Converting urea into high value-added 2-oxazolidinones under solvent-free conditions

Article abstract of DOI:10.1039/c6ra21809a

Zn-modified mesoporous Mg-Al nanoplates oxides were prepared by co-precipitation and further characterized and used in the synthesis of 2-oxazolidinones from urea and epoxides under solvent-free conditions. The characterization results suggested that Zn_1.1Mg_2.0AlO_4.6, which featured more accessible active medium basic sites, were favorable for obtaining superior catalytic activity. This synthetic process is mild, convenient, simple and gives good yields up to 80%.

Full text of DOI:10.1039/c6ra21809a

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DOI: 10.1039/C6RA21809A
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ARTICLE
Converting urea into high value-added 2-oxazolidinones under
solvent-free conditions
Peixue Wang,a,b Qinghe Li, a Shimin Liu, a and Youquan Deng*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
Zn-modified mesoporous Mg–Al nanoplates oxides were prepared by co-precipitation method, characterized and used in
the synthesis of 2-oxazolidinones from urea and epoxides under solvent-free conditions. The characterization results
suggested that Zn1.1Mg2.0AlO4.6 with more accessible active medium basic sites were favorable for obtaining much superior
catalytic activity. This synthetic process is mild, convenient, simple and gives good yields to 80%.
DOI: 10.1039/x0xx00000x
www.rsc.org/
pressure or high temperature or some stoichiometric
dehydrating reagents. To the best of our knowledge, little
work appears to have been done on the reaction of urea and
1. Introduction
Carbon dioxide (CO2) is an abundant, renewable and
economical carbon resource, so the direct use of CO2 as a raw
material in chemical industry should be an ideal choice.1-4
However, carbon atom in CO2 is in its most oxidized form and
is a relatively unreactive molecule, therefore, the activation
of CO2 normally needs the usage of a highly energy starting
materials or to shift the equilibrium to the right by removing
a compound.5-6 The use of urea as an active form of carbon
dioxide is a feasible way in the chemical industry. Until now,
manufacturing urea is very mature7-9 and currently supply of
urea in China is exceeding demand. In this sense, using urea
as feedstock to produce higher valuable chemicals opens a
new window for CO2 utilization. This easily accessible
chemical has been used as building block in the synthesis of
epoxides, and the reaction could be carried out without any
solvent. In such process, urea can be used as the
carbonylation and amination agent, which is a nontoxic
material and has abundant resources with lower price.
Meanwhile, NH3 co-produced in the reaction can be easily
converted to urea, since the urea synthesis from CO2 and NH3
has been established. Hence, this route could be considered
as indirect utilization of CO2 to produce the green chemicals
of 2-oxazolidinones.
In this study, Zn-modified mesoporous Mg–Al mixed oxides
were prepared by calcinations of Zn-Mg-Al hydrotalcite-like
compounds, which were found to catalyze the 2-
oxazolidinones synthesis from urea and epoxides with high
efficiency. TG, XRD, BET, SEM and CO2-TPD studies were
conducted to explore the relationship between structure and
performance.
N-carbamoyl-L-amino
acids,10
carbamates,11-14
cyclic
carbonates or dialkyl carbonates, 11-13 diphenyl carbonate,15-16
urea derivatives17-18 and many nitrogen-containing
heterocycles.19-26 Green chemistry can be defined as the
synthesis of important chemical and biological products from
readily available and inexpensive starting materials through
atom economical and environmentally benign strategies. In
this work, urea was used for the synthesis of 2-oxazolidinones
(Scheme 1), which are important cyclic compounds in both
fine chemicals and synthetic organic chemistry. They are
widely used in the synthesis of pharmaceuticals, pesticides,
cosmetics, and so on.27-29 From economical and
environmental views, the direct synthesis of 2-oxazolidinones
from CO2 and aziridines30-33 or β-aminoalcohols34-36 is more
attractive. Nevertheless, most of these works require high
Scheme 1 Synthesis of 2-oxazolidinones from urea and epoxides.
