Catalytic synthesis of methylene diphenyl dicarbamate from methyl phenyl carbamate and trioxane over sulfuric acid catalyst

Article abstract of DOI:10.1007/s11458-009-0207-4

Methylene diphenyl dicarbamate (MDC) was synthesized from methyl phenyl carbamate (MPC) and trioxane using sulfuric acid (H₂SO₄) as catalyst. The effects of reaction temperature, reaction time, molar ratio of reactants and the content of catalyst have been studied in details. The results showed that H₂SO₄ exhibited high catalytic activity with the merits of moderate reaction velocity. Under the conditions of n(MPC)/n(trioxane) = 3:1, reaction temperature of 95°C, reaction time of 3.5 h and 30% H₂SO₄, the conversion of MPC reached 99.0% with the selectivity of MDC 81.6%. Moreover, the H₂SO₄ catalyst was reused five times without obviously activity decrease. Based on the identification of byproducts, a possible reaction mechanism was proposed. Higher Education Press and Springer-Verlag Berlin Heidelberg 2010.

Full text of DOI:10.1007/s11458-009-0207-4

Front. Chem. China 2010, 5(1): 99–103
DOI 10.1007/s11458-009-0207-4
RESEARCH ARTICLE
hydrochloride as byproduct. For the purpose of eliminating all
these drawbacks, several non-phosgene routes [4–6] have
been developed. Among them, thermal decomposition of
methylene diphenyl dicarbamate (MDC) to obtain MDI is
thought to be one of the most attractive ways.
Catalytic synthesis of
methylene diphenyl
dicarbamate from methyl
phenyl carbamate and
trioxane over sulfuric acid
catalyst
Recently, studies on MDC synthesis have been mainly
focused on the condensation of methyl phenyl carbamate
(
MPC), which can be synthesized by many methods.
However, there are still some challenges to be overcome for
the industrialization of this reaction. Takeshita et al. [7,8]
studied the condensation of MPC using HCHO as methylene
agent with sulfuric acid (H SO ) (96%) as catalyst. The MDC
2
4
was obtained in 73% yield with large amount of byproducts
that was caused by superfluous HCHO. In the presence of
acid, trioxane was decomposed to HCHO with appropriate
rate to avoid the superfluous HCHO. Therefore, Wada et al.
1
2
1
Tao LI , Huiquan LI (✉) , Guotong QIN (✉) ,
2
2
Haitao LIU and Yixia PEI
[
9–11], Fukuoka et al. [12] and Clerici et al. [13] did lots of
Methylene diphenyl dicarbamate (MDC) was synthe-
sized from methyl phenyl carbamate (MPC) and
trioxane using sulfuric acid (H SO ) as catalyst. The
effects of reaction temperature, reaction time, molar
ratio of reactants and the content of catalyst have been
research on the condensation reaction using trioxane as
methylene agent with different catalysts. The selectivity of
MDC has been improved, but the highest MPC conversion
was no more than 45%. Li et al. [14] worked on the MDC
synthesis from trioxane and MPC using mixed acid as
catalyst, resulting an MDC yield of 75.8%. However, harsh
reaction conditions such as high reaction temperature (105°C)
and long reaction time (5 h) are required.
In previous studies, there was neither further report about
the detailed research on the condensation of MPC and
trioxane nor the reaction mechanism. This prompted us to
undertake the present work. In this paper, the effects of
reaction conditions, the recycle of catalyst and the identifica-
tion of major byproduts were studied. Based on the
identification of byproducts, a possible reaction mechanism
was proposed.
2
4
studied in details. The results showed that H SO4
2
exhibited high catalytic activity with the merits of
moderate reaction velocity. Under the conditions of
n(MPC)/n(trioxane) = 3:1, reaction temperature of
9
5°C, reaction time of 3.5 h and 30% H SO , the
2 4
conversion of MPC reached 99.0% with the selectivity
of MDC 81.6%. Moreover, the H SO catalyst was
2
4
reused five times without obviously activity decrease.
