Hydrotalcites Promoted by NaAlO2 as Strongly Basic Catalysts with Record Activity in Glycerol Carbonate Synthesis

Article abstract of DOI:10.1002/cctc.201701726

A new type of highly basic catalysts is obtained by promoting Mg–Al layered double hydroxides with sodium aluminate. The Mg–Al mixed oxides obtained by the calcination of pristine hydrotalcites are poorly active in the synthesis of glycerol carbonate from glycerol and dimethyl carbonate (DCM). Pure sodium aluminate on the other hand is highly active in this reaction, but it is also highly corrosive, making its handling problematic. Remarkably, promoting hydrotalcites with low amounts of sodium aluminate is sufficient to reach high yields. At 90 °C, with 3 wt % catalyst and with a DMC/glycerol ratio of 2:1, a glycerol conversion of 92 % was achieved after 30 min over the 10 wt % NaAlO₂/hydrotalcite catalyst with almost 100 % selectivity towards glycerol carbonate. The texture and the crystallinity of the catalysts were strongly affected by the addition of NaAlO₂. The high activity was clearly correlated with the boost in basicity brought about by sodium aluminate promotion. Whereas pristine hydrotalcites possess only weak basic sites, the basicity of the catalysts increased drastically upon promotion with NaAlO₂, both in amount and strength. Diffuse reflectance infrared spectroscopy coupled with CO₂ adsorption measurements revealed the presence of surface carbonates arising from strongly basic sites. Importantly, our study demonstrates that these basic catalysts are truly heterogeneous, stable, and reusable.

Full text of DOI:10.1002/cctc.201701726

Accepted Article
Title: Hydrotalcite promoted by NaAlO2 as strongly basic catalysts with
record activity in glycerol carbonate synthesis
Authors: Sreerangappa Ramesh, François Devred, Ludivine van den
Biggelaar, and Damien P. Debecker
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Hydrotalcite promoted by NaAlO2 as strongly basic catalysts with
record activity in glycerol carbonate synthesis
Sreerangappa Ramesh,[a] François Devred,[a] Ludivine van den Biggelaar,[a] and Damien P.
Debecker*[a]
A new type of highly basic catalysts is obtained by promoting Mg-
Al layered double hydroxides with sodium aluminate. The Mg-Al
mixed oxides obtained by the calcination of pristine hydrotalcites
are poorly active in the synthesis of glycerol carbonate from glycerol
and dimethylcarbonate. Pure sodium aluminate on the other hand is
highly active in this reaction, but it is also highly corrosive, making
its handling problematic. Remarkably, promoting hydrotalcites with
low amount of sodium aluminate is sufficient to reach high yields.
At 90°C, with 3 wt.% catalyst and with a DMC:glycerol ratio of 2:1,
a glycerol conversion of 92% was achieved after 30 min over the 10
wt. % NaAlO2/hydrotalcite catalyst with almost 100% selectivity
towards glycerol carbonate. The texture and the crystallinity of the
catalysts were strongly affected by the addition of NaAlO2. Yet the
high activity was clearly correlated with the boost in basicity
brought about by sodium aluminate promotion. While pristine
hydrotalcites possess only weak basic sites, the basicity of the
catalysts increased drastically upon promotion with NaAlO2, both in
amount and strength. Diffuse reflectance infra-red spectroscopy
coupled with CO2 adsorption show the presence of surface
carbonates arising from strongly basic sites. Importantly, our study
demonstrates that these basic catalysts are truly heterogeneous,
stable, and reusable.
