Methyl N -phenyl carbamate synthesis over Zn/Al/Ce mixed oxide derived from hydrotalcite-like precursors

Article abstract of DOI:10.1039/c9ra09642f

Methyl N-phenyl carbamate (MPC) is an important intermediate for the green synthesis of methylene diphenyl diisocyanate (MDI) as well as many other important products. In the present work, Zn/Al/Ce mixed oxides derived from hydrotalcite-like precursors were employed as effective and recoverable heterogeneous catalyst for MPC synthesis via DMC aminolysis. Zn/Al/Ce hydrotalcite-like precursors prepared via coprecipitation method and the resulting catalysts were characterized by means of XRD, BET, SEM and XPS. Strong interactions within the Zn/Al/Ce mixed oxides were observed via the addition of appropriate amount of cerium. The mixed oxides containing 2.5% cerium showed high DMC aminolysis activity giving aniline conversion of 95.8%, MPC selectivity of 81.6% and MPC yield of 78.2%. Moreover, as a heterogeneous catalyst, it also exhibited superiorities of easy recovery and recyclable stability for MPC synthesis.

Full text of DOI:10.1039/c9ra09642f

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Methyl N-phenyl carbamate synthesis over Zn/Al/
Ce mixed oxide derived from hydrotalcite-like
precursors†
Cite this: RSC Adv., 2019, 9, 42474
a
a
a
a
a
Min Kang, Hai Zhou, * Dajiang Tang, Xiaomei Chen,
Ying Guo
b
and Ning Zhao
*
Methyl N-phenyl carbamate (MPC) is an important intermediate for the green synthesis of methylene
diphenyl diisocyanate (MDI) as well as many other important products. In the present work, Zn/Al/Ce
mixed oxides derived from hydrotalcite-like precursors were employed as effective and recoverable
heterogeneous catalyst for MPC synthesis via DMC aminolysis. Zn/Al/Ce hydrotalcite-like precursors
prepared via coprecipitation method and the resulting catalysts were characterized by means of XRD,
BET, SEM and XPS. Strong interactions within the Zn/Al/Ce mixed oxides were observed via the addition
of appropriate amount of cerium. The mixed oxides containing 2.5% cerium showed high DMC
aminolysis activity giving aniline conversion of 95.8%, MPC selectivity of 81.6% and MPC yield of 78.2%.
Moreover, as a heterogeneous catalyst, it also exhibited superiorities of easy recovery and recyclable
stability for MPC synthesis.
Received 19th November 2019
Accepted 16th December 2019
DOI: 10.1039/c9ra09642f
rsc.li/rsc-advances
is very attractive, because the byproducts methanol could be
used for the production of DMC. However, due to the high
1
Introduction
15
Methyl N-phenyl carbamate (MPC) is a key intermediate that reactivity of the intermediates decomposed from DMC, carba-
can be used for the synthesis of many important products, such moylation and methylation proceed simultaneity during the
1
as pesticides, pharmaceuticals, dyestuffs and so on. More aminolysis reaction, which, respectively, leads to the formation
0
importantly, MPC can also be used for the green synthesis of of MPC and N-methylaniline (NMA) and/or N,N -dimethylani-
2
,16–18
methylene diphenyl diisocyanate (MDI), which is an important line (DMA), as shown in Scheme 1.
precursor for the synthesis of polyurethane (Scheme 1).
Thus, developing
catalysts with high activity is quiet necessary so as to enhance
Traditionally, MDI is prepared industrially by using phosgene the selectivity as well as yield of MPC.
as one of the feedstocks, which could undoubtedly lead to
So far, various metal salts including Zn,
1
–3
11,19–22
8,11
23
Pb,
Sn,
1
24
serious environmental pollution and safety issues. As a conse- Na, etc. had been successfully employed as homogeneous
quence, green synthesis of MPC is of great signicance with catalysts for the synthesis of MPC and other carbamates.
respect to the non-phosgene synthesis of MDI and other Despite the high activity, these homogeneous catalysts also
isocyanates.
MPC could be synthesized via the reactions of phenylurea tion and catalysts recovery. In addition, Zn(OAc)
bring about troublesome problems such as products purica-
may dissolve
and dimethyl carbonate (DMC) and/or methanol, aniline and and even gradually transform into ZnO, leading to decreased
2
2
4
5
2
,6–10
3,11
DMC
and C
synthetic paths have also been developed recently. Among these synthesis of carbamates is very attractive. Recently, CeO
34
and/or methyl carbamate (MC),
and even aniline catalytic performance of Zn(OAc)
2 2
/SiO during repeatable use.
