Calcined Mg-Al layered double hydroxide as a heterogeneous catalyst for the synthesis of urea derivatives from amines and CO2

Article abstract of DOI:10.1002/cjoc.201201143

The calcined Mg-Al layered double hydroxides (LDHs) with a Mg/Al molar ratio of 3:1 were synthesized and characterized thoroughly by X-ray diffraction (XRD), temperature-programmed desorption (TPD) of CO₂, and thermogravimetric analysis (TGA). Thus the calcined Mg-Al LDHs were used as catalyst for the catalytic synthesis of disubstituted ureas from amines and CO₂. The effects of reaction time, reaction temperature, pressure, solvent and calcined temperature on activity have been investigated. The results indicated that aliphatic amines, cyclohexylamine and benzylamine can be converted to the corresponding ureas selectively over the calcined Mg-Al LDHs catalysts with N-methyl-2-pyrrolidone (NMP) as solvent without using any dehydrating regent. The catalyst can be recycled several times with only slight loss of activity. Copyright

Full text of DOI:10.1002/cjoc.201201143

FULL PAPER
DOI: 10.1002/cjoc.201201143
Calcined Mg-Al Layered Double Hydroxide as a Heterogeneous
Catalyst for the Synthesis of Urea Derivatives from
†
Amines and CO2
Ran Zhang, Li Hua, Li Guo, Baoning Song, Jizhong Chen, and Zhenshan Hou*
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of
Science and Technology, Shanghai 200237, China
The calcined Mg-Al layered double hydroxides (LDHs) with a Mg/Al molar ratio of 3∶1 were synthesized and
characterized thoroughly by X-ray diffraction (XRD), temperature-programmed desorption (TPD) of CO , and
2
thermogravimetric analysis (TGA). Thus the calcined Mg-Al LDHs were used as catalyst for the catalytic synthesis
of disubstituted ureas from amines and CO . The effects of reaction time, reaction temperature, pressure, solvent
2
and calcined temperature on activity have been investigated. The results indicated that aliphatic amines, cyclo-
hexylamine and benzylamine can be converted to the corresponding ureas selectively over the calcined Mg-Al
LDHs catalysts with N-methyl-2-pyrrolidone (NMP) as solvent without using any dehydrating regent. The catalyst
can be recycled several times with only slight loss of activity.
Keywords carbon dioxide, 1,3-disubstituted urea, hydrotalcites, layered double hydroxide, heterogeneous cataly-
sis
Introduction
bon dioxide is environmentally benign and conveniently
available, therefore more and more concerns have been
received to use carbon dioxide as direct carbon source
to synthesize 1,3-disubstituted urea derivatives. Re-
cently, the direct synthesis of 1,3-disubstituted urea de-
There is currently necessary for alternatives to chem-
ical feedstocks for the fine chemicals industry that is
independent of oil. Carbon dioxide has emerged as a
renewable candidate for its abundance, non-toxicity and
non-inflammability. However, its use as a C-1 reagent
poses big challenges for its thermodynamic and kineti-
cal stability. On the other hand, as we have known,
N-containing compounds such as isocyanates, car-
bamates and 1,3-disubstituted urea derivatives are im-
portant chemicals. 1,3-Disubstituted urea derivatives are
good chemical intermediates in the synthesis of agricul-
tural chemicals, pharmaceuticals, and also widely used
as antioxidants in gasoline and additives as well as in
rivatives from amines and CO has been reported. Sev-
2
eral types of catalysts, including strong inorganic bases
[
8]
Cs CO , homogeneous catalysts such as RuCl •H O/
2
3
3
2
[9]
[10]
Bu3P and titanium imido complexes, the ionic liq-
uid [Bmim]OH, and supported base catalysts such as
CsOH/IL and KOH/ PEG1000 have been utilized
for this reaction. Although 1,3-disubstituted urea can
also be synthesized from carbonylation of amines with
[
2]
[
11]
[12]
CO in absence of any solvent and catalyst under opti-
2
[
13]
[
1,2]
mal conditions,
the reaction conditions were harsh
dye chemistry.
and remained to be improved.
Highly toxic phosgene is used as raw material in the
traditional synthetic process of 1,3-disubstituted urea
derivatives. Because of the high toxicity of phosgene,
some alternative routes have been developed. Oxidative
carbonylations of amines with CO as carbonyl resource
The previously reported catalysts are normally ex-
pensive and difficult to be recycled. Clearly, to develop
an efficient, cheaper and reusable catalytic system
which can transform carbon dioxide and a series of or-
ganic amines into 1,3-disubstituted urea directly still
remains challenges.