Based on the identification of byproducts, a possible
reaction mechanism was proposed.
Keywords methylene diphenyl dicarbamate, methyl
phenyl carbamate, trioxane, condensation
2
Materials and methods
1
Introduction
2.1 Materials
Diphenylmethane diisocyanate (MDI) is an important inter-
mediate for the manufacture of polyurethanes, which can be
used as a material of metal-adhesive, fiberglass, dope,
elastomer-fiber and all kinds of leatheroid, etc. [1–3].
Commercially, MDI is manufactured by the reaction of
phosgene and methylene dianiline. However, the phosgene
route is not environmentally friendly due to the high
toxicity of phosgene and hence generates highly corrosive
Trioxane was purchased from Sinopharm Chemical Reagent
Co. Ltd. H SO was purchased from Beijing shiji Co. Ltd.
MPC was synthesized from 1,3-diphenyl urea and DMC,
which had been deeply studied in our laboratory [6] with the
purity of 99.8%.
2
4
2.2 Experimental method
In a typical procedure for MDC synthesis, MPC (6.04 g,
Received November 16, 2009; accepted December 28, 2009
1
1
2
. School of Chemistry and Environment, Beihang University, Beijing 0.04 mol), trioxane (1.20 g, 0.0133 mol), water (36 mL) and
00191, China
H SO (16.06 g, 98.0%) were charged into a 100 mL round-
bottom flask with an oil bath. Trioxane would not be added
2
4
. Key Laboratory of Green Process and Engineering, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, China
E-mail: hqli@home.ipe.ac.cn, qingt@buaa.edu.cn
until the mixture was heated to a certain temperature with
©
Higher Education Press and Springer-Verlag Berlin Heidelberg 2010

1
00
Tao LI, Huiquan LI, Guotong QIN, Haitao LIU and Yixia PEI
vigorous stirring. The reactant solution was cooled to room to the temperature. When the temperature is above 95°C, the
temperature after a certain amount of time, then filtered to decreasing of MDC selectivity is caused by the augment in
separate solid from liquid. In the stability test of recycled byproducts because the higher temperature is favor for the
catalyst, the H SO solution was purified two times by active byproducts formation. This result is coincident with literatures
2
4
carbon. Then the loss of the H SO was balanced by adding [15–17] in different reaction system. As a conclusion, the
2
4
fresh H SO .
optimum temperature should be controlled at 95°C.
2
4
2
.3 Analytical methods
3.2 Effect of reaction time
The products were analyzed by high performance liquid The investigation of reaction time is illustrated in Figure 2. It
chromatography (HPLC, Agilent 1100 series) equipped is fully consistent with the expected results. The conversion of
with a UV detector and Zorbax Extented-C18. The mobile MPC only increases with the rising of reaction time and
phase of CH OH/H O = 55/45 (v/v) was used at a flow of reaches 100% after 3 h. The MDC yield first goes through a
3
2
–
1
1
.0 mL$min . The yield of MDC was calculated based on high value and then drops. The maximum (81.6%) is achieved
MPC.
at 3.5 h. The selectivity of MDC appears to be rather
Mass spectroscopy (MS) analysis was run on Waters complicated and varies between 75% and 81%. Fukuoka et
micromass ZQ-4000 (USA). An HPLC coupled with tandem al. [4] suggested that the conversion of MPC to MDC maybe
mass spectrometry (HPLC-MS/MS) was run on a Q-Trap LC- go through a certain intermediate before MDC formation. The
MS system with a turbo ion spray source and a Zorbax SB- detail about the intermediate is presented in the discussion part
C18 column (radius of 2.1 mm, length of 150 mm). of the reaction mechanism. According to our results, the
Parameters of electrospray ionization were: ion spray voltage optimum time should be controlled at 3.5 h.
of 5500 V, source temperature of 623 K, Cur Gas 25 V, Gas1
5
5 V, Gas2 45 V, DP 90 V, respectively.
3
Results and discussion
3
.1 Effect of reaction temperature
Figure 1 shows the relationship between the temperature and
the conversation, selection and yield. The conversion of MPC
increases with the rising of the temperature. Complete con-
version of MPC can be achieved at temperatures over 95°C.