1
2–14
3 2
source (e.g. CH OH and CO ),
etc. And some new Thus, the development of heterogeneous catalysts for the
1
16,25–30
,
2
31
32
33
synthetic procedures, the aminolysis of DMC for MPC synthesis Au/CeO
,
MnO
–CeO
,
TiO
–Cr
O
/SiO
,
2
ZnO–MgO, and
2
x
2
2
2
3
6
zinc alkyl carboxylate covalently bonded on silica, etc. have
been developed as heterogeneous catalysts for the synthesis of
carbamates. And substantial improvements have been achieved
in the aspects of reactivity, selectivity, stability as well as the
a
Department of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi
5
63002, China. E-mail: 591658314@163.com; Fax: +86-851-28924799; Tel: +86-
51-28924799
8
b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese reaction mechanism. Laursen et al. investigated the dissocia-
Academy of Sciences, Taiyuan 030001, China. E-mail: Zhaoning@sxicc.ac.cn; Fax: tion process of DMC on CeO
via rst-principles calculation,
2
+
86-351-4041153; Tel: +86-351-4063121
which showed that the selectivity of CeO toward carbamoyla-
tion strongly depend on its crystal shape, and CeO2 nano-
octahedra exposing (111) facets was more active and selective
2
†
Electronic supplementary information (ESI) available: XPS of Zn 2p, Al 2p and
Ce 3d core-level of catalysts, results of the optimization of reaction conditions,
and the SEM image of Zn/Al/Ce2.5 aer times cyclic test. See DOI:
0.1039/c9ra09642f
5
16
than CeO
2
nano-rods and nano-cubes. Zinc alkyl carboxylate
1
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the above mentioned compositions were denoted as Zn/Al/Ce0,
Zn/Al/Ce1, Zn/Al/Ce2.5, Zn/Al/Ce5, Zn/Al/Ce7.5 and Zn/Al/Ce10,
respectively.
2.2 Characterization
The precursors and catalysts were characterized by X-ray
diffraction (XRD) on a Rigaku D/max-rB diffractometer with
Cu Ka radiation. Scanning electron microscopic (SEM, Scios,
FEI) was used for observing the morphology of samples at
acceleration voltages of 20 kV. Nitrogen adsorption–desorption
properties of the samples were measured on Quantachrome
Autosorb-IQ2-MP-XR-VP at 77 K, the specic surface area was
obtained via Brunauer–Emmett–Teller (BET) method from the
N2 adsorption–desorption isotherms. X-ray photoelectron
spectroscopy (XPS) data was collected on an ESCALAB250Xi
Scheme 1 The possible reaction paths of DMC aminolysis and MDI
synthesis using MPC as feedstock. The symbol “ꢃ” shows the unse- spectrometer (Thermo Fisher Scientic). All the binding ener-
lective byproducts of DMC aminolysis.
gies were calibrated internally by adventitious carbon deposit C
1s peak at 284.8 eV. The Thermo Avantage soware was used to
analyze the XPS data.
covalently bonded on silica could show almost unchanged
activity during repeated evaluation for 12 times under
a moderate conversion of aniline.
2
.3 Catalytic evaluation
6
MPC synthesis was carried out in a 50 mL Teon-lined autoclave
equipped with a magnetic stirrer. In a typical experiment,
It is widely reported that hydrotalcite-like compounds
(HTlcs) are excellent precursors for preparing mixed oxides
19.35 g DMC, 0.8 g aniline (the molar ratio of DMC to aniline
catalysts, because catalysts derived from calcined HTlcs possess
superior properties of high dispersion of metallic active sites
was 25) and 0.253 g catalyst were added into the reactor. Under
a nitrogen atmosphere, the reaction proceeded at 200 C for 7 h.
Aer reaction, the catalyst was separated by centrifugation and
the liquid product was analyzed by high performance liquid
chromatography (HPLC).
ꢂ
35–38
and unique surface acid–base properties.
Thus, considering
the fact that metal cations in HTlcs are highly adjustable, and
zinc and cerium are two of the main active components, herein,
we demonstrate the design and fabrication of Zn/Al/Ce mixed
oxides derived from calcined Zn/Al/Ce HTlcs precursors for
MPC synthesis via DMC aminolysis. The results show that Zn/
Al/Ce mixed oxides possessing 2.5% cerium is highly active
and stable, giving aniline conversion of 95.8%, MPC selectivity
of 81.6% and MPC yield of 78.2%.