[
3]
are studied by using Au/polymer, metallomacrocyclic
[
4]
[5]
[6]
compounds, Ru(CO)
3
(PPh
3
)
2
,
W(CO)
6
,
etc. as
catalysts. However, CO is toxic and the mixture of car-
bon monoxide and air is dangerous. Thus expensive
methylating reagent like dimethyl carbonate has also
It was reported that solid base catalysts like LDHs
could show an excellent performance in various base-
catalyzed organic transformations and also be used as
[7]
[14,15]
been used as alternative carbon source. Because car-
adsorbents for carbon dioxide or other acidic gases.
*
E-mail: houzhenshan@ecust.edu.cn; Tel.: 0086-021-64251686
Received November 18, 2012; accepted January 31, 2013; published online March 14, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201201143 or from the author.
Dedicated to the 60th Anniversary of East China University of Science and Technology.
†
Chin. J. Chem. 2013, 31, 381—387
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
381

Zhang et al.
FULL PAPER
They can be seen as possessing a pair of strong basic
pressure gauge and thermocouple. The desired amounts
of amine, catalyst, and the organic solvent were loaded
into autoclave, successively. Then the reactor was
2
−
sites (O ions) and weak Lewis acid sites (co-ordina-
3
＋
[16,17]
tively unsaturated Al ions).
Inspired by the pre-
[18,19]
vious work,
we prepared a series of the calcined
2
closed and purged three times with dry CO . The reac-
Mg-Al LDHs with Mg-Al LDH as precursor and then
attempted to use the calcined Mg-Al LDHs as the cata-
lyst for carbonylation of amine with CO . Herewith, the
2
LDH materials have been characterized and then ap-
plied in the synthesis of 1,3-disubstituted urea deriva-
tives from direct carbonylation of various amines and
CO with 1-methyl-2-pyrrolidinone (NMP) as solvent.
2
The reaction conditions have been optimized to achieve
high catalytic performance.
tion was magnetically stirred and heated in a tempera-
ture-controlled heating jacket. After reaction, the reactor
was cooled under ice bath condition, and then carefully
depressurized. The product was recovered by adding
3
CH OH to reaction mixture, and the catalyst was sepa-
rated from the organic liquid phase by centrifuging. The
liquid phase was analyzed by gas chromatograph (Shi-
madzu GC-2014, KB-50 capillary column, 30 m×0.32
mm×0.50 μm) and GC-MS (Agilent-6890GC-5973MS,
HP-5MS, 30 m×0.25 mm×0.25 μm). Conversion and
selectivity of products were then both obtained.
Experimental
For the recycling of catalysts, the recovered Mg-Al
LDH was regenerated by a thermal regeneration as re-
General
[20,21]
All chemicals were analytical grade and commer-
cially available. They were used without further purifi-
ported in the previous literature.
After each run, the
catalyst was recovered and then calcined according to
the previous procedure.
cation, unless otherwise stated. CO
Shanghai Shangnong Gas Factory with a purity of
99.95%.
2
was supplied by
Catalyst characterization
＞
The LDH structure and its evolution were identified
by X-ray diffraction (XRD). The XRD patterns were
recorded using a Rigaku D/max 2550VB/PC X-ray dif-
fractometer (with a variable slit width). Graphite mono-
chromator, Cu Kα target (λ＝0.15056 nm), tube voltage
40 kV, tube current 40 mA, emission slit (1/6)°, scan
rate 6 (°)/min, scan area 10－80°. The samples were
also characterized by thermogravimetric analysis (TGA)
Synthesis of Mg-Al LDH
The Mg-Al LDHs used in this work were prepared
via a co-precipitation technique. Briefly, 0.09 mol
Mg(NO ) •6H O and 0.03 mol Al(NO ) •9H O were
3 2 2 3 3 2
dissolved in 120 mL deionized water. The solution was
then added, dropwise with vigorous stirring, to a 120
mL solution containing 0.24 mol NaOH and 0.03 mol
Na
with continuous stirring and the pH was maintained at
0. The resultant mixture was then aged at 70 ℃ for 18
2
CO
3
. The mixture was then refluxed at 70 ℃ for l h
2
and N adsorption-desorption method. Thermal decom-
position diagrams of the raw LDH and calcined samples
were investigated through TGA instrument. All samples
were heated at a constant heating rate of 10 ℃/min in
flowing anhydrous air (20 mL/min) from room tem-
perature to 800 ℃. The specific surface area (BET) was
1
h. The resulting precipitate was separated by vacuum
filtration, washed with deionized water repeatedly and
then dried at 80 ℃ in a vacuum drying oven overnight.