Glycerol carbonate can be synthesized from glycerol by four
main routes (Scheme 1), namely, by reaction with carbon
dioxide,[5] phosgene,[6] urea [7], or alkyl carbonates.[8] The use of
phosgene as carbonylating agent is inadequate due to its
environmental hazardous nature. The utilization of carbon dioxide
to synthesize glycerol carbonate is the most appealing option, but
CO2 activation is challenging and requires harsh reaction
conditions.[5a] Glycerol carbonylation with urea can, in principle
provide high yields of glycerol carbonate in the presence of
heterogeneous acid catalysts, but deactivation of the active sites
due to the formation of ammonia as a co-product of the process
is a challenge.[7c] A more promising option seems to be available
with basic catalysts. For example, Lari et al., reported the
continuous production of glycerol carbonate from urea and
glycerol.[9] Similarly, the catalytic reaction of glycerol with alkyl
carbonates is a pertinent approach for glycerol carbonate
synthesis. In particular, dimethyl carbonate (DMC) is a non-toxic,
biodegradable reactant that can be produced via relatively green
routes[10] and its transesterification with glycerol is
thermodynamically favoured.[8b]
Introduction
The extensive production of biodiesel to fulfill the worldwide
demand for greener fuels has resulted in the production of large
amounts of glycerol as a byproduct (10 wt.% referred to the
amount of biodiesel produced).[1] Since glycerol finds limited
applications as a chemical, it has to be converted into value added
products in order to make the biodiesel industry more profitable.[2]
Glycerol carbonate, for example, has a market value ~10 times
higher than crude glycerol. As a nontoxic and water soluble liquid
that can be obtained from biomass, glycerol carbonate has
become an important platform chemical that can find many
applications in personal care products, solvent industry, polymer
industry or as a chemical intermediate various other products.[3]
Thus, glycerol carbonate synthesis by environmentally benign
process and starting from a bio-based product of low value like
glycerol is of great interest.[4]
Scheme 1. Glycerol carbonate synthesis by different routes.
Several homogeneous and heterogeneous catalysts were
reported in the literature for the transesterification of glycerol with
DMC to produce glycerol carbonate.[6] Homogeneous bases, such
as KOH, NaOH and K2CO3 are efficient catalysts to carry out the
above conversion. However, the separation or neutralization of
the catalyst from the products is problematic, and large amount of
waste water would also be produced during the process.[2b, 3a, 11]
Enzyme catalysts showed excellent yields for glycerol carbonate,
but they suffer from many drawbacks, such as their high
production cost and low reaction rates currently making them
unrealistic candidates for such industrial process.[12] Replacement
of the traditional homogeneous catalysts, such as KOH, by
heterogeneous basic catalysts could reduce waste streams,
facilitate catalyst separation from the product, and allow the
catalyst to be reused.
[a]
Dr. S. Ramesh, Dr. F. Devred, v.d.B. Ludivine,
Prof. Dr. D. P. Debecker
Institute of condensed matter and Nanoscience,
Université catholique de Louvain Place
Louis Pasteur, 1, box L4.01.09, 1348 Louvain la-Neuve, Belgium.
E-mail: damien.debecker@uclouvain.be
Supporting information for this article is given via a link at the end of
the document
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10.1002/cctc.201701726
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21]
Recently, various mixed metal oxides and hydrotalcites-
derived compounds have been applied as heterogeneous basic
catalysts for the upgrading of glycerol to glycerol carbonate.[13]
However, in order to get high yields of glycerol carbonate, the
reaction has to be carried out either at relatively high temperature
(e.g. above 100°C) or with a large excess of DMC to push the
mixed oxides with higher surface area.[16a,
Indeed, upon
calcination at 400°C (sample denoted “HT400”), the solid reached
a SSA of 226 m².g- 1 and a Vp of 0.76 cm³.g-1. The calcined sample
showed N2 physisorption isotherms of type IV (Figure 1)
according to the IUPAC classification, characteristic of a
mesoporous solids.[22] The formation of mesopores is known to be
due to the removal of CO2 and H2O from the HT structure.[22-23]
Sodium aluminate was incorporated by impregnation before
calcination, with a loading of 5, 10 or 20 wt.% (catalysts denoted
5 SAHT, 10 SAHT and 20 SAHT). In the calcined NaAlO2-
promoted catalysts, textural properties were drastically affected.
Mesopores were still present and their average size tended to
increase (Table1). In parallel, the total pore volume and specific
surface area dropped as the NaAlO2 loading increased.
equilibrium and the kinetics (e.g. molar ratio of DMC to glycerol of
14]
5).[13c,
The basic sites on heterogeneous catalysts are the
active centres for the transesterification reaction.[15] Usually, a
correlation is found between the basic property (basic strength
and amount of basic sites) of the catalysts and their catalytic
activity. Hence the development of fine-tuned heterogeneous
catalyst exhibiting abundant and strong basic sites is the key for
the design of a suitable glycerol carbonate synthesis process.
