CO2-Fixation on Aliphatic a,w-Diamines to Form Cyclic Ureas
amines towards nucleophilic addition or -substitution reac-
tions. In the case of diethylenetriamine, the reaction formed
a five-membered ring in which one of the nitrogen atoms of
the urea contained a 2-aminoethyl substituent. Increasing the
length of the a,w-diaminoalkene substantially lowered the
yield of the corresponding urea, owing to the much-higher dif-
ficulty of forming larger rings.
Concerning the reusability of the catalyst, we observed
a gradual decrease in the catalytic activity of the sample,
which almost completely disappeared after three uses. To reac-
tivate the deactivated catalyst, we submitted the used sample
to different treatments, including calcination in air at 4508C,
washing with acids and bases, etc. However, the most-efficient
reactivation procedure, which consisted of washing the used
catalyst with 0.05 m HCl in water to remove the basic amines
and byproducts derived thereof, followed by drying at 608C,
was still not fully satisfactory and the ceria catalyst only recov-
ered 40% of the activity of the fresh material.
Figure 3. Yield of imidazolidin-2-one from the reaction of ethylendiamine
^
with CO2 in the presence of np-CeO2 (8 nm; ) and commercial CeO2
~
(40 nm; ). Reaction conditions: ethylendiamine (2 mmol), EtOH (35 mmol),
CO2 (7 bar), CeO2 (75 mg), 1608C. Inset: Arrhenius plot of the natural loga-
rithm of the estimated initial reaction rate versus the inverse of the absolute
temperature for the reaction between ethylenediamine and CO2 in the pres-
ence of CeO2 (8 nm).
at intermediate conversion whilst the reaction mixture was still
hot, indicated that the reaction stopped completely after the
removal of the solid, thus indicating that the process was truly
heterogeneous and was not owing to the presence of some
species that had leached from the solid into the solution that
could act as the “real” catalytic species.
One reason for this drop in activity could be that carbonate-
like species develop on the surface of the CeO2 nanoparticles
and block the active sites on the surface. This possibility was
supported by the fact that IR spectroscopic analysis showed
the presence of inorganic carbonates on the used CeO2 cata-
lyst. However, our attempts to thermally decompose these car-
bonate-like species at moderate temperatures to restore the
catalytic activity to that of fresh ceria samples were not com-
pletely successful.
After having shown the superior efficiency of the nanoparti-
culate ceria catalyst that was prepared by the porous-biopoly-
mer-templation method, we expanded the scope of the reac-
tion to include other primary- and secondary amines (Table 1).
Secondary amines that were structurally related to ethylenedia-
mine were formed in much-lower yields; this result was in
agreement with the expected lower reactivity of secondary
In conclusion, herein, we have used mesoporous alginate
aerogel as a natural polymeric template to obtain nanoparticu-
late ceria with small particle size. This nanoparticulate ceria
was active in promoting the cycloaddition of CO2 to aliphatic
amines to form cyclic ureas. Ethylenediamine was a suitable
substrate, thereby forming a five-membered ring. The yield of
CO2-fixation on a,w-diamines decreased for larger cycles or
when secondary amines were used. The catalyst became deac-
tivated and gradually lost its catalytic activity. Attempts to re-
activate used catalysts only led to a partial recovery of the ini-
tial activity of the fresh materials.
Table 1. Yield of product from the insertion reaction between a,w-di-
amines and CO2 in the presence of np-CeO2 as the catalyst.
Entry Reaction
Conditions[a] Yield
[%]
1
2
3
4
5
A
B
C
D
E
11
23
14
22
37
Experimental Section
6
7
E
E
36
5
Materials
CeO2 (average size <50 nm) was obtained from Aldrich. Nanoparti-
culate CeO2 (np-CeO2, 8 nm) was prepared by the dropwise addi-
tion of 1 equiv (w/w) of sodium alginate to a stirred 0.1m aqueous
solution of (NH4)2Ce(NO3)6 at RT by using a syringe (internal diame-
ter: 0.8 mm). The gel beads were stirred overnight at RT and then
filtered and washed with distilled water. Next, the hydrogel beads
were dehydrated by a series of consecutive washings with EtOH/
water solutions of increasing alcohol concentration (10, 30, 50, 70,
90, and 100%) for 15 min each. The alcogel beads were dried
under supercritical CO2 conditions (slightly above 73 bar and
318C), thereby yielding aerogel beads. To remove all of the organic
matter and to obtain nanoparticulate cerium dioxide, the beads
were calcined in air at 5408C.
8
9
E
E
9
19
[a] General conditions: a,w-diamine (2 mmol), EtOH (35 mmol), CO2
(7 bar), catalyst (75 mg), 8 h. Specific conditions A: np-CeO2 (40 nm, Al-
drich), 1608C; B: np-CeO2 (8 nm), 1608C, triethylamine; C: np-CeO2
(8 nm), 1308C; D: np-CeO2 (5 nm), 1458C; E: np-CeO2 (8 nm), 1608C.
ChemCatChem 0000, 00, 1 – 5
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