Similarly, Zn-Al LDH was also prepared and character-
ized by XRD (Figure 1S).
2
calculated by the N adsorption-desorption method at 77
K using a Quantachrome Instrument. Prior to the ex-
periments the catalysts were outgassed at 200 ℃ for 12
h under vacuum. Surface concentrations of the basic
centers were determined by temperature-programmed
Synthesis of the calcined Mg-Al LDHs
The Mg-Al LDHs were placed in a ceramic crucible
with a lid and heated in muffle furnace at a ramp-rate of
desorption of carbon dioxide (CO
carried under argon as carrier gas with flowing rate 20
mL/min. Prior to CO adsorption, all prepared catalysts
were pretreated at 100 ℃ for 1 h and exposed to a flow
of CO for 2 h. Desorption proceeded by heating (10 ℃/
2 2
-TPD). CO -TPD was
5
℃/min to a specified temperature such as 550 ℃.
The material was held at this temperature for 6 h before
being allowed to cool to room temperature at a rate of
ca. 10 ℃/min. The material obtained was stored ready
for use in a dessicator filled with activated molecular
sieve and color silica gel so as to minimise the uptake of
atmospheric water. The material was recorded as Mg-Al
LDH550. In the same way, a series of the calcined
Mg-Al LDHs were prepared by calcining Mg-Al LDH
at 450, 650, 750 and 850 ℃, which were denoted as
Mg-Al LDH450, Mg-Al LDH650, Mg-Al LDH750 and
Mg-Al LDH850, respectively.
2
2
min) up to the final temperature of 650 ℃.
Results and Discussion
The solvent effect
At first, we chose the direct carbonylation of butyl-
amine (BA) and CO as a model reaction to examine the
2
effect of solvents on the reaction (Scheme 1).
It was found that the butylamine conversion was
only 10.1% (Table 1, entry 1) when the reaction was
carried out in the absence of any solvent and catalyst
under the present conditions. The activity was improved
Procedure for the carbonylation reactions
Reactions were performed in 50 mL stainless steel
autoclaves (32 mL with PTFE liner), equipped with
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Chin. J. Chem. 2013, 31, 381—387

Heterogeneous Catalyst for the Synthesis of Urea Derivatives from Amines and CO
2
Scheme 1 Carbonylation of butylamine with CO
2
44.5% and 48.2%, respectively (Table 2, entries 3 and
), which demonstrated that Mg-Al LDH has an obvious
advantage for this reaction. It has been reported that the
5
O
CO + n-Bu NH2
n-Bu
N
N
n-Bu + H O
basicity of the catalysts can play an important role in the
reaction. Therefore, the CO -TPD was carried out to
2
2
2
H
H
[8]
investigate further the distribution of the surface basic
slightly but still was relatively low if Mg-Al LDH550
was used as catalyst without any solvent (Table 1, entry
centers. Figure 1 showed the CO -desorption curves of
2
different catalysts. When considering the desorption
2
). In the next step, the different solvents such as terti-
temperature of CO , it appeared that only weak basic
2
ary butanol, acetonitrile and NMP have been screened
for the reaction. It was observed that the moderate con-
version was afforded with NMP as solvent in the ab-
sence of catalyst (25.0%, Table 1, entry 3). While
centers existed on surfaces of Al O and MgO, while
2
3
Mg-Al LDH and TiO showed weak basic centers and
2
some moderately strong basic centers. It should be no-
ticed that Zn-Al LDH showed more amounts of very
strong basic centers, in comparison with Mg-Al LDH.
Combined with the catalytic performance shown above,
it can be concluded that weak and moderate strong basic
centers are critical to the reaction, while too strong basic
centers are unfavorable to the reaction.
(
CH
tylamine conversion was obtained (Table 1, entry 4).
Although CH CN could afford higher butylamine con-
3 3
) COH was used as reaction solvent very low bu-
3
version, the selectivity was too low (Table 1, entry 5).