2. Experimental
2.1 Catalysts preparation and characterization
The Mg-Zn-Al hydrotalcites (HTMg–Zn–Al) were prepared
by co-precipitation method (See supporting information).13
Elemental quantitative analyses of Mg, Zn and Al were
performed with a CCD-based inductively coupled plasma-
atomic emission spectrometer (ICP-AES, Agilent 725-ES). The
morphological structures were examined by field emission
scanning electron microscopy (FE-SEM, JSM-6701F). Thermal
analyses were carried out using a METTLER TG1 model. X-ray
diffraction (XRD) was measured on a Siemens D/max-RB
powder X-ray diffract meter. The BET surface areas of the
catalysts were obtained with physisorption of N2 using a
a.State Key Laboratory for Oxo Synthesis and Selective Oxidation, State Key
Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou, China, 730000. E-mail: ydeng@licp.cas.cn; Fax:
+86-931-4968116; Tel: +86-931-4968116
b. Qingdao Center of Resource Chemistry & New Materials, Qingdao, China,
266100.
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Micromeritics ASAP 2010. The surface base properties of the
catalysts were measured by temperature programmed
desorption (TPD) of CO2 and carried out on TPD flow system
equipped with a TCD detector.
Zn3.2AlO4.7
103
155
19
12
0.5
0.6
15
31
-
-
15
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DOI: 10.1039/C6RA21809A
Zn1.1Mg2.0AlO4.6
21
10
-
-
52
45
Zn1.1Mg2.0AlO4.6
reused
-
-
-
-
35
2.2 2-oxazolidinones synthese from urea and epoxides
a: Determined by ICP. B: Determined by TPD.
Typically, 10 mmol epoxide, 20 mmol urea and 0.05 g
catalyst were added into the stainless steel autoclave with a
glass tube inside. The reactions were carried out at 120-150
oC for 2-8 h with stirring. After reaction, the autoclave was
cooled down to room temperature. The reaction mixture was
diluted with methanol to 15 ml and then the qualitatively
identified by HP-7890 GC-MS equipped with a SE-54 capillary
column. The quantitative analysis of the product was
determined by Agilent 7890 GC equipped with a SE-54
capillary column and a FID detector (biphenyl was used as
internal standard).
Fig. 1 TGA curve of the hydrotalcite-like compounds.
3. Results and discussion
3.1 Results of the catalysts characterization
The typical TG (Fig. 1), XRD (Fig. 2) patterns demonstrated
the formation of layered double hydroxides (LDHs). All the
hydrotalcites showed three similar stages of weight loss in TG
curves. For HTZn1.1Mg2.0Al, the first step, ranging from 50 °C
to 180 °C, is due to the evaporation of surface water on the
sample. In the following stage, interlayer water molecules are
lost up to 381 °C. Dehydroxylation, removal of the hydroxyl
groups from the layers as water vapour, and decomposition
of the interlayer anions occur at higher temperatures up to
600 °C. As shown in Fig. 2, the HTMg3.0Al and HTZn3.2Al
showed the typical XRD patterns of the hydrotalcite structure
with intense sharp and symmetric peaks.37 When the Zn
component was introduced to the HTMg3.0Al, the diffraction
peaks of HTZn1.1Mg2.0Al is still consisted with the peaks of HTs,
demonstrating that the obtained sample kept the layered
structure. However, the layered structure destroyed
thoroughly in HTZn1.1Mg2.0Al and ZnO diffraction peaks are
observed after calcination. The obtained composite oxides
show high specific surface areas (>100 m2/g) owing to the
formation of a significant porosity (Table 1). All of the oxides
displayed a mesoporous material (the average pore size
ranging from about 9 to 19 nm). Besides, the sheet-like
structure can also be visualized from the SEM images (Fig. 3A),
and it exhibited mainly nanoplate-shaped crystals. After
calcination at 400 °C (Fig. 3B), the discriminable nanoplates-
like features was still exist, suggesting that 400 °C was the
appropriate treatment conditions in order to completely
decompose the interlayer anions without exceeding the
critical temperature at which phase segregation and sintering
take place.13
Fig. 2 XRD patterns of (a) HTZn3.2Al; (b) HTZn1.1Mg2.0Al; (c) HTMg3.0Al; (d)
Zn1.1Mg2.0AlO4.6 and (e) Zn1.1Mg2.0AlO4.6-reused :HTs; ZnO;
Zn(NH3)2(NCO)2).
