J. Li et al. / Catalysis Communications 12 (2011) 1224–1227
1225
Table 1
Catalyst screening for synthesis of methyl carbamate.
Entry
Catalyst
Time/h
Tem./°C
Yield/%a
Sel./%
TONb
1
2
3
4
5
6
7
8
9
–
12
12
12
12
12
12
12
12
12
12
12
12
12
2
4
24
4
4
4
4
4
6
4
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
170
180
220
200
200
180
200
b2
4
6
9
11
8
–
–
MgO
CaO
Bi
CeO
TiO
ZrO
99
99
99
98
99
99
99
99
98
98
98
98
98
98
96
99
98
76
99
98
96
98
6
Scheme 1. One-pot syntheses of alkyl carbamates from CO
catalyzed by in-situ synthesized (NH from V
2 2 4
, NH COONH and alcohols
10
14
18
13
13
10
16
30
27
34
40
34
38
38
21
35
33
18
29
101
29
)
4 2
V
3
O
8
2 5
O .
2
O
3
2
2
2
8
6
with the alcohol. Each experiment was performed in triplicate and the
standard deviations are indicated with error bars in the appropriate
figures.
Cr O
2
3
Fe
ZnO
Co
NiO
V
V
V
V
V
V
2
O
3
10
19
17
21
25
21
24
24
13
22
21
11
18
63
18
10
11
12
13
14
15
16
17
18
2
O
3
2
.3. Identification
2
2
2
2
2
2
O
O
O
O
O
O
5
5
5
5
5
5
5
5
5
5
5
Qualitative analysis was conducted with a HP 6890/5973 GC–MS.
Yields of alkyl carbamates were measured with an Agilent GC-6820
equipped with a FID detector and an AT.SE-54 capillary column using
1
,4-dioxane as an internal standard. The 1H NMR spectra were
19
20
V O
2
c
d
e
f
recorded on a Bruker AMX FT 400-MHz NMR spectrometer. X-ray
diffraction (XRD) was performed on a Siemens D/max-RB powder X-
ray diffract meter. Diffraction patterns were recorded with Cu Kα
radiation over a 2θ range of 10° to 80° and a position-sentient detector
using a step size of 0.017°. X-ray photoelectron spectroscopy (XPS)
was performed with a VG ESCALAB 210 instrument. Mg Kα radiation
at an energy scale calibrated versus adventitious carbon (C1s peak at
V
V
V
V
2
2
2
2
O
O
O
O
2
2
2
1
2
3
NH
2
COONH
4
80 mmol (6.24 g, 160 mmol ammonia),
2 5
V O 1 mmol, methanol
6
40 mmol, initial CO pressure 5 MPa.
2
a
GC yield based on ammonia.
Mol of MC produced per mol of catalyst.
b
c
2
85.00 eV) was used.
Initial CO
2
pressure 1 MPa.
2
pressure 3 MPa.
d
e
Initial CO
Trimethoxymethane (160 mmol) as dehydrant, tetrabutylammonium bromide as
2
.4. Theoretical calculations
cocatalyst.
f
Catalyst was recovered and used for the sixth time.
All the theoretical calculations were performed with the Gaussian
3 programs using the B3LYP functional [13]. The full electron basis
0
set 6-31+G(d) was used for geometry optimization and frequency
calculation. The optimized geometry was confirmed by the frequency
calculation to be a real minimum without any imaginary vibrational
frequency. And the Zero-point vibrational energy (ZPE) corrections
were obtained using unscaled frequencies. The single-point electronic
energies were calculated with the B3LYP/6-311++G(2df, 2p)
method. With the theoretical gas-phase results in hand, we utilized
the C-PCM solvation model at the B3LYP/6-31+G(d) level to estimate
the solvation effect.