In contrast, good conversion and excellent selectivity
can be achieved when NMP was used as solvent (Table
1
, entry 6). The results above showed that different sol-
Table 2 Synthesis of 1,3-disubstituted urea in presence of dif-
ferent catalysts
a
vents affected the conversion considerably. Acetonitrile
reacted with the substrate and thus the selectivity was
poor. The basic solvent NMP gave best results. In con-
trast, tertiary butanol seemed to inhibit the reaction (Ta-
ble 1, entries 2 and 4) due to its very weak solvation. It
was reported that NMP experienced a volume expansion
of ~30% when exposed to 2.5 MPa of CO
which indicated that CO can be dissolved in NMP con-
siderably. In addition, NMP could also stabilize the
b
Entry
Catalyst
MgO
Conv./%
5.7
Sel. /%
≥99
≥99
≥99
≥99
≥99
1
2
3
4
5
Al
2
O
3
36.8
[
8,22]
TiO2
44.5
2
at 25 ℃,
Zn-Al LDH
Mg-Al LDH
29.2
2
48.2
[
23]
ionic intermediates by the deprotonation, which was
favorable to the reaction. Thus NMP as solvent afforded
the highest reaction activity.
a
Reaction conditions: BA 32 mmol, pressure 4.0 MPa, tempera-
b
ture 170 ℃, time 4 h, catalyst 0.300 g, NMP 2 mL. A trace of
dibutylamine was detected.
Table 1 Effect of different solvents on the carbonylation of
a
butylamine with CO2
b
Entry
1
Catalyst
None
Solvent
None
Conv./%
10.1
Sel. /%
≥99
Mg-Al
LDH550
2
3
4
None
NMP
30.2
25.0
5.2
≥99
≥99
≥99
None
Mg-Al
LDH550
Mg-Al
LDH550
Mg-Al
(CH ) COH
3
3
c
5
6
CH
3
CN
92.0
67.0
35.0
NMP
≥99
LDH550
Figure 1 Temperature-programmed desorption of CO
dicated samples. (a) Al ; (b) TiO ; (c) MgO; (d) Mg-Al LDH;
e) Zn-Al LDH.
2
for in-
a
Reaction conditions: BA 32 mmol, pressure 3.0 MPa, tempera-
2
O
3
2
ture 170 ℃, reaction time 6 h, NMP: 2 mL. 0.300 g Mg-Al
LDH550 catalyst was charged if needed. Selectivity to 1,3-di-
substituted urea. By-product was detected as N-butylacetamide.
(
b
c
Effect of calcination temperature
For the sake of comparison, the different catalysts
have been used with NMP as solvent for the reaction.
The Mg-Al LDH samples calcined at different tem-
peratures were characterized by XRD. As shown in Fig-
ure 2, the Mg-Al LDH exhibited some common features
of layered materials (e.g. narrow, symmetric, strong
lines at low 2θ values and weaker, less symmetric lines
at high 2θ values) (Figure 2a). The LDHs samples cal-
As shown in Table 2, MgO, Al
only 5.7%, 36.8%, and 29.2% of butylamine conversion,
respectively (Table 2, entries 1, 2 and 4). TiO and
Mg-Al LDH catalysts afforded moderate conversion as
2 3
O and Zn-Al LDH gave
2
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383

Zhang et al.
FULL PAPER
cined at 450 ℃ (Figure 2b) caused the crystallinity
decreasing with no any other significant changes in the
structures. On the other hand, a calcination temperature
of 550 ℃ destroyed the LDH structure and led to the
formation of a periclase MgO phase in addition to
amorphous aluminium oxides (Figure 2c). These results
are consistent with previous findings that periclase MgO
is the sole crystalline phase expected to occur at this
[24,25]
calcination temperature.
Raising the calcination
temperature to 650 and 750 ℃ (Figure 2d and 2e) re-
sulted in no change in the phases and only a slowly
sharpening of the MgO reflections in the samples. There
2 4
occurred reflections for spinel MgAl O when calcined
at 850 ℃ (Figure 2f). Obviously, the structure calcined
at 550 ℃ was transient state from layered double hy-
droxides to layered double oxides, and some amorphous
phases occurred.
Figure 3 Temperature-programmed desorption of CO for in-
2
dicated samples. All samples were calcined for 6 h. (a) no calci-
nation; (b) Mg-Al LDH450; (c) Mg-Al LDH550; (d) Mg-Al
LDH650; (e) Mg-Al LDH750; (f) Mg-Al LDH850.