(

▼
:
:
Table 1 Physical and base properties of the composite oxides derived from HTs
Basic sites (μmol/g)a
SBET
Dp
Vp
Fig. 3 SEM images of (A) HTZn1.1Mg2.0Al and (B) Zn1.1Mg2.0AlO4.6
.
Catalyst
(m2/g) (nm) (cm3/g)
Weak Medium Strong
<100 °C 100-300 °C >300 °C
Total
127
Mg3.0AlO4.5
158
9
0.4
60
52
15
2 | J. Name., 2012, 00, 1-3
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The surface basic properties of these composite oxide
catalysts were studied by CO2-TPD (Fig. 4) and the basic
species was assigned according to the temperature at which
peaks appeared (Table 1). Obviously, the weak strength basic
sites peak of CO2 desorption appeared at about 75 °C was
observed over all the catalysts. For the Mg3.0AlO4.5, the
desorbed CO2 peak at 120 °C was attributed to the metal–
oxygen pairs (such as Mg-O, Zn-O and Al-O). The peak of CO2
desorption at 120 °C ascribed to medium strength basic sites
was disappeared in Zn3.2AlO4.7 catalyst. The CO2-TPD profile of
the reused catalyst Zn1.1Mg2.0AlO4.6-reused has more weak
strength basic sites and the medium strength basic sites
declined. Moreover, it should be mentioned that among all
the catalysts, Mg3.0AlO4.5 showed the highest desorption
temperature of the strong basic sites at about 400 °C, which
was attributed to unsaturated oxygen atoms.
from thermal decomposition of hydrotalcites were used as
the catalysts (entries 5-7), the ternary composite oxide
View Article Online
DOI: 10.1039/C6RA21809A
Zn1.1Mg2.0AlO4.6 displayed the highest catalytic activity and
82% yield of 5-methyl-2-oxazolidinone was obtained. The
previous reports38 illustrated that the nucleophilic ring
opening of the PO at the less substitute site under basic
conditions, followed by intramolecular cyclization to produce
the corresponding 5-methyl-2-oxazolidinone. So, it can be
conjectured that the yields of 5-methyl-2-oxazolidinone were
mainly affected by the strength of the basic sites. Previously,
the present authors measured the basic properties of the
present oxide catalysts by temperature programmed
desorption (TPD) of adsorbed CO2 (Table 1). It was shown that
Zn3.2AlO4.7 only has weakly basic sites, while the Mg3.0AlO4.5
has strongly basic sites. The lower activity of the Zn3.2AlO4.7
might be due to its weak basicity, which was unfavorable for
the nucleophilic ring opening of the PO. The catalyst
Mg3.0AlO4.5 with large amounts of medium basic sites shown
relatively lower catalytic activity was considered to be related
to the presence of strong basic sites, which would promote
the by-product formation. Therefore, incorporation of a zinc
component into Mg–Al oxides remarkably affected the
basicity strength and the amount of basic sites. Based on the
preliminary results, it was concluded that the high activity
and selectivity of Zn1.1Mg2.0AlO4.6 for 5-methyl-2-
oxazolidinone synthesis result from its basic properties.
However, the catalyst morphology has little effect on the
catalytic performance for such a catalytic system.
Table 2 Synthesis of 5-methyl-2-oxazolidinone from PO and urea with different
catalysts and reaction conditions a
Yield (%)b
T
t
Fig.
