Ionic liquid-mediated solvothermal synthesis of 4,4′-methylenediphenyl diisocyanate (MDI): An efficient and environment-friendly process

Article abstract of DOI:10.1039/c8nj01175c

4,4′-Methylenediphenyl diisocyanate (4,4′-MDI) is an immensely important intermediate employed in the manufacturing of polyurethanes. Many synthetic routes have been developed over the decades for the synthesis of 4,4′-MDI compounds on a large scale; these compounds are highly toxic and hazardous in nature. In this study, an environment-friendly route is proposed for the synthesis of 4,4′-MDI using 4,4′-diaminodiphenylmethane and dimethyl carbonate (DMC) as starting materials, which are nontoxic in nature. The synthesis of ionic liquids (ILs) and their utilization in the decomposition reaction are systematically investigated. Imidazole-functionalized ionic liquids were prepared for the synthesis of 4,4′-MDI, and their thermal performances were evaluated by TGA. We found that in comparison with other imidazole-functionalized ionic liquids, 1-ethoxycarbonylmethyl-3-methylimidazolium tetrafluoroborate ([EAmim]BF₄) exhibited preferable thermal activity for the decomposition of 4,4′-methylenediphenyl dimethylcarbamate (4,4′-MDC). Moreover, these ILs were more effective when they were combined with zinc as a catalyst, which enhanced the decomposition of MDC. Under optimal conditions, the yield of MDI compared to that of Zn(OAc)₂-[EAmim]BF₄ catalyst increased up to 96%. The mechanism of the enhanced performance of ionic liquids by catalytic activity of zinc acetate was also investigated.

Full text of DOI:10.1039/c8nj01175c

NJC
View Article Online
View Journal
PAPER
Ionic liquid-mediated solvothermal synthesis
of 4,40-methylenediphenyl diisocyanate (MDI):
Cite this:DOI: 10.1039/c8nj01175c
an efficient and environment-friendly process†
C. W. Duan, *a J. You,b B. Liu,b J. L. Ma,c H. P. Zhou,d H. B. Zhangd and
J. Zhang*e
4,40-Methylenediphenyl diisocyanate (4,40-MDI) is an immensely important intermediate employed in the
manufacturing of polyurethanes. Many synthetic routes have been developed over the decades for the
synthesis of 4,40-MDI compounds on a large scale; these compounds are highly toxic and hazardous
in nature. In this study, an environment-friendly route is proposed for the synthesis of 4,40-MDI using
4,40-diaminodiphenylmethane and dimethyl carbonate (DMC) as starting materials, which are nontoxic in
nature. The synthesis of ionic liquids (ILs) and their utilization in the decomposition reaction are systematically
investigated. Imidazole-functionalized ionic liquids were prepared for the synthesis of 4,40-MDI, and their
thermal performances were evaluated by TGA. We found that in comparison with other imidazole-
functionalized ionic liquids, 1-ethoxycarbonylmethyl-3-methylimidazolium tetrafluoroborate ([EAmim]BF4)
exhibited preferable thermal activity for the decomposition of 4,40-methylenediphenyl dimethylcarbamate
(4,40-MDC). Moreover, these ILs were more effective when they were combined with zinc as a catalyst, which
enhanced the decomposition of MDC. Under optimal conditions, the yield of MDI compared to that of
Zn(OAc)2–[EAmim]BF4 catalyst increased up to 96%. The mechanism of the enhanced performance of ionic
liquids by catalytic activity of zinc acetate was also investigated.
Received 13th March 2018,
Accepted 14th June 2018
DOI: 10.1039/c8nj01175c
rsc.li/njc
In a simple way, 4,40-MDI can be synthesized by the reaction
of phosgene with methylene dianiline in dichloromethane
Introduction
Diisocyanates such as toluene diisocyanate (TDI) and methylene- as a solvent (eqn (1)). However, this conventional method for
diphenyl diisocyanate (MDI) are considered important raw producing isocyanates is an extremely hazardous process
materials in the manufacture of polyurethanes, pesticides and because of the high toxicity of phosgene and corrosive nature
herbicides.1,2 4,40-MDI is one of the most important isocyanates of the co-product HCl.6–8 There have been persistent efforts
widely used in polyurethane production as it is a non-volatile for developing alternative processes avoiding phosgene such
material, which is safe for handling.3,4 Thus, MDI has emerged as as the reductive carbonylation of nitrobenzene,9–11 oxidative
an alternative to conventional pure TDI, and it has been developed carbonylation of aniline, and aminolysis of dimethyl carbonate
to produce elastomers and flexible polyurethane foams.5 (DMC).12–16
(1)
a Department of Environmental Science and Engineering, North China Electric Power University, Baoding, 071003, China. E-mail: duancw@ncepu.edu.cn;
Tel: +86 0312 7525228
b Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering,
Harbin University of Science and Technology, Harbin, 150001, China
c School of Information Science and Engineering, Hebei University of Science and Technology, Shijiazhuang, 050018, China
d College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao, 266590, China
e Department of Computed Tomography and Magnetic Resonance Imaging, Baoding First Center Hospital/First Aid Center, Baoding, 071003, China.
E-mail: Zhangji163361@163.com
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj01175c
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
In 1996, Tafesh demonstrated synthesis of 4,40-MDI utilizing which does not produce any toxic byproducts other than methanol
4,40-dinitrodiphenylmethane and carbon monoxide (CO) as and water. In the oxidative carbonylation process, the amino-
reagents and palladium metal as the catalyst (eqn (2)),9 but lysis of DMC can be completed under mild conditions, and
there are a few shortcomings of this method: it cannot be DMC can also be reproduced by the oxidative carbonylation of
applied at room temperature due to the low thermal energy methanol, which increases the overall efficiency of the process
associated with this process, which is not sufficient for this and lowers the cost of the production of DMC. This non-
reaction as it requires high activation energy. Even at elevated phosgene route uses nontoxic reagents and produces non-
temperatures of 160–210 1C, the required pressure for reductive polluting co-products and therefore, this process has been
carbonylation of dinitrodiphenylmethane is at least 29 MPa.10,11 explored to a great extent;18–20 the aminolysis of methylene
It is apparent from eqn (2) that only one-third of CO can be used diphenyl dicarbamate (MDC) is believed to be one of the most
effectively, and recycling of unreacted CO mixed with the efficient processes for thermal decomposition by the removal of
co-product CO2 can increase the production cost. Although the low-boiling alcohols.21–23 It was demonstrated by Chen et al.21
palladium metal exhibits excellent catalytic activity during the that the yield of MDI could be 52% with thermal decomposi-
synthesis of MDI, it is not cost-effective as the process for tion of 3% MDC in dibutyl phthalate at 260 1C. Guan reported
recycling the catalyst is complicated. Therefore, this process is that the yield could be increased to 67% in the presence
never adopted for the commercial production of MDI.
