Ceria nanoparticles as heterogeneous catalyst for CO2 fixation by ω-aminoalcohols

Article abstract of DOI:10.1039/c001955k

In contrast to γ-Al₂O₃, TiO₂, ZrO₂, MgO and Y₂O₃, CeO₂ is a reusable catalyst for the reaction of CO₂ with ω-aminoalcohols to form cyclic carbamates; the highest yield (68%) was obtained for the preparation of N-alkyl 1,3-oxazolidin-2-ones from N-alkyl ethanolamines.

Full text of DOI:10.1039/c001955k

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Ceria nanoparticles as heterogeneous catalyst for CO2 fixation
by x-aminoalcoholsw
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Raquel Juarez, Patricia Concepcion, Avelino Corma* and Hermenegildo Garcıa*
Received 28th January 2010, Accepted 31st March 2010
First published as an Advance Article on the web 21st April 2010
DOI: 10.1039/c001955k
In contrast to c-Al2O3, TiO2, ZrO2, MgO and Y2O3, CeO2 is a
reusable catalyst for the reaction of CO2 with x-aminoalcohols
to form cyclic carbamates; the highest yield (68%) was obtained
for the preparation of N-alkyl 1,3-oxazolidin-2-ones from
N-alkyl ethanolamines.
for this selection is that diethanolamine is widely used as
solvent to trap CO2. Considering transalkylation of organic
cyclic carbonates as a related precedent,8 we selected nano-
particulated metal oxides as solid catalysts, including g-alumina,
P-25 titania, nanoparticulated ZrO2 (40 nm average particle
size), high surface area MgO nanoparticles and Y2O3 nano-
particles. Another series of catalysts were prepared by deposition
of Au nanoparticles on nanoparticulated CeO2. The catalytic
data obtained for oxazolidinone formation from diethanolamine
are summarized in Table 1.z
The use of CO2 as C1 feedstock (‘‘CO2 fixation’’) replacing
methane and CO is a hot topic in Green Chemistry.1–3
Compounds developed from CO2 trapping can enjoy the
advantage of ‘‘CO2 neutrality’’, minimizing the impact of
many chemical syntheses having a positive CO2 balance.1
The main problem in CO2 fixation reactions is the high
thermodynamic stability of CO2 that renders most of the
equilibria unfavorable toward product formation.4 Alcohols
and amines react with CO2 forming dialkyl carbonates and
N,N0-dialkyl ureas that can be used as the starting materials
for green alkylations and for the formation of carbamates.3,5
In the context of new processes for CO2 fixation and compared
to simple alcohols, o-aminoalcohols should react with CO2
with somewhat more favorable equilibrium toward product
formation due to favorable intramolecular trapping of the
hydroxyl group of the carbamic acid intermediate leading to
the formation of cyclic carbamates (Scheme 1).
As it is indicated in Table 1, negative results were obtained
in all cases (footnote a), except in the case of ceria. Two
samples of ceria differing in the average particle size were
tested and it was found that nanoparticulated ceria of 5 nm
average size (np-CeO2) is most active, but less selective than
the commercial non-nanoparticulated ceria sample of larger
average particle size, CeO2 (40 nm). The data of Table 1,
showing that yields between 24 and 29% of oxazolidinone can
be obtained by reaction with CO2, are remarkable and much
higher than those obtained for dimethyl carbonate formation
by reaction of CH3OH and CO2 that are around 5%.6 It is
known that as the ceria particle size decreases in the nano-
metre length scale, there is an increasing density of defects in
ceria mainly consisting in framework oxygen vacancies and
ceria atoms in +3 oxidation state.15 Apparently the presence
of these defects is positive for the catalytic activity of CeO2 for
the carboxylation process (see later for the proposed reaction
mechanism). IR spectroscopy has shown that upon admitting
CO2 into dehydrated np-CeO2 or CeO2 instantaneous appearance
of strong bands attributable to metal carbonates is
observed.6,8
The present work describes the activity of ceria as hetero-
geneous catalyst for the reaction of CO2 with primary and