Shimadzu LC-20A HPLC equipped a Shimadzu C-18 (4.6 mm
ꢃ 150 mm, 5 mm) column was used to analyze the liquid
3 2
product. The mobile phase was CH OH/H O (70/30, volume-to-
ꢀ
1
volume), the ow rate of mobile phase is 0.4 mL min . The
column temperature was 40 C, and the detection wavelength
was 254 nm.
ꢂ
2
Experimental
2.1 Catalysts preparation
All the regents were of analytic grade and used without further
purication. A series of HTlcs with Zn–Al–Ce atomic ratio of
2
+
3+
3+
7
5 : 25 ꢀ x : x (Zn : (Al + Ce ) ¼ 3 : 1) were prepared via
coprecipitation method, where x was equal to 0, 1, 2.5, 5, 7.5
and 10. The typical preparation process of HTlcs was as follows:
calculated amount of Zn(NO
Ce(NO $6H O were dissolved in distilled water, forming
solution A. Then NaOH and Na CO with the molar ratio of 3
3 2 2 3 3 2
) $6H O, Al(NO ) $9H O and
3
)
3
2
2
3
were dissolved in distilled water, forming solution B. Solutions
A and B were added dropwise into another baker that contained
100 mL distilled water under vigorous stirring. During copre-
cipitation, the pH of solution was maintained at 10 ꢁ 0.5. Aer
ꢂ
aged for 24 h at 80 C, the suspension was ltered and washed
several times with distilled water. The lter cake was dried at
ꢂ
ꢂ
8
0 C and calcined at 500 C for 4 h. The resulting catalysts with Fig. 1 XRD patterns of the uncalcined Zn/Al/Ce HTlcs precursors.
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the diffraction peaks of CeO
amount of cerium, which indicates the high crystallinity and
2
increases with the increase
3
Results and discussion
3.1 Characterization
agglomeration of CeO at high cerium content. Moreover, as for
2
XRD patterns of uncalcined Zn/Al/Ce precursors are displayed the Ce-containing catalysts, the diffraction peaks of ZnO (101)
ꢂ
in Fig. 1. Apparently, all samples show a series of strong facet located at ca. 36.5 shi toward lower angles (the inset of
ꢂ
4+
diffraction peaks at ca. 11.6, 23.3, 34.4, 39.1 and 46.5 , corre- Fig. 2), which may be caused by the incorporation Ce cations
36
sponding to Zn
006), (012), (015) and (018), respectively. This suggests that the in all the XRD patterns, indicating that aluminum oxide mainly
as-prepared precursors show the typical characteristics of Zn/Al exists in the form of amorphous phase.
HTlcs (JCPDS: 38-0486). In accordance with previous literatures,
In order to investigate the interactions between the different
6 2 3 2
Al (OH)16CO $4H
2 3
O crystal facets of (003), with larger ionic radius. No crystalline Al O can be observed
(
the peak intensities of the Ce-containing samples decrease with oxides within the Zn/Al/Ce catalysts, they are characterized by
the increase of cerium content, revealing the decreased crys- means of XPS. It is found that both the binding energy of Zn 2p
35–37
tallinity.
No apparent diffraction peaks shi could be and Al 2p shi toward higher binding energy aer the intro-
observed over the Ce-containing samples (the inset of Fig. 1), as duction of cerium. And catalysts with relative low cerium
compared with pure Zn/Al HTlcs. Even though, the XRD results content (e.g. Zn/Al/Ce1 and Zn/Al/Ce2.5) exhibit larger shi than
still suggest that the addition of cerium signicantly affect the the rest three Ce-containing catalysts, as shown in Fig. S1 and
formation process of the as-prepared Zn/Al HTlcs precursors S2.† Correspondingly, the binding energy of Ce 3d for Zn/Al/Ce1
(
e.g. structure distortion and low crystallinity), due to the larger and Zn/Al/Ce2.5 shi toward lower binding energy as compared
3
+
3+
ionic radius of Ce (0.101 nm) as compared with that of Al
to Zn/Al/Ce2.5, Zn/Al/Ce7.5 and Zn/Al/Ce10 (Fig. S3†). Fig. 3
0.054 nm). Meanwhile, previous report also proved that high shows the high resolution XPS spectra of Ce 3d. According to
36
(
2
+
3+
Zn /Al ratio would result in deformation of the layered literatures, it can be deconvoluted into eight peaks corre-
37
39,40
structure, which was not favor the formation of HTlcs. As for sponding to Ce 3d3/2 and 3d5/2
.