of ZnO/Zn catalyst.22 When Zn was used as the catalyst and
(2)
Another method developed by several groups for the synthesis of nitrobenzene–tetrahydrofuran as the solvent during thermal
MDI is the catalytic oxidative carbonylation of aniline with CO and decomposition, the yield of MDI increased to 87%. Although
O2 in the presence of an alcohol and appropriate catalyst, followed this method can reduce polymerization of MDI to give higher
by decomposition of the resultant carbamate, as delineated in yield, the removal of large amounts of solvents to concentrate
eqn (3) and (4).12–16 Kim demonstrated that by adding SeO2/K2CO3 the product may pose technical challenges and create hindrance
to the reaction system, 49% selectivity could be achieved for to the large-scale production of MDI. Therefore, our focus is to
carbamate.12 In the process, oxidative carbonylation of aniline was explore a better method for synthesizing MDI by the decomposi-
performed even at a high pressure of 6.8 MPa. It was reported later tion of MDC in low amount of solvents, which can improve the
by Chen et al.13 that the yield of carbamate could reach 43.5% when product concentration process.
(5)
(6)
(7)
the reaction was performed at a reduced pressure of 3.8 MPa over
Ionic liquids (ILs) have emerged as environment friendly media,
CuCl2–NaI during 1 h of batch reaction at 438 K. However, several and they can be used as alternatives to traditional volatile organic
problems were observed for the above-mentioned process such solvents due to their nonflammability, low volatility, low toxicity
as the hazards associated with reaction conditions and low and high thermal and chemical stabilities.24,25 Therefore, ILs are
selectivity of the intermediate for manufacturing MDI.
2RNH2 + 2CO + O2 + 2R0OH - 2RNHC(QO)OR0 + 2H2O
RNHC(QO)OR0 - RNCO + R0OH
widely used in various capacities: they are used as solvents, catalysts
in chemical reactions, and in separation and manufacturing
processes,26–30 especially as heat transfer fluids31–33 and in fuel
cells.34–36 In the past few years, a large number of ionic liquids
have been reported.37 The most common ones are imidazole-
(3)
(4)
Recently, a greener approach has been adopted for the synthesis based ionic liquids such as 1-butyl-3-methylimidazolium
of MDI, as shown in eqn (5)–(7).17 It has been documented that ([bmim]) and 1-allyl-3-methylimidazolium ([amim]) ferric chloride,
dimethyl carbonate (DMC) is an environmentally benign reagent, which are used for the study of thermo-chemical and other
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
thermodynamic properties.38–46 The decomposition temperatures were synthesized according to previously reported methods.56,57
of these ILs are in the range of 360 1C to 424 1C.43–46 Thus, there is 1-alkyl-3-methylimidazolium bromides ([Cnmim]Br) were pre-
growing interest regarding their application for the reactions that pared by the reaction of N-methylimidazole and bromobutane
require high temperatures. An example is imidazole-based ionic or N-methylimidazole and bromooctane. A white precipitate of
liquid, which is used as a thermal fluid in chemical reactions NaBr appeared immediately when [Cnmim]Br was added to
performed at high temperatures. It is important to find an ionic NaBF4 solution. The mixture was filtered after stirring for 4 h,
liquid with extended thermal stability because these reactions are and the filtrate was removed by vacuum distillation at 393 K after
usually carried out at elevated temperatures. Qu et al. reported the 4 h. Finally, the resultant neutral ionic liquid [Cnmim]BF4 was
syntheses and thermochemical properties of 1-butyl-3-methyl- dried in a vacuum dryer. The completion of anion exchange was
imidazolium tetrafluoroborate and 1-ethoxycarbonylmethyl-3- monitored with silver nitrate. Both the products were liquid
methylimidazolium tetrafluoroborate;47,48 the results demonstrated with yields of 90% and 92%. 1-Ethoxycarbonylmethyl-3-methyl-
that these ILs with BF4 anions possessed long-term stabilities and imidazolium tetrafluoroborate ([EAmim]BF4) was synthesized
excellent thermo-durabilities.47,48 These imidazole-functionalized following a two-step method reported by Qu et al.47,58 The first
ionic liquids were used for the synthesis of MDI; it is well-known step was nucleophilic substitution by N-methylimidazole on
that an organic cation, an inorganic anion and the length of the ethylchloroacetate to obtain a 1-ethoxycarbonylmethyl-3-methyl-
alkyl chain in the cation can influence its properties such as imidazole-containing halide salt ([EAmim]Cl). The second step
melting point, density, viscosity and thermostability.49–51 To involved sodium tetrafluoroborate-mediated anion exchange
study the decomposition of MDC, three kinds of imidazole- reaction with the intermediate chloride salt to produce the
functionalized ionic liquids were synthesized and investigated final product 1-ethoxycarbonylmethyl-3-methylimidazolium tetra-
systemically. Imidazole-functionalized ionic liquids are also used as fluoroborate ([EAmim]BF4). Anion exchange was monitored as
catalysts in many reactions. For example, Zhang et al.52 prepared an [Cnmim]BF4 was formed, and the yield of the product was 90%.