The distribution of the surface basic centers was also
monitored by CO
2
-TPD (Figure 3). As shown in Figure
was used and afforded 67.0% conversion of butylamine,
which is higher than 59.0% conversion reported with
3
, there mainly existed weak basic centers in Mg-Al
[2]
LDH precursor (Figure 3a) or its products calcined at
either 450 ℃ (Figure 3b) or 850 ℃ (Figure 3f). In
contrast, a large amount of basic centers appeared at
calcination temperature ranged from 550 to 750 ℃
[Bmim]OH as catalyst. The catalytic activity was
consistent with the special structure of Mg-Al LDH550
as shown in Figure 2c, where low crystalline phase
formed along with structure transition from layered
double hydroxides to layered double oxides compared
with other samples. Perhaps it is just the amorphous
phase with more exposed defects that promotes the re-
(
Figure 3c, 3d and 3e). It can be seen that the material
calcined at 550 ℃ harbored the largest amount of
moderately strong base.
2
action. In addition, the CO -TPD in Figure 3c has illus-
trated that Mg-Al LDH550 possessed broad distribution
of moderately strong basic centers, which can also con-
tribute to the relatively higher catalytic activity. The
Mg-Al LDH 550 and Mg-Al LDH 650 showed higher spe-
cific surface area (Table 1S), as comparison with those
samples calcined at other temperatures, resulting in
higher catalytic activity (Figure 4).
Figure 2 XRD patterns of Mg-Al LDH and its products cal-
cined at different temperatures. (a) Mg-Al LDH; (b) Mg-Al
LDH450; (c) Mg-Al LDH550; (d) Mg-Al LDH650; (e) Mg-Al
LDH750; (f) Mg-Al LDH850. The phases were indicated by the
different symbols: ● hydrotalcite, ƒ periclase, ▼ spinel.
The pattern of typical TG also substantiated the for-
mation of Mg-Al LDHs (Figure 2S). The mass loss in
the range from the room temperature to around 200 ℃
can be attributed to the removal of interlayered water,
the gradual mass loss from 200 to 600 ℃ can be at-
tributed to the decomposition of hydroxyl groups from
the inorganic layers. These phenomena were consistent
with structure evolution as shown in Figure 2.
Figure 4 Influence of calcination temperature on the reaction
activity. BA 32 mmol, pressure 3.0 MPa, time 6 h, temperature
1
70 ℃, Mg-Al LDH550 0.300 g, NMP 2 mL. All samples were
calcined for 6 h. The selectivity to 1,3-disubstituted urea was
more than 99% in all cases.
Effect of reaction conditions
Figure 4 showed the effect of calcination tempera-
ture on the reaction activity. We can see that a maxi-
mum can be observed when Mg-Al LDH550 catalyst
It was observed that a suitable reaction temperature
was crucial for the reaction to reach a reasonable con-
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Chin. J. Chem. 2013, 31, 381—387

Heterogeneous Catalyst for the Synthesis of Urea Derivatives from Amines and CO
2
version (Figure 5). The conversion reached a maximum
dissolved in the liquid phase and fully contacted with
[13]
at 170 ℃, i.e. 67.0%. This reaction is exothermic and
the substrate, facilitating the reaction.
On the other
[
8]
reversible. If the reaction temperature is too low, reac-
tion activation energy cannot be achieved, resulting in
low activity. When the temperature was increased up
too high, the reaction equilibrium was not favored to
shift towards the right and the conversion decreased.
Thus a maximum occurred as the temperature increased.
hand, too high CO pressure could cause a dilution ef-
2
fect and also make the surface of the Mg-Al LDH550 to
be adsorbed with an excess of carbon dioxide and there
were few and even no active centers accessible to sub-
[
26]
strate molecules. Thus a maximum occurred with the
changes of CO pressure.
2
Figure 5 Effect of reaction temperature on the activity. BA 32
mmol, pressure 3.0 MPa, time 6 h, Mg-Al LDH550 0.300 g,
NMP 2 mL. The selectivity to 1,3-disubstituted urea was more
than 99% in all cases.
Figure 7 The dependence of the conversion of butylamine on
the reaction pressure. BA 32 mmol, temperature 170 ℃, time 6 h,
Mg-Al LDH550 0.300 g, NMP 2 mL. The selectivity to 1,3-di-
substituted urea was more than 99% in all cases.