The selectivity and yield of MDC increase while the tem-
perature rise. They all reach maximum values at 95°C, then
the selectivity and yield of MDC decrease with the further
rising of the temperature. At the lower temperature range, the
Figure 2 Effect of reaction time on condensation reaction
n(MPC)/n(trioxane) = 3, WH2SO4= 30%, T = 95°C).
(
rising of MDC yield indicates that MDC formation is sensitive 3.3 Effect of MPC to trioxane molar ratio
The effect of molar ratio of MPC to trioxane is also studied in
the range of 2.5 to 12. Results are shown in Figure 3. From
Figure 3, it can be found that the conversion of MPC
decreases with the rising of the molar ratio, whereas the
selectivity and yield of MDC passes a maximum of 79.4% and
7
8.5% at the ratio of 3. When the molar ratio is lower than 3,
polycondensations of the byproducts are easily formed
14,18], which attributes to the higher concentration of
[
HCHO decomposed by trioxane. When the ratio is more
than 3 (which means the concentration of trioxane is low in
reaction system), the yield and selectivity of MDC decrease
due to the lack of methylene agent. Therefore the optimal
molar ratio of MPC to trioxane is 3∶1.
Figure 1 Effect of temperature on condensation reaction
n(MPC)/n(trioxane) = 3, WH2SO4= 30%, t = 3.5 h).
(
Frontiers of Chemistry in China Vol. 5, No. 1, 2010

Catalytic synthesis of methylene diphenyl dicarbamate from methyl phenyl carbamate and trioxane over sulfuric acid catalyst
101
selectivity is steady on the whole. In a word, the H SO
2
4
catalyst shows a good stability without significant decrease in
the activity after recycling for five times.
Figure 3 Effect of the molar ratio of MPC to trioxane on
condensation reaction (WH2SO4= 30%, T = 95°C, t = 3.5 h).
3
.4 Effect of the H SO concentration
2
4
2 4
Figure 5 Effect of recycled H SO (n(MPC)/n(trioxane) = 3, T =
9
5°C, t = 3.5 h, WH2SO4= 30%).
Figure 4 shows the relationship between the results of reaction
and the concentration of H SO . The conversation of MPC
2
4
3
.6 Analysis on byproducts and mechanism
increases with the amount rising of H SO . The curve of
2
4
MDC-yield appears to be a reversed V-shape against the
Since the byproducts are complexes in the condensation of
MPC with trioxane, there is little work on the identification of
byproducts. In order to have an explicit understanding on the
reaction process, the byproduct identification is emphatically
studied by HPLC-MS/MS and the results are listed in Figure 6
and Figure 7. The liquid chromatogram (Figure 6) shows that
over six kinds of compounds have been detected in the
products. The MPC and MDC can be directly identified by
pure samples. Their corresponding times are 8.21 min (MPC)
and 30.32 min (MDC), respectively. The major byproduct is
detected at 19.79 min. The MS spectra are shown in Figure 7.
The high abundance is at m/z = 313.10 via the loss of H. It
indicates that the molecular weight is 314. Further reaction/
fragmentation of the ion at m/z = 281.09 is from (313.10 –
amount of H SO . The proper concentration of H SO is 30%.
2
4
2
4
In the condensation process of MPC and trioxane, H SO acts
2
4
as two roles: one is the catalysis for synthesis of MDC, the
+
other is contribution H to the decomposition of trioxane.
+
Hence the low amount of H is not enough to realize these
two roles at the same time. When the concentration of H SO
2
4
is over 30%, the byproducts are easily formed in the form of
polynuclear compounds and N-phenmethyl compounds. The
HPLC analysis results also conform that the amount of
byproducts increases as the H SO amounts increases. Similar
2
4
results are also confirmed by Ref. [19].
