The Induction Period and Novel Active Species in Zn(OAc)2 Catalyzed Synthesis of Aromatic Carbamates

Article abstract of DOI:10.1007/s10562-017-2055-z

Abstract: Zn(OAc)₂ is known to exhibit excellent catalytic activity for aromatic carbamate synthesis by the reaction of aromatic amine and dimethyl carbonate (DMC). It is determined that an induction period exists during the synthesis of dimethyl-4,4′-methylenediphenyl dicarbamate (MDC) and methyl N-phenyl carbamate (MPC), although this period is more evident in the case of MDC. Zn(OAc)₂ is able to catalyze the reaction of amine and DMC, but also react with DMC to form Zn₄O(OAc)₆, which also shows excellent catalytic activity for carbamate synthesis. As for MDC synthesis, Zn₄O(OAc)₆ plays the important role. However, Zn(OAc)₂ plays the leading role in MPC synthesis. The deactivation of Zn₄O(OAc)₆ was also investigated. Zn₄O(OAc)₆ was initially transformed into Zn₅(OH)₈(OAc)₂·2H₂O (LHZA) and then into ZnO. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2055-z

Catal Lett
DOI 10.1007/s10562-017-2055-z
The Induction Period and Novel Active Species in Zn(OAc)2
Catalyzed Synthesis of Aromatic Carbamates
Fang Li1 · Xi Wang1 · Hongqin Li1 · Shufang Wang1 · Wei Xue1 · Yanji Wang1
Received: 21 January 2017 / Accepted: 7 April 2017
© Springer Science+Business Media New York 2017
Abstract Zn(OAc)2 is known to exhibit excellent cata-
lytic activity for aromatic carbamate synthesis by the reac-
tion of aromatic amine and dimethyl carbonate (DMC). It
is determined that an induction period exists during the
synthesis of dimethyl-4,4′-methylenediphenyl dicarbamate
(MDC) and methyl N-phenyl carbamate (MPC), although
this period is more evident in the case of MDC. Zn(OAc)2
is able to catalyze the reaction of amine and DMC, but
also react with DMC to form Zn4O(OAc)6, which also
shows excellent catalytic activity for carbamate synthe-
sis. As for MDC synthesis, Zn4O(OAc)6 plays the impor-
tant role. However, Zn(OAc)2 plays the leading role in
MPC synthesis. The deactivation of Zn4O(OAc)6 was also
investigated. Zn4O(OAc)6 was initially transformed into
Zn5(OH)8(OAc)2·2H2O (LHZA) and then into ZnO.
Graphical Abstract
R-NH2
+
O
H3C
O
C
O
CH3
R-NHCOOCH3
+ CH3OH
Zn(OAc)2
Zn4O(OAc)6
Keywords Zn(OAc)2 · Dimethyl carbonate · Methyl
N-phenyl carbamate · Dimethyl-4,4′-methylenediphenyl
dicarbamate · Zn4O(OAc)6 · Induction period
1 Introduction
Organic carbamates are widely used as important inter-
mediates in the synthesis of agrochemicals, dyestuffs and
pharmaceuticals. They also play a key role in synthetic
chemistry in the protection of amino groups [1–4] and, in
addition, are important intermediates in isocyanates pro-
duction by their thermal decomposition [5]. Isocyanates
can be divided into aromatic and aliphatic types and several
of the aromatic isocyanates, such as 4,4′-diphenyl-methane
diisocyanate and 2,4-toluene diisocyanate, are widely used
in polyurethane production.
* Wei Xue
weixue@hebut.edu.cn
* Yanji Wang
yjwang@hebut.edu.cn
1
The most common method of synthesizing aromatic
isocyanates begins with phosgene. Phosgene, however, is
very hazardous and its use in large scale production entails
Hebei Provincial Key Lab of Green Chemical Technology
& High Efficient Energy Saving, School of Chemical
Engineering & Technology, Hebei University of Technology,
Tianjin 300130, China
Vol.:(0123456789)
1 3

F. Li et al.
risks related to worker exposure, equipment corrosion and
environmental damage upon the release of waste streams.