Layer double hydroxides (LDH), also called hydrotalcites (HT),
belong to a class of two dimensional clay materials which received
much attention in recent years as catalysts and catalyst supports
due to their tunable texture and basicity.[16] It has been shown that
activated hydrotalcites can be used as catalysts or catalyst
500
400
300
200
100
0
17]
supports in a wide range of base-catalyzed reactions.[16a,
However, pristine hydrotalcites or the calcined mixed oxides
obtained from them only exhibit moderate activity in the synthesis
of glycerol carbonate from glycerol and DMC.[14a]
Sodium aluminate (NaAlO2, or “SA”) is a highly basic material
and cheaply available in the solid form or as concentrated solution.
Being highly insoluble in various organic solvents, NaAlO2 can be
used as a true heterogeneous catalyst, especially alcohols. Only
a small number of publications have reported NaAlO2 as an active
catalyst for a limited number of base-catalyzed reactions
(transesterification, isomerisation and condensation).[18] Recently,
we showed that NaAlO2 is an excellent active phase for the
production of glycerol carbonate, especially when produced by
spray drying, in the form of aggregates of small crystallites
featuring a high density of strong basic sites.[19] The catalyst
exhibited record activity in the classical reaction conditions and
was also very active at room temperature.
0
0.2
0.4
0.6
0.8
1
Relative pressure (p/p0)
Figure 1. Nitrogen adsorption (full symbols) and desorption (empty symbols)
isotherms obtained on HT400 (circles), 5 SAHT (squares), 10 SAHT (tirangles)
and 20 SAHT (diamonds).
In its pure solid form, however, NaAlO2 is highly hygroscopic
and corrosive, making its handling problematic. Also, the
commercially available form of NaAlO2 or the solid obtained by
simple drying of NaAlO2 solution has very low surface area (below
5 m².g-1).[19-20] Blending, supporting or dispersing this highly
reactive component would be a relevant strategy to optimize both
catalytic activity and workability.
Table 1. Texture and basicity
Catalyst
SSA
Vp
(cm3 g–1
Dp
(nm)
Total amount
of CO2
Total amount
of CO2
desorbed
(µmol.m–2
)
(m2 g–1
)
)
desorbed
(mmol g–1
)
In the present work, we investigate NaAlO2-promoted Mg-Al
mixed oxide catalysts obtained by impregnating NaAlO2 onto
hydrotalcites, followed by calcination. The new solids are
characterized in terms of texture, structure and basic properties.
Their performance is measured in the synthesis of glycerol
carbonate by transesterification of glycerol with DMC, and their
recyclability is studied.
HT
138
226
54
23
5
0.50
0.76
0.25
0.10
0.03
0.01
15
13
19
18
22
21
--
HT400
5 SAHT
10 SAHT
20 SAHT
NaAlO2
0.5
2.2
0.7
13.0
47.8
260
400
1.1
1.3
2
0.8
Results and Discussion
The morphology of the catalysts was inspected by SEM (Figure
2). The hydrotalcite calcined at 400°C showed loose aggregates
of the typical platelet structure of calcined hydrotalcites, with a
sponge-type aspect. The mesopores described above
correspond to the inter particle interstices.[24] Visually, the
structure seemed preserved on 5 SAHT but the roughness of the
Texture, morphology and crystallinity
The pristine hydrotalcite (denoted “HT”) had a Mg: Al ratio of 2, a
specific surface area (SSA) of 138 m2g-1 and a pore volume (Vp)
of 0.50 cm³.g-1 (Table 1). Calcination of hydrotalcites is known to
induce dehydration, decarboxylation, loss of compensating
anions, and decomposition of lamellar structure leading to MgAl
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10.1002/cctc.201701726
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aggregates seemed to decrease when the NaAlO2 loading was
further increased, leading to relatively smooth structures at higher
NaAlO2 loading. Also, the aggregated were more compact and
inter particle spaces appeared to decrease, indicating that
porosity shrunk upon loading SA. Thus, observations from the
SEM images were in good agreement with nitrogen physisorption
measurements.