acid–base bifunctional ionic liquid, 1-(2-(1-piperidinyl)ethyl)-3- Handling of materials and tests were conducted in an argon-filled
methylimidazolium trichlorolead ([PEmim]PbCl3), and they used glove box to ensure accuracy of measurements for determining
it successfully as a catalyst for the synthesis of MPC. It was also the thermal behaviour and temperatures, which allowed correct
demonstrated by Wang53 that a mixture of [bmim]BF4 and tetra- and safe application. The argon-filled glove box was free of oxygen
methyl guanidine (TMG) can be used as a catalyst in the synthesis contamination and water vapor with levels controlled at less than
of MPC. For the synthesis of MDI from methylene diphenyl 5 and 2 ppm, respectively. More details about the synthesis and
dicarbamate, Zn(OAc)2 and Zn(OAc)2/acetate have been used as characterization of ionic liquids can be seen in the ESI.†
catalysts.22,54,55 The above-mentioned catalysts can be easily dis-
Reaction apparatus and operation
solved in the reaction medium, which accelerates the reaction rate
and improves the product yield. Ionic liquids can effectively assist The reaction of diaminodiphenylmethane and DMC was
zinc catalysts to improve the reaction rate, and they can be carried out in a 500 mL stainless steel autoclave fitted with a
successfully separated from the catalysts after completion of the magnetic stirrer bar. A typical operation for MDC synthesis is as
reaction; moreover, due to their low volatility, they are more stable follows: aniline (20 g), Zn(OAc)2 (0.7 g), and DMC (200 g) were
at higher temperatures. Three kinds of imidazole-functionalized charged and then stirred at 140–180 1C for 3 h. After the
ionic liquids were synthesized and employed for the decomposition reaction was complete, the intermediate was cooled to room
of 4,40-MDC. After complete decomposition of MDC, the recovery temperature, and the remaining DMC was recycled. MDI was
and reuse of catalysts and ILs were investigated systemically.
prepared by thermal decomposition of 4,40-MDC by adding
certain amounts of 4,40-MDC (5 g), 1-ethoxycarbonylmethyl-3-
methylimidazolium tetrafluoroborate (20 g) and catalysts such
as Zn(OAC)2, ZnO, zinc and phenylacetate in a three-neck
bottle. Before the decomposition process started, a continuous
flow of nitrogen gas at a rate of 10 mL minꢀ1 was purged into
the reaction system. The operational procedure included the
introduction of a heat carrier into the reactor to raise the
temperature, and the pressure was adjusted for the vacuum
system. The pressure was maintained at 0.5 kPa during the
whole decomposition process. As the temperature increased
rapidly from 25 to 230 1C, the product MDI was formed, and it
flowed into the receiving flask rapidly. The reaction was com-
pleted in 15 min, and the corresponding reactions are shown in
eqn (8) and (9). Finally, the product MDI was weighed and
Experimental section
Materials and reagents
N-Methylimidazole (99%) was obtained from Sinopharm Chemical
Reagent Co., Ltd. Bromobutane, bromooctane, ethylchloroacetate
and sodium tetrafluoroborate (NaBF4) were obtained from Tianjin
Kermel Chemical Reagent Co. Zn(OAc)2, ZnO, zinc phenylacetate
and silver nitrate were purchased from Tianjin Damao Chemical
Co. All other chemicals used in the experiment were obtained
commercially and used without further purification.
Preparation of ionic liquids
The purity of ionic liquids is very important, and it has a analyzed by IR and 1H-NMR. It is well-known that the –NCO
significant impact on thermal decomposition. Ionic liquids such group in MDI is highly moisture-sensitive and therefore, it had
as 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) to be stored in an argon-filled glove box before application and
and 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim]BF4) testing. After complete decomposition of MDC, the by-product
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
methanol was removed under reduced pressure to obtain the Results and discussion
product MDI in the receiving flask. The reaction mixture
containing Zn(OAc)2–[EAmim]BF4, methanol and [EAmim]BF4
Preparation of MDC
The reactions of diaminodiphenylmethane and DMC for the
synthesis of 4,40-MDC over various zinc catalysts were con-
ducted at different temperatures, and the results are shown
in Fig. 1. When no catalyst was added to the system, the yield of
4,40-MDC was low, around 54% at 145 1C, which indicated that
there was scope for improvement by employing a suitable
catalyst in terms of yield and reaction time. Among the catalysts
was filtered under vacuum and then, non-toxic dimethyl carbo-
nate having a small amount of activated charcoal was added to
the filtrate. Finally, the filtrate was distilled at 90 1C under
reduced pressure and subsequently dried in a vacuum dryer at
120 1C for 4 h to afford [EAmim]BF4. It should be noted that an
infrared vortex-evaporator connected to the PoleStar Coldtrap
System was used for drying the ionic liquids under vacuum.
(8)
(9)
Analysis and characterization of the product
mentioned in Fig. 1, Zn(OAc)2 exhibited excellent catalytic
activity during 4,40-MDC synthesis. In comparison with ZnO
and zinc, phenylacetate and Zn(OAc)2 produced 4,40-MDC in
88% yield at 145 1C. The catalytic activity of Zn(OAc)2 for
methoxycarbonylation of diaminodiphenylmethane with DMC
was investigated by Baba;59 Zn(OAc)2 was dissolved in DMC to
achieve a homogenous reaction system. The transformation of
Zn(OAc)2 catalyst in methoxycarbonylation was supported by
the formation of methyl acetate, which was presumably pro-
duced by the reaction of acetate anions coordinated to Zn2+
with DMC (or methanol formed by the decomposition of DMC).
The effect of various temperatures on the reaction of methoxy-
carbonylation for diaminodiphenylmethane was also studied
(Fig. 1). Fig. 1 shows that the conversion of diaminodiphenyl-
methane increased with the increase in temperature from
150 to 180 1C, whereas the yield of MDC increased below
160 1C and then decreased dramatically with further increase
Fourier transform infrared spectroscopy (FT-IR) was carried out on
a Nicolet 380 FT-IR spectrometer. The FT-IR spectra of reagents,
MDC and the ILs were recorded on a Vector 22 infrared
spectroscope at frequency ranges of 400–4000 and 400–2000 cmꢀ1
.
The intermediate MDC and product MDI were identified by
Mercury 300 nuclear magnetic resonance (1H-NMR) at 293 K in
CDCl3 solvent. The yields of MDC and MDI were calculated
based on stoichiometric MDC or MDI. The equation is as
follows:
Yield of MDC or MDI
moles of MDC or MDI formed
¼
ꢁ 100%
stoichiometric MDC or MDI
After the reaction of diaminodiphenylmethane and DMC was
complete, the reaction system was cooled to room tempera-
ture. The intermediate MDC, which was precipitated from the
solvent of DMC, was weighed using an electronic balance.
Based on the theoretical yield of methoxycarbonylation
between diaminodiphenylmethane and DMC as well as the
above-mentioned results, the yield of MDC was calculated.