Owing to the environmental hazards of phosgene, efforts
are being made to develop non-phosgene synthetic routes to
isocyanates. Among the non-phosgene routes, the synthesis
of carbamates by aminolysis of dimethyl carbonate (DMC)
followed by the thermal decomposition of these carbamates
to generate isocyanates is becoming more attractive [6].
The reaction scheme for the synthesis of isocyanates from
DMC is described in Scheme 1. In this process, the only
by-product is methanol. In addition, the methanol gener-
ated as a by-product may be used as the raw material for the
subsequent production of DMC by oxidative carbonylation,
thus reducing the overall production costs.
also exists in the reaction of MDA or aniline with DMC
and what may cause the induction period? Additionally,
we have determined that, during the catalytic process,
Zn(OAc)2 can react with DMC to form a new compound
that shows better catalytic performance, and so which plays
the important role in the catalytic reaction: Zn(OAc)2 or
the new compound? To investigate this catalytic process
in more detail, the present study examined the reactions of
DMC with aniline and MDA over a Zn(OAc)2 catalyst and
the associated reaction mechanism is discussed herein.
2 Experimental
In the first step of Scheme 1, a catalyst is required to
achieve a suitably high aromatic carbamate yield. At pre-
sent, lead [7–10] or zinc [11–17] compounds and other
catalysts [18–22] are often applied in this reaction. Among
these catalysts, zinc compounds, especially Zn(OAc)2,
often give the best activity for aromatic carbamate syn-
thesis. As an example, Gurgiolo [11] prepared methyl
N-phenyl carbamate (MPC) using DMC and aniline as raw
ingredients over a Zn(OAc)2 catalyst, and the selectivity for
MPC from aniline was found to be 99.8% at 140°C. Addi-
tionally, Baba [13] investigated the methoxycarbonylation
of 2,4-toluene diamine (TDA) and 4,4′-diphenylmethane
diamine (MDA) with DMC to their corresponding dicarba-
mates using Zn(OAc)2 as a catalyst at 180°C for 2 h. The
yield of dimethyl toluene-2,4-dicarbamate was 96% while
the yield of dimethyl-4,4′-methylenediphenylcarbamate
(MDC) was 98%. Zn(OAc)2 also shows excellent cata-
lytic activity for the reaction of TDA or MDA with diethyl
carbonate, generating yield of the corresponding carba-
mate greater than 93% [15]. Other zinc compounds, such
as N,N-substituted carbamate complexes of zinc [12] and
Zn4O(OAc)6 [16], actually exhibit superior catalytic perfor-
mance for the reaction of aromatic amines and DMC, but
are challenging to prepare and thus more expensive.
2.1 Chemicals and Materials
Dimethyl carbonate (99.5%) was purchased from Tangshan
Chaoyang Chemical Co. Ltd. Aniline (99.5%) and zinc
acetate (99.5%) was purchased from Sinopharm Chemi-
cal Reagent Co. Ltd. MPC (>98.0%) and MDI (>98.0%)
was purchased from Tokyo Chemical Industry Co. Ltd.
MDA(>97.0%) was purchased from Acros.
4,4′-MDC standard was prepared by the reaction of 4,4′-
MDI (>97.0%) and excessive methanol (>99.9%), then by
recyrstallization using ethanol as solvent.
2.2 Catalytic Evaluation
The reactions to generate either MPC or MDC were car-
ried out in a stainless steel autoclave. In this process, the
aromatic amine, DMC and Zn(OAc)2 (prepared by the
dehydration of Zn(OAc)2·H2O at 120°C) were added to the
autoclave and allowed to react at the desired temperature
for the desired time span. Owing to the different solubility
of MPC or MDC in DMC, the two reaction mixtures were
treated differently. Following MPC synthesis, the reaction
mixture was simply filtered to separate the catalyst, and the
filtrate was analyzed by high performance liquid chroma-
tography (HPLC). Following MDC synthesis, the reaction
mixture was also filtrated but, owing to the poor solubility
of MDC in DMC, some MDC that had precipitated from
the DMC was contained in the filter cake along with the
catalyst. In order to separate the MDC from the catalyst, the
filter cake was washed with acetone and both the resulting
acetone solution and the filtrate were analyzed by HPLC.
A Mettler Toledo Labmax system equipped with a
1L glass reactor was used to observe the solubility of
Zn(OAc)2 in DMC and also to prepare Zn4O(OAc)6.
