One-pot efficient synthesis of methyl carbamate by M5Ca5(PO4)6F2 (M?=?Co, Ni, Cu, Zn) catalysts

Article abstract of DOI:10.1016/j.catcom.2019.04.023

A facile and efficient one-pot synthesis of methyl carbamate (MC) by urea and methanol was studied using metal modified fluorapatite as a catalyst. M₅Ca₅(PO₄)₆F₂ (M₅FAP, M = Co, Ni, Cu, Zn) was synthesized by co-precipitation. Among the catalysts, Zn₅Ca₅(PO₄)₆F₂ (Zn₅FAP) displayed the higher activity, selectivity and excellent stability after five times of reuse. A possible synthesis reaction mechanism over Zn₅FAP catalyst was discussed.

Full text of DOI:10.1016/j.catcom.2019.04.023

Accepted Manuscript
One-pot efficient synthesis of methyl carbamate by
M5Ca5(PO4)6F2 (M=Co, Ni, Cu, Zn) catalysts
Wei Wang, Feng Xu, Wenqiao Du, Long Zhang
PII:
S1566-7367(19)30137-2
DOI:
https://doi.org/10.1016/j.catcom.2019.04.023
CATCOM 5691
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
17 December 2018
24 April 2019
27 April 2019
Please cite this article as: W. Wang, F. Xu, W. Du, et al., One-pot efficient synthesis
of methyl carbamate by M5Ca5(PO4)6F2 (M=Co, Ni, Cu, Zn) catalysts, Catalysis
Communications, https://doi.org/10.1016/j.catcom.2019.04.023
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ACCEPTED MANUSCRIPT
One-pot efficient synthesis of methyl carbamate by
M Ca (PO ) F (M= Co, Ni, Cu, Zn) catalysts
5
5
4 6
2
Wei Wang, Feng Xu, Wenqiao Du, Long Zhang*
Jilin Provincial Engineering Laboratory for the Complex Utilization of
Petro-resources and Biomass, School of Chemical Engineering, Changchun
University of Technology, Changchun, Jilin, 130012, P. R. China.
Corresponding author: Tel. or Fax: +86-431-85716785. E-mail address:
zhanglongzhl@163.com

ACCEPTED MANUSCRIPT
Abstract
A facile and efficient one-pot synthesis of methyl carbamate (MC) by urea and
methanol was studied using metal modified fluorapatite as a catalyst. M Ca (PO ) F
4 6 2
5
5
(M5FAP, M= Co, Ni, Cu, Zn) was synthesized by co-precipitation. Among the
catalysts, Zn5Ca5(PO4)6F2 (Zn5FAP) displayed the higher activity, selectivity and
excellent stability after five times of reuse. A possible synthesis reaction mechanism
over Zn5FAP catalyst was discussed.
Keywords: M5Ca5(PO4)6F2; urea; methanol; methyl carbamate.

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1
. Introduction
Methyl carbamate (MC, H2NCOOCH3) is one of the simplest carbamate
compounds which was widely used as intermediate in the synthesis of microbicides,
pesticides, herbicides and pharmaceutical agents [1-5]. At present, the synthesis
methods of carbamates include: phosgene [6], carbonylation [7-9], urea alcoholysis
and carbon dioxide direct synthesis [10]. Conventional synthesis methods of
carbamates use toxic and strong corrosive raw materials and catalysts are expensive
and easily deactivated. Although CO2 is readily available, cheap raw materials have
been widely investigated for the direct synthesis of carbamates through an alcohol.
However, the latter process is still far from satisfactory due to the difficulty in the
activation of CO2 and deactivation problems of the catalysts used. Urea is another
abundant, low- cost and less toxic resource material for the synthesis of carbamate via
urea alcoholysis, if a reusable and highly active catalyst would be developed.
Early research showed lower yields to MC when BF3 [11] and (CH3COO)2Cu [12]
were used as catalysts, and no more attention was paid for several decades. J. D.
Slater [13] disclosed a process using phosphoric acid as a catalyst at 1.034 MPa and
4
23 K, where after 1 h of reaction the yield to MC was 88.2% but by-product
separation and reaction equipment was easily corroded and made it inconvenient for
industrial production. In order to enhance the yield of carbamate, a semi-continuous
process has been investigated, where for n(urea):n(methanol)=6:1, a flow rate of fresh
methanol of 4 mL/min and use of 800 rpm, provided a 98.7% yield for MC [14].
Current attempts for the synthesis of MC from urea used highly efficient and