The peaks located at ca. 885.2
3
+
the present study, the content of aluminum decreases with the and 903.4 eV suggest the presence of Ce , and the rest peaks
increase of cerium, the low aluminum content would also lead located at ca. 882.1, 888.4, 898, 900.6, 907.2 and 916.4 eV can be
4
+
3+
to the decrease of HTlcs phase. As a result, besides the Zn/Al assigned to Ce . The amount of Ce could be calculated
3
+
HTlcs as the dominate phase, trace amount of ZnO (JCPDS: according to the relative contribution of Ce features to the
6-1451) can also be observed owing to the decreased crystal- spectrum. As displayed in Table 1, the ratio of Ce /Ce almost
linity and the relative high aging temperature (80 C). Simi- increases with the decrease of cerium content within the Zn/Al/
larly, the diffraction peak at 28.6 may suggest that a portion of Ce mixed oxides, and Zn/Al/Ce2.5 possesses the highest Ce ,
3
+
4+
3
ꢂ
37
ꢂ
3+
cerium transforms into CeO
process.
2
(JCPDS: 34-0394) aer the aging which is somewhat in accordance with the binding energy shi
of Zn 2p and Al 2p. It should be mentioned that the identi-
ꢂ
3+
The precursors are then calcined at 500 C for 4 h, so as to cation of Ce is just simplication since it only corresponds to
40
2 3
ensure the obtained catalysts possessing relative high specic the center of most intense peak for Ce O . Eve though, it could
35,36
area and strong interactions within the mixed oxides.
The still reect the surface electron rich state of CeO
2
within the
XRD patterns of the resulting Zn/Al/Ce catalysts are presented in
Fig. 2. Aer calcination, the Zn/Al HTlcs phase disappears and
transforms into mixed oxide phases, which include the species
2
of ZnO (JCPDS: 36-1451) and CeO (JCPDS: 34-0394). Apparently,
Fig. 2 XRD patterns of the Zn/Al/Ce catalysts.
Fig. 3 XPS of Ce 3d core-level of the Zn/Al/Ce catalysts.
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Table
catalysts
1
Physicochemical properties of the resulting Zn/Al/Ce size. The specic surface area (SSA) of Zn/Al/Ce catalysts is in
2
ꢀ1
the range of 30–45 m g , and no regular changes in SSA could
be drawn due to the low values, as shown in Table 1. However,
the average pore size of the catalysts increases with the
increasing of cerium content, which may result from the
decomposition of Zn/Al/Ce HTlcs precursors with high distor-
tion and low crystallinity.
a
Atomic
ratio (%)
Average pore
size (nm)
2
ꢀ1
a
3+
4+
Catalysts
SSA (m g
)
Zn
Al
Ce Ce /Ce
Zn/Al/Ce0
Zn/Al/Ce1
Zn/Al/Ce2.5 31.3
Zn/Al/Ce5 40.2
Zn/Al/Ce7.5 29.8
Zn/Al/Ce10 43.5
44.1
35.3
7.6
8.2
8.9
9.6
9.3
80.3 19.7
0
79.3 19.8 0.8 14.86%
79.2 19.0 1.8 15.10%
76.0 20.0 3.9 14.53%
75.8 17.6 6.6 13.96%
76.4 14.8 8.8 11.08%
3.2 Catalytic performance
The above results suggest that the addition of cerium have
remarkable inuence on the phase, microstructure and even
10.8
a
3+
4+
The atomic ratio and the ratio of Ce /Ce are determined by XPS chemical state of Zn/Al/Ce precursors as well as the resulting
results.
catalysts. Accordingly, strong interactions between the Zn/Al/Ce
mixed oxides within the resulting solid catalysts are expected.
As a result, reaction conditions of reaction temperature, reac-
tant molar ratio and reaction time are optimized over Zn/Al/
mixed oxides, which suggests the transferring of electron from
Ce2.5.
zinc and aluminum atoms to cerium atoms, especially for Zn/Al/
As for DMC aminolysis, carbamoylation and methylation are
Ce1 and Zn/Al/Ce2.5.