CH OH), the fragmentation at m/z = 249.08 (281.09 –
3
CH OH) appears in the spectrum. It means that the byproduct
3
is one of the isomers of MDC. It is well known that there are
three isomers of MDC: 4,4′-MDC, 2,4′-MDC, and 2,2′-MDC.
In our work, the MDC mentioned above is 4,4′-MDC, other
isomers are byproducts relative to it. But when the ion
+
+
H C OH attacks MPC to substitute H , the first location
2
2 4
Figure 4 Effect of concentration of H SO on condensation
reaction (n(MPC)/n(trioxane) = 3, T = 95°C, t = 3.5 h).
3
.5 Recycling of H SO
2
4
The effect of recycled H SO is studied under optimum
2
4
conditions. The results are shown in Figure 5. The conversa-
tion of MPC has a little fluctuation, but the MDC yield and
Figure 6 The liquid chromatogram of byproducts.
Frontiers of Chemistry in China Vol. 5, No. 1, 2010

1
02
Tao LI, Huiquan LI, Guotong QIN, Haitao LIU and Yixia PEI
the second step, HCHO is transformed into proton alcohol
Eq. (2)). Furthermore, the proton alcohol attacks the benzene
(
ring of MPC to form an intermediate (a) (Eq. (3)) at the third
step. Then intermediate (a) is transformed to an intermediate
(b) (Eq. (4)). Finally, the intermediate (b) combines with other
MPC to produce 4,4′-MDC or 2,4′-MDC (Eq. (5)).
Among the five steps, the first step has been studied a lot
and has been proven by Cui et al. [20] and Wang et al. [21].
This step is very important because it is relatively slow and
hence controls the rate of the condensation reaction. Besides,
MPC can be perfectly transformed into MDC under such
condition. The products are pulverous so that the reaction
materials can be mixed very well. As a result, these greatly
overcome the adhesive phenomenon and slack the byproducts
formation in whole process. Meanwhile, two kinds of
intermediates (a) and (b) are present in the third and forth
step. The formation of intermediate products requires a certain
amount of reaction time which is the main reason for the
minimum selectivity of MDC at 2 h in the effect of reaction
time (Figure 2). On the contrary, the phenomenon is a good
evidence that the third and forth steps are the only way for the
MDC synthesis. The mechanisms of the third and forth steps
are also supported by Hou et al. [22,23].
Figure 7 MS spectra of 2,4′-MDC.
+
+
is 4-H and the second is 2-H . What’s more, the group
“
–NHCOOCH ” is bulky, the steric hindrance is huge so that
3
2
,2′-MDC is difficult to form. Thus we can confirm that the
major byproduct at 19.79 min is 2,4′-MDC. As for other
byproducts, their total amount is very tiny so that the analysis
is difficult. Therefore, the detailed research about them isn’t
discussed in the section.
Based on our work, a possible mechanism is proposed as 4 Conclusions
follows. The process of products formation includes five
steps, as shown in Scheme 1. The first step is that trioxane is In this work, MDC was synthesized from MPC using trioxane
+
depolymerizated into HCHO in the present of H (Eq. (1)). In as methylene agent over H
SO catalyst. Since trioxane can be
4
2
Scheme 1 Proposed reaction mechanism in five steps.
Frontiers of Chemistry in China Vol. 5, No. 1, 2010

Catalytic synthesis of methylene diphenyl dicarbamate from methyl phenyl carbamate and trioxane over sulfuric acid catalyst
103
decomposed to HCHO and hence avoid the parcel of
superfluous HCHO, it shows kinds of merits in moderate
References
reaction velocity and great improvement of the MDC yield.
Under suitable reaction conditions, the yield of MDC reached
1
. Cai, Z.; Zhou, M.; Hu, W.; Yang, Z., Chin. J. Synth. Chem. 2002,
2, 164–166
8
1.6%. Moreover, H SO can be reused for five times while
2. Feng, G.; Zhang, H., Hebei Inst. Technol. 2001, 23, 65–67
2
4
no significant loss in catalytic activity was observed. The
major byproduct 2,4′-MDC was identified and a possible
reaction mechanism was proposed in the end.