Importantly, the EDX probe mounted on the SEM allowed to
map the different elements on the catalyst and to show that
NaAlO2 impregnation on the hydrotalcite support was
homogeneous, leading to a uniform distribution of the active
phase throughout the sample (Supporting Information, Figure S1).
XRD patterns are shown in Figure 3. The pristine hydrotalcite
(HT) showed the typical diffractograms of such layered double
hydroxide materials, with the peaks at 2θ = 11.2°, 23.0°, and 35.2°,
HT400
attributed to the d(0 0 3), d(0 0 6), and d(0 1 2) diffraction
25]
planes.[16a,
After calcination of the pristine support, the
crystalline domains of the hydrotalcite are no longer observed and
the obtained broad diffraction line of low intensity are ascribed to
the MgO-like phase typically formed in calcined LDH.[23] Upon
impregnation with NaAlO2 solution, the layered hydrotalcite
structure is preserved (10SAHT uncalcined). The destruction of
the layered structure is observed upon calcination exactly like in
the absence of NaAlO2 (Figure 3). NaAlO2 promoted Mg-Al mixed
oxide catalysts exhibited similar diffraction patterns as that of
HT400 (Figure 3). At 5 wt.% SA, no additional crystalline species
is detected, indicating that sodium aluminate was highly
dispersed. The onset of crystalline sodium aluminate was
observed for the catalysts with 10 wt.% of NaAlO2. In 20 SAHT,
the characteristic peaks of sodium aluminate at 2θ = 20.8°, 30.2°,
33.6°, 35.0°, 45.8°, 48.4°, 51.8°, 58.1°, 61.1°, 63.0° (JCPDS 00-
019-1179 and 01-083-0316) were clearly observed.
5 SAHT
10 SAHT
20 SAHT
* NaAlO2
*
*
*
*
# MgO
20 SAHT
10 SAHT
*
*
*
5 SAHT
Uncalcined
10 SAHT
#
#
HT 400
Pristine HT
10
20
30
40
2 theta / °
50
60
70
80
Figure 3. XRD patterns of the pristine and calcined hydrotalcite materials and
of the NaAlO2-promoted catalysts with different loading.
Basicity
It is commonly accepted that the catalytic activity for glycerol
carbonate synthesis is related to the catalyst basicity; the more
basic the catalyst, the higher the activity.[19] The basicity of the
prepared catalysts was evaluated using CO2-TPD measurements
(Figure 4). The amount of desorbed CO2 and the temperature of
maximum desorption are the criteria for the amount and strength
of basic sites, respectively.[26] In order to evaluate the amount of
different types of basic sites, the desorption curve can be
arbitrarily differentiated into two regions corresponding to weak
and strong basic sites (below or above 400°C respectively). The
weak basic sites are related to CO2 molecules interacting with
Figure 2. SEM micrographs of the catalysts.
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10.1002/cctc.201701726
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surface hydroxyl groups and to Mg+2-O−2 and Al+3-O−2 pairs. The
strong basic sites are related to CO2 molecules interacting with
isolated O2− anions.[16a, 27] CO2-TPD experiments showed a drastic
increase in total basicity upon addition of NaAlO2 (Table 1). In
particular, the incorporation of NaAlO2 appears to convert part of
the weak basic sites found in bare hydrotalcites into strong basic
sites.
Surface analysis
XPS analysis has been carried out in order to obtain information
about the surface composition of the catalyst (Table 2). On the
bare substrate (HT 400) the presence of Al, Mg, and O was
measured in the expected proportions, along with the usual C
contamination and with traces of Cl as impurities. The binding
energies for Mg (50.0 eV), Al (74.0 eV) and O (531.0 eV)
correspond to the expected values for the oxide forms.[30] After
NaAlO2 promotion, the surface Na/Mg atomic ratio expectedly
increased with the NaAlO2 loading (Table 2).
NaAlO2
1246
1308
1684
1636
1669
20 SAHT
20 SAHT
20 SAHT
10 SAHT
5 SAHT
5 SAHT
HT400
HT400
100 200 300 400 500 600 700 800
Temperature / 0C
1800 1700 1600 1500 1400 1300 1200 1100
Wavenumbers / cm-1
Figure 4. CO2-TPD of the calcined hydrotalcite (HT400), “x SAHT” catalysts
with different NaAlO2 loading, and pure NaAlO2.