SEM images of the HTlcs precursors are shown in Fig. S4.† In
accordance with literatures,
the two main reactions, which are strongly affected by reaction
2,16–18
temperatures.
It was generally reported that incorrect
36,37
all the precursors exhibit
increasing of reaction temperature could enhance methylation,
resulting in a decrease of MPC selectivity. Thus, reaction
temperature is optimized rstly (the molar ratio of DMC to
aniline is 20, and reaction time is 7 h, Fig. S5†). The conversion
of aniline increases sharply with the increase of temperature
nanoplates structure with the diameter of several hundred
nanometers. The size of the nanoplates increases with the
increasing of cerium content. It further conrms the inuence
of cerium on the formation of Zn/Al/Ce HTlcs, which is in
accordance with the XRD results. The SEM images of the
resulting Zn/Al/Ce catalysts are presented in Fig. 4. As can be
seen, Zn/Al/Ce catalysts still keep the nanoplates structure, and
the decreased size of the catalysts suggests the shrinkage and
fraction of the nanoplates during the thermal decomposition of
basic carbonate precursor. Similar with the precursors, the Zn/
Al/Ce catalysts with higher cerium content still possess larger
ꢂ
from 150 to 210 C. The selectivity and yield of MPC increase
with the increasing of temperature, reach the maximum value
ꢂ
of 66.4% and 60.4%, respectively, at 200 C. Aerwards, both
the selectivity and yield of MPC decrease sharply, which is
probably caused by the methylation of aniline at elevated
2,6,19
temperature.
Subsequently, the molar ratio of DMC to
aniline is also optimized, as shown in Fig. S6.† It should be
Fig. 4 SEM images of the resulting catalysts (a) Zn/Al/Ce0, (b) Zn/Al/Ce1, (c) Zn/Al/Ce2.5, (d) Zn/Al/Ce5, (e) Zn/Al/Ce7.5 and (f) Zn/Al/Ce10.
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4
+
mentioned that the catalytic performance enhances when the as well as the increased surface Ce sites. And CeO
2
particles
molar ratio of DMC to aniline increases to 25, giving aniline enclosed by unfavorable low energy facets (e.g. (100)) may also
16,30,42
conversion of 95.8%, MPC selectivity of 81.6% and MPC yield of form.
The above two aspects may promote the unselective
7
8.2%. It is reported that DMC acts as solvent and reactant dissociation of DMC, leading to the increased selectivity of NMA
19
during DMC aminolysis reaction. Thus, excessive DMC would and DMA. As a result, the catalytic performance of Zn/Al/Ce2.5,
promote the conversion of aniline as well as the selectivity of Zn/Al/Ce5 and Zn/Al/Ce7.5 decrease gradually, suggesting the
MPC by suppressing the side reactions. The inuence of reac- adverse inuence of cerium under high content. Moreover, it
tion time is further investigated (Fig. S7†). The conversion of should be mentioned that high cerium content is benet for the
aniline increases with the increasing of reaction time, reaching formation of larger pore size (Table 1), and the surface acid–
ca. 95% at 5 h and remaining unchanged with further increase base sites might be more effectively turned with the addition of
of reaction time. The maximal selectivity and yield of MPC are relative larger amount of cerium, thus Zn/Al/Ce5 and Zn/Al/
obtained at 7 h and then decrease sharply. As a result, the Ce7.5 possess than Zn/Al/Ce1. Consequently, MPC synthesis
chosen reaction time is 7 h.
over the present Zn/Al/Ce heterogeneous catalyst is affected by
3
+
Table 2 displays the catalytic performance over the series of many factors, such as Ce fraction, SSA, pore size, surface acid–
Zn/Al/Ce catalysts with different cerium content under the base sites and so on. And much work is needed before clearly
optimized reaction conditions. As can be seen, without the discerning the activation mechanism of DMC and/or amine
addition of cerium Zn/Al/Ce0 shows a certain catalytic activity, over this kind of heterogeneous catalyst.
the aniline conversion is 91.0%, MPC selectivity is 63.3% and
Properties of the stability and recyclability of heterogeneous
the MPC yield is 57.6%. The activity of Zn/Al/Ce0 might origi- catalysts is crucial for industrial application. In this study, the
38
nate from the surface acidic sites of Zn/Al mixed oxide as well cycling stability of Zn/Al/Ce2.5 was also investigated, which was
as zinc element that is generally used as the active component conducted as follows: aer each reaction, the solid catalyst was
2
,6,22,41
for carbamates synthesis.