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environmentally-friendly Zn-based catalysts. Joe et al. [15] reported
ZnO(0.64)-CeO2(0.26)-Ga2O3(0.1) as a catalyst via the urea methanolysis route, and
°
the yield of MC was 65.6% at 170 C, 20 bar and after 4 h of reaction. Wang and
co-workers [16] used ZnO and Zn(OH)2 as catalysts for the synthesis of MC from
urea and methanol, and the obtained yields of MC were 48 and 88%, respectively.
Fluorapatite, FAP (Ca10(PO4)6F2), as one of the most promising bone replacement
materials, has gained interest for its catalytic applications due to its ion- exchange
performance, tunable acid-base properties in various reactions, and lower costs. Over
2
+
the past decade, FAP loaded with metal ions [17-19], such as MxCa10-x(PO4)6F2: Mn ,
2
+
2+
3+
Cu , Zn and Sb were prepared and applied in Knoevenagel reaction [20], Michael
addition [21], Diels-Alder [22], organosilane oxidation [23] and Friedel-Crafts
reaction [24]. FAP with various Ca/P ratios were prepared with the addition of anions
and cations to adjust the acid/base character of the solid and used in the methanol
mercaptan reaction [25]. With HAP-Na-CO3 as catalyst, methanol conversion was 100%
and the selectivity to methane thiol was 60%. In our previous work, MFAP (M = Co,
Ni, Cu, Zn) catalysts were prepared for the synthesis of methyl N-phenyl carbamate
(MPC) from urea, methanol and aniline, with 94.4% aniline conversion and 75.8%
selectivity to MPC [26].
In this work, we report a facile and efficient process for the direct synthesis of
MC from urea and methanol using M5Ca5(PO4)6F2 (M5FAP, M = Co, Ni, Cu, Zn) as
catalyst, and a possible mechanism is also discussed.
2
. Experimental

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2
.1. Catalyst preparation
All chemical reagents were of analytical purity. Calcium nitrate, cobaltous nitrate,
nickel nitrate, copper nitrate, zinc nitrate, methanol, urea, β-cyclodestrin and the
standard sample MC were purchased from Shanghai aladdin Biochemical Technology
Co., Ltd.
The catalyst M5Ca5(PO4)6F2 (M5FAP, M = Co, Ni, Cu, Zn) and pure FAP were
prepared by the co-precipitation method (Scheme 1), uaing β-cyclodestrin as template:
1
50 mL solution containing 1.2 g of β-cyclodestrin and certain amounts of
Ca(NO3)2·4H2O and metal nitrate (M(NO3)2·xH2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O,
Cu(NO3)2·3H2O, Zn(NO3)2·6H2O,) equivalent to a molar ratio 5/5, were added
dropwise to 100 mL deionized water containing 7.7 g of (NH4)2HPO4 and 0.8 g of
NH4F unter continuous stirring followed by the addition of 20 mL NH3 solution to
°
maintain the pH of solution to 11. The suspension was then refluxed at 120 C for 24
h. The crystallites of M5FAP and pure FAP formed were then washed with distilled
°
water three times and filtered, and the precipitate was then dried at 70 C for 24 h. The
obtained M5FAP and pure FAP powders were heated in a muffle furnace at the
°
°
°
heating rate of 10 C/min up to 550 C and then calcined at 550 C for 3 h followed by
cooling to room temperature and seiving to 80-100 mesh size before use.
2
.2. Catalyst characterization
The content of Ca, Co, Ni, Cu, Zn and P elements in the synthesized M5FAP solid
powders and pure FAP were determined by Leeman Prodigy Spec inductively