During the Zn(OAc)2-catalyzed reaction of m-phenylen-
ediamine or 2,4-toluene diamine with DMC, an induc-
tion period existed [13]. Whether this induction period
2.3 Product Analysis
Waters HPLC was used to analyze the reaction liquid.
The analytical conditions for MPC synthesis were as
Scheme 1 General synthesis of isocyanates from DMC and amines
113×63 mm
1 3

The Induction Period and Novel Active Species in Zn(OAc)2 Catalyzed Synthesis of Aromatic…
follows: A Kromasil C-18 (Φ4.6 mm × 150 mm, 5 μm)
chromatographic column was used at a flow rate of
0.4 mL/min. CH3OH/H2O (70/30, V/V) was used as the
mobile phase and the detection wavelength was 254 nm.
A quantitative analysis was carried out using nitroben-
zene as the internal standard. The analytical conditions
for MDC synthesis were the same as that of MPC, except
the mobile phase and the flow rate. CH3OH/H2O (60/40,
V/V) was used as the mobile phase and the flow rate was
0.8 mL/min for MDC analysis.
3 Results and Discussion
3.1 Induction Period in Zn(OAc)2 Catalyzed MDC
or MPC Synthesis
Effect of reaction time on the catalytic activity of Zn(OAc)2
was shown in Table 1. During the reaction of aniline with
DMC, aniline conversion was 68% at a reaction time of
1 h and reached to 97% at 2 h. Accordingly, MPC yield
increased from 68 to 97%. With regard to the MDC synthe-
sis, MDA conversion and MDC yield also increased with
reaction time. The MDA conversion of 47% and the MDC
yield of 2% at a reaction time of 2 h increased to 100 and
84% at a reaction time of 3 h, respectively. Thus the induc-
tion period existed in the reactions of MDA and aniline
with DMC.
2.4 Catalyst Characterization
X-ray diffraction patterns were recorded on a Rigaku D/
MAX2500 with Cu Kα radiation at 40 kV and 100 mA.
The scanning range was 3°–90°.
It was also observed that the induction period was short-
ened by pretreatment of the Zn(OAc)2 with DMC, based on
the results shown in Table 1 (Entry 2), especially for MDC
synthesis. The MDC yield reached 92% at a reaction time
of 2 h, following pretreatment of the Zn(OAc)2 with DMC
at 130°C for 1 h. In contrast, the original MDC yield was
only 2% at a reaction time of 2 h when the Zn(OAc)2 was
not pretreated with DMC. As for MPC synthesis, MPC
yield was 68% at a reaction time of 1 h without the treat-
ment of Zn(OAc)2 with DMC. After the pretreatment,
FTIR spectra were obtained with a NICOLET
NEXUS-470 spectrometer. A DTGS detector was used
and 32 scans were accumulated at a spectral resolution
of 4 cm−1
.
Element analysis was performed for carbon and
hydrogen with a Thermo Scientific Flash EA 1112 ele-
mental analyzer.
NMR spectra were recorded on a Bruker Avance 400
spectrometer at ambient temperature (1H 400 MHz, 13C
100 MHz).
Table 1 Effect of the DMC
Entry Catalyst
Reaction of DMC and aniline
Reaction of DMC and MDA
pretreatment on the catalytic
performance of Zn(OAc)2
Time (h) Aniline con- MPC
Time (h) MDA con- MDC
version (%) yield
(%)
version (%)
yield (%)
1
Zn(OAc)2
1a
2a
3a
68
97
98
80
98
67
98
99
68
97
97
80
98
59
94
99
1b
2b
3b
1d
2d
1f
2f
3f
36
47
100
66
100
46
100
100
0
2
84
7
92
2
86
89
2
3
Zn(OAc)2 pretreated 1c
2c
The acquired solidc
0.5e
1e
2 e
aReaction conditions: Zn(OAc)2 0.56 g (3 mmol), aniline 2 mL (22 mmol), DMC 40 mL (475 mmol),
130°C
bReaction conditions: Zn(OAc)2 0.20 g (1 mmol), MDA 0.82 g (4 mmol), DMC 14 mL (166 mmol), 130°C
c0.56 g (3 mmol) Zn(OAc)2 was pretreated with 40 mL DMC (475 mmol) at 130°C for 1 h, then cool to
room temperature and 2 mL aniline (22 mmol) was added to continue the reaction
d0.20 g (1 mmol) Zn(OAc)2 was pretreated with 14 mL (166 mmol) DMC at 130°C for 1h, then cool to
room temperature and 0.82 g MDA (4 mmol) was added to continue the reaction
eReaction conditions: the acquired solid 0.56 g (0.9 mmol), aniline 2 mL (22 mmol), DMC 40 mL
(475 mmol), 130°C
fReaction conditions: the acquired solid 0.20 g (0.3 mmol), MDA 0.82 g (4 mmol), DMC 14 mL
(166 mmol), 130°C
1 3