N-substituted o-aminoalcohols. CeO2 and ZrO2 are the most
widely solid catalysts for various CO2 fixation reactions.3,6–8
In a prior study, we found that CeO2 catalyses transalkylation
of propylene carbonate with linear alcohols.8
Ethanolamines can react with CO2 giving rise to the corres-
ponding N-alkyl 1,3-oxazolidin-2-ones. Oxazolidinones have a
wide range of applications as suitable polar solvents, as
synthetic intermediates and chiral inductors.9–11 The synthesis
of these pentacyclic heterocycles can be achieved by direct
reaction with CO2 as indicated in Scheme 1. Analogously, the
reaction of 3-aminopropanol and 4-aminobutanol would
render 1,3-oxazinan-2-ones and 1,3-oxazepan-2-ones that
are useful as reaction intermediates for the synthesis of
compounds with biological activity.12–14
We have found that the main reason of the moderate
selectivity to 1,3-oxazolidin-2-one is its instability under the
reaction conditions, undergoing decomposition probably by
further reaction with diethanolamine giving unidentified
oligomeric materials. To provide a better comparison between
the activity of the two ceria samples (np-CeO2 and CeO2
(40 nm)) and considering the occurrence of consecutive reactions,
it is convenient to compare time conversion plots rather than
In the first stage of our work we selected diethanolamine as
substrate and performed the screening of a series of metal
oxides to promote the CO2 insertion (Scheme 2). The reason
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Instituto de Tecnologıa Quımica, CSIC-UPV, Universidad Politecnica
de Valencia, Av de los Naranjos s/n, 46022, Valencia, Spain.
Fax: (+34)963877809
w Electronic supplementary information (ESI) available: Character-
ization of surface sites based on FT-IR spectroscopy of adsorbed
methanol. See DOI: 10.1039/c001955k
Scheme 1 Reaction of o-aminoalcohols with CO2 with the formation
of cyclic carbamates.
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Table 2 Conversion and selectivity for the reaction of N-alkyl
ethanolamines with CO2. Reaction conditions: N-alkyl ethanolamine =
2 mmol, ethanol = 35 mmol, CO2 = 7 bar, catalyst = 75 mg, reaction
time = 8 h, reaction temperature = 160 1C
Scheme 2 Reaction of diethanolamine with CO2 with formation of
N-2-hydroxyethyl substituted oxazolidinone.
Table 1 Catalytic activity for the reaction of diethanolamine with
CO2. Reaction conditions: diethanolamine = 2 mmol, ethanol =
35 mmol, CO2 = 7 bar, metal oxide = 75 mg, reaction time = 8 h,
reaction temperature = 160 1Ca
Catalyst
R
R0
Conv. (%)
Sel. (%)
np-CeO2 (5 nm)
CeO2 (40 nm)
ZrO2
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Et
n-Pr
n-Pr
H
68
48
19
12
80
77
45
32
88
90
87
60
85
83
89
56
Catalyst
Conv.b (%)
Sel.c (%)
np-CeO2 (5 nm)
CeO2 (not-np)
Au (0.01 wt%)/np-CeO2 (5 nm)
66
36
48
36
82
46
Al2O3
np-CeO2 (5 nm)
np-CeO2 (5 nm)
CeO2 (40 nm)
np-CeO2 (5 nm)
a
Other solid catalysts tested under the same conditions were g-Al2O3,
P-25 TiO2, ZrO2, MgO and Y2O3 as well as Au/np-CeO2 (5 nm) at
gold loadings from 0.79 to 2.46 wt%. In all these cases no product
b
formation was observed. Conversion (%) = (initial mol diethanol-
amine ꢁ mol diethanolamine at 8 h) ꢂ 100/initial mol diethanolamine.
After having found that nanoparticulated ceria promotes
the reaction of CO2 with diethanolamine we have expanded
the scope of the catalytic CO2 fixation by performing analogous
reactions with ethanolamine derivatives. The results are
presented in Table 2. As it can be seen, high conversions of
the N-alkyl ethanolamines with good selectivity to N-alkyl
1,3-oxazolidin-2-one were obtained in all three cases, showing
that the process is general.
c
Selectivity (%) = (mol product formed/mol of converted diethanol-
amine) ꢂ 100.
single selectivity data at different conversion. Fig. 1 shows to
the evolution of the reaction of diethanolamine with CO2 in
the absence and in the presence of np-CeO2 and CeO2 (not np).