The yield of MDI was calculated based on the same method.
The pressure was fixed at 0.5 kPa during the whole decom-
position process. As the temperature was increased rapidly
from room temperature to 190 1C, the product MDI started
forming and flowed into the receiving flask rapidly. After
complete decomposition, the product MDI was obtained in
the receiving flask, and it was washed with methanol and
weighed using an electronic balance. Finally, the yield of
MDC was calculated according to the theoretical yield of the
decomposition of MDC and the amount of MDI that was
formed.
Fig. 1 Influence of temperature and catalyst on the reaction for MDC
synthesis.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
Fig. 2 IR spectrum of MDC, which was synthesized at 155 1C with Zn(OAc)2 catalyst.
in the temperature. For MDC, the maximum yield was 94% at liquids were systemically investigated by TGA. As shown in
155 1C. This observation corroborated the findings of Guan, Fig. 4, the TGA curve of [C4mim]BF4 declined from 368 1C (this
who mentioned that higher temperature did not facilitate the value was obtained from TGA as identified by Tdecomp; Tdecomp was
production of MDC.22 The decrease in the yield of MDC above calculated using a thermal analysis software, which provided the
160 1C was due to the effect of higher temperature, which translation of the intercept of two linear functions: the baseline
favored the production of a trimer or tetramer of MDC.
of zero weight loss and tangent of weight vs. temperature upon
A freshly prepared MDC sample was analyzed by FT-IR and decomposition), indicating that these ILs were suitable for
1H-NMR, and the results are shown in Fig. 2 and 3, respectively. The MDC decomposition. These results agreed with the results
spectral data agreed with the values of the standard, indicating that provided by Erdmenger, who proved that conventional ionic
the intermediate was MDC. In the IR spectrum, the peaks centered liquids such as [C4mim]BF4 start decomposing at a temperature
at 3330 cmꢀ1 and 2950 cmꢀ1 were assigned to the stretching of about 380 1C.60–62 However, it should be noted that the
vibrations of N–H and C–H bonds, respectively. The peaks centered actual degradation starts before at a lower temperature than
63
at 1710, 1240 and 1070 cmꢀ1 were assigned to the stretching Tdecomp
.
Valkenburg demonstrated that [C2mim]BF4 has a
vibration of CQO bond in the ester group. The peaks at around Tdecomp value of 455 1C, but at least 1.37 wt% is decomposed
1600, 1540, 1450 and o1000 cmꢀ1 corresponded to the benzene at 200 1C.46 Hence, the maximum operating temperature was
1
ring. The H-NMR spectrum of MDC was consistent with a strong set considerably lower than Tdecomp. It can also be seen in Fig. 4
singlet peak centered at around 3.78 ppm, corresponding to 3H in that the Tdecomp value of [C8mim]BF4 was 322 1C, which was
OCH3 group. Fig. 3 also shows that nine moderate peaks were slightly lower than the value for [C4mim]BF4. It is known that
observed around 7.07–7.40 ppm in the spectrum, which corre- the length of the alkyl chain of the cation affects the thermal
sponded to 5H in the benzene ring. The moderate broad singlet stability of ionic liquids. Kosmulski64 demonstrated that the
peak at around 6.66 ppm corresponded to 1H in NH group. The thermal stabilities of ionic liquids decrease with the increase in
peaks at around 3.89 ppm and 3.79 ppm were assigned to Hs in the length of alkyl chain, especially for imidazolium ionic
–CH2 and –OCH3, respectively. Based on the above-mentioned liquids with basic [Cl] and coordinating [BF4] and [PF6] anions.
analysis, it was concluded that pure MDC was formed with Thus, [C8mim]BF4 was found to have lower thermal stability than
molecular formula of C17H18O4N2. Therefore, this result can be [C4mim]BF4.60,65 In comparison with the decomposition tempera-
used as an indicator for the completion of methoxycarbonylation. ture of [Cnmim]BF4, it was worth noting that the decomposition
temperature of 1-ethoxycarbonylmethyl-3-methylimidazolium
Synthesis of ionic liquids used for decomposition of MDC
tetrafluoroborate occurred at 425 1C, which was higher than
To study the effect on the decomposition of MDC, the thermo- the decomposition temperature of [Cnmim]BF4. Due to over-
dynamic activities of different imidazole-functionalized ionic estimation and limitations posed by the non-isothermal TGA
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
Fig. 3 1H-NMR of MDC, which was synthesized at 155 1C with Zn(OAc)2 catalyst.
Based on the above-mentioned analysis, 1-ethoxycarbonyl-
methyl-3-methylimidazolium tetrafluoroborate ([EAmim]BF4)
was selected and applied for the decomposition of MDC for
synthesizing MDI.
Fig. 5 shows the FT-IR spectra of 1-ethoxycarbonylmethyl-3-
methylimi-dazolium tetrafluoroborate ([EAmim]BF4) in the range of
4000–400 cmꢀ1. The bands at 1440 cmꢀ1 and 1580 cmꢀ1 were
assigned to hydrogen-bonded imidazole. The characteristic
peaks centered at 3127 and 3166 cmꢀ1 were assigned to the
stretching vibration of C–H bond of sp2-hybridized carbon. The
band at 1751 cmꢀ1 was due to the presence of carbonyl group.
The peaks centered at 1440 and 1377 cmꢀ1 were assigned
to C–H bending in CH3 group. The characteristic peak
of BF4 appeared at 1054 cmꢀ1. To determine the structure of
1-ethoxycarbonylmethyl-3-methylimidazolium tetrafluoroborate
accurately, [EAmim]BF4 was analysed using 1H-NMR, and the
results are presented in Fig. 6. 1H-NMR analysis was performed
after drying the sample under high vacuum for a long time. The
1H-NMR (CDCl3, 300 MHz) data of d = 9.03 (s, 1H), 7.69 (t, 2H),
5.21 (s, 2H), 4.21 (m, 2H), 3.96 (s, 3H), 2.49 (t, 1H) and 1.23
(m, 3H) corresponded to the hydrogen atoms marked as 1, 2, 3,
4, 5, 6 and 7 in Scheme 1, which inserts in Fig. 6.
