and selectivity to glycerol
3
932 | Dalton Trans., 2011, 40, 3927–3937
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Scheme 2 Possible reaction pathways for the reaction between glycerol and urea.
carbonate after the ten cycles (see Table S1 in ESI for full set of
data†).
Despite the apparent simplicity of the reaction of glycerol with
urea, this reaction can take a large number of possible pathways
Scheme 2). One might expect that at the given reaction conditions,
(
many of the pathways displayed in Scheme 2 will be possible
and this adds complexity of the reaction and mechanistic details.
There have been a number of reports that describe the presence of
several of the products displayed (Scheme 2), but these are not in
full agreement. The presence of specific by-products is important
to understand the effect of the catalyst in the reaction, and to
elucidate some of the mechanisms occurring with or without
catalyst. Hence, a set of time on line measurements was performed
1
3
Fig. 7 Time online analysis for 2.5 wt.% Au/MgO (I) and calcined
for 7 h using FT-IR, C NMR, MS and HPLC to fully identify
and quantify all reaction products.
◦
at 400 C in the reaction of glycerol with urea. Reaction conditions:
◦
glycerol/urea molar ratio: 1 : 1.5, temperature: 150 C, catalyst: 0.25 g,
The reaction profile (Fig. 7) indicates that after 2 h ca. 70%
of glycerol has been converted and that conversion levels off.
The reaction is carried out in pure glycerol and, as the reaction
progresses, the concentration of glycerol decreases slowing the
kinetics of the reaction (see Fig. S3 in ESI for molar composition
of the reaction mixture†). The presence of large amounts of 2,3-
dihydroxypropyl carbamate (5) in the initial stages of the reaction
suggests that it is formed in the first step. While compounds 7 and
reaction time: 7 h. Key: ᭜ selectivity to glycerol carbonate (6), ꢀselectivity
to 2,3-dihydroxyproyl carbamate (5), ᭡ selectivity to 4-(hydroxymethyl)
oxazolidin-2-one (7), ¥ selectivity to (2-oxo-1,3-dioxolan-4-yl) methyl
carbamate (9) and, ᭹ glycerol conversion.
product of the reaction. This is in agreement with observations by
30
Aresta et al. for their phosphated zirconia systems, who similarly
claim that in the presence of their catalysts the reaction mechanism
9
were also detected in small percentages, but the selectivity of
glycerol to form these products drops immediately for compound
and after 2 h for compound 9. On the other hand, selectivity
does not proceed via isocyanic acid as found for ZnSO , but via
2,3-dihydroxypropyl carbamate (5) instead. This is also important
4
7
because the formation of isocyanic acid and its oligomers would
represent the unselective use of urea. Fig. 8 also shows the bands
to glycerol carbonate increases with conversion over the range of
time studied.
-
1
-1
corresponding to urea at 1620 cm and 1670 cm after 1 h
reaction, and how they decrease in intensity with time on line. This
is in agreement with the values of glycerol conversion calculated,
indicating that conversion increases rapidly for the first hours of
the reaction and level off after 5–7 h, when urea and glycerol
FT-IR spectra of samples taken at 1 and 4 h are displayed
in Fig. 8. The isocyanic acid (4) N
C
O stretching band
at 2250 cm- is not present and we can therefore exclude the
1
mechanism in which isocyanic acid (4) is produced as the primary
This journal is © The Royal Society of Chemistry 2011
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◦
Fig. 8 FT-IR spectra of reaction samples taken after 1 and 4 h for the reaction of glycerol with urea at 150 C in the presence of 2.5 wt.% Au/MgO (I).
-1
The region between 2300–2200 cm is expanded and illustrated within the figure to evidence the absence of isocyanic acid.
concentrations become significantly lower. The band correspond-
ing to 2,3-dihydroxypropyl carbamate (5) at 1710 cm- is also
evident at early TOL but diminishes with time as the intermediate
is transformed in to glycerol carbonate. The spectroscopic analysis
also shows that glycerol carbonate concentration increases with
time as expected. All the features observed in the IR spectra
but they did not fully identified. We propose that this peak is due
1
to the formation of (2-oxo-1,3,-dioxolan-4-yl)methyl carbamate,
this is because glycerol carbonate has a primary hydroxyl group
available to react with urea to form the 9 (Fig. 9). This is following
the same mechanism that favours the formation of 5 and it is
therefore not surprising that this reaction occurs. The presence
of compound 9 was later confirmed by MS analysis, which was
also definitive in the assignments of the other reaction products.