F. Li et al.
MPC yield reached 80% at a reaction time of 1 h. It can be
seen that the pretreatment of the Zn(OAc)2 with DMC can
shorten the reaction time.
Zn(OAc)2. Thus the two amino groups in MDA cannot
be esterified simultaneously, but are rather successively
catalyzed by heterogeneous Zn(OAc)2. When homoge-
neous Zn(OAc)2 is present, however, the probability of
the two amino groups simultaneously being esterified
is increased, owing to the reduced steric hindrance. The
aforementioned factors result in the different induction
periods observed for the reactions of MDA and aniline
with DMC.
In order to find the cause for the induction period, 4.3 g
of Zn(OAc)2 was pretreated with 300 mL DMC at 130°C
for 1 h (nDMC:nZn(OAc)2 =151:1) in a 1 L glass reactor. It
was found that Zn(OAc)2 could not completely dissolve
in DMC during the pretreatment, which indicates that the
solubility of Zn(OAc)2 in DMC was limited at this tem-
perature. Thus it is presumed that the possible cause of the
induction period might be the time required for Zn(OAc)2
dissolution in the DMC at 130°C. Due to the limited sol-
ubility of Zn(OAc)2 in the DMC, Zn(OAc)2 exists in two
forms at the start of the reaction; homogeneous Zn(OAc)2
and heterogeneous Zn(OAc)2, with the latter present in a
large proportion. Both the homogeneous and heterogeneous
Zn(OAc)2 exhibit good catalytic performance for MPC or
MDC synthesis. In the case of the heterogeneous catalyst,
the reactants must diffuse and adsorb on the catalyst sur-
face before the reaction can occur, which leads to an induc-
tion period.
Additionally, it is possible that the above pretreat-
ment step could promote the transformation of Zn(OAc)2
to other catalytic species. To assess this possibility, the
aforementioned pretreatment reaction mixture was cooled
to room temperature and filtered under vacuum. The fil-
trate was distilled under vacuum and a white solid pre-
cipitated after the distillation. This solid was evaluated
for the reaction of DMC with aniline and MDA, with the
result shown in Table 1 (Entry 3). The solid exhibited
excellent catalytic activity for MPC synthesis, giving a
MPC yield of 94% at a reaction time of 1 h. Similarly,
this solid exhibited excellent catalytic performance for
MDC synthesis, generating an MDC yield of 86% at a
reaction time of 2 h.
Furthermore, the induction period is more obvious dur-
ing the reaction of MDA with DMC compared with that of
aniline with DMC, possibly owing to the reaction mecha-
nism associated with using Zn(OAc)2 as the catalyst. The
reaction of an amine with DMC is nucleophilic, since the
carbonyl group of DMC will attract nucleophiles owing to
its positive polarization and the sp2 hybridization of the C
atom [23, 24]. When Zn(OAc)2 is used as the catalyst, the
d10 electrons of Zn2+ can coordinate with the oxygen atom
of the carbonyl to form an intermediate (Scheme 2) that can
activate the carbonyl group of DMC by the nucleophilic
attack of amine [14].