Nevertheless, methylation collected by centrifugation, washed with methanol under
ꢂ
proceeds obviously, giving the NMA and DMA selectivity of ultrasonic for several times and dried at 60 C for 12 h. Because
10.9% and 24.3%, respectively. Aniline conversion, MPC selec- some catalyst was lost during the recovery process, a certain
tivity and MPC yield increase over the Ce-containing Zn/Al/Ce amount of fresh catalyst (ca. 20%) was added so as to ensure the
catalysts, among which Zn/Al/Ce2.5 shows the highest carba- mass of catalyst (0.253 g) in the reaction system unchanged. The
moylation catalytic activity. Both the selectivity and yield of results are shown in Fig. 5. To our delight, the conversion of
MPC decrease gradually with the further increasing of cerium aniline is kept at about 95%, and MPC selectivity and yield
content, while the selectivity of methylation byproducts decrease very slowly during the 5 repeated times. The selectivity
enhances apparently. As a consequence, the catalytic perfor- of the main methylation byproducts (DMA) is 13.3% in the rst
mance of Zn/Al/Ce10 is very close to that of Zn/Al/Ce0.
use, which gradually increases to 24.6% aer the h cycles,
The above results suggest that high performance heteroge- while selectivity of NMA and DPU remain at ca. 4% and 0.6%,
neous catalysts could be obtained via the addition of appro- respectively. The MPC selectivity and yield are found to be
priate amount of cerium into Zn/Al mixed oxides. As evidenced 68.6% and 65.2%, respectively, aer 5 cycles. As shown in
3
+
by XPS, Zn/Al/Ce2.5 possesses the highest Ce fraction owing to Fig. S8,† Zn/Al/Ce2.5 still retains the nanoplates structure,
the strong interactions within the mixed oxides. Thus, it can be suggesting the stability of its microstructure. The deactivation
3
+
deduced that a proper interactions between Ce of this solid might originate from the decrease of pore volume and diameter
catalysts and DMC would be generated, which would then result
34,38
in higher catalytic activity for MPC synthesis.
However, high
agglomeration of CeO is expected under high cerium content,
2
which leads to the nonuniform dispersion of metallic elements
a
Table 2 Catalytic performance of the series of Zn/Al/Ce catalysts
Selectivity (%)
Aniline conversion MPC yield
Catalysts
(%)
(%)
MPC NMA DMA DPU
Zn/Al/Ce0
Zn/Al/Ce1
Zn/Al/Ce2.5 95.8
Zn/Al/Ce5 94.6
Zn/Al/Ce7.5 95.6
Zn/Al/Ce10 92.6
91.0
94.5
57.6
63.1
78.2
72.0
72.6
57.5
63.3 10.9 24.3 0.7
66.8
81.6
76.1
75.9
62.1
7.1 24.9 0.6
4.3 13.3 0.4
4.1 18.4 0.7
5.3 17.7 0.6
6.6 29.8 0.8
a
Reaction conditions: DMC 19.35 g, aniline 1 g (molar ratio of DMC to
ꢂ
Fig. 5 Recycling test of Zn/Al/Ce2.5. Reaction conditions: molar ratio
aniline is 25), catalyst 0.253 g, reaction temperature 200 C, reaction
ꢂ
time 7 h.
of DMC to aniline is 25, catalyst 0.253 g, reaction temperature 200 C,
reaction time 7 h.
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of catalyst caused by the blocking of organic substances, and
the formation of carbonate like species on the surface of cata-
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lyst. Moreover, XRD pattern of Zn/Al/Ce2.5 aer the cyclic test
is almost the same as the uncalcined precursor (Fig. S9†), which
conrms the reconstruction of the hydrotalcite structure due to
43
its memory effect. As a result, the catalytic performance of Zn/
Al/Ce2.5 may be recovered via simple thermal annealing.
The catalytic performance and cycling stability of the present
Zn/Al/Ce2.5 are superior to previously reported heterogeneous
2
catalysts, such as Zn(OAc) /SiO , ZnO–TiO (ref. 9) and ordered
2
2
2
10
AlSBA-15. As far as we know, there is no report for the reaction
of DMC and amine for the synthesis of carbamates (e.g. MPC)
catalyzed over mixed oxides derived from hydrotalcite-like
precursors. Considering the merits of the HTlcs precursors,
and the advantages of feasible preparation, low cost, high
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
We are grateful for grants from Technology Foundation of the
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