MS indicated the presence of the following masses in the reaction
mixture; 135, 118, 117 and 161, which are in agreement of the
30
have been assigned with the help of the literature with the
-
1
exception of an unknown band at ca. 1780 cm . It must be noted
30
that Aresta et al. also reported an unknown product in their
1
3
catalytic studies, which was present in their C NMR spectra,
13
◦
Fig. 9
C NMR spectroscopy of the reaction mixture obtained after a 4 h reaction catalysed by 2.5 wt.% Au/MgO (I) and calcined at 400 C. Key: (1) =
glycerol; (6) = glycerol carbonate; (7) = 4-(hydroxymethyl)oxazolidin-2-one; (9) = (2-oxo-1,3,-dioxolan-4-yl)methyl carbamate. Small unlabelled peaks
belong to an impurity which concentration did not change during reaction.
3
934 | Dalton Trans., 2011, 40, 3927–3937
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reaction products 5, 6, 7 and 9 respectively. From the above, it
is evident that glycerol and urea readily react to produce 2,3-
dihydroxypropyl carbamate (5) as the only initial product, which
can subsequently undergo an intramolecular cyclisation, either
eliminating ammonia to yield glycerol carbonate (6) or eliminating
water resulting in the formation of 4-(hydroxymethyl)oxazolidin-
Table 2 Catalytic data obtained from an uncatalysed reaction with
various glycerol/urea molar ratios
Selectivity (%)
Glycerol : urea Conv. Glycerol
Entry (molar ratio) (%) carbonate 5
Glycerol carbonate
yield (%)
7
9
2
-one (7), this compound also having been identified by other
1
2
3
4
2 : 1
1 : 1
1 : 1.5
1 : 2
40
57
59
67
70
41
36
34
27 1
2
28
17
researchers at concentration levels of ca. 2%. It must be noted
that mass 117 also corresponds to the isomer compound 12,
but complementary NMR and MS studies confirm that none
of compound 11 was formed and therefore we can completely
exclude the reaction pathway whereby urea reacts with the primary
hydroxyl of glycerol eliminating water and creating a C–N bond.
Glycerol carbonate is therefore formed preferentially, and it then
it can subsequently react with urea to yield (2-oxo-1,3-dioxolan-4-
yl)methyl carbamate (9), which was also been detected by Climent
42 7 10 24
44 7 13 21
37 9 22 21
a
Reaction conditions: glycerol/urea molar ratio: 1 : 1.5, temperature:
50 C, catalyst: 0.25 g, time: 4 h. Product (5) = 2,3-dixydroxylpropyl
◦
1
carbamate, product (7) = 4-(hydroxymethyl) oxazolidin-2-one, product
(
9) = (2-oxo-1,3-dioxolan-4-yl) methyl carbamate.
Table 3 Comparison between the product distributions obtained from
an uncatalysed and catalysed (2.5 wt.% Au/MgO) reaction with a
glycerol/urea molar ratio of 2
17
et al. at similar concentration levels. It must be noted that
Climent et al. have also evaluated magnesium and zinc oxides and
17
a
improved the yield of glycerol carbonate up to 72% by using a Zn–
Selectivity (%)
Al hydrotalcite catalyst using more favourable reaction conditions
◦
(
glycerol/urea molar ratio = 1, reduced pressure of 30 Torr, 145 C,
Time Conv Glycerol
Glycerol carbonate
yield (%)
Entry Catalyst
(h)
(%) carbonate 5
7
9
5
wt% catalyst at 5 h reaction time).
In order to corroborate the mechanism of formation of com-
pound 9, glycerol carbonate was heated at 150 C under a flow of
1
Blank
0
0
1.0
2
4
0
0
0
0
0
0
◦
.5
10
18
29
40
0
11
25
35
57
0
78 0 11 1
N
2
and in the absence of urea. Glycerol carbonate was quite stable
66 3
55 5
37 3
6
5
3
0
4
.0
.0
10
23
0
and only ca. 5% of the compound was lost in 24 h. However,
when urea was added and the same experiment was performed
high conversions of glycerol carbonate were observed (see Fig. S4
in ESI†). Under these conditions glycerol carbonate readily reacts
with urea to yield (2-oxo-1,3-dioxolan-4-yl)methyl carbamate (9)
with almost 50% conversion after only 30 min. A total of 70%
of glycerol carbonate had been converted after 4 h and the major
product detected by MS was still compound (9). Of course, this
fast reaction does not normally occur to great extent during the
synthesis of glycerol carbonate because it must compete with
the faster reaction of glycerol with urea and urea concentration
decreases significantly by the time that high concentrations of
glycerol carbonate are present in the reaction mixture. This is in
agreement with the reaction mechanism proposed and emphasises
the importance of controlling the glycerol/glycerol carbonate
ratios during reaction to avoid consecutive undesired reactions.