Figure 5. CO2 DRIFT Spectra of the calcined hydrotalcite (HT400) and of the
“x SAHT” catalysts with different NaAlO2 loading.
To explore further the types of basic sites present on the
catalyst, CO2 adsorption was studied by DRIFTS (Figure 5). The
formation of different species of adsorbed CO2 (monodentate or
unidentate, bidentate, and bridged species) stems from the
presence of different types of surface basic sites.[28] The CO2
adsorption modes are shown in Scheme 2. CO2 adsorbed on
oxygen ions with the lowest coordination number (usually
monodentate) leads to strong basic sites.[28-29] Bidentate
carbonate and bridged carbonate adsorption can be assigned to
moderate or weak basic sites. The weak basic sites may also be
associated with the surface hydroxyl groups over which the CO2
adsorption species is present in the form of bicarbonate.[29a]
Calcined hydrotalcites show intense peaks at 1636, 1669 and
1684 cm-1 which correspond to bridged carbonates which
contribute to weak basic sites. Upon addition of sodium aluminate,
the bands at 1246 cm-1 and 1308 cm-1 increased noticeably. The
latter correspond to monodentate carbonate attributed to strong
basic sites. Strong basic sites clearly increased with the increase
in sodium aluminate loading. This is in good agreement with CO2-
TPD results.
Table2. XPS analysis of hydrotalcite supported by SA catalysts
Catalyst
Na
Mg
Al
O
C
Cl
Na /Mg
ratio
HT400
24.0
21.8
18.5
10.8
12.4
14.3
13.5
12.6
50.8
48.5
45.6
45.2
12.5
10.5
12.9
18.4
0.3
0.9
1.8
2.1
-
5 SAHT
10 SAHT
20 SAHT
3.9
7.7
10.9
0.18
0.42
1.01
Glycerol carbonate synthesis
The catalysts were used in the transesterification of DMC
with glycerol to obtain glycerol carbonate at 90 °C. Because an
excess of DMC drives the reaction toward the transesterification
product, a DMC/glycerol molar ratio of 2 was applied. In all case,
100% selectivity to glycerol carbonate was obtained, as verified
by GC and NMR (Supporting Information, Figure S2).
The glycerol conversion as a function of the time is reported
in Figure 6. In the absence of a catalyst, only a negligible glycerol
carbonate formation was observed after 4 h (yield ∼ 5%). Pristine
hydrotalcite showed moderate activity, yielding only 26% glycerol
conversion after 30 minutes of reaction time at 90°C. Upon
calcination, both the textural properties and the basicity are
altered. It is known that Brønsted basic sites are converted into
Lewis basic sites.[16a] Despite these modifications only slightly
higher glycerol conversion is reached (34% after 30 min).
Scheme 2. Different mode of CO2 adsorption.[28]
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As reported recently,[19] pure NaAlO2 showed much higher
as 14% with the calcined hydrotalcite. By promoting the
hydrotalcite with only 5 wt.% NaAlO2 (5 SAHT), glycerol
conversion doubled to 28%. Glycerol conversion increased
further to 38% and 42% with further increased in SA concentration
to 10 wt.% and 20 wt.% respectively. Thus, intrinsic activity clearly
increases with the NaAlO2 loading, consistent with the idea that
the main active sites are brought by the strongly basic NaAlO2
active phase. In fact, a correlation is observed between the initial
activity and the surface concentration of basic sites (Figure 7).
activity, almost reaching equilibrium (~93%) after 20 min only.