Figure 6 showed the reaction kinetics curve over
Mg-Al LDH550 catalyst. The carbonylation reaction
between amines and CO can be performed smoothly
2
without any dehydrating agents. With the increasing
reaction time, the butylamine conversion first sharply
increased and then kept constant after reaction for 6 h.
Recycling of Mg-Al LDH550 catalyst
Table 3 showed that Mg-Al LDH550 can be recy-
cled at least 4 times with only slight loss of catalytic
activity, which might result form the mechanical loss of
catalyst for each catalytic run (Table 3, entries 1－4).
This can be clarified that if the ratio of substrate to the
recovered catalyst was kept constant for the consecutive
recycles, the catalytic activity can be restored (Table 3,
entry 5).
Table 3 Recyling of catalyst in the carbonylation of amine with
a
CO2
b
Run
1
m
catalyst /g
0.300
0.291
0.249
0.219
0.204
Conv./%
68.7
Sel./%
≥99
≥99
≥99
≥99
≥99
2
65.7
3
62.0
4
58.8
c
5
65.8
Figure 6 Effect of reaction time on the activity. BA 32 mmol,
pressure 3.0 MPa, temperature 170 ℃, Mg-Al LDH550 0.300 g,
NMP 2 mL. The selectivity to 1,3-disubstituted urea was more
than 99% in all cases.
a
Reaction condition: BA 32 mmol, time 6 h, temperature 170 ℃,
b
pressure 3.0 MPa, NMP 2 mL. Catalyst was regenerated after
every run through calcining at 550 ℃ for 6 h. Less substrate
c
(
22.4 mmol) was used.
The reaction pressures also have a significant influ-
ence on the reaction. As shown in Figure 7, the bu-
tylamine conversion firstly increased with the reaction
pressure and then started to decrease when pressure is
above 3.0 MPa. At pressures below 3.0 MPa, with
As reported previously, DMSO can make LDH form
a transparent or semi-transparent colloidal system and
[
27]
allowed the partial exfoliation of LDHs, which fol-
[
28]
lowed a similar mechanism to that of formamide,
presumably because of their strong hydrogen-bonding
interaction with the hydroxyl groups of the LDH layers.
2
pressure increasing, more compressed CO could be
Chin. J. Chem. 2013, 31, 381—387
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385

Zhang et al.
FULL PAPER
Interestingly, in our work we observed that NMP phase
became semi-transparent colloidal and viscous after
reaction. Observed from X-ray diffraction patterns in
Figure 8b, the structure of Mg-Al LDH550 disappeared
after reaction and there occurred three new peaks at low
angles with one sharp peak and two broad peaks. It has
been known that the evolution of an amorphous-like
halo in a 2θ range of 3°－20° is the characteristics of
amine as a substrate (Table 4, entry 1), it indeed exhib-
ited comparable activity with that of homogenous cata-
lyst as reported when long chain aliphatic amine, ben-
zylamine and cyclohexylamine were used as substrates
[
8]
(Table 4, entries 2－4).
a
Table 4 Carbonylation of other organic amines with CO
2
Entry
Substrate
Product
Conv./%
[29-32]
exfoliation.
The broad halo at 2θ＝7.5° (arrow po-
O
sition) in Figure 8b is due to scattering from an aggre-
gate of partially exfoliated LDH sheets, and the sharp
diffraction was perhaps arisen from structure fracture in
the reaction-induced exfoliation. Interestingly, adding
amounts of water restored its structure and further iden-
tified its exfoliation (Figure 8c). The catalyst after reac-
tion was calcined at 550 ℃ for 6 h, the structure of
Mg-Al LDHs was regenerated unambiguously (Figure
i-Pr NH2
1
1a
i-Pr
N
H
N
i-Pr
22.7
H
2
a
O
n-He NH2
n-He
N
H
N
H
n-He
2
3
51.8
58.9
1
b
2b
O
8
d).
Cy NH2
Cy
N
H
N
H
Cy
1
c
2
c
O
Bn NH2
4
Bn
N
H
N
H
Bn
70.2
1d
2
d
a
Reaction condition: amine 32 mmol, pressure 3.0 MPa, time 10
h, temperature 170 ℃, Mg-Al LDH550 0.300 g, NMP 2 mL. The
selectivity to 1,3-disubstituted urea was more than 99% in all
cases.