The observations above suggest that variations in the ratio
between glycerol and urea will have important effects on the
product distribution. The effect of the glycerol/urea ratio in the
uncatalysed reaction is displayed in Table 2. In these experiments,
the reaction parameters were kept constant but the initial amount
of urea was varied. Increasing the glycerol/urea molar ratio has
the obvious consequence of reducing glycerol conversion, as not
enough urea is present and becomes the limiting reactant. So
that with a glycerol to urea ratio of 2 : 1 only 40% of glycerol
conversion was achieved (entry 1) out of a maximum possible
conversion of 50%, whereas when more urea is available we achieve
2
2.5wt%Au/ 0
MgO
0 0
0
1
2
4
.5
.0
.0
.0
22
30
41
41
20
38
67
85
73 5
55 5
31 0.02 28
13 0.02 35
2
2
4
11
a
Reaction conditions: glycerol/urea molar ratio: 1 : 1.5, temperature:
50 C, catalyst: 0.25 g, time: 4 h. Product (5) = 2,3-dixydroxylpropyl
◦
1
carbamate, product (7) = 4-(hydroxymethyl) oxazolidin-2-one, product
(9) = (2-oxo-1,3-dioxolan-4-yl) methyl carbamate.
with excess glycerol (2 : 1 glycerol : urea molar ratio) also produced
less of compound 7 (see Fig. S6 in ESI for full time on line reaction
profile†).
The reaction of glycerol with urea was also carried out using
an excess of glycerol in the presence of the 2.5 wt% Au/MgO
catalyst (see Fig. S6 in ESI†). Table 3 compares the full time on
line conversion and selectivities for the glycerol-rich reactions both
in the presence and absence of the catalyst (2.5% Au/MgO). It is
remarkable how the selectivity to 2,3-dihydroxypropyl carbamate
(5) decreases after 4 h reaction from 37% to 13% in the presence
of the catalyst. Moreover, hardly any formation of compound (7)
was detectable when the catalyst was present, thus suggesting that
the catalyst is selective to the formation of the carbonate at the
expense of the nitrogen insertion products. It also appears that
less glycerol carbonate reacts with a second equivalent of urea
molecule yielding (2-oxo-1,3-dioxolan-4-yl)methyl carbamate (9)
in the presence of a catalyst. Comparison of the catalysed and
uncatalysed reactions in Table 3 suggests that the major differences
in the two reaction profiles is that 2,3-dihydroxypropyl carbamate
(5) reacts faster to give the desired carbonate in the presence of
our catalyst. Overall, for reactions performed with excess glycerol
high activity (referred to urea) was found, but the main role of
6
7% conversion (entry 4). More importantly, the selectivity of
the reaction gradually changes with the molar ratio of the two
reactants, in particular, selectivities to glycerol carbonate now
range from 70% (entry 1) to 34% (entry 4). Also, the increase in
urea concentration results in an enhancement of the consecutive
carbonylation reaction producing 9, and selectivity to 9 increases
from 2% to 22% (entries 1 and 4). The blank reaction carried out
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the catalyst appears to be on improving selectivity by promoting
glycerol carbonate formation at the expense of other by-products.
acids for the formation of cyclic acetals from glycerol resulting
in the formation of the 5-membered ring dioxolane derivatives.
42
In our particular reaction, the results suggest that the main effect
of the catalyst is indeed in promoting the intramolecular reaction
of the 2,3-dihydroxypropyl carbamate (5) to glycerol carbonate
(6) which seems a very good candidate for Lewis acid catalysis.
Gold might coordinate to the carbonyl group of the reaction
intermediate (5) and facilitate the nucleophilic attack by the
secondary hydroxyl group into the electrophilic carbon. The amine
leaving group would then accept the proton from the hydroxyl
which releases the ammonia and forces the closure of the ring as a
carbonate. The origin of the enhanced selectivity might originate
from the absorption of 5 favouring the desired intermolecular
interaction that yields glycerol carbonate. The role of the support
could be many, and future work will provide new clues whether
the support is merely ensuring that gold is in the suitable active
state or there are other metal support interaction effects. Previous
work in gold catalysis research indicates that the support plays
a fundamental role in the activity and selectivity for many other
reactions, such as in hydrogen peroxide synthesis where the activity
of the metal could be correlated with the isoelectric point of the
Mechanistic considerations and role of the catalyst. In view of
these results, we propose that the first step of the reaction proceeds
uncatalysed via a homogeneous reaction and this first step is very
favourable with no evidence of competition with the other possible
reaction pathways under our reactions conditions, as neither
compounds 4, 3 nor 11 have been detected by us or in other recent
17,30
reports.