Fig. 4 TGA curves of various ionic liquids between 30 1C and 650 1C.
method, it was necessary to perform long-term isothermal (static)
TGA measurements for a prolonged period of time; moreover,
compared with normal TGA, it could provide more information
on stability at higher temperatures. Isothermal analyses showed
that in 120 min, just 1% was lost at 230 1C (Fig. S1, ESI†). As the
temperature was raised to 430 1C, the weight loss of 1-ethoxy-
Decomposition of MDC
carbonylmethyl-3-methylimidazolium tetrafluoroborate was only Chen found that by utilizing ultrafine ZnO as a catalyst, the
4.8% after heating for the same time. This suggested that yield of MDI can be increased to 52% at 260 1C.21 Wang et al.
1-ethoxycarbonylmethyl-3-methylimidazolium tetrafluoroborate, reported a method that gave high yield of MDI at 300 1C
which showed excellent thermostability at higher temperatures, can without adding any solvent.66 It was clear that the solvent
be used as the solvent in the applications of MDC decomposition. method can reduce side reactions such as polymerization to
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
Fig. 5 FT-IR spectra of 1-ethoxycarbonylmethyl-3-methylimidazolium tetrafluoroborate in the range of 4000–400 cmꢀ1
.
Fig. 6 1H-NMR spectra of 1-ethoxycarbonylmethyl-3-methylimidazolium tetrafluoroborate.
give high yield. In addition, it was reported by Chen et al.21,67 due to its thermosensitivity. Based on the analysis, the opti-
that high temperature was favorable for polymerization of MDI mum temperature for decomposition was found to be 230 1C.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
Table 1 Influence of different catalysts on decomposition of MDC at
230 1C in liquid paraffin and [EAmim]BF4 solvents
hydrolyzed during the process of analysis. Moreover, the absorp-
tion peaks that appeared at 1721–1725 cmꢀ1, 1640–1646 cmꢀ1
and 1533–1545 cmꢀ1 corresponded to CQO bending, CQN
stretching, and C–H bending in Ar–H, indicating the formation
of MDI.
To establish the structure and purity of the product conclu-
sively, 1H-NMR spectra (Fig. 8) were obtained, which confirmed
the structure of MDI. As expected, –CH2 protons appeared
at 3.91 ppm, and the peaks that appeared between 7.11 and
6.99 ppm corresponded to the C–H protons of the benzene ring,
which was consistent with MDI structures. It is known that
Temperature (1C) Cat
Solvent
Yield of MDI (%)
230
230
230
230
230
230
230
230
—
Liquid paraffin 37
Liquid paraffin 88
Zn(OAc)2
Zinc phenylacetate Liquid paraffin 58
ZnO
—
Zinc phenylacetate [EAmim]BF4
ZnO
Liquid paraffin 43
[EAmim]BF4
37
69
52
96
[EAmim]BF4
[EAmim]BF4
Zn(OAc)2
To obtain high yield of MDI, various zinc-related catalysts were the solvent peak from CDCl3 appears at 7.26 ppm, and the low-
tested. The activities of various catalysts for the decomposition intensity peak around at 3.76 ppm is associated with the
of MDC are shown in Table 1. As shown in Table 1, liquid residual methanol. Based on IR and 1H-NMR analyses, the final
paraffin and [EAmim]BF4 without the addition of any catalyst product was identified as MDI, which was the result of the
exhibited very low activity for MDI synthesis, and the yield decomposition of MDC.
obtained was only 37%. It is believed that the behavior of
[EAmim]BF4 is similar to that of a normal solvent, and it was
Catalytic activity and mechanism of Zn(OAc)2–[EAmim]BF4
only used as the thermal medium in the decomposition. After Based on the above-mentioned analysis, it was apparent that
loading the zinc species as catalysts at 230 1C for 15 min, the there were differences in the yields of MDI when Zn(OAc)2 was
catalytic activity increased sharply, and the yield of MDI added to liquid paraffin or [EAmim]BF4 separately. Zn(OAc)2
enhanced several fold. Among these above-mentioned catalysts, with water-soluble ionic liquids such as [EAmim]BF4 showed
Zn(OAc)2 produced better MDI yield than other zinc catalysts. excellent catalytic activity. It was concluded that the integration
The yields of MDI reached the highest values of 88% and 96% of Zn(OAc)2 and [EAmim]BF4 occurred during the decomposition,
in the presence of Zn(OAc)2 in the solvents liquid paraffin and which facilitated the formation of MDI. To study the catalytic
[EAmim]BF4, respectively.
performance of Zn(OAc)2–[EAmim]BF4 more systematically, effi-
MDI prepared by this process was characterized by both FT-IR cacy tests of the reaction regarding temperature and amount of
and 1H-NMR spectroscopies. In the FT-IR spectra of MDI (Fig. 7), the the catalyst were conducted, and the results are listed in Tables 2
absence of absorption bands at about 2290–2250 cmꢀ1 (identified by and 3. Table 2 shows the effect of reaction temperature on the
NCO groups) confirmed the existence of MDI. It was also observed catalytic performance of Zn(OAc)2–[EAmim]BF4. It was clear that
that narrow bands that appeared at 3290–3320 cmꢀ1 were due to the the yield of MDI decreased with the decrease in reaction tempera-
presence of N–H stretching vibration. It appeared that MDI was ture from 230 to 190 1C. When the temperature was maintained at
Fig. 7 IR spectrum of MDI that was synthesized at 230 1C with Zn(OAc)2–[EAmim]BF4 catalyst.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
Fig. 8 1H-NMR of MDI, which was synthesized at 230 1C with Zn(OAc)2–[EAmim]BF4 catalyst.
Table 2 The effect of reaction temperature on the catalytic performance sufficient for the reaction to cross the barrier of activation energy.
Thus, the maximum yield of 4,40-MDI was found to be 96% at a
temperature of 230 1C. The effect of the amount of the catalyst
of Zn(OAc)2–[EAmim]BF4 utilized in the decomposition of MDC in
[EAmim]BF4
on the synthesis of MDI was also studied (Table 3). It is shown
in Table 3 that the yield of MDI increased with the increase in
weight percentage of Zn(OAc)2–[EAmim]BF4, and a maximum
yield of 96% was achieved by employing 5 wt% (Zn(OAc)2–
[EAmim]BF4/[EAmim]BF4). Further increase in the amount of
the catalyst eventually decreased the yield of MDI to 82%. Based
on this observation, it was concluded that the optimum catalyst
amount for the decomposition of MDC was 5 wt%.