4 The CO2-TPD profiles of (a) Mg3.0AlO4.5 (b) Zn3.2AlO4.7 (c)
Entry
Catalyst
(oC)
(h)
Mg1.1Zn2.0AlO4.6 (d) Zn1.1Mg2.0AlO4.6-reused
3.2 2-oxazolidinones synthesis from urea and epoxides
--
140
140
140
140
140
140
140
120
130
150
140
140
140
140
140
4
4
4
4
4
4
4
4
4
4
2
8
4
4
4
35
50
57
70
65
75
82
7
3
14
9
15
16
11
10
10
11
5
1
2
In our initial study, the reaction of urea and propylene
oxide (PO) was chosen as the model reaction to explore the
catalyst system, Table 2. Generally, the reaction was carried
out at 140 oC for 4 h in the presence of different catalysts. In
the blank test (entry 1), the yield of 5-methyl-2-oxazolidinone
was only 35%, which indicated that catalyst was essential for
such reaction proceeded successfully. Yields of 5-methyl-2-
oxazolidinone were raised to 50%, 57% and 70% with ZnO,
Al2O3 and MgO catalyst, respectively (entries 2-4). On the
basis of the result of GC–MS, it can be found that the main
by-products with these catalysts are 1-amino-2-propanol and
hydroxyl functionalized 2-oxazolidinone. As shown in Scheme
2, the 1-amino-2-propanol was formed by the side reaction
between NH3 and PO, which could also be reacted with urea
to 5-methyl-2-oxazolidinone. When the 5-methyl-2-
oxazolidinone concentration in the reaction mixture was
relatively higher, further ring opening of epoxide with the
formed 5-methyl-2-oxazolidinone would be occurred and
generate the hydroxyl functionalized 2-oxazolidinone.
Moreover, trace amount of 4-methyl-2-oxazolidinone was
also detected due to the nucleophilic ring opening of the PO
at methyl substitute site. When the composite oxides derived
ZnO
Al2O3
3
MgO
3
4
Zn3.2AlO4.7
6
5
Mg3.0AlO4.5
Zn1.1Mg2.0AlO4.6
Zn1.1Mg2.0AlO4.6
Zn1.1Mg2.0AlO4.6
Zn1.1Mg2.0AlO4.6
Zn1.1Mg2.0AlO4.6
Zn1.1Mg2.0AlO4.6
7
6
2
7
2
1
8
56
45
65
82
81
81
80
5
16
41
2
9
14
15
1
10
11
12
13
14
15
6
1st
Zn1.1Mg2.0AlO4.6
3
5
2nd
Zn1.1Mg2.0AlO4.6
Zn1.1Mg2.0AlO4.6
3
6
3rd
4
6
a Reaction conditions: 10 mmol PO; 20 mmol urea; 0.05 g catalysts; 120-150 °C;
2-8 h.
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b Determined by gas chromatography.
attack at the carbon atom at which phenyl substitute was
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connected to afford 4-substituted-2-oxDaOzoI:l1id0.i1n0o3n9/eCs6.3R9A2W18h0e9nA
the cyclohexene oxide (entry 8) was used for the synthesis of
2-oxazolidinones, 80% conversion of disubstituted epoxide
and 64% selectivity the corresponding 2-oxazolidinones were
obtained, the lower activity towards the synthesis of 2-
oxazolidinones compared with other epoxides might be due
to the high hindrance of cyclohexene oxide. From the
preliminary results, it can be conjectured that the basic
Zn0.8Mg1.9AlO4.2 catalyst has good catalytic performance for
various 5-substituted-2-oxazolidinones synthesis by the
epoxide opening reaction with urea.
H2N NH2
NH4+NCO-
NH3 + HNCO
O
O
R
HO
NH2
H
N
R
(1)
C
O
R
H2N NH2
O
O
O
O
O
-
+
Olat
M
R
HN
O
N
O
HO
R
R
(2)
R
(3)
Lewis base
M=Mg, Zn, Al
H
N
Table 3 Synthesis of 2-oxazolidinones from various epoxides and urea a
C
O
O
O
Epoxide
Major product
Conv. (%)
99
Sel. (%)b
89
HN
R
O
Entry
1
R
-
+
Olat
M
(4)
Scheme 2 Possible reaction pathways for the reaction of urea and PO.