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coupled plasma atomic emission spectroscopy (ICP-AES) and an Elemental Analyzer
(Vario EL III). All quantitative determinations were conducted at least in triplicate.
Powder X-ray diffraction (XRD) patterns of M FAP solids and pure FAP were
5
detected on a Rigaku D/MAX 2200PC diffractometer. The scanning rate used was
º
°
°
5
/min over the scan range 20 -60 2 theta.
The surface area of the M5FAP and pure FAP powders was determined using the
Brunauer-Emmett-Teller (BET) method of N2 adsorption-desorption using a
Micromeritics Tristar 3020 analyzer. Samples were outgassed for 6 h under vacuum at
°
2
60 C before adsorption.
The surface basicity of the M5FAP and pure FAP powders was measured by
selective CO2-TPD: 0.1 g of M5FAP or pure FAP powder was placed in a U-type
°
quartz tube. The solid sample was then pretreated under 20 mL/min He gas at 180 C
for 30 min to remove weakly adsorbed water and CO from the sample’s exposure to
2
the atmosphere before loaded to the micro-reactor. A 10 mL/min flow of 95% CO2/He
gas mixture was subsequently passed into the reactor for 60 min. Then weakly
°
adsorbed CO2 was removed at 80 C for 120 min under the flow of He (20 mL/min)
°
and the sample was then cooled in He flow to 30 C. The temperature of the M5FAP
°
°
powder solid was then increased from 30 to 600 C at the rate of 10 C/min under 10
mL/min He. The surface acidity of M5FAP and pure FAP powder solids was
measured by NH3-TPD. The procedures were identical to those for CO2-TPD except
that the flow of 95% CO2/He gas was replaced by the equivalent one of NH3. The

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NH3-TPD measurements were conducted using the Micromeritics Auto Chem. II
2
920 Chemisorption Analyzer.
2
.3. Catalyst activity performance
The catalytic reaction for the synthesis of MC was conducted in a stainless-steel
autoclave reactor system of 50 mL equipped with electric heating and stirring. In a
typical operation, 1.0 g of urea, 10 mL of methanol and 0.05 g of catalyst were added
to the reactor followed by a CO2 gas purge three times to displace the air in it. The
°
reactor was then heated and maintained at 140 C for 3 h using a 900 rpm stirring rate.
After completion of reaction, the mixture was filtered to remove the catalyst, and the
remaining solution was identified by GC-MS and analyzed by gas chromatography
(Agilent GC6890-5973I) to determine quantitatively the composition of the products.
The selectivity and yield of MC, and the conversion of urea were calculated based
on the following Eqs. (1)-(3):
amount (mol) of MC
Yield of MC (%)=
×100%
(1)
×100% (2)
×100% (3)
initial amount (mol) of urea
amount (mol)of MC
Selectivity to MC (%)=
Conversion of urea(%)=
amount (mol)of MC+amount (mol)of byproduct
amount (mol)of MC+amount (mol) of byproduct
initial amount (mol)of urea
3
. Results and discussion
The chemical composition of the prepared M5FAP is listed in Table 1. The
theoretical value of Ca/P ratio in FAP is 1.67. The substituted amount of different

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transition metals was between 18.46 and 19.20 wt%. The (Ca+M)/P value of M5FAP
(M = Co, Ni, Cu, Zn) compared to pure FAP was found to decrease.
The structure of M FAP (M = Co, Ni, Cu, Zn) powders was analysed by X-ray
5
diffraction, and the recorded diffractogram was compared with that of FAP standard
profile (JCDD card No.: 15-0876). The FAP crystallizes in the hexagonal system with
the space group P63/m. The standard data for the pure FAP are: a=9.364 Å and
c=6.893 Å, which are in agreement with the lattice parameters of the prepared FAP
(see Table 1). The powder XRD patterns of Co5FAP, Ni5FAP, Cu5FAP, and Zn5FAP
appear similar to that of pure FAP (Fig. 1 and Table 1). The M5FAP (M = Co, Ni, Cu,
Zn) did not change the crystalline structure of pure FAP. The intensity of typical
diffraction peaks has not obviously changed, indicating no disorganization of the
crystalline structure of FAP.
FAP is known to possess a rather low porosity structure, and its surface area is less
2
than 15 m /g. β-cyclodextrin is easily available, cheap, and has a hydrophilic exterior
and a hydrophobic internal cavity. According to research works reported [32, 33],
β-cyclodextrin as template can increase the surface area of hydroxyapatite, leading to
a greater proportion of its atoms or molecules on the surface rather than in the interior
structure of it. In our research, β-cyclodextrin was used as template to increase the
2
specific surface of FAPs, and indeed, the FAP surface area increased to 40.62 m /g;
2
other M5FAP powders have shown larger surface areas (90-100 m /g) as shown in
Table 1.