Actually, 58% conversion is already reached after only 5 min of
reaction. In the transesterification of DMC with glycerol, the main
function of the solid catalyst is to support the abstraction of H+
from glycerol on the basic sites so as to form glycerol anion.[8b]
The higher the catalyst basicity, the more negative the charge of
−
the glyceroxide anion (C3H7O3 ), and consequently, the lower the
activation energy of the reaction. Hence, the concentration of
surface basic sites and strength is known to play an important role
in this catalytic reaction.[8b, 19]
Yet, NaAlO2 in its pure solid form is highly hygroscopic and
corrosive, and thereby difficult to handle. Hence supporting
NaAlO2 on a suitable support was inspected. The 10 SAHT and
20 SAHT catalysts also allows to reach the equilibrium conversion,
after a very short reaction time of 30 min. This clearly
outcompetes other catalysts from the literature, which sometimes
showed similar yield in similar reaction conditions but
systematically used either higher catalyst amount or longer
reaction time. For example, Granados-Reyes et al. used exactly
the same reaction conditions and reached the same equilibrium
conversion after 3h with a hydrotalcite catalyst.[31] Similarly, Pan
et al. run the reaction at only 70°C but had to use 10 wt.%
hydrotalcite catalyst and 4 h reaction time to reach equilibrium.[32]
Song et al. reached the same after 4 h of reaction at 95°C and
with 10 wt.% of Li/ZnO catalyst.[33] MgO catalyst was also
proposed by Simanjuntak et al., reaching 76% conversion in 30
min, at 90°C, with 5 wt.% catalyst.[34] Thus, NaAlO2 blended with
hydrotalcite is an excellent alternative, almost allowing to reach
the record reaction rates reported for pure NaAlO2 while
circumventing the downsides associated with its handling.
60
100SA
40
20 SAHT
10 SAHT
5 SAHT
20
HT400
0
0
100
200
300
400
Surface basic site density / µmol.m-2
Figure 7. Correlation between the initial catalytic activity and the surface basic
site density: calcined hydrotalcite (HT400, □), pure NaAlO2 (100SA, ◊)[19] and
NaAlO2/hydrotalcite catalysts (5 SAHT (▲), 10 SAHT (▲), 20 SAHT (▲)).
Stability, reusability and scalability
100
80
60
40
20
0
In order to investigate the heterogeneous nature of the
SAHT catalysts, a hot filtration test was conducted to check for
the absence of reactivity in solution. The reaction was carried out
using 10 SAHT and interrupted after 10 min (conversion = 58%,
far enough from the thermodynamic equilibrium). The hot reaction
mixture containing the catalyst was filtered to remove the catalyst.
The filtrate was refluxed at 90 °C for an additional 50 min in the
absence of the solid catalyst. Analysis of the final reaction mixture
showed that the conversion did not evolve (Figure 8). This test
confirms that there was no leaching of the active sites into the
reaction mixture or in other words that the catalysis is truly
heterogeneous.
Additionally, to evaluate the recyclability and reusability of
the SAHT catalysts, we performed several cycles of reaction.
After each run, the catalyst was filtered off, washed twice with 5
mL of methanol to remove the adsorbed reactants on the catalyst
surface, and dried at 120 °C for 4 h. The results presented in
Figure 9 indicate that the catalyst can be recycled and reused up
to three times with no significant loss in catalytic activity. The
structural, textural and chemical integrity of the spent catalyst was
also examined by X-ray diffraction, N2-physisorption and CO2-
TPD. XRD diffractograms matched well with the characteristic
peaks of fresh catalyst indicating no change in the structure of the
catalyst even after three consecutive cycles (Figure S3). Texture
and basicity of the used sample (10 SAHT) were not affected
either (Table S1).
0
10
20
Time / min
30
Figure 6. Catalytic activity of fresh pristine hydrotalcite (HT, ○), calcined
hydrotalcite (HT400, □), pure NaAlO2 (100SA, ◊)[19] and NaAlO2/hydrotalcite
catalysts: 5 SAHT (▲), 10 SAHT (▲) and 20 SAHT (▲). Reaction conditions:
glycerol (20 mmol), DMC (40 mmol), 3 wt. % of catalyst (with respect to glycerol),
90°C.
Since all NaAlO2 catalysts similarly allow reaching a high
yield after 30 min, it is necessary to look at their initial activity to
compare them among each other in terms of intrinsic activity. We
approximate initial activity by looking at the glycerol conversion
after 5 min of reaction. In these conditions, conversion is as low
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Pure sodium aluminate showed high activity for the reaction but
is very difficult to handle due to its highly hygroscopic and
corrosive nature. Supporting this highly reactive component on
hydrotalcite is shown to be a relevant strategy to optimize catalytic
activity and practicability. The method presented here is very
simple, based on an impregnation method, followed by calcination.