3. Qian, B.; Zhu, J., Chem. Propellants Polym. Mater. 2006, 4, 16–
21
4. Fukuoka, S.; Chono, M.; Watanabe, T.; Kohno, M., US Patent,
4
547322, 1985-10-15
Acknowledgements This work was supported by the Ministry of
Science and Technology of China (Grant No. 2006BAC02A08) and
Knowledge Innovation Program of the Chinese Academy of
Sciences (Grant No. KGCX2-YW-215-2).
5. Li, S.; Wang, G., Chin. J. Synth. Chem. 2004, 2, 138–141
6
. Gao, J.; Li, H.; Zhang, Y.; Zhang, Y., Green Chem. 2007, 9, 572–
76
5
7
8
. Takeshita, N.; Takano, T., JP Patent, 01135758, 1989-05-12
. Takeshita, N.; Yoneda, T.; Takano, T., JP Patent, 01261358,
Huiquan LI is a professor at Institute of
Process Engineering, Chinese Academy of
Sciences. He was born in Liaoning Province
in 1970 and received his Ph.D. degree in
Chemical Engineering Department from
Dalian University of Technology in 1999. His
1
989-10-18
9
. Wada, H., JP Patent, 8157760, 1981-05-20
1
0. Wada, H., JP Patent, 8157761, 1981-05-20
1. Wada, H.; Kasori, Y., JP Patent, 57114563, 1982-07-16
2. Fukuoka, S.; Watanabe T., JP Patent, 5896054, 1983-06-7
1
1
research is focused on the field of green 13. Clerici, M. G.; Giuseppe, B.; Ugo, R., US Patent, 5241119,
chemical engineering and circular economy
1993-08-31
technology, including (1) green catalysis and 14. Li, S.; Yao, J.; Wang, Y.; Wang, G.; Li, S.; Xue, S., Natur. Gas
process intensification of CO2 capture, storage and utilization,
Chem. Ind. 2004, 29, 33–36
specially catalytic conversion; (2) high value-added utilization and 15. Pei, Y.; You, Z.; Li, H.; Guan, W.; Zhang, Y., Modern Chem. Ind.
pollution control of solid residua; (3) synthesis of chemical product
from biomass in ionic liquids; and (4) system integration and
ecological industry in chemical or metallurgical industry.
2007, 27, 357–360
1
6. Xie, K.; Xiang, S.; Han, F., Chem. Intermed. 2008, 7, 1–12
7. Zhao, X.; Hu, L.; Geng, Y.; Wang, Y., J. Mol. Catal. A 2007,
1
2
76, 168–173
Guotong QIN is an associate professor at School
of Chemistry and Environment, Beihang Uni-
versity. He was born in Hebei Province, China, in
1
1
8. Fukuoka, S.; Watanabe, T., US Patent, 4552974, 1985-11-12
9. Wang, K.; Yao, J.; Wang, Y.; Wang, G., Natur. Gas Chem. Ind.
2
006, 31, 19–21
0. Cui, S.; Lu, B.; Xiao, X.; Han, Y.; Cai, Q., Catal. Lett. 2007, 119,
77–282
1. Wang, K.; Yao, J.; Wang, Y.; Wang, G., J. Mol. Catal. 2007, 21,
24–228
1
968 and received his Ph.D. degree from School
2
2
of Chemical Engineering of Dalian University of
Technology in 1999. His research is focused on
porous materials, including (1) synthesis and
application of aerogels; (2) synthesis of low cost
2
2
inorganic membrane and membrane separation; 22. Hou, Z.; Okuhara, T., Appl. Catal. A Gen. 2001, 216, 147–155
and (3) hybrid membrane process.
23. Hou, Z.; Okuhara, T., J. Mol. Catal. A 2003, 206, 121–130
Frontiers of Chemistry in China Vol. 5, No. 1, 2010