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The surface basic and acidic properties of the investigated solids were probed by
CO2-TPD and NH3-TPD, respectively. Fig. 2 shows the CO2-TPD profile of the
M FAP solid, where the strength of surface basic sites of M FAP follows the order:
5
5
Zn5FAP > Cu5FAP > Ni5FAP > Co5FAP > FAP. No acidity was observed based on
the NH3-TPD measurements (Table 1). These results strongly suggest that the surface
of M5FAP possesses basic in nature catalytic sites.
Among the catalysts tested, the yield to MC over M5FAP catalysts increased with
increasing surface basicity of the catalysts. This means that the latter surface paramter
of the catalyst plays an important role in determining the catalytic performance of this
solid for the reaction of MC formation. Zn5FAP with the strongest surface basicity
showed the best catalytic performance in the synthesis of MC via the urea
methanolysis route, where 98.9% selectivity and yield of MC after 3 h of reaction at
°
1
40 C was achieved (conversion of urea was 100%). On the other hand, FAP,
Co5FAP, Ni5FAP and Cu5FAP show lower activity. The activity of M5FAP follows
the order: Zn5FAP > Cu5FAP > Ni5FAP > Co5FAP > FAP.
The effect of reaction time on the synthesis of MC using M5FAP catalysts is shown
in Fig. 3. Increasing the reaction time, the yield to MC increases gradually. When the
reaction time is 3 h, the yield of MC reaches the highest value when Zn5FAP is used
as catalyst, whereas by further increasing the reaction time, the yield of MC starts to
decrease as the byproduct dimethyl carbonate concentration increases. The stability of
Zn5FAP catalyst (Fig. 4 and Fig. 1) was measured as well, where the separated
catalyst was washed three times with ethanol and deionized water, respectively,

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°
before the next catalytic test. The catalyst was then dried at 100 C for 24 h and reused
at the denoted reaction conditions. The selectivity and yield of MC are both kept over
9
4% in the fifth recycling experiment. In order to further check the stability of
Zn5FAP, the used catalysts were characterized by powder XRD and ICP-AES. As
shown in Fig. 1 (f), the position of the peaks in the XRD patterns for Zn5FAP were
not changed, which shows that the crystal structure of the solid remained intact after
reaction; the (Ca+Zn)/P ratio determined was 1.66. The latter results suggest that the
developed Zn5FAP catalyst appears promising for an extended stability performance
in practical applications.
The catalytic mechanism for urea methanolysis to MC is presented in Fig. 5 and can
be described as follows. Urea is easily decomposed into isocyanic acid (CONH). A
base catalyst is expected to promote the dissociative chemisorption of CH3OH to form
CH O-, which is a much stronger nucleophile species than CH OH itself, and which
3
3
can easily attack the C atom of isocyanic acid carbonyl [32]. According to the
literature [33], as a result of the strong basic character of Zn5FAP, the lone-pair
electrons of methoxy group attacks the carbon atom of Zn-N=C=O group, whereas the
N atom of the metal isocyanato group seizes the H atom of methoxy group. The
intermediate is thus generated. The chemical bonds of Zn-N and O-H of the
intermediate brake and the MC is formed accompanied by the regeneration of the
catalyst.
4
. Conclusions