The second step of the reaction is however crucial
to determine selectivity, and this is precisely where our catalysts
are playing a key role. In this second step, 2,3-dihydroxypropyl
carbamate (5) can form a number of compounds, but only a
few were detected. Importantly, the catalyst is more selective to
glycerol carbonate than towards 4-(hydroxymethyl)oxazolidin-2-
one (7), the 6-ring carbonate (8) or the insertion of a second urea
into the primary OH group of the primary product (3). Therefore,
it is evident that a general trend exists whereby any of the hydroxyl
groups in glycerol may act as nucleophiles, eliminating ammonia,
and this is supported by the fact that compounds 5, 6 and 9 are
the only major products of the reaction. Each of these is a product
of a nucleophilic attack of a hydroxyl group on the electrophilic
carbon in the carbonyl group. The amine group only acts as a
nucleophile to eliminate water in very small proportion and only
in the second step of the reaction where compound 5 was already
formed and the leaving group comes from a secondary carbon. The
only primary product detected from the reaction of glycerol with
urea was compound 5, which indicates that the primary hydroxyl
group in glycerol is a much better nucleophile than the amine
43
support utilised. It must be noted that the most basic support was
more effective and, we consider that the combination of this with
the Lewis acidity of the supported gold results in the improvement
in yields and selectivity. This approach is in agreement with the
17
studies by Climent et al. and also finds some parallelism with
observations that a collaborative effect exists between gold and
44
the support to induce the selective oxidation of alcohols.
groups in urea. This becomes evident when comparing the pK
of urea (0.10) with that of glycerol (14.15) and considering that a
very low pK is also expected for the intermediate carbamate (5).
a
Conclusion
a
A number of heterogeneous catalysts have been discovered to
promote the formation of glycerol carbonate from the solvent-
free reaction of glycerol with urea. In particular Zn, Ga and
Au supported in ZSM-5 promoted the reaction in the following
order Au > Ga > Zn. Additionally, a vanadyl pyrophosphate
catalyst was also very active. The activity of gold supported on
titania, carbon, niobium oxide, zinc oxide and magnesium oxide
prepared via a simple impregnation technique was evaluated.
It is also noteworthy that the 6-membered ring (8) was not
detected in any of the experiments performed. One might expect
that the intramolecular cyclisation reaction of compound (5) to
form glycerol carbonate (6) would compete with the formation of
the 6-membered ring isomer (8). Both intramolecular cyclisation
reactions are classified as exo-trig following Baldwin’s rules as in
both cases the bond broken during the cyclisation step is outside
the ring and in both cases the electrophilic carbon is sp2 (trigonal).
Following Baldwin’s rules both 5 and 6-membered rings should be
favourable. In the present case however, the usual preference for
However, the most active catalysts were obtained when gold
was deposited on magnesium oxide and calcined at 400
◦
C
and, the catalyst did not lose its activity even after 10 re-cycles.
The yields of glycerol carbonate obtained are superior to those
obtained with homogenous zinc sulfate salts previously reported
and, the catalyst increases selectivity to glycerol carbonate. This
might occur by promoting the intramolecular cyclisation of the
carbamate intermediate of the reaction to form glycerol carbonate.
5
-ring formation over 6 reaches something of a maximum.
The most favoured hypothesis on the role of catalysts for this
reaction is that the catalyst provides a suitable combination of
17
Lewis basicity and acidity. Some authors have implied that the
role of the catalyst could be that of co-adsorbing urea and glycerol
to favour the formation of glycerol carbonate. However, it has also
been shown that the first step of the reaction is very fast and occurs
uncatalysed. Careful observation of the preliminary experiments
performed in his work leads us to favour that the role of the
catalyst is related to the slower second step of the reaction, to the
intramolecular reaction forming the carbonate. Gold can interact
with carbonate groups as it shows a promotional effect over ceria in
the transalkylation of propylene carbonate to dimethyl carbonate,
Acknowledgements
The authors thank the EPSRC for financial support
EP/H007679/1.
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