The white solid was subsequently characterized
by Fourier Transform Infrared (FTIR) spectroscopy
and the results were shown in Fig. 1. The absorption
bands (Fig. 1a) at 1559 and 1447 cm−1 are attributed to
the asymmetric and symmetric stretching vibration of
COO−, while the band at 697 cm−1 is attributed to the
deformation vibration of COO−. Figure 1b shows the
As for heterogeneous Zn(OAc)2, DMC must diffuse to
and adsorb on the heterogeneous Zn(OAc)2 surface. And
this is not the case for homogeneous Zn(OAc)2. As noted,
however, aniline has one amino group, while MDA has
two. The difference between these two species, therefore,
occurs owing to the lower probability of the two MDA
amino groups simultaneously attacking the carbonyl
group of DMC during catalysis by the heterogeneous
Fig. 1 FTIR spectrum of a Zn(OAc)2 and b the acquired solid
Scheme 2 The intermediate in Zn(OAc)2 catalyzed the reaction of
210×148 mm
DMC with amine 92×42 mm
1 3

The Induction Period and Novel Active Species in Zn(OAc)2 Catalyzed Synthesis of Aromatic…
FTIR spectrum of the acquired solid, which matches
that of Zn4O(OAc)6 [16, 25]. The bands at 1602 and
1440 cm−1 correspond to the asymmetric and symmet-
ric stretching vibrations of COO−, while those at 1058
and 1035 cm−1 are attributed to C–C stretching. The
bands at 617 and 671 cm−1 are assigned to the deforma-
tion vibration of COO− and that at 528 cm−1 is due to the
asymmetric stretching vibration of Zn4O−. Additionally,
even at 1 h. This indicated that the induction period was
shortened. The cause is that, at higher temperature, the
solubility of Zn(OAc)2 in DMC increased and the for-
mation of Zn4O(OAc)6 is accelerated. Moreover, higher
temperature can also promote the reaction rate of MDC
synthesis. Therefore, higher temperature shortens the
induction period. Unlike the temperature, the molar ratio of
Zn(OAc)2/MDA and DMC/MDA showed less influence on
the induction period of Zn(OAc)2 within 3 h. This could be
also explained by the fact that they had less influence on
the reaction rate and the solubility of Zn(OAc)2 in DMC.
1
the white solid was also characterized by H NMR, 13C
NMR and elemental analyses. The results are as follows:
1H NMR (400 MHz, DMSO-d 6) δ 1.878 (CH3); 13C
NMR (100 MHz, DMSO-d ) δ 23.1 (CH3), 178.5(CO).
Anal. Calcd for Zn4O(OAc)66: C, 22.81; H, 2.87. Found:
C, 22.86; H, 2.82. Thus it can be seen that after the pre-
treatment of Zn(OAc)2 with DMC part of the Zn(OAc)2
is transformed into Zn4O(OAc)6, which has a tetrahedral
Zn4O− center with bridging carboxyl groups along each
of the six edges of the tetrahedron [26].
3.2 Catalytic Stability of Zn4O(OAc)6
The stability of Zn4O(OAc)6 in MDC and MPC synthe-
sis was studied, with the results shown in Table 2. After
the second use, the catalytic performance of Zn4O(OAc)6
for MDC and MPC synthesis decreased significantly. To
investigate the deactivation, Zn4O(OAc)6 was character-
ized after the first and second uses by XRD. Figure 3 shows
the XRD patterns of deactivated Zn4O(OAc)6 in MDC
synthesis. Weak ZnO diffraction peaks are seen in Fig. 3a,
indicating the presence of a small amount of ZnO after
the initial usage of the Zn4O(OAc)6. Additionally, diffrac-
tion peaks appear at 2θ values of 5.8°, 33.2° and 59.6° in
the XRD pattern of Zn4O(OAc)6 after its first use. These
can be assigned to the (001), (100) and (110) reflections
of layered basic zinc acetate, Zn5(OH)8(OAc)2·2H2O
(LHZA) [27–29]. Figure 3b presents the XRD pattern of
Zn4O(OAc)6 after its second use. It can be seen that the
intensities of the LHZA diffraction peaks are obviously
weakened while the ZnO peaks are strengthened, demon-
strating that the majority of the LHZA was transformed
into ZnO. Figure 4 shows the XRD patterns of deactivated
Zn4O(OAc)6 in MPC synthesis. Similarly, the diffraction
peaks of LHZA appear (shown in Fig. 4a) after the first
use of Zn4O(OAc)6. After its second use, the diffraction
peaks of LHZA disappear and the diffraction peaks of ZnO
appear. Therefore, the deactivation of Zn4O(OAc)6 is evi-
dently due to the formation of ZnO. During the formation
of ZnO, Zn4O(OAc)6 is initially transformed into LHZA
and then the LHZA is transformed into ZnO.