The catalytic mechanism of Zn(OAc)2–[EAmim]BF4 was
investigated for the decomposition of MDC, and the result is
shown in Fig. 9. Schreiner68 investigated the effect of ionic
liquids on the Diels–Alder reaction between cyclopentadiene
and methacrylate, and it was found that a hydrogen bond can
be formed between the carbonyl oxygen of methacrylate and
C–H–C on the imidazolium ring of the ionic liquids, which is a
crucial factor for facilitating the reaction. The strongest hydro-
Temperature (1C)
Cat
Solvent
Yield of MDI (%)
190
190
210
230
—
[EAmim]BF4
[EAmim]BF4
[EAmim]BF4
[EAmim]BF4
—
43
74
96
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
Table 3 The effect of catalyst amount on the catalytic performance of
Zn(OAc)2–[EAmim]BF4 utilized in the decomposition of MDC at 230 1C in
solvent of [EAmim]BF4
Temperature (1C) Cat amount (%) Solvent
Yield of MDI (%)
230
230
230
230
—
1
5
[EAmim]BF4 37
[EAmim]BF4 78
[EAmim]BF4 96
[EAmim]BF4 82
10
210 1C, the yield of MDI reduced by 22%. As the temperature gen bond is observed in ionic liquids that have the strongest
further declined from 210 to 190 1C, the yield was merely 43%. It hydrogen-donating cations coupled with the weakest hydrogen-
is well-known that temperature plays a significant role in the accepting anions.69 Ionic liquids with [mim] cations can form
ꢀ
decomposition reaction.22 It was concluded that the activity of strong hydrogen bonds. In addition, the BF4 anions are
Zn(OAc)2–[EAmim]BF4 decreased dramatically with further decrease weaker than hydrogen-bond acceptors.24 Thus, the hydrogen
in the temperature. It was observed that without the addition of the bond between C–H–C on an imidazolium ring of ILs and the
catalyst to the reaction system, the product MDI could not be formed carbonyl oxygen atom of zinc acetate is strong. Thus, strong
at 190 1C. It was apparent that thermal energy alone was not hydrogen bonding between ILs and zinc acetate was formed,
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
Fig. 9 Mechanism for MDI synthesis via decomposition of MDC catalyzed by Zn(OAc)2–[EAmim]BF4.
and zinc acetate changed from a bidentate to a unidentate
ligand. Simultaneously, this change promoted the coordination
of –OCH3 in the MDC molecule with zinc atom in the zinc
acetate molecule, facilitating the abstraction of a hydrogen
atom from –NH and methoxy group, which resulted in the
formation of MDI (Fig. 9). Meanwhile, the leaving OCH3 group
was protonated before leaving the MDC molecule to form a
methanol molecule. Eventually, C–H–C on the imidazolium
ring of IL combined with the carbonyl oxygen atom of zinc
acetate to form a new hydrogen bond.
Based on the FT-IR analysis of the product obtained from
the reaction between aniline and DMC catalyzed by anhydrous
zinc acetate, it was found by Zhao70 that anhydrous zinc acetate
changed from a bidentate to a monodentate ligand during the
process of complexation. Similar results were also proposed by
Guo,71 who found that the transformation of zinc carboxylate
from a bidentate ligand to a monodentate ligand occurred
during methoxycarbonylation of MDA with DMC. To determine
Fig. 10 IR spectra of (a) zinc acetate and (b) Zn(OAc)2–[EAmim]BF4,
Zn(OAc)2–[EAmim]BF4 were obtained after filtration of
a mixture of
[EAmim]BF4, which was heated at 230 1C for 15 min.
whether ionic liquids could alter the structure of zinc acetate, was caused by a bidentate coordination structure. In the case of
anhydrous zinc acetate and Zn(OAc)2–[EAmim]BF4 (Zn(OAc)2– Zn(OAc)2–[EAmim]BF4, it was found from Fig. 10b that the
[EAmim]BF4) were collected after filtration from the mixture above-mentioned two bands appeared at 1396 and 1572 cmꢀ1
,
.
of [EAmim]BF4 and Zn(OAc)2, which was heated at 230 1C for and the gap between the two bands widened from 96 to 176 cmꢀ1
15 min, and these samples were analyzed by IR separately, as These results agreed with the results reported by Zhao et al.70 Thus,
shown in Fig. 10. It can be seen from Fig. 10 that the bands at it was inferred that the structure of anhydrous zinc acetate had
1452 and 1548 cmꢀ1 corresponded to the asymmetric stretch been altered from bidentate to monodentate coordination struc-
peak and symmetric peak of the carbonyl group in a zinc ture, and Zn(OAc)2–[EAmim]BF4 complex was formed during the
acetate molecule. The space of 96 cmꢀ1 between the two bands decomposition of MDC.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
Fig. 11 IR spectra of the recovered [EAmim]BF4; the inset is the picture of [EAmim]BF4 before and after purification.
Recycling of the catalyst and ILs
Table 4 The catalytic performance of recovered Zn(OAc)2–[EAmim]BF4
for the decomposition of MDC at 230 1C in [EAmim]BF4 solvent
It is a fact that ionic liquids as solvents darken quickly when
heated to high temperatures, which can be regenerated with
additional purification processes. After complete decomposition
of the reagent MDC, a large amount of methanol was produced
Temperature (1C) Recovery times Solvent
Yield of MDI (%)
230
230
1
2
3
4
5
[EAmim]BF4 96
[EAmim]BF4 94.5
[EAmim]BF4 93
[EAmim]BF4 92.6
[EAmim]BF4 92.4
as a byproduct, and it was removed under reduced pressure to 230
230
230
afford the product MDI in excellent yield. It was demonstrated by
Zhang72 that extractive distillation is an efficient method for the
separation of methanol, dimethyl carbonate and imidazole-
functionalized ionic liquids from the reaction mixture. To isolate
the catalyst and residual methanol in [EAmim]BF4, the reaction
system containing Zn(OAc)2–[EAmim]BF4 and [EAmim]BF4 was
filtered, and the filtrate was added to nontoxic dimethyl carbo-
nate having a small amount of activated charcoal. The mixtures
were distilled at 90 1C under reduced pressure and subsequently
dried in a vacuum drier at 120 1C for 4 h to afford [EAmim]BF4.