(entries 14–21). Under above conditions, 25% of MC yield with a
selectivity of 98% can be achieved. Moreover, in order to obtain higher
´
˚
MC yield, an attempt of using several dehydrants was also made. If 4 A
molecular sieves, anhydrous sodium sulfate, and anhydrous magne-
sium sulfate were used, however, no any improvement in MC yield
could be observed. When trimethoxymethane as dehydrant and
tetrabutylammonium bromide as cocatalyst were used, ca. 63% of MC
yield with 96% selectivity could be obtained (entry 22). The reusability
of the catalyst showed that V
obvious activity loss (entry 23).
In order to find out true catalytically active species, the catalytic
activity of V and (NH was comparatively investigated.
When the reaction proceeded for 0.5 h, only b1% MC yield was
obtained, while ca. 4% MC was produced over the (NH under
the same conditions (Fig. 1). After 1 h, the MC yield increased to 7% for
the V and a small amount of (NH formed. After 4 h, a
similar MC yield of about 24–25% could be obtained over both
catalysts, indicating that in-situ produced (NH was the true
catalytically active species for this reaction. Moreover, the catalytic
syntheses of EC and BC over V were also studied. Under optimized
2 5
O could be recycled six times without
3
. Results and discussion
2
O
5
4 2 3 8
) V O
The main purpose of this work is to develop a new process of CO
2
and NH
control the amount of the charged gases for each time if CO
are introduced into the autoclave together. Fortunately, it has been
reported that a white solid NH COONH can easily form when NH
and CO are fixed. Therefore, in this paper NH COONH was used as
the feed for the purpose of ensuring the NH amount consistent for
3
for syntheses of valuable chemicals. However, it is difficult to
4 2 3 8
) V O
2
and NH
3
O
2 5
4 2 3 8
) V O
2
4
3
2
2
4
4 2 3 8
) V O
3
each time. Firstly, MC production was used as a model reaction to
evaluate the activity of different metal oxides, and the results were
shown in Table 1. The blank experiment showed that only 2% of MC
yield could be obtained (entry 1). Then a series of metal oxides were
screened as the catalysts for the MC synthesis (entries 2–13). 4–25%
MC yields could be obtained while the MC selectivity is up to 98%.
Only trace of methyl N-methyl carbamate as byproduct, which may be
derived from the side reaction between MC and methanol, was
2 5
O
conditions, 7% of yields for both EC and BC could be obtained after 4 h
(Table 2, entries 1 and 2). After 12 h, 11% of EC yield and 14% of BC
yield could be obtained.
Fig. 2. gave the XRD patterns of three materials. From Fig. 2a, seven
distinct reflection peaks in the range of 15° to 35° can be attributed to
the characteristic orthorhombic phase of V
2 5
O , i.e. d200, d001, d101, d110,
d301, d011 and d310 spacing (ICSD Powder Diffraction File, card No. 77-
detected by GC–MS. Among the metal oxides tested, V
highest catalytic activity. XRD and XPS analysis showed that
NH was in-situ produced from V during the reaction.
The reaction conditions including the reaction time, temperature, and
CO pressure were optimized after the catalysts screening. The most
optimized conditions is as follows: NH COONH 80 mmol, V
mmol, methanol 640 mmol, initial CO pressure 5 MPa, 220 °C, 4 h
2
O
5
showed the
2418, cell parameters a=11.51, b=3.564, c=4.368 Å) [14]. Fig. 2b
gives the d001, d210, d201, d211, d310, d311 and d312 spacing of the typical
(
)
4 2
V O
3 8
2
O
5
tetragonal phase of (NH
Diffraction File, card No. 78-1229, cell parameters a =8.885,
c=5.564 Å) [15]. Fig. 3. gave the XP spectra of the fresh V and
in-situ formed (NH . The C, O, V elements for V and C, N, O,
V elements for (NH can be observed. In Fig. 3A, the binding
4 2 3 8
) V O at 15°b2θb46° (ICSD Powder
2
2 5
O
2
4
2
O
5
)
4 2
V O
3 8
2 5
O
1
2
4 2 3 8
) V O