The above results noted an induction period during the
synthesis of MDC or MPC using Zn(OAc)2 as the catalyst.
However, after the pretreatment of Zn(OAc)2 with DMC,
this induction period was significantly shortened. Owing to
the limited solubility of Zn(OAc)2 in DMC, the amount of
dissolved Zn(OAc)2 increases as the reaction time is pro-
longed. The dissolved Zn(OAc)2 will catalyze the reaction,
but also react with DMC to form Zn4O(OAc)6. Therefore,
the joint catalytic effects of the dissolved Zn(OAc)2 and the
Zn4O(OAc)6 rapidly increase the MDC yield. Then, which
compound plays the important role in the catalytic reaction:
Zn(OAc)2 or Zn4O(OAc)6. As for MDC synthesis (shown
in Table 1, Entry 1), MDA conversion and MDC yield
were found to be 36 and 0% at 1 h, respectively. As afore-
mentioned, part of Zn(OAc)2 could be transformed into
Zn4O(OAc)6 when Zn(OAc)2 was pretreated with DMC for
1 h at 130°C. MDA conversion and MDC yield increased
promptly to 100 and 84%, respectively, at 3 h. So it is rea-
sonable to presume that Zn4O(OAc)6 plays the major role
in MDC synthesis. However, with regard to the MPC syn-
thesis, MPC yield was 68% at a reaction time of 1 h and it
increased to 97% at 2 h. So Zn(OAc)2 plays the leading role
in MPC synthesis.
Additionally, the effects of reaction conditions on the
induction period of Zn(OAc)2 in MDC synthesis were
evaluated and the results are shown in Fig. 2. There was
a distinct induction period within 3 h when the reaction
temperature was 130°C, as shown in Fig. 2a, b. How-
ever, the induction period didn’t appear at 110°C. When
the temperature was 110°C, MDA conversion and MDC
yield were only 27 and 1% for 3 h, respectively. It meant
that the induction period was probably prolonged at lower
temperature. When the temperature increased to 150°C,
MDA conversion and MDC yield reached 100 and 83%
4 Conclusion
(1) An induction period exists during the synthesis of
MDC and MPC, possibly owing to the time lag associ-
ated with dissolution of Zn(OAc)2 in DMC.
1 3

F. Li et al.
(a)
(b)
(c)
(e)
(d)
(f)
Fig. 2 Effects of reaction conditions on induction period in MDC synthesis reaction conditions: a, b nDMC: nMDA =40, nZn(OAc) : nMDA =0.3; c, d
2
nDMC: nMDA =40, 130°C; e, f nZn(OAc) : nMDA =0.3, 130°C 206×158 mm
2
Table 2 Stability of Zn4O(OAc)6 for MDC and MPC synthesis
(2) Zn(OAc)2 can react with DMC to form Zn4O(OAc)6,
which also shows excellent catalytic activity for car-
bamate synthesis. As for MDC synthesis, Zn4O(OAc)6
plays the important role in the catalytic reaction. How-
ever, Zn(OAc)2 plays the leading role in MPC synthe-
sis.
Times
Reaction of DMC and anilinea Reaction of DMC and
MDAb
Aniline conver- MPC yield MDA conver- MDC
sion (%)
(%)
sion (%)
yield
(%)
(3) During the catalytic synthesis of aromatic carbamate
Zn4O(OAc)6 initially transforms into LHZA and then
into ZnO.
1
2
99
2
99
2
100
50
89
3
aReaction conditions: catalyst 0.56 g (0.9 mmol), aniline 2 mL
(22 mmol), DMC 40 mL (475 mmol), 130°C, 2 h
bReaction conditions: catalyst 0.20 g (0.3 mmol), MDA 0.82 g
(4 mmol), DMC 14 mL (166 mmol), 130°C, 3 h
1 3

The Induction Period and Novel Active Species in Zn(OAc)2 Catalyzed Synthesis of Aromatic…
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Acknowledgements This work was supported by the National
Natural Science Foundation of China (Nos. 21236001, 21106031,
21576069), and the Programme for 100 Excellent Talents in Univer-
sity of Hebei Province (II) (BR2-208).
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