Subsequently, the impact of the reaction temperature on
the reaction of urea and PO was investigated (entries 8-10).
The 5-methyl-2-oxazolidinone yield sharply increases in the
2
3
4
99
99
92
81
88
80
o
temperature range of 120–140 C and decreases slightly
when the reaction temperature exceeds 140 oC, probably due
to higher temperature promoted the side reaction reactions
(between PO and NH3 or 5-methyl-2-oxazolidinone), which
lead to the selectivity of 1-amino-2-propanol and hydroxyl
o
functionalized 2-oxazolidinone increased. So, 140 C should
5
6
94
96
87
88
be the proper reaction temperature for obtaining higher yield
of 5-methyl-2-oxazolidinone. Finally, the impact of reaction
time on 5-methyl-2-oxazolidinone synthesis was investigated
(entries 11-12). It can be seen that after 4 h reaction the yield
of 5-methyl-2-oxazolidinone can reach 82%. Further
prolonging the reaction time has no remarkable effect on
increasing the yield of 5-methyl-2-oxazolidinone. Apart from
the catalytic activity, the reusability of catalyst is another
important issue. The results obtained (entries 13-15)
indicated that the yield of 5-methyl-2-oxazolidinone slightly
decreased to 80% after 3 cycles. This may due to the catalyst
leaching (about 36%wt Mg, 11%wt Zn and <0.1%wt Al were
leached after 3 recycles), the decline of medium strength
basic sites (Table 1) and phase transformations of ZnO during
the reaction (a new phase of Zn(NH3)2(NCO)2 was observed
over spent catalysts in XRD patterns, Fig. 2e).
O
HN
O
O
7
8
99
80
55
64
a Reaction conditions: 10 mmol epoxides; 20 mmol urea; 0.05 g Zn1.1Mg2.0AlO4.6
140 °C; 8 h. Conversion and selectivity were determined by gas chromatography.
;
b Selectivity of the 5-substituted-2-oxazolidinone.
3.3 Scope of the catalysts
To investigate the limitation and scope of this protocol, the
reaction between several different epoxides with urea for
corresponding 2-oxazolidinones was further tested using the
optimized conditions, and the results are given in Table
3. >90% conversion of epoxides and >80% selectivity of the
corresponding 2-oxazolidinones were obtained with terminal
epoxides (entries 1–6). The reaction of styrene oxide and
urea under 140 °C produces merely 55% selectivity of 5-
substituted-2-oxazolidinones, and a very large quantity of 4-
substituted-2-oxazolidinones (45%) was also formed. The low
selectivity of the 5-substituted-2-oxazolidinones for the
styrene oxide (entry 7) might be attributed to the conjugative
effect derived from the aromatic ring, which would beneficial
4. Conclusions
Production of value-added chemicals using urea as an
active form of carbon dioxide is favorable to the sustainable
development of the chemical industry. In this work, various 2-
oxazolidinones were successfully synthesized from urea and
epoxides in the absence of any solvent. This efficient and
green protocol has the advantages of environmental
friendliness, high yields and operational simplicity.
Acknowledgments
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37 M. J. Climent, A. Corma, P. D. Frutos, S. Iborra,VMiew. NArtoicyle,OAn.line
This work was supported by National Natural Science
Foundation of China (No. 21173240).
Velty, P. Concepci ´on, J. Catal., 2010,DOI: 10.1039/C6RA21809A
269, 140-149.
38 S. Bonollo, F. Fringuelli, F. Pizzo and L. Vaccaro, Synlett,
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39 S. Bonollo, D. Lanari and L. Vaccaro, Eur. J. Org. Chem., 2011,
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DOI: 10.1039/C6RA21809A
Solvent-free synthesis of 2-oxazolidinones from urea and epoxides over Zn-modified mesoporous
Mg–Al nano-oxides.