The texture of the hydrotalcite is strongly affected after blending
with NaAlO2. The structure of the composite catalyst resembles
that of calcined hydrotalcites, with the typical destruction of the
layered structure and the appearance of an MgO-like phase. In
addition, NaAlO2 crystallites are detected when the loading
reaches 10 wt.% or more. Most importantly, the blending with
NaAlO2 results in the genesis of abundant strong basic sites. All
in all, the catalysts showed full selectivity and high activity in the
synthesis of glycerol carbonate from glycerol and DMC. They
outperform previously reported heterogeneous catalysts used in
similar reaction conditions. They truly act as a heterogeneous
catalyst and show excellent reusability and stability. Being both
easy to handle and highly active, they constitute a promising
alternative to current heterogeneous catalysts for this reaction
and other base-catalyzed reactions.
100
80
60
Catalyst removed
40
20
0
0
10 20 30 40 50 60 70
Time / min
Figure 8. Glycerol carbonate yield as a function of time with 10 SAHT catalyst
(circles) and results of the hot filtration test where the catalyst was removed by
filtration after 10 min (triangle). Reaction conditions: glycerol (20 mmol), DMC
(40 mmol), 3 wt. % of 10 SAHT catalyst (with respect to glycerol), 90°C.
Experimental Section
Finally, the reaction was conducted at a larger scale to verify
if similar levels of performance can be obtained upon scale up.
200 mmol of glycerol (18.4 g) were reacted with 400 mmol of DMC
(36.0 g) at 90°C in the presence of 0.55 g of 10 SAHT catalyst (i.e.
keeping the same ratios of reactants and catalyst but multiplying
all quantities by 10). Exactly the same glycerol conversion was
obtained (93%), which can be seen as encouraging in the
perspective of scale up to commercial applications of the catalyst
for this reaction.
Chemicals
The pristine hydrotalcite sample (Mg: Al ratio 2) was kindly provided by
Kisuma chemicals (The Netherlands). Sodium aluminate solution (25
wt. %) was kindly provided by Dequachim (Belgium). Glycerol (99 %),
Ethanol (99 %), Butanol (99 %), Dimethyl carbonate (99 %) and Glycerol
carbonate (90%) were obtained by Sigma Aldrich and were used as
received.
60
50
40
30
20
10
0
Catalyst preparation
A series of NaAlO2-promoted Mg-Al mixed oxides catalysts with NaAlO2
content from 0 to 20 wt.% was prepared by an impregnation method. The
required quantity of NaAlO2 solution (25 wt.% in water) was diluted in
distilled water (10 ml) and this solution was added to 5 g of hydrotalcite
support with continuous mixing, to form a thick paste. Excess water was
removed by evaporation on water bath. The resulting solid was dried at
100 °C overnight and finally calcined at 400 °C for 4 h in a static air muffle
furnace. These catalysts were denoted as "xSAHT", where x indicates the
weight content of NaAlO2 (“SA”) in the final mixed oxide (in wt.%). The
pristine hydrotalcites material is denoted "HT" and the calcined
hydrotalcites (that has not undergone the impregnation steps) is denoted
"HT400". A pure NaAlO2 catalysts (100SA) was prepared by spray drying,
as recently reported.[19]
Fresh
Cycle 1
Cycle 2
Cycle 3
Number of catalytic cycles
Catalyst characterization
Figure 9. Catalyst reusability studies. Reaction conditions: glycerol (20 mmol),
DMC (40 mmol), 3 wt. % of 10 SAHT catalyst (with respect to glycerol), 90°C
for 10 min.
X-ray diffraction (XRD) powder patterns were collected on a Siemens
diffractometer model D5000 fitted with a Cu Kα (1.541˚) radiation source.
Data were recorded over a 2Ɵ range of 10–80° with an angular step of
0.05° at 3 s/step which resulted in a scan rate of 1°/min. Patterns were
identified using files from the Joint Committee on Powder Diffraction
Standards (JCPDS). Textural properties were determined by nitrogen
physisorption at 77 K using a Micromeritics Tristar equipment. Samples
were previously degassed in situ at 393 K under vacuum for overnight.