[EAmim]BF4 was transformed into a pale-yellow liquid, and it
was similar to crude [EAmim]BF4 obtained after re-purification,
as shown in the inset of Fig. 11. By this process, the recovery of
in the synthesis of MPC could be recycled up to 5 times and even
then, the catalytic performance was excellent. Based on the analysis
shown in Table 4, no significant loss in the catalytic activity of
Zn(OAc)2–[EAmim]BF4 was observed during the decomposition
process. Therefore, it was considered that the recovered
Zn(OAc)2–[EAmim]BF4 can be recycled at least thrice.
Conclusions
[EAmim]BF4 was 86%. The recovered [EAmim]BF4 sample was Conventional methods for synthesizing MDI involve the application
also characterized by IR, and the result from Fig. 11 shows that of unsuitable organic solvents. Thus, 4,40-methylenediphenyl
the structure of the recovered [EAmim]BF4 had not changed diisocyanate (MDI) was synthesized by the reaction of
(the corresponding characterization of the recovered ionic liquids is 4,40-diaminodiphenylmethane and non-toxic dimethyl carbonate
given in ESI†). To study the purity of recycled [EAmim]BF4, 1H-NMR (DMC) and subsequently, the decomposition of MDC was com-
analysis was conducted, and the results are presented in Fig. S2 pleted in ionic liquids. It was found that at a fixed temperature of
(ESI†). Based on IR and 1H-NMR analyses, the recovered ionic 230 1C, the highest yield of 96% of MDI was obtained in the
liquids were identified as [EAmim]BF4 with a higher purity of 99%. presence of Zn(OAc)2 as a catalyst and 1-ethoxycarbonylmethyl-3-
The catalytic performance of the recycled Zn(OAc)2–[EAmim]BF4 methylimidazolium tetrafluoroborate as a solvent. Compared with
sample for the decomposition of MDC was also studied. Results in other catalysts, Zn(OAc)2–[EAmim]BF4 showed excellent catalytic
Table 4 indicate that the yield of MDI decreased slightly with the performance for the decomposition of MDC, and its catalytic
increase in the number of recycling processes. After five purifica- mechanism was investigated systemically. After complete decompo-
tions, the yield of MDI was 92.4%. This result corroborated the sition, a maximum recovery of 86% was achieved for [EAmim]BF4
observation of Zhao,70 who found that Zn(OAc)2–[bmim]PF6 utilized through re-purification. The catalyst Zn(OAc)2–[EAmim]BF4 retained
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.

View Article Online
Paper
NJC
22 X. Guan, H. Q. Li and H. T. Liu, J. Beijing Univ. Chem.
Technol., 2009, 4, 12.
23 X. Zhao, Y. Wang and S. Wang, Ind. Eng. Chem. Res., 2002,
41, 5139.
its activity even after recycling five times, and the yield of MDI
achieved was 92.4%. Thus, a greener method has been developed,
which can be useful for large-scale production of MDI.
24 M. Ishikawa, T. Sugimoto and M. Kikuta, J. Power Sources,
2006, 162, 658.
Conflicts of interest
25 F. Ouyang, Z. Z. Wang, Y. Zhou, Z. Cheng, Z. H. Lu, Z. Yang
and J. T. Duan, Appl. Catal., A, 2015, 492, 177.
26 F. F. Chen, K. Huang, Y. Zhou, Z. Q. Tian, X. Zhu, D. J. Tao
and S. Dai, Angew. Chem., Int. Ed., 2016, 55, 7166.
27 W. Huang, D. J. Tao, F. F. Chen, W. Hui, J. Zhu and Y. Zhou,
Catal. Lett., 2017, 147, 1347.
28 C. W. Duan, L. X. Hu and J. L. Ma, J. Mater. Chem. A, 2018,
6, 6309.
29 K. Yin, D. K. Chu, X. R. Dong, C. Wang, J. A. Duan and J. He,
Nanoscale, 2017, 9, 14229.
There are no conflicts to declare.
Acknowledgements
The present work was supported financially by the Natural Science
Foundation of Hebei Province (Grant E2018502054). This work was
also supported financially by the Fundamental Research Funds for
the Central Universities (Grant 2017MS141), the National Nature
Science Foundation of China (Grant 21506043), and the National
Major Science and Technology Program for Water Pollution
Control and Treatment (Grant 2017ZX07101-001-007).
30 R. L. Souza, S. P. M. Ventura, C. M. F. Soares, J. A. P.
´
Coutinhob and A. S. Lima, Green Chem., 2015, 17, 3026.
31 Z. Liu, K. Cheng, H. Li, G. Cao, D. Wu and Y. Shi, Environ.
Sci. Pollut. Res., 2018, 25, 3510.
32 F. Liu, Y. D. Deng, X. P. Han, W. W. Hu and C. Zhong,
J. Alloys Compd., 2016, 654, 163.
33 S. N. Shah, L. K. Chellappan, G. Gonfa, M. I. A. Mutalib,
R. B. M. Pilus and M. A. Bustam, Chem. Eng. J., 2016, 284, 487.
Notes and references
1 P. T. Anastas and M. M. Kirchhoff, Acc. Chem. Res., 2002,
35, 686.
´
2 M. Ramırez, J. Vargas and M. Springborg, J. Phys. Chem. A,
´
34 M. Dıaz, A. Ortiz and I. Ortiz, J. Membr. Sci., 2014, 469, 379.
2016, 120, 4256.
35 R. S. Malik, P. Verma and V. Choudhary, Electrochim. Acta,
2015, 152, 352.
3 A. Blencowe, A. Clarke and M. G. B. Drew, React. Funct.
Polym., 2006, 66, 1284.
36 Y. Li, Y. Jiang, Z. Ruan, K. Lin, Z. Yu, Z. Zheng and Y. Yuan,
J. Mater. Chem. A, 2017, 5, 21300.
4 Y. Ono, Appl. Catal., A, 1997, 155, 133.
5 G. Sabbioni, L. R. Vanimireddy and Z. L. Lummus, Arch.
Toxicol., 2017, 91, 1809.
6 H. Babad and A. G. Zeiler, Chem. Rev., 1973, 73, 75.
7 H. Eckert and B. Forster, Angew. Chem., Int. Ed. Engl., 1987,
26, 894.