As can be seen in this figure, ceria is required in order for
diethanolamine to react with CO2. The activity of np-CeO2 is
higher than that of the commercial not-np CeO2. However, as
commented earlier, the resulting N-(2-hydroxyethyl) oxazoli-
dinone is not stable under the reaction conditions, with
decreased selectivity as the conversion increases.
Considering that N-methyl aminoethanol was the compound
giving higher yields of the corresponding oxazolidinone, we
re-screened the activity of the other metal oxides (g-Al2O3,
P-25 TiO2, ZrO2, MgO and Y2O3). While N-methyl amino-
ethanol conversion in the presence of TiO2, MgO and Y2O3
was below 6% after 8 h, Al2O3 and ZrO2 render the penta-
cyclic carbamate with moderate yield (Table 2). These data
with N-methyl aminoethanol reconfirms the superiority of
np-CeO2 over the other oxides.
As commented earlier, previous reports stated that Au
nanoparticles increase the inherent activity of ceria for the
transalkylation of organic carbonates with linear alcohols or
for the carbamoylation of aromatic amines with dimethyl
carbonate.8,16 In the present case, however, it was found that
Au nanoparticles, even at the smallest loading (0.01 wt%),
significantly decreases np-CeO2 activity that is suppressed at
higher loadings (Table 1). We propose that this negative
influence of Au arises from the fact that Au nanoparticles
are covering the ceria defects (see mechanistic proposal below).
Thus, the results presented in Table 1 constitute an example of a
negative interaction of ceria with Au nanoparticles.
To test the heterogeneous nature of the catalysis, the
reaction of N-methyl aminoethanol using np-CeO2 as catalyst
was initiated under the typical reaction conditions and the
solid filtered from the hot solution after 90 min when the
conversion was already 38%. The resulting clear solution after
filtration was heated under the reaction conditions observing a
further 11% N-methyl aminoethanol conversion. This indicates
that leached Ce is responsible of about 22% of the conversion.
Furthermore, chemical analysis of the filtered solution shows
the presence of Ce corresponding to 0.2% of the initial Ce in
the solid catalyst. Thus, the hot filtration test and chemical
analysis shows that under the reaction conditions some Ce is
dissolved and promotes the reaction in the liquid phase.
Nevertheless, the np-CeO2 solid recovered after the reaction
of N-methyl aminoethanol with CO2 at 160 1C was reused
three times without noting any significant activity decay or
selectivity change towards N-methyl-1,3-oxazolidin-2-one.
This is not surprising considering the small percentage of
leached Ce. Apparently, although a minute fraction of
np-CeO2 becomes dissolved and acts in the homogeneous
phase, np-CeO2 can be almost completely recovered without
losing activity for the CO2 fixation by o-aminoalcohols.
In Table 2, the results of the reaction of 2-amino-1-propanol
with CO2 are also included. It is interesting to note that
Fig. 1 Conversion (m, E and ’) and selectivity (n and }) for the
reaction of diethanolamine with CO2 in the presence of np-CeO2
(5 nm) (m and n), commercial CeO2 (40 nm) (E and B) and CeO2
(non-np) (’). Reaction conditions: Diethanolamine = 2 mmol,
ethanol = 35 mmol, CO2 = 7 bar, metal oxide = 75 mg, reaction
temperature = 160 1C.
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Table 3 Conversion and selectivity for the reaction of o-amino-
alcohols with CO2 in the presence of np-CeO2 as catalyst. Reaction
In conclusion, in the present work we describe that fixation
of CO2 by bidentate o-aminoalcohols can be carried out
using nanoparticulated ceria as catalyst. High conversions
with good selectivity towards oxazolidinone were achieved
for N-alkyl substituted aminoethanols. Our report is of inter-
est in the context of Green Chemistry trying to develop new
processes using CO2 as a C1 feedstock.
conditions: o-aminoalcohol
= 2 mmol, ethanol = 35 mmol,
CO2 = 7 bar, np-CeO2 = 75 mg, reaction temperature = 160 1C,
reaction time 6h.