Surface areas were calculated using the Brunauer–Emmet–Teller (BET)
Conclusions
We designed a new class of heterogeneous highly basic solids by
blending sodium aluminate with hydrotalcite derived mixed oxides.
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10.1002/cctc.201701726
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FULL PAPER
methods over a p/p0 range where a linear relationship was maintained
(0.05-0.30).
mixture. After mixing for 3 min, the reaction mixture was centrifuged to
separate the solid catalyst.
To evaluate the basicity of the prepared catalysts, CO2 temperature-
programmed desorption (TPD) experiments were conducted using a
CATLAB instrument, from Hiden equipped with QGA mass spectrometer
for gas analysis. Approximately 60 mg of each sample were loaded in a
quartz micro-reactor supported by quartz wool and degassed at 500°C for
1h using a heating rate of 10°C/min in flowing helium (50 cm3min-1). Next,
the samples were cooled to 50°C and exposed to flowing 15% CO2-He (50
cm3min-1) for 1.5h and finally purged in flowing helium for 3h at 50°C. In
the TPD experiments, the samples were heated up to 800°C using a
heating rate of 5°Cmin-1 and a He flow of 50 cm3min-1. The amounts of
desorbed CO2 was obtained by integration of the desorption profiles and
referenced to the signals calibrated for known volumes of analyzed gases.
The reaction products were analyzed by gas chromatography (GC-456
SCION BRUKER) equipped with a flame ionization detector, split/splitless
injection unit and a capillary column (DB-WAX, 30 m, 0.25 mm, 0.25 m).
Helium was used as the carrier gas. The injection was performed in split
mode with a split ratio of 100:1. Initially, the oven temperature was set at
100°C and was increased at the rate of 15°C min-1 until it reached 240°C
and then it was maintained at this temperature for 15 min. The FID and
injector temperatures were fixed at 270°C and 300°C, respectively. The
products were confirmed by proton NMR studies (Bruker NMR with 300
mHz instrument by using TMS as standard). The experimental runs were
repeated three times and showed good repeatability (maximum deviation
of 3% in relative for the conversion).
CO2 adsorption was explored by diffuse reflection infrared Fourier
transform spectroscopy (DRIFTS), with
a BRUKER EQUINOX 55
Acknowledgements
spectrometer. The sample was heated to 400 °C in He flow and held at
this temperature for 1 h prior to experiment in order to remove absorbed
water. Then, it was cooled to 30 °C and 140 scans were recorded and
averaged. High-purity carbon dioxide was introduced to the cell at 50
cm3min-1 for 1 h. Then, a He flow of 50 cm3min-1 was admitted and spectra
of adsorbed CO2 were recorded under using a resolution of 4 cm-1
(average of 140 scans).
Authors gratefully acknowledge the Walloon Region for the
financial support (BEWARE programme, convention n° 1410279).
Conflict of interest
The authors declare no conflict of interest.
Scanning Electron Microscope (SEM) images were used to determine the
morphology of the studied samples. SEM images were taken with a JEOL
7600 F with a 15.0 kV voltage. Samples were dried under vacuum at 60 °C
for 24 h and then placed on a piece of carbon black tape on an aluminium
stub. A chromium sputter coating of 10 nm was applied under vacuum with
a Sputter Metal 208 HR (Cressington).
Keywords: Sodium aluminate • Layered double hydroxide •
Solid base catalysts • Glycerol carbonate
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Entry for the Table of Contents
Layout 2:
FULL PAPER
Sreerangappa Ramesh, François
Devred, van den Biggelaar Ludivine,
Damien P. Debecker*
+ NaAlO2 (SA)
Calcination
Page No. – Page No.
Hydrotalcite promoted by sodium
aluminate as highly basic catalysts
for glycerol carbonate synthesis
SAHT
Hydrotalcites (HT)
Hydrotalcites are known basic materials but perform poorly in the synthesis of
glycerol carbonate. NaAlO2 is highly active but not convenient. Promoting
hydrotalcites with low amount sodium aluminate allows to overcome these limitations,
leading to excellent catalysts which combine the qualities of the two partners.
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