8 L. Cotarca, P. Delogu and A. Nardelli, Synthesis, 1996, 553.
9 A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035.
10 S. M. Islam, D. Mal and B. K. Palit, J. Mol. Catal. A: Chem.,
1999, 142, 169.
11 P. Toochinda and S. S. C. Chuang, Ind. Eng. Chem. Res.,
2004, 43, 1192.
12 H. S. Kim, Y. J. Kim and H. Lee, J. Catal., 1999, 184, 526.
13 B. Chen and S. S. C. Chuang, J. Mol. Catal. A: Chem., 2003,
195, 37.
14 G. Sabbioni, N. Dongari and A. Kumar, Toxicol. Lett., 2016,
260, 46.
37 P. C. Marr and A. C. Marr, Green Chem., 2016, 18, 105.
38 M. Sattari, F. Gharagheizi, P. Ilani-Kashkouli, A. H. Mohammadi
and D. Ramjugernath, J. Therm. Anal. Calorim., 2014, 115, 1863.
39 W. Guan, L. Li, H. Wang, J. Tong and J. Z. Yang, J. Therm.
Anal. Calorim., 2008, 94, 507.
40 V. Kamavaram and R. G. Reddy, Int. J. Therm. Sci., 2008,
47, 773.
41 Y. Hao, J. Peng, S. W. Hu, J. Li and M. L. Zhai, Thermochim.
Acta, 2010, 501, 78.
42 A. Akc, V. Balci and A. Uzun, Thermochim. Acta, 2014,
589, 131.
43 C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki
and J. F. Brennecke, J. Chem. Eng. Data, 2004, 49, 954.
44 F. Wendler, L.-N. Todi and F. Meister, Thermochim. Acta,
2012, 528, 76.
45 C. K. Maaike, B. Wim, J. P. Cor and W. Geert-Jan, Thermochim.
Acta, 2007, 465, 40.
´
´
15 A. Orejon, A. M. Masdeu-Bulto and P. Salagre, Ind. Eng.
Chem. Res., 2008, 47, 8032.
46 M. E. Van Valkenburg, R. L. Vaughn, M. Williams and
J. S. Wilkes, Thermochim. Acta, 2005, 425, 181.
47 M. Qu, Y. Yao, J. He, X. Ma, S. Liu, J. Feng and L. Hou,
Tribol. Int., 2016, 104, 166.
48 W. Q. Feng, Y. H. Lu, Y. Chen, Y. W. Lu and T. Yang,
J. Therm. Anal. Calorim., 2016, 125, 143.
49 E. A. Turner, C. C. Pye and R. D. Singer, J. Phys. Chem. A,
2003, 107, 2277.
50 P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000,
39, 3772.
16 F. Ragaini, Dalton Trans., 2009, 6251.
17 G. Lewandowski and E. Milchert, J. Hazard. Mater., 2005,
119, 19.
18 A. A. Kelkar, D. S. Kolhe and S. Kanagasabapathy, Ind. Eng.
Chem. Res., 1992, 31, 172.
19 P. Giannoccaro, I. Tommasi and M. Aresta, J. Org. Chem.,
1994, 476, 13.
20 J. Gao, H. Li and Y. Zhang, Green Chem., 2007, 9, 572.
21 D. Chen, L. M. Liu and Y. Wang, Chin. J. Catal., 2005,
26, 987.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018

View Article Online
NJC
Paper
51 T. Sima, S. Guo, F. Shi and Y. Deng, Tetrahedron Lett., 2002, 62 T. Erdmenger, J. Vitz, F. Wiesbrock and U. S. Schubert,
43, 8145. J. Mater. Chem., 2008, 18, 5267.
52 L. Zhang, Y. Yang, Y. Xue, X. Fu, Y. An and G. Gao, Catal. 63 J. M. Crosthwaite, M. J. Muldoon, J. K. Dixon, J. L. Anderson
Today, 2010, 158, 279. and J. F. Brennecke, J. Chem. Thermodyn., 2005, 37, 559.
53 N. Wang, Y. Geng, X. Zhao, H. An and Y. Wang, Shiyou 64 M. Kosmulski, J. Gustafsson and J. B. Rosenholm, Thermochim.
Huagong, 2008, 37, 1255. Acta, 2004, 412, 47.
54 T. Baba, A. Kobayashi and T. Yamauchi, Catal. Lett., 2002, 65 H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and
82, 193. M. Watanabe, J. Phys. Chem. B, 2005, 109, 6103.
55 F. Li, W. B. Li, J. Li, W. Xue, Y. J. Wang and X. Q. Zhao, Appl. 66 Q. Wang, W. Kang and Y. Zhang, Chin. J. Catal., 2013,
Catal., A, 2014, 475, 355.
34, 548.
56 J. Zhu, L. Bai and B. Chen, Chem. Eng. J., 2009, 147, 58.
57 T. Joseph, S. Sahoo and S. B. Halligudi, J. Mol. Catal. A:
Chem., 2005, 234, 107.
67 Q. H. Zhang, H. Q. Li, H. T. Liu and Y. X. Pei, Chem. J. Chin.
Univ., 2011, 32, 1106.
68 P. R. Schreiner and A. Wittkopp, Org. Lett., 2002, 4, 217.
58 C. Li, Z. Li, A. Wang, J. Yin, J. Wang, H. Li and Q. Liu, RSC 69 A. Aggarwal, N. L. Lancaster and A. R. Sethi, Green Chem.,
Adv., 2013, 3, 6356. 2002, 4, 517.
59 T. Baba, A. Kobayashi and Y. Kawanami, Green Chem., 2005, 70 X. Zhao, L. Kang, N. Wang, H. An, F. Li and Y. Wang, Ind.
7, 159. Eng. Chem. Res., 2012, 51, 11335.
60 J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. 71 X. Guo, Z. Qin, W. Fan, G. Wang, R. Zhao, S. Peng and
Willauer, G. A. Broker and R. D. Rogers, Green Chem.,
2001, 3, 156.
61 J. D. Holbrey and K. R. Seddon, Dalton Trans., 1999, 2133.
J. Wang, Catal. Lett., 2009, 128, 405.
72 Z. Zhang, H. Xu, Q. Zhang, A. Zhang, Y. Li and W. Li, Fluid
Phase Equilibr., 2017, 435, 98.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018
New J. Chem.