Substrate
Conv. (%)
Sel. (%)
2-Aminoethanol
3-Aminopropanol
4-Aminobutanol
58
30
20
87
79
76
Financial support by the Spanish Ministry of Science and
Innovation (CTQ2009-11586) is gratefully acknowledged.
R. J. thanks the Spanish Ministry of Education for a scholarship.
2-amino-1-propanol is widely available by dehydration and
amination of glycerol.17 Glycerol is a by-product of biodiesel
and a surplus of glycerol is expected. Thus, there is a large
incentive in developing products derived from glycerol such as
4-methyloxazolidin-2-one that is an nitrogenated analog of
glycerol carbonate.
Notes and references
z Materials. g-Al2O3, P25 TiO2, MgO, CeO2 (40 nm average size),
non-nanoparticulated CeO2 (non-np-CeO2) and amino alcohols were
commercial samples (Aldrich). nanoparticulated CeO2 (np-CeO2) and
ZrO2 were prepared by hydrolysis in aqueous solution of the corres-
ponding soluble salts at controlled pH. as reported in the literature.8
Au/CeO2 was obtained from HAuCl4 by a deposition–precipitation
method as reported.16
Reaction procedure. Catalytic experiments were performed in
reinforced glass reactors equipped with temperature and pressure
controllers. For each reaction, the reactant mixtures were dissolved
in ethanol (35 mmol) and placed into the reactor (2 mL capacity)
together with the corresponding catalyst (75 mg). n-Decane was used
as internal standard to determine conversions and selectivity. The
reactors were sealed and deeply introduced into the silicone bath
preheated at the required temperature. The reactions were conducted
at autogeneous pressure. During the experiment, the magnetic stirring
rate was fixed at 1000 rpm. Aliquots were taken from the reactor at
different reaction times. Once the catalyst particles were removed from
the solution by centrifuging at 12 000 rpm, the products were analyzed
by GC-MS (Hewlett-Packard, 35 m capillary column of cross-linked
5% phenyl methylsilicone).
Finally, the influence of the ring size of the resulting cyclic
carbamate was studied by using as starting materials the
parent 2-aminoethanol, 3-aminopropanol and 4-aminobutanol
to give 1,3-oxazolidin-2-one, 1,3-oxazinan-2-one and
1,3-oxazepinan-2-one, respectively. Table 3 show the data
corresponding to the highest product yield. Further increase
in o-aminoalcohol conversion leads to a decrease in the
selectivity toward the cyclic carbamate.
Table 3 shows that as the size of the resulting ring increases,
the corresponding heterocycle becomes disfavoured. It is
interesting to note that for 2-aminoethanol and 3-aminopropanol,
formation of a secondary product corresponding to a dimeric
condensation was observed. Detection of this by-product
supports our previous claims about decomposition of the
cyclic carbamate under the reaction conditions (Scheme 3).
Apparently in the case of secondary ethanolamines shown in
Table 2, self-decomposition is disfavoured with respect to
primary amines for steric reasons and the corresponding
oxazolidinones are more stable. In any case, the data of
Table 3 are again remarkable in the context of related CO2
fixation reactions with yields between 50 and 15%.
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carbamates and the differences among the ceria samples were
studied by IR spectroscopy (see ESIw). It was observed that
after CO2 chemisorption and evacuation at 160 1C (the reaction
temperature) using CH3OH as probe to characterize the active
sites, np-CeO2 is the only sample having acid sites able to form
CH3+, while commercial CeO2 (40 nm) or Au/np-CeO2
lack these sites. These acids sites are required to effect the
esterification of carbamic acid into the cyclic carbamate.
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Scheme 3 Proposed mechanism of cyclic carbamate decomposition
and formation of dimeric diamines.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4181–4183 | 4183