Conclusions
In summary, the calcined Mg-Al layered double hy-
droxide (Mg-Al LDH550) is an efficient heterogeneous
catalyst for the synthesis of 1,3-disubstituted urea from
Figure 8 XRD patterns of Mg-Al LDH550 catalyst. (a) before
reaction; (b) after reaction; (c) adding water after reaction; (d)
after one catalytic recycle and calcined at 550 ℃ for 6 h. Arrow
position: broad halo.
different amines and CO in NMP without any dehy-
2
drating agents. This catalytic process is cost-effective
because the catalyst is cheap, easily available and reus-
able. In addition, NMP played an important role in the
reaction. Although the calcined Mg-Al LDH was exfoli-
ated in the course of reaction, the structure of LDH can
be regenerated only thorough simple calcination, which
offered the good recyclability for present catalytic sys-
tem.
According to the characterization of XRD (Figure 8),
NMP and the bulky product molecular could intercalate
into the layers and partial exfoliation of LDH emerged
in the course of reaction. It has been observed that the
exfoliated catalyst afforded low butylamine conversion
(
38.4%) because some water was present in exfoliated
samples, and also strong interaction between the inter-
calated molecules and basic centers, which could hinder
the adsorption and activation of substrate molecules on
the catalyst. However, the calcined materials can restore
the catalytic activity perfectly (Table 3). This implied
that although the exfoliation might destroy to some ex-
tent the structure of Mg-Al LDHs, the layered double
hydroxide structure can be recovered easily and af-
forded the almost same catalytic activity as the fresh
calcined LDH catalyst.
Acknowledgement
The authors are grateful for the support from the Na-
tional Natural Science Foundation of China (No.
2
1073058), the Research Fund for the Doctoral Program
of Higher Education of China (20100074110014), and
the Fundamental Research Funds for the Central Uni-
versities.
Finally, the scope of the substrates was examined
and the results were displayed in Table 4. It can be seen
that although the present Mg-Al LDH catalyst still af-
forded low activity with the branch chain aliphatic
References
[1] Bigi, F.; Maggi, R.; Sartori, G. Green Chem. 2000, 2, 140.
[2] Jiang, T.; Ma, X. M.; Zhou, Y. X.; Liang, S. G.; Zhang, J. C.; Han, B.
386
www.cjc.wiley-vch.de
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2013, 31, 381—387

Heterogeneous Catalyst for the Synthesis of Urea Derivatives from Amines and CO
2
X. Green Chem. 2008, 10, 465.
3] Shi, F.; Deng, Y. Q. J. Catal. 2002, 211, 548.
4] Leung, T. W.; Dombek, B. D. J. Chem. Soc., Chem. Commun. 1992,
Kim, T. W.; Sahimi, M.; Tsotsis, T. T. Ind. Eng. Chem. Res. 2008, 47,
6150; (h) Wang, X. P.; Yu, J. X.; Cheng, J.; Hao, Z. P.; Xu, Z. P.
Environ. Sci. Technol. 2008, 42, 614.
[
[
3
, 205.
[16] Shen, J. Y.; Kobe, J. M.; Chen, Y.; Dumesic, J. A. Langmuir 1994,
10, 3902.
[17] Tichit, D.; Bennani, M. N.; Figueras, F.; Ruiz, J. R. Langmuir 1998,
14, 2086.
[18] Talawar, M. B.; Jyothi, T. M.; Sawant, P. D.; Rajab, T.; Rao, B. S.
Green Chem. 2000, 2, 266.
[19] Climent, M. J.; Corma, A. S.; Iborra, S.; Primo, J. J. Catal. 1995,
151, 60.
[
[
5] Cenini, S.; Pizzotti, M.; Crotti, C.; Porta, F.; La Monica, G. J. Chem.
Soc., Chem. Commun. 1984, 19, 1286.
6] McCusker, J. E.; Main, A. D.; Johnson, K. S.; Grasso, C. A.;
McElwee-White, L. J. Org. Chem. 2000, 65, 5216.
7] Nagaraju, N.; Kuriakose, G. Green Chem. 2002, 4, 269.
8] Ion, A.; Parvulescu, V.; Jacobs, P.; De Vos, D. Green Chem. 2007, 9,
[
[
1
58.
[
9] Fournier, J.; Bruneau, C.; Dixneuf, P. H.; Lécolier, S. L. J. Org.
Chem. 1991, 56, 4456.