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A series of M5Ca5(PO4)6F2 (M5FAP, M = Co, Ni, Cu, Zn) catalysts were prepared
and their outstanding catalytic performance for the synthesis of MC by urea and
methanol was reported. The Zn FAP catalyst had the highest catalytic activities
5
among all the M5FAP catalysts tested. The conversion of urea was 100%, the
selectivity to MC was 98.9% and the yield of MC was also 98.9%. The Zn5FAP
catalyst was easily recovered and reused. Zn5FAP is therefore a new type of green and
efficient catalyst for the synthesis of MC from urea and methanol.
Acknowledgments
The authors gratefully acknowledge Changchun University of Technology for all of
the supports.
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Table 1. Chemical composition, surface area and acidity of the different catalysts
prepared.
a
M
(Ca+M)/
P
Surface
Basicity
Acidity
(mmol
a (Å)
c (Å)
(
wt%)
area
2
(mmol CO /g
2
(
m /g)
catalyst)
3
NH /g
catalyst)
0.12
FAP
Co FAP
Ni FAP
-
1.67
1.65
1.66
1.64
1.66
9.353
9.356
9.357
9.362
9.360
6.871
6.879
6.880
6.882
6.883
40.62
90.44
92.63
97.80
0.22
0.41
0.46
0.48
5
19.11
18.46
19.20
19.03
-
-
-
-
5
Cu
5
5
FAP
FAP
Zn
100.22
0.58
FAP.
a
M refers to Co in Co
5
FAP, Ni in Ni
5
FAP, Cu in Cu
5
FAP, and Zn in Zn
5

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Figure Captions
Figure 1. XRD patterns of the M5FAP and pure FAP solid powders prepared.
Figure 2. CO -TPD curves of the M FAP and pure FAP powders prepared.
2
5
Figure 3. The effect of reaction time on the MC yield.
Figure 4. Reusability of the Zn5FAP catalysts for the MC synthesized.
Figure 5. Proposed mechanism of MC synthesis over Zn5FAP catalyst.
Scheme 1. Synthesis M5FAP by the co-precipitation method.

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Figure 1 XRD Patterns of the M5FAP and pure FAP powders prepared.
a: FAP; b: Co
5 5 4 6 2 5 5 4 6 2 5 5 4 6 2 5 5 4 6 2
Ca (PO ) F ; c: Ni Ca (PO ) F ; d: Cu Ca (PO ) F ; e: Zn Ca (PO ) F ; f: reused
Zn Ca (PO ) F
5
5
4 6 2

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Figure 2 CO2-TPD curves of M5FAP and pure FAP powders prepared

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Figure 3 The effect of reaction time on the MC yield
All reactions were performed under the following conditions: urea 1.0 g, catalyst 50 mg,
°
urea/methanol molar ratio 1:10, temperature 140 C.

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Figure 4 Reusability of the Zn5FAP catalysts for MC synthesized
5
All reactions were performed under the following conditions: urea 1.0 g, Zn FAP 50 mg,
°
urea/methanol molar ratio 1:10, temperature 140 C, time 3 h.

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Figure 5 The mechanism over Zn5FAP catalyst for MC synthesis
O
CH3
C
O
O
P
P
O
O
O
P
P
Zn
Zn
H
N
H
O
C
Zn
(
cat)
(cat)
H2N
NH2
CH3OH
O
P
O
P
-
NH3
O
C
N
OCH3
H
O
C
H
H2N
OCH3
Zn
O
P
O
P
O
O
P
Zn
P
(cat)

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Scheme 1. Synthesis M5FAP by co-precipitation.

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O
CH3
C
N
O
O
P
P
O
O
P
P
O
Zn
Zn
H
H
O
C
Zn
(cat)
(cat)
H2N
NH2
CH3OH
O
P
O
P
-
NH3
O
C
N
OCH3
H
O
C
H
H2N
OCH3
Zn
O
P
O
P
O
O
P
Zn
P
(cat)
Graphical abstract

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Highlights

Prepared a series of metal modified fluorapatite by
co-precipitation.


One-pot synthesis of methyl carbamate from urea and
methanol by Zn FAP was efficient.
5
The selectivity and yield were satisfied and the possible
mechanism was discussed.