[20] Zhang, F.; Ni, Z.; Xia, S. J.; Liu, X. M.; Wang, Q. Q. Chin. J. Chem.
2009, 27, 1767.
[
[
10] James, C.; Anderson; Rafael, B. M. Org. Biomol. Chem. 2012, 10,
334.
11] Shi, F.; Deng, Y. Q.; SiMa, T. L.; Peng, J. J.; Gu, Y. L.; Qiao, B. T.
Angew. Chem., Int. Ed. 2003, 42, 3257.
[21] Crepaldi, E.; Tronto, J.; Cardoso, L. P.; Valim, J. B. Colloids Surf., A:
Physicochem. Eng. Aspects 2002, 211, 103.
[22] Rajasingam, R.; Lioe, L.; Pham, Q. T.; Lucien, F. P. J. Supercrit.
Fluids 2004, 31, 227.
1
[
[
12] Kong, D. L.; He, L. N.; Wang, J. Q. Synlett 2010, 8, 1276.
13] Wu, C. Y.; Cheng, H. Y.; Liu, R. X.; Wang, Q.; Hao, Y. F.; Yua, Y. C.;
Zhao, F. Y. Green Chem. 2010, 12, 1811.
[23] Vaidya, P. D.; Mahajani, V. V. Ind. Eng. Chem. Res. 2005, 44, 1868.
[24] Reichle, W. T.; Kang, S. Y.; Everhardt, D. S. J. Catal. 1986, 101,
352.
[
[
14] (a) Kagunya, W.; Hassan, Z.; Jones, W. Inorg. Chem. 1996, 35, 5970;
[25] Aramendía, M. A.; Avilés, Y.; Borau, V.; Luque, J. M.; Marinas, J.
M.; Ruiz, J. J. Mater. Chem. 1999, 9, 1603.
[26] Zhao, F. Y.; Fujita, S.; Akihara, S. J.; Arai, M. J. Phys. Chem., A
2005, 109, 4419.
[27] Zhao, Y.; Yang, W. D.; Xue, Y. H.; Wang, X. G.; Lin, T. J. Mater.
Chem. 2011, 21, 4869.
[28] Gordijo, C. R.; Constantino, V. R. L.; Silva, D. D. J. Solid State
Chem. 2007, 180, 1967.
[29] Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682.
[30] Wu, Q. L.; Sjåstad, A. O.; Ørnulv, B.; Knudsen, K. D.; Roots, J.;
Pedersen, J. S.; Norby, P. J. Mater. Chem. 2007, 17, 965.
[31] Ma, R. Z.; Liu, Z. P.; Li, L.; Iyi, N.; Sasaki, T. J. Mater. Chem. 2006,
16, 3809.
(b) Carlino, S.; Hudson, M. J.; Husain, S. W.; Knowles, J. A. Solid
State Ionics 1996, 84, 117; (c) Kaneda, K.; Ueno, S.; Ebitani, K.
Curr. Top. Catal. 1997, 1, 91; (d) Velu, S.; Swamy, C. S. React.
Kinet. Catal. Lett. 1997, 62, 339; (e) Othman, M. R.; Rasid, N. M.;
Fernando, W. J. N. Chem. Eng. Sci. 2006, 61, 1555.
15] (a) Ram Reddy, M. K.; Xu, Z. P.; Lu, G. Q.; Diniz da Costa, J. C. Ind.
Eng. Chem. Res. 2006, 45, 7504; (b) Hutson, N. D.; Attwood, B. C.
Adsorption 2008, 14, 781; (c) Horiuchi, T.; Hidaka, H.; Fukui, T.;
Kubo, Y.; Horio, M.; Suzuki, K.; Mori, T. Appl. Catal., A: General
1
998, 167, 195; (d) Meis, N. N. A. H.; Bitter, J. H.; de Jong, K. P.
Ind. Eng. Chem. Res. 2010, 49, 8086; (e) Yavuz, C. T.; Shinall, B. D.;
Iretskii, A. V.; White, M. G.; Golden, T.; Atilhan, M.; Ford, P. C.;
Stucky, G. D. Chem. Mater. 2009, 21, 3473; (f) Zou, Y.; Mata, V.;
Rodrigues, A. E. Purif. Technol. 2002, 26, 195; (g) Dadwhal, M.;
[32] Román, M. S. S.; Holgado, M. J.; Jaubertie, C.; Rives, V. Solid State
Sci. 2008, 10, 1333.